Phospholipase A2 activation by melittin enhances spontaneous glutamatergic excitatory transmission in rat substantia gelatinosa neurons.

In order to know a role of phospholipase A2 in modulating nociceptive transmission, the effect of a secreted phospholipase A2 activator melittin on spontaneous glutamatergic excitatory transmission was investigated in substantia gelatinosa neurons of an adult rat spinal cord slice by using the whole-cell patch-clamp technique. Bath-applied melittin at concentrations higher than 0.5 microM increased both the amplitude and the frequency of spontaneous excitatory postsynaptic current in a manner independent of tetrodotoxin; the latter effect of which was examined in detail. In 80% of the neurons examined (n = 64), melittin superfused for 3 min gradually increased spontaneous excitatory postsynaptic current frequency (by 65+/-6% at 1 microM; n = 51) in a dose-dependent manner (effective concentration for half-maximal effect = 1.1 microM). This effect subsided within 3 min after washout. The spontaneous excitatory postsynaptic current frequency increase produced by melittin was reduced by the phospholipase A2 inhibitor 4-bromophenacryl bromide (10 microM) while being unaffected by the cyclooxygenase inhibitor indomethacin (100 microM) and the lipoxygenase inhibitor nordihydroguaiaretic acid (100 microM). A similar increase in spontaneous excitatory postsynaptic current frequency was produced by exogenous arachidonic acid (50 microM); this effect was also unaffected by the cyclooxygenase or lipoxygenase inhibitor. Melittin failed to increase spontaneous excitatory postsynaptic current frequency in a nominally Ca2+-free or La3+-containing Krebs solution. We conclude that melittin increases the spontaneous release of L-glutamate to substantia gelatinosa neurons by activating secreted phospholipase A2 and increasing Ca2+ influx through voltage-gated Ca2+ channels in nerve terminals, probably with an involvement of arachidonic acid but not its metabolites produced by cyclooxygenase and lipoxygenase. Considering that the substantia gelatinosa plays an important role in regulating nociceptive transmission, it is suggested that this transmission may be positively modulated by secreted phospholipase A2 activation in the substantia gelatinosa.