Development of opiate tolerance in the chick embryo.

Tolerance to morphine was produced in the chick embryo. Eggs were injected with morphine sulfate (MS) (20 mg/kg egg) or H2O daily starting on incubation day 12. On day 16, embryo activities were recorded and eggs were injected with either MS or naloxone. Activity of H2O-pretreated controls decreased after both MS and naloxone. Embryos treated with MS from incubation days 12-15 showed no activity change after morphine and responded to naloxone with increases in activity. Baseline rates of distress vocalizations (DV) of 1-2 day old chicks were not affected by MS pretreatment during incubation days 12-19. However, 1 mg/kg MS decreased the rate of DV of control chicks by 90% whereas MS-pretreated chicks were unaffected. At age 4-5 days, the baseline rate of DV and rate after MS were higher in MS-pretreated chicks. However, all chicks showed significant decreases in rate of DV after MS injection. Naloxone increased the rate of DV of paired 1-2 day old chicks, but response of MS-pretreated chicks was significantly greater than controls.