The effect of light-dark cycle alteration on thermal and chemical pain in the male rat

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.