McNally and his colleagues conducted four experiments with rats, injecting them with either morphine or saline solution every day for 10 days. Then, either one or seven days after the final injection, they gently restrained each rat for 30 minutes as a form of stress.
The team then measured the rats' biological responses to the restraint stress. They also studied behaviors that reflect anxiety, checking the rats' levels of social interaction and general activity. The researchers tested anxiety responses for three different dose levels and different durations of exposure (0, 1, 5 or 10 days).
In the absence of stress, the opiate-treated rats were exactly the same as the control rats. Only when the animals were exposed to a stressor were there marked differences in nervous-system and behavioral responses. For example, in terms of anxiety, the impact of stress was twice as great for the morphine-treated rats as for the saline-treated rats. Whereas stress reduced social interaction by about 31 percent in the saline-treated animals, it reduced social interaction by 68 percent in the morphine-treated animals.
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