MECHANOSENSORY NEURONS INNERVATING APLYSIA SIPHON ENCODE NOXIOUS STIMULI AND DISPLAY NOCICEPTIVE SENSITIZATION

Numerous studies of learning and memory in Aplysia have focused on pri mary mechanosensory neurons innervating the siphon and having their so mata in the left E (LE) cluster of the abdominal ganglion. Although sy stematic 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 pre sent study demonstrates that LE mechanosensory thresholds in the freel y moving siphon are much higher than in the pinned siphon. Light tacti le stimuli adequate to activate central neurons and reflexive siphon m ovements often fail to activate the LE cells when the siphon is unrest rained. Because the LE cells display increasing discharge to increasin g pressures, with maximal activation by crushing or tearing stimuli th at cause tissue injury, they satisfy accepted definitions of nocicepto r. Indeed, they show similarities to vertebrate A delta nociceptors, i ncluding a property apparently unique (among primary afferents) to noc iceptors-sensitization by noxious stimulation of their receptive field , Either pinching or pinning the siphon decreases LE cell mechanosenso ry threshold and enhances soma excitability. Such stimuli reduce effec tive tissue compliance and cause neuromodulation that enhances sensory responsiveness, These results, and recent descriptions of predatory a ttacks on Aplysia, suggest that LE sensory neurons are tuned to graspi ng 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 protec t against subsequent threats.