The mechanism underlying the modulatory effect of substance P(SP) on GABA-activated response in rat dorsal root ganglion(DRG) neurons was investigated. In freshly dissociated rat DRG neurons, whole-cell patch-clamp technique was used to record GABA-activated current and sharp electrode intracellular recording technique was used to record GABA-induced membrane depolarization. Application of GABA(1–1000 μmol/L) induced an inward current in a concentration-dependent manner in 114 out of 127 DRG neurons(89.8 %) examined with whole-cell patch-clamp recordings. Bath application of GABA(1–1000 μmol/L) evoked a depolarizing response in 236 out of 257(91.8%) DRG neurons examined with intracellular recordings. Application of SP(0.001–1 μmol/L) suppressed the GABA-activated inward current and membrane depolarization. The inhibitory effects were concentration-dependent and could be blocked by the selective neurokinin 1(NK1) receptors antagonist spantide but not by L659187 and SR142801(1 μmol/L, n=7), selective antagonists of NK2 and NK3. The inhibitory effect of SP was significantly reduced by the calcium chelator BAPTA-AM, phospholipase C(PLC) inhibitor U73122, and PKC inhibitor chelerythrine, respectively. The PKA inhibitor H-89 did not affect the SP effect. Remarkably, the inhibitory effect of SP on GABA-activated current was nearly completely removed by a selective PKCε inhibitor epilon-V1-2 but not by safingol and LY333531, selective inhibitors of PKCα and PKCβ. Our results suggest that NK1 receptor mediates SP-induced inhibition of GABA-activated current and membrane depolarization by activating intracellular PLC-Ca2+-PKCε cascade. SP might regulate the excitability of peripheral nociceptors through inhibition of the "pre-synaptic inhibition" evoked by GABA, which may explain its role in pain and neurogenic inflammation.
