Effects of Naloxone on Morphine Analgesia and Spinal c-fos Expression in Rat Formalin Test

Abstract

Background: This study was performed to evaluate the dose-related effects of naloxone on morphine analgesia in the rat formalin test, and observe the correlation of pain behavior and spinal c-fos expression induced by a formalin injection. Methods: Fifty rats were divided into five groups; control, morphine (morphine pre-treated, intra-peritoneal injection of 0.1 mg of morphine 5 min prior to formalin injection), and three naloxone groups, which were divided according to the administered dose-ratio of naloxone to morphine 20 : 1 (5), 10 : 1 (10), and 1 : 1 (100) representing the low-, medium-, and high-dose naloxone groups, respectively, were injected intra-peritoneally 16 min after a formalin. A fifty ul of 5% formalin was injected into the right hind paw. All rats were observed for their pain behavior according to the number of flinches during phases 1 (2 3, 5 6 min) and 2 (1 min per every 5 min from 10 to 61 min). The spinal c-fos expression was quantitatively analyzed at 1 and 2 hours after the formalin injection using a real-time PCR. Results: The morphine pre-treated (morphine and three naloxone) groups during phase 1, and the morphine, low- and medium-dose naloxone groups during phase 2, showed significantly less flinches compared to those of the control (P