A comparison between wheat germ agglutinin-and choleragenoid-horseradish peroxidase as anterogradely transported markers in central branches of primary sensory neurones in the rat with some observations in the cat

Light- and electron-microscopic localization of primary dental afferents to medullary dorsal horn (pars caudalis)

Light-microscopic (LM) and ultrastructural (electron-microscopic, or EM) identification of primary dental afferents to medullary dorsal horn (MDH) was demonstrated in the cat following injections of horseradish peroxidase (HRP) into pulpal chambers of unilateral maxillary and mandibular posterior teeth, including the cuspids. Use of a new osmication protocol improved and simplified the EM localization of reaction product within the brain stem terminals. LM examination showed that the projection pattern varied between the different levels of MDH. At caudal levels, the labeling was primarily confined to a narrow band consisting of a dense projection to the dorsomedial portion of laminae I and superficial II and a less intense projection to lamina V. The pattern to rostral levels became increasingly more dense and extensive within these same laminae. LM examination of the tooth apex region showed that a limited spread to the periodontal ligament occurred in some cases. EM investigation of the ipsilateral MDH demonstrated reaction product in terminals with synaptic vesicles that are presynaptic to small and medium-sized dendrites. Labeled axonal endings in close association with cell bodies were also observed. No labeled structures were identified in the contralateral MDH. Some of the reaction product found with EM was below the LM limit of resolution, and thus ultrastructural investigation is necessary for a complete analysis of any pathway when using HRP.

Transganglionic transport of wheat germ agglutinin-HRP and choleragenoid-HRP in rat trigeminal primary sensory neurons

Horseradish peroxidase conjugates of either the lectin wheat germ agglutinin (WGA-HRP) or choleragenoid (B-HRP) have been shown to be sensitive neuroanatomical tracers. In the present study a comparison was made between these two conjugates as transganglionic tracers in trigeminal primary sensory neurons following injection into the rat mystacial vibrissae skin. Differences between the two tracers were observed in the labeling of cell bodies in the trigeminal ganglion. Injection of WGA-HRP resulted in labeling of predominantly small cell bodies, whereas B-HRP gave rise to labeling of somewhat larger cell bodies. By increasing the concentration of the injected WGA-HRP solution the number of labeled cells increased substantially, while a corresponding increase in the concentration of B-HRP resulted in a relatively small increase in the number of labeled cells. WGA-HRP injection resulted in labeling of primary afferents mainly in the substantia gelatinosa of the trigeminal subnucleus caudalis. When the concentration of the injected WGA-HRP solution was increased, labeling was also observed in the marginal and magnocellular zones. Following B-HRP injection, labeling was only observed in the magnocellular zone and innermost part of the substantia gelatinosa. This general pattern of labeling was the same when the concentration of the B-HRP solution was increased.

Further indications for enhancement of retrograde transneuronal transport of WGA-HRP by synaptic activity

Factors affecting the retrograde transneuronal transport of wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP), from spinal motoneurones to interneurones, have been studied in the cat. To this end, the location of transneuronally labelled interneurones was compared in animals which were awake or remained anaesthetized after WGA-HRP had been injected into the semitendinosus or the medial gastrocnemius nerve. In the anaesthetized animals motor axons of the injected nerves were stimulated selectively, to activate only Renshaw cells, or together with group I afferents, to activate also laminae V-VI interneurones with input from these muscles. The interneurones labelled in this study were distributed in different spinal cord regions than the interneurones labelled in preparations in which group I afferents of antagonist muscles were stimulated, as described in a previous study. The reported observations extend evidence of Harrison et al. that the retrograde transneuronal transport of WGA-HRP is facilitated by synaptic activity.