Intermittent access training produces greater motivation for a non-drug reinforcer than long access training

Abstract

It has recently been proposed that the intermittent access (IntA) drug self-administration procedure better produces behavioral changes relevant to addiction than the long access (LgA) procedure. In this version of the IntA procedure, the drug is made available for a 5-min period during each half hour of a 6-h session. In contrast, on the LgA procedure, the drug is available continuously for 6 h. Previous studies have found that IntA drug self-administration produces greater drug motivation, measured by increased progressive ratio breakpoints, than LgA self-administration. It has been hypothesized that this effect is due to the rapid, "spiking" brain levels of the drug, and consequent neuroadaptations, experienced by rats during IntA sessions. However, no study has compared the effects of IntA versus LgA training on reinforcer motivation when using a non-drug reinforcer. The present study compared motivation for a saccharin reinforcer after IntA or LgA training. In Experiment 1, separate groups of rats lever-pressed for saccharin on the IntA or LgA procedures. In Experiment 2, a within-subjects design was used where rats pressed one lever on the IntA procedure and another lever on the LgA procedure for saccharin. In both experiments, IntA training produced greater breakpoints than LgA training. As no drug was used here, spiking drug levels could not have been responsible for the increased saccharin motivation observed after IntA training. Instead, it is proposed that differences in stimulus-reinforcer associations learned during IntA versus LgA training may be responsible for the effect. Future research is needed to determine the extent to which such learning factors may contribute to the increased motivation observed after IntA training with drug reinforcers.

Keywords: Animal models of addiction; Behavioral momentum; Intermittent access; Long access; Saccharin.

Fig. 1

(a) Mean (± SEM) reinforcers earned per session during the ten long access (LgA) or intermittent access (IntA) training sessions of Experiment 1. (b) Mean (± SEM) percentage change in reinforcers obtained from session 1 to session 10 in the LgA and IntA groups. (c) Mean (± SEM) rate of reinforcement during lever insertion for the LgA and IntA groups averaged over the final three LgA or IntA sessions. ** indicates p < 0.01, * indicates p < 0.05

Fig. 2

Representative cumulative records from the final training session from two rats in the long access (LgA) group (top panels) and two rats in the intermittent access (IntA) group (bottom panels) in Experiment 1. Slashmarks indicate when saccharin reinforcers were earned

Fig. 3

(a) Mean (± SEM) reinforcers earned during each hour on the first and last intermittent access (IntA) or long access (LgA) training session. (b) Mean (± SEM) reinforcers earned per 10-min bin of the first hour of the session (first and last training session) for the LgA group

Fig. 4

(a) Mean (± SEM) breakpoint on the progressive ratio test for the long access (LgA) and intermittent access (IntA) groups in Experiment 1. (b) Mean (± SEM) response rates observed in the LgA and IntA groups during the ratios corresponding to the first nine reinforcers of the test. The numbers in parentheses on the x-axis indicate the ratio in effect for a particular reinforcer number during the test. (c) and (d) Scatterplots showing relation between last training session responses and breakpoint on the progressive ratio test for the LgA and IntA groups, respectively. Note that the x- and y-axes scales differ over panels c and d. * indicates p < 0.05

Fig. 5

(a) Mean (± SEM) reinforcers earned per session during the final two long access (LgA) and intermittent access (IntA) sessions in Experiment 2. (b) Mean (± SEM) rate of reinforcement during lever insertion for the LgA and IntA levers averaged over the final two LgA or IntA sessions. *** indicates p < 0.001

Fig. 6

Representative cumulative records from the final long access (LgA) and intermittent access (IntA) training sessions for two rats in Experiment 2

Fig. 7

Mean (± SEM) breakpoint on the long access (LgA) and intermittent access (IntA) progressive ratio tests in Experiment 2. (b) Mean (± SEM) response rates observed on the LgA and IntA levers during the ratios corresponding to the first seven reinforcers of the test. The numbers in parentheses on the x-axis indicate the ratio in effect for a particular reinforcer number during the test. (c) and (d) Scatterplots showing relation between last training session responses and breakpoint on the progressive ratio test for the LgA and IntA groups, respectively. Note that the x-axes scales differ over panels c and d. * indicates p < 0.05