The Actions of Prostaglandin E ₂, Naloxone and Testosterone on Starvation-Induced Suppression of Luteinizing Hormone-Releasing Hormone and Luteinizing-Hormone Secretion

Abstract

In man and other mammals, starvation is accompanied by a severe suppression of luteinizing hormone-releasing hormone (LHRH) and luteinizing-hormone (LH) secretion, which is caused by unknown alterations in hypothalamic functions. Prostaglandin E₂ (PGE₂), endorphins and testosterone (T) are know to be strongly involved in the regulation of LHRH release. The present study examined whether the influence of these substances on LHRH and LH secretion was affected by starvation. In vitro experiments checked the release of PGE₂ and LHRH from median eminences (ME) of fed male rats and ones starved for 5 days. Stimulation with potassium (80 m M) induced an equally strong release of PGE₂ and LHRH from the MEs of both fed and starved rats. When PGE₂ (10 ⁴ M) was added to the superfusion medium, the potassium-stimulated release of LHRH was significantly enhanced in both groups of animals. The results clearly showed that in the terminal region of the hypothalamic LHRH system the release of this hormone and the action of PGE₂ were not altered by starvation. In vivo experiments tested whether the effects of LHRH, PGE₂, naloxone (NAL), or T on LH secretion were different in intact or castrated male’rats fed or starved for 3 and 5 days. LHRH (250 ng/kg) stimulated the same amount of LH secretion in fed and starved rats. The starvation-induced LH suppression was not due to a dysfunction at the pituitary level. The stimulatory action of PGE₂ (1 mg/kg) on LH was gradually reduced throughout the starvation period. NAL (5 mg/kg) had little, respectively, no effect on LH release on the 3rd or 5th day of starvation. When given simultaneously, NAL did not affect the action of PGE₂ in any of the three nutritional states. The interference of factors other than endorphins or PGE₂ is assumed for starvation-induced suppression of LH secretion. Starvation did not alter the influence of T (ca. 3 ng/ml plasma) on LH secretion at the pituitary level. The stimulatory effect of PGE2 on LH release was completely suppressed by T in fed, as well as in castrated rats starved for 3 or 5 days.