Is the inhibitory action of estradiol on luteinizing hormone pulse frequency in anestrous ewes mediated by noradrenergic neurons in the preoptic area?

This study tested the hypothesis that estradiol inhibits luteinizing hormone (LH) pulse frequency in anestrous ewes by increasing the activity of an inhibitory noradrenergic (NE) system that acts in the ovine preoptic area (POA). The effects of estradiol on the release of NE and other neurotransmitters in the POA were determined using intracranial microdialysis. Microdialysis probes (5 mm membrane length) were inserted via chronic guide tubes into the POA. Ringer's solution was pumped through the probes at a rate of 2 microliters/min for 8 h, the alpha-adrenergic antagonist phenoxybenzamine (PBZ; 60 micrograms/ml of Ringer's solution) was then administered via the probe from hours 8 to 12, and Ringer's solution alone was given the last 4 h. The outflow from the dialysis probes was collected every 20 min from 2 to 16 h and concentrations of aminergic transmitters and gamma aminobutyric acid determined by high-performance liquid chromatography. Blood samples were collected every 10 min throughout the experiment and the LH pulse patterns determined. Dialysis was done in the same neural area twice in each ewe, without (ovariectomy only) and with estradiol treatment (ovariectomy and a 1.5-cm-long Silastic capsule filled with crystalline estradiol placed subcutaneously for 2 days); the order of dialysis was randomized. As expected, estradiol decreased the LH pulse frequency. This negative feedback action of estradiol was associated with a decrease in mean NE concentrations in dialysate samples and an increase in the intra-animal variability of NE. Estradiol had no effect on any other neurotransmitter measured.(ABSTRACT TRUNCATED AT 250 WORDS)