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The central projections of rat trigeminal primary afferent neurons to various "non-trigeminal" areas of the central nervous system were examined by labeling the fibers with wheat germ agglutinin-horseradish peroxidase (WGA-HRP) transported anterogradely from the trigeminal ganglion. This technique produced a clear and comprehensive picture of trigeminal primary afferent connectivity that was in many ways superior to that which may be obtained by using degeneration, autoradiography, cobalt labeling, or HRP transganglionic transport techniques. Strong terminal labeling was observed in all four rostrocaudal subdivisions of the trigeminal brainstem nuclear complex, as well as in the dorsal horn of the cervical spinal cord bilaterally, numerous brainstem nuclei, and in the cerebellum. Labeling in the ipsilateral dorsal horn of the cervical spinal cord was very dense at C1, moderately dense at C2 and C3, and sparse at C4-C7. Numerous fibers crossed the midline in the medulla and upper cervical spinal cord and terminated in the contralateral pars caudalis and dorsal horn of the spinal cord from C1-C5. The latter axons terminated most heavily in the mandibular and ophthalmic regions of the contralateral side. Extremely dense terminal labeling was observed in the ipsilateral paratrigeminal nucleus and the nucleus of the solitary tract, moderate labeling was seen in the supratrigeminal nucleus and in the dorsal reticular formation, and small numbers of fibers were observed in the cuneate, trigeminal motor, lateral and superior vestibular nuclei, and in the cerebellum. The latter fibers entered the cerebellum in the superior cerebellar peduncle and projected to the posterior and anterior lobes as well as to the interposed and lateral deep cerebellar nuclei. Most projections in this study originated from fibers in the dorsal part of the spinal tract of V, suggesting a predominantly mandibular origin for these fibers. Projections from the ophthalmic and maxillary divisions, in contrast, were directed mainly to the cervical spinal cord bilaterally, to contralateral pars caudalis, and to certain areas of the reticular formation. In conclusion, this study has demonstrated that somatosensory information from the head and face may be transmitted directly to widespread and functionally heterogeneous areas of the rat central nervous system, including the spinal cord dorsal horn, numerous brainstem nuclei, and the cerebellum.(ABSTRACT TRUNCATED AT 400 WORDS)

The central projections of five peripheral branches of the trigeminal nerve were investigated by the method of transganglionic transport of horseradish peroxidase (HRP). In separate animals, the corneal, supraorbital, infraorbital, mental, or inferior alveolar branches were transected and soaked in concentrated solutions of HRP. Forty-eight to 72 hours after surgery, the brain-stem, upper cervical spinal cord, and trigeminal ganglia were perfusion-fixed and processed according to the tetramethylbenzidine technique. The results show that trigeminal primary afferent neurons which innervate the cornea project mainly to the levels of caudal pars interpolaris and caudal pars caudalis. In contrast, trigeminal primary afferent neurons whose peripheral processes course through the supraorbital, infraorbital, or mental nerves project most heavily to the trigeminal main sensory nucleus, pars interpolaris, and the rostrocaudal middle three-fifths of pars caudalis. Trigeminal primary afferent neurons which give origin to the inferior alveolar nerve project heavily and in approximately equal numbers of all rostrocaudal levels of the trigeminal brainstem nuclear complex (TBNC). A small number of fibers from each of the latter four cell populations project directly to the contralateral C1-C2 dorsal horn. A small number of fibers from each cell population studied end in the reticular formation immediately adjacent to the spinal nucleus of V. It is concluded that the cornea and facial skin regions of the cat are represented nonuniformly along the rostrocaudal length of the TBNC.

Horseradish peroxidase-wheat germ agglutinin (HRP-WGA) applied to proximal stumps of tooth pulp primary afferent neurons in rats was taken up and transported transneuronally to neurons in the ipsilateral trigeminal brainstem nuclear complex. The results of this study suggest that HRP-WGA transport may be a novel means of labeling both primary and higher order neurons that transmit tooth pulp sensory information in the rat and may be used to investigate the fine structure and synaptic contacts of central nervous system neurons that receive tooth pulp afferent input.