The role of left and right amygdala on unilateral pain induced by formalin test in rats

Pain is one of the most important processes in the body and it is controlled by many pathways in CNS. Many supraspinal nuclei are involved in the modulation of nociception. Amygdala is a neural structure that in addition to being a key region involved in the modulation of emotional and defensive responses; it is also associated with pain modulation. The central nucleus of the amygdala (CeA) has been called the “nociceptive amygdala”. In this study 60 male wistar rats (280+30g) in six groups (3 test groups and 3 control groups) were used. In test groups, 0.5 microliter lidocaine was injected in the left, right or both amygdala to make local anesthesia. In control groups 0.5 microliter of normal saline were injected as test groups. After 15 minutes 50 microliter of 2.5% of formalin were injected subcutaneously to right hind paw of rats and nociceptive score were detected every 15 seconds for one hour. The results showed that after formalin induced pain in the right paw; the local anesthesia in left amygdala (ipsilateral) can increase nociceptive score more than right amygdala. ‍ The anesthetic effect of lidocaine injection in both amygdala increased the nociception, more than the left or right nucleus alone. It is also showed that ipsilateral amygdala has more control on pain, than the left one in the unilateral induced pain by formalin test.