A new study, published in Nature by researchers from McGill University, shows that increased sodium concentration in the blood may influence the circadian rhythm of mice via an excitatory GABAergic pathway.
Our circadian rhythm is controlled by the suprachiasmatic nucleus (SCN), which coordinates physiological and behavioral changes to anticipate changes related to our 24-hour cycle. For example, it allows the secretion of an anti-diuretic hormone as well as the drop in body temperature in order to limit water loss during the night. While light, the main regulating factor, has long been well known, there are other factors that can influence the SCN.
In this study, the researchers injected mice with saline solution during a period when the neurons of the SCN are inactive. They then witnessed a decrease in thermogenesis and body temperature in the mice. This result was confirmed by optogenetic stimulation of the neurons of the SCN .
By combining anatomy and electrophysiology, they were able to show that the saline solution activated neurons sensitive to sodium, located in a particular region of the brain called organum vasculosum lamina terminalis. These GABAergic neurons relay this information to the neurons of the SCN via an excitatory effect of GABA.
These results suggest that physiological factors other than simple light may be able to regulate our circadian rhythm, via yet unknown pathways.
Source: Gizowski C, Bourque CW. Sodium regulates clock time and output via an excitatory GABAergic pathway. Nature. 2020;583(7816):421-424. doi:10.1038/s41586-020-2471-x