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.