Comparison of kappa 2-opioid receptors in guinea pig brain and guinea pig ileum membranes.

The presence of kappa-opioid receptor subtypes has been clearly established in guinea pig brain. Using [3H]bremazocine in the presence of reversible blockers of mu, delta and kappa 1 receptors, two additional binding sites can be determined in guinea pig brain membranes. The site with higher affinity for the opioid ligands represents kappa 2, while the other site has low affinity and is poorly characterized. The kappa 2 site has high affinity for ethylketocyclazocine and other benzomorphans, as well as for the dynorphin gene products tested. The dynorphin analogs have no appreciable affinity for the low affinity site, so this site should not be called a kappa receptor. With an appropriate membrane preparation, kappa 2 binding can also be demonstrated in the guinea pig ileum. Binding affinities for selected ligands at kappa 2 in guinea pig ileum membranes are very similar to affinities found in brain membranes.