Possible role of inhibitory glycinergic neurons in the regulation of lordosis behavior in the rat.

Strychnine sulfate (3.9 or 27 micrograms in 0.5 microliter saline) was bilaterally infused into the ventromedial hypothalamic nucleus (VMH) of ovariectomized sexually inexperienced rats primed 40 hr earlier with 4 micrograms of estradiol benzoate (EB). This dose of EB induced only weak lordosis behavior in 25% of the subjects (Ss). Strychnine at the 3 and 9 micrograms dosages, but not at 27 micrograms, induced intense lordosis behavior, but no proceptivity, in most estrogen-primed Ss (69% in 3 micrograms, 94% in 9 micrograms). Ovariectomized adrenalectomized EB-primed Ss also displayed significant lordosis behavior (59%) following infusion of 9 micrograms of strychnine into the VMH. Strychnine (9 micrograms) failed to stimulate lordosis in ovariectomized Ss that were not estrogen-primed. Administration of 5 micrograms EB followed 40 hr later by 2 mg of progesterone (P) elicited intense lordosis behavior in most Ss. Bilateral injections into the VMH of glycine (100 micrograms), beta-alanine (100 micrograms) or taurine (50 micrograms) to rats that were already displaying estrous behavior (greater than 80 LQ) in response to the sequential administration of EB and P failed to depress lordosis when tested between 5 min and 60 min postinjection. Similarly, glycine (20 or 100 micrograms) injected into the VMH of estrogen-primed, ovariectomized rats within 15 minutes of a 2 mg SC injection of P failed to interfere with the subsequent response to this steroid when tested 2 and 4 hr after P. The results suggest that strychnine injected into the VMH facilitates lordosis behavior in estrogen-primed rats by removing a tonic inhibitor effect exerted by glycinergic neurons on VMH neurons.(ABSTRACT TRUNCATED AT 250 WORDS)