Role of Midbrain Central Gray in Pain-Induced Defensive Boxing of Rats
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Lesions of the central gray altered the performance of animals in the boxing, escape, and open field tests, but did not alter the occurrence or the threshold of flinching in response to shock. Feeding behavior was also disrupted in many of the operated rats. The distributions of scores of animals one week after surgery for boxing, escape, open field, and flinch are shown in Figs. 1-4, and are compared to the distributions of scores from comparable tests on control animals with no lesions. The criteria for performance deficits were determined from visual inspection of these distributions.

The criterion for a boxing deficit was defined as a level of performance at which the rat boxed on no more than 5% of shocks given in the test. Eight of the 10 rats with 50% or more damage to the central gray (extensive damage group) showed a boxing deficit, while only 1 of 9 lesioned rats with less than 50% damage (restricted damage group) and none of the control animals on their second test showed boxing deficits. Statistical probabilities of the differences between proportions of each group with boxing deficits were computed by the Fisher Exact Probability Test: extensive vs. restricted groups, p = 0.004; extensive vs. control group, p<0.001.

An escape deficit was defined as a level of performance at which the animal did not reach the escape criterion within 25 trials. All control animals learned to escape. Eight of 9 rats with extensive central gray damage which were tested for escape and 1 of the 7 rats with restricted damage showed escape deficits. The extensive damage group was statistically more likely to show a deficit: extensive vs. restricted, p=0.006; extensive vs. controls, p<0.001 (Fisher Exact Test).

The criterion for an open field deficit was defined as a level of performance at which the rat crossed 50 intersections or more during the 4 min test. All 6 of the rats with extensive damage which were tested in the open field showed deficits, 3 of the 5 rats with restricted damage showed deficits, and none of the controls showed deficits. The proportion of the extensive damage group was significantly higher than that of the control group (p< 0.001 by Fisher Exact Test). Among lesioned animals there was a significant positive correlation between percentage of central gray damaged and open field score (r = 0.76, p<0.005). Freezing postures were not observed in animals with open field deficits, whereas control rats adopted freezing or crouching postures for about 75% of the time in the open field.

There was no difference in the thresholds for flinching in response to footshock between the lesioned animals (threshold 0.12 mA) and the controls (threshold 0.11 mA). Feeding deficits occurred in 10 animals, all with 49% or more central gray damage.

The above deficits did not appear to be due to general motor debilitation. Animals with boxing or escape deficits had greatly increased ambulation in the open field. Of the 9 rats with escape deficits, 7 showed themselves physically capable of performing the task by mounting the platform one or more times during inter-trial intervals or after the 5 sec escape criterion. Of the 6 rats with escape deficits which were tested for passive avoidance, only one animal reached a criterion of 3 one-min stays on the platform in 10 trials, again indicating that an inability to mount the platform was not the cause of the escape deficit. Similarly, boxing deficits did not seem to result from an inability to remain in the upright posture, for all of these animals frequently reared up on the wall of the shock-box and on the wall of the open field enclosure.

Sensory systems important for boxing and escape [6, 10] were also intact in animals with test deficits. Pain sensitivity was normal according to the flinch test, and tactile sensation was noted to be heightened since rats gave an exaggerated startle reaction to light tactile stimulation. Also, rats with lesions did not run into walls in the open field test, and, prior to weighing sessions, they were observed to veer around objects in their path. Hence, visual and/or vibrissae sensations appeared intact.

Boxing and escape deficits apparently were not due to competing behaviors related to hyperactivity. Despite abnormally high ambulation in the open field test, one rat learned the escape task and two others had normal or increased boxing.

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