What can we learn on rodent fearfulness/anxiety from the genetically heterogeneous NIH-HS rat stock?

The “National Institutes of Health” genetically heterogeneous (NIH-HS) rat stock was created in the 1980s through an eight-way cross of as much as possible separate inbred rat strains (i.e. the MR/N, WN/N, WKY/N, M520/N, F344/N, ACI/N, BN/SsN and BUF/N strains) which were readily available at that time. Hansen and Spuhler [1] developed a more naturalistic, genetically heterogeneous rat stock with the aim of optimizing the distribution of genotypic frequencies and recombination and under the hypothesis that the NIH-HS stock could yield a broad-range distribution of responses (broader than commonly used laboratory rat strains) to experimental conditions, and thus serve as a base population for selection studies. Along the last decade, in a series of studies we have phenotypically characterized the NIH-HS rat stock (a colony exists at our laboratory since 2004) for their anxiety/fearfulness profiles (using a battery of both unconditioned and conditioned tests/tasks), as well as regarding their stress-induced hormonal responses, coping style under inescapable stress and spatial learning ability. We have also compared the phenotypic profiles of NIH-HS rats with those of the low anxious RHA-I and the high anxious RLA-I rat strains. The NIH-HS rat stock is, as a population, a rather anxious type of rat, with predominantly reactive/passive coping style in unlearned and learned anxiety/fear tests, and elevated stress hormone responses (as well as enhanced “depressive” symptoms in the forced swimming test). Genetic studies currently under way have thus far revealed that the genetically heterogeneous NIH-HS rat stock constitutes a unique tool for fine mapping of QTL (for multiple behavioural and biological complex traits) to megabase resolution levels, thus enabling candidate gene identification. We give some examples of this in the present paper, while also highlighting that microarray gene expression studies reveal that HPA-axis- and prolactin-related genes (among others) in the amygdala appear to be related with (or associated to) the coping style and anxiety/fearfulness responses of NIH-HS rats.

Helplessness-like escape deficits of NIH-HS rats predict passive behavior in the forced swimming test:Relevance for the concurrent validity of rat models of depression

The genetically heterogeneous NIH-HS rat stock has been characterized by its response to anxiety- and fear-inducing situations, thus leading to the conclusion that they are a rather anxious and passive coping type of rats. Taking advantage of these profiles, and knowing that they show very poor performance in the two-way active (shuttle box) escape/avoidance task, we have tested NIH-HS rats (n = 80) in the forced swimming test (FST) as well as we have studied escape response deficits (i.e. response failures) of the same animals in the two-way shuttle box task. They were also tested for anxiety in the elevated zero-maze. The goal of such a study was that of investigating whether there are associations or relationships among helplessness-like or passive coping responses between both models of depression, i.e. the FST and the helplessness-like escape deficits in the shuttle box task. The results for the first time show associations among responses from both depression models and that selecting rats for displaying extreme (active or passive) responses in one of the models predict in a coherent manner (according to the hypothesis) their behaviour in the other model. These findings are discussed in the context of the concurrent validity of both models of depression as well as concerning the possible relevance of NIH-HS rats as a tool for future studies on this field.