Morphological types of neurons located at taste‐responsive sites in the solitary nucleus of the hamster

Abstract

HRP histochemistry and microelectrode mapping were combined to study the sizes, shapes, and orientations of neuronal cell bodies and dendrites located at sites of taste-elicited single unit activity in the nucleus of the solitary tract (NST). Cells responding to sapid stimulation of the anterior tongue were extracellularly recorded using micropipettes containing HRP. Iontophoretic injection of the marker at the recording sites resulted in small (50-200 microns diameter) opaque zones bordered by a small number (2-15) of neurons with Golgi-like filling of their cell bodies, dendrites, and to some extent, their axons. The cell bodies were near (50-250 microns) the injection sites into which they sent labelled dendrites. Two broad categories of neurons were typically filled. Elongate cells had oval- to spindle-shaped cell bodies oriented mediolaterally. Two primary dendrites extended 100-300 microns from the cell body, one medially and one laterally, and branched within a cylindrical dendritic field oriented mediolaterally. A minority of the HRP-filled elongate cells had unusually long rostrally or caudally directed dendritic branches. Stellate cells had oval, round, triangular, or polygonal cell bodies and 3-5 primary dendrites coursing 200-300 microns in all directions and branching as unoriented, spheroidal fields. A minority of stellate cells had relatively unbranched wavy dendrites, resembling tentacles, while others had unusually small cell bodies (10-15 microns diameter), small dendrites, and locally arborizing axons. Of 151 labelled cells, all but 12 were remarkably confined to the rostral NST. Nearly 90% were concentrated in the rostral central cytoarchitectonic subdivision, where stellate cells predominated, or in the rostral lateral subdivision, where elongate cells predominated. These morphological types of neurons, filled at neurophysiological recording sites, are compared with cell types identified in previous light and electron microscopic studies of the cytoarchitecture, connections, and synaptic organization of the gustatory NST.