Reduction of extracellular dopamine and metabolite concentrations in rat striatum by low doses of acute cyamemazine

The low incidence of extrapyramidal effects with atypical neuroleptics has been ascribed to their 5-HT2A- and 5-HT2C-serotonin receptor antagonistic proper-ties. On the other hand, the acute increase in striatal dopamine release by submaximal dopamine D-2 autoreceptor blockade can be respectively reduced and increased by 5-HT2A- and 5-HT2C-antagonists. Cyamemazine is a neuroleptic D-2- and 5-HT2A-receptor antagonist, with small antagonistic activity at 5-HT2C receptors and low incidence of extrapyramidal side effects. Therefore, submaximal cyamemazine was tested in rats for its acute action on the extracellular concentrations of dopamine and dopamine metabolites (DOPAC: 3,4,dihydroxyphenylacetic acid and HVA: 4-hydroxy-3-methoxy-phenyl-acetic acid) in the corpus striatum. The serotonin metabolite 5-HIAA (5-hydroxy-indoleacetic acid) was measured in parallel. Rats prepared for microdialysis (striatum) were intraperitoneally given cyamemazine 1 mg/kg, 5 mg/kg or vehicle (n=4 in each group). Dopamine, DOPAC, HVA and 5-HIAA concentrations in perfusates under basal conditions and after stimulation by high K+ were measured by HPLC coupled to electrochemical detection. Cyamemazine I mg/kg significantly reduced extracellular concentrations of basal dopamine (-77%), DOPAC (-54%), HVA (-54%) and 5-HIAA (-65%). No such effects were seen with the dose of cyamemazine 5 mg/kg or for K+-evoked dopamine release. In conclusion, submaximal cyamemazine can acutely reduce basal dopamine release and metabolism in the rat striatum. Such unusual action can be explained by the original pharmacological profile of cyamemazine (potent D-2- and 5-HT2A-antagonist, with small antagonistic activity at 5-HT2C receptors). Further experiments are required to explain the low incidence of extrapyramidal side actions with cyamemazine.