L-Glutamate (Glu) is the main excitatory neurotransmitter in the mammalian central nervous system, and it is involved in most aspects of normal brain function, including cognition, memory and learning, plasticity, and motor movement. Although microdialysis techniques have been used to study Glu, the slow temporal resolution of the technique may be inadequate to properly examine tonic and phasic Glu. Thus, our laboratory has developed an enzyme-based microelectrode array (MEA) with fast response time and low detection limits for Glu. We have modified the MEA design to allow for reliable measures in the brain of awake, freely moving mice. In this study, we chronically implanted the MEA in prefrontal cortex (PFC) or striatum (Str) of awake, freely moving C57BL/6 mice. We successfully measured Glu levels 7 days postimplantation without loss of MEA sensitivity. In addition, we determined resting (tonic) Glu levels to be 3.3 M in the PFC and 5.0 M in the Str. Resting Glu levels were subjected to pharmacological manipulation with tetrodotoxin (TTX) and DL-threo-␤-hydroxyaspartate (THA). TTX significantly (p Ͻ 0.05) decreased resting Glu by 20%, whereas THA significantly (p Ͻ 0.05) increased resting Glu by 60%. Taken together, our data show that chronic recordings of tonic and phasic clearance of exogenously applied Glu can be carried out in awake mice for at least 7 days in vivo, allowing for longer term studies of Glu regulation.

J. Neurochem. (2010) 115, 1608–1620. Abstract 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 (∼40%) decline in resting glutamate levels. In addition significant reductions in extracellular glutamate were seen with locally applied ω‐conotoxin (MVIIC; ∼50%; calcium channel blocker), and the mGluR2/3 agonist, LY379268 (∼20%), and a significant increase with the mGluR2/3 antagonist LY341495 (∼40%), effects all consistent with a large neuronal contribution to the resting glutamate levels. Local administration of D,L‐threo‐β‐benzyloxyaspartate (glutamate transporter inhibitor) produced an ∼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.

Chronic implantation of neurotransmitter measuring devices is essential for awake, behavioral studies occurring over multiple days. Little is known regarding the effects of long term implantation on surrounding brain parenchyma and the resulting alterations in the functional properties of this tissue.

The current pandemic caused by severe acute respiratory syndrome coronavirus (SARS-CoV)-2 has created an unparalleled health crisis. Besides the acute respiratory infection, CoVs are neuroinvasive causing additional inflammation and neurodegeneration. This is likely also true of SARS-CoV-2 given reports of neurological manifestations in coronavirus disease 2019 (COVID-19) positive patients. Older adults > 65 years of age constitute a high-risk group prone to severe infection and death. Despite the higher mortality rate, a majority of cases are expected to recover and survive from this viral outbreak. But, the long-term consequences of SARS-CoV-2 neuroinfection are unknown. We discuss these potential chronic changes to the central nervous system (CNS) in relation to accelerated brain aging and age-related neurodegenerative disorders.

The symptomologies of Alzheimer’s disease (AD) develop over decades suggesting modifiable life-style factors may contribute to disease pathogenesis. In humans, hyperinsulinemia associated with type 2 diabetes mellitus increases the risk for developing AD and both diseases share similar age-related etiologies including amyloidogenesis. Since we have demonstrated that soluble Aβ42 elicits glutamate release, we wanted to understand how diet-induced insulin resistance alters hippocampal glutamate dynamics, which are important for memory formation and consolidation. Eight to twelve week-old C57BL/6J and AβPP/PS1 mice were placed on either a low-fat diet (LFD) or high-fat diet (HFD) for eight months. A HFD led to significant weight increases as well as impaired insulin sensitivity, glucose tolerance, and learning in both C57BL/6J and AβPP/PS1 mice. AβPP/PS1 LFD mice had elevated hippocampal basal as well as stimulus-evoked glutamate release that was further increased with consumption of a HFD. Immunohistochemistry indicated an increase in vesicular glutamate transporter 1 and glial fibrillary acidic protein density in hippocampal subregions corresponding with this elevated extracellular glutamate. While no differences in hippocampal plaque load were observed, the elevated astrogliotic response surrounding the plaques in AβPP/PS1 HFD mice may have been a compensatory mechanism to control plaque accumulation. These data support that AβPP/PS1 mice have chronically elevated extracellular glutamate that is exacerbated by a HFD and that modifiable life-style factors such as obesity-induced insulin resistance can contribute to AD pathogenesis.