Autogenic modulation of mechanoreceptor excitability by glutamate release from synaptic-like vesicles: evidence from the rat muscle spindle primary sensory ending

Fifty-nanometre diameter, clear, synaptic-like vesicles (SLVs) are found in primary mechanosensory nerve terminals of vertebrate and invertebrate animals. We have investigated their role in mechanosensory function using the muscle spindle primary endings of rat Ia afferents as a model. Uptake and release of the synaptic vesicle marker FM1-43 indicated that SLVs recycle like synaptic vesicles and do so in a Ca2+-sensitive manner. Mechanical stimulation increased SLV recycling, increasing both dye uptake and release. Immunogold/electronmicroscopy showed that, like the central synaptic endings, la peripheral endings are enriched with glutamate. Moreover, exogenous glutamate enhanced stretch-induced la excitability. Enhanced excitability persisted in the presence of antagonists to the commonest ionotropic and metabotropic glutamate receptors (kynurenate, MCPG, CPPG and MAP4). However, excitation by glutamate was abolished by (R,S)-3,5-dihydroxyphenylglycine (DHPG), and rather more effectively by (2R,1'-S,2'-R,3'-S)-2-(2'-carboxy-3'-phenylcyclopropyl) glycine (PCCG-13). PCCG-13 also significantly reduced stretch-activated excitability in the absence of exogenous glutamate. These data indicate that SLVs recycle at rest, releasing glutamate, and that mechanical activity increases this process. The blockade with DHPG and PCCG-13 suggests that endogenous glutamate release acts, at least in part, through the recently described phospholipase D-linked metabotropic Glu receptor to maintain the excitability of the sensory endings.