ULTRASTRUCTURAL OBSERVATIONS OF SYNAPTIC CONNECTIONS OF VIBRISSA AFFERENT TERMINALS IN CAT PRINCIPAL SENSORY NUCLEUS AND MORPHOMETRY OF RELATED SYNAPTIC ELEMENTS

Previous work. suggests that slowly adapting (SA) periodontal afferent s have different synaptic arrangements in the principal (Vp) and oral trigeminal nuclei and that the synaptic structure associated with tran smitter release may be related directly to bouton size. The present st udy examined the ultrastructures of SA and fast adapting (FA) vibrissa afferents and their associated unlabeled axonal endings in the cat Vp by using intra-axonal labeling with horseradish peroxidase and a morp hometric analysis. All SA and FA afferent boutons contained clear, rou nd, synaptic vesicles. All the FA and most SA boutons were presynaptic to dendrites, but a few SA boutons were axosomatic. Both types of bou ton were frequently postsynaptic to unlabeled axonal ending(s) contain ing pleomorphic, synaptic vesicles (P-ending). The size of labeled bou tons was larger in FA than SA afferents, but the size of dendrites pos tsynaptic to labeled boutons was larger for SA than FA afferents. Larg e-sized FA and SA boutons made synaptic contacts with small-diameter d endrites. The size of FA and SA boutons was larger than that of their associated P-endings. A morphometric analysis made on the pooled data of SA and FA boutons indicated that apposed surface area, active zone number, total active zone area, vesicle number, and mitochondrial volu me were highly correlated in a positive linear manner with labeled bou ton volume. These relationships were also applicable to unlabeled P-en dings, but the range of each parameter was smaller than that of the la beled boutons. These observations provide evidence that the two functi onally distinct types of vibrissa afferent manifest unique differences but share certain structural features in the synaptic organization an d that the ultrastructural ''size principle'' proposed by Pierce and M endell ([1993] J. Neurosci. 13:4748-4763) for Ia-motoneuron synapses i s applicable to the somatosensory system. (C) 1997 Wiley-Liss, Inc.