Group rearing abolishes hyperdefensiveness induced in weanling rats by lateral septal or medial accumbens lesions but not by medial hypothalamic lesions

Aggression in humans: what is its biological foundation?

Although human aggression is frequently inferred to parallel aggression based on testosterone in nonprimate mammals, there is little concrete support for this position. High- and low-aggression individuals do not consistently differ in serum testosterone. Aggression does not change at puberty when testosterone levels increase. Aggression does not increase in hypogonadal males (or females) when exogenous testosterone is administered to support sexual activity. Similarly, there are no reports that aggression increases in hirsute females even though testosterone levels may rise to 200% above normal. Conversely, castration or antiandrogen administration to human males is not associated with a consistent decrease in aggression. Finally, changes in human aggression associated with neuropathology are not consistent with current knowledge of the neural basis of testosterone-dependent aggression. In contrast, human aggression does have a substantial number of features in common with defensive aggression seen in nonprimate mammals. It is present at all age levels, is displayed by both males and females, is directed at both males and females, and is not dependent on seasonal changes in hormone levels or experiential events such as sexual activity. As would be expected from current knowledge of the neural system controlling defensive aggression, aggression in humans increases with tumors in the medial hypothalamus and septal region, and with seizure activity in the amygdala. It decreases with lesions in the amygdala. The inference that human aggression has its roots in the defensive aggression of nonprimate mammals is in general agreement with evidence on the consistency of human aggressiveness over age, with similarities in male and female aggressiveness in laboratory studies, and with observations that some neurological disturbances contribute to criminal violence. This evidence suggests that human aggression has its biological roots in the defensive aggression of nonprimate mammals and not in hormone-dependent aggression based on testosterone.

Medial accumbens lesions attenuate testosterone-dependent aggression in male rats

Male hooded rats were castrated and implanted with testosterone-filled Silastic tubes appropriate for maintaining a normal average serum testosterone concentration. They were then given lesions of the medial accumbens nucleus or sham lesions. Twenty-four hours postoperatively each male was housed with a female. Beginning 7 days following pairing and continuing once each week for 4 weeks, each lesioned or sham-lesioned male was observed for aggression toward an unfamiliar male intruder. On the day following each test of aggression toward an unfamiliar male, each lesioned and sham-lesioned male was assessed for defensiveness toward an experimenter. Rats with medial accumbens lesions displayed significantly less aggression toward an unfamiliar male intruder during each of the weekly tests than did sham-lesioned animals. The attenuation was most pronounced in animals with lesions damaging the posterior part of the medial accumbens nucleus (also designated as anterior portion of the bed nucleus of the stria terminalis) in the region of the crossover of the anterior commissure. Although medial accumbens lesions are known to make individually housed rats hyperdefensive toward an experimenter, lesion-induced hyperdefensiveness was not observed in the pair-housed animals in the present experiment. It is argued that the medial accumbens/bed nucleus of the stria terminalis area is an important region in the anterior forebrain for the modulation of hormone-dependent aggression.

Effects of lesions of the cerebellar vermis on VMH lesion-induced hyperdefensiveness, spontaneous mouse killing, and freezing in rats

In a series of independent experiments, we showed that lesions of the vermis of the cerebellum in rats blocked the hyperdefensiveness induced by lesions of the ventromedial hypothalamus (VMH), attenuated spontaneous mouse killing, and reduced unconditioned freezing and other signs of fear in the presence of a cat. The vermal lesions did not significantly affect foot-shock conditioned freezing. Control lesions of the cerebellar hemispheres did not affect VMH lesion-induced hyperdefensiveness or freezing in the presence of a cat. The hemispheric lesions did attenuate foot-shock conditioned freezing. The data are discussed in terms of the striking similarities and differences between the behavioral effects of cerebellar vermal lesions and amygdala lesions and the interaction of a number of brain areas in modulating agonistic behaviors. The results leave no doubt that the medial cerebellum is significantly involved in the control of species-specific agonistic behaviors. The specific dimension of agonistic behaviors and the details of the interactions with other brain areas remain a puzzle which we approached here by expanding the behavioral profile of animals with lesions of the cerebellar vermis.

Intermale social aggression in rats: suppression by medial hypothalamic lesions independently of enhanced defensiveness or decreased testicular testosterone

Medial hypothalamic lesions or sham lesions were made in castrated adult male rats with subcutaneous implants of testosterone-filled silastic capsules. Seven days following surgery all animals were given a test of defensiveness (reactivity) toward an experimenter. The following day, groups composed of one lesioned male rat, one sham-lesioned male rat, and one intact female rat were placed in large cages. Beginning two weeks later, unfamiliar intruders were introduced into each colony on a weekly basis and the aggressive behavior of the residents recorded. All 12 of the sham-lesioned animals but only 2 of 12 lesioned animals displayed substantial intermale social aggression toward intruders. Analysis of individual elements of intermale social aggression indicated that the lesioned animals were deficient in attack, bite, and piloerection but not in on-top behavior. The deficit in intermale social aggression was not correlated with defensiveness toward the experimenter or body weight of the lesioned animals. It is argued that the medial hypothalamus plays a role in the modulation of intermale social aggression which is independent of its role in modulating defensiveness or testosterone production. These results also demonstrate that intermale social aggression develops even when testosterone levels are held relatively constant by replacing testicular testosterone with an artificial testosterone source.

Intermale social aggression: suppression by medial preoptic area lesions

The intermale social aggressive behavior of male rats cohabiting with a female rat was quantitatively scored weekly in response to the introduction of an unfamiliar intruding male. Resident male rats whose aggressiveness toward an intruder reached a criterion level were subjected to either sham lesions or bilateral lesions in the region of the medial preoptic area. The lesioned rats continued to exhibit levels of piloerection and lateral attack that were not significantly lower than those of sham-lesioned animals. However, the lesioned animals did emit significantly fewer bites and spent significantly less time in the "on-top" position than did sham-lesioned animals. The lesioned animals also displayed significantly less sexual behavior than the sham-lesioned animals but were not different in terms of defensiveness toward the experimenter. It is suggested that bilateral lesions in the region of the medial preoptic area cause a decrease in the intensity of intermale social aggression but do not prevent external stimuli from eliciting the aggression.