Ultrastructural analysis of dynorphin B-immunoreactive cells and terminals in the superficial dorsal horn of the deafferented spinal cord of the rat

Light microscopic studies have demonstrated important differences in the distribution of enkephalin and dynorphin cells and terminals in the dorsal horn. Most importantly, dynorphin neurons are located in regions almost exclusively associated with the transmission and/or control of nociceptive messages (laminae I, IIo, and V); enkephalin neurons, although located in the same regions, are also found in areas involved in the transmission of nonnociceptive messages, e.g., laminae IIi and III. To determine whether there are also differences in the synaptic organization of the two opioid peptides, we have examined the distribution of dynorphin B immunoreactivity at the ultrastructural level. The studies were performed in colchicine-treated rats that underwent dorsal rhizotomy so that the relationship of dynorphin terminals and cells to primary afferent terminals could be established. Dynorphin B-immunoreactive cell bodies and dendrites in laminae I and IIo receive convergent primary and nonprimary afferent input, which suggests that dynorphin neurons receive a small-diameter, nociceptive input. Dynorphin terminals predominantly contain round, agranular vesicles; some terminals also contain a few dense core vesicles. Most dynorphin terminals are presynaptic to unlabelled dendrites; both asymmetric and symmetrical axonal contacts were noted. Dynorphin-immunoreactive boutons are also presynaptic to unlabelled cell bodies and spines. Twenty-nine percent of dynorphin terminals were associated with axonal profiles, including degenerating primary afferent terminals; only rarely could a synaptic density be detected. Although some degenerating primary afferent terminals were clearly presynaptic to dynorphin-immunoreactive terminals, in most cases, the polarity of the relationship between primary afferents and dynorphin terminals could not be established. These data indicate that synaptic interactions made by and with dynorphin-immunoreactive cells and terminals in the superficial dorsal horn are not very different from those that were previously reported for enkephalin cells and terminals. Thus, it is unlikely that dynorphin terminals provide a significant presynaptic input to primary afferent fibers. On the other hand, the presence of a primary afferent input to dynorphin cell bodies and dendrites in the superficial dorsal horn suggests that dynorphin cells receive a direct input from small-diameter, nociceptive primary afferents. That connection might contribute to the increased levels of dynorphin message and peptide that have been reported in rats experiencing a chronic inflammatory condition.(ABSTRACT TRUNCATED AT 400 WORDS)     

Distribution of the piriform cortical terminals to cells in the central segment of the mediodorsal thalamic nucleus of the rat.

A Golgi electron microscopic study was undertaken to investigate the distribution of terminals from the piriform cortex that synapse on identified dendrites of neurons in the central segment of the mediodorsal thalamic nucleus of the rat. The piriform cortical terminals were identified as degenerating terminals following lesions in the cortex. They consisted of two types, i.e., large (LR type) and small (SR type) presynaptic terminals, both of which had round synaptic vesicles and formed asymmetric synaptic contacts. SR boutons terminated preferentially onto distal dendrites and never synapsed on primary dendrites. LR terminals synapsed preferentially on proximal dendrites, but were also found on more distal dendritic segments.     

Synaptic organization of the rat parafascicular nucleus,with special reference to its afferents from the superior colliculus and the pedunculopontine tegmental nucleus

The synaptic organization of afferents to the parafascicular nucleus (Pf) of the thalamus was studied in rats. In the Pf, three types of axon terminals were identified: the first type was a small terminal with round synaptic vesicles forming an asymmetric synapse, the second type was a large terminal with round synaptic vesicles forming an asymmetric synapse, and the third type was a terminal with pleomorphic vesicles forming a symmetric synapse. They were named SR, LR and P boutons, respectively. In order to determine the origin of these axon terminals, biotinylated dextran amine (BDA) was injected into the main afferent sources of the Pf, the superior colliculus (SC) and the pedunculopontine tegmental nucleus (PPN). Axon terminals from the SC were both SR and LR boutons which made synaptic contacts with somata and dendrites. PPN afferents were SR boutons, which made synaptic contacts with somata and smaller dendrites. Double-labeled electron microscopic studies, in which a retrograde tracer (wheat germ agglutinin conjugated to horseradish peroxidase: WGA-HRP) was injected into the striatum and an anterograde tracer (BDA) into the SC revealed that SC afferent terminals made synapses directly with Pf neurons that projected to the striatum. Another experiment was performed to find out whether two different afferents converged onto a single Pf neuron. To address this question, two different tracers were injected into the SC and PPN in a rat. Electron microscopically, both afferent terminals from the SC and PPN made synaptic contacts with the same dendrite. Our results prove that a single neuron of the rat Pf received convergent projections from two different sources.     

Ultrastructure of chemically defined neuron systems in the dorsal horn of the monkey. III. Serotonin immunoreactivity

Enkephalinergic axons and terminals were identified by the PAP immunohistochemical method in lamina I (marginal zone) and lamina II₀ (outer substantia gelatinosa) in the dorsal horn of the monkey spinal cord. Synaptic profiles with enkephalin-like immunoreactivity (MELI) contained clear, round, vesicles, sometimes a few large granular vesicles, and usually formed asymmetrical contacts.MELI terminals forming synaptic contacts with various sizes of dendrites and with dendritic spines were the most common type of relationship found; axosomatic contacts were few. Additionally, two types of complexes were observed in which an MELI terminal formed a specialized apposition with an unlabelled terminal. The contact of often resembled a synapse and in most cases the MELI terminal was suspected to be presynaptic. One complex consisted of a MELI terminal apposing the LGV type terminal (containing large granular vesicles), which in turn was presynaptic to a dendrite. (The identify of the LGV terminal could not be determined, but it had some characteristics similar to those described for substance P terminals and for a class of primary afferents in the monkey dorsal horn). The other type of complex consisted of a MELI terminal apposing an R-type terminal (containing small, round, clear vesicles) which was in turn presynaptic to a dendrite. Often, the MELI terminal also formed a synapse onto the same dendrite.The axodendritic, axospinous and axosomatic contacts of MELI terminals in the superficial dorsal horn may produce some of the depressive postsynaptic-like effects of enkephalin iontophoresis onto dorsal horn neurons. In these cases the responses of dorsal horn neurons to both low threshold and nociceptive primary afferents is suppressed. However, the opiate receptor-dependent PAD of C-fibers observed in the dorsal horn may be mediated by the MELI complexes formed with LGV and R terminals found in lamina I.     

Synaptic organization of the nucleus gracilis of the cat. Experimental identification of dorsal root fibers and cortical afferents

The synaptic organization throughout the nucleus gracilis has been investigated in unoperated cats. Axon terminals of variable size can establish synaptic contacts with neuronal somata, dendritic processes, initial segment of axons or with other axon terminals at “complex synaptic arrangements.” Large boutons with rounded vesicles are regularly associated with smaller boutons containing flattened vesicles; the latter type of bouton forms frequently a double synapse being presynaptic to the large bouton and to the element postsynaptic to this (“complex synaptic arrangements”). Medium-sized to small axon terminals of the “isolated” type contain primarily either rounded or flattened vesicles. These boutons are surrounded by a thin glial process which also wraps the postsynaptic element, mostly represented by a small dendritic profile. The “isolated” type of bouton seems to be more abundant in the rostral than in the caudal part of the nucleus. In all unoperated control animals altered axons and axon terminals are present. They are enlarged and display hyperplasia and dilatation of tubular profiles of smooth endoplasmic reticulum as well as proliferation of microtubules and various aspects of mitochondrial degeneration. In cats sacrificed 48 hours after section of lumbo-sacral dorsal roots a high number of “dark” boutons are observed in various stages of degeneration. These terminals are identifiable with the large boutons containing rounded vesicles and postsynaptic to the smaller boutons with flattened vesicles. The morphology of dorsal root terminals in the nucleus gracilis is discussed in relation to that of primary afferent terminals in other central structures and to the functional aspects of axo-axonic contacts. The sensori-motor cortex was removed in another series of animals which were sacrificed after one to four days. As a consequence of such lesions cortical fiber terminals in the nucleus gracilis may undergo either the “dark” or the “light” type of degeneration. These terminals are of smaller size than those of primary afferents, they usually synapse on dendritic profiles of small diameter, are not involved in axo-axonic contacts and seem to contain rounded vesicles. Therefore they can be identified with at least some of the small and medium-sized boutons of the “isolated” type.     

Projections to the inferior colliculus from the dorsal column nuclei. An experimental electron microscopic study in the cat.

The projections from the dorsal column nuclei (DCN) to the inferior colliculus (IC) were investigated in cats by means of electron microscopy. The DCN were destroyed unilaterally by electrocoagulation and the animals survived 3 or 4 days. A variable number of degenerating synaptic boutons were observed bilaterally in the IC-external nucleus and in the intercollicular area. The greatest number of degenerating terminals was encountered in the rostral portions of the IC-external nucleus and in the intercollicular area contralateral to the DCN lesion. In these regions some series of ultrathin sections the degenerating boutons comprised 6-13% of the entire synaptic bouton population of the examined area. In the caudal portion of the contralateral IC-external nucleus the number of degenerating boutons greatly diminished and the latter showed a patchy distribution. Ipsilateral to the DCN lesion the number of degenerating terminals was low, and a more substantial number was found only in the rostroventral portion of the IC-external nucleus and in the intercollicular area. The degenerating synaptic boutons displayed markedly diverse patterns of alterations. Most often was the dark (electron-dense) degeneration pattern, followed by the filamentous degeneration. More rare were the light (electron-lucent) degenerating boutons, and a very limited number of terminals displayed pinocytotic degeneration changes. The DCN boutons contained round and oval synaptic vesicles and terminated mainly on proximal dendritic trunks, followed by the perikarya of the efferent neurons, and smaller, distal dendrites. Rarely the degenerating terminals contacted dendritic spines, and no contacts were found with the small (interneuronal) perikarya and with other vesicle containing profiles. The axodendritic contacts were asymmetrical and the axosomatic--from the intermediate type. In agreement with previous light microscopic data, the present findings demonstrate the existence of substantial excitatory projection from the DCN to the polysensory nuclei of the IC that integrate converging auditory and tactile information, and are involved in acoustico-motor behavior.     

Origin and ultrastructural identification of dorsal column nuclear synaptic terminals in the basilar pontine gray of rats

The ultrastructural characteristics of HRP-WGA-labeled or degenerating axon terminals arising from neurons in the dorsal column nuclei (DCN) were identified within the contralateral basilar pontine nuclei (BPN) following unilateral HRP-WGA injections or ablations of the DCN. The cells of origin of these projections were also identified through the application of the retrograde tracer HRP-WGA. Two groups of degenerating DCN-pontine terminals were identified. Both formed asymmetrical synaptic contacts with dendritic shafts and/or dendritic appendages of pontine neurons. One group of degenerating terminals contained small, round synaptic vesicles, while the other exhibited a mixture of dense core and pleomorphic vesicles. The former group, which clearly represented the majority of degenerating terminals observed, was interpreted to progress from an early filamentous form of degeneration to a later electron-dense variety and to originate from dorsally located DCN cells distributed primarily at the level of and just caudal to the area postrema. Other DCN-labeled neurons were more ventrally located and were postulated to give rise to those degenerative boutons that contained a mixture of dense core and pleomorphic-shaped vesicles. The present study also identified the cells of origin of two additional projections to the basilar pons: one from cells in the external cuneate nucleus and another from neurons of the medullary reticular formation.     

Quantitative ultrastructure of slowly adapting lingual afferent terminals in the principal and oral nuclei in the cat

Previous studies provide evidence that a structure/function correlation exists in the cytoarchitectonically different zones of the trigeminal sensory nuclei. To extend this relationship, we examined the ultrastructural features of trigeminal primary afferent neurons in the cat dorsal principal nucleus (Vpd) and the rostrodorsomedial oral nucleus (Vo.r) using intra-axonal labeling with horseradish peroxidase and morphometric analyses. All labeled boutons contained round synaptic vesicles. Eighty-two percent of the boutons in the Vo.r and 99% of the boutons in the Vpd were presynaptic to nonprimary dendrites. The remaining boutons in the Vo.r were presynaptic to somata (8%) or primary dendrites (10%). The average number of postsynaptic profiles per labeled bouton did not differ in the Vpd and Vo.r. Most labeled boutons in the two nuclei were postsynaptic to unlabeled axon terminals with pleomorphic vesicles (p-ending). The number of p-endings per labeled bouton was higher in the Vpd than Vo.r A morphometric analysis indicated that labeled bouton volume and apposed surface area were larger in the Vpd than Vo.r while active zone area and vesicle number did not differ. All these parameters were larger than those of p-endings in each nucleus. In both labeled boutons and p-endings, the parameters were positively correlated with bouton size. These results suggest that sensory information conveyed through trigeminal afferents is more strongly controlled at the level of the first synapse by presynaptic mechanisms in the Vpd than in the Vo.r, while the efficacy of transmission at primary afferent synapses does not differ.     

The fine structure of the inferior colliculus in the cat. II. Synaptic organization.

The synaptic organization of the central nucleus of the inferior colliculus (ICc) of the cat has been investigated by means of electron microscopy. On the basis of the following criteria: the size and the shape of the synaptic vesicles, the distribution and density of the vesicular population, the size and the shape of the synaptic boutons, their origin, and the characteristics of the active synaptic zones, several types of synaptic boutons in the ICc have been discriminated: LR1, LR2, SR, SSB, F1, F2, P, DCV-terminals, and "d"-profiles. The LR1, LR2, SR and SSB bouton types contain clear, round or slightly oval synaptic vesicles and form asymmetrical synapses mainly with middle sized and small dendrites and dendritic spines. LR2-terminals not rarely contact also the neuronal perikarya, whilst the SR-boutons form exclusively axodendritic and axospinous synapses. The P, F1 and F2-boutons contain a pleomorphic vesicular population (P-boutons), with an increased degree of vesicle flattening (F1 and F2-boutons) and form symmetrical axosomatic, axodendritic and axospinous contacts. Especially often the F1-boutons form axosomatic synapses, whilst the F2-terminals end mainly on dendrites. The DCV-boutons contain a mixed population of clear round synaptic vesicles and large dense core vesicles. The DCV-boutons terminate mainly on spines and small distal dendrites by means of asymmetrical synaptic specializations. The "d"-profiles originate from dendrites, and are identical to the thalamic "d"-profiles but are far more rarely observed in the ICc. The "d"-profiles are postsynaptic mainly to the LR-types, and are presynaptic to conventional dendrites, thus participating in synaptic triads. The axonal hillocks and the initial axonal segments of the larger perikarya in the ICc are substantially innervated mainly by LR and P-boutons. Glomerulus-like formations are fairly often, especially around the LR1-terminals, contacting several small postsynaptic targets. True synaptic glomeruli are only rarely observed. Branching myelinated axons are found mainly within the fibrodendritic laminae, whilst unmyelinated collaterals, emitted by myelinated axons are especially often encountered outside the laminae. Various types of myelinated axons form nodal synapses.     

GABA and glycine-like immunoreactivity at axoaxonic synapses on 1a muscle afferent terminals in the spinal cord of the rat

The object of this study was to analyze the synaptic interactions of identified muscle spindle afferent axon terminals in the spinal cord of the rat. Group 1a muscle afferents supplying the gastrocnemius muscle were impaled with microelectrodes in the dorsal white matter of the spinal cord and stained by intracellular injection with Neurobiotin. Postembedding immunogold techniques were used to reveal GABA- and glycine-like immunoreactivity in boutons presynaptic to afferent terminals in the ventral horn and the deep layers of the dorsal horn. Serial-section reconstruction was used to reveal the distribution of synaptic contacts of different types on the afferent terminals. The majority of afferent boutons received axoaxonic and made axodendritic or axosomatic synaptic contacts. In the ventral horn, 91% of boutons presynaptic to the afferent terminals were immunoreactive for GABA alone and 9% were immunoreactive for both GABA and glycine. The mean number of axo-axonic contacts received per terminal was 2.7, and the mean number of synaptic contacts at which the terminal was the presynaptic element was 1.4. In the deep layers of the dorsal horn, 58% of boutons presynaptic to afferent terminals were immunoreactive for GABA alone, 31% were immunoreactive for GABA and glycine, and 11% for glycine alone. The mean number of axoaxonic contacts received per afferent terminal in this region was 1.6 and the mean number of synaptic contacts at which the terminal was the presynaptic element was 0.86. This clearly establishes the principle that activity in 1a afferents is modulated by several neurochemically distinct populations of presynaptic neuron.     

Deafferentation effects in lateral cuneate nucleus of the cat: correlation of structural alterations with firing pattern changes

A correlated anatomical and physiological investigation of the effects of unilateral cervicothoracic dorsal rhizotomies upon lateral cuneate nucleus of the cat (LCN) is reported. Pairs of adult cats with identical survival times were selected to correlate structural and functional changes. Two phases are described in the development of alterations of neuronal firing patterns. In the first phase, a relative silence within LCN was associated with depletion of round synaptic vesicles in the presynaptic profiles (LR boutons) of primary dorsal root afferents. The second phase was characterized by a development of spontaneous electrical hyperactivity which corresponded anatomically to the presence of denuded postsynaptic specializations, transient increase of adjacent extracellular space and an apparent decrease in the number of dendritic spines. There was a persistence of an unaltered population of small presynaptic boutons with flattened vesicles (SF boutons). The LCN neuronal membrane is viewed as having an intrinsic tendency for repetitive firing which is enhanced by the functional effects of denuded postsynaptic specialization. A marked similarity was found between some of the spontaneous firing patterns of normal animals (doublets) and the high frequency bursting firing pattern in deafferented preparation. Three models for repetitive spike production are considered in our analysis: oscillator-produced spikes; EPSP-produced spikes; and spike-evoked spikes. The spike-evoked spikes model is considered to be the origin of normal doublet activity and a candidate for the deafferented burst activity. Abnormal hyperactivity after deafferentation may be a function of changes in the membrane characteristics occurring at or near the denuded postsynaptic specializations.     

Axoaxonic synapses on terminals of group II muscle spindle afferent axons in the spinal cord of the cat.

The purpose of the present study was to determine if terminals of identified group II muscle spindle afferents participate in axoaxonic synaptic arrangements and, if so, to investigate the transmitter content of presynaptic terminals in these arrangements. Group II muscle afferents supplying the gastrocnemius-soleus or semitendinosus muscles were identified in adult cats and stained intra-axonally with horseradish peroxidase. In total, three group II axons were labelled and processed for combined light and electron microscopy. Group II axons gave rise to collaterals which characteristically descended through the superficial dorsal horn and formed relatively sparse terminal arborizations in the dorsal horn (laminae IV and V) and more profuse arbors in the intermediate grey matter (laminae VI-VII). Forty boutons were examined through series of ultrathin sections and all but four were postsynaptic to other axon terminals. Occasionally, more than one axon was presynaptic to a single group II terminal. Immunogold studies showed that all axons in presynaptic apposition to group II boutons contained gamma-aminobutyric acid (GABA) and also that glycine was colocalized in the majority of these axons. This evidence suggests that transmission from group II muscle afferents is under strong presynaptic inhibitory control and that it is mainly the subgroup of GABAergic interneurons with colocalized glycine which mediate this inhibition. Seventeen group II boutons were components of synaptic triads where the presynaptic axoaxonic bouton formed a synapse with the same dendrite as the group II axon. Therefore, a proportion of the interneurons which form axoaxonic synapses with group II axons are also likely to have postsynaptic inhibitory actions on target neurons of group II afferents.     

Ultrastructural evidence related to presynaptic inhibition of primary muscle afferents in Clarke's column of the cat

As part of an investigation on excitatory synaptic transmission in the mammalian CNS, we have examined ultrastructural details of the synaptic connection between primary afferent fibers and dorsal spinocerebellar tract (DSCT) neurons in Clarke's column of the cat spinal cord. Single primary muscle afferents (group Ia and Ib) and DSCT neurons were identified and stained intracellularly with HRP. The terminations of these afferent fibers were examined in serial sections under the EM. Five of 6 Ib boutons and 1 of 14 Ia boutons were contacted by small presynaptic boutons. An example was illustrated in which only 1 out of 7 boutons arising from the same Ia fiber and contacting the same postsynaptic DSCT neuron was contacted by a presynaptic bouton. It is likely that the presynaptic contacts are responsible for presynaptic inhibition of synaptic transmission between primary afferents and DSCT neurons. We have proposed that the observed differences in presynaptic contacts from bouton to bouton may be one of the causes of a nonuniformity in the probability of transmitter release between release sites at this connection.     

Morphology of physiologically identified slowly adapting lung stretch receptor afferents stained with intra-axonal horseradish peroxidase in the nucleus of the tractus solitarius of the cat. II. An ultrastructural analysis

The nucleus of the tractus solitarius is a site for termination of primary afferents originating from a variety of visceral receptors. The localization of bouton terminals of slowly adapting lung stretch (SAR) afferent fibers originating from the tracheobronchial tree have been described in the companion paper (Kalia and Richter, '85). The most conspicuous finding regarding the location of SAR terminals is that they are concentrated within specific subnuclear groups of the nucleus of the tractus solitarius (nTS) and are distributed widely in the rostrocaudal plane of the medulla oblongata. These light microscopic features have provided us with valuable information with regard to the organization of visceral afferents in the central nervous system. The synaptic profiles formed by the 476 bouton terminals of these HRP-labeled afferents have been described in this paper in serial thin sections. All of the bouton terminals examined under the electron microscope were found to contain round synaptic vesicles. Synaptic boutons (1.0-3.0 microns in diameter) were usually of the en passant variety and made contact with different structures depending upon the subnucleus which was examined. In the ventral (v) and the ventrolateral (vl) subnuclei of the nTS, asymmetrical (type I) synaptic contacts containing round, clear synaptic vesicles of 35-50 microns in diameter were found and these contacts were made with (1) the soma of cell bodies located in that subnucleus; (2) spiny dendrites in that nucleus; (3) vesicle-containing axon terminals that were presynaptic to the HRP-labeled bouton terminal; and (4) vesicle-containing dendrites in which the HRP profile was presynaptically located. The terminal axon remained myelinated till the last 1 micron before the bouton terminal was formed. There was no distinct, unmyelinated portion of the terminal axon. The synaptic bouton received axon-axonal synapses from unlabeled bouton terminals containing round, clear vesicles. This is the first report of the localization of these afferent fibers as well as of the regional variations in the ultrastructure of boutons of physiologically identified terminals. It appears likely that the lung stretch afferent fibers, by having axon-axonal as well as axon-somatic contact in the ventral, ventrolateral, and intermediate subnuclei of the nTS, can interact in a variety of different ways in this region. The significance of these features in relation to the precise influence of respiratory afferents on central respiratory mechanisms needs to be evaluated further.     

Ultrastructure and synaptic organization of axon terminals from brainstem structures to the mediodorsal thalamic nucleus of the rat.

The ultrastructural characteristics and synaptic organization of afferent terminals from the brainstem to the mediodorsal thalamic nucleus (MD) of the rat have been studied with the electron microscope, by means of anterograde transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP). Labeled fibers were seen predominantly in the lateral portion of MD after the injections of WGA-HRP into the substantia nigra pars reticulata (SNr), the superior colliculus (SC), and the dorsal tegmental region (DT). The boutons arising from the SC were relatively small (less than 1.5 microns in diameter), formed asymmetric synaptic contacts with small dendrites and dendritic spines, and contained round synaptic vesicles. The axon terminals from the DT were mostly large boutons (2-4.5 microns) with asymmetric synaptic specializations and round vesicles. These boutons and their postsynaptic targets formed synaptic glomeruli that were entirely or partially ensheathed by glial lamellae. The ultrastructural features are almost identical to those of boutons in the medial and central segments of MD that were previously shown to originate from the basal amygdaloid nucleus and the piriform cortex. The boutons from the SNr had a wide range in size, but the majority were medium-sized to large (1.5-4 microns). The nigral boutons established symmetric synaptic contacts with dendritic shafts and occasionally with somata, and contained pleomorphic vesicles. However, like the DT terminals, they participated in glomerular formations. The nigral terminals closely resemble previously described terminals in the medial part of MD from the ventral pallidum, except that the nigral terminals formed en passant and axosomatic synapses as well as axodendritic synapses. A combined immunohistochemistry and WGA-HRP tracing study revealed that the nigral inputs were immunoreactive for glutamic acid decarboxylase and the axon terminals from the DT were immunoreactive for choline acetyltransferase. In a separate study, the colliculothalamic fibers have been shown to take up and transport the transmitter specific tracer [3H]-D-aspartate, and are therefore putatively glutamatergic and/or aspartatergic. Taken together with this, the present results suggest that the collicular afferents are excitatory and glutamatergic and/or aspartatergic, that the inputs from the DT are also excitatory and cholinergic, while the nigral inputs are inhibitory and GABAergic.     

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.     

Ultrastructure of ascending cholinergic terminals in the anteroventral thalamic nucleus of the rat: a comparison with the mammillothalamic terminals

In this study, to identify the ultrastructure and distribution of ascending cholinergic afferent terminals in the anteroventral thalamic nucleus, we used an anti-vesicular acetylcholine transporter antibody as marker of cholinergic afferents, and characterized the immunoreactive terminals at the ultrastructural level. We then compared the distribution pattern of the cholinergic terminals and that of the mammillothalamic terminals identified by anterograde transport of a tracer injected into the mammillary body. The cholinergic terminals were small, and formed both symmetrical and asymmetrical synaptic contacts throughout the dendritic arborizations, particularly in the distal region. This distribution pattern differed from that of mammillothalamic terminals, that were of LR (large terminal containing round synaptic vesicles) type and were preferentially distributed in the proximal region of dendrites. We also found relatively numerous cholinergic terminals making contact directly with immunonegative excitatory terminals, both LR and SR (small terminal containing round vesicles) terminals, without clear postsynaptic specialization. A few cholinergic terminals even seemed to form a synaptic complex with the LR or SR terminals. These findings suggest that the ascending cholinergic afferents in the anteroventral thalamic nucleus can effectively modulate excitatory inputs from both the mammillothalamic and corticothalamic terminals, in close vicinity to a synaptic site.     

A correlative HRP,Golgi, and EM study of the intrinsic organization of the feline dorsal column nuclei

The retrograde transport of horseradish peroxidase (HRP), Golgi impregnations, and electron microscopic (EM) observations have been employed to investigate the intrinsic organization of one cytoarchitectonic subdivision of the feline dorsal column nuclei (DCN): the “clusters” region. Previous studies have demonstrated that neurons arranged in typical cell clusters in the dorsal two-thirds of the feline DCN project to the ventrobasal complex (VB) of the thalamus. Following injections of HRP in the VB of adult cats, over 90% of the neurons in this region contain detectable reaction product. These thalamic projecting neurons (TPN) are typically round, range from 20 to 35 μm in diameter, and have round nuclei and abundant cytoplasm. In Golgi preparations, TPN are identified by their characteristic arrangement in the cell clusters. Observations of their cytological characteristics in gold-toned preparations in both 1-μm-thick sections, and in thin sections at the EM level, supplement the data from HRP material. The dendrites of TPN in the clusters region characteristically converge in the perikarya-free center of each cluster, and appendages originate from both proximal and distal portions of the major dendrites of these neurons. These appendages vary in morphology from short-stalked, occasionally multi-lobed, bulbous thorns to long-stalked processes terminating in a single or multilobed swelling. Other appendages often display beaded swellings along their length (moniliform appendages). Neurons in the clusters region unlabelled by HRP injected in the VB have cytological charateristics quite distinct from those of TPN in the same region. Typically located at the periphery of the cell clusters, unlabelled neurons have fusiform perikarya, range in diameter from 8 to 12 μm along their short axis, and have highly indented nuclei and sparse cytoplasm. In Golgi preparations, small fusiform neurons located at the periphery of cell clusters typically have two major dendritic trunks originating from opposite poles of their perikarya. Observations in 1-μm-thick plastic sections of re-embedded Golgi-impregnated neurons of this type support their identifications as neurons which are unlabelled after HRP injection in VB. Dendritic appendages are present also on this type of neuron, although they do not appear to be as frequent as in the case of the TPN. Although the axon of the small, fusiform neurons have not been impregnated beyond the initial segment in the present Golgi material, thus precluding the classification of this type of neuron as a Golgi type II neuron, a combination of the HRP and Golgi observations suggest that these neurons are distinct from TPN, and are possibly interneurons. In addition to the previously described synaptic complexes of primary afferent terminals associated with, or postsynaptic to, small boutons with flattened vesicles (F-boutons, of presumed interneuronal origin), EM observations of serial sections demonstrate triadic synaptic arrangements similar to those encountered in other CNS nuclei. In such instances, a primary afferent terminal is presynaptic to a pale profile containing polymorphicvesicles (P-bouton), and both boutons are presynaptic to a dendrite. In serial sections, P-boutons are demonstrated to be of dendritic origin, while F-boutons are observed to arise frommyelinated fibers. By comparison with Golgi observations, P-boutons are believed to corre-spond to the dendritic appendages of TPN, suggesting that TPN, as well as interneurons, contribute to the intrinsic synaptology of the cell clusters region.