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The dopamine hypothesis proposes that antipsychotic drugs act primarily through limbic cortical D2/D2-like dopamine receptor blockade.
To evaluate this hypothesis with the D2/D3-selective SPET probe [123I]-epidepride.
[123I]-epidepride SPETscans were performed on 12 patients with schizophrenia treated with antipsychotics and 11 age-matched healthy controls. [123I]-epidepride specific binding to D2/D3 dopamine receptors was estimated, and relative percentage D2/D3 receptor occupancy by typical antipsychotic drugs determined.
Mean (s.d.) daily dose was 669.12 (516.8) mg chlorpromazine equivalents. Mean percentage D2/D3 receptor occupancy was 81.6 (8.1) and 73.2 (13.9) in the temporal cortex and striatum respectively.
Typical antipsychotic drug treatment is associated with substantial temporal cortical D2/D3 receptor occupancy. The relationship between this and efficacy is poor in patients with treatment-resistant schizophrenia.
5-HT2A receptor antagonism may be crucial to the action of atypical antipsychotics. Previous work has related 5-HT2A receptor blockade to clinical efficacy and protection from extrapyramidal side-effects.
We developed a SPET imaging protocol for assessing 5-HT2A receptor binding using the selective ligand 1231-5-1-R91150. Six healthy volunteers, five clozapine- and five risperidone-treated subjects with DSM–IV schizophrenia were studied. Multi-slice SPET was performed on each subject.
Cortex: cerebellum ratios were significantly lower in both clozapine-and risperidone-treated subjects compared with the healthy volunteers in all cortical regions. There was no difference in occupancy between the two drug-treated groups. No correlation was found between the percentage change in the Global Assessment Scale (GAS) and 5-HT2A receptor binding indices in the drug-treated groups.
Clozapine and risperidone potently block 5-HT2A receptors in vivo. The lack of relationship between receptor binding indices and change in GAS suggests that 5-HT2A receptor blockade may be unrelated to clinical improvement. Future studies will substantiate this finding by studying 5-HT2A receptor binding in large groups of patients treated with both typical and novel atypical antipsychotics.
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