Basal anxiety-like behavior predicts differences in dendritic morphology in the medial prefrontal cortex in two strains of rats

Abstract

Basal differences in the brain may account for why some individuals are more vulnerable to stress than others. Although trait anxiety behavior varies greatly in human populations, most animal models of anxiety disorders tend to focus on the development of anxiety after a stressful experience. In this study, adult male Sprague-Dawley and Lewis rats were grouped according to baseline anxiety-like behavior in the open field, measured by time spent and distance traveled in the center. Individuals that fell one standard deviation above and below the mean, approximately the top and bottom 15%, were selected for the Low and High Anxiety groups. Pyramidal neurons from layer II/III of the prelimbic region of the medial prefrontal cortex were iontophoretically loaded with Lucifer yellow dye and reconstructed. In both strains, animals in the High Anxiety group had smaller apical dendrites than those in the Low Anxiety group. No difference was found in basal dendrites. Sholl analysis revealed a strain difference in the distribution of dendritic material between anxiety groups. These results illustrate significant variability in dendritic morphology in the prefrontal cortex of healthy adult male rats prior to experimental manipulation that correlates with baseline levels of anxiety-like behavior.

Introduction

Stressful experiences have been shown to alter both behavior and brain structures in animal models as well as in human patient populations. Basal anxiety levels may also play a role in determining an individual's susceptibility or resilience toward future stressors. In humans, basal or trait anxiety is linked to subtle structural differences in the prefrontal cortex, a highly plastic region that is necessary for the inhibition of many basic emotional responses, particularly fear. Healthy adults that score higher on trait anxiety questionnaires have smaller prefrontal cortical volume [1], and exhibit less connectivity between the prefrontal cortex and amygdala [2] compared to their low trait anxiety counterparts. Additionally, healthy individuals who are better able to recall an extinction memory, i.e. those that are able to inhibit their fear responses, have increased cortical thickness in the prefrontal cortex [3]. There is evidence that people with a predisposition toward increased anxiety, which often corresponds with genetic polymorphisms and/or physical differences in the brain, are more likely to have heightened anxiety responses [4], [5] or develop anxiety-based pathology [6] after stressful or traumatic experiences. It is possible that these individual differences may hold a key to preventing the development of anxiety disorders and may help to pinpoint targets for future therapeutics.

Although, in humans, individual variability in trait anxiety and mood are seen as important factors in determining stress vulnerability, little emphasis has been placed on basal anxiety levels in animal models. Commonly used laboratory rodents with similar genetic backgrounds and life experiences can exhibit vastly different levels of anxiety-like behaviors. This is represented in the literature by the large range of behavioral baselines for control groups in conventional measures of anxiety, like the open field and elevated plus maze [7], [8], and the number of strains developed by crossbreeding individuals with extreme anxiety phenotypes; e.g. novelty seeking high and low responders [9], high and low fear conditioners [10], Fischer and Lewis rat strains, BALB/c and C57BL/6 mouse strains.

Similar to humans, the rodent prefrontal cortex also modulates anxiety-like behaviors. The medial prefrontal cortex (mPFC) is involved in rodent extinction learning and memory; stimulation of mPFC enhances extinction memory of a conditioned fear response while inactivation impairs learning and increases later fear expression [11]. While there is strong evidence that the more ventral, infralimbic region (IL) of the rodent mPFC is primarily responsible for extinction learning (for review, see Ref. [12]), the role of the mPFC in modulating other anxiety-like behaviors such as risk assessment, avoidance, and defensive responses is not as clear. Lesion studies in the literature examining the prelimbic (PL) and/or IL regions of mPFC have produced mixed results – with some experiments finding increased anxiety-like behaviors after lesion, such as reduced time spent in the center of an open field and in the open arms of the elevated plus maze [13] and increased latency to enter and interact with a novel environment [14], and others showing decreased anxiety-like behavior after lesion using the same measures [15], [16], or even hemisphere- and region-specific differences [17], [18].

Like the well-characterized IL region, the PL receives information from many limbic structures, including the ventral hippocampus, basolateral amygdala and other areas of mPFC, which may allow it to attribute emotional relevance to cognitive tasks [19]. PL also projects to limbic regions, including the paraventricular nucleus of the hypothalamus, where it has an inhibitory effect on the hypothalamic stress response during acute stress; lesions to PL enhance corticotrophin releasing hormone production while lesions to IL inhibit hormone production [20]. PL is highly vulnerable to chronic stressors. Chronic stress that has been shown to increase anxiety-like behavior in both rats and mice [21], [22], [23], also induces dendritic remodeling of PL pyramidal neurons [23], [24], [25], [26], [27], and can modify connectivity between PL and limbic circuits [28], [29].

With human trait anxiety in mind, we examined individual differences in basal anxiety-like behavior in the open field in two rodent strains known to differ in anxiety-like behavior, the outbred Sprague-Dawley (SD), and the inbred Lewis strain. Compared to the SD strain, Lewis rats produce higher basal levels of anxiety-like behavior, such as reduced time in the open arms of the elevated plus maze, and increased anxiety-like behaviors after stressful events [30], [31], [32]. To better understand the relationship between mPFC structure and basal anxiety, individuals from SD and Lewis strains that fell on the upper and lower ends of the anxiety-like behavioral spectrum were chosen for morphological analysis of pyramidal neurons within the PL region of mPFC.