EFFECTS OF IMMUNE SYSTEM CHRONIC CHALLENGE ON BRAIN MORPHOLOGY AND COGNITIVE ABILITIES IN A CICHLID FISH

Mounting an immune response is an energetically demanding process. However, its impact on expensive tissues such as the brain, and consequently on cognitive abilities, remains largely unexplored. Here, we investigated whether a chronic immune challenge, which is expected to increase energetic constraints, negatively affected brain morphology and cognitive performance. As a study system, we used the African cichlid fish, Neolamprologus pulcher. We induced a chronic immune activation through repeated treatment with bacterial lipopolysaccharide (LPS). We first evaluated cognitive performance using classic paradigms for associative and reversal learning in a two-colour discrimination task, as well as inhibitory control using an obstacle detour test, and then collected brain and body measurements. Preliminary analyses show no effect of the treatment on cognitive performance across all three tests. Interestingly, LPS treatment affected brain allometry, altering how brain size scales with body size, both at the level of total brain size and of individual brain regions. These effects were body-size dependent, with small-bodied fish being affected differently than large-bodied fish. Additional analyses examining individual-level correlations between performance and brain morphology as a function of LPS treatment revealed potential relationships between several brain regions and performance only in the detour test, suggesting region-specific trade-offs. Together, although preliminary, these results suggest that energy allocation to brain development under prolonged immune challenge is size-dependent and likely mediated by body energetic reserves, without detectable effects on the tested cognitive abilities. Our integrative approach shows that energetic imbalance can affect brain development even in the absence of measurable cognitive deficits.