Activation of a Remote (1-year old) Emotional Memory Interferes with the Retrieval of a Newly Formed Hippocampus-Dependent Memory in Rats

Hippocampal and prefrontal cortical functioning can be impaired in people with post-traumatic stress disorder thus creating traumatic memories. There have been inconsistent findings in research studying people with post-traumatic stress disorder (PTSD). Some people with PTSD experience the event again when remembering it and they exhibit impairment of the hippocampal and prefrontal cortical functioning; this is shown in problems with working memory, attention, and concentration. Other studies have shown no cognitive impairment in people with PTSD. This current study used adult male rats to test. Spatial memory of the rats was tested with a water maze apparatus. This consists of several different swimming sections and lights strategically placed so the rats remember different sections. The goal is to reach the section with the platform right underneath the water. If the rat did not reach the goal platform within 60 seconds, then the rat was guided to the platform by the researcher. The rats were trained in this water maze apparatus before a 30 minute rest period, then spatial memory was tested by putting the rat back into the maze and the researchers looked to see if the rats went to the goal platform. Inhibitory avoidance (IA) was tested by placing the rats in an apparatus that was half lighted and the other half dark. Avoidance training was conducted by putting the rat on the lighted side and allowing it to explore both sides. Once the rat entered the dark side a footshock was administered until it went back to the light side. Rats in the control group were allowed to explore both sides without shock. The measure of interest was how long it took each rat to cross from the light side to the dark side of the apparatus. Experiment 1 examined the effect of (IA) and 24 hour retrieval on memory. Two groups of rats were trained in the water maze. Then one group was also put into the footshock apparatus. This experiment showed that the no shock group had impaired short term memory. Experiment 2 used 16 rats that had not been used in experiment 1. In this experiment, the researchers implanted the memory of the shock experience before any water maze training. One, 6, or 12 months later, the same rats were given water maze training. This series of experiments tested the hypothesis that re-exposure of the rats to the IA apparatus—at 1, 6 and 12 months after IA training—would reactivate the memory of the shock experience, which would interfere with the rats’ ability to retrieve the memory of the newly learned location of the hidden platform. Experiment 3 tested rats that were re-exposed to the IA apparatus in Experiments 1 and 2. It found that the re-exposed rats spent more time on the lighted side of the apparatus than the untrained rats did. This series of experiments found two primary findings. Firstly, inhibitory avoidance training right after the water maze impaired short term memory of the location of the hidden platform. Secondly, the activation of a rat’s memory of the IA experience impaired retrieval of the memory of the location of the hidden platform. These findings are relevant to people with in PTSD where one memory interferes with the retrieval of another memory.

ZOLADZ, P. R., WOODSON, J. C., HAYNES, V. F., & DIAMOND, D. M. (2010). Activation of a remote (1-year old) emotional memory interferes with the retrieval of a newly formed hippocampus-dependent memory in rats. Stress: The International Journal on the Biology of Stress, 13(1), 36-52. doi:10.3109/10253890902853123

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