Orexin A and B are neuropeptides involved in sleep control, appetite, energy homeostasis and thermoregulation. Recently, a study using transgenic mice knocked-out the orexin receptors (OX1R and OX2R), and overexpressed orexin, discovering that orexin also has effects on bone metabolism; acting centrally to increase bone formation, and peripherally to suppress bone formation. However, there is limited information on the direct role of orexin on the cells of the bone. Therefore, this study investigated the effects of orexin on primary osteoblasts and an osteoclast-like cell line, in vitro.
The effect of orexin A and B on primary rat osteoblast proliferation was assessed by 3H-thymidine incorporation following 24hr treatment. Osteoblasts were differentiated for 21 days in the presence of orexin A and B, with Von Kossa staining used to visualise the mineralised area. The presence of orexin receptors were evaluated by real-time PCR in rat osteoblasts at different stages of differentiation. Osteoclastogenesis was evaluated by the number of multinucleated TRAP+ve RAW264.7 cells following 48hr treatment.
Orexin A and B (10-8M and 10-9M) significantly increased osteoblast proliferation (p<0.01). Preliminary differentiation experiments showed an increased trend in mineralised area following treatment with orexin A and B, however, high variation between replicates suggested this was not statistically significant. Interestingly, there was no evidence of orexin receptor expression in the primary osteoblasts at any stage of differentiation. Neither orexin A nor B had any effect on osteoclastogenesis in the RAW264.7 cells.
Contrary to previously reported data, both orexin A and B had direct effects on osteoblast activity, although the mechanism of action is unknown as there was no evidence of the presence of the orexin receptors. Thus, further study needs to be done to evaluate its direct and indirect action on the cells of the bone, as well as the signalling pathway of these peptides.