Selective Protection of Benzomorphan Binding Sites Against Inactivation by N-Ethylmaleimide. Evidence for k-Opioid Receptors in Frog Brain

Abstract

Selective binding of [³H]bremazocine and [³H]-ethylketocyclazocine to k-opioid receptor sites in frog (Rana esculenta) brain membranes is irreversibly inactivated by the sulfhydryl group alkylating agent N-ethylmale-imide (NEM). Pretreatment of the membranes with k-selective compounds [ethylketocyclazocine (EKC), dy-norphin (1–13), or U-50,488H] but not with [D-Ala²,N-Me-Phe⁴,Gly⁵-ol]enkephalin (DAGO; μ specific ligand) or [d-Ala²,N-Me-Phe⁴,Gly⁵-ol]enkephalin (DADLE; δ specific ligand) strongly protects the binding of the radioligands against NEM inactivation. These results provide more evidence for the existence of k-opioid receptors in frog brain. The relatively high concentrations of NEM that are needed to decrease the specific binding of [³H]bremazocine together with the observation of an almost complete protection of its binding sites by NaCl suggest that bremazocine may act as an opioid antagonist in frog brain.