Pre- and post-synaptic switches of GABA actions associated with Cl− homeostatic changes are induced in the spinal nucleus of the trigeminal nerve in a rat model of trigeminal neuropathic pain

Although trigeminal neuropathic pain is one of the most common chronic pain syndromes, the etiology is still unknown. Here, a rat model was generated using chronic constrictive injury (CCI) with ligation of the infraorbital nerve to test the hypothesis that collapse of chloride homeostasis in trigeminal neurons causes impairment of γ-aminobutyric acid-ergic (GABAergic) inhibition and induces trigeminal allodynia. Rats showed a reduction and increase in pain threshold and pain response scores, respectively, to mechanical stimulation, 1 and 3 weeks after CCI. In situ hybridization and immunohistochemical analysis showed that inward-directed Na+, K+-2Cl− cotransporter (NKCC1) mRNA and protein were upregulated in the small-sized and large-sized primary neurons in the injured side of the trigeminal ganglion and in the peripherin-positive terminal, respectively, for the first 2 weeks, while outward-directed K+-Cl− cotransporter (KCC2) mRNA and protein were downregulated in secondary relay neurons on the injured side of the spinal trigeminal nucleus caudalis (Sp5C). Optical imaging of evoked synaptic responses using a voltage-sensitive dye revealed that pre- and post-synaptic GABA actions were disinhibited and excitatory in the injured side, respectively, but inhibited in the sham-operated side of the Sp5C. This downregulation of KCC2 in the Sp5C may result in an excitatory switch by impairing postsynaptic GABA inhibition. GABA-mediated presynaptic disinhibition was attenuated by bumetanide, suggesting that NKCC1 upregulation in primary neurons may facilitate pain transmission by presynaptic GABAergic depolarization. Such Cl− homeostatic disruption resulting in perturbation of the inhibitory system possibly increases pain transmission, which may underlie the pathophysiology of trigeminal neuropathic pain.

147Highlights► We made neuropathic pain model by chronic constrictive infraorbital nerve injury. ► KCC2 downregulated in relay neurons in spinal trigeminal nucleus caudalis (Sp5C). ► NKCC1 immunoreactivity increased in primary afferent terminals in the Sp5C. ► Pre- and post-synaptic GABAergic inhibition was lost and reversed, respectively. ► Cl− homeostatic collapses in trigeminal relay/primary neurons underlie allodynia.