The Environment versus Genetics in Controlling the Contribution of MAP Kinases to Synaptic Plasticity

Abstract

A challenge in biomedical research is to design experimental paradigms that reflect a natural setting. Even when freshly isolated tissues are used, they are almost always derived from animals housed in cages that poorly reflect the animal's native environment. This issue is highlighted by studies on brain function, where mice housed in a more natural "enriched environment" display enhanced learning and memory and delayed onset of symptoms of neurodegenerative diseases compared to mice housed conventionally. How the environment mediates its effects on brain function is poorly understood. We show that after exposure of adolescent mice to an "enriched environment," the induction of long-term potentiation (LTP), a form of synaptic plasticity that is thought to contribute to learning and memory, involves a novel signal transduction pathway that is nonfunctional in comparable mice housed conventionally. This environmentally gated signaling pathway, which rescues defective LTP induction in adolescent Ras-GRF knockout mice, consists of NMDA glutamate receptor activation of p38, a MAP kinase that does not contribute to LTP in mice housed conventionally. Interestingly, the same exposure to environmental enrichment does not have this effect in adult mice. This study reveals a new level of cell signaling control whereby environmental factors gate the efficacy of a specific signaling cascade to control how LTP is induced in adolescent animals. The suppression of this gating mechanism in mature animals represents a new form of age-dependent decline in brain plasticity.