Fig. 1: Effects of chronic social stress (CSS) on reward learning and effort.

a Experimental design. BBW + FC: measurement of baseline body weight and food consumption; 90–95% BBW/Conditioning: conditioning under food restriction that reduced BW to 90–95% BBW; CSS/CON: CSS protocol or control handling; re-BBW + FC: BW and food consumption under ad libitum feeding on days 1–12 of CSS/CON provided re-baseline values; 95–100% re-BBW: mice were mildly food restricted to be tested at 95–100% re-BBW. b CSS mice were placed in the cage of a dominant, aggressive CD-1 mouse to receive a 30–60 s physical attack followed by 24 h sensory exposure through a divider; this was repeated with a different CD-1 mouse on each of 15 days. CON mice were kept in littermate pairs and were handled for 1 min on each of 15 days. c–g Discriminative reward learning-memory (DRLM) test. c Tone discriminative stimulus (DS) signalled chocolate sucrose pellet (gustatory reward) availability following a feeder response; maximum DS duration was 30 s per trial and inter-trial intervals (ITIs) were 20–80 s (mean = 50 s). Mice received 3 daily tests of 40 trials each and trials 1–20 per test were used for data analysis. Data are shown as mean + SEM per test and per mouse scores. Statistical analysis was conducted using 2-way mixed-model ANOVA; 1 CSS mouse was a low-responder outlier and excluded from the analysis. d Number of chocolate pellets obtained, i.e. DS trials with a response, across tests (left) and individual and overall mean scores (right). e Median DS response latency. f Median ITI response interval. g Median learning ratio (ITI response interval/DS response latency), across tests (left) and individual median scores for test 3 (right). Test days indicated by different letters were significantly different in Tukey’s multiple comparisons test: a vs. b p < 0.05 or lower. h–m Reward-to-effort valuation (REV) test. Data are shown as individual values and group mean values. Statistical analysis was conducted using t-tests. h Nose-poke responses at an operant stimulus triggered chocolate sucrose pellet delivery on a progressive ratio (PR) schedule (5 trials at PR1, 5 × PR5, 5 × PR9, 5 × PR13, etc.), signalled by a 1 s tone DS. Mice received 2 daily tests and data for test 2 are shown, as individual scores and group means; normal food was provided as a low-reward/low-effort choice. Data analysis was conducted using unpaired t-tests. i Number of operant responses. j Number of chocolate pellets earned. k Final ratio attained. l Latency to retrieve pellet after completion of ratio. m Post-reinforcement pause to resume operant responding. Images b, c and h were created with BioRender.com.