ABSTRACT
There is compelling evidence that glutamate can act as a cotransmitter in the mammalian brain. Interestingly, the third vesicular glutamate transporter (VGLUT3) is primarily found in neurons that were anticipated to be nonglutamatergic. Whereas the function of VGLUT3 in acetylcholinergic and serotoninergic neurons has been elucidated, the role of VGLUT3 in neurons releasing gamma-aminobutyric acid (GABA) is not settled. We have previously shown that VGLUT3 is found together with the vesicular GABA transporter (VIAAT) on synaptic vesicle membranes in the hippocampus. Now we provide novel electron microscopic data from the rat hippocampus suggesting that glutamate is enriched in inhibitory nerve terminals containing VGLUT3 compared to those lacking VGLUT3. The opposite was found for GABA; VGLUT3-positive inhibitory terminals contained lower density of GABA labeling compared to VGLUT3-negative inhibitory terminals. In addition, semiquantitative confocal immunofluorescence showed that N-methyl-D-aspartate (NMDA)-receptor labeling was present more frequently in VGLUT3-positive/VIAAT-positive synapses versus in VGLUT3-negative/VIAAT-positive synapses. Electron microscopic immunogold data further suggest that NMDA receptors are enriched in VGLUT3 containing inhibitory terminals. Our data reveal new chemical characteristics of a subset of GABAergic interneurons in the hippocampus. The analyses suggest that glutamate is coreleased with GABA from hippocampal basket cell-synapses to act on NMDA receptors.
Subject(s)
Brain/cytology , Brain/metabolism , GABAergic Neurons/metabolism , Glutamic Acid/metabolism , Vesicular Glutamate Transport Proteins/metabolism , Animals , GABAergic Neurons/ultrastructure , Male , Microscopy, Confocal , Microscopy, Electron, Transmission , Presynaptic Terminals/metabolism , Presynaptic Terminals/ultrastructure , Rats , Rats, Wistar , Receptors, AMPA/metabolism , Vesicular Inhibitory Amino Acid Transport Proteins/metabolismABSTRACT
BACKGROUND: Metabolic impairment contributes to development of Parkinson's disease (PD). Mitochondrial dysfunction is involved in degeneration of nigral dopamine neurons. Also, in PD there are alterations in glucose metabolism in nigro-striatal pathways, and increased cerebral lactate levels have been found. OBJECTIVES: We raise the question of whether changes in the amount transporters of energy substrates are involved in the pathogenesis of PD. METHODS: We have used confocal immunofluorescence and immunogold postembedding electron microscopic techniques to study whether there are altered levels of the transporters for monocarboxylates (MCT1 and MCT2) and glucose (GLUT1) in the MPTP mouse model of PD. RESULTS: We found that MCT1 and GLUT1 were densely located in blood vessel endothelium, while MCT2 was present in perivascular astrocytic end feet processes in the substantia nigra and the striatum of control mice. We found that the localisation and densities of MCTs and GLUT1 were unaltered in the PD model. DISCUSSION: This is the first study reporting on the distribution of metabolic transporters in PD. Our results suggest that, although there are metabolic impairments in PD, the levels of MCT1, MCT2 and GLUT1 is not changed following dopaminergic neurodegeneration. This is in contrast to findings in other neurodegenerative disease, such as mesial temporal lobe epilepsy, where there are large alterations in MCT levels.
