Pituitary adenylate cyclase-activating polypeptide directly modulates the activity of proopiomelanocortin neurons in the rat arcuate nucleus.

Pituitary adenylate cyclase-activating polypeptide (PACAP) and the proopiomelanocortin (POMC)-derived peptide alpha-melanocyte-stimulating hormone (alpha-MSH) both regulate multiple neuroendocrine functions and feeding behavior. Two subtypes of PACAP receptor mRNAs, pituitary adenylate cyclase-activating polypeptide-specific receptor (PAC1-R) and pituitary adenylate cyclase-activating polypeptide/vasoactive intestinal polypeptide mutual receptor (VPAC2-R), are actively expressed in the arcuate nucleus of the hypothalamus, where POMC cell bodies are located. This observation led us to investigate the possible regulatory action of PACAP on rat POMC neurons. Double-labeling in situ hybridization histochemistry revealed that approximately 50% of POMC-producing neurons express PAC1-R and/or VPAC2-R mRNAs. The proportion of POMC neurons that also contain PAC1-R mRNA was homogeneous along the rostro-caudal axis of the arcuate nucleus while POMC-positive cell bodies expressing the VPAC2-R subtype were more abundant in the rostral region. Incubation of mediobasal hypothalamic explants with PACAP (10(-7) M; 30 min) increased POMC mRNA expression, and this effect was blocked by PACAP6-38 (10(-6) M). In contrast, incubation with vasoactive intestinal polypeptide (10(-7) M) did not affect POMC mRNA level. Incubation of hypothalamic fragments with PACAP (10(-7) M) caused a significant increase in alpha-MSH content in the tissue and in the incubation medium. Altogether, the present results reveal that exogenous PACAP, acting probably through PAC1-R, regulates the activity of POMC neurons in the rat hypothalamus. These data suggest that the effects of PACAP on the gonadotropin-releasing hormone neuroendocrine axis and the regulation of feeding behavior may be mediated, at least in part, through modulation of POMC neurons.