Presynaptic suppression of dorsal horn inhibitory transmission by mu-opioid receptors.

Opioids modify sensory experience at many levels in the CNS. The mechanisms of this action, including the ways opioid receptors affect synaptic transmission, are not yet fully understood. Here we show that the selective activation of mu-opioid receptors suppressed inhibitory transmission between spinal cord dorsal horn neurons in vitro. mu-Opioid receptor activation reduced evoked inhibitory postsynaptic current (eIPSC) amplitude by acting presynaptically, because it altered the paired-pulse ratio, did not affect GABA-evoked currents, and decreased miniature IPSC (mIPSC) frequency. The mechanism of this effect was independent both of presynaptic Ca(2+) entry and of the pathway linking presynaptic kainate (KA) receptors to suppression of inhibitory transmission in the same cells. These data identify mu-opioid receptors as important presynaptic modulators of dorsal horn inhibitory transmission.