Spinal and peripheral adenosine A₁ receptors contribute to antinociception by tramadol in the formalin test in mice

Tramadol, an analgesic used alone or combined with acetaminophen, has a complex mechanism of action involving opioid and amine mechanisms. In this study, we explored the involvement of spinal and peripheral adenosine A ₁ receptors in antinociception by tramadol, and determined whether spinal serotonin 5-HT₇ receptors were linked to spinal A₁ receptor actions. Antinociception was examined using the 2% formalin test in mice. Tramadol was administered systemically (intraperitoneal) or peripherally (intraplantar). Caffeine (non-selective A₁/A_2A receptor antagonist) and SCH58261 (selective A_2A receptor antagonist) were given systemically, while DPCPX (selective A₁ receptor antagonist) was given systemically, spinally (lumbar puncture), or peripherally. Systemic tramadol 35 mg/kg produced antinociception against phase 2 formalin-evoked flinching behaviors, particularly in the earlier parts (phase 2A). Systemic caffeine (10 mg/kg) and DPCPX (1 mg/kg), but not SCH58261 (3 mg/kg), inhibited antinociception by systemic tramadol. Spinal DPCPX 3 μg also inhibited the action of systemic tramadol. Spinal SB269970 (selective 5-HT₇ receptor antagonist) 3-10 μg did not alter the effect of systemic tramadol. Intraplantar tramadol produced antinociception against flinching behaviors, and this action was reversed by intraplantar DPCPX 4.5 μg administered on the ipsilateral, but not contralateral, side. Intraplantar DPCPX also reversed antinociception by systemic tramadol. These results indicate that adenosine A₁ receptors contribute to antinociception by tramadol in the mouse formalin model, and that spinal and peripheral sites are involved in these actions. 5HT₇ receptors in the spinal cord do not appear to be involved in the recruitment of A₁ receptor mechanisms when tramadol is given systemically in this model.