Faculty Bibliography

OBJECTIVE: This study was undertaken to measure serotonergic modulation of dopamine in vivo by using positron emission tomography (PET), a radiotracer for the striatal dopamine D2 receptor ([11C]raclopride), and a pharmacologic challenge of the serotonin system (d,l-fenfluramine). METHOD: Two PET studies using [11C]raclopride were performed in 11 normal male subjects before administration of the serotonin-releasing agent and reuptake inhibitor fenfluramine (60 mg p.o.) and 3 hours afterward. A graphical analysis method was used with the [11C]raclopride data to derive the distribution volume of D2 receptors. Plasma levels of fenfluramine, norfenfluramine, homovanillic acid (HVA), cortisol, and prolactin were determined. RESULTS: Levels of fenfluramine and prolactin were elevated 2 hours after fenfluramine administration and remained significantly elevated during the second scan, while levels of HVA and cortisol were not altered significantly during the time of scanning. A significant decrease in the specific binding (striatum) and the nonspecific binding subtracted from the specific binding (striatum minus cerebellum) of [11C]raclopride was observed. The rate of metabolism of [11C]raclopride and the nonspecific binding (cerebellum) were not significantly altered by the fenfluramine intervention. CONCLUSIONS: The observed decrease in [11C]raclopride binding is consistent with an increase in dopamine concentrations and with the ability of serotonin to stimulate dopamine activity. The ability to measure serotonergic modulation of dopamine in vivo may have implications for the study of etiologic and therapeutic mechanisms in schizophrenia, major depressive disorder, obsessive-compulsive disorder, and substance abuse

Positron emission tomography and the fluorodeoxyglucose method were used to measure regional brain metabolism before and 2 h after haloperidol (5 mg, i.m.) in 11 young normal men. These data were compared with measures obtained from nine previously studied normal men who had received no drug intervention. Although a previously published study had demonstrated significantly decreased metabolism in whole brain, neocortex, limbic cortex, thalamus, and caudate nucleus 12 h after a 5-mg dose of haloperidol, the present 2-h study did not show significant metabolic changes despite the fact that significant extrapyramidal effects occurred. Taken together, these studies demonstrate differences in the temporal organization of behavioral and metabolic responses to haloperidol challenge

Buprenorphine (BPN) is a mixed opiate agonist-antagonist used as an analgesic and in the treatment of opiate addiction. We have used [6-O-[11C]methyl]buprenorphine ([11C]BPN) to measure the regional distribution in baboon brain, the test-retest stability of repeated studies in the same animal, the displacement of the labeled drug by naloxone in vivo, and the tissue distribution in mice. The regional distribution of radioactivity in baboon brain determined with PET was striatum > thalamus > cingulate gyrus > frontal cortex > parietal cortex > occipital cortex > cerebellum. This distribution corresponded to opiate receptor density and to previously published data (37). The tracer uptake in adult female baboons showed no significant variation in serial scans in the same baboon with no intervention in the same scanning session. HPLC analysis of baboon plasma showed the presence of labeled metabolites with 92% +/- 2.2% and 43% +/- 14.4% of the intact tracer remaining at 5 and 30 min, respectively. Naloxone, an opiate receptor antagonist, administered 30-40 min after tracer injection at a dose of 1.0 mg/kg i.v., reduced [11C]BPN binding in thalamus, striatum, cingulate gyrus, and frontal cortex to values 0.25 to 0.60 of that with no intervention. There were minimal (< 15%) effects on cerebellum. Naloxone treatment significantly reduced the slope of the Patlak plot in receptor-containing regions. These results demonstrate that [11C]BPN can be displaced by naloxone in vivo, and they affirm the feasibility of using this tracer and displacement methodology for short-term kinetics studies with PET. Mouse tissue distribution data were used to estimate the radiation dosimetry to humans. The critical organ was the small intestine, with a radiation dose estimate to humans of 117 nrad/mCi

OBJECTIVE: The purpose of this report was to determine 1) the effects of chronic haloperidol treatment on cerebral metabolism in schizophrenic patients, 2) the relation between negative symptoms and haloperidol-induced regional changes in cerebral glucose utilization, and 3) the relation between metabolic change and clinical antipsychotic effect. METHOD: Cerebral glucose utilization, as determined by position emission tomography (PET), was studied in 18 male schizophrenic subjects before and after chronic treatment with haloperidol at a standardized plasma level. RESULTS: Overall, haloperidol caused a widespread decrease in absolute cerebral glucose metabolism. The cerebral metabolic response to haloperidol was blunted in patients with high pretreatment negative symptom scores. CONCLUSIONS: Taken together with the results from a previously reported PET study of the effects of an acute amphetamine challenge (in which 14 of the current subjects participated), these data suggest that the negative symptom complex is associated with diminished cerebral response to change in dopaminergic activity. This deficit cannot be solely accounted for by structural differences

Functional brain imaging with positron emission tomography (PET) has opened up new avenues for the investigation of possible functional disturbances related to psychiatric disease as well as pharmacodynamic assessment of drug treatment in vivo. Different strategies to study pharmacologic effects on the brain have been developed in recent years. The basic methods are to measure (a) blood flow or glucose metabolism, (b) parameters of specific receptor binding, or (c) neurotransmitter metabolism. Each of these can be performed either in a resting state or after perturbation with a pharmacologic challenge. Our group has developed a general strategy for investigating pharmacologic effects on brain function: (a) determining indirect drug-induced metabolic changes with fluorodeoxyglucose PET and (b) characterizing functional interactions of neurotransmitter systems by assaying drug-induced displacement of specific receptor ligands. These study designs reflect a paradigm shift where functional coupling of brain regions and interaction of different neurotransmitter systems are seen as the basis for a multitransmitter hypothesis of schizophrenia. In this view, any disturbance in the self-regulatory process is reflected in the loss of functional interaction between systems. An overview of recent studies and their possible clinical importance will be presented

Positron emission tomography and in vivo microdialysis were used to study serotonin's role in modulating striatal dopamine. Serial PET studies were performed in adult female baboons at baseline and following drug treatment, using the dopamine (D2) selective radiotracer, 11C-raclopride. The serotonergic system was manipulated by administration of the selective 5-HT reuptake inhibitor, citalopram, or by serotonergic (5-HT2) receptor blockade (using altanserin, a 5-HT2 antagonist). 11C-Raclopride time-activity data from striatum and cerebellum were combined with plasma arterial input functions and analyzed by calculating a distribution volume as described previously (Logan et al., 1990). Additionally, in vivo microdialysis studies were performed in awake freely moving rats using similar pharmacologic challenges plus SR 46349B, a new highly selective 5-HT2 receptor antagonist. Altanserin and SR 46349B increased extracellular striatal dopamine concentrations (35% and 910%, respectively) while altanserin decreased striatal 11C-raclopride binding (37%). Citalopram, however, decreased extracellular striatal dopamine concentrations (50%) and increased 11C-raclopride binding (33%). These data demonstrate that 5-HT-selective drugs produce changes in striatal dopamine that can be measured noninvasively with PET. Furthermore, the PET data obtained from anesthetized baboons is consistent with in vivo microdialysis data obtained from awake and freely moving rats. Finally, these studies have implications for understanding the therapeutic efficacy of atypical neuroleptics and their utility for treating schizophrenia and affective disorders.

We had previously demonstrated extrastriatal uptake of [18F]N-methylspiroperidol (18F-NMS) in the human brain. This study evaluates the effect of haloperidol on 18F-NMS binding in extrastriatal brain regions. Six schizophrenic patients on haloperidol underwent two PET scans with 18F-NMS at 12 h and at 6 days after haloperidol withdrawal. There was a significant increase in 18F-NMS uptake in striatal, thalamic, and temporal regions but not in frontal, occipital, or cerebellar regions, following drug withdrawal. Haloperidol's ability to block the uptake of 18F-NMS is an indication of the specificity of the radioligand's binding in these regions and supports the postmortem data demonstrating the presence of dopamine D2 receptors in the thalamus and temporal cortex of the human brain