Quantitative ultrastructure of slowly adapting lingual afferent terminals in the principal and oral nuclei in the cat

Previous studies provide evidence that a structure/function correlation exi sts in the cytoarchitectonically different zones of the trigeminal sensory nuclei. To extend this relationship, we examined the ultrastructural featur es of trigeminal primary afferent neurons in the cat dorsal principal nucle us (Vpd) and the rostrodorsomedial oral nucleus (Vo.r) using intra-axonal l abeling with horseradish peroxidase and morphometric analyses. All labeled boutons contained round synaptic vesicles. Eighty-two percent of the bouton s in the Vo.r and 99% of the boutons in the Vpd were presynaptic to nonprim ary dendrites. The remaining boutons in the Vo.r were presynaptic to somata (8%) or primary dendrites (10%). The average number of postsynaptic profil es per labeled bouton did not differ in the Vpd and Vo.r. Most labeled bout ons in the two nuclei were postsynaptic to unlabeled axon terminals with pl eomorphic vesicles (p-ending). The number of p-endings per labeled bouton w as higher in the Vpd than Vo.r A morphometric analysis indicated that label ed bouton volume and apposed surface area were larger in the Vpd than Vo.r while active zone area and vesicle number did not differ. All these paramet ers were larger than those of p-endings in each nucleus. In both labeled bo utons and p-endings, the parameters were positively correlated with bouton size. These results suggest that sensory information conveyed through trige minal afferents is more strongly controlled at the level of the first synap se by presynaptic mechanisms in the Vpd than in the Vo.r, while the efficac y of transmission at primary afferent synapses does not differ. (C) 2001 Wi ley-Liss, Inc.