Circadian changes in respiratory responses to acute hypoxia and histamine H1 receptors in mice.

Central histamine has crucial roles in circadian rhythm, ventilation, and the balance of energy metabolism via H1 receptors. We focused on the variation in ventilatory responses to hypoxia between light and dark periods, and the requirement of histamine H1 receptors for the circadian variation, using wild-type (WT) and histamine H1 receptor-knockout (H1RKO) mice. In WT mice, minute ventilation (V(E)) during hypoxia was higher in the dark period than in the light period. In H1RKO mice, changes in V(E) between photoperiods were minimal because V(E) increased relative to VO(2) (particularly in the light period). H1RKO mice showed metabolic acidosis, and increased levels of ketone bodies in blood during the light period. These data suggested that changes in V(E) during hypoxia vary between light and dark periods, and that H1 receptors have a role in the circadian variation in V(E) through control of acid-base status and metabolism in mice.