ASSOCIATION BETWEEN PITUITARY ADENYLATE CYCLASE-ACTIVATING POLYPEPTIDE AND THYROTROPIN-RELEASING-HORMONE IN THE RAT HYPOTHALAMUS

Pituitary adenylate cyclase-activating polypeptide (PACAP) is present in many regions of the hypothalamus including the paraventricular nucl eus (PVN). In this study the anatomical relationship between PACAP- an d thyrotropin-releasing hormone (TRH)-immunoreactive neuronal elements was investigated in the rat hypothalamus. Using a well-characterized mouse monoclonal antibody against PACAP and a rabbit polyclonal antise rum against TRH, we found numerous nerve fibers with PACAP-immunoreact ivity (ir) closely apposed to TRH neurons in the PVN suggesting synapt ic contacts. Electron microscopy confirmed the presence of synapses be tween PACAP-ir terminals and TRH-ir perikarya and various dendritic pr ofiles as well as between PACAP-ir terminals and unlabeled perikarya a nd small-to medium-sized dendrites. Coexistence of the two peptides in perikarya of the PVN was limited to only a few neurons in the periven tricular subdivision, but PACAP-ir and TRH-ir extensively coexisted in perikarya of the perifornical cell group, medial preoptic area, later al hypothalamus and dorsomedial nucleus. The interactions between PACA P-containing neuronal processes and TRH neurons in the PVN raise the p ossibility that PACAP modulates the secretion of TRH destined for regu lation of anterior pituitary TSH. The more general association between PACAP and TRH in other regions of the hypothalamus suggests a further role for PACAP as a cofactor in the function of TRH neurons. (C) 1997 Elsevier Science B.V.