Role of Midbrain Central Gray in Pain-Induced Defensive Boxing of Rats
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Despite considerable research on the pain-induced defensive boxing of rats, its basic neural mechanism is still obscure. This stereotyped behavior, characterized by upright posture with pawing movements of the forelimbs, is easily elicited by administering inescapable shock to paired rats in a small enclosure [26]. Lesions of the amygdala and hippocampus [8], cerebral cortex [5], far lateral hypothalamus [22], and midbrain reticular formation lateral to the central gray [15], have all been shown to reduce pain-induced boxing behavior, but no lesions have yet been shown to abolish it altogether.

The neural organization of affective defense behavior in the cat is much better understood, however, and may be homologous to that of boxing in the rat. Both affective defense in the cat and defensive boxing in the rat function to threaten or repulse another attacking animal. Each behavior shields the vulnerable backside of the animal from its opponent and positions the limbs and jaws for attack. Each behavior may be elicited either by the infliction of pain or the confrontation with another threatening animal. The motor patterns of each behavior involve stance adjustments and striking movements with the forelimbs. Each may lead into a biting attack eventually, but not invariably. Several differences should be noted, however: while hissing or growling vocalization is characteristics of the cat's affective defense, squealing or hissing is only sometimes present in the rat; and while the stance adjustment of the rat involves an upright posture, the cat usually rolls sideways instead, raising only one foreleg into striking position.

Studies of the neural mechanisms of affective defense in the cat have shown that the behavior is organized at three levels, in the amygdala, medial hypothalamus, and midbrain central gray, and that of the three levels only the central gray is necessary for the occurrence of the behavior [9].

The present study was designed to test the hypothesis that lesions of the midbrain central gray would abolish shock-induced defensive boxing in the rat. Since it has been shown in previous studies [12, 17, 19] that central gray lesions in the rat also affect various other responses to fear or pain, the experimental animals were also tested for freezing in the open field, flinching in response to footshock, and escape from footshock after the placement of lesions.

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