GABA and glutamate changes underlying altered autonomic function in obstructive sleep apnea

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PROJECT SUMMARY At least 10% of the population has obstructive sleep apnea (OSA), a condition where breathing is interrupted hundreds of times a night due to collapse of the upper airway. The disorder is accompanied by psychological impairments and cardiovascular morbidity and mortality, but symptoms are only partially addressed with continuous positive airway pressure (CPAP), the gold standard intervention, indicating that mechanisms other than breathing interruptions contribute to health problems in OSA. Impaired autonomic function due to neural alterations is a likely source of symptoms in OSA, since structural and functional deficits are present in brain regions that regulate such functions. Symptoms are sex-specific, with females typically showing greater severity despite lower prevalence than males. However, neuroimaging findings have not distinguished the nature of the brain pathology. Based on structural, functional, and preliminary findings of altered neurotransmitter levels, and animal models of OSA, we hypothesize that brain alterations impact limbic regions, and the insula in particular. The insula plays key roles in autonomic regulation as well as mood and some cognitive functions, and there may be reorganization that underlies altered function in the structure. Furthermore, since some symptoms do not resolve with CPAP, the reorganization may remain even with treatment. We propose measuring neurotransmitter levels in the insula in OSA, and relating these levels to the extent of structural and functional deficits in the condition, with the hypothesis that the altered function is associated with altered GABA and glutamate, a theory supported by our preliminary data. We will also test whether such changes are present with CPAP. We will use two-dimensional magnetic resonance spectroscopy (MRS) to measure GABA and glutamate, in conjunction with structural measures sensitive to cellular size and integrity (with diffusion tensor imaging, DTI) and water content (free water with T2 relaxation time, and extracellular water with DTI-derived free water fraction [FW]). FW is important since extracellular rather than intracellular glutamate leads to excitotoxicity. Function will be measured with functional magnetic resonance imaging (fMRI) during an autonomic challenge, the Valsalva maneuver. We will study 60 untreated, newly-diagnosed OSA, 60 matched controls, and 60 OSA patients with 1 year of compliant CPAP use. We will assess whether GABA and glutamate in the insula are altered in OSA vs. controls, and in treated vs. untreated patients. We will relate these neurotransmitter levels to DTI indices of tissue structure, mean diffusivity (MD), and fractional anisotropy (FA). We will also relate levels of GABA and glutamate to the magnitude of fMRI responses during the autonomic challenge. The findings will help explain mechanisms of brain injury and reorganization in OSA, and suggest possible treatment options to address common symptoms in OSA not addressed by CPAP, including possible manipulation of brain function through altering GABA or glutamate.