Traumatic brain injury produces delay-dependent memory impairment in rats

Memory impairment following traumatic brain injury (TBI) is common in both humans and animals. A noteworthy feature of memory dysfunction in human TBI is impaired memory performance that is dependent on the delay between initial learning and recall of information. However, previous studies of TBI-induced memory impairment in animals have failed to control for the initial amount of learning between sham and injured animals. The present study demonstrates that experimental TBI in rats produces delay-dependent memory impairment, even when the initial degree of learning is controlled for. Animals were injured at a moderate level of lateral fluid percussion (LFP) injury (n = 10) or received a sham injury (n = 9), and then trained in a water T-maze version of the delayed-non-matching-to-place (DNMP) task beginning 10 days post-injury. Acquisition training consisted of 15 trials per day on post-injury days 11-15 using a minimal (7-sec) delay between the sample and choice phases of the task. Following acquisition, the delay between the sample and choice phases of the task was progressively increased to 15, 30, and 120 sec. Injured animals acquired the task at the same rate as sham animals and performed equally well at the 15-sec delay (p > 0.05). However, as the delay increased to 30 and 120 sec, the performance of the injured animals deteriorated (p < 0.05). These results indicate that LFP injury produces delay-dependent memory impairments in rats. This is therefore a valid model of an important feature of memory impairment in human TBI, and should be a useful addition to the available methods for assessing memory impairment and the effect of therapeutic interventions after TBI.