Variability and temporal dynamics of novel object recognition in aging male C57BL/6 mice

Despite its wide application in studies on memory and disease-related cognitive impairment, the use of the novel object recognition (NOR) test in research on aging has been limited and produced conflicting results. The purpose of this study was to characterize NOR across the lifespan of male C57BL/6 mice (3, 6, 9, 12, 15, and 24 months), the most popular rodent model of aging. NOR, measured after a delay of 24 hours, was analyzed with respect to variability, its dependence on locomotor and exploratory activity, and with an emphasis on its temporal dynamics. The latter has been recognized in rats, but never been investigated in mice. We find that although locomotor activity decreases monotonically above 3 months of age, exploratory activity in our setup remains constant and sufficient for NOR testing up to 12 months. As a major finding, we show that NOR depends on exploration time in the test phase in an age-dependent manner. Whereas NOR in our paradigm remains significant up to at least 20 seconds total exploration time in 3 and 6 months old mice, novel object preference at 9 and 12 months of age is detectable after 10 seconds exploration, but decreases rapidly and is missed when the first 20 seconds are analyzed. Analysis of the first 2 minutes of the test phase does not detect NOR except for 3 months old mice. We conclude that temporal aspects of NOR have been neglected and must be considered when making comparisons between or within mice at different ages.