Brain Mechanisms

Tentative analysis of apomorphine-induced intraspecific aggressive behavior in the rat according to Adams's classification. Administration of apomorphine in rats induces signs of intraspecies aggression. Three categories of rats have been thus distinguished: (a) those in which these signs are severe, long-lasting, and reproducible; (b) those in which these signs are moderate, briefer, and inconstant; and (c) those in which these signs are absent (Senault 1968; 1970).

During these aggressive manifestations the animals rear on their hind legs, vocalize, and lunge and paw at the opponent's face with their front paws or teeth. This behavior may be quite damaging, and seems to correspond to the "defensive behavior" among the different categories described by Adams.

Experiments performed on two extreme aggressive versus non aggressive animals (Senault 1973; 1977) have shown the following effects of destruction by electrolytic lesions (or aspiration in case of olfactory bulbs);

(1) Septum, putamen and ventromedial hypothalamus; no affect on aggression; There is no inhibition in the aggressive rats, nor is there facilitation in nonaggressive rats.

(2) Lateral hypothalamus, globus pallidus, substantia nigra and amygdala can reduce or sometimes inhibit completely the occurrence of aggressive behavior in aggressive rats.

(3) Olfactory bulbs, caudate nucleus can elicit appearance of these signs in nonaggressive rats.

A comparison of these results with the schema for the neural circuitry of defense and offense proposed by Adams, shows that apomorphine-induced aggressive behavior differs:

(i) from both schemes in terms of the absence of the role of septum.

(ii) from the defense scheme by the fact that the ventromedial hypothalamus has no role in it. We have shown (Senault 1977) that the absence of influence of the ventromedial nucleus distinguishes this aggressive behavior from that induced by electric shocks, which is favored by lesions of this structure (Panksepp 1971a; Eichelman 1971; Grossman 1972).

(iii) from the offense scheme by the fact that the influence of olfactory bulbs seems the opposite: facilitative in offense behavior, inhibitory in apomorphine- induced aggressiveness. This is also the case because of the essential role of the globus pallidus in this aggressive behavior: apomorphine seems to have an attack function at this site since injections of this substance into this structure can elicit aggressive behavior (Senault 1977). The results, in agreement with those reported by MacLean (1978), provide one example of the role of the globus pallidus in intraspecies aggressive behavior in the rat.

These data would be an argument for considering this behavior as an offense behavior. It is also this offense system that represents the neural ciruitry the most similar to that of apomorphine-induced behavior. The observed differences are referred to globus pallidus, septum and olfactory bulbus: As a result of our studies and those of McLean, the globus pallidus might be included in the "offense" system; and as Adams stated, the data reported in the literature related to the septum are not univocal. The same holds true for the olfactory bulbs In point of fact, considering only rats, and setting aside the behavior of mice-killers - which is favored by olfactory bulbectomy (Vergnes and Karli 1963) as well as hypermotility syndrome, similar to the septal syndrome induced by bulbectomy (Douglas, Issacson. and Moss 1969; Kumadaki, Hitomi, and Kumadi 1967: Ueki, Nurimoto, and Ogawa 1972) - the variability of the effects obtained after this operation is striking. It can depend on the size of the lesion (Bandler and Chi 1972), the strain of the experimented animal (Thorne and Linder 1971), and the aggressive behavior under study (intra or extraspecies aggressiveness in regard to manipulator or object) (Bernstein and Moyer 1970; Bugbee and Eichelman 1972) - which means that aggressiveness is not a unitary phenomenon and that the neural structures involved vary as a function of the observed behaviors.

In his tentative study Adams has dealt only with intraspecies aggression, but we should note that apomorphine-induced aggression is indeed a type of intraspecies aggression, one that seems to be akin to the behavioral postures observed in the defense scheme on the one hand and to the neural profile of the offense scheme on the other. This form of aggression does not seem to integrate with Adams's proposed classification, and may well call into question its underlying principles.