Dissociation of the inhibitory and facilitatory effects of norepinephrine on the release of LH by premammillary lesions.

A dual action of the adrenergic system on the release of LH has been demonstrated: facilitation mediated by alpha-adrenergic receptors and inhibition mediated by beta-adrenergic receptors. These two actions can be dissociated either by treatment with agents which block-specific adrenergic receptors or by cuts placed just in front of the mammillary bodies (PM cuts) interrupting caudal afferents to the medial basal hypothalamus. These cuts affected the release of luteinizing hormone (LH) in a way similar to that of treatment with the beta-adrenoblocker, propranolol. Thus, in animals with PM cuts (1) the release of LH following the injection of norepinephrine into the third ventricle was enhanced and (2) the blocking action of intraventricular injection of isoproterenol on the release of LH was suppressed whereas the facilitatory effect of clonidine was not changed. Furthermore, whereas the release of LH induced by intraventricular injection of norepinephrine in intact rats was enhanced by treatment with propranolol, in rats with PM cuts the already enhanced induced release was no further raised by propranolol treatment. On the other hand, PM cuts did not affect the suppressive action of the alpha-adrenoblocker phenoxybenzamine on the norepinephrine-induced release of LH. It is concluded that adrenergic mechanisms inhibiting the release of LH were suppressed by PM cuts whereas those facilitating the release were not affected by these cuts.