Hypoxia alters posterior cingulate cortex metabolism during a memory task: A ¹H fMRS study.

Area of Science:

• Neuroscience
• Physiology
• Medical Imaging

Background:

• Environmental hypoxia (low inspired oxygen) globally increases cerebral blood flow (CBF), but regionally, responses are heterogeneous.
• The posterior cingulate cortex (PCC) shows decreased CBF and reversed task-evoked BOLD signals under hypoxia, questioning neurovascular coupling.
• Previous findings suggest altered neural activity or vascular changes, necessitating a non-haemodynamic measure to differentiate origins.

Purpose of the Study:

• To investigate the neural origin of altered PCC activity during hypoxia using functional magnetic resonance spectroscopy (fMRS).
• To measure glutamate response to memory recall in the PCC under normoxia and hypoxia.
• To confirm reduced CBF in the PCC during hypoxia using arterial spin labeling (ASL).

Main Methods:

• Utilized fMRS to measure PCC glutamate changes during memory recall under normoxia (FiO2=0.209) and hypoxia (FiO2=0.120).
• Acquired ASL-derived CBF measurements to assess regional blood flow.
• Compared neurometabolite concentrations (glutamate, glucose) between normoxic and hypoxic conditions at rest and during cognitive tasks.

Main Results:

• Hypoxia reduced CBF in the PCC and default mode network regions, consistent with prior studies.
• Memory recall increased PCC glutamate by 8% during normoxia, but this change was absent during hypoxia.
• Exploratory analysis revealed reduced PCC glucose during hypoxia, both at rest and during tasks.

Conclusions:

• Hypoxia alters the activity-induced increase in PCC glutamate, potentially indicating reduced oxidative metabolism.
• Reduced PCC glucose levels during hypoxia suggest ongoing metabolism, possibly non-oxidative, due to decreased CBF.
• Findings suggest that observed changes in PCC activity under hypoxia are linked to metabolic and vascular alterations rather than solely neurovascular uncoupling.