Mesencephalic central gray lesions and fear-motivated behavior in rats

Abstract

In Experiment 1, the performance of 11 rats with central gray lesions including complete destruction of the ventrolateral edge, 11 rats with comparable lesions but with only partial ventrolateral damage and 14 sham-operated controls was compared in an operant, shock-food conflict task. The animals with complete ventrolateral destruction accepted more shocks and accepted shocks at a higher intensity than controls, while the group with partial destruction showed intermediate results. In another test, the spontaneous activity of control animals was totally suppressed for 2 min following noxious shock but was unaltered in animals with complete ventrolateral destruction. In animals with partial lesions, post-shock activity was directly related to the amount of ventrolateral destruction. Both lesion groups explored significantly more than controls in an open field. In Experiment 2, employing different animals, 9 lesioned rats with complete or near-complete ventrolateral central gray destruction and 6 sham-operated controls were tested in a step-through passive avoidance task. The performance of the lesioned animals was greatly impaired. The open field and post-shock activity results of the first experiment were replicated in Experiment 2. When placed in the open field or on a high perch most controls but no lesioned animals defecated. Lesioned animals were significantly more active than controls during a 17-h observation period on an activity platform. It was concluded that the mesencephalic central gray matter, and in particular its ventrolateral aspect, is critical to the normal expression of fear in rats. Several alternative hypotheses were discussed.