Light and electron microscopic autoradiographic study of the dorsal root projections to the cat dorsal horn

Central projections and connections of lumbar primary afferent fibers in adult rats: effectively revealed using Texas red-dextran amine tracing

Signals from lumbar primary afferent fibers are important for modulating locomotion of the hind-limbs. However, silver impregnation techniques, autoradiography, wheat germ agglutinin-horseradish peroxidase and cholera toxin B subunit-horseradish peroxidase cannot image the central projections and connections of the dorsal root in detail. Thus, we injected 3-kDa Texas red-dextran amine into the proximal trunks of L₄ dorsal roots in adult rats. Confocal microscopy results revealed that numerous labeled arborizations and varicosities extended to the dorsal horn from T₁₂–S₄, to Clarke's column from T₁₀–L₂, and to the ventral horn from L_1–5. The labeled varicosities at the L₄ cord level were very dense, particularly in laminae I–III, and the density decreased gradually in more rostral and caudal segments. In addition, they were predominately distributed in laminae I–IV, moderately in laminae V–VII and sparsely in laminae VIII–X. Furthermore, direct contacts of lumbar afferent fibers with propriospinal neurons were widespread in gray matter. In conclusion, the projection and connection patterns of L₄ afferents were illustrated in detail by Texas red-dextran amine-dorsal root tracing.

Visualization of lamina I of the dorsal horn in live adult rat spinal cord slices

The superficial dorsal horn of the spinal cord, particularly lamina I, plays a key role in the integration and relay of pain related sensory input. To study the physiology of lamina I neurons in slices, a clear delineation of this layer can be greatly advantageous. Yet, it has remained difficult to distinguish this layer in live tissue in conventional transverse spinal slices because of its very narrow thickness at the edge of the dorsal horn. We describe here the criteria we used to delineate lamina I in live tissue using gradient contrast videomicroscopy in 400 microm-thick parasagittal spinal cord slices from adult rats (30-60-day-old). Because of the longitudinal orientation of the neurons in this layer, the resulting distinctive reticulated appearance of lamina I made it possible to readily distinguish it from lamina II. The usefulness of this distinguishing parameter is demonstrated by our ability to contrast synaptic properties of neurons in lamina I from those in lamina II. Complete morphological identification of lamina I neurons however also requires visualization of the cell in the horizontal plane. To maintain compatibility with the parasagittal slice, we used 3D reconstructions from confocal images of the recorded neurons. Rotation of the neuron in space allowed for its morphological characterization in all three planes (horizontal, parasagittal, and transverse). This approach therefore presents optimal conditions for systematic electrophysiological recording from visually identified lamina I neurons.

A quantitative study of neurons which express neurokinin-1 or somatostatin sst2a receptor in rat spinal dorsal horn

The neurokinin-1 and somatostatin sst2a receptors have both been identified on spinal cord neurons. In this study we have estimated the proportions of neurons in different parts of the spinal cord which express these receptors, by using a monoclonal antibody against a neuronal nuclear protein named NeuN and combining the optical disector method with confocal microscopy. The NeuN antibody was initially tested on over 3200 neurons identified with antisera against a variety of compounds, including neuropeptides, enzymes and receptors, and also on astrocytes and oligodendrocytes. All of the neurons, but none of the glial cells that were examined possessed NeuN-immunoreactivity, which suggests that NeuN is a reliable marker for all spinal cord neurons. We found that approximately 45% of neurons in lamina I, 23-29% of those in laminae IV-VI and 18% in lamina X possessed the neurokinin-1 receptor, while the receptor was present on a smaller proportion of neurons in laminae II and III (6% and 11%, respectively). Thirteen percent of lamina I neurons and 15% of those in lamina II expressed the sst2a receptor. To provide further information about the types of neuron which possess the sst2a receptor, we searched for possible co-existence with the neurokinin-1 receptor as well as with GABA and glycine. sst2a and neurokinin-1 receptors were not co-localized on neurons in laminae I and II. All of the sst2a-immunoreactive neurons examined were also GABA-immunoreactive, and 83.5% were glycine-immunoreactive, indicating that the receptor is located on inhibitory neurons in the superficial dorsal horn. These results demonstrate the proportions of neurons in each region of the spinal cord which can be directly activated by substance P or somatostatin acting through these receptors. Levels of receptors can change in pathological states, and this method could be used to determine whether or not these changes involve alterations in the number of neurons which express receptors. In addition, the method can be used to estimate the sizes of neurochemically-defined populations of spinal cord neurons.

Ultrastructural observations of synaptic connections of vibrissa afferent terminals in cat principal sensory nucleus and morphometry of related synaptic elements

Previous work suggests that slowly adapting (SA) periodontal afferents have different synaptic arrangements in the principal (Vp) and oral trigeminal nuclei and that the synaptic structure associated with transmitter release may be related directly to bouton size. The present study examined the ultrastructures of SA and fast adapting (FA) vibrissa afferents and their associated unlabeled axonal endings in the cat Vp by using intra-axonal labeling with horseradish peroxidase and a morphometric analysis. All SA and FA afferent boutons contained clear, round, synaptic vesicles. All the FA and most SA boutons were presynaptic to dendrites, but a few SA boutons were axosomatic. Both types of bouton were frequently postsynaptic to unlabeled axonal ending(s) containing pleomorphic, synaptic vesicles (P-ending). The size of labeled boutons was larger in FA than SA afferents, but the size of dendrites postsynaptic to labeled boutons was larger for SA than FA afferents. Large-sized FA and SA boutons made synaptic contacts with small-diameter dendrites. The size of FA and SA boutons was larger than that of their associated P-endings. A morphometric analysis made on the pooled data of SA and FA boutons indicated that apposed surface area, active zone number, total active zone area, vesicle number, and mitochondrial volume were highly correlated in a positive linear manner with labeled bouton volume. These relationships were also applicable to unlabeled P-endings, but the range of each parameter was smaller than that of the labeled boutons. These observations provide evidence that the two functionally distinct types of vibrissa afferent manifest unique differences but share certain structural features in the synaptic organization and that the ultrastructural "size principle" proposed by Pierce and Mendell ([1993] J. Neurosci. 13:4748-4763) for Ia-motoneuron synapses is applicable to the somatosensory system.

Morphology and distribution of spinothalamic lamina I neurons in the monkey

Lamina I spinothalamic tract (STT) neurons were identified by retrograde labeling with cholera toxin subunit b (CTb) in monkeys. On the basis of the criteria of somatal shape and dendritic orientation in horizontal sections used in prior work in the cat, three distinct morphological types were recognized: fusiform (F) cells with spindle-shaped somata and two main longitudinal dendritic arbors; pyramidal (P) cells with triangular somata and three main dendrites oriented primarily longitudinally; and multipolar (M) cells with polygonal somata and four or more dendrites directed longitudinally and mediolaterally. Some cells had transitional shapes, but cells with indeterminate shapes and a few with small round, unipolar, or eccentric somata were grouped as unclassified (U). Greater variation appeared in the monkey than had been seen in the cat, and more subtypes were noted. The overall proportions of these cell types were: 47% F, 27% P, 22% M, and 5% U. Differential longitudinal distributions were found over the length of the spinal cord (from the second cervical through the first coccygeal segments). Pyramidal and multipolar cells together predominated in the enlargements, whereas fusiform cells predominated in thoracic segments. We conclude that three distinct morphological types of lamina I STT cells are present in the monkey as in the cat. Considered with other recent findings, the present results support the possibility that these three cell types may correspond to distinct physiological classes of nociceptive and thermoreceptive lamina I STT cells.

Morphological classes of spinothalamic lamina I neurons in the cat

We examined the morphology and distribution of retrogradely labeled spinothalamic tract (STT) neurons in lamina I (the marginal zone) of the spinal dorsal horn after large injections of cholera toxin subunit B (CTb) or Fast Blue (FB) into the contralateral thalamus of cats. Based on the shape and orientation of the somata and proximal dendrites in horizontal sections, three distinct cell types were identified: (1) fusiform cells with small, spindle-shaped somata and bipolar, longitudinal dendritic arbors; (2) pyramidal cells with triangular somata and three main dendritic origins with primarily longitudinal arborizations; and (3) multipolar cells with larger, multiangular somata and four or more radiating dendritic arbors directed both longitudinally and mediolaterally. These three morphological types differed significantly in the number of primary dendrites and the size of the somata. Subclasses of multipolar cells were noted. Nearly all cells could be categorized into these three classes consistently in horizontal sections. A small number of cells with transitional shapes or with small, round somata were unclassified. The proportional distributions of these cell types were found to vary over the length of the spinal cord (from the third cervical through the coccygeal segments) in three cats. The overall proportions of cell types were 34% fusiform, 36% pyramidal, 25% multipolar, and 5% unclassified. The proportions of pyramidal and multipolar cells were strikingly higher within the C7-8 and L6-7 segments and lowest in the thoracic segments. In contrast, fusiform cells formed about 20% of the labeled lamina I STT population in the C7-8 and L6-7 segments but more than 60% in thoracic segments. Across all nine cats, the proportions were similar within the cervical (C5-8) and lumbosacral (L5-S1) enlargements, although considerable interanimal variability was noted. These distinct morphological types of lamina I STT cells with differential longitudinal distributions probably have different functional roles. They may correspond to the three main physiological classes of lamina I STT cells.

Morphology and synaptic connections of slowly adapting periodontal afferent terminals in the trigeminal subnuclei principalis and oralis of the cat

Previous studies suggest that sensory information from primary afferent fibers is processed in a distinct manner in the individual subnuclei of trigeminal sensory nuclear complex. The present study has addressed this issue by using intra-axonal labeling with horseradish peroxidase to examine the ultrastructure and synaptic organization of axon terminals from slowly adapting (SA) periodontal afferents in the ventral subdivision (Vpv) of principalis and the rostro-dorsomedial part (Vo.r) of oralis. Our observations are based on complete or near-complete reconstructions of 139 synaptic boutons in Vpv and 105 in Vo.r. All the labeled boutons contained clear, spherical, synaptic vesicles and were presynaptic to unlabeled dendrites, and they were frequently postsynaptic to unlabeled axon terminals containing pleomorphic synaptic vesicles (P-endings). The P-endings frequently formed axodendritic synapses on dendrites which received axodendritic synapses from labeled boutons (synaptic triads). On the basis of the number of contacts, synaptic arrangements associated with the labeled boutons could be subgrouped into simple (one or two contacts), intermediate (three or four contacts), and complex (five or more contacts) types. The labeled boutons varied from round to elongated forms with smooth to more irregular or scalloped contours. The boutons with scalloped contour were much more frequent in the complex type. The boutons of the intermediate type were significantly smaller than the complex type and larger than the simple type. The SA periodontal afferent terminals participated in each type of synaptic arrangements in Vpv, but were mostly of the simple type in Vo.r. The size of labeled boutons was significantly larger in Vpv than in Vo.r. The total number of axodendritic and axoaxonic contacts per labeled bouton was significantly higher in Vpv than in Vo.r. Another difference was the more frequent occurrence of synaptic triads in Vpv than in Vo.r. These observations provide evidence that sensory information from primary afferent fibers is processed in a different manner in the two subnuclei.

Synaptic connections of a low-threshold mechanoreceptive primary neuron within the trigeminal subnucleus oralis

The central axon of a primary afferent neuron that responded to indentation of the glabrous skin of the lower lip in a slowly adapting fashion was intra-axonally injected with horseradish peroxidase. The labeled terminal within the subnucleus oralis was examined electron microscopically. The labeled ending had a pale axoplasm and contained clear spherical synaptic vesicles. The labeled ending formed a synaptic triad with a dendrite and an unlabeled axonal ending with pleomorphic vesicles (a mixture of oval, flattened and dense core vesicles). The labeled primary ending was presynaptic only to the dendrite, while the unlabeled ending was presynaptic to both the dendrite and the labeled primary ending.

Fine structure and synaptic architecture of HRP-labelled primary afferent terminations in lamina IIi of the rat dorsal horn

The fine structure and synaptic architecture of the afferent terminations in dorsal horn lamina II are studied using a combined light and electron microscopic procedure after anterograde labelling with horseradish peroxidase. Vibratome parasagittal sections, stained with heavy metal intensified diaminobenzidine after tracer application to the dorsal roots, were flat-embedded in Epon. The five types of labelled terminal arbors occurring in lamina IIi (Cruz et al., '87: J. Comp. Neurol. 261:221-236) were drawn and relocated in 5-microns sections cut serially from the thick sections. Ultrathin sections were then cut from the 5-microns sections so that the terminal fibers and swellings observed in the light microscope could be traced in the electron microscope. The flame-shaped arbors arose from fine myelinated stem fibers. Terminal strands generated large oval central terminals of type II synaptic glomeruli (CII), which established frequent axoaxonal contacts. Similar terminals have been labelled in the cat after tracer injections into hair-follicle fibers (Réthelyi et al., '82: J. Comp. Neurol. 207:381-393). The other four plexuses arose from unmyelinated stem fibers. The swarms of ultrafine boutons consisted of extremely thin terminal fibers generating very small, round, or polygonal glomerular terminals containing tightly packed agranular synaptic vesicles of variable size and one mitochondrion at best. The terminal strands of the bouquet plexus bore long and scalloped central varicosities of type I synaptic glomeruli (CI) with pleomorphic agranular vesicles and a relative abundance of dendroaxonal contacts. These features, together with the location in dorsal lamina IIi, suggest their belonging to the fluoride resistant acid phosphatase (FRAP)-reactive population. The boutons of the undulating fibers and those of the lateral plexus were, like those of the bouquets, scalloped and elongated rostrocaudally (CI), but contained a few large granular vesicles. The occurrence of the swarm, undulating, and lateral plexuses in ventral lamina IIi, which seems to lack FRAP or peptidergic terminals, suggests an origin from other, still unidentified neurochemical populations of fine primary afferents.

Plasticity of spinal systems after unilateral lumbosacral dorsal rhizotomy in the adult rat

Plasticity of spinal systems in response to lumbosacral deafferentation has previously been described for the cat, by using immunocytochemistry to demonstrate plasticity of tachykinin systems and degeneration methods to demonstrate plasticity of descending systems. In this study, we describe the response to lumbosacral deafferentation in the adult rat. Application of immunocytochemical methods to visualize tachykinins (predominantly substance P magnitude of SP), serotonin (5-HT), and dopamine B-hydroxylase (DBH), the synthesizing enzyme for norepinephrine, permits us to compare the response of SP systems in rat and cat spinal cord and to examine the response of two descending systems, serotoninergic and noradrenergic, to deafferentation. We used image analysis of light microscopic preparations to quantify the immunoreaction product in the spinal cord in order to estimate the magnitude, time course and localization of changes induced by the lesion. The distribution of SP, serotoninergic (5-HT), and noradrenergic staining in the spinal cord of rat is very similar to that of the cat. Unilateral lumbosacral rhizotomy elicits a partial depletion, followed by a partial replacement of tachykinin immunoreactivity in laminae I and II. This response was similar to that described for the cat, although characterized by a longer time course, and, as in the cat, is likely due to plasticity of tachykinin containing interneurons. The same lesion elicits no depletion but a marked and permanent increase in 5-HT immunoreactivity in laminae I and II, which develops more rapidly than the response by the SP system. These results indicate sprouting or increased production of SP and 5-HT in response to deafferentation. No change was seen in DBH immunoreactivity, indicating that the noradrenergic system does not show plasticity in response to deafferentation. Our results demonstrate that dorsal rhizotomy evokes different effects in different systems in the adult spinal cord of the rat and thus suggests that the response of undamaged pathways to partial denervation of their target is regulated rather than random.

High resolution radioautographic localization of [125I]FK-33-824-labelled mu opioid receptors in the spinal cord of normal and deafferented rats

Recent data have shown that [125I]D-Ala2, MePhe4, Met(o)ol5-enkephalin (FK-33-824) is a highly selective and specific mu opioid receptor ligand [Moyse et al. (1986) Peptides 7, 351-355]. This probe was used here to investigate the detailed radioautographic distribution of mu sites at various levels of the spinal cord. [125I]FK-33-824 binding sites were localized by both tritium-sensitive film and liquid emulsion radioautography in the spinal cord of naive and deafferented rats. In naive animals, high densities of mu sites were apparent within laminae I-II at all levels of the dorsal horn, with higher levels of labelling seen in layer IIi as compared to IIo in the lumbar segment. Laminae III-IV contained about half the quantities of binding observed in superficial layers. Relatively high densities of sites were also seen over lamina VI in the upper cervical cord and throughout Clarke's column. Within the latter, [125I]FK-33-824 binding clearly spared the large perikarya of the spinocerebellar neurons. In the ventral horn, [125I]FK-33-824 binding was mainly concentrated in layer IX, at the level of cervical and lumbar enlargements. Labelled sites were confined to the neuropil, mostly sparing the soma of motoneurons. Significant decreases in [125I]FK-33-824 binding in laminae I-II (55%) and III-IV (28%) were detected four days following cervical (C3-C7) or lumbar (L1-L6) rhizotomies. These decrements were most evident at seven days post-lesion at C3-C7 levels (93 and 76% in laminae I-II and III-IV, respectively) and recovered slightly thereafter up to 28 days post-lesion. In contrast, dorsal rhizotomies did not influence mu labelling in either the ventral horn or Clarke's column. These results confirm the association of mu opioid binding sites with dorsal primary afferent fibres and demonstrate the presence of mu sites in Clarke's column and lamina IX of the ventral horn. These findings suggest that endogenous opioids in the spinal cord play a role in sensory motor integration as well as in the modulation of primary nociceptive inputs.

Analysis of calcitonin gene-related peptide-like immunoreactivity in the cat dorsal spinal cord and dorsal root ganglia provide evidence for a multisegmental projection of nociceptive C-fiber primary afferents

Recent studies have suggested that calcitonin gene-related peptide (CGRP) can be used as a marker for a subpopulation of nociceptive primary afferents. Consequently, CGRP-immunoreactive (CGRP-IR) primary afferents have been reported to project many segments rostral to their segment of entry and to send collaterals into the superficial and deep laminae of the dorsal horn. This study reports that some CGRP-IR primary afferents of sacral origin project rostral through the ipsilateral lumbar enlargement in the cat. The ultrastructure of these multisegmentally projecting primary afferent axons and terminals identified in a partially denervated cat was examined and compared to the ultrastructure of CGRP-IR afferents from an intact cat. Retrograde transport of wheatgerm agglutinin-colloidal gold injected into the cat L4 spinal cord resulted in labeling of primary afferent cell bodies in the ipsilateral L4-S1 dorsal root ganglia (DRG). Analysis of every fourth section through the ipsilateral S1 DRG revealed as many as 1,000 retrogradely labeled neuronal cell bodies. One third of these cell bodies were double labeled for CGRP-like immunoreactivity. The number of single- and double-labeled cells increased in ganglia closer to the injection site (L4-L7). At the ultrastructural level, in the lumbosacral dorsal spinal cord of a normal cat, most CGRP-IR axons were unmyelinated, while the rest were small myelinated axons. In both the superficial dorsal horn and lamina V, CGRP-IR varicosities were dome shaped, scallop shaped, or elongated. The CGRP-IR varicosities contained small agranular vesicles and frequently a few dense core vesicles. These labeled varicosities formed asymmetric synapses on unlabeled dendritic spines, shafts, or neuronal somata. One cat received multiple unilateral dorsal rhizotomies (S1-L4) and an ipsilateral hemisection (mid L4). CGRP-IR axons and terminals were found within each of the rhizotomized segments, although their density was greatly reduced compared to that in the intact animals. In Lissauer's tract the majority (greater than 90%) of CGRP-IR fibers were unmyelinated. In laminae I and V, the remaining CGRP-IR varicosities were mainly the dome-shaped varicosities morphologically similar to those observed in the normal spinal cords. They contained small agranular vesicles and a few dense core vesicles and formed asymmetric synapses on unlabeled dendritic shafts and spines. These data demonstrate that unmyelinated, presumably C-fiber nociceptive primary afferents and some small myelinated A-delta nociceptive primary afferents of sacral origin project rostral through the cat lumbar enlargement and make synaptic connections in both the superficial and deep laminae of the cat dorsal spinal cord.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Neonatal capsaicin treatment induces invasion of the substantia gelatinosa by the terminal arborizations of hair follicle afferents in the rat dorsal horn

Capsaicin, administered on the day of birth, was found to alter laminar distribution, but not the receptive field properties or the morphology of the collateral arborizations of hair follicle afferents (HFAs) intra-axonally injected with horseradish peroxidase (HRP). Of the 65 HFA terminal arbors in capsaicin treated rats, 46 (71%) were found to enter the substantia gelatinosa (in control rats, 44/165, 27%). All of the collaterals projected to somatotopically normal areas of cord. Dorsal horn shrinkage (21%), as estimated by planimetric measurements of Nissl and acetylcholinesterase-stained material, was only a partial explanation of this result. This idea was supported by the statistically significant increase (27%, P less than 0.05) in the absolute dorsoventral length of collaterals. The results show that the destruction of unmyelinated fibres during the early postnatal period by capsaicin induces HFA invasion into the area that C fibres normally occupy. This invasion suggests that the laminar termination sites for different primary afferent fibres are not altogether specified and that intact neonatal primary afferents have the capacity to sprout into denervated regions of spinal cord.

Ultrastructural organization of regenerated adult dorsal root axons within transplants of fetal spinal cord

It has previously been demonstrated that the severed central branches of adult mammalian dorsal root ganglion cells regenerate into transplants of fetal spinal cord. The aim of this study was to determine whether these regenerating axons form synapses, and, if they do, to characterize them morphologically. Embryonic day 14 or 15 spinal cord was transplanted into the lumbar enlargement of adult Sprague-Dawley rats, and the L4 or L5 dorsal root was cut and then juxtaposed to the transplant. One to 3 months later the regenerated dorsal roots were labeled by anterograde filling with wheat germ agglutinin-horseradish peroxidase (WGA-HRP) or by immunocytochemistry for calcitonin gene-related peptide (CGRP). Dorsal root labeling with WGA-HRP demonstrated that regenerated axon terminals made synaptic contacts within transplants, and stereological electron microscopic analysis demonstrated that CGRP-immunoreactive axon terminals occupied an average of 9% of the neuropil within 2 mm of the dorsal root-transplant interface. The majority of synapses were axodendritic, but a significant percentage were axosomatic or axoaxonic. Since axoaxonic synapses were observed in transplants in which both pre- and postsynaptic profiles of axoaxonic synapses were labeled for CGRP, some regenerated axons apparently form synapses with each other. Approximately 90% of synaptic contacts were simple, 9% were complex, and 25% of the complex terminals were immunopositive for CGRP. Glia occupied 25% of the neuropil within 1 mm of the dorsal root-transplant interface, but only 6% of the neuropil 1-2 mm from the interface. We also performed a stereological analysis of the neuropil in lamina I. The area fractions of neuropil occupied by myelinated axons, perikarya, and dendrites were similar in transplants and in lamina I. However, the area fraction occupied by unmyelinated axons was significantly smaller in transplants, and the area fraction occupied by axon terminals was significantly larger in transplants compared with lamina I. Regenerated CGRP-immunoreactive synaptic terminals in transplants were significantly larger than in normal lamina I, and their synaptic contact length was also increased, suggesting that a compensatory mechanism for increasing synaptic efficiency might occur within the transplants. Synaptic density, however, was significantly reduced in transplants, indicating a smaller number of synaptic terminals per unit area. In lamina I, as in the transplant, most synapses were axodendritic, but the percentage of axosomatic and axoaxonic terminals was lower in lamina I than in the transplants.(ABSTRACT TRUNCATED AT 400 WORDS)

Central boutons of glomeruli in the spinal cord of the cat are enriched with L-glutamate-like immunoreactivity

Several lines of evidence indicate that L-glutamate may be a neurotransmitter of fine myelinated and unmyelinated primary afferent fibres in the spinal cord. The aim of the present study was to determine if L-glutamate was enriched in the terminals of these fibres. We performed the post-embedding immunogold technique on sections taken from the superficial regions of the lumbar cord in two cats. An antiserum, raised against protein-conjugated L-glutamate, was employed. Several tests on tissue and on a model system indicated that the antiserum recognized a glutaraldehyde-fixed L-glutamate-like substance. Terminals of fine afferent fibres were identified in the substantia gelatinosa as central boutons of synaptic glomeruli. Central boutons were examined through serial sections following immunogold reactions and were found to be heavily labelled with gold particles in consecutive sections. Quantitative analysis indicated that central boutons were more than two and a half times as densely labelled with gold particles than the tissue average. It was concluded that this represents a genuine enrichment of L-glutamate in these structures. Comparisons were made between L-glutamate-immunoreactive properties of central terminals and immunoreactivity for GABA, aspartate and glutamine. Statistical analysis revealed that central boutons in sections incubated in GABA antiserum and glutamine antiserum were associated with significantly lower densities of gold particle labelling than the average for the same tissue. Particle densities of central boutons in sections incubated in aspartate antiserum were not significantly different from average tissue densities. It was concluded that central boutons were not enriched with these three amino acids. Central boutons of synaptic glomeruli were classified into three groups on morphological criteria: (1) dense sinusoidal boutons; (2) large dense-core vesicle-containing boutons; and (3) regular synaptic vesicle-containing boutons. Quantitative analysis revealed that all of these groups were enriched in glutamate immunoreactivity, however, there were differences between the groups; large dense-core vesicle-containing boutons were associated with significantly lower densities of particles than regular synaptic vesicle-containing and dense sinusoidal terminals. The evidence indicates that central boutons, which most probably originate from fine myelinated and unmyelinated primary afferent fibres, are enriched with L-glutamate which may serve as a neurotransmitter in such fibres.

Ultrastructure of somatostatin-immunoreactive nerve terminals in laminae I and II of the rat trigeminal subnucleus caudalis

The morphology and distribution of somatostatin-immunoreactive synaptic boutons was studied in the rat trigeminal subnucleus caudalis using pre-embedding electron microscopic techniques. Immunoreactive terminals were found in lamina I and throughout lamina II but were more concentrated in outer lamina II. All immunoreactive terminals contained many round or pleomorphic agranular small synaptic vesicles and some large dense-cored vesicles. Lamina I terminals were all simple dome-shaped and relatively small. They established one asymmetric or slightly asymmetric synapse over a dendritic spine or a small, medium or large dendritic shaft. The large dendrites are probably derived from Waldeyer neurons. Many lamina II immunoreactive terminals were also simple dome-shaped terminals and established asymmetric synaptic contacts with one postsynaptic structure, usually a dendritic spine or a small to medium-sized dendritic shaft. However, other lamina II immunoreactive terminals were larger and displayed more complex morphology and synaptology. Complex immunoreactive terminals had scalloped or smooth contours and made synaptic contacts with more than one postsynaptic profile. In outer lamina II they sometimes constituted the central terminals of typical glomerular synaptic complexes. We conclude that many of the immunoreactive simple terminals probably originate from intrinsic somatostatin-immunoreactive interneurons while some of the more complex ones and the central glomerular terminals are likely to originate from primary afferents. These results are consistent with our accompanying light microscopic study (Alvarez and Priestley, Neuroscience 38, 343-357, 1990) which indicates that somatostatin-immunoreactive primary afferents project preferentially to outer lamina II while the lamina I somatostatin-immunoreactive plexus is likely to originate largely from laminae I and II interneurons. In addition somatostatin-immunoreactive cell bodies were found in lamina II. The heaviest immunoreactivity in these cells was in the Golgi apparatus. Also some vesicles containing dendrites were immunostained, and these were most abundant in inner lamina II. Thus, in trigeminal subnucleus caudalis, somatostatin may be derived from primary afferent synaptic boutons, interneuron synaptic boutons and interneuron dendrites. However, each of these sites probably makes a proportionately different contribution to the total amount of somatostatin released in each lamina or sublamina.

Cells in laminae III and IV of rat spinal dorsal horn receive monosynaptic primary afferent input in lamina II

In order to determine how information conveyed by fine primary afferent fibres might reach the deeper laminae of the spinal dorsal horn, 5 Golgi-stained neurones with somata in lamina III or IV and dendrites that entered lamina II were examined by electron microscopy. Three of the cells were from animals in which 2 or 3 dorsal roots had been cut 26 or 30 hours previously. These cells received numerous synapses in lamina II, and between 13 and 16% of these (24-31% of asymmetric synapses) were from degenerating axons. Synapses with degenerating axons were found throughout the depth of lamina II, including the dorsal part, which receives primary afferent input from myelinated nociceptors and from unmyelinated axons. In addition, all 3 cells were postsynaptic to degenerating axons within lamina III. The 2 cells from unoperated animals also received many synapses within lamina II and at some of these the presynaptic axon was the central terminal of a glomerulus. Only one example of a dendrodendritic synapse involving a stained dendrite was seen. Cells of laminae III and IV may therefore not be a major target for presynaptic dendrites of cells in lamina II. It is concluded that one way in which information carried by primary afferents passes from the superficial dorsal horn to the deeper laminae is through monosynaptic contacts between these afferents and the dorsal dendrites of some cells whose somata are situated in laminae III and IV. If the axons of these cells generate local collaterals, this may account for some of the activation of cells whose dendrites do not enter lamina II.

Cells of origin of spinothalamic tract projections to the medial and lateral thalamus in the cat

The double fluorescent retrograde labeling method was used to examine the distribution of spinothalamic tract (STT) cells that project to the medial and lateral thalamus in the cat. Injections of one fluorescent tracer (Fast Blue or Diamidino Yellow) were made throughout the lateral thalamus and injections of the other tracer were made in the medial thalamus at sites extrapolated from recording track coordinates. Survival times were successively extended (up to 5 weeks) in order to maximize labeling in both the cervical and lumbosacral spinal cord. On average, over 2,000 labeled contralateral STT cells were counted in serial sections from segments C5-7 and L5-S2. Numerical variability of the order of a factor of two was attributable to inherent differences between individual animals. The total number of cells labeled with fluorescent tracers was comparable to the number labeled with horseradish peroxidase in control cases, although there were significant differences between the laminar distributions of labeling produced by the two methods. Injections made anterior to the thalamus to control for labeling due to leakage or passing fibers did not produce substantial spinal labeling. The laminar distribution of fluorescent dye-labeled STT cells was consistent; about half (47%) were located in lamina I, 8% were in lamina V, 5% in lamina VI, 20% in lamina VII, and 20% in lamina VIII. The proportions of STT cells in laminae I and V were higher in cervical segments (57% and 12%, respectively) than in lumbosacral segments (38% and 6%). The dominant contribution of lamina I cells to the STT thus revealed by the fluorescent tracers is striking. The proportions of STT cells labeled from the medial and the lateral thalamus varied with segmental and laminar location and with injection placement. The majority (62%) of STT cells in most cases projected only to the medial thalamus, 25% projected only to the lateral thalamus, and 13% projected to both. The STT cell populations in laminae I, VII, and VIII each displayed this common projection pattern. In contrast, cells in laminae V and VI projected predominantly to the lateral thalamus. Twice as many STT cells in lamina I (19%) projected to both the medial and the lateral thalamus as from other laminae. A greater proportion of laminae V-VIII STT cells in segments L5-6 projected to the lateral thalamus, and in S1-2, more projected to the medial thalamus.(ABSTRACT TRUNCATED AT 400 WORDS)

Cytoarchitectonic organization of the spinal cord in the rat: II. The cervical and upper thoracic cord

A laminar cytoarchitectonic scheme of the cervical and upper thoracic segments of the rat spinal cord is presented in which Rexed's principles for the cat are applied. The material examined in the current investigation consists of 50-80 microns-thick unstained or Nissl-stained sections, and 2 microns-thick plastic-embedded sections stained with paraphenylenediamine. The cytoarchitectonic organization was found to be basically similar to that of the cat. As in our previous study of the cytoarchitectonic organization of the lower thoracic and lumbosacral spinal cord (Molander et al.; J. Comp. Neurol. 230:133-141, '84), the borderlines between the laminae were often found to be ambiguous, suggestive of zones of transition rather than sharp borders. In addition to the laminar scheme, the distribution of certain important cell groups, including the column of Clarke, the central cervical nucleus, the lateral cervical nucleus, the lateral spinal nucleus, and the internal basilar nucleus, is described.

Ultrastructure of primary afferent fibres and terminals expressing alpha-galactose extended oligosaccharides in the spinal cord and brainstem of the rat

The ultrastructural characteristics of primary afferent fibres, which express alpha-galactose extended oligosaccharides recognized by LD2 and LA4 monoclonal antibodies, and the subcellular localization of these oligosaccharides were studied. LD2 and LA4 antibodies both label intensely the plasma membrane of primary afferent fibres, and with LD2 antibody all immunoreactive profiles also possessed strong intracellular staining. In contrast, intracellular staining with LA4 antibody was observed in only a subpopulation of stained profiles. LD2-immunoreactive fibres were detected in trigeminal and Lissauer tracts and in lamina I (LI) and lamina II (LII), and appeared as a mixture of unmyelinated and myelinated fibres. The highest density of LD2-immunoreactive synaptic boutons was found in lamina II outer (LIIo). Many of the terminals were simple dome-shaped terminals, making single asymmetric synapses over small and medium-sized dendritic shafts and dendritic spines. All LA4-immunoreactive fibres were unmyelinated. In addition, some small scalloped central-glomerular terminals contacting two or three dendrites were found. LA4-immunoreactive fibres were found more frequently than terminals and appeared most heavily immunostained in trigeminal and Lissauer tracts. In the neuropil of LI and LII, LA4 profiles were generally very weakly immunostained, although a small sample of immunostained synaptic boutons was detected. All LA4-immunoreactive terminals were found in lamina II inner (LIIi) and made simple asymmetric axodendritic synapses. In addition to axons and terminals, some dendrites exhibited LD2 immunoreactivity and this was most intense in the region of synaptic vesicles. In addition to neurons, some endothelial cells were immunostained with LD2 antibody and astrocytes were immunostained with LA4 antibody.

Spatial distribution of pre- and postsynaptic sites of axon terminals in the dorsal horn of the frog spinal cord

Axon terminals which could be interpreted as dorsal root boutons, were photographed from a series of 98 ultrathin sections with a Jeol 100B electron microscope. A total of 13 boutons were recovered for computer reconstruction. Two of them were terminal boutons, eight en passant boutons and three boutons were only partially recovered. All boutons contained multiple synaptic sites (maximum 33 and minimum seven) at which axodendritic and axoaxonic synapses were established. Axodendritic synapses were of the asymmetric type and they were directed toward adjacent dendrites. In axoaxonic synapses, which were of the symmetric type, the boutons were invariably on the postsynaptic side. Among the presynaptic profiles axons with spherical and pleomorphic vesicles and dendrites with flattened vesicles could be discerned. On average, each 2.67-microns2 bouton surface area contained one presynaptic site at which an axodendritic synapse was established, and each 7-microns2 surface area contained one postsynaptic site for an axoaxonic (or dendroaxonic) contact. A tendency of grouping of synaptic sites was observed. Distance measurements between the closest neighbours of all synaptic sites were made in four combinations in boutons with the original and with a random distribution of synaptic sites. The arithmetic mean of distances measured between the presynaptic and the closest postsynaptic sites was almost twice as big as that measured in the reverse direction. The difference between these values became greatly reduced in the case of random distribution. The arithmetic mean of distances between the closest neighbours of presynaptic sites was about the same as that between the closest neighbours of postsynaptic sites. This latter value was considerably increased with randomly distributed synaptic sites. The results suggest a non-random distribution of synaptic sites on the surface of boutons. The analysis of cluster formation of synaptic sites performed with a numerical taxonomy technique revealed that the majority of the 153 synaptic sites were comprised in 27 clusters containing both pre- and postsynaptic sites within the 1-micron similarity level. All postsynaptic sites were within 1 micron of one or more presynaptic sites. On the basis of the assumption that the postsynaptic sites are occupied by inhibitory axoaxonic synapses, it is suggested that the transmitter release from the presynaptic sites can be individually controlled in this structural arrangement. A probable mechanism of this function may be the passive invasion of the bouton by the impulse propagating actively along the dorsal root fibre.

Spinal and supraspinal terminations of primary afferent fibers from the gastrocnemius-soleus muscle in the cat

The central distribution of the terminations of primary afferent fibers from the gastrocnemius-soleus muscle in the cat was examined with the method of transganglionic transport of horseradish peroxidase. At the segmental level, the main projection areas were found to be laminae I and V-VII; in the rostrocaudal direction, the terminations extended from the third sacral segment to nucleus Z. A 40% solution of horseradish peroxidase in 2% dimethylsulfoxide was applied to the central cut ends of the muscle nerves in an open pool for several hours and subsequently was removed. A capsule of tracer solution applied during the survival period of the animals was found to result in additional labeling due to peripheral leakage. The tissue sections were processed with tetramethylbenzidine. Termination fields were consistently observed ipsilaterally in: lamina I from the L4 through S3 segments, being most dense in L6 and S1; lateral lamina V in L6 and S1-3; medial laminae VI-VII from L5 through S3; medial Clarke's column from L1 through L4; the ventral aspect of the gracile nucleus; and, nucleus Z. Little or no labeling was found in laminae II-IV in experiments in which peripheral leakage of tracer solution was prevented. The distribution of reaction product in laminae VI-VII and Clarke's column corresponds to the projections of large-diameter afferent fibers from the gastrocnemius-soleus muscle. The projections to laminae I and V, which are attributed to small-diameter sensory fibers, indicate involvement of these laminae in sensory modalities mediated by slowly conducting muscle afferent fibers, e.g. deep nociception. This pattern contrasts strongly with the central projections of cutaneous fibers, which terminate heavily in laminae II-IV, but resembles the central distribution of fibers from tooth pulp and viscera.

Rapidly adapting pulmonary receptor afferents: II. Fine structure and synaptic organization of central terminal processes in the nucleus of the tractus solitarius

The nucleus of the tractus solitarius (nTS) is a site for termination of primary afferents originating from a variety of different visceral sensory endings (Kalia and Mesulam: J. Comp. Neurol. 193:523-553, '80). The light and electron microscopic evaluation of bouton terminals of slowly adapting lung stretch (SAR) afferent fibers originating from the tracheobronchial tree has been described previously (Kalia and Richter: J. Comp. Neurol. 241:503-520, 521-535, '85). The companion article (Kalia and Richter: J. Comp. Neurol. 273:000-000, '88) describes details of the light microscopic organization of a second group of pulmonary afferents, the rapidly adapting receptors (RARs), that are known to signal transient volume changes in airways (Sellick and Widdicombe: J. Physiol. (Lond.) 203: 359-381, '69; Q.J. Exp. Physiol. 55:153-163, '70). Terminals from RAR afferents are concentrated within two specific subnuclear groups of the nTS (dnTS and nI) and are distributed over 4 mm of the medulla oblongata rostrocaudally. Within the nTS, axon collaterals of RAR afferents remain myelinated up to a diameter of 0.4-1.0 microns. Preterminal processes are always unmyelinated and range in diameter from 0.15 to 0.3 microns. Bouton terminals (1.0-2.0 microns) are of both the en passant and end terminal varieties. The synaptic profiles formed by 143 bouton terminals of RAR afferents, were examined in uninterrupted sequential sections and are described in this paper. All the bouton terminals examined under the electron microscope were found to contain clear, round synaptic vesicles. Boutons made synaptic contact with different profiles in each of the two subnuclei (dnTS and nI) examined. Contacts were usually asymmetrical (type I) containing clear, round synaptic vesicles 35-50 nm in diameter. In the dorsal subnucleus of the nTS (dnTS), the synaptic arrangement of RAR boutons did not appear to be complex. The RAR bouton terminal was usually located in juxtaposition to unlabeled axon terminals of similar morphological characteristics. Typically, the RAR bouton terminal made synaptic contact with a medium-sized spiny dendrite. No axosomatic contacts involving RAR afferents were observed in this subnucleus. In the intermediate subnucleus of the nTS (nI), the most common synaptic arrangement of RAR bouton terminals was in the form of a "glomerulus," which was formed by five to seven different types of neuronal profiles surrounding the labeled RAR bouton terminal.(ABSTRACT TRUNCATED AT 400 WORDS)

Opiate receptors in the human spinal cord: a detailed anatomical study comparing the autoradiographic localization of [3H]diprenorphine binding sites with the laminar pattern of substance P, myelin and nissl staining

The anatomical localization of opiate receptors in the human spinal cord has been examined in six cases aged 7-41 years using quantitative autoradiographic methods following the incubation of fresh, unfixed cryostat sections with [3H]diprenorphine. In order to precisely localize the distribution of receptors in the spinal cord, the laminar anatomy of the spinal grey was demonstrated at each spinal level examined using 50-microns sections stained for myelin, Nissl substance and substance P. In all cases, autoradiograms demonstrated that opiate receptors were distributed in a similar fashion in the grey matter of the cervical, thoracic, lumbar, sacral and coccygeal regions of the human spinal cord. At all 25 spinal levels examined, opiate receptors were mainly localized within the upper laminae of the dorsal horn (laminae I-III) and within the tract of Lissauer. The highest density of opiate receptors was localized within the inner segment of lamina II where the receptors formed a very dense band lying immediately dorsal to lamina III. The density of receptors in this inner region of lamina II (33 +/- 2 fmol/mg) was more than two-and-one-half times greater than that in the remaining upper laminae which showed moderate receptor densities: lamina I (12 +/- 4 fmol/mg) and outer lamina II (13 +/- 3 fmol/mg) both showed similar receptor densities which were higher than those in lamina III (10 +/- 3 fmol/mg) The tract of Lissauer (11 +/- 2 fmol/mg) also showed a moderate density of opiate receptors which was intermediate between the densities in laminae I/IIo and the density of lamina III. The density of receptors in the remaining laminae of the spinal cord varied from moderately low to virtually zero. Moderately low densities of receptors were found in laminae V, VI, VIII, IX and X with very low levels within laminae IV and VII. In particular, in lamina VII opiate receptors were unable to be detected above normal background levels in the dorsal nucleus of Clarke. These results show that, as in other mammalian species, opiate receptors in the human spinal cord are mainly concentrated in the upper laminae of the dorsal horn and in the tract of Lissauer. The possible role of these receptors in modulating spinal nociceptive information is discussed with respect to previous findings on the relationship of opiate receptors to primary afferent fibres in the spinal cord.

Exocytosis from large dense cored vesicles outside the active synaptic zones of terminals within the trigeminal subnucleus caudalis: a possible mechanism for neuropeptide release

It has been hypothesized that chemical interactions between neurons in the central nervous system can occur in the absence of well defined synaptic complexes, but morphological correlates have been difficult to find. The present study demonstrates exocytotic release from large (70-130 nm) dense cored vesicles at structurally nonspecialized areas along the plasmalemma of structurally different categories of terminals and occasionally from dendrites and axons within the neuropil of the trigeminal subnucleus caudalis. In rats, the marginal (lamina I) and substantia gelatinosa (lamina II) layers contain the central terminals of primary afferent fibers from the infraorbital nerve that supply the skin and whiskers (vibrissae). Different types of interneurons are also present and may modify the input being relayed to higher centers. While exocytotic profiles were present in control animals, they increased significantly (P less than 0.01) on the ipsilateral side 1-24 h after a unilateral skin lesion in the vibrissae area. A second increase (P less than 0.001) occurred 14-15 days after the lesion. Virtually all examples of large vesicle exocytosis were observed at structurally nonspecialized sites while those at the active synaptic zones involved small clear vesicles. Substance P-like immunofluorescence, present in controls and on the ipsilateral side during the first 6 days, subsequently declined until 4 weeks after surgery when some recovery was noted. The increase in large vesicle exocytosis and the decrease in substance P are interpreted to reflect functional adjustments of different neurons in response to the lesion. The exocytosis involving large dense cored vesicles may serve to deliver transmitters and/or neuropeptide modulators to appropriate receptors in a wider area than release into a specialized synaptic cleft would allow.

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.

Synaptic architecture of glomeruli in superficial dorsal horn of rat spinal cord, as shown in serial reconstructions

Using serial section EM analysis, synaptic organization of glomeruli in lamina II of the dorsal horn of the rat has been examined. Four C1-terminals (small, dark and sinuous), four CIIa (large, light and regular, without neurofilaments) and four CIIb (with neurofilaments) at the centres of synaptic glomeruli of types I, IIa and IIb, respectively, were serially sectioned and reconstructed. Asymmetrical synapses between the central terminal (C) and dendritic profiles without synaptic vesicles (D) prevailed in all types of glomeruli. Symmetrical dendroaxonic contacts with presynaptic dendrites (V1----C) occurred practically only in type I glomeruli in which there were also more asymmetrical C----V1 contacts than in type II glomeruli. Symmetrical axoaxonic synapses V2----C were more abundant in type IIa and IIb glomeruli. Type IIa glomeruli had a significantly larger number of C----D synapses and of all synapses per unit area of C surface, than type IIb glomeruli. Triadic systems with C and D postsynaptic to V2 were nearly as numerous as those involving V1 in type I glomeruli. Triads with V2 were however largely preponderant in type IIa and virtually exclusive in type IIb. It thus seems that each of the three types of glomerulus has its own pattern of synaptic interactions which might reflect specific complexes of feed-forward and feed-back mechanisms. In type I glomeruli, excitation of second-order neurons by nociceptive C1 terminals may be controlled in similar proportions by presynaptic dendrites excited within the glomerulus by the C terminal itself, or by peripheral axons excited from outside the glomerulus. This kind of control is likely to prevail in type IIa glomeruli and to be the only efficient modulatory mechanism in type IIb glomeruli.

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

Horseradish peroxidase conjugates of either the lectin wheat germ agglutinin or choleragenoid, the binding subunit of cholera toxin, were injected into the L5 spinal ganglion of adult rats. This enabled comparison of these two conjugates as anterograde tracers in the primary sensory system. After a postoperative survival of 4 h to 30 days, the rats were perfused and frozen sections from spinal and medullary regions receiving primary afferents were processed for horseradish peroxidase histochemistry with tetramethylbenzidine as the chromogen. Additional observations were made in two adult cats. Following injection of wheat germ agglutinin-horseradish peroxidase the labelling appeared mostly as small-sized granules. The concentration of labelled primary afferents in the grey matter of the spinal cord was greatest in the marginal zone and the substantia gelatinosa and less pronounced in the deep parts of the dorsal horn. Labelling was also found in a region lateral to the central canal and in the ventral horn. Following injection of choleragenoid-horseradish peroxidase the labelling appeared mainly as larger-sized granular profiles. The concentration of labelled primary afferents was greatest in the deep part of the dorsal horn and pronounced in a region lateral to the central canal and in the ventral horn. All these regions are known to receive large calibre fibres. The marginal zone and the substantia gelatinosa, known to receive fine calibre fibres, showed almost no labelling in the rat. In the cat, however, there was somewhat more labelling in the substantia gelatinosa. Labelling of neuronal cell bodies indicating transneuronal transport was seen after injection of wheat germ agglutinin-horseradish peroxidase. Transneuronal labelling did not seem to occur after injection of choleragenoid-horseradish peroxidase. The present findings show that wheat germ agglutinin-and choleragenoid-horseradish peroxidase give rise to markedly different labelling patterns. A possible explanation for the different labelling in the marginal zone and substantia gelatinosa could be that certain primary sensory neurones lack either receptors for choleragenoid on their neuronal plasma membrane or the ability to transport the choleragenoid-horseradish peroxidase complex.

Peptide- and non-peptide-containing unmyelinated primary afferents: the parallel processing of nociceptive information

Primary afferent C fibres can be subdivided into a number of subgroups on the basis of cytochemistry or receptor binding characteristics. Numerous peptides have been localized to dorsal root ganglia, yet these appear to be only found in approximately 50% of small perikarya. A large proportion of the remaining small cells do not contain peptides but are identifiable in rodents by their content of a fluoride resistant acid phosphatase. Attempts have been made to correlate particular biochemical types with particular receptive field profiles, with rather modest success. As an alternative we suggest, principally from an analysis of skin afferents, that peptide- and non-peptide-containing afferents are two distinct C fibre pathways innervating similar peripheral structures and conveying similar information, but to different areas within the dorsal horn. Morphological evidence also suggests that these two subsystems form either glomerular or simple synaptic arrangements in the dorsal horn. The significance of parallel pathways for the processing of nociceptive information is briefly discussed.

An electron microscopic study of terminals of rapidly adapting mechanoreceptive afferent fibers in the cat spinal cord

The intra-axonal horseradish peroxidase technique was used to examine the central terminals of 7 A beta primary afferent fibers from rapidly adapting (RA) mechanoreceptors in the glabrous skin of the cat's hindpaw. At the light microscopic level, labelled collaterals were seen to bear occasional boutonlike swellings, mostly (75-82%) of the en passant type. These swellings were distributed more or less uniformly from lamina III to a dorsal part of lamina VI in the dorsal horn, over a maximum longitudinal extent of about 4 mm. At the electron microscopic level, we observed that labelled boutons of RA afferent fibers were 1.0 to 3.3 micrometers in longest sectional dimension, and contained clear, round synaptic vesicles. They frequently formed asymmetric axospinous and axodendritic synapses and commonly appeared to receive contacts from unlabelled structures containing flattened or pleomorphic vesicles plus occasional large dense-cored vesicles. The examination of synaptic connectivity over the entire surface of individual boutons indicated that RA afferent boutons each made contacts with an average of one spine and one dendrite and, in addition, appeared to be postsynaptic to an average of two unlabelled vesicle-containing structures. This synaptic organization was, in general, more complex than that we had seen previously in Pacinian corpuscle (PC) and slowly adapting (SA) type I mechanoreceptive afferent fibers. Our findings indicate that RA, SA, and PC afferent terminals, while displaying some differential synaptic organizations, have many morphological and synaptological characteristics in common. These afferent terminals, in turn, seem to be generally distinguishable from the terminals of muscle spindle Ia afferents or unmyelinated primary afferents.

The noradrenergic innervation of vasopressin neurons in the paraventricular nucleus of the hypothalamus: an ultrastructural study using radioautography and immunocytochemistry

Immunocytochemical and radioautographic procedures were combined at the ultrastructural level to study the noradrenergic synaptic input to vasopressin neurons in selected portions of the paraventricular nucleus of the hypothalamus (PVN) of the rat. Radioactive norepinephrine (NE) was infused into the lateral ventricle or applied topically to the region of the PVN. After appropriate survival times, brain tissues were processed for ultrastructural immunocytochemical demonstration of vasopressin using a monoclonal antibody. [3H]NE varicosities were detected by electron microscopic radioautography. In the periventricular zone of the PVN, radioactive varicosities were numerous accounting for 20-30% of all nerve terminals in this zones. These NE terminals primarily innervated dendritic processes of non-vasopressinergic neurons. Although an occasional axosomatic synapse was observed, input to vasopressin positive neurons was exclusively to their dendrites. In the lateral magnocellular sub-nucleus of the PVN (designed pvl2), noradrenergic terminals were fewer in number accounting for only 1-2% of the total. These terminals were found predominately but not exclusively making axodendritic synapses onto non-vasopressin processes. In both regions, many of the radiolabeled terminals had well-defined membrane appositions with their post-synaptic partners which included a synaptic cleft and post-synaptic density of varying thickness. In both the periventricular zone and the lateral magnocellular regions, noradrenergic varicosities were seen in close proximity to numerous blood vessels.

Effects of dorsal rhizotomy on the several types of primary afferent terminals in laminae I-III of the rat spinal cord. An electron microscope study

After cervical dorsal rhizotomy, small dark central terminals (C1) of glomeruli underwent electron dense changes at 8 h and were all degenerated at 36 h; their number persisted, though slightly diminished, up to 15 days, glial engulfment being negligible. Light large central terminals without neurofilaments (CIIa) showed electron-lucent or electron-dense degeneration from 14 to 36 h, while those with neurofilaments (CIIb) exhibited increased neurofilamentous areas, with depletion and presynaptic concentration of synaptic vesicles as in the electron-lucent change, at the 8-36 h postrhizotomy periods. Both CII-varieties were all degenerated at 36 h and became electron dense at 48 h; glial phagocytosis was intense and no terminals were present after 4 days. It is concluded that in the rat the 3 types of central glomerular terminals are primary axons, and that each type undergoes a different pattern of degeneration which points to a separate primary afferent origin. Numerous nonglomerular axodendritic endings began showing electron-dense degeneration at 8 h which rapidly masked their normal structure, although most appeared to contain round agranular vesicles, and some of them dense-cored vesicles (in lamina I). A few endings exhibited electron-lucent degeneration. Labeling methods seem preferable for studying the primary origin of nonglomerular terminals, due to the difficulty in recognizing the normal predegenerative structure of these profiles.

The cytoarchitectonic organization of the spinal cord in the rat. I. The lower thoracic and lumbosacral cord

A laminar cytoarchitectonic scheme of the lower thoracic and lumbosacral segments of the rat spinal cord is presented in which Rexed's principles for the cat are applied. The material consists of 80-micron-thick sections stained with toluidine blue or according to van Gieson and 2-micron-thick sections stained with p-phenylenediamine or toluidine blue. The cytoarchitectonic organization of the rat spinal cord was found to be basically similar to that of the cat, although certain differences exist--for example, in the extension of the laminae. In addition to the laminar scheme, the distribution of certain cell groups, Lissauer's tract, and the pyramidal tract were investigated.

Axons which take up [3H]serotonin are presynaptic to enkephalin immunoreactive neurons in cat dorsal horn

Through combined ultrastructural localization of ENK immunoreactivity and [3H]5-HT uptake sites, it was found that 5-HT radiolabeled axons contact ENK immunoreactive cell bodies and small dendrites in the cat superficial dorsal horn. While some contacts displayed both pre- and postsynaptic specializations, the majority lacked a definitive synaptic cleft and postsynaptic density. These results suggest that 5-HT-containing axons, presumably derived from the medullary nucleus raphe magnus, directly influence spinal opiod antinociceptive activity.

An electron microscopic study of primary afferent terminals from slowly adapting type I receptors in the cat

Primary afferent fibers transmitting impulses from slowly adapting (SA) Type I receptors in the glabrous skin of the hind paw of the cat were injected intraaxonally in the spinal cord with horseradish peroxidase (HRP). At the light microscopic level, terminal arborizations were observed in the medial dorsal horn extending up to 6 mm rostrocaudally in and near the seventh lumbar segment. Boutonlike swellings labelled with HRP were distributed in clusters in Rexed's laminae III-VI. There was a tendency for the most dorsal clusters from an individual fiber to be located rostrally and for the most ventral clusters to be located caudally. At the electron microscopic level, a combination of morphometric analysis and serial reconstruction revealed the following: (1) All the boutons labelled with HRP contained predominantly clear, round synaptic vesicles, 40-50 nm in diameter. (2) Labelled boutons (n = 75) had cross-sectional longest dimensions of 1.72 +/- 0.53 micron (Mean +/- S.D.), perimeters of 4.95 +/- 1.52 micron, and areas of 1.18 +/- 0.59 micron 2. Their shapes in section varied from rounded to elongated forms. (3) The sizes of labelled boutons decreased significantly and linearly with depth from lamina IV to VI. The shapes of the bouton cross sections also became rounder with depth in the dorsal horn. (4) About 72% of synaptic contacts associated with HRP-filled boutons were with dendritic spines and shafts; most of these synapses were of the asymmetric type. (5) The remainder (28%) of the appositions were synapselike contacts between labelled boutons and unlabelled structures containing flattened or pleomorphic vesicles, and occasional dense-cored vesicles. (6) We observed no unequivocal axosomatic contacts made by labelled boutons. (7) The lengths of synaptic appositions with dendritic spines (0.46 +/- 0.20 micron) or with dendritic shafts (0.51 +/- 0.18 micron) were significantly greater than the synapselike contacts with vesicle-containing unlabelled structures (0.29 +/- 0.09 micron). (8) Complex neuropilar organization was occasionally seen with labelled boutons as central elements, although simpler organizations were much more common. In summary, HRP-labelled fibers ended predominantly in boutons containing clear, round vesicles forming axospinous and axodendritic synapses.(ABSTRACT TRUNCATED AT 400 WORDS)

The distribution of afferent fibers from the gastrocnemius-soleus muscle in the dorsal horn of the cat, as revealed by the transport of horseradish peroxidase

The transganglionic transport of horseradish peroxidase (HRP) was used to examine the distribution of afferent fibers from the gastrocnemius-soleus (GS) muscle in the dorsal horn of the cat. Intense labeling was consistently observed in lamina I (in segments L4 to S3) and in the lateral portion of lamina V (segments L6 and S1-3), but not to any significant extent in laminae II-IV. These terminal fields were ascribed to the small-diameter (group III/IV) GS afferent fibers.

Opioid neurons and pain modulation: an ultrastructural analysis of enkephalin in cat superficial dorsal horn

To clarify the circuitry through which opioid compounds modulate spinal and trigeminal nociceptive transmission, we have examined the synaptic associations formed by leucine-enkephalin-containing (enkephalin) neurons in the superficial dorsal horn of the cat. As described previously, punctate enkephalin immunoreactivity is concentrated in the marginal layer (lamina I) and in both the outer and inner layers of the substantia gelatinosa (lamina IIo and IIi). In colchicine treated cats, enkephalin perikarya are most numerous in lamina I and at the border between laminae I and II. Ultrastructural analysis reveals that enkephalin cells receive a diverse afferent input. The majority of afferent inputs are presynaptic to the enkephalin dendrites; few axosomatic synapses are seen. Among these presynaptic axonal profiles are unlabeled axons which resemble primary afferent terminals, including the characteristic central axonal varicosity. Enkephalin dendrites are also postsynaptic to enkephalin immunoreactive axons. Two types of enkephalin axonal profiles appear in the superficial dorsal horn. Class I profiles are only found in lamina I. These are large profiles which form few synapses; those synapses made are axodendritic. Class II enkephalin axons are smaller and are distributed in both layers I and II. While Class II axons most commonly form axo-dendritic synapses, they also form axo-axonic synapses with flat vesicle-containing profiles; the latter are generally presynaptic to the enkephalin terminals. Serial analysis further revealed that both the enkephalin and the flat vesicle-containing profile synapse onto a common dendrite. Although enkephalin axons frequently lie adjacent to round vesicle-containing profiles, anatomical evidence that opioid axons form synapses with this type of ending was not found. An additional type of enkephalin vesicle containing-profile is found in layer IIi; its morphological features do not clearly distinguish its axonal or dendritic origin. These endings are typically postsynaptic to unlabelled central endings, and provide minimal presynaptic input to other elements in the neuropil. Like some class II axons, these labelled profiles contain vesicles which cluster at the membrane immediately adjacent to unlabelled central axons. These results indicate that spinal enkephalin neurons receive a variety of synaptic inputs. These include inputs which may derive from primary afferent axons. Enkephalin neurons, in turn, influence nociceptive transmission predominantly through postsynaptic mechanisms. Finally, while we did not observe enkephalin terminals presynaptic in an axoaxonic relationship, the possibility that enkephalin neurons modulate the excitability of fine fiber nociceptive and nonnociceptive afferents via "nonsynaptic interactions" is discussed.