DIFFERENTIAL 5-HT-MEDIATED REGULATION OF STRESS-INDUCED ACTIVATION OFPROOPIOMELANOCORTIN (POMC) GENE-EXPRESSION IN THE ANTERIOR AND INTERMEDIATE LOBE OF THE PITUITARY IN MALE RATS

The purpose of the present study was to examine the role of 5-hydroxyt ryptamine (5-HT) neurons in mediating the effects of stress on proopio melanocortin (POMC) gene expression in the anterior and intermediate l obes of the pituitary gland. To this aim, the effects of 5-HT depletio n induced by administration of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT; 200 mu g/rat; i.c.v.; 7 days) were investigated on POMC mRNA levels in the anterior and intermediate lobe of control and restraint -stressed rats. Three hours after brief exposure to diethyl ether (2 m in) followed by 60 min of restraint stress increased POMC mRNA levels in the anterior and intermediate lobe of the pituitary. 5,7-DHT neurot oxic lesion, which resulted in a marked depletion of 5-HT (below the l evel of sensitivity of the neurochemical assay, 6 pg/sample) but not o f dopamine or norepinephrine concentrations in the periventricular nuc leus of the hypothalamus, had no effect on basal POMC mRNA levels in t he anterior or intermediate lobe of the pituitary. However, 5-HT deple tion further increased POMC mRNA levels in the anterior pituitary and completely blocked POMC mRNA level enhancement induced in the intermed iate lobe of stressed rats. These results suggest a possible inhibitor y 5-HT tone on POMC gene expression in the anterior pituitary and a st imulatory 5-HT tone in the intermediate lobe of the pituitary under th ese experimental conditions of stress. It appears, therefore, that 5-H T exerts a differential regulation of stress-induced activation of POM C gene expression in the anterior and intermediate lobes of the pituit ary in male rats. (C) 1997 Elsevier Science B.V.