Motivation of heroin-seeking elicited by drug-associated cues is related to total amount of heroin exposure during self-administration in rats.

Conditioned stimuli (CSs) previously associated with heroin are critically involved in activating long-lasting relapse and compulsive drug seeking. This study examined the magnitude of heroin seeking induced by drug-related cues in relation to the total amount of drug exposure during training. Five groups of male Sprague-Dawley rats (n=6/group) were trained by the nose-poking response to self-administer different doses of heroin (0, 0.01, 0.025, 0.05, and 0.1 mg/kg per infusion respectively, one 4-h session daily, limited to 25 infusions per session) under an identical progressive ratio schedule with gradual incremental response requirements. All the rats established stable heroin self-administration within 14 days of self-administration training, and the time needed to obtain all the 25 heroin infusions decreased across sessions. After 14 days of abstinence, heroin seeking elicited by contextual cues (self-administration chamber) or discrete contingent CSs previously associated with heroin infusions was measured in two consecutive 1-h test phases. During both test phases, the rats trained with heroin even at the lowest dose (0.01 mg/kg) showed higher active responses than saline controls, and the active responses were also higher in rats trained with doses of 0.025, 0.05, and 0.10 mg/kg in comparison with those trained with a dose of 0.01 mg/kg. There was no observable dose-dependence increase of responses at doses above 0.025 mg/kg. The results suggested that an increased motivation to seek heroin induced by drug-related cues is associated with the total amount of heroin intake.