Studies on circadian susceptibility rhythm to haloperidol

Rats were given haloperidol in a variety of doses and time combinations and the sedation period were then measured. A clear-cut daily fluctuation in the sedative effect was observed, and the pattern of fluctuation differed depending on the dosage. In an attempt to elucidate the mechanism of this phenomenon, haloperidol was administered at two different times between which there was a significant difference in the sedation period. No difference was found. Thus it is presumed that daily fluctuation in the sedative effect of haloperidol may be ascribed not to the daily fluctuation in the levels of absorption, excretion, metabolism, or distribution of this drug, but rather to the daily fluctuation at the level of catecholamine receptors in the brain. There was a daily fluctuation in the antiapomorphine effect of haloperidol in a variety of doses and time combinations, and the pattern of fluctuation almost equaled that of the sedative effects of haloperidol. The daily fluctuation of the sedative effect is probably due to effects of circadian rhythm in brain dopamine receptors.