8-OH-DPAT as a 5-HT7 agonist: phase shifts of the circadian biological clock through increases in cAMP production.

Neurons in the suprachiasmatic nucleus (SCN), the site of the endogenous biological clock in mammals, fire spontaneously, peaking in firing rate near ZT6 or at the midpoint of the light phase in a 12:12 light-dark cycle. In rat hypothalamic slices, tissue incubations with drugs can produce a shift in this daily rhythm, the magnitude of which is dependent upon dose and the time of treatment. Previous work with 8-OH-DPAT had noted its ability to produce a phase advance, an earlier occurrence of the peak in neuronal firing, when applied at ZT6. Activation of 5-HT7 receptors was thought to be responsible for the shift, despite the clear preference of 8-OH-DPAT for 5-HT1A sites in terms of receptor binding affinity. In the present work, the actions of 8-OH-DPAT in SCN slices were confirmed and expanded to include additional dose-response and antagonist treatments. By itself, 8-OH-DPAT produced a concentration-dependent phase advance that was sensitive to co-application with 5-HT7 antagonists (ritanserin, mesulergine, SB-269970), but not to 5-HT1A antagonists (WAY-100,635, UH-301). Assignment of the receptor mechanisms for the antagonists employed was accomplished in experiments measuring binding affinities and the generation of cAMP, the latter monitored in a HEK-293 cell line expressing the r5-HT7 receptor and in tissue derived from rat SCN. The results indicate that the increases observed in cAMP levels are small but appear to be sufficient to produce a pharmacological resetting of the clock pacemaker. By aiding in the identification of the 5-HT receptor subtype responsible for the observed phase shifts and cAMP changes, 8-OH-DPAT represents an important pharmacological tool for 5-HT7 receptor activation, essentially broadening its role as the prototypical 5-HT1A agonist to one combining these two receptor activities.