Background: Adenosine has shown the analgesia, which was mediated through spinal adenosine receptors. The presence of four subtypes of adenosine receptors like A₁, A_2A, A_2B and A₃ in the spinal cord has been demonstrated. The authors determined the role of spinal adenosine receptors for the nociceptive modulation by examination of effect of intrathecal adenosine subtype agonists for formalin-induced pain. Methods: Lumbar intrathecal catheterization was done with male Sprague-Dawley rats. Formalin- induced pain developed with injection of formalin solution (5%, 50μl) into the hindpaw of rats. Adenosine subtype agonists (A₁, CPA; A_2A, DPMA; A₃, IB-MECA) were intrathecally given 10 min before the formalin injection. Afnd then, formalin-induced pain behavior (flinching response) was observed for 60 min. Results: Intrathecal CPA and IB-MECA produced a minimal or no effect during phase 1 response of the formalin test, respectively, but both agents reduced the phase 2 flinching response. Intrathecal DPMA suppressed both phase responses evoked by formalin injection. Conclusions: Spinal A₁ and A_2A receptors may play an important role in the modulation of the facilitated state as well as acute nociception and, while A₃ receptor being relevant to the modulation of only the facilitated state.

KeywordsAdenosine, Adenosine receptor subtype, Formalin test, Intrathecal, Rat