Circadian Rhythms in Locomotor Activity of the Hagfish, Eptatretus burgeri VI. The Effects of Cutting the Spinal Cord

The present study is designed to clarify the mechanism by which the circadian pacemaker controls the locomotor activity of the hagfish and also to estimate the role of brain and spinal cord in the swimming behavior of the animal. We examined the effect of cutting the spinal cord at the 6 different positions on the circadian rhythm and the locomotor behavior of the animal. The most frontal cut was located between the brain and spinal cord, and the other 5 cuts were given to every 1/6 the length of the spinal cord. The relation between the locomotor activity and the cut position of spinal cord was summarized as follows. (1) When the ratio of frontal part before the cut was 0/6-1/6, the animal locomoted under initiative of caudal part, in random direction at the bottom and showed neither nocturnal rhythm in LD nor circadian rhythm in DD. (2) When the ratio of the frontal part before the cut was 4/6-5/6, the animal swam up to the surface under initiative of frontal part, and showed both nocturnal rhythm in LD and circadian rhythm in DD. (3) When the frontal ratio of spinal cord was 2/6 or 3/6, the animal showed both kinds of swimming behavior of (1) and (2). These results suggest that the descending system from the brain enable the hagfish to swim up to the surface and to express the rhythmicity of locomotor activity under control of the circadian pacemaker when at least frontal 2/6 of the spinal cord is connected to the brain by neuronal networks not by humoral factors.