Specific, high affinity [3H]ethylketocyclazocine binding in rat central nervous system: lack of evidence for kappa receptors.

The binding properties and distribution of the tritiated benzomorphan, [3H]ethylketocyclazocine, were studied in rat central nervous system (CNS). Specific binding is saturable and represents 75 to 90% of total binding. Scatchard analysis is consistent with the existence of two classes of binding sites with Kd values of 0.5 and 10.5 nM, respectively. Ethylketocyclazocine binding is increased in the presence of sodium chloride. This characteristic resembles that of opiate antagonists rather than that of other agonists. The distribution of ligands. The IC50 values of various opioid agonists and antagonists for competition with [3H]ethylketocyclazocine correlate well with their IC50 values for competition with [3H]naloxone. This evidence does not lend support to the existence of a separate kappa receptor in rat CNS.