Tehran University Medical Journal

Background: Animals have an internal biological clock with melatonin hormone that helps them to adapt to light/dark circles. Since melatonin is associated with an alteration in the expression and production of opioid receptors, this study aimed to evaluate the effect of changes in the light/dark circles on pain sensation in rats.
Methods: This research study in order to investigate the thermal and chemical pain sensation using tail flick and formalin tests, 35 Wistar rats were randomly divided into five groups of seven animals, including 24 hours of light (24L), 16 hours of light / 8 hours of darkness (16L/8D), 12 hours of light / 12 hours of darkness (control), 8 hours of light / 16 hours of darkness (8L/16D) and 24 hours of darkness (24D) were tested. The study was conducted at the Department of Biology of Ferdowsi University of Mashhad, Iran, from April to September 2015. Also besides the Rotarod test was performed to determine the general motor activity of animals.
Results: In the tail flick test, an increase in the time of darkness elevated the threshold of thermal pain and subsequently resulted in analgesic effect in the 24 hours of darkness (24D) group (P=0.03), while reducing the dark period in the group of 16 hours of brightness / 8 hours of darkness caused a reduction in the threshold of thermal pain, resulting in hyperalgesia (P=0.002). In the formalin test, the chemical pain score at the end of the chronic phase was significantly increased in the experimental group of 16 hours of brightness / 8 hours of darkness compared to control, indicating hyperalgesia (P=0.03).
Conclusion: Perhaps, alterations in light duration may change the production of melatonin and opioids and their receptors. Therefore, it is expected that reduction of the duration of darkness and thus shortening the period of increased production of melatonin and the subsequent lower expression of opioid receptors, in this group, resulting in a lower thermal pain threshold and analgesic response.

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.