Strain-dependent morphine-induced analgesic and hyperalgesic effects on thermal nociception in domestic fowl (Gallus gallus).

The effects of morphine (30 mg/kg), naloxone (5 mg/kg), and morphine with naloxone on distress vocalizations and thermal nociception were examined in different strains of domestic fowl. Naloxone by itself did not significantly affect vocalizations or thermal nociception. Morphine produced a naloxone-reversible attenuation of vocalizations that was not strain dependent. Morphine produced a strain-dependent analgesic (Rhode Island Red strain) and hyperalgesic (White Leghorn and Cal-White strains) effect on tests of thermal nociception. Both the analgesic and hyperalgesic effects were reversed by naloxone. These opposite effects on thermal nociception may reflect the effects of selective breeding on opioid receptor subtype.