Naloxone inhibits gastric emptying in the rat

Naloxone is generally considered to be a pure antagonist, but it may produc e several behavioral effects, such as hyperalgesia or stimulation of respir ation. We studied the effect of naloxone on gastric emptying and gastrointe stinal transit in rats. Six to eight Wistar rats (200 - 250 g) were used fo r each experiment. Either saline or naloxone (0.01-10 mg/kg) was injected i ntraperitoneally at 0 min. At 30 min, radiolabeled saline or milk 1 mi, was infused into the stomach. At 60 min, gastric emptying and gastrointestinal transit were calculated by measuring the radioactivity in the gastrointest inal tract. Naloxone significantly inhibited gastric emptying of saline (P = 0.002) and of milk (P < 0.05), but not the gastrointestinal transit of ei ther (P > 0.05). Gastric emptying of saline showed a significant peak (P < 0.05) in the dose-response curve at 0.7 mg/kg. Therefore, naloxone signific antly inhibits gastric emptying of saline and milk, but not the gastrointes tinal transit of either. Implications: Although naloxone is generally consi dered to be a pure opioid receptor antagonist, it delays gastric emptying o f saline or milk, as does morphine in the rat. However, it is uncertain fro m our results whether naloxone inhibited gastric emptying by antagonizing t he effects of endogenous opioids.