Alpha-conotoxin Im1 Makes Leeches Swim Erratically
36th Annual Meeting of the Society for Neuroscience, Atlanta, GA, 2006. Prog. no. 253.2
Medicinal leeches (Hirudo medicinalis) move quickly through water by swimming, a behavior that is driven by an elegant and fluid-dynamically efficient whole-body motion. Previous work has characterized the neuronal basis of swimming in detail. We have found that injecting an alpha-conotoxin, Im1, into intact leeches disrupts-but does not destroy-swimming behavior. To understand better how activity in the nervous system generates swimming, we have been studying this disruption. Following injection of Im1, leeches become more likely to swim spontaneously, and they no longer follow their usual straight path, which greatly reduces their forward motion. We have studied the neuronal basis of this effect by bath-applying Im1 to isolated nerve cords and simultaneously recording from up to 7 motor nerves along the ventral nerve cord using suction electrodes. We found that the toxin modified neuronal output during swimming in a least three ways: (1) Bursts of action potentials in cells DE-3-motor neurons that drive the contraction of the dorsal longitudinal muscles-were severely disrupted. Although the median burst duration and period remained largely unchanged, the distribution of durations and periods of individual bursts became much more variable, suggesting a loss of whole-body control. (2) Fictive swim episodes were interspersed with episodes of generally elevated motor neuron activity, a phenomenon never seen in control conditions. (3) Im1 made interganglionic propagation delays less tightly controlled, and shorter on average, which helps to explain the reduction of stroke efficacy. We are currently investigating where in the swim central pattern generator the disruption originates by comparing changes elicited in different phases of the swim cycle as they are reflected in activity recorded from pairs of dorsal and ventral nerves along the nerve cord, and by imaging interneuronal activity using voltage-sensitive dyes.