Effects of continuous nicotine infusion on nicotine self-administration in rats: relationship between Continuously infused and self-administered nicotine doses and serum concentrations

Rationale: The efficacy of nicotine replacement therapy (NRT) for smoking cessation is limited. One reason for this limited efficacy may be that typical serum nicotine concentrations provided by NRT do not match the peak arterial nicotine concentrations achieved from smoking. Objective: The purpose of the present study was to determine whether continuous nicotine infusion at a rate producing serum nicotine concentrations that match the estimated peak arterial nicotine concentrations associated with nicotine self-administration (NSA) in rats produces greater suppression of NSA than lower infusion rates. Methods: The effects of continuous nicotine infusion were studied by intravenously administering nicotine at various rates (1.0, 3.0, and 8.0 mg/kg per day) to rats concurrently self-administering nicotine (0.03 mg/ kg per infusion) during 23-h sessions or cocaine (0.17 mg/ kg per infusion) during 2-h sessions. Results: Continuous nicotine infusion suppressed NSA in a rate-related fashion. NSA was suppressed by 17, 50, and 73% at infusion rates of 1.0, 3.0 and 8.0 mg/kg per day, respectively. The 8.0-mg/kg per day infusion rate, which provided venous serum nicotine concentrations equaling the peak arterial concentrations associated with NSA, suppressed NSA to a greater extent than lower infusion rates. The 8.0-mg/kg per day nicotine infusion rate had no effect on cocaine-maintained responding, demonstrating that its effects were specific for suppression of NSA. This infusion rate provided a mean percentage replacement of nicotine from NSA of more than 700%. Reacquisition of NSA after suppression by the two highest infusion rates was delayed compared with reacquisition after saline extinction. Conclusions: Continuous nicotine infusion produced an infusion rate-related suppression of NSA that was greatest when the infusion provided nicotine doses and venous serum concentrations substantially higher than those typically associated with NRT in humans.