THE CENTRAL PROJECTIONS OF THE MONKEY TOOTH-PULP AFFERENT NEURONS

Transganglionic transport of horseradish peroxidase conjugated to whea tgerm agglutinin (HRP:WGA) entrapped in hypoallergenic polyacrylamide gel was used to study the patterns of termination of primary afferents that innervate the upper and lower tooth pulps within the trigeminal sensory nuclear complex (TSNC) of the monkey. HRP:WGA injections were also made into the lower incisors and molars, in order to examine the topographic arrangement of pulpal afferent projections. HRP-labeled pu lpal afferents innervating lower and upper teeth projected ipsilateral ly to the rostral subnucleus dorsalis (Vpd) and caudal subnucleus vent ralis (Vpv) of the nucleus principalis (Vp); the rostrodorsomedial (Vo .r) and dorsomedial (Vo.dm) subdivisions of the nucleus oralis (Vo); t he dorsomedial subdivision of the nucleus interpolaris (Vi); and lamin ae I-II and/or V of the nucleus caudalis (Vc) at its rostralmost level . The HRP-labeled terminals from upper and lower pulpal afferents form ed a rostrocaudal column from the midlevel of Vp to the rostral tip of Vc. The label in Vp and Vo was considerably dense, but the column of terminals was interrupted at the Vpd-Vpv transition. The label in Vi a nd Vc was much less dense compared to that in the rostral nuclei, and the column of terminals was interrupted frequently. The representation of the upper and lower teeth in TSNC was organized in a somatotopic f ashion that varied from one subdivision to the next, though their term inal zones overlapped within Vpd. The upper and lower teeth were repre sented in Vpv, Vo.r, Vo.dm, Vi, and Vc in a ventrodorsal, dorsoventral , lateromedial, lateromedial, and lateromedial sequence, respectively. Topographic arrangement was also noticed for the projections of pulpa l afferents from the lower incisors and molars: The representations of the lower incisors and molars in Vpv, Vo.r, Vo.dm, Vi, and Vc were or ganized in a lateromedial, dorsoventral, ventrodorsal, ventrodorsal, a nd lateromedial sequence, respectively. The present results indicating sparse projections from pulpal afferents in the monkey's Vc are in go od correspondence with a clinical report that trigeminal tractotomy ju st rostral to the obex has no significant effect on dental pain percep tion in patients. Furthermore, the present study indicates that projec tion patterns of pulpal afferents-which include the termination sites, the density of terminations between nuclei, and topographic arrangeme nt-differ among animal species.