THE ESTRADIOL-INDUCED LUTEINIZING-HORMONE SURGE IN THE EWE IS NOT ASSOCIATED WITH INCREASED GONADOTROPIN-RELEASING-HORMONE MESSENGER-RIBONUCLEIC-ACID LEVELS

This experiment was undertaken to determine whether the estrogen-induc ed LH and GnRH surge in the ewe is associated with activation of a spe cific subpopulation of neurons in the mid-brain of the ewe as indicate d by a change in GnRH mRNA levels. Fifteen ovariectomized ewes were as signed to treatment groups 3-4 wk after ovariectomy. One group of ewes served as controls (n = 2); 50 mu g estradiol-17 beta (E-2) was admin istered to the remaining ewes. Blood samples were collected from all e wes before treatment (2-h period at 10-min intervals) and continued at 30-min intervals until tissue was collected. At 6, 12, 18, and 24 h a fter E-2 (n = 3 for each time point), brains were collected and proces sed for localization and measurement of GnRH mRNA by in situ hybridiza tion histochemistry. Serum was analyzed for LH concentrations. Serum L H was pulsatile in controls and decreased at 6 h after E-2, and by 12 h the LH surge was initiated. LH levels peaked at 18 h after E-2 and r eturned to basal levels 24 h after E-2 treatment. A cRNA probe corresp onding to the GnRH-associated peptide region of ovine GnRH prepropepti de mRNA was used to identify GnRH mRNA. Associated with the onset and peak of the LH surge were decreased levels (p < 0.1) of GnRH mRNA in n eurons of the preoptic area (POA). Neither the number nor mRNA content of GnRH neurons in the diagonal band of Broca, septal area, or medial basal hypothalamus (MBH) changed during the LH surge. In contrast to E-2-induced increases in GnRH secretion during the LH surge, our data indicate that E-2 decreases steady-state amounts of GnRH mRNA and that GnRH neurons in the POA are influenced to the greatest extent during the E-2-induced GnRH surge.