Interspecific aggressiveness and reactivity in mouse-killing and nonkilling rats: Compared effects of olfactory bulb removal and raphe lesions

The experiments aimed at uncovering possible correlations between inter-specific aggressiveness and general and emotional responsiveness (appraised by means of the open-field technique) in the rat. Killer rats showed a higher level of emotional responsiveness than nonkillers. Removal of the olfactory bulbs induced an increased reactivity both in the rats which were converted into killers in those whose behavior toward mice remained unchanged. Destruction of the dorsal and medial nuclei of the raphé induced a clear hyperreactivity in most lesioned animals, but provoked initiation of mouse-killing behavior in only one-third. When produced in rats which had remained nonkillers following olfactory bulb removal, the raphé lesion clearly enhanced both the general and the emotional responsiveness; it provoked initiation of mouse-killing behavior in about 75% of the lesioned animals. The discussion bears on the correlations between interspecific aggressiveness and experimentally induced hyperreactivity in the rat.     

Interspecific aggression and reactivity in rats: Effects of selective raphe lesions and additional olfactory bulb ablation

Large depletion of brain 5 HT has been shown to induce mouse-killing behavior in the rat. Selective lesions of the raphe nuclei have been investigated in order to determine whether the various components of the 5 HT system exert some specific control over this aggressive behavior. Electrolytic lesions of the dorsal or the median raphe nucleus do not induce mouse killing, whereas combined lesions of these nuclei elicit this behavior in about 40% of naive rats. Consequently, it appears that serotonergic neurons originating in the dorsal and median raphe nuclei work synergistically in mediating inhibitory control over mouse-killing behavior. Loco-motor activity is increased in novel environments by each of the selective lesions and to a larger extent by combined raphe lesions; 24 hours activity in resting conditions is unchanged during the light period, and increased during the dark period of the daily cycle by the various lesions. As it has been shown previously that hyper-activity in response to novelty following raphe lesions is not directly related to the 5 HT decrease in the brain, it appears that interspecific aggression and motor responsiveness must not be dependent on the same neural substrate within the raphe nuclei. The raphe lesions do not facilitate the elicitation of mouse killing by further olfactory bulb ablations, in contrast to earlier results where bulbectomy facilitated the induction of this behavior by raphe lesions.