Synaptic connections of GABA-containing boutons in the lateral cervical nucleus of the cat: an ultrastructural study employing pre- and post-embedding immunocytochemical methods

Q&A: Array tomography

Array tomography encompasses light and electron microscopy modalities that offer unparalleled opportunities to explore three-dimensional cellular architectures in extremely fine structural and molecular detail. Fluorescence array tomography achieves much higher resolution and molecular multiplexing than most other fluorescence microscopy methods, while electron array tomography can capture three-dimensional ultrastructure much more easily and rapidly than traditional serial-section electron microscopy methods. A correlative fluorescence/electron microscopy mode of array tomography furthermore offers a unique capacity to merge the molecular discrimination strengths of multichannel fluorescence microscopy with the ultrastructural imaging strengths of electron microscopy. This essay samples the first decade of array tomography, highlighting applications in neuroscience.

Ultrastructural analysis of the synaptic connectivity of TRPV1-expressing primary afferent terminals in the rat trigeminal caudal nucleus

Trigeminal primary afferents that express the transient receptor potential vanilloid 1 (TRPV1) are important for the transmission of orofacial nociception. However, little is known about how the TRPV1-mediated nociceptive information is processed at the first relay nucleus in the central nervous system (CNS). To address this issue, we studied the synaptic connectivity of TRPV1-positive (+) terminals in the rat trigeminal caudal nucleus (Vc) by using electron microscopic immunohistochemistry and analysis of serial thin sections. Whereas the large majority of TRPV1+ terminals made synaptic contacts of an asymmetric type with one or two postsynaptic dendrites, a considerable fraction also participated in complex glomerular synaptic arrangements. A few TRPV1+ terminals received axoaxonic contacts from synaptic endings that contained pleomorphic synaptic vesicles and were immunolabeled for glutamic acid decarboxylase, the synthesizing enzyme for the inhibitory neurotransmitter γ-aminobutyric acid (GABA). We classified the TRPV1+ terminals into an S-type, containing less than five dense-core vesicles (DCVs), and a DCV-type, containing five or more DCVs. The number of postsynaptic dendrites was similar between the two types of terminals; however, whereas axoaxonic contacts were frequent on the S-type, the DCV-type did not receive axoaxonic contacts. In the sensory root of the trigeminal ganglion, TRPV1+ axons were mostly unmyelinated, and a small fraction was small myelinated. These results suggest that the TRPV1-mediated nociceptive information from the orofacial region is processed in a specific manner by two distinct types of synaptic arrangements in the Vc, and that the central input of a few TRPV1+ afferents is presynaptically modulated via a GABA-mediated mechanism.

Neurons in the dorsal column white matter of the spinal cord: complex neuropil in an unexpected location

It is common to think of gray matter as the site of integration in neural circuits and white matter as the wires that connect different groups of neurons. The dorsal column (DC) white matter, for example, is the spinal cord axonal pathway through which a topographic map of the body is conveyed to the somatosensory cortex. We now describe a network of neurons located along the midline of the DCs. The neurons are present in several mammals, including primates and birds, and have a profuse dendritic arbor that expresses both the neuron-specific marker, microtubule-associated protein-2, and the neurokinin-1 receptor, a target of the neuropeptide, substance P. Electron microscopy and double immunostaining for synaptophysin and a marker of gamma-aminobutyric acid-ergic terminals documented a rich synaptic input to these neurons. Finally, injection of a gamma-aminobutyric acid type A receptor antagonist or of substance P into the cerebrospinal fluid of the rat spinal cord induced Fos expression and internalization of the neurokinin-1 receptor in these neurons, respectively, indicating that the DC neurons are under tonic inhibitory control and can respond to neurotransmitters that circulate in the cerebrospinal fluid.

Pre-embedding staining for GAD67 versus postembedding staining for GABA as markers for central GABAergic terminals

Pre-embedding immunocytochemistry for the active form of glutamate decarboxylase (GAD67) and postembedding staining for gamma-aminobutyric acid (GABA) were compared as markers for central GABAergic terminals in the phrenic motor nucleus, in which phrenic motor neurons had been retrogradely labeled with cholera toxin B-horseradish peroxidase. Nerve terminals with or without GAD67 immunoreactivity were identified in one ultrathin section. GABA was localized with immunogold in an adjacent section after etching and bleaching. GABA labeling density was assessed over 519 GAD67-positive and GAD67-negative nerve terminals in the phrenic motor nucleus. Frequency histograms showed that statistically higher densities of gold particles occurred over most GAD67-positive terminals. However, some GAD67-negative terminals also showed high densities of gold particles, and some GAD67-positive terminals showed low densities. Preabsorption of the anti-GABA antibody with a GABA-protein conjugate, but not with other amino acid-protein conjugates, significantly reduced gold labeling over both GAD67-positive and GAD67-negative terminals. These results show that the presence of GAD67 immunoreactivity correlates strongly with high densities of immunogold labeling for GABA in nerve terminals in the phrenic motor nucleus. Preabsorption controls indicate that authentic GABA was localized in the postembedding labeling procedure. Only a small proportion of intensely GABA-immunoreactive terminals lack GAD67, suggesting that both GAD67 and GABA are reliable markers of GABAergic nerve terminals.

The raccoon lateral cervical nucleus: mediolateral organization of GABA-positive and GABA-negative neurons and fibers

In the lateral cervical nucleus (LCN) of the cat, GABA-immunoreactive neurons and substance P-immunoreactive fibers are concentrated in the medial part of the nucleus, whereas in the monkey LCN no preferential locations have been identified. In raccoons, substance P-immunoreactive fibers display a distribution pattern similar to that in cats. However, the presence and distribution of GABA-immunoreactive neurons in the raccoon LCN has not been examined, and it is therefore not known whether raccoons are similar to cats or primates in this respect. Thus, in the present study, the raccoon LCN was examined for the presence and distribution of GABA-immunoreactive cells with respect to their numbers, locations, and sizes. The distribution of GABA-positive fibers and varicosities within the LCN was also investigated. The results of measurements of cross-sectional areas of LCN neurons indicate a trend toward decreasing cell size along the dorsolateral to medial axis of the raccoon LCN. Compared to neurons of the centrally located ventromedial division, neurons are statistically significantly larger in the dorsolateral division and smaller in the medial division of the nucleus. Cell counts in post-embedding-stained semithin sections through the nucleus revealed an average of 8,700 neurons per LCN. Approximately 4% of LCN neurons are GABA-immunoreactive. These neurons are small and most (80%) of them are located in the medial third of the LCN. In contrast, GABA-immunoreactive fibers and varicosities are present in about equal density throughout the raccoon LCN. Thus, the distributions of GABA-immunoreactive neurons and neuron sizes in the raccoon LCN conform closely to those in cats. Together with previous observations in cats and raccoons, the present findings support the notion that these small GABA-immunoreactive neurons may be local circuit inhibitory neurons and indicate the presence of a mediolateral segregation that may be of fundamental importance for the functional organization of the carnivore LCN.

Electron microscopy of immunoreactivity patterns for glutamate and gamma-aminobutyric acid in synaptic glomeruli of the feline spinal trigeminal nucleus (Subnucleus Caudalis)

We studied the ultrastructure of the synaptic organization in the feline spinal trigeminal nucleus, emphasizing specific neurotransmitter patterns within lamina II of the pars caudalis/medullary dorsal horn. Normal adults were perfused, and Vibratome sections from pars caudalis were processed for electron microscopy. Ultrathin sections were reacted with antibodies for the excitatory neurotransmitter glutamate (Glu) and for the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) by using postembedding immunogold techniques. Both single- and double-labeled preparations were examined. Results with single labeling show that Glu-immunoreactive terminals have round synaptic vesicles and form asymmetric synaptic contacts onto dendrites. GABA-immunoreactive axon terminals and vesicle-containing dendrites have pleomorphic vesicles, and the axon terminals form symmetric contacts onto dendrites and other axons. Double labeling on a single section shows glomeruli with central Glu-immunoreactive terminals that are presynaptic to dendrites, including GABA+ vesicle-containing dendrites. These Glu+ terminals are also postsynaptic to GABA+ axon terminals, and these GABA-immunoreactive terminals may also be presynaptic to the GABA+ vesicle-containing dendrites. Quantitative analyses confirm the specificity of the Glu and GABA immunoreactivities seen in the various glomerular profiles. The results suggest that a subpopulation of Glu-immunoreactive primary afferents (excitatory) may be under the direct synaptic influence of a GABA-immunoreactive intrinsic pathway (inhibitory) by both presynaptic and postsynaptic mechanisms.

Immunocytochemical evidence for vesicular storage of glutamate in cat spinocervical and cervicothalamic tract terminals

The densities of synaptic vesicles and gold particles, signaling fixed glutamate, were examined in spinocervical and cervicothalamic tract terminals. Statistically significant positive correlations between these parameters were detected in both terminal populations, whereas presumed inhibitory profiles displayed insignificant or negative correlations. These findings indicate a vesicular storage of glutamate in spinocervical and cervicothalamic tract terminals, and thus provide further evidence for glutamate as a neurotransmitter in the spinocervicothalamic pathway.

GABAergic innervation of rat abducens motoneurons retrogradely labelled with HRP: quantitative ultrastuctural analysis of cell bodies and proximal dendrites

In this quantitative electron microscopic study we investigated the distribution of GABA axon terminals on rat abducens motoneurons by combining retrograde labelling of montoneurons with post-embedding immunodetection of GABA. We analysed the synapses on 13 cell bodies and 60 proximal dendritic profiles distributed along the entire rostro-caudal extent of the nucleus. For each of these two compartments, we analysed 1754 and 1176 axon terminals in contact with 6042 and 3299 microns of postsynaptic membrane. The axon terminals were classified as Sv-type (containing spherical vesicles) or Pv-type (containing pleomorphic vesicles). The GABAergic terminals contained pleomorphic vesicles and established mainly symmetrical synaptic contacts. Their apposition lengths were greater than those of unlabelled terminals. On cell bodies, the percentage of GABAergic synaptic covering varied from 2.5% to 14.1% and the synaptic frequency of GABAergic axon terminals varied from 0.6% to 8.9%. These two parameters were significantly correlated with the diameter of the motoneurons. The percentage of synaptic covering and synaptic frequency were smaller on dendrites of small motoneurons than on those of large ones. The proximal dendrites of small motoneurons had a lesser GABAergic innervation than large ones. The total synaptic covering and frequency were smaller on somata than on dendrites. However, the percentage of synaptic covering by GABA terminals was higher on cell bodies than on proximal dendrites.

Catecholaminergic innervation of the lateral cervical nucleus: a correlated light and electron microscopic analysis of tyrosine hydroxylase-immunoreactive axons in the cat

The organization of catecholamine-containing axons in the cat lateral cervical nucleus was examined by immunocytochemical methods using a specific tyrosine hydroxylase antiserum. Light microscopic examination revealed numerous tyrosine hydroxylase-immunoreactive axons and varicosities throughout this nucleus, and some of these structures were found in contact with neuronal cell bodies. Correlated ultrastructural analysis showed that these varicosities were synaptic boutons which formed symmetric synaptic junctions with dendrites and somata. This evidence suggests that catecholamines exert a postsynaptic action upon neurons within the lateral cervical nucleus.

Neurotransmitters in subcortical somatosensory pathways

Investigations during recent years indicate that many different neuroactive substances are involved in the transmission and modulation of somesthetic information in the central nervous system. This review surveys recent developments within the field of somatosensory neurotransmission, emphasizing immunocytochemical findings. Increasing evidence indicates a widespread role for glutamate as a fast-acting excitatory neurotransmitter at different levels in somatosensory pathways. Several studies have substantiated a role for glutamate as a neurotransmitter in primary afferent neurons and in corticofugal projections, and also indicate a neurotransmitter role for glutamate in ascending somatosensory pathways. Other substances likely to be involved in somatosensory neurotransmission include the neuropeptides. Many different peptides have been detected in primary afferent neurons with unmyelinated or thinly myelinated axons, and are thus likely to be directly involved in primary afferent neurotransmission. Some neurons giving rise to ascending somatosensory pathways, primarily those with cell bodies in the dorsal horn, are also immunoreactive for peptides. Recent investigations have shown that the expression of neuropeptides, both in primary afferent and ascending tract neurons, may change as a result of various kinds of peripheral manipulation. The occurrence of neurotransmitters in intrinsic neurons and neurons providing modulating inputs to somatosensory relay nuclei (the dorsal horn, the lateral cervical nucleus, the dorsal column nuclei and the ventrobasal thalamus) is also reviewed. Neurotransmitters and modulators in such neurons include acetylcholine, monoamines, GABA, glycine, glutamate, and various neuropeptides.

Compartmentation of glutamate and glutamine in the lateral cervical nucleus: further evidence for glutamate as a spinocervical tract neurotransmitter

Previous observations indicate that spinocervical tract terminals contain relatively high levels of glutamate. To examine whether these high glutamate levels are likely to represent a neurotransmitter pool or an elevated metabolic pool, the distributions of glutamate- and glutamine-like immunoreactivities were examined in adjacent immunogold-labeled sections of the lateral cervical nucleus. Spinocervical tract terminals were identified by anterograde transport of horseradish peroxidase and wheat germ agglutinin-horseradish peroxidase conjugate from the spinal cord. Spinocervical tract terminals were found to contain significantly higher levels of glutamate-like immunoreactivity than other examined tissue compartments (large neuronal cell bodies, terminals with pleomorphic vesicles, astrocytes, and average tissue level). In contrast, the highest levels of glutamine-like immunoreactivity were detected in astrocytes. The different analyzed tissue elements formed three groups with respect to glutamate:glutamine ratios: one high ratio group including spinocervical tract terminals, a second group with intermediate ratios consisting of neuronal cell bodies and terminals containing pleomorphic synaptic vesicles, and a third low ratio group including astrocytes. Our findings indicate the presence of a compartmentation of glutamate and glutamine in the lateral cervical nucleus, similar to that postulated in biochemical studies of the central nervous system. The results also show that spinocervical tract terminals have high glutamate: glutamine ratios, similar to those previously observed in putative glutamatergic terminals in the cerebellar cortex. Thus, spinocervical tract terminals display biochemical characteristics that would be expected of glutamatergic terminals and the present findings therefore provide further evidence for glutamate as a spinocervical tract neurotransmitter.

Direct observations of synapses between L-glutamate-immunoreactive boutons and identified spinocervical tract neurones in the spinal cord of the cat

Four spinocervical tract cells in lumbosacral spinal cords of adult cats were physiologically characterized and intracellularly labelled with horseradish peroxidase. The neurones were examined with a light microscope and reconstructed. Selected regions were chosen for ultrastructural analysis. Thin sections were treated to reveal the presence of L-glutamate by using the postembedding immunogold method. Two antisera, which specifically recognise the presence of fixed glutamate in tissue, were used in the study. Somata, proximal, and distal dendrites of all four neurones received synaptic contacts from boutons which displayed an obvious immunogold reaction. These boutons formed between 35% and 48% of all synaptic contacts onto spinocervical tract cells. Glutamate-enriched boutons were associated with gold particle densities which were 2-3 times greater than the average densities associated with the surrounding neuropil. Their profiles had a mean diameter of 1.68 microns, contained round agranular synaptic vesicles, and formed asymmetrical synaptic junctions. However, not all boutons displaying these characteristics were enriched with glutamate. Immunogold studies of alternate thin sections, which were incubated with glutamate or GABA antiserum, demonstrated that synaptic boutons on spinocervical tract cells were either enriched with GABA or with glutamate and formed two separate populations which had distinct morphological characteristics. GABA-containing boutons contained irregularly shaped agranular vesicles and formed symmetrical synaptic junctions, whereas glutamate-enriched boutons corresponded to those described above. A further population of boutons, containing highly flattened vesicles, was not immunoreactive for GABA or glutamate. The evidence supports the idea that much of the excitatory transmission into the SCT is mediated by L-glutamate.

Role of cervical neurons in propriospinal inhibition of thoracic dorsal horn neurons

We previously reported that electrical or glutamate stimulation of the cervical spinal cord elicits a 40-60% decrease in renal sympathetic nerve activity (RSA) in the anesthetized rats. This sympatho-inhibition was possible, however, only after transection of the spinal cord at C1 or GABAergic inhibition of neurons in the rostral ventrolateral medulla. We postulated that cervical neurons inhibit RSA by inhibiting the activity of spinal interneurons that are antecedent to sympathetic preganglionic neurons (SPNs), and that these interneurons may be, in turn, excited by afferent signals. In this study, we tested the hypothesis that cervical neurons can inhibit visceroceptive thoracic spinal neurons. We recorded the spontaneous and evoked activity of 45 dorsal horn neurons responsive to splanchnic stimulation before, during, and after chemical or electrical stimulation of the cervical spinal cord in chloralose-anesthetized spinal rats. Cervical spinal stimulation that inhibited RSA also inhibited the spontaneous and/or evoked activity of 44 dorsal horn neurons. In addition to inhibiting splanchnic-evoked neuronal responses, cervical stimulation also inhibited responses, in the same neurons, evoked by noxious heat or light brushing of receptive dermatomes. We concluded that cervical neurons participate in propriospinal inhibition of afferent transmission and that this inhibitory system may be involved in controlling the access of afferent information to SPNs.

GABA-immunoreactive terminals synapse on primate spinothalamic tract cells

Gamma-aminobutyric acid (GABA) is a putative inhibitory neurotransmitter in the vertebrate nervous system. Several lines of evidence suggest that GABA plays an important role in the processing and modulation of sensory input in the spinal cord dorsal horn. In the present study, the relationship between GABA-immunoreactive (GABA-IR) terminals and spinothalamic tract (STT) cells in the monkey lumbar cord was investigated. Physiologically characterized STT cells, one located in lamina V and two located in lateral lamina IV, were intracellularly injected with horseradish peroxidase (HRP). A fourth STT cell, located in lamina I, was retrogradely labeled following injection of HRP into the contralateral thalamus. Immunogold labeling of ultrathin sections through the cell bodies and proximal dendrites of the STT neurons demonstrated that the percentage of the GABA-IR terminals in contact with these profiles was 24.7% and 36%, respectively. The average STT surface length contacted by GABA-IR terminals for cell bodies and proximal dendrites was 18.2% and 26.7%, respectively. For the lamina I cell, 7 out of 35 (20%) of the terminals were GABA-IR and they covered 9.6% of the surface analyzed. These data demonstrate that GABA-IR terminals synapse directly on STT cells, constituting a substantial proportion of the terminal population on these cells. Furthermore, compared to the cell bodies, a greater percentage of the input on the proximal dendrites is GABAergic. These anatomical data are consistent with the findings of a previously published iontophoretic study that demonstrated that GABA can exert a strong inhibitory influence on STT cells. These findings are discussed in relation to GABAergic involvement in tonic and phasic inhibition of STT neurons.

Peripheral inflammation is associated with increased glutamic acid decarboxylase immunoreactivity in the rat spinal cord

We have examined the frequency and distribution of neuron profiles immunoreactive for glutamic acid decarboxylase, a biosynthetic enzyme for the putative inhibitory neurotransmitter, gamma aminobutyric acid, in the lumbar spinal cord of colchicine-treated rats with unilateral inflammation of a hindpaw. Ipsilateral to the inflamed hindpaw, there was an apparent increase in the levels of glutamic acid decarboxylase, as indicated by significant increases in the number of visible glutamic acid decarboxylase-like immunoreactive profiles within the superficial dorsal horn, neck of the dorsal horn and the deep gray matter at L4. An increase limited to the deep gray matter at L6 was also seen. No alteration was identified at L2. These results are the first to demonstrate that peripheral inflammation is associated with altered levels of glutamic acid decarboxylase-like immunoreactivity.

Direct observations of synapses between GABA-immunoreactive boutons and identified spinocervical tract neurons in the cat's spinal cord

Three spinocervical tract neurons in adult cats were physiologically characterized and intracellularly labelled with horseradish peroxidase. The neurons were reconstructed and examined with the light microscope and were prepared for postembedding immunochemical analysis by using an antiserum which specifically recognizes GABA in glutaraldehyde-fixed tissue. Semithin sections were tested and examined with the light microscope. Somata, proximal, and distal dendrites of all three cells were associated with numerous punctate GABA-immunoreactive structures. Immunoreactive perikarya of small neurons in the vicinity of spinocervical tract cells were also observed. Ultrastructural analysis, with the immunogold technique, revealed that somata and proximal dendrites of all three neurons received synaptic contacts (about 37% of total synapses) from GABA-immunoreactive boutons and that distal dendrites were also associated with substantial numbers of immunoreactive structures (about 27% of synapses). Immunoreactive boutons were small (about 1 micron in diameter), contained irregularly shaped agranular vesicles, and formed symmetrical synaptic junctions with identified neurons. An additional group of immunoreactive boutons was observed to be associated with one of the cells only; these contained many large dense-core vesicles in addition to small agranular vesicles. Boutons containing round agranular vesicles and flattened agranular vesicles were not observed to be immunoreactive. The evidence supports the idea that much of the postsynaptic inhibition observed in spinocervical tract neurons is mediated by GABA and that even the most distal dendrites of these neurons receive inhibitory inputs.

Direct observations of synapses between GABA-immunoreactive boutons and muscle afferent terminals in lamina VI of the cat's spinal cord

Single group Ia muscle afferent fibres in the lumbar spinal cord of the cat were impaled with microelectrodes and labelled with horseradish peroxidase. Two collateral axons were prepared for combined light and electron microscopy. Arbors selected from lamina VI were processed by the postembedding immunogold technique with antiserum which specifically recognizes GABA in glutaraldehyde-fixed tissue. Twelve Ia boutons were examined through series of thin sections with the electron microscope and all of them were associated with presynaptic axon terminals which were positively labelled for GABA. Some Ia boutons received synaptic contacts from several GABAergic terminals. The present study establishes that a GABA-like substance is present in axon terminals presynaptic to Ia afferent boutons in lamina VI of the spinal cord. This evidence provides a morphological basis for presynaptic inhibition of Ia afferent input into lamina VI.

Terminals of group Ia primary afferent fibres in Clarke's column are enriched with L-glutamate-like immunoreactivity

Group Ia muscle spindle afferent fibres form giant terminals in Clarke's column which can be identified by morphological criteria. Postembedding Immunogold reactions were performed on tissue from the column using antiserum which recognized fixed L-glutamate in tissue. Giant terminals were heavily labelled with gold particles and quantitative analysis revealed that they contained significantly higher concentrations of L-glutamate in comparison with adjacent structures. L-Glutamate-enrichment of giant boutons is further evidence supporting the idea that this amino acid is a neurotransmitter at Ia synapses.

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.