Agonists at μ-
opioid receptors (MORs) represent the gold standard for the treatment of severe
pain. A key
element of
opioid analgesia is the depression of nociceptive information at the first synaptic relay in spinal
pain pathways. The underlying mechanisms are, however, largely unknown. In spinal cord slices with dorsal roots attached prepared from young rats, we determined the inhibitory effect of the selective MOR agonist [d-Ala(2), N-Me-Phe(4), Gly(5)-ol]-
enkephalin (
DAMGO) on monosynaptic Aδ- and C-fiber-evoked EPSCs in lamina I neurons.
DAMGO depressed presynaptically Aδ- and C-fiber-mediated responses, indicating that MORs are expressed on central terminals of both fiber types. We next addressed the mechanisms of presynaptic inhibition. The effect of
DAMGO at both Aδ- and C-fiber terminals was mainly mediated by an inhibition of N-type voltage-dependent Ca(2+) channels (VDCCs), and to a lesser extent of P/Q-type VDCCs. Inhibition by
DAMGO was not reduced by K(+) channel blockers. The rate of miniature EPSCs was reduced by
DAMGO in a dose-dependent manner. The
opioid also reduced Ca(2+)-dependent,
ionomycin-induced EPSCs downstream of VDCCs.
DAMGO had no effect on the kinetics of vesicle exocytosis in C-fiber terminals, but decreased the rate of unloading of Aδ-fiber boutons moderately, as revealed by two-photon imaging of styryl
dye destaining. Together, these results suggest that binding of
opioids to MORs reduces nociceptive signal transmission at central Aδ- and C-fiber synapses mainly by inhibition of presynaptic N-type VDCCs. P/Q-type VDCCs and the transmitter release machinery are targets of
opioid action as well.