Melatonin facilitates extinction, but not acquisition or expression, of conditional cued fear in rats.

Fulian Huang,Chang-Qi Li,Xiaoyan Liu,Zehua Yang

doi:10.1186/1471-2202-15-86

Fulian Huang, Chang-Qi Li + Show 2 more

Open Access

https://doi.org/10.1186/1471-2202-15-86

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Abstract

BackgroundPrevious studies have shown that melatonin is involved in the processes that contribute to learning and memory. At present study, we tested the effects of exogenous melatonin (2.5 mg/kg) on the acquisition, expression and extinction of cued fear in rats.ResultsResults showed that a single afternoon administration 30 min before conditioning has no effect on the acquisition of cued fear. Compared to rats injected with vehicle, rats injected with melatonin 30 min before extinction training presented a significant lower freezing during both extinction training and extinction test phases, however, freezing response did not differ for the initial four trials during extinction training. Melatonin injected immediately after extinction training was ineffective on extinction learning.ConclusionsThese results suggest that melatonin, at the dose applied in this study, facilitates the extinction of conditional cued fear without affecting its acquisition or expression, and melatonin facilitates cued fear extinction only when it is present during extinction training. These findings extend previous research on the melatonin effects on learning and memory and suggest that melatonin may serve as an agent for the treatment of anxiety disorders such as posttraumatic stress disorder (PTSD).

Highlights

Previous studies have shown that melatonin is involved in the processes that contribute to learning and memory
Experiment 1: Effects of melatonin on the acquisition or retention of conditional cued fear First, we investigated the effects of melatonin on cued fear acquisition
There was no a significant effect of group (F (1, 10) = 0.049, p > 0.05) and no significant trial-by-group interaction (F (13, 130) = 1.566, p > 0.05). These results suggest that melatonin administration immediately after extinction training has no effect on the extinction of conditional cued fear

Summary

Introduction

Previous studies have shown that melatonin is involved in the processes that contribute to learning and memory. We tested the effects of exogenous melatonin (2.5 mg/kg) on the acquisition, expression and extinction of cued fear in rats. It is thought to be involved in modulating complex processes such as learning and memory [7] by binding to receptors [MT(1)/MT(2)] which are widely distributed in the brain [8]. Melatonin was shown to possess memory facilitating effects in the novel object recognition task [9,10] and the olfactory social memory test in rats [11]. Other reports showed that melatonin enhances performance in a verbal association task [12] and improves memory acquisition under stress [13]. Mice lacking melatonin MT2 receptors showed impaired performance

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