Tramadol and Tramadol+Caffeine Synergism in the Rat Formalin Test Are Mediated by Central Opioid and Serotonergic Mechanisms

Figure 2

Antinociceptive effects of 3.2 mg/kg of caffeine (CAF), 8.8 and 27.8 mg/kg of tramadol (TRAM), and 8.8 + 3.2 mg/kg of tramadol and caffeine, respectively (TRAM+CAF). Antinociception was expressed as the percent of maximum possible effect (%MPE) in the second phase of the formalin test. Data are presented as mean ± SEM of six rats. () combination-treated group versus tramadol or caffeine alone.