The distribution of tachykinin-like immunoreactivity (LI) was studied in the adrenal gland of the frog Rana ridibunda using the immunofluorescence technique. A dense network of varicose fibers immu- noreactive to both substance-P (SP) and neurokinin-A (NKA) was found in the adrenal tissue. In contrast, no positive fibers could be detected using antineurokinin-B (NKB) antibodies. At the electron microscope level, the immunogold technique revealed that tachykinin- LI was sequestered in dense core vesicles of 50-70 nm. Bilateral transection of either splanchnic or vagus nerves or total lesion of celiac sympathetic ganglion did not suppress tachykinin-LI. A combination of HPLC analysis and RIA detection was used to characterize tachykinin-LI in frog adrenal extracts. Two major peaks were resolved, which coeluted, respectively, with synthetic ranakinin, a novel tachykinin previously isolated from the frog brain, and [Leu3, Ile7]NKA previously isolated from the frog gut. No NKB could be detected in the extracts. The effects of various synthetic tachykinins on corticosteroid secretion were studied using perifused frog adrenal slices. For concentrations ranging from 10-8-10-4 M, SP induced a dose-dependent stimulation of corticosterone and aldosterone release. A desensitization phenomenon was observed when iterative or prolonged infusions of SP were administered to the tissue. All mammalian or amphibian tachykinin-related peptides tested in our model also enhanced corticosteroid production. The effectiveness of the tachykinins tested was: [Pro1] NKB > NKA > ranakinin > [Pro9]SP > SP > kassinin > physalaemin > NKB > [Leu3, lie7] NKA. SP also enhanced prostaglandin E2 and prostacyclin release in the effluent perifusate and the response preceded by 10-15 min the increase in corticosteroid output. Indomethacin (5 x 10-6 m), a specific blocker of cyclooxygenase activity, totally suppressed SP-evoked steroid secretion. These data indicate that tachykinin- induced stimulation of steroidogenesis was mediated through activation of the arachidonic acid cascade. Taken together, our results show that the frog adrenal gland is innervated by a dense network of peptidergic fibers containing both ranakinin and [Leu3, lie7] NKA, which, in vitro, stimulates corticosteroid secretion by adrenocortical cells through a prostaglandin-dependent mechanism. The present results support the view that tachykinins released by nerve fibers exert a neuroendocrine control on corticosteroid release in amphibians.
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