Influence Of Inhibition Of GABAergic Neurons In Spinal Cord Dorsal Horn On Neuropathic Pain

Purpose:There are many inhibitory GABAergic neurons in spinal cord dorsal horn.These inhibitory neurons tonically control the activities of nociceptive transmission neurons.The reduction of inhibitory GABAergic synaptic responses is considered as the important mechanism underlying pathological pain.Given that the inhibitory neurotransmitter GABA is converted from excitatory neurotransmitter glutamate,some of the GABAergic neurons in the central nervous system belong to dual neurons that are able to co-release GABA and glutamate.This study aimed to investigate the possible influence of nerve injury on the function of spinal GABAergic neurons.Methods:We established the spared nerve injury(SNI)model of neuropathic pain in GAD2-Cre or PV-Cre mice.Chemogenetic ablation or inhibition of GAD2-positive(GAD2~+)and Parvalbumin-positive(PV~+)GABAergic neurons was achieved by intraspinal viral injection,and the changes of painful behaviors were measured.Immunohistochemical staining was conducted to examine the changes of GABA synthetase.Electrophysiological recordings were performed to investigate the changes of synaptic transmission mediated by GABAergic neurons.Results:(1)Spinal GABAergic neurons exert gating control over pain transmission.The removal of GABAergic inhibition by chemogenetic ablation of GABAergic neurons evoked dynamic mechanical allodynia,punctate mechanical allodynia and mechanical hyperalgesia in intact GAD2-Cre mice.(2)However,the chemogenetic ablation of spinal GABAergic neurons in the nerve-injured GAD2-Cre mice didn’t exaggerate neuropathic pain but alleviated the pain responses,suggesting that the nerve injury dramatically altered the function of GABAergic neurons.(3)To consolidate the observation above,chemogenetic inhibition of GABAergic neuron activity was performed,which led to mechanical allodynia and hyperalgesia in intact GAD2-Cre mice but generated analgesic action against neuropathic pain.(4)Spinal GABAergic neurons are neurochemcially divided into several subtypes,of which PV~+neurons play an important role in the control of allodynia transmission.Our data showed that the ablation of PV~+neurons also evoked mechanical allodynia and hyperalgesia in intact PV-Cre mice but alleviated neuropathic pain in the nerve-injured mice.(5)Similar results were obtained with chemogenetic inhibition of PV~+neurons,which caused mechanical allodynia and hyperalgesia in intact PV-Cre mice and produced analgesic action against neuropathic pain.(6)The inhibitory transmitter GABA is catalyzed by glutamic acid decarboxylase(GAD)to excitatory transmitter glutamate.We found that the nerve injury significantly reduced the content of GAD2 in PV~+neurons,which might lead to the reduction of GABA and the increase of glutamate.(7)Our electrophysiological recordings demonstrated that the nerve injury not only reduced the GABA release from PV~+neurons but drove PV~+neurons to release glutamate and generate excitatory postsynaptic currents on postsynaptic neurons,suggesting that the nerve injury operated to switch the inhibitory PV~+neurons to be excitatory glutamatergic neurons.Conclusion:The inhibitory function of spinal GABAergic neurons was switched by nerve injury to be excitatory.Inhibition of GABAergic neuron activity was able to attenuate neuropathic pain.