Sexual dimorphism of GABA release in the medial preoptic area and luteinizing hormone release in gonadectomized estrogen-primed rats

We showed marked sex differences in the GABA outflow in the medial preoptic area of intact rats. To further determine the sexually dimorphic effects of estrogen on the GABA outflow, an in vivo microdialysis study was performed in gonadectomized rats 3-5 days after the estrogen- or cholesterol-priming. Dialysates and sequential blood samples (150 microl each) were simultaneously collected under freely moving conditions. Serum estradiol concentrations at 72 and 84 h after the estrogen capsule implantation were approximately 75 pg/ml in both sexes. Ovariectomized estrogen-primed (OVX+E(2)) rats showed high GABA outflow from the late night through the morning, which was significantly declined until the onset of surge like secretion of luteinizing hormone (LH) in the afternoon (N=7). Ovariectomized cholesterol-primed (OVX+C) rats consistently showed low GABA outflow and high serum LH concentration (N=8). Conversely, orchidectomized estrogen-primed (ORX+E(2)) rats showed high and episodic GABA outflow without any daily changes (N=7), which was significantly greater than orchidectomized cholesterol-primed (ORX+C; N=8) and OVX+C rats. Only OVX+E(2) rats showed significant daily changes in the GABA outflow and serum LH concentration. Fitting with the double cosinor method demonstrated that the acrophase of the GABA outflow in OVX+E(2) rats occurs in the early morning, whereas the acrophases in OVX+C, ORX+C, and ORX+E(2) rats occur at various times of day. The present findings suggest that sex-specific effects of estrogen on the daily GABA release in the medial preoptic area may be involved in the sex difference of LH release in rats.