Developing in a polluted atmosphere: A link between long-term exposure to elevated atmospheric CO2 and hyperactivity.

Abstract

The World Health Organisation (WHO) estimates that 99% of people worldwide breathe polluted air on a day-to-day basis (i.e. air with pollutant levels higher than WHO guidelines) and that ambient air pollution causes more than 4 million premature deaths per year. These numbers reflect the effect of particulate matter on cardiovascular and respiratory disease (Cohen et al. 2017) and probably underestimate the importance of air pollution on health because the role of atmospheric carbon dioxide (CO2) has not been considered thus far. Furthermore, air pollution has neurotoxic effects that are increasingly recognised and elevated atmospheric CO2 has the potential to cause renal and bone disease, expanding the latent health impact of air pollution globally. In this issue of The Journal of Physiology, Wyrwoll et al. (2022) address this timely question, exploring both systemic and neurological outcomes of a life lived in a high CO2 environment. Comparing life-time exposure to current atmospheric CO2 (460 ppm) with the higher CO2 level of 890 ppm, a level predicted to occur by the year 2100, Wyrwoll et al. (2022) showed that mice exposed to elevated CO2 during gestation had a higher birth weight than their control counterparts. Abnormal birth weight can increase risk of later morbidity, although, in this case, weight normalised over the postnatal period and developmental milestones, such as righting reflex or eye opening, were not delayed. Dimorphism occurred between males and females at adulthood, with high CO2 exposed females having lower body weight compared to age-matched controls. It is well known that substantial acute increases in inspired, or decreases in expired, CO2 result in respiratory acidosis, where the pH of the blood falls below the normal physiological range. Short-term compensations include increased respiration or altered renal excretion of hydrogen and bicarbonate ions. Longer-term, excess hydrogen can be buffered by bicarbonate released from bone. This bicarbonate release may lead to osteoporosis, whereas the accompanying release of calcium and other ions can contribute to renal calcification. It has been unclear until now whether increased atmospheric CO2 could drive these pathologies. This question is pertinent given that the world is experiencing unprecedented increases in atmospheric CO2. Surprisingly, no pathological changes were observed in kidney structure at adulthood following exposure to the high CO2 environment for 3 months (i.e. from gestation to young adulthood) or using measures of renal function (Wyrwoll et al. 2022). Nor were differences in bone structural density identified, as would be expected if substantial physiological compensation was occurring. This may reflect the relatively short experimental period and the fact that blood pH was only borderline acidotic in this model, although it was sufficient to change the structure and function of the lungs, as detailed in a companion study (Larcombe et al. 2021). Particulate matter 2.5 (PM2.5) from air pollution has now been added to the list of environmental pollutants with neurotoxic capacity (e.g. pesticides, heavy metals). A plethora of recent studies show that PM2.5 can produce inflammation and oxidative stress throughout the brain, linked to increased risk of neurodevelopmental disorders and neurodegenerative disease (Costa et al. 2020). Acute increases in CO2, as may arise in a poorly ventilated indoor setting, can reduce cognitive function, particularly decision making and executive function (Karnauskas et al. 2020). This reduction is probably a result of altered neuronal excitability, a phenomenon that could affect brain development, where neural activity is essential for co-ordinating normal developmental events. The study by Wyrwoll et al. (2022) makes the case that global rising CO2 levels could confer a risk similar to other components of air pollution. High life-time CO2 exposure produced significant differences in dopamine receptors in the brain of adult offspring (Wyrwoll et al. 2022). Increased expression of the dopamine receptor D1 (DRD1) was found in males, whereas DRD2 expression decreased in females. Increased DRD1 activity has been associated with hyperactivity, a behaviour observed in both male and female mice in the study by Wyrwoll et al. (2022). Plasma corticosterone levels also increased with CO2 exposure, possibly contributing to the hyperactivity. Expression of the gene for 11β-hydroxysteroid dehydrogenase type 1 (Hsd11b1), an enzyme that converts cortisone to cortisol, was increased in the hippocampus, further supporting this possibility. Chronic elevation of brain glucocorticoids associates with age-related neurodegeneration and cognitive impairment, and could harm the developing brain. Further exploration is needed to determine how elevated CO2 may affect pregnancy, ageing and the mechanisms underlying behavioural change. It is unclear from the study by Wyrwoll et al. (2022) whether the reported hyperactivity is a result of changes in brain structure and function. Elucidating both behaviours and their neurological basis in this paradigm will not be trivial. The interpretation of behaviour in mice is always complicated and, in the study by Wyrwoll et al. (2022), CO2 exposed mice appeared to interact abnormally with some behavioural testing paradigms in a manner not reflected in the test metrics. One limitation of the study by Wyrwoll et al. (2022) was the choice to use 460 ppm CO2 as the control condition. Average CO2 in the recent past was lower, ∼370 ppm at the turn of the century, and, although 460 ppm is a feasible estimate of current ambient CO2 within an urban environment (average atmospheric CO2 of 420 ppm plus local urban production), this choice may reflect an already