A role for glutamate in regulating mechanosensory sensitivity: Further evidence from rat muscle spindle primary afferent terminals

Muscle spindles are proprioceptive sensory organs that constantly report the length and movements of skeletal muscle. The complicated process of mechanotransduction underlying their function is still poorly understood. The stretch-sensitive annulospiral terminals have populations of small (50 nm), clear synaptic-like vesicles (SLVs). We have shown (Bewick et al., 2005) SLVs contain glutamate, which when released, increases spindle firing by acting on a non-canonical, phospholipase D-coupled metabotropic glutamate receptor (PLD-mGluR). For a better understanding of how glutamate modulates afferent firing, we examined the effects of DL-TBOA, a non-selective excitatory amino acid transporter (EAAT) inhibitor and Rose Bengal, a blocker of the vesicular glutamate transporters (VGLUTs). Adult Sprague-Dawley rats (male, 300-370g) were killed and 4th lumbrical nerve-muscle preparations excised from both hind legs and stored under gassed (95%O2-5%CO2) saline. Spindle discharges were recorded at room temperature during 1 mm stretch-and-hold cycles (~10% muscle length). Data are expressed as mean ± SD. Differences between the pre-drug control and with-drug mean firing frequencies (impulses per second, imp/sec) were evaluated by paired t-test, with a significance threshold of P = 0.05. 100 µM DL-TBOA increased afferent firing from 199.0 ± 48.3 to 322.9 ± 48.9 imp/sec (n = 8; P < 0.0001) after 3 hr incubation, indicating that EAATs are important regulators of the effects of endogenously-released glutamate. This excitation involved activation of PLD-mGluRs, since it could be counteracted by PCCG-13, a specific PLD-mGluR blocker. Thus, when lumbricals were incubated in DL-TBOA with 10 µM PCCG-13, firing initially increased, from 214.6 ± 19.4 to 273.6 ± 27.7 imp/sec (2 hr, n = 2; P < 0.002), but then decreased to 123.5 ± 28.6 imp/sec (3 hr, n = 2; P < 0.003). All effects were fully reversible on washing. However, the non-selective VGLUT inhibitor Rose Bengal had no significant effect on the afferent discharge at 100 nM (n = 2; P < 0.51) or 1 µM (n = 2; P < 0.11). These data indicate tonic endogenous glutamate release from SLV recycling constantly modulates firing in muscle spindle afferents, with EAATs limiting its effects by re-uptake. The absence of perturbation from the VGLUT inhibitor, despite the presence of VGLUT1 (Wu et al., 2004), requires further study. It may indicate a large pool of pre-loaded SLVs must first be emptied before glutamate depletion effects are observed.