Changes in motor nerve terminals during bupivacaine-induced postsynaptic deprivation

The local anaesthetic agent bupivacaine induces a rapid degeneration of muscle cells when injected in the sternocleidomastoid muscle of the adult rat with no damage to intramuscular nerves. Nevertheless, silver impregnation and electron microscopy reveal a sequence of retraction and sprouting of the motor nerve endings during the period of the bupivacaine-induced muscle cell deprivation by necrosis. These morphological changes seem to be qualitatively similar to the retraction and growth phenomena described as forming part of the remodelling process affecting normal untreated neuromuscular junctions of the adult amphibia and mammals. The results are discussed in the context of the regulatory mechanisms governing the maintenance of motor nerve terminals.

The cytoskeletal architecture of the presynaptic terminal and molecular structure of synapsin 1

We have examined the cytoskeletal architecture and its relationship with synaptic vesicles in synapses by quick-freeze deep-etch electron microscopy (QF.DE). The main cytoskeletal elements in the presynaptic terminals (neuromuscular junction, electric organ, and cerebellar cortex) were actin filaments and microtubules. The actin filaments formed a network and frequently were associated closely with the presynaptic plasma membranes and active zones. Short, linking strands approximately 30 nm long were found between actin and synaptic vesicles, between microtubules and synaptic vesicles. Fine strands (30-60 nm) were also found between synaptic vesicles. Frequently spherical structures existed in the middle of the strands between synaptic vesicles. Another kind of strand (approximately 100 nm long, thinner than the actin filaments) between synaptic vesicles and plasma membranes was also observed. We have examined the molecular structure of synapsin 1 and its relationship with actin filaments, microtubules, and synaptic vesicles in vitro using the low angle rotary shadowing technique and QF.DE. The synapsin 1, approximately 47 nm long, was composed of a head (approximately 14 nm diam) and a tail (approximately 33 nm long), having a tadpole-like appearance. The high resolution provided by QF.DE revealed that a single synapsin 1 cross-linked actin filaments and linked actin filaments with synaptic vesicles, forming approximately 30-nm short strands. The head was on the actin and the tail was attached to the synaptic vesicle or actin filament. Microtubules were also cross-linked by a single synapsin 1, which also connected a microtubule to synaptic vesicles, forming approximately 30 nm strands. The spherical head was on the microtubules and the tail was attached to the synaptic vesicles or to microtubules. Synaptic vesicles incubated with synapsin 1 were linked with each other via fine short fibrils and frequently we identified spherical structures from which two or three fibril radiated and cross-linked synaptic vesicles. We have examined the localization of synapsin 1 using ultracryomicrotomy and colloidal gold-immunocytochemistry of anti-synapsin 1 IgG. Synapsin 1 was exclusively localized in the regions occupied by synaptic vesicles. Statistical analyses indicated that synapsin 1 is located mostly at least approximately 30 nm away from the presynaptic membrane. These data derived via three different approaches suggest that synapsin 1 could be a main element of short linkages between actin filaments and synaptic vesicles, and between microtubules and synaptic vesicles, and between synaptic vesicles in the nerve terminals.(ABSTRACT TRUNCATED AT 400 WORDS)

Modification of diurnal changes in the ultrastructure of synaptic ribbons of the turtle

Synaptic ribbons of rod cells of the red-eared turtle under light-dark (LD) cycle, continuous dark (DD) and continuous light (LL) were examined by electron microscopy. In the LD cycle, several stick-shaped (plate-shaped, three-dimensionally) ribbons formed multilayered ribbon complexes at 1000 hr, and they turned into aggregates of rounded ribbons at 1900 hr (1 hr after light-off). In DD, stick-shaped ribbons were situated singly near presynaptic membranes. Large ribbon complexes consisting of 7-8 parallel layers, as shown in the LD cycle, were not observed. Aggregates of rounded ribbons, or club-shaped ones could not be observed. In LL, aggregates of irregular ribbon fragments were noted. None of the ribbons in this regimen showed sharp contours. Cyclic LD illumination is necessary to form multilayered ribbon complexes and rounded ribbons. Internal circadian rhythm contributes little part to the diurnal changes in the structure of the synaptic ribbons.

Ultrastructural localization of GABA-immunoreactive terminals in the anteroventral cochlear nucleus of the guinea pig

The immunocytochemical distribution of gamma-aminobutyric acid (GABA) was studied by electron microscopy in the anteroventral cochlear nucleus (AVCN) of the guinea pig using affinity-purified antibodies made against GABA conjugated to bovine serum albumin. Our observations confirm that spherical cells are the predominant cell type in the guinea pig AVCN and receive numerous axosomatic contacts (Schwartz and Gulley, (1978) J. Anat. 153, 489-508). Stellate cells receive few axosomatic contacts. Electron microscopic immunocytochemistry shows that GABA immunoreactivity is present in synaptic terminals in the AVCN. Of the several classes of presynaptic terminals present in the AVCN as characterized by vesicle type (large round; oval/pleomorphic; flat; small round) only those containing oval/pleomorphic vesicles were GABA-immunoreactive. However, GABA immunoreactivity may not be present in all these terminals because some oval/pleomorphic terminals are unlabelled. Immunoreactive terminals are widespread in the AVCN; they are abundant on spherical cell bodies, rarely seen on stellate cell bodies and are also found scattered throughout the neuropile.

Deafferentation-induced alterations in the rat dorsal horn: I. Comparison of peripheral nerve injury vs. rhizotomy effects on presynaptic, postsynaptic, and glial processes

Light microscopical degeneration and ultrastructural alterations in the rat spinal dorsal horn were studied following either cutting of the sciatic nerve or rhizotomy at L4 and L5; survival time for both procedures was 3 weeks. Fink-Heimer silver methods showed minimal degeneration of afferent central processes after sciatic section, and limited ultrastructural changes were present. Both rhizotomy and nerve section resulted in degenerating terminals. Most were swollen and electron lucent, with loss of vesicles; some electron-dense terminals were present, particularly after rhizotomy. Both procedures also produced significant degeneration of postsynaptic dendrites and soma, evidenced by either increases in electron density, or loss of organelles and cavitation, or accumulation of osmiophilic floccular material. Glial processes frequently were expanded and extended to engulf single degenerating terminals and dendrites, or terminal-dendrite units; in other cases glial tongues separated terminals from their postsynaptic dendrite. Glial processes often wrapped around degenerating profiles or groups of profiles in several layers, sometimes forming complex labyrinths. These results confirm past descriptions of pre- and postsynaptic changes resulting from peripheral nerve section, but newly reveal that dendritic destruction and increased glial activity are also significant following rhizotomy. Documentation of these changes is relevant for studies of reorganization following nerve and spinal cord damage, as well as providing an ultrastructural basis for evaluation of effects of neurotoxins that affect primary afferents, as described in a companion paper.

[Cytochemical detection of actin in the structure of the synaptic apparatus of hippocampal field CA3]

Ultrastructural distribution of actin in dendrites, dendritic spines and presynaptic boutons of the hippocampal area CA3 of the guinea pig was investigated using decoration and immunocytochemical methods. The distribution of actin was non-homogeneous in all the parts of neurons. The highest concentration of this contractile protein was revealed in the spine cytoplasm. Here actin forms a dense cytoskeleton meshwork and is present also in postsynaptic densities. An intimate interaction between the spine actin cytoskeleton and the postsynaptic actin densities has been revealed. This feature may indicate the involvement of actin cytoskeleton in the organization and maintenance of dimensions, location and geometry of active zones.

The glycine receptor deficiency of the mutant mouse spastic: evidence for normal glycine receptor structure and localization

Homozygotes of the mutant mouse spastic exhibit reduced binding of 3H-strychnine to homogenates from various regions of the CNS compared with unaffected littermates (White and Heller, 1982). Here we report evidence that the spastic mutation coincides with a reduced concentration and an unaltered structure of the glycine receptor in spinal cord. Scatchard analysis of 3H-strychnine binding revealed a single binding site with a Bmax of 267 +/- 62 fmol/mg protein for spastic and of 864 +/- 220 fmol/mg protein for control mice; no difference was found for the corresponding KD values. Also Ki values of glycine for 3H-strychnine binding and displacement of 3H-strychnine by beta-alanine and taurine were indistinguishable for both preparations. Photoaffinity labeling of synaptic membranes with 3H-strychnine identified an Mr = 48,000 polypeptide in both control and spastic mouse membranes. Tryptic digestion of these membranes produced radiolabeled peptide fragments of identical molecular weights, suggesting that the proteolytic cleavage sites around the antagonist binding site are conserved in the mutant glycine receptor protein. Glycine receptors from both control and mutant mice were purified by affinity chromatography on aminostrychnine agarose. SDS/PAGE revealed three polypeptides of Mr = 48,000, 58,000, and 93,000 in both receptor preparations. Monoclonal antibodies directed against different subunits of the glycine receptor were applied to an enzyme-linked immunosorbent assay. The same pattern of immunoreactivity was obtained for glycine receptor from spinal cord of spastic homozygotes, control mice, and rats, suggesting conservation of the antigenic epitopes in the mutant receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of the amino acids of substance P in the recognition of its receptor: affinities of synthesized SP analogues for the specific 125I-BHSP binding site on rat brain synaptosomes

Substance P analogues have been synthesized, by solid-phase methodology, in order to get a better knowledge of the structural requirements for the 125I-BHSP binding on rat brain synaptosomes. Assuming that the core of SP exists in an alpha-helicoidal structure three major points should be underlined: the SP receptor recognizes probably the side of the helix bearing the two side chains of Phe and Phe; the arginine guanidinium interacts with either a carboxylate or a phosphate function of the binding site; the C-terminal tripeptide undergoes a conformational change allowing the interactions of the C-terminal amide with a carboxylate and that of the sulfur atom with an electrophile of the binding site. The specificity of these peptides have been further estimated by comparing their binding potencies to those observed for the 125I-BHE specific binding on rat cortical synaptosomes and their bioactivities on guinea-pig ileum.

Ultrastructural correlates of transmitter release in presynaptic areas of lamprey reticulospinal axons

The ultrastructure of presynaptic areas of lamprey reticulospinal axons was studied before, during, and after periods of elevated transmitter release produced either by repetitive action potential activity or depolarization by elevated extracellular potassium. Controls for possible effects of these procedures per se were done by replacing extracellular Ca with Mg to block transmitter release. In some experiments the time course of ultrastructural changes during K depolarization and subsequent recovery were studied by fixing tissue samples at various times. Transmitter release produced by action potential activity (20/sec for 15 min) in the presence of extracellular Ca significantly and reversibly decreased the number of synaptic vesicles, the area occupied by the vesicles, and the density of synaptic vesicles. An unexpected finding was a reversible decrease in the length of the differentiated membrane during periods of increased transmitter release. Transmitter release significantly and reversibly increased the number of coated vesicles, expanded the presynaptic membrane, and increased the number of pleomorphic vesicles. K depolarization (50 mM K for 15 min) produced identical, reversible effects, except that the expansion of the presynaptic membrane, although significant, was relatively small and there was no change in the number of pleomorphic vesicles. Raising the temperature of the saline from 2 degrees C (K depolarization experiments) or 7 degrees C (action potential experiments) to 20 degrees C did not change the results qualitatively but did produce somewhat larger effects during stimulation and appeared to increase the speed of recovery. Action potential activity or K depolarization in control experiments with the Ca in the saline replaced by Mg had little or no effect on synaptic ultrastructure. Synaptic vesicles in lamprey reticulospinal axons never contacted the axonal membrane anywhere other than at the differentiated membrane. During periods of elevated transmitter release, although the absolute number of vesicles in contact with the differentiated membrane decreased, the percentage of total vesicles in contact with the differentiated membrane increased dramatically. This suggests that the differentiated membrane is the site of vesicle release and there is an active process of vesicle movement to this membrane. In the course of this work it was observed that presynaptic areas closer than approximately 2 mm to the site of axonal transection, regardless of the composition of the saline or the experimental conditions, showed ultrastructural changes typical of increased transmitter release.(ABSTRACT TRUNCATED AT 400 WORDS)

Fine structure of synapses of the central nervous system in resinless sections

The cytoskeleton has been implicated in neuronal function, particularly in axonal transport, excitability at axonal membranes, and movement of synaptic vesicles at preganglionic endings. The present study demonstrates the presence of a pre- and postsynaptic cytoskeleton in resinless sections of CNS tissue by use of the polyethylene glycol (PEG) technique of Wolosewick (1980) viewed by conventional transmission EM, scanning transmission EM, and surface scanning EM. The PEG technique permits visualization of the cytoskeletal network unobscured by the electron scattering properties of epoxy embedment. In the presynaptic process, synaptic vesicles appear to be suspended in a filamentous network that is contiguous with the synaptic vesicle membrane and with the presynaptic plasma membrane and its dense material. In the postsynaptic process, the postsynaptic density (PSD) is seen in intimate contact with the postsynaptic membrane. En face images of the PSD in some synapses appear as a torus. Emanating from the filamentous web of the PSD are filaments which extend to the adjacent plasma membrane. We conclude that membranous synaptic elements are contiguous with a three-dimensional lattice network that is similar to that described in whole unembedded cells (Wolosewick and Porter, 1976). Moreover, the synaptic densities represent a specialized elaboration of the cytoskeleton.

Pre- and post-synaptic structures in insect CNS: intramembranous features and sites of alpha-bungarotoxin binding

The central neuropile of thoracic ganglia in the central nervous system (CNS) of the cockroach Periplaneta americana contains synapses with characteristic pre- and post-synaptic membrane specializations and associated structures. These include dense pre-synaptic T-bars surrounded by synaptic vesicles, together with post-synaptic densities of varying electron opacity. Exocytotic release of synaptic vesicles is observed only rarely near presynaptic densities, but coated pits are seen at variable distances from them, and may be involved in membrane retrieval. After freeze-fracture, paralinear arrays of intramembranous articles (IMPs) are detected on the P face of many presynaptic terminals, with associated dimples indicative of vesicular release. The E face of these membranes exhibits protuberances complementary to the P face dimples, as well as scattered larger IMPs. Post-synaptic membranes possess dense IMP aggregates on the P face, some of which may represent receptor molecules. Electrophysiological studies with biotinylated alpha-bungarotoxin reveal that biotinylation does not inhibit the pharmacological effectiveness of the toxin in blocking acetylcholine receptors on an identified motoneurone in the metathoracic ganglion. Preliminary thin section ultrastructural analysis of this tissue post-treated with avidin-HRP or avidin-ferritin indicates that alpha-bungarotoxin-binding sites are localized at certain synapses in these insect thoracic ganglia.

On the use of lesions of afferents to localize neurotransmitter receptor sites in the striatum

Rats received lesions of either the cortico-striatal or the nigrostriatal projections. Two to three days after either type of lesion, evidence for degenerating dendritic spines that were postsynaptic to degenerating striatal boutons was often encountered. This type of transsynaptic degeneration of spines indicates that it is inappropriate to conclude that decreases in the number of neurotransmitter receptors after these types of lesions necessarily indicates that these receptors are located presynaptically on striatal afferents.

Quantitative densitometry of neurotransmitter receptors

An autoradiographic procedure is described that allows the quantitative measurement of neurotransmitter receptors by optical density readings. This procedure is a modification of the method of Young and Kuhar (1979a). Frozen brain sections are labeled in vitro with [3H]ligands under conditions that maximize specific binding to neurotransmitter receptors. The labeled sections are then placed against the 3H-sensitive LKB Ultrofilm to produce the autoradiograms. These autoradiograms resemble those produced by [14C]deoxyglucose autoradiography (Sokoloff, 1977) and are suitable for quantitative analysis with a densitometer. Muscarinic cholinergic receptors in rat and zebra finch brain and 5-HT receptors in rat brain were visualized by this method. When the proper combination of ligand concentration and exposure time are used, the method provides quantitative information about the amount and affinity of neurotransmitter receptors in brain sections. This was established by comparisons of densitometric readings with parallel measurements made by scintillation counting of sections.

Light and electron microscopic identification of monoaminergic terminals in the central nervous system

A brief critical survey of methods used for light and electron microscopic examination of amine-containing pathways within the CNS. Light microscopic techniques such as fluorescence histochemistry, immunocytochemistry, autoradiography, silver degeneration techniques, and retrograde tracing technique are suitable for studying the topography of pathways but, due to limits of resolution, they are inadequate for identifying terminals. Electron microscopy which is adequate to visualize terminals does not provide an overall view. This review considers various methods which have been devised to specifically detect aminergic nerve terminals. Electrolytic and chemical induced degenerations are described in noradrenergic, dopaminergic, and serotoninergic terminals. Although the individual degenerative alterations are not specific for aminergic terminals, the degenerative process when considered as a whole can be informative. At present no single technique can provide complete information about the origin, course, connections, and terminals of aminergic systems. Concurrent application of light and electron microscopy, experimental surgery, histochemistry, and microsample biochemistry would provide a complete description.

Internal organization of membranes at end bulbs of Held in the anteroventral cochlear nucleus

The end of bulb of Held in the rostral ventral cochlear nucleus of the chinchilla and guinea pig was studied with the freeze-fracture technique. The end bulb has multiple, small active zones which are uniformly distributed within the calyceal portion of this terminal. Single or small groups of active zones are surrounded by enlarged channels of extracellular space often containing processes of astrocytes. Small plasmalemmal deformations occur at these active zones. The number of these deformations is thought to be indicative of exocytotic transmitter release because they are more frequent in animals fixed in a noisy environment compared to animals fixed in a quiet environment. Thus, our study provides a basis for the quantitative study of changes in transmitter secretion at a central nervous system synapse driven by a controllable natural stimulus. The postsynaptic active zone at end bulbs resembles other excitatory synapses in the central nervous system in having an aggregate of large particles on the external membrane leaflet. This junctional aggregate of particles is coextensive with the presynaptic active zone and with the postsynaptic density seen in thin sections. Several perisynaptic aggregates of particles are deployed around each active zone on the external membrane leaflet. These irregularly-shaped aggregates occur preferentially opposite the channels of enlarged extracellular space and along the edge of the end bulb and are not components of intercellular junctions or plasmalemmal contacts with cytoplasmic organelles. Although the function of the different particle aggregates on the postsynaptic membrane is not clear, our findings provide a basis for studying the factors controlling and maintaining their structure as well as more evidence that a consistent relationship exists between types of synaptic action and structure of the postsynaptic membrane.

Serotonin nerve terminals in the locus coeruleus of adult rat: a radioautographic study

Serotonin (5-HT) nerve terminals in the locus coeruleus (LC) of adult rat were visualized by high-resolution radioautography, in order to examine their distribution, fine structural features and intimate relationships with norepinephrine neurons. In animals pretreated with a monoamine oxidase inhibitor, prolonged intraventricular perfusion of 10(-4) M [3H]5-HT resulted in a specific identification of most if not all 5-HT axonal varicosities in LC. These terminals were equally distributed between the dorsal and ventral divisions of the nucleus. Their density was approximated at 10(7) per cu.mm within the middle third of the LC. In electron microscope radioautographs, the labeled 5-HT varicosities averaged 0.9 micron in diameter. They all exhibited a distinctive storage organelle, in the form of microvesicles and microcanaliculi (15-25 nm in diameter) partly filled with electron-dense material and usually aggregated in association with several large dense-core vesicles. While this finding of intrinsic morphological characteristics appeared compatible with a special cellular origin or regional differentiation, it was also suggestive of particular functional properties and/or mode of action. In a sample comprised of some 500 sectional profiles from labeled 5-HT varicosities in LC, a small proportion only (less than 10%) exhibited morphologically defined synaptic junctions. These rare contacts were invariably made with dendritic processes and never observed on the noradrenergic perikarya. It is therefore concluded that, in the LC, non-synaptic as well as synaptic mechanisms might be involved in the modulation and transneuronal regulation of norepinephrine neurons by 5-HT afferents.

Tissue culture analysis of neurogenesis: myelination and synapse formation are retarded by serum deprivation

Explants from newborn mouse cerebellum were cultured in nutrient media containing either adequate (30%) or low (15% or 7.5%) serum content. By light microscopic observation, delayed and inhibited myelination was detected in cultures fed with low serum media (experimental cultures). Specific activities of two enzymes related to myelin synthesis, 2',3'-cyclic nucleotide-3'-phosphohydrolase and 3-hydroxy-3-methylglutaryl coenzyme A reductase, were also reduced in experimental cultures. Morphometric analysis of electron micrographs showed that the size of presynaptic endings and total area occupied by synapses in experimental cultures were substantially reduced, while synaptic density per unit area increased. Reflecting the results of synaptic underdevelopment, the levels of two neurotransmitter enzymes, choline acetyltransferase and glutamic acid decarboxylase, were also decreased in experimental cultures.