Origin of pituitary adenylate cyclase-activating polypeptide (PACAP)-immunoreactive fibers innervating guinea pig parasympathetic cardiac ganglia

Abstract

The present study investigated the origin of pituitary adenylate cyclase-activating polypeptide (PACAP) -immunoreactive (IR) fibers innervating guinea pig cardiac ganglia. Immunohistochemistry was performed on whole-mounts containing cardiac ganglia, and sections of stellate, nodose, and dorsal root ganglia (DRG, thoracic levels 1-4), and caudal medulla. In control preparations, only 4% of the cardiac neurons were PACAP-IR, although most cardiac ganglion cells were surrounded by a network of PACAP-IR fibers. After 3-7 days in explant culture, the number of PACAP-IR cardiac neurons increased approximately eightfold. However, virtually all PACAP-IR fibers surrounding the cardiac neurons had degenerated, demonstrating that the major source of the PACAP-IR fibers was extrinsic to the cardiac ganglia preparation. PACAP- and choline acetyltransferase (ChAT) immunoreactivity were colocalized in fibers within the stellate ganglia but not within neuropeptide Y (NPY) -IR cell bodies and fibers. PACAP-IR cells and fibers were present in the nodose ganglia. PACAP immunoreactivity also was present in fibers and primarily small neurons in thoracic DRGs. In situ hybridization demonstrated the presence of proPACAP mRNA within neurons in the region of the dorsal motor nucleus of the vagus and nucleus ambiguus. PACAP immunoreactivity was colocalized with ChAT immunoreactivity, but not with NPY immunoreactivity or SP immunoreactivity, in fibers surrounding neurons within cardiac ganglia. We conclude that PACAP-containing fibers innervating the postganglionic parasympathetic neurons in guinea pig cardiac ganglia are primarily preganglionic parasympathetic axons.