Melatonin impaired acquisition but not expression of contextual fear in rats

Melatonin has been shown to be 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 contextual fear conditioning (experiment 1) and fear expression (experiment 2) in rats. Behavior procedure involved three training phases: habituation, fear conditioning and test. Melatonin was injected either 30 min before conditioning (experiment 1) or 30 min before testing (experiment 2). Results showed that rats injected melatonin 30 min before conditioning presented a significant lower freezing during both fear conditioning and test phases. Melatonin injected 30 min before testing was ineffective on fear expression. These results suggest that melatonin, at the dose applied in this study, impaired fear acquisition but not fear expression. These findings extend previous research on the melatonin effects on learning and memory.