Comparison of sigma- and kappa-opiate receptor ligands as excitatory amino acid antagonists

Using the technique of microelectrophoresis in pentobarbitone-anaesthetized cats and rats, the effects of benzomorphans, with known actions at sigma- and kappa- opioid receptors, were tested on responses of spinal neurones to amino acids and acetylcholine. The racemic mixture and both enantiomers of the sigma opiate receptor agonist, N-allylnormetazocine (SKF 10, 047), and the dissociative anaesthetic, ketamine, reduced or abolished excitation evoked by N-methyl-aspartate (NMA) with only small and variable effects on responses to quisqualate or kainate. (+)-SKF 10, 047 was 1.2 +/- 0.7 times more potent than the (-)-enantiomer in antagonizing NMA. On Renshaw cells, (+)-SKF 10, 047 enhanced responses to acetylcholine whereas the (-) enantiomer produced only a small reduction. The kappa- opiate receptor agonist, ethylketocyclazocine, had no selective effects on responses to amino acids or to acetylcholine. We conclude that actions at sigma- but not kappa-, opiate receptors are responsible for the NMA antagonism observed with benzomorphans.