Circadian rhythm in the responsiveness of central 5-HT2A receptor to DOI in rats.

The present study investigated the circadian variation in the behavioral response to the selective 5-HT2A/2C receptor agonist (+/-)-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI). Rats were subcutaneously injected with DOI at 0000, 0400, 0800, 1200, 1600, or 2000 hours. The wet-dog shake (WDS) response was counted following administration of DOI. A circadian rhythm that peaked during the late light period (0400 hours) and reached the lowest point during the late dark period (1600 hours), was observed in the DOI-induced WDS response. In a separate experiment, DOI was administered intra-cerebroventricularly at either 0400 or 1600 hours. The WDS response to the drug at 0400 hours was significantly higher than the response at 1600 hours. These results suggest that the function of the central 5-HT2A receptor exhibits a circadian rhythm.