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The role of hippocampal nitric oxide in passive avoidance learning

Introduction: Nitric oxide (NO) is a retrograde messenger in hippocampal synaptic plasticity which is involved in learning and memory processes. Previous studies revealed that hippocampal pyramidal cells contain NO synthase (NOS) enzyme, which produces NO and could be a promising target to evaluate the role of NO in brain cognitive functions. In this study, we conducted an experiment to assess the role of NO in passive avoidance learning by using a NOS inhibitor (L-NAME). Methods: Adult male Wistar rats (200-250 gr) were bilaterally implanted into the CA1 region of the hippocampus. Animals were subjected to behavioral tests one week after surgery. Rats received different doses of L-NAME (5, 10 and 15 µg/0.5µl/side) into the CA1 of hippocampus 25 min before training. Retrieval tests were performed in 3 different stages after training including working or immediate (immediately after training), short-term (90 min after training) and long-term (24 h after training) memories. Results: Our findings showed that pre-training injection of L-NAME 15 µg/0.5µl/side significantly increased the number of step-through into the dark chamber for immediate memory, while decreased step-through latency for shortterm memory. Conclusion: These results suggest that hippocampal NOS inhibition impairs both immediate and short-term memory, but have no significant effect on long-term memory. Thereby hippocampal NO may affect early on learning and memory in passive avoidance task.