Enhancing neuronal chloride extrusion rescues \u03b12/\u03b13 GABAA-mediated analgesia in neuropathic pain

Louis-Etienne Lorenzo,Isabel Plasencia-Fernandez,Irenej Kianicka,Alexandre A Girard,Francesco Ferrini,Alfredo Ribeiro-Da-Silva,Yves De Koninck,Karine Bachand,Simon Labrecque,Martin Gagnon,Nicolas Doyon,Antoine G Godin,Dominic Boudreau

doi:10.1038/s41467-019-14154-6

Abstract

Spinal disinhibition has been hypothesized to underlie pain hypersensitivity in neuropathic pain. Apparently contradictory mechanisms have been reported, raising questions on the best target to produce analgesia. Here, we show that nerve injury is associated with a reduction in the number of inhibitory synapses in the spinal dorsal horn. Paradoxically, this is accompanied by a BDNF-TrkB-mediated upregulation of synaptic GABAARs and by an α1-to-α2GABAAR subunit switch, providing a mechanistic rationale for the analgesic action of the α2,3GABAAR benzodiazepine-site ligand L838,417 after nerve injury. Yet, we demonstrate that impaired Cl- extrusion underlies the failure of L838,417 to induce analgesia at high doses due to a resulting collapse in Cl- gradient, dramatically limiting the benzodiazepine therapeutic window. In turn, enhancing KCC2 activity not only potentiated L838,417-induced analgesia, it rescued its analgesic potential at high doses, revealing a novel strategy for analgesia in pathological pain, by combined targeting of the appropriate GABAAR-subtypes and restoring Cl- homeostasis.

Highlights

Spinal disinhibition has been hypothesized to underlie pain hypersensitivity in neuropathic pain
To perform an unbiased quantitative analysis of the changes in markers of GABAA receptors (GABAARs)- and glycine receptors (GlyRs)-containing synapses following peripheral nerve injury (PNI), it was important to objectively define the territory in the superficial dorsal horn (SDH) that corresponded to the central projections of the lesioned afferents
A total of 82 rats were used for subsequent quantitative immunocytochemical analyses of markers of GABAAR and GlyR synapses

Summary

Introduction

Spinal disinhibition has been hypothesized to underlie pain hypersensitivity in neuropathic pain. We found a decrease in GABAergic terminals in the SDH after peripheral nerve injury (PNI)[15], but no systematic quantitative studies on the nature and degree of receptor expression at SDH synapses are available This information is important to assess the net functional impact of a deficit in Cl− homeostasis resulting from KCC2 hypofunction[19]. The ensuing impaired Cl− extrusion capacity of the cells renders them prone to a collapse of the transmembrane Cl− gradient upon strong Cl− load induced by enhanced GABAA receptor activity[26,27] To challenge this hypothesis, we tested the effect of rescuing Cl− homeostasis with the KCC2-enhancer CLP25728 on the analgesic action of the benzodiazepine site ligand. These findings point to a novel therapeutic strategy for treatment of neuropathic pain, by combined targeting of the appropriate GABAAR subtypes and restoring Cl− homeostasis

Methods

Results

Conclusion