When it comes to animal behavior research, scientists understand that the highest level of animal care is not only an ethical mandate but also a necessary component of high quality research. It is this understanding, which has motivated researchers, and animal care staff to adopt a high standard for rodent housing conditions. In many instances, this has resulted in a push toward creating an “enriched environment” for rodent housing, above standard housing conditions. However, there is no one standard definition or procedure that is used to “enrich” an environment. In fact, the type and level of enrichment can (and does) vary greatly.
For example, and enriched environment (or EE) has been defined as a set of housing conditions that contain “natural” and ecologically relevant features (such as nesting material and bedding that rodents can use for burrowing). An enriched environment can be defined, also, as a set of conditions that facilitate increased sensory and motor stimulation (such as a running wheel or objects to climb on). In some instances, an enriched environment is defined as any change to a facility’s standard housing environment that meets minimum criteria (i.e. a different type of bedding instead of a steel grid floor). As Musch et al, points out in the paper Environmental Enrichment of Laboratory Rodents: The Answer Depends on the Question, it’s not that different types of EE do not share mutually overlapping goals or features (1). However, the goals of applying the EE (even when the conditions themselves are largely overlapping) can be very different. This means the choice of EE for a given scientist needs to be contextual, based on the current evidence, and applicable to the research question.
Importantly, the effects of an enriched environment is typically evaluated both physiologically (for changes in core temperature, mean body weight, etc) and behaviorally (using various maze and stress tests).
Enriched Environment: Experimental Results
In a behavioral study comparing the effects of EE between different mouse strains (specifically C57BL/6 and 129S6/ SvEv) researchers found that EE actually increased strain differences in locomotor activity, hot plate, and forced swim test (2). In contrast, strain differences in the elevated plus maze and resident-intruder test was not retained across environments. Lastly, 129S6/ SvEv mice showed increased hot-plate latencies and reduced aggression whereas C57BL/6 mice showed increased locomotor activity and reduced habituation in the elevated plus maze.
In addition to defining the specific enrichment, it is important to account for how and when the EE is introduced. For example, in a transgenic mouse model of Alzheimer’s disease, the simple introduction of a running wheel was insufficient to prevent disease progression altogether. Instead, the protective outcome depended upon how far along the disease had progressed before the wheel was introduced. In fact, in some cases, it was noted that wheel running has a stereotypic quality, which may actually increase brain pathology (3). On the other hand, when it comes to mouse models of depression, environmental enrichment has been associated with increased neurogenesis, a putative mechanism for the therapeutic action of anti-depressants (4).
Lastly, although EE can be a useful tool in certain situations, the resulting effects need to be carefully interpreted. With regards to the relationship between EE and depression, for example, human research has shown that the combination of antidepressants and a stimulating environment positively influence cognition and plasticity (5). Yet, the chronic administration of antidepressants results in changes that do not overlap with changes conferred by EE, demonstrating that while both yield positive outcomes they likely operate via differing mechanisms.
When it comes to environmental enrichment, the bottom line is this: one size does not fit all. In a way, the utility and the limitations of EE as a tool are inseparable. It’s a double-edged sword: on the one hand, standardization would decrease variability and increase generalizability across studies. On the other hand, too much standardization leaves no room for flexibility and renders the tool potentially ineffective (such as in cases when too much enrichment introduces confounds). In all instances the decision of what EE is and when it should be introduced should hinge upon the research question at hand.
Toth LA, Kregel K, Leon L, & Musch TI (2011). Environmental enrichment of laboratory rodents: the answer depends on the question. Comparative medicine, 61 (4), 314-21 PMID: 22330246
Abramov U, Puussaar T, Raud S, Kurrikoff K, & Vasar E (2008). Behavioural differences between C57BL/6 and 129S6/SvEv strains are reinforced by environmental enrichment. Neuroscience letters, 443 (3), 223-7 PMID: 18687379
Richter, H., Ambrée, O., Lewejohann, L., Herring, A., Keyvani, K., Paulus, W., Palme, R., Touma, C., Schäbitz, W., & Sachser, N. (2008). Wheel-running in a transgenic mouse model of Alzheimer’s disease: Protection or symptom? Behavioural Brain Research, 190 (1), 74-84 DOI: 10.1016/j.bbr.2008.02.005
Laviola G, Hannan AJ, Macri S, Solinas M, Jaber M. 2008. Effects of enriched environment on animal models of neurodegenrerative diseases and psychiatric disorders. Neurobiol Dis 31:159–168.
Bjartmar L, Alkhori L, Ruud J, Mohammed AH, Marcusson J, & Hallbeck M (2010). Long-term treatment with antidepressants, but not environmental stimulation, induces expression of NP2 mRNA in hippocampus and medial habenula. Brain research, 1328, 25-33 PMID: 20223226