We have shown recently that leptin modulates at least two aspects of anterior pituitary LH release in ruminants: basal and GnRH-mediated release. To test the hypothesis that leptin directly affects basal and GHRH-mediated GH secretion from the adenohypophysis, we examined the effects of various doses of recombinant ovine leptin (oleptin) on perifused adenohypophyseal (AP) explants and compared responses of tissues from control and fasted cows. Ten mature, ovariectomized and estradiol-implanted cows were assigned to one of two dietary groups: (1) normal-fed (n=5) and (2) fasted for 72 h (n=5). At the end of the fasting period, cows were euthanized and pituitaries were collected. Adenohypophyseal explants were perifused for a total of 6·5 h, including a 2-h treatment at 2·5 h with Krebs-Ringer bicarbonate buffer containing 0, 5, 10, 50, or 100 ng/ml oleptin, and a challenge with GHRH at 4·5 h. All doses of oleptin greater than 5 ng/ml decreased (P,0·01) basal GH secretion compared with controls in tissues collected from normal-fed cows. In contrast, GH release from AP explants from fasted cows treated with the lowest dose of oleptin was 28% (P,0·002) higher than control explants, but larger doses had no effect. Leptin caused an inversely related, dose-dependent increase in GHRH-mediated GH release in tissues from normal-fed cows. Marked increases (P,0·01-P,0·001) in GH release were observed for the 5 and 10 ng/ml oleptin, with lesser (P,0·08) and no effects observed at the 50 and 100 ng/ml doses respectively. In fasted cows, oleptin had no stimulatory effect on GHRH-induced GH release. Results show that leptin can act directly at the anterior pituitary level to modulate GH release, and this effect is dependent upon nutritional history.

Studies assessed, either directly or indirectly, the role of GnRH in leptin-mediated stimulation of LH release in cattle before and after sexual maturation. In experiment 1, the objectives were to determine whether leptin could acutely accelerate the frequency of LH pulses, and putatively GnRH pulses, in prepubertal heifers at different stages of development. In experiment 2, we determined directly whether acute, leptin-mediated increases in LH secretion in the fasted, mature female are accompanied by an increase in GnRH secretion. Ten-month-old prepubertal heifers (experiment 1) fed normal- (n = 5) and restricted-growth (n = 5) diets received three injections of saline or recombinant ovine leptin (oleptin; 0.2 microg/kg body weight, i.v.) at hourly intervals during 5-h experiments conducted every 5 wk until all normal-growth heifers were pubertal. Leptin increased mean concentrations of circulating LH regardless of diet, but pulse characteristics were not altered at any age. In experiment 2, ovariectomized, estradiol-implanted cows (n = 5) were fasted twice for 72 h and treated with either saline or oleptin i.v. (as in experiment 1) on Day 3 of each fast. Leptin increased plasma concentrations of LH and third ventricle cerebrospinal fluid concentrations of GnRH, and increased the amplitude of LH and the size of GnRH pulses, respectively, on Day 3 of fasting compared to saline. Overall, results indicate that leptin is unable to accelerate the pulse generator in heifers at any developmental stage. However, leptin-mediated augmentation of LH concentrations and pulse amplitude in the nutritionally stressed, mature female are associated with modifications in GnRH secretory dynamics.

The increased sensitivities of the LH and FSH assays enabled the detection of gonadotrophins that were < 0.4% of pretreatment serum levels. Using these methods, serum LH was suppressed to nondetectable or near nondetectable levels while serum FSH was highly suppressed but still detectable in the majority of men undergoing steroidal treatment for contraceptive purposes.