Abstract
Behavioural tests based on the spontaneous recognition paradigm have been used extensively for examining the memory capacity of rodents. By exploiting their innate preference to investigate novel stimuli, inferences can be drawn about the perceived familiarity of encountered objects. Olfaction is the dominant sense used by mice to navigate their environment, yet these tests are often conducted using visual objects. By employing odors, one can reduce the high level of variability commonly observed between subjects. In this paper, we describe a protocol for assessing context-dependent odor memory by probing spatial and temporal associations separately or in conjunction with each other. We also detail a context-independent novel odor recognition protocol. These tests offer a simple and effective method for measuring odor memory in rodents using cheap and easily obtained materials.
Keywords: Memory, Olfactory, Behaviour, Spontaneous, Novelty, Context
Background
When presented with a novel and familiar odor, mice behaviorally express memory for the familiar odor by spending a greater proportion of time investigating the unknown stimulus (Ennaceur and Delacour, 1988). Previously encountered odors can also instigate an increase in investigation if it is found in a novel position in space or temporal sequence (Mitchell and Laiacona, 1998; Eacott and Norman, 2004; Dere et al., 2005; Hunsaker et al., 2008; Barker et al., 2017; Aqrabawi and Kim, 2018). This sensitivity to the contextual information associated with an odor can be useful for evaluating different aspects of memory which may manifest from unique neural substrates. This is particularly useful when investigating with animal models of memory disorders where one form of memory is affected to a greater extent. Notably, none of the behavioral paradigms require extensive training and can be easily modified to suit experimental needs.
Materials and Reagents
Equipment
Software
Procedure
Data analysis
Discrimination ratios can be determined using the investigation time for each odor presented during the final exposure of each test. This ratio was calculated as the difference between the time spent exploring the novel and familiar odor-context configurations divided by the total amount of time investigating both odors: Here a value of zero represents equal investigation of both odors. Positive values up to one indicate preference for the novel odor configuration, whereas a negative value indicates greater investigation of the familiar stimulus. Each discrimination ratio should be compared to zero (chance performance) using a two-tailed one-sample t-test. Performance between groups can be compared using a t-test or a one-way ANOVA with testing group as a factor if analyzing multiple treatment conditions.
Here percent investigation time should be analyzed using a two-way ANOVA with testing group and odor spatiotemporal configuration as factors. Where appropriate, Tukey’s multiple comparisons test can be used for post hoc comparisons.
Notes
Acknowledgments
This research was funded by operating grants to J.C.K. from the Canadian Institutes for Health Research (CIHR) (MOP 496401) and the Natural Sciences and Engineering Council of Canada (NSERC) (MOP 491009). These protocols were adapted from previous experiments published in Aqrabawi and Kim (2018).
Competing interests
The authors declare no competing financial interests.
Ethics
All procedures are in accordance with the guidelines of the Canadian Council on Animal Care (CCAC) and the University of Toronto Animal Care Committee.
References
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