Effects of melatonin on macrophages/microglia in postnatal rat brain

The present study examined the response of macrophages/microglia to multipl e injections of melatonin in the pineal gland and different regions of the brain. The macrophages;microglia showed a significant increase in cell numb ers and upregulation of complement type 3 receptors (CR3), major histocompa tibility complex class I (MHC I) and class II (MHC II) antigens, and antige ns of monocyte/macrophage lineage, as detected by the antibodies OX-42, OX- 18, OX-6, and EDI, respectively. The upregulation of the above antigens was observed in 1-d-old rats given daily injections of melatonin and killed at 7-11 d of age; no noticeable change was observed at earlier time intervals . The macrophages/microglia expressing the above antigens appeared round an d showed a vacuolated cytoplasm compared with ramified cells in the control rats. Upregulation of CD4 antigens as detected with the antibody W3/25 was also observed in macrophages/microglia in the corpus callosum and epiplexu s cells in the lateral ventricles, but not in the pineal gland and the cere bral cortex in the same age group. In rats killed between 2 and 5 d, and at 14 d of age after melatonin treatment, the immunoreactivities of macrophag es/microglia with the above mentioned antibodies were comparable to cells i n the control rats. Immunoreactive cells were not detected in ally of the a ge groups in melatonin-treated or control rats with the antibodies W3/13 an d OX-33, which are markers for T and B lymphocytes. It is concluded that CR 3 receptors, MHC antigens, and CD4 antigens on macrophages/microglia are up regulated following melatonin administration. On the other hand, once the m elatonin treatment is discontinued the expression of the various antigens/r eceptors returns to normal levels, suggesting that increased immune potenti ality and its maintenance in these cells require the continuous action of t he drug.