Topographic representation of lower and upper teeth within the trigeminal sensory nuclei of adult cat as demonstrated by the transganglionic transport of horseradish peroxidase

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Abstract
Transganglionic transport of horseradish peroxidase‐wheat germ agglutinin conjugate (HRP‐WGA) entrapped in hypoallergenic polyacrylamide gel was used to study the patterns of termination of primary afferents that innervate the lower and upper tooth pulps within the trigeminal sensory nuclear complex (TSNC). HRP injections were made into the inferior and superior alveolar nerves in order to compare the central projections of the whole nerve with those from tooth pulps. In addition, the relationship between the distribution of the trigeminothalamic tract cells and the projection sites of the tooth pulp afferents was investigated by injecting HRP into the posterior ventral thalamus. HRP‐labeled tooth pulp afferent fibers innervating the lower and upper teeth projected to the subnucleus dorsalis (Vpd) of pars principalis, the rostrodorsomedial part (Vo.r) and nucleus dorsomedialis (Vo.dm) of pars oralis, the medial regions of pars interpolaris, and laminae I, II, and V of pars caudalis. Terminal fields of the lower tooth pulp afferents formed a rostrocaudally running, uninterrupted column from the midlevel of Vpd to the caudal tip of caudalis. In contrast, the column of termination of upper tooth pulp afferents was discontinuous at the Vpd/Vo.r transition, and ended at the more rostral level of the caudalis than that of the lower tooth pulp afferents. The representation of the lower and upper teeth in the TSNC was organized in a somatotopic fashion which varied from one subdivision to the next, although terminal zones of the inferior and superior alveolar nerves overlapped within the Vo.r, Vo.dm, and dorsomedial part of rostral pars interpolaris. The lower and upper teeth were represented in the Vpd, Vo.r, Vo.dm, medial region of pars interpolaris, and laminae I, II, and V, in a ventrodorsal or caudorostral, dorsoventral, lateromedial, dorsoventral, and mediolateral or dorsomedial‐ventrolateral sequence, respectively. The smaller, more focal terminal areas of the teeth contrasted sharply with more extensive terminal fields of the alveolar nerves. The HRP injections within the thalamus indicated that neurons in Vpd, the caudal pars inter polaris, and laminae I/V of caudalis, which are subdivisions of TSNC that receive pulpal projections, sent their axons to the ipsilateral and contralateral posterior ventral thalamus. The present data reconfirmed our previous suggestion (Shigenaga et al.: J. Comp. Neurol. 243:388–408, '86a, J. Comp Neurol. 244:1–18, '86b) of a functional segregation among the cytoarchitectonically different zones of the TSNC, and further suggested that the sub nucleus dorsalis of the principal nucleus in addition to the caudal parts of interpolaris and laminae W of the caudalis plays an important role in the relay of pain from the oral regions.