The effect of leptin on the tonic secretion of gonadotropins in the female rats.

OBJECTIVES: This study is an attempt to determine, the in vivo action of leptin on this hypophysiotropic hypothalamic area, by evaluating the concentrations of luteinizing hormone (LH) and follicular stimulating hormone (FSH) in the serum.

METHODS: The experiments were done by stereotaxic injection of recombinant rat leptin (rrleptin) into the third cerebral ventricle (V3) of adult female Wistar rats. Subjects were divided into five groups. Group A included normal intact animals. In Group B, the rats were stereotaxically administered with rrleptin in the V3. The rats, in Groups C, D and E, were subjected to electrolytic lesion of the arcuate nucleus (ARC), of the ventromedial nucleus (VMH) and of both of these hypothalamic nuclei, respectively. Immediately after the electric lesion, they were intracranially injected with rrleptin. Blood samplings for serum LH and FSH levels estimation were performed three times: 1) just before any stereotaxic procedure, 2) six hours, and 3) twenty-four hours after leptin administration.

RESULTS: The results showed that serum LH levels increased dramatically in group B, six hours after leptin administration. The LH levels in Groups C, D and E presented the same pattern with a lower peak. The FSH levels were doubled six hours after leptin administration in all groups without any exception. Both LH and FSH serum levels reverted to the initial basic levels after 24 hours.

DISCUSSION: The significant conclusion derived from this study is that ARC and VMH, which are responsible for controlling the tonic secretion of gonadotropins, respond in a different way for the FSH and LH secretion. This also suggests that some other mechanism(s) or factor(s) may additionally participate in the control of the tonic component of FSH secretion.