Locomotor response to an open field during C57BL/6J active and inactive phases: differences dependent on conditions of illumination.

Time of day has proven to be a source of variability in diverse behavioral measures. Knowledge of the pattern of this temporal effect as well as its origin (exogenous or endogenous) is essential for a precise description of any behavior. This study analyzed the effect of the external light-dark cycle and the internal rest-activity rhythm on the response of C57BL/6J mice to a novel environment. In a first experiment, animals maintained in a 12:12-h light-dark cycle were tested in an open field at six different times of day. A diurnal rhythm of ambulation in the open field was observed with greater levels of activity exhibited by those groups tested at night. Long-term and short-term behavioral habituation to spatial novelty were also affected by phase of the light-dark cycle. A second experiment was designed to control for any direct effect of the light-dark cycle by keeping the animals in dim green light where entrainment was maintained by a skeleton photoperiod (two 15-min bright-light pulses separated by 12 hours of green, dim light). This second group of animals was tested at two different circadian phases under the same conditions of illumination. One group was tested during the subjective night and another group during the subjective day, i.e., 2 or 14 h after the onset of the active phase, as assessed by wheel-running behavior. No effect of circadian phase on ambulation or habituation of this response to the open field was observed in these animals. Taken together, these results suggest that spatial novelty is equally arousing regardless of circadian phase and that the conditions of illumination can dramatically alter the response to a novel environment.