MODULATION OF REPRODUCTIVE HORMONE-SECRETION BY NUTRITIONAL INTAKE - STRESS SIGNALS VERSUS METABOLIC SIGNALS

States of chronic undernutrition can cause a profound suppression of r eproductive function. To begin to determine the time course and the na ture of the mechanism by which undernutrition suppresses the activity of the reproductive axis we have examined the effects of brief periods of fasting on reproductive hormone secretion in men and male rhesus m onkeys. In monkeys there is a significant suppression of pulsatile lut einizing hormone (LH) and testosterone secretion after a single day of fasting, that is apparent within the first 4 h after a meal is missed . The suppression of pulsatile LH secretion on a day of fasting does n ot appear to be caused by the psychological stress experienced when mo nkeys are deprived of their daily meal in that monkeys who are maintai ned in a metabolically fed state (by feeding a large excess of food on the day prior to fasting), but are deprived of a meal and displayed b ehavioural agitation associated with fasting, have no suppression of L H secretion. The suppression of LH secretion on a day of fasting canno t be reversed by naloxone infusion, indicating that increased secretio n of opioid peptides is not the primary mechanism causing the decrease in the central drive to the reproductive axis during fasting. In addi tion, although there is a mild activation of the adrenal axis, with sl ight rises in plasma adrenocorticotropic hormone (ACTH) and cortisol c oncentrations on a day of fasting, the increased activity of the adren al axis does not appear to cause the fasting-induced suppression of LH secretion, since there is no correlation between the magnitude of the rise in cortisol secretion and the degree of suppression of LH secret ion during fasting, and dexamethasone injections, which inhibit the ce ntral drive to the adrenal axis, fail to prevent fasting-induced suppr ession of LH secretion. Cumulatively these findings argue against the hypothesis that a general stress response to fasting causes the fastin g-induced suppression of reproductive axis activity. Alternatively, it seems more likely that signals specifically occurring as a result of changes in the metabolic state of the body may be causing the suppress ion of reproductive hormone secretion that occurs during the early sta ges of fasting.