Pharmacological characterization of the binding of [3H]bremazocine in guinea-pig brain: evidence for multiplicity of the κ-opioid receptors

Abstract

In guinea-pig brain, [3H]bremazocine has a binding capacity of 27.2 pmol/g wet tissue, which is statistically different from that of [3H]ethylketazocine (14.7 pmol/g wet tissue) or the sum of the individual binding capacities of μ-, δ-, and κ-selective ligands (15.0 pmol/g wet tissue). Saturation studies of [3H]bremazocine performed in the presence of unlabelled μ-, δ-, and κ-blockers still reveal a homogeneous population of binding sites. [3H]Bremazocine under suppressed conditions displays at these sites a Kd of 2.51 nM with a binding capacity of 9.15 pmol/g wet tissue. We have performed the pharmacological characterization of these additional opioid binding sites. Displacement curves measured with a number of opioid substances were all best fitted to a one-site model. The stereoselectivity of these additional sites was demonstrated by using two groups of stereoisomers. Oripavine and benzomorphan opioids were among the most potent drugs at the [3H]bremazocine sites (μ + δ + κ suppressed). Diprenorphine, bremazocine, cyclazocine, and ethylketazocine displayed apparent affinities constants (1/Ka) of 8.66, 7.57, 21.4, and 38.0 nM, respectively at those sites. The κ-selective drugs U50488, U69593, PD117302, and tifluadom were inhibitors of the binding of [3H]bremazocine at these sites with apparent affinities of 113, 268, 76.9, and 47.9 nM. All μ- or δ-selective drugs tested in this study have caused weak or no inhibition of the binding. Correlation analyses were done between the different affinities measured at the [3H]bremazocine sites (μ + δ + κ suppressed) and those observed at the known μ-, δ-, and κ-sites of the guinea-pig brain. The results were highly and significantly correlated with the κ-binding profile, while no correlation could be established with the μ,- or δ-binding profiles. Moreover, the results reported here do not correspond to a pharmacological profile that belongs to the σ- or ε-receptors. Thus, it is likely that in the guinea-pig brain the [3H]bremazocine sites (μ + δ + κ suppressed) are a subtype of κ-site family, namely κ2-sites. Finally, apparent affinities displayed at those sites are correlated with the biological activity of opioids in the guinea-pig ileum and rat vas deferens, suggesting that those sites could be considered as receptors.Key words: opioid receptors, kappa subtype, guinea-pig brain.