Morphine-induced analgesia subsequent to formalin injection in female Wistar rat

Background and Objectives: Previous studies have shown the antinociceptive effect of morphine in animal models, but the specific anti-pain consequence of the abuse drug in female animals is unclear. The present research showed the morphine antinociception in female Wistar rats using formalin test.
Materials and Methods: Subjects were 40 female Wistar rats purchased from Pasteur Institute of Iran. They were treated intraperitoneally (i.p.) with morphine (3-12 mg/kg) 10 min before of the formalin test. In order to induce the chemical pain, the rats received intraplantar injection of formalin (2.5% / 50 μL) and given the test. The control rats received saline solution (1 mL/kg, i.p.) instead of the morphine. The nociceptive response was divided into two phases: phase acute (0-15 min) and phase chronic (15-60 min). The animals were graded by a 4-point scale each 15 sec continuously throughout the test.
Results: This study revealed the morphine-induced antinociception at the early as well as the late phases of the formalin test. Moreover, the response to the lower doses of morphine in the early phase was considerable in comparison to the late phase.
Conclusion: The data indicate a definite morphine antinociception role in female Wistar rats. The female rats may exhibit less anti-pain effect of morphine in the late phase of the chemical pain unless if receiving the effective doses of the drug.