Abstract
The effects of the circadian light/dark cycle on ventilatory responses to chemical stimuli have rarely been studied in experimental animals, despite evidence that the cycle may be a factor in respiratory results. We measured the ventilatory response to hypoxia (HVR) in unanesthetized wild-type and histamine type-1 receptors knockout (H1RKO) mice in the light and dark periods with a whole-body, single-chamber plethysmograph. Animals were subjected to a 10-min hypoxic exposure (7% O2 and 3% CO2 in N2) after acclimatization to the chamber for 90 min. In both groups of mice, minute ventilation increased in response to the hypoxia and declined gradually after the peak response regardless of when HVR was determined. However, we found differences in the HVR between wild-type and H1RKO mice. In wild-type mice, the minute ventilation response was higher in the dark period than in the light period, which was due to differences in the tidal volume response rather than the respiratory frequency response. Conversely, in H1RKO mice, minute ventilation responses did not differ between the two periods, which were similar to the response of wild-type mice determined in the dark period. In summary, the circadian light/dark cycle altered the HVR in wild-type mice, whereas, in H1RKO mice, the cycle difference in the HVR disappeared. These results suggest that H1R contributes to the circadian light/dark cycle differences in the HVR in unanesthetized mice. [J Physiol Sci. 2006;56 Suppl:S137]
