Anti-nociceptive Activity of L-citrulline in Mice Using Formalin and Hot Plate Tests: Possible Mechanism of Action

Background: Peripheral pain regulation is a very complex phenomenon due to the numerous neural pathways responsible for it. Objectives: The current study aimed to determine the anti-nociceptive activity of L-citrulline and the possible role of opioidergic, nitric oxide (NO), and serotoninergic systems in mice using the formalin and hot plate tests. Methods: In this study, ۳۰۰ male NMRI mice were divided into ۲ groups: ۱۵۰ mice were used for the formalin test and ۱۵۰ for the hot plate test (tests ۱-۶) with ۴ sub-groups in each (n=۵۰). The formalin test determined pain caused by the injection of formalin in the hind paw, and the hot plate test recorded pain reactions caused by heat stimulation as response latency time. Further, time mice capable of staying on the rotarod bar were determined. Results: Morphine reduced licking and biting time and latency time in the hot plate (P<۰.۰۵), while L-citrulline (۵۰ and ۱۰۰ mg/kg) decreased pain response and increased latency time compared to the control (P<۰.۰۵). Further, pre-treatment with naloxone following L-citrulline increased licking and biting time in the formalin test and decreased latency time in the hot plate test (P<۰.۰۵). In addition, pretreatment with L-NAME following L-citrulline diminished licking and biting time in the formalin test and increased latency time in the hot plate test (P<۰.۰۵). Likewise, ad pre-treatment with ritanserin following L-citrulline reduced licking and biting time and latency time compared to control mice (P<۰.۰۵). Similarly, licking and biting time and latency time were decreased by pre-treatment with ondansetron following L-citrulline. Finally, no significant disturbance was observed in motor coordination by L-citrulline (۲۵, ۵۰, and ۱۰۰ mg/kg) at P>۰.۰۵. Conclusion: It seems that L-citrulline has anti-nociceptive effects, and its role is mediated by opioidergic, NO systems, as well as ۵-HT۲ and ۵-HT۳ receptors. Further, L-citrulline did not disturb motor coordination.