ABSTRACT
Effects of endomorphin-1 (EM-1) and endomorphin-2 (EM-2) on synaptic transmission were investigated on neurons in substantia gelatinosa (SG) of the spinal dorsal horn by whole-cell voltage clamp recording. Both EM-1 (1 μmol/L) and EM-2 (1 μmol/L)remarkably reduced the frequency but not the amplitude of miniature excitatory postsynaptic currents (mEPSCs) and miniature inhibitory postsynaptic currents (mIPSCs). These effects were antagonized by 3-funaltrexamine ( β-FNA, 10 μmol/L), a selective μ-opioid receptor antagonist. Noticeably, EM-1 showed higher potency in decreasing the frequency of mEPSCs and mIPSCs than that of EM-2. These results indicate that EMs suppress both excitatory and inhibitory synaptic transmission by activating presynaptic μ-opioid receptors in the SG and EM-1, compared with EM-2, might be a more potent endogenous analgesic at the spinal cord level.