Operant effort-based decision-making task reveals sex differences in motivational behavior but no long-term effects of adolescent intermittent ethanol in Sprague Dawley rats

Abstract

Loss of motivated behavior, or apathy, is a key feature across multiple affective disorders, and is assessed via operant effort-based decision-making (EBDM). The mechanisms of amotivation have been connected to pro-inflammatory signaling which can directly impact dopamine signaling. Chronic alcohol exposure is associated with altered immune signaling and impaired goal-directed behavior, so the present studies assessed the impact of adolescent intermittent ethanol (AIE) on EBDM in adulthood across sex. Adolescent male and female (N = 32/n = 8 per group) Sprague-Dawley rats were exposed to ethanol (4 g/kg) intragastrically on a 3 days on/2 days off schedule during postnatal days ~30-50 or given vehicle, and allowed to age into adulthood (P80+). All rats were then trained on the operant EBDM concurrent FR5/chow task, after which we tested the impact of sex and AIE history on responding 1) during breakpoint challenge raising the FR requirement in a log₂ pattern, 2) 90 min after immune challenge (2 μg/kg IL-1β), 3) 18 h after 3.5 g/kg intraperitoneal ethanol challenge (hangover), and 4) immediately after a 30-min restraint stress challenge. Immune challenge disrupted motivated behavior without affecting appetite. No effects of AIE emerged and sex differences were evident throughout all challenges. Females responded less for pellets yet persisted responding until a higher breakpoint. This work indicates that AIE does not alter baseline or evoked EBDM as can be measured with this approach. Testing across aging and using other modalities should be performed to continue examining the effects of chronic alcohol on apathy.

Keywords: Adolescent intermittent ethanol; Effort-based decision making; Hangover; IL-1; Sex differences; Stress.

Figure 1.. Experimental schematic and background information.

(A) The schematic for this experiment shows that all experimental rats were born and raised in our colony until adolescent postnatal (P) day ~30, when AIE was initiated, consisting of four cycles of three days on (4 g/kg intragastric ethanol) and two days off (rest) and totaling 12 exposures by P50 (VEH animals received water intubations). After a period of abstinence, adult rats (~P85) began training on EBDM, with the schedule of tests outlined sequentially with interspersed periods of return to baseline. Figures for (B) males and (C) females show weight comparisons to previous studies across AIE and the ensuing food deprivation for the experiment.

Figure 2.. Baseline measures between challenges.

This figure shows the baselines that were achieved for (A) active lever responses, (B) nosepokes, (C) pellets, and (D) chow between challenges (written in grey), showing sex differences in baseline measures across the experiment and relative stability across time. Interaction post hocs at p < 0.05 are represented with * for within-sex differences between periods, and # for sex difference during a specific period. Total N = 32 / group n = 8 for the first two baselines, then N = 31 / n = 7-8 for the rest.

Figure 3.. Challenge 1: breakpoint.

This figure shows results for the breakpoint challenge for (A) active lever responses, (B) nosepokes, (C) pellets, and (D) chow, revealing sex differences in breakpoint reached during this test (total N = 32 / group n = 8). Interaction post hocs at p < 0.05 are represented with letters for within-sex differences for males (m) and females (f) as compared to their own FR5 day, a # for sex differences during a period, and & in the instance where male AIE rats ate more pellets than other groups.

Figure 4.. Challenge 2: immune.

This figure shows results of the Interleukin (IL)-1β challenge for (A) active lever responses, (B) nosepokes, (C) pellets, and (D) chow, indicating higher overall responding by males and lower responding following challenge for both sexes (total N = 31, group n = 7-8). Significance at p < 0.05 is represented with @ for a main effect of challenge and # for a main effect of sex.

Figure 5.. Challenge 3: ethanol hangover.

This figure shows results for the “hangover” challenge for (A) active lever responses, (B) nosepokes, (C) pellets, and (D) chow, revealing higher responding by males overall and lower responding after challenge (total N = 30, group n = 7-8). Interaction post hocs at p < 0.05 are represented with ^ signifying an effect of challenge for both sexes, and a # showing a sex difference during a specific period. Main effect of challenge is represented with @.

Figure 6.. Challenge 4: restraint stress.

This figure shows results for the restraint stress challenge for (A) active lever responses, (B) nosepokes, (C) pellets, and (D) chow, revealing higher responding by males overall and lower responding after challenge(total N = 30, group n = 7-8). An interaction post hoc at p < 0.05 is represented with ^ signifying an effect of challenge for both sexes. Main effects of challenge (@) and sex (#) are also shown.