Long-term memory in Aplysia modulates the total number of varicosities of single identified sensory neurons.

The morphological consequences of long-term habituation and sensitization of the gill withdrawal reflex in Aplysia california were explored by examining the total number of presynaptic varicosities of single identified sensory neurons (a critical site of plasticity for the biochemical and biophysical changes that underlie both types of learning) in control and behaviorally trained animals. Sensory neurons from habituated animals had 35% fewer synaptic varicosities than did sensory neurons from control animals. In contrast, sensory neurons from sensitized animals had twice as many varicosities per sensory neuron compared to controls, as well as enlarged neuropil arbors. These changes suggest that modulation of synapse number may play a role in the maintenance of long-term memory.