Evidence in favor of such a hypothesis is given by Jurgensen et al. who have first reported the fact that an alkaloid fraction from Uncaria tomentosa (Willd.) DC, a vine used by Peruvian Indians to treat several diseases, given by the intraperitonneal route, dose-dependently suppressed the behavioral response to the chemical stimuli in the models indicated and increased latencies in the thermal stimuli models (33). The antinociception caused by the fraction in the formalin test was significantly attenuated by intraperitoneal injection of mice with ketanserin (5-HT2 receptor antagonist), but was not affected by naltrexone (opioid receptor antagonist), atropine (a nonselective muscarinic antagonist), L-arginine (precursor of nitric oxide), prazosin (^-adrenoceptor antagonist), yohimbine (a2-adrenoceptor antagonist), and reserpine (a natural monoamine depletory alkaloid from Rauvolfia serpentina [Rubiaceae]). Together, these results indicate that the fraction abrogates, dose-dependently, pain via interaction with 5-HT2 receptors.

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