Robert W. Rhoades

Trigeminal primary afferents project bilaterally to dorsal horn and ipsilaterally to cerebellum, reticular formation, and cuneate, solitary, supratrigeminal and vagal nuclei.

Abstract Horseradish peroxidase (HRP) applied to the transected mandibular division of the trigeminal ganglion was transported anterogradely to previously well known primary afferent terminal zones in the dorsal brainstem trigeminal nuclear complex of the rat, and retrogradely to cell bodies in the trigeminal motor, supratrigeminal and mesencephalic nuclei. Primary trigeminal afferents were also visible in the ipsilateral cerebellar cortex and paraflocculus, and the dentate, cuneate, solitary, supratrigeminal, and dorsal motor vagal nuclei, parvicellular reticular formation, area postrema, and C1–C6 dorsal horn. The contralateral medulla and cervical dorsal horn were also innervated by primary afferents which crossed in the posterior commissure. These projections were also labeled when HRP was applied to individual sensory branches of the mandibular nerve.

Morphology, response properties, and collateral projections of trigeminothalamic neurons in brainstem subnucleus interpolaris of rat.

SummaryIntracellular recording, electrical stimulation and horseradish peroxidase (HRP) injection techniques were used to delineate the structural and functional characteristics of trigeminothalamic projection neurons in subnucleus interpolaris of the trigeminal brainstem nuclear complex in rat. Eleven such neurons were successfully characterized and recovered. All were medium to large multipolar neurons in the ventral part of interpolaris and all except one also projected to the superior colliculus. Six of these cells also sent axon collaterals to subnucleus principalis and the medullary parvicellular reticular formation and had local collaterals within interpolaris. None of these trigeminothalamic cells were antidromically activated from the cerebellum. All but one of the recovered cells were responsive to deflection of any one of a number (4–19) of vibrissae. The remaining cell was discharged by displacement of mystical guard hairs. Analysis of electrophysiological and anatomical data revealed significant correlations between receptive field size and dendritic area, thalamic conduction latency and axon diameter, and number of targets innervated and axon diameter.

Superior collicular projection to intralaminar thalamus in rat

The superior collicular (SC) cells which project to the intralaminar thalamus (IT; nuclei centralis lateralis, CL; paracentralis, PC; parafascicularis, Pf) in the rat were identified by means of retrograde transport of wheatgerm agglutinin conjugated horseradish peroxidase (WGA-HRP). SC-IT cells were located throughout the mediolateral and rostrocaudal extents of the tectum ipsilateral to the thalamic injection. In this SC, they had a primarily bilaminar distribution in the lower one-half of the stratum griseum intermediale (SGI) and upper portion of the stratum griseum profundum (SGP). In these laminae, SC-IT cells were arranged in clusters or patches similar to those which have been described for many inputs to the deep SC laminae. A small number of SC-IT cells were also observed in the deep laminae of the tectum contralateral to the thalamic injection. Double labelling experiments using True Blue (TB) and Diamidino Yellow (DY) demonstrated that less than 1% of the contralaterally projecting SC-IT cells also innervated ipsilateral IT. Anterograde tracing with [3H]leucine demonstrated further that SC projected heavily to CL, PC and Pf. This projection also extended into the medial portion of the posterior thalamus (PO).

Reorganization of the peripheral projections of the trigeminal ganglion following neonatal transection of the infraorbital nerve.

Two different anatomical techniques were used to obtain evidence that transection of the infraorbital (IO) nerve on the day of birth would result in reorganization of the peripheral projections of the trigeminal nerve. In 14 of 19 neonatally nerve-damaged adult rats, injection of horseradish peroxidase (HRP) directly into the IO nerve, proximal to the point of the neonatal transection, resulted in labeled cells in the ophthalmic-maxillary portion of the ganglion and labeled fibers in mandibular sensory nerves. In an additional 28 neonatally nerve-damaged adult rats, double-labeling techniques were employed to document the reorganization suggested by the HRP tracing experiments. In these experiments, one fluorescent tracer, diamidino yellow (DY), was injected directly into the regenerate IO nerve, proximal to the point of the neonatal transection; a second tracer, true blue (TB), was deposited into peripheral ophthalmic and/or mandibular fields. These combinations of injections invariably resulted in the demonstration of a small number (46-401) of double-labeled cells that were located in the ophthalmic-maxillary part of the ganglion. Identical combinations of injections in normal adult rats and the intact sides of nerve-damaged animals never produced more than 6 double-labeled cells per ganglion. In two additional series of experiments, sequential double-labeling techniques were employed to demonstrate that the multiply projecting ganglion cells probably arose in at least two ways: (1) development of non-IO projections by ganglion cells that contributed axons to the IO nerve at the time of the lesion; (2) elaboration of IO axon branches by primary afferent neurons that had non-IO projections at the time of the lesion. A final two-stage double-labeling experiment demonstrated that approximately 75% of the ganglion cells that projected to the whisker pad at birth, and survived transection of the IO nerve on the first postnatal day, regenerated axons into this trigeminal branch.

Dendrites of deep layer, somatosensory superior collicular neurons extend into the superficial laminae

Intracellular recording and horseradish peroxidase (HRP) injection techniques were used to delineate the structural and functional properties of superior collicular (SC) neurons in hamsters. Of 34 cells recovered from the deep laminae (those ventral to the stratum opticum--SO), 26 were exclusively somatosensory and 10 of these extended dendrites into the superficial layers, the stratum griseum superficiale (SGS) and SO. In 2 instances, dendrites extended only to the SO, but in 8 others they reached the SGS. Three of the latter cells had dendrites which terminated just beneath the pial surface. These findings show that an anatomical substrate for communication from superficial to deep layer cells exists in the hamster SC, but that such communication may not necessarily be reflected in the response of deep layer neurons.

Preventing regeneration of infraorbital axons does not alter the ganglionic or transganglionic consequences of neonatal transection of this trigeminal branch

Retrograde and transganglionic tracing with a combination of horseradish peroxidase (HRP) and wheatgerm agglutinin (WGA)-conjugated HRP (WGA-HRP) was employed to determine whether transection of the infraorbital (IO) nerve on the day of birth and prevention of regeneration by retransecting it at weekly intervals until the time of a terminal anatomical experiment had effects upon ganglion cell survival and innervation of the brainstem by this trigeminal (V) branch that differed from those which followed a single transection of the same nerve on the day of birth without any attempt to prevent peripheral regeneration of the cut axons. Counts of labelled ganglion cells and examination of the brainstem labelling produced by application of HRP and WGA-HRP to the IO nerve proximal to the point of transection(s) at 6 weeks of age demonstrated no differential effects of preventing regeneration of the cut nerve. In animals subjected to a single transection of the nerve (n = 9), we counted an average of 5001.2 (S.D. = 1286.9) labelled ganglion cells and these had an average diameter of 22.7 micron (S.D. = 6.3). In the rats (n = 9) that sustained multiple nerve cuts, the average number of labelled ganglion cells was 4447.8 (S.D. = 1060.9). The mean diameter for these primary afferent neurons was 21.5 micron (S.D. = 6.6). Neither of these values were significantly different from those from the rats subjected to a single nerve cut. The cell counts from both of these groups were significantly lower than those obtained after application of HRP and WGA-HRP to the IO nerve in normal rats (n = 3, X = 12,553.3, S.D. = 1454.8), but the average cell diameter in the normals (X = 23.2, S.D. = 6.6) was not significantly greater than that in the nerve-damaged animals. The pattern of brainstem labelling observed in the rats subjected to multiple nerve cuts was the same as that in the rats which sustained a single transection of the IO nerve on the day of birth. Very little terminal labelling was observed in nucleus principalis, subnucleus oralis, subnucleus interpolaris or the magnocellular portion of caudalis. There was, however, very heavy labelling in laminae I and II of the latter nucleus.

Organization of the intercollicular pathway in rat

The intercollicular pathway of the rat was studied using autoradiographic (ARG) and horseradish peroxidase (HRP) tracing techniques. The HRP experiments demonstrated that the cells of origin of the intertectal pathway were located primarily in the rostral stratum griseum intermediale ( SGI ), stratum album intermedium (SAI) and stratum griseum profundum (SGP). Intertectal neurons were in most cases multipolar and had average somal diameters which ranged between 8 and 33 micron. Only a small number of superficial layer neurons contributed axons to the intercollicular pathway. ARG tracing showed that the intertectal pathway terminated in the deep layers of the rostral one half of the colliculus. The primary terminal zone was SGP. In addition, labeled axons left this region and coursed dorsally to terminate in a series of patches in the lower SGI and upper SAI. A small number of labeled fibers also reached the stratum opticum (SO) and lower stratum griseum superficiale (SGS).

Reorganization of trigeminal primary afferents following neonatal infraorbital nerve section in hamster

The infraorbital nerve was sectioned and the ipsilateral whisker follicles were cauterized in hamsters within 12 h of birth. Sixty to ninety days later application of HRP to the proximal stumps of the ipsilateral lingual, inferior alveolar, mylohyoid and auriculotemporal nerves resulted in increased numbers of labeled somata in trigeminal ganglion regions which contain primarily infraorbital cell bodies in normal animals. The labeled central processes of mandibular nerves also occupied portions of the brainstem trigeminal complex normally innervated by infraorbital axons. These findings represent the first anatomical demonstration of trigeminal primary afferent plasticity.

A comparison of visual callosal organization in normal, bilaterally enucleated and congenitally anophthalmic mice

SummaryVisual callosal connections were examined using the horseradish peroxidase (HRP) technique in normal, neonatal and adult C57BL mice, and in adults of this strain which were bilaterally enucleated within 12 h of birth. In addition, callosal connections were also delineated in two strains of congenitally anophthalmic mice, ZRDCTan and orJ. Material from 129/J mice served as controls for the latter strain. In normal adults anterograde labelling and HRP labelled cells were visible primarily at the borders of area 17. In the 17–18a border region, labelled neurons were located primarily in layers II–III and V. In the medial striate cortex, a small number of labelled cells were present, primarily in lamina VI. Anterograde HRP labelling in the normal adults was also located primarily at the borders of area 17. At the 17–18a border, it was very heavy in layers V and VI, somewhat lighter in layer IV, and fairly dense in layers II–III and the lower half of lamina I. Labelling indicative of anterograde HRP transport was also visible in lowermost lamina V and layer VI across the entire mediolateral extent of area 17. In normals injected with HRP on postnatal day 2 and perfused 24 h later, callosal neurons were distributed throughout the dorsal posterior neocortex, primarily in layers V and VI. Only a very few labelled cells were visible in the supragranular laminae. In adult mice blinded at birth, the zone of callosal cells and terminals extended much further into area 17 than in normals, but aside from the anterograde labelling in layer VI and lowermost lamina V, the medial one-third of the striate cortex was still for the most part devoid of callosal cells and fibers. The laminar distributions of the labelled cells and anterograde transport in the blinded animals were the same as in the normal mice. In both strains of anophthalmic mice the pattern of callosal connections was unlike that in either the normals or neonatal enucleates. In the caudal “visual” cortex, callosal cells and anterograde transport indicative of terminal labelling were visible primarily in the 17–18a border area. Rostrally, however, they were both distributed in multiple (two-three) patches within area 17. Serial reconstructions demonstrated that these patches tended to be aligned in stripes which ran parallel to the 17–18a border. One of these was always located at the 17–18a border, and here the laminar distribution of labelled cells and anterograde labelling was the same as in the normals. In the more medial patches, however, labelled cells and anterograde labelling were confined almost completely to layers II and III. The distribution of callosal cells in neonatal ZRDCT-an mice was not appreciably different from that in C57BL mice of the same age.

Axon arbors of functionally distinct whisker afferents are similar in medullary dorsal horn

Using the intra-axonal horseradish peroxidase (HRP) technique, we have found that the central axon arbors of functionally distinct mystacial whisker primary afferents in rat medullary dorsal horn were all similar with respect to their shape and density of terminal boutons. Arbors from slowly adapting, rapidly adapting, velocity or nociceptive biased whisker afferents all appeared to form a series of highly localized terminal aggregates within a specific rostrocaudal segment of the superficial or deep half of the lamina III-IV magnocellular region. Other trigeminal afferents adhered to previously described somatosensory structure-function relationships in the cat lumbosacral spinal cord. These findings suggest that topographic constraints, as well as functional considerations, are important in determining primary afferent terminal arbor patterns in the medullary dorsal horn.