Characterisation of κ-opioid binding sites in rat and guinea-pig spinal cord

The binding of radiolabelled ligands with high affinity for κ-opioid binding sites has been studied in homogenates of lumbo-sacral spinal cord from the rat. The unselective ligands [ 3H]bremazocine and [ 3H]diprenorphine labelled a large number of sites which could not be fully resolved in terms of μ-, δ- and κ-types by displacement assays. In particular binding at the κ-site appeared anomalous in that sites which could be identified as high affinity κ-type represented only 40% of total κ-binding, defined using the unselective [ 3H]ligands. This was confirmed by the low levels of binding seen with the κ-agonists [ 3H]dynorphin A(1–9) and [ 3H]U-69593. In guinea-pig cord, under conditions in which binding to μ- and δ-sites was suppressed, [ 3H]dynorphin A(1–9) and [ 3H]U-69593 labelled only 60% of the κ population, defined by the [ 3H]unselective ligands. The reasons for the observed discrepancies are discussed.