Gamma aminobutyric acid mediates ventromedial hypothalamic mechanisms controlling the execution of lordotic responses in the female rat.

To study the participation of gamma-aminobutyric acid (GABA) in the control of sexual behavior in the female rat, the effect of GABA and picrotoxin injections in the ventromedial hypothalamic nucleus (VMN) upon lordosis frequency and multiunit spike activity (MUSA) was determined. Infusion of 100 micrograms GABA into conscious rats reduced lordotic responsiveness within 15 min after injection; with a similar time course, the same dose markedly reduced MUSA in urethane anesthetized rats. Forty-five min after injection lordotic responsiveness recuperated to preinjection levels; at this time MUSA showed a rebound increase in neuron firing frequency. The possible relation between ventromedial hypothalamic neuronal activity and capacity for lordotic responses was further tested with injections of a local anesthetic: 1 microliter of 2% Xylocaine infused into the VMN produced similar results, suppressing MUSA and lordotic responsiveness for ca. 45 min beginning immediately after injection. Microinjections of GABA antagonist picrotoxin had the opposite effects: 0.1 microgram increased MUSA and lordotic responsiveness at 5 and 45 min; however at 20 min, when MUSA was at its highest, lordosis frequency was not elevated. Injections of solvent had no consistent effects on either measure. Two conclusions many be tentatively drawn from these data: (a) the VMN is the origin of a neural signal which exerts a moment-to-moment gating control on the execution of lordosis, and (b) the generation and/or the output of this signal is under the control of a GABAergic hypothalamic mechanism which normally exerts an inhibitory effect on the display of lordotic responses.