Objective: To investigate the peripheral effect of cimetidine on formalin induced pain and morphine analgesia. Design: Experimental study.
Animals: A total of 72 male mice weighing between 23-26 gr.
Procedure: The animals placed in the formalin test chambers. Intraperitoneal (IP).injections of cimetidine at dose rates of 5, 10 and 20 mg/Kg, morphine (5 mg/Kg) and cimetidine (20 mg/Kg) before morphine (5 mg/Kg), Subcutaneus(SC). injection of naloxone
(5 mg/Kg) alone or before (IP). injectionof cimetidine(20 mg/Kg) were performed. Following intraplantar injection of formalin
(20 µ1,5%) with a 28-guage syringe, the paw licking and biting durations as pain response were measured at five minutes intervals for total 1 hour.
Statistical analysis: Paired t-test, one way and repeated measures ANaYA and duncan test.
Results: Intrapaw injection of normal saline induced pain at 0-5 minutes after injection. Formalin injection by the same route produced biphasic pain response (first phase: 0-5 and second phase: 20-40 min after injection)(IP). injection of cimetidine at dose rates of 5 and 10 mg/Kg had no effect, and cimetidine (20 mg/Kg) without any effect on first phase, reduced the second phase of pain (IP). injection of morphine
produced analgesia by reducing the first and second phases and total 1 h of pain response. Cimetidine (20 mg/Kg) injection before morphine (5 mg/Kg) had no effect on morphine analgesia injection (SC) of naloxone (5 mg/Kg) didn't changed the formalin induced pain phases. Cimetidine injection after naloxone without any effect on first phase, reduced the second phase of noloxane hyperalgesia.
Clinical implication :By the present study it is concluded that cimetidine (histamine Hz antagonist) produced an analgesic effect on inflammatory but not on neurogenic phase of formalin-induced pain. Then, histamine Hz receptor may be involved in flammatory pain. It seems that in pain
mechamisms, there may be not a relationship between Hz receptor and opiate system.J.Fac. Vet.Med.Univ.Tehran. 59,4:373-378,2004