Importance of early lighting conditions in maternal care by dam as well as anxiety and memory later in life of offspring

Rodent studies have revealed that the early rearing environment plays an important role in the development of stress vulnerability, memory and cognition. Although early lighting conditions (ELC) are involved in these neuronal developments through both maternal and offspring behavior, their influence has not been fully elucidated. Thus, by using Sprague-Dawley rats, we examined whether ELC affected maternal care by the dam and the subsequent neurodevelopment of the offspring. Prolonged dark phase conditions (PDC) (light/dark, 6/18 h) and prolonged light phase conditions (light/dark, 18/6 h) were administered from postnatal day 2 to postnatal day 14. Throughout this period, maternal care and the circadian rhythmicity of dams were investigated. In adolescence and adulthood of the offspring, we measured anxiety-like behavior, social interaction, object recognition memory, activity rhythm and corticosterone response to stress with hippocampal expression of N-methyl-D-aspartate and glucocorticoid receptor mRNAs. PDC altered maternal care and circadian rhythmicity in the dam compared with normal lighting conditions and prolonged light phase conditions. PDC markedly increased anxiety-like behavior, decreased social interaction and object recognition memory, and inhibited corticosterone feedback in offspring later in life. Furthermore, hippocampal levels of glucocorticoid receptor mRNA and N-methyl-D-aspartate receptor 2B mRNA in rats subjected to PDC were significantly lower than in animals subjected to normal lighting conditions. In the adult offspring, the circadian rhythm of locomotor activity was not affected. These findings suggested that ELC affect mother-infant interactions and subsequently at least partially alter the neurobehavioral development of offspring.