Balanced plasticity and stability of the electrical properties of a molluscan modulatory interneuron after classical conditioning: a computational study

The Cerebral Giant Cells (CGCs) are a pair of identifi ed modulatory interneurons in the Central Nervous System of the pond snail Lymnaea stagnalis with an important role in the expression of both unconditioned and conditioned feeding behavior. Following single-trial food-reward classical conditioning, the membrane potential of the CGCs becomes persistently depolarized. This depolarization contributes to the conditioned response by facilitating sensory cell to command neuron synapses, which results in the activation of the feeding network by the conditioned stimulus. Despite the depolarization of the membrane potential, which enables the CGGs to play a key role in learning-induced network plasticity, there is no persistent change in the tonic fi ring rate or shape of the action potentials, allowing these neurons to retain their normal network function in feeding. In order to understand the ionic mechanisms of this novel combination of plasticity and stability of intrinsic electrical properties, we fi rst constructed and validated a Hodgkin-Huxley-type model of the CGCs. We then used this model to elucidate how learninginduced changes in a somal persistent sodium and a delayed rectifi er potassium current lead to a persistent depolarization of the CGCs whilst maintaining their fi ring rate. Including in the model an additional increase in the conductance of a high-voltage-activated calcium current allowed the spike amplitude and spike duration also to be maintained after conditioning. We conclude therefore that a balanced increase in three identifi ed conductances is suffi cient to explain the electrophysiological changes found in the CGCs after classical conditioning. © 2010 Vavoulis.