Abstract
Mammals have autonomic thermoregulatory systems, which induce shivering for thermoregulation. But it is not known when such a system appeared in vertebrates. Here we show that newborn zebrafish has gained an autonomic thermoregulatory system that shakes body and tail fin for thermoregulation. When temperature was lowered from 28.5 oC to 9–18 oC in newborn fish 3 days post-fertilization (3 dpf), shaking of body and tail fin occurred. In spinalized fish, this response remained intact, indicating that shaking is reflex. To ask whether shaking induces thermogenesis, we measured water temperature (250 μl) in a test tube which was initially held at 12 oC and heated by shaking of fish placed in the tube. When five fish were placed, temperature increased by 0.005 oC * s−1. When ten fish were placed, temperature increased by 0.010 oC * s−1. These results show that cooling-induced shaking induces intensive thermogenesis. To study motor control, we recorded EMG with cell-attached patch mode in spinalized fish embedded in agar. When temperature was decreased from room temperature to 12 oC, alternating spikes of right and left muscles occurred continuously below ∼18 oC. These spike trains were consistent with shaking in video images, but different from that of swimming. The present study shows the evolutionary path from heterotherms to homeotherms. [J Physiol Sci. 2008;58 Suppl:S101]
