The purpose of this study was to determine the significance of chronic opioid exposure on the level of heroin self-administration in the rat. Rats were divided into morphine (M, subcutaneous morphine pellets) and placebo (P, subcutaneous placebo pellets) groups and self-administered several different doses of heroin during daily limited access 1-h sessions and prolonged access 8-h sessions. No effects on heroin self-administration occurred when the rats were implanted with morphine pellets and allowed to self-administer heroin in a limited access paradigm (1-h group). However, rats with morphine pellet implantation showed a rapid escalation (Days 0-3 post-pellet) in heroin self-administration in the more prolonged access group (8 h group) compared to placebo-pelleted animals also with 8-h access. Ultimately, placebo-pelleted 8-h exposed animals showed an escalation in heroin self-administration but this effect was delayed until Days 16-18 post-pellet. These results suggest that passive administration of morphine sufficient to produce and maintain dependence facilitates escalation in heroin intake.
