Tonabersat Blocks Gap Junctions and Alleviates Thermal Pain Behavior in Mice

Gap junctions (GJs) are channels that enable exchange of ions and small molecules between cells, and have been implicated in the development and maintenance of neuropathic pain. Injury-induced glial activation in sensory ganglia is associated with increased coupling by GJs, which in turn enhances neuronal excitability, contributing to pain signaling. Tonabersat (TON) was suggested to act as a GJ blocker with analgesic actions, but these claims have been disputed. Here we examined whether TON blocks GJs and whether it influences pain behavior in a mouse pain model. Gap junctional coupling was assayed by the dye coupling method in mouse liver and trigeminal ganglia. Pain behavior was tested in a mouse model of chemotherapy-induced pain, using von Frey filaments (tactile sensitivity), the acetone method (cold sensitivity) and hot plate (heat sensitivity). Intracellular dye injection showed that TON and the GJ blocker carbenoxolone (both 50 µM) inhibited gap junctional coupling by 70% and 82%, respectively. In the trigeminal ganglia, TON and carbenoxolone inhibited gap junctional coupling by 74 and 77%, respectively. TON selectively reduced heat and cold hypersensitivity, but not mechanical threshold. Carbenoxolone reduced all hypersensitivity types. No sex differences were observed. We conclude that both TON and carbenoxolone reduced coupling, indicating their potential to influence GJ-mediated coupling. In behavioral tests TON showed a selective effect for thermal hypersensitivity, but the underlying mechanism is unclear. Carbenoxolone produced a somewhat greater reduction in coupling compared with TON. These findings suggest that GJs play a role in pain pathways and highlight the need to explore whether other pain-relieving drugs act by blocking GJs.