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.
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