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-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, ß-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.

Increased behavioral and neuronal responses to a hallucinogenic drug in PACAP heterozygous mutant mice.

Accumulating evidence from human genetic studies implicates the pituitary adenylate cyclase-activating polypeptide (PACAP) gene as a risk factor for psychiatric disorders, including schizophrenia and stress-related diseases. Mice with homozygous disruption of the PACAP gene display profound behavioral and neurological abnormalities that are ameliorated with the atypical antipsychotic and dopamine D2 and serotonin (5-HT)2 antagonist risperidone and the 5-HT2 receptor antagonist ritanserin; however, the underlying mechanisms remain unknown. Here, we investigated if PACAP heterozygous mutant (PACAP(+/-)) mice, which appear behaviorally normal, are vulnerable to aversive stimuli. PACAP(+/-) mice were administered a 5-HT2 receptor agonist, (±)-2,5-dimethoxy-4-iodoamphetamine (DOI), a hallucinogenic drug, and their responses were compared with the littermate wild-type mice. After DOI injection, PACAP(+/-) mice showed increased head-twitch responses, while their behavior was normal after saline. DOI induced deficits in sensorimotor gating, as determined by prepulse inhibition, specifically in PACAP(+/-) mice. However, other 5-HT2 receptor-dependent responses, such as corticosterone release and hypothermia, were similarly observed in PACAP(+/-) and wild-type mice. c-Fos expression analysis, performed in various brain regions, revealed that the DOI-induced increase in the number of c-Fos-positive cells was more pronounced in 5-HT2A receptor-negative cells in the somatosensory cortex in PACAP(+/-) mice compared with wild-type mice. These results indicate that PACAP(+/-) mice exhibit specific vulnerability to DOI-induced deficits in cortical sensory function, such as exaggerated head-twitch responses and sensorimotor gating deficits. Our findings provide insight into the neural mechanisms underlying impaired behavioral responses in which 5-HT2 receptors are implicated.