Assessing the Effect of Lidocaine Injection into the Nucleus Paragigantocellularislateralis on Formalin Test and Hot Plate Test Induced Nociceptive Behaviors in Rats

Background and Objectives: There have been many studies done to recognize the mechanism of controllingpain. However, there are still a lot of un-answered questions about initiatory nervous centers in expressing nociceptive behaviors and pain modulation. The nuclei ofparagigantocellularislateralis (LPGi) participate in different functions like adjustment of blood circulation, withdrawal syndrome, and pain perception. In this study, we investigated the effect of transient inactivation of the LPGi nucleus on formalin-induced nociceptive behaviors as a tonic pain model, and also the hot plate test as an acute pain model in rats. Material and Methods: A total of ۳۰ adult male Wistar rats (۲۰۰-۲۵۰ g) were used in the study. The formalin test (۵۰ μl, ۲%) was used to evaluate the effects of temporary inactivation by lidocaine into the LPGi nucleus on formalin-induced nociceptive behaviors and hot plate test (adjusted in ۵۲+۰.۱ centigrade). Animals were divided into four groups: control, sham, vehicle, treatment for the formalin test, and vehicle plus treatment for the hot-plate test. A week after the surgery, saline and lidocaine were injected into the LPGi, and the nociceptive behaviors for the formalin test and hot plate were recorded. For analysis of the data, we used t-test and ANOVA followed by theTukey post hoc test. Results: Lidocaine injection decreased nociceptive behaviors in phase ۱, interphase, and the first part of phase ۲ in the formalin test compared to the saline group. Also, in the hot plate test, lidocaine injection into the LPGinuclei caused a significant increase in the noxious response compared to the saline group in the minutes ۳۰ and ۶۰. Conclusion: Our results suggest the probability of involvement of the LPGi nucleus incausing formalin induced nociceptive behaviors and also acute pain.

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Keywords: Nucleus paragigantocellularislateralis ، Lidocaine ، Formalin test ، Hot plate test