REGULATION OF PREOPTIC AREA GONADOTROPIN-RELEASING-HORMONE (GNRH) MESSENGER-RNA 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) concentra tions, 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 aft er 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 mec hanisms are involved in regulating its expression. Accordingly, in sit u hybridization was undertaken using a S-35-labelled antisense oligonu cleotide probe complementary to bases 102-149 of the rat GnRH cDNA to quantify cellular GnRH mRNA expression in the medial septum (MS), diag onal band of Broca (DBB) and rostral preoptic area (rPOA) of intact ma les, rats gonadectomized for 7 weeks and gonadectomized animals implan ted with silastic capsules containing testosterone (T), estrogen (E) o r dihydrotestosterone (DHT). Ws found no difference between any of the treatment groups in the number of cells expressing GnRH mRNA in the M S/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 castr ation (intact: 0.95 +/- 0.07 silver grains/mu m(2)/cell; gonadectomize d: 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) treatme nt. DHT replacement had no effect on cellular levels of GnRH mRNA in g onadectomized rats (1.26 +/- 0.03). Frequency analysis of relative GnR H mRNA expression/cell showed that the rostral preoptic GnRH populatio n responded to the steroid treatment in an homogeneous manner. These r esults show that GnRH mRNA expression is elevated specifically within the rPOA of the long-term gonadectomized male rat when LH secretion ha s stabilized at a constant high level. Further, we show that the gonad al steroid regulation of cellular GnRH mRNA content at such time occur s only through an estrogen receptor-mediated pathway.