RESUMO
Visceral pain is one of the most common forms of pain which requires new therapeutical drugs. The aim of this study is to investigate the inhibitory effect of gabapentin on induced abdominal contractions and to examine the effect of its co- administration with morphine. In this study, 96 mice received acetic-acid intraperitoneally after administration of saline, gabapentin [1, 5, 10, 50 and 100 mg/ kg], morphine [0.25, 0.5, 1, 3 and 5 m/g kg] or a combination of subanalgesic dose of morphine with subanalgesic and the lowest effective dose of gabapentin. In one group naloxone [5mg/kg/i.p] was injected 20 minutes prior to the injection of acetic acid. Then the number of writhes were counted for 45 minutes. Both gabapentin and morphine reduced writhing in a dose-dependent manner. The number of writhes decreased significantly by gabapentin [50 and 100/ mg/kg] and morphine [0.5, 1, 3, 5 mg/kg] comparing to the control group [P<0.001]. Also, the sub-analgesic dose of morphine [0.25mg/kg] with sub-analgesic and low effective dose of gabapentin [50mg/kg and 10mg/kg, respectively] significantly decreased the number of writhes [P<0.005]. The combination of low effective dose of gabapentin [50mg/kg] and sub-analgesic dose of morphine decreased the number of writhings by 94% as compared to the controls [P<0.005]. The antinociceptive effect of combinational administration was not reversed by naloxone [opioid antagonist]. These data demonstrated the comparable efficacy of gabapentin [50 and 100/ mg/ kg; i.p.] with 0.5mg/kg morphine in visceral pains. Also the combination of subanalgesic doses of gabapentin and morphine, which were ineffective alone, produced significant analgesic effect in writhing model of pain. Therefore, combination of low doses of morphine and gabapentin, due to lower rate of side effects, may be clinically considered as a safer treatment in the management of visceral pains