Effects of a temporary period on pasture on the transcriptomic signature of horses housed in individual boxes

AO_SCPLOWBSTRACTC_SCPLOWPasture access is widely recognized for its welfare, behavioural, and health benefits in horses compared to individual stabling. Our recent studies have shown that even short-term pasture exposure enhances welfare and induces lasting changes in gut microbiota, promoting health-associated bacterial populations. This study investigated the transcriptomic signature of 22 horses following a standardized pasture protocol, with peripheral blood gene expression analysed before pasture access (T0) and three months after returning to individual stabling (T1). First, using sPLS regression, we correlated gene expression profiles with behavioural welfare indicators measured at T0 and T1. Horses with pasture access exhibited significantly altered blood transcriptome compared to controls, with aggressiveness towards humans emerging as the strongest behavioural correlate of gene expression. Bioinformatic analyses revealed that aggressiveness-associated genes were linked to inflammation, apoptosis, and cell differentiation/growth. Then, horses with pasture access were divided in resilient and non-resilient according to the improvement of their behaviour after time on pasture. Differentially expressed genes post versus pre- pasture, compared between resilient and non-resilient horses for aggressiveness showed that inflammatory signals were downregulated in both subgroups, with a more pronounced effect in resilient horses. This suggests that resilient individuals are better equipped to modulate inflammatory responses in low-stress environments like pasture. Surprisingly, for unresponsiveness to the environment--a trait linked to depressive-like states--resilient horses displayed increased inflammatory signaling (e.g., IRF and CD40 activation) post-pasture, while non-resilient horses showed activation of anti-inflammatory PPAR signaling. Notably, non-resilient horses exhibited molecular signatures associated with organismal death, morbidity, and growth failure, indicating maladaptive physiological states. In contrast, resilient horses demonstrated activation of growth-related pathways (e.g., BMP2 and BMPR1A), suggesting a shift toward anabolic and developmental processes. This study underscores the behavioural and molecular benefits of pasture access for horses, particularly in reducing aggressiveness and inflammatory signaling in resilient individuals. The findings highlight the complex interplay between behaviour, inflammation, and resilience, with pasture access promoting adaptive physiological and molecular responses. Further research is needed to elucidate the long-term implications of these transcriptomic changes and their broader relevance to equine welfare.