In the present study we characterized action impulsivity and delay tolerance in a group of 188 rats using the VDS test paradigm. The entire protocol lasts 8 days, but the VDS evaluation requires only a single session making it ideal to study short-lived phenomena or transient life stages such as adolescence or estrous cycle phases. An important attribute of the task is that it simultaneously captures both action impulsivity and delay tolerance. Task acquisition, reflected in the progressive decline of omission trials until complete fading, was achieved within 2–6 sessions. All animals were able to learn the task and from training sessions 7 to 10 virtually no omissions were recorded. Older animals typically required more sessions, an observation previously reported in other operant behavior protocols41,42,43,44.

Premature responses during the training phase reflect impulsive action. The behavioral construct is similar to that of the 5-choice serial reaction time task (5-csrtt)45,46. This type of impulsive behavior is also captured in the early moments of the 3si delays40. In both instances, impulsive action decreased with age. While a previous study in a 2-csrtt has shown that 1 m.o. rats were more impulsive than >3 m.o. animals25, the opposite was observed in a single instrumental nose poke task using animals of similar ages34. Interestingly, this last task bears some resemblances with the VDS test and results are probably more akin to delay intolerance rather than behavioral inhibition (see below). In a study confined to an older population, Muir and colleagues observed that 10–11 m.o. rats were more impulsive than 23–24 m.o. rats, though this observation was restricted to a specific condition (longer delays)24.

In the present study, sex was associated with distinct action impulsivity behavior, with females performing more premature responses than males. Similar findings have been reported in 3 m.o. rats using the 2-csrtt task, although the behavioral differences were manifested specifically in delays of longer duration25. The available literature indeed suggests that sex-related differences are specific of delay conditions – see for instance25,35,47.

The VDS test also includes a delay (in)tolerance component which correlates with delay discounting (DD) behavior and manifests as an increment of impulsive response rate upon exposure to large delays to signal/reward40 – see also48,49,50. In our population, 2–6 m.o. animals demonstrated increased PR rate in the 3sf block compared to baseline rate in 3si, while the remaining groups maintained (1–2 and 6–12 m.o.) or even slightly decreased (12–18 m.o.) their response rate. Our observations are consistent with DD protocols, which have demonstrated that 1 m.o. rats were less impulsive than 2 m.o. animals34,51 and that 25 m.o. rats were less impulsive than 6 m.o.27. Lukkes and colleagues also have observed that early adolescent female (but not male) rats were less impulsive in a DD task than were young adult/adult females31. It is difficult to construct a meaningful framework for the DD studies reported in the literature. On one hand, no age-related differences were demonstrated in one spatial adjusting-delay task (5, 9 and >27 m.o.)33, while in another study, 1 m.o. rats made more impulsive choices than 2 m.o.29. As a further confounder, 1 m.o. animals in a spatial (T-maze) DD task were shown to be more impulsive than 3 m.o. but only under very specific conditions (10 and 15 second delay)30. One major difference between DD and VDS paradigms is that, in the former, delay and reward-size effects cannot be isolated. When the amount of reward is controlled, and the indifference point calculated over an option between an adjusted delay and a variable (random) delay, adolescent animals are found to be less impulsive than young adult animals51, as also observed in the VDS – see also34. In this context, the relevant conclusion appears to be that reward-driven behavior in adolescents is more directed by an exogenous stimulus, while it is more goal-directed in adults52 – see also53. Indeed, adolescent and adult rats differ in their reward-evoked activities of the dorsal striatum and orbitofrontal cortex54,55 – see also for review56. Incongruence in the published results may also be partially attributed to differences in procedure, i.e. adjusting vs increasing delay57, ascending vs descending delay58 and magnitude of reinforcement59.

Female and male rats had similar rates of premature response in all blocks of the VDS paradigm, with exception of 3si. During this period females were seen to have a higher PR rate than males, potentially reflecting differences in action impulsivity previously observed in the training phase. Consistent with our overall findings, a number of earlier DD studies also found no significant sex-associated differences39,60. Other studies have reported small differences between male and female subjects under very particular experimental conditions37, while yet others have demonstrated an effect (females > males36), or even an opposite relationship (males > females38)36. In females, another variable which might contribute to the behavior under evaluation is phase of the estrous cycle. Our results show a trend towards increased tolerance to delay during diestrus compared to proestrus. Several studies in humans also have demonstrated diminished impulsivity during the mid phase of the menstrual cycle61,62. One interpretation of these differences is that they are related to the modulating effect by sex steroids upon dopaminergic tone63,64,65, which is known to affect impulsive behavior4,5,66.

An additional potential source of inter-study variability is the animals’ strain. It has for example been shown that male Lewis rats discount faster than Fisher 34467,68,69,70,71,72, although differences can be attenuated or eliminated with repeated assessment, perhaps as an effect of learning and/or aging73. Two other studies using additional rat strains74,75, support inter-strain differences. In our study, we restricted the analysis to the 12–18 m.o. group in order to evaluate groups comparable to one another. WH animals were found to have increased PR in comparison with SD rats, both during training and test, demonstrating strain differences in both action and delay intolerance components of impulsivity.

Finally, considering the heterogeneity of our population, differences in motivation and/or motor performance have necessarily to be accounted. Indeed, sex and age were associated with statistically significant differences in latency to feed, although these were in general small – maximum 244.6 ms (2–6 vs 12–18 m.o). Interestingly, younger (1–2 and 2–6 m.o.) and older (2–12 and 12–18 m.o.) groups differed substantially regarding response latencies (max. 2717.4 ms; 2–6 vs 12–18 m.o.). This is to some extent paradoxical in the sense that in the former animals need to move in a more elaborate manner (i.e. between the nosepoking and feeding orifices), while the latter requires minimal movement (the animal can perform sequential premature responses and eventually one final correct response). Such suggests that response latencies essentially reflect animals’ premature responding, i.e. animals responding at higher rates also have shorter latencies. In fact, females present both statistically significant higher PR and lower response latencies specifically at the 3si stage.

In conclusion, in this study of a sample of substantial size, we confirmed decreased action impulsivity and delay tolerance with age. Of interest was our finding that, in contrast to the prevailing view mainly derived from human studies17,18,19,20,21,76 – see also for review22,23, delay intolerance appears to be maximal at early adulthood, not in adolescence. In our analysis of action impulsivity, we found that females demonstrated a significantly greater number of premature responses than males. No similar difference was evident for delay tolerance. Our consideration of the influence of strain suggested that, in general, WH animals acted more impulsively than SD.