Inhibition of lordosis behavior by intrahypothalamic implants of pargyline

Hypothalamic sites responsible for monoaminergic of gonadal hormone-facilitated female sexual behavior (the lordosis response) were investigated. Pargyline, an irreversible inhibitor of monoamine oxidase, was applied stereotaxically to the hypothalamus of ovariectomized, estrogen-primed females 2 h prior to progesterone administration. Application of pargyline dorsal to or within the lateral aspect of the ventromedial nucleus led to a reduction in lordosis quotients and quality scores 5-7 and 29-31 h later. Implantation dorsal to or within the dorsomedial nucleus did not inhibit lordosis responding 5-7 h later but did inhibit the response 29-31 h later. In both implant sites, lordosis responding returned to prepargyline levels within 55 h after drug placement. The effects of the pargyline cannulae were verified biochemically by measuring activity of monoamine oxidase in preoptic-hypothalamic nuclei. Enzyme activity was inhibited to some extent in all nuclei sampled. The ability of the implants to antagonize lordosis responding was related to the extent to which they inhibited monoamine oxidase activity in the hypothalamus. Results suggest that localized perturbations in hypothalamic cells whose monoamine metabolism is known to be affected by gonadal hormones is sufficient to affect female sexual behavior.