The Analgesic Effects of Celecoxib on the Formalin-induced Short- and Long-term Inflammatory Pain.

The non-steroidal anti-inflammatory drug celecoxib has long been used for reducing pain, in spite of moderate gastrointestinal side effects. In previous studies, it has been shown that celecoxib can inhibit formalin-induced spontaneous pain and secondary hyperalgesia. Injecting formalin into a rodent's hind paw not only induces acute pain behaviors, but also produces long-lasting hyperalgesia. Whether celecoxib can also have long-lasting effects is still unknown. Our results show that pretreatment with an intraperitoneal injection of celecoxib at one hour before formalin injection induced inhibition on the spontaneous flinch and licking behaviors in the second phase but not the first phase. Meanwhile, FOS expressions were also reduced with celecoxib pretreatment. Consecutive administration of celecoxib also protects the hind paw from hypoalgesia and relieves formalin-induced, long-lasting hyperalgesia in the ipsilateral hind paw. These analgesic effects may be related to suppression of the activation of neurons and astrocytes indicated by FOS and GFAP expressions. Based on the above findings, celecoxib demonstrated analgesic effects not only on acute spontaneous pain behavior but also on long-lasting hyperalgesia induced by formalin injection. The inhibition of neurons and astrocytes by celecoxib may be possible reasons for its analgesia.