Newby-Schmidt M B, Norton S
Pharmacol Biochem Behav. 1983 May;18(5):817-20. doi: 10.1016/0091-3057(83)90028-x.
Morphine tolerance and dependence were produced in chick embryos by injecting 20 mg/kg into the air space of the egg daily from incubation day 12. Starting on day 16, two groups of eggs were withdrawn from morphine by either substituting water or by treating with naloxone. Chick embryo activities were monitored on incubation day 18. Activities of the embryos withdrawn from morphine did not differ from control, but naloxone injection produced a substantial increase in activity. Neither regimen of morphine withdrawal affected the hatchability of the eggs. When these chicks were 1 day old, the effect of 1 mg morphine/kg on the rate of distress vocalization was measured. The withdrawn chicks were 1 day old, the effect of 1 mg morphine/kg on the rate of distress vocalization was measured. The withdrawn chicks vocalized at a rate that was intermediate between that of controls and that of nonwithdrawn chicks indicating that tolerance was still present one week after the last morphine injection.
