Two distinct serotonin receptors co-mediate non-photic signals to the circadian clock

Several lines of evidence indicate that serotonin type 7 (5-HT₇) receptors play a critical role for non-photic resetting of the mammalian circadian clock; however, the contributions of other types of 5-HT receptors to non-photic entrainment are not yet clarified. The present study demonstrates that MKC-242, a selective 5-HT_1A receptor agonist, can evoke a non-photic-like phase-response in hamsters in vivo. This phase-shifting response to MKC-242 was antagonized not only by the selective 5-HT_1A receptor blocker WAY100635 but also by the selective 5-HT₇ receptor blocker DR4004. These suggest that synchronous activation of 5-HT_1A and 5-HT₇ receptors mediates non-photic signals to the hamster circadian clock.