Effect of the triakontatetraneuropeptide (TTN) on corticosteroid secretion by the frog adrenal gland

Abstract

Diazepam-binding inhibitor (DBI) was initially isolated from the rat brain as a result of its ability to compete with benzodiazepines for their receptors. Immunohistochemical studies have recently shown the presence of peripheral-type benzodiazepine receptor (PBR)-and DBI-like immunoreactivity in the frog adrenal gland. The aim of the present study was to investigate the effect of two biologically active DBI-derived peptides, the triakontatetraneuropeptide [TTN; ] and the octadecaneuropeptide [ODN; DBI(33-50)], on corticosteroid secretion by frog adrenocortical cells. Exposure of frog adrenal explants to graded concentrations of TTN (3·16 10 8 to 3·16 10 6 M) induced a dose-related increase in corticosterone and aldosterone secretion. In contrast, 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 corticosteroid secretion, indicating that TTN acts directly on adrenocortical cells. The central-type benzodiazepine receptor (CBR) agonist, clonazepam, did not stimulate corticosteroid secretion and the CBR antagonist, flumazenil, did not block the stimulatory action of TTN. Similarly, the PBR agonist, Ro5-4864, did not mimic the stimulatory effect of TTN and the PBR antagonist, flunitrazepam, did not affect the stimulatory action of TTN. The present study provides the first evidence for a stimulatory effect of TTN on intact adrenocortical cells. The receptor mediating the corticotropic action of TTN is not related to central-or peripheral-type benzodiazepine receptors. Our data suggest that TTN, released by chromaffin cells, may act as a paracrine factor regulating the activity of adrenocortical cells.

Ontogeny of Diazepam Binding Inhibitor/Acyl‐CoA Binding Protein mRNA and Peripheral Benzodiazepine Receptor mRNA Expression in the Rat1

“…The high expression levels of both DBI/ACBP mRNA and PBR mRNA in placenta and foetal adrenal cortex fit with the concept of an involvement of one or both of these proteins in steroid hormone synthesis. As mentioned earlier, most data support the notion of a stimulatory effect of DBI/ACBP (or one of its fragments) on steroid synthesis, while the link with the PBR does not appear to be entirely clear ( 7, 14). In foetal adrenal (G16/G18), the cortex exhibited high levels of both mRNA species, whereas the medulla expressed PBR mRNA but was almost devoid of DBI/ACBP mRNA.…”

“…However, it should be noted that data on PBR are, in part, conflicting. While the PBR appears to be involved in the stimulation of neurosteroid synthesis by the DBI/ACBP fragment TTN in frog hypothalamus ( 13), this did not appear to be the case for corticosterone secretion in frog adrenal ( 14), in contrast to the data on mammalian adrenal cells ( 7, 10). The mitochondrial PBR is composed of a 18 kDa isoquinoline carboxamide binding protein and a 34 kDa voltage dependent anion channel (VDAC), which may be transiently or permanently associated with additional proteins such as a 30 kDa adenine nucleotide carrier (ADC).…”

Regulation of Neurosteroid Biosynthesis by Neurotransmitters and Neuropeptides

“…TTN is a selective ligand of TSPO (Slobodyansky et al, 1989; Papadopoulos et al, 2006) while ODN acts as an inverse agonist of CBRs (Ferrero et al, 1984). Endozepines have been shown to regulate steroid secretion by adrenocortical cells (Yanagibashi et al, 1989; Papadopoulos, 1993; Lesouhaitier et al, 1996, 1998) and Leydig cells (Papadopoulos et al, 1990, 1991a,b; Garnier et al, 1993, 1994; Duparc et al, 2003). Concurrently, it has been found that TTN stimulates Δ 5 P biosynthesis by isolated mitochondria from C6 glioma cells (Papadopoulos et al, 1992) indicating that endozepines may be involved in the regulation of neurosteroid production.…”

“…TTN also stimulates steroidogenesis by mitochondria isolated from adrenocortical and testicular Leydig cells (Besman et al, 1989; Yanagibashi et al, 1989; Papadopoulos et al, 1991a). In frog adrenal tissue, TTN stimulates corticosterone and aldosterone secretion in vitro (Lesouhaitier et al, 1996, 1998). Since TSPO is expressed not only in peripheral organs but also in the CNS (Braestrup and Squires, 1977; Benavides et al, 1983a,b; Anholt et al, 1984; Richards and Möhler, 1984; Gehlert et al, 1985), these observations suggest that TTN may be involved in the regulation of neurosteroidogenesis.…”

GABA inhibits endozepine release from cultured rat astrocytes

“…Consistent with their broad distribution, endozepines appear to exert multiple biological activities such as modulation of mela-notropin release from pituitary cells (Tonon et al, 1989), stimulation of steroid synthesis in Leydig and adrenocortical cells (Garnier et al, 1993;Lesouhaitier et al, 1998) and in neuronal cells (Do-Rego et al, 1997), inhibition of glucose-induced insulin release from the pancreas (Ostenson et al, 1994), and stimulation of cholecystokinin secretion from the intestine (Hertzig et al, 1996).…”