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Objective: To investigate the effect of intraperitoneal injection of Chlorpheniramine on formalin - induced pain and morphine analgesia.
Design: Experimental study.
Animals: Seventy - two male mice weighing between 23- 26gr.
Procedure: Animals were placed in the formalin test chambers. Interplanetary injection of formalin (20 1, 5%) with a 28-guage injection needle was performed. The durations of the licking and biting of the injected paw was measured every five mm for lh. Intraperitoneal injections of Chlorpheniramine at doses of 5, 10 and 20 mg/kg, morphine (5 mg/kg) and subcutaneous injection of naloxone (5 mg/kg) were performed. Furthermore, Chlorpheniramine was also injected (i.p., 20 mg/kg) after morphine (i.p., 5 mg/kg) and before naloxone (s.c., 5mg/kg).
Statistical analysis: One - way and repeated measures ANOVA and Duncan test.
Results: Intrapaw injection of normal saline induced a weak response only in the first five mm. Formalin injection by the same route produced a biphasic pain response (first phase: 0-5 and second phase: 20-40 mm after injection).
Intraperitoneal injection of Chlorpheniramine (5, 10 and 20 mg/kg) without any effect on first phase, suppressed the second phase of pain. Morphine (i.p. 5mg/Kg) produced
analgesia by reducing both phases of pain. Naloxone (s.c., 5mg/kg) did not change the formalin - induced pain.
Chlorpheniramine injection after morphine potentiated the morphine analgesia, but it’s injection before naloxone did not prevent the naloxone - induced hyperalgesia. Conclusion: Based on the present results it is concluded that Chlorpheniramine (H1 antagonist) produced antinociception in the second phase (inflammatory phase) of formalin - induced pain. Hence, Hi receptors may have a role in inflammatory pain. In this regard, antinociception induced by a Hi antagonist may be dependent on Opioid
system.