Differential presynaptic effects of opioid agonists on Adelta- and C-afferent glutamatergic transmission to the spinal dorsal horn.

BACKGROUND Although intrathecal administration of opioids produces antinociceptive effects in the spinal cord, it has not been established whether intrathecal opioid application more effectively terminates C fiber-mediated pain than A fiber-mediated pain. Here, the authors focus on the differences in opioid actions on Adelta- and C-afferent responses. METHODS Using the whole cell patch clamp technique, the authors investigated the presynaptic inhibitory actions of micro-, delta-, and kappa-opioid receptor agonists on primary afferent-evoked excitatory postsynaptic currents (EPSCs) in substantia gelatinosa neurons of adult rat spinal cord slices. RESULTS The micro agonist DAMGO (0.1, 1 microM) reduced the amplitude of glutamatergic monosynaptic Adelta- or C fiber-evoked EPSCs. C fiber-evoked EPSCs were inhibited to a greater extent than Adelta fiber-evoked EPSCs. The delta agonist DPDPE (1, 10 microM) produced modest inhibition of Adelta- or C fiber-evoked EPSCs. In contrast, the kappa agonist U69593 (1 microM) did not affect the amplitude of either Adelta or C fiber-evoked EPSCs. CONCLUSIONS These results indicate that opioids suppress excitatory synaptic transmission mainly through activation of micro receptors on primary afferent C fibers. Given that the substantia gelatinosa is the main termination of Adelta and C fibers transmitting nociceptive information, the current findings may partially explain the different potency of opioid agonists.