Problems of understanding the substructure of synapses

The Structural Basis of Long-Term Potentiation in Hippocampal Synapses, Revealed by Electron Microscopy Imaging of Lanthanum-Induced Synaptic Vesicle Recycling

Hippocampal neurons in dissociated cell cultures were exposed to the trivalent cation lanthanum for short periods (15–30 min) and prepared for electron microscopy (EM), to evaluate the stimulatory effects of this cation on synaptic ultrastructure. Not only were characteristic ultrastructural changes of exaggerated synaptic vesicle turnover seen within the presynapses of these cultures—including synaptic vesicle depletion and proliferation of vesicle-recycling structures—but the overall architecture of a large proportion of the synapses in the cultures was dramatically altered, due to large postsynaptic “bulges” or herniations into the presynapses. Moreover, in most cases, these postsynaptic herniations or protrusions produced by lanthanum were seen by EM to distort or break or “perforate” the so-called postsynaptic densities (PSDs) that harbor receptors and recognition molecules essential for synaptic function. These dramatic EM observations lead us to postulate that such PSD breakages or “perforations” could very possibly create essential substrates or “tags” for synaptic growth, simply by creating fragmented free edges around the PSDs, into which new receptors and recognition molecules could be recruited more easily, and thus, they could represent the physical substrate for the important synaptic growth process known as “long-term potentiation” (LTP). All of this was created simply in hippocampal dissociated cell cultures, and simply by pushing synaptic vesicle recycling way beyond its normal limits with the trivalent cation lanthanum, but we argued in this report that such fundamental changes in synaptic architecture—given that they can occur at all—could also occur at the extremes of normal neuronal activity, which are presumed to lead to learning and memory.

The synaptic life of microtubules

In neurons, control of microtubule dynamics is required for multiple homeostatic and regulated activities. Over the past few decades, a great deal has been learned about the role of the microtubule cytoskeleton in axonal and dendritic transport, with a broad impact on neuronal health and disease. However, significantly less attention has been paid to the importance of microtubule dynamics in directly regulating synaptic function. Here, we review emerging literature demonstrating that microtubules enter synapses and control central aspects of synaptic activity, including neurotransmitter release and synaptic plasticity. The pleiotropic effects caused by a dysfunctional synaptic microtubule cytoskeleton may thus represent a key point of vulnerability for neurons and a primary driver of neurological disease.

Activity-Dependent Nucleation of Dynamic Microtubules at Presynaptic Boutons Controls Neurotransmission

Control of microtubule (MT) nucleation and dynamics is critical for neuronal function. Whether MT nucleation is regulated at presynaptic boutons and influences overall presynaptic activity remains unknown. By visualizing MT plus-end dynamics at individual excitatory en passant boutons in axons of cultured hippocampal neurons and in hippocampal slices expressing EB3-EGFP and vGlut1-mCherry, we found that dynamic MTs preferentially grow from presynaptic boutons, show biased directionality in that they are almost always oriented toward the distal tip of the axon, and can be induced by neuronal activity. Silencing of γ-tubulin expression reduced presynaptic MT nucleation, and depletion of either HAUS1 or HAUS7-augmin subunits increased the percentage of retrograde comets initiated at boutons, indicating that γ-tubulin and augmin are required for activity-dependent de novo nucleation of uniformly distally oriented dynamic MTs. We analyzed the dynamics of a wide range of axonal organelles as well as synaptic vesicles (SVs) relative to vGlut1⁺ stable presynaptic boutons in a time window during which MT nucleation at boutons is promoted upon induction of neuronal activity, and we found that γ-tubulin-dependent presynaptic MT nucleation controls bidirectional (SV) interbouton transport and regulates evoked SV exocytosis. Hence, en passant boutons act as hotspots for activity-dependent de novo MT nucleation, which controls neurotransmission by providing dynamic tracks for bidirectional delivery of SVs between sites of neurotransmitter release.

Structure activity relationship of synaptic and junctional neurotransmission

Chemical neurotransmission may include transmission to local or remote sites. Locally, contact between 'bare' portions of the bulbous nerve terminal termed a varicosity and the effector cell may be in the form of either synapse or non-synaptic contact. Traditionally, all local transmissions between nerves and effector cells are considered synaptic in nature. This is particularly true for communication between neurons. However, communication between nerves and other effectors such as smooth muscles has been described as nonsynaptic or junctional in nature. Nonsynaptic neurotransmission is now also increasingly recognized in the CNS. This review focuses on the relationship between structure and function that orchestrate synaptic and junctional neurotransmissions. A synapse is a specialized focal contact between the presynaptic active zone capable of ultrafast release of soluble transmitters and the postsynaptic density that cluster ionotropic receptors. The presynaptic and the postsynaptic areas are separated by the 'closed' synaptic cavity. The physiological hallmark of the synapse is ultrafast postsynaptic potentials lasting milliseconds. In contrast, junctions are juxtapositions of nerve terminals and the effector cells without clear synaptic specializations and the junctional space is 'open' to the extracellular space. Based on the nature of the transmitters, postjunctional receptors and their separation from the release sites, the junctions can be divided into 'close' and 'wide' junctions. Functionally, the 'close' and the 'wide' junctions can be distinguished by postjunctional potentials lasting ~1s and tens of seconds, respectively. Both synaptic and junctional communications are common between neurons; however, junctional transmission is the rule at many neuro-non-neural effectors.

Axon hillocks and initial segments in spinal trigeminal nucleus with emphasis on synapses including axo-axo-axonic contacts

As a part of a continuing study of the feline spinal trigeminal nucleus, the fine structure and synaptic arrangements on the axon hillock and axon initial segment of neurons in this region are described here. Transmission electron microscopy has been used to characterize qualitatively the axon hillock and initial segment and associated synapses in pars interpolaris. Axon hillocks and initial segments are easily identified in continuity with somata or as isolated profiles in the neuropil, and they receive synaptic contacts: these we regard as axo-axonic. The presynaptic terminals contain either mainly round or mainly flattened synaptic vesicles and have Type I (asymmetric) or Type II (symmetric) thickenings respectively at their contacts with the axon hillock or initial segment. I report here also the unusual arrangement of three separate axons in a serial synaptic complex. Some of the round vesicle Type I contacts onto the axon hillock-initial segment region also receive Type II contacts from one or more flattened vesicle terminals, thus forming an axo-axo-axonic complex. These flattened vesicle terminals lack the usual features of a presynaptic dendrite. It has been shown that in this nucleus some round vesicle terminals, especially those postsynaptic to flattened vesicle terminals, are primary afferents from the periphery. Therefore the round vesicle terminal presynaptic to the axon hillock-initial segment region, some of which are included in the axo-axo-axonic complex may also be a primary afferent directly contacting the spike generator area of the relay neuron and under presynaptic control of a flattened vesicle synapse. The latter may possibly be an intrinsic contact. This strategic situation of round vesicle terminals and the axo-axo-axonic complex at the axon hillock or initial segment has major implications relevant to the overall output of these neurons.

Ultrastructural characteristics of zebrafish spinal motoneurons innervating glycolytic white, and oxidative red and intermediate muscle fibers

Spinal motoneurons in the zebrafish were classified using morphological criteria. Dorsomedial white motoneurons which innervate the fast, glycolytic white muscle fiber compartment were distinguished from ventrolateral red and intermediate motoneurons which innervate the slow, oxidative, red and intermediate muscle fiber compartments. Synapses on cell somata and cell organelles were studied in detail. The motoneurons which innervate white muscle fibers (W motoneurons) are considerably larger than those which innervate red and intermediate muscle fibers (RI motoneurons; W > RI). Significant differences were also found in the size of the nucleus (W > RI) and in the ratio size nucleus/size soma (W < RI); small differences were found regarding endoplasmic reticulum (W > RI) and mitochondria (W < RI). There were no differences in synaptic apposition length or percentage of terminals with flat vesicles. Small differences were discerned with regard to covering percentages (W < RI) and percentage of terminals with round vesicles (W > RI). Terminals with dense cored vesicles appeared on W motoneuron somata only. Within the motoneuron population, there was a positive correlation between the coverage of terminals containing flat vesicles and the perimeter of the cell soma. In RI motoneurons, there was a positive correlation between the perimeter of the cell and the amount of endoplasmic reticulum. A negative correlation was found between the RI cell perimeter and mitochondria, which is in line with a high succinate dehydrogenase activity in small cells.

Further evidence for the dynamic formation of transmitter quanta at the neuromuscular junction

Fatt and Katz (Nature 166:597-598, 1950; J Physiol 117:109-128, 1952) attributed miniature endplate potentials (MEPPs) to the action of a standard quantity of transmitter, the quantum (Del Castillo and Katz, J Physiol 124:560-573, 1954). Quantal packets of transmitter were proposed to be preformed (Del Castillo and Katz, In CNRS Paris (Ed): "Microphysiologie comparée des éléments excitables" 67:245-258, 1957) and stored in large numbers in the motor nerve terminal. Statistical analyses of intervals between MEPPs and numbers of quanta composing small endplate potentials indicated that quantal release was a random process and that release sites functioned independently of each other. With the discovery of synaptic vesicles it was proposed that each contained one quantum of transmitter. The quantal-vesicular hypothesis (Del Castillo and Katz, as cited above) fails, however, to explain amplitude distributions of MEPPs that are skewed and/or that show multiple peaks (Kriebel et al., Brain Res Review 15:167-178, 1990). The drop formation process (Shaw, "The Dripping Faucet as a Model Chaotic System," Santa Cruz, CA: Aerial Press, Inc., 1984) was shown to generate amplitude classes of drops that were similar to classes of MEPPs which suggested that rapid changes in quantal size and ratios of skew- to bell-MEPPs could be explained with a simple dynamic process which determines quantal size at the moment of release (Kriebel et al., as cited above, 1990). Further similarities between miniature endplate currents (MEPCs) and the formation of drops are reported here. We found that rapid changes in MEPC amplitudes and time courses, which accompany an increase in frequency, mimic changes in drop sizes that accompany increases in flow rate. MEPC intervals have a minimum and their distributions are comparable to those of drop intervals. During an increased rate of transmitter release, MEPP amplitudes and intervals were positively correlated. The results suggest that spontaneously released transmitter "packets" are formed at the moment of release and that transmitter supply to the process that forms packets is continuous.

Age-related remodeling of glutamic-acid decarboxylase-labeled elements in deafferented piriform cortex of rats

Olfactory bulb (OB) removal has been shown to result in plasticity in the piriform cortex (PC) that is age dependent. We are studying this phenomenon using immunoelectron microscopy of glutamic acid decarboxylase immunoreactivity (GAD, the enzymatic precursor for GABA) at selected postnatal ages and in adults with emphasis on short survival times of 4-7 days after OB ablation. Normally GAD-labeled synaptic terminals form type II symmetric contacts onto unlabeled dendrites and GAD-labeled dendrites receive type I, asymmetric contacts from unlabeled terminals (Westenbroek, et al., 1988a). The OB lesion results in degenerating terminals with type I contacts onto unlabeled and onto GAD-labeled dendrites. Type I postsynaptic sites may be seen partially contacted by or entirely devoid of degenerating terminals and occasionally may be apposed to variable degrees by normal unlabeled or by GAD-positive terminals. Subsequently, some GAD-labeled terminals may form asymmetric type I contacts usually with unlabeled dendrites and rarely with GAD-labeled dendrites. The findings are most common in the youngest subjects and essentially absent in the adult subjects. A sequence of reinnervation of deafferented type I sites by GAD-labeled terminals is suggested for the formation of this "atypical" synapse and the sequelae of this reorganization are discussed.

Transmitter release: prepackaging and random mechanism or dynamic and deterministic process

Stepwise variations in end-plate potential amplitudes that are also multiples of spontaneous miniature end-plate potentials (MEPPs) demonstrate a quantal nature of evoked transmitter release at the vertebrate neuromuscular junction. Both the number of quanta which form relatively small end-plate potentials (EPPs) and the time intervals between MEPPs were found to fit Poisson statistics. These observations suggested that the release process randomly liberates uniform quantities of transmitter. Initial studies showed that quantal size remained stable after seemingly high rates of release which was interpreted to indicate that a large store of equally sized, equally available, and independently releasable quanta are present in the nerve terminals. The observation of numerous presynaptic vesicles that contain transmitter provided a morphological basis for prepacked transmitter (i.e., quanta). However, physiological studies over the last 15 years have yielded data that are difficult to incorporate into the quantum-vesicle hypothesis. With normal conditions and during most treatments which increase the rate of release, two classes of MEPPs have been found and both show a substructure. The bell-MEPP class was characterized by Fatt and Katz and the smaller skew-MEPP class has been studied by Kriebel. The ratio of the two classes and substructure compositions of both classes are variable. Short series of MEPPs and unitary EPPs (U-EPPs) show preferred amplitudes and longer series of MEPPs and U-EPPs show stepwise variations in amplitude. Slow-MEPPs and giant MEPPs belong to the skew class and represent nearly synchronous bursts of smaller MEPPs. Transmitter packet formation, preferred amplitudes, stepwise variations in amplitudes, random-like distributions and organized bursts can be simulated by a simple deterministic system, the drop formation process, that is known for its periodic and chaotic behaviors which are determined by the single parameter of flow rate. MEPP intervals, sizes and classes, are also dependent on rates of release which demonstrate that the release process(es) is highly organized and sensitive to different conditions. We demonstrate that the processes of drop formation and release of a packet of transmitter have similar properties and that deterministic characteristics describe MEPP and U-EPP time dependencies and amplitude substructures. The data and model presented here suggest that packet size of acetylcholine may be determined at the moment of release.

The use of specimen tilt in transmission electron microscopy of the central nervous system

Thin sections of nervous tissue were viewed at different tilt angles using a transmission electron microscope equipped with a eucentric goniometer stage. In a comparison study of various degrees of tilt, one can observe additional morphological features within synaptic profiles, define subsynaptic structures such as Taxi-bodies, and clearly see the crystalline formation of cytochemical tracers. This study demonstrates the value of tilting thin-sections in the analysis of synapses and other biological material at the ultrastructural level.

Ultrastructural localization of immunoreactivity in the developing piriform cortex

The purpose of this study was to determine the ultrastructural basis for the immunoreactivity patterns in synaptic structures during development in layers I and II of the piriform cortex (PC) of rats. Antisera to cholecystokinin (CCK) and glutamic acid decarboxylase (GAD) were used at several different postnatal days (PN) and in adults to describe the distribution, characteristics, and relative frequency of labeled profiles--especially axons and terminals--with emphasis on details of the synaptic contacts. GAD-positive terminals occur from PN 2 to adulthood but only form contacts in deeper sublayers (Ib and II) initially. Contacts increase in layer I after PN 6 and are reduced in layer II after PN 21 when the GAD-labeled terminals and synapses take on adult features with flattened vesicles and symmetric contacts. CCK-labeled terminals are present in deeper sublayers at PN 2 but are few and rarely form contacts. Both terminals and contacts increase between PN 2 and 9, taking on distinctive shapes and vesicle morphology by PN 13. At PN 21 and older, CCK terminals have mainly flattened vesicles and mostly form symmetric contacts onto dendrites and somata in deeper layers (Ib and II). Superficial sublayer Ia has very few CCK-labeled synapses and axons. Thus immunoreactivity occurs in terminals prior to synapse formation; labeling of the presynaptic specializations precedes subsequent maturation; synaptic vesicle morphology and membrane specializations are similar for the vast majority of both CCK and GAD terminals; inhibitory (GABA) synapses are established sooner than the possibly excitatory CCK synapses; a deep to superficial gradient of synaptogenesis is associated with GAD-positive terminals in the PC; and the labeling patterns may be related to critical developmental or synaptogenic periods.

Ultrastructure of synaptic remodeling in piriform cortex of adult rats after neonatal olfactory bulb removal: an immunocytochemical study

The purpose of this investigation was to study possible remodeling in synaptic structures of the piriform cortex (PC) of adult rats following neonatal deafferentation by removal of the olfactory bulb (OB) at birth. Emphasis was placed on possible qualitative changes in the ultrastructure and immunocytochemical localization of cholecystokinin (CCK, a possible excitatory neurotransmitter or modulator) and glutamic acid decarboxylase (GAD, precursor enzyme to the inhibitory transmitter GABA) in axons, terminals, and synaptic complexes. Light microscopic results in normal adult material show that GAD-positive terminals form a dense band subjacent to the lateral olfactory tract (LOT), become less dense in deeper Ib, and are rare in layer II. Following deafferentation, GAD-positive terminals appear denser and more homogeneously distributed throughout layer I and are also more prevalent in layer II. Ultrastructural results of normals and controls indicate GAD-positive terminals normally contain pleomorphic or flattened vesicles and form symmetric contacts onto dendritic shafts and branches throughout layer I. In deafferented layer I not only do there appear to be greater numbers of symmetric GAD-positive contacts, but in contrast to normals, asymmetric contacts mainly onto spines are now present. Light microscopic results from deafferented material also show an apparent proliferation with spread or sprouting of CCK-positive fibers or axonlike structures mainly into layer Ia, whereas these fibers are normally observed only in the LOT and are generally few in number. Also in normals the few CCK-positive terminals in the area subjacent to the LOT contain flattened or pleomorphic vesicles and form symmetric contacts. Deafferentation results in CCK-positive terminals throughout layer I with a greater frequency of synaptic contacts which now also include a few asymmetric contacts onto spines. The findings clearly show modifications in synaptic patterns of immunocytochemical-labeled terminals that might be compatible with the process of atypical reinnervation of deafferented postsynaptic sites and possible ingrowth of new axons.

Ultrastructure of degenerative changes following ricin application to feline dental pulps

The ultrastructure of degenerative changes within the ipsilateral trigeminal ganglion, and partes caudalis and interpolaris of the spinal trigeminal nucleus in the cat is described following the application of the potent toxin ricin to the tooth pulps of unilateral maxillary and mandibular posterior teeth, including the cuspids. Survival times ranged from 6 to 10 days. Typical changes identified within the ipsilateral trigeminal ganglion included myelin fragmentation and 'compartmentalization' of the axoplasm of medium-sized myelinated axons, while small myelinated and unmyelinated axons underwent a more variable response ranging from electron-lucent to electron-dense changes. The affected cell body was characterized by the presence of swollen, electron-lucent mitochondria, a reduction of cytoplasmic ribosomes and a filamentous hyperplasia. Other changes often included an eccentric nucleus and satellite cell proliferation. Degenerative changes often occurred in isolated elements surrounded by normal profiles, suggesting specificity of ricin within the trigeminal ganglion. Changes within brainstem axons showed both an electron-dense and a lucent, fragmenting type of axonal alteration. Terminal changes ranged from electron-dense to lucent and also included filamentous hyperplasia and 'hyperglycogenesis'. The altered axonal knobs contained round synaptic vesicles that were presynaptic to dendritic profiles and postsynaptic to terminals containing flattened synaptic vesicles. The above brainstem alterations were identified specifically in the following areas: ventrolateral, medial and dorsomedial pars interpolaris; the ventrolateral and mid-dorsal to dorsomedial areas of the marginalis and outer substantia gelatinosa layers of pars caudalis; and in ventral pockets corresponding to lamina V of the medullary dorsal horn. Dense alterations within terminals containing flattened synaptic vesicles that are typically presynaptic to primary afferents in these areas were rare findings, but along with vacuolization of dendritic profiles suggest a trans-synaptic effect possibly due to the exocytosis of ricin. The results are discussed in relation to different reports of dental projections and with regards to patterns of transganglionic degeneration.

New observations on the substructure of the active zone of brain synapses and motor endplates

This study offers a new concept on the origin and function of the hitherto enigmatic presynaptic dense projections (dps) of neurons and motor endplates. After a deuterium oxide-albumin pretreatment (da), brain tissue and motor endplate of rat and frog reveal an intricate association of smooth endoplasmic reticulum (ser), microtubules (mts) and synaptic vesicles (sv) at the presynaptic grid-active zone of synapses. The ser entwines the mts, which are clothed in svs, and impinges directly onto the presynaptic membrane as sacs or 'tubular-fibrillar' extensions. Since no dps are seen in these sections, whereas they do occur in conventionally processed material (i.e. without da pretreatment), it is suggested that the dps of conventional material may, in part, originate from improperly fixed ser at these points. Thus for the first time we demonstrate an in vivo system of ser which, because its 'finger' processes come into intimate contact with the presynaptic membrane, may be implicated in Ca2+ ion translocation, presumably out of the presynaptic bulb. Since no such tubular ser has been demonstrated in what are claimed to be sophisticated techniques (i.e. high-speed slam-freezing-freeze substitution) the actual sophistication of such methods is questioned.

Microtubule organization and synaptogenesis in the vestibular sensory cells

Microtubule organization in type I hair cells has been investigated during the synaptogenesis of vestibular receptors in mammals. The different steps in the maturation of the synapse between the hair cell and the nerve chalice were: a slight symmetrical membrane densification; the appearance of synaptic bodies alongside microtubules closely associated with densified presynaptic membranes; the disappearance of synaptic bodies and the persistence of microtubules. During this development, microvesicules were never seen to be associated with microtubules.

A morphometric study of the effects of maturation and aging on synaptic patterns in the spinal trigeminal nucleus of the cat

Morphometric methods have been used to study the synaptic and terminal patterns in cat trigeminal nucleus, pars interpolaris, during development and aging. Ages 1, 3, 6, 11, 16, 21, 27, 110, 600 days and 8 and 11 years were studied. Both proportions and densities (number per unit area) of certain terminals and synapses showed significant changes with age. Axoaxonic synapses especially showed two major periods of increase (3-6 days and 21-27 days). The values of most parameters increased in the 21-27 day period to peak levels and then decreased gradually with age. The results indicate two separate critical synaptogenic periods of development and a loss of synaptic elements in aging. Factors contributing to these changes are discussed as is the potential for plasticity in the different afferents at each period.

Ultrastructure of transganglionic HRP transport in cat trigeminal system

The ultrastructure of transganglionic transport of horseradish peroxidase (HRP) from the inferior alveolar (IA) nerve to the brainstem is being studied in the cat. The IA nerve was soaked in an HRP solution and following a two-day survival the animal was perfused transcardially with a paraformaldehyde-glutaraldehyde solution. The tissue was immediately dissected and postfixed for 1-3 h in perfusate. Sections of 75 micron thickness were cut with a Vibratome and reacted utilizing tetramethyl benzidine (TMB) as the chromagen. Optimum results for electron microscopy were obtained by osmication in a pH 6.0, 1% osmium tetroxide solution for 45 min at 45 degrees C, followed by rapid dehydration and embedment in Epon. The resulting HRP-TMB reaction product was characterized and identified ultrastructurally in ganglion cells, peripheral and central axons and in brainstem terminals. The HRP-TMB reaction product varied in density but had consistent crystalline-like laminations of a repeating unit and characterized by a membrane 4-5 nm in diameter. Some of the HRP-TMB reaction product found in terminals and axons was below the limit of resolution of the light microscope.

Effects of ethanol on the rat brain: ultrastructural alterations in the temporal cortex and in the hippocampus

Male rats were submitted either to an oral alcohol intoxication or to chronological aging. Nervous morphometry shows that chronic alcohol consumption induces an increase in the proportion of neurons with dense cytoplasm and an increase of the synaptic cleft affecting principally synapses with spherical vesicles. The cerebrovascular morphometry revealed that the vascularity enhances with chronic alcohol consumption in young animal. The same enhancement is observed in aged animals showing thus a parallelism between alcoholised and aged animals.

Asymmetric and symmetric synaptic junctions in the dorsal lateral geniculate nucleus of cat and monkey

We have investigated the applicability of traditional classifications of synaptic junctions in the dorsal lateral geniculate nucleus (DLGN) of the cat and monkey. Our principal sample is restricted to synapses made by the retinal terminal (round vesicles) in DLGN and to synapses made by flattened vesicle processes that were postsynaptic to the retinal terminal. We inspected consecutive thin sections through 250 synaptic junctions that showed clear synaptic clefts in every section. The thickness of the membrane and postsynaptic density (PSD) was measured on each section and an average thickness was computed for each synaptic junction. In both species the frequency distribution for these measurements forms an uninterrupted progression from the absence of a continuous PSD through the presence of a heavy density with most synaptic contacts falling in the midrange. Twenty-two of the round vesicle profiles and 40 of the flat vesicle profiles we studied had very modest densities (13-16 nm) and exhibited a continuous PSD on some sections, but only small puffs or a complete lack of density on others. We concluded that this group which constituted 25% of the synaptic contacts we studied could not be classified as asymmetric or symmetric. As a group the round vesicle synaptic junctions exhibited a heavier PSD than the flat vesicle contacts. The difference between the mean thickness in both species was statistically significant. However, we hesitate to describe the round vesicle synapses as asymmetric and the flat vesicle contacts as symmetric because such a large proportion of the former made synaptic contacts with a PSD thickness within the range of the latter.

The in-series and in-parallel components in rat hindlimb tendon organs

The structure of the tendon organs was studied in the shank muscles of adult rats both under the light- and electron-microscope. The rat tendon organs measure on the average about 500 microns in length and 60 microns in diameter. Most tendon organs are surrounded by muscle fibres and their short individual tendons, and insert into the aponeuroses or intramuscular tendons. Each tendon organ consists of a neurotendinous core composed of collagen bundles that represent tendons of 5-10 muscle fibres; it is innervated by a Ib sensory fibre that branches and terminates among the loose collagen fascicles of the core. Sensory terminals are oriented both transversely and longitudinally. Their position and relation to collagen bundles indicate that, during tendon organ activation, the terminals are probably depolarized both by lateral compression and elongation. The core is enclosed in a capsule that consists of about 5 lamellar layers of capsular cells and closely resembles the perineurium. The majority of the tendon organs also comprise a purely tendinous compartment in the lumen or within the capsular wall. These tendinous components remain separated from the neurotendinous core and do not come into contact with axon terminals. The collagen fibrils of the tendinous compartments are densely packed and larger in diameter than those of the neurotendinous core. The sensory terminals of the tendon organ lie in series with those muscle fibres and collagen bundles that constitute the neurotendinous core, but they are in parallel with the purely tendinous tendon organ components and their respective muscle fibres. Thus, one tendon organ may comprise both in series and in parallel components, which is apparently reflected in its function. It is suggested that the purely tendinous tendon organ compartments account for the in-parallel effects upon the function of tendon organs described in some recent electrophysiological studies.

Immunocytochemical evidence for tubulin in the presynaptic terminal of synaptosomes

Ultrastructural studies of intact tissue rarely show presynaptic microtubules, and immunocytochemical studies on tissue sections have previously been unable to demonstrate tubulin in the nerve terminal. In contrast, a microtubular coil can be readily detected in the presynaptic nerve terminal of synaptosomes. We have developed an immunocytochemical procedure on the synaptosome preparation and demonstrated, using monoclonal antibodies, that in the presynaptic terminal alpha and beta tubulin subunits are specifically restricted to the equatorial microtubular coil.

Neurotransmitter release mechanisms and microtubules

The morphological mechanisms involved in translocation of the synaptic vesicle to the presynaptic membrane, release of transmitter from the vesicle and recycling of the vesicle membrane are still far from understood. However, there is strong evidence that vesicles move along the surfaces of a specific set of highly labile presynaptic microtubules that direct the vesicles to the active zones. These microtubules are focused in a precise geometrical array, which is in register with and in contact with presynaptic dense projections of the central nervous system synapse or presynaptic dense bars of the motor endplate. These dense complexes constitute the presynaptic grid or active zones. The regular arrays of dense projections or bars are in turn coincident with rings or chains of synaptic vesicles mobilized at release sites on the presynaptic membrane (having arrived at these precise points by microtubule translocation). Thus it is suggested that the presynaptic microtubules not only translocate synaptic vesicles, but because of their ordered arrays determine, in ontogeny, the ordered structure of the presynaptic grid.

The enigma of microtubule coils in brain synaptosomes

When synaptosomes are prepared from rat brain and incubated in Krebs solution, the presynaptic bulb develops a coil of microtubules (mts). Various considerations indicate that the coil does not have a cytoskeletal supportive function. Synaptosome coil mts show certain peculiarities, e.g. they thrive during incubation in Krebs solution (dendritic mts are depolymerized in Krebs solution) and they show no protofilament molecular substructure with tannic acid. Dendritic mts show clearly a 13 protofilament substructure when processed in the same way. Synaptosomal coil mts are sensitive to micromolar calcium and are depolymerized by treatment of the synaptosomes with veratridine or A23187. Our evidence indicates that coil mts of synaptosome and synaptic vesicle clothed mts of 'intact' albumin-treated synapses are different morphological and functional entities. As mentioned above, the function of coil mts remains enigmatic, while the mts seen in albumin-treated synapses could well have a role in synaptic vesicle translocation.

Cross-linking of synaptic vesicle proteins. Effect of ATP

Synaptic vesicles isolated from bovine brain were subjected to cross-linking with the bifunctional amino group reagent dimethyl adipimidate. The resulting proteins were analyzed by SDS-polyacrylamide gel electrophoresis. The reagent (10 mM) caused partial or complete disappearance from the SDS gel of most of the major polypeptides of the vesicles and the formation of new polymeric species with molecular weights greater than 500000. Using lower concentrations of the adipimidate (2-5 mM) a more selective cross-linking occurred, with the disappearance of a group of protein bands having apparent molecular weight values of 60000-68000, 40000-41000 and 25000-30000. The extent of cross-linking was independent of vesicle concentration in the range 0.3-3.0 mg protein per ml. Addition of ATP or AMP to the cross-linking reaction mixture resulted in a marked reduction in cross-linking of all of the major vesicle polypeptides (apparent molecular weight values 160000, 77000, 55000, 42000, 32000, 28000 and 26000). Several proteins were less affected by ATP or AMP; these were mostly the same vesicle proteins as those which had become cross-linked with low concentrations (2 mM) of dimethyl adipimidate. The ATP effect was markedly reduced if the vesicles were pretreated prior to the cross-linking reaction with alkaline buffer (pH 8.5) in either the presence or the absence of ATP. In the presence of 32P-labeled ATP, several of the vesicle protein bands became phosphorylated, but the extent of their cross-linking did not depend upon the state of phosphorylation of the major phoshorylated proteins. The results are consistent with the presence of aggregated protein complexes and of stabilized arrays of the major proteins within the vesicle membrane.

Fine structural analysis of the synaptic junction of Merkel cell-axon-complexes

The ultrastructure of synaptic contact areas in Merkel cell-axon-complexes from sinus hair follicles and touch domes of various mammals (nude mice, rats, cats, rabbits, opossums and monkeys) was investigated by electron microscopy of ultrathin sections from perfusion fixed tissue. Synapses between Merkel cells and axons were a common feature in all analyzed species. Special staining with digallic acid and goniometric tilting facilitated the resolution of the membranous and paramembranous synaptic elements. The synaptic contact revealed the typical characteristics of a chemical synapse, except for presynaptic clear vesicles: a postsynaptic membrane thickening and dense projections at the presynaptic membrane (i.e., the Merkel cell membrane). The cleft material was resolved as a fuzzy coating of the outer leaflets of the synaptic membranes with occasional bridges across the synaptic cleft. The presence of a synapse in the Merkel cell-axon-complexes emphasizes the receptor function of the Merkel cell besides other possible functions of this cell.

Cold stable microtubules in brain studied in fractions and slices

Microtubules were shown to remain intact in brain slices and subfractions maintained at 0 degrees C for 1 h. Under the same conditions, microtubules isolated from brain by warm assembly-cold disassembly methods, disassemble into their constituent subunit proteins. No selective depletions of microtubules were seen when brain slices were incubated in homogenizing buffer at either 0 degrees C or 37 degrees C. The response of native microtubules in brain slices in incubation in other solutions showed that their properties were otherwise the same as those of assembled microtubules. The separated alpha and beta subunits of isolated cold labile and cold stable microtubules were compared by electrophoresis and isoelectric focusing and were shown to possess the same mobilities. The results suggest that native microtubules are temperature insensitive and that isolated microtubules are assembled from pre-existing pools of subunit proteins. The results further suggest that native microtubules possess a factor, lacking in isolated assembled microtubules, which confers temperature stability on the former.

Microtubules, dendritic spines and spine appratuses

Using techniques for enhanced microtubular preservation, including albumin pretreatment (Gray, 1975), occipital cortex of rats was studied electron microscopically at various ages of development. A close structural relationship was seen between microtubules, sacs of SER and the postsynaptic "thickening" in primordial spines and with the dense "plate" material of spine apparatuses. Stereoscopic preparations in addition show a more complicated substructure than previously described for the "plate". Microtubules may contribute to the formation of the "plate" of the spine apparatus which in turn is associated with the postsynaptic "thickening" of the mature spine. Possible functional correlates are discussed.

An ultrastructural study of nerve and glial cells by freeze-substitution

The ultrastructure of nerve and glial cells in the cerebral and cerebellar cortices of mice was studied after rapid freezing followed by substitution fixation. The cerebral and cerebellar cortices were frozen by bringing them into contact with a polished pure copper block cooled at a temperature of about -196 degrees C. The tissues were fixed and substituted in acetone containing 2-4% OsO4 at -78 degrees C for 2-3 days and then prepared for electron microscopy. Tissue fixed by this method displayed the following characteristics. (1) The contour of cells, processes and intracellular membrane systems was smooth. (2) The extracellular spaces were of variable widths. (3) Microtubules were well preserved and were often observed to extend into nerve terminals and to run close to presynaptic membranes. (4) The matrix of cytoplasm and mitochondria was electron dense. Dense granules, possibly binding sites of divalent cations, were often found in the mitochondrial matrix. (5) The plasma membrane of neuronal processes was thicker than that of glial processes. (6) The plasma membranes of nerve fibres and glial processes appeared asymmetrical, the inner leaflet being slightly thicker than the outer leaflet, whereas membranes of cell organelles such as smooth endoplasmic reticulum. Golgi bodies, lysosomes, multivesicular bodies, mitochondria and synaptic vesicles, were symmetrical.

Aggregations of synaptic vesicles on the exposed inner membrane of presynaptic mitochondria in brain

Fragments of rat cerebral cortex have been incubated under various conditions. When divalent cations are present, patches of the external membrane of some mitochondria are disassembled leaving the inner mitochondrial membrane exposed to the cytoplasm. Sometimes the entire external membrane is missing. In presynaptic bulbs the synaptic vesicles are attracted and adhere to the exposed outer face of the inner mitochondrial membrane. The mode of attraction and adhesion of the vesicles is discussed. Possibly this could serve as a model for further investigation of the attraction of vesicles to the active zone of the presynaptic membrane.

Morphological changes in afferent vestibular hair cell synapses during the postnatal development of the cat

In the course of postnatal development in the cat, there is a decrease of about 93% in the total number of synaptic bodies (synaptic balls and synaptic bars) in type I hair cells. In type II hair cells, there is no change in the number of synaptic balls. Simultaneously, the length of specialized neuroepithelial contact increases by approximately 300% during type I hair cell maturation. Only the synaptic bars displaying a polylamellar ultrastructure persist in the type I hair cells of the adult animal. It is suggested that the afferent vestibular synapses of the type I hair cell are transformed during ontogeny.

Paramembranous densities of 'C' terminal-motoneuron synapses in the spinal cord of the rat

A category of large boutons forming synapses with the soma and proximal dendrites of spinal motoneurons was studied in glutaraldehyde-fixed, non-osmicated tissue stained with uranyl acetate and lead citrate. The identity of these boutons with 'C' boutons was indicated by their shape, frequency and distribution as well as by the ultrastructural characteristics of the boutons and the associated postsynaptic structures. In contrast to previous descriptions based on osmicated tissue, this study demonstrates that paramembranous densities are a feature of 'C' terminal-motoneuron synapses.

Synaptic vesicles and microtubules in frog motor endplates

Motor endplates of the cutaneous pectoris skeletal muscle of the frog have been examined by electron microscopy using a new technique. This involves pretreatment with an albumin solution, followed by fixation with 4% unbuffered tetroxide. A small proportion of the endplate axonal ramifications show microtubules clothed in synaptic vesicles and focused on the presynaptic membrane, in particular on the active zones. The microtubules run in the presynaptic cytoplasm either parallel to or across the active zones. These two sets of microtubules cross each other at the active zones, which lie opposite the dips in the post-junctional folds. The possibility that the microtubules are involved in the translocation of synaptic vesicles to the active zone is discussed.

An association between mitochondria and microtubules in synaptosomes and axon terminals of cerebral cortex

A substantial number of synaptosomes prepared from rat cerebral cortex were found by electron microscopy to contain a horseshoe-shaped mitochondrion flanked by an arc of three to ten microtubules opposite the synaptic membrane specializations. The microtubules were replaced by characteristic paracrystals following the incubation of synaptosomes with vinblastine sulfate. A similar spatial organization of microtubules and mitochondria was also observed in some axon terminals of the cerebral cortex in situ. The significance of this novel observation is discussed with regard to the role of microtubules in axonal transport at the synaptic terminal and previous reports on the identification of tubulin in synaptosomes and subsynaptic structures.

Microtubules in synapses of the retina

Using a new method, microtubules can be seen running up to, and lying in close relationship with, the synaptic ribbons in the outer and inner plexiform layers of the frog retina. In the inner plexiform layer microtubules can be seen running up to the terminal membrane in the non-ribbon synapses. Unlike non-ribbon C.N.S. synapses (frog and rat) processed by the same method. There is no clear association between synaptic vesicles and microtubules in the approach regions.

Microtubules associated with nuclear pore complexes and coated pits in the CNS

Using a new albumin prefixation technique, microtubules have been observed in close association with the nuclear pores of neurons and glia. Thus, microtubules may be involved in such phenomena as anchoring, migration or rotation of the nucleus or in chemical messenger transport between nucleus and cytoplasm. Microtubules are also seen running close to the coated pits of dendrites. The implications are discussed.