Mice housed in groups of 4-6 exhibit a diurnal surge in their platelet count.

ABSTRACT

The number of circulating platelets in humans exhibits diurnal rhythmicity, with lowest numbers often recorded in the morning. It has been demonstrated that a similar diurnal rhythmicity in the platelet count exists in mice. In this brief communication, it is reported that husbandry conditions affect the diurnal rhythm of platelet abundance in mice. The platelet count in mice, housed one per cage and entrained to a 12 hour 12 hour light dark cycle, fluctuated over 24 hours, with peak counts occurring during the animals' rest period. In contrast, this pattern was dramatically altered in mice housed as groups of 4-6 mice per cage. In group-housed mice, there was a transient surge in both platelet and reticulated platelet numbers at the transition from light to dark, corresponding to the time that animals initiate daily locomotor activity. It is speculated that this difference may reflect the circadian regulation of a stress response experienced by group-housed mice, possibly upon sampling. This finding highlights a new component to the mammalian platelet count that has not been reported before. This is an important observation because the surge in platelet and reticulated platelet numbers and the mechanism controlling it, may contribute to the diurnal incidence of cardiovascular events seen in humans.