Alterations of chick locomotion produced by morphine treatment in ovo

Analysis of walking of young chicks showed that morphine treatment of the chick embryo resulted in motor difficulties that depended on the timing of initial morphine exposure. Morphine sulfate was injected daily at a dose of 20 mg/kg egg into the air sac of viable eggs. Groups of eggs were treated starting on either day 12 or day 16 of incubation with either morphine or sterile water. Injections were continued through incubation day 19 and the eggs were allowed to hatch. The chicks started on morphine on day 16 showed signs of neuromuscular weakness at the posthatching age of 1-2 days. At 7 days of age they had a wider stride and greater angle of placement of the feet compared to both the controls, and to the chicks that were started on morphine on day 12. Since tolerance to morphine can develop in the chick embryo after only 4 daily injections, the chicks receiving morphine from day 12 may have been tolerant to the deleterious effects produced by morphine on locomotion at a critical period of motor development around day 16.