Effect of curcumin on morphine-induced antinociception in acute corneal pain in rats

In the present study, the effects of the acute and chronic oral administrations (po) of curcumin in the absence and presence of morphine and naloxone was investigated on the sensation of acute corneal pain in rats. Acute corneal pain was induced by the local application o fhypertonic saline (5 M NaCl) on the corneal surface, and the number of eye wipes was then counted for 30 s. Subcutaneous (sc) injections of morphine (1, 2 and 4 mg/kg) significantly suppressed corneal pain (p < 0.05). Naloxone (2 mg/kg, sc) did not change the intensity of pain when used alone, but pretreatment of the rates with naloxone (2 mg/kg, sc) significantly prevented the antinociceptive effect induced by morphine (2 mg/kg, sc; p < 0.05). The short-term and long-term oral administration of curcumin (either a single dose of 200 mg/kg or dosages of 6.25, 12.5 of 25 mg/kg for 15 days each, respectively) both significantly decreased the number of eye wipes (p < 0.05). The antinociceptive effect induced by morphine was significantly (p < 0.05) enhanced by both the acute (200 mg/kg, once, po) and chronic (25 mg/kg, 15 days, po) curcumin treatments. Naloxone (2 mg/kg, sc) did not change the antinociception that was induced by acute (200 mg/kg, once, po) and chronic (25 mg/kg, 15 days, po) treatments of curcumin. The present findings suggest that morphine produced an analgesic effect through a naloxone-sensitive mechanism in acute corneal pain. Both high-dose acute and lower-dose chronic oral administrations of curcumin suppressed corneal pain. Moreover, curcumin enhanced morphine-induced antinociception.