PITUITARY ADENYLATE CYCLASE-ACTIVATING PEPTIDE GENE-EXPRESSION IN CORTICOTROPIN-RELEASING FACTOR-CONTAINING PARVICELLULAR NEURONS OF THE RAT HYPOTHALAMIC PARAVENTRICULAR NUCLEUS IS INDUCED BY COLCHICINE, BUT NOT BY ADRENALECTOMY, ACUTE OSMOTIC, ETHER, OR RESTRAINT STRESS

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a newly discovered neuropeptide that is present in high amounts in hypothalami c neuroendocrine neurons and potently stimulates the accumulation of c AMP within cells of the anterior pituitary. We have employed several s pecific antisera recognizing different parts of the PACAP precursor to elucidate the distribution of PACAP-Like immunoreactivities in the hy pothalamic components of the hypothalamopituitary-adrenocortical axis in sections obtained from normal and colchicine-treated rats. Using im munohistochemistry with avidinbiotin-coupled peroxidase as a reporter system, high numbers of PACAP-immunoreactive perikarya were found in c olchicine-pretreated rats in many of the parvicellular subdivisions of the hypothalamic paraventricular nucleus (PVN). A few cells were also found in the magnocellular subdivisions of the nucleus, and a similar small population of cells was observed in the dorsolateral aspect of the supraoptic nucleus. Using indirect immunofluorescence, the relatio n between CRF- and PACAP-containing neurons in the various parvicellul ar subnuclei of the PVN was studied, and a high degree of colocalizati on was demonstrated in the neurons of the medial parvicellular part of PVN. To further study the functional implications of PACAP in the hyp othalamo-pituitary-adrenocortical axis, we examined the expression of PACAP messenger RNA (mRNA) in the PVN in response to five different st imulatory paradigms that previously have been shown to stimulate CRF m RNA expression in the medial parvicellular part of the PVN. The stimul atory challenges of adrenalectomy, restraint stress, ip injection of h ypertonic saline, ether stress, and intracerebroventricular injection of colchicine induced significant elevations of CRF mRNA expression in the medial parvicellular part of the PVN. In contrast, the expression of PACAP mRNA, which is hardly detectable within the medial parvicell ular part of the PVN, was induced only by colchicine treatment (from u ndetectable levels to 177 +/- 21 dpm/g; mean +/- SEM), whereas PACAP m RNA remained undetectable in this region of the PVN after exposure to any of the other stimulatory paradigms. The onset of colchicine-induce d PACAP mRNA expression in the PVN was rapid (3 h), and PACAP mRNA lev els remained elevated throughout the 48-h observation period. Consider ing the different topography and connections of the parvicellular subn uclei of the PVN, the current observations suggest that PACAP present in parvicellular neurons of the PVN may act not only as a neuroendocri ne transmitter/modulator in the hypothalamopituitary-adrenocortical ax is, but also as transmitter mediating neurotransmission conveyed from the PVN to preganglionic neurons of the autonomic system. However, the physiological stimuli triggering the increased demand for PACAP synth esis within these neurons remain to be elucidated.