Peripheral and preemptive opioid antinociception in a mouse visceral pain model

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Abstract
Recent studies suggest that opioids can produce analgesia through peripheral mechanisms following inflammation of peripheral tissue. This study examined whether opioids administered prior to inflammation can produce antinociception by peripheral mechanisms in a model of visceral pain. Mice were injected intraperitoneally (i.p.) with 1% acetic acid to evoke abdominal writhing, a standard model of visceral pain. The number of writhes that occurred during 30 min after acetic acid were determined. Intraperitoneal injection of morphine sulfate (60, 90, 100 or 120 μg/0.3 ml) or the peripherally acting opioid loperamide (0.12, 0.36, 1.2 or 3.6 mg/0.3 ml) given 5 min after acetic acid decreased writhing in a dose-dependent fashion. Morphine (100 μg) produced an 70% attenuation in the number of writhes while loperamide (1.2 mg) decreased writhing by 56%. These antinociceptive effects were blocked by pretreatment with the opioid receptor antagonists naloxone (10 mg/kg) and its quarternary version naloxone methiodide (10 mg/kg). To determine whether opioids produced preemptive antinociception via peripheral mechanisms, mice received i.p. injections of morphine (1, 5, and 10 μg/0.3 ml) or vehicle 5 min before acetic acid. Doses of 5 and 10 μg morphine inhibited the number of writhes by 51 and 93%, respectively. The highest dose (10 μg) was ineffective when given intravenously 5 min before acetic acid, suggesting that antinociception following i.p. administration was acting via peripheral mechanisms. These data demonstrate that low doses of opioids, given before or after acetic acid, produce visceral antinociception through peripheral mechanisms. This may be clinically relevant for the management of postoperative abdominal pain.