Pituitary adenylate cyclase-activating polypeptide (PACAP) is a newly discovered neuropeptide that is present in high amounts in hypothalamic neuroendocrine neurons and potently stimulates the accumulation of cAMP within cells of the anterior pituitary. We have employed several specific antisera recognizing different parts of the PACAP precursor to elucidate the distribution of PACAP-like immunoreactivities in the hypothalamic components of the hypothalamo-pituitary-adrenocortical axis in sections obtained from normal and colchicine-treated rats. Using immunohistochemistry with avidin-biotin-coupled peroxidase as a reporter system, high numbers of PACAP-immunoreactive perikarya were found in colchicine-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 relation between CRF- and PACAP-containing neurons in the various parvicellular subnuclei of the PVN was studied, and a high degree of colocalization was demonstrated in the neurons of the medial parvicellular part of PVN. To further study the functional implications of PACAP in the hypothalamo-pituitary-adrenocortical axis, we examined the expression of PACAP messenger RNA (mRNA) in the PVN in response to five different stimulatory paradigms that previously have been shown to stimulate CRF mRNA expression in the medial parvicellular part of the PVN. The stimulatory challenges of adrenalectomy, restraint stress, ip injection of hypertonic 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 parvicellular part of the PVN, was induced only by colchicine treatment (from undetectable levels to 177 +/- 21 dpm/g; mean +/- SEM), whereas PACAP mRNA remained undetectable in this region of the PVN after exposure to any of the other stimulatory paradigms. The onset of colchicine-induced PACAP mRNA expression in the PVN was rapid (3 h), and PACAP mRNA levels remained elevated throughout the 48-h observation period. Considering the different topography and connections of the parvicellular subnuclei of the PVN, the current observations suggest that PACAP present in parvicellular neurons of the PVN may act not only as a neuroendocrine transmitter/modulator in the hypothalamo-pituitary-adrenocortical axis, but also as transmitter mediating neurotransmission conveyed from the PVN to preganglionic neurons of the autonomic system.(ABSTRACT TRUNCATED AT 400 WORDS)