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 o rigin (exogenous or endogenous) is essential for a precise description of a ny behavior. This study analyzed the effect of the external light-dark cycl e 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 d ay. A diurnal rhythm of ambulation in the open field was observed with grea ter levels of activity exhibited by those groups tested at night. Long-term and short-term behavioral habituation to spatial novelty were also affecte d by phase of the light-dark cycle. A second experiment was designed to con trol for any direct effect of the light-dark cycle by keeping the animals i n dim green light where entrainment was maintained by a skeleton photoperio d (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 phase s under the same conditions of illumination. One group was tested during th e 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 beha vior. No effect of circadian phase on ambulation or habituation of this res ponse to the open field was observed in these animals. Taken together, thes e results suggest that spatial novelty is equally arousing regardless of ci rcadian phase and that the conditions of illumination can dramatically alte r the response to a novel environment. (C) 2000 Elsevier Science Inc. All r ights reserved.