Consumer demand experiments in group-housed mice with light as a negative reinforcer for environmental enrichment

Due to legislation and ethics environmental enrichment has become an important issue in laboratory animal science. Many studies using preference tests prove that animals prefer enriched cages to non-enriched and that a more complex environment affects the mice’ development and behaviour beneficially. As preference tests only lead to a hierarchy of preferred items and the animals’ long-term welfare is not necessarily assured they always bare the possibility of misinterpretation. In addition to that, several studies showed that environmental enrichment influences the results of animal experiments. Therefore it is essential to standardize housing conditions using enrichment items because only in that case the enhancement of the animal’s well-being as well as the accuracy of experimental results will be assured. The consumer demand theory -a means to measure the strength of a preference- is considered to be an appropriate method for the objective evaluation of enrichment objects. So far, these test have not been carried out with group-housed mice and the purpose of this study was to evaluate light as a negative reinforcer for group-housed mice and to test enrichment objects using this method. At the age of three weeks male and female BALB/c mice, originated from Harlan Winkelmann, were marked and randomly allocated into Makrolon® Type III cages (single cage period = SC) in groups of four of the same sex. After two weeks they were transferred into the ‘double cage’, an apparatus consisting of two Makrolon® Type III cages connected by a Perspex tube. Both cages were equipped with food, tab water and wood shavings. The animals again had two weeks to adapt to the new environment (double cage period = DC). Afterwards the contents of the cages as well as the light regime changed (aversion period = AV). One cage (C1) was exposed to a 12:12 dark/light cycle while the light intensity in the other cage (C2) was increased logarithmically from 100 to 3200 lux and the illumination was permanent. Each light intensity lasted for three days. Afterwards the next light intensity was selected. Both cages were divided by a photo-resist wall. In experiment 1 C1 and C2 both were supplied with food, tap water and wood shavings. In experiment 2 C1 contained food and wood shavings whereas C2 only contained a tap water bottle. In experiment 3 C1 was equipped with food and water and C2 contained wood shavings. In experiment 4 both cages were supplied with food, tap water and wood shavings and C2 additionally contained one mouse house. All mice had to accept the increasing light intensities in order to get access to the resource placed in C2. After 18 days of AV the mice were euthanized and morphological parameters were compared with respect to the four different housing situations in the AV. Experiment 5 and 6 investigated on the possibility that the additional cage itself can be considered as enrichment by mice. The SC was the same as in experiments 1 to 4. The double cage consisted of one Makrolon® Type III cage (C3) equipped with food, water and wood shavings and either another Makrolon® Type III cage or a Makrolon® Type II long cage left empty (C4). After two weeks in this double cage the animals were transferred back into a single Makrolon® Type III cage where they stayed for three days (single cage period 2 = SC2) before they were euthanized. In all experiments at the end of each period blood samples were taken in order to determine the plasma corticosterone levels. The results of this study show that light is a possible negative reinforcer to evaluate enrichment items for mice. According to other studies the male as well as the female mice accepted the negative reinforcer light in order to get access to the water, the wood shavings and the mouse house, respectively, whereas other behavioural patterns were performed less frequently. The mice showed an inelastic demand for water, wood shavings as well as for the mouse house. Regarding the corticosterone concentrations, male mice seem either to have different strategies to cope with aversive situations than females because during the AV the corticosterone titers of male mice decreased whereas in female mice the concentrations increased compared to the values of the DC. The different housing conditions of the experiments led to significant changes in morphological parameters such as the heart weight, the spleen weight and the adrenal weight.