EFFECT OF THE TRIAKONTATETRANEUROPEPTIDE (TTN) ON CORTICOSTEROID SECRETION BY THE FROG ADRENAL-GLAND

Diazepam-binding inhibitor (DBI) was initially isolated from the rat b rain as a result of its ability to compete with benzodiazepines for th eir receptors. Immunohistochemical studies have recently shown the pre sence of peripheral-type benzodiazepine receptor (PBR)- and DBI-like i mmunoreactivity in the frog adrenal gland. The aim of the present stud y was to investigate the effect of two biologically active DBI-derived peptides, the triakonta-tetraneuropeptide [TTN; DBI(17-50)] and the o ctadecaneuropeptide [ODN; DBI(33-50)], on corticosteroid secretion by frog adrenocortical cells. Exposure of frog adrenal explants to graded concentrations of TTN (3.16 x 10(-8) to 3.16 x 10(-6) M) induced a do se-related increase in corticosterone and aldosterone secretion. In co ntrast, ODN did not modify corticosteroid output. When repeated pulses of TTN (10(-6) M) were administered at 2-h intervals, the response of the adrenal explants to the second dose of TTN was markedly reduced, suggesting the existence of a desensitization phenomenon. Exposure of dispersed adrenal cells to TTN also induced a marked stimulation of co rticosteroid secretion, indicating that TTN acts directly on adrenocor tical cells. The central-type benzodiazepine receptor (CBR) agonist, c lonazepam, did not stimulate corticosteroid secretion and the CBR anta gonist, flumazenil, did not block the stimulatory action of TTN. Simil arly, the PBR agonist, Ro5-4864, did not mimic the stimulatory effect of TTN and the PBR antagonist, flunitrazepam, did not affect the stimu latory action of TTN. The present study provides the first evidence fo r a stimulatory effect of TTN on intact adrenocortical cells. The rece ptor mediating the corticotropic action of TTN is not related to centr al- or peripheral-type benzodiazepine receptors. Our data suggest that TTN, released by chromaffin cells, may act as a paracrine factor regu lating the activity of adrenocortical cells.