Multiple feedback regulatory loops upon rat hypothalamic corticotropin-releasing hormone secretion in vitro: Potential clinical implications

To examine whether the hypothalamic corticotropin-releasing hormone (CRH) neuron is regulated by CRH, by products of the proopiomelanocortin (POMC) gene, and/or by glucocorticoids, we used a rat hypothalamic organ culture system in which rat CRH secretion from single explanted hypothalami was evaluated by an RIA (iCRH) specific for rat CRH. The effects of graded concentrations of ovine CRH (oCRH), adrenocorticotropin hormone (ACTH), beta-endorphin (beta-EP), alpha-melanocyte-stimulating hormone (alpha-MSH), corticotropin-like intermediate lobe peptide (CLIP), ovine beta-lipotropin (ovine beta-LPH), and dexamethasone (DEX) upon unstimulated and serotonin- (5HT), acetylcholine- (ACh), and norepinephrine-(NE) stimulated CRH secretion were determined. oCRH and DEX inhibited unstimulated iCRH secretion with ID50 at the 10(-8) M range. ACTH had no detectable suppressive effect at 10(-8) M. oCRH, ACTH, and DEX inhibited 5HT-, ACh-, and NE-stimulated iCRH secretion in a dose-dependent fashion. beta-EP, alpha-MSH, and CLIP also inhibited 5HT-induced iCRH secretion. Of the latter peptides, the strongest inhibitor was beta-EP and the weakest was CLIP. Ovine beta-LPH had only a weak inhibitory effect on 5HT-induced iCRH secretion. Generally, the concentrations required for 50% suppression of neurotransmitter-stimulated iCRH secretion were significantly lower than those required for a similar suppression of unstimulated iCRH secretion. In conclusion, these data suggest the presence of multiple negative feedback loops involved in the regulation of the hypothalamic CRH neuron: an ultrashort CRH-mediated loop, a short, hypothalamic POMC-derived peptide loop, and a long, glucocorticoid-mediated negative feedback loop. The potency of these negative feedback loops may be determined by the state of activation of the CRH neuron.