Agnieszka Sergiel, Đuro Huber, Slaven Reljić, Antonio Bradić, Jasna Aladrović

The brainstem is essential for transmitting and regulating signals between different brain regions and regulating vital, life-sustaining functions. Fatty acids (FA) are structural components of brainstem neuronal cells; they influence membrane fluidity and contribute to brain energy metabolism. A key regulator of FA metabolism in the brain, stearoyl-CoA desaturase (SCD), controls the conversion of saturated FAs (SFA) into monounsaturated FAs (MUFA) with the same number of carbon atoms, thereby modulating membrane properties and cellular function. SCD activity index had not previously been determined in the brain of brown bears. We aimed to determine FA composition and SCD indices using samples collected from 69 free-ranging brown bears (23 females, F; 46 males, M) in the Croatian part of the Dinaric-Pindos population range during spring and autumn from 2022 to 2025, while attending to human-induced mortalities. Samples from the medulla or pons regions were homogenized, total lipids extracted, and FA composition was determined by gas chromatography. In spring 2022, SFA predominated in the brainstem of both sexes, whereas polyunsaturated FA (PUFA) predominated in autumn. In 2023, across both seasons and sexes, SFA predominated, while in 2024 and 2025 PUFA predominated. In spring 2024, F and M had significantly higher SCDi18 index compared to same sex in autumn (79.34±0.40 vs 72.85±0.55; 82.28±1.73 vs 75.40±3.02). In spring 2024, F had significantly lower SCDi18, while M had significantly higher SCDi16 index compared to same sex in autumn (79.34±0.40 vs 82.28±1.73; 4.83±0.69 vs 3.53±0.34). In spring 2023, F and M had significantly higher ratio of unsaturated FA (UFA) to SFA compared to same sex in autumn (1.35±0.13 vs 1.05±0.13; 1.60±0.10 vs 1.20±0.14). Results indicate interannual and seasonal shifts between SFA and PUFA predominance, accompanied by sex- and season-specific modulation of SCD activity probably in response to physiological and environmental demands.