Actions of endomorphins on synaptic transmission of A delta-fibers in spinal cord dorsal horn neurons

The effects of endogenous mu-opioid ligands, endomorphins, on A delta-affer ent-evoked excitatory postsynaptic currents (EPSCs) were studied in substan tia gelatinosa neurons in spinal cord slices. Under voltage-clamp condition s, endomorphins blocked the evoked EPSCs in a dose-dependent manner. To det ermine if the block resulted from changes in transmitter release from gluta matergic synaptic terminals, the opioid actions on miniature excitatory pos tsynaptic currents (mEPSCs) were examined. Endomorphins (1 mu M) reduced th e frequency but not the amplitude of mEPSCs, suggesting that endomorphins d irectly act on presynaptic terminals. The effects of endomorphins on the un itary (quantal) properties of the evoked EPSCs were also studied. Endomorph ins reduced unitary content without significantly changing unitary amplitud e. These results suggest that in addition to presynaptic actions on interne urons, endomorphins also inhibit evoked EPSCs by reducing transmitter relea se from A delta-afferent terminals. Copyright (C) 2000 National Science Cou ncil, ROC and S. Karger AG, Basel.