Involvement of muscarinic system of the dorsal hippocampus on acute stress-induced spatial learning and memory enhancement in male mice

Introduction: The effects of stimulation and inhibition of muscarinic acetylcholine receptors in the dorsal hippocampus on spatial learning and memory in male NMRI mice after acute stress were investigated. Materials and Methods: Animals were divided into two subsets of stress and non-stress. Each subset consisted of: saline, atropine (a muscarinic acetylcholine receptor antagonist) (1, 5, and 10 µg/mouse), and pilocarpine (a muscarinic acetylcholine receptor agonist) (1, 5, and 10 µg / mouse) groups. In the stress subset, the animals received an electro foot shock 5 minutes before each drug or saline injections. One day after drug injection with or without stress (respectively), the animals' spatial learning and memory were tested in the Barnes maze. In this study, the time and distance traveled to reach the target chamber, and the number of errors in reaching the target chamber were studied as variables of spatial learning and memory. Results: The time of arrival and the distance traveled to reach the target hole was reduced in the stress group.  The number of errors in these animals was also lower.  Atropine (1, 5, and 10 μg / mouse) inhibited the improving effect of acute stress on spatial memory, but pilocarpine (1, 5, and 10 μg / mouse) improved the stress effect. Atropine (1, 5 and 10 µg/mice) in non-stressed animals only had memory impacts in the first and second days, but pilocarpine (1, 5 and 10 µ/ mice) in non-stressed animals improved the spatial memory. Conclusion: Acute stress enhanced spatial learning and memory in mice. Considering the effectiveness of atropine and pilocarpine in inhibiting or enhancing the effects of stress (respectively), it seems that the effect of acute stress on improving spatial learning and memory, at least in part,  might be mediated through the activation of cholinergic muscarinic in the dorsal hippocampus.