The effects of environmental switchovers between enrichment and impoverishment on brain and behavior in mice

Rearing rodents or primates in environments that are comparatively enriched or impoverished results in often impressive physiological and anatomical changes after a certain period of exposure. However, the anatomical and biochemical characteristics that respond to environmental experience (Bennett, Diamond, Krech, & Rosenzweig, 1964; Greenough & Volkmar, 1973; Walsh, Budtz-Olsen, Penny, & Cummins, 1969) have been subjected to a more sophisticated level of analysis than have the less consistent changes in behavioral tendencies. The specific nature of environmentally induced modifications in behavioral tendencies must be better understood, if substantive predictions about brain-behavior relationships can be made. Furthermore, the long-term effects of particular environmental experiences have not been made clear. This question of permanence would seem to have direct relevance to the current problems in human child development. There are two basic methods of assessing the permanence, or stability, of environmental effects on brain and behavior. The first involves the introduction of a long temporal interval between treatment and testing, which introduces numerous practical and experimental confounds. The second method involves “switching” animals between environmental enrichment and impoverishment, and assessing the degree to which effects of the first environmental exposure are maintained. This second approach has rarely been applied, and the few reports have been somewhat contradictory. One of the first reported that rats switched into enrichment (EC) for 48 days, after 33 days in “isolation” (IC), were similar to totally enriched rats in weight of total cortex, but similar to isolated rats in dorsal cortex weight (Rosenzweig, Krech, Bennett, & Zolman, 1962). Additional brain weight measures did not discriminate the groups; however, the switched group retained an intermediate ranking on most measures. Zolman and Morimoto (1962) reported persistent enrichment effects on enzyme activity and brain weights in rats after 30 days in impoverishment. In contrast to these findings, Rosenzweig, Bennett and Diamond (1967) found that cortical weight differences induced by 80 days in EC or IC did not persist through an additional 50 days in the opposite condition. Geller (1971) found a diminution in neurotransmitter level differences between EC and IC after 30 days in standard laboratory conditions. Enzyme activity differences in occipital cortex between EC and standard laboratory reared rats disappeared within three days after the EC rats were placed in standard laboratory housing (Brown, 1971). Finally, Bennett, Rosenzweig, Diamond, Morimoto, and Hebert (1974) manipulated the time rats spent in EC, IC, and after being switched from EC into IC. Cerebral differences persisted up to 21 days after rats were switched into IC; enzyme differences maintained for 47 days. Although persistence, or stability, of enrichment induced cerebral changes is suggested by these findings, the influence of temporal parameters is still unclear. Most importantly, perhaps, the issue of behavioral persistence or stability under these conditions has not been addressed. I think that there are two main questions that frame this issue, one of immediate experimental interest and one of long-term importance to future applications of this knowledge. First, it is of experimental importance to better understand the relationship, if any, between a level of cerebral persistence and a corresponding stability of a behavioral tendency. Knowledge of probable behavioral stabilities is necessary to establish possible physiological mechanisms mediating persistent changes in behavior. Second, the problem of the stability of environmentally induced modifications of brain and behavior is basic to the proposed utility of particular environmental “therapies” in the ultimate rehabilitation of nonadaptive behavioral tendencies produced by earlier life experiences. The second question addresses the issue of early experience in a somewhat unique way, i.e. by considering not only the discrete effects of a particular early environment on brain and behavior, but also the general effects on the organism\u27s total developmental tendency, such that its future behavior is biased in a particular, determined direction by these early experiences. The following experiments are the first attempts to better understand these kinds of problems in applying environmental programs to the modification of physiological and behavioral tendencies