The reinforcement enhancing effects of nicotine have been postulated as a major contributor in tobacco addiction. Previous work has suggested that females may be reinforced more by the sensory elements of smoking than males. Differential sensitivity to sensory reinforcement and/or the value enhancing effects of nicotine may contribute to sex differences in rates of smoking and successful cessation. The present study investigated the possibility of sex differences in the baseline value of sensory reinforcement and in the value enhancing effects of nicotine and the smoking cessation aid bupropion (Zyban). Male and female rats (n=12/sex) were trained to lever press for brief changes in chamber illumination. The effects of nicotine (0.4 mg/kg), bupropion (10 and 20 mg/kg), and saline on lever pressing were assessed across a range of response requirements. Changes in reinforcement value were also quantified using the reinforcement demand model proposed by Hursh and Silberberg (2008). Responding maintained by the sensory stimulus was greater in females than males under all conditions. Nicotine and bupropion enhanced operant responding, and this enhancement was greater in females than males. The present findings suggest that sensory reinforcement and reinforcement enhancement by nicotine may be greater contributors to tobacco addiction in females than in males.
Some recent theories about drug dependence attribute a core role of Pavlovian stimulus-stimulus (S-S) contingencies in maintaining drug consumption and drug seeking. According to this view, conditioned stimuli to the drug elicits responses that increased the frequency of drug-reinforced responses by a process called Pavlovian to instrumental transfer. On the other hand, response-stimulus (R-S) contingencies are not considered key determinants of the addict behavior. However, standard experimental procedures to study drug-seeking behavior consists on pairing stimulus to drug delivery under some schedule of reinforcement, therefore, they do not allow to analyze Pavlovian and operant processes in drug seeking. We present data from two experiments that evaluate drug-seeking behavior after discrimination or Pavlovian conditioning. On both experiments, rats were allocated to one of three groups: operant (O), Pavlovian (P) and control (C). First, rats from group O received infusions of morphine (0.75 mg/kg) after turn a wheel under FR2 when a light was on (S+), but not when was off (S-). P and C groups were yoked to O group, so when S+ was presented, P received an infusion of morphine and C of saline. No operandum was available for P and C Group. Then, all rats were trained to press a bar for morphine under a FR5 schedule in the absence of both stimuli (Experiment 1) or in the absence of S+ (Experiment 2). Finally, S+ and S- were reintroduced, and bar presses were measure under extinction. The results showed that just animals from O group obtained discrimination indexes values of 80% or more at Phase 3, indicating that stimulus control of drug seeking behavior was established under the discrimination training, but no under the Pavlovian training. Results are discussed regarding other experimental data that indicate that drug dependence develops after self-administration of a drug but not under its hetero-administration. Implications for actual suppositions about drug-consumption and drug-seeking behavior are also discussed.