A POPULATION OF SCP-CONTAINING NEURONS IN THE BUCCAL GANGLION OF APLYSIA ARE RADULA MECHANOAFFERENTS AND RECEIVE EXCITATION OF CENTRAL ORIGIN

The rostral cluster of SCP-immunoreactive cells, originally identified in each buccal hemiganglion of juvenile Aplysia, was examined in matu re specimens. Immunohistochemical and dye-fill experiments showed that each rostral cluster consists of approximately 40 cells. Although the se 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 t he food-grasping portion of the chitinous radula. This tissue contains SCP-immunoreactive nerve fibers and varicosities in regions correspon ding to the projections of dye-filled neurons. Several observations in dicate that rostral cluster neurons transduce tactile stimuli applied to the radula surface: (1) each cell responds to touch of a circumscri bed receptive field with a rapidly adapting burst of action potentials , (2) the evoked spikes arise abruptly from the resting potential with out prepotentials, and (3) the responses persist when central and peri pheral synaptic transmission is blocked in high Mg2+, low Ca2+ artific ial seawater solutions. These cells, designated radula mechanoafferent (RM) neurons, do not respond to chemical stimuli including NaCl, glut amate, and seaweed extract. The highest density of receptive fields is found on the posterodorsal edges of the radula halves, areas most dir ectly involved in grasping food. The RM neurons are electrically coupl ed cells, with coupling coefficients ranging from 0.006 to 0.22. They fire phasically during buccal motor programs, even in the absence of p eripheral feedback from the radula or other portions of the buccal mas s. In radiolabeling studies the RM cells were found to synthesize auth entic SCPA and SCPB. Sensorin-A, a peptide that is localized to other Aplysia mechanoafferent neurons, was not detected immunohistochemicall y in these cells.