“Unintended (positive) consequences”: Response-independent reinforcement reduces delay discounting during a test of delay-exposure and maturation effects

Delay Discounting (DD) refers to the tendency for behavioral consequences (rewards or punishers) to be devalued as they occur more remotely in the future. DD has become a fruitful model of impulsive behavior and thus there is growing interest in developing interventions that could reduce it. Delay Exposure (DE) training has shown promising results in rodents. It consists of rats pressing a lever to produce a cue stimulus for a 17.5 s interval, at the end of which a reinforcer is delivered (Renda et al., 2021). The present study aimed to address some limitations in DE research: extensive number of training sessions required (60 or more), exclusive use of group designs, and potential confounding maturation effects. Using a within-subjects design, a variation of DE and its standard control condition (Immediate Exposure-IE) were evaluated in DD of rats tested in adolescence and adulthood. All DE and IE standard aspects remained the same except that the delay cues and reinforcers were response independent due to equipment limitations that prevented the use of a third operandum. Eighteen male Sprague-Dawley rats were daily exposed to a delay-discounting task (DDT, Liley et al., 2019) until steady responding was obtained at late adolescence (starting postnatal week – PW– 14, Ghasemi et al., 2021) and early adulthood (PW 25-27). Eight of the 18 rats (Main group) continued daily DD assessment plus different experimental manipulations added to the DDT across six conditions. The remaining 10 rats were kept as a Waitlist control group (no DD assessment/treatment between PW 28-42). The first manipulation added to the DDT controlled the additional access to pellets and time spent in the operant chamber entailed in subsequent DE/IE conditions. This 60Pellets condition consisted of 1-hr pre-DDT-session response-independent access to the same number of reinforcers delivered during each session of the DE/IE manipulations (i.e., 60 pellets). The third and fifth conditions consisted of DE and IE (counterbalanced across half of the animals) with 60Pellets reversals interspersed between them (fourth and sixth conditions). Based on subjects’ performance during the first six conditions, a second control manipulation was introduced. This Wait condition entailed rats waiting in the operant chamber during the 60-min before the daily DDT session without operanda, delay cue, or pellets. All 18 rats, including waitlist, were exposed to a DDT retest during PW 43-49 (i.e., middle adulthood). Waitlist rats were then exposed to 60Pellets and DE conditions between PW 46-51. Unexpectedly, we found that exposure to the 60Pellets condition during early adulthood produced significant reductions in DD, with an average of 16 sessions needed to produce steady low DD. These rats maintained low DD levels throughout the remaining conditions of the experiment, regardless the manipulation, including DE/IE and two reversals to 60Pellets. DD of Waitlist rats did not change between early- and middle-adulthood tests separated by ~20 weeks, and not significantly decreased under 60Pellets and DE conditions. These preliminary and promising findings suggest early- but not middle-adulthood response-independent access to reinforcement reduces impulsive choice long-term with a relatively brief intervention. The specific contribution of response-independent DE/IE, extensive and repetitive DD assessment, and maturation on this effect is unclear. Further efforts aimed at reproducing these effects and identifying the processes involved are warranted.