Cell-type-specific reorganization of VGSCs in auditory cortex and therapeutic potential of Nav1.6 blockade for tinnitus

Neuronal hyperexcitability resulting from an inhibitory-excitatory imbalance in the primary auditory cortex (A1) is a key pathological feature of tinnitus. Voltage-gated sodium channels (VGSCs) are crucial in regulating neuronal excitability by facilitating action potential generation and propagation. However, the specific involvement of VGSC subtypes in tinnitus-related hyperexcitability within the A1 cortex remains poorly understood. Previous studies have shown that acute and chronic salicylate administration can induce stable tinnitus in rats. In this study, we investigated the distribution and expression profiles of four VGSC subtypes (Nav1.1, Nav1.2, Nav1.3, and Nav1.6) in the A1 cortex of rats following systemic salicylate administration. Immunohistochemical staining and quantitative PCR analyses revealed dynamic and subtype-specific changes in VGSC expression. Notably, while the expression of Nav1.1 and Nav1.2 was significantly reduced in GAD67-immunoreactive GABAergic neurons, both Nav1.3 and Nav1.6 showed substantial upregulation, particularly in VGLUT2-immunoreactive glutamatergic neurons in the A1 cortex. Among these, Nav1.6 exhibited the most pronounced changes, suggesting it could be a key player in the altered excitatory-inhibitory balance observed in tinnitus. Furthermore, Nav1.6 knockout mice displayed reduced central gain enhancement following salicylate administration, further implicating Nav1.6 in tinnitus pathology. Treatment with NBI-921352, a selective Nav1.6 inhibitor, alleviated tinnitus-like behaviors induced by both acute and chronic salicylate treatments, concomitant with a suppression of salicylate-induced central gain enhancement. These findings suggest that the bidirectional regulation of VGSC subtypes contributes to tinnitus-associated excitatory-inhibitory imbalances in the A1 cortex, with Nav1.6 representing a promising therapeutic target for tinnitus.