The ultrastructure of prosternal sensory hair afferents within the locust central nervous system

The sensory neurones innervating long prosternai hairs of Locusta migratorioides were backfilled with horseradish peroxidase through their dendrites. The neurones' central projections in and around the medial ventral tract were examined with electron microscopy. Most synapses occur on axon collaterals which ramify through the neuropile around the tract where both input and output synapses were observed. Serial sectioning methods were used to determine the relative distribution of inputs and outputs which often lie in close proximity to one another on the axon terminals. The prosternai hair terminals contain agranular synaptic vesicles approximately 37 nm in diameter. Surrounding unidentified neuropilar profiles contain vesicles which are either statistically indistinguishable in size, or are larger, 45 nm diameter agranular vesicles. Neurones which are pre- or postsynaptic to labelled terminals generally contain vesicles of the second type.Input synapses onto the central terminals of primary afferent neurones can be recognised as a widespread phenomenon in the nervous systems of both invertebrates and vertebrates which will allow a fine degree of control of sensory inflow into the central nervous system.     

Post-embryonic development of rectifying electrical synapses in the crayfish: ultrastructure

The post-embryonic development of the rectifying Giant Fibre-Motor Giant (GF-MoG) synapse and the Giant Fibre-Segmental Giant (GF-SG) synapse has been investigated using electron-microscopy. In adults, the MoG and SG neurons make contact with the GFs by sending narrow 'finger-like' processes through the glial and connective tissue sheath surrounding each GF. The junctional region is characterized by closely apposed membranes (approximately 4 nm separation) traversed by regularly spaced connections, and large (60-80 nm) spherical vesicles in the presynaptic fibre. In newly hatched crayfish junctional contact is made over extensive areas of flat membrane apposition, due to the absence of a thick connective sheath around the giant fibres. Initially the junctional region is dominated by contacts which are morphologically indistinguishable from chemical synapses, i.e. 1. The apposed membranes are separated by a cleft of approximately 20-30 nm (an order of magnitude larger than the cleft distance at electrotonic synapses). 2. There is pre- and post-synaptic thickening of the junctional membranes with a dense cytoplasmic material. 3. Small (25-40 nm) pleomorphic vesicles are found on the presynaptic side of the junction, commonly in association with a dense presynaptic bar. Regions of junctional contact displaying the adult electronic-type morphology first appear at approximately one week post-hatching. At this age they are limited in distribution and occupy a central position in the area of contact surrounded by a broad 'chemical-like' annulus. During subsequent development these sites with electrotonic-type morphology grow in relative size, so that the 'chemical-like' sites become compressed towards the edges of the regions of contact. The adult type of morphology, in which the 'chemical-like' regions are vestigial, is achieved approximately two months after hatching.     

On the identification of the afferent axon terminals in the nucleus lateralis of the cerebellum an electron microscope study

Summary Axon terminals in the neuropil of the lateral nucleus can be divided into six classes, each with a specific constellation of characteristics that consistently occur together. Two of these classes have synaptic varicosities with elliptical synaptic vesicles, one in a dense, the other in a sparse matrix, and both make axosomatic and axodendritic synapses. The remaining four classes all have round synaptic vesicles and do not make axosomatic synapses. In the first of these four, the vesicles are tightly packed in a dense matrix, in another they are loosely dispersed, and in the third they are clustered. In the fourth, large granular vesicles predominate. Of these six classes, the most numerous belong to the axons of the Purkinje cell terminal arborization. These boutons resemble their counterparts in the cerebellar cortex, the recurrent collaterals of the Purkinje axon. They have elliptical and flat synaptic vesicles in a dark matrix. The varicosities terminate on somata and dendrites of large and small neurons and constitute the majority of their input. Purkinje axons constitute 86% of the total population of terminals on large neuronal perikarya and 50% of those on their dendrites, but only 78% on the somata of small neurons and 31% on their dendrites. The terminals of climbing fiber collaterals are recognized by their resemblance in electron micrographs to the terminals of the climbing fiber arborization in the cerebellar cortex. They bear round synaptic vesicles packed into a dense axoplasmic matrix and make Gray's type 1 axodendritic synapses with large and small neurons. These axons are restricted to the lateral and ventral aspects of the nucleus and constitute 5% of the terminals on large cell dendrites and 6% of those on small neurons. The axons tentatively identified as collaterals of mossy fibers are myelinated fibers with a light axoplasm containing round synaptic vesicles, dispersed throughout their varicosities. They make Gray's type 1 synapses and constitute a fair percentage of the total axodendritic contacts in the neuropil, 22% on large neurons and 28% on small neurons. The bases for these tentative identifications are discussed in detail, as are the various synaptic relationships undertaken by each class of axon. The remaining 4 classes of axons of the neuropil will be described in subsequent papers.Supported in part by U.S. Public Health Service grants NS 10536 and NS 03659, Training grant NS 05591 from the National Institute of Neurological Diseases and Stroke, and a William F. Milton Fund Award from Harvard University.     

Ultrastructure of marginal zone during prenatal development of human spinal cord

Summary Electron microscopic studies were conducted in the marginal zone (lamina I) in human fetuses ranging from 8–25 weeks of gestational age. At 8 weeks the neurons have indented nuclei and sparse organelles in the cytoplasm. The neuropil shows contacts between the axons and dendritic profiles. Some of them are well defined synapses with post synaptic thickening and agranular spherical vesicles in the presynaptic terminal. At 18 weeks compactly packed organelles with long cisternae of rough endoplasmic reticulum could be visualized in the neuronal cytoplasm. At 25 weeks the neurons have heterochromatin patches in the nuclei. Axosomatic, dendrodendritic, axoaxonic, symmetrical, asymmetrical and multisynaptic contacts with agranular and dense core vesicles are seen at different sequential age periods.     

Fine structural study of the abdominal muscle receptor organs of the crayfish (Procambarus clarkii). Sensory endings and synaptic structures

Summary The sensory endings, neuromuscular junctions and interneuronal synapses in the crayfish muscle receptor organ have been studied by electron microscopy. The dendrites of the receptor neuron terminate as endings which are either free in the connective tissue matrix of the central region of the receptor strands, or abut on the muscle membrane forming a specialized junction with a narrow cleft of about 18 nm. Efferent nerve endings are classified into three types on the basis of their fine structural features. Type 1 endings contain mainly spherical vesicles with a diameter of about 55 nm and a few large granular vesicles with a diameter of about 100 nm, and synapse exclusively on muscle fibres. Type 2 endings have a high proportion of elongated vesicles measuring about 30 × 80 nm and a few large granular vesicles, and synapse on both sensory neurons and muscle. Type 3 endings are characterized by the high electron density of the axoplasm and numerous large granular vesicles with a diameter of about 100 nm; they synapse only on the sensory neuron of the slow receptor unit.It is suggested that Type 1 endings are excitatory, and Type 2 and 3 endings are inhibitory. Several differences in postsynaptic structure were observed between the putative excitatory and inhibitory neuromuscular junctions. Axo-axonal synapses between endings of Type 1 and Type 2, the latter being presynaptic to the former, are also found. Functional implications and possible roles of these structures are discussed.     

Axon terminals of the intrinsic neurons in the nucleus lateralis of the cerebellum an electron microscope study

Summary The large projection neurons of the lateral nucleus have long axons, which leave the cell mass in the superior cerebellar peduncle. These axons emit myelinated recurrent collaterals which have synaptic varicosities en passant. The varicosities are 2–5 m in diameter and contain round, agranular synaptic vesicles ranging between 280 and 480 Å with diameters of approximately 400 Å. The vesicles lie in a moderately dark axoplasmic matrix with a mean packing density of 281/m². The varicosities synapse through Gray's type 1 junctions with dendrites and thorns of large and small neurons. They constitute 22% of the total axonal population on dendrites of large neurons and 10% on dendrites of small neurons. The recurrent collateral system may provide a means for positive feedback to the same neuron and other neurons of the neuropil.The small neuron or interneuron has a short axonal plexus. The axon is myelinated, and is distinctive with a light axoplasmic matrix and varicosities containing elliptical synaptic vesicles. The vesicles are loosely dispersed with a mean population density of 44/m². These varicosities synapse through an intermediate type of junction upon the somata of certain large and small neurons and they consitute 14% and 22% of the axosomatic synapses respectively. They also make synapses on dendrites, constituting 12% and 25% of the total population of axons synapsing with dendrites of large neurons and those of small neurons respectively. It is suggested that these are the inhibitory interneurons of the lateral nucleus.The corticonuclear input through Purkinje axons is the dominant influence on the lateral nucleus neurons. This inhibitory input is considerably larger on the large neurons than on the small ones. It is speculated that the axosomatic synapses are inhibitory. Excitatory influences, through the collaterals of mossy and climbing fibers and the recurrent collaterals of the large intrinsic neurons, impinge upon the dendrites, where the axons of both Purkinje cells and interneurons also terminate.Supported in part by U.S. Public Health Service grants NS10536, NS03659, Training grant NS05591 from the National Institute of Neurological Diseases and Stroke, and a William F. Milton Fund Award from Harvard University.     

GABA-immunoreactivity in processes presynaptic to the terminals of afferents from a locust leg proprioceptor

Summary Individually labelled sensory neurons from the femoral chordotonal organ, a proprioceptor at the femoro-tibial joint of a locust hindleg, were analysed by intracellular recording, and by electron microscopical immunocytochemistry to reveal the arrangement of their input and output synapses and to determine whether the input synapses were GABAergic. Intracellular recordings from these sensory neurons show spikes superimposed on a barrage of synaptic potentials during movements of the femoro-tibial joint. These synaptic inputs can be mimicked by GABA. Input synapses are made onto the vesicle-containing terminals of afferents and are often closely associated with the output synapses. By contrast, the axons of the afferents in the neuropil have no vesicles and neither make nor receive synapses. The input synapses to the afferent terminals are made from processes typically a few microns in diameter, whereas the output synapses are made onto much smaller processes of only 0.1–0.2 m. Input synapses at which an afferent terminal is the only postsynaptic element are common. Where the synapse is dyadic the second postsynaptic element does not usually appear to be a chordotonal afferent. The output synapses from the afferent terminals are usually dyadic. At 78% of the input synapses, the presynaptic neurite showed immunoreactivity to a GABA antibody, supporting the physiological evidence that the presynaptic effects can be mediated by the release of GABA. The remaining (22%) immunonegative synapses are intermingled with those showing GABA immunoreactivity, but their putative transmitter is unknown. These morphological observations suggest that the presynaptic control of the chordotonal afferents is largely mediated by GABAergic neurons, but because other types of neuron also appear to be involved, presynaptic modulation may be more complex than has yet been revealed by the physiology.     

Ultrastructural features of neurons and synaptic contacts in the posterodorsal medial amygdala of adult male rats

The aim of the present study was to describe the ultrastructure of neurons (from eight animals) and to analyse the synaptic terminal distribution (from two animals) in the posterodorsal subnucleus of the medial amygdala (MePD) of adult male rats. Using transmission electron microscopy, it was possible to identify many spiny and aspiny dendrites, unmyelinated axonal bundles, single axonal processes, a few myelinated axons, blood vessels and glial processes in the neuropil. Axodendritic synapses were the most frequently observed (67.5%), appearing to be of either the inhibitory or the excitatory types. The presynaptic region contained round or flattened vesicles that occurred either singly or with dense-cored vesicles (DCVs). The dendrites often received many synapses on a single shaft, and axon terminals displayed synaptic contacts with one or more postsynaptic structures. Dendritic spines showed different morphologies and the synapses on them (23.1%) formed a single and apparently excitatory synaptic contact with round, electron-lucid vesicles alone or, less frequently, with DCVs. Inhibitory and excitatory axosomatic synapses (8.2%) and excitatory axoaxonic synapses (1.2%) were also identified. The present report provides new findings relevant to the study of the MePD cellular organization and could be combined with other morphological data in order to reveal the functional activity of this area in male rats.     

Synaptic organization of the dorsal lateral geniculate nucleus in the adult hamster

Summary The synaptic organization of the sector of the dorsal lateral geniculate nucleus has been examined by electron microscopy in normal adult hamsters and in adult hamsters subjected to unilateral eye enucleation or intravitreal injection of horseradish peroxidase.Two types of neuropil are apparent. Islands of complex neuropil partially enclosed by astrocyte processes (synaptic glomeruli) are surrounded by a sea of simpler non-glomerular neuropil. The latter is dominated by small axon terminals with spherical synaptic vesicles and Gray type 1 axodendritic contacts (SR-boutons) and also contains axon terminals with flattened synaptic vesicles (F-boutons). The glomerular neuropil contains (i) exclusively postsynaptic dendrites and dendritic protrusions of presumptive projection cells; (ii) pre- and postsynaptic pleomorphic-vesiclecontaining P-boutons (interpreted as appendages of the dendrites of interneurons); (iii) large axon terminals containing spherical synaptic vesicles and large pale mitochondria (R-boutons) which were experimentally identified as retinal terminals and which are presynaptic to both projection cell dendrites and P-boutons at Gray type 1 contacts; (iv) F-boutons (minority component). F-boutons and P-boutons are presynaptic to both projection cell dendrites and P-boutons and P-boutons are the intermediate elements of various serial synapses including triplet (triadic) synapses. Medium-large terminals with spherical synatpic vesicles and dark mitochondria (RLD-boutons) which were commonly invaginated by dendritic spines of projection cells in small glomerulus-like formations were also identified. The origin of RLD-boutons is unknown but SR-boutons probably derive chiefly from ipsilateral visual cortex and possibly also from superior colliculus, and non-glomerular F-boutons probably originate in the ipsilateral thalamic reticular nucleus.No differences in synaptic organization were found between the part of the nucleus which receives uncrossed retinal input and the part which receives crossed input, nor were differences seen in the size, fine structure or relationships between the terminals of identified crossed and uncrossed retinal axons.     

A GABA immunocytochemical study of rat motor thalamus: light and electron microscopic observations.

A light and electron microscopic study of GABA-immunoreactive neurons and profiles in the ventroanterior-ventrolateral and ventromedial nuclei of rat dorsal thalamus was conducted using antiserum raised against GABA. Less than 1% of the neurons in these motor-related nuclei exhibited GABA immunoreactivity, confirming previous reports that these nuclei are largely devoid of interneurons. Immunoreactive neurons in the ventral anterior-ventral lateral complex and ventromedial nucleus were bipolar or multipolar in shape, and tended to be smaller than non-immunoreactive neurons. GABA immunoreactivity in the neuropil consisted of labeled axon terminals and myelinated and unmyelinated axons, and was lower in the ventral anterior-ventral lateral complex and ventromedial nucleus than in neighboring thalamic nuclei. The density of neuropil immunolabeling was slightly higher in ventral anterior-ventral lateral complex than in ventromedial nucleus. GABA-immunoreactive axon terminals, collectively termed MP boutons for their medium size and pleomorphic vesicles (and corresponding to "F" profiles of some previous studies of thalamic ultrastructure), formed symmetric synapses and puncta adhaerentia contacts predominantly with large and medium-diameter (i.e. proximal) non-immunoreactive dendrites. Approximately 12 and 18% of boutons in the ventral anterior-ventral lateral complex and ventromedial nucleus, respectively, were GABA-immunopositive. Many of these immunoreactive profiles probably arose from GABAergic neurons in the thalamic reticular nucleus, substantia nigra pars reticulata and entopeduncular nucleus. Two types of non-immunoreactive axon terminals were distinguished based on differences in morphology and synaptic termination sites. Boutons with small ovoid profiles and round vesicles that formed prominent asymmetric synapses onto small-diameter dendrites were observed. Mitochondria were rarely observed within these boutons, which arose from thin unmyelinated axons. These boutons composed approximately 82 and 74% of boutons in the ventral anterior-ventral lateral complex and ventromedial nucleus, respectively, and were considered to arise predominantly from neurons in the cerebral cortex. In contrast, boutons with large terminals that contained round or plemorphic vesicles and formed multiple asymmetric synapses predominantly with large-diameter dendrites were also observed. Puncta adhaerentia contacts were also common. Mitochondria were numerous within large boutons with round vesicles, which arose from myelinated axons. Many of the large boutons were likely to have originated from neurons in the cerebellar nuclei. Approximately 6% of the boutons in the ventral anterior-ventral lateral complex and 8% in ventromedial nucleus were of the large type.(ABSTRACT TRUNCATED AT 400 WORDS)     

A newly recognized element in the monkey dorsal lateral geniculate nucleus exhibiting both presynaptic and postsynaptic sites

Summary The dorsal lateral geniculate nucleus (LGNd) of four normal monkeys (Macaca mulatta) and of two other such animals with total unilateral ablation of the visual cortices (4–6 days survival) were examined in serial thin sections with the electron microscope. In these materials we have observed a new neuropil component which has the cytologic characteristics of principal cell (P-cell) dendrites, i.e. large and dark mitochondria, smooth endoplasmic cisterns and filamentous, non-synaptic contacts with retinal terminals. In addition, these elements contain large round synaptic vesicles and can be seen forming asymmetric synapses exclusively with presynaptic dendrites belonging to interneurons (I-cells). Occasionally, a reciprocal synapse is formed between the two profiles. The novel elements are postsynaptic to various vesicle-containing profiles, i.e. axonal boutons of presumably retinal and cortical origin, and I-cell presynaptic dendrites. They are found more frequently in the specimens with cortical ablations, although their number is still much lower than that of the other classic components of the neuropil. Measurements made on × 80 000 electron micrographs of spheroid vesicles within presumptive retinal terminals, cortical endings and the new profile described in this report, result in mean diameters of 38.6nm, 33.3nm and 44.3 nm, respectively. The differences between the means are statistically significant.Although the profile with large dark mitochondria and large round vesicles may represent a dendrite of a different I-cell type, or a recurrent axon collateral of a P-cell, it appears more probable that it is a presynaptic dendrite of a P-cell. The infrequent but consistent occurrence of these elements suggests that at least some P-cells can develop presynaptic sites on their dendrites, a property which contributes to the synaptic complexity of the LGNd.     

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

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

Neuronal structure and synaptic distribution of a uropod doser motor neuron in the crayfish terminal ganglion

Summary One of the uropod closer muscles in the crayfish, the adductor exopodite, is innervated by two large identified motor neurons. They were injected intracellularly with horseradish peroxidase or nickel chloride to reveal the structure and distribution of the input and output synapses using electron microscopy. The development of nickel with rubeanic acid greatly improved the tissue preservation at the ultrastructural level compared with ammonium sulphide. Cell bodies of the motor neurons lying in the ventro-lateral cortex of the ganglion are extensively invaginated by glial cells. Input synapses occur directly upon the primary neurite within the neuropil or upon the major anterior neurite. They are most abundant, however, upon the numerous smaller neurites of the motor neuron. The primary neurite in the dorsal region of the neuropil, upon which no synapses were made, is wrapped with glial cells. Occasionally, these two adductor exopodite motor neurons were found as adjacent postsynaptic profiles at the same synapse when both cells were stained simultaneously in the same preparation. In the present study we could not locate any sites of synaptic output which strictly fulfil the structural criteria of a synapse on the processes of the motor neuron. This result is inconsistent with physiological evidence which suggests that spikeless interactions occur between the two adductor exopodite motor neurons and their synergists. This might be the result of two possible features of the interaction: the sites of synaptic output may be limited to a few restricted branches, and the interaction between these motor neurons may depend largely upon electrical synapses.     

Synaptic organization in the olivary pretectal nucleus of the adult rat

The fine structure of the glomerular neuropil of the olivary pretectal nucleus has been studied in adult rats. Four presynaptic components and two postsynaptic components of the neuropil are described and the synaptic relationships established between these components, including complex serial synapses, are defined. A conspicuous feature of this neuropil is the presence of triplet (triadic) synapses involving retinal afferents, appendages of the presynaptic dendrites of intrinsic neurones, and projection cell dendrites.     

Organization of crustacean neuropil. II. Distribution of synaptic contacts on identified motor neurons in lobster stomatogastric ganglion

Summary Identified neurons in the stomatogastric ganglion of the lobster were examined and reconstructed by serial section electron microscopy. Each motor neuron consists of a soma, a primary process leading directly from the soma to the motor axon which leaves the ganglion, and a group of secondary processes which branch from the primary process and ramify within the neuropil. Synapses are found only on small processes in the synaptic neuropil, never on the primary processes or on larger secondary processes in the coarse neuropil. Nearly every secondary process of every neuron examined makes both pre- and postsynaptic contacts. Hence these neurons are not polarized into distinct pre- and postsynaptic regions but have both input and output distributed over each of the secondary processes in the neuropil. The connection between a specific pair of neurons is also distributed over several branches of both the pre- and the postsynaptic neurons.The restriction of synapses to the more distal portions of the secondary processes suggests that no single contact or localized group of contacts can exert an overriding influence on the neuron by virtue of an especially advantageous position. The close proximity of input and output on most secondary processes suggests that synaptic input may be capable of directly influencing output without the intervention of action potentials. The distribution of specific synapses over several branches of both pre- and postsynaptic neurons suggests that each neuron functions as a whole without differentiation into specialized branches.     

GABAergic and pallidal terminals in the thalamic reticular nucleus of squirrel monkeys

The ultrastructure of synaptic terminals from the external segment of the globus pallidus and of other synaptic terminals positive for gamma-aminobutyric acid (GABA) was examined in the thalamic reticular nucleus (TRN) of squirrel monkeys. Two GABA-positive terminals types were commonly encountered within the TRN neuropil. The most common type of GABAergic terminals (F terminals) are filled with dispersed pleomorphic synaptic vesicles and clusters of mitochondria. These terminals establish multiple symmetric synapses upon the somata and dendrites of TRN neurons. The external pallidal terminals, labeled with WGA-HRP, arise from thinly myelinated axons and correspond to the medium to large F terminals. A less prevalent population of smaller GABAergic synaptic profiles was also identified. The synaptic profiles in this second group contain considerably fewer pleomorphic synaptic vesicles in small irregular clusters and fewer mitochondria, establish symmetric synapses, are postsynaptic to other axonal terminals, are presynaptic to dendrites and soma, and are unlabeled following pallidal injections of WGA-HRP.     

GABAergic neurons in the mouse superficial dorsal horn with special emphasis on their relation to primary afferent central terminals

Immunocytochemical studies were carried out on the morphological relation between primary afferent central terminals (C-terminals) and GABAergic neurons in the mouse superficial dorsal horn. The superficial dorsal horn is composed of many synaptic glomeruli comprising two types: Type I with centrally located CI-terminals surrounded by several dendrites and few axonal endings, and Type II with centrally located CII-terminals surrounded by several dendrites and a few axonal endings. The CI-terminals are sinuous or scalloped with densely packed agranular synaptic vesicles, a few granular synaptic vesicles and mitochondria, and show an electron dense axoplasm, whereas the CII-terminals are large and round or rectangular with evenly distributed agranular synaptic vesicles, a number of granular synaptic vesicles and mitochondria, and show an electron opaque axoplasm. The immunoreaction of GABA was remarkable in the superficial laminae of the dorsal horn. Many interneuronal somata in the substantia gelatinosa showed GABAergic immunoreactivity. The immunoreaction was seen in the entire GABAergic neuroplasm, but not in the nucleus and its envelope. Most GABAergic features appeared as dendrites making postsynaptic contact with CI- or CII-terminals; i.e., numerous C-terminals made presynaptic contact with GABAergic dendrites. GABA immunoreactivity was seen over round synaptic vesicles and mitochondrial membranes. A few CII-terminals made presynaptic contact with GABAergic interneuronal somata. Previous physiological and anatomical studies have suggested that not only the cutaneous nociceptive primary afferent C-terminals but also mechanoreceptive primary afferent C-terminals make presynaptic contact with the GABAergic dendrites, boutons and soma. The presynaptic relation of these primary afferents with GABAergic neurons seems to provide morphological support for the essential feature of the gate control theory: primary afferent fibers may play a part in the modulation of nociceptive information via GABAergic neurons in the superficial dorsal horn. Small GABAergic terminals were found to make contact with blood capillaries suggesting the release of GABA into circulation.     

Immunocytochemical localization of the GABAB2 receptor subunit in the glomeruli of the mouse main olfactory bulb

The olfactory input to the brain is carried out by olfactory nerve axons that terminate in the olfactory bulb glomeruli and make synapses onto dendrites of glutamatergic projection neurons, mitral and tufted cells, and GABAergic interneurons, periglomerular cells. The dendrites are reciprocally connected through asymmetric synapses of mitral/tufted cells with periglomerular cells and symmetric synapses of the opposite direction. Transmission at the first synapse in the olfactory pathway is regulated presynaptically, and this regulation is mediated, in part, by metabotropic GABAB receptors that, when activated, inhibit transmitter release from the olfactory nerve. Functional GABAB receptors are heterodimers composed of the GABAB1 and GABAB2 subunits. Studies using double immunofluorescence have shown colocalization of both subunits in the glomerular neuropil, and ultrastructural studies have localized GABAB1 to extrasynaptic, synaptic, and perisynaptic sites on the plasma membrane of olfactory nerve terminals. We studied the subcellular localization of GABAB2 in the mouse olfactory glomeruli using a subunit-specific antibody and preembedding immunogold labeling. Immunoreactivity for GABAB2 was associated with symmetric dendrodendritic synapses of periglomerular cells with mitral/tufted cells and was localized to the extrasynaptic plasma membrane of presynaptic dendrites, and extrasynaptic, synaptic, and perisynaptic sites on the plasma membrane of postsynaptic dendrites. The results suggest that postsynaptic, and perhaps presynaptic, GABAB receptors may be expressed at GABAergic synapses between dendrites of periglomerular interneurons and projection neurons. Immunolabeling was observed at junctions of the olfactory nerve with mitral/tufted cell dendrites, providing ultrastructural evidence for the expression of the GABAB2 subunit at the primary olfactory synapse.     

Membrane ultrastructure of the giant synapse of the squidLoligo pealei

The ultrastructure of the ‘giant synapse’ of the stellate ganglion of the squid was studied with freeze-fracture and thin-sectioning techniques. A sheath of glial cells separates the pre- and post-synaptic axons. At intervals, round-topped processes of the postsynaptic axon pierce the sheath to contact the presynaptic axon. This area of synaptic contact is marked by a widened intercellular cleft containing electron-dense material and by a cluster of synaptic vesicles within the presynaptic cytoplasm. The number of synaptic vesicles in such clusters was greatly reduced by electrical stimulation of the synapse during fixation. Freeze-fracture reveals a roughly circular patch (0.3 μm diameter) of 10 nm particles on the cytoplasmic leaflet of the presynaptic membrane. A similar patch of particles lies on the external leaflet of the apposed postsynaptic membrane.The squid giant synapse thus consists of multiple small pre- and postsynaptic active zones where neurotransmitter is released from the presynaptic terminal and sensed by postsynaptic receptors. Comparison of the structure of these postsynaptic active zones with those at synapses where the transmitter or transmitter action is known suggests that the excitatory transmitter at this synapse is an amino acid.Presumptive gap junctions, marked by particles in the cytoplasmic leaflet, are found between small-diameter axons in the stellate ganglion but not at the giant synapse. Glial-cell membranes contain aggregates of particles and pits suggestive of gap junctions. The aggregates of pits are embedded within linear arrays of particles which somewhat resemble tight junctions.