Circadian changes in PACAP type 1 (PAC1) receptor mRNA in the rat suprachiasmatic and supraoptic nuclei.

A receptor for pituitary adenylate cyclase activating polypeptide (PACAP), denoted as PAC1, is expressed in the suprachiasmatic nuclei (SCN). Since the circadian clock demonstrates phase-dependent sensitivity to PACAP, we have used in situ hybridization histochemistry to examine whether PAC1 mRNA is differentially expressed in the rat SCN across the 24-h cycle. There was a significant variation in PAC1 mRNA within the SCN and supraoptic nuclei during the light-dark cycle and in constant darkness, with peaks at the middle of both the real and subjective day and night; no significant variation was observed in the cingulate cortex. The results suggest that the phase-dependent actions of PACAP on the clock may involve phase-specific changes in the availability of PAC1 receptors within the SCN.