Early resource scarcity alters motivation for natural rewards in a sex- and reinforcer-dependent manner

Abstract

Rationale: Early life adversity impacts reward-related behaviors, including reward seeking for drugs of abuse. However, the effects of early stress on natural rewards, such as food and social rewards, which have strong implications for symptoms of psychiatric conditions such as major depressive disorder (MDD), are understudied. To fill this gap, we used the limited bedding and nesting (LBN) procedure to assess the impact of early resource scarcity on motivational drive for both food and social rewards in rats.

Methods: Male and female Long Evans rats were reared in either an LBN environment, with limited nesting materials and no enrichment, from their postnatal day 2-9 or control environment with ample nesting materials and enrichment. As adults, they were tested for reward-seeking behavior on progressive ratio operant tasks: food reward (sucrose) or social reward (access to a same-sex/age conspecific).

Results: We observed sex differences in the impact of LBN on motivation for natural rewards. In males, LBN increased motivation for both a sucrose and social reward. In females, LBN reduced motivation for sucrose but had no effect on social reward.

Conclusions: These results suggest that the effects of LBN on motivation for natural rewards are both sex- and reinforcer-dependent, with males and females showing differential motivation for food and social rewards following early scarcity. Our previous data revealed an LBN-driven reduction in motivation for morphine in males and no effect in females, highlighting the reinforcer-dependent impact of early resource scarcity on motivated behavior more widely.

Keywords: Early life stress; Food reward; LBN; Natural reward; Sex difference; Social motivation.

Figure 1.. Effects of LBN on sucrose self-administration.

Timeline of experiment: rats undergo LBN during PND 2–9. In adulthood (e.g., PND 60), they begin 10 days of training on an FR1 schedule. Following 10 days of training, they are run through 1 day of PR (A). Number of rewards earned on an FR1 schedule over 10 days (B). Number of active lever presses on an FR1 schedule over 10 days (C). Breakpoint ratio on a PR schedule (D). Number of active lever presses on a PR schedule (E). N=9–15/group. *p<.05, **p<.01, ***p<.001 compared to same-sex control, †=p<0.05, ††=p<0.01 compared to male control. Data are represented as means ± SEMs. CTRL, control housing; F, female; FR1, fixed ratio 1; LBN, limited bedding and nesting housing; M, male; PR, progressive ratio

Fig. 2.. Effects of LBN on social self-administration.

Timeline of experiment: rats undergo LBN during PND 2–9. In adulthood (e.g., PND 60), they begin 6 days of training on a trial based FR1 schedule. Following 6 days of training, they are run through 1 day of PR (A). Rewards earned on trial based FR1 schedule (B). Average latency to receive a reward on day 1 vs. day 6 while on a trial based FR1 schedule (C). Breakpoint ratio on a PR schedule (D). Number of active lever presses on a PR schedule (E). N=7–8/group. *p<.05, **p<.01. Data are represented as means ± SEMs. CTRL, control housing; F, female; FR1, fixed ratio 1; LBN, limited bedding and nesting housing; M, male; PR, progressive ratio