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Abstract:
Glutamate and γ‐aminobutyric acid (GABA) are the most common neurotransmitters in the central nervous system. By exciting, inhibiting, and modulating neural elements and microcircuits, these chemicals critically regulate brain information processing and energy metabolism at different spatiotemporal scales. However, the exact relationship between the extracellular concentration of these molecules and emergence of specific patterns in neuronal ensemble activity remains elusive. Partly this is due to the fact that recording of the mean extracellular field potentials (mEFP) concurrently with a quantitative assessment of alterations in the concentration of such neurochemicals are currently unavailable. Here, we present a silicon‐based implantable ultrafine microelectrode array (35 µm diameter) composed of several iridium‐stabilized electrochemical and electrophysiological contacts. The electrophysiological electrodes have an average impedance of 0.5 MΩ at 1 kHz. The amperometric electrochemical channels, divided into two groups of glutamate‐ and GABA‐responsive electrodes, show a sensitivity of 0.39 nA/µM for glutamate and 0.38 nA/µM on the adjacent channel for GABA. This novel multimodal microelectrode was used to simultaneously monitor extracellular glutamate and GABA concentrations, spikes, multi‐unit neuronal activity (MUA) and local field potentials (LFP) in the lateral geniculate nucleus (LGN) of anaesthetized rats (n = 5). Retinal stimulation with flickering monochromatic light, emphasizing the simplest form of feedforward processing in thalamus, induced neuronal response patterns in LGN that were highly correlated with the temporal alterations in glutamate concentrations. GABA responses, while similar in profile to MUA and LFP recordings, were found to be event‐selective, suggesting network‐level processes. Our findings suggest that this multimodal method may greatly contribute into our understanding of microcircuit organization, by reducing the inherent ambiguity in the mEFP through neurotransmitter‐release‐tracking.
