Naltrexone modulates dopamine release following chronic, but not acute amphetamine administration: a translational study.

The opioid antagonist naltrexone has been shown to attenuate the subjective effects of amphetamine. However, the mechanisms behind this modulatory effect are currently unknown. We hypothesized that naltrexone would diminish the striatal dopamine release induced by amphetamine, which is considered an important mechanism behind many of its stimulant properties. We used positron emission tomography and the dopamine D2-receptor radioligand [11C]raclopride in healthy subjects to study the dopaminergic effects of an amphetamine injection after pretreatment with naltrexone or placebo. In a rat model, we used microdialysis to study the modulatory effects of naltrexone on dopamine levels after acute and chronic amphetamine exposure. In healthy humans, naltrexone attenuated the subjective effects of amphetamine, confirming our previous results. Amphetamine produced a significant reduction in striatal radioligand binding, indicating increased levels of endogenous dopamine. However, there was no statistically significant effect of naltrexone on dopamine release. The same pattern was observed in rats, where an acute injection of amphetamine caused a significant rise in striatal dopamine levels, with no effect of naltrexone pretreatment. However, in a chronic model, naltrexone significantly attenuated the dopamine release caused by reinstatement of amphetamine. Collectively, these data suggest that the opioid system becomes engaged during the more chronic phase of drug use, evidenced by the modulatory effect of naltrexone on dopamine release following chronic amphetamine administration. The importance of opioid-dopamine interactions in the reinforcing and addictive effects of amphetamine is highlighted by the present findings and may help to facilitate medication development in the field of stimulant dependence.

Positron emission tomographic analysis of central D1 and D2 dopamine receptor occupancy in patients treated with classical neuroleptics and clozapine. Relation to extrapyramidal side effects.

Positron emission tomography and selective radioligands were used to determine D1 and D2 dopamine receptor occupancy induced by neuroleptics in the basal ganglia of drug-treated schizophrenic patients. In 22 patients treated with conventional dosages of classical neuroleptics, the D2 occupancy was 70% to 89%. Patients with acute extrapyramidal syndromes had a higher D2 occupancy than those without side effects. This finding indicates that neuroleptic-induced extrapyramidal syndromes are related to the degree of central D2 occupancy induced in the basal ganglia. In five patients treated with clozapine, the prototype atypical antipsychotic drug, a lower D2 occupancy of 38% to 63% was found. This finding demonstrates that clozapine is also "atypical" with respect to the central D2 occupancy in patients. During treatment with clozapine, there is a low frequency of extrapyramidal syndromes, which accordingly may reflect the comparatively low D2 occupancy induced by clinical doses of clozapine. Classical neuroleptics, like haloperidol or sulpiride, did not cause any evident D1 occupancy, but the thioxanthene flupentixol induced a 36% to 44% occupancy. In four patients treated with clozapine, the D1 occupancy was 38% to 52%. The D1 occupancy induced by clozapine and flupentixol may contribute to the antipsychotic effect of these drugs.

D2 dopamine receptors in neuroleptic-naive schizophrenic patients. A positron emission tomography study with [11C]raclopride.

Several groups have reported increased densities of D2 dopamine receptors in the basal ganglia of schizophrenic brains postmortem. The significance of this finding has been questioned, since an upregulation of receptor number may be a neuronal response to neuroleptic drug treatment. We have used positron emission tomography and [11C]raclopride to examine central D2 dopamine receptor binding in 20 healthy subjects and 18 newly admitted, young, neuroleptic-naive patients with schizophrenia. An in vivo saturation procedure was applied for quantitative determination of D2 dopamine receptor density (Bmax) and affinity (Kd). When the two groups were compared, no significant difference in Bmax or Kd values was found in the putamen or the caudate nucleus. The hypothesis of generally elevated central D2 dopamine receptor densities in schizophrenia was thus not supported by the present findings. In the patients but not in the healthy controls, significantly higher densities were found in the left than in the right putamen but not in the caudate nucleus.

Central D2-dopamine receptor occupancy in relation to antipsychotic drug effects: a double-blind PET study of schizophrenic patients.

The relationship between central D2-dopamine receptor occupancy and antipsychotic drug effects was examined in a double-blind study. Raclopride was the compound used to induce a selective occupancy of the D2-dopamine receptors. In addition, 11C-labeled raclopride was the radioligand used to measure occupancy by positron emission tomography (PET). Seventeen schizophrenic patients were randomly assigned to one of three parallel groups treated for 4 weeks with daily doses of 2, 6, or 12 mg of raclopride. D2-receptor occupancy was determined by PET at steady-state conditions in 13 patients who completed the study. A statistically significant relationship was demonstrated between antipsychotic effect and degree of D2-receptor occupancy (p < 0.05). Patients with extrapyramidal side effects had significantly higher D2-receptor occupancy than those without (p = 0.02). The finding of a relationship between selective occupancy of the D2-dopamine receptors and clinical effects in schizophrenic patients principally provides new support for the dopamine hypothesis of antipsychotic drug action.

Initial studies of embryonic transplants of human hippocampus and cerebral cortex derived from schizophrenic women.

Human fetal brain tissue was obtained from first-trimester elective abortions of two women who also had schizophrenia. Portions of the embryonic hippocampus or cerebral cortex were transplanted into the anterior eye chamber of immunologically compromised athymic nude rats. In this environment, embryonic brain tissue derived from normal women generally continues organotypic growth and development for many months. Although initial survival after transplantation was normal, the tissue derived from schizophrenic women manifested less robust growth. However, cells in the transplants showed typical neuronal differentiation, with development of different neuronal types, such as pyramidal cells, granule cells, and gamma-aminobutyric acid (GABA)-containing interneurons. Rhythmic electrical activity was also observed, indicative of some local synaptic organization. The presence of messenger RNA (mRNA) for brain-derived neuronotrophic factor (BDNF) was observed using in situ hybridization. The reason for the decreased rate of growth of these transplants remains unknown and the significance of the finding cannot be assessed from only two fetuses. However, these preliminary findings suggest that fetal transplants may be a useful model system for the detection of developmental pathogenic processes in the expression and transmission of schizophrenia.

D1, D2, and 5-HT2 receptor occupancy in relation to clozapine serum concentration: a PET study of schizophrenic patients.

OBJECTIVE: Central D1, D2, and 5-HT2 receptor occupancy in schizophrenic patients treated with clozapine was determined and related to clozapine serum concentrations. METHOD: Seventeen patients treated with clozapine (125-600 mg/day) were examined with positron emission tomography (PET) and one to three of the following selective radioligands: [11C]SCH23390 (N = 11), [11C]raclopride (N = 16), and [11C]N-methylspiperone (N = 5). Clozapine concentration in serum was determined by gas chromatography/mass spectrometry. RESULTS: D2 receptor occupancy (20%-67%) was lower than that previously determined in patients treated with classical neuroleptics (70%-90%). D1 receptor occupancy (36%-59%) was higher than that induced by classical neuroleptics (0%-44%). 5-HT2 receptor occupancy was very high (84%-94%), even at low clozapine doses. Despite a 20-fold range in clozapine serum concentration (105-2121 ng/ml) at the time of PET examination, D2 receptor occupancy was low in all patients and was not described by the curvilinear relationship between serum drug concentration and receptor occupancy that has been demonstrated for classical antipsychotics. CONCLUSIONS: The results confirm in an extended series of patients that clozapine is atypical with regard to degree of D2 receptor occupancy, a finding that may explain the lack of extrapyramidal side effects. The combination of relatively high D1, low D2, and very high 5-HT2 receptor occupancy values is unique to clozapine. Clozapine serum concentrations have not been unequivocally shown to predict clinical effects. In this study, concentration did not predict degree of occupancy in brain. Thus, careful clinical titration cannot be replaced by monitoring of drug concentrations for optimization of clozapine treatment in individual patients.

No support for regional selectivity in clozapine-treated patients: a PET study with [(11)C]raclopride and [(11)C]FLB 457.

OBJECTIVE: The authors' goal was to test the hypothesis of extrastriatal D(2) receptor selectivity as the mechanism of action of clozapine. METHOD: Positron emission tomography (PET) was used to examine extrastriatal as well as striatal dopamine D(2) receptor occupancy in four patients treated with clozapine and three patients treated with haloperidol. The reference radioligand [(11)C]raclopride was used for determination of D(2) receptor occupancy in the striatum. The radioligand [(11)C]FLB 457 was chosen for determination of D(2) receptor occupancy in the thalamus, the temporal cortex, and the frontal cortex. RESULTS: In patients treated with haloperidol the D(2) receptor occupancy was high in all examined brain regions. In clozapine-treated patients the D(2) receptor occupancy was relatively low in both the striatum and the extrastriatal regions. CONCLUSIONS: The results from the present study give no support for the hypothesis of regional selectivity as the mechanism of action for clozapine.

Pharmacokinetics of raclopride formulations. Influence of prolactin and tolerability in healthy male volunteers.

The pharmacokinetic and pharmacodynamic properties of raclopride, a new antipsychotic, were investigated in 16 healthy men. Single 4 mg doses were administered as intravenous infusion, oral solution and 2 extended release (ER) formulations. Total plasma clearance was about 100 ml/min (6.0 L/h), of which renal clearance accounted for 0.2 ml/min, indicating extensive metabolism. The volume of distribution was 1.5 L/kg; mean absolute bioavailability was 65 to 67% following the oral solution and the ER formulations. A transient increase in plasma prolactin levels followed both the intravenous infusion and the oral solution. The ER formulations resulted in a lower increase, which appeared later. However, the area under the prolactin level curve was similar after administration of all dosage forms. The frequency and severity of the most commonly reported side effects (tiredness and restlessness) were higher after the intravenous infusion than after the ER capsules. These findings indicate that such capsules may be advantageous for clinical antipsychotic treatment with raclopride.

D1-, D2-, and 5-HT2-receptor occupancy in clozapine-treated patients.

Positron emission tomography (PET) and selective radioligands were used to determine central D1- and D2-dopamine- and 5-HT2-receptor occupancy in clozapine-treated patients with schizophrenia. In 16 patients, the mean D2-receptor occupancy was 47% (range, 20%-67%), which is lower than that previously demonstrated in patients treated with classical neuroleptics (range, 70%-89%). In 11 patients, the mean D1-receptor occupancy was 44% (range, 33%-59%), which is high when compared with that for classical neuroleptics. In a group of 5 patients, including some treated with low doses of clozapine, the mean 5-HT2-receptor occupancy was a high 89% (range, 84%-94%). By comparison to classical neuroleptics, clozapine is atypical with regard to central binding characteristics in the brain of treated patients.

High 5-HT2 receptor occupancy in clozapine treated patients demonstrated by PET.

The clinical benefit of the atypical antipsychotic drug clozapine may be related to a combined effect on D2 and 5-HT2 receptors. To examine the basis for this hypothesis, positron emission tomography (PET) and the radioligand [11C]N-methylspiperone were used to determine cortical 5-HT2 receptor occupancy in three psychotic patients treated with 125 mg, 175mg and 200mg clozapine daily. The uptake of [11C]N-methylspiperone in the frontal cortex was very low compared to that in neuroleptic naive schizophrenic patients. 5-HT2 receptor occupancy calculated in the clozapine treated patients was 84%, 87% and 90%. The results show that clinical treatment with clozapine induces a high 5-HT2 receptor occupancy in psychotic patients at a low dose level.

Time course of D2-dopamine receptor occupancy examined by PET after single oral doses of haloperidol.

Central D2-dopamine receptor occupancy was followed by repeated PET experiments after administration of single oral doses of haloperidol to four healthy men. D2-dopamine receptor occupancy was high already 3 h after administration of 4 and 7.5 mg haloperidol and remained high for at least 27 h. Akathisia appeared when D2-dopamine receptor occupancy was maximal. After initiation of neuroleptic drug treatment several days or weeks may elapse before antipsychotic effect is evident. The results of this study do not indicate that any late onset of the antipsychotic effect is related to an insufficient D2-dopamine receptor occupancy during the first days of treatment.

An open clinical and biochemical study of ritanserin in acute patients with schizophrenia.

The effect of the selective serotonin-2 antagonist ritanserin was investigated in an open study of patients with schizophrenia. The patients were in an acute psychotic state considered to require neuroleptic medication. No neuroleptic drug was allowed during the study or during the last month preceeding the study. Oxazepam or nitrazepam were allowed for sedation or sleep inducement. Safety, tolerability, potential antipsychotic effect, and drug effects on monoamine metabolites in serum and CSF and prolactin in serum were evaluated. Central D2-dopamine receptor occupancy was determined by positron emission tomography. Ten male patients (mean age 32.4) fulfilling DSM-III-R criteria for schizophrenia were included in the study. Nine of these patients completed 4 weeks' treatment with ritanserin 10 mg b.i.d. The clinical effect was evaluated by means of CPRS and SANS and significant improvement was seen after 4 weeks' treatment both in positive and negative symptoms. Ritanserin was well tolerated and no extrapyramidal symptoms or akathisia were seen. Concentrations of monoamine metabolites and prolactin did not change during treatment. Ritanserin did not occupy D2-dopamine receptors. Thus, no indications of any D2-dopamine-antagonistic activity were obtained. All patients had expected ritanserin levels in plasma during the whole study. This first study of a selective serotonin-2 antagonist in the treatment of acute schizophrenic patients demonstrated significant clinical effects. However, the open design of the study does not allow us to conclude with any certainty that the patients' improvement was due to a specific blockade of serotonin-2 receptors or unspecific factors, although a direct D2-dopamine blockade could be ruled out.

D1- and D2-dopamine receptor occupancy during treatment with conventional and atypical neuroleptics.

Using positron emission tomography and the selective ligands 11C-SCH23390 and 11C-raclopride, central D1- and D2-dopamine receptor occupancy was determined in schizophrenic patients treated with clinical doses of classical and atypical neuroleptics. Treatment with ten chemically distinct classical neuroleptics resulted in a 65-89% occupancy of D2-dopamine receptors. This finding represents strong support for the hypothesis that the mechanism of action of antipsychotic drugs is indeed related to a substantial degree of D2-dopamine receptor occupancy. In two patients treated with the atypical neuroleptic clozapine, 300 mg b.i.d. and 150 mg b.i.d., the D2-dopamine receptor occupancy was 65 and 40%, respectively. D1-dopamine receptor occupancy was determined in six antipsychotic drug-treated patients. No D1-dopamine receptor occupancy was found in patients treated with sulpiride and perphenazine, compounds known to be selective D2-dopamine receptor antagonists. The highest D1-dopamine receptor occupancy, 42%, was found in the patient treated with clozapine 150 mg b.i.d. The effects of the atypical neuroleptic clozapine may be related to a combined effect on both D1- and D2-dopamine receptors.

High 5HT2A receptor occupancy in M100907-treated schizophrenic patients.

RATIONALE: Selective drugs are required to test the hypothesis whether antipsychotic effects may be induced or modulated by 5HT(2A) receptor antagonism. M100907 (previously known as MDL 100,907) is a highly selective 5HT(2A) antagonist in clinical development. OBJECTIVE: To test if the suggested clinical dose of 20 mg M100907 daily induces high 5HT(2A) receptor occupancy in patients with schizophrenia. METHODS: The 5HT(2A) receptor occupancy was determined in two patients with schizophrenia treated with M100907, 20 mg once a day. Positron emission tomography (PET) with (11)C-labeled M100907, was performed prestudy and under steady state conditions. Clinical ratings were performed weekly. RESULTS: Clinical treatment with M100907, 20 mg daily induced a very high 5HT(2A) receptor occupancy in the frontal cortex of both patients (>90%). M100907 was well tolerated. One patient improved minimally and one patient became minimally worse during treatment. CONCLUSIONS: The results confirm that an oral dose of 20 mg per day ensures adequate 5HT(2A) receptor occupancy for clinical proof of concept. The sample is too small to allow conclusions about the clinical effect.

Neuronal development in embryonic brain tissue derived from schizophrenic women and grafted to animal hosts.

The distribution of schizophrenia in families supports the hypothesis of heritable risk factors in schizophrenia, but there is as yet no identification of an inherited neurobiological defect. Human embryonic brain tissue fragments, derived from first trimester abortions, can be transplanted into rat hosts, where they continue neuronal development and are accessible for neurobiological investigation. Hippocampal transplants derived from three schizophrenic women and a larger series of normal women have been studied. If there are heritable neuronal defects associated with schizophrenia, a proportion of the transplants from schizophrenic women would be expected to carry these defects. The transplants from the first two schizophrenic women showed profound abnormalities in survival and growth, compared to the series of transplants from normal women. The transplants from the third schizophrenic woman showed normal growth and development, as well as typical histological and electrophysiological features. The data must be regarded as preliminary, because of the small number of subjects that have been studied. However, they are consistent with the transmission of a defect in neuronal development to some of the offspring of schizophrenic women, a possibility consistent with other studies of the pathogenesis of schizophrenia. The mechanism of the defect in development remains to be identified.

Central D1- and D2-receptor occupancy during antipsychotic drug treatment.

1. It has been unequivocally shown that antipsychotic compounds reduce dopaminergic transmission. A relationship in vitro between the potency for the antipsychotic effect and the blockade of D2-dopamine receptors has been shown. No such relationships have been demonstrated for any other central receptor population. 2. Positron emission tomography (PET) has made it possible to investigate interactions of psychotropic drugs with central receptors in the living human brain. Using the selective D2 receptor antagonist raclopride labelled with positron emitting isotope 11C, it has been shown that chemically distinct classical neuroleptics in conventional doses occupy a high degree (65-89%) of the D2-receptors in the human brain. The results substantiate the opinion that the antipsychotic effects is mediated by a blockade of D2-dopamine receptors. 3. The degree of binding to D1-receptors using the 11C-labelled D1-antagonist from Schering (SCH 23390) as the ligand was also determined. The D1-receptor occupancy seemed to be dependent on the type of the antipsychotic compound studied. 4. The atypical neuroleptic compound clozapine demonstrated a different binding profile than the classical neuroleptics. Thus, clozapine in conventional doses occupied D2-receptors to a smaller extent (40%, 40%, 65%) than classical neuroleptics. The occupation of D1-receptors was higher (40%, 42%) than that of classical compounds (0-36%). 5. The unique clinical profile of clozapine may be related to its potency on both D1- and D2-receptors. The distribution of D1-receptors varies from that of D2-receptors in the human brain which may be one reason for the importance of blocking both D1- and D2-receptors for a full antipsychotic response.

PET imaging of neuroreceptors in schizophrenia.

Among the brain imaging techniques developed during the past two decades positron emission tomography has the highest sensitivity, allowing the analysis of specific neurotransmitter mechanisms in the living human brain. By using a combination of selective ligands labelled with positron emitting isotopes, D1 and D2 dopamine, serotonin 5HT2 and benzodiazepine receptors were examined in schizophrenic patients (DSM-IIIR) and healthy control subjects. With this technique receptor populations could be excellently visualized and quantified with regard to number and binding characteristics in several brain regions. The characteristics of total D1 and D2 dopamine receptor populations in the caudate and putamen did not differ significantly in young drug naive schizophrenic patients and age matched control subjects. On the other hand, there was a highly significant reduction of the D1 signal in high intensity regions of the basal ganglia when [11C]SCH 23390, a selective D1 dopamine receptor antagonist, was used. These results suggest the possibility of a reduced D1 dopamine receptor density in the patch compartment of the basal ganglia in schizophrenia. For 5HT2 and benzodiazepine receptors no major alteration of receptor characteristics was observed in several neocortical and limbic brain regions.

Positron emission tomography studies on D2 dopamine receptor occupancy and plasma antipsychotic drug levels in man.

Positron emission tomography (PET) and the selective radioligand [11C]raclopride were used to measure D2 receptor occupancy in several studies in schizophrenic patients and healthy subjects. Clinical effects of the treatment were recorded, and drug plasma concentrations measured. The hypothesized curvilinear relationship between receptor occupancy and drug plasma concentrations was supported experimentally. We suggest that there may be a distinct threshold for antipsychotic effect at about 70% D2 receptor occupancy, and another threshold for extrapyramidal side effects above 80% D2 receptor occupancy. Receptor occupancy was high even at low plasma drug concentrations. Our data indicate a need to establish dose-response curves for the conventional neuroleptics. Such a dose-response relationship might be expected for a much lower dose interval than those previously examined.

A PET study of D2 dopamine receptor density at different phases of the menstrual cycle.

Behavioral and biochemical studies in animals indicate that central dopaminergic neurotransmission may be modulated by sex steroids. This may be the mechanism underlying the suggested association between estrogen and schizophrenia. The aim was to examine if different levels of sex steroids during the menstrual cycle are associated with variations in D2 dopamine receptor density as measured with positron emission tomography (PET) and [11C]raclopride. Five healthy women were examined, one during two subsequent follicular phases and four during two different phases of their menstrual cycle. In none of the women did the difference in putamen to cerebellum (P/C) ratios (-11 to 10%) exceed the difference in P/C ratios previously reported in test-retest analyses in men (-11 to 9%). The findings do not support the conclusion that there is a menstrual-cycle-dependent variation in D2 receptor density detectable with single PET examinations and [11C]raclopride. Furthermore, a stable P/C ratio throughout the menstrual cycle indicated a stable D2 receptor occupancy in schizophrenic women treated with antipsychotic drugs. Repeated PET examinations of schizophrenic women known to deteriorate during particular phases of their menstrual cycle may further contribute to our understanding of the association between schizophrenia and sex steroids.

No elevated D2 dopamine receptors in neuroleptic-naive schizophrenic patients revealed by positron emission tomography and [11C]N-methylspiperone.

The dopamine hypothesis of schizophrenia received strong support when a two- to three-fold elevation of D2 receptor densities was demonstrated by positron emission tomography (PET) and [11C]N-methylspiperone ([11C]NMSP). In the present study, the reproducibility of this finding was examined by application of a similar method in seven normal comparison subjects and seven neuroleptic-naive schizophrenic patients examined by PET before and after administration of haloperidol, 7.5 mg. After haloperidol, the specific binding of [11C]NMSP was reduced by 80-90%, resulting in a signal-to-noise ratio that was unfavorably low for reliable quantification. No significant difference was found between normal subjects and patients in a descriptive analysis of the time-activity curves or in a nonequilibrium graphical determination of D2 receptor densities in the basal ganglia. The results are consistent with those of a previous quantitative PET study of [11C]raclopride binding, which showed normal densities of D2 receptors in the striatum of neuroleptic-naive schizophrenic patients.