Comparative pharmacological properties and autoradiographic distribution of [3H]ethylketocyclazocine binding sites in rat and guinea pig brain.

[3H]Ethylketocyclazocine (EKC) binds to a single class of sites under various incubation conditions in both rat and guinea pig brain. Ligand selectivity patterns strongly suggest that [3H]EKC binds to kappa opiate receptors in both species and under both incubation conditions. Moreover, the autoradiographic distribution of [3H]EKC binding sites is very similar under both incubation conditions. In guinea pig brain, [3H]EKC binding sites are highly concentrated in deep layers of the cortex, caudate-putamen, and nucleus accumbens. In rat brain, kappa opiate receptors are mainly found in caudate-putamen, nucleus accumbens, amygdala, and thalamus. [3H]EKC binding sites are not concentrated in deep layers of the cortex in rat brain. Thus, the autoradiographic distribution of kappa opiate receptors is species-dependent.