Mechanisms for Ovariectomy-Induced Hyperalgesia and Its Relief by Calcitonin: Participation of 5-HT1A-Like Receptor on C-Afferent Terminals in Substantia Gelatinosa of the Rat Spinal Cord

Chronic treatment with calcitonin in osteoporotic patients alleviates the pain associated with this condition by an unknown mechanism. In ovariectomized rats that develop osteoporosis and hyperalgesia, we examined whether a functional change in serotonergic systems in the spinal dorsal horn was involved, using whole-cell recordings from substantia gelatinosa neurons in spinal cord slices and [3H]8-hydroxy-2(di- n -propylamino)tetralin ([3H]8-OH-DPAT) binding. Hyperalgesia could be attributed to the elimination of presynaptic inhibition by 5-HT of glutamatergic primary C-afferent terminals and an associated decrease in the density of [3H]8-OH-DPAT binding sites whose receptors are neither 5-HT1A- nor 5-HT7-subtype. These changes in serotonergic systems were restored after chronic treatment with calcitonin. Reversal of 5-HT receptor changes by calcitonin treatment may provide an explanation for its analgesic actions in patients.