Positron emission tomography reveals elevated D2 dopamine receptors in drug-naive schizophrenics

PET imaging studies of dopamine D2 receptors: comparison of [18F]N-methylspiperone and the benzamide analogues [18F]MABN and [18F]MBP in baboon brain

A series of positron emission tomography (PET) imaging studies was conducted in a baboon with the benzamide derivatives [18F]2,3-dimethoxy-N-[9-(4-fluorobenzyl)-9-azabicyclo[3.3.1]non an-3 beta-yl]benzamide ([18F]MABN) and [18F]2,3-dimethoxy-N-[1-(4-fluorobenzyl)piperidin-4-yl]be nza mide ([18F]MBP). Studies were also conducted with the butyrophenone [18F]N-methylspiperone (NMSP) for comparison. Tissue-time activity curves of [18F]MABN are similar to those of [18F]NMSP since both compounds displayed approximately the same uptake in the basal ganglia and displayed irreversible binding kinetics in vivo. However, the rapid rate of clearance from the cerebellum and high basal ganglia:cerebellum ratio of [18F]MABN indicate that this compound has a much lower amount of nonspecific binding than [18F]NMSP. [18F]MBP displayed a higher uptake in the basal ganglia relative to [18F]NMSP and [18F]MABN and exhibited reversible binding kinetics in vivo. This property of [18F]MBP is desirable since the uptake of radioactivity in D2-rich ligands is less likely to be influenced by changes in cerebral blood flow. The current data suggest that both [18F]MABN and [18F]MBP are promising ligands for studying dopamine D2 receptors with PET.

D2-receptor imaging with [123I]IBZM and single photon emission tomography in psychiatry: a survey of current status

D2-dopamine receptors can be visualized in the human brain in vivo by Single Photon Emission Tomography (SPECT) and the radiolabeled benzamide [123I]IBZM. The present paper reviews the current status of this type of functional brain imaging with respect to basic methodological aspects, data analysis and quantification. The results from published clinical studies in different psychiatric patient populations and normal controls with [123I]IBZM are reviewed. [123I]IBZM-SPECT is a powerful tool for the investigation of D2-dopamine receptor status in psychiatric disorders, different types of drug treatment as well as therapeutic and side effects of pharmacologic agents. However, there still is a need for standardized imaging times and image-processing procedures. Advantages and disadvantages of SPECT with special regard to Positron Emission Tomography (PET) are also discussed.

Quantitative autoradiography of dopamine-D1 receptors, D2 receptors, and dopamine uptake sites in postmortem striatal specimens from schizophrenic patients

A number of previously published homogenate receptor binding studies have postulated that dopaminergic dysfunction in schizophrenia may be related to abnormalities in dopamine receptors. In this study, postmortem striatal specimens from patients with schizophrenia, normal controls, and psychiatric controls that had received neuroleptics were studied with quantitative autoradiography for dopamine receptors. Autoradiography with single concentrations of [3H]-SCH 23390 for D1 receptors, [3H]-raclopride for D2 receptors, and [3H]-CFT for dopamine uptake sites failed to define significant differences between the study groups. [3H]-CFT bound in a patchy distribution in the striatum that is believed to correspond to striosomal and matrix striatal compartments. There were no differences between groups when [3H]-CFT binding density was examined in the striosomal and matrix compartments. There were also no differences between groups in the percentage of striatal area occupied by striosomal or matrix compartments as defined by [3H]-CFT binding. We conclude that abnormalities of these dopamine receptor subtypes are probably not primary features of the schizophrenic syndrome in the brain collection examined. Previous reports of elevated D2 receptor binding in schizophrenia may have been related to drug treatment effects. Alternatively, the relatively high affinity of ligands used in previous studies for D4 receptors may explain the discrepancy in our findings. Unchanged [3H]-CFT binding in the schizophrenic group also suggests that the density of mesostriatal neuronal terminals is not altered in schizophrenia.

Elevated dopa decarboxylase activity in living brain of patients with psychosis

The hypofrontality theory of the pathogenesis of schizophrenia predicts that cortical lesions cause psychosis. During a search for abnormalities of catecholaminergic neurotransmission in patients with complex partial seizures of the mesial temporal lobe, we discovered an increase of the rate of metabolism of an exogenous dopa tracer (6-[18F]fluoro-L-dopa) in the neostriatum of a subgroup of patients with a history of psychosis. When specifically assayed for this abnormality, patients with schizophrenia revealed the same significant increase of the rate of metabolism in the striatum. The finding is consistent with the theory that a state of psychosis arises when episodic dopamine excess is superimposed on a trait of basic dopamine deficiency in the striatum. The finding is explained by the hypothesis that cortical insufficiency, a proposed pathogenetic mechanism of both disorders, causes an up-regulation of the enzymes responsible for dopa turnover in the neostriatum as well as the receptors mediating dopaminergic neurotransmission.

Ketamine increases the striatal N-[11C]methylspiperone binding in vivo: positron emission tomography study using conscious rhesus monkey

A system for positron emission tomography study of conscious monkeys was newly developed. By use of this system in combination with a microdialysis technique, the effect of ketamine on the binding and release of dopamine was investigated. The administration of ketamine (5 mg/kg) caused sedation accompanied by psychotic symptoms such as nystagmus and stereotyped movements of extremities. During this psychotomimetic period produced by ketamine, a significant increase in the accumulation of the dopamine D2 receptor ligand N-[11C]methylspiperone was observed in the striatum compared with the level in the conscious state, while no significant change was observed in the frontal cortex and cerebellum. In contrast to the use of ketamine as the anesthetic, pentobarbital (25 mg/kg), which produced deeper anesthesia but no psychotic symptoms, caused a decrease in the accumulation of N-[11C]methylspiperone in the striatum. Kinetic analysis, conducted by a graphical method, revealed that the value of the association constant (K3) for N-[11C]methylspiperone binding in the striatum was increased to approximately 130% by ketamine and decreased to approximately 70% by pentobarbital compared with the control values. Furthermore, the release of dopamine from the striatum measured by microdialysis was not affected by ketamine anesthesia. These results indicate that ketamine facilitates striatal dopaminergic neurotransmission through increasing the binding activity of dopamine D2 receptors in the striatum, and suggest that these changes may be related to the psychotomimetic behavioral symptoms of this drug.

Forskolin-induced up-regulation and functional supersensitivity of dopamine D2long receptors expressed by Ltk- cells

Mouse fibroblast Ltk- cells, stably expressing the human dopamine D2long receptor, were grown in the presence of forskolin (100 microM) for 4 or 16 h. The 16 h treatment resulted in a significant up-regulation of the dopamine D2long receptors by 43-96% as measured with [3H]raclopride with no change in the Kd value. A significant increase in the maximal inhibition of acute forskolin-stimulated cAMP accumulation by dopamine (0.1 nM-3 microM) was found both at 4 and 16 h. No such D2long-receptor-coupled response to dopamine could be detected in wild-type, untransfected, Ltk- cells with or without forskolin treatment. Furthermore, basal cAMP levels as well as the maximal response to acute forskolin stimulation decreased in the D2long receptor expressing cells with the treatment, by 33% and 23% respectively. The results indicate that persistent maintenance of high cAMP levels in transfected Ltk- cells may lead to adaptive quantitative and functional changes of the dopamine D2long receptor reminiscent of receptor supersensitivity induced by chronic antagonist treatment in vivo where the receptor targeted is inhibitorily coupled to adenylyl cyclase, as is the D2long receptor. This may provide a model for studying mechanisms underlying dopamine D2 receptor up-regulation and receptor supersensitivity, not readily induced in cell lines.

Pharmacological management of schizophrenia in older patients

Although antipsychotic drugs are the mainstay of treatment in older patients with schizophrenia, much of the theoretical work underpinning their use is based on evidence gained from younger patients. With respect to dosages, there has been little work comparing plasma concentrations of antipsychotics in older patients with those of younger patients. However, there are well documented changes in the pharmacokinetics of these drugs in the elderly, particularly in their hepatic metabolism and renal excretion. There is also evidence that older patients experience more adverse effects from antipsychotics than younger patients. Such effects include extrapyramidal symptoms, postural hypotension and falls. For these reasons it is recommended that starting doses of antipsychotic drugs in older patients should be in the region of 25 to 50% of that recommended for younger patients, and should be slowly increased. Selection of a particular antipsychotic agent is best made on the basis of individual patient characteristics and the adverse effect profiles of particular drugs.

Radiopharmaceuticals for brain imaging

Brain imaging is performed using radiopharmaceuticals by single photon emission computed tomography (SPECT) and positron emission tomography (PET). SPECT and PET radiopharmaceuticals are classified according to blood-brain-barrier permeability, cerebral perfusion and metabolism receptor-binding, and antigen-antibody binding. The blood-brain-barrier (BBB) SPECT agents, such as 99mTcO4-, [99mTc]DTPA, 201TI and [67Ga]citrate are excluded by normal brain cells, but enter into tumor cells because of altered BBB. These agents were used in the earlier period for the detection of brain tumors. SPECT perfusion agents such as [123I]IMP, [99mTc]HMPAO, [99mTc]ECD are lipophilic agents and therefore, diffuse into the normal brain. These tracers have been successfully used to detect various cerebrovascular diseases such as stroke, Parkinson disease, Huntington's disease, epilepsy, dementia, and psychiatric disorders. Xenon-133 and radiolabeled microspheres have been used for the measurement of cerebral blood flow (CBF). Important receptor-binding SPECT radiopharmaceuticals include [123I]QNE, [123I]IBZM, and [123I]iomazenil. These tracers bind to specific receptors in the brain, thus displaying their distribution in various receptor-related cerebral diseases. Radioiodinated monoclonal antibodies were used for the detection of brain tumors. PET radiopharmaceuticals for brain imaging are commonly labeled with positron-emitters such as 11C, 13N, 15O, and 18F, although other radionuclides such as 82Rb, 62Cu and 68Ga also were used. The brain uptake of [13N]glutamate, [68Ga]EDTA and [82Rb]RbCl depends on the BBB permeability, but these are rarely used for brain imaging. Several cerebral perfusion agents have been introduced, of which [15O]water, [13N]ammonia, and [15O]butanol have been used more frequently. Regional CBF has been quantitated by using these tracers in normal and different cerebral disease states. Other perfusion agents include [15O]O2, [11C]CO, [11C]CO2, [18F]fluoromethane, [15O]O2, [11C]butanol, and [62Cu]PTSM. Among the PET cerebral metabolic agents, [18F]fluorodeoxyglucose (FDG) is most commonly used to detect metabolic abnormalities in the brain. Various brain tumors have been graded by [18F]FDG PET. This technique was used to detect epileptic foci by showing increased uptake in the foci during the ictal period and decreased uptake in the interictal period. Differentiation between recurrent tumors and radiation necrosis and the detection of Alzheimer's disease have been made successfully by [18F]FDG PET. Other PET metabolic agents such as [11C]deoxyglucose, and [11C]methylmethionine have drawn attention in the detection of brain tumors. [18F]fluorodopa is a cerebral neurotransmitter agent, which has been found very useful in the detection of Parkinson disease that shows reduced uptake of the tracer in the striatum of the brain.(ABSTRACT TRUNCATED AT 400 WORDS)

Dopamine D2 receptor occupancy measured by single photon emission computed tomography with 123I-Iodobenzamide in chronic schizophrenia

Single photon emission computed tomography (SPECT) with 123I-iodobenzamide (123I-IBZM) was used to study 22 chronic schizophrenic patients. The patients, who were receiving maintenance therapy with typical neuroleptics, had not shown any significant improvement since their admission to the hospital. Basal ganglia/frontal cortex ratios of the uptake of 123I-IBZM did not show significant differences on the basis of neuroleptic dosage in chlorpromazine equivalents. There were, however, significant differences in 123I-IBZM uptake in the basal ganglia among patients characterized by negative, mixed, and positive symptoms of schizophrenia. Although only a small number of patients had shown a positive response to treatment by the time of discharge, D2 receptor blockade was significantly higher in responders than in nonresponders. In addition, there was an inverse correlation between reduced activation as measured by the Brief Psychiatric Rating Scale and the basal ganglia/frontal cortex ratio. These findings suggest a complex pathogenetic link between the blockade of dopamine D2 receptors and psychopathology in chronic schizophrenic patients. SPECT studies with 123I-IBZM appear to have prognostic value in identifying chronic schizophrenic patients who respond poorly to neuroleptic treatment.

Stereotypy, schizophrenia and dopamine D2 receptor binding in the basal ganglia

Animal models suggest a relationship between disturbed striatal dopaminergic function and stereotyped behaviour. Several studies show increased stereotypy in schizophrenic patients compared to normal controls. We investigated the performance of 12 antipsychotic-drug-free schizophrenic patients, and 15 healthy control subjects on a neuropsychological measure of stereotypy--the two-choice guessing task--and correlated this with in vivo striatal dopamine D2 receptor binding, as measured by 123I-iodobenzamide single photon emission tomography. Patients and controls did not differ with respect to the measures of stereotypy derived from the task. However, there was a significant correlation between one of these measures (RR Information) and the degree of striatal D2 receptor binding asymmetry in the patient group only. In view of research connecting striatal and frontal lesions with stereotypy in animals and cognitive inflexibility in humans, these data could suggest a similar disturbance underlying the phenomenon in schizophrenia.

Antipsychotic drug mechanisms and neurotransmitter systems in schizophrenia

The dopamine hypothesis of schizophrenia has been central to the understanding of the mechanism of action of the antipsychotic drugs. Certainly the close correlation between drug dosage and affinity for D2 dopamine receptors indicates the importance of these receptors in the antipsychotic response, although D2 antagonism can also induce acute extrapyramidal side effects. The antipsychotic drugs also act at other neurotransmitter receptors, and these actions contribute to the unwanted side effects of treatment. The use of selective D2 antagonists can diminish such effects, although effects at other receptors are reported to be of value in minimizing extrapyramidal syndromes as well as in enhancing response either directly or by diminishing negative symptoms. The present understanding of these receptor mechanisms is reviewed.

Striatal dopamine receptor binding in epileptic psychoses

In order to study the nature of dopaminergic activity in epileptic psychoses we investigated striatal dopamine receptor binding in 14 patients with epilepsy. Seven of the patients were acutely psychotic when studied, having recently developed a periictal schizophreniform psychosis. The remaining patients were not psychotic. All patients were scanned using single photon emission tomography (SPET) with 123I-IBZM, a specific dopamine D2 receptor ligand. A region of interest analysis was performed. Comparison of mean basal ganglia to occipital cortex activity ratios in the two groups demonstrated significantly reduced specific binding of 123I-IBZM to striatal D2 receptors in the psychotic patients compared to those without psychosis.

Genetic study of dopamine D1, D2, and D4 receptors in schizophrenia

The goal of this study was to define precisely the involvement of the dopamine D1, D2, and D4 receptor genes in the etiology of schizophrenia. A linkage analysis using the lod score method was performed in 37 families originating from France (n = 14) and from the Island of La Réunion in the Indian Ocean (n = 23). No evidence of linkage between schizophrenia and genetic markers located at these loci was found. A simulation study was carried out to gauge the significance of these results. The conclusions of a nonparametric linkage test (i.e., the affected pedigree member method) were equally negative. For each genetic marker, an allelic association with the disease was also sought: 80 unrelated patients and 80 healthy control subjects were tested, and no significant association was found. These results, which are in agreement with those obtained by other groups, do not support the involvement of the dopamine D1, D2, and D4 receptor genes in the pathogenesis of schizophrenia.

A theoretical and neurophysiological consideration on the pathogenesis of positive symptoms of schizophrenia: implications of dopaminergic function in the emotional circuit

The implications of the emotional circuit and the gating mechanism by dopamine (DA) proposed by Maeda in the pathogenesis of positive symptoms of schizophrenia were reconsidered based upon recent advances and findings in the fields of neurophysiology and neuropharmacology and in biological studies of schizophrenia. The gating mechanism by DA was partly supported by new evidence that glutamatergic or GABAergic neurotransmission, which mediates the hippocampo-lateral septal or the piriform cortico-amygdaloid neuronal connections, is likely to be modulated by DA. The compensation-facilitating or gating functions of DA was considered again to play an important role in producing positive symptoms in schizophrenics, who have been suggested to have morphological abnormalities in the limbic system or in the prefrontal cortex prior to the appearance of positive symptoms.

New perspectives on the clinical neurobiology of treatment response in schizophrenia

A structured approach is used to review the rapid progress in neurobiology related to treatment response in schizophrenia. Findings are presented and discussed according to the molecular, cellular and regional levels of brain organization. The genotype-phenotype structure is used to consider potential interactions between genes, clinical manifestations of the illness and treatment response. The integration of neurobiological research into the future development of new treatment strategies for schizophrenia holds significant promise.

D2 dopamine receptor binding in the basal ganglia of antipsychotic-free schizophrenic patients. An 123I-IBZM single photon emission computerised tomography study

We used SPECT to examine striatal D2 receptor binding in 20 antipsychotic-free DSM-III-R schizophrenic patients and 20 age- and sex-matched normal controls. Dynamic single-slice SPECT, at a slice chosen to include the basal ganglia, began immediately following intravenous injection of 185 MBq of 123I-IBZM. A semiquantitative approach was used to generate indices of specific D2 receptor binding in the basal ganglia. There was no overall elevation of D2 receptor binding between patients and controls. A male sex-specific left lateralised asymmetry of striatal D2 receptor binding was found in the patient group. Age-dependent decline of striatal D2 receptors was confirmed in controls, but not in patients. These results suggest that alterations in striatal D2 receptor distribution and density do occur in schizophrenia, and possibly reflect wider disruptions in prefrontal-striatal-limbic circuits.

Central D2 receptors and negative symptoms of schizophrenia

Most in vivo studies of striatal D2 receptor (SD2R) density with positron emission tomography in schizophrenia have attempted to relate this variable to the diagnosis of the illness. In the present study, a relationship between SD2R and clinical dimensions of this psychosis was searched for in a highly selected group of young negative schizophrenics (8 drug-naïve and 2 drug-free). The SD2R density index measured in vivo using 76Br-bromolisuride and PET correlated negatively (r = 0.80, P < 0.01) with a psychomotor dimension of schizophrenia involving blunted affect and alogia. The mean SD2R index of the patients did not differ from that of age-matched control subjects. Therefore, this behavioural dimension accounts for the variance of the SD2R, suggesting that the striatal dopamine system modulates symptoms such as flattened affect and alogia.

Effects of central cholinergic blockade on striatal dopamine release measured with positron emission tomography in normal human subjects

Previously we demonstrated that positron emission tomography (PET) can be used to measure changes in the concentrations of synaptic dopamine and acetylcholine. Whether induced directly or indirectly through interactions with other neurotransmitters, these studies support the use of PET for investigating the functional responsiveness of a specific neurotransmitter to a pharmacologic challenge. In an extension of these findings to the human brain, PET studies designed to measure the responsiveness of striatal dopamine release to central cholinergic blockade were conducted in normal male volunteers using high-resolution PET and [11C]raclopride, a D2-dopamine receptor antagonist. [11C]Raclopride scans were performed prior to and 30 min after systemic administration of the potent muscarinic cholinergic antagonist, scopolamine (0.007 mg/kg). After scopolamine administration, [11C]raclopride binding decreased in the striatum (specific binding) but not in the cerebellum (nonspecific binding) resulting in a significant decrease, exceeding the test/retest variability of this ligand (5%), in the ratio of the distribution volumes of the striatum to the cerebellum (17%). Furthermore, scopolamine administration did not alter the systemic rate of [11C]raclopride metabolism or the metabolite-corrected plasma input function. These results are consistent not only with the known inhibitory influence that acetylcholine exerts on striatal dopamine release but also with our initial 18F-labeled N-methylspiroperidol and benztropine studies. Thus these data support the use of PET for measuring the functional responsiveness of an endogenous neurotransmitter to an indirect pharmacologic challenge in the living human brain.

Dopamine D2 and serotonin S2 receptors in susceptibility to methamphetamine psychosis detected by positron emission tomography

Positron emission tomography (PET) was used to assess the role of dopamine D2 receptors in the striatum and serotonin S2 receptors in the frontal cortex in the susceptibility to methamphetamine-induced psychosis. Subjects were six men who had previously experienced methamphetamine psychosis (methamphetamine subjects) and 10 age- and sex-matched control subjects. The radiotracer used was 11C-N-methylspiperone. Although binding availability, assessed by dynamic analysis, in the two regions did not differ between the two groups, the ratio of binding availability in the striatum to that in the frontal cortex significantly decreased in the methamphetamine subjects as compared with the control subjects. These findings suggest that an imbalance in the activity of these two receptors may be related to the susceptibility to methamphetamine psychosis.

Dopamine D2 receptor density estimates in schizophrenia: a positron emission tomography study with 11C-N-methylspiperone

Positron emission tomography (PET) with 11C-N-methylspiperone as the radioligand was carried out in 25 chronic schizophrenic patients to determine dopamine D2 receptor density estimates in the corpus striatum. The sample included 18 neuroleptic-naive and 7 neuroleptic-free patients. Dopamine D2 receptor density estimates (Bmax) were obtained using a two-scan/four-compartment model. The Bmax estimates for the entire group (33.39 +/- 3.43 pmole/g) were significantly elevated when compared with estimates for the control group (Bmax = 15.63 +/- 2.38). The Bmax values for the entire group of schizophrenic patients showed a significant decline as a function of age. The Bmax values were significantly related to duration of illness (y = 13.2 + 10.3795x - 0.7931x2; r = 0.48). Thirteen patients and seven control subjects were added to our original publication sample (Wong et al., 1986c). The patients' Bmax values, when adjusted for age and sex effects, were significantly different compared with those of control subjects. Clinical data from the entire group were compared with published data from other research groups that have estimated dopamine D2 receptor density using different radioligands and different methods of data analysis. Comparisons of the clinical characteristics of the published studies show significant differences in patient populations, suggesting that discrepancies among published studies may reflect, in part, heterogeneity among groups of schizophrenic patients. The D2 receptor abnormality described in this study may be a late manifestation of disease, and the implications of this observation are discussed.

Pressure-induced disorders in neurotransmission and spontaneous behavior in rats: an animal model of psychosis

Disorders in neurotransmission and spontaneous behavior in rats exposed to a high pressure helium-oxygen mixture that shows interesting parallels with the dopaminergic hypothesis of schizophrenia at both the biochemical and the therapeutic responding levels are reviewed. Furthermore, as human subjects exposed to a very high pressure have shown psychotic episodes, we conclude that the pressure-induced disorders in neurotransmission and spontaneous behavior in rats could constitute a valid animal model of schizophreniform psychosis and a useful tool for both the investigation of the biological mechanisms underlying schizophrenia and the development of new antipsychotic drugs.

Dopamine D4 receptors elevated in schizophrenia

Although the biological basis of schizophrenia is not known, possible causes include genetic defects, viruses, amines, brain structure and metabolism, neuroreceptors, and G proteins. The hypothesis of dopamine overactivity in schizophrenia is based on the fact that neuroleptics block dopamine D2 receptors in direct relation to their clinical antipsychotic potencies. Moreover, dopamine D2 or D2-like receptors are elevated in postmortem schizophrenia brain tissue. This elevation, however, is only found in vivo using [11C]methylspiperone but not [11C]raclopride. The dopamine D4 receptor gene has not yet been excluded in schizophrenia because the 21 gene variants of D4 have not yet been tested. Because the link between D1 and D2 receptors is reduced in schizophrenia tissue, we tested whether one component of this link was sensitive to guanine nucleotide. We report here that the binding of [3H]raclopride to D2 receptors in schizophrenia was not sensitive to guanine nucleotide. This finding permitted analysis of data on the binding of [3H]emonapride to the D2, D3 and D4 receptors. We conclude that the combined density of D2 and D3 receptors (labelled by [3H]raclopride) is increased by only 10% in schizophrenia brain, as found by Farde et al., but that it is the density of dopamine D4 receptors which is sixfold elevated in schizophrenia. These findings resolve the apparent discrepancy, mentioned above, wherein the density of [11C]methylspiperone-labelled sites (D2, D3 and D4), but not that of [11C]raclopride-labelled sites (D2 and D3), was found elevated in the schizophrenia striatum.

Electrical sensitization of the meso-limbic dopaminergic system in rats: a pathogenetic model for schizophrenia

To study whether it was possible to modify mesolimbic dopaminergic activity by intermittent electrical stimulations (IES), 44 rats were either electrically stimulated or sham-stimulated in the ventral tegmental area (VTA) once daily for 70 days. This was done through chronically implanted intracranial electrodes. The intensity of electrical stimulation was determined by the lowest current that elicited a definite motor response. Stimulated rats demonstrated a significantly potentiated behavioral response after 70 stimulations. Seven months after IES rats still demonstrated an increased sensitivity to electrical stimulations in the VTA. A new stimulation period only resulted in a modest additional fall in threshold values. There was a highly significant difference between the current needed to provoke a given response in sensitized rats and in sham-stimulated rats. The behavioral response to stimulation was suppressed both by the dopamine (DA) D2 receptor antagonists haloperidol and raclopride and by the DA D1 receptor antagonist SCH 23390. Furthermore, stimulated rats showed an enhanced response to stimulation with amphetamine and to a lesser extent with apomorphine. Between stimulation periods sensitized animals demonstrated a reduced social interaction. In conclusion intermittent electrical stimulations of the VTA resulted in a syndrome characterized by a hypersensitive response to electrical and pharmacological DA provocation combined with abnormal social interaction. This animal model has points of resemblance with recent interpretations of the DA hypothesis for schizophrenia.

Acute effects of neuroleptics on unmedicated schizophrenic patients and controls

Acute administration of haloperidol (0.2 mg/kg) produced many more side effects in normal controls than in unmedicated schizophrenic patients. Prior to the neuroleptic challenge, both groups were on the peripheral monoamine oxidase inhibitor, debrisoquin, for at least 1 week, in order to enhance the relative contribution of CNS catecholamine metabolites to those measured in both plasma and urine. The patient group had higher plasma levels of methoxyhydroxyphenylglycol (MHPG) and homovanillic acid (HVA) and higher urinary MHPG output than controls, but there were no effects of haloperidol challenge, compared to placebo challenge. In both groups there were significant declines in plasma HVA levels from 8:30 AM to 12 NOON. These declines were unaffected by the haloperidol challenge. Explanations for the marked differences in behavioral effects of haloperidol on patients and controls include the possibility that dopamine receptor numbers were increased in the brains of the schizophrenic patients.

Dopamine D4 versus D2 receptor selectivity of dopamine receptor antagonists: possible therapeutic implications

The dopamine D4 receptor, which is considered a close variant of the dopamine D2 receptor, has recently been cloned. Receptor binding studies demonstrated that clozapine, which is an effective antipsychotic agent but atypical in that it lacks the usual side effects of other antipsychotic agents, has high selectivity for the dopamine D4 receptor versus the dopamine D2 receptor. Comparative binding affinity studies have been carried out for a number of interesting dopaminergic agents using membranes prepared from cloned dopamine D2 and D4 receptor containing cells. It was found that clozapine is selective for the dopamine D4 vs. the D2 receptor by a factor of 2.8. Other compounds with dopamine D4 receptor selectivity were (+)-apomorphine (8.7), (+)-N-propyl-norapomorphine (NPA) (2.4) and melperone (1.3). Compounds with considerable selectivity for the dopamine D2 receptor were haloperidol (0.31), chlorpromazine (0.084), trifluoperazine (0.034) and raclopride (0.001). Overall, the results with the antipsychotic agents tested, support the concept that dopamine D4 receptor selectivity may confer clozapine-like antipsychotic efficacy and furthermore that dopamine D2 receptor selectivity may confer side effect liability (extrapyramidal side effects and tardive dyskinesia).

Comparative single and double immunolabelling with antisera against catecholamine biosynthetic enzymes: criteria for the identification of dopaminergic, noradrenergic and adrenergic structures in selected rat brain areas

Immunodetection of catecholamine biosynthetic enzymes is frequently used for the visualization of central nervous catecholaminergic systems. Because of the method's limited specificity for the different catecholamines, interpretation of the results often presents difficulties. To determine criteria for the identification of dopaminergic, noradrenergic, and adrenergic afferents to the rat amygdaloid complex, comparative immunolabelling for tyrosine hydroxylase (TH), dopamine-beta-hydroxylase (DBH), and phenylethanolamine-N-methyl-transferase (PNMT) was carried out using single- and double-labelling for fluorescence, light- and electron microscopy. The observations were complemented by findings in brainstem and hypothalamic areas. The results indicated that TH-labelling detected preferentially dopaminergic afferents in the lateral central and intercalated amygdaloid nuclei. DBH-labelling detected noradrenergic axons in nuclei lacking PNMT-immunoreactive fibres, and PNMT was a marker for adrenergic axons in the entire complex. For nuclei with combined dense dopaminergic, noradrenergic and/or adrenergic innervation, morphological and immunolabelling characteristics were described which, to a certain extent, enabled identification of the different afferents in anti-TH or anti-DBH-preparations. Using a monoclonal TH-antiserum, noradrenergic and adrenergic axons displayed weaker immunoreactivity than dopaminergic ones, and possessed characteristic morphological features. TH-immunoreactivity in noradrenergic axons differed depending on their origin, and showed intra-axonal compartmentalization. The present study provides a basis for the use of the detection of biosynthetic enzymes in future investigations into the ultrastructure and connectivity of the catecholaminergic amygdala innervation.

Differential induction of neurotensin and c-fos gene expression by typical versus atypical antipsychotics

Precise neural mechanisms underlying the pathophysiology and pharmacotherapy of psychotic disorders remain largely unknown. Present studies investigated the effects of various antipsychotic drugs on expression of the gene encoding the purported endogenous antipsychotic-like peptide neurotensin (NT) in striatal regions of the rat brain. The results demonstrate that several clinically efficacious antipsychotic drugs selectively and specifically increase expression of NT/neuromedin N (NT/N) mRNA in the shell of the nucleus accumbens, a region of the forebrain associated with limbic systems. On the other hand, only typical antipsychotics that cause a high incidence of acute motor side effects increased the expression of NT/N mRNA in the dorsolateral striatum, an extrapyramidal region primarily involved in motor control. In addition, it appears that distinct mechanisms may be involved in the effects of antipsychotics on NT/N gene expression in the dorsolateral striatum versus the accumbal shell. Thus neuroleptic-induced increases in NT/N mRNA expression in the dorsolateral striatum were preceded by a rapid and transient activation of c-fos mRNA, whereas none of the antipsychotics affected c-fos mRNA expression in the accumbal shell. The anatomical characteristics of NT/N gene expression induced by typical versus atypical antipsychotics raise the possibility that increased activity of specific NT neurons may contribute to the therapeutic effects (NT neurons in the accumbal shell) or motor side effects (NT neurons in the dorsolateral striatum) of these drugs.

Striatal binding of the PET ligand 11C-raclopride is altered by drugs that modify synaptic dopamine levels

Bilateral decreases in striatal 11C-raclopride binding were observed in adult female baboons with high resolution PET following administration of drugs that act centrally on dopaminergic neurons. At baseline and following administration of d-amphetamine (a dopamine-releasing drug), GBR-12909 (a potent dopamine reuptake inhibitor), or tetrabenazine (a biogenic amine depleting drug) PET scans of 11C-raclopride binding were obtained in a CTI 931 positron tomograph. In all studies, the ratio of the distribution volumes for the striatum to the cerebellum for 11C-raclopride binding decreased significantly by an average of 16.2% for d-amphetamine, 22.1% for GBR-12909, and 28.3% for tetrabenazine while there were no significant changes observed in the cerebellum or in the rate of systemic metabolism of the radiotracer. These decreases exceed the test/retest variability of striatal 11C-raclopride binding measured in the same animals under identical experimental conditions (Dewey et al., 1992b). Together these studies demonstrate that PET measurements of striatal 11C-raclopride binding can be used to indirectly and non-invasively monitor changes in synaptic dopamine concentrations that result from a variety of neurophysiologic mechanisms.

Cholinergic lesions of mouse striatum induced by AF64A alter D2 dopaminergic behavior and reduce D2 dopamine receptors and D2 dopamine receptor mRNA

To determine whether dopamine receptors are expressed in acetylcholine-containing neurons intrinsic to the striatum, and to study further the interactions between the dopaminergic and cholinergic systems, the irreversibly acting cholinergic neurotoxin, ethylcholine mustard aziridinium ion (AF64A), was injected unilaterally into the mouse corpus striatum, and rotational behavior induced by dopamine agonists and certain molecular events associated with this lesion were determined 7 days after lesioning. Brains were analyzed for D2 dopamine receptors by autoradiography, using [3H](-)sulpiride as a ligand, and for D2 dopamine receptor mRNA and glutamic acid decarboxylase mRNA by Northern blot analysis, using selective radiolabelled oligonucleotide probes. Choline uptake sites were determined by binding assays using [3H]hemicholinium-3, a selective choline reuptake blocker, as a ligand. Mice with intrastriatal injections of AF64A showed ipsilateral rotational responses to the non-selective dopamine agonist apomorphine and to the D2 dopamine agonists, pergolide and quinpirole, but not to the D1 dopamine agonist SKF 38393. This was associated with a significant reduction in D2 dopamine receptors in the ipsilateral striatum and a significant decrease in the amount of D2 dopamine receptor mRNA. That AF64A produced a relatively selective cholinergic deficit was supported by the evidence showing that AF64A lesions significantly reduced [3H]hemicholinium-3 binding sites but did not alter glutamic acid decarboxylase (GAD) mRNA. Further, hemicholinium-3, prevented the AF64A-induced changes in rotational behavior. These results suggest that striatal cholinergic interneurons contain D2 dopamine receptors and express the D2 dopamine receptor gene, and that these interneurons are involved in dopamine-mediated rotational behavior.

Neurocognitive subtypes of chronic schizophrenia

Performance on four key neurocognitive tasks was used to search for subtypes in 104 DSM-IIIR schizophrenic patients. The tasks were the Wisconsin Card Sorting Test to index executive prefrontal cerebral function, intrusion errors from the California Verbal Learning Test to tap hippocampal-diencephalic mnestic function, bilateral hand performance on the Purdue Pegboard to index fine motor-basal ganglial function, and a pro-rated IQ from the Wechsler Adult Intelligence Scale-Revised to measure general cognitive-cerebral function. Neurocognitive data were analyzed using hierarchical and disjoint clustering procedures with Euclidean distance. A five cluster solution was considered optimal. Cluster 1 (n = 24) comprised patients with selective executive-prefrontal dysfunction; cluster 2 (n = 16) suggested normative function; cluster 3 (n = 20) involved patients with executive-motor/cortico-basal ganglial deficit; cluster 4 (n = 25) suggested dementia/multi-focal disturbance; and cluster 5 (n = 19) consisted of patients with selective motor-basal ganglial deficit. The subtypes differed significantly in age, duration of illness, and extent of hospitalization. Suggestive trends in sex composition and anti-Parkinsonian medication patterns were noted. Neurocognitive tasks combined with cluster analysis have promise in reducing and clarifying the heterogeneity of schizophrenia.

Backwardness of human neuroanatomy

To interpret the activity of living human brains, their neuroanatomy must be known in detail. New techniques to do this are urgently needed, since most of the methods now used on monkeys cannot be used on humans.

Dose dependent occupancy of central dopamine D2 receptors by the novel neuroleptic CP-88,059-01: a study using positron emission tomography and 11C-raclopride

Positron emission tomography (PET) and 11C-raclopride were used to measure the occupancy of central dopamine D2 receptors by a new neuroleptic, CP-88,059-1. In a double blind dose escalation study, seven healthy male subjects received a predose of between 2 mg and 60 mg CP-88,059-1, 5 h before PET scanning. One additional subject was assigned to placebo predose. Receptor occupancy was defined as the percentage reduction in binding potential compared with that seen in the subject predosed with placebo and with that seen in seven unmedicated normal volunteers previously studied. Binding of 11C-raclopride decreased in a dose dependent manner, and 85% dopamine D2 receptor occupancy was achieved with the highest dose of CP-88,059-1. The findings confirm that brain dopamine D2 receptors are blocked by CP-88,059-1 and suggest that an effective antipsychotic dose will be between 20 mg and 40 mg. The study high-lights the potential of positron emission tomography in the preclinical evaluation of new drugs.

The cloned dopamine D2 receptor reveals different densities for dopamine receptor antagonist ligands. Implications for human brain positron emission tomography

Since [3H]emonapride ([3H]YM-09151-2), a benzamide neuroleptic, consistently detects more dopamine D2 receptors than [3H]spiperone in the same tissue, we tested whether this property was inherent in the cloned dopamine D2 receptor. We found that the density of dopamine D2 receptors labelled by [3H]emonapride was 1.5-fold to 2-fold (mean of 1.8-fold) higher than the density of dopamine D2 receptors labelled by [3H]spiperone in cells expressing cloned dopamine D2 receptors (either the short form (from rat) or the long form (from human)), matching similar findings in anterior pituitary tissue (rat or pig) or in post-mortem human caudate nucleus tissue. The situation was similar for another benzamide, [3H]raclopride, which revealed 1.3-fold to 1.8-fold (mean of 1.5-fold) more binding sites than that for [3H]spiperone in cell membranes containing cloned dopamine D2 receptors. The apparently different dopamine D2 receptor densities revealed by these two types of 3H-ligands (i.e. [3H]spiperone and the [3H]benzamides), therefore, arise from an inherent property of the dopamine D2 receptor protein. These findings for the cloned dopamine D2 receptor, therefore, partly explain the higher dopamine D2 receptor density measured in human brain (by positron emission tomography) when using radioactive raclopride compared to results using radioactive methylspiperone.(ABSTRACT TRUNCATED AT 250 WORDS)

Study of chronic schizophrenics using 31P magnetic resonance chemical shift imaging

Phosphorus-31 chemical shift imaging showed regional abnormalities of in vivo 31P NMR spectra in the brains of chronic schizophrenic patients. In the left temporal region, the level of % phosphodiesters (PDE) was increased and the level of % gamma alpha beta-ATP (obtained by summation of gamma-ATP, alpha-ATP, and beta-ATP) was decreased. In the basal ganglia, the levels of % PDE were decreased and the level of % phosphomonoesters was increased. The levels of % gamma alpha beta-ATP were increased in the right basal ganglia. The level of % phosphocreatine was decreased in the frontoparietal region. These findings may represent different patterns of dysfunction of membrane phospholipid bilayers and high-energy phosphate metabolism in the specific cerebral regions.

In vivo studies of [125I]iodobenzamide and [11C]iodobenzamide: a ligand suitable for positron emission tomography and single photon emission tomography imaging of cerebral D2 dopamine receptors

Iodobenzamide (IMB) labeled with either [11C] or [125I] was studied in mice and baboons. Pharmacological studies demonstrated an in vivo binding profile compatible with D2 dopamine receptors. Mouse biodistribution studies with both [11C]IMB and [125I]IMB showed a similar brain distribution of radioactivity. Mouse [125I]IMB studies with amphetamine and reserpine pretreatment suggested that IMB may be less susceptible to endogenous dopamine competition for D2 receptor binding in vivo as compared to raclopride. Preliminary baboon studies showed haloperidol competition for IMB binding sites.

Neuropsychiatric disorders: investigation of schizophrenia and substance abuse

Because neuropsychiatric disorders involve functional and neurochemical cerebral abnormalities, positron emission tomography (PET) is ideally suited for their investigation. The use of tracers to measure regional brain glucose metabolism and/or blood flow has allowed the evaluation of brain function in psychiatric patients. The use of radioligands to assess receptor concentration has enabled an evaluation of the extent to which specific neurotransmitter systems are involved in the pathogenesis of mental illness. This article reviews the application of PET technology to the understanding of schizophrenic disorders and substance abuse.

Phenomenology. Its place in schizophrenia research

In some quarters schizophrenia has gained the reputation of a graveyard of research. Few findings stand the test of time, most of the pieces of this particular jigsaw seem to be missing, and it is not easy to make sense of those that are available. Even ‘hard’ scientific findings fail to be replicated, an example being the status of D2receptors in drug-naive schizophrenics (Wonget al, 1986; Fardeet al, 1987).

Quantitation of carbon-11-labeled raclopride in rat striatum using positron emission tomography

Using conventional autoradiographic and tissue counting techniques, the experimental quantitation of in vivo kinetics of prospective or established radioligands for PET is animal and labour intensive. The present study tested the feasibility of using PET itself to quantitate the specific binding of [11C]raclopride to rat striatum and to study the effects of experimental manipulation of endogenous dopamine on binding parameters. Carbon-11-labeled raclopride was given i.v. to anaesthetised rats, positioned in a PET camera and dynamic emission scans acquired over 60 min. Time-activity curves were generated for selected regions of interest, representing striatum and cerebellum and the striatal data fitted to a compartmental model, using cerebellum as the input function, thus circumventing the need for individual metabolite-corrected plasma curves. In control rats, the binding potential (BP), defined as the ratio of the rate constants for transfer from "free to bound" and "bound to free" compartments, was of the order of 0.6. This was reduced threefold by predosing with nonradioactive raclopride. Increasing extracellular dopamine levels by predosing with d-amphetamine resulted in a significant decrease in BP whereas reducing extracellular dopamine by predosing with gamma-butyrolactone caused a significant increase. Thus, despite the limitation in spatial resolution of PET, specific binding of raclopride could be assessed from regional time-activity curves from individual rats. The system was sufficiently sensitive that changes in BP could be detected following modulation of endogenous dopamine levels, a finding of potential relevance to the interpretation of clinical PET data.