Codeine analgesic and morphine hyperalgesic effects on thermal nociception in domestic fowl

The effects of codeine phosphate and morphine sulfate (2.5, 15.0, and 30 mg/ml/kg; IM) on latency of a jump response elicited by a noxious (61 degrees C) thermal stimulus were studied in White Leghorn cockerels at 15-16 days posthatch. Codeine induced a significant dose-dependent increase in jump response latency (analgesic effect), whereas morphine at each dose induced a significant decrease in jump response latency (hyperalgesic effect). Naloxone (5 mg/ml/kg) reversed the hyperalgesic effect of morphine (30 mg/ml/kg) and potentiated codeine analgesic effects. It is unlikely that codeine analgesic effects in domestic fowl reflect demethylation of codeine to morphine. These opposite codeine and morphine effects may reflect the interaction of these opiates at different populations of opioid receptors or at different substrates.