The effect of oxycodone on thermal hyperalgesia in streptozotocin-induced diabetic mice was examined. The antinociceptive response was assessed by recording the latency in the tail-flick test using the radiant heat from a 50-W projection bulb on the tail. The tail-flick latency in diabetic mice was significantly shorter than that in non-diabetic mice. When diabetic mice were treated with oxycodone (5 mg/kg, s.c.), the tail-flick latency in diabetic mice was prolonged to the level considerably longer than the baseline latencies of non-diabetic mice. However, s.c. administration of morphine (5 mg/kg) did not produce a significant inhibition of the tail-flick response in diabetic mice. Oxycodone, at doses of 1.25-5.0 mg/kg administered s.c., produced a dose-dependent increase in the tail-flick latencies in both diabetic and non-diabetic mice. The antinociceptive effect of oxycodone was antagonized by pretreatment with a selective δ-opioid receptor antagonist, β-funaltrexamine (20 mg/kg, s.c.), in both non-diabetic and diabetic mice. In non-diabetic mice, pretreatment with a selective κ-opioid receptor antagonist, nor-binaltorphimine (20 mg/kg, s.c.) had no effect on the peak antinociceptive effect of oxycodone observed 30 min after administration, however, it slightly but significantly reduced oxycodone-induced antinociception observed 60 and 90 min after administration. On the other hand, pretreatment with nor-binaltorphimine practically abolished the peak (30 min) and persistent (60 and 90 min) antinociceptive effects of oxycodone in diabetic mice. Naltrindole (35 mg/kg, s.c.), a selective δ-opioid receptor antagonist, had no effects on the antinociceptive effect of oxycodone in both non-diabetic and diabetic mice. These results suggest that the antinociceptive effects of oxycodone may be mediated by μ- and κ-opioid receptors in diabetic mice, whereas it may interact primarily with μ-opioid receptors in non-diabetic mice.
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