Insulin-Like Growth Factor-2 (IGF-2) Does Not Improve Memory in the Chronic Stage of Traumatic Brain Injury in Rodents

Persistent cognitive impairment(s) can be a significant consequence of traumatic brain injury (TBI) and can markedly compromise quality of life. Unfortunately, identifying effective treatments to alleviate memory impairments in the chronic stage of TBI has proven elusive. Several studies have demonstrated that insulin-like growth factor-2 (IGF-2) can enhance memory in both normal animals and in experimental models of disease. In this study, we questioned whether IGF-2, when administered before learning, could enhance memory performance in the chronic stage of TBI. Male C57BL/6 mice (n = 7 per group) were injured using an electronic cortical impact injury device. Four months later, mice were tested for their cognitive performance in the fear memory extinction, novel object recognition (NOR), and Morris water maze tasks. Twenty minutes before each day of training, mice received a subcutaneous injection of either 30 μg/kg of IGF-2 or an equal volume of vehicle. Memory testing was carried out 24 h after training in the absence of the drug. Uninjured sham animals treated with IGF-2 (or vehicle) were trained and tested in the fear memory extinction task as a positive control. Our data show that although IGF-2 (30 μg/kg) improved memory extinction in uninjured mice, it was ineffective at improving fear memory extinction in the chronic stage of TBI. Similarly, IGF-2 administration to chronically injured animals did not improve TBI-related deficits in either NOR or spatial memory. Our results indicate that IGF-2, administered in the chronic stage of injury, is ineffective at enhancing memory performance and therefore may not be a beneficial treatment option for lingering cognitive impairments after a TBI.