The current study tests whether different exposures to carbon dioxide (CO2) can safely result in the increased movement of proteins from the brain into the blood. The investigators believe that this would be a proxy for the brain clearing waste products more effectively. The current study will use a counter-balanced design, in which individuals with and without a history of traumatic brain injury (TBI) will receive different levels of CO2 (targeted changes of approximately 5 or 10 mmHG in end-tidal CO2) approximately one week apart. The counter-balanced design means that each participant receives a single dose of CO2 at each visit, and different doses of CO2 at each visit. The order in which participants receive the dose is randomized, and the participant will not be informed of the dose.

Impaired clearance of metabolic waste and cellular debris is a hallmark of TBI and other neurodegenerative conditions. Clearance primarily occurs through glymphatic/lymphatic pathways, which is partially dependent on the influx of cerebrospinal fluid (CSF). CSF flow is greatest during sleep, when low-frequency oscillations in cerebral blood volume are most prominent. The investigators propose that changing levels of cerebral blood volume via the administration of CO2 will drive CSF flow and ultimately promote brain waste clearance. The proposed study is significant because it examines whether prescribed CO2 can enhance protein efflux (i.e., a surrogate for waste clearance), and the biological mechanisms that may mediate this mechanism in both health and disease. The first study aim is therefore to determine whether the administration of CO2, a potent vasodilator, can be prescribed to mimic global changes in cerebral blood volume in a dose-dependent fashion. Basal protein levels and efflux (i.e., change from baseline) are quantified using high-sensitivity proteomic platforms. The second aim is to examine how individual differences in cerebrovascular function and other disease factors such as atrophy affect CO2-induced protein efflux.

Using a counter-balanced (AB/BA) design, individuals (aged 18-82 years) with chronic TBI and individuals without a history of TBI (healthy subjects) will be dosed to achieve either 5 or 10 mmHG changes in end-tidal CO2. Importantly, the proposed cerebrovascular mechanisms and surrogate markers of waste clearance are readily quantified in humans using advanced MR-imaging and commercially available proteomic platforms, exponentially increasing their clinical translation.

Inclusion criteria for TBI cohort: 1) Able to give valid informed consent, 2) 18-82 years old, 3) history of TBI of any severity level (mild, moderate or severe) that was sustained in adulthood (age 18 and older), which will be confirmed using a semi-structured identification method (includes asking about loss of consciousness, posttraumatic amnesia, and feeling dazed/confused).

Inclusion criteria for Healthy Subjects cohort: 1) Able to give valid informed consent and 2) 18-82 years old years old.

Exclusion criteria for TBI cohort: 1) contraindications to MRI scanning including pregnancy or claustrophobia, 2) unable to give valid informed consent, incarcerated, 3) diagnosed with Alzheimer's, ADRD or mild cognitive impairment, 4) pre-existing history of autism spectrum disorders, intellectual disability, serious neurological (e.g., epilepsy, tumors, other conditions requiring neurosurgery) or psychiatric disorders (requiring hospitalization) prior to TBI onset, 5) current or previous diagnosis of a psychosis spectrum disorder or bipolar disorder, 6) respiratory diseases or pulmonary conditions that may increase the risk of study procedures (e.g., severe asthma, chronic obstructive pulmonary disease or other significant respiratory disorders), 7) a history of cardiac arrythmias 8) history of a current substance use disorder, 9) non-English fluency (based on screener), 10) Weigh under 110 pounds for blood draw.

Exclusion criteria for Healthy Subjects cohort: 1) contraindications to MRI scanning including pregnancy or claustrophobia, 2) unable to give valid informed consent, incarcerated, 3) diagnosed with Alzheimer's, ADRD or mild cognitive impairment, 4) history of developmental, neurological, or psychiatric disorders requiring hospitalization, 5) regular use of any substance in the past six months (i.e., no current use confirmed with urine screening), 6) previous head injury with greater than 30 minutes loss of consciousness, 7) respiratory diseases or pulmonary conditions that may increase the risk of study procedures (e.g., severe asthma, chronic obstructive pulmonary disease or other significant respiratory disorders), 8) a history of cardiac arrythmias, 9) non-English fluency (based on screener), 10) Weigh under 110 pounds for blood draw.