STRUCTURE AND FUNCTION OF GUSTATORY NEURONS IN THE NUCLEUS OF THE SOLITARY TRACT - III - CLASSIFICATION OF TERMINALS USING CLUSTER-ANALYSIS

In sensory systems, insight into synaptic arrangements on cells of kno wn physiological response properties has helped our understanding of t he structural basis for these properties. To carry out these types of studies, however, synaptic types in the region of interest must be def ined. Unfortunately, defining synaptic types in the brainstem has prov ed to be a challenging enterprise. Our study was done to classify syna pses in the gustatory part of the nucleus solitarius using objective q uantitative criteria and a cluster analysis procedure. Cluster analysi s allows classification of a population of objects, such as synaptic t erminals, into groups that exhibit similar characteristics. Six termin al types were identified using cluster analysis and subsequent analyse s of variance and post hoc tests. Unlike classification schemes used f or the cerebral cortex, where synaptic apposition density thickness an d shape of vesicles is useful (Gray's Type I and II synapses), the con centration of vesicles in a terminal was a more useful measurement wit h which to classify terminals in the nucleus solitarius, To validate t hat vesicle density (vesicles/mu m(2)) is a useful defining characteri stic to classify terminals in the nucleus solitarius, terminals of a k nown type were used. GABAergic terminals were identified using postemb edding immunohistochemical techniques, and their vesicle density was d etermined. GABAergic terminals fall into the range of two of the termi nal types defined by the cluster analysis and, based on vesicle densit y, two types of GABAergic terminals were identified. We conclude that vesicle density is a helpful means to identify synapses in this brains tem nucleus.