Conditioned withdrawal drives heroin consumption and decreases reward sensitivity

Aspects of drug withdrawal may become conditioned to previously neutral environmental stimuli via classical conditioning processes. Nevertheless, the significance of conditioned withdrawal effects in motivating drug intake remains largely unexplored. Here, we investigated the effects of conditioned withdrawal in modulating heroin consumption and brain reward sensitivity in rats. Rats intravenously self-administered heroin (20 microg/infusion) during 0 h (control), 1 h (nondependent), or 23 h (dependent) sessions and had daily intracranial self-stimulation (ICSS) thresholds assessed. ICSS thresholds remained stable and unaltered in control rats. In nondependent rats, heroin self-administration induced a transient activation of reward systems, reflected in lowering of ICSS thresholds. In dependent rats, heroin intake escalated across sessions and was associated with a gradual decrease in reward sensitivity, reflected in progressively elevated ICSS thresholds. Thus, as dependence develops, heroin may be consumed not only for its acute reward-facilitating effects, but also to counter persistent deficits in reward sensitivity. In nondependent rats, the opioid receptor antagonist naloxone (30 microg/kg) increased heroin consumption and reversed heroin-induced lowering of ICSS thresholds, effects resistant to classical conditioning. In contrast, in dependent rats naloxone (30 microg/kg) increased heroin consumption and also elevated ICSS thresholds above their already elevated baseline levels (i.e., precipitated withdrawal). Most importantly, stimuli repeatedly paired with naloxone-precipitated withdrawal provoked heroin consumption and elevated ICSS thresholds in dependent rats. Thus, conditioned stimuli predicting the onset of heroin withdrawal, and hence the reward deficits coupled with this state, may play a critical role in provoking craving and relapse in human opiate addicts.