GABA_A-5-HT_1A receptor interaction in the mediobasal hypothalamus

Both serotonin (5-HT) and γ-aminobutyric acid (GABA) modulate female rat lordosis behavior and appear to interact in their control of the behavior. The current experiments were designed to investigate the interaction between these two neurotransmitters in sexually receptive female rats. Ovariectomized female rats, with bilateral cannulae directed toward the ventromedial nucleus of the hypothalamus (VMN), were hormonally primed with 10 μg estradiol benzoate and 500 μg progesterone. Sexual behavior was examined after intracranial infusion with 200 ng (±)-8-hydroxy 2-(di-n-propylamino)tetralin (8-OH-DPAT), 25 ng (5-aminomethyl-3-hydroxyisoxazole)hydrobromide (muscimol), 10 ng bicuculline or a combination of the drugs. As expected, 8-OH-DPAT reduced lordosis behavior and muscimol attenuated this inhibition in a bicuculline-sensitive manner. Muscimol alone also reduced lordosis behavior. These findings contrast with several reports that muscimol facilitates lordosis behavior of suboptimally hormonally primed female rats. The current outcome is discussed in terms of procedural differences between the present experiment and earlier studies. It is suggested that muscimol may enhance lordosis responding in suboptimally hormonally primed rats by activation of GABA_A receptors located dorsal to the VMN. In contrast, activation of receptors located more ventrally within the mediobasal hypothalamus (MBH) may inhibit the behavior of rats that are already sexually receptive.