Circadian rhythm in the hypothermic response to serotonin(1A) receptor agonist 8-OH-DPAT in rats

In a recent study, we found a circadian rhythm in the serotonin (5-HT) syndrome (forepaw treading, head weaving, and flat body posture) in the response to the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). The present study examined the rhythm in the responsiveness to 8-OH-DPAT by observing hypothermia, another index of the 5-HT1A receptor function. Rats standardized to L(1800-0600) :D(0600-1800) were subcutaneously administered with 0.16 mg/kg 8-OH-DPAT at one of the following times of day: 00:00, 04:00, 08:00, 12:00, 16:00, 20:00h. Two parameters of assessing the hypothermic response, the maximum decrease in temperature after the administration (MAXDT) and the area under the curve of the decrease in temperature (AUCDT), displayed significant circadian rhythms with the peaks in the middark phase (12:00h). The waveform of the rhythms observed was similar to that of the rhythms that we observed previously in the 5-HT syndrome. The evidence presented here further supports that the central postsynaptic 5-HT1A receptor function exhibits a circadian rhythm.