STRUCTURE AND FUNCTION OF GUSTATORY NEURONS IN THE NUCLEUS OF THE SOLITARY TRACT .1. A CLASSIFICATION OF NEURONS BASED ON MORPHOLOGICAL FEATURES

Prior investigations in other laboratories have provided convincing ev idence that the neurons of the rostral nucleus of the solitary tract ( rNST) can be grouped according to their physiological response propert ies or morphologic features. The present study is based on the premise that the response properties of gustatory neurons are related to, and perhaps governed by, their morphology and connectivity. In this first phase of our ongoing investigation of structure-function relationship s in the rNST of the rat, we have used intracellular injection of neur obiotin to label individual physiologically characterized gustatory ne urons. A total of 63 taste-sensitive neurons were successfully labeled and subjected to three-dimensional quantitative and qualitative analy sis. A cluster analysis using six morphologic features (total cell vol ume, soma area, mean segment length, swelling density, spine density, and number of primary dendrites) was used to identify six cell groups. Subsequent analyses of variance and posthoc comparisons verified that each of these six groups differed from all others with respect to at least one variable, so each group was ''typified'' by at least one of the six morphologic features. Neurons in group A were found to be the smallest neurons in the sample. The cells in group B had small somata and exhibited the highest swelling density of any group. Group C neuro ns were distinguished by dendrites with long, spine-free branches. The se dendrites were significantly longer than those of any other group e xcept Group F. The neurons in group D had more primary dendrites than any other group. Group E neurons possessed dendrites with the lowest s welling density but the most spines of any group. The cells in group F were the largest neurons in our sample and possessed the largest soma ta of any group. Thus overall cell size and density of dendritic spine s and swellings were found to be particularly important variables in t his classification scheme. Our preliminary results suggest that the nu mber and density of dendritic spines (as well as other morphologic fea tures) may be related to a given neuron's most effective stimulus, ind icating that it will indeed be possible to use the criteria establishe d in the present investigation to derive structure-function relationsh ips for gustatory neurons in the rNST. (C) 1994 Wiley-Liss, Inc.