PARENT SESSION
TRAINEE RESEARCH COMPETITION

AN IMMUNOCYTOCHEMICAL INVESTIGATION INTO THE ROLE OF ESTROGEN IN THE GROWTH HORMONE AXIS OF THE EWE.

Scanlan, Niamh¹, Skinner, Donal¹, ¹

ABSTRACT- Growth hormone (GH) is influenced by the interplay of two hypothalamic neuropeptides: the stimulatory effects of GH-releasing hormone (GHRH) and the inhibitory effects of somatostatin (SS). It has recently been hypothesized that, together with estrogen, GH may function as a neuroendocrine regulator of ovulation. It is of note, therefore, that in response to estrogen, a surge of GH is observed in coincidence with surging luteinizing hormone (LH) in the ewe. Others reports suggest that GH may play a critical role in the efficacy of LH action. Initial studies mapped the distribution of GHRH immunoreactive (-ir) cells in the hypothalamus of ovariectomized ewes that were pretreated with estrogen and progesterone. Relative to other hypothalamic neuropeptides, the most obvious feature of GHRH-ir perikaryia was their restricted distribution to the arcuate nucleus and the ventromedial hypothalamic nucleus, extending rostro-cadually but with fewer neurons caudally. Subsequent double-labeling experiments investigated whether GHRH and SS neurons in the hypothalamus and GH cells in the pituitary expressed estrogen receptors (ER). Although GHRH-ir and SS-ir neurons were surrounded by many immunostained ERs, they did not themselves possess ERs. In the pituitary, however, some (4.1%) GH-ir cells were found to express ER-ir cells, but this was much lower than the number of LH-ir cells found to express ER-ir cells (71.2%). These data provide strong evidence that estrogen does not act directly on GHRH and SS neurons in the hypothalamus to induce the preovulatory GH surge, but could suggest that there is a common higher center in the brain controlling the release of both GH and LH at this time. Estrogen may act on GH input, but whether this contributes to the induction of the GH surge is currently under investigation.

KEY WORDS: Growth hormone axis, Estrogen, LH, Sheep