The Effect of Melatonin on Thrombopoiesis in Irradiated Mice.

We have reported that serotonin (5-hydroxytryptamine, 5-HT) significantly stimulates megakaryocytopoiesis. It has been known that platelet count can be increased in stress conditions and serotonin has an effect on actin re-organization and promotes nitric oxide (NO) synthesis via 5-HT2A receptors. NO may induce apoptosis of mature megakaryocytes to produce platelets. Taken together, we proposed a possible mechanism of serotonin on megakaryocyte differentiation and platelet formation: serotonin enhances the differentiation and maturation of megakaryocytes via 5-HT2BR and actin re-organization; and serotonin is involved in the process of platelet formation by promoting the synthesis of NO to induce cell apoptosis via 5-HT2AR. Serotonin is the precursor of melatonin. The effect of melatonin on thrombopoiesis in irradiated mice (4 Gy) was investigated in this study. Eighteen mice were randomly divided into three groups (6 in each). Group 1 (normal control, N) received no irradiation or melatonin; Group 2 (model control, C) and Group 3 (melatonin, M) received 4 Gy total body irradiation for 3 minutes. After irradiation, melatonin (10mg/kg.d) was injected intraperitoneally into Group M for consecutively 21 days. And Group C was administered normal saline instead of melatonin. Peripheral blood platelets, white blood cells (WBC), and red blood cells (RBC) were analyzed from the three groups on day 0, 7, 14, and 21. All the mice were executed to collect bone marrow cells for the assay of CFU-MK and CFU-F (fibroblastoid). Our results showed that melatonin enhanced the recovery of platelets and WBC counts. Moreover, melatonin also promoted CFU-MK (20 ± 3 vs 11 ± 2 colonies/2 x10⁵ cells) and CFU-F formation (28 ± 10 vs 15 ± 3 colonies/2 x10⁶ cells). We also investigated the in-vitro effect of melatonin on CFU-MK formation at different doses (0–500 nM). The results showed that melatonin significantly promoted CFU-MK formation at a dose dependant manner. The maximal concentration was at 200 nM (P<0.01, N=6) compared with the control. The size of CFU-MK with melatonin treatment was much larger and each MK cell was more mature. Our studies showed that melatonin had a promoting effect on thrombopoiesis in this model and also stimulated in vitro CFU-MK formation. Thus, melatonin could be developed as an alternative drug for the treatment of thrombocytopenia.