Recruitment of motor neuronal persistent inward currents shapes withdrawal reflexes in the frog

The details of behaviour are determined by the interplay of synaptic connectivity within neuronal circuitry and the intrinsic membrane properties of individual neurones. One particularly dramatic intrinsic property displayed by neurones in many regions of the nervous system is membrane potential bistability, in which transient excitation of a neurone results in a persistent depolarization outlasting the initial excitation. Here we characterize the contribution of such intrinsic bistability, also referred to as plateau properties and mediated by persistent inward currents (PICs), in spinal motor neurones to the production of withdrawal behaviours 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 behaviour and intracellular neuronal recordings. We found that PICs, following their potentiation by serotonin (5-HT), are recruited and contribute to the production of withdrawal behaviours. These properties conferred a voltage-dependent prolongation to the duration of motor neuronal activity. Consistent with this potentiation of motor neuronal PICs, 5-HT also increased the duration of evoked muscle activations. This behavioural potentiation, as well as the expression of PICs in individual neurones, was reduced following antagonism of L-type Ca²⁺ channels. These results demonstrate that PICs in motor neurones can be recruited during the production of behaviour and play a role in specifying the temporal details of motor output.