Proceedings of The Physiological Society

Europhysiology 2018 (London, UK) (2018) Proc Physiol Soc 41, PCB262

Poster Communications

Aberrant Changes in Oxygen Levels in the Brain during Seizures in Awake Freely Behaving Rodents

A. Ledo1, C. F. Lourenço1, J. Laranjinha2,1, G. A. Gerhardt3, R. M. Barbosa2,1

1. Center for Neuroscience and Cell Biology, University of Coimbra, Coimbra, Portugal. 2. Faculty of Pharmacy, University of Coimbra, Coimbra, Portugal. 3. Department of Neuroscience, Center for Microelectrode Technology, University of Kentucky Medical Center, Lexington, Kentucky, United States.

Introduction: Temporal lobe epilepsy (TLE) is a form of acquired epilepsy characterized as an electro-clinical syndrome in which seizures emanate from the limbic system (1, 2). TLE is particularly disabling due to the unpredictable and recurrent nature of seizures and incidence of antiepileptic drug resistance (3) many times an indication for resection surgery (4).Seizures are paroxysmal events in which increased neuronal activity is accompanied by an increase in localized energetic demand, which is followed by a hemodynamic response which overwhelms the tissue with energetic substrates such as oxygen and glucose. Both monitoring of electrical activity and hemodynamic response have been explored extensively in the identification of seizure onset zones as well as in epilepsy diagnosis. This underlies the importance of understanding brain energetic and cerebrovascular response during seizures, which we here address by concurrent recording of local field potential and brain tissue pO2 using high-frequency amperometric oximetry. Methods: we used multisite Pt microelectrode arrays (MEA) (5) to perform high-frequency amperometric recording of pO2 and local field potential (LFP)-related currents in the CNS of chronically implanted freely-moving rats during pilocarpine-evoked status epilepticus. Fast Fourier transform (FFT) filtering of the raw electrochemical signal allowed extraction of the low frequency component (<1 Hz), corresponding to the electrochemical reduction of O2 and high (>1 Hz) frequency component, corresponding to the LFP currents. Results: We found that resting levels of pO2 in the rodent brain varied between 6.63 ± 0.74 µM in the DG region of the hippocampus and 22.1 ± 4.91 µM in the cerebral cortex. Induction of status epilepticus by intrahippocampal injection of pilocarpine produced typical and graded behavior correlate of temporal lobe seizures, from oro-alimentary automatisms to forelimb clonus, clonic extension and wild running and ultimately generalized tonic-clonic seizure. Reading of LFP currents in the hippocampus confirmed temporal lobe seizure with initial hyper-synchronization at low frequency followed by increased power at high frequency. Regarding interstitial levels of pO2, we observed a biphasic change in the hippocampus. The initial dip at seizure onset (ΔO2= -4.5 ± 0.7 µM) was followed by a prolonged hyperoxygenation phase (ΔO2 = +10.4 ± 2.9 µM). More interesting was the observation that the initial dip appeared to precede a more pronounced alteration in LFP, suggestion that increased oxidative metabolism in response to initial increase in neuronal firing may be a valuable feature in seizure prediction. Conclusions: This strategy revealed that a single sensor can simultaneously report chemical (pO2) and electrophysiological (LFP currents) information, allowing concurrent monitoring of electrical and neurovascular/neurometabolic activities.

Where applicable, experiments conform with Society ethical requirements