PACAP–PAC1 Signaling Regulates Serotonin 2A Receptor Internalization

Abstract

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-HT2A) antagonism. Heterozygous Pacap mutant mice show a significantly higher hallucinogenic response than wild-type mice to a 5-HT2A agonist. Endogenous PACAP may, therefore, affect 5-HT2A signaling; however, the underlying neurobiological mechanism for this remains unclear. Here, we examined whether PACAP modulates 5-HT2A signaling by addressing cellular protein localization. PACAP induced an increase in internalization of 5-HT2A but not 5-HT1A, 5-HT2C, dopamine D2 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 PACAP6-38, and PAC1 silencing. In addition, the levels of endogenous 5-HT2A 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-HT2A agonist-induced head twitch responses in mice. These results suggest that PACAP–PAC1 signaling increases 5-HT2A internalization resulting in attenuation of 5-HT2A-mediated signaling, although further study is necessary to determine the relationship between behavioral abnormalities in Pacap−/− mice and PACAP-induced 5-HT2A internalization.

Serotonin Receptor and Transporter Endocytosis Is an Important Factor in the Cellular Basis of Depression and Anxiety

“…Interestingly, mutation of the Ser280 PKC phosphorylation site abolishes 5-HT 2A receptor signaling in response to LSD and DOI treatment, but not serotonin treatment (Karaki et al, 2014 ), highlighting the differential signaling induced by serotonin and putatively therapeutic hallucinogenic compounds. Finally, a recent study by Hayata-Takano et al ( 2021 ) identified that pituitary adenylate cyclase-activating polypeptide (PACAP) regulated the PKC and β-arrestin-dependent endocytosis of human 5-HT 2A , but not 5-HT 1A or 5-HT 2C , in HEK293 cells. In cortical neurons isolated from PACAP knockout mice, 5-HT 2A expression on the cell surface was increased, and these mice displayed a higher response to DOI compared to wild-type mice.…”

“…In cortical neurons isolated from PACAP knockout mice, 5-HT 2A expression on the cell surface was increased, and these mice displayed a higher response to DOI compared to wild-type mice. Importantly, Karaki et al ( 2014 ) and Hayata-Takano et al ( 2021 ) reveal the importance of PKC phosphorylation and receptor internalization in the signaling response of 5-HT 2A to serotonin and hallucinogenic compounds.…”