A detailed study on the role of sex steroid milieu in determining plasma leptin concentrations in adult male and female rats

We examined the effects of sex steroid milieu on plasma leptin levels in ad ult male and female rats. Since the body weight is known to influence lepti n concentrations, the hormone was measured in rats with a very similar body weight (about 250 g) throughout this study. Plasma leptin levels were sign ificantly higher in female than in male rats. Orchidectomy (ODX) caused a s ignificant rise in leptin, and replacement of a physiological dose of testo sterone (T) completely abolished the effect of ODX. Since the effect of tam oxifen (estrogen antagonist) coadministered with T on leptin levels in ODX rats was the same as that of T alone, it was suggested that the suppressive effect of T on leptin may be mediated by the androgenic potency of T, but not by its aromatized product, estradiol. In female rats, plasma leptin con centrations did not change significantly during the estrous cycle. Furtherm ore, leptin levels were not affected either by ovariectomy alone or by the administration after ovariectomy of physiological doses of estradiol, proge sterone, or both. This is the first study to demonstrate in rats with a ver y similar body weight the existence of a clear sexual difference in plasma leptin levels, and also a suppressive action of T on the adipocyte hormone concentrations. (C) 1999 Academic Press.