Topical and systemic administrations of ketanserin attenuate hypersensitivity and expression of CGRP in rats with spinal nerve ligation

The aim of this study was to examine the involvement of 5-HT and 5-HT2A receptors in neuropathic pain and their possible cellular mechanism. To evaluate a potential therapy the 5-HT2A receptor antagonist ketanserin was administered topically or subcutaneously in rats with L5 nerve ligation. Unilateral spinal nerve ligation induced hypersensitivity to thermal and mechanical stimuli in the ipsilateral hindpaw and an increase in calcitonin gene-related peptide (CGRP) immunoreactivity (CGRP-IR) in small and medium diameter neurons in dorsal root ganglia (DRG) at L4 and L6, but not at L5. Topical application of ketanserin (3%) delivered in a mixture of gelatin, glycerol and kaolin onto skin over the hindpaw produced significant elevation of nociceptive threshold in the paw. Daily injection of ketanserin inhibited the thermal hyperalgesia in a dose dependent manner (0.1 and 0.3 mg/kg, s.c.). The inhibition occurred at 1–3 days after the injection started and persisted for at least 2 weeks without decline. Injection of ketanserin (0.3 mg/kg) also suppressed tactile allodynia. Moreover, ketanserin (0.3 mg/kg) reversed the increase in CGRP-IR expression in L4 and L6 DRG neurons. These results support the hypothesis that adaptive change in CGRP expression that occurred in the DRG adjacent to the DRG containing injured neurons underlies the nerve ligation-induced hypersensitivity. This study suggests that 5-HT and 5-HT2A receptors contribute to the maintenance of neuropathic pain by up-regulating the expression of CGRP-IR, and that topical and systemic administrations of ketanserin are promising therapies for relieving neuropathic pain without tolerance.