Role Of Tonic Inhibition Mediated By ?₅ Subunit-containing GABA_A Receptors In Nociceptive Synaptic Transmission And Plasticity

Background: ?-Aminobutyric acid type A receptors?GABA_AR?are distributed at both inhibitory postsynaptic membrane and extrasynaptic sites.The synaptic GABA_AR mediate the phasic inhibition,while ?₅ subunit-containing GABA_AR??₅-GABA_AR?are located at extrasynaptic membrane and generate tonic inhibition.Convincing evidence has indicated that the phasic inhibition is critical for the processing of nociceptive signals.However,the role of tonic inhibition produced by extrasynaptic ?₅-GABA_AR in nociceptive transmission and plasticity remains largely unknown.The current study used ?₅-GABA_AR-selective reverse agonist L-655708 to investigate the possible influence of ?₅-GABA_AR on glutamatergic conveyance and synaptic plasticity.Methods: Extracellular recordings were performed in superficial dorsal horn of L5-L6 spinal cord to record the C-fiber evoked field potentials,and the effects of L-655708 on C fiber transmission and Long-Term Potentiation?LTP?were investigated.The possible regulation by L-655708 of synaptic glutamate receptors was assayed by western blot.The paw withdrawal latency in response to thermal stimulation was measured to investigate the effect of L-655708 on nociceptive behaviors.Results:?1?Suboptimal stimuli?60 V,0.5-ms duration,2 Hz,1 min?of sciatic nerves was unable to elicit LTP in normal rats;?2?When L-655708?0.5 pmol/25 mL?was used to pretreat the spinal dorsal horn for 60 min,subsequent suboptimal stimuli succeeded in inducing LTP,which showed the facilitatory effect of L-655708 on LTP;?3?Treatment of spinal cord slices in vitro with L-655708?20 n M?didn't affect the total protein contents of NMDA receptor Glu N1?Glu N2 B and Glu N2 A subunit as well as AMPA receptor Glu A1 subunit,which,however,significantly increased the synaptic concentration of Glu N1?Glu N2 B and Glu A1 subunit;?4?Treatment of spinal cord slices in vitro with L-655708?20 n M?dramatically boosted the phosphorylation levels of NMDA receptor Glu N2 B subunit at Ser1303,Glu N1 subunit at Ser896,and AMPA receptor Glu A1 subunit at Ser831;?5?Glu N2B-specific antagonist Ifenprodil?6 nmol/25 mL?,but not Glu N2 A antagonist TCN-201?0.2 nmol/25 mL?,blocked LTP facilitation caused by L-655708;?6?Intrathecal injection of L-655708 reduced the paw withdrawal latency of native mice;?7? Intrathecal application of L-655708?0.5 pmol/5 mL?had no effect on the inflammatory hyperalgesia induced by Complete Freund's Adjuvant?CFA?.Conclusions: ?₅-GABA_AR in spinal dorsal horn played a tonic inhibition of glutamatergic nociceptive transmission and synaptic plasticity.The removal of ?₅-GABA_AR-mediated tonic inhibition evoked hyperalgesia in intact animals.