High affinity binding of [3H]ethylketocyclazocine to rat brain homogenate.

Ethylketocyclazocine (Ekc), a potent kappa-receptor agonist, binds to opiate receptor sites in rat brain tissue. The binding was saturable with respect to the concentration of [3H]Ekc. The dissociation constant of Ekc-receptor complex was 1.8 nM and the maximum number of binding sites in the whole brain was 9.6 pmol/g of tissue. Opiate agonists and antagonists have a high affinity for [3H]Ekc binding sites, but results of present investigations failed to differentiate opiates thought to be specific for the mu- and kappa-receptors.