Abstract
The rostral cluster of SCP-immunoreactive cells, originally identified in each buccal hemiganglion of juvenile Aplysia, was examined in mature specimens. Immunohistochemical and dye-fill experiments showed that each rostral cluster consists of approximately 40 cells. Although these neurons exhibit heterogeneity of size and shape, all cells project an axon into the radula nerve. Tracing of dye-filled cells showed that they project to the layer of tissue that lines the inner surface of the food-grasping portion of the chitinous radula. This tissue contains SCP- immunoreactive nerve fibers and varicosities in regions corresponding to the projections of dye-filled neurons. Several observations indicate that rostral cluster neurons transduce tactile stimuli applied to the radula surface: (1) each cell responds to touch of a circumscribed receptive field with a rapidly adapting burst of action potentials, (2) the evoked spikes arise abruptly from the resting potential without prepotentials, and (3) the responses persist when central and peripheral synaptic transmission is blocked in high Mg2+, low Ca2+ artificial seawater solutions. These cells, designated radula mechanoafferent (RM) neurons, do not respond to chemical stimuli including NaCl, glutamate, and seaweed extract. The highest density of receptive fields is found on the posterodorsal edges of the radula halves, areas most directly involved in grasping food. The RM neurons are electrically coupled cells, with coupling coefficients ranging from 0.006 to 0.22. They fire phasically during buccal motor programs, even in the absence of peripheral feedback from the radula or other portions of the buccal mass. In radiolabeling studies the RM cells were found to synthesize authentic SCPA and SCPB. Sensorin-A, a peptide that is localized to other Aplysia mechanoafferent neurons, was not detected immunohistochemically in these cells.