Introduction

Baseline blood flow (CBF), blood oxygen saturation (sO2), and vascular density are lower in the hippocampus than the visual cortex (V1), despite similar oxygen consumption rates. The hippocampus is therefore predicted to exist at a watershed whereby neurons furthest from the blood vessels only just receive enough oxygen to function properly in a normal healthy brain. Consequently, the hippocampus is likely to be especially vulnerable to even mild decreases in brain blood or oxygen supply, as seen in Alzheimer’s disease (AD) and acute mountain sickness (AMS), for example.

Aims

Our aim was therefore to determine if a mild decrease in brain oxygen supply differently affects function in hippocampus compared to neocortex (V1).

Methods

To achieve this, we mildly reduced the fraction of inspired oxygen (FiO2) using an altitude generator. The generator supplied mice with air of 21-11% FiO2 via a nose cone or tent enclosing the mouse’s cage. In awake head-fixed mice that previously underwent surgery to place a cranial window over the hippocampus or neocortex, we recorded haemodynamic changes using a combined laser-doppler flowmetry haemoglobin spectroscopy (oxy-CBF) probe, and neuronal calcium signalling using 2-photon microscopy. We also performed biochemical assays and mass spectrometry to measure the effects of hypoxia on metabolism and nutrient homeostasis in post-mortem brain tissue collected from mice exposed to 1hr at 21% or 11% FiO2.

Statistical Analyses

Robust linear mixed models assessed effects of FiO2, brain region and their interaction, with animal ID as the random factor (n=4-9 mice per condition).

Ethics

All experimental procedures involving animals were approved by the UK Home Office in accordance with the 1986 Animals in Scientific Procedures Act, as well as by the University of Sussex animal welfare ethical review board.  

Results

Baseline CBF and sO2 were lower in hippocampus than neocortex. Mild hypoxia briefly increased CBF but decreased sO2 more in hippocampus than in neocortex. This caused hippocampal pyramidal cells to fire more frequently but with less synchronisation and coordination within and between neurons. Conversely, neocortical pyramidal neurons fired less frequently but synchronisation and coordination were preserved. The increased frequency of neuronal calcium events in hippocampal pyramidal cells likely resulted from disinhibition since the frequency of neuronal calcium events decreased in hippocampal inhibitory interneurons. Neurovascular coupling was preserved in both regions. Mild hypoxia reduced iron levels in the hippocampus but not in neocortex.

Conclusions

Overall, mild decreases in brain oxygen supply produce lower oxygenation in hippocampus than neocortex. This causes different effects on neuronal activity and nutrient homeostasis and may help explain why the hippocampus is one of the most affected regions in AD and AMS.

Future Directions

Existing data are being extracted to determine whether hypoxia-sensitive TRPA1 channels may be responsible for neuronal calcium signalling changes in hippocampus. Current experiments are testing the effects of more chronic (2 week) mild hypoxia exposure on hippocampal neuronal and vascular function and amyloid beta accumulation a mouse model of AD.