Evidence for Involvement of α-Adrenergic Receptors in Norepinephrine-Induced Prostaglandin E_< 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 α- or β-adrenergic receptors. Incubation of median eminence fragments from adult male rats with different concentrations of phentolamine, an α-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 × 10^-7 and 0.9 × 10^-7 M for PGE₂ and LHRH, respectively. Complete suppression of the NE effects was observed with a phentolamine concentration of 5 × 10^-6 M. This dose also reduced basal PGE₂ and LHRH release. In contrast to phentolamine, the β-receptor antagonist propranolol, tested at a concentration of 5 × 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 α-adrenergic receptor.