Rapid microelectrode measurements and the origin and regulation of extracellular glutamate in rat prefrontal cortex

P>Glutamate in the prefrontal cortex (PFC) plays a significant role in several mental illnesses, including schizophrenia, addiction and anxiety. Previous studies on PFC glutamate-mediated function have used techniques that raise questions on the neuronal versus astrocytic origin of glutamate. The present studies used enzyme-based microelectrode arrays to monitor second-by-second resting glutamate levels in the PFC of awake rats. Locally applied drugs were employed in an attempt to discriminate between the neuronal or glial components of the resting glutamate signal. Local application of tetrodotoxin (sodium channel blocker), produced a significant (similar to 40%) decline in resting glutamate levels. In addition significant reductions in extracellular glutamate were seen with locally applied omega-conotoxin (MVIIC; similar to 50%; calcium channel blocker), and the mGluR(2/3) agonist, LY379268 (similar to 20%), and a significant increase with the mGluR(2/3) antagonist LY341495 (similar to 40%), effects all consistent with a large neuronal contribution to the resting glutamate levels. Local administration of D,L-threo-beta-benzyloxyaspartate (glutamate transporter inhibitor) produced an similar to 120% increase in extracellular glutamate levels, supporting that excitatory amino acid transporters, which are largely located on glia, modulate clearance of extracellular glutamate. Interestingly, local application of (S)-4-carboxyphenylglycine (cystine/glutamate antiporter inhibitor), produced small, non-significant bi-phasic changes in extracellular glutamate versus vehicle control. Finally, pre-administration of tetrodotoxin completely blocked the glutamate response to tail pinch stress. Taken together, these results support that PFC resting glutamate levels in rats as measured by the microelectrode array technology are at least 40-50% derived from neurons. Furthermore, these data support that the impulse flow-dependent glutamate release from a physiologically -evoked event is entirely neuronally derived.