Stress vulnerability promotes an alcohol-prone phenotype in a preclinical model of sustained depression.

SDPS vulnerability increases preference for alcohol. A, Control rats and the two SDPS groups were habituated to progressively increasing concentrations of alcohol (2%‐12%) in the home cage, for a period of 5 weeks, using a two‐bottle paradigm. B, Consumption of alcohol during 24‐hour free access, normalized for weight, revealed that, starting from 6% onwards, SDPS‐prone rats displayed a relative increase in consumption of alcohol, albeit that no significant overall differences were observed between the three groups. C, Analysis of alcohol preference over water during 24‐hour free access, depicted as percentage of alcohol/total liquid consumed. A clear facilitation of alcohol consumption selectively in the SDPS‐prone rats was observed. Between‐group differences were most prominent at 2% alcohol concentration. SDPS‐resilient and control groups showed similar preference for alcohol in all concentrations provided. D, Consumption of 12% alcohol during 1‐hour limited access (average of 10 sessions). No difference in alcohol intake was observed between the three groups. E, Similarly, no between‐group difference in preference for the alcohol solution was observed, as all three groups drank similar amounts of alcohol vs water during the 1‐hour sessions. Repeated measures analysis of variance (ANOVA) main time (t) and group (g) effects are indicated; one‐way ANOVA post hoc group comparisons are depicted (c); controls, n = 15, SDPS prone, n = 10, SDPS resilient, n = 10; *P < 0.05.

SDPS facilitates acquisition of operant alcohol self‐administration. A, At approximately 4 months from the last defeat episode and following alcohol habituation at the home cage, all rats were subjected to a cue‐coupled alcohol self‐administration paradigm, starting with acquisition at fixed ration (FR). Different reinforcement schedules were used (FR1‐FR3). B, Analysis of the number of responses to the active, alcohol‐delivering hole during FR1 revealed significant training and group effects, as both SDPS groups displayed increased responding as compared with controls. Similarly, during FR2 and FR3 training schedules, the two SDPS groups exhibited enhanced responding for alcohol vs controls. Although SDPS‐prone animals showed relatively higher response rates, no group difference between the two SDPS groups was observed. C, Similar to active responding, significant effects of training were observed in all three FRs for the number of rewards gained. Both SDPS groups obtained significantly more rewards as compared with controls, in all three FR schedules provided, while no group differences were seen between the two SDPS groups. Repeated measures ANOVA across the three reinforcement schedules, main time (t) and group (g) effects are depicted; pairwise group comparisons are indicated (vertical lines, black, SDPS prone vs controls; grey, SDPS resilient vs controls); controls, n = 15, SDPS prone, n = 10, SDPS resilient, n = 10; *P < 0.05; **P < 0.01.

SDPS vulnerability increases motivation for alcohol intake. A, Following acquisition of alcohol self‐administration (SA), all animals were subjected to five progressive ratio sessions in which motivation for alcohol was assessed. B, Analysis of active responding revealed a main group effect, as SDPS‐prone animals displayed significantly higher number of responses vs controls. No difference between the two SDPS groups or between the SDPS‐resilient and control animals was observed. C, Similarly, break points (maximum fixed ration [FR] reached) confirmed an SDPS‐induced increase in motivation for alcohol. Importantly, this effect was seen only in the SDPS‐prone rats, as SDPS‐resilient animals did not differ from controls. Repeated measures ANOVA across the five progressive ratio (PR) sessions main group (g) effect and pairwise group comparisons are depicted (vertical lines, b); one way ANOVA main group (g) effects are indicated (c); controls, n = 15, SDPS prone, n = 10, SDPS resilient, n = 10; *P < 0.05; **P < 0.01.

SDPS vulnerability induces persistent alcohol seeking. A, After progressive ratio (PR) training, all animals were subjected to retraining in FR1 for 13 1‐hour sessions (cf Figure S3). During sessions 4 to 7, a 30‐second time‐out interval was implemented following each alcohol reward, doubling the original time‐out period. B, Analysis of the active responses over these four sessions revealed a main group effect, as SDPS‐prone rats displayed significantly increased responses vs controls and a trend vs SDPS‐resilient rats. No difference between SDPS‐resilient and control groups was observed. C, To correct for preexisting group differences in the chance of time‐out responding, the ratio of time‐out responses to actual rewards was calculated. Analysis of the time‐out/reward ratio over four reFR1 sessions showed that the SDPS‐prone group reached significantly higher ratio, when compared with both control and SDPS‐resilient groups. No difference between control and SDPS‐resilient groups was observed. Repeated measures ANOVA across the four reFR1 sessions main time (t) and group (g) effects and pairwise group comparisons (vertical lines, black, SDPS prone vs controls; grey, vs SDPS resilient) are indicated; controls, n = 15, SDPS prone, n = 10, SDPS resilient, n = 10; *P < 0.05; **P < 0.01.

SDPS vulnerability delays extinction learning and facilitates reinstatement of alcohol seeking. A, After retraining in FR1, all animals were provided with 15 1‐hour daily extinction sessions, during which uncoupling of the context (operant chambers) and the alcohol delivery was achieved. Following extinction training, all animals were subjected to cue‐induced reinstatement, in absence of alcohol. B, Main training and group effects indicated differential extinction performance across the training period and among the three groups. Analysis of responding per training bin (3 × 5 extinction sessions) showed that the SDPS‐prone group exhibited delayed extinction learning in the first 2 weeks (sessions 1‐5 and 6‐10), as reflected in increased active responses vs controls and SDPS‐resilient rats. Controls and SDPS‐resilient animals responded similarly, illustrating that the observed effects on extinction were SDPS prone specific. Analysis of the remaining extinction sessions (11‐15) showed that all three groups were successfully extinguished by the end of the training period. C, During the saline test, presentation of cues previously associated with reward delivery reinstated alcohol seeking in all three groups, as compared with their average responding during the last three extinction sessions (indicated in b, dashed square). SDPS‐prone animals showed increased relapse when compared with controls. No between‐group differences in SDPS‐resilient vs control or SDPS‐prone vs SDPS‐resilient groups were observed. Guanfacine pretreatment abolished alcohol seeking in all three groups and normalized the number of active responses in SDPS‐prone animals. Repeated measures analysis of variance (ANOVA) main time (t) and group (g) effects (b, c) and pairwise group comparisons (vertical lines, black, SDPS‐prone vs controls; grey, vs SDPS resilient, b) are indicated; One‐way ANOVA main group effect is indicated (c); controls, n = 15, SDPS prone, n = 10, SDPS resilient, n = 10; P < 0.10; *P < 0.05; **P < 0.01.