Regulatory Effects Of Extrasynaptic GABA_A Receptors On Nociceptive Synaptic Plasticity In Spinal Cord Dorsal Horn Of Rats

Objective: In adult animals, γ-aminobutiric acid（GABA） is one of the important inhibitory neurotransmitters that influence the neuronal excitability by activation of A type GABA receptor（GABA_A receptor; GABA_AR）. Synaptic GABA_A receptor mediates the phasic inhibition, while extrasynaptic GABA_A receptor produces tonic inhibition. Previous studies have demonstrated that extrasynaptic GABA_A receptor contains either α₅ subunit（referred to as α₅-GABA_A receptor） or δ subunit（referred to as δ-GABA_A receptor）. The role of GABAergic phasic inhibition in glutamatergic neurotransmission has been extensively studied. However, it remains elusive what role the tonic inhibition plays in nociceptive synaptic transmission and plasticity in spinal cord dorsal horn. The current study investigated the regulatory effects of GABAergic tonic inhibition on the long-term potentiation（LTP） of C fiber-evoked field potentials.Method: We performed the electrophysiological recordings of C fiber–evoked field potentials in spinal dorsal horn and delivered the low frequency stimulation（LFS）onto sciatic nerves to induce LTP. The α₅-GABA_A receptor inhibitor L-655,708（20 nM） or δ-GABA_A receptor agonist THIP（2 μM） was spinally applied to explore the possible changes in the amplitudes of C-fiber responses and LTP and the underlying mechanisms.Results:（1） LTP was stably induced in spinal dorsal horn of rats following LFS（0.5-ms duration, 60 V, 2 Hz, 2 minutes） delivery onto sciatic nerves, which lasted for at least 3h. Spinal application of saline did not affect the magnitudes of basal C-fiber responses as well as LTP.（2） α₅-GABA_A receptor antagonist L-655,708（20 nM）, when applied locally at the recording spinal segments, augmented the basal C-fiber responses, suggesting that α₅-GABA_A receptor inhibition increased the synaptic efficacy between spinal dorsal horn neurons and primary afferent C-fibers.（3） L-655,708, when spinally applied to pretreat the spinal cord for 1 h, caused a significant increase of LTP magnitude, suggesting that the removal of α₅-GABA_A receptor-mediated inhibition facilitated LTP induction.（4） Pretreatment with ifenprodil（10 μg）, a selective antagonist against NMDA-subtype glutamate receptors containing GluN2 B subunit（GluN2B receptor）, prevented the facilitory effect of L-655,708 on spinal LTP, suggesting that α₅-GABA_A receptor might regulate LTP through GluN2 B signaling.（5） Pretreatment with TCN-201（10 μM）, a selective antagonist against NMDA receptors containing GluN2 A subunit（GluN2A receptor）, didn’t prevent L-655,708 from facilitating spinal LTP.（6） Spinal application of L-655,708 at 0.5 h or 3 h after LFS caused a further increase of the magnitudes of early-phase and late-phase LTP.（7） Selective activation of δ-GABA_A receptor by THIP（2 μM） before LFS delivery significantly attenuated the LTP magnitudes of C fiber-evoked field potential, although it didn’t influence the basal transmission.（8） THIP, when locally given at 3 h after LFS, repressed the magnitude of late-phase LTP.Conclusion: Extrasynaptic α₅-GABA_A receptor and δ-GABA_A receptors that generate tonic inhibition were involved in the regulation of LTP in spinal cord dorsal horn.