Regulation of intravenous cocaine self-administration in rats selectively bred for high (HiS) and low (LoS) saccharin intake.

Rationale: Rats selectively bred for high saccharin (HiS) intake consume more alcohol and acquire intravenous (i.v.) cocaine self-administration more rapidly than their low saccharin (LoS)-consuming counterparts. The present experiment was designed to determine whether HiS and LoS rats differed in other aspects of drug abuse.
Objective: The purpose of the present experiment was to use a two-lever dose self-selection procedure to investigate the regulation/dysregulation of i.v. cocaine self-administration in female HiS and LoS rats.
Materials and Methods: HiS and LoS rats were trained to self-administer eight different doses of cocaine during daily 5-h sessions, with the cocaine doses ranging from 0.2 to 1.6 mg/kg in steps of 0.2 mg/kg. The dose size increased after a response on one lever (infusion duration lengthened by 3 s) and decreased after a response on the other lever (infusion duration shortened by 3 s), with a lower limit of 0 s and with an upper limit of 24 s (a corresponding range of 0 to 1.6 mg/kg); the animals increased or decreased their self-administered dose in nine discrete steps.
Results: The HiS rats showed less precise regulation of their postinfusion interval (PII) than LoS rats based on the size of the previously self-administered cocaine dose. Correlations between these variables were lower for HiS than for LoS rats during the acquisition and maintenance phases, and HiS rats had lower PIIs than LoS rats after many of the cocaine doses. The HiS rats also self-administered significantly more cocaine infusions during the maintenance phase than the LoS rats, especially at the highest dose.
Conclusions: These data indicate that HiS rats are more likely to self-administer more cocaine infusions, at higher doses, and with less precise dose-time regulation than LoS rats. Thus, rats selectively bred for HiS showed less inhibitory control over their cocaine intake than LoS rats, suggesting that a genetic predisposition for saccharin preference is related to the rewarding effects of i.v. cocaine.