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

The present study investigated the origin of pituitary adenylate cyclase-ac tivating polypeptide (PACAP) -immunoreactive (IR) fibers innervating guinea pig cardiac ganglia. Immunohistochemistry was performed on whole-mounts co ntaining cardiac ganglia, and sections of stellate, nodose, and dorsal root ganglia (DRG, thoracic levels 1-4), and caudal medulla. In control prepara tions, 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 d ays in explant culture, the number of PACAP-IR cardiac neurons increased ap proximately eightfold. However, virtually all PACAP-IR fibers surrounding t he 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 nodo se ganglia. PACAP immunoreactivity also was present in fibers and primarily small neurons in thoracic DRGs. In situ hybridization demonstrated the pre sence of proPACAP mRNA within neurons in the region of the dorsal motor nuc leus of the vagus and nucleus ambiguus. PACAP immunoreactivity was colocali zed with ChAT immunoreactivity, but not with NPY immunoreactivity or SP imm unoreactivity, in fibers surrounding neurons within cardiac ganglia. We con clude that PACAP-containing fibers innervating the postganglionic parasympa thetic neurons in guinea pig cardiac ganglia are primarily preganglionic pa rasympathetic axons. (C) 2000 Wiley-Liss, Inc.