Demonstration of the heterogeneity of the kappa-opioid receptors in guinea-pig cerebellum using selective and nonselective drugs

In guinea-pig cerebellum, saturation studies reveal that the nonselective opioid 3H]ethylketazocine has a binding capacity (R) of 6.79 pmol/g tissue which is similar to the sum of the individual R values of the mu-, delta- and kappa 1-selective opioids. Conversely, the binding parameters of the nonselective opioid 3H]bremazocine are best-fitted to a two-site model (Kd1 = 0.12 nM, R1 = 11.3 pmol/g tissue; Kd2 = 6.03 nM, R2 = 9.09 pmol/g tissue) with an R TOTAL value of 20.3 pmol/g tissue which is statistically different from the R value of 3H]ethylketazocine or the sum of R mu + R delta + R kappa 1. This suggests that 3H]bremazocine labels additional opioid binding sites. After suppression of the mu-, delta- and kappa 1-receptors, 3H]bremazocine binding is then best-fitted to a one-site model with a Kd value of 1.48 nM and an R value of 11.2 pmol/g tissue. Competition studies done against the binding of 3H]U69593 indicate that the opioid receptors labelled with this ligand are related to the kappa 1-receptor subtype. However, competition studies performed against the binding of 3H]bremazocine (under suppressed conditions) display a pharmacological profile related to another subtype of kappa-receptors previously described in guinea-pig brain as the kappa 2-receptors.