Neonatal novelty exposure ameliorates anoxia-induced hyperactivity in the open field

Post-weaning social isolation modifies neonatal anoxia-induced changes in energy metabolism and growth of rats

Neonatal oxygen deficiency in rats may disturb growth and long-term metabolic homeostasis. In order to facilitate metabolic evaluation, the subjects are usually housed individually. However, social isolation associated with individually housed conditions alters animal behavior, which may influence the experimental results. This study investigated the effects of social isolation on neonatal anoxia-induced changes in growth and energy metabolism. Male and female Wistar rats were exposed, on postnatal day 2 (P2), to either 25-min of anoxia or control treatment. From P27 onward, part of the subjects of each group was isolated in standard cages, and the remaining subjects were housed in groups. At P34 or P95, the subjects were fasted for 18 h, refeed for 1 h, and then perfused 30 min later. Glycemia, leptin, insulin, and morphology of the pancreas were evaluated at both ages. For subjects perfused at P95, body weight and food intake were recorded up to P90, and the brain was collected for Fos and NeuN immunohistochemistry. Results showed that male rats exposed to neonatal anoxia and social isolation exhibited increased body weight gain despite the lack of changes in food intake. In addition, social isolation (1) decreased post-fasting weight loss and post-fasting food intake and (2) increased glycemia, insulin, and leptin levels of male and female rats exposed to anoxia and control treatments, both at P35 and P95. Furthermore, although at P35, anoxia increased insulin levels of males, it decreased the area of the β-positive cells in the pancreas of females. At P95, anoxia increased post-prandial weight loss of males, post-fasting food intake, insulin, and leptin, and decreased Fos expression in the arcuate nucleus (ARC) of males and females. Hyperphagia was associated with possible resistance to leptin and insulin, suspected by the high circulating levels of these hormones and poor neuronal activation of ARC. This study demonstrated that continuous social isolation from weaning modifies, in a differentiated way, the long-term energy metabolism and growth of male and female Wistar rats exposed to neonatal anoxia or even control treatments. Therefore, social isolation should be considered as a factor that negatively influences experimental results and the outcomes of the neonatal injury. These results should also be taken into account in clinical procedures, since the used model simulates the preterm babies' conditions and some therapeutic approaches require isolation.

Adult aggression during an initial social encounter: effects of neonatal anoxia and relation to juvenile open-field activity

In male Long-Evans hooded rats, we examined: (1) combined effects of neonatal anoxia and novelty exposure on aggression during adulthood; (2) open-field activity before juvenility as a predictor for adult aggression. Litters of neonates were exposed to either 100% N2 gas (Anoxia) or room air (Control) for 25 min on postnatal Day 1 (P1). Within each of the Anoxia and Control conditions, one half of the neonates were individually exposed to a non-home cage for 3 min daily during P2-21 (Novel: N(Anoxia)=15; N(Control)=13) while the other half remained in the home cage (Home: N(Anoxia)=15; N(Control)=13). Prior to the onset of juvenility (P25), open-field activity was measured during four 20-s trials. At the onset of adulthood (P100-101), we measured the occurrence of biting during four 5-min sessions of social interaction between pairs of rats. Neonatal anoxia and novelty exposure had contrasting effects on adult aggression with the former increasing aggression and the latter having no statistically significant effect. The open-field measures before the onset of juvenility were significantly correlated with the occurrence of biting behavior during adulthood, suggesting that activity in a novel environment observed very briefly in early life may serve as a predictor for adult aggression.

Early life modulators and predictors of adult synaptic plasticity

Early life experience can induce long-lasting changes in brain and behaviour that are opposite in direction, such as enhancement or impairment in regulation of stress response, structural and functional integrity of the hippocampus, and learning and memory. To explore how multiple early life events jointly determine developmental outcome, we investigated the combined effects of neonatal trauma (anoxia on postnatal day 1, P1) and neonatal novelty exposure (P2-21) on adult social recognition memory (3 months of age) and synaptic plasticity in the CA1 of the rat hippocampus (4.5-8 months of age). While neonatal anoxia selectively reduced post-tetanic potentiation (PTP), neonatal novel exposure selectively increased long-term potentiation (LTP). No interaction between anoxia and novelty exposure was found on either PTP or LTP. These findings suggest that the two contrasting neonatal events have selective and distinct effects on two different forms of synaptic plasticity. At the level of behaviour, the effect of novelty exposure on LTP was associated with increased social memory, and the effect of anoxia on PTP was not accompanied by changes in social memory. Such a finding suggests a bias toward the involvement of LTP over PTP in social memory. Finally, we report a surprising finding that an early behavioural measure of emotional response to a novel environment obtained at 25 days of age can predict adult LTP measured several months later. Therefore, individual differences in emotional responses present during the juvenile stage may contribute to adult individual differences in cellular mechanisms that underlie learning and memory.