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The UFAW 3Rs Liaison Group Research Studentship

Anjanette Harris, University of Edinburgh

Stress, sex and memory in laboratory rats

Ms Anjanette Harris was awarded a 3Rs Liaison Group Scholarship in 2005 for her project looking into stress, sex and memory in laboratory rats. Anjanette undertook her project at the University of Edinburgh and was supervised by Dr Susan Healy and Dr Rick D’Eath. 

In 2004, of the 2.85 million scientific procedures carried out, 16% were on rats (mice were the most commonly used animal species and were the animal model of choice in 67% of experimental procedures). Typically, rats are housed in small, barren, ‘standardised housing’ which is used to reduce environmentally-induced variation both across and within laboratories. Additionally, rats may be housed in isolation, which can lead to behavioural and physiological changes indicative of impaired welfare. Importantly, factors that influence laboratory rat welfare may also undermine the reliability of experimental data.  For example, rodents housed under stressful conditions may be cognitively impaired, which can invalidate research on learning and memory, which uses these animals.

Much of our understanding of spatial cognition (e.g. the cues animals use to navigate and the neurobiological basis of spatial cognition) comes from studying learning and memory in laboratory rats. Anjanette was interested in exploring whether there are sex differences in rats undergoing spatial cognition tests, since, in a number of mammalian species, males outperform females in tests of spatial ability. In particular, Ms Harris was interested in studying whether a variation in stress levels may be the cause of any differences as there is some evidence to suggest that females respond more poorly to acute stress, such as is imposed by a test situation, and yet their spatial performance may be unchanged or enhanced by chronic stress.  Male cognitive abilities, on the other hand, may be adversely affected under conditions of chronic stress.

Anjanette’s research focussed on the impact of housing conditions (isolation, enrichment) on spatial cognition in male and female rats. Specifically, Anjanette sought to answer four questions: 1) does isolation housing impair cognition in rats? And are the sexes affected differently? 2) Is isolation without visual contact more stressful than isolation with visual contact? 3) Does environmental enrichment reduce stress during testing? And 4) Is it stressful to remove social enrichment? And can physical enrichment alone ameliorate stress caused by the removal of social enrichment? If the sexes do differ in the way they respond to housing, then it may be beneficial for males and females to be housed in different ways.

Ms Harris tested cognition using a Morris water maze (MWM; see Figure 1), which is one of the most frequently used spatial tests. Anjanette aimed to determine whether chronic isolation stress acts in concert with acute stress (associated with the MWM) and whether males and females are affected to the same degree. She hoped to determine if housing conditions could cause sufficient stress so as to affect the outcome of a cognitive experiment i.e. looking for sex differences in spatial cognition. Anjanette used behavioural measures of stress e.g. thigmotaxis (wall-hugging), to assess acute stress during testing, and bar biting in the home cage, to assess stress associated with different housing conditions.  Body weight and food intake were also monitored as additional measures of welfare (both typically increase in isolated rats). 

Figure 1: The Morris water maze (MWM) consists of a circular pool (approx 2m in diameter) filled with opaque luke-warm water.  Located under the surface of the water is an escape platform (8cm in diameter).  A rat is released into the maze at the edge of the tank and swims around until it encounters the platform.  Upon re-release the rat uses extra-maze cues posters on the wall etc) to locate the platform.  The most commonly used measure of performance in an MWM is the time taken to reach the platform.  

Anjanette found that there were no significant effects of isolation housing on behaviour in the home cage or on cognitive performance in the MWM in six experiments. Furthermore, visual contact between neighbouring cages and the holding room seemed to be more important to laboratory rats than social housing. Crucially, Anjanette considered that isolation housing did not explain the inconsistencies in reports of sex differences in spatial ability in the literature. However, stress due to the test situation could explain why males performed better than females in the MWM in her experiments. Indeed, since thigmotaxis confounds MWM performance, Anjanette proposed that measuring thigmotaxis may be a non-invasive refinement of MWM procedures and that MWM data should not be presented without consideration of thigmotaxis. Anjanette suggested that environmental enrichment (social and physical) enhanced cognitive performance in the MWM because it reduced thigmotaxis during testing. However, it was unclear which component of enrichment (physical or social) had the greatest beneficial impact on welfare or if the effects were the same for different strains, ages and sex of rat. These questions would need answering before encouraging widespread use of enrichment as a tool to improve welfare outwith the home cage. 

Dr Harris was awarded her PhD in 2009.

Published papers arising from Anjanette’s work supported by UFAW:

Harris AP, D'Eath RB & Healy SD. 2010. A cage without a view increases stress and impairs cognitive performance in rats. Animal Welfare. 19(3): 235-241.

Harris AP, D'Eath RB & Healy SD. 2009. Environmental enrichment enhances spatial cognition in rats by reducing thigmotaxis (wall hugging) during testing. Animal Behaviour. 77(6): 1459-1464. http://dx.doi.org/10.1016/j.anbehav.2009.02.019

Healy SD, Bacon IE, Haggis O. Harris AP & Kelley LA. 2009. Explanations for variation in cognitive ability: behavioural ecology meets comparative cognition. Behavioural Processes. 80(3): 288-294. http://dx.doi.org/10.1016/j.beproc.2008.10.002

Harris AP, D'Eath RB & Healy SD. 2008. Sex differences in spatial cognition are not caused by isolation housing. Behaviour. 145(6): 757-778. http://dx.doi.org/10.1163/156853908783929142.

Harris AP, D'Eath RB & Healy SD. 2008. Sex differences, or not, in spatial cognition: acute stress is the key. Animal Behaviour. 76(5): 1579-1589. http://dx.doi.org/10.1016/j.anbehav.2008.07.016.