Development of home cage social behaviors in BALB/cJ vs. C57BL/6J mice

Abstract

BALB/cJ and C57BL/6J inbred mouse strains have been proposed as useful models of low and high levels of sociability (tendency to seek social interaction), respectively, based primarily on behaviors of ∼30-day-old mice in the Social Approach Test (SAT). In the SAT, approach and sniffing behaviors of a test mouse toward an unfamiliar stimulus mouse are measured in a novel environment. However, it is unclear whether such results generalize to a familiar environment with a familiar social partner, such as with a littermate in a home cage environment. We hypothesized that C57BL/6J mice would show higher levels of social behaviors than BALB/cJ mice in the home cage environment, particularly at 30 days-of-age. We measured active and passive social behaviors in home cages by pairs of BALB/cJ or C57BL/6J littermates at ages 30, 41, and 69 days. The strains did not differ robustly in their active social behaviors. C57BL/6J mice were more passively social than BALB/cJ mice at 30 days, and C57BL/6J levels of passive social behaviors declined to BALB/cJ levels by 69 days. The differences in passive social behaviors at 30 days-of-age were primarily attributable to differences in huddling. These results indicate that different test conditions (SAT conditions vs. home cage conditions) elicit strain differences in distinct types of behaviors (approach/sniffing vs. huddling behaviors, respectively). Assessment of the more naturalistic social interactions in the familiar home cage environment with a familiar littermate will provide a useful component of a comprehensive assessment of social behaviors in mouse models relevant to autism.

Introduction

The high sociability of typically developing infants is evident in their preference for visually investigating faces more than objects, as well as in their development of social smiling, reciprocal and communicative vocalization, and joint attention [1], [2]. This high sociability during infancy and childhood enables children to expand their repertoire of social skills and social cognition as they share their interests, learn social rules of interaction, form relationships, and develop social reciprocity and judgment [3]. By contrast, children with autism spectrum disorders (ASD) often show reduced sociability starting in early childhood, which may hinder the subsequent development of social cognition and social skills [4], [5], [6]. Moreover, the profile of social behaviors of individuals with ASD may vary according to the familiarity of the other person and the environment [7], [8], [9]. Thus, understanding the altered developmental trajectory of social behaviors in ASD requires understanding how fundamental social interest and behaviors typically develop early in life in multiple environmental contexts and with both familiar and unfamiliar people.

Because of the experimental control they allow, mouse models are useful for investigating the biological basis of fundamental social behavior development. They may also be used to elucidate genetic and environmental influences on these behaviors, and the underlying molecular mechanisms of social behavior development [10]. The Social Approach Test provides a way to measure sociability, or the tendency of a mouse to approach and investigate another mouse [11], [12], [13], [14]. In this behavioral assay, which has become perhaps the most widely used test of sociability for assessing mouse models relevant to ASD [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], a “test” mouse is placed in a novel arena and allowed to sniff and otherwise investigate an unfamiliar “stimulus” mouse that is confined to a small part of the arena. Mice of the inbred strain C57BL/6J near 30 days of age are highly sociable in this test, while BALB/cJ mice of the same age show relatively low sociability [11], [14], [21], [22], [47].

The Social Approach Test is, by design, a highly controlled test. The test and stimulus mice are separated by a transparent, air-permeable barrier and the stimulus mouse is restricted from moving about the arena, both of which limit naturalistic interaction between the two mice (though in some variants of the Social Approach Test, a period of free interaction is included at the end of the test). The rationale for this high level of control is that it allows the investigator to measure the sociability of a specific mouse, the test mouse, while holding the behavior of the stimulus mouse relatively constant. Furthermore, the test involves handling the mice to place them into a novel environment and to expose them to an unfamiliar stimulus mouse. These elements of stress and novelty may alter the social behaviors of the test mouse relative to a context in which the mouse has not been handled, is in a more familiar environment, and is interacting with a familiar cagemate. Thus, the kind of social behaviors measured in the Social Approach Test, while valid and important in their own right, might be rather different from those that occur in a more familiar environment with a familiar partner and when more naturalistic behaviors are allowed. To conduct a more complete profile of social behaviors in mouse models relevant to ASD, varying the degree of familiarity of both the environment and the stimulus mouse (i.e., the partner, or target, mouse) may be particularly important, because one of the prominent features of ASD is sensitivity to the stress of novel environments and people, or changes in familiar routines [48], [49], [50].

Based on previous studies, strain differences in social affiliative behaviors – which excludes aggressive, reproductive, and maternal behaviors – between C57BL/6J and BALB/cJ mice evidently manifest in contexts beyond the Social Approach Test. BALB/cJ mice show lower levels of allogrooming in a novel arena [51], lower social interaction in an assay similar to a resident–intruder paradigm [52], and lower social reward [53] than C57BL/6J mice. In the familiar environment of their home cages with familiar cagemates, BALB/cJ mice allogroom less and sleep and rest alone more than C57BL/6J mice [54]. In the same situation, BALB/cJ mice also do not barber each other or other strains [55], whereas C57BL/6J barber extensively [56] (barbering occurs during formation of dominance hierarchies or around the time of mating). However, the number of studies that have examined home cage social behaviors of these strains is relatively small, and nearly all of these studies, including all of the observations of home cage behaviors, used adult mice. Hence, little is known about the development of social behaviors of C57BL/6J and BALB/cJ mice in home cage environments.

To better understand the development of home cage social behaviors, we observed mice at 30, 41, and 69 days of age. We hypothesized that BALB/cJ mice would show less frequent active and passive social behaviors than C57BL/6J mice and that these strain differences would be more pronounced at 30 days of age, which would parallel results across development in the Social Approach Test [14].