Ultrastructural changes in acetylcholinesterase activity in the deafferented spinal trigeminal nucleus

Association of extracellular acetylcholinesterase with gustatory nerve terminal fibers in the nucleus of the solitary tract

Acetylcholinesterase (AChE) staining is associated with terminal fields of the glossopharyngeal and chorda tympani nerves in the nucleus of the solitary tract (NST). To address AChE function at these sites, the location of the staining was examined at the fine structural level in combination with the labeling of chorda tympani nerve fibers with biotinylated dextran in golden Syrian hamsters. AChE staining was located in the endoplasmic reticulum of geniculate ganglion neuronal somata, and extracellularly, surrounding labeled chorda tympani terminal fibers and boutons in the NST. Neuronal profiles adjacent to these labeled fibers were stained less intensely, whereas most non-adjacent profiles were unstained. The location of staining is consistent with the secretion of AChE into the extracellular space by primary afferent chorda tympani fibers. AChE staining was reduced in the dextran-labeled chorda tympani fibers and terminals as well as adjacent non-labeled profiles 2 weeks following nerve transection and dextran application. The distribution of staining outside synapses and the loss of staining following denervation is suggestive of a non-cholinergic role for AChE in the intact gustatory system.

Experimental trigeminal nerve injury

The successful reinnervation of peripheral targets after injury varies with the axonal population of the nerve that is injured and the extent of the dislocation of its central component from the peripheral endoneurial tube. Larger-diameter axons such as those supplying mechanoreceptors recover more readily than narrower axons such as those supplying taste. A complex, bi-directional interaction between lingual epithelium and sprouting nerve results in the redifferentiation of taste buds after denervation. Dentin and the dental pulp provide a strong attraction to sprouting nerves and will become reinnervated from collateral sources if recovery of the original innervation is blocked. The most effective repair technique for transected lingual nerves is one which brings the cut ends together rather than one that provides a temporary bridge. Injuries can result in cell death in the trigeminal ganglion but only if the injury is severe and recovery is prevented. Lesser damage results in chromatolysis and the increased expression of neuropeptides. All nerve injuries bring about changes in the trigeminal nucleus. These occur as changes in receptive field and the incidence of spontaneously active neurons, effects which are consistent with the unmasking of existing afferents. These functional changes are short-lived and reversible. Morphologically, nerve injury results in terminal degeneration in the nuclei and an increased expression of the c-Fos gene and some neuropeptides. Only a chronic constriction injury induces behavioral changes. The adult trigeminal system retains considerable plasticity that permits it to respond successfully to nerve injury. Much remains to be learned about this response, particularly of the trophic factors that control peripheral recovery and the central response to more severe injuries.

Contralateral degeneration in the cat spinal trigeminal nucleus following unilateral retrogasserian trigeminal rhizotomy

Retrogasserian trigeminal rhizotomy was used to study the central projections and patterns of degeneration in the spinal trigeminal nucleus (STN). At survival times of 3-20 days, reduced silver stains show extensive degeneration throughout the ipsilateral STN and in addition, well delineated degeneration was identified in the periobex region of the contralateral STN that varied with survival time. The results suggest that primary afferents may contribute to this contralateral projection.