Taming of wild Rattus norvegicus by lesions of the mesencephalic central gray

Abstract

Wild-trapped Rattus norvegicus show a consistent pattern of fear and defensive behavior to nonpainful stimuli such as an approaching experimenter, an anesthetized conspecific, or tactile stimulation of the back and vibrassae, as well as to painful stimuli. This reactivity to a range of stimuli, and the different behaviors by which such fear or defensiveness may be expressed, including flight, freezing, vocalization, the jump-attack, and specific biting patterns, make wild rats very appropriate subjects for the analysis of brain mechanisms underlying fear. Lesions of the mesencephalic central gray dramatically lowered these defensive reactions to both painful and nonpainful threat stimuli, reducing or eliminating each of the defensive reactions measured. The subjects showed no evidence of significant motor impairment or disturbance of primary sensory mechanisms. Because these experimental lesions involved considerable damage to the superior colliculi, a second study used wild rats with damage only to the colliculi. These animals displayed some deficits in visually guided behavior and in reactivity to certain tactile stimuli, but most of their fear reactions were intact. These findings suggest that the mesencephalic central gray, traditionally regarded as involved in reactivity to pain, may be one component of a mechanism underlying fear and defensive behaviors to nonpainful as well as noxious stimuli.