GABAergic drugs alter hypothalamic serotonin release and lordosis in estrogen-primed rats

The effects of muscimol, a GABA(A) agonist, and phaclofen, a GABA(B) antagonist, on serotonin (5HT) release in the mediobasal hypothalamus and lordosis behavior were studied in freely moving rats using in vivo microdialysis. Two days after implantation of bilateral guide cannulae directed towards the ventromedial nucleus of the hypothalamus (VMH), ovariectomized rats were primed with estradiol (E(2)). The rats were implanted with microdialysis probes 24 h later. Following a pretest for lordosis, perfusate 5HT was measured at 20-min intervals until the baseline was stable. The rats were treated with 10, 30 or 100 microM muscimol or 30 and 100 microM phaclofen in artificial CSF delivered via reverse dialysis for 40 min. Control animals were continuously perfused with artificial CSF. Behavior was tested 20, 60 and 180 min after introduction of the drug. Decreased hypothalamic 5HT (40-60% of baseline) and marked facilitation of lordosis were present 20 min after administration of either drug. The effects of 10 and 30 microM muscimol and 30 microM phaclofen on both 5HT and lordosis were reversed after 180 min. Reversal of the behavioral and neurochemical effects were not evident in either the 100 microM muscimol or 100 microM phaclofen groups at the time-points tested. Proceptive responses were observed in phaclofen-treated rats but not in rats treated with muscimol. Levels of hypothalamic 5HT and lordosis quotients in control rats did not significantly differ from initial values. These results suggest that GABAergic effects on lordosis may be mediated through an interaction with 5HT in the mediobasal hypothalamus.