Environmental conditions modulate stress-induced behavioural and immuno-metabolic outcomes through sex-dependent effects

Major depressive disorder is a chronic and potentially life-threatening condition that affects up to 10% of the global population [1]. A robust body of evidence highlights persistent immune activation alongside alterations in glucose, lipid, and energy metabolism in depressed patients. These immuno-metabolic abnormalities are associated with specific symptom clusters and appear to characterize a subgroup of patients with reduced responsiveness to conventional antidepressants [2]. In this context, interventions targeting inflammatory pathways or lifestyle factors may offer greater efficacy, supporting a precision psychiatry approach.

To explore the mechanisms underlying the interaction between immune-metabolic alterations and vulnerability to depression, we exposed male and female mice fed a high-fat diet (HFD; 58% fat)—which induces systemic chronic low-grade inflammation [3–5]—to a period of chronic mild stress. Following stress exposure, mice were chronically treated with either vehicle or the anti-inflammatory drug ibuprofen (p.o.), while being housed under either enriched or stressful environmental conditions. Behavioral outcomes were assessed before and after stress exposure, as well as following treatment, using the automated high-throughput IntelliCage system. At the end of the experimental protocol, peripheral levels of adipokines and pro- and anti-inflammatory markers were quantified and integrated with metabolomic analyses of brain tissue samples. Pre- versus post-stress differences were assessed using t-tests, while group effects were examined using two-way ANOVA with treatment and environmental condition as factors, conducted separately for each sex. Tukey’s post hoc tests were performed when appropriate. Finally, principal component analysis (PCA) was applied to metabolomic data to identify patterns of variation and sample clustering.

Chronic stress reduced saccharin preference in both sexes, indicating a depression-like phenotype. Following two weeks of treatment, a significant main effect of environmental condition was observed only in female mice, with enriched animals displaying higher saccharin preference compared to stressed animals (p < 0.05). Consistently, in females, motivation was influenced by environmental conditions, with enriched animals showing significantly higher motivation compared to stressed groups (p < 0.0001).

Although no alterations in circadian activity patterns were observed throughout the experiment, in both sexes stress-exposed groups exhibited higher levels of locomotor activity. By contrast, pharmacological treatment did not significantly affect these behavioral outcomes.

Regarding metabolic pathways, environmental enrichment increased both adiponectin and leptin expression levels in females, while a selective increase in leptin was also observed in enriched males. Ibuprofen exerted a buffering effect on leptin levels (p < 0.05), specifically in females. In contrast, the expression of selected inflammatory markers was differentially modulated in a sex-dependent manner: males were more sensitive to ibuprofen (e.g., CXCL10, FLT-3L), whereas in females modulation was mainly driven by environmental conditions (e.g., increased CXCL1/KC and FLT-3L in enriched animals). Although preliminary, metabolomic data support these findings, suggesting more pronounced effects in females, where the influence of environmental conditions is further confirmed. All together, these findings suggest that environmental conditions play a key role in modulating stress-related behavioral, metabolic, and inflammatory outcomes, with a greater impact in females.

Funding: European Union -Next Generation EU-NRRP M6C2-Investment 2.1 Enhancement and strengthening of biomedical research in the NHS.

References

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