MORPHOLOGY AND SYNAPTIC CONNECTIONS OF SLOWLY ADAPTING PERIODONTAL AFFERENT TERMINALS IN THE TRIGEMINAL SUBNUCLEI PRINCIPALIS AND ORALIS OFTHE CAT

Previous studies suggest that sensory information from primary afferen t fibers is processed in a distinct manner in the individual subnuclei of trigeminal sensory nuclear complex. The present study has addresse d this issue by using intra-axonal labeling with horseradish peroxidas e to examine the ultrastructure and synaptic organization of axon term inals from slowly adapting (SA) periodontal afferents in the ventral s ubdivision (Vpv) of principalis and the rostro-dorsomedial part (Vo.r) of oralis. Our observations are based on complete or near-complete re constructions of 139 synaptic boutons in Vpv and 105 in Vo.r. All the labeled boutons contained clear, spherical, synaptic vesicles and were presynaptic to unlabeled dendrites, and they were frequently postsyna ptic to unlabeled axon terminals containing pleomorphic synaptic vesic les (P-endings). The P-endings frequently formed axodendritic synapses on dendrites which received axodendritic synapses from labeled bouton s (synaptic triads). On the basis of the number of contacts, synaptic arrangements associated with the labeled boutons could be subgrouped i nto simple (one or two contacts), intermediate (three or four contacts ), and complex (five or more contacts) types. The labeled boutons vari ed from round to elongated forms with smooth to more irregular or scal loped contours. The boutons with scalloped contour were much more freq uent in the complex type. The boutons of the intermediate type were si gnificantly smaller than the complex type and larger than the simple t ype. The SA periodontal afferent terminals participated in each type o f synaptic arrangements in Vpv, but were mostly of the simple type in Vo.r. The size of labeled boutons was significantly larger in Vpv than in Vo.r. The total number of axodendritic and axoaxonic contacts per labeled bouton was significantly higher in Vpv than in Vo.r. Another d ifference was the more frequent occurrence of synaptic triads in Vpv t han in Vo.r. These observations provide evidence that sensory informat ion from primary afferent fibers is processed in a different manner in the two subnuclei. (C) 1994 Wiley-Liss, Inc.