暗纹东方鲀应激逃避行为及其Mauthner细胞的形态学观察

Observation of fast escape behavior and morphology of Mauthner cells in pufferfish(Takifugu obscurus)

探讨暗纹东方鲀(Takifugu obscurus)这种具有胀气、佯死行为的鱼类是否具有快速逃跑行为,以及脑干Mauthner细胞是否是该行为的指令性神经元。研究发现,给予骤发的声音刺激(100 Hz,110 dB),暗纹东方鲀幼鱼和斑马鱼(Danio rerio)均出现快速逃跑行为。但是,与斑马鱼相比,暗纹东方鲀的快速逃跑行为发生概率低,且延迟时间长(P<0.01)。经10μmol/L GABAA受体抑制剂药浴处理30 min后,与对照组相比,暗纹东方鲀快速逃跑行为发生概率显著增加(P<0.05),反应延迟时间也显著降低(P<0.01)。组织学观察发现,暗纹东方鲀的Mauthner细胞形态特殊,胞体为椭圆形,细胞的长短轴比显著低于斑马鱼Mauthner细胞的长短轴比,且未见其与第八神经的直接联系。并且,在暗纹东方鲀延脑中未观察到Mid2cm、Mid3cm等神经元。因此,推测由于暗纹东方鲀Mauthner细胞欠发达,导致其快速逃跑能力较弱,故在应激逃避方面,需要进化出其他的防御方式,如胀气等,以保证种群的存活率。本研究为深入研究鱼类的应激反应的生理机制及经济鱼类的健康养殖提供了理论依据。

The startle response is a response to sudden, startling stimuli, such as sudden noise or threatening disturbance. Fast escape is a common type of stress response behavior in teleost and amphibian larvae. Pufferfish （Takifugu obscurus）, an important aquaculture species in southern China, typically exhibit an energy-dissipative inflation behavior in response to a stressor. It is unknown whether pufferfish have a fast escape capacity and under what kind of stressful situation. Understanding the stress responses and neuro-mechanisms of avoidance behavior provides a theoretical basis for improving aquaculture practices. We exposed larval pufferfish and zebrafish （Danio rerio） to a sudden sound stimuli （100 Hz, 110 dB） and recorded their behavior using a high-speed （1 000 frames per second） digital camera. Both species attempted to escape when exposed to sudden auditory stimuli. However, the probability of exhibiting fast escape behavior was significantly lower in pufferfish than in zebrafish, and the latency of the behavior was significantly longer in the former than in the latter（P〈0.01）. However, the behavior probability was significantly increased and the latency reduced in pufferfish by treatment with a GABAA receptor inhibitor（P〈0.05）. The Mauthner cell in the hindbrain is believed to be the commander neuron of this behavior. Together with the reticulospinal neurons （e.g., Mid2cm and Mid3cm）, Mauthner cells form the brainstem escape network that regulates the fast escape behavior. Our histological evaluation revealed that the morphological characters of the Mauthner cells in pufferfish differed from those in zebrafish and most other fish. The cells were oval-shape, and the dorsoventral/mediolateral axial ratio was significantly smaller than that in the zebrafish. In the pufferfish medulla, we were unable to define neurons that are readily visible in zebrafish, including the Mid2cm, Mid3cm, or the synaptic connections between the eighth nerve and Mauthner cell. Our results suggest that the Mauthner cell in pufferfish was less developed and the fast escape behavior may not be as important as in zebrafish. Instead, the pufferfish has developed a unique behavior characterized by sudden inflation and floating on the surface of the water as a defense mechanism under sudden stressful stimulation. Our results provide a basis for further ohvsiological study of the neural mechanism underlying the stress response in oufferfish.