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 thePOA 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 µl/min for 8 h, the α-adrenergic antagonist phenoxybenzamine (PBZ; 60 µg/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 estadiol 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. Perfusion of PBZ through the dialysis probe increased NE levels, presumably by blocking adrenergic autoreceptors. However, this effect was only observed when the probes were located rostral to the decussation of the anterior commissure. Thus the autoregulatory control of NE release appears to vary in different hypothalamic areas. PBZ significantly increased the LH pulse amplitude, independently of any effects on NE release, raising the possibility that noradrenergic neurons act via α-adrenergic receptors to inhibit the LH pulse amplitude. In conclusion, these data demonstrate that estradiol decreases the mean NE levels in the POA of anestrous ewes, but may increase its episodic release.

Keywords:
Gamma aminobutyric acid, Gonadal steroids, Gonadotropins, episodic secretion, Noradrenalin
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© 1995 S. Karger AG, Basel
1995
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