EVIDENCE FOR KAPPA-1 OPIOID RECEPTOR MULTIPLICITY IN THE GUINEA-PIG CEREBELLUM

Both [H-3]U69,593 alone and [H-3]ethylketocyclazocine (EKC) in the presence of mu and delta-blockers ([D-Ala2,MePhe4,Glyol5]enkephalin (DAMGO) and [D-Pen2,D-Pen5]enkephalin (DPDPE)) label kappa-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 kappa-classification of binding. However, saturation studies revealed that the B(max) for [H-3]EKC binding was 45% greater than that for [H-3]U69593, suggesting that [H-3]EKC might be labeling more than one site. Although nonlinear regression analysis of dynorphin A(1-17) and nor-BNI competition of [H-3]EKC binding best fit the curves with a one site model, competitions by dynorphin B, dynorphin A(1-9) and alpha-neoendorphin revealed Hill coefficients less than unity and were best fit to a two site model. Kinetic analysis also supported [H-3]EKC binding heterogeneity. Together, these studies imply that under these conditions [H-3]EKC labels more than one site in the guinea pig cerebellum. The sensitivity of all specific [H-3]EKC binding to the selective kappa ligands dynorphin A(1-17) and nor-BNI indicates that both component are kappa while the differing sensitivities of dynorphin B, alpha-neoendorphin and dynorphin A(1-9) for these components support our previous hypothesis of kappa-1a and kappa-1b binding subtypes.