Improved learning and memory with theta-burst stimulation of the fornix in rat model of traumatic brain injury

Objective: Learning and memory deficits are a source of considerable morbidity after traumatic brain injury (TBI). We investigated the effect of different patterns of hippocampal stimulation via a fornix electrode on cognitively demanding tasks after TBI. Methods: Male Sprague-Dawley rats underwent fluid-percussion injury and were compared with sham-operated rats. Electrodes were implanted into the fornix and hippocampus, and stimulation of the fornix produced robust evoked potentials in the hippocampus. A 60-s delayed non-match-to-sample (DNMS) swim T-maze was serially performed using four stimulation patterns: no stimulation (No Stim), low-frequency stimulation (LFS, 5 Hz), high-frequency stimulation (HFS, 130 Hz), and theta-burst stimulation (TBS, 200 Hz in 50 ms trains, five trains per second; 60 µA biphasic pulses). In a separate cohort of sham and injured animals, Morris water maze (MWM) was performed with or without TBS. Results: In the DNMS swim T-maze, LFS and HFS did not significantly improve performance after TBI. However, there was a significant difference in performance between TBI + No Stim and TBI + TBS groups (P