Persistent increase in the motivation to take heroin in rats with a history of drug escalation

The transition from stable to escalated levels of cocaine self administration has been shown to depend upon drug availability. The generality of this phenomenon is assessed here by studying the effects of availability on heroin self-administration. Two groups of rats were trained on a 1-h continuous schedule of self-administration, after which, access to heroin (40 mu g/injection) was increased to 11 h in one group (long access or LgA mts) or kept to I h in the other group (short access or ShA mts). After 18 sessions on this regimen, both ShA and LgA rats were tested for extinction and stress-induced reinstatement of heroin-seeking behavior. In LgA mts, both total and first hour intake gradually escalated over time. After escalation, LgA rats were slower to extinguish heroin-seeking behavior and responded more to the reinstating effect of stress after extinction. These findings show that: (1) the escalation process in drug consumption is common to both opiate and stimulant self-administration; (2) escalation in heroin consumption is associated with a persistent increase in the motivation for taking heroin. (C) 2000 American College of Neuropsychopharmacology. Published by Elsevier Science Inc.