Mechanosensory neurons innervating Aplysia siphon encode noxious stimuli and display nociceptive sensitization

Numerous studies of learning and memory in Aplysia have focused on primary mechanosensory neurons innervating the siphon and having their somata in the left E (LE) cluster of the abdominal ganglion. Although systematic analyses have been made of the responses of these LE cells to mechanical stimulation of the tightly pinned siphon, little is known about corresponding responses when the siphon is unrestrained. The present study demonstrates that LE mechanosensory thresholds in the freely moving siphon are much higher than in the pinned siphon. Light tactile stimuli adequate to activate central neurons and reflexive siphon movements often fail to activate the LE cells when the siphon is unrestrained. Because the LE cells display increasing discharge to increasing pressures, with maximal activation by crushing or tearing stimuli that cause tissue injury, they satisfy accepted definitions of nociceptor. Indeed, they show similarities to vertebrate Adelta nociceptors, including a property apparently unique (among primary afferents) to nociceptors-sensitization by noxious stimulation of their receptive field. Either pinching or pinning the siphon decreases LE cell mechanosensory threshold and enhances soma excitability. Such stimuli reduce effective tissue compliance and cause neuromodulation that enhances sensory responsiveness. These results, and recent descriptions of predatory attacks on Aplysia, suggest that LE sensory neurons are tuned to grasping and crushing stimuli that threaten or produce bodily harm. LE cell sensitization has effects, resembling hyperalgesia and allodynia, that compensate for loss of sensory function during injury and help protect against subsequent threats.