5-hydroxytryptamine (5-HT) released in inflammatory tissues plays a pivotal role in
pain hypersensitivity. However, it is not clear whether 5-HT2A receptors in the inflamed tissues mediate this effect. The present study investigated the contribution of 5-HT2A receptors in the periphery to chronic inflammatory
pain. Complete
Freund's adjuvant (CFA) was injected subcutaneously in the hindpaw of rats. The selective
5-HT2A receptor antagonist
ketanserin was given in the inflamed site. Paw withdrawal latency responding to heat or mechanical stimuli was measured. Expression of
neuropeptide Y (NPY) in the spinal dorsal horn and dorsal root ganglia (DRG) was assayed using immunohistochemistry technique. The results showed that
ketanserin administered in the inflamed site inhibited
thermal hyperalgesia in a dose-dependent manner (20, 40 and 80 µg) induced by the intraplantar injection of CFA.
Ketanserin given once per day at a dose of 80 µg abolished heat
hyperalgesia and also attenuated
mechanical allodynia on the third day. CFA injection increased the expression of NPY in superficial laminae of the spinal cord, but not in the DRG. The local treatment of
ketanserin completely inhibited CFA-induced increase in NPY expression in superficial laminae of the spinal cord. These results indicated that activation of 5-HT2A receptors in the inflamed tissues was involved in the pathogenesis of inflammatory
pain and the blockade of 5-HT2A receptors in the periphery could relieve
pain hypersensitivity and normalize the cellular disorder in the spinal dorsal horn associated with pathological
pain. The present study suggests that the peripheral 5-HT2A receptors can be a promising target for
pharmaceutical therapy to treat chronic inflammatory
pain without central nervous system side effects.