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Motor Axon Synapses on Renshaw Cells Contain Higher Levels of Aspartate than Glutamate
PLOS ONE
  • Dannette Shanon Richards, Wright State University
  • Ronald W. Griffith
  • Shannon H. Romer, Wright State University - Main Campus
  • Francisco J. Alvarez, Wright State University - Main Campus
Document Type
Article
Publication Date
5-9-2014
Abstract
Motoneuron synapses on spinal cord interneurons known as Renshaw cells activate nicotinic, AMPA and NMDA receptors consistent with co-release of acetylcholine and excitatory amino acids (EAA). However, whether these synapses express vesicular glutamate transporters (VGLUTs) capable of accumulating glutamate into synaptic vesicles is controversial. An alternative possibility is that these synapses release other EAAs, like aspartate, not dependent on VGLUTs. To clarify the exact EAA concentrated at motor axon synapses we performed a quantitative postembedding colloidal gold immunoelectron analysis for aspartate and glutamate on motor axon synapses (identified by immunoreactivity to the vesicular acetylcholine transporter; VAChT) contacting calbindin-immunoreactive (-IR) Renshaw cell dendrites. The results show that 71% to 80% of motor axon synaptic boutons on Renshaw cells contained aspartate immunolabeling two standard deviations above average neuropil labeling. Moreover, VAChT-IR synapses on Renshaw cells contained, on average, aspartate immunolabeling at 2.5 to 2.8 times above the average neuropil level. In contrast, glutamate enrichment was lower; 21% to 44% of VAChT-IR synapses showed glutamate-IR two standard deviations above average neuropil labeling and average glutamate immunogold density was 1.7 to 2.0 times the neuropil level. The results were not influenced by antibody affinities because glutamate antibodies detected glutamate-enriched brain homogenates more efficiently than aspartate antibodies detecting aspartate-enriched brain homogenates. Furthermore, synaptic boutons with ultrastructural features of Type I excitatory synapses were always labeled by glutamate antibodies at higher density than motor axon synapses. We conclude that motor axon synapses co-express aspartate and glutamate, but aspartate is concentrated at higher levels than glutamate.
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© 2014 Richards et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

DOI
10.1371/journal.pone.0097240
Citation Information
Dannette Shanon Richards, Ronald W. Griffith, Shannon H. Romer and Francisco J. Alvarez. "Motor Axon Synapses on Renshaw Cells Contain Higher Levels of Aspartate than Glutamate" PLOS ONE Vol. 9 Iss. 5 (2014) ISSN: 19326203
Available at: http://works.bepress.com/francisco_alvarez/94/