Objective: To evaluate the effect of short- and long-term administration of melatonin on central brain-derived neurotrophic factor (BDNF) levels in rats with acute and chronic inflammatory pain. Methods: The animals were allocated to one of two experiments: experiment 1 or experiment 2. In experiment 1, all animals were injected with complete Freund's adjuvant (CFA) to induce inflammation and were randomly allocated to receiving melatonin (60 mg/kg) or vehicle. Injections were administered 1 h after CFA and once daily for 2 more days (for a total of 3 days of melatonin administration). In experiment 2, fifteen days after CFA injection, the animals were treated with melatonin (50 mg/kg) or vehicle for 8 days. The animals were killed by decapitation 24 h after the last melatonin or vehicle administration, and an ELISA assay was performed to detect BDNF expression in the spinal cord, brainstem, and prefrontal cortex of the rats in both groups. Data were analyzed using Student's t test and the results are expressed as means ± SEM. Results: In the first experiment, the BDNF levels of the melatonin group were reduced in the prefrontal cortex (Student's t test, p = 0.01) and increased in the spinal cord (Student's t test, p = 0.04). In experiment 2, BDNF levels were similar in both groups for all structures (Student's t test, p > 0.00 for all). A two-way ANOVA reveled a significant effect of structures (p = 0.0001) but not of treatment (p > 0.05). The prefrontal cortex presented higher BDNF levels than other structures (ANOVA/Student-Newman-Keuls test, p = 0.0001). Considering the relationship between BDNF levels in all three structures, we found an effect of central nervous system structures (p = 0.01) and an interaction between treatment and structures (p = 0.04). Conclusion: The high spinal cord BDNF levels and the low prefrontal cortical BDNF levels observed in rats with acute CFA-induced inflammation following short-term melatonin administration may be related to the pain-modulating and neuroprotective effects of this protein. Long-term melatonin administration did not alter BDNF levels in chronic inflammation.

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