Abstract
Even when trained under exactly the same conditions outbred male Sprague-Dawley (SD) rats vary in the form of the Pavlovian conditioned approach response (CR) they acquire. The form of the CR (i.e. sign-tracking vs. goal-tracking) predicts to what degree individuals attribute incentive salience to cues associated with food or drugs. However, we have noticed variation in the incidence of these two phenotypes in rats obtained from different vendors. In this study, we quantified sign- and goal-tracking behavior in a reasonably large sample of SD rats obtained from two vendors (Harlan or Charles River), as well as from individual colonies operated by both vendors. Our sample of rats acquired from Harlan had, on average, more sign-trackers than goal-trackers, and vice versa for our sample of rats acquired from Charles River. Furthermore, there were significant differences among colonies of the same vendor. Although it is impossible to rule out environmental variables, SD rats at different vendors and barriers may have reduced phenotypic heterogeneity as a result of genetic variables, such as random genetic drift or population bottlenecks. Consistent with this hypothesis, we identified marked population structure among colonies from Harlan. Therefore, despite sharing the same name, investigators should be aware that important genetic and phenotypic differences exist among SD rats from different vendors or even from different colonies of the same vendor. If used judiciously this can be an asset to experimental design, but it can also be a pitfall for those unaware of the issue.
Highlights
Even the most experimentally reproducible behaviors show consistent inter-individual variation
It can be seen that rats supplied from Harlan had significantly higher Pavlovian conditioned approach (PCA) index scores than rats supplied from Charles River over the five training sessions (Fig. 1; main effect of vendor; F(1,642.21) = 55.63, p = 2.85610213)
It has been demonstrated previously that sign-tracking behavior differs between two inbred strains of rats (i.e., Lewis and Fischer) [27]; this study is the first to demonstrate that the incidence of sign- and goal-tracking behavior reliably varies within an outbred stock of rats at the level of vendors and individual colonies
Summary
Even the most experimentally reproducible behaviors show consistent inter-individual variation. Understanding the sources of this variation allows better control of experimental conditions in order to reduce ‘‘noise’’ and increase the probability of obtaining replicable results. Environmental factors, such as lighting, odors, and housing conditions, are routinely recognized as important variables and standardized in behavioral experiments. Genetic factors are well-known, and in animals studies are typically controlled by limiting analyses to a particular strain or stock of animals. Both inbred and outbred rats can be used in studies investigating rodent behavior, and each has a different level of genetic diversity. Outbred stocks, such as Sprague-Dawley (SD), are often popular research models for translational studies of behavior and pharmacology due to their genetic diversity, which may more closely mirror the genetic and behavioral heterogeneity observed in human populations [2]
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