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.

Importance of early environment in the development of post-traumatic stress disorder-like behaviors.

A number of clinical studies in which early adversities were defined retrospectively, demonstrated that early adverse experiences increased the morbidity rate of post-traumatic stress disorder (PTSD) in later life. However, no prospective studies have yet been conducted to elucidate whether early adversity affects the risk or severity of PTSD. Thus, we examined whether early adversity would strengthen the severity of PTSD symptoms in later life by using neonatal isolation (NI) and single prolonged stress (SPS) as an animal model of PTSD. We measured anxiety-like behavior in the elevated plus maze (EPM), contextual freezing in the contextual fear (CF) test, and analgesia in the flinch-jump and hot-plate tests in four groups of adult rats (sham, NI, SPS, and NI+SPS). NI significantly enhanced the SPS-induced decrease in the percentage of open arm time and open arm entries in the EPM, enhanced the SPS-induced increase in contextual freezing, and strengthened SPS-induced analgesia, without any changes in locomotor activity in the open field locomotor test. In addition, we examined the effect of environmental enrichment (EE). Repeated exposure to EE ameliorated the NI-induced enhancement of contextual freezing, but not anxiety-like behavior or analgesia, in response to SPS. The results of the present study demonstrated that while early adversity strengthened PTSD-like symptoms, EE alleviated the enhanced contextual freezing by NI and SPS. These findings suggest that early adversity may worsen dysfunction of the amygdala and hippocampus in PTSD, and an early intervention may alleviate the early adversity-mediated enhancement of hippocampal dysfunction in PTSD.