Background and Aim : : Many behavioral and physiological processes exhibit periodic oscillations termed biological rhythms that are differentially influenced by either central or peripheral clocks in mammals. Photoperiods plays an essential role in the synchronization of metabolism, physiology and behavior to the cyclic variations of the environment. Variation in the production of opioid receptors over a 24-h period and melatonin association as a biomarker of physiological and behavioral rhythmic changes with opioid system, is considered to contribute to photoperiodic alterations in pain perception. Despite numerous investigations on the effect of circadian rhythms on pain perception, findings related to this issue remain inconsistent. This study aimed to assess the effect of duration of different photoperiods on pain perception in rat. In this study, the possible changes in the photo period on chemical and thermal pain was investigated...

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