Inhibition of guinea pig lordosis behavior by the phenylethanolamine N-methyltransferase (PNMT) inhibitor SKF-64139: Mediation by α noradrenergic receptors

Experiments were undertaken to determine whether the steroid-dependent lordosis response of female guinea pigs is under adrenergic control. In initial experiments, treatment with the centrally active phenylethanolomine N-methyltransferase (PNMT; the enzyme catalyzing methylation of norepinephrine to epinephrine) inhibitor SKF-64139 inhibited lordosis behavior induced by estradiol-17β benzoate plus progesterone. SKF-29661, a PNMT inhibitor that does not cross the blood-brain barrier, did not affect lordosis. However, no detectable epinephrine was found in brain or spinal cord of drug- or vehicle-treated guinea pigs. This suggests that epinephrine neuronal systems do not exist in the guinea pig CNS. In agreement with this idea, the inhibitory effects of SKF-64139 on lordosis were found to be primarily attributable to the blockade of α noradrenergic receptors rather than to PNMT inhibition. Two lines of evidence support this conclusion. First, using in vitro receptor binding techniques, SKF-64139 was found to have a relatively high affinity for α₁ and particularly α₂ receptors in guinea pig forebrain. Second, presumably through competitive inhibition of SKF-64139 binding to α receptors, treatment with clonidine (an α receptor agonist) overrode the inhibitory effects of SKF-64139 on lordosis. Taken together, these findings indicate the possible absence of epinephrine neuronal systems in guinea pig brain and reemphasize the importance of α receptors in regulating steroid-dependent lordosis behavior in this species.