Evidence for κ1 opioid receptor multiplicity in the guinea pig cerebellum

Abstract

Both [ 3H]U69,593 alone and [ 3H]ethylketocyclazocine (EKC) in the presence of μ and δ blockers ([D-Ala 2,MePhe 4, Glyol 5] enkephalin (DAMGO) and [D-Pen 2,D-Pen 5]enkephalin (DPDPE)) label κ receptors in the guinea pig cerebellum. Dynorphin A(1–17) and nor-binaltorphimine (nor-BNI) potently competed the binding of both radioligands with Hill coefficients of approximately unity, strongly supporting a κ classification of binding. However, saturation studies revealed that the B max for [ 3H]EKC binding was 45% greater than that for [ 3H]U69593, suggesting that [ 3H]EKC might be labeling more than one site. Although nonlinear regression analysis of dynorphin A(1–17) and nor-BNI competition of [ 3H]EKC binding best fit the curves with a one site model, competitions by dynorphin B, dynorphin A(1–9) and α-neoendorphin revealed Hill coefficients less than unity and were best fit to a two site model. Kinetic analysis also supported [ 3H]EKC binding heterogeneity. Together, these studies imply that under these conditions [ 3H]EKC labels more than one site in the guinea pig cerebellum. The sensitivity of all specific [ 3H]EKC binding to the selective κ ligands dynorphin A(1–17) and nor-BNI indicates that both component are κ while the differing sensitivities of dynorphin B, α-neoendorphin and dynorphin A(1–9) for these components support our previous hypothesis of κ 1a and κ 1b binding subtypes.