Brain Mechanisms

The offense motivational mechanism. The neural basis of offense has not been investigated in the cat (Flynn 1976), and only occasionally in the rat; therefore, there are few data available on the question of the neuroanatomical locus of the hypothetical offense motivational mechanism. It is probably not located in the forebrain, since, as noted earlier from studies by Olivier (1977) and Adams (1971), all of the motor patterns of offense have been displayed in coordinated fashion by the rat following destruction of the hypothalamus. It probably lies in the midbrain in a location that receives projections from the lateral hypothalamus, since lesions of the latter abolish offense (Adams 1971), while electrical stimulation produces offense (Panksepp 1971; Woodworth 1971; Koolhaas 1978). There is one report on a midbrain lesion that abolishes offense in rats; the locus was the ventral raphe nucleus (Kostowski & Valzelli 1974).

Motivating stimuli for offense. In muroid rodents there are three primary types of facilitative motivating stimuli for offense (Adams, submitted for publication). The first is effective in males only and consists of pheromonal stimuli that depend upon testosterone in the opponent. The filter for these stimuli is presumably activated by testosterone and, for that reason, is not normally present in females. The second is effective in both males and females and consists of stimuli, principally olfactory, that identify the opponent as an unfamiliar conspecific. The third is present in both males and females, and consists of a complex stimulus situation that elicits competitive fighting. This type of fighting is elicited in animals who have been deprived of food or water and are given a limited supply such that they must compete with an opponent for acquisition (Zook & Adams 1975). [See also Toates: "Homeostasis and Drinking" BBS 2(1) 1979.]

Laboratory rats and mice differ from wild muroid rodents and other laboratory species such as gerbils and hamsters in depending primarily upon the motivating stimulus of testosterone-dependent pheromones. For that reason, laboratory rat and mouse females show little offense except in a competitive fighting situation, whereas among wild rats and mice, gerbils, hamsters, and so forth, females are often as likely as males to attack unfamiliar conspecifics. Also, laboratory rats and mice generally do not show intermale fighting after destruction of the olfactory bulbs, while other species continue to show fighting with unfamiliar conspecifics. The reason for these differences is not known. It may depend upon a selection against offense in the breeding of laboratory mice and rats, which has reduced the effectiveness of olfactory filters for motivating stimuli tuned to unfamiliar conspecific odors. It may also depend, in part, upon the relatively homogeneous diet and bacterial flora in laboratory colonies, which may reduce the variability in odors among laboratory animals, making all the animals in the colony relatively "familiar."

The sensory filters for the motivating stimuli of offense tuned to unfamiliar conspecific odors are apparently located in the amygdala, while those tuned to testosterone-dependent odors are apparently located in more medial structures such as the septum or preoptic area. This conclusion is based upon apparently contradictory data concerning the effects of amygdaloid lesions upon offense

Lesions of the amygdala disrupt offense in some types of muroid rodents but not others. They disrupt offense in those animals that do not depend upon testosterone-dependent pheromones as motivating stimuli for offense: in male hamsters (Bunnell et al. 1970; Shipley & Kolb 1977), and in wild rats (Galef 1970). They do not disrupt offense in those animals that depend primarily upon testosterone-dependent pheromones as motivating stimuli: male laboratory rats (Busch & Barfield 1974; Bunnell 1966). The role of the amygdala as a sensory filter for unfamiliar conspecific stimuli that activate offense is complementary to its role as a sensory filter for consociate stimuli activating the hypothalamic switching circuit for defense and submission. The critical input to the amygdala for offense may come from the vomeronasal organ and the accessory olfactory bulb, since these structures project to the corticomedial amygdala, where lesions abolish offense in the hamster (Lehman et al. 1978). The output pathway may be the stria terminalis through the medial preoptic area to the lateral hypothalamus; this might explain why lesions of the medial preoptic area, but not of the anterior hypothalamus, disrupt offense in female hamsters (Hammond & Rowe 1976).

In muroid rodents the sensory filters for the offense stimuli that are dependent upon testosterone and tuned to testosterone-dependent pheromones may be located in the septum or preoptic hypothalamus. In one study, implantation of testosterone in the septum reinstated intermale fighting (offense) in laboratory mice after it had been abolished by castration (Owen et al, 1974). A more recent study, employing a similar experimental design in the laboratory rat, found that implantations in the preoptic area were more effective (Bermond 1978).

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