Mechanisms regulating spill-over of synaptic glutamate to extrasynaptic NMDA receptors in mouse substantia nigra dopaminergic neurons

• Published in: The European journal of neuroscience Vol. 42; no. 9; pp. 2633 - 2643
• Main Authors: Wild, A. R., Bollands, M., Morris, P. G., Jones, S.
• Format: Journal Article
• Language: English
• Published: France Blackwell Publishing Ltd 01.11.2015; John Wiley and Sons Inc
• Subjects: Amino Acids - pharmacology; Animals; Aspartic Acid - pharmacology; Dizocilpine Maleate - pharmacology; Dopaminergic Neurons - drug effects; Dopaminergic Neurons - physiology; Electric Stimulation; Excitatory Amino Acid Antagonists - pharmacology; Excitatory Postsynaptic Potentials - drug effects; glutamate transporter; Glutamic Acid - physiology; Male; metabotropic glutamate receptor; Mice; Mice, Inbred C57BL; Molecular and Synaptic Mechanisms; NMDA glutamate receptor; Receptors, N-Methyl-D-Aspartate - antagonists & inhibitors; Receptors, N-Methyl-D-Aspartate - physiology; Substantia Nigra - physiology; substantia nigra pars compacta; Synapses - drug effects; Synapses - physiology; Xanthenes - pharmacology; substantia nigra pars compacta; NMDA glutamate receptor; glutamate transporter; metabotropic glutamate receptor
• ISSN: 0953-816X; 1460-9568
• DOI: 10.1111/ejn.13075

N‐Methyl‐d‐aspartate glutamate receptors (NMDARs) contribute to neural development, plasticity and survival, but they are also linked with neurodegeneration. NMDARs at synapses are activated by coincident glutamate release and depolarization. NMDARs distal to synapses can sometimes be recruited by ‘spill‐over’ of glutamate during high‐frequency synaptic stimulation or when glutamate uptake is compromised, and this influences the shape of NMDAR‐mediated postsynaptic responses. In substantia nigra dopamine neurons, activation of NMDARs beyond the synapse during different frequencies of presynaptic stimulation has not been explored, even though excitatory afferents from the subthalamic nucleus show a range of firing frequencies, and these frequencies change in human and experimental Parkinson's disease. This study reports that high‐frequency stimulation (80 Hz/200 ms) evoked NMDAR‐excitatory postsynaptic currents (EPSCs) that were larger and longer lasting than those evoked by single stimuli at low frequency (0.1 Hz). MK‐801, which irreversibly blocked NMDAR‐EPSCs activated during 0.1‐Hz stimulation, left a proportion of NMDAR‐EPSCs that could be activated by 80‐Hz stimulation and that may represent activity of NMDARs distal to synapses. TBOA, which blocks glutamate transporters, significantly increased NMDAR‐EPSCs in response to 80‐Hz stimulation, particularly when metabotropic glutamate receptors (mGluRs) were also blocked, indicating that recruitment of NMDARs distal to synapses is regulated by glutamate transporters and mGluRs. These regulatory mechanisms may be essential in the substantia nigra for restricting glutamate diffusion from synaptic sites and keeping NMDAR‐EPSCs in dopamine neurons relatively small and fast. Failure of glutamate transporters may contribute to the declining health of dopamine neurons during pathological conditions. In substantia nigra dopamine neurons, high frequency stimulation evoked larger and longer lasting NMDAR‐EPSCs than single low frequency stimuli. Following synaptic block with MK‐801, a proportion of NMDARs could be activated by high frequency stimulation; these were inhibited by memantine and may represent NMDARs distal to synapses. Glutamate transporter block significantly increased high frequency‐evoked NMDAR‐EPSCs, particularly when metabotropic glutamate receptors were also blocked.