SYNAPTIC RELATIONSHIPS BETWEEN THE CHORDA TYMPANI AND TYROSINE HYDROXYLASE-IMMUNOREACTIVE DENDRITIC PROCESSES IN THE GUSTATORY ZONE OF THE NUCLEUS OF THE SOLITARY TRACT IN THE HAMSTER

The toxic lectin ricin was applied to the hamster chorda tympani (CT), producing anterograde degeneration of its terminal boutons within the gustatory zone of the nucleus of the solitary tract (NST). Immunocyto chemistry was subsequently performed with antiserum against tyrosine h ydroxylase (TH), and the synaptic relationships between degenerating C T terminal boutons and either TH-immunoreactive or unlabeled dendritic processes were examined al the electron microscopic level. Degenerati ng CT terminal boutons formed asymmetric axodendritic synapses and con tained small, clear, spherical synaptic vesicles that were densely pac ked and evenly distributed throughout the ending, with no accumulation at the active synaptic. The degenerating CT terminated on the dendrit es of TH-immunoreactive neurons in 36% (35/97) of the cases. The most frequent termination pattern involved the CT and two or three other in puts in synaptic contact with a single immunoreactive dendrite, result ing in a glomerular-like structure that was enclosed by glial processe s. In 64% (62/97) of the cases, the degenerating CT was in synaptic co ntact with unlabeled dendrites, often forming a calyx-like synaptic pr ofile that surrounded much of the perimeter of a single unlabeled dend rite. These results indicate that the TH-immunoreactive neurons of the gustatory NST receive direct input from the CT and taste receptors of the anterior tongue and that the termination patterns of the CT vary with its target neuron in the gustatory NST. The glomerular-like struc ture that characterizes many of the terminations of the CT provides an opportunity for the convergence of several functionally distinct inpu ts (both gustatory and somatosensory) onto putative dopaminergic neuro ns that may shape their responsiveness to the stimulation of the oral cavity. (C) 1998 Wiley-Liss, Inc.