Recovery from brain injury in animals: relative efficacy of environmental enrichment, physical exercise or formal training (1990–2002)
Introduction
Great progress has been achieved over the last decades showing that the brain displays plasticity (e.g., Kolb, 1999, Rosenzweig and Bennett, 1996), a term which refers to the capacity of a system to achieve new functions by transforming, on a long-term basis and under environmental constraint, either its constituting elements or its internal connectivity network (Paillard, 1976). It was also shown that both brain and spinal cord have a regenerative potentiality, especially in young but even, though to a lesser degree, in adult and senescent mammals (Will et al., 1998). A major focus of research is the attempt to find to what extent behavioral plasticity and recovery of function are mediated by changes in the nervous system. Indeed, as we will stress, most non-invasive rehabilitation treatments that are used at present also affect brain function and structure, modulating in particular the plastic processes that may underlie sparing or recovery of function following brain damage.
This review considers primarily the period of the last 13 years, roughly speaking the “decade of the brain”. For the preceding decades one may refer to previous reviews (e.g., Will, 1981, Will and Kelche, 1992). Since 1990, there were also some interesting reviews published on related topics (Edgerton et al., 2001, Feldman and Knudsen, 1998, Hall, 1998, Kolb and Gibb, 1991, Mohammed et al., 1993, Puurunen and Sivenius, 2002, Robbins et al., 1996, Rosenzweig and Bennett, 1996, Schrott, 1997, Taub et al., 2002, Van Praag et al., 2000, Weiler and Rijntjes, 1999), but the present review is the first to compare directly, in both functional and structural terms, the three main non-invasive therapeutic strategies used for achieving rehabilitation after brain damage, namely (1) environmental enrichment, (2) physical exercise, and (3) specific formal training (Fig. 1). The comparison between these strategies will be based mainly on animal studies that try to model some kind of brain damage observed in humans. The comparison will also be based on similar studies carried out in intact animals.
A first section will consider the behavioral effects of the three kinds of treatment and a second section will list their impact on the central nervous system (CNS). These latter effects may underlie their behavioral effects. The whole review will contribute to improve our understanding of the effectiveness of enriched experience, relative to physical exercise and to specific formal training. In focussing on our theme, we refrain from reviewing several related topics of current interest, including the following: (a) Do standard laboratory environments impair the utility of animals for behavioral and biomedical research (e.g., Knight, 2001, Wurbel, 2001)? (b) Do automated human-free environments provide superior housing for laboratory animals (Bohannon, 2002)? (c) How do stimulant drugs, in conjunction with enriched environment or training, enhance recovery from brain injury (e.g., Feeney, 1997, Johansson et al., 1997, Puurunen and Sivenius, 2002, Rosenzweig, 2002)?
Section snippets
Behavioral effects
An important question in terms of functional recovery is whether disruption of CNS structure by lesion or disruption of CNS development, secondary to either genetic or chromosomal anomalies, results in a profound deficiency of the plasticity required to respond to environmental enrichment, physical exercise or specific formal training. In other words, what is the generality of the previously reported behavioral data indicating recovery of function: are they limited or even invalidated by some
Effects on CNS
Recovery of behavioral function may be due to changes in non-damaged CNS areas and, it is worth, therefore, to take into account not only the effects of enrichment and of its major components on lesion-dependent plastic processes in damaged CNS areas, but also on those observed in intact CNS areas after injury as well as those observed in normal brains.
Conclusion
In most studies, it is not clear whether enriched experience, physical exercise or training promotes compensatory effects or genuine recovery. There are, however, some studies, in addition to those already mentioned (Biernaskie and Corbett, 2001, Johansson and Belichenko, 2002, Kelche and Will, 1982), which demonstrate that compensatory effects are most probably the adequate explanation of the observed recovery, whether considering the behavioral level or the cerebral level (e.g., Ip et al.,
Acknowledgements
The authors are indebted to Dr. Jean-Christophe Cassel for his stimulating comments on a first version of this review. Special thanks also to Catherine Krieger for her invaluable technical assistance.
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