Circadian rhythm in the function of central 5-HT1A receptors is endogenous in nature

In a recent study, we found a circadian rhythm in the response of central serotonin (5-HT)1A receptors to 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). In the present study, the 8-OH-DPAT-induced 5-HT behavioural syndrome was examined in rats kept under continuous dark (DD) conditions for 6 days. The results revealed a circadian rhythm in the behavioural response to 8-OH-DPAT under DD conditions. The pattern of this rhythm was similar to that observed under the light-dark conditions, except for the phase delay that corresponds to the period of free-running rhythm. These results strongly suggest that the rhythm in the function of central 5-HT1A receptors is driven by the endogenous oscillator.