Neuroanatomical and Physiological Evidence for the Involvement of Pituitary Adenylate Cyclase‐Activating Polypeptide in the Regulation of the Distal Lobe of the Frog Pituitary

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Abstract
Pituitary adenylate cyclase-activating polypeptide (PACAP) is a 38 amino-acid peptide which belongs to the glucagon/secretin/vasoactive intestinal peptide superfamily. The sequence of PACAP is identical in all mammalian species studied so far but frog PACAP differs by one amino-acid from mammalian PACAP. The aim of the present study was to investigate the presence of PACAP in the hypothalamo-pituitary complex of the frog Rana ribibunda and to determine the biological activity of frog PACAP on homologous pituitary cells. The distribution of PACAP-containing neurons and fibers was examined by the indirect immunofluorescence method using an antiserum raised against the N-terminal region of the peptide. In the hypothalamus, PACAP-immunoreactive perikarya were localized in the preoptic nucleus and the dorsal and ventral infundibular nuclei. Beaded nerve fibers were observed coursing from the ventral infundibular nucleus to the external vascular layer of the median eminence. A dense network of immunoreactive axons terminated in the vicinity of the capillaries of the hypophysial portal system. The neurointermediate lobe and the distal lobe of the pituitary were devoid of immunoreactive elements. The amount of PACAP-like immunoreactive material in hypothalamus extracts was measured by radioimmunoassay; the apparent concentration of PACAP was 4.5 ng/mg protein. Synthetic frog PACAP38 and PACAP27 induced a similar dose-dependent stimulation of cAMP production in isolated frog distal lobe pituitary fragments (ED50 = 2 x 10(-8) M). At the maximum dose tested (5 x 10(-6) M), both frog PACAP38 and PACAP27 produced a 4-fold increase in cAMP production.(ABSTRACT TRUNCATED AT 250 WORDS)

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