The dark side of light exposure: impacts on object recognition memory and spatial associative memory in Wistar rats

Permanent exposure to constant light (LL) induces arrhythmicity in locomotor activity and impairs spatial memory, an effect that has been associated with reduced hippocampal neurogenesis in rodents. However, it remains unclear whether these memory deficits result from circadian disruption or from a sustained (tonic) effect of light exposure. In this study, rats were exposed to LL during lactation (21 days) to prevent the development of arrhythmicity in adulthood. Subsequently, animals were maintained under a 12:12 light and dark (LD) cycle for 39 days and then re-exposed to LL. Behavior assessments were conducted after 3 (LL₃) and 7 (LL₇) weeks after LL re-exposure. Control groups maintained in a 12:12 LD cycle (LD₃ and LD₇) were tested at the same time points. LL re-exposed rats exhibited a free-running locomotor rhythm with a reduced percentage of variance. Compared to LD rats, LL re-exposed rats showed impairments in object recognition memory and spatial associative memory in the LL₇, but not in LL₃. Immunohistochemical analyses of parvalbumin- and neuronal nucleus protein-positive cells in the dorsal hippocampus and prefrontal cortex, performed 7 weeks after LL re-exposure, revealed no differences between groups. These findings suggest that the negative impacts of the LL exposure on learning performance may be due to a tonic effect of the light that emerges after a prolonged LL re-exposure.