Aggression and anxiety: social context and neurobiological links

Abstract

Psychopathologies such as anxiety- and depression-related disorders are often characterized by impaired social behaviours including excessive aggression and violence. Excessive aggression and violence likely develop as a consequence of generally disturbed emotional regulation, such as abnormally high or low levels of anxiety. This suggests an overlap between brain circuitries and neurochemical systems regulating aggression and anxiety. In this review, we will discuss different forms of male aggression, rodent models of excessive aggression, and neurobiological mechanisms underlying male aggression in the context of anxiety. We will summarize our attempts to establish an animal model of high and abnormal aggression using rats selected for high (HAB) vs. low (LAB) anxiety-related behaviour. Briefly, male LAB rats and, to a lesser extent, male HAB rats show high and abnormal forms of aggression compared with non-selected (NAB) rats, making them a suitable animal model for studying excessive aggression in the context of extremes in innate anxiety. In addition, we will discuss differences in the activity of the hypothalamic-pituitary-adrenal axis, brain arginine vasopressin, and the serotonin systems, among others, which contribute to the distinct behavioural phenotypes related to aggression and anxiety. Further investigation of the neurobiological systems in animals with distinct anxiety phenotypes might provide valuable information about the link between excessive aggression and disturbed emotional regulation, which is essential for understanding the social and emotional deficits that are characteristic of many human psychiatric disorders.

Keywords: HAB rats; HPA axis; LAB rats; abnormal aggression; serotonin; trait anxiety; vasopressin; violence.

Figure 1

(A) Anxiety-related behaviour on the elevated plus-maze in 9-week-old HAB and LAB rats between 2003 and 2008; *p < 0. 05 LAB vs. HAB rats (2003–2008); (B) Aggressive behaviour during the RI test in 16- to 22-week-old male HAB, LAB, and NAB rats between 2004 and 2008. *p < 0.05 NAB vs. HAB (2005–2007) and LAB (2004–2008), +p < 0.05 LAB vs. HAB. Data are presented as mean ± SEM.

Figure 2

(A) Anxiety-related behaviour on the EPM as reflected by the percentage of time on the open arms measured in 9-week-old male HAB and LAB rats (*p < 0. 05 vs. HAB at any time, ∼p < 0.05 vs. other 3 seasons), and (B) aggressive behaviour of 16- to 22-week-old male HAB, LAB, and NAB rats (*p < 0.05 vs. NAB, #p < 0.05 vs. summer and fall) during the four seasons. Numbers in columns indicate group size. Data are presented as mean + SEM.

Figure 3

Social behaviours of HAB, LAB and NAB male rats during the 10-min RI test: (A) aggressive behaviour, (B) social investigation, (C) lateral threat, and (D) offensive upright. (E) Lateral threat and (F) offensive upright displayed by the resident towards the intruder (intruder is marked with black lines). *p < 0.05 vs. NAB and HAB, #p < 0.05 vs. NAB. Data are presented as mean + SEM.

Figure 4

Allocation of HAB, LAB, and NAB male in low-, medium- and high-aggressive. Rats are categorized according to their level of aggressive behaviour expressed as percentage of time in the RI test (low-aggressive: less than 15% aggressive behaviour; medium-aggressive: between 15 and 55% aggressive behaviour; high-aggressive: more than 55% aggressive behaviour).

Figure 5

Line-specific differences in abnormal aggression as indicated by the percentage of HAB, LAB and NAB male rats attacking vulnerable body parts of a male intruder (left) or a female intruder (right) during the RI test in their home cage.