ReviewTherapeutic and protective effect of environmental enrichment against psychogenic and neurogenic stress
Introduction
The modern concept of environmental enrichment (EE) was adopted from the early work of pioneers in the field that suggested that environmental manipulation can alter neurobiology and behavior [33], [81], [83]. The original studies constituted the template for the EE design with relatively spacious cages housing rodents in large groups and with ready access to a variety of stimulus-objects.
EE provides enhanced opportunities to interact with complex inanimate and social stimuli and to explore the environment [82], [84]. EE has been described as a continuous enhancement in cognitive, sensorimotor and/or physical activity [60]. In the context of enrichment, the continual introduction of new stimulus objects [99] enables the animal to acquire sensory and cognitive experiences. The availability of running wheels, ropes, ladders, tunnels or bridges enables the animal not only to exercise but also to undergo acrobatic training [50]. Furthermore, EE animals are housed in relatively large cages, typically in groups of 6–12 animals, providing opportunity for social interactions [72], [106].
Research has increasingly indicated that EE induces morphological, neurobiological, physiological and behavioral changes [31], [51], [80], [99], [107]. The environment can affect behavior negatively or positively. Enrichment has been shown to improve cognition, reduce anxiety [4], [10], [67] and facilitate recovery from brain lesions [107]. Even an increase in cage size has been shown to improve general well being and reduce anxiety level of animals [56]. The contribution of inanimate stimulation and social interaction has been investigated [89]. The importance of social stimulation was made even more evident by studies that reported the behaviorally detrimental effect of isolation [74], [108].
EE is beneficial for the physiological well being of the animals. With the exception of one report [11], EE animals gained weight faster and had higher body weight than those in standard conditions [35], [84], [96]. Conversely, removal of enrichment features had adverse effects. Enriched mice lost relatively more body weight than controls after being placed in individual cages for only one night [96]. Animals with septal lesions reared in an enriched environment showed an immediate increase in reactivity when switched to a restricted environment [30]. Altogether, EE appears to be beneficial for the general psychological and physical well being of animals [5], [32], [57].
The purpose of the present manuscript is to review the animal research literature pertaining to the role of EE in attenuating the behavioral and physiological expression of anxiety-type responses. The literature reviewed below suggests that EE may not only protect from the detrimental effects of psychogenic and neurogenic stressors, but may even reverse their effects. Possible mechanisms of action of the therapeutic effect of enrichment are also discussed.
Section snippets
Anxiolytic effects of environmental enrichment
Stress is defined as a perceived or actual threat to an organism that is accompanied with a sense of unpredictability or lack of control [87]. Stress may exert positive or negative effects on an organism, depending on its intensity and length. The detrimental effects of stress are long-term and range from behavioral/cognitive deficits to brain damage such as hippocampal atrophy [61], [65], [91]. Even when presented prenatally or during early life, stress can still have a detrimental impact on
Variability in environmental enrichment implementations
Due to the large variety of stimulus objects and settings that have been used in the previous enrichment studies, there seems to be little consensus on what constitutes EE inter- or intra-species [2]. The variance in the degree of complexity of the enriched environments may account for variance among results reported by different experimenters [96]. The standards for enrichment vary between studies [5] and what is referred to as super-enrichment in one study [96] could well be defined as
Suggested mechanisms
The mechanisms by which EE may engender stress resiliency range from cognitive/behavioral to cellular and molecular processes. Some of the proposed mechanisms of action of EE are reviewed below. Particular attention is paid to the mechanisms contributing to the anxiolytic actions of EE. It seems that further research is necessary to determine how various mechanisms may interact to impart the potential therapeutic advantage.
As mentioned earlier, EE provides various types of stimulation including
Conclusions
Pronounced variability in enrichment protocols may account for some of the inconsistencies noted in the effects of EE on behavior and physiology. One major inconsistency in the literature pertains to the effect of enrichment on plasma corticosterone levels [3], [4], [84], [98]. Behaviorally, the majority of the studies indicated that EE reduced resting or basal anxiety levels. Similarly, in response to a stressor, most (but not all) studies reported that animals exposed to EE expressed
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