Bone Abstracts

Melatonin is a neuro-hormone released primarily from the pineal gland, which has been shown to have bone anabolic effect, although it is still unclear whether its skeletal action is directly mediated by receptors expressed on bone cells or is indirect. In this study, we examined melatonin’s effects on bone cellular activities in vitro and tested whether it modifies angiogenesis and blood flow to bone, which are both essential for bone formation.

Primary osteoclasts from mouse bone marrow cells and osteoblasts from mouse calvariae were cultured in increasing concentrations of melatonin (0.1–100 μg/ml) to evaluate its effect on bone cell numbers and activities. RNA was extracted from mouse primary osteoblasts and osteoclasts for detection of melatonin receptors MT1 and MT2 using RT-PCR.

Bone blood flow of the hind limb was measured by laser Doppler imaging after intraperitoneal injection of melatonin in C57Bl6 mice (1 and 10 mg/kg, n=5). The effect of melatonin (1 100 μg/ml) on angiogenesis was assessed using the chick chorioallantoic membrane assay (CAM).

Both MT1 and MT2 receptors were expressed in mouse primary osteoclasts and osteoblasts. Quantitative RT-PCR showed a higher expression of MT2 in osteoblasts, however MT1 was more expressed in osteoclasts. Results show that melatonin dose-dependently increased bone nodule formation from 10 μg/ml (P<0.0001). Melatonin had no significant effect on osteoclast number and bone resorption activity in vitro. In vivo, melatonin did not affect the hindlimb blood flow in C57BL6 mice compared to control and did not stimulate neovascularisation in vitro, using the CAM assay.

In conclusion, our results confirm that melatonin is a potent bone anabolic in vitro and suggest that its skeletal effect could be directly mediated by MT2 expressed in osteoblasts rather than by increasing bone blood supply.