Additive inhibition of lordosis by simultaneous treatments with GABA_A and GABA_B receptor agonists, muscimol and baclofen, in female rats

In order to investigate the relationship between GABA_A and GABA_B receptors in the induction of lordosis behavior, agonists of these receptor subtypes were injected simultaneously to estrogen-treated ovariectomized rats and lordosis behavior was observed before and after the injections. The GABA_A receptor agonist, muscimol (MUS), at a dose in the range from 1.0 to 1.4 mg/kg body weight (bw) or the GABA_B receptor agonist, baclofen (BAC) at a dose in the range from 1 to 10 mg/kg bw, was injected intraperitoneally. The lordosis quotient (LQ) decreased after treatments with MUS or BAC and a dose-dependent decrease of LQ was observed in MUS or BAC-treated rats. When 1.2 mg/kg bw MUS and 5 mg/kg bw BAC were injected simultaneously, the mean LQ decreased strongly and was significantly lower than the values obtained after single injections of the agonists at these doses (P < 0.05). In addition, to ascertain the time-course of changes, a behavioral test was carried out 7 times from 15 to 180 min after the injection of agonists. The low LQ in the rats injected with both MUS and BAC continued longer than in rats given single injections. These results indicate that both GABA_A and GABA_B receptors are involved in lordosis-inhibiting mechanisms by the GABA neuron and operate independently.