Takeshi Yoshimura

Background: The polypeptide hormone calcitonin is clinically well known for its ability to relieve neuropathic pain such as spinal canal stenosis, diabetic neuropathy and complex regional pain syndrome. Mechanisms for its analgesic effect, however, remain unclear. Here we investigated the mechanism of anti-hyperalgesic action of calcitonin in a neuropathic pain model in rats. Results: Subcutaneous injection of elcatonin, a synthetic derivative of eel calcitonin, relieved hyperalgesia induced by chronic constriction injury (CCI). Real-time reverse transcriptase-polymerase chain reaction analysis revealed that the CCI provoked the upregulation of tetrodotoxin (TTX)-sensitive Nav.1.3 mRNA and downregulation of TTX-resistant Nav1.8 and Nav1.9 mRNA on the ipsilateral dorsal root ganglion (DRG), which would consequently increase the excitability of peripheral nerves. These changes were reversed by elcatonin. In addition, the gene expression of the calcitonin receptor and binding site of ¹²⁵I-calcitonin was increased at the constricted peripheral nerve tissue but not at the DRG. The anti-hyperalgesic effect and normalization of sodium channel mRNA by elcatonin was parallel to the change of the calcitonin receptor expression. Elcatonin, however, did not affect the sensitivity of nociception or gene expression of sodium channel, while it suppressed calcitonin receptor mRNA under normal conditions. Conclusions: These results suggest that the anti-hyperalgesic action of calcitonin on CCI rats could be attributable to the normalization of the sodium channel expression, which might be exerted by an unknown signal produced at the peripheral nerve tissue but not by DRG neurons through the activation of the calcitonin receptor. Calcitonin signals were silent in the normal condition and nerve injury may be one of triggers for conversion of a silent to an active signal.

In addition to its regulatory effect on bone mass, calcitonin has been shown to relieve pain and alleviate peripheral circulatory disturbance in patients with Raynaud’s syndrome and complex regional pain syndrome. In the present study, we investigated whether calcitonin ameliorates diminished blood flow and enhanced arterial contraction in response to noradrenaline in chronic constriction injury (CCI) of the sciatic nerve in rats. Following surgically induced CCI, laser Doppler flowmetry studies showed a significant decrease in plantar skin blood flow of the ipsilateral hind paw compared to the contralateral side. A subcutaneous bolus injection of elcatonin (20 U/kg), a synthetic derivative of eel calcitonin, significantly improved decreased skin blood flow in the ipsilateral side. In vitro analysis of plantar arteries isolated from the ipsilateral hind paw 7–13 days after the CCI procedure showed higher sensitivity to noradrenaline than the plantar arteries from the contralateral side. Elcatonin (0.1–10 nm) significantly reduced noradrenaline‐induced contraction in the arteries of the ipsilateral side, whereas it had little effect on those of the contralateral side. These results suggest that calcitonin selectively ameliorates enhanced arterial contractility in CCI neuropathic rats, thus leading to its alleviating effect on peripheral circulatory disturbance.