Norepinephrine infusions into the medial preoptic area inhibit lordosis behavior

Neurotransmitters, including norepinephrine, have been implicated in the mediation of ovarian steroid induced lordosis behavior in ovariectomized rats. In this study we have found that norepinephrine (NE) infusions into the medial preoptic area (MPOA) reduced lordosis frequencies of estrogen-progesterone treated (0.5 micrograms estradiol benzoate for three days followed by 500 micrograms progesterone 4-5 hours before testing) receptive rats. Norepinephrine doses of 2 micrograms or more per animal infused into the MPOA significantly reduced lordosis levels within five minutes. Infusions of 10 and 20 micrograms doses of NE suppressed lordosis levels for 15 minutes after infusion. At the lowest inhibitory dose (2 micrograms/animal) simultaneous infusion of 5 micrograms/microliter of the noradrenergic antagonist yohimbine, but not of phentolamine or propranolol, blocked the reduction in lordosis resulting from NE infusion. Preoptic infusions of epinephrine and clonidine were also effective in reducing lordosis quotients, while methoxamine, phenylephrine and isoproterenol did not alter receptivity. These findings are consistent with the conclusion that the direct effect of norepinephrine infusions into the MPOA is inhibition of lordosis responding. There is some evidence that this inhibitory influence is mediated by alpha 2-noradrenergic receptors.