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

Peripheral injury and anterograde transport of wheat germ agglutinin-horse radish peroxidase to the spinal cord

Previous observations have revealed labeling in the extracellular space surrounding boutons and unmyelinated fibers in superficial laminae of the spinal cord after injection of the tracer wheat germ agglutinin conjugated to horseradish peroxidase in dorsal root ganglia. The degree of extracellular labeling appeared related to the extent of the damage to the ganglia at the time of the injection. To determine whether injury might produce extracellular labeling, we investigated the effects of unilateral nerve crush or transection on spinal labeling after bilateral injections of the tracer into sciatic nerves. Confirming previous reports, labeling was confined to small dorsal root ganglion cells and to spinal laminae I and II, suggesting a selective affinity of this tracer for unmyelinated fibers. Labeling of both ganglion neurons and superficial spinal laminae was increased on the injured side, probably as a result of increased efficiency of receptor-mediated endocytosis. Electron microscopical observations revealed that the tracer was largely confined to unmyelinated dorsal root fibers bilaterally; a higher percentage of these fibers were labeled on the injured side. In the dorsal horn, the tracer was predominantly within unmyelinated axons and their terminals on the control side, whereas most of the labeling was extracellular and transneuronal on the injured side. The extracellular labeling surrounded unmyelinated fibers and their terminals in the spinal cord, but was excluded from the synaptic cleft. The demonstration that injury is accompanied by significantly increased release of this tracer from the terminals of unmyelinated fibers into the extracellular space suggests that endogenous substances may be released after peripheral lesions as a central signal of injury.

Long-range afferents in the rat spinal cord. II. Arborizations that penetrate grey matter

1. The caudal extent of the collateral arborizations of entering sensory fibres in rat spinal cord was investigated by two methods: bulk labelling of peripheral nerves by injection of horseradish peroxidase conjugated to cholera toxin (B-HRP) and by antidromic stimulation using small currents from microelectrodes in the spinal cord while recording from single units in peripheral nerve or dorsal root. 2. The results show that injection of B-HRP into the sural or sciatic nerve labelled sural afferents in the grey matter three to four segments caudal to their root entry and sciatic nerve fibres were located in S4, the most caudal segment examined, four to six segments caudal to their root entry. 3. Detailed mapping with microelectrode stimulation showed that the parent descending fibres from filaments dissected from the L1 dorsal root coursed more than 20 mm, seven to eight segments caudal to the entry point in the dorsal columns and sent branches into the grey matter. Single units from the sural nerve were also followed caudally into the S2 and S3 spinal cord segments and also issued collateral branches into the grey matter. 4. The present results suggest that there is close agreement in the caudal penetration of long-ranging afferents by using complementary anatomical and electrophysiological methods.

Demonstration of transganglionically transported choleragenoid in rat spinal cord by immunofluorescence cytochemistry

Injections of the B-subunit of cholera toxin (CTB) were made into the sciatic nerve of the rat. Following a survival of 2-3 days, the fluorescent antibody technique was used to show that CTB can be utilized as a highly sensitive immunocytochemically detectable transganglionic tracer for primary afferent fibers in the spinal cord. CTB-labeled fibers as well as fibers containing calcitonin gene-related peptide- (CGRP-) or substance P-like immunoreactivity were visualized simultaneously by using different fluorochromes. However, no double labeled fibers were found.

Physiology and morphology of neurons in the dorsal motor nucleus of the vagus and the nucleus of the solitary tract that are sensitive to distension of the small intestine

We have recently shown that distension-sensitive vagal afferents are part of a neural circuit affecting absorption of water in the rat small intestine. Our results indicated that vagal afferent activity directly or indirectly influences the activity of neurons in the dorsal motor nucleus of the vagus (DMNV). In the present study we pursued this interaction by examining the structure and function of neurons in the DMNV and nucleus of the solitary tract (NST) that responded to moderate distension of the small intestine. Distension-sensitive cells were filled by intracellular iontophoretic injection of horseradish peroxidase. A total of 43 distension-sensitive brainstem neurons were successfully characterized and labeled. Sixteen of the 17 NST neurons were excited by distension of the small intestine. Ten of the seventeen were restricted to the ipsilateral NST. Only two NST neurons possessed axons that terminated in the subjacent DMNV. In contrast to the response profile of the NST neurons, 24 of 26 DMNV neurons were inhibited by intestinal distension. Fourteen of the DMNV neurons appeared to contribute to the vagus nerve and 15 extended dendrites into the overlying NST. We propose that distension-induced inhibition of DMNV activity is accomplished by inhibitory NST neurons, which synapse on the dendrites of DMNV neurons in the NST.

WGA-HRP and choleragenoid-HRP as anterogradely transported tracers in vagal visceral afferents and binding of WGA and choleragenoid to nodose ganglion neurons in rodents

The axonal and terminal labelling pattern in the brain stem resulting from the injection of horseradish peroxidase (HRP) conjugate of wheat germ agglutinin (WGA) or choleragenoid into the nodose ganglion of guinea pigs was examined. In addition, the binding profiles of WGA and choleragenoid in the nodose ganglion of guinea pig and rat were examined. The results show that WGA-HRP and choleragenoid-HRP (B-HRP) produce almost identical distribution of axonal and terminal labelling, the difference being some contralateral fibre labelling present only with B-HRP. However, WGA-HRP shows the strongest labelling at short survival times, whereas B-HRP requires longer postoperative survival times to reach maximum labelling intensity. All nodose ganglion neurons appear to bind WGA as well as choleragenoid although to a varying degree. The results of this and previous studies support the view that visceral sensory ganglion cells and the large light subpopulation of somatic dorsal root ganglion cells both bind choleragenoid, whereas the small dark somatic cells show affinity for WGA but rarely for choleragenoid.

Structural changes of the soleus and the tibialis anterior motoneuron pool during development in the rat

The morphological development of motoneuron pools of two hindlimb muscles of the rat, soleus (SOL) and tibialis anterior (TA), was studied in rats ranging in age between 8 and 30 postnatal days (P8-P30). Motoneurons were retrogradely labelled by injecting a cholera toxin B subunit solution directly into the muscles. This resulted in extensive labelling of motoneurons as well as their dendritic trees. The distribution of cross sectional areas of neuronal somata was determined for both muscles at various ages. Somal size increased considerably between P8 and P12, whereas growth was moderate between P12 and P20. The size distribution of SOL motoneurons was bimodal from P20, whereas the size distribution of TA motoneurons remained largely unimodal. The morphological development of the dendritic tree was studied qualitatively. The development of dendritic arborization within the SOL and the TA motoneuron pool showed major differences. The arborization pattern of dendrites of TA motoneurons was basically multipolar at all ages. In contrast, dendrites of SOL neurons tended to line up with the rostro-caudal axis and became organized in longitudinal bundles from P16 onwards. The relatively late appearance of dendrite bundles in the soleus motoneuron pool suggests that they might be related to the fine-tuning of neuronal activity rather than patterning of motor activity. The occurrence of dendrite bundles in SOL and not in TA motoneuron pools suggests that they may be related to the different afferent organization of this postural muscle or to its tonic activation pattern.

Central projections of the sensory innervation to the middle cerebral artery in the squirrel monkey

To analyze the brainstem projections of the innervation to the middle cerebral artery (MCA) in the squirrel monkey, transganglionic tracing of wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) was used. After application of WGA-HRP to the middle cerebral artery (MCA), labelled cell bodies were identified in the ipsilateral trigeminal and superior cervical ganglia. In the brainstem, positive labelling indicative of preterminals and terminals occurred in a discontinuous pattern throughout the trigeminal brainstem nuclear complex. At the level of the obex, nerve terminations were identified in the nucleus tractus solitarius, nucleus motorius dorsalis nervi vagi and the nucleus nervi hypoglossi. Positive WGA-HRP profiles were also observed in the periaqueductal gray matter.

Organization of cutaneous primary afferent fibers projecting to the dorsal horn in the rat: WGA-HRP versus B-HRP

Primary afferent projections from cutaneous afferents in the forelimb and hindlimb digits to the dorsal horn (DH) were examined using 4 tracers: (1) 25% free horseradish peroxidase (HRP), (2) 2.5% wheat-germ agglutinin conjugated to horseradish peroxidase (WGA-HRP), (3) a mixture of 25% free HRP and 2.5% WGA-HRP (WGA-HRP/HRP) or (4) 0.1% HRP conjugated to cholera toxin (B-HRP). The tracer was injected intracutaneously into the digits. Three to 4 days later, the rats were perfused transcardially, transverse sections (60-microns thick) were cut and the HRP was reacted using the tetramethyl benzidine (TMB) method. The location of the label was reconstructed by camera lucida drawings. In rats which received an injection of HRP alone, no label was detected in the DH. Rats injected with WGA-HRP had projection patterns similar to those injected with WGA-HRP/HRP. Patterns of labelling with WGA-HRP differed markedly from those with B-HRP. WGA-HRP labelled cutaneous afferents projecting to Rexed's laminae I-III, with the densest label in lamina II; in contrast, B-HRP labelled cutaneous afferents projecting to laminae II-V, with the densest label in laminae III-IV. These results indicate that, for cutaneous primary afferents projecting to the DH, WGA-HRP and B-HRP labelled different subpopulations of fibers, with the B-HRP-labelled subpopulation biased toward afferents of larger diameter. Rostrocaudally, the extent of the densest fiber projections, whether labelled by WGA-HRP or by B-HRP, was essentially the same, but the extent of the less densely labelled projections was much greater with B-HRP than with WGA-HRP. Comparisons of the projection maps from each of the five digits, using either WGA-HRP or B-HRP, indicated that, as seen in transverse sections through the DH, there was extensive overlapping among the labelled cutaneous afferent fibers from adjacent, or even non-adjacent digits.

Peripheral nerve injury triggers central sprouting of myelinated afferents

The central terminals of primary afferent neurons are topographically highly ordered in the spinal cord. Peripheral receptor sensitivity is reflected by dorsal horn laminar location: low-threshold mechanoreceptors terminate in laminae III and IV (refs 2, 3) and high-threshold nociceptors in laminae I, II and V (refs 4,5). Unmyelinated C fibres, most of which are nociceptors, terminate predominantly in lamina II (refs 5, 7). There is therefore an anatomical framework for the transfer of specific inputs to localized subsets of dorsal horn neurons. This specificity must contribute to the relationship between a low-intensity stimulus and an innocuous sensation and a noxious stimulus and pain. We now show that after peripheral nerve injury the central terminals of axotomized myelinated afferents, including the large A beta fibres, sprout into lamina II. This structural reorganization in the adult central nervous system may contribute to the development of the pain mediated by A-fibres that can follow nerve lesions in humans.

Evidence for dorsal root projection to somatostatin-immunoreactive structures in laminae I–II of the spinal dorsal horn

In order to determine if somatostatin (SOM)-immunoreactive (I) cell bodies and processes in lamina (L) II of the rat spinal cord receive dorsal root input, the latter were anterogradely labeled by wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). SOM-I structures were demonstrated by immunohistochemistry. Cell bodies labeled transscellularly or transsynaptically by WGA-HRP and immunohistochemically for SOM were present in L II. In addition, a L I cell was double labeled. These results suggest that some dorsal root axons innervate SOM-I neurons in L I-II of the spinal cord. In addition to confirming immunohistochemical observations in published reports, we have revealed SOM-I central terminals in the type II glomerulus. Further, a SOM-I CI-terminal, presumed to be of primary afferent origin, contacted a SOM-I dendrite in L II. Since SOM has been implicated in nociceptive function in the dorsal horn, it is possible that some of the SOM-I structures identified are involved in nociceptive processing.

Somatotopic organization of hindlimb skin sensory inputs to the dorsal horn of hatchling chicks (Gallus g. domesticus)

The somatotopic organization of skin sensory nerve projections to the lumbosacral dorsal horn of hatchling chickens was determined with the aid of transganglionic transport of horseradish peroxidase (HRP) processed with tetramethylbenzidine histochemistry. A total of eight hindlimb nerves were studied, five of which were purely cutaneous. When combined, the innervation fields of these nerves covered most of the hindlimb surface, allowing a nearly complete somatotopic map of the hindlimb to be generated. This report describes a novel pattern of cutaneous nerve projections to the dorsal horn. Unlike other vertebrates, cutaneous nerves of chickens formed two separate, somatotopically organized projections across the mediolateral axis of the dorsal horn; when serially reconstructed and superimposed, these projections produced two nonoverlapping somatotopic maps of the skin surface lying side by side. Each of these separate maps was nearly identical to the other in overall topology. These two separate maps appear to represent distinct modalities of sensory information, as projections composing the medial map were preferentially labeled by choleragenoid-HRP, whereas those composing the lateral map were preferentially labeled by wheat germ agglutinin-HRP. In mammals, these HRP ligands selectively label the central projections of myelinated and unmyelinated cutaneous afferents, respectively. The present study, therefore, strongly supports the cytoarchitectonic findings of Brinkman and Martin (Brain Res. 56:43-62, '73) that lamina III lies medial, rather than ventral, to lamina II in the chicken dorsal horn. Further, the present studies also suggest that laminae II and III of chickens are homologous to the homonymous laminae in the dorsal horn of mammals.

Somatotopic organization of the dorsal column nuclei in the rat: transganglionic labelling with B-HRP and WGA-HRP

To analyze the patterns of cutaneous primary afferent fibers projecting to the dorsal column nuclei in the rat, horseradish peroxidase (HRP)-based tracers were injected intracutaneously into a number of discrete regions of the forelimbs and hindlimbs. Three-4 days following the HRP injections, the rats were perfused transcardially; 60 microns transverse sections were cut, and the HRP was reacted using the tetramethyl benzidine method. Comparisons were made of projections following injections with choleragenoid-conjugated horseradish peroxidase (B-HRP) or with wheat-germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). B-HRP and WGA-HRP produced similar patterns of labelling, but B-HRP produced greater intensity of labelling and slightly larger projection areas. In the cuneate nucleus (CN), HRP labelling of primary afferents from small, delimited regions, e.g., from a portion of the skin of a single digit, appeared to be precisely restricted in rostrocaudally oriented columns, with little or no overlap (in the mediolateral and dorsoventral plane) into adjacent regions. With respect to rostrocaudal organization, a region in the CN containing a dense population of cutaneous primary afferent fibers appeared to be similar to the middle, or cluster, region in cats and in raccoons and the pars rotunda in primates. Projection patterns were very consistent from rat to rat, but their somatotopic organization differed from that suggested by electrophysiological studies: cutaneous afferents from forelimb digit 1 projected near the ventral border of the CN; those from digit 5 projected dorsomedially to those from digit 1; the projections from the remaining digits formed a crescent between the projections from digits 1 and 5. In the gracile nucleus, the organization of cutaneous afferent projections from hindlimb digits was more variable and complex than that found in the CN.

Long-range afferents in the rat spinal cord. 1. Numbers, distances and conduction velocities

The caudal extent of the penetration of primary afferent axons from the T12 and L1 dorsal roots and sural nerve has been investigated in adult decerebrate spinal rats. Microelectrode stimulation at the root entry zone (REZ) and at further caudal points in the spinal cord was used to generate antidromic action potentials in single fibres recorded in dorsal roots or peripheral nerves. A total of 209 units were recorded in T12 and L1 dorsal roots and 27% of these could be antidromically activated 10 mm caudal to the REZ. Fifteen percent of the units could be stimulated at the L4-5 border, 15 mm caudal to the T12 segment whereas 4.5% of the axons could be stimulated 25 mm caudally in the S4 segment, 11 segments caudal to the entry segment. Similar recordings made from units in the sural nerve showed that of all the sural axons that penetrated to the L6 segment 50%, 18% and 2% of these reached the S1, S2 and S4 segments respectively. The conduction velocities of these units were clearly in the A-beta range when recorded in the nerve but decreased on entering the spinal cord and were reduced by 83% at their caudal end point. The results show that substantial numbers of primary afferents have long-ranging caudal branches in areas beyond the regions of known postsynaptic effects. The functions of these caudal projections are unclear but they may represent a potential substrate for the development of functional connections under conditions of disease or denervation.

Central projections of the sciatic, saphenous, median, and ulnar nerves of the rat demonstrated by transganglionic transport of choleragenoid-HRP (B-HRP) and wheat germ agglutinin-HRP (WGA-HRP)

The central projections of the rat sciatic, saphenous, median, and ulnar nerves were labeled by injecting each nerve with 0.05 mg B-HRP, or 0.5 mg WGA-HRP, or a mixture of both. The B-HRP labeled large dorsal root ganglion cells (30-50 microns) and, correspondingly, 98% of axons labeled in a rootlet were meyelinated; although all sizes of myelinated axons were labeled, a greater proportion fell in the large ranges (2-6.5 microns axon diameter) than in the small ranges (0.5-2 microns). Primary afferents labeled with B-HRP were distributed in laminae I, III, IV, and V of the dorsal horn and extended into the intermediate grey and the ventral horn; Clarke's column and the respective dorsal column nuclei were also densely labeled. Motoneurons of the nerve were densely labeled by B-HRP, including extensive regions of their dendritic trees. In contrast, WGA-HRP labeled small dorsal root ganglion cells (15-25 microns) and in the dorsal rootlets, 84% of the labeled axons were nonmyelinated; the small population of labeled myelinated afferents mainly fell within the smaller ranges (0.5-2.0 microns). Terminal fields of WGA-HRP labeled afferents were restricted to the superficial dorsal horn (laminae I-III), and to limited regions in the dorsal column nuclei. Sciatic nerve projections traced by labeling with B-HRP alone or in combination with WGA-HRP were more extensive than previously described when using either native HRP or WGA-HRP. Afferents to the dorsal horn extended from L1-S1, to Clarke's nucleus from T8-L1, to the ventral horn from L2-L5, and extended throughout the medial and dorsal region of the gracilie nucleus. Motoneurons were found from L4-L6. Using the same tracers, saphenous projections extended in the superficial dorsal horn from caudal L1 to rostral L4, in the deep dorsal horn to mid L4 and along the length of the central part of the gracilie nucleus. The median nerve projected to the internal basilar nucleus from C1-C6, the dorsal horn from C3-T2, Clarke's nucleus from T1-T6, the external cuneate nucleus, and a large central area throughout the length of the cuneate nucleus. Motoneurons were located in dorsolateral and ventrolateral nuclear groups from C4 through C8. The ulnar nerve projections were less extensive but also included the internal basilar nucleus from C1-C6, the medial region of the dorsal horn from C4-T1, Clarke's nucleus from T1-T6, the external cuneate nucleus, and the medial part of the cuneate nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

Central projections from the skin of the hand in squirrel monkeys

Central termination patterns of afferents from the hands of squirrel monkeys were studied after subdermal injections of wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) or cholera toxin subunit B conjugated to HRP (BHRP). WGA-HRP more effectively labeled axons terminating in the superficial dorsal horn of the spinal cord, while BHRP more effectively labeled axons terminating in the deeper layers. Injections of both tracers, when restricted to parts of glabrous digits, palm, or dorsal hand, revealed somatotopic patterns in the spinal cord and pars rotunda of the cuneate nucleus that were, in some respects, similar and, in other respects, quite different from those previously reported for macaque monkey (Florence et al., J. Comp. Neurol. 286:48-70, '89). As in macaques, injections in digits 1-5 produced a rostrocaudal sequence of foci of terminations in the cervical spinal cord. However, inputs from the palm were located medial to those from the digits, whereas the palm is represented lateral to the digits in macaque monkeys. Since inputs from the palm is also medial in the dorsal horn in cats (Nyberg and Blomqvist, J. Comp. Neurol. 242:28-39, '85), the condition in squirrel monkeys may be similar to the generalized state. In the cuneate nucleus, single injections in the hand produced dense label in the pars rotunda, and sparse label in the rostral and caudal poles. As in macaque monkeys, inputs from specific parts of the hand related to rostrocaudal clusters of cells that are cytochrome oxidase dense. The representation of the digits differed from macaques in that the digits were represented dorsal to the palm, rather that ventral to the palm as in macaques. Again, comparisons with cats suggest that squirrel monkeys have the more generalized pattern. Finally, inputs from the hair, dorsal surfaces of the digits terminated on the same clusters as the inputs from the glabrous, ventral surfaces, apparently overlapping somewhat. The proximity of these terminations from dorsal and ventral surfaces of the digits may be related to observations that cortical representations of the glabrous surfaces of digits become responsive to dorsal surfaces of the same digits when inputs from glabrous skin are chronically deactivated (e.g., Merzenich et al., Neuroscience 3:33-55, '83).

Rat tooth pulp projections to spinal trigeminal subnucleus caudalis are glutamate-like immunoreactive

It has been shown that glutamate-like immunoreactive axon terminals are present within the spinal trigeminal nucleus, including subnucleus caudalis. The morphology of many of these terminations is consistent with their identification as primary afferents. To establish whether primary afferent projections to subnucleus caudalis are glutamate-like immunoreactive, we injected an anterograde tract tracer into rat incisor tooth pulp, histochemically visualized this tracer within subnucleus caudalis, and then used an immunocytochemical technique to label glutamate-like immunoreactive profiles within these same sections. The anterograde tract tracer used, the B subunit of cholera toxin conjugated to horseradish peroxidase (B-HRP), is transported transganglionically and can be used to localize tooth pulp projection fibers in the spinal trigeminal nucleus. A majority of B-HRP projection fibers from rat lower incisors terminated ipsilaterally in axon terminals in the dorsal region of subnucleus caudalis. Labeled axon terminals were both scallop-shaped and smooth in profile. Small numbers of fibers containing B-HRP extended into laminae I-III caudally and were present in both the border zone between laminae IV and V and the most lateral region of lamina V rostrally. Approximately 75% of the B-HRP-labeled projection fibers were glutamate-like immunoreactive, providing evidence that the excitatory amino acid glutamate functions as a neurotransmitter in a subpopulation of these fibers. Terminals reactive for both B-HRP and glutamate-like immunoreactivity contained small, spherical round vesicles, formed asymmetric synapses, and participated in axoaxonic and axodendritic synaptic junctions. These results support the hypothesis that glutamate may be a transmitter of A delta and C fibers involved in relaying nociceptive information from the tooth pulp.

Populations of rat spinal primary afferent neurons with choleragenoid binding compared with those labelled by markers for neurofilament and carbohydrate groups: a quantitative immunocytochemical study

A quantitative comparison was made of the population of L5 dorsal root ganglion neurons that bind choleragenoid and the population of neurons labelled by RT97, an anti-neurofilament antibody, SSEA3, an antibody to a globoseries carbohydrate group and 1B2, an antibody to a lactoseries carbohydrate group. Neurons with choleragenoid binding formed a population of 37% of all the cells and 94% of these were also immunoreactive to RT97. The choleragenoid-and RT97-positive groups of cells showed the same size distribution. Two small populations of cells existed which were choleragenoid negative/RT97 positive and choleragenoid positive/RT97 negative. SSEA3 immunoreactivity was found in 8% of the dorsal root ganglion neurons which were of all sizes. The 1B2 immunoreactivity was found in 32% of the dorsal root ganglion cells, the majority of which were in the small size range. More than half of the SSEA3-positive cells but only a few of the 1B2-positive neurons were labelled for choleragenoid. These results demonstrate that choleragenoid labels the majority of the light (RT97-positive) cell population and that light cells unlabelled by choleragenoid are small in number and spread across a wide size range.

Distribution of lumbar dorsal root fibers in the lower thoracic and lumbosacral spinal cord of the rat studied with choleragenoid horseradish peroxidase conjugate

Spinal cord projections from lumbar dorsal root ganglia (DRGs) were investigated in adult rats following injections of choleragenoid horseradish peroxidase (B-HRP) into each of the six lumbar DRGs. This method is known to label primarily thick fibers. Labeling was found in all laminae except in the outer part of lamina II. Labeled fibers and terminal-like structures were found 8-13 segments rostral and 1-5 segments caudal to the injected DRGs. A somatotopic organization was revealed in laminae III, where the labeling seemed to be organized in mediolateral zones. Some of these protruded as fingerlike processes through segments rostral and caudal to the root entry level. An interdigitating pattern for these fingerlike processes was seen between some DRGs, while an extensive overlap was found between other DRGs. Many zones were found to correspond to the known central projections of peripheral sensory nerves supplied by the injected ganglion. This suggests that the central projection of a DRG is highly related to the projections of the peripheral nerves included in the DRG. The projections to lamina IV were organized in a similar manner as in lamina III, even though the projections showed a higher degree of overlap than in lamina III. No clear somatotopic organization was found in laminae V-IX. Provided that the topographical relationship between central projections of single peripheral nerves and of DRGs correspond to their peripheral projections, the results of this study, together with results of earlier studies suggest that the outlines of dermatomes are highly related to the territories of peripheral nerves included in the dermatomes.

Plasticity of dorsal root and descending serotoninergic projections after partial deafferentation of the adult rat spinal cord

Plasticity of dorsal root (DR) and descending serotoninergic (5-HT) projections following dorsal rhizotomy from L2 to S1 sparing L5 was studied by means of an intra-animal comparison in the adult rat spinal cord. Projections of the chronically and acutely spared root were compared by cholera-toxin conjugated horseradish peroxidase (CT-HRP) injected into the sciatic nerves as the transganglionic tracer. Projections in unoperated controls, operated controls (acute bilateral spared root), and in experimental animals (chronic spared root on one side and acute spared root on the other) were mapped and the density was measured with an image analysis system. Labeled DRG cells and motor neurons were counted to determine if there were differences in the delivery of the label between the two sides. Measurements of the area of the dorsal horn and, separately, of the superficial laminae were made to control for shrinkage. DR projections were symmetrical in operated and unoperated controls, but a significant increase in DR projection density was found from L6 to L3 in the dorsal horn and Clarke's nucleus at L1 on the chronic spared root side in animals in which an equal number of DRG cells was labeled on the two sides. Density of 5-HT immunoreactivity was symmetrical in controls. Ipsilateral to chronic spared root rhizotomy, the area fraction occupied by 5-HT projections increased in Clarke's nucleus and in the superficial dorsal horn of all partially deafferented segments except L5, the spared root segment. Partial deafferentation of the adult rat lumbosacral spinal cord may therefore elicit sprouting from the spared dorsal root and, outside of the dorsal root projection zone, sprouting from the spared descending 5-HT system. Plasticity of dorsal root projections and of 5-HT projections occur in different regions; in regions of the increased spared root projection, no increase in seen in 5-HT projections, suggesting that sprouting in the adult rat spinal cord is regulated, perhaps by competitive or hierarchical mechanisms.

Extracellular labeling of unmyelinated dorsal root terminals after WGA-HRP injections in spinal ganglia

Wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) is a widely used neuroanatomical tracer. When compared with other tracers, WGA-HRP may preferentially label unmyelinated fibers. In agreement with this hypothesis, injections of WGA-HRP in cervical and lumbar dorsal root ganglia resulted in more prominent light microscopical labeling in superficial than deep laminae of the dorsal horn. However, ultrastructural examination of these laminae reveals a paucity of terminal labeling in contrast to the abundance of extracellular tracer in the space surrounding unmyelinated fibers and their terminals, and to the widespread occurrence of transneuronal labeling. These results bear upon the mechanism of preferential labeling in the spinal cord and have implications for the interpretation of the labeling obtained when using WGA-HRP.

Anterograde transsynaptic transport of WGA-HRP from spinal afferents to postganglionic sympathetic cells of the stellate ganglion of the guinea pig

After injection of wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) or choleragenoid conjugated HRP (B-HRP) into lower cervical and upper thoracic dorsal root ganglia (DRG), HRP reaction product was observed in peripheral fibers of spinal afferents and in postganglionic cell bodies of the stellate ganglion (SG) in the guinea pig. After WGA-HRP injection into C8-T3 or T5 DRG, HRP-labelled cells were observed to cluster at the rami within the SG, with peak labelling observed 36 h after injection. SG cell labelling occurred with B-HRP as well, but not with native HRP after injection into thoracic DRG. Injection of this tracer in C8 DRG gave rise to a small number of labelled cells. In contrast to the labelling pattern following thoracic or C8 DRG injections, injection of WGA-HRP or native HRP into C6 DRG, led to random SG cell labelling. We conclude that the anterograde transsynaptic transport, following injection of WGA-HRP into thoracic DRG, provides a method to selectively label a population of postganglionic sympathetic neurons within the SG. A combination of transsynaptic and retrograde transport appears to be responsible for labelling after injection into C8 DRG, whereas labelling after C6 DRG injections seems to be due primarily to retrograde transport.

Transneuronal transport of wheat germ agglutinin conjugated horseradish peroxidase into last order spinal interneurones projecting to acromio- and spinodeltoideus motoneurones in the cat. 1. Location of labelled interneurones and influence of synaptic activity on the transneuronal transport

Transneuronal transport of wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) was used to define the location of last order spinal interneurones projecting to deltoideus motoneurones in C5-C8 of the cat. Labelled interneurones were found bilaterally from rostral C1 to caudal Th5 and from L3 to the L4/5 border. Ipsilaterally they were located in laminae V-IX, while contralaterally they were confined to lamina VIII except for a few cells in laminae VII and IX. To estimate the degree to which interneuronal activity facilitates the transneuronal transport from deltoideus motoneurones, the numbers of labelled interneurones were compared under different experimental conditions after WGA-HRP injection. The number of labelled last order interneurones was larger in one awake and active cat than in one awake but inactive cat and also larger in six anaesthetized animals in which spinal pathways were stimulated to evoke antidromic and synaptic activation of the interneurones, than in two anaesthetized animals without stimulation. It is concluded that the transneuronal transport of WGA-HRP is considerably facilitated by increased activity in the last order interneurones. An overall tendency was observed for a positive correlation between the number of labelled interneurones and the number of primarily stained deltoideus motoneurones. In order to reach a detectable concentration of WGA-HRP in the last order interneurones a certain number of motoneurones has to be labelled to the extent that they appear homogenously black.

Central projections of C4-C8 dorsal root ganglia in the rat studied by anterograde transport of WGA-HRP

Injections of WGA-HRP were made in the rat C4-C8 dorsal root ganglia (DRGs) individually to study the central projections and their relations to each other. The main dorsal horn projections from these DRGs to the dorsal horn lamina II extended for about two segments rostrally and caudally to the injected DRG, whereas the projections to laminae I, III, and IV were less restricted rostrocaudally. Comparisons of the dorsal horn projections from the DRGs investigated indicated a tendency for a somatotopic organization, which was most prominent in lamina II. Labeled central branches from the C4-8 DRGs could be traced in the dorsal column as far caudally as 12-17 segments caudal to the level of entrance. Most of these fibers appeared to end in the medial dorsal horn base, including the column of Clarke. Labeling of primary afferents in the ventral horn generally extended for at least 3-4 segments rostral and caudal to the level of the injected DRG. Projections to the central cervical nucleus were most prominent from the C4 DRG and gradually became less prominent from the more caudal DRGs. Heavy projections to the cuneate nucleus (Cun) originated from the C7 and C8 DRG, whereas those from the C4-C6 DRGs were less extensive. The Cun projections from the different DRGs appeared to overlap, and the same was true for the projections to the external cuneate nucleus. Projections to the gracile nucleus, the vestibular nuclear complex, including nucleus X, and to trigeminal sensory nuclei were seen from all DRGs investigated.

The expression of different cytochemical markers in normal and axotomised dorsal root ganglion cells projecting to the nucleus gracilis in the adult rat

Rat lumbar dorsal root ganglion neurones projecting to the nucleus gracilis in the brainstem were retrogradely labelled with Fluoro-Gold and analysed immunocytochemically for their expression of substance P-, calcitonin gene-related peptide-, galanin-, galanin message-associated peptide-, neuropeptide Y-, nitric oxide synthase- and carbonic anhydrase-like immunoreactivity as well as affinity to Griffonia (bandeiraea) simplicifolia lectin I--isolectin B4, RT97 and to choleragenoid. The analysis was made both in uninjured rats and in rats which had been subjected to unilateral sciatic nerve transection and partial resection 3 weeks earlier. The data showed that 6% of the L4 and L5 lumbar dorsal root ganglion cells that projected to the nucleus gracilis showed substance P-like immunoreactivity. Following nerve injury, none of the nucleus gracilis-projecting dorsal root ganglion cells showed substance P-like immunoreactivity. Nineteen per cent of the investigated cell population showed calcitonin gene-related peptide-like immunoreactivity in uninjured rats, but no nucleus gracilis-projecting calcitonin gene-related peptide-positive cells were found after nerve injury. Galanin- and galanin message-associated peptide-like immunoreactivity were found in 2% and 3%, respectively, of the Fluoro-Gold-labelled cell population normally and in 22% and 14%, respectively, after injury. No neuropeptide Y-positive cells were found in the Fluoro-Gold-labelled cell population normally, but after nerve injury, 96% of this population became neuropeptide Y-positive. Nitric oxide synthase-like immunoreactivity was found in 2% of the Fluoro-Gold-labelled cells normally and in 10% after injury. Two per cent of the Fluoro-Gold-labelled cells in the normal cases were stained by Griffonia (bandeiraea) simplicifolia lectin I--isolectin B4. After injury, however, no such double labelling was found. Thirty-four per cent of the Fluoro-Gold-labelled cell population was carbonic anhydrase positive normally, and 42% after injury. Seventy-five per cent of the Fluoro-Gold-labelled cells showed RT97 immunoreactivity normally and 12% after injury. Choleragenoid-like immunoreactivity was found in 99% of the Fluoro-Gold-labelled dorsal root ganglion cells normally and 81% after injury. Immunohistochemical visualisation of choleragenoid transganglionically transported from the injured sciatic nerve combined with neuropeptide Y immunocytochemistry showed that primary afferent fibres and terminals in the nucleus gracilis contain neuropeptide Y following peripheral nerve transection. Taken together, the results indicate that peripherally axotomised nucleus gracilis-projecting neurones undergo marked alterations in their cytochemical characteristics, which may be significant for the structural and functional plasticity of this system after injury.

Somatotopic organization of the cuneate nucleus in the rat: transganglionic labelling with WGA-HRP

A novel somatotopic map of primary cutaneous afferents projecting to the cuneate nucleus in the rat was determined by transganglionic transport of wheat-germ agglutinin conjugated to horseradish peroxidase and free horseradish peroxidase. Intracutaneous injections of tracer into different limited regions of the forelimb resulted in discrete areas of label for each injection site, with little or no overlap into other projection areas. The map of cutaneous projections onto the cuneate nucleus revealed by our anatomical tracing provided much more detail than any previous study in the rat, and demonstrated some significant differences from earlier maps based on electrophysiological recordings.

Trigeminal projections to contralateral dorsal horn: central extent, peripheral origins, and plasticity

Prior studies have documented a trigeminal (V) mandibular primary afferent projection to the dorsomedial portion of the contralateral medullary and cervical dorsal horns in cat, hamster, and rat. We now report the existence of a much more substantial V ophthalmic primary afferent projection to the ventrolateral portion of contralateral medullary and cervical dorsal horns in rat. Horseradish peroxidase (HRP) injections into the V ganglion or V brainstem complex anterogradely labeled a fascicle of primary afferent axons that exited the caudal ventrolateral V spinal tract to form a rostrocaudally continuous, transversely oriented, V primary afferent decussation. These fibers terminated most heavily in laminae III-V of the ventrolateral dorsal horn in contralateral caudal medulla and the first and second cervical segments. Retrograde tracing with diamidino yellow (DY) or fluorogold and anterograde tracing with Phaseolus vulgaris leucoagglutinin also demonstrated a substantial commissural projection of central origin in medullary dorsal horn laminae I-VII. The latter projection had a more diffuse trajectory and termination pattern than that of the V primary afferent decussation. Unilateral HRP injections into medullary and cervical dorsal horns also retrogradely labeled V primary afferent collaterals contralateral to the injection site in corresponding regions of dorsal horn, and also in ventromedial interpolaris, oralis, and principalis, rostral to their decussation. Axons (1.5 +/- 0.8 microns mean diameter; 0.4-3.9 microns range) therefore terminated both ipsi- and contralateral to their cells of origin. These HRP injections also labeled an average of 40.4 +/- 13.0 V ganglion cells (mean +/- SD, corrected for split somata) in dorsomedial, ophthalmic regions of the contralateral ganglion. Their mean diameter was slightly larger than that of cells labeled ipsilaterally (29.9 vs. 26.3 microns). Double-labeling studies assessed possible ophthalmic receptor surfaces innervated by centrally crossing primary afferents. DY was injected into right medullary and cervical dorsal horns, and HRP was applied to either the left cornea, the ethmoid nerve, or the dura overlying cerebral cortex. Though DY labeled from 75 to 125 left ganglion cells per animal, no cells were double-labeled. All of these findings suggest that nociceptive-specific ganglion cells are not a source of the crossed ophthalmic primary afferent projection. Unilateral transection of the infraorbital nerve on the day of birth did not alter the crossed primary afferent projection to the partially deafferented side of the brainstem. This is further evidence of an absence of central sprouting in spared V primary afferents following neonatal V deafferentation.

Deafferentation-induced expansion of saphenous terminal field labelling in the adult rat dorsal horn following pronase injection of the sciatic nerve

We have examined the effect of the degeneration of sciatic nerve afferents on the distribution of saphenous terminals in the adult rat dorsal horn. Deafferentation was produced by injection into the sciatic nerve of pronase, a combination of proteolytic enzymes, which causes death of ganglion cells and degeneration of their terminal fields. The saphenous terminal fields were labelled by exposing the cut nerve to a combination of horseradish peroxidase (HRP) and wheat germ agglutinin-horseradish peroxidase (WGA-HRP). Terminals were mainly found in the superficial dorsal horn, indicating that small-diameter afferents were heavily labelled. In one group of control animals, the normal sciatic and normal saphenous terminal fields were shown to be bilaterally symmetrical. In the experimental group, the initial injection of one sciatic nerve with pronase was followed 4 months later by bilateral HRP/WGA-HRP labelling of both saphenous nerves. In each animal, the terminal field of the saphenous nerve on the lesioned side was expanded in the medial, lateral, and caudal directions. Medially and laterally, the expanded terminal field overlapped more of the sciatic territory than normal; caudally, saphenous terminals were found in the rostral portion of the L5 segment, in an area normally filled by sciatic terminals and devoid of saphenous terminals. The expansion resulted in a total saphenous area 26% larger than the control side. Electron microscopy demonstrated that the label in both the normal and expanded territories was primarily contained in axons and terminals, with minor transneuronal labelling. Labelled terminals in the expanded areas were both simple terminals with round, clear vesicles, and glomerular terminals with multiple synaptic contacts; these terminal types resemble those previously described for primary afferents in the superficial dorsal horn. Although the preexistence of "silent" synaptic terminals in the expanded areas cannot be disproven, the data support the hypothesis that primary afferents in the adult have the potential to sprout and establish synapses when the conditions of the deafferentation are favorable.

Dose- and time-dependent selective and non-selective effects of ricin (RCA 120) on rat primary sensory neurons

The distribution of degenerating fibers in the spinal cord was studied in Fink-Heimer-stained sections following treatment of the tibial nerve with ricinus communis agglutinin (RCA 120). The ricin was either injected into the nerve or applied in a capsule on the transected nerve. Short survival times and low doses of ricin resulted in degeneration in somatotopically appropriate parts of the medial dorsal horn. Longer survival times and higher doses resulted in degeneration which progressively expanded into inappropriate areas in the central and lateral parts of the dorsal horn and in deeper laminae regardless of the mode of application. Furthermore, the effect of a ricin injection into the tibial nerve on transganglionic transport of choleragenoid horseradish peroxidase (B-HRP) in the peroneal nerve was studied following a simultaneous or delayed B-HRP injection. A simultaneous ricin and a B-HRP injection resulted in primary afferent HRP labeling in the gray matter, regardless of the dose of ricin. Following a delayed B-HRP injection almost no primary afferent labeling was seen in the gray matter, unless a very low dose of ricin was injected. This study shows that treatment of a peripheral nerve with a high dose of ricin and a long survival time may result in a considerable non-selective degeneration of fibers in the spinal cord. A selective degeneration may, however, be obtained by using lower doses or shorter survival times.

Somatotopic organization of inputs from the hand to the spinal gray and cuneate nucleus of monkeys with observations on the cuneate nucleus of humans

Central termination patterns of primary afferents from the hand and forelimb were studied following subdermal injections of HRP conjugates in macaque monkeys. In the middle layers of the dorsal horn of the spinal cord, afferents from digits 1-5 terminated in a rostrocaudal sequence in separate, elongated columns at cervical levels 5-7. Afferents from the glabrous digits extended to the medial margin of the dorsal gray, while afferents from the dorsal skin of the digits terminated more laterally. Afferents from the dorsal hand and palm terminated lateral to those from the digits, while inputs from the forearm occupied tissue rostral and caudal to the representation of the hand. In the cuneate nucleus, terminations from each digit formed an elongated column that was densely labelled in the central pars rotunda and sparsely labelled in both the rostral and caudal reticular poles. Within the pars rotunda, digits 1-5 were represented in order from lateral to medial. Inputs from the digit tips terminated ventral to inputs from the proximal digits. Afferents from the dorsal skin of the digits terminated in an even more dorsal position, while the most dorsal portion of the pars rotunda related to the glabrous and dorsal hand. Within the pars rotunda, terminations from specific parts of the hand overlapped parcellated clusters of neurons. These clusters were densely reactive for cytochrome oxidase (CO) and were surrounded by myelinated fibers. Much sparser label in the reticular poles was found consistently only after injections in the glabrous digits. Inputs to the poles appeared diffuse and overlapping while preserving some somatotopic order. When treated for CO or stained for Nissl substance or myelin, the pars rotunda of humans showed parcellation patterns that closely resembled the patterns seen in monkeys. From the relationship of inputs to the CO dense cell clusters in monkeys, it was possible to postulate in detail the somatotopic organization of inputs to pars rotunda of humans. The present results provide a comprehensive description of the somatotopic patterns of termination of afferents from the skin of the hand and forearm in the spinal cord and cuneate nucleus of macaque monkeys. A direct relationship of afferent somatotopy and identifiable cell clusters in the pars rotunda of the cuneate nucleus is further demonstrated. Finally, the patterns of cell clusters in the pars rotunda of macaque monkeys and humans suggest that the somatotopic organization of the cuneate nucleus may be very similar in human and nonhuman primates.

Central distribution of cervical primary afferents in the rat, with emphasis on proprioceptive projections to vestibular, perihypoglossal, and upper thoracic spinal nuclei

The projections of primary afferents from rostral cervical segments to the brainstem and the spinal cord of the rat were investigated by using anterograde and transganglionic transport techniques. Projections from whole spinal ganglia were compared with those from single nerves carrying only exteroceptive or proprioceptive fibers. Injections of horseradish peroxidase (HRP) or wheat germ agglutinin-horseradish peroxidase conjugate (WGA-HRP) were performed into dorsal root ganglia C2, C3, and C4. Free HRP was applied to the cut dorsal rami C2 and C3, greater occipital nerve, sternomastoid nerve, and to the C1/2 anastomosis, which contains afferents from suboccipital muscles and the atlanto-occipital joint. WGA-HRP injections into ganglia C7 and L5 were performed for comparative purposes. Injections of WGA-HRP or free HRP into rostral cervical dorsal root ganglia and HRP application to C2 and C3 dorsal rami produced labeling in dorsal and ventral horns at the level of entrance, the central cervical nucleus, and in external and main cuneate nuclei. From axons ascending to pontine and descending to upper thoracic spinal levels, medial collaterals were distributed to medial and descending vestibular, perihypoglossal and solitary nuclei, and the intermediate zone and Clarke's nucleus dorsalis in the spinal cord. Lateral collaterals projected mainly to the trigeminal subnucleus interpolaris and to lateral spinal laminae IV and V. Results from HRP application to single peripheral nerves indicated that medial collaterals were almost exclusively proprioceptive, whereas lateral collaterals were largely exteroceptive with a contribution from suboccipital proprioceptive fibers. WGA-HRP injections into dorsal root ganglia C7 and L5 failed to produce significant labeling within vestibular and periphypoglossal nuclei, although they demonstrated classical projection sites within the brainstem and spinal cord. The consistent collateralisation pattern of rostral cervical afferents along their whole rostrocaudal course enables them to contact a variety of precerebellar, vestibulospinal, and preoculomotor neurons. These connections reflect the well-known significance of proprioceptive neck afferents for the control of posture, head position, and eye movements.

Immunocytochemical evidence for the localization of the GM1 ganglioside in carbonic anhydrase-containing and RT 97-immunoreactive rat primary sensory neurons

Localization of GM1 ganglioside, the receptor for cholera toxin, and choleragenoid, which is the binding subunit of cholera toxin, was studied in the rat L5 dorsal root ganglion. Sections were incubated with choleragenoid and treated immunocytochemically. Choleragenoid-like immunoreactive cells were then examined for possible co-localization with carbonic anhydrase-like, RT 97 (antibody to neurofilament proteins), substance P-like, somatostatin-like and calcitonin gene-related peptide-like immunoreactivity and fluoride-resistant acid phosphatase (FRAP) activity, using adjacent sections. A subpopulation of dorsal root ganglion neurons exhibited choleragenoid-like immunoreactivity. The majority of these were medium-sized and large neurons. The strongest immunoreactivity was found in the area of the plasma membrane, but strong reactivity was also seen in the cytoplasm. The majority of the choleragenoid-like immunoreactive cells showed carbonic anhydrase-like and RT 97 immunoreactivity. Cells showing co-localization of choleragenoid-like and neuropeptide-like immunoreactivity or activity for FRAP were rarely observed. Our results suggest that the GM1 receptor is localized primarily on carbonic anhydrase-containing and RT 97-immunoreactive primary sensory neurons.

Simultaneous immunohistochemical demonstration of intra-axonally transported markers and neuropeptides in the peripheral nervous system of the guinea pig

Projections and peptide neurotransmitter/neuromodulator content of autonomic and visceral afferent neurons of the guinea pig were studied after application of the subunit B of cholera toxin (CTB) with or without horseradish peroxidase (HRP) as retrograde and anterograde tracers and subsequent immunohistochemical processing for double staining using antibodies raised to CTB, HRP and various neuropeptides. The results demonstrate that substance P (SP)- and calcitonin gene-related peptide (CGRP)-containing dorsal root ganglion cells project to the pylorus as well as to the celiac superior mesenteric and stellate ganglia as demonstrated with both retrograde and anterograde transport methodology. Binding studies revealed that a small number of the CTB-binding dorsal root ganglion cells contains immunoreactivity to SP and CGRP. The majority of the CTB-binding cells is SP- and CGRP-negative and terminate in the deeper parts of the dorsal horn. After injection of CTB conjugated to HRP (B-HRP) into the nodose ganglion, both motor and sensory elements were labeled in the medulla oblongata. Some of the CTB labeled vagal sensory nerve fibers in the nucleus tractus solitarii (NTS) were also found to contain immunoreactivity to SP or CGRP. The tracer was also transported through the peripheral branch of the nodose ganglion cells and labeled terminals in the esophagus.

Distribution of cardiac sympathetic afferent fibers in the guinea pig heart labeled by anterograde transport of wheat germ agglutinin-horseradish peroxidase

Anterogradely transported wheat germ agglutinin-horseradish peroxidase (WGA-HRP) was used to selectively label the distribution within the guinea pig heart of cardiac sympathetic afferent fibers whose cell bodies lie in the dorsal root ganglia (DRGs) of C6, C8, T1-3. The majority of fibers were seen in the posterior atrial wall, the pulmonary arterial walls, and along the major branches of the coronary arteries. Labeled fibers were also found in the parietal pericardium and associated with the atrioventricular and aortic valves. The labeling pattern was dependent upon segmental level: the most general labeling followed upper thoracic DRG injection, while labeled fibers associated with the coronary arteries were nearly absent after lower cervical DRG injection. Comparison of heart labeling among chemically sympathectomized and untreated animals demonstrated no difference in the distribution of frequency of WGA-HRP labeled fibers, indicating the specificity of this technique. The present findings indicate that the spinal sensory innervation of the heart has its major origins in the uppermost thoracic dorsal root ganglia and has a highly selective regional distribution. The implications of these findings in relation to cardiac autonomic dysfunction and pain are discussed.

Studies of peptidergic input to the lateral spinal nucleus

Lesions were made to interrupt potential sources of peptidergic input to the lateral spinal nucleus (LSn) in rats. Rhizotomies and spinal transections, as well as lesions of the lateral funiculus, failed to reduce immunohistochemical staining for substance P, dynorphin, Met-enkephalin, somatostatin and FMRF-amide in the LSn at lumbar levels. Thus, all examined peptidergic afferent input to the LSn appears to originate locally within the spinal cord.

Tracing of neuronal connections with cholera toxin subunit B: light and electron microscopic immunohistochemistry using monoclonal antibodies

Subunit B of cholera toxin was used as a tracer substance in the central nervous system after being injected into various brain regions, mainly somatosensory relay structures. The tracer was localized with an immunoperoxidase technique, using monoclonal antibodies raised in mouse hybridomas. This method, which is applicable in both light and electron microscopic studies, is characterized by high contrast between specific labeling and unspecific background activity. It yields excellent retrograde labeling of the dendritic tree and is thus suitable for studying the neuronal cytoarchitecture and, on the ultrastructural level, the synaptic organization of identified projection neurons.

Regeneration of adult dorsal root axons into transplants of embryonic spinal cord

Transplants of the embryonic rat spinal cord survive and differentiate in the spinal cords of adult and newborn host rats. Very little is known about the extent to which these homotopic transplants can provide an environment for regeneration of adult host axons that normally terminate in the spinal cord. We have used horseradish peroxidase injury filling and transganglionic transport methods to determine whether transected dorsal roots regenerate into fetal spinal cord tissue grafted into the spinal cords of adult rats. Additional transplants were examined for the presence of calcitonin gene-related peptide-like immunoreactivity, which in the normal dorsal horn is derived exclusively from primary afferent axons. Host animals had one side of the L4-5 spinal cord resected and replaced by a transplant of E14 or E15 spinal cord. Adjacent dorsal roots were sectioned and juxtaposed to the graft. The dorsal roots and their projections into the transplants were then labeled 2-9 months later. The tracing methods that used transport or diffusion of horseradish peroxidase demonstrated that severed host dorsal root axons had regenerated and grown into the transplants. In addition, some donor and host neurons had extended their axons into the periphery to at least the midthigh level as indicated by retrograde labeling following application of tracer to the sciatic nerve. Primary afferent axons immunoreactive for calcitonin gene-related peptide were among those that regenerated into transplants, and the projections shown by this immunocytochemical method exceeded those demonstrated by the horseradish peroxidase tracing techniques. Growth of the host dorsal roots into transplants indicates that fetal spinal cord tissue permits regeneration of adult axotomized neurons that would otherwise be aborted at the dorsal root/spinal cord junction. This transplantation model should therefore prove useful in studying the enhancement and specificity of the regrowth of axons that normally terminate in the spinal cord.

Central distribution of trigeminal and upper cervical primary afferents in the rat studied by anterograde transport of horseradish peroxidase conjugated to wheat germ agglutinin

Injections of WGA-HRP were made in the rat trigeminal ganglion and C1-3 dorsal root ganglia (DRGs) to study the central projection patterns and their relations to each other. Trigeminal ganglion injections resulted in heavy terminal labeling in all trigeminal sensory nuclei. Prominent labeling was also observed in the solitary tract nucleus and in the medial parts of the dorsal horn at C1-3 levels, but labeling could be followed caudally to the C7 segment. Contralateral trigeminal projections were found in the nucleus caudalis and in the dorsal horn at C1-3 levels. The C1 DRG was found to be inconstant in the rat. When it was present, small amounts of terminal labeling were found in the external cuneate nucleus (ECN) and the central cervical nucleus (CCN). No dorsal horn projections were seen from the C1 DRG. Injections in the C2 DRG resulted in heavy labeling in the ECN, nucleus X, CCN, and dorsal horn, where it was mainly located in lateral areas. Labeling could be followed caudally to the Th 7 segment. C2 DRG projections also appeared in the cuneate nucleus (Cun), in all the trigeminal sensory nuclei, and in the spinal, medial, and lateral vestibular nuclei. A small C2 DRG projection was observed in the ventral cochlear nucleus. C3 DRG injections resulted in heavy labeling in both medial middle and lateral parts of the dorsal horn, in the ECN, and in nucleus X, whereas the labeling in the CCN was somewhat weaker. Smaller projections were seen to trigeminal nuclei, Cun, and the column of Clarke. Comparisons of the central projection fields of trigeminal and upper cervical primary afferents indicated a somatotopic organization but with a certain degree of overlap.

Evidence for the existence of a lateral cervical nucleus in mice, guinea pigs, and rabbits

The lateral cervical nucleus of carnivores is large and is thought to play a prominent role in somatosensory processing. In contrast, early studies indicated that rats, mice, guinea pigs, and rabbits did not have a lateral cervical nucleus. However, we reported the existence of a lateral cervical nucleus in rats as a result of studies using retrograde transport techniques. In the present study, similar techniques were used to examine the possibility that early studies also overlooked the lateral cervical nucleus in mice, guinea pigs, and rabbits. In each of these species, a retrograde tracer was injected into the thalamus. These injections labeled a small number of neurons contralaterally in the dorsal part of the lateral funiculi of rostral cervical segments. Mice had the greatest number of neurons projecting from the lateral cervical nucleus to the thalamus, and rabbits had the fewest.

Spinal cord projections from hindlimb muscle nerves in the rat studied by transganglionic transport of horseradish peroxidase, wheat germ agglutinin conjugated horseradish peroxidase, or horseradish peroxidase with dimethylsulfoxide

The spinal cord projections of four different groups of hindlimb muscle nerve branches--the medial and lateral gastrocnemius nerves, muscle branches of the deep peroneal nerve, muscle branches of the femoral nerve, and a nerve to the hamstring muscles--were studied with transganglionic transport of horseradish peroxidase (HRP) in the rat. The influence of varying the postoperative survival (3, 6, and 10 days) and of using wheat germ agglutinin-HRP conjugate (WGA-HRP), or HRP with dimethylsulfoxide (DMSO) instead of free HRP was studied for the gastrocnemius nerves. After 3 days' survival following application of HRP to the gastrocnemius nerves, fine granular labeling was found mainly in lamina V in L4-5, and coarse granular labeling was found in Clarke's column as far caudally as L2, and in laminae VI and VII predominantly in Th12-L2. After 6 or 10 days' survival, the fine labeling in lamina V was sparse or absent, whereas the coarse labeling appeared to remain or to be only slightly reduced in Clarke's column and in laminae VI and VII. No labeling suggestive of terminals was observed in laminae I-III from the gastrocnemius nerves. Except for sparse labeling in lamina I in some of the cases and some minor differences rostrocaudally, the spinal distribution of labeling was similar to that from the other nerves investigated. The distribution of labeling obtained after application of WGA-HRP or HRP with DMSO to the gastrocnemius nerves was very similar to that obtained with free HRP after 3 days' survival. The results indicate that the spinal cord projections of hindlimb muscle nerves in the rat distribute mainly in the deep part of the dorsal horn and in the intermediate zone. Furthermore, the lack of labeling suggestive of terminals in laminae I-III from the gastrocnemius nerves suggests, in conflict with earlier findings in the cat, that primary afferent fibers from muscles do not necessarily terminate in these laminae in the rat. The results suggest, furthermore, that fine granular labeling found in lamina V represents fine-calibered afferent fibers. Finally, the similar spinal projection patterns of the different muscle nerves investigated suggest either a less developed or an essentially different somatotopic organization for muscle afferents compared to cutaneous afferents, as revealed in earlier studies.

Distribution of primary afferent fibres from the forelimb of the rat to the upper cervical spinal cord in relation to the location of spinothalamic neuron populations

Half the monosynaptic spinothalamic neurons in the rat spinal cord lie in the first 4 cervical segments, predominantly in the internal basilar column and ventral horn. The present study shows that both these regions receive primary afferents from the forelimb. The superficial dorsal horn receives few, and the region of the lateral cervical nucleus no primary afferents from the brachial plexus. We suggest that internal basilar column cells may represent localised relays concerned with the limbs, and that the gracile and cuneate nuclei are not unique, but are paralleled by other neuron populations in the upper cord.

Anterograde transsynaptic transport of WGA-HRP in rat olfactory pathways

The transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) was studied in rat olfactory pathways. After applications of tracer to the vomeronasal organ, the olfactory epithelium or injections into the olfactory bulb, WGA-HRP reaction product was observed in second-order neuron terminal areas of each pathway, e.g. within posteromedial cortical amygdaloid nucleus, primary olfactory cortex and contralateral primary olfactory cortex, respectively. The results indicate that anterograde transsynaptic transport of WGA-HRP occurs in olfactory pathways, as has been shown in visual, somatosensory and limbic systems, and thus, anterograde transsynaptic transport may be a mechanism for neurons to exchange materials and/or messages.

Peripheral axotomy of the rat mandibular trigeminal nerve leads to an increase in VIP and decrease of other primary afferent neuropeptides in the spinal trigeminal nucleus

In the vasoactive intestinal polypeptide (VIP)-rich lumbosacral spinal cord, VIP increases at the expense of other neuropeptides after primary sensory nerve axotomy. This study was undertaken to ascertain whether similar changes occur in peripherally axotomised cranial sensory nerves. VIP immunoreactivity increased in the terminal region of the mandibular nerve in the trigeminal nucleus caudalis following unilateral section of the sensory root of the mandibular trigeminal nerve at the foramen orale. Other primary afferent neuropeptides (substance P, cholecystokinin and somatostatin) were depleted and fluoride-resistant acid phosphatase activity was abolished in the same circumscribed areas of the nucleus caudalis. The rise in VIP and depletion of other markers began 4 days postoperatively and was maximal by 10 days, these levels remaining unchanged up to 1 year postoperatively. VIP-immunoreactive cell bodies were absent from trigeminal ganglia from the unoperated side but small and medium cells stained intensely in the ganglia of the operated side after axotomy. These observations indicate that increase of VIP in sensory nerve terminals is a general phenomenon occurring in both cranial and spinal sensory terminal areas. The intense VIP immunoreactivity in axotomised trigeminal ganglia suggests that the increased levels of VIP in the nucleus caudalis are of peripheral origin, indicating a change in expression of neuropeptides within primary afferent neurons following peripheral axotomy.

Afferent projections from the mammary glands to the spinal cord in the lactating rat--I. A neuroanatomical study using the transganglionic transport of horseradish peroxidase-wheatgerm agglutinin

Horseradish peroxidase-wheatgerm agglutinin was injected subcutaneously into one or more nipples of lactating rats to determine the spinal organization of sensory afferents emanating from the mammary glands. After survival periods of 45-96 h, dorsal root ganglia and segments of the spinal cord and/or medulla oblongata were sectioned and reacted histochemically with tetramethylbenzidine to reveal the transganglionically transported tracer. For each nipple injected, the peroxidase reaction product was found in somata, ranging in diameter from 15 to 60 microns, and fibres in 5-11 contiguous dorsal root ganglia. The number of labelled profiles was highest in the 2-4 central-most ganglia of the series and generally decreased progressively rostrally and caudally. After separate injections into each of the six ipsilateral nipples, labelling occurred in all ipsilateral dorsal root ganglia between the 5th cervical and 6th lumbar spinal segments. Substantial overlap of the spinal projections from adjacent mammary glands was seen, a given dorsal root ganglion innervating 2-3 different glands. Label in the spinal cord was restricted to the medial portion of the superficial dorsal horn. It occurred in what appeared to be terminal fields and fibres essentially in the substantia gelatinosa, but was also seen to extend into the marginal zone and sometimes into deeper regions of the dorsal horn. Label was found in both the gracile and cuneate nuclei of the medulla oblongata, though only occasionally and then only very sparsely. The substantial spread and segmental overlap of labelled mammary afferents, and the fact that most labelled afferents terminated in the dorsal horn, suggest that this spinal region may be an important site for the integration of sensory input from the mammary glands that may play a role in the sensory induction of reflex milk ejection.

Somatotopic organization of cutaneous afferent terminals and dorsal horn neuronal receptive fields in the superficial and deep laminae of the rat lumbar spinal cord

The somatotopic organization of A- and C-afferent fibre terminals in the dorsal horn of the rat lumbar spinal cord was compared with the spatial location of second-order dorsal horn neuronal mechanoreceptive fields. The central terminal fields of the sural, saphenous, and tibial nerve were mapped by labelling the nerves with horseradish peroxidase (HRP). A previous study used the transganglionic transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) to produce a somatotopic map of high-threshold C-fibre terminal fields in lamina II (Swett and Woolf: J. Comp. Neurol. 231:66-77, '85). In the present study the terminal fields of low-threshold A beta afferents that terminate in laminae III and IV were mapped by using unconjugated HRP at prolonged survival times (72 hours). Unfixed tissue was used to increase the sensitivity of the tetramethylbenzidine reaction, thus allowing these afferent terminals to be clearly seen. The general spatial arrangement of the terminal fields in laminae III/IV closely resembled that found in lamina II in the mediolateral and rostrocaudal planes but because of a dorsoventral obliquity of the afferent terminals, the superficial and deeper fields are not in strict vertical register. The input to laminae II-IV of the dorsal horn may therefore be viewed as two horizontally arranged sheets of afferent terminals both accurately representing the skin surface, the more superficial sheet representing the high-threshold C-afferents and the deeper sheet, low-threshold A-beta afferents. The spatial organization of high-threshold A-delta afferents in laminae I and V appears to be quite different, with a transverse rather than a longitudinal orientation. To study dorsal horn cell receptive field organization two single units with mechanoreceptive fields were recorded extracellularly in each of 87 vertical tracks in the lumbar spinal cord, one unit in the superficial dorsal horn and the second in the deep dorsal horn. In general the somatotopic organization of the receptive fields of both sets of units followed that of the afferent terminal fields but there were cells with receptive fields that were anomalous relative to the recording site. No evidence of any vertical relation or columnar arrangement in receptive field size, threshold, or location on the body surface was found when comparing the two units in a pair. Furthermore, no laminar functional specialization was found, the majority of neurones having both low- and high-threshold inputs.(ABSTRACT TRUNCATED AT 400 WORDS)

Laminar distribution and somatotopic organization of primary afferent fibers from hindlimb nerves in the dorsal horn. A study by transganglionic transport of horseradish peroxidase in the rat

Primary afferent nerve fibers to the spinal cord in the adult rat were labeled by applying horseradish peroxidase to the cut end of one of the following hindlimb nerves; the tibial, medial plantar, lateral plantar, common peroneal, saphenous, sural, lateral femoral cutaneous or obturator nerve. Maximal labeling intensity was found in the dorsal horn after 36-72 h survival. Labeling was observed in different dorsal horn laminae at different levels within the L1-S1 spinal cord segments, depending on which nerve horseradish peroxidase had been exposed to, probably reflecting the individual composition of afferent fiber types. Although a certain overlap was found, the central projections of the eight different nerves investigated formed well delineated three dimensional compartments within the medial 2/3 to 3/4 of the dorsal horn. This was most clearly discernible in lamina II. Although interindividual differences were present, bilaterally identical operations gave symmetrical projection patterns in the dorsal horn. The results indicate that dorsal horn projections of hindlimb nerves are organized in a highly ordered somatotopic fashion.

Different neuron populations in the feline lateral cervical nucleus: a light and electron microscopic study with the retrograde axonal transport technique

A comparative light and electron microscopic study was made of the two major neuron populations in the feline lateral cervical nucleus (LCN) that have an ascending projection--namely, the cervicothalamic and cervicomesencephalic neurons. The small population of neurons in the LCN that remained unlabelled after extensive injections into all known areas of LCN termination was also investigated. The intra-axonal retrograde tracing technique was employed and three different tracers were used: native horseradish peroxidase, horseradish peroxidase conjugated to wheat germ agglutinin, and cholera toxin subunit B (CTb). The diaminobenzidine and tetramethylbenzidine methods were used for the peroxidase reaction, and an immunocytochemical staining method with monoclonal antibodies was employed to localize CTb. The light microscopic cell count was performed on plastic-embedded semithin (2 micron) sections, and a section-embedding technique was used for sampling regions of interest for electron microscopy. The total number of neurons in the LCN was calculated to be 8,300, which is higher than numbers reported from other studies on frozen sections. It was concluded from control experiments that many neurons fail to counterstain in frozen sections. The average proportion of labelled cervicothalamic neurons was 94%, which accords with earlier reports. The frequency of labelled neurons projecting to the midbrain was 25-49%, depending on the method used. The highest number of labelled neurons was found with the immunocytochemical method with use of CTb as a tracer. In the electron microscopic study, the cell area, form factor, nuclear area, bouton covering ratio, mean appositional length of boutons, and bouton density were measured. There were no significant differences in these respects between the cervicothalamic and cervicomesencephalic neurons, but the unlabelled neurons differed significantly from the labelled ones, with a smaller somal area, a lower bouton covering ratio, and a larger relative nuclear area. The ascending neurons were scattered over the entire LCN, but the neurons that remained unlabelled after the extensive injections was found in its ventromedial part. It is suggested that these are interneurons. In the light microscopic study of the animals injected with CTb, the ventromedial part of the LCN displayed labelled structures resembling terminals. Electron microscopic examination of the sections revealed boutons synapsing on unlabelled neurons and dendrites. Occasionally labelled neurons and dendrites were also contacted.(ABSTRACT TRUNCATED AT 400 WORDS)

Transneuronal labelling of neurones projecting to forelimb motoneurones in cats performing different movements

Electrophysiologically identified propriospinal neurones (PNs) in the C3-C4 segments mediate disynaptic excitation and inhibition from higher centres to forelimb motoneurones. In behavioural experiments it was shown that C3-C4 PNs can mediate the command for target-reaching. We have investigated if the C3-C4 PNs which project monosynaptically to forelimb motoneurones are specifically active during target-reaching by using the technique of transneuronal uptake of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) into last order neurones which appears to be activity-dependent. The nerve branches to the spino- and acromio-deltoid muscles were injected with WGA-HRP. Cats preoperatively trained, performed either target-reaching or walking movements. Transneuronally labelled neurones in C2 to C4, located ipsilaterally in laminae V-VIII and contralaterally in lamina VIII, were found in target-reaching cats but not in walking cats. The results show a functional specificity of the C3-C4 PNs and support the proposal of activity dependent transneuronal labelling. It is suggested that this method can be used as a tool for functional identification of last order neurones activated in different movements.

Primary sensory afferents in the inferior mesenteric ganglion and related nerves of the guinea pig. An experimental study with anterogradely transported wheat germ agglutinin-horseradish peroxidase conjugate

Peripheral visceral afferents in the guinea pig were labeled by injections of wheat germ agglutinin-horseradish peroxidase conjugate (WGA-HRP) into the L2 and L3 dorsal root ganglia bilaterally. After anterograde transport of the tracer the following areas were examined for the presence of HRP-labeled fibers: the inferior mesenteric ganglion (IMG), the inferior mesenteric artery (IMA) with surrounding tissue, the hypogastric nerves, parts of the descending and sigmoid colon as well as the urinary bladder. Large numbers of heavily labeled fibers were found in the IMG, in the colonic nerves around the IMA and in the hypogastric nerves. In the IMG, profiles suggestive of being labeled axon terminals were observed. Labeled fibers were observed in the muscle layers of the colon and in the bladder wall. The results show that anterograde tracing with WGA-HRP can be used successfully in analyzing the morphology and structural organization of visceral afferents in the periphery.

Labeling of cutaneous sensory nerve endings with axonally transported horseradish peroxidase and wheat germ agglutinin-horseradish peroxidase conjugate: a methodological study in the rat

Practical aspects on the use of horseradish peroxidase (HRP) and wheat germ agglutinin-horseradish peroxidase conjugate (WGA-HRP) to trace peripheral cutaneous nerve endings have been studied. The parameters studied included application site of the tracer, post-application survival time, tracer concentration and tracer volume. These parameters were examined in the glabrous skin of the rat hindpaw. The best results were obtained with injections of 1 microliter WGA-HRP (20 micrograms/microliter) in dorsal root ganglia innervating the examined cutaneous region and a postinjection survival time of 18-36 h. With this approach extensive and heavy labeling was achieved of epidermal nerve endings, nerve endings in Merkel cell-neurite complexes and Meissner corpuscles. Useful, but less extensive labeling of these types of peripheral nerve endings, was obtained with injections of HRP in the lumbar spinal cord dorsal horn.