Interactions of nicotinamide with dopamine receptors in vivo

[3H]-Spiperone (20 microCi/kg, 0.0003 mg/kg, SC) was administered to mice. Relative decreases in the 2-hr ratio of accumulation of this dopamine receptor radioligand in the dopaminergic corpus striatum ("specific" plus "nonspecific binding") and the nondopaminergic cerebellum ("nonspecific binding" only) were used to evaluate nicotinamide for possible effects on the dopamine receptor. The nicotinamide-treated animals were also observed for signs of catalepsy. Pretreatment for 30 min with IP doses of 200 and 500 mg/kg reduced accumulation in both areas approximately the same as judged from striatum:cerebellum ratios, which did not differ significantly from controls. However, at 1000 mg/kg, although nicotinamide decreased [3H]-spiperone accumulation in both striatum and cerebellum, "specific binding" was affected more than "nonspecific binding," as judged from a statistically significant decrease in the striatum:cerebellum ratio. This dose also produced a cataleptic state. Nicotinamide at high doses might have some antagonistic effect on dopamine receptors in mice as judged from the greater effect on accumulation of [3H]-spiperone in striatum ("specific binding") than in cerebellum ("nonspecific binding") which appeared to correlate somewhat with the production of a cataleptic state.