Calcitonin alleviates hyperalgesia in osteoporotic rats by modulating serotonin transporter activity

Summary: Calcitonin may relieve pain by modulating central serotonin activity. Calcitonin partly reversed the hypersensitivity to pain induced by ovariectomy. This suggests that the anti-nociceptive effects of calcitonin in the treatment of osteoporosis may be mediated by alterations in neural serotonin transporter (SERT) activity. Introduction: This study used a rat model of osteoporosis to evaluate the role of the cerebral serotonin system in the anti-nociceptive effect of calcitonin, a drug used to treat post-menopausal osteoporosis. Methods: Osteoporosis was induced in rats by ovariectomy (OVX). Rats were then randomized to the following four groups: sham operation, OVX, OVX plus calcitonin, or OVX plus alendronate. Results: OVX led to alterations in bone micro-architecture; alendronate strongly reversed this effect, and calcitonin moderately reversed this effect. OVX increased hyperalgesia (determined as the time for hind paw withdrawal from a heat source); calcitonin reduced this effect, but alendronate had no effect. OVX increased the expression of c-Fos (a neuronal marker of pain) in the thalamus; calcitonin strongly reversed this effect, and alendronate moderately reversed this effect. OVX also reduced SERT but increased 5-HT1A receptor expression and activity; calcitonin aggravated this effect, but alendronate had no effect on recovery of SERT/5-HT1A activity and expression. Conclusions: Our study of a rat model of osteoporosis suggests that OVX-induced enhancement of the serotonergic system may protect against hyperalgesia. However, the anti-nociceptive effects of calcitonin in osteoporosis may be mediated by decreased neural SERT activity and increased activation of 5-HT1 receptors in the thalamus.