Effects of early exposure to intermale aggression on the aggressiveness of adult male mice varying in their genetic disposition for aggressive behavior

Medial amygdala lesions modify aggressive behavior and immediate early gene expression in oxytocin and vasopressin neurons during intermale exposure

The medial amygdala and neuropeptides oxytocin (OXT) and vasopressin (VSP) have been associated aggressive behavior regulation. However, the specific mechanism involved in OXT and VSP modulation in distinct brain regions during hostile intermale aggressive behavior is undetermined. A retrograde tracer mouse model was employed using male C57BL/6 mice injected with rhodamine-conjugated latex microsphere suspensions in the right hypothalamic paraventricular nucleus. Adult male C57BL/6 mice (aged 14-16 weeks) were subjected to resident-intruder testing using juvenile intruder mice (aged 3 weeks) or adult intruder mice (aged 8 weeks). Following exposure, Fos protein expression was increased in the medial amygdala neurons of resident mice receiving the retrograde tracer. Thus, medial amygdala neurons projecting to or localized in the vicinity of the hypothalamic paraventricular nucleus showed immediate early gene (IEG) expression following resident-intruder testing that was considered an indirect marker of activation. Additionally, intermale aggression-related behaviors were inhibited or modified by exposure to juvenile or adult intruders, respectively, in mice that underwent medial amygdala lesioning. Furthermore, Fos protein expression in OXT-positive neurons was attenuated. Thus, ablation of medial amygdala neurons prevented immediate early gene expression in OXT- and VSP-positive neurons in the hypothalamus, bed nucleus of stria terminalis, and medial preoptic area during intermale exposure. These findings indicate that the medial amygdala likely modulates hostile aggressive behavior associated with immediate early gene expression in OXT and VSP neurons in specific brain areas, which may actually be instrumental in beneficial social interaction-related aggressive responses associated with mating, territorial defense, and offspring protection.

Escalated Aggressive Behavior: New Pharmacotherapeutic Approaches and Opportunities

Psychopharmacologic studies of aggressive behavior in animals under controlled laboratory conditions have been instrumental in developing and evaluating specific and effective novel drug treatments that reduce aggressive behavior. An initial contribution of this research is to create experimental conditions that enable the display of aggressive and defensive acts and postures in species that engage in either dominance or territorial or maternal aggression. Quantitative ethological analyses allow the precise delineation of the sequential organization of aggressive bursts, providing a benchmark for assessing excessive or pathological forms of aggressive behavior. A second contribution of preclinical research is the development of experimental models of escalated forms of aggressive behavior, such as focusing on genetic predispositions or social provocations and frustrative experiences. A critical role of preclinical research is in the pharmacological and neurochemical analysis of aggressive behavior; for example, a host of undesirable side effects prompted a shift from classic dopaminergic neuroleptic compounds to the more recently developed atypical neuroleptics with effective and more specific anti-aggressive effects. The long-established role of brain serotonin in impulsive and escalated forms of aggressive behavior continues to be a focus of preclinical studies. New evidence differentiates dynamic state changes in corticolimbic serotonergic neurons during the termination of aggressive behavior from the deficient-serotonin trait in violence-prone individuals. It can be anticipated that currently developed tools for targeting the genes that code for specific subtypes of serotonin receptors will offer new therapeutic options for reducing aggressive behavior, and the 5-HT(1B) receptor appears to be a promising target. The modulation of GABA and GABA(A) receptors by 5-HT in corticolimbic neurons promises to be particularly relevant for specific forms of escalated aggressive behavior such as alcohol-heightened aggression.

Selective breeding for isolation-induced intermale aggression in mice: associated responses and environmental influences

Aggressive (TA) and nonaggressive (TNA) lines of mice were established by selective breeding for isolation-induced intermale aggression. This paper summarizes and updates studies performed on the TA and TNA lines. The genetic analysis revealed that in these lines the genes for aggression are located on the autosomes and demonstrate a Mendelian segregation. The genes are expressed only in the presence of androgens which are normally present only in males. Behavioral and biological responses associated with high and low levels of aggression in TA and TNA mice are reviewed. Line differences have been found in olfactory communication and marking behavior, in maternal and predatory aggression in females, in locomotor activity, and in learning abilities. Also, correlated neurochemical and endocrinological responses to the selection have been detected. Maternal factors during the preweaning period do not significantly affect the development of aggression in TA and TNA males, while early postweaning exposure to aggression or sex enhanced later aggressive and sexual activity. Early experience and genetic disposition for aggression are correlated, with TA males showing the greatest increase in the behaviors studied.