Hydrogen sulfide exposure reduces thermal set point in zebrafish

Behavioural flexibility allows ectotherms to exploit the environment to
govern their metabolic physiology, including in response to environmental
stress. Hydrogen sulfide (H2S) is a widespread environmental toxin that
can lethally inhibit metabolism. However, H2S can also alter behaviour and
physiology, including a hypothesised induction of hibernation-like states
characterised by downward shifts of the innate thermal setpoint
(anapyrexia). Support for this hypothesis has proved controversial because
it is difficult to isolate active and passive components of
thermoregulation, especially in animals with high resting metabolic heat
production. Here, we directly test this hypothesis by leveraging the
natural behavioural thermoregulatory drive of fish to move between
environments of different temperatures in accordance with their current
physiological state and thermal preference. We observed a decrease in
adult zebrafish (Danio rerio) preferred body temperature with exposure to
0.02% H2S, which we interpret as a shift in thermal setpoint. Individuals
exhibited consistent differences in shuttling behaviour and preferred
temperatures, which were reduced by a constant temperature magnitude
during H2S exposure. Seeking lower temperatures alleviated H2S-induced
metabolic stress, as measured by reduced rates of aquatic surface
respiration. Our findings highlight the interactions between individual
variation and sublethal impacts of environmental toxins on behaviour.

行为灵活性赋予外温动物（ectotherms）借助环境调控自身代谢生理的能力，该调控机制同样可响应环境胁迫。硫化氢（Hydrogen sulfide, H₂S）是一类分布广泛的环境毒素，可通过抑制代谢通路引发致死效应。然而，硫化氢亦可改变动物的行为与生理状态，其中包括一项被提出的假说：硫化氢可诱导类冬眠状态，该状态以内生性体温调定点（thermal setpoint）下移为特征，即无热觉低温症（anapyrexia）。针对该假说的验证始终存在争议，难点在于难以分离体温调节的主动与被动组分，尤其针对静息代谢产热水平较高的动物。本研究利用鱼类基于当前生理状态与热偏好，在不同温度环境间自主移动的自然行为性体温调节驱动力，直接对该假说开展检验。我们观察到，成年斑马鱼（Danio rerio）在暴露于0.02%浓度的硫化氢环境时，其偏好的体表体温出现下降，我们将此现象解读为体温调定点的偏移。个体在穿梭行为与偏好体温方面均表现出稳定的个体差异，且在硫化氢暴露期间，该差异的温度跨度被恒定压缩。通过寻求更低温度可缓解硫化氢诱导的代谢胁迫，该效应可通过水生表面呼吸（aquatic surface respiration）速率的降低得到量化验证。本研究结果揭示了个体变异与环境毒素亚致死效应之间的相互作用，且该相互作用体现在动物行为层面。