Cellular correlates of long-term sensitization in Aplysia

Although in vitro analyses of long-term changes in the sensorimotor connection of Aplysia have been used extensively to understand long-term sensitization, relatively little is known about the ways in which the connection is modified by learning in vivo. Moreover, sites other than the sensory neurons might be modified as well. In this paper, several different biophysical properties of sensory neurons, motor neurons, and LPI17, an identified interneuron, were examined. Membrane properties of sensory neurons, which were expressed as increased excitability and increased spike afterdepolarization, were affected by the training. The biophysical properties of motor neurons also were affected by training, resulting in hyperpolarization of the resting membrane potential and a decrease in spike threshold. These results suggest that motor neurons are potential loci for storage of the memory in sensitization. The strength of the connection between sensory and motor neurons was affected by the training, although the connection between LPI17 and the motor neuron was unaffected. Biophysical properties of LPI17 were unaffected by training. The results emphasize the importance of plasticity at sensory-motor synapses and are consistent with the idea that there are multiple sites of plasticity distributed throughout the nervous system.