Brain cooling marginally increases acute upper thermal tolerance in Atlantic cod

Physiological mechanisms determining thermal limits in fishes are
debated but remain elusive. It has been hypothesised that motor function loss, observed as loss of equilibrium during acute warming, is
due to direct thermal effects on brain neuronal function. To test this, we
mounted cooling plates on the heads of Atlantic cod (Gadus morhua) and quantified whether local brain cooling increased
whole-organism acute upper thermal tolerance. Brain cooling reduced brain
temperature by 2-6°C below ambient water and increased thermal tolerance by 0.5
and 0.6°C on average relative to instrumented and uninstrumented controls, respectively,
suggesting that direct thermal effects on brain neurons may contribute to setting
upper thermal limits in fish. However, the improvement in thermal tolerance with
brain cooling was small relative to the difference in brain temperature,
demonstrating that other mechanisms (e.g., failure of spinal and peripheral
neurons, or muscle) may also contribute to controlling acute thermal tolerance.

决定鱼类热耐受上限的生理机制迄今仍存在诸多争议，且尚未完全阐明。已有假说指出，急性升温过程中出现的运动功能丧失（表现为平衡失调），系脑神经元功能受到直接热效应影响所致。为验证该假说，本研究将冷却贴片安装于大西洋鳕（*Gadus morhua*）头部，通过定量实验探究局部脑部降温是否可提升整体个体的急性高温耐受上限。相较于安装了配套监测装置的对照组与未安装任何装置的对照组，脑部降温可使脑温较环境水温降低2~6℃，并分别将热耐受极限平均提升0.5℃与0.6℃；这提示脑神经元受到的直接热效应，可能是决定鱼类高温耐受上限的关键因素之一。但相较于脑温的变化幅度，脑部降温对热耐受上限的提升幅度相对有限，这表明其他机制（例如脊髓与外周神经元功能衰竭，或肌肉功能异常）同样可能参与调控鱼类的急性高温耐受能力。