We have previously shown that pretreatment with a 5-HT(7) receptor antagonist, SB-269970-A, attenuated phase shifts induced by microinjections of serotonergic agonists in the hamster dorsal raphe (Duncan, M.J., Grear, K.E., Hoskins, M.A.; Brain Research 1008:40-48, 2004). Although SB-269970-A is highly selective for the 5-HT(7) receptors, it has moderate affinity for the 5-HT(5A) receptors, which are present in the hamster dorsal raphe. To further test whether the 5-HT(7) receptors mediate the phase shifting effect of serotonergic agonists in the dorsal raphe, we investigated the role of cAMP because this second messenger is increased by activation of the 5-HT(7) receptors but inhibited by activation of the 5-HT(5A) or 5-HT(1A) receptors. As an additional control experiment, the effect of WAY-100,635, an antagonist to the 5-HT(1A) receptors, was tested. The results showed that local administration of Rp-cAMPS (1 microM), a cAMP antagonist, significantly reduced the phase shift induced by the 5-HT(1A/5A/7) agonist, (R)-(+)8-hydroxy-2-(di-n-propylamino)tetralin (10 microM), microinjected into the dorsal raphe 6 h before lights off. Furthermore, microinjection of 8-bromo-cAMP (50 microM) induced significantly larger phase shifts than vehicle. In the last experiment, microinjection of the dorsal raphe with WAY-100,635 (50 nM) before the 5-HT(1A/5A/7) agonist, 5-carboxyamidotryptamine (100 nM), did not significantly affect the phase shift. These results show that activation of cAMP-dependent kinase by cAMP is necessary and sufficient for induction of phase shifts by serotonergic drugs in the hamster dorsal raphe. Furthermore, these findings are consistent with the hypothesis that the 5-HT(7) but not the 5-HT(5A) or 5-HT(1A) receptors mediate serotonergic phase shifts.