The Role of Glutamate in Maintaining Muscle Spindle Afferent Excitability in Adult Mice

Abstract

Group Ia and II muscle spindle afferents are sensory neurons critical to detecting muscle movement and length, which is important in proprioception and motor control. Glutamate is released by synaptic‐like vesicles (SLVs) at muscle spindle afferent nerve endings and exogenous glutamate increases sensory neuron firing rate during stretch (Bewick, et al., 2005). However, the activity of individual muscle spindle afferents was not measured, allowing for the possibility that this increased firing also included other sensory neurons known to have glutamate receptors, like nociceptors. Here we test the hypothesis that glutamate increases the firing rate of individual muscle spindle afferents. We isolated the extensor digitorum longus muscle and the sciatic nerve from adult mice (8–12 wks old) and recorded the activity of individual muscle spindle afferents during exogenous glutamate treatment. The muscle‐nerve preparation was perfused in an oxygenated synthetic interstitial fluid tissue bath, an extracellular recording electrode was placed on the nerve to measure the activity of individual muscle spindle afferents, and a battery of 4s ramp‐and‐hold stretches were performed on the muscle. Glutamate addition led to a significant increase in muscle spindle afferent firing rate during stretch (1mM; n=12, 18.8%). We also decreased glutamate release by blocking the vesicular glutamate transporter 1 (VGLUT1) with the inhibitor xanthurenic acid (XA). We found a significant decrease in muscle spindle afferent firing rates (3mM; n=17, −39%), with 5 afferents exhibiting a complete absence of firing. However, we noticed heterogeneity in muscle spindle afferent responses to glutamate and XA treatment, with some afferents not responding at all to the drug treatments. To confirm our pharmacological findings, we obtained a transgenic mouse line lacking a single copy of vesicular glutamate transporter 1 (VGLUT1; B6.129X1‐Sic17a7tm1Edw/MmcD; Fremeau, 2004). We observed two muscle spindle afferents responding normally to stretch, while another two failed to sustain firing at the end of stretch, something that is only rarely seen in wild type animals. This heterogeneity may be due to differences in VGLUT1 protein levels in individual afferents. We are currently increasing our sample size, but our preliminary results support our hypothesis that glutamate is critical to maintaining muscle spindle afferent firing during maintained stretch. These studies will lead to a better understanding of how muscle spindle afferent stretch sensitivity is regulated. Support or Funding Information This work was supported by NIH SC3 GM127195 (KAW) and NIH R25 GM071381 (SO and NV).