Regulation of serum leptin levels by gonadal function in rats

The aim of this study was to investigate the regulation of serum leptin lev els by gender and gonadal steroid milieu. Thus, we measured serum leptin le vels by radioimmunoassay in (a) intact male and female rats, (b) female rat s at different stages of the estrous cycle and (c) ovariectomized or orchid ectomized rats. Gonadectomized groups were or were not implanted with silas tic capsules (10 or 30 mm in length, 1.519 mm internal diameter; 3.06 mm ex ternal diameter) containing estradiol or testosterone and decapitated two w eeks later. We found (i) intact female rats weighing 50 g, 250 g and 300 g exhibited hi gher serum leptin concentrations than intact male rats of similar body weig ht; (ii) leptin concentrations were not affected by the phase of the estrou s cycle; (iii) two weeks after gonadectomy serum leptin concentrations: inc reased in both male (from 4.47 +/- 1.87 to 8.76 +/- 1.24 ng/ml) and female (from 1.97 +/- 0.46 to 5.29 +/- 0.51 ng/ml) rats. The ovariectomy-induced i ncrease in serum leptin levels was not dependent, at least completely, on c hanges in body weight since it could be observed when comparisons were made between ovariectomized rats and intact rats in estrus matched for body wei ght. In contrast the effect of orchidectomy on serum leptin levels appears to be dependent on changes in body weight since it was no longer observed w hen comparisons were made with a group of intact male rats matched for body weight. In conclusion, these results suggest that serum leptin concentrations are c ontrolled by gonadal function either directly or as a consequence of change s in body weight.