Effects of Intrathecal administration of capsaicin and intraperitoneal administration of naloxone on anti-nociceptive function of heated Cannabis sativa female flowers hydroalcoholic extract

Background: The endocannabinoid system interacts with the vanilloid and opioid systems. The current study aims to investigate the effects of the extract obtained from the heated Cannabis Sativa female flower base either with capsaicin at the spinal cord level or naloxone at the systemic level on the intensity of chemical and thermal pain sensation. Methods: This experimental study was performed in the Department of Biology at Ferdowsi University of Mashhad from April 2014 to March 2015 using adult male Wistar rats (200-250 g) categorized into groups of 7 animals. In addition to the control and sham (solvent of chemicals) groups, groups with intraperitoneal administration of 50 mg/kg of the hydroalcoholic extract, 2 mg/kg of naloxone alone and naloxone together with extract were investigated. Moreover, intrathecal administration groups including the concentration of 0.01 mg/ 10 μl of extract, the concentration of           0.002 mg/10 μl of capsaicin alone and the extract together with capsaicin were evaluated. To measure the thermal pain threshold, a tail-flick test was used and to measure the chemical pain intensity, the formalin test was utilized. The obtained data were analyzed statically. Results: Intrathecal administration of the extract together with capsaicin led to a significant reduction of thermal hyperalgesia (P<0.001) and chemical hyperalgesia (P<0.001) induced by intrathecal administration of capsaicin. On the other hand, intraperitoneal administration of naloxone together with the extract did not effect on the thermal pain threshold. While the administration of naloxone increased the severity of chemical pain during the acute phase compared to the group treated with the extract alone (P<0.01). Conclusion: The phytocannabinoids of the flower extract may have inhibited capsaicin-induced hyperalgesia via cannabinoid receptors activation and the TRPV1 receptor desensitization. Naloxone administration has also been able to attenuate the analgesic effect of hydroalcoholic flower extract during the acute phase of chemical pain. Probably the extract is thought to exert part of its effect on pain through opioid receptors.