Current Concepts and State of Knowledge

Effects on Learning and Memory

The amount of stimulation received during childhood and adolescence has profound effects on behavioral and neurobiological development (Renner and Rosenzweig 1987). In rats, enriched environments produce numerous neuroanatomical changes, including increases in cortical weight, cortical thickness, size of neuronal cell bodies and nuclei, number of glial cells, number of dendritic spines, and number of synapses per neuron. Most likely as a result of these neurobiological changes, enrichment also improves performance on several measures of learning and memory. Differences are evident in some of the most basic behaviors, including activity in an inescapable novel environment and startle reactivity. In general, as task complexity increases, the difference in performance increases between EC and IC rats (Renner and Rosenzweig 1987). For example, while EC and IC rats do not differ in the acquisition of operant lever press behavior, EC rats perform better than IC rats in tests of spatial memory. EC rats process contextual conditioning cues more rapidly and display better discrimination between conditioned stimuli when compared to SC rats (Barbelivien etal. 2006).

Early research on environmental enrichment examined a variety of control conditions to determine if any one component of enrichment was most critical for producing the neurobiological effects related to learning and memory. Social stimulation alone is not sufficient to produce neurobiological changes similar to those observed with enrichment. Overall, this research suggests that cohorts, novel objects, and handling are all essential for robust enrichment. While neurobiological effects of enrichment are evident after just four days of enrichment, the effects are more consistent and reliable when rats are assigned to their respective environments after weaning and housed for 30 or more days.

Effects on Drug Action

In addition to enrichment altering performance on a variety of measures of learning and memory, enrichment also alters the effects of a variety of drugs of abuse (Bardo and Dwoskin 2004). Several studies have examined the effects of enrichment on amphetamine-induced hyperac-tivity. EC rats display greater amphetamine-induced hy-peractivity than IC rats following an acute injection of

► amphetamine. However, with repeated injections of amphetamine, IC rats display greater locomotor ► sensi-tization than EC rats. Similar results have been observed with cocaine and nicotine.

Interestingly, enrichment also appears to enhance conditioning to an amphetamine-paired context. Using the ► conditioned place preference (CPP) procedure, which is a measure of drug reward, EC rats display greater amphetamine CPP than IC rats. Similarly, using a low dose of amphetamine (0.3 mg/kg), context-dependent conditioned hyperactivity is obtained in EC rats, but not IC rats. Moreover, when the rate of ► extinction of conditioned hyperactivity is measured, EC rats extinguish at a faster rate than IC rats across a range of amphetamine doses. These results suggest that, in general, EC rats display a greater ability than IC rats to acquire and extinguish contextual conditioning of drug effects.

Numerous studies have examined the effects of enrichment on self-administration of stimulants, opiates, and sedatives. When drugs are available via the oral route, male EC rats increase their ► barbiturate consumption, but not their amphetamine consumption, when compared to male IC rats. Interestingly, female EC rats decrease their barbiturate consumption when compared to female IC rats, suggesting that sex can moderate the effects of enrichment. EC males also consume orally more

► cocaine and ► ethanol than male IC rats. The amount of oral ethanol consumption in male EC, SC, and IC rats also has been examined using a two-lever operant procedure in which one lever delivered 10% ethanol and the other lever had no programmed consequence. EC rats responded less than IC rats for oral 10% ethanol; SC rats were intermediate between EC and IC rats (Deehan et al. 2007, see Fig. 1), thus suggesting that enrichment may alter the reinforcing effects of oral ethanol itself.

Intravenous ► drug self-administration offers an advantage over the oral route because it eliminates the possibility that differences in taste reactivity may complicate interpretation of the results obtained. Several studies indicate that environmental enrichment decreases the self-administration of low unit doses of amphetamine and cocaine. In one initial study in which rats were trained to lever press for a low unit dose of amphetamine

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