Abstracts

Rationale:
Glutamate (GLU) and gamma-aminobutyric acid (GABA) are the primary excitatory (E) and inhibitory (I) neurotransmitters in the mammalian brain, respectively. Dysregulation of the GLU/GABA balance (E/I) is linked to neurological disorders, including epilepsy. Microdialysis can measure GLU and GABA over time, but it has low spatiotemporal resolution. Enzyme-based microelectrode biosensors have markedly improved spatiotemporal resolution. However, enzymes degrade over time. We developed a novel system employing replaceable microwire biosensor arrays to study changes in GLU and GABA periodically over 16 wk in a rat model of temporal lobe epilepsy.
Method:
To overcome recording time limitations due to degradation of biosensor enzymes, we created a microwire biosensor array that inserts into a permanently implanted cannula. It is periodically replaced to facilitate longitudinal recording using rodent models of brain disorders and injury. The GLU and reagent-free GABA biosensors[1], as well as a sentinel sensor to subtract interferent signals[2], are located at the tips of the microwires which are positioned in the same plane. Placement of biosensors in the same plane is advantageous for recording signals in laminar structures in the brain and in brain slices.  Procedures involving rats were conducted according to a Louisiana Tech University IACUC protocol.
Results:
We first used the novel probes to measure the responses of GLU and GABA to different stimulation frequencies (10-, 50-, and 140-Hz) in rat hippocampal brain slices. We observed a significant, nonlinear shift in the E/I ratio from excitatory to inhibitory dominance as the stimulation frequency increased. This shift is indicative of an increase in GABAergic signaling that suppresses glutamatergic excitotoxicity and the relative insensitivity of GLU receptors to high frequency stimulation. Next, we recorded GLU and GABA in real time over 16 weeks in an epileptic rat, before and after epileptogenesis. GLU and GABA dynamics were consistent with expected behavior, including during spontaneous seizure activity. Figure 1 shows how GLU (A) and GABA (B) dynamics changed before and during a spontaneous seizure. Interictal epileptiform discharges (IED, 450-ms duration, flat bar), were followed by walking and grooming. GABA decreases slowly during grooming (sloped bar) and then suddenly (left arrow) before a seizure. Forelimb clonus (right arrow) was followed by rearing and falling with increased amplitude. Afterward, facial twitches and head bobbing predominated with an increase in GABA (inclined bar) near the end of the seizure. Vertical and horizontal scale bars represent 1 nA and 20 s, respectively. The E/I ratio was dominated by excitation during IED (1.2) and seizure (2.9); otherwise, E/I < 1.
Conclusion:
We have created a powerful new tool for longitudinal studies of epileptogenesis. References: [1] Hossain et al.,  Front. Neurosci. 2018, 12: 500; [2] Scoggin et al., Biosens. & Bioelect. 2019, 126: 751-7.  
Funding:
:National Science Foundation, OIA/EPSCoR 1632891, L. Iasemidis, PI.