THE RICHNESS OF THE ENVIRONMENT AFFECTS VISUAL SYSTEM DEVELOPMENT: A ROLE FOR IGF-1

A pre-weaning enriched experience is sufficient to affect maturation of the visual system, suggesting that factors influenced by the environment during the first days of life might prime visual circuits towards a faster development. Here, we showed that IGF-1 levels are increased in the visual cortex of EE rats as early as P6 and this is a crucial event for setting in motion the developmental program induced by EE. Early intracerebroventricular (i.c.v.) infusion of IGF-1 in standard rats was sufficient to mimic the action of EE on visual acuity development, whereas blocking IGF-1 signaling by means of i.c.v. injections of the IGF-1 receptor antagonist JB1 prevented the deployment of EE effects. Early IGF-1 decreased the ratio between expression of NKCC1 and KCC2 cation/chloride transporters, indicating that IGF-1 is a crucial factor in accelerating the maturation of GABAergic neurotransmission and promoting the developmental switch of GABA polarity from excitation to inhibition. On the other hand, we showed for the first time that impoverished environment (IE) affects visual cortical development. We analyzed both a gross index of development, body weight, and two indexes of functional development, visual acuity, assessed by VEPs, and VEP latency: development of all these indexes was delayed in IE animals. In addition we found that IGF-1 levels are decreased in the visual cortex of IE rats in parallel with a downstream molecule rp S6. Also the development of GABAergic intracortical inhibition and cortical myelination are delayed in IE rats. These results suggest that richness of early environment regulates the developmental trajectories of the visual system in a bidirectional way, EE accelerates them while IE slows down them, and that IGF-1 levels are involved in both effects.