Effects of stress and long-term potentiation (LTP) on subsequent LTP and the theta burst response in the dentate gyrus

Exposure to an aversive and stressful events is reported to have similar effects of hippocampal plasticity and behavior as does exposure to high-frequency stimulation of the hippocampus. Here we directly compared the effects of exposure to a stressor vs. a previous induction of LTP on a subsequent induction of LTP and the extracelllular response to a tetanus patterned after endogenous theta rhythms. Stimulating the dentate gyrus via the perforant path, Sprague-Dawley rats (n = 65) were tetanized 2 h after exposure to a stressor consisting of restraint and 60, 1 s, 1 mA tail shocks. Unstressed controls were tetanized once and then again 2 h later. Exposure to the stressor impaired LTP of the EPSP 2 h later, as did a previous induction of LTP. In addition, exposure to the stressor altered the extracellular response to subsequent theta burst stimulation (10, 40 ms bursts at 100 Hz, each separated by 200 ms), as did a previous induction of LTP. Whereas unstressed rats exposed to the first tetanus exhibited a marked decline in the amplitude across successive bursts, stressed rats exhibited no such decline, a response pattern similar to that observed in unstressed rats exposed to a second tetanus. The similarity between the effects of stress and tetanic stimulation on hippocampal plasticity support the hypothesis that stress and LTP are converging on similar neuronal mechanisms.