Garcia de Yebenes E, Li S, Pelletier G
MRC Group in Molecular Endocrinology, CHUL Research Center, Québec, Canada.
Brain Res. 1997 Mar 7;750(1-2):277-84. doi: 10.1016/s0006-8993(96)01367-4.
The neurotransmitter gamma-aminobutyric acid (GABA) exerts a tonic inhibitory influence on proopiomelanocortin (POMC) neurons in the hypothalamus as well as on the melanotrope cells of the intermediate lobe (IL) of the pituitary gland. Moreover, the activation of the GABAA receptor complex by different ligands has been shown to exert a negative influence on the POMC gene expression at the hypothalamic level. In order to elucidate the in vivo regulation of the POMC mRNA levels in the intermediate lobe of the pituitary by endogenous ligands of the GABAA receptor complex, we have studied the effect of intravenous (i.v.) and intracerebroventricular (i.c.v) injections of octadecaneuropeptide (ODN), a peptide derived from diazepam-binding inhibitor (DBI). The possible involvement of neurosteroids in the action of ODN on melanotropic cells was evaluated following inhibition of two enzymes involved in the biosynthesis of neurosteroids known as activators of G3BAA receptor complex: trilostane, an inhibitor of 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD), and MK-906, an inhibitor of 5 alpha-reductase. The i.v. injection of ODN produced a dose-dependent inhibition of POMC gene expression in the IL. The i.c.v. injection of ODN also depressed POMC mRNA. These effects were completely reversed by the concomitant administration of the GABAA antagonist picrotoxin. Similar results were obtained in POMC neurons in the arcuate nucleus (AN) of the hypothalamus. Trilostane administration induced an increase in POMC mRNA and also prevented the inhibitory influence of ODN. The neurosteroid pregnenolone-sulfate, a negative modulator of the GABAA receptor, also stimulated POMC gene expression. On the other hand, MK-906 produced a decrease in mRNA levels and could not reverse the effect of ODN. The results indicate that activation of the GABAA receptor complex by the endogenous benzodiazepine receptor ligand ODN can induce a negative regulation of POMC gene expression in the IL of the pituitary and neurons in the AN. The present results do not provide clear evidence that neurosteroids are involved in the action of ODN on POMC gene expression in the IL.
神经递质γ-氨基丁酸（GABA）对下丘脑的阿片促黑素皮质素原（POMC）神经元以及垂体中间叶（IL）的促黑素细胞发挥着持续性抑制作用。此外，不同配体对GABAA受体复合物的激活已被证明会对下丘脑水平的POMC基因表达产生负面影响。为了阐明GABAA受体复合物的内源性配体对垂体中间叶POMC mRNA水平的体内调节作用，我们研究了静脉注射（i.v.）和脑室内注射（i.c.v）源自地西泮结合抑制剂（DBI）的十八肽（ODN）的效果。在抑制两种参与神经甾体生物合成的酶（已知为G3BAA受体复合物激活剂）后，评估了神经甾体在ODN对促黑素细胞作用中的可能参与情况：曲洛司坦，一种3β-羟基类固醇脱氢酶（3β-HSD）抑制剂；以及MK-906，一种5α-还原酶抑制剂。静脉注射ODN会对IL中的POMC基因表达产生剂量依赖性抑制。脑室内注射ODN也会降低POMC mRNA水平。这些作用被同时给予的GABAA拮抗剂印防己毒素完全逆转。在下丘脑弓状核（AN）的POMC神经元中也获得了类似结果。给予曲洛司坦会导致POMC mRNA增加，并且还能防止ODN的抑制作用。神经甾体孕烯醇酮硫酸盐，一种GABAA受体的负性调节剂，也刺激POMC基因表达。另一方面，MK-906会导致mRNA水平下降，并且不能逆转ODN的作用。结果表明，内源性苯二氮䓬受体配体ODN对GABAA受体复合物的激活可诱导垂体IL和AN中神经元的POMC基因表达的负调节。目前的结果没有提供明确证据表明神经甾体参与了ODN对IL中POMC基因表达的作用。
