RESUMEN
Recently, it was demonstrated that inbred strains of mice have a clearcut circadian rhythm of pineal and serum melatonin. Moreover, it is known that melatonin is involved in many immunoregulatory functions. Among them, hematopoiesis is influenced by the action of melatonin via melatonin-induced opioids on kappa-opioid receptors, which are present on stromal bone marrow cells. Therefore, the present study was carried out to investigate the presence of melatonin in the bone marrow in which immunocompetent cells are generated. Specifically, we aimed at answering the following question: are bone marrow cells involved in melatonin synthesis? In the present study, we demonstrate that (1) bone marrow cells contain high concentrations of melatonin; (2) bone marrow cells have a N-acetyltransferase activity and they express the mRNA encoding hydroxy-O-methyltransferase and (3) bone marrow cells cultured for a prolonged period exhibited high levels of melatonin. Results presented here suggest that mouse and human bone marrow and bone marrow cells are capable of de novo synthesis of melatonin, which may have intracellular and or paracrine functions.