Depression-like behavior corresponds with cardiac changes in a rodent model of spinal cord injury

In previous studies we have shown that approximately 1/3 of male Sprague Dawley rats develop symptoms of depression following a spinal cord injury (SCI). Using established behavioral tests to measure depression in rodents, we found that after SCI, subjects characterized as depressed had decreased sucrose preference, open field activity, social exploration, and burrowing behavior. As some of these tests of depression could be affected by the compromised motor function inherent to the SCI condition, the current study examined whether non-subjective, physiological differences in heart rate and heart rate variability were also associated with depression, as seen in humans. Male Sprague Dawley rats were implanted with radiotelemetry devices and either received a moderate contusion injury or remained intact. The implanted telemetry devices recorded home cage activity, body temperature, heart rate, and heart rate variability for 5 min/h throughout a 30-day post-injury assessment period. Depression behavior was evaluated using a battery of tests conducted on days 9-10 and 19-20 post-injury. Locomotor recovery and pain reactivity were also examined. Hierarchical clustering, based on the behavioral scores collected on the tests of depression, revealed that 28% of the SCI subjects displayed symptoms of depression, relative to the remaining 72% of SCI subjects. The subjects characterized as depressed had significantly lower social interaction and burrowing activity than the group that was not depressed. Interestingly, the subjects behaviorally characterized as depressed also had significantly lower heart rate variability than the not-depressed intact group. There was no difference between not-depressed SCI and intact rats on this measure. Therefore, in addition to behavior, depressed and not-depressed rats differ on measures of physiological function that are associated with depression in humans. These physiological differences further validate the rodent model of depression after SCI.