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Received July 27, 2004
Revised September 9, 2004
Accepted after revision November 2, 2004
1 Copenhagen University
2 M.I.T.
* To whom correspondence should be addressed. E-mail: mtresch{at}mit.edu.
The details of behavior are determined by the interplay of synaptic connectivity within neuronal circuitry and the intrinsic membrane properties of individual neurons. One particularly dramatic intrinsic property displayed by neurons in many regions of the nervous system is membrane potential bistability, in which transient excitation of a neuron results in a persistent depolarization outlasting the initial excitation. Here we characterize the contribution of such intrinsic bistability, also referred to as plateau properties, in spinal motor neurons to the production of withdrawal behaviors in the frog. We performed experiments on the isolated frog spinal cord with attached hindlimb. This preparation allowed the simultaneous monitoring of muscle activations during motor behavior and intracellular neuronal recordings. We found that plateau properties, following their potentiation by serotonin, are recruited and contribute to the production of withdrawal behaviors. These properties conferred a voltage-dependent prolongation to the duration of motor neuronal activity. Consistent with this potentiation of motor neuronal plateau properties, serotonin also increased the duration of evoked muscle activations. This behavioral potentiation, as well as the expression of plateau properties in individual neurons, was reduced following antagonism of L-type Ca2+ channels. These results demonstrate that plateau properties mediated by L-type Ca2+ channels in motor neurons can be recruited during the production of behavior and play a role in specifying the temporal details of motor output.
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