|Ventromedial Tegmental Lesions Abolish Offense
Without Disturbing Predation or Defense
In the ten rats that fulfilled pre-operative testing criteria, brain lesions were placed bilaterally in the ventromedial tegmentum. The lesions were placed by stereotaxic procedure using the coordinates 0.5 mm anterior, 0.7 mm vertical and 1.0 mm left and right and passing radio frequency current of 6 mA through the tip of an insulated electrode for 40 sec. Surgery was performed only after the animal had been fully anesthetized with Chloropent anesthesia, using 140 mg per kg. At the conclusion of the experiment, the lesions were reconstructed from serial frozen sections of the brain referring to the stereotaxic atlas of Paxinos and Watson . It should be noted that a discrepancy was found between the labeling of the sections in the atlas and the stereotaxic coordinates employed in the study, such that is would appear that the atlas sections are mislabeled almost a millimeter too far anterior for our strain of rats (i.e. , the section labeled 1.2 mm in the atlas describes the anatomical plane where we placed lesions using a setting of 0.5 mm anterior to the ear bar). In the figure, the lesions are illustrated in terms of the brain structures of that section rather than the anterior-posterior level which is given only for reference to the atlas.
Post-operative tests of offense and predation were conducted in eight rats (two had died from surgical complications) in the same manner as pre-operative tests. Three post-operative tests for offense and predation, respectively, were conducted on at least three separate days from 2-13 days after surgery. In one case, offense testing was continued for a total of six tests over 21 days to ensure that there was no recovery of offense in that period. Following all offense and predation testing, the rat was tested for defense according to the method of shock-elicited boxing, and thresholds were determined by the titration method . The testing for defense was not done until after all testing for offense and predation because of the possibility of interaction between offense and defense systems .
Offense was abolished in five rats whose lesions are illustrated in Fig. 1. These lesions destroyed much of the ventromedial tegmentum at a level including and slightly anterior to the reticular tegmental nucleus. The median raphe was damaged or destroyed in three of the animals and spared in two others. The lesions were tear-shaped and extended at their maximal width for 0.7 mm anterior and posterior to the shaded area shown in Fig. 1 which represents the largest cross-sectional area of the lesion in the stereotaxic plane. In addition to these five rats, there were three other rats in which offense was not abolished; it was found that their lesions were all posterior to the ones illustrated here and they involved no damage at the tegmental levels illustrated in Fig. 1. Data from those animals are not considered in this study. Lesions were not placed in other regions of the tegmentum in this study, since our pilot work has shown that other such tegmental lesions do not abolish offense.
The behavioral results were the same in all five rats shown in Fig. 1: offense was abolished, while predation remained unchanged and the rats were capable of showing shock-elicited defensive boxing at the conclusion of the testing. Not only was the bite-and-kick attack of offense abolished, but also there was no offensive sideways posture or offensive upright posture. Other social behaviors remained unaffected by the lesion: there were no decrements in the frequency of olfactory investigation or grooming of the intruder.