High and abnormal forms of aggression in rats with extremes in trait anxiety – Involvement of the dopamine system in the nucleus accumbens
Introduction
Although aggressive behavior is important for assuring survival of an organism, strict species-specific rules have established in order to minimize injuries and killing. However, in human society, excessive aggression is a major health and social problem. More than 700,000 people die worldwide each year because of aggressive assault (Bartolomeos et al., 2007). Violent forms of aggressive behavior can occur as a symptom of psychopathologies, such as personality disorders, schizophrenia or depressive illnesses (Eronen et al., 1998, Haller and Kruk, 2006), but underlying neurobiological mechanisms of excessive and abnormal forms of aggression are largely unknown. It has been hypothesized that alterations of the reward system play an important role in individuals with psychopathic traits showing violent behavior. More specifically, hyper-activity of the mesocorticolimbic dopamine system was recently reported in psychopathic patients (Buckholtz et al., 2010). These studies have been supported by recent rodent studies which implicated dopamine and the reward circuitry in the regulation of aggression (Ferrari et al., 2003, Couppis and Kennedy, 2008). Other brain factors likely to be involved in the regulation of high levels of aggression include, e.g., serotonin, vasopressin, γ-aminobutyric acid and glucocorticoids (Miczek et al., 2002, Mikics et al., 2004, Ferris, 2005, Haller and Kruk, 2006, Neumann et al., 2010, Veenema et al., 2010).
We could recently establish an animal model for high aggressive behavior with the aim to further study the underlying neurobiological mechanisms (Veenema et al., 2007b). Rats selectively bred for low (LAB) and high (HAB) anxiety-related behavior, respectively (Liebsch et al., 1998a, Landgraf and Wigger, 2002, Bosch et al., 2006), were found to substantially differ in various aspects of social behavior (Neumann et al., 2010), including inter-male aggression, with LAB rats displaying an extremely high and HAB males displaying a rather intermediate level of aggression compared with non-selected (NAB) rats (Veenema et al., 2007b).
The pronounced aggressive behavior of LAB rats is accompanied by an elevated level of neuronal activity within the paraventricular nucleus and an elevated corticotropin response to social stimuli such as the resident-intruder (RI) test (Veenema et al., 2007b) suggesting an association between high aggression and high social stress responsiveness. Additionally, LAB rats show a reduced arginine vasopressin response and neuronal activation within the lateral septum during the RI test. Manipulation of this neuropeptide system within the septum did not affect aggression while increasing anxiety, indicating rather an indirect effect of septal vasopressin on aggression in LAB rats (Beiderbeck et al., 2007). However, a possible involvement of the reward system, specifically the dopaminergic system of the nucleus accumbens (NAc) has not been studied in the context of high aggression in these rats.
In the present study, we generated a detailed behavioral profile of LAB, HAB and NAB male rats and investigated the potential association between anxiety and aggression. We further studied whether the high level of aggression of LAB and HAB rats is accompanied by different types of abnormal aggression and ignorance of species-specific rules reflected by attacking vulnerable body parts, non-estrus female rats or a narcotized rat. In this context, we quantified the mRNA expression of two immediate-early genes (c-fos, zif268) using in situ hybridization in response to the display of aggression. As we could show an increased neuronal activation in the NAc specifically of high-aggressive LAB rats, we tested the hypothesis that an increased activation of the reward system contributes to the high level of aggression in these rats. Therefore, we monitored local dopamine release during the RI test using intracerebral microdialysis comparing high aggressive LAB and low aggressive NAB rats. The elevated neuronal activation together with an increased dopamine release within the NAc of LAB rats prompted us to investigate the behavioral consequences of manipulation of the dopamine system within the NAc of LAB rats. Thus, by using bilateral infusion of either an anesthetic or of a selective dopamine D1 or D2 receptor antagonist, we aimed to reveal detailed brain mechanisms underlying hyper-aggression in LAB rats.
Section snippets
Animals
Experiments were carried out on male Wistar rats, selectively bred for low (LAB) or high (HAB) anxiety-related behavior in the animal facilities of the University of Regensburg, Germany, (Liebsch et al., 1998a, Landgraf and Wigger, 2002, Neumann et al., 2010) and on non-selected male Wistar rats (NAB; Charles River, Sulzfeld, Germany). Rats were constantly kept under controlled laboratory conditions (12:12 h light/dark cycle; lights on at 6:00 h, 21 ± 1 °C, 60% ± 5% humidity, standard rat nutrition
Aggression profile of LAB, HAB and NAB rats and correlation with trait anxiety
The total aggressive behavior differed between LAB, HAB and NAB breeding lines during the last five years (factor line, F(2,348) = 66.6; p < 0.001). LAB rats were consistently more aggressive than NAB (p < 0.001) and HAB (p < 0.01) males, while HAB rats displayed a higher level of aggression than NAB males (p < 0.001; Fig. 1A). Among the elements of aggressive behavior, line-dependent differences were found in the display of lateral threat (F(2,348) = 76.7; p < 0.001; lateral threat as percentage of total
Discussion
LAB and HAB male rats differ with respect to various aggressive and non-aggressive social behaviors, although breeding selection has exclusively been based on anxiety-related behavior. Specifically, low anxious LAB males show extremely high inter-male aggression compared with the very low aggressive NAB males, whereas highly anxious HAB males show a rather intermediate level of aggression. Furthermore, LAB and HAB, but not NAB, rats display abnormal forms of aggression which are more pronounced
Role of the funding source
This study was supported by the Deutsche Forschungsgemeinschaft (I.D.N.), and BMBF (I.D.N.).
Conflict of interest
The authors declare that, except for income received from their primary employer, no financial support or compensation has been received from any individual or corporate entity over the past three years for research or professional service and there are no personal financial holdings that could be perceived as constituting a potential conflict of interest.
Acknowledgments
The authors are grateful to Dr. József Halász for his assistance with the behavioral scoring of attacks directed towards vulnerable body parts, to Dr. Osnat M. Ben-Shahar for her assistance with establishing the dopamine D1- and D2-like receptor autoradiography in our laboratory, and to Dr. Michael Numan for critically reading the manuscript. The authors also like to thank Laura Traulsen, Gabriele Schindler, Nicole Grunwald and Teresa Dintenfelder for excellent technical and experimental
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Current address: Department of Psychology, Boston College, McGuinn 522, 140 Commonwealth Ave, Chestnut Hill, MA 02467, USA.