Short-term leptin infusion does not affect circulating levels of LH, testosterone or cortisol in food-restricted pubertal male rhesus macaques

OBJECTIVES Although the adipocyte protein leptin has been implicated in the control of reproductive function in rodents, its role in primate reproductive physiology is poorly understood. Because primates in puberty show nighttime LH secretion and there is considerable evidence that the fertile state requires adequate nutrition, we reasoned that animals on the verge of reproductive competence would respond to leptin infusions by secreting LH. Food restriction reduces circulating leptin levels and slows or stops the GnRH pulse generator. Therefore, we examined the endocrine effects of leptin infusions in food-restricted male pubertal primates during the night when they normally secrete LH. In addition, we investigated the effect of leptin on in vitro testosterone production by Leydig cells. SUBJECTS Four pubertal male rhesus macaques (Macaca mulatta), 4-5.5 kg in weight (2.5-4-year-old) were examined in this study. Leydig cells from adult male rats were to investigate in vitro effects of leptin, DESIGN To document that animals had entered puberty, blood samples were collected from each of the four animals at 15-minute intervals for 15h both during the day and at night. Since at this age animals secrete LH mainly at night, blood samples were collected at 15-minute intervals from each of the four animals on two separate occasions for 15h between 1500 and 0600h. During the experiment, animals were feeding from 0800 to 0830h, cages were completely cleaned of food at 0900h and the afternoon meal was not given to individual animals on the day they were studied. One of the studies served as the control (food restricted group) and during the other, 2 mg (n=4) or 0.3mg (n=3) of recombinant human leptin was administered intravenously during 2000-0100h (food restricted plus leptin group), Blood samples (1 ml) were collected through the indwelling catheter and immediately transferred from the plastic syringe into chilled glass tubes containing 10 mu l 14% EDTA. The samples were centrifuged at 5-h intervals and the plasma withdrawn and stored frozen at -20 degrees C in polypropylene vials until assays were performed. MEASUREMENTS Bioactive LH was determined and testosterone, cortisol and leptin were measured by radioimmunoassay. RESULTS During daytime experiments in these animals, LH pulses were sometimes observed late in the day and generally continued for 12-15h, Food-restricted pubertal males showed delayed or absent LH pulses. Short-term leptin administration to food-restricted male rhesus macaques had no effect on LH, testosterone, or cortisol levels either during or after the infusion. Leptin also had no direct effect on basal or LH-stimulated testosterone production in Leydig cells. CONCLUSIONS Our data support the notion that leptin is not the missing signal for the acute suppression of reproductive hormones secretion in food-restricted primates.