Estrogen and estrogen receptor-β (ERβ)-selective ligands induce galanin expression within gonadotropin hormone-releasing hormone-immunoreactive neurons in the female rat brain

Among the many factors that integrate the activity of the GnRH neuronal system, estrogens play the most important role. In females, estrogen, in addition to the negative feedback, also exhibits a positive feedback influence upon the activity and output of GnRH neurons to generate the preovulatory LH surge and ovulation. Until recently, the belief has been that the GnRH neurons do not contain estrogen receptors (ERs) and that the action of estrogen upon GnRH neurons is indirect involving several, estrogen-sensitive neurotransmitter and neuromodulator systems that trans-synaptically regulate the activity of the GnRH neurons. Based on our recent findings that GnRH neurons of the female rat coexpress galanin, that galanin is a potent GnRH-releasing peptide, and that ER beta is present in GnRH neurons, we have evaluated the effect of 17 beta-estradiol and two ER beta-selective agonists (WAY-200070, WAY-166818) on the expression of galanin within GnRH neurons. By combining immunocytochemistry for GnRH and in situ hybridization histochemistry for galanin, we demonstrate that 17 beta-estradiol ( 20 mu g/kg, sc) stimulates galanin expression within GnRH-immunoreactive neurons in a time-dependent manner. A significant increase was observed 2 h after its administration to ovariectomized rats. However, a more robust expression required 3-d treatment regimen. Treatment with the beta-selective ligands resulted in similar observations, although no statistical analysis is available for the 2 hr survival. These observations strongly suggest that estrogen and the ER beta-selective ligands stimulate galanin expression within GnRH neurons via ER beta, although an indirect mechanism via interneurons still cannot be ruled out.