Drug addiction is characterized by compulsive drug seeking despite negative consequences. Research has focused on the reward system, but understanding the inability to cease drug use despite adverse outcomes is crucial. Animal studies often use punishment (e.g., electric shocks) paired with reward-seeking actions to investigate insensitivity to negative outcomes. Differences in sensitivity to punishment are linked to the transition from controlled to uncontrolled drug use. However, the temporal aspect of punishment is often overlooked. Emotional impact of future events is discounted; their value diminishes with time. Individuals vary in discounting rates, with rapid reward discounting linked to impulsivity and addiction risk. Addictive behavior's negative consequences are often delayed; rapid punishment discounting may prevent consideration of future consequences, increasing vulnerability to addiction. The study uses a progressive shock strength (PSS) procedure, a self-adjusting method calibrating punishment strength based on animal behavior. This offers advantages over other methods by reducing exposure to high shock levels and aligning with the 3Rs principles of animal research. The researchers hypothesize that introducing delays between reward-seeking action and punishment will reveal individual differences in resistance to punishment and highlight potential vulnerability to addiction.

Extensive research explores the motivational aspects of addiction and the role of the reward system. However, the inability to cease drug use despite negative consequences has only recently garnered significant attention. Animal models frequently employ punishment procedures (electric footshocks) to investigate insensitivity to negative outcomes and the transition from controlled to uncontrolled drug use. These studies demonstrate that differences in sensitivity to punishment can serve as a marker of addiction. The emotional impact of future events, known as temporal discounting, is also a key factor in addiction. Individuals vary in their discounting rates, and those who discount rapidly are considered impulsive and at higher risk for addiction. The negative consequences of addictive behaviors are often delayed, leading individuals with rapid punishment discounting to disregard future consequences, thereby increasing their vulnerability. Therefore, understanding the role of delayed punishment is crucial.

Forty-eight male Sprague-Dawley rats underwent food restriction and were divided into two cohorts with varying delay presentation orders. Experiments followed European Union directives and ethical committee approvals. After fixed-ratio 1 (FR1) food training, rats underwent PSS sessions with delays (0–12 s) between lever presses and punishment. Progressive ratio (PR) sessions were interspersed to measure food motivation. Anxiety-like behavior and pain sensitivity were assessed after operant sessions. The PSS procedure involved increasing shock duration with successful trials, measuring resistance to punishment via breakpoints (total shock intensity). PR sessions measured motivation by requiring increasingly more responses per reward. Anxiety was assessed using an open field test, and pain sensitivity using a hot plate test. Data analysis employed nonparametric tests (Friedman, Mann-Whitney) and two-way ANOVA due to non-normal distribution of PSS data. Median split at delay 0s categorized rats as shock-sensitive or shock-resistant.

Increasing delay significantly increased PSS breakpoints (both responses and electrical charge), indicating that delayed punishment is less effective. However, this effect varied widely among rats. High correlation between PSS breakpoints across delays suggested that relative sensitivity to punishment was consistent despite delays. No correlation was found between PSS and PR breakpoints, indicating different behavioral processes. The median split revealed two distinct groups: shock-sensitive and shock-resistant rats. Shock-sensitive rats showed significantly lower breakpoints and strong conditioned suppression (reduced responding even in non-punishment sessions). No differences in pain sensitivity were observed between groups, but shock-sensitive rats displayed significantly higher anxiety-like behavior, suggesting that this behavior might be a consequence of the repeated fear experience in the operant context. The introduction of delay seemed to exacerbate existing differences, as shock-resistant rats responded strongly to delays at 3 seconds, while shock sensitive rats responded only at 12 seconds. Importantly, shock-sensitive animals displayed a conditioned suppression of responding even in sessions without shocks, which was evident across all delay conditions.

The findings confirm that delayed punishment reduces its effectiveness, consistent with temporal discounting principles. Importantly, individual differences in delay discounting strongly influenced responses to delayed punishment. Shock-resistant rats displayed stronger temporal discounting of aversive consequences. This highlights the value of the PSS procedure with delay manipulation in identifying individuals vulnerable to addiction (low aversion sensitivity and excessive discounting). Shock-sensitive rats, who showed anxiety and response suppression, may represent a population vulnerable to opportunity loss due to excessive anxiety, suggesting a different type of vulnerability. The study's unexpected finding of conditioned suppression in shock-sensitive rats raises questions about the interaction of Pavlovian and operant conditioning in the PSS procedure. The lack of correlation between PSS and PR suggests that these measures assess distinct behavioral processes, one related to response inhibition under aversive conditions and the other to reward seeking motivation. The absence of progressive loss of control over food-seeking supports the notion that sensitivity to punishment, in the PSS procedure, is a stable trait. These results suggest that, at least for food reward, sensitivity to punishment may be a stable individual trait, not significantly altered by repeated experience.

This study demonstrates that incorporating delayed punishment into the PSS procedure effectively identifies distinct rat populations with varying vulnerabilities to addiction-like behaviors towards food. Shock-resistant rats, exhibiting higher temporal discounting of aversive consequences, represent a profile prone to addiction. Conversely, shock-sensitive rats show heightened anxiety and suppressed responding, suggesting a vulnerability linked to opportunity loss. Future research should explore these findings in female rats and investigate whether the PSS procedure can predict vulnerability to drug addiction. Further studies should clarify the interplay between Pavlovian and operant conditioning in shaping the observed behavioral responses.

The study's limitations include the use of only male rats, limiting the generalizability to females, and the use of a hot plate test as a relatively crude measure of pain sensitivity. Although the hot plate test is common in such studies, it does not measure sensitivity to electrical shocks specifically. Future studies should address these limitations by including female rats and employing more refined measures of pain sensitivity. The absence of a control group and the relatively small sample size should also be noted. Although unpublished results in the lab show a trend of slightly higher PSS break points in repeated exposures, the correlation observed across the experiment suggests that this increase does not confound the results significantly.