Microglia, the sentinels of the central nervous system, are responsible for the surveillance and the innate defense against pathogen or danger/damage-associated molecular patterns. The response is fine-tuned to restrain pro-inflammatory responses, preserving neighboring cells. At the injured area, microglia temporarily shift to a pro-inflammatory phenotype (M1), followed by anti-inflammatory (M2) phenotypes. The duration and magnitude of the pro-inflammatory phase are finely regulated to avoid unnecessary loss of brain tissue.

The present study shows that melatonin synthesized by microglia plays a key role in the transformation of M1 to M2 phenotypes. In a mixed rat cerebellar glia culture, the percentage of activated microglia did not vary significantly with the treatments, while the role of melatonin synthesized by microglia in promoting the end of the pro-inflammatory phase, and the initiation of the regulatory/phagocytic phases was inferred by using pharmacological tools. Total microglia were identified by the expression of CD11b/c, whereas positive to IBA-1 microglia were considered activated, independent of the phenotype. M1 and M2 phenotypes were distinguished with the biomarkers NOS-2 and ARG-1, as these enzymes act on the same substrate (L-arginine), producing pro-inflammatory (NO) or anti-inflammatory (polyamines and proline) end products, respectively. Luzindole, a blocker of melatonin receptors, impaired the conversion of M1 to M2 phenotypes and zymosan phagocytosis.

Thus, melatonin content synthesized by cerebellar microglia determines the extension of the pro-inflammatory phase of defense response.