Environmental enrichment for laboratory rats and mice: endocrine, physiological, and behavioral benefits of meeting rodents' biological needs

Adriana Domínguez-Oliva¹Ismael Hernández-Avalos^2*Antonio Bueno-Nava³Cuauhtémoc Chávez⁴Antonio Verduzco-Mendoza⁵Adriana Olmos-Hernández⁶Dina Villanueva-García⁷Alberto Avila-Luna⁸PATRICIA MORA-MEDINA⁹Julio Martínez-Burnes^10,11Arturo Gálvez-Rosas¹²Daniel Mota-Rojas^1*

• ¹Metropolitan Autonomous University, Mexico City, Mexico
• ²Facultad de estudios Superiores Cuautitlán. UNAM., Universidad Nacional Autonoma de México, Cuautitlán, Mexico
• ³Luis Guillermo Ibarra Ibarra. División Neurociencias., Instituto Nacional de Rehabilitación, Mexico, Mexico
• ⁴Unidad Lerma., Metropolitan Autonomous University, Mexico City, Mexico
• ⁵Luis Guillermo Ibarra Ibarra. Department Bioterio., Instituto Nacional de Rehabilitación, Mexico, Mexico
• ⁶Department Bioterio., Instituto Nacional de Rehabilitación, Mexico, Mexico
• ⁷Hospital Infantil de México Federico Gómez, Mexico, Mexico
• ⁸División de Neurociencias., Instituto Nacional de Rehabilitación, Mexico, Mexico
• ⁹UNAM. FESC, Cuautitlán, Mexico
• ¹⁰Universidad Autonoma de Tamaulipas, Victoria, Mexico
• ¹¹Facultad de Medicina Veterinaria y Zootecnia, Universidad Autonoma de Tamaulipas, Mexico, Mexico
• ¹²Luis Guillermo Ibarra Ibarra., Instituto Nacional de Rehabilitación, Mexico, Mexico

Standard housing for laboratory rodents is characterized by cages that do not always provide an adequate environment to meet the animal’s behavioral needs. When animals are reared under impoverished conditions, negative states such as boredom and distress might arise. Environmental enrichment (EE) is an alternative to expose rodents to physical, sensory, cognitive, and/or social stimulation greater than the one received under standard housing conditions. The present review aims to discuss the main physiological, endocrine, and behavioral effects of environmental enrichment in murine research models. The positive and negative effects will be addressed, as well as factors including enrichment-related (i.e., type of EE, duration of EE) and animal-related aspects (i.e., strain, sex, or age) that need to be considered by researchers when adopting EE for laboratory rodents. It was observed that EE decreases corticosterone concentrations in rodents, an indication of lower levels of stress. Likewise, tachycardia, hypertension, and shorter heart rate variability are ameliorated with the implementation of EE (reflecting a beneficial effect). Among the behavioral benefits, rodents reared under EE have anti-anxiety characteristics, increased exploratory behavior, and less fear-related responses than standard-housed animals. However, in some cases, increased aggression has been reported. Although there is no standardization for EE, to properly adopt EE in experimental facilities, researchers must consider enrichment- and animal-related factors to improve the welfare of laboratory rodents.