Social reinforcement as alternative to sucrose reinforcement is increased by nicotine and methylphenidate in male Fischer-344 rats

Abstract

Background: Stimulant drugs such as nicotine (NIC) and methylphenidate (MPH) are hypothesized to increase the reinforcing value of sensory stimuli, thus increasing the effectiveness of such reinforcers as alternatives to sucrose reinforcers.

Methods: Inbred Fischer-344 rats (n = 30) were assigned to three groups: saline (SAL; n = 10), nicotine (NIC; n = 10), or methylphenidate (MPH; n = 10). Testing was done in three phases: sucrose only, (SUC), sucrose and drug (SUC/DRUG), and sucrose, drug, and social reinforcement (SUC/DRUG/SOC). During the SUC phase, rats were trained on a progressive ratio 5 (PR5) reinforcement schedule for sucrose (20% solution). In the SUC/DRUG phase, animals were treated with SAL, NIC (0.4 mg/kg, n = 10 SC), or MPH (2.0 mg/kg, n = 10 IP) 30 min prior to testing. In the SUC/DRUG/SOC phase, animals continued receiving drug treatment, and social reinforcement was introduced concurrently with the sucrose reinforcer. The progressive ratio for each reinforcer ran independently of the others. Reinforcing value was measured as break point (BP), the highest number of responses resulting in a reinforcer.

Results: SAL-treated animals showed no significant change in sucrose BP. MPH-treated animals showed decreased sucrose BP in the SUC/DRUG phase, with a further reduction in the SUC/DRUG/SOC phase. NIC-treated animals decreased sucrose BP only when a social alternative was offered.

Conclusion: Both NIC and MPH reduce the sucrose BP in the presence of a social alternative. The decrease in sucrose responding, coupled with increased social responding, suggests that the social alternative acted as an effective alternative reinforcer to sucrose. From a translational perspective, these results suggest that stimulant drugs such as NIC and MPH may increase the effectiveness of treatments that use alternative social reinforcers to decrease eating.

Keywords: Behavioral economics; Operant; Progressive ratio; Social behavior.

Fig. 1

Social approach apparatus. The testing apparatus consisted of a round central test chamber with three solenoid operated sliding doors. The animals could operate the sliding doors (observation ports) by snout poking. The left and right observation ports provided access to the left and right stimulus chambers. During testing, the test rat was placed in the central test chamber and a stimulus rat was placed inside one of the stimulus chambers. Snout poking to observe the stimulus rat was a measure of social reinforcement. The opposite stimulus chamber was empty and served as a control. The central observation port provided a sucrose reinforcer. The social and sucrose reinforcers were concurrently available on separate progressive ratio 5 schedules of reinforcement. See text and Supplemental Video for a more detailed description

Fig. 2

Sucrose BP, across all phases of testing. Within subject comparisons across phases showed that SAL (open circle) had no effect on sucrose BP and that introduction of the social reinforcers also had no effect on sucrose BP in the saline group. For NIC (open triangle), within subject comparisons showed that NIC decreased sucrose BP only in the presence of a concurrently available social reinforcer. MPH (open square) decreased sucrose BP when given alone and in conjunction with a social reinforcer. Note—a number sign indicates within subject differences in comparison to the preceding phase (#p < 0.05, ##p < 0.01, ###p < 0.001). An asterisk indicates between-subject differences in comparison to saline within the same phase (**p < 0.01, ***p < 0.001)

Fig. 3

Sucrose, social, and control BP during SOC phase. BP during the concurrent social and sucrose reinforcement phase. In comparison to SAL-treated animals, NIC and MPH decreased the value of the sucrose reinforcer (gray bar) and increased the value of the social reinforcer (white bar; black bar shows control side). Note—an asterisk indicates significant between group difference (*p < 0.05, **p < 0.01, ***p < 0.001)