Environmental control of morphine withdrawal: Context specificity or stimulus novelty?

Past investigations of context-specific morphine withdrawal have demonstrated that when rats are tested while undrugged, withdrawal behaviors are more pronounced in an environment previously paired with drug administration than in an environment previously paired with saline administration (even in rats that have had exposure to only low doses of morphine prior to testing). In these studies, rearing is a commonly used index of morphine withdrawal. Rearing is also an exploratory behavior. The higher levels of rearing displayed in a morphine-paired, compared with a saline-paired, environment may be a manifestation of a tendency to explore this environment, rather than a tendency to display withdrawal symptoms in this environment. We evaluated the extent to which rearing and other measures of morphine withdrawal are displayed in drug-paired, saline-paired, and novel environments. The results suggest that, in some cases, apparent context specificity of morphine withdrawal actually results from novelty-elicited exploration.