The goal of this study was to characterize the contribution of a population of low-threshold mechanoreceptors to short-term habituation of siphon-elicited reflex responses in Aplysia californica. Since the location of their somata is unknown, we refer to them as the Unidentified Low-Threshold mechanoreceptors (ULTs). The ULTs operate in parallel to the higher-threshold and well-characterized LE sensory neurons, yet little is known regarding their contribution to behavioral plasticity. Using extracellular recordings from the siphon nerve, we found that habituation training that favors ULT activation resulted in a significant decrease in afferent activity at training intervals up to 1 min per stimulus (1 min ISI). To determine how this reduction impacts responses at other sites of the reflex network, we used intracellular recordings to measure training-induced changes in either L29 excitatory interneurons or LFS siphon motor neurons. With a 30s ISI, changes at both locations were training site-specific and matched the rate of change of afferent activity, implicating regulated sensory activity as a primary mechanism. With a shorter training interval (1s ISI), site-specificity of training was not observed in the L29s, but was still preserved in the motor neurons. For both, the rate of change during training was faster than the rate of change of afferent activity. Taken together, we conclude that regulation of low-threshold sensory neuron activity can play a significant role in short-term habituation, but other network processes may be recruited at more rapid training intervals.
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