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amphetamine, EC rats show a greater release of dopamine in the nucleus accumbens measured by in vivo ► micro-dialysis compared to IC rats. This enrichment-induced change is not specific to amphetamine, but appears to occur across various drugs of abuse. Alterations in reward-relevant brain regions likely play a role in the differential sensitivity of EC and IC rats to self-administer drugs of abuse.

In addition to dopamine, enrichment alters drug-induced glutamate release as measured by microdialysis (Rahman and Bardo 2008). ► Glutamate is an excitatory amino acid that has been implicated in drug reward. When challenged with amphetamine, EC rats show greater extracellular levels of glutamate in the nucleus accumbens compared to IC rats. Within the mPFC, EC rats also have increased glutamatergic tone compared to IC rats. This latter finding may be important from a drug abuse perspective because glutamatergic activity is not only important in drug reward, it may also be involved with behavioral inhibitory processes that become dysfunctional during the addiction cycle. While the effects of enrichment on the dopaminergic and glutamatergic systems contributes to the reinforcing effect of drugs of abuse, additional research is necessary to determine the interaction between these systems and the mechanism for the ability of environmental enrichment to decrease sensitivity to drugs of abuse.

Application to Humans

Teachers and educators are generally aware of the important role of enriching environments for promoting learning in children, adolescents, and young adults. In addition, there is considerable evidence that enriching stimulation, as provided by exercise and learning, protects against many neurodegenerative diseases that can occur later in life. While preclinical research indicates that enrichment also protects against drug abuse vulnerability, there is a paucity of information that addresses this issue specifically in humans. Nonetheless, genetic studies using twin concordance or familial assessment strategies indicate that the heritability of drug abuse is only around 40-60%, thus implicating a substantial role for environmental factors.

A recent study examined various summer programs implemented to promote healthy development among school-aged children and adolescents in order to determine the features that were important to demonstrate effectiveness (Bell and Carrillo, 2007). While a number of specific features were identified, a general conclusion was that the instructional techniques were most effective when academic learning was embedded in enriching activities. These results suggest that enrichment accelerates academic achievement and promotes positive development.

A more direct assessment of the effects of enrichment on human development and health-related risk was conducted by Raine et al. (2003). Children aged 3-5 years received enrichment or control treatment and were assessed subsequently in their young adulthood for personality disorders. Enrichment consisted of training in physical health, exercise, and multimodal enrichment provided by toys, art, handicrafts, drama, and music to improve verbal skills, visuospatial coordination, and memory. As young adults, enriched subjects had lower scores for schizotypal personality, antisocial behavior, and criminal behavior (including drug-related charges) compared to control subjects. These results suggest that environmental enrichment during development protects against the emergence of drug abuse and associated personality disorders later in life. Further research is needed to determine what neurobiological alterations are associated with these long-term behavioral effects of enrichment.

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