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We have studied the effect of glutamate and the glutamatergic agonists N‐methyl‐d‐aspartate (NMDA), kainate, and α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionic acid (AMPA) on [3H]GABA release from the external plexiform layer of the olfactory bulb. The GABA uptake blocker nipecotic acid significantly increased the basal [3H]GABA release and the release evoked by a high K+ concentration, glutamate, and kainate. The glutamate uptake blocker pyrrolidine‐2,4‐dicarboxylate (2,4‐PDC) inhibited by 50% the glutamate‐induced [3H]GABA release with no change in the basal GABA release. The glutamatergic agonists NMDA, kainate, and AMPA also induced a significant [3H]GABA release. The presence of glycine and the absence of Mg2+ have no potentiating effect on NMDA‐stimulated release; however, when the tissue was previously depolarized with a high K+ concentration, a significant increase in the NMDA response was observed that was potentiated by glycine and inhibited by the NMDA receptor antagonist 2‐amino‐5‐phosphonoheptanoic acid (AP‐7). The kainate and AMPA effects were antagonized by the non‐NMDA receptor antagonist 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX) but not by AP‐7. The glutamate effect was also inhibited by CNQX but not by the NMDA antagonist 2‐amino‐5‐phosphonopentanoic acid (AP‐5); nevertheless, in the presence of glycine, [3H]GABA release evoked by glutamate was potentiated, and this response was significantly antagonized by AP‐5. Tetrodotoxin inhibited glutamate‐ and kainate‐stimulated [3H]GABA release but not the NMDA‐stimulated release. The present results show that in the external plexiform layer of the olfactory bulb, glutamate is stimulating GABA release through a presynaptic, receptor‐mediated mechanism as a mixed agonist on NMDA and non‐NMDA receptors; glutamate is apparently also able to induce GABA release through heteroexchange.

The objective of this study was to identify the genetic cause for progressive peripheral nerve disease in a Venezuelan family. Despite the growing list of genes associated with Charcot-Marie-Tooth disease, many patients with axonal forms lack a genetic diagnosis. Methods: A pedigree was constructed, based on family clinical data. Next-generation sequencing of mitochondrial DNA (mtDNA) was performed for 6 affected family members. Muscle biopsies from 4 family members were used for analysis of muscle histology and ultrastructure, mtDNA sequencing, and RNA quantification. Ultrastructural studies were performed on sensory nerve biopsies from 2 affected family members. Results: Electrodiagnostic testing showed a motor and sensory axonal polyneuropathy. Pedigree analysis revealed inheritance only through the maternal line, consistent with mitochondrial transmission. Sequencing of mtDNA identified a mutation in the mitochondrial tRNA Val (mt-tRNA Val) gene, m.1661A>G, present at nearly 100% heteroplasmy, which disrupts a Watson-Crick base pair in the T-stem-loop. Muscle biopsies showed chronic denervation/reinnervation changes, whereas biochemical analysis of electron transport chain (ETC) enzyme activities showed reduction in multiple ETC complexes. Northern blots from skeletal muscle total RNA showed severe reduction in abundance of mt-tRNA Val , and mildly increased mt-tRNA Phe , in subjects compared with unrelated age-and sex-matched controls. Nerve biopsies from 2 affected family members demonstrated ultrastructural mitochondrial abnormalities (hyperplasia, hypertrophy, and crystalline arrays) consistent with a mitochondrial neuropathy. Conclusion: We identify a previously unreported cause of Charcot-Marie-Tooth (CMT) disease, a mutation in the mt-tRNA Val , in a Venezuelan family. This work expands the list of CMT-associated genes from protein-coding genes to a mitochondrial tRNA gene.

The effect of L-cysteine sulfinic acid (CSA) and L-homocysteic acid (HCA) on the release of tritiated gamma-amino butyric acid ([3H]GABA), from the external plexiform layer (EPL) of the rat olfactory bulb, was compared with that of glutamate. These amino acids induced release of GABA was strongly inhibited by the glutamate uptake blocker, pyrrolidine-2,4-dicarboxylate (2,4,PDC) (50 microM), while it was not inhibited by the specific GABA uptake blockers nipecotic acid (0.5 mM) or NO-711 (5 microM). Only the HCA induced GABA release was 60% inhibited by beta-alanine (0.5 mM), a glial GABA uptake blocker and 78% by the NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (AP-5) (100 microM). The non-NMDA receptor antagonists 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline (CNQX) up to 500 microM had no effect on HCA or CSA stimulated GABA release. These results bring evidence for an excitatory role of HCA and CSA together with glutamate on GABAergic neuronal or glial elements, in the olfactory bulb. This role could be mediated through the reversal of the glutamate or/and the glial GABA transporter and through the activation of a NMDA type receptor.