Aspects of central and peripheral regulation of reproduction in mammals.

To increase our knowledge concerning the central and peripheral regulation of reproduction in mammals a series of studies were performed. In the first experiment, we found that exogenous leptin altered the activity of the hypothalmo-pituitary-gonadotropic axis in sheep during insufficient feeding. The action of leptin appears to be mediated by changes in GnRH and LH secretion as well as NPY immunoreactivity. The aim of the second experiment was to investigate the role of the adipoinsular axis hormones during pregnancy in rats. The elevated levels of plasma leptin as wells as the increased mRNAs expression of the leptin receptors in placenta indicate the significant role of the hormone in fetal growth and development. On the other hand, a decrease in leptin receptors mRNA content within hypothalamus and pituitary together with unchanged plasma insulin level may suggest that during rat pregnancy leptin resistance was developed in the hypothalamus, pituitary and pancreatic islets. The third experiment was carried out to establish the role of opioids and glucocorticoids in the regulation of the hypothalmo-pituitary-gonadal axis in ewes during natural or synchronized estrous cycle. Prolonged treatment with progesterone resulted in significant changes in plasma levels of Met-enkephalin, cortisol and steroids and altered the expression of proenkephalin mRNA in the hypothalamus, pituitary, ovary and adrenals. Injections of Met-enkephalin or naltrexone (blocker of opioid receptors) modulated the progesterone influence in tested tissues. The data clearly suggest that opioids are involved in the regulation of the estrous cycle at the hypothalamo-pituitary-gonadal/adrenal axes.

Acute in vivo effects of leptin and leptin fragments on corticosteroid hormone secretion and entero-insular axis in the rat.

Leptin is a 147-amino acid adipose tissue-secreted hormone, which acts via several subtypes of receptors, the main and better known variants of which are named Ob-Ra and Ob-Rb. Structure-activity relationship studies pointed out the importance of the N-terminal and C-terminal amino-acid sequences 22-115 and 116-166, respectively, for the biological and receptor binding activities of leptin. Evidence has been provided that leptin affects corticosteroid-hormone and insulin secretion, and therefore we have investigated in the rat the expression of leptin receptor expression in adrenal cortex and pancreatic islets, as well as the effects of the acute treatment with leptin and leptin fragments 150-167, 138-167, 93-105, 22-56 and 26-39 on the plasma concentrations of aldosterone, corticosterone, insulin and glucagon. Reverse transcription-polymerase chain reaction showed the expression of both Ob-Ra and Ob-Rb mRNAs in adrenal cortex and pancreatic islets, the Ob-Rb expression in pancreas being 2-fold higher than in adrenals. Radioimmuno assay demonstrated that leptin enhanced plasma aldosterone and corticosterone concentrations, decreased plasma insulin concentration, and did not significantly affect glucagon plasma concentration. All leptin fragments tested exerted a corticosteroid-hormone secretagogue action, while only leptin fragments 116-130, 138-167 and 93-105 elicited a sizeable insulin antisecretagogue effect. Taken together these findings suggest that: i) the in vivo acute stimulating effect of leptin on adrenocortical hormone secretion is not connected to specific sequences of its molecule, while the insulinostatic effect is probably mediated by the sequence 93-105; and ii) the secretagogue and antisecretagogue effect of leptin are prevalently mediated by Ob-Ra and Ob-Rb, respectively.

Estrus cycle-dependent action of leptin on basal and GH or IGF-I stimulated steroid secretion by whole porcine follicles.

OBJECTIVE: To determine the levels of leptin in the follicular fluid and using culture of whole ovarian follicles, to test the hypothesis that leptin may directly influence GH and IGF-I stimulated ovarian function. METHODS: Porcine follicles were recovered from ovaries during early, middle, and preovulatory stage of the follicular phase of the estrus cycle. They were cultured in the presence of the recombinant ovine leptin (oLEP) added either alone or with oGH or hIGF-I. Steroid concentrations in the media were determined after 48 h of culture. RESULTS: The respective values for leptin in follicular fluid from small, medium and large follicles were 1.98, 2.18 and 1.96 ng/ml, respectively. Leptin added alone at a dose of 2 ng/ml had no effect on basal steroid secretion by small and medium follicles. However, in small follicles a synergic action of GH and IGF-I was noted. Leptin did not influence the secretion of progesterone by follicles collected during the early and middle follicular phases. In preovulatory follicles, leptin added alone to the culture media caused a decrease in basal estradiol secretion with a concomitant increase in progesterone secretion. Moreover, it acted synergistically with IGF-I and GH causing further stimulation of progesterone secretion. CONCLUSIONS: The presented data show a direct, maturation dependent action of leptin on GH and IGF-I stimulated follicular steroidogenesis. During follicular growth they acted synergistically with GH and IGF-I in estradiol production, while in preovulatory follicles, they acted with both investigated hormones in luteinization process, which starts before follicular disruption.