Synaptic Apposition Research Articles

Distribution of GABA immunoreactivity in the optic tectum of the frog: A light and electron microscopic study

GABA immunoreactivity was studied in the optic tectum of the frog, Rana esculenta, by postembedding immunohistochemical methods at the light and electron microscopic levels. Nearly one-third of the total population of tectal cells appeared to be GABA-immunoreactive. The proportion of stained neurons was highest in layer 9 (61%), and they occurred less frequently in layers 7 (21%) and 6 (27%). Stained perikarya represented a population of small neurons with a diameter of 8–10 μm. Large cell bodies in layer 7 or at the top of layer 6, and cells of origin of the mesencephalic trigeminal tract in layer 2, were devoid of labelling. Axon terminals and dendrites displaying immunoreactivity for GABA were observed in all of the plexiform layers. On the basis of ultrastructural characteristics two types of GABA-positive axon terminals and two variations of GABA-immunoreactive dendrites were distinguished. Synaptic relations of GABA-immunoreactive and GABA-negative axons as well as dendrites were also studied. Besides a wide variety of axodendritic synapses, dendrodendritic synaptic appositions were also revealed. The results suggest that various inhibitory mechanisms are involved in tectal circuits, which have to be incorporated into future neuronal models concerning visual information processing in the optic tectum of the frog.

Organization of neurons labeled by antibodies to gamma-aminobutyric acid (GABA) in the superior colliculus of the Rhesus monkey

The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) is found in the superior colliculus (SC) of many mammalian species. In cat, several distinct classes of putative GABAergic neuron have been identified using antibodies directed against the neurotransmitter. It is not known whether these classes are found in other species. To study this, we examined the distribution, morphology, ultrastructure, and synaptic organization of GABA immunoreactive neurons in the SC of the Rhesus monkey (Macaca mulatta). Antibody-labeled neurons were distributed throughout the monkey SC, but were most densely concentrated within the zonal and superficial gray layers (32.5% of the total). These neurons were all small cells ranging from 6.6-16.3 microns in average diameter, and had granule, pyriform, and horizontal morphologies. Four types of labeled profile were identified in single ultrathin sections with the electron microscope. Presynaptic dendrites (PSDs) contained pleomorphic vesicles, received synaptic input from unlabeled axon terminals, and sometimes formed symmetric synaptic contacts with postsynaptic profiles. Two subtypes were found. One type contained loose accumulations of synaptic vesicles throughout the profile and had a distinctive varicose shape. The other type contained small discrete clusters of synaptic vesicles near the site of synaptic apposition. The former were much more common. Profiles with typical axon terminal morphology were also found. These profiles usually contained numerous flattened vesicles and formed symmetric synapses with postsynaptic profiles, both dendrites and cell bodies. Some conventional dendrites and myelinated axons were also labeled. Serial ultrathin section reconstructions revealed that PSDs formed complex synaptic relationships with other elements. Retinal terminals, identified by their characteristic pale mitochondria, established synaptic contacts with both types of PSD. These PSDs also established contact with each other, providing a possible anatomical substrate for disinhibition. We conclude that the monkey SC has multiple GABAergic cell types, similar to those found in cat, and may represent an organization common to both mammals and some other vertebrate species. The circuitry established by these cell types may provide a mechanism for disinhibition as well as inhibition in the mammalian SC.

Rat cerebral cortical synaptoneurosomal membranes. Structure and interactions with imidazobenzodiazepine and 1,4-dihydropyridine calcium channel drugs

Small angle x-ray scattering has been used to investigate the structure of synaptoneurosomal (SNM) membranes from rat cerebral cortex. Electron micrographs of the preparation showed SNM with classical synaptic appositions intact, other vesicles, occasional mitochondria, and some myelin. An immunoassay for myelin basic protein placed the myelin content of normal rat SNM at less than 2% by weight of the total membrane present. X-Ray diffraction patterns showed five diffraction orders with a unit cell repeat for the membrane of 71 to 78 A at higher hydration states. At lower hydration, 11 orders appeared; the unit cell repeat was 130 A, indicating that the unit cell contained two membranes. Electron density profiles for the 130-A unit cell were determined; they clearly showed the two opposed asymmetrical membranes of the SNM vesicles. SNM membrane/buffer partition coefficients (Kp) of imidazobenzodiazepine and 1,4-dihydropyridine (DHP) calcium channel drugs were measured; Kp's for DHP drugs were approximately five times higher in rabbit light sarcoplasmic reticulum than in SNM. Ro 15-1788 and the DHP BAY K 8644 bind primarily to the outer monolayer of vesicles of intact SNM membranes. Nonspecific equilibrium binding of Ro 15-1788 occurs mainly in the upper acyl chain of the bilayer in lipid extracts of SNM membrane.

Sexual Dimorphism in the Synaptic Input to Gonadotropin Releasing Hormone Neurons*

GnRH neurons form the final common pathway regulating the secretion of gonadotropins from the anterior pituitary. Since the patterns of gonadotropin release display profound sexual dimorphism among mammals including the rodent, we undertook an ultrastructural analysis to determine whether these neurosecretory cells were differentially innervated between the sexes. As a further exploration of the organization of the neurocircuitry integrating GnRH neurons with the central nervous system, we also determined the degree to which GnRH cells and their processes were innervated by terminals containing either the endogenous opiate, beta-endorphin (BE) or GnRH itself. Sections from the diagonal band of Broca and the preoptic area of adult male and diestrus II female rats were immunocytochemically processed for dual localization of GnRH and BE. GnRH neurons cut through the plane of the nucleus were identified in 1 micron sections. Serial ultrathin sections were made and analyzed for 1) total synaptic input to both cell bodies and dendrites; 2) BE input; and 3) input arising from GnRH itself. We report that GnRH neuronal cell bodies in females received approximately twice the number of synapses as did those of males. The input to the GnRH dendrites, when measured as percent of plasma membrane in synaptic contact, also showed a profound sexual dimorphism with the female having a larger proportion of the dendrite in synaptic apposition. BE innervation contributed to this dimorphism at the level of both the cell body and dendrite. In contrast, the distribution and number of GnRH terminals did not differ between the sexes. In both they were confined to the dendritic arbor. We hypothesize that the capacity of the female rodent GnRH system to show neurogenic derived alterations in GnRH output not seen in the male may be due in part to these anatomical differences.

The effect of thyroxine, 3,5-dimethyl-3'-isopropyl-L-thyronine and iodized oil on fetal brain development in the iodine-deficient sheep

Studies have been carried out to investigate the role of maternal and fetal thyroid function in the effects of iodine deficiency on fetal brain development in sheep. Iodine deficiency was established with an especially prepared low-iodine diet of maize and pea pollard. The iodine-deficient sheep were mated and the end of the second trimester of pregnancy (100 days gestation) were divided into groups which received either a sc injection of T4 or 3,5-dimethyl-3-isopropyl-L-thyromine or an injection of iodized oil. AT 140 days gestation (10 days prior to parturition) comparison of the fetuses delivered by hysterotomy revealed that the retarded fetal brain development observed in iodine deficiency was greatly improved by T4 and by iodized oil. However, T4 and iodized oil failed to correct the reduction in the number and the increase in the length of synaptic appositions which were observed in the fetal cerebral cortex after iodine deficiency. In addition, the histological appearance of the fetal thyroid gland and the levels of plasma thyroid hormones were restored to normal. The administration of 3,5-dimethyl-3'-isopropyl-L-thyronine had no effect on the retarded fetal brain and body development of the iodine-deficient fetuses. The lack of response may be due to the ability of 3,5-dimethyl-3'-isopropyl-L-thyronine to cross the ovine placenta as no reduction in the abnormally elevated fetal plasma TSH observed in spite of a fall in maternal plasma TSH and apparent restoration of maternal thyroid function.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences in structure and distribution of the molecular forms of acetylcholinesterase.

Two structurally distinct molecular forms of acetylcholinesterase are found in the electric organs of Torpedo californica. One form is dimensionally asymmetric and composed of heterologous subunits. The other form is hydrophobic and composed of homologous subunits. Sequence-specific antibodies were raised against a synthetic peptide corresponding to the COOH-terminal region (Lys560-Leu575) of the catalytic subunits of the asymmetric form of acetylcholinesterase. These antibodies reacted with the asymmetric form of acetylcholinesterase, but not with the hydrophobic form. These results confirm recent studies suggesting that the COOH-terminal domain of the asymmetric form differs from that of the hydrophobic form, and represent the first demonstration of antibodies selective for the catalytic subunits of the asymmetric form. In addition, the reactive epitope of a monoclonal antibody (4E7), previously shown to be selective for the hydrophobic form of acetylcholinesterase, has been identified as an N-linked complex carbohydrate, thus defining posttranslational differences between the two forms. These two form-selective antibodies, as well as panselective polyclonal and monoclonal antibodies, were used in light and electron microscopic immunolocalization studies to investigate the distribution of the two forms of acetylcholinesterase in the electric organ of Torpedo. Both forms were localized almost exclusively to the innervated surface of the electrocytes. However, they were differentially distributed along the innervated surface. Specific asymmetric-form immunoreactivity was restricted to areas of synaptic apposition and to the invaginations of the postsynaptic membrane that form the synaptic gutters. In contrast, immunoreactivity attributable to the hydrophobic form was selectively found along the non-synaptic surface of the nerve terminals and was not observed in the synaptic cleft or in the invaginations of the postsynaptic membrane. This differential distribution suggests that the two forms of acetylcholinesterase may play different roles in regulating the local concentration of acetylcholine in the synapse.

Plasticity of postsynaptic density material in optic synapses of the suprachiasmatic nucleus in the senescent rat

Optic synapses in the suprachiasmatic nucleus were studied in senescent rats after 2 weeks of constant light influence (light rats) or darkness (dark rats). The amount of postsynaptic density material was significantly greater in dark rats compared with light rats. Dark rats also showed a higher percentage of asymmetric synapses than light rats. The sizes of synaptic appositions and boutons were also compared. The phenomena observed may indicate supersensitivity and a higher number of excitatory synapses in dark rats and subsensitivity and lower number of excitatory synapses in light rats.

Ultrastructural alterations in caudate nucleus in aged cats

These studies provide information on the changes in the ultrastructure in the caudate nucleus of aged cats. The major findings was that there was a decrease in the density of synapses in caudate neuropil. This decrease occurred in animals after 3 years of age and remained relatively constant in older animals. In conjunction with this change a population of unusually long synapses also occurred. These larger synaptic appositions were associated with enlarged spine heads. The caudate also showed a number of qualitative ultrastructural alterations. Many neurons contained accumulations of lipofuscin or lipopigment granules in aged animals. These inclusions occurred in both soma and dendrites of neurons and all types of glial cells. A unique configuration of collapsed agranular cisterns also was observed in aged animals. The present results indicate that decreases in synaptic density may by one morphological event underlying functional alterations observed in caudate neurons in aged cats.

The dendrites of single brain-stem motoneurons intracellularly labelled with horseradish peroxidase in the cat. An ultrastructural analysis of the synaptic covering and the microenvironment

Two laryngeal motoneurons intracellularly stained with horseradish peroxidase were studied ultrastructurally. The precise position of the ultrastructural observations made along the dendrites was obtained from the computer-reconstruction of the motoneurons in three dimensions. The shape and the size of the synaptic boutons, the percentage of membrane covered by bouton appositions and active zones, the number of boutons per 100μm 2 (packing density) were analysed on the soma and on the labelled dendrites at different distances from the soma up to 1000 μm. The results revealed no important regional differences in the mean length of synaptic apposition. The packing density was in the range of 9.3–14.9 boutons per 100 μm 2 and was not correlated with the distance from the soma. The percentage apposition covering was higher on the soma and the proximal part of the dendrites than on the remaining part of the dendritic arborization. Close appositions between labelled dendrite and unlabelled somata and/or dendrites together with dendro-dendritic synapses suggested the possibility that the dendrites may be involved in local cell-to-cell communication. Microdendrites emerging from the soma or the proximal dendrites were contacted by synaptic boutons which may be more efficient as revealed by computation.

The effect of chronic ethanol consumption on the fine structure of the CA 1 stratum oriens in short-sleep and long-sleep mice: short-term and long-term exposure

The effect of chronic ethanol administration on the fine structure of the hippocampal CA 1 stratum oriens was examined in two lines of mice selectively bred for their differential sensitivity to acute ethanol exposure (long-sleep, LS and short-sleep, SS mice). Two experiments were performed. In the first experiment, mice received a liquid diet for 3 weeks with the final amount of ethanol being 35% ethanol-derived calories. In the second experiment, mice received 23.5% ethanol-derived calories for 3 months. Quantitative electron microscopy of the dendritic spines and synaptic appositions in the stratum oriens of CA 1 revealed an interaction between diet and line of mice, but only in the 3-month exposure condition. This difference was due to a significant decrease in the density of spines and synaptic appositions in the LS mice receiving ethanol. Additionally, baseline differences between lines indicate that the lines are differing in the density of spine synapses in the absence of ethanol treatment. The possible interaction between acute sensitivity to ethanol and differences in fine structure are examined.

Plasticity of dendrodendritic microcircuits following mitral cell loss in the olfactory bulb of the murine mutant Purkinje cell degeneration.

Mitral cells of the olfactory bulb typically form reciprocal dendrodendritic synapses with anaxonic interneurons, granule cells, within a sublamina of the external plexiform layer. As a result of mitral cell loss in the murine mutant Purkinjie cell degeneration (PCD), subpopulations of these granule cells are denervated. The present report examines the capacity of these denervated interneurons to form new dendrodendritic microcircuits with a second population of olfactory bulb neurons, tufted cells. Quantitative ultrastructural assessments were made of the morphology and distribution of dendrodendritic circuits in the olfactory bulbs of normal heterozygous littermates and affected homozygous recessive PCD mice following mitral cell loss. There were no apparent morphological characteristics that distinguished the reciprocal synaptic connections formed by mitral cells from those formed by tufted cells. However, the segregation of mitral cell dendrodendritic circuits in the deep sublamina of the external plexiform layer (EPL) and tufted cell circuits in the superficial sublamina provided the basis for a comparative analysis of synaptic organization following mitral cell loss. Following mitral cell loss there was a significant reduction in the area occupied by characteristic mitral cell dendrites within the deep sublamina of the EPL. A slight but nonsignificant increase in the area occupied by granule cell spines was also observed. The number of synaptic appositions involving granule cells decreased slightly, the number involving tufted cells increased significantly in the mutant mice. This indicates that many granule cell spines survive denervation and establish new reciprocal dendrodendritic synapses at available sites on tufted cells. In both the control and mutant mice the ratios of symmetrical:asymmetrical dendrodendritic synapses closely approached 1. This demonstrates that not only do the denervated spines receive new afferent input from tufted cell dendrites, but they also establish the reciprocal efferent projection. These data are discussed in terms of the sublaminar organization of dendrodendritic microcircuits in the olfactory bulb and their capacity of plasticity and reorganization following pertubation.

Effect of chronic ethanol consumption on the fine structure of the dentate gyrus in long-sleep and short-sleep mice

The effect of short-and long-term chronic ethanol consumption on the fine structure of the dentate gyrus was examined in two lines of mice selected for their differential sensitivity to acute ethanol administration. Quantitative electron microscopic analysis of dendritic spines, axon terminals, and synaptic appositions revealed significant differences between the long-sleep and short-sleep mice. In control preparations, long-sleep mice were found to have larger spine areas and perimeters, larger axon terminals, and longer synaptic appositions than short-sleep mice. In addition, the shape of dendritic spines in the long-sleep mice was significantly more complex than those of short-sleep mice. Ethanol tended to increase this complexity in long-sleep mice only. Ethanol had only a limited effect on the other anatomical measures. The results provide evidence for ultrastructural differences between the nervous systems of these lines of mice which may have a role in their differential sensitivity to acute ethanol administration.

Age-related morphological rearrangements of synaptic junctions in the rat cerebellum and hippocampus

A quantitative investigation has been carried out on the age-related morphological changes of the synaptic junctions in the cerebellar glomeruli and in the supragranular layer of the hippocampal dentate gyrus of young (3 months), adult (12 months) and old (28–30 months) rats. The numerical (Nv) and surface (Sv) density as well as the average length ( L ) of E-PTA stained synapses was calculated by means of morphometric methods. The results we obtained showed a similar trend in both these areas of the CNS. Nv significantly increased between young and adult rats and significantly decreased in the old group, when compared both to young and adult animals. Sv appeared to be unchanged comparing young and adult animals, whereas it was significantly reduced in the old group. L showed a decrease between 3 and 12 months of age and appeared to be markedly increased in the old animals when compared to adult values. From the present findings it can be inferred that number (Nv) and size ( L ) of the synapses are in a close inverse relationship which, through the organism's life span, aims to maintain the constancy of the surface contact area (Sv) among the dendritic network. By considering Nv, L and Sv altogether per age group, we were able to obtain a reliable measurement of the morphological aspect of synaptic plasticity through different periods of the life. With regard to aging we found that, despite the ‘compensative synaptogenesis’ brought about by the increased size ( L ) of the synaptic appositions, the reactive capacity of old nerve cells is seriously impaired.

A possible mechanism of morphometric changes in dendritic spines induced by stimulation.

A number of experimental procedures which induce increased electrical activity (including long-term potentiation) were shown to be accompanied by morphometric changes in dendritic spines. These changes include an enlargement of the spine head, shortening and widening of the spine stalk, and an increase in the length of synaptic apposition. A possible mechanism is suggested which takes into account specific cytological features of the spine and the existence of contractile proteins in neurons. Dendritic spines are defined as special domains of the neuron which have a unique organization of the cytoplasm. Actin filaments form a very dense network in the spine head, and they are longitudinally organized within the spine stalk. Spines were also shown to contain myosin and other actin-regulatory proteins. The high density of the actin network could explain the characteristic absence of the cytoplasmic organelles from dendritic spines. In analogy with other cells, such an actin organization indicates low levels of free cytosolic calcium. Even in the resting state, calcium levels may be unevenly distributed through the neuron, being lowest within the subplasmalemmal region. Due to the high surface-to-volume ratio in spines, the cytoplasm is formed mostly by the subplasmalemmal region. The spine apparatus or the smooth endoplasmic reticulum, which is recognized as a calcium-sequestering site in spines, may also contribute to the low calcium levels there. However, when in the stimulated spine the voltage-dependent calcium channels open, then, given the spine's high surface-to-volume ratio, the concentration of calcium may very quickly attain levels that will activate the actin-regulatory proteins and myosin and thus trigger the chain of events leading to the enlargement of the spine head and to the contraction (i.e., widening and shortening) of the spine stalk. The increased free cytosolic calcium may also activate the protein-producing system localized at the base of the spine, which, under certain conditions, could stabilize the morphometric changes of the spine.

Structural plasticity of developing optic synapses under different lighting conditions

Four-month-old male hooded rats were reared from birth under constant light and darkness conditions. Changes in the amount of postsynaptic density material in the optic synapses in the suprachiasmatic nucleus of these rats were compared with animals maintained under routine light-dark (12 h) cycles. The thickness of postsynaptic density material was found to be significantly greater in dark-reared rats relative to light-reared animals. The plasticity of this structure may have functional implications in the sensitivity of postsynaptic response. There was no significant difference in the lengths of the synaptic apposition and the size of boutons.

Activity dependent plasticity of postsynaptic density structure in the ventral cochlear nucleus of the rat

Young adult male rats were anaesthetised with urethane and exposed to either 24 h of silence or 24 h of repetitive 77 dB tones in a sound-proofed anechoic chamber. The influence of these two conditions on the ultrastructure of the synaptic appositions made by auditory afferents in the anterior ventral cochlear nucleus (end bulbs of Held) was compared. Both the cross-sectional area and the mean thickness of the postsynaptic density (PSD) in the rats exposed to tones were significantly reduced when compared with rats maintained in in silence. Similarly, the degree of curvature of the apposition was significantly reduced. The results of these experiments provide further evidence that the postsynaptic density material is a plastic structure significantly influenced by the amount of activity in the presynaptic element.

Synaptic remodelling during development and maturation: Junction differentiation and splitting as a mechanism for modifying connectivity

Morphological variation of the synaptic active zone during later development and maturation (15–224 days) has been studied in the molecular layer of the rat occipital cortex. Both E-PTA stained and osmicated tissue have been used. In the E-PTA stained material the degree of specialization of the presynaptic thickening is directly related to junction length. Junctions with well-developed dense projections (Types A and B) are longest and continue to increase in length with maturation, suggesting that active remodelling of the synaptic apposition is an ongoing process. The presence of perforated junctions, possessing two or more regions with specializations of different maturity and different curvature, raises the possibility that these junctions may arise by the addition and differentiation of new paramembranous material at an existing junction. In osmicated tissue, presynaptic terminals possessing multiple active zones have been quantitated. The maturational increase in number of simple perforated junctions (Type 1), is paralleled by a smaller increase in the number of multiple perforated junctions (Type 2). A spine apparatus is frequently observed in these perforated terminals, suggesting that it is intimately involved in the reorganization. The direction of curvature of the closely apposed junctions is predominantly negative (indenting the postsynaptic process). Other types of arrangement, with separate postsynaptic processes, are described (Types 3–5), and micrographs suggestive of sequential stages in pre- and postsynaptic terminal splitting are presented. The total length of the postsynaptic thickening of perforated terminals is twice the mean synaptic length of non-perforated terminals, again suggestive that duplication of the active zone may have occurred. Division of existing synaptic terminals by duplication and subsequent splitting would readily account for the increased dendritic spinal numbers seen in Golgi preparations of animals raised under enriched conditions. This would be a straight-forward mechanism by which reinforcement of neuronal connections could occur in response to use.

Suprachiasmatic nucleus: numbers of synaptic appositions and various types of synapses. A morphometric study on male and female rats.

The suprachiasmatic nucleus (SCN) of male rats was estimated contain 16 X 10(6) synaptic appositions (unilaterally) or 250 X 10(6) appositions in 1 mm3 tissue of the nucleus with an average of 1404 appositions per neuron. There are significantly fewer synaptic appositions in the suprachiasmatic nucleus of female rats (15 X 10(6) per SCN or 236 X 10(6) in 1 mm3 tissue of SCN with 1264 appositions per neuron on an average). Additionally, numbers of various types of synapses (axo-somatic, invaginated, dendrodendritic and optic) are estimated.

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)

Synaptic interaction of vagal afferents and catecholaminergic neurons in the rat nucleus tractus solitarius

Combined radioautography and immunocytochemistry were used to define the ultrastructure and synaptic relations between vagal sensory afferents and catecholaminergic (CA) neurons of the A2 group located within the nucleus tractus solitarius (NTS) of rat brain. The vagal afferents were radioautographically labeled by tritiated amino acids anterogradely transported from the nodose ganglion. Immunocytochemical labeling for tyrosine hydroxylase (TH) served for the identification of catecholaminergic neurons. The radiographically labeled axons seen by light microscopy were widely distributed throughout the more caudal NTS. The reduced silver grains were more densely distributed within the NTS located homolateral to the injected nodose ganglion. The radioautographically labeled processes were localized in regions containing catecholaminergic neurons as indicated by immunoreactivity for TH. Electron microscopic analysis of the medial NTS at the level of the obex demonstrated that the reduced silver grains were localized within axon terminals. The radioautographically labeled terminals were 2–3 μm in diameter, contained numerous small, clear and a few large, dense vesicles, and formed predominately axodendritic synapses. Many of the recipient dendrites contained immunoreactivity for TH. In rare instances, vagal afferents formed synaptic appositions with both TH-labeled and unlabeled axon terminals and neuronal soma. This study provides the first ultrastructural evidence that the catecholaminergic neurons within the NTS receive direct synapses from sensory neurons in the nodose ganglion.