LACK OF EVIDENCE OF KAPPA(2)-SELECTIVE ACTIVATION OF G-PROTEINS - KAPPA-OPIOID RECEPTOR STIMULATION OF [S-35] GTP-GAMMA-S BINDING IN GUINEA-PIG BRAIN

Although only one gene for kappa opioid receptors has been cloned to d ate, kappa(1) and kappa(2) receptors have been defined pharmacological ly, with drugs such as bremazocine binding to both putative kappa rece ptor subtypes. To examine whether kappa receptor subtypes can be disti nguished at the level of the G-protein, the ability of the kappa(1) ag onist N-[2-(1-pyrrolidinyl)cyclohexyl]-benzeneacetamide) methane sulfo nate (U-50488H) to stimulate [S-35]guanosine-5'-O-(gamma-thio)-triphos phate ([S-35]GTP gamma S) binding in guinea pig brain was compared wit h that of bremazocine and dynorphin. In membranes prepared from guinea pig striatum, both bremazocine and U-50488H stimulated [S-35]GTP gamm a S binding with the same relative; efficacy, while dynorphin produced at least two-fold greater efficacy than the other two agonists. In vi tro autoradiography of agonist-stimulated [S-35]GTP gamma S binding re vealed similar regional distributions of bremazocine- and U-50488H-act ivated G-proteins. In striatal membranes, the kappa antagonist nor-bin altorphimine (nor-BNI) blocked both bremazocine- and U-50488H-stimulat ed [S-35]GTP gamma S binding with similar K-e values. In agonist addit ivity experiments, the stimulation of [S-35]GTP gamma S binding by the delta agonist [D-pen(2,5), p-Cl-Phe(4)]enkephalin (p-Cl-DPDPE) was ap proximately additive with the two kappa agonists. Stimulation of [S-35 ]GTP gamma S binding by the mu agonist [D-Ala(2), N-Me-4, Gly(5)-ol]-e nkephalin (DAMGO) was additive with U-50488H, but not with bremazocine , reflecting the mu antagonist properties of this compound. The combin ation of bremazocine and U-50488H together produced no greater stimula tion of binding than either agonist alone, indicating that they were b inding to the same site. These results demonstrate that bremazocine an d U-50488H activate G-proteins in guinea pig brain through the same re ceptor, and suggest that kappa(2) receptors are not coupled through th e same signal transduction mechanisms as kappa(1) receptors. BIOCHEM P HARMACOL 56;1:113-120, 1998. (C) 1998 Elsevier Science Inc.