Interactions between opioids and cocaine on locomotor activity in rats: influence of an opioid's relative efficacy at the mu receptor.

Rationale: Cocaine and mu opioid agonists increase central dopamine concentrations and produce robust interactions at both neurochemical and behavioral levels. Although the interactions between cocaine and high-efficacy mu opioids have been well characterized, the interactions between cocaine and lower efficacy opioids have not been as extensively examined.

Objective: The purpose of this study was to examine the interactions between cocaine and opioids possessing a range of relative efficacy at the mu receptor.