Regulation of preoptic area gonadotrophin-releasing hormone (GnRH) mRNA expression by gonadal steroids in the long-term gonadectomized male rat.

Testosterone exerts important feedback actions on the hypothalamus and pituitary of the male rat to control reproductive hormone secretion. Marked fluctuations occur in plasma-luteinizing hormone (LH) concentrations, hypothalamic gonadotrophin-releasing hormone (GnRH) content and GnRH mRNA expression following castration and it appears as though a stable post-castration equilibrium is not attained until 3-4 weeks after gonadectomy. In the present study, we have investigated the effects of long-term (7 week) gonadectomy on GnRH mRNA expression in the male rat and determined whether estrogen or androgen receptor-mediated mechanisms are involved in regulating its expression. Accordingly, in situ hybridization was undertaken using a 35S-labelled antisense oligonucleotide probe complementary to bases 102-149 of the rat GnRH cDNA to quantify cellular GnRH mRNA expression in the medial septum (MS), diagonal band of Broca (DBB) and rostral preoptic area (rPOA) of intact males, rats gonadectomized for 7 weeks and gonadectomized animals implanted with silastic capsules containing testosterone (T), estrogen (E) or dihydrotestosterone (DHT). We found no difference between any of the treatment groups in the number of cells expressing GnRH mRNA in the MS/DBB or rPOA. Similarly, the GnRH mRNA content of cells in the MS/DBB was not different between the treatment groups. In contrast, cellular GnRH mRNA expression in the rPOA was elevated 7 weeks following castration (intact: 0.95 +/- 0.07 silver grains/microm2/cell; gonadectomized: 1.26 +/- 0.03; mean +/- S.E.M., P < 0.05) and this was restored to intact levels by either T (1.02 +/- 0.07) or E (1.02 +/- 0.08) treatment. DHT replacement had no effect on cellular levels of GnRH mRNA in gonadectomized rats (1.26 +/- 0.03). Frequency analysis of relative GnRH mRNA expression/cell showed that the rostral preoptic GnRH population responded to the steroid treatment in an homogeneous manner. These results show that GnRH mRNA expression is elevated specifically within the rPOA of the long-term gonadectomized male rat when LH secretion has stabilized at a constant high level. Further, we show that the gonadal steroid regulation of cellular GnRH mRNA content at such time occurs only through an estrogen receptor-mediated pathway.