Na_V1.1 and Na_V1.6 Play Unique and Essential Roles in Muscle Spindle Afferent Function in Adult Mice

Voltage gated sodium channels (NaVs) are crucial for the generation and propagation of action potentials. There are 9 different isoforms found in mammals (NaV1.1-1.9), and NaV1.1, NaV1.6, and NaV1.7 are found in the muscle proprioceptors that sense muscle stretch, the muscle spindle afferents. This sensory information is critical for motor reflexes and the sense of body and limb position in space. We previously found that mice lacking one or both copies of NaV1.1 in peripheral sensory neurons displayed motor deficits, including intention tremors. Muscle spindle afferents in these mice also had unstable static stretch sensitivity but normal dynamic sensitivity, suggesting a specific deficit in the maintenance of proprioceptor excitability when the Piezo2 channel was likely to be closed (Espino, et al., 2022). We hypothesized that NaV1.6 would also be necessary for normal motor behavior and muscle spindle afferent function. We generated sensory neuron specific deletions of NaV1.6 by crossing NaV1.6fl/fl mice with mice expressing PirtCre (NaV1.6cko) and observed severe motor deficits on the open field and rotarod test only in NaV1.6cko (n=15) and not NaV1.6Het (n=12) mice compared to NaV1.6fl/fl controls (n=8). In contrast to mice lacking NaV1.1, we did not observe intention tremors. We used an ex vivo muscle nerve preparation consisting of the extensor digitorum longus muscle and peroneal branch of the sciatic nerve. The muscle was maintained at the length of maximal twitch contraction (Lo) and given a battery of 9 ramp-and-hold stretches (2.5, 5, and 7.5% Lo repeated 3 times each) and 16 sinusoidal vibrations (5-100 μm amplitude; 10-100 Hz frequency). Preliminary results include recordings from 18 adult mice of both sexes (NaV1.6fl/fl n=8; NaV1.6cko n=10). We were only able to find stretch sensitive responses in 3 of 10 NaV1.6cko mice, and those were severely impaired with firing rates less than 5 Hz. In contrast, we found robust slowly adapting responses to stretch in all 8 NaV1.6fl/fl mice. Only 1 of 10 NaV1.6cko mice had any firing in response to vibration, in contrast to the normal dynamic sensitivity observed in NaV1.6fl/fl and NaV1.1cko afferents. This suggests that NaV1.6 plays a critical role in initiating muscle spindle afferent transmission, whereas NaV1.1 is essential for maintaining proprioceptor excitability during static stretch. Future studies will determine whether loss of NaV1.7, which does not result in motor deficits, alters muscle spindle afferent function. This work was supported by NIH Grant 5SC3GM127195 (KAW) a RISE Fellowship 5R25GM71381 (SO), NIH 5T32GM099608-10 and 1T32GM1144303-01A1 (CE). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.