Endogenous galanin modulates the gonadotropin and prolactin proestrous surges in the rat

Abstract

Galanin (GAL) has recently emerged as an important neuroendocrine regulator which participates in the control of several pituitary and hypothalamic hormones. Our earlier observation that GAL stimulates LHRH release from nerve terminals of the median eminence as well as basal LH and LHRH-induced LH secretion from pituitary cells in vitro prompted us to evaluate whether endogenous GAL plays a role in regulation of the physiologically occurring preovulatory surges of gonadotropins and PRL. Proestrous female rats were passively immunized against GAL using a high affinity sheep antirat GAL serum (FMS-FJL 17-5). Animals were implanted during diestrus with indwelling atrial cannulae. On the expected day of proestrus, rats received 1 ml of either normal sheep serum or GAL antiserum (GAL-AS), iv, 1 h before blood sampling started. Blood samples (0.5 ml) were collected at hourly intervals from 1400-2300 h, and plasma levels of LH, FSH, and PRL measured by RIA. At several time intervals after GAL-AS administration, the maximum binding ability of the rat plasma was evaluated using standard saturation assays. High neutralizing levels of immunoglobulins were present throughout the experimental period. GAL passive immunization blunted the LH preovulatory surge by 30%. Although maximum LH levels were unaffected by the treatment, the area under the secretory curve and LH levels at 1700, 1900, and 2000 h were significantly reduced. Conversely, FSH secretion was not significantly altered for either maximum FSH levels or area under the curve. However, FSH levels were significantly diminished in GAL-AS-treated rats at 1700 h. GAL passive immunization selectively reduced the plateau phase of the preovulatory surge of PRL. No significant differences were observed in the initiation of the surge or maximum PRL levels, whereas PRL levels were significantly reduced from 1700 to 2200 h. In addition, the area under the PRL curve was diminished in GAL-AS-treated animals by 40%. In conclusion, our results clearly demonstrate that endogenous GAL is involved in control of the preovulatory surges of LH and PRL without altering the FSH surge. In addition, they provide, for the first time, evidence of an important role for endogenous GAL in the regulation of physiological events leading to ovulation.