The progesterone-induced LH surge in the estrogen-primed rat is thought to be mediated through interneurons since LHRH neurons reportedly lack steroid receptors. Along these lines, glutamate and gamma-aminobutyric acid (GABA) neurons are considered to be two of the major interneurons in the hypothalamus involved in the control of LHRH and LH secretion. Glutamate is a major excitatory amino acid neurotransmitter while GABA is a major inhibitory amino acid neurotransmitter in the control of LH secretion. Glutamate is converted in the brain to GABA by the rate-limiting enzyme glutamic acid decarboxylase (GAD). Regulation of GAD by steroid hormones would therefore appear to be a logical mechanism for the control of LHRH and LH secretion based on the changes in glutamate and GABA production which would result from such regulation. Therefore, the purpose of the present study was to determine whether the steroid hormone, progesterone, regulates the GAD enzyme in the hypothalamus at a time when it induces an LH surge. To accomplish this aim, northern and dot blot analysis were used to measure GAD65 and GAD67 mRNA levels in the preoptic area (POA) and medial basal hypothalamus (MBH) in ovariectomized (OVX) immature rats, OVX rats primed with 2 mu g of estradiol (E2) for 2 days, and OVX rats primed with 2 mu g of E2 for 2 days and administered progesterone (1 mg/kg, 09.00 h) to induce an LH surge. The results of the study show that GAD67 mRNA levels were suppressed by progesterone in the POA at 12.00 h (at the start of the surge) (24.4% suppression vs. E2 only; p <0.01) in the first experiment. This suppression was confirmed in a second experiment as progesterone suppressed GAD67 mRNA levels in the POA at 12.00 h (31.8% vs. E2 only; p <0.05) and also at 14.00 h (40% vs. E2 only; p <0.05) which correlates to the peak of the LH surge. The changes in GAD67 by progesterone were restricted to the POA, as GAD67 mRNA levels in the MBH were unaffected by the steroid treatment. Furthermore, the effect of progesterone on POA GAD67 mRNA levels was specific as it was blocked by prior treatment with the antiprogestin, RU486 (400 mu g). In the MBH, both GAD67 and GAD65 mRNA levels were increased by estradiol treatment at 16.00 h and this increase was prevented by progesterone administration in the presence of estrogen priming. In conclusion, these studies demonstrate that the mRNA for the key enzyme responsible for the conversion of the major excitatory amino acid neurotransmitter in the hypothalamus (glutamate) to the major inhibitory amino acid neurotransmitter (GABA), is regulated by the steroid hormone progesterone. Progesterone suppression of GAD67 mRNA levels in the POA, where LHRH neurons reside, and at a time immediately preceding and during the LH surge, could provide a mechanism for greater stimulation of LHRH neurons through enhanced excitatory glutamate production concomitant with deceased inhibitory GABA production in the POA.