EXPERIMENTAL STUDY In vitro pituitary and testicular effects of the leptin-related synthetic peptide leptin(116–130) amide involve actions both similar to and distinct from those of the native leptin molecule in the adult rat

The obese gene (ob) product, leptin, has recently emerged as a key element in body weight homeostasis, neuroendocrine function and fertility. Identification of biologically active, readily synthesized fragments of the leptin molecule has drawn considerable attention, as they may provide a powerful tool for detailed characterization of the biological actions of leptin in different experimental settings. Recently, a fragment of mouse leptin protein comprising amino acids 116–130, termed leptin(116–130) amide, was shown to mimic the effects of the native molecule in terms of body weight gain and food intake, and to elicit LH and prolactin (PRL) secretion in vivo. As a continuation of our previous experimental work, the present study reports on the effects of leptin(116–130) amide on basal and stimulated testosterone secretion by adult rat testis in vitro. In addition, a comparison of the effects of human recombinant leptin and leptin(116–130) amide at the pituitary level on the patterns of LH, FSH, PRL and GH secretion is presented. As reported previously by our group, human recombinant leptin(10– 10 M) significantly inhibited both basal and human chorionic gonadotrophin (hCG)-stimulated testosterone secretion in vitro. Similarly, incubation of testicular tissue in the presence of increasing concentrations of leptin(116–130) amide (10–10 M) resulted in a dose-dependent inhibition of basal and hCG-stimulated testosterone secretion; a reduction that was significant from a dose of 10 M upwards. In addition, leptin(116–130) amide, at all doses tested (10–10 M), significantly decreased LH and FSH secretion by incubated hemi-pituitaries from adult male rats. In contrast, in the same experimental protocol, recombinant leptin(10–10 M) was ineffective in modulating LH and FSH release. Finally, neither recombinant leptin nor leptin(116–130) amide were able to change basal PRL and GH secretion in vitro. Our results confirm the ability of leptin, acting at the testicular level, to inhibit testosterone secretion, and map the effect to a domain of the leptin molecule that lies between amino acid residues 116 and 130. In addition, we provide evidence for a direct inhibitory action of leptin(116–130) amide on pituitary LH and FSH secretion, a phenomenon not observed for the native leptin molecule, in the adult male rat. European Journal of Endocrinology 142 406–410