Binding characteristics of kappa opioids in rat brain. A comparison of in vitro binding paradigms

Putative kappa binding was investigated in homogenates of the brain of the rat using [3H]ethylketocylazocine and [3H]diprenorphine under conditions where mu and delta sites were blocked. Under blocked conditions, the binding of [3H]ethylketocyclazocine labelled a single site, as defined by kinetic and equilibrium analysis, which showed high affinity for dynorphin 1-17, U50,488H, and benzomorphan compounds. However, blocked binding of [3H]diprenorphine showed a biphasic dissociation curve, and did not show high affinity for the specific kappa agonists dynorphin 1-17 or U50,488H. It is proposed that blocked [3H]ethylketocyclazocine is the more appropriate paradigm to study putative kappa binding, while blocked [3H]diprenorphine may label additional non-mu/non-delta sites.