Evidence for involvement of alpha-adrenergic receptors in norepinephrine-induced prostaglandin E2 and luteinizing hormone-releasing hormone release from the median eminence.

Studies were conducted to determine if the stimulatory effect that norepinephrine (NE) exerts on the release of prostaglandin E2 (PGE2) and LHRH from the median eminence is mediated by alpha- or beta-adrenergic receptors. Incubation of median eminence fragments from adult male rats with different concentrations of phentolamine, an alpha-receptor antagonist, resulted in a dose-related inhibition of the release of both PGE2 and LHRH induced by NE, with an IC50 of 3.5 X 10(-7) and 0.9 X 10(-7) M for PGE2 and LHRH, respectively. Complete suppression of the NE effects was observed with a phentolamine concentration of 5 X 10(-6) M. This dose also reduced basal PGE2 and LHRH release. In contrast to phentolamine, the beta-receptor antagonist propranolol, tested at a concentration of 5 X 10(-6) M, was completely ineffective in altering the stimulatory effect of NE on either PGE2 or LHRH release. Moreover (and as previously observed for LHRH), blockade of dopaminergic receptors with Pimozide (10(-6) M) failed to inhibit the release of PGE2 induced by NE, thus indicating that the stimulatory effect of NE is not mediated by a dopaminergic mechanism. It is concluded that NE stimulates the release of PGE2 and LHRH from nerve terminals of the median eminence by first interacting with an alpha-adrenergic receptor.