The Role of Somatostatin in the Regulation of Gonadotropin Secretion in Sheep

Two modes of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) secretion are necessary for female fertility: surge and episodic secretion. However, the neural systems that regulate these GnRH secretion patterns are still under investigation. The neuropeptide somatostatin (SST) inhibits episodic luteinizing hormone (LH) secretion in humans and sheep, and several lines of evidence suggest SST may regulate secretion during the LH surge. Neither a SST receptor 2 (SSTR2) agonist (octreotide) or antagonist (CYN154806; CYN) altered the amplitude or timing of the LH surge. Administration of CYN to intact ewes during either the breeding season or anestrus increased LH secretion and increased c-Fos in a subset of GnRH and kisspeptin cells during anestrus. To determine if these stimulatory effects are steroid-dependent or -independent, we administered CYN to ovariectomized ewes. This SSTR2 antagonist increased LH pulse frequency in ovariectomized ewes during anestrus, but not during the breeding season. The results demonstrate that SST, acting through SSTR2, inhibits episodic LH secretion, likely acting in the mediobasal hypothalamus, but action at this receptor does not alter LH surge secretion. Additionally, these data provide evidence that SST contributes to the steroid-independent suppression of LH pulse frequency during anestrus.;Potential sites for SST action in the ovine hypothalamus were investigated using immunohistochemistry to determine whether cells that produce kisspeptin (KNDy cells in the arcuate nucleus) or GnRH receive direct synaptic contact from SST fibers, and whether the amount of these inputs changes throughout the estrous cycle or by season. The majority of KNDy cells receive synaptic input (evident by presynaptic synaptophysin immunostaining) from SST cells, but the amount of these inputs did not differ among groups. A subset of GnRH cells in both the preoptic area (POA) and mediobasal hypothalamus also receive synaptic input from SST cells. A greater percentage of POA GnRH cells had SST synapses during the surge than in anestrus, the luteal or early follicular phase of the estrus cycle. The total number of synaptic inputs onto GnRH and KNDy cells was altered by phase of the estrous cycle and season, extending the hypothesis that changes in the GnRH and KNDy cell synaptic connectivity may contribute to altered gonadotropin secretion throughout the estrous cycle and between seasons.;Several lines of evidence support the hypothesis that somatostatin (SST) cells in the ventral medial nucleus (VMN) may be involved in the generation of the LH surge in sheep. In this study, we confirmed that SST cells in the VMN are activated during the LH surge using c-Fos as a marker for cellular activation. One explanation for the discrepancy between the observations of SST cell activation during the LH surge, but no effect of pharmacological manipulation of SSTR2, is that the cells that contain SST in the VMN also produce additional signaling molecules, such as nitric oxide. We used immunohistochemistry to determine that a high percentage (70-80%) of these SST cells also contain neuronal nitric oxide synthase (nNOS). Triple-label immunohistochemistry was used to determine that a greater percentage of the dual labeled SST and nNOS cells contain c-Fos during the LH surge compared to the early follicular phase. In contrast, the percentage of single labeled nNOS or SST cells that contained c-Fos did not differ between the LH surge and early follicular phase. Thus we propose that this population of SST cells in the VMN also release nitric oxide and that this transmitter contributes to the generation of the LH surge in sheep.