The role of the principal sensory nucleus in central trigeminal pattern formation

NGF augmentation rescues trigeminal ganglion and principalis neurons, but not brainstem or cortical whisker patterns, after infraorbital nerve injury at birth

Prior studies indicate that neonatal nerve injury kills many trigeminal (V) first- and second-order cells, and interrupts pattern formation in the brainstem and cerebral cortex. Yet it is not known whether effects upon cell survival and pattern formation are causally related. To determine whether axotomized V ganglion cells can be rescued by an exogenous trophic agent, rats received 5 mg/kg of nerve growth factor (NGF) prior to, and every day after, infraorbital nerve section on the day of birth until sacrifice on postnatal day (PND) 1, 3, 5, 7, or 14. Other animals received identical lesions without NGF. Ganglion cell numbers were significantly reduced by PND1 in pups not given NGF, while NGF-treated rats displayed no significant cell loss through PND7. However, NGF did not permanently rescue V neurons because ganglion cell numbers were reliably reduced by PND14. Cell numbers in V nucleus principalis were reduced by PND1 in pups not given NGF, while NGF-treated animals displayed no cell loss through PND14. NGF's rescue of second-order cells is probably an indirect effect of NGF action upon V ganglion cells because, in other newborns, NGF failed to maintain principalis cells after direct lesion of the left V ganglion. To determine whether preventing cell death permits whisker-related pattern formation, other rats also received NGF prior to and after infraorbital nerve section at birth. After 3-14 days, patterns were assessed in the brainstem and cortex with cytochrome oxidase histochemistry and serotonin immunocytochemistry. Whisker-related patterns failed to develop as in cases not given NGF. These data indicate that communication with the periphery is necessary for the maintenance of central whisker-related patterns. They also suggest that V ganglion cells can be rescued, albeit temporarily, from rapid injury-induced death by NGF, thereby delaying injury-induced cell death in nucleus principalis. However, the mechanism(s) responsible for injury-induced pattern alterations in the developing V system remains to be elucidated.

2-DG uptake patterns related to single vibrissae during exploratory behaviors in the hamster trigeminal system

Stimulation of one or several whiskers activates discrete foci throughout the trigeminal (V) neuraxis. These foci contribute to patterns, corresponding to the patterns of vibrissae, that have been directly related to aggregates of cells and axon terminals in the "barrel" cortex. Here, we combine high-resolution, 2-deoxyglucose (2DG) mapping and cytochrome oxidase (CO) staining to determine whether the known pattern of V primary afferent projections is sufficient to deduce the functional activation of their targets during exploratory behavior. Four adult hamsters had all of their large mystacial vibrissae trimmed acutely, except for C3 on the left, and B2 and D4 on the right; in two others, the left C3 and right A1 and E4 whiskers were spared. After fasting overnight, 2DG was injected and the animals behaved freely in the dark for 45 minutes. The brainstem, thalamus, and cortices were sectioned, then processed for both CO staining and 2DG autoradiography. Image-processing microscopy was used to separate the autoradiographic silver grains from the histochemical staining. CO patches were patterned in a whisker-like fashion in the full rostrocaudal extent of V nucleus principalis and in caudal portions of spinal V subnuclei interpolaris and caudalis, but absent in subnucleus oralis. 2DG silver grains were densest above those CO patches in the pattern corresponding to the active whiskers. There were no consistent 2DG foci in subnuclei oralis or rostral caudalis. In these same cases, prominent 2DG labeling was restricted to the appropriate barrels in the contralateral cortex. Only one case, however, displayed a clear and appropriate region of heightened 2DG uptake in contralateral ventroposteromedial thalamus (VPM) and the adjacent part of the reticular thalamic nucleus. Patterns of increased glucose utilization with single whisker stimulation are well matched to the CO patterns that mirror distributions of neurons associated with a vibrissa in the V brainstem complex, thalamus, and cortex. Single whiskers are represented by relatively homogeneous longitudinal columns of 2DG labeling in the V brainstem nuclei. The columns are not continuous through the axial extent of the V brainstem complex; rather, they occur separately within principalis, interpolaris, and caudalis. While whisker columns were consistently labeled in interpolaris and caudalis in all animals, the labeling was increasingly variable in principalis, barrel cortex, and VPM, respectively. This suggests that the behaving animal can and does significantly modulate activity in this major, synaptically secure pathway.

Effects of neonatal transection of the infraorbital nerve upon the structural and functional organization of the ventral posteromedial nucleus in the rat

The present study examined the way in which an indirect partial deafferentation of the medial portion of the ventrobasal complex (VPM/VPL) induced by neonatal transection of the infraorbital nerve (ION) altered the structural and functional properties of its constituent neurons. This manipulation significantly reduced the volume of the contralateral VPM/VPL. In addition, cell counts in Nissl-stained material revealed a significant reduction of the number of VPM/VPL neurons contralateral to neonatal ION transection. We also analyzed the effect of neonatal ION transection on the soma-dendritic morphology of individual neurons in the ventral posteromedial nucleus of the thalamus (VPM) by intracellular injection of horseradish peroxidase (HRP) in vivo and Lucifer yellow in fixed slices. Neonatal transection of the ION resulted in increased dendritic length, area, and volume of VPM neurons in both preparations; however only the changes observed in fixed slices reached statistical significance. Alterations in the functional characteristics of VPM neurons were also observed following neonatal nerve damage. There was a significant decrease in the percentage of vibrissae-sensitive neurons and a corresponding increase in the percentages of neurons responsive to guard hair deflection or that were unresponsive to peripheral stimulation. Neonatal nerve damage also resulted in significantly longer latencies of VPM cells after stimulation of either trigeminal nucleus principalis or subnucleus interpolaris. The present results indicate that the development of normal response properties and soma-dendritic morphology of VPM neurons is dependent upon intact afferent input during development. Indirect partial deafferentation of VPM/VPL by neonatal transection of the ION results in reduced neuron number, which may result in decreased competition among the dendrites of these neurons. This proposal is consistent with observations of increased dendritic dimensions of VPM neurons contralateral to neonatal ION damage.

Differential effects of peripheral damage on vibrissa-related patterns in trigeminal nucleus principalis, subnucleus interpolaris, and subnucleus caudalis

Histochemistry for cytochrome oxidase reveals a vibrissa-related pattern in trigeminal nucleus principalis, subnucleus interpolaris, and the magnocellular portion of subnucleus caudalis. This pattern is apparent in late fetal animals and is disrupted by transection of the infraorbital nerve on the day of birth. We recently reported results suggesting that the cytochrome oxidase pattern reflects primary afferent-induced clustering of second order neurons in all of these nuclei. If this conclusion is correct, it should follow that primary afferent lesions made after the cytochrome oxidase pattern became established in the brainstem might have little effect upon it. Accordingly, we transected the infraorbital nerve (the trigeminal branch that supplies the vibrissae) on postnatal days 0-10 and evaluated the vibrissa-related pattern in the brainstem with cytochrome oxidase histochemistry at varying intervals after these lesions. If the infraorbital nerve was sectioned on postnatal days 0-2, the vibrissa-related pattern was absent in trigeminal nucleus principalis, and both subnucleus interpolaris and caudalis. If such lesions were made after postnatal day 9, there was no appreciable effect upon the cytochrome oxidase pattern in any portion of the trigeminal brainstem complex. However, if lesions were made between postnatal days 3 and 8, the density and clarity of the cytochrome oxidase staining pattern were reduced in interpolaris and caudalis, but not in principalis. This difference was not due to differential transganglionic degeneration in these nuclei. Tracing with horseradish peroxidase demonstrated qualitatively equivalent primary afferent losses in principalis, interpolaris, and caudalis. Immunocytochemistry with a monoclonal antibody directed against parvalbumin also demonstrated a vibrissa-related pattern of cell bodies in principalis and interpolaris in rats killed on postnatal day 9 or later ages. The combination of retrograde tracing and immunocytochemistry revealed that the parvalbumin-immunoreactive neurons in principalis projected to thalamus while those in interpolaris were not labelled by tracer injections into the thalamus, midbrain, cerebellum or spinal cord. Infraorbital nerve transections made as late as postnatal day 8 resulted in a sharp decrease in the staining of parvalbumin-positive neurons in interpolaris, but not in principalis. Lesions made on postnatal day 10 had no qualitative effect upon parvalbumin-positive neurons in any portion of the trigeminal brainstem complex. The results of this study support the conclusion that the vibrissa-related cytochrome oxidase pattern in principalis becomes independent of primary afferent input at a very short interval after its initial appearance. In contrast, the patterns in more caudal portions of the trigeminal brainstem complex require maintenance of primary afferent input for a much longer postnatal period.(ABSTRACT TRUNCATED AT 400 WORDS)

Current hypotheses of structural pattern formation in the somatosensory system and their potential relevance to humans

Current hypotheses of structural pattern formation in the mammalian somatosensory system are modeled on experimental findings from the trigeminal system of rodents. The present results show that, like rodents, the trigeminal nucleus principalis of humans contains a parcellated pattern of cytochrome oxidase dense patches. These results provide an indication of the potential usefulness of rodent-based hypotheses for understanding pattern formation in human somatosensory connections.

The source of the transient serotoninergic input to the developing visual and somatosensory cortices in rat

For approximately the first two weeks of life, dense serotonin immunoreactivity closely matches the pattern of thalamocortical axons innervating both the granular portion of the primary somatosensory cortex and area 17 in rodents [D'Amato et al. (1987) Proc. natn. Acad. Sci. 84, 4322-4326; Fujimiya et al. (1986) J. comp. Neurol. (1986) 246, 191-201; Rhoades et al. (1990) J. comp. Neurol. 293, 190-207]. This serotonin immunoreactivity is not contained in thalamocortical axons [Rhoades et al. (1990) 293, 190-207] but its source has never been demonstrated. In the present study, a variety of approaches were used to address this issue. The combination of electron microscopy and immunocytochemistry showed that all serotonin immunoreactivity in the developing cerebral cortex was contained in axons and that the terminals of many of these fibers made synapses with the dendrites of cortical cells. Treatment with fluoxetine, a specific inhibitor of serotonin uptake, did not result in a loss of the cortical pattern of serotonin immunoreactivity, indicating that immunoreactive fibers were not labeled solely as a result of serotonin uptake. The combination of retrograde tracing from the primary somatosensory cortex and area 17 with immunocytochemistry demonstrated numerous double-labeled cells in nucleus raphe dorsalis and the median raphe nucleus. Smaller numbers of double-labeled neurons were located in the B9 cell group and the region of the lateral midbrain tegmentum. Large electrolytic lesions that included most of the nucleus raphe dorsalis and median raphe nucleus, but which left the B9 group and more caudal serotoninergic cells undamaged, caused either a substantial reduction in density or complete disappearance of the serotonin pattern in both hemispheres. Unilateral electrolytic lesions of the medial forebrain bundle resulted in a loss of the pattern only on the side of the damage. Injection of the neurotoxin 5,7-dihydroxytryptamine directly into the mesencephalon either abolished or substantially reduced the density of the cortical serotonin immunoreactivity. Injections that produced substantial cell loss in the median raphe nucleus, but only minor cell loss in the nucleus raphe dorsalis had little effect upon the cortical pattern of serotonin immunoreactivity. These results indicate that the dense serotonin immunoreactivity which appears transiently in the visual and somatosensory cortices of perinatal rodents is contained in serotoninergic axons that arise from cells in the nucleus raphe dorsalis and perhaps also the median raphe nucleus.

Callosal projections in rat somatosensory cortex are altered by early removal of afferent input

During the first postnatal week, the distribution of callosal projection neurons in the rat somatosensory cortex changes from a uniform to a discontinuous pattern. To determine if this change is influenced by afferent inputs to the somatosensory cortex, the effect of both early unilateral infraorbital nerve section and unilateral removal of the dorsal thalamus on the distribution of callosal projections in rat somatosensory cortex was examined. One month after either of the above manipulations at birth, the tangential distribution of callosal projections in the somatosensory cortex was examined using the combined retrograde and anterograde transport of horseradish peroxidase. Both manipulations alter the distribution of callosal projection neurons and terminations in the somatosensory cortex. After infraorbital nerve section, the distribution of callosal projections is altered in the contralateral primary somatosensory cortex. The abnormalities observed are consistent with the altered distribution of thalamocortical projections. In addition, consistent abnormalities were observed in the pattern of callosal projections of the second somatosensory area of both hemispheres. Most notably, they are absent in a portion of the region that contains the representation of the mystacial vibrissae and sinus hairs in this area. Thalamic ablation resulted in highly aberrant patterns of callosal projections in the somatosensory cortex on the operated side, where abnormal bands and clusters of callosal projections were observed in apparently random locations. These results are interpreted as evidence that both peripheral and central inputs influence the maturational changes in the distribution of callosal projection neurons.

Maintenance of discrete somatosensory maps in subcortical relay nuclei is dependent on an intact sensory cortex

Lesions of the rat barrelfield cortex drastically alter the discrete representations of the somatosensory periphery in the central nervous system. We have found that lesions placed in the parietal cortex, after the formation of barrels (postnatal day 5), can irreversibly abolish vibrissae- and extremity-related patterns of cytochrome oxidase activity in the principal sensory nucleus of the trigeminal nerve and in the dorsal column nuclei. Furthermore, abnormal patterns of enzymatic activity occur in the remaining primary somatosensory cortex and the ventrobasal nucleus of the thalamus. We conclude that cortical barrels are essential in maintenance of periphery-related discrete morphological organization in the rodent somatosensory system.

Structure and function of barrel 'precursor' cells in trigeminal nucleus principalis

Intracellular recording, electrical stimulation, receptive field mapping, HRP injection, and computer reconstruction techniques were used to study principalis cells in rat. They (n = 80) responded within 1.2 +/- 0.2 ms of trigeminal ganglion shocks and 69% were antidromically activated by thalamic shocks; 69% were vibrissa-sensitive, of which 80% responded to only a single vibrissa. The remainder responded only to guard hairs, skin, teeth, or nociceptors. Stained thalamic-projecting cells with one vibrissa receptive fields had stereotyped morphologies. Small somata gave rise to dendrites which extended only a short distance from the soma, where they branched extensively. Each tree was polarized, spanning no more than a hemisphere around the soma; however, there was no consistent direction of polarity. Dendritic trees extended 68 +/- 14, 95 +/- 48, and 91 +/- 29 micron in the transverse, sagittal and horizontal planes, respectively. Dendritic spines were rare, yet swellings were common. Axons never branched locally.

Common fur and mystacial vibrissae parallel sensory pathways: 14 C 2-deoxyglucose and WGA-HRP studies in the rat

Stimulation of mystacial vibrissae in rows A,B, and C increased (14C) 2-deoxyglucose (2DG) uptake in spinal trigeminal nucleus pars caudalis (Sp5c) mostly in ventral portions of laminae III-IV with less activation of II and V. Stimulation of common fur above the whiskers mainly activated lamina II, with less activation in deeper layers. The patterns of activation were compatible with an inverted head, onion skin Sp5c somatotopy. Wheatgerm Agglutinin-Horseradish Peroxidase (WGA-HRP) injections into common fur between mystacial vibrissae rows A-B and B-C led to anterograde transganglionic labeling only of Sp5c, mainly of lamina II with less label in layer V, and very sparse label in III and IV. WGA-HRP skin injections appear to primarily label small fibers, which along with larger fibers, were metabolically activated during common fur stimulation. Mystacial vibrissae stimulation increased 2DG uptake in ventral ipsilateral spinal trigeminal nuclei pars interpolaris (Sp5i) and oralis (Sp5o) and principal trigeminal sensory nucleus (Pr5). Common fur stimulation above the whiskers slightly increased 2DG uptake in ventral Sp5i, Sp5o, and possibly Pr5. The most dorsal aspect of the ventroposteromedial (VPM) nucleus of thalamus was activated contralateral to whisker stimulation. Stimulation of the common fur dorsal to the whiskers activated a region of dorsal VPM caudal to the VPM region activated during whisker stimulation. This is consistent with previous data showing that ventral whiskers and portions of the face are represented rostrally in VPM, and more dorsal whiskers and dorsal portions of the face are represented progressively more caudally in VPM. Mystacial vibrissae stimulation activated the contralateral primary sensory SI barrelfield cortex and a separate region in the second somatosensory SII cortex. Common fur stimulation above the whiskers activated a cortical region between the SI and SII whisker activated regions of cortex. It is proposed that this region represented the combined SI and SII common fur regions of somatosensory neocortex. Both whisker and common fur stimulation activated all layers of cortex, with layer IV being most activated followed by II-III, V, and VI. These data indicate that sensory input from the mystacial vibrissae in the adult rat is processed in brainstem, thalamic, and cortical pathways which are predominantly parallel to those which process information from the neighboring common fur sensory receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Organization of the projections from barrel cortex to thalamus in mice studied with Phaseolus vulgaris-leucoagglutinin and HRP

In order to elucidate the geometric organization of projections from the barrel cortex to the thalamus, iontophoretic injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin were made. The injections were confined to one barrel column (i.e. barrel in layer IV + cortical tissue above and below it). Axonal terminations could be demonstrated in three thalamic nuclei: reticularis (RT), ventrobasalis (VB) and posterior (PO). Anterograde terminal labelling was obtained in RT + VB; in PO only; or in RT + VB + PO. The terminals labelled in PO were much larger than those in RT and VB. The termination areas in RT, VB and PO were shaped like rods which have a rostro-caudal orientation. These cortico-thalamic projections are discretely and topographically organized. The clearest such arrangement was found in VB. Here, projections from the A row of barrels in BF terminate dorsally, whereas those from the C row end ventrally. Barrel A1 projects to the lateral part of VB, whereas A4, to more medial parts; other rows are arranged similarly. These results were compared with the distribution of thalamo-cortical projection neurons that were labelled after iontophoretic HRP injections in individual barrels. We concluded that the corticothalamic projections originating from one barrel column contact an are of barreloids in VB.

A quantitative comparison of stimulus-response relationships of vibrissa-activated neurons in subnuclei oralis and interpolaris of the rat's trigeminal sensory complex: receptive field properties and threshold distributions

Electrical activity of single vibrissa-activated neurons was recorded in pars interpolaris and pars oralis of the nucleus of the trigeminal spinal tract of rats. Stimuli consisted of quantitatively controlled deflections of individual mystacial vibrissae. The evoked spike trains were analyzed as point-process time series with a variety of quantitative procedures. Most second-order trigeminal neurons were directionally sensitive. About one-third of interpolaris neurons and over half of oralis neurons responded to axial push of the hair shaft. About half of the neurons of both oralis and interpolaris had receptive fields that included more than one vibrissa. The upper-quartile receptive field size of neurons of interpolaris was about half that of oralis, although the distributions overlapped. In interpolaris, almost one-fifth of the sample discharged in the absence of an intentionally applied stimulus; in oralis, over one-third of the sample displayed background activity. The ranges of angular displacement thresholds of both samples exceeded three orders of magnitude. The distributions differed quantitatively. They were, nevertheless, similar in shape, and exhibited considerable overlap. The median threshold of oralis neurons was about half that of interpolaris neurons, and about one-fifth that of first-order neurons. About one-fourth of the second-order neurons exhibited a nonmonotonic relationship between pulse displacement and the number of evoked spikes. Neurons of interpolaris tended to be more severely nonmonotonic than those of oralis, and some "turned off" at stimulus magnitudes well above threshold. Nevertheless, the number of spikes evoked in either entire sample was a monotonically increasing function of pulse displacement. The range of angular velocity thresholds observed in both second-order samples exceeded three orders of magnitude. As with the angular displacement thresholds, the distributions of angular velocity thresholds were quantitatively different, although their shapes and extremes were similar, and they overlapped extensively. The observed differences of stimulus-response relationships of the neurons in interpolaris and oralis reflect differences in the ways different trigeminal nuclei receive, process, and distribute information to the circuits in which they participate.

Morphological alterations in subcortical vibrissal relays following vibrissal follicle destruction at birth in the mouse

Morphological modifications of two subcortical vibrissal relays were analyzed, following destruction of vibrissal follicles in newborn mice. The volume of the nucleus interpolaris (NI) of the trigeminal nuclear complex in the brainstem decreased by 33%, while the number of its neuronal perikarya decreased only moderately. Vibrissal deafferentation caused no shrinkage of the ventrobasal complex (VB). In the damaged medial vibrissal part of VB (VBm), however, neuronal density was higher than normal, indicating the prevention or retardation of physiologically programmed cell death in the afferentation deprived thalamic somatosensory relay station. It is suggested that the difference in neuron density produced by deafferentation is related to the states of maturation at birth of the two subcortical vibrissal relays. Following vibrissal deafferentation the basic organization of the synaptic neuropil appeared to be similar to the control. Quantitative electron microscopic (EM) analysis revealed, however, an increased number of axon terminals with ovoid synaptic vesicles in both deafferented relay stations. The increased density of gamma-aminobutyric acid (GABA)-immunostained boutons observed in the VBm following vibrissal deprivation suggested a compensatory increase most probably of the inhibitory axon endings. Quantitative EM analysis also provided evidence that many or most of the specific afferent terminals in the damaged VBm were not identical with but were substitutes for the original "vibrissal" specific afferents. Forty percent of all "specific" afferents were shown to be modified corticothalamic terminals. The modification and the resemblence of some cortical endings to specific afferents demonstrated the morphogenetic plasticity of synaptogenesis in these terminals during development as well as the importance and inductive potential of the postsynaptic target in the differentiation of presynaptic axon terminals.