VIPR2 and Huntington disease: Preclinical: The role of PACAP and the PAC1 receptor in HD is not well understood, but with post-mortem human HD hippocampal samples showing specific decreases of PAC1 without changes in VPAC1 or VPAC2, its involvement and possible therapeutic efficacy have been explored in recent years by Xifró and colleagues in R6/1 mouse models of HD.