Monosynaptic connections made by the sensory neurons of the gill- and siphon-withdrawal reflex in Aplysia participate in the storage of long-term memory for sensitization.

We have found that in the gill- and siphon- withdrawal reflex of Aplysia, the memory for short-term sensitization grades smoothly into long-term memory with increased amounts of sensitization training. One cellular locus for the storage of the memory underlying short-term sensitization is the set of monosynaptic connections between the siphon sensory cells and the gill and siphon motor neurons. We have now also found that these same monosynaptic connections participate in the storage of the memory underlying long-term Sensitization. We examined the amplitudes of the direct synaptic connections produced by siphon sensory neurons on the gill motor neuron L7 in nervous systems removed from control and from long-term sensitized animals 1 day after the end of long-term sensitization training. The connections were significantly larger in long-term sensitized animals than in control animals. The finding that long-term memory occurs at the same synaptic locus as the short-term memory should facilitate study of the cellular and molecular mechanisms involved in the conversion of short-term to long-term memory.