The Effects Of Melittin On Spontaneous Glutamatergic Transmission In Rat Substantia Gelatinosa Neurons

Melittin is an algesic peptide derived from bee venom. To investigate its effects on spontaneous glutamatergic transmission, whole–cell clamp-patch recordings were performed on 117 SG (substantia gelatinosa) neurons of adult rat spinal horn. In 64 examined SG neurons, at a holding potential of –70 mV, perfusion of melittin (1 ?M) enhanced the spontaneous excitatory postsynaptic current (sEPSC) frequency by 65±6% (n=51) and the sEPSC amplitude by 43±5% (n=20). Both actions of melittin were resistant to TTX, but could be completely blocked by a PLA2 inhibitor 4-BPB (4-bromophenacyl bromide, 10 ?M). Perfusion of arachidonic acid (50 ?M) also enhanced the sEPSC frequency by 88±18% (n=6) but had no effect on the sEPSC amplitude ( 102±1% of control, n=4). The action of melittin on the sEPSC frequency was dose-dependent (0.2-5 ?M, EC50=1.1 ?M) and could be inhibited by Ca2+-free Krebs solution and LaCl3 (a voltage-dependent calcium channel inhibitor, 30 ?M), but not by endomethacin (a cyclooxygenase inhibitor, 100 ?M) or nordihydroguaiaretic acid (NDGA, a lipooxygenase inhibitor, 100 ?M). We conclude that melittin, by activating endogenous PLA2, can enhance the release of glutamate form the primary fibers and the sensitivity of AMPA receptors on membrane of SG neurons, both of which can facilitate pain transmission. The former is mediated by arachidonic acid and depends on the influx of extracellular Ca2+, and the latter probably depends on the modifications of lipid environment of AMPA receptors.