Novel ex vivo model for studying the neuronal synaptic response in precisely cut brain tissue slices subjected to controlled hypoxic patterns

Normal brain function is highly dependent on oxygen and its availability can be compromised under some chronic respiratory diseases. In fact, the neurocognitive impairments associated to chronic obstructive pulmonary disease (COPD) and obstructive sleep apnea (OSA) have been extensively studied. However, how different levels of oxygen can affect the synaptic activity it is unknown. Our objective is to develop and characterize a novel ex vivo model to induce controlled hypoxia and accurately measure neuronal activity through electrophysiological recordings in precisely cut brain tissue slices. To this end, fresh tissue slices (300–400 μm thick) from mice brains were placed on a well whose bottom consisted of a permeable silicon membrane. The chamber beneath the membrane was subjected to controlled levels of hypoxia and the oxygen concentration within the tissue was measured with an oxygen microsensor. The well was adapted to an electrophysiological device to record field excitatory postsynaptic potentials (fEPSPs) in real time. Our model was able to determine the reduction of neuronal activity in response to different levels of oxygen and its recovery when oxygen levels were restored to physiological values. The fEPSPs remained unaltered from 21% O2 (hyperoxia) to 6 % O2 (physioxia). However, lower levels of O2 (<6%) were translated into a reduction of fEPSPs reaching to 40% at 2% O2. This system will provide the opportunity to determine how different levels and types of hypoxia (chronic and intermittent) can alter the neuronal activity, which could help to explain the neurocognitive impairment associated to COPD and OSA.