Neural basis of the stress response in a pufferfish, Takifugu obscurus.

When faced with danger, pufferfish react with both a fast-start escape response and inflation behavior. The neural basis of these stress responses in the pufferfish has not been described. The present study reveals that during inflation behavior, the buccal cavity expands and compresses as a pump to control the direction of water flow and draws water in and out. The inflation involves a series of major anatomical modifications of the head. The greatly enlarged first branchiostegal ray and its associated hyohyoideus abductor muscle are the key mechanisms responsible for this behavior. The nerve branch innervating the hyohyoideus abductor muscle is highly developed, and its central origin at the motor nucleus of the VIIth cranial nerve was revealed by tract-tracing using the carbocyanine dye DiI. The central connections from its origin were found to be several motor nuclei in the medulla and the spinal cord, the nuclei of cranial nerve III and IV in the mesencephalon, and the pretectalis superficialis and periventricular preoptic nuclei in the diencephalon. Both the sympathetic cells and the parvocellular part of the periventricular preoptic nucleus might play a neuro-endocrine role in the rapid movement of the buccal cavity during puffing behavior in this species of pufferfish. The central circuit revealed by this study is hypothesized to mediate the C-start escape behavior and puffing behavior.