Simulations of Implanted Glutamate Sensor Performance in Three Spatial Dimensions and Time Inform Sensor Design and...

Simulations in three dimensions and time provide guidance on implantable, electroenzymatic glutamate sensor design; relative placement in planar sensor arrays; feasibility of sensing synaptic release events; and interpretation of sensor data. Electroenzymatic sensors based on the immobilization of oxidases on microelectrodes have proven valuable for the monitoring of neurotransmitter signaling in deep brain structures; however, the complex extracellular milieu featuring slow diffusive mass transport makes rational sensor design and data interpretation challenging. Simulations show that miniaturization of the disk-shaped device size below a radius of ∼25 μm improves sensitivity, spatial resolution, and the accuracy of glutamate concentration measurements in vivo based on calibration factors determined in vitro. Calculations also show that crosstalk between glutamate sensors in a planar array due to lateral H₂O₂ diffusion is unimportant at micron-scale separation distances, but that the glutamate depletion zone in the vicinity of an individual sensor may limit site spacing to ≳40 μm. Deposition of immobilized glutamate oxidase layers that extend beyond the edge of the underlying microelectrode can improve sensitivity modestly, but H₂O₂ generated by the enzyme at the periphery may not be captured efficiently by the microelectrode, thereby resulting in potentially toxic local H₂O₂ concentrations of ∼25 μM. While simulations predict that detection of glutamate from single-vesicle release events in a synapse would result in current signals that are challenging to detect with conventional equipment, electroenzymatic sensors arranged in planar arrays on biocompatible probes will continue to be a powerful tool for neuroscientists.