Physical activity stimulates clock neurons of the day-active rodent <i>Arvicanthis ansorgei</i>

Our biological clock, located in the suprachiasmatic nucleus (SCN), controls behavioral activity rhythms by producing circadian rhythms in SCN electrical activity. Behavioral studies in humans suggest that the clock is sensitive not only to light but also to physical activity. Here, we examined the effect of physical activity on the brain's clock in the diurnal rodent, <i>Arvicanthis ansorgei</i>. We found that the electrical activity of SCN neurons in vitro is high during the day and low during the night. Recordings via stationary microelectrodes in freely moving <i>Arvicanthis</i> revealed that the SCN baseline rhythm in discharge was superimposed by increments in electrical activity. These increments in electrical activity occurred during brief (seconds) or long (hours) periods of spontaneous activity of the animal and were observed at each phase of the cycle, i.e., both day and night. To establish the causal relation, we manipulated the animal's activity by providing it with a running wheel. The voluntary use of the wheel resulted in direct and significant increments in SCN electrical activity. We conclude that behavioral activity triggers the increments in SCN electrical activity, rather than vice versa. Consequently, physical activity during the day will raise the amplitude of the SCN electrical discharge rhythm, thereby strengthening clock function. In contrast, night-time activity will be countereffective and attenuate the rhythm in electrical activity. The data elucidate the route via which daytime exercise supports clock function.