Morphological And Functional Study Of The Mauthner Cell In The Pufferfish (Takifugu Obscurus) Brain

The startle response or startle reflex is a response of vertebrates tosudden, startling stimuli, such as sudden noise or threatening disturbance.In human，it’s a brainstem reflectory reaction that serves to facilitate escapefrom the sudden stimuli. It has been found across the human lifespan and inmany other vertebrates. Fast escape is a common type of startle response inteleost fishes and amphibian lavae. The processes of the behavior includetwo stages: fish bends itself into C-type (stage1); then, swims away fromthe stimuli source quickly (stage2). In the present study, we found that thepufferfish (Takifugu obscurus) and zebrafish (Danio rerio) intended toescape when exposed to a sudden auditory stimulus (100Hz,110dB). Theprobability of the fast escape behavior in the pufferfish was obviouslylower than in the zebrafish, and the latency of the behavior wassignificantly longer in the former than in the latter.However, the behavior probability significantly increased and thelatency was reduced in the pufferfish by Gabazine, a GABAAreceptorinhibitor (10μM). GABAAis fast-responding GABA receptors. The GABA receptors are a class of receptors that respond to the neurotransmitter,GABA, the chief inhibitory neurotransmitter in the vertebrate centralnervous system.The Mauthner cell in the hindbrain is believed to be the commanderneuron of the fast escape behavior. Together with the reticulospinal neuron,such as Mid2cm and Mid3cm, Mauthner cells that form the brainstemescape network. We found that the morphological characters of theMauthner cells in the pufferfish were different from those in the zebrafishand most other fish. The cell was oval-shape, and itsdorsoventral/mediolateral axial ratio was significantly smaller in thezebrafish. We were neither able to define the neurons such as Mid2cm、Mid3cm in medulla of the pufferfish, nor to define the synaptic connectionsbetween the eighth nerve and Mauthner cell such as in the zebrafish.In order to determine the excitory characteristics of the Mauthner cellsin the pufferfish, glutamate receptor potentiators--Aniracetam and NMDAwere used to treat the pufferfish to observe its behavioral andimmunochemical responses. Aniracetam and NMDA targeted on the AMPAand the NMDA receptor, respectively. Both AMPA and NMDA receptorsbelong to iontropic glutamate receptor which is the excitory receptor incentral nervous system. Our results indicate that the probability of C-startling behavior significantly increased, and the latency was reduced in the pufferfish by the treatment of Aniracetam and NMDA. Other behavioralparameters were changed significantly as well, such as body angle at theend of stage1and the duration of stage1. The effect of Aniracetam seemedto be stronger than that of NMDA on altering behavior.Immunochemical results suggest that both the AMPA receptor andNMDA receptor were located on the membrane of the Mauthner cell in thepufferfish. However, the number of NMDA receptors was less than theAMPA receptors. After the Aniracetam treatment, the number of NMDAreceptor signals increased, indicating that the NMDA receptor can beactivated by AMPA receptor agonist, and the glutamate receptors in theMauthner cell glutamatergic synapses play an important role in initiatingthe fast escape in Takifugu obscurus. The low probability of the behaviormight be due to a small number of glutamate receptors on the Mauthnercells in Takifugu obscurus.Our results point out that the Mauthner cell in the pufferfish was underdeveloped, and the fast escape behavior might not be as important in thepufferfish as that in the zebrafish. Instead, the pufferfish has developed analternative defensive behavior to deal with sudden stressful stimulation, i.e.sudden inflating the body and floating on water surface. This studyprovides new understanding of the pufferfish’s special defensive behaviorand neurology, and fundamental data for further physiological studies on neural mechanism of response to stress in the pufferfish.Since the pufferfish is an important aquaculture species in China,understanding the stress stimulation and the neuro-mechanisms ofavoidance behavior of the fish would provide basic information fordeveloping healthy aquaculture protocol for the species.