By K. Carlos. State University of New York Institute of Technology at Delhi. 2019.
The utility of the Wisconsin Card in gene expression for NMDA receptor subunits in prefrontal Sorting Test in detecting and localizing frontal lobe lesions . Rules for neuropsychological diagnosis: receptors and expression of N-methyl-D-aspartate receptor sub- classification of brain function in older children . J Consult Clin units in subregions of human hippocampus: effects of schizo- Psychol 1979;47:258–264 . Effects of prenatal protein procedural learning and problem solving in schizophrenic pa- malnutrition on mossy fibers of the hippocampal formation in tients by Tower of Hanoi type tasks . Defective corticogenesis prefrontal cortex subregions in strategy switching . BehavNeu- and reduction in Reelin immunoreactivity in cortex and hippo- rosci 1999;113:32–41 . Psychiatry Res 1992;44: nol acetate-induced abnormalities in the entorhinal cortex of 141–151 . Schizophrenia is associated stantia innominata on attentional function in the rat: further with elevated amphetamine-induced synaptic dopamine con- implications for the role of the cholinergic neurons of the nu- centrations: evidence from a novel positron emission tomogra- cleus basalis in cognitive processes . The use of psychoactive drugs as a neuro- zation in the pathophysiology of schizophrenia: deficits and dys- psychological tool in studies of attention in man . In: Uhr I , function in neuronal regulation and plasticity. Gating and habituation of the 700 Neuropsychopharmacology: The Fifth Generation of Progress startle reflex in schizophrenic patients. Arch Gen Psychiatry N-methyl-D-aspartate antagonist-induced psychosis: a study 1992;49:206–215. Bennett Research models of sensorimotor gating and habituation deficits in schiz- Award. Reversal of phencyclidine-induced effects by glycine and ophrenia. Comparison of ketamine- of prepulse inhibition produced by amphetamine in mice. J induced thought disorder in healthy volunteers and thought Neurosci 1999;19:4627–4633. Effective neuroleptic medica- mediate systemic dizocilpine (MK-801)-induced hyperlocomo- tion removes prepulse inhibition deficits in schizophrenia pa- tion and dopamine release in the nucleus accumbens. Corticolimbic dopamine neurotrans- processing deficits in schizophrenia with clozapine. Am J Psy- mission is temporally dissociated from the cognitive and loco- chiatry 1999;156:1046–1051. Increased startle respond- clinical and experimental amphetamine (model) psychoses. Gouzoulis-Mayfrank E, Habermeyer E, Hermle L, et al. Hallu- after long-term administration of phencyclidine. Science 1997; cinogenic drug-induced states resemble acute endogenous psy- 277:953–955. Spring- phine-induced stereotypic behaviors in the rat. Brain Res Dev field, IL: Charles C Thomas Publisher, 1968. Genetic variation in vulnerability experimental psychoses. Springfield, IL: Charles C Thomas Pub- to the behavioral effects of neonatal hippocampal damage in lisher, 1971. To model a psychiatric disorder in biological and subjective effects of four closely related spaced animals: schizophrenia as a reality test. Neuropsychopharmacology doses of N,N-dimethyltryptamine in humans. The effect of N,N- neonatal nitric oxide synthase inhibition in rats: a potential dimethyltryptamine in human subjects tolerant to lysergic acid neurodevelopmental model of schizophrenia. Regulation of the information flow in serotonin receptors are altered on the limbic system of schizo- the nucleus accumbens: a model for the pathophysiology of phrenics. Classification of typical and and development of schizophrenia: advances in experimental psy- atypical antipsychotic drugs on the basis of dopamine D-1, D- chopathology. Washington, DC: American Psychological Associ- 2 and serotonin-2 pKi values. Evidence for 5-HT2 of rats produces a deficit in prepulse inhibition of acoustic startle involvement in the mechanism of action of hallucinogenic similar to that in schizophrenia. Behavioral studies of hallucinogenic drugs in ani- 92. Isolation rearing affects mals: implications for schizophrenia research. Pharmacopsychia- sequential organization of motor behavior in post-pubertal but try 1998;2:73–79. Effects of isolation rearing on startle 100,907, clozapine, risperidone and haloperidol. BehavBrain reactivity, habituation, and prepulse inhibition of male Lewis, Res 1997;88:43–49. HT2A receptor antagonist for the treatment of schizophrenia. Examination of drug-induced and isola- CNS Drug Rev 1997;3:49–67. Recent advances in the phencyclidine screen antipsychotic drugs. Social isolation the noncompetitive NMDA antagonist, ketamine, in humans: produces developmentally specific deficits in prepulse inhibition psychotomimetic, perceptual, cognitive, and neuroendocrine re- of the acoustic startle response but does not disrupt latent inhibi- sponses. Ontogeny of isolation rearing-induced defi- Chapter 50: Animal Models Relevant to Schizophrenia Disorders 701 cits in sensorimotor gating in rats. Rats reared in social isola- BehavNeurosci 2000;114:374–388. Inbred strain differences in prepulse macol Biochem Behav 1997;58:1031–1036. M100907 on pharmacological and developmental animal 105. Social interaction and models of prepulse inhibition deficits in schizophrenia. Neuro- sensorimotor gating abnormalities in mice lacking Dvl1. Genetics, haloperi- tor subunits are not necessary for hippocampal-dependent learn- dol-induced catalepsy and haloperidol-induced changes in ing or sensorimotor gating: a behavioral characterization of acoustic startle and prepulse inhibition. Mice with re- standing the biology of a complex phenotype: rat strain and duced NMDA receptor expression display behaviors related to substrain differences in the sensorimotor gating-disruptive schizophrenia. Modulation domain of the NMDA receptor 2B subunit: analysis of associa- of the startle response and startle laterality in relatives of schizo- tion with schizophrenia. Study of a new schizo- evidence of inhibitory deficits. Am J Psychiatry 2000;157: phrenomimetic drug—sernyl. Subanesthetic doses of SUS rats in animal models with construct validity for schizo- ketamine stimulate psychosis in schizophrenia. BRAFF ROBERT FREEDMAN The power and appeal of the molecular biology mantra, chiatric illness, but it assumes that the effect (i. Based on this man- the population is sufficiently homogeneous (e. An tra, the genomes of viruses, bacteria, fruit flies, and now attractive feature of this approach is that the search for genes humans are being mapped and sequenced, so that all the is not constrained by preexisting hypotheses about the biol- genes and, ultimately, their corresponding biological activ- ogy of the illness, which, in the case of schizophrenia, is ity can be identified. A second commonly applied strategy is, in fact, reasonable to ask how this information can be related to the opposite approach; an assumption is made about the the inheritance of risk for psychiatric illness. For a bacterial biology of the illness and then candidate genes associated enzyme, genetic coding of the amino acid sequence of pro- with that biology are examined to determine if they are teins can be closely associated with a functional change in mutated. Both approaches have been successful to a limited enzymatic activity. For a complex psychiatric illness, as de- extent for explicating the genetics of schizophrenia. Replica- fined by DSM-IV criteria, the relationship is obviously not ble linkages for schizophrenia have been obtained at several as straightforward. Psychiatric illnesses such as schizophre- locations (e. On the other hand, DNA mutations have been both genetic and nongenetic factors. Furthermore, there is found in candidate genes such as NURR1, the gene for the no reason to presuppose that only one gene is responsible receptor for retinoic acid, a pathway critical in neuronal for a complex psychiatric disorder such as schizophrenia, as development, but these mutations seem to be found in only there is in some simple mendelian illnesses. Persons who a small proportion of schizophrenic patients (3). Thus, how best to use by identifying brain dysfunctions that may be caused by a the power of molecular genetics to understand the inheri- single genetic abnormality. The rationale comes from the tance and pathophysiology of complex genetic psychiatric mantra itself. If discrete genetic abnormalities are associated illnesses remains an enigma that is only now beginning to with schizophrenia, then each of them should cause a spe- be solved. Even if several genes are ab- molecular genetics that is applied to complex psychiatric normal, along with additional environmental factors, the disorders, it is assumed that the distribution of illness in a functional abnormality resulting from each gene should family represents the effect of a single gene, and techniques generally be identifiable. Theoretically, the relationship be- of genetic analysis are used to identify that gene. This ap- tween these functional abnormalities and genes, discovered proach does not necessarily overlook the complexity of psy- either by genetic linkage or by candidate gene analysis, should be stronger than the association to the illness itself because the illness itself results from a mixture of genetic and David L. Braff: Department of Psychiatry, University of California at nongenetic abnormalities that may vary between different San Diego, La Jolla, California. Robert Freedman: Department of Psychiatry and Pharmacology, Univer- individuals and families. As is true for the other approaches sity of Colorado, Denver, Colorado. Nevertheless, the In this context, even if endophenotypes turn out to be mul- strategy has been useful for gene discovery in other complex tiple, rather than single, gene phenomena, their genetic ar- illnesses, such as colon cancer and hemochromatosis. In chitecture, even as complex endophenotypes, may turn out colon cancer, the formation of multiple polyps, rather than to be simpler than schizophrenia in certain families. The cancer itself, has been found to be the genetically heritable sections below outline the stage of investigation for a num- trait (4), and in hemochromatosis, a high serum level of ber of putative phenotypes, from presence in schizophrenia iron, rather than the clinically recognized illness, has been probands and their relatives to statistically significant ge- found to be the more penetrant heritable trait (5). Endophenotype is often used as the descriptive term for Several points must be considered in the assessment of these discrete, genetically determined phenotypes that may endophenotypes. First, because these are putative genetic be part of a complex illness. The search for endophenotypes traits, their biology begins at conception, so that by the is not straightforward because no a priori criterion can be time they are measured in adulthood, their expression may used to decide if a particular element of schizophrenia or have been modified by such factors as development, aging, any other psychiatric illness reflects the effect of a single brain injury, and medication and substance abuse and. Putative endophenotypes have ranged from clinical ond, most genes expressed in the brain are expressed in characterizations, such as the presence of schizotypy in rela- many different brain areas, so that their ultimate functional tives of schizophrenic patients (6), to the neurophysiologic expression may involve much more than the simple pheno- and neuropsychological measures described in this chapter, type being measured. Third, many genes expressed in the to structural measures of specific, functionally important brain are also involved in the development of neurons, so regions of the brain and ventricular size. Because none of that their most important functional effects may have oc- these phenotypes has yet led to the identification of a spe- curred prenatally. ENDOPHENOTYPES AND THE GENETICS OF SCHIZOPHRENIA: EFFECT SIZE DIFFERENCES BETWEEN SCHIZOPHRENIA SPECTRUM GROUPS AND NORMAL COMPARISON SUBJECTS Clinically Unaffected Schizophrenia Relatives of Schizotypal Personality Phenotypes Patients Schizophrenia Patients Disorder Patients References P50 Suppression 0. These effect sizes were computed by using the means and standard deviations for the normal comparison subjects and the means of the patient groups. The range of values differs from study to study because different investigators used different patient populations taking different types and amounts of medications; also, the experimental paradigms, although similar, often differed in terms of stimulus parameters. Of course, in some of these studies, multiple conditions were used, some of which were needed to establish floor and ceiling effects. In these cases, we generally cite the most robust effect sizes. Chapter 51: Endophenotypes in the Genetics of Schizophrenia 705 so that if schizophrenia is a multifactorial trait, some siblings 'accounting' for, at most, 50% to 70% of the variance of should express specific phenotypes independently of other the disorder; the remaining variability resides in nongenetic phenotypes. These siblings may be better subjects for char- 'second hits,' such as neonatal or in utero neural damage acterizing the phenotype than the patients themselves, to the developing hippocampus (12–15) or other factors. A plethora of studies indicate that in addition to mutant Finally, because the aim of genetics generally is to identify genes, a second level of environmental or other generalized affected individuals who have or do not have a particular or specific stressor probably must act as a second hit in the genetic abnormality, the measurement of the putative phe- central nervous system. An example of the result of this notype must clearly separate most affected and unaffected need for a second hit is illustrated by the fact that clinically individuals, regardless of whether a quantitative or discrete 'unaffected' relatives of patients with schizophrenia have variable is used. The range of effect sizes for several putative endophenotypic markers of abnormalities in some or all of endophenotypes is shown in Table 51. The measurement of endophenotypes is in itself schizophrenia. Therefore, it appears clear that some nonge- a complex endeavor in which modest-appearing paradig- netic contributions (not necessarily reflected by these endo- matic manipulations lead to significant shifts in the signal phenotypes) are crucially important in the expression of of the dependent measure being assessed. In searching for non–diagnosis- netic strategies to evaluate the current state of understanding based 'candidate endophenotypes,' we are not alone in the pathophysiology of schizophrenia.
The latter possibility has received very recent cocaine users in comparison with that in controls . In a further study of the related to D2-receptor availability . The D2-receptor levels same patient sample , reductions in orbitofrontal cortex and for normal subjects who liked methylphenidate were signifi- cingulate metabolism were particularly profound , and these cantly lower than those for normal subjects who did not reductions were correlated with reductions in (striatal) DA like the drug , and they were strikingly similar to those of D2-receptor availability (33) . However , craving (on a scale long-term cocaine users in earlier studies by the same inves- of 0 to 10) during the week of the study did not correlate tigators (51) . Reduced D2 receptors may thus be a marker with striatal D2-receptor availability; correlations of craving for vulnerability to stimulant misuse in addition to , or per- with metabolic rates were not reported . Paralleling the metabolic findings of Volkow, Childress et al. However, neither baseline cocaine crav- Administering cocaine in the laboratory is a reliable and ing nor withdrawal (self-rated on a scale of 0 to 9) at the robust trigger of cocaine desire (4), and cocaine users com- time of the scan correlated with rCBF in these regions. Almost two decades ago, Eikelboom and Stew- art (52) modeled this behavior in rats, showing that small The neuroimaging data clearly show a number of differences drug 'primes' could motivate drug seeking and reinstate between the brains of cocaine patients undergoing cessation extinguished responding for drug. According to the priming in comparison with controls not using drugs. The observed hypothesis, the initial drug effect always precedes the full differences often are clearly linked to brain DA systems. The state has powerful positive tion craving' has been variable. At this time, correlative incentive properties, 'pulling' the organism back to the evidence from studies in early ( 1 week) cessation, but not drug. In this 'opponent frontal and prefrontal cortex metabolism, -opioid process' view, the brain responds to cocaine with homeo- binding). The opponent re- often conducted in separate populations, and with impor- sponse (i. Clinically, patients do complain about the jittery offset must be drawn with caution. Powerful within-subject de- of the cocaine high, and they recognize that taking another signs, including frequent scans and subjective measures dose of the drug will alleviate this state. If such istration more closely related to a brain state that occurs at longitudinal studies were to confirm a lack of a relationship the onset or at the offset of the drug response? Although the between craving and later resting hypoactivity (by metabo- question is posed as a choice, these possibilities (unfortu- 1578 Neuropsychopharmacology: The Fifth Generation of Progress nately for the task of developing medications) are not mu- anterior cingulate). Signal change in the amygdala during tually exclusive. Craving of the positive, appetitive, cocaine administration was initially reported as heterogene- 'primed' variety may map onto brain responses associated ous (some patients showed increases and others showed de- with the initial effect of the drug and be followed shortly creases), not correlated with rush, and negatively correlated thereafter by the dysphoric craving of offset, which may with craving ratings. However, in a follow-up study with map onto a later set of brain responses that are opposite in cardiac gating of the fMRI signal (see below), the direction direction to those of drug. The exquisite temporal sensitivity of signal change in amygdala was positive for all subjects. Although the brain regions that correlated with 'crav- What are the likely neuroanatomic and neurochemical ing' and 'rush' overlapped substantially, a clear dissocia- features of the craving state(s) associated with cocaine ad- tion was also noted. More than two decades of animal research with early maximal, short-duration signals from the ventral (see refs. On the other hand, 'craving,' but not 'rush,' cor- in cocaine reinforcement and motivation. Thus, a priori related with an early-onset but sustained signal from the neuroanatomic predictions include the familiar projections nucleus accumbens/subcallosal cortex. All the activated re- of the DA cells in the ventral tegmental area of the midbrain gions showed early onset to cocaine. Thus, the primary dif- to the ventral striatum (nucleus accumbens), amygdala, ference between 'craving' and 'rush' (euphoria) substrates basal forebrain, orbitofrontal cortex, and medial prefrontal/ was not a matter of which regions were activated, but of anterior cingulate cortex. Put another way, a full orchestra is playing from and human (57) research leaves room for the contribution the outset of cocaine administration. As the 'rush' wanes, of other brain systems, DA neuronal elements (DATs, post- some instruments drop out. How do these data fit with the 'priming' and 'opponent process' hypotheses of craving in response to cocaine? Un- fortunately, the fit is not completely straightforward for Data either view. At the first level of examination, this did not obtain a craving measure (58–61), did not analyze finding seems consistent with a priming effect; the signal the craving item (62), or analyzed a craving item but did occurs very early and therefore looks like a direct drug effect. The remaining four studies discussed below have been should map better onto the brain correlates (ventral tegmen- published since 1997. In terms of clear evidence for a simple oppo- nology with a BOLD (blood oxygen level-dependent) scan nent process view, no later-occurring activations opposite to to map the brain circuitry activated during a period 5 min- the direction of the 'direct' drug effects in ventral tegmen- utes before, and 13 minutes after, cocaine (0. Subjective ratings cause the direct drug effect was a positive signal change in ('rush,' 'high,' 'craving,' and 'low') were taken each virtually all brain areas, detecting opposite direction effects minute throughout the experiment. Because of the lated with the group-averaged temporal pattern of signals physiologic basis of the BOLD signal, the meaning of 'neg- from each brain region meeting specified threshold and ex- ative signal change' is an ongoing research challenge for tent criteria for differential activation by cocaine. No activity in any single brain region precisely echoed the * fMRI is extremely vulnerable to movement artifact. Signals from the onset and late peak of 'craving' ratings. However, signifi- amygdala and other structures near the base of the brain can be affected cant positive correlations were obtained with regions having by the slight movement, at each heartbeat, of blood entering the brain early-onset (during euphoria) but sustained activations. Cardiac gating of the fMRI signal allows the fMRI scanner to be controlled These included the nucleus accumbens/subcallosal cortex by the heartbeat, and images are collected in the intervals between beats. Chapter 110: Neuroimaging of Cocaine Craving States 1579 with a 15O bolus performed during and after cocaine admin- ingly, both the 'high' and the DA response to methylpheni- istration offers sufficient temporal resolution (15O has a date (measured by raclopride binding) in the striatum were half-life of 128 seconds) that it can be used to sort out greater in the controls than in the cocaine users, as though 'early/direct' from any 'later/opposed' effects of cocaine. In experienced users (the which metabolism (measured by PET and 18F-fluorodeox- only subjects who can be given cocaine in human studies), yglucose), D2-receptor availability (measured by 11C-raclo- distinguishing 'direct' from 'opposed' effects could be pride), and subjective responses (27 minutes after each infu- very difficult. Animal research mapping the temporal corre- sion) were determined (66). The actions of methylphenidate lates of brain response to cocaine and its signals during the on brain metabolism showed substantial variability across course of initial and repeated administrations could clarify subjects that correlated with striatal D2-receptor availabil- these relationships; of course, such experiments cannot ethi- ity; the stimulant increased metabolism in subjects with a cally be conducted in humans. Although stimulant administration to cocaine users and measures of methylphenidate induced metabolic increases in several craving have been conducted by the Volkow team at Brook- areas (cingulate, thalamus, cerebellum), it increased right haven Laboratories. In one of these experiments, cocaine orbitofrontal and right striatal metabolism only in the sub- was used as the stimulant probe; in the other two, methyl- jects who experienced cocaine craving ('desire for cocaine phenidate was used. In the cocaine study, cocaine users were and perception of loss of control over cocaine'). This obser- given intravenous cocaine in doses of 0. Subjective self-ratings were taken every sufficient to induce the metabolic increases in the frontal minute for the first 20 minutes post infusion, and then at regions. As in the prior study, ratings of 'high' at 27 min- 10-minute intervals for the next 40 minutes. The ratings utes post methylphenidate did not correlate with the effects of cocaine 'high' and 'rush' were positively correlated with of the drug on metabolism or D2-receptor availability. DAT occupancy, but 'craving' (the desire for cocaine, rated on a scale from 1 to 10) and 'restlessness' were not. Thus, Summary as in the earlier study of Breiter et al. Although several brain regions were commonly ac- tion suggest that a simple DA hypothesis of stimulant effects tivated by both craving and rush, rush-associated ventral (including craving) may be insufficient. One study com- tegmental area/basal forebrain signals rapidly declined while pared the subjective and brain DA response of cocaine users the craving-associated signals in the nucleus accumbens/ (3 to 6 weeks post cocaine) with that of controls after an subcallosal cortex persisted. This partial dissociation sug- injection of methylphenidate, which (like cocaine) blocks gests at least some independence of substrates for these two DA reuptake (51). The early onset of brain activation in cocaine-induced methylphenidate per kilogram was followed by an injection craving is consistent with the priming hypothesis; the partial of 11C-raclopride, a dopamine D2 ligand sensitive to com- dissociation with rush/euphoria is not. Regions of interest were the 'offset' activations correlated with craving is inconsistent striatum, thalamus, and cerebellum (as a comparison region with a simple opponent process view, but conditioned op- devoid of D2 receptors). Subjective measures were taken ponent effects could occur very early in highly experienced 5 minutes before and 27 minutes after methylphenidate. This dual time course of effects may account for lation may have been compromised by taking the subjective some of the variability or inconsistency in effects reported. Interest- With regard to which dopaminergic neuronal elements 1580 Neuropsychopharmacology: The Fifth Generation of Progress are involved in the craving response, the results are mixed. Is cue-elicited cocaine craving more 'druglike' or 'drug- Cocaine-induced craving was unrelated to DAT occupancy. Do the multiple cues surrounding the cocaine However, methylphenidate-induced cocaine craving was experience become linked predominantly to the brief, in- correlated with right striatal and right orbitofrontal in- tense, orgasmic euphoria... These findings could support a postulated predictions for neuroimaging and pharmacotherapies. If DA dysfunction in the striatal–thalamic–orbitofrontal cir- craving to common external (paraphernalia, other users, cuit in cocaine addiction (33,67). One similarity between drug-buying locations, drug talk) or internal (e. As mesolimbic–mesocortical DA system activated by cocaine discussed in the final section, the orbitofrontal cortex is thus itself are also activated during cue-induced craving. In ani- far the only region linked to craving in all three paradigms: mals, cues for cocaine can indeed trigger mesolimbic DA (early) cessation, stimulant administration, and exposure to overflow in nucleus accumbens and amygdala (70) and can drug-related cues. Patients ministration, a role of transmitters other than DA in meth- often describe cocaine-like effects in response to cues, in- ylphenidate-induced effects (both high and craving) seems cluding heart pounding, ear ringing, head buzzing, stomach likely. Methylphenidate alone was insufficient to increase 'flipping,' mild euphoria, a 'taste' of cocaine in the back frontal metabolism, and in other studies by the Brookhaven of the throat, even the 'smell' of cocaine in the room... But what do the brains of methylphenidate did not always result in a subjective high in cocaine users say about the nature of cue-induced cocaine healthy controls (craving was not probed) (68). It tested whether video-induced cocaine IMAGING OF CRAVING DURING COCAINE craving might increase endogenous DA, as indexed by com- CUES 123I-iodobenzamide, in SPECT. After this initial effort, imaging studies ad- (5,6). According to this view, cocaine cues trigger cocaine- dressed the neuroanatomic rather than the neurochemical related subjective and physiologic responses, including crav- substrates of cue-induced craving. Drug-related conditioning can result cocaine cues since 1992. These studies cover a range of 15 18 in both responses similar to those produced by the drug imaging technologies (PET with O bolus, PET with F- itself ('druglike') and responses opposite to those of the fluorodeoxyglucose, fMRI) and include several variations drug ('drug-opposite') (69), likely reflecting a conditioned on the method of presenting drug cues to induce craving. Both kinds of The variations are useful because results obtained in only responses may be of motivational significance. Druglike re- one laboratory, or with one method of cue presentation, sponses to cues reminiscent of the drug ('Wow... Conversely, any replication and ing the user back to the drug. If drug-opposite responses convergence of findings across multiple laboratories and to the cues are uncomfortable (in opiate users, these include methods are very encouraging. BRAIN IMAGING OF CUE-INDUCED COCAINE CRAVING Imaging Cocaine Days of Laboratory Technology Population Cessation Cue Description Resultsa U. Nac/SCC +,^(–) each, same order) OFC 0 DLPFC 0 Cerebellum 0 Medical College of Wisconsin Garavan et al. When controls were studied, the summary reflects significant difference between groups for the drug cue vs. When no controls were studied, the effects are only for drug cue vs. Because of space constraints, results are summarized and “no difference” regions are presented only as relevant to discussion in the text. Please refer to the original articles for a complete listing of neuroanatomic regions studied. Abbreviations: SPECT, single-photon emission computed tomography; 123I-IBZM, iodobenzamide, a D2-receptor ligand; PET, positron emission tomography; ROI, region of interest analysis; OFC, orbitofrontal cortex; DLPFC, dorsolateral prefrontal cortax; VLPFC, ventrolateral prefrontal cortex; fMRI, functional magnetic resonance image; BOLD, blood oxygen level-dependent technique; SPM, statistical parametric mapping technique; FDG, 12F-fluorodeoxyglucose; NAc/SCC, nucleus accumbens/subcallosal cortex; l. In the first activated by cocaine itself (summarized above). Because of PET study of craving, predicted increases in amygdala rCBF the number of studies, the findings reviewed below are or- (measured with 15O-water as the perfusion tracer) were ganized according to the anatomic structures that have fre- found in cocaine patients viewing videos that induced crav- quently been activated during cue-induced craving. This effect was not evident in controls without a cocaine history. Inter- Limbic-Related Structures estingly, baseline rCBF in the amygdala of cocaine users Several of the structures activated during cue-induced crav- tended to be lower than in controls, such that increased ing are parts of an interconnected rostral limbic system, rCBF in response to the cocaine cues did not exceed the important in motivation and affective experience. Devinsky amygdala rCBF in control subjects under the same condi- et al. Activation of the amygdala in this study was initially 'the amygdala and septum, and orbitofrontal, anterior in- documented by a region-of-interest (ROI) analysis and has sula, and anterior cingulate cortices, the ventral striatum in- recently been confirmed by statistical parametric mapping cluding the nucleus accumbens, and several brainstem motor (SPM) of the group data (Fig. The amygdala is critical in signal surrounding the ventral amygdala, in response to cocaine learning for biologically significant (pleasant or unpleasant) versus nature video. The PET camera used in the recent events (76,77) and has been activated by cue-induced crav- 18F-fluorodeoxyglucose study by Wang et al. Anterior Cingulate The anterior cingulate is located in the dorsomedial prefron- tal cortex and is interconnected with other rostral limbic structures, including the amygdala and nucleus accumbens. Multiple roles of the anterior cingulate include selective at- tention and emotional reactivity to significant stimuli (75, 87). In parallel with the amygdala findings, the anterior cingulate was differentially activated during video-induced cocaine craving in our initial PET study with 15O bolus (by ROI analysis and later confirmation by SPM analysis of the group data) (Fig. As with the amyg- dala effect, the cue-induced rCBF increase in anterior cingu- late was from a resting baseline that was hypoactive relative to the baseline of controls. These studies used fMRI (84,86) and PET with 15O bolus (82), FIGURE110.
This comparative effectiveness review (CER) covers the first two areas . Rate Control Whether or not a rhythm-control strategy is adopted , current treatment guidelines suggest that adequate rate control should be achieved in all patients with AF to prevent myocardial 1 infarction (if significant coronary artery disease is present) , exacerbation of heart failure , and tachycardia-induced cardiomyopathy; to alleviate symptoms; and to improve exercise tolerance and quality of life . Thus , the 2006 Guidelines for the Management of Patients with Atrial Fibrillation—prepared jointly by the American College of Cardiology (ACC) , the American Heart Association (AHA) , and the European Society of Cardiology (ESC)—highlight the need for adequate rate control in patients with AF and designate measurement of the heart rate at rest and control of the rate with pharmacological agents (either a beta blocker or a nonhydropyridine calcium channel blocker in most patients) as a Class I recommendation (condition for which there is evidence and/or general agreement that a given procedure or treatment is useful and 14 effective) . However , since the development of the ACC/AHA/ESC Guidelines, many additional studies have been published on the comparative safety and effectiveness of the different available medications used for ventricular rate control in clinical practice. Thus, an updated review of published studies is timely. If pharmacological therapy is insufficient for rate control and symptom management, or is associated with side effects, the 2006 ACC/AHA/ESC Guidelines recommend ablation of the atrioventricular node (AVN) in conjunction with permanent pacemaker implantation to control 14 heart rate. As the latter involves implantation of an indwelling device that is not reversible, it is considered a treatment of last resort for patients for whom initial pharmacotherapy was ineffective. However, the most recent systematic review on this topic was published more than a decade ago. This review will synthesize the evidence that has been published since then to better define the role of this procedure in contemporary clinical practice and in specific subpopulations where it might be more or less effective. Another clinical dilemma is whether patients with AF do better with strict or lenient rate control. In theory, strict control could reduce symptoms and prevent complications. However, stricter control requires more intensive use of medications which carry their own side effects. The 2011 Focused Update on the Management of Patients with Atrial Fibrillation by the American College of Cardiology Foundation (ACCF), the AHA, and the Heart Rhythm Society 16 (HRS) addressed the issue of strict versus lenient rate control in patients with AF. Specifically, these guidelines emphasized the following Class III recommendation (conditions for which there is evidence and/or general agreement that the procedure/treatment is not useful/effective and in some cases may be harmful): “Treatment to achieve strict rate control of heart rate (<80 bpm at rest or <110 bpm during a 6-minute walk) is not beneficial compared to achieving a resting heart rate <110 bpm in patients with persistent AF who have stable ventricular function (left 16 ventricular ejection fraction >0. For pharmacological cardioversion of AF, the 2006 ACC/AHA/ESC Guidelines recommend flecainide, dofetilide, propafenone, and ibutilide as Class I recommendations, and amiodarone as a Class IIa recommendation (weight of 14 evidence/opinion is in favor of usefulness/efficacy). To enhance direct-current cardioversion, the 2006 ACC/AHA/ESC Guidelines recommend pretreatment with amiodarone, flecainide, ibutilide, propafenone, or sotalol. For maintenance of sinus rhythm after cardioversion, the 2006 ACC/AHA/ESC Guidelines list different antiarrhythmic medications for different clinical settings. The 2011 ACCF/AHA/HRS Focused Update builds upon the recommendations in the 2006 ACC/AHA/ESC Guidelines using published data on new antiarrhythmic medications. Therefore, this report will review existing evidence and summarize current evidence gaps on the comparative safety and effectiveness of available antiarrhythmic agents for conversion of AF to sinus rhythm, for facilitating successful electrical cardioversion, and for maintaining sinus rhythm after successful conversion of AF to sinus rhythm. Recommendations for maintenance of sinus rhythm in patients with recurrent paroxysmal or persistent AF from the 2011 ACCF/AHA/HRS Focused Update on the Management a of Patients With Atrial Fibrillation (Updating the 2006 Guideline) aFrom Wann, 2011;16 reprinted with permission, Circulation. Abbreviations: ACCF=American College of Cardiology Foundation; AHA=American Heart Association; HRS=Heart Rhythm Society; LVH=left ventricular hypertrophy In addition to pharmacological and direct current cardioversion, a number of surgical interventions are used for rhythm control. Catheter ablation for the treatment of AF (with pulmonary vein isolation [PVI] being the most commonly used ablation) has evolved rapidly from an experimental procedure to a commonly performed procedure that is widely regarded as a useful treatment option for symptomatic patients with AF in whom medications are not effective 14,16,18 or not tolerated. These studies vary from small and large single-center nonrandomized studies to multicenter prospective randomized controlled trials (RCTs). However, the relatively small number of patients included in each trial makes definitive conclusions about the safety and efficacy of pulmonary vein isolation based on an individual study difficult and does not permit meaningful analyses of key subgroups of patients (e. Antiarrhythmic Drug Therapy for AF (CABANA) study will provide important information on the effect of catheter ablation on final outcomes, this trial is 18 not expected to end until several years from now. The present review will increase the power of existing studies by synthesizing the evidence on this procedure by pooling data from existing studies and by exploring whether other types of studies or comparative effectiveness research would be helpful. Several other procedures have been investigated in the treatment of AF. One such procedure is the surgical Maze procedure, which appears to confer some benefit to selected patients with 19 AF. Implantation of a cardiac resynchronization therapy (CRT) device is another procedure that may decrease the burden of AF in patients who are eligible for this device based on a left ventricular ejection fraction ≤35 percent, a wide QRS complex, and heart failure symptoms despite optimal medical therapy. Secondary analyses of major clinical trials have provided 20,21 conflicting findings on the effect of CRT on AF burden. This report will review and synthesize current published data on these novel procedures and will help to better define their risks and benefits in contemporary clinical practice. Rate Control Versus Rhythm Control Although several studies of rate- and rhythm-control strategy exist, to date no study has shown that maintaining patients with AF in sinus rhythm provides a long-term survival benefit. We also do not know whether the risks and benefits of different therapies vary by AF type. Our review seeks to systematically review the comparative risks and benefits of specific outcomes to allow patients and providers to assess the patient-specific tradeoffs of the differing strategies. Scope and Key Questions Scope of the Review This CER was funded by AHRQ and is designed to evaluate the comparative safety and effectiveness of a wide range of pharmacological and procedural rate- and rhythm-control strategies for the treatment of adult patients with paroxysmal, persistent, or permanent AF (includes atrial flutter). Rate-control and rhythm-control strategies for patients with AF have been evaluated in numerous studies. Despite these studies, several uncertainties remain, and comparative safety and effectiveness analyses of the available management strategies for patients with AF are needed. Existing systematic reviews of the evidence either do not include the most recent clinical evidence or are inconclusive; moreover, for some important clinical and policy questions of interest, systematic reviews have not yet been performed. This new review of the available data not only addresses existing uncertainties, but also defines gaps in knowledge and identifies future research needs. The first three KQs considered in this CER focus on rate-control therapies. Specifically: • KQ 1: What are the comparative safety and effectiveness of pharmacological agents used for ventricular rate control in patients with atrial fibrillation? Do the comparative safety and effectiveness of these therapies differ among specific patient subgroups of interest? Do the comparative safety and effectiveness of these therapies differ among specific patient subgroups of interest? Do the comparative safety and effectiveness of these therapies differ among specific patient subgroups of interest? The next two KQs focus specifically on rhythm-control therapies: • KQ 4: What are the comparative safety and effectiveness of available antiarrhythmic agents and electrical cardioversion for conversion of atrial fibrillation to sinus rhythm? Do the comparative safety and effectiveness of these therapies differ among specific patient subgroups of interest? Do the comparative safety and effectiveness of these therapies differ among specific patient subgroups of interest? The final KQ seeks to evaluate the comparison of the available rate- and rhythm-control therapies. Does the comparative safety and effectiveness of these therapies differ among specific patient subgroups of interest? Analytic Framework Figure 2 depicts the analytic framework for this project. Analytic framework Abbreviations: AF=atrial fibrillation; CV=cardiovascular; KQ=Key Question This figure depicts the KQs within the context of the PICOTS described elsewhere in this document. The patient population of interest is adults with AF. Interventions of interest are procedural and nonpharmacological therapies for rate control (KQs 3 and 6), pharmacological therapies for rate control (KQs 1, 2, 3, and 6), pharmacological therapies for rhythm control (KQs 4, 5, and 6), electrical cardioversion (KQs 4, 5, and 6), and procedural and nonpharmacological therapies for rhythm control (KQs 5 and 6). Strict versus more lenient pharmacological therapies for rate control are considered in a separate question (KQ 2). Intermediate outcomes of interest are restoration of sinus rhythm, maintenance of sinus rhythm, recurrence of AF at 12 months, ventricular rate control, and development of cardiomyopathy. Final outcomes of interest are mortality (all-cause and cardiovascular), myocardial infarction, cardiovascular hospitalizations (including AF hospitalizations), heart failure symptoms, control of AF symptoms (e. Also of interest are the following adverse events associated with pharmacological treatment: hypotension, hypo/hyperthyroidism, arrhythmias, allergic reactions, hepatotoxicity, neurotoxicity, pulmonary toxicity, ophthalmological toxicity, and dermatological toxicity. Procedural complications of interest include pulmonary vein stenosis, left atrial esophageal fistula, phrenic nerve palsy, cardiac tamponade, and other complications (such as infection, bleeding, and thromboembolic events). For all six KQs, we will attempt to determine whether the comparative safety and effectiveness of the various therapies investigated differ among specific patient subgroups of interest. Patient characteristics to be assessed here include age, comorbidities, type of AF, previous pharmacological therapy failure, sex, enlarged left atrium, and high risk for stroke and bleeding events. The main sections in this chapter reflect the elements of the protocol established for the CER; certain methods map to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses 23 (PRISMA) checklist. Topic Refinement and Review Protocol During the topic refinement stage, we solicited input from Key Informants representing medical professional societies/clinicians in the areas of general internal medicine, geriatrics, cardiology, electrophysiology, and primary care; patients; scientific experts; Federal agencies; and payers to help define the Key Questions (KQs). The KQs were then posted for public comment for 4 weeks from September 27 to October 25, 2011, and the comments received were considered in the development of the research protocol. We next convened a Technical Expert Panel (TEP) comprising clinical, content, and methodological experts to provide input to the draft protocol in defining populations, interventions, comparisons, and outcomes, and in 24 identifying particular studies or databases to search. Before involvement in the CER process, the Key Informants and members of the TEP were required to disclose any financial conflicts of interest greater than $10,000 and any other relevant business or professional conflicts. Any potential conflicts of interest were balanced or mitigated. Neither Key Informants nor members of the TEP performed analysis of any kind, nor did any of them contribute to the writing of this report. Literature Search Strategy Search Strategy ® ® To identify relevant published literature, we searched PubMed , Embase , and the Cochrane Database of Systematic Reviews (CDSR), limiting the search to studies published from January 1, 2000, to August 1, 2012. We believe that the evidence published from 2000 on represents the current standard of care for patients with atrial fibrillation (AF) and relevant comorbidities. In addition, a 2001 AHRQ report on the management of new onset AF summarized the evidence 25-27 prior to 2000. Where possible, we used existing validated search filters (such as the Clinical Queries Filters in PubMed). An experienced search librarian guided all searches. We supplemented the electronic searches with a manual 16,19,25-135 search of citations from a set of key primary and systematic review articles. We also considered studies identified through suggestions from external peer and public reviewers. Final updating of all database searches was performed during the review period. All citations were ® imported into an electronic database (EndNote X4; Thomson Reuters, Philadelphia, PA). We used several approaches to identify relevant grey literature including requests to drug and device manufacturers for scientific information packets and searches of study registries and conference abstracts for relevant articles from completed studies. Grey literature databases searched included ClinicalTrials. Search terms used for all of the above sources are provided in Appendix A. Inclusion and Exclusion Criteria The PICOTS (Populations, Interventions, Comparators, Outcomes, Timings, and Settings of interest) criteria used to screen articles for inclusion/exclusion at both the title-and-abstract and full-text screening stages are detailed in Table 1. Inclusion and exclusion criteria PICOTS Element Inclusion Criteria Exclusion Criteria Populations • Humans • Patients who have known • Adults (age ≥ 18 years of age) reversible causes of AF (including • Patients with AF (includes atrial flutter) but not limited to postoperative, o Paroxysmal AF (recurrent episodes that self- postmyocardial infarction, terminate in less than 7 days) hyperthyroidism) o Persistent AF (recurrent episodes that last more • All subjects are <18 years of age, than 7 days) or some subjects are under <18 o Permanent AF (an ongoing, long-term episode) years of age but results are not • Subgroups of potential interest include: broken down by age o Patients stratified by age (≤ 40, 41–64, 65–74, 75–84, 85+) o Patients with different types of AF (paroxysmal, persistent, permanent) o Patients with specific comorbidities (heart failure, coronary artery disease, kidney disease, hypertrophic cardiomyopathy, thyroid disease, pulmonary disease) o Patients for whom a prior rate- (KQ 3) or rhythm- control (KQ 5) pharmacological strategy was ineffective o Women o Patients with an enlarged left atrium o Patients at high risk for stroke and bleeding events (patients with diabetes, heart failure, and hypertension) 8 Table 1. Inclusion and exclusion criteria (continued) PICOTS Element Inclusion Criteria Exclusion Criteria Interventions • Pharmacological agents for rate control (KQ 1, KQ 2, • Studies comparing different KQ 3, KQ 6): imaging or mapping techniques o Beta blockers (e. Inclusion and exclusion criteria (continued) PICOTS Element Inclusion Criteria Exclusion Criteria Outcomes Study assesses a patient-centered outcome of interest: Study does not include any outcomes • Intermediate outcomes: of interest o Restoration of sinus rhythm (conversion) o Maintenance of sinus rhythm o Recurrence of AF at 12 months o Ventricular rate control o Development of cardiomyopathy a • Final outcomes: o Mortality (all-cause, cardiovascular) o Myocardial infarction o Cardiovascular hospitalizations (including AF hospitalizations) o Heart failure symptoms o Control of AF symptoms (e. Abbreviations: AF=atrial fibrillation; AVN=atrioventricular node; CRT=cardiac resynchronization therapy; KQ=Key Question; ICD=implantable cardioverter defibrillator; PICOTS=Populations, Interventions, Comparators, Outcomes, Timing, Settings; RCTs=randomized controlled trials 10 Study Selection Using the prespecified inclusion and exclusion criteria described in Table 1, two investigators independently reviewed titles and abstracts for potential relevance to the KQs. Articles included by either reviewer underwent full-text screening. At the full-text review stage, paired researchers independently reviewed the articles and indicated a decision to “include” or “exclude” the article for data abstraction. When the two reviewers arrived at different decisions about whether to include or exclude an article, they reconciled the difference through review and discussion, or through a third-party arbitrator if needed. Full-text articles meeting our eligibility criteria were included for data abstraction. Relevant systematic review articles, meta-analyses, and methods articles were flagged for manual searching of references and cross-referencing against the library of citations identified through electronic database searching. For citations retrieved by searching the grey literature, the above-described procedures were modified such that a single screener initially reviewed all search results; final eligibility of citations for data abstraction was determined by duplicate screening review. All screening decisions were made and tracked in a Distiller SR database (Evidence Partners Inc. Data Extraction The research team created data abstraction forms and evidence table templates for abstracting data for each KQ. Based on clinical and methodological expertise, a pair of investigators was assigned to abstract data from each eligible article. One investigator abstracted the data, and the second reviewed the completed abstraction form alongside the original article to check for accuracy and completeness. To aid in both reproducibility and standardization of data collection, researchers received data abstraction instructions directly on each form created specifically for this project within the DistillerSR database. We designed the data abstraction forms to collect the data required to evaluate the specified eligibility criteria for inclusion in this review, as well as demographic and other data needed for determining outcomes (intermediate, final, and adverse events outcomes). We paid particular attention to describing the details of treatment (e. In addition, we described comparators carefully, as treatment standards may have changed during the period covered by this review. The safety outcomes were framed to help identify adverse events, including those from drug therapies (e. Data necessary for assessing quality and applicability, as described in the 22 Methods Guide, were abstracted. Before the data abstraction form templates were used, they were pilot-tested with a sample of included articles to ensure that all relevant data elements were captured and that there was consistency/reproducibility between abstractors.
Stutt- of human cerebral cortex using a surface-based atlas . Functional and struc- paracingulate sulci: pattern , variability , asymmetry , and probabi- tural mapping of human cerebral cortex: solutions are in the listic map . Cerebellar mor- neural pathway in children and adolescents . Science 1999;283: phology as a predictor of symptom and psychosocial outcome in 1908–1911 . Quantifying variability in anatomical differences in human primary auditory cortex: proba- the planum temporale: a probability map . Cerebral Cortex 1999; bilistic mapping and volume measurement from MR scans . Volumetry of hippocampus statistical analysis for CBF activation studies in human brain . J and amygdala with high-resolution MRI and three-dimensional Cereb Blood Flow Metab 1992;12:900–918. A voxel-based method longitudinal magnetic resonance imaging study. Arch Gen Psy- for the statistical analysis of gray and white matter density applied chiatry 1999;56(7):649–654. Automatic quantification struction of the human central sulcus reveals a morphological of multiple sclerosis lesion volume using stereotaxic space. Proceedings of the 4th International Conference on Visualization in 66. A comparison of retrospective Biomedical Computing, VBC 1996. SHULMAN In the last 5 years there has been a renewed interest in the tory and excitatory neuronal function requiring energy. Ob- role of metabolism in supporting brain function. Much of servation of a regional increase or decrease of the functional this interest is based on the development of functional posi- imaging signal is not sufficient to distinguish these possibili- tron emission tomography (PET) and magnetic resonance ties. Glia also requires energy, and the relationship between imaging (MRI). Although often incorrectly described as di- its energy demands and neuronal activity remains to be es- rectly mapping neuronal activity, both functional PET and tablished. Given these uncertainties about the meaning of MRI actually measure changes in either glucose metabolism the signal at a neuronal level, the validity of functional imag- or physiologic parameters coupled to glucose metabolism ing as a tool for studying mental processes has been largely such as blood flow and volume (1). A major limitation in established based on agreement with prior expectation from interpreting functional imaging is that the relationship be- psychological paradigms (3,5). It differs by allowing the measurement of the concen- cesses are involved in short-term neuronal information trations and synthesis rates of individual chemical com- transfer, and the relative distribution of energy among them pounds within precisely defined regions in the brain. There is also uncertainty as to basis of its chemical specificity is that the resonance fre- how the different classes of neurons in a region contribute quency of an MRS active nucleus depends not only on the to the overall energy consumption. While an increase in the local magnetic field strength, but also on its chemical envi- imaging signal is usually assigned to an increase in neuronal ronment, a phenomenon referred to as chemical shift. MRS measurements of the 1H nucleus are the most commonly excitation, this interpretation is confounded by both inhibi- used for in vivo studies due to 1H being the most sensitive nucleus present in biological systems. Metabolites that can be measured by 1H MRS include aspartate, -aminobutyric Douglas L. Shulman: Yale University School acid (GABA), glucose, glutamate, glutamine, and lactate. These metabolites play critical roles in neuroenergetics, Nicola Sibson: University of Oxford, Oxford, United Kingdom. KlineInstitute forPsychiatric Research,Orangeburg, amino acid neurotransmission, and neuromodulation. Schematic representations of the glutamate/glutamine cycle between neurons and astrocytes and the detoxification pathway of glutamine synthesis. A: The glutamatine/glutamate cycle between neurons and astrocytes. Released neurotransmitter glutamate is transported from the synaptic cleft by surrounding astrocytic end processes. Once in the astrocyte, glutamate is converted to glutamine by glutamine synthetase. Glutamine is released by the astrocyte, trans- ported into the neuron, and converted to glutamate by phosphate-activated glutaminase (PAG), which completes the cycle. B: Including the ammonia detoxification (or anaplerotic) pathway of glutamine synthesis. The net rate of glutamine synthesis reflects both neurotransmitter cycling (Vcycle) and anaplerosis (Vana). The stoichiometric relationships required by mass balance between the net balance of ammonia and glutamine and Vana are given in Eq. C: An alternative model for neuronal glutamate repletion in which the astrocyte repletes the lost neuronal glutamate by providing the neuron with -ketoglutarate [or equivalently other tricarboxylic acid cycle (TCA) intermediates] (32–34). Glc, glucose; a-KG, -ketoglutarate: Vtrans, net rate of net ammonia transport into the brain (VNH4 in the text); Vefflux, rate of glutamine efflux from the brain; Vana, anaplerotic flux; V , rate of the glutamate/glutamine cycle; V , rate of glutamine synthesis. Using [2-13C] cycle gln glucose (27) and [2-13C] acetate precursors these pathways may now be distinguished. The natural abundance of the 13C isotope is This chapter reviews these findings and discusses some of 1. Substrates labeled with for studying neurotransmitter systems of approximately 1 13 to 4 mm3 in animal models and 7 to 40 mm3 in human the nonradioactive, stable, C isotope have been employed in vivo to study metabolic flux, enzyme activity, and meta- brain. Even in the best case the MRS signal is the sum of bolic regulation in the living brain of animals and humans the signal from a large number of neurons and glia including (6–39). Enhanced sensitivity may be achieved by measuring many different subtypes. Fortunately, nature has localized the 13C enrichment of a molecule through indirect detection key enzymes and metabolites involved in neurotransmitter through 1H MRS. From these measurements the flux cycling in specific cell types, which greatly simplifies the through specific metabolic pathways may be calculated (17, interpretation of the MRS measurements. As with any new tech- and the relationship of amino acid metabolism to functional nique there are still uncertainties due to methodologic is- neuroenergetics. Studies performed to validate the MRS measurements glutamine have been shown to be localized within gluta- will be reviewed, and present limits in measurement accu- matergic neurons, GABAergic neurons, and glia, respec- racy and interpretation delineated. Under nonfasting conditions glucose is the almost exclusive source of energy for the brain. By following the flow of 13C label from glu- 13 IN VIVO C MRS MEASUREMENTS OF THE cose into these metabolites, MRS has been used to deter- PATHWAYS OF GLUCOSE OXIDATION: mine the separate rates of glucose oxidation in these cell FINDINGS AND VALIDATION types. The metabolism of glutamatergic neurons, GABAer- gic neurons, and glia is coupled by neurotransmitter cycles. This section reviews studies in which MRS was used to In the glutamate/glutamine cycle, glutamate released from measure the pathways of glucose oxidation in the cerebral nerve terminals (by either vesicular release or transport re- cortex. Glucose oxidation under nonfasting conditions is versal) is transported into surrounding glial cells, and con- almost the exclusive source of energy for the brain. Glutamine in then transported out of localization of key enzymes involved in GABA and gluta- the glia and into the neurons, where it is converted back mate metabolism in specific cell types provides the capabil- to glutamate, thereby completing the cycle (Fig. By ity for MRS to study their separate neuroenergetic require- following the flow of 13C label from glutamate into gluta- 13 ments. Through a similar strategy the cal specificity of MRS allows the flow of 13C label from GABA/glutamine cycle may be measured. The major finding of these stud- cycling to neuroenergetics have provided several new and ies is that in normal conditions in nonactivated human cere- controversial insights into the relationship of brain metabo- bral cortex and in rodent models, glucose oxidation in gluta- lism and function. Contrary to the previous view of a sepa- matergic neurons accounts for between 60% and 80% of rate metabolic and neurotransmitter pool of glutamate, glu- cerebral cortex energy consumption. The remaining 20% tamate release and recycling have been shown to be a major to 40% is primarily distributed between GABAergic neu- metabolic pathway. Another key finding is that the gluta- mate/glutamine cycle in the cerebral cortex is coupled in a Oxidation in Glutamatergic Neurons close to 1:1 ratio to neuronal (primarily glutamatergic) glu- The initial use of MRS to study brain metabolism was to cose oxidation above isoelectricity. This finding, in combi- measure glucose oxidation by following the flow of 13C nation with cellular studies, has led to a model for the coup- isotope from [1-13C] glucose into the C4 position of gluta- ling between functional neuroenergetics and glutamate mate (2,6). The coupling between neurotransmis- a [1-13C] glucose precursor to C4-glutamate and subse- sion and neuroenergetics provides a linkage between the quently C4-glutamine. Glucose is metabolized to pyruvate functional imaging signal and specific neuronal processes. The 318 Neuropsychopharmacology: The Fifth Generation of Progress The rate of neuronal glucose oxidation has been deter- mined in several studies from 13C MRS and 1H-13CMRS measurements of cerebral cortex glutamate turnover from a [1-13C] glucose precursor in animal models (2,14–17, 21,22,25–27) and humans (12,13,18,19,29,31,35,43,44). Comparison of the rates of neuronal glucose oxidation mea- sured in these studies with conventional arteriovenous (AV) difference and PET measurements of total glucose con- sumption found that the majority (between 70% and 90%) of total glucose oxidation in the rat and human brain is associated with the large glutamate pool, believed to reflect glutamatergic neurons, measured by MRS. In two recent 13C MRS studies of resting awake human occipital parietal FIGURE 25. The figure shows a 50- cortex, in which other pathways of glucose metabolism were minute accumulation 13C MRS spectrum obtained at 4 T approxi- directly measured, a similar range of between 60% (35) and mately 60 minutes after the start of a 1-13C glucose infusion. The 80% (29) of total glucose oxidation was calculated for the spectrum was obtained from a 72-mL volume centered on the midline in the occipital/parietal lobe. The large percentage of cortical syn- sion of regions of the bottom trace. Labeled resonances include apses that are glutamatergic and the high electrical activity the C2, C3, and C4 positions of glutamate, glutamine, aspartate, of glutamatergic pyramidal cells (4,45) may explain why and -aminobutyric acid (GABA) and the C3 position of lactate. As described in the text (see In Vivo 13C MRS Measurements of such a large fraction of total glucose oxidation is associated the Pathways of Glucose Oxidation: Findings and Validation), the with glutamatergic neurons. Local- ized in vivo 13C-NMR of glutamate metabolism in the human other neuron types, particularly GABAergic, the assignment brain: initial resultsat 4 tesla. DevNeurosci 1999;20:380–388, with of the fraction of glucose oxidation occurring in gluta- permission. In the future, the fraction of glutamate in glia may be measured more accurately through dynamic 13C MRS measurements of glutamate and glutamine labeling during the infusion of label is then transferred to the tricarboxylic acid cycle (TCA) labeled acetate that is incorporated into the brain selectively by the actions of pyruvate dehydrogenase (PDH) and citrate in the glia (28,38,39). When the label reaches C4- -ketoglutarate it is transferred to the large neuronal glutamate pool by the high MRS Measurements of the Rate Glucose activity exchange reactions of the amino acid transaminases Oxidation in GABAergic Neurons and mitochondrial/cytosolic transporters. The large gluta- mate pool was first identified in 14C tracer studies (40). GABA is the major inhibitory neurotransmitter and may Based on kinetic and immunohistochemical staining stud- represent over 30% of the synapses in the cerebral cortex (4, ies, it is believed to correspond to the glutamate pool of 46,47). GABA is synthesized from glutamate in GABAergic glutamatergic neurons (18,41,42). Due to the rate of these neurons by the enzyme glutamic acid decarboxylase (GAD). Almost all of the brain GABA pool tion of 13C label into the trapping glutamate pool, and the is localized to GABAergic neurons under normal condi- kinetic curves analyzed by metabolic modeling to calculate tions. The labeling of the GABA pool from [1-13C] glucose the rate of the neuronal TCA cycle (18). The trapping pool provides a minimum estimate of the rate of glucose oxida- assumption is not essential to determine the rate of the tion in the GABAergic neuron. The estimate is a minimum TCA cycle because subsequent labeling in the C3 position because label may bypass GABA and continue from -keto- of glutamate can be measured to allow calculation of the glutarate/glutamate into the TCA cycle directly. Because glucose is the primary MRS analysis of cerebral cortex from extracts of rats infused fuel for neuronal oxidation, the measurements of the TCA with [1-13C] glucose has been used to measure the time cycle may be converted to measurements of glucose oxida- course of labeling in the GABA and glutamate pools (24, tion using known stoichiometries (17,18). Isotopic labeling of C4-glutamine by the glutamate/glutamine cycle from a [1-13C] glucose precursor. Infused [1-13C] glucose labels neuronal C3-pyruvate. This label is then incorpo- rated via the combined action of pyruvate dehydrogenase and the TCA cycle into -ketoglutarate, which is in rapid exchange with glutamate due to the action of several transaminases. The large glutamate pool in the neuron acts as a label trap with [4-13C]-glutamate accumulating at the rate of the neuronal TCA cycle. Released [4-13C] glutamate from the nerve terminal is taken up by glial transport and the 13C label is transferred to [4-13C] glutamine through the action of glutamine synthetase at the rate of the glutamate/glutamine cycle. Interpretation of glutamine labeling is complicated by 13C label entering by the astrocyte pyruvate dehydrogenase reaction. MRS studies using 15N ammonia, [2-13C] glucose, and [2-13C] acetate, as well as comparison with traditional measurementsof theuptake ofnet glutamineprecursors, haveshown thatthe majorityof labeling in glutamine from [1-13C] glucose is from the glutamate/glutamine cycle (27,36–39). Under conditions of -chloralose anesthesia studies of human cerebral cortex (13,29,35), the rate of the rate of glucose oxidation in GABAergic neurons was GABA synthesis, and by inference glucose oxidation in the estimated to be between 10% and 20% of total neuronal GABAergic pool, was estimated to be on the order of 10% glucose oxidation. This value is similar to previous estimates of total glucose oxidation, although no rates were given. In obtained using isotopic methods and by inhibiting the de- the future, with the higher sensitivity available using inverse gradative enzyme GABA transaminase (24). It should be MRS methods in combination with the development of noted that determination of the rate of GABA synthesis ultrahigh field magnets for human studies, measurements from isotopic methods depends on the assumption that the of the rate of glucose oxidation in GABAergic neurons glutamate precursor pool for GABA is severalfold lower in should be possible in humans. A long-term controversy in brain metabolism studies has been the rate of glucose oxidation in glial cells. Early esti- IN VIVO MRS MEASUREMENTS OF THE mates range from 10% to over 50% of glucose oxidation RATE OF THE GLUTAMATE/GLUTAMINE (49). MRS may be used to measure the rate of glial glucose CYCLE: FINDINGS AND VALIDATION oxidation based on the localization of the enzyme glutamine synthetase in the glia (50). This localization allows the rate The function of the glutamate/glutamine cycle is to prevent of the glial TCA cycle to be calculated from the labeling of depletion of the nerve terminal glutamate pool by synaptic glutamine from glial glutamate. Glial cells have a high capacity for transporting glu- findings were by Van den Berg and co-workers (40), who, tamate from the synaptic cleft in order to maintain a low using 14C isotopic labeling strategies, assigned a rate to glial ECF (extracellular fluid) concentration of glutamate (50, pyruvate dehydrogenase, which they referred to as the small 51).
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