Actions of opioids on excitatory and inhibitory transmission in substantiagelatinosa of adult rat spinal cord

1. The actions of opioid receptor agonists on synaptic transmission in subs tantia gelatinosa (SG) neurones in adult (6- to 10-week-old) rat spinal cor d slices were examined by use of the blind whole-cell patch-clamp technique . 2. Both the mu-receptor agonist DAMGO (1 mu M) and the S-receptor agonist D PDPE (1 mu M) reduced the amplitude of glutamatergic excitatory postsynapti c currents (EPSCs) which were monosynaptically evoked by stimulating A delt a afferent fibres. Both also decreased the frequency of miniature EPSCs wit hout affecting their amplitude. 3. In contrast, the K-receptor agonist U-69593(1 mu M) had little effect on the evoked and miniature EPSCs. 4. The effects of DAMGO and DPDPE were not seen in the presence of the mu-r eceptor antagonist CTAP (1 mu M) and the delta-receptor antagonist naltrind ole (1 mu M), respectively. 5. Neither DAMGO nor DPDPE at 1 mu M affected the responses of SG: neurones to bath-applied AMPA (10 mu M). 6. Evoked and miniature inhibitory postsynaptic currents (IPSCs), mediated by either the GABA(A) or the glycine receptor, were unaffected by the mu-, delta- and kappa-receptor agonists. Similar results were also obtained in S G neurones in young adult (3- to 4-week-old) rat spinal cord slices. 7. These results indicate that opioids suppress excitatory but not inhibito ry synaptic transmission, possibly through the activation of mu- and delta- but not kappa-receptors in adult rat spinal cord SG neurones; these action s are presynaptic in origin. Such an action of opioids may be a possible me chanism for the antinociception produced by their intrathecal administratio n.