PACAP–PAC1 Signaling Regulates Serotonin 2A Receptor Internalization

Mice lacking pituitary adenylate cyclase-activating polypeptide (PACAP) display psychomotor abnormalities, most of which are ameliorated by atypical antipsychotics with serotonin (5-HT) 2A receptor (5-HT(2A)) antagonism. Heterozygous Pacap mutant mice show a significantly higher hallucinogenic response than wild-type mice to a 5-HT(2A) agonist. Endogenous PACAP may, therefore, affect 5-HT(2A) signaling; however, the underlying neurobiological mechanism for this remains unclear. Here, we examined whether PACAP modulates 5-HT(2A) signaling by addressing cellular protein localization. PACAP induced an increase in internalization of 5-HT(2A) but not 5-HT(1A), 5-HT(2C), dopamine D(2) receptors or metabotropic glutamate receptor 2 in HEK293T cells. This PACAP action was inhibited by protein kinase C inhibitors, β-arrestin2 silencing, the PACAP receptor PAC1 antagonist PACAP(6-38), and PAC1 silencing. In addition, the levels of endogenous 5-HT(2A) were decreased on the cell surface of primary cultured cortical neurons after PACAP stimulation and were increased in frontal cortex cell membranes of Pacap(−/−) mice. Finally, intracerebroventricular PACAP administration suppressed 5-HT(2A) agonist-induced head twitch responses in mice. These results suggest that PACAP–PAC1 signaling increases 5-HT(2A) internalization resulting in attenuation of 5-HT(2A)-mediated signaling, although further study is necessary to determine the relationship between behavioral abnormalities in Pacap(−/−) mice and PACAP-induced 5-HT(2A) internalization.