Endomorphin-1 is more potent than endomorphin-2 in inhibition of synaptic transmission in substantia gelatinosa of adult rat spinal cord

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. Both EM-1 (1 mu M) and EM-2 (1 mu M) 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 beta-funaltrexamine (beta-FNA, 10 mu M), a selective mu-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 mu-opioid receptors in the SG and EM-1, compared with EM-2, might be a more potent endogenous analgesic at the spinal cord level.