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Enrichment in the husbandry of laboratory rodents

Enrichment is considered an essential factor in providing refinement and animal welfare in laboratory animal husbandry. The aim of enrichment is to make the environment as species-appropriate as possible in order to allow the animals to live out their natural needs and behavior patterns. However, this should not endanger the laboratory animals themselves, the people who work with them, or the research.1

Enrichment is often neglected for reasons of economy or ergonomics. Nevertheless, in addition to the moral obligation, there is also a legal requirement to meet the needs of "animals used for scientific purposes." Article 33 "on care and accommodation" of Directive 2010/63/EU of the European Parliament and Council states that for all animals "appropriate accommodation, environment, necessary food, water and care" must be provided for their health and welfare. In addition, "all factors which restrict an animal in the satisfaction of its physiological and ethological needs" must be minimized.2

A basic distinction is made between social enrichment and environmental enrichment. Since laboratory rodents such as mice and rats are a social species, they should preferably be kept in groups. Environmental enrichment mainly includes the equipment and construction of the cage but also the diet of the animals. For example, material for nest building is an important part of providing rodents with a species-appropriate environment. Because it is known that both female animals, regardless of their cycle phase, but also male animals build nests. Similarly, nail logs or a multi-level cage provide a more diverse environment for laboratory rodents.3

Enrichment has been shown to influence, depending on its duration, mode of presentation, and type, factors such as behavior, physiology, and even brain structure in laboratory animals. In some studies, for example, of therapeutic options for neurodegenerative diseases such as Alzheimer's or strokes, diverse enrichment is used selectively. It is known that animals are promoted by enrichment with regard to their cognitive and motor skills and also show a lower level of anxiety behavior. Furthermore, species-appropriate husbandry leads to a significant reduction in stress and development of behavioral stereotypies.4

This figure from the article titled "Enriched environments, experience-dependent plasticity and disorders of the nervous system," illustrates the brain regions engaged by various types of enrichment. Somatosensory (red) and visual stimuli (orange) stimulate the corresponding section in the cortex. The hippocampus is stressed by cognitive occupational material (blue). Stimulation of the motor cortex and cerebellum is provided by activities such as bicycle running (green).5

 

Source: https://www.nature.com/articles/nrn1970

A common argument against the use of diverse enrichment is the reproducibility of the research. However, variance in results occurs even in traditional husbandry systems. L. Lewejohann et al. prove, among others in a behavioral study on mice, that it is not the variability due to environment or even experimenter that is decisive for differences in the results. Rather, a general standardization of the experiment is important to ensure reproducibility.6

Accordingly, a species-appropriate, diverse environment should not be a luxury, but should be used in a standardized manner for the benefit of the animals. Mieske et al. in their review "Bored at home? A systematic review on the effect of environmental enrichment on the welfare of laboratory rats and mice" looked at enrichment to improve the welfare of laboratory rodents and reduce behaviors comparable to boredom in humans. This graph shows the effects of enrichment with respect to various parameters such as activity, corticosterone, etc. inc stands for increase, dec for decrease, and neutr for no change.

Source: https://www.frontiersin.org/articles/10.3389/fvets.2022.899219/full

 

Sources:

1) Baumans V. Environmental enrichment for laboratory rodents and rabbits: requirements of rodents, rabbits, and research. ILAR J. 2005;46(2):162-70. doi: 10.1093/ilar.46.2.162. PMID: 15775025
2) RL 2010/63/EU
3) Lewejohann L. Enrichment für Versuchstiere. Leipziger Blaue Hefte, S.452-457 (2019)
4) van Praag H, Kempermann G, Gage FH. Running increases cell proliferation and neurogenesis in the adult mouse dentate gyrus. Nat Neurosci. 1999 Mar;2(3):266-70. doi: 10.1038/6368. PMID: 10195220 Olsson IAS, Dahlborn K. Improving housing conditions for laboratory mice: a review of ‘environmental enrichment’. Lab Anim 2002; 36: 243–270. Lewejohann L. Enrichment für Versuchstiere. Leipziger Blaue Hefte: 452-457 (2019) Benaroya-Milshtein N, Hollander N, Apter A, Kukulansky T, Raz N, Wilf A, et al. Environmental enrichment in mice decreases anxiety, attenuates stress responses and enhances natural killer cell activity. Eur J Neurosci. (2004) 20:1341– 7. doi: 10.1111/j.1460-9568.2004.03587.x Garner JP. Stereotypies and other abnormal repetitive behaviors: Potential impact on validity, reliability, and replicability of scientific outcomes. ILAR J. (2005) 46:106–17. doi: 10.1093/ilar.46.2.106
5) Nithianantharajah J, Hannan A. Enriched environments, experience-dependent plasticity and disorders of the nervous system. Nat Rev Neurosci 7, 697–709 (2006). https://doi.org/10.1038/nrn1970)
6) Lewejohann L, Reinhard C, Schrewe A, Brandewiede J, Haemisch A, Görtz N, Schachner M, Sachser N. Environmental bias? Effects of housing conditions, laboratory environment and experimenter on behavioral tests. Genes Brain Behav. 2006 Feb;5(1):64-72. doi: 10.1111/j.1601-183X.2005.00140.x. PMID: 16436190. Würbel, H. Behavioral phenotyping enhanced – beyond(environmental) standardization. Genes Brain Behav1, 3–8 (2002)

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