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 qamage 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.

The 2-deoxyglucose method was used to compare regional brain activities of unrestrained wild Norway rats engaged in fear-based defensive behavior (n = 81, and that of solitary controls (n = 81. After infusion with 100 micrograms/kg of ('4CI-deoxyglucose via jugular catheters, experimental rats spent the 45-min uptake period in flight, boxing, and defensive attack to painless threat stimuli. Coronal sections of brains were exposed to X-ray film, and the resultant global maps of regional brain activity for the two groups were quantitated by high-resolution fiber optic densitometry at 86 cerebral points and analyzed statistically by computer. Significant group differences in regional brain glucose uptake were found at 16 loci. The considerable agreement between these structures and those previously identified in the literature as involved in defensive behavior provides evidence for the potential of this method of investigation of brain correlates of specific behavioral patterns.