Melatonin suppresses the plasma calcium level in female goldfish during the late and post-reproductive stages.

In vertebrate females, plasma calcium concentrations decrease after the reproductive period to prevent calcium from leaking from the body. Goldfish scales store calcium internally, which is released into the blood by osteoclast resorption. Previously, we found that melatonin increases calcitonin (CT), an osteoclast inhibitor, through melatonin receptors in goldfish scales, and inhibits osteoclast activity. However, the physiological function of the melatonin-dependent suppression of osteoclast activity is unclear. In this study, we found that plasma calcium levels increased during the reproductive stage in female goldfish and decreased during the post-reproductive stage. Histological and mRNA expression analyses of osteoclast marker molecules revealed that osteoclasts in the scales were inactivated during the late and post-reproduction stages when the expression of CT and its receptor increased in female goldfish scales. Moreover, exogenous melatonin suppressed plasma calcium levels in female goldfish during their late reproductive stage in cases of increased melatonin receptor expression. Taken together, these results suggest that melatonin inactivates scale osteoclast resorption by increasing CT levels in the scales and prevents calcium leakage from the body during the late and post-reproductive stages of female goldfish.