Elsevier

Behavioural Brain Research

Volume 286, 1 June 2015, Pages 300-307
Behavioural Brain Research

Research report
Ephrin-A5 regulates inter-male aggression in mice

https://doi.org/10.1016/j.bbr.2015.03.001Get rights and content

Highlights

  • Offensive and defensive aggression were studied in ephrin-A5 mutant (ephrin-A5−/−) mice.

  • In the resident-intruder test for offensive aggression, ephrin-A5−/− mice exhibit severe reduction in aggression toward an intruder male.

  • Defensive aggression in the form of target biting was higher in ephrin-A5−/− mice.

  • Olfaction and testosterone levels were comparable between mutant and wild-type controls.

Abstract

The Eph family of receptor tyrosine kinases play key roles in both the patterning of the developing nervous system and neural plasticity in the mature brain. To determine functions of ephrin-A5, a GPI-linked ligand to the Eph receptors, in animal behavior regulations, we examined effects of its inactivation on male mouse aggression. When tested in the resident-intruder paradigm for offensive aggression, ephrin-A5-mutant animals (ephrin-A5−/−) exhibited severe reduction in conspecific aggression compared to wild-type controls. On the contrary, defensive aggression in the form of target biting was higher in ephrin-A5−/− mice, indicating that the mutant mice are capable of attacking behavior. In addition, given the critical role of olfaction in aggressive behavior, we examined the ability of the ephrin-A5−/− mice to smell and found no differences between the mutant and control animals. Testosterone levels in the mutant mice were also found to be within the normal range. Taken together, our data reveal a new role of ephrin-A5 in the regulation of aggressive behavior in mice.

Introduction

Aggressive behavior is defined as behavior that occurs when a conflict between the interest of two individuals exists [1], [2]. Appropriate levels of aggression may be viewed as a universal fitness trait which enables survival, whereas exaggerated levels can inappropriately harm or even cause death of the individual involved [3]. Animal studies classified male aggression into two major categories: offensive and defensive, which differ in their motive, site and intensity of attack, and outcomes [4], [5]. Offensive aggression is also known as inter-male aggression and occurs in response to challenges over resources (i.e., territory). It involves attack toward the back and flank of the opponent [5], [6]. In rodents, offensive aggression is used to gain dominant status and access to sexually active females [1]. In laboratory research, the resident-intruder (RI) model is commonly used to study offensive aggression [7]. Defensive aggression, also known as fear-induced aggression [7], occurs in the presence of a stimulus that is considered dangerous to the animal. Here, the animal will first try to avoid the threat and will attack only if escape is not possible. This type of aggression may elicit submissive posture or, if the threat persists, attacks directed toward the nearest offending body parts, which are usually the head and snout [8]. The target biting test has been used to measure this type of aggressive behavior in rodents [9], [10].

Different brain regions and signaling molecules are linked to aggression including the hypothalamus, medial amygdala (MEA), lateral septum (LAS), periaqueductal gray (PAG) and the bed nucleus of the stria terminalis (BNST) [1], [2], [4]. Studies in rats identified a broadly distributed “hypothalamic attack area” (HAA) from which electrical and pharmacological stimulation elicited attacks, and lesions reduced it [4], [11]. The HAA includes the lateral part of the anterior hypothalamus (AH), the ventromedial nucleus of the hypothalamus (VMN) and the ventral part of the lateral hypothalamus [1]. It has been suggested that under normal conditions, this area controls whether agonist behavior is appropriate or not, but when stimulated the animal will attack even when not suitable [12]. Recently, Lin et al. [13] identified an aggression locus in the mouse ventrolateral subdivision of the ventromedial hypothalamus (VMHvl) that corresponds to the HAA of the rat.

In this study, we found that ephrin-A5, a ligand of the Eph receptor tyrosine kinase family, is important for the development of aggressive behavior in mice. The Eph receptors and their ephrin ligands are the largest family of receptor tyrosine kinases with 14 receptors and 8 ligands in mammals [14]. Members of this family are divided into the EphA/ephrin-A and EphB/ephrin-B subclass based on structural homology and binding affinities [15]. In general A-class receptors bind to all A ligands and B-class receptors bind to all B ligands. However, some exceptions exist, specifically, EphB2 can bind to ephrin-A5 and EphA4 can bind to all the class B ligands [16], [17]. Both receptor and ligand are anchored to the membrane resulting in signal transduction that can propagate into both receptor and ligand-expressing cells. These singling events have been implicated in various biological responses including proper development of the central nervous system and blood vessel formation [17], [18], [19]. In addition, several Eph receptors have been shown to regulate the proper development of motor and social behavior in mice [20], [21], [22]. We have previously shown that EphA5 inactivation caused a decrease in aggressive behavior in mice [23]. In the current study, we report that inter-male, offensive aggression is severely reduced in male mice lacking ephrin-A5 (ephrin-A5−/−). This does not appear to be related to an inability to attack, since during the target biting test, ephrin-A5−/− mice exhibited increased target biting. In addition, testosterone levels and general olfaction were normal in the null mice indicating that their ability to smell and recognize the presence of the intruder is intact. Taken together our data reveal an important role of ephrin-A5 in aggressive behavior.

Section snippets

Animals

Both wild-type and ephrin-A5−/− animals used for this study were generated from litter mates on a mixed background (C57BL/6 and 129/SV) as described previously [24], [25], since backcrossing into pure C57BL/6 background leads to embryonic lethality (data not shown). Mice were maintained on a 12 h light/dark reverse cycle (lights off from 07:00 to 19:00 h), and had free access to food and water. The temperature was maintained at 25 °C. All behavioral experiments were performed during the first

Reduced inter-male (offensive) aggression in ephrin-A5−/− mice

In order to evaluate roles of ephrin-A5 in animal behavior regulations, we examined effects of its inactivation on mouse motor activity, spatial learning, and aggression. We found no defects in motor activity except a mild hyperactivity [25] and no significant differences in spatial learning between ephrin-A5−/− and wild type control mice (data not shown). In contrast, our analysis revealed a striking absence of fighting in ephrin-A5−/− male mice (Fig. 1). Wild-type male mice took an average of

Discussion

In this study we observed that inactivation of ephrin-A5 in male mice results in a major reduction in offensive aggressive behavior toward an intruder male. When tested with age and genotype matched intruders, none of the ephrin-A5−/− animals engaged in attack behavior. It has been reported that the level of aggressive behavior is influenced by the intruder; changes in social investigation, movement and pheromones led to different responses from the resident [7], [32]. For example, castrated

Acknowledgements

Research partially supported by 2PO1HD023315 (RZ), RO1EY019012 to RZ, P30ES005022, T32007148, and R01ES015991 to JRR; and Charles and Johanna Busch Memorial Fund to GCW.

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