Cyclic AMP mediates circadian phase shifts induced by microinjection of serotonergic drugs in the hamster dorsal raphe nucleus

We have previously shown that pretreatment with a 5-HT₇ 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-HT₇ 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-HT₇ 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 μM), 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 μM), microinjected into the dorsal raphe 6 h before lights off. Furthermore, microinjection of 8-bromo-cAMP (50 μM) 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-HT₇ but not the 5-HT_5A or 5-HT _1A receptors mediate serotonergic phase shifts.