Study confirms how the body regulates high levels of CO2 in the blood

Karina Toledo/Agência FAPESP/DICYT In a recently published study in the journal Experimental Physiology, Brazilian researchers have confirmed the importance of a specific group of neurons found in a region of the brain known as the retrotrapezoid nucleus (RTN) in detecting changes in carbon dioxide (CO2) levels and in modulating the activity of the neuronal groups that control respiratory activity.

Scientists from the Biomedical Sciences Institute of the University of São Paulo (USP) and the School of Dentistry at the São Paulo State University (Unesp) in Araraquara participated in the study.

“CO2 is important for regulating the acid-base balance of the blood. When the concentration of this gas becomes higher than normal, the blood tends to become more acidic, which promotes the activation of specialized sensors called chemoreceptors,” said Eduardo Colombari, professor at the School of Dentistry at Unesp in Araraquara and coordinator of the thematic project, “Neural mechanisms involved in chemoreception.”

“Some of these chemoreceptors are located in the central nervous system; more precisely, on the ventrolateral surface of the medulla oblongata [the region of the brain responsible for neurovegetative control that forms the interface between the spinal cord and the mesencephalon] in the RTN,” he explained.

According to Colombari, the neurons in this region express a specific marker that allows them to be identified. This marker consists of a transcription factor called Phox2b, which is involved in the cell differentiation of autonomic and respiratory neurons.

“These neurons communicate with other neural groups responsible for controlling respiratory activity in order to keep CO2 levels within the physiological range,” said the researcher.

Previous studies in the scientific literature, said Colombari, have suggested that various neuronal groups, such as the nucleus of the solitary tract, the raphe nuclei (which secrete serotonin), and the pontine and hypothalamic areas, were involved in the control of chemoreception (in this case, the detection and modulation of CO2 levels).

The group’s work has demonstrated, however, that the respiratory changes caused by the increase in CO2 levels are compromised during the occurrence of selective destruction of the RTN neurons that express Phox2b.

“The neurons that express Phox2b have receptors in their membranes for a neurotransmitter called substance P. This neurotransmitter is involved in several neural communications that maintain the appropriate functioning of several physiological functions,” said Colombari.

To induce neuronal injury, the researchers used a toxin known as saporin, which binds to the main receptor for substance P. The toxin was directly injected into the RTN region of test animals.

“Previous studies had already shown that the process of neuron destruction by saporin takes 10 to 14 days. And, in fact, in measurements taken during the initial days following the injections, no major differences were observed between the control group and the group that had the neuronal lesion in the RTN region. But after the 14th day, the respiratory response of the animals that had received saporin was quite compromised,” Colombari explained.

The animals were placed in a chamber in which it was possible to measure the quantity of air they breathed in and out during each respiratory cycle. Then, the researchers introduced a combination of gases comprising nearly 7% CO2, a much higher concentration than that found in atmospheric air (approximately 0.03%), which was sufficient to cause a condition called hypercapnia (high levels of CO2 in the blood) and promote chemoreflex activation.

Ten minutes after being exposed to CO2, the group of animals that received the toxin presented significantly reduced breathing capacity in ambient air, as well as during exposure to high levels of CO2.

According to Colombari, the response was mainly compromised by the reduction in the volume of air current in the group that received saporin. “These animals practically lost the ability to eliminate excess CO2 and could have died from poisoning if the experimental conditions were maintained any longer,” he said.

The researcher further explained that the work illustrated how a small region of the brain contains neurons with a classic biochemical signature (Phox2b) which are involved in detecting and maintaining adequate levels of CO2, thus allowing the maintenance of homeostasis.

Leptin and control of ventilation

Another line of research within the thematic project showed that the responses triggered by the central chemoreflex could be compromised by a deficiency of leptin, the hormone produced by adipose tissue that is involved in the sensation of satiety.

“Some cases of obesity are caused by a deficiency in the production of leptin. In mice that have been genetically modified to not express leptin, we have shown that the replacement of this hormone improves the central chemoreflex response. But we’re still investigating how leptin is involved in this control,” said Colombari.

The group’s findings resulted in articles published in the European Journal of Physiology and the journal Acta Physiologica.

According to Colombari, advances in understanding the mechanisms involved in the perception of CO2 levels in the central nervous system could help prevent cases of sudden death in infants and adults in the future.

“There are cases of infants whose neurons that express Phox2b in the RTN region, in other words, the neurons that detect variations in CO2 levels, are compromised, harming the breathing process during sleep and wakefulness,” he said.

“This is an extremely rare syndrome called Congenital Central Hypoventilation Syndrome or Ondine’s Syndrome. When these mechanisms are better explained, we can think about developing tests to identify individuals who would need better monitoring of CO2 levels for adaptation to the pathophysiological conditions,” the researcher said.