Microtubule-macrotubule transformations induced by volatile anesthetics. Mechanism of macrotubule assembly

Insights into the interaction dynamics between volatile anesthetics and tubulin through computational molecular modelling

General anesthetics, able to reversibly suppress all conscious brain activity, have baffled medical science for decades, and little is known about their exact molecular mechanism of action. Given the recent scientific interest in the exploration of microtubules as putative functional targets of anesthetics, and the involvement thereof in neurodegenerative disorders, the present work focuses on the investigation of the interaction between human tubulin and four volatile anesthetics: ethylene, desflurane, halothane and methoxyflurane. Interaction sites on different tubulin isotypes are predicted through docking, along with an estimate of the binding affinity ranking. The analysis is expanded by Molecular Dynamics simulations, where the dimers are allowed to freely interact with anesthetics in the surrounding medium. This allowed for the determination of interaction hotspots on tubulin dimers, which could be linked to different functional consequences on the microtubule architecture, and confirmed the weak, Van der Waals-type interaction, occurring within hydrophobic pockets on the dimer. Both docking and MD simulations highlighted significantly weaker interactions of ethylene, consistent with its far lower potency as a general anesthetic. Overall, simulations suggest a transient interaction between anesthetics and microtubules in general anesthesia, and contact probability analysis shows interaction strengths consistent with the potencies of the four compounds.Communicated by Ramaswamy H. Sarma.

Anesthetic Alterations of Collective Terahertz Oscillations in Tubulin Correlate with Clinical Potency: Implications for Anesthetic Action and Post-Operative Cognitive Dysfunction

Anesthesia blocks consciousness and memory while sparing non-conscious brain activities. While the exact mechanisms of anesthetic action are unknown, the Meyer-Overton correlation provides a link between anesthetic potency and solubility in a lipid-like, non-polar medium. Anesthetic action is also related to an anesthetic's hydrophobicity, permanent dipole, and polarizability, and is accepted to occur in lipid-like, non-polar regions within brain proteins. Generally the protein target for anesthetics is assumed to be neuronal membrane receptors and ion channels, however new evidence points to critical effects on intra-neuronal microtubules, a target of interest due to their potential role in post-operative cognitive dysfunction (POCD). Here we use binding site predictions on tubulin, the protein subunit of microtubules, with molecular docking simulations, quantum chemistry calculations, and theoretical modeling of collective dipole interactions in tubulin to investigate the effect of a group of gases including anesthetics, non-anesthetics, and anesthetic/convulsants on tubulin dynamics. We found that these gases alter collective terahertz dipole oscillations in a manner that is correlated with their anesthetic potency. Understanding anesthetic action may help reveal brain mechanisms underlying consciousness, and minimize POCD in the choice and development of anesthetics used during surgeries for patients suffering from neurodegenerative conditions with compromised cytoskeletal microtubules.

Neuroprotective Effects Against POCD by Photobiomodulation: Evidence from Assembly/Disassembly of the Cytoskeleton

Postoperative cognitive dysfunction (POCD) is a decline in memory following anaesthesia and surgery in elderly patients. While often reversible, it consumes medical resources, compromises patient well-being, and possibly accelerates progression into Alzheimer's disease. Anesthetics have been implicated in POCD, as has neuroinflammation, as indicated by cytokine inflammatory markers. Photobiomodulation (PBM) is an effective treatment for a number of conditions, including inflammation. PBM also has a direct effect on microtubule disassembly in neurons with the formation of small, reversible varicosities, which cause neural blockade and alleviation of pain symptoms. This mimics endogenously formed varicosities that are neuroprotective against damage, toxins, and the formation of larger, destructive varicosities and focal swellings. It is proposed that PBM may be effective as a preconditioning treatment against POCD; similar to the PBM treatment, protective and abscopal effects that have been demonstrated in experimental models of macular degeneration, neurological, and cardiac conditions.

Heuristic consequences of a load of oxygen in microtubules

The current cell oxygen paradigm shows some major gaps that have not yet been resolved. Something seems to be lacking for the comprehensive statement of the oxygen distribution in the cell, especially the low cytoplasmic oxygen level. The entrapment of oxygen in microtubules (MTs) resolves the latter observation, as well as the occurrence of an extensive cytoplasmic foam formation. It leads to a novel oxygen paradigm for cells. During the steady-state treadmilling, the mobile cavity would absorb oxygenated cytoplasm forward, entrap gas nuclei and concentrate them. A fluorescence method is described to confirm the in vitro load of oxygen in MTs during their periodic growths and shrinkages. The latter operating mechanism is called the gas dynamic instability (GDI) of MTs. Several known biosystems could rest on the GDI. (1) The GTP-cap is linked with the gas meniscus encountered in a tube filled with gas. The GTP hydrolysis is linked to the conformational change of the GTPase domain according to the bubble pressure, and to the shaking of protofilaments with gas particles (soliton-like waves). (2) The GDI provides a free energy water pump because water molecules have to escape from MT pores when foam concentrates within the MT. Beside ATP hydrolysis in motor proteins, the GDI provides an additional driving force in intracellular transport of cargo. The water streams flowing from the MT through slits organize themselves as water layers between the cargo and the MT surface, and break ionic bridges. It makes the cargo glide over a water rail. (3) The GDI provides a universal motor for chromosome segregation because the depolymerization of kinetochorial MTs is expected to generate a strong cytoplasmic foam. Chromosomes are sucked up according to the pressure difference (or density difference) applied to opposite sides of the kinetochore, which is in agreement with Archimedes' principle of buoyancy. Non-kinetochorial MTs reabsorb foam during GDI. Last, the mitotic spindle is imagined as a gas recycler. (4) The luminal particles within MTs (called MIPs) are imagined as a foam organizer, the luminal proteins being part of the borders and edges of identical bubbles. (5) Last, volatile anesthetics could destabilize MTs through anesthetic-induced bubble nucleation between protofilaments, and therefore causing shear stress and the opening of MT. The load of oxygen in MTs might provide a major advance in this area of research.

Isoflurane affects the cytoskeleton but not survival, proliferation, or synaptogenic properties of rat astrocytes in vitro

BACKGROUND

More than half of the cells in the brain are glia and yet the impact of general anaesthetics on these cells is largely unexamined. We hypothesized that astroglia, which are strongly implicated in neuronal well-being and synapse formation and function, are vulnerable to adverse effects of isoflurane.

METHODS

Cultured rat astrocytes were treated with 1.4% isoflurane in air or air alone for 4 h. Viability, proliferation, and cytoskeleton were assessed by colorimetric assay, immunocytochemistry, or a migration assay at the end of treatment or 2 days later. Also, primary rat cortical neurones were treated for 4 days with conditioned medium from control [astrocyte-conditioned media (ACM)], or isoflurane-exposed astrocytes (Iso-ACM) and synaptic puncta were assessed by synapsin 1 and PSD-95 immunostaining.

RESULTS

By several measures, isoflurane did not kill astrocytes. Nor, based on incorporation of a thymidine analogue, did it inhibit proliferation. Isoflurane had no effect on F-actin but reduced expression of α-tubulin and glial fibrillary acidic protein both during exposure (P<0.05 and P<0.001, respectively) and 2 days later (P<0.01), but did not impair astrocyte motility. ACM increased formation of PSD-95 but not synapsin 1 positive puncta in neuronal cultures, and Iso-ACM was equally effective.

CONCLUSIONS

Isoflurane decreased expression of microtubule and intermediate filament proteins in astrocytes in vitro, but did not affect their viability, proliferation, motility, and ability to support synapses.

George M, Freedman H, Barakat KH, Damaraju S, Hameroff S, Tuszynski JA. Computational predictions of volatile anesthetic interactions with the microtubule cytoskeleton: implications for side effects of general anesthesia

The cytoskeleton is essential to cell morphology, cargo trafficking, and cell division. As the neuronal cytoskeleton is extremely complex, it is no wonder that a startling number of neurodegenerative disorders (including but not limited to Alzheimer's disease, Parkinson's disease and Huntington's disease) share the common feature of a dysfunctional neuronal cytoskeleton. Recently, concern has been raised about a possible link between anesthesia, post-operative cognitive dysfunction, and the exacerbation of neurodegenerative disorders. Experimental investigations suggest that anesthetics bind to and affect cytoskeletal microtubules, and that anesthesia-related cognitive dysfunction involves microtubule instability, hyper-phosphorylation of the microtubule-associated protein tau, and tau separation from microtubules. However, exact mechanisms are yet to be identified. In this paper the interaction of anesthetics with the microtubule subunit protein tubulin is investigated using computer-modeling methods. Homology modeling, molecular dynamics simulations and surface geometry techniques were used to determine putative binding sites for volatile anesthetics on tubulin. This was followed by free energy based docking calculations for halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) on the tubulin body, and C-terminal regions for specific tubulin isotypes. Locations of the putative binding sites, halothane binding energies and the relation to cytoskeleton function are reported in this paper.

Anesthesia and the old brain

The perioperative period may have long-term consequences on cognitive function in the elderly patient. In this special article, we summarize the rationale and evidence that the anesthetic per se is a contributor. The evidence at this point is considered suggestive and further research is needed, especially in humans.

n-Butanol induces depolymerization of microtubules in vivo and in vitro

The effects of butanol on microtubules (MTs) were examined by immunofluorescence microscopy. Fragmentation of cortical MTs was induced by n-butanol, but not by s- and t-butanols, in cultured tobacco BY-2 cells. Taxol prevented n-butanol-induced MT fragmentation. Fragmented cortical MTs were still attached to the inner face of the plasma membrane when n-butanol-treated protoplasts were ruptured on the slide glass. Moreover, MTs were depolymerized in the presence of n-butanol in vitro. Therefore, n-butanol is not only an activator of phospholipase D but also an effective MT-depolymerizing agent.

Inhibitory effects of halothane on the thermogenic pathway in brown adipocytes: localization to adenylyl cyclase and mitochondrial fatty acid oxidation

Volatile anesthetics such as halothane efficiently inhibit nonshivering thermogenesis as well as the cellular manifestation of that phenomenon: norepinephrine-induced respiration in brown adipocytes. To identify the molecular site(s) of action of such anesthetics, we have examined the effect of halothane on the sequential intracellular steps from the interaction of norepinephrine with isolated brown adipocytes to the stimulation of mitochondrial respiration (=thermogenesis). We did not identify an inhibition at the level of the adrenergic receptors, but a first site of inhibition was identified as the generation of cAMP by adenylyl cyclase; this led to inhibition of norepinephrine-induced expression of the uncoupling protein-1 (UCP1) gene and reduced norepinephrine-induced lipolysis as secondary effects. Although an inhibition of lipolysis in itself would inhibit thermogenesis, circumvention of this inhibition revealed that a second, postlipolytic, site of inhibition existed: halothane also inhibited the stimulatory effect of exogenous fatty acids on cellular respiration. This inhibition was independent of the presence of UCP1 in the mitochondria of the cells and was thus not directly on the thermogenic uncoupling mechanism. Since not only fatty acid oxidation but also pyruvate oxidation were inhibited by halothane in isolated mitochondria, whereas glycerol-3-phosphate oxidation was not, the second site of action of halothane, evident when cyclase/lipolytic inhibition was circumvented, was located to the respiratory chain, complex I. The results thus explain the inhibition of nonshivering thermogenesis by identifying two sites of action of halothane in brown adipocytes. In addition, the results may open for new formulations of the molecular background to anesthesia.

Tubulin assembly in the presence of calcium ions and taxol: microtubule bundling and formation of macrotubule-ring complexes

It has been confirmed that taxol is able to prevent Ca(2+)-induced inhibition of microtubule formation from tubulin in the presence of microtubule-associated proteins. However, by means of electron microscopy and scanning force microscopy it could be demonstrated that assembly in the presence of Ca2+ and taxol leads to structural aberrations. The kind of aberration depends on the order of addition of taxol and Ca2+ to tubulin. When taxol was added first, microtubules were formed preferentially. But, these microtubules typically associated with each other by close wall-to-wall alignments or they formed complexes with some C-shaped protofilament ribbons, resulting in microtubule bundles or doublet- and triplet-like microtubule structures, respectively. When Ca2+ was added first, macrotubules, rings, and ring crystals were the dominant assembly products. Mostly, the macrotubules were also bundled or they enclosed rings in their lumen. The findings clearly demonstrate the potency of Ca2+ to induce different polymorphic assemblies with additional protofilament associations, not realized in microtubules.

Gamma-tubulin can both nucleate microtubule assembly and self-assemble into novel tubular structures in mammalian cells

alpha-, beta-, and gamma-tubulins are evolutionarily highly conserved members of the tubulin gene superfamily. While the abundant members, alpha- and beta-tubulins, constitute the building blocks of cellular microtubule polymers, gamma-tubulin is a low abundance protein which localized to the pericentriolar material and may play a role in microtubule assembly. To test whether gamma-tubulin mediates the nucleation of microtubule assembly in vivo, and co-assembles with alpha- and beta-tubulins into microtubules or self-assembles into macro-molecular structures, we experimentally elevated the expression of gamma-tubulin in the cell cytoplasm. In most cells, overexpression of gamma-tubulin causes a dramatic reorganization of the cellular microtubule network. Furthermore, we show that when overexpressed, gamma-tubulin causes ectopic nucleation of microtubules which are not associated with the centrosome. In a fraction of cells, gamma-tubulin self-assembles into novel tubular structures with a diameter of approximately 50 nm (named gamma-tubules). Furthermore, unlike microtubules, gamma-tubules are resistant to cold or drug induced depolymerization. These data provide evidence that gamma-tubulin can cause nucleation of microtubule assembly and can self-assemble into novel tubular structures.

Structural diversity and dynamics of microtubules and polymorphic tubulin assemblies

Tubulin, the main protein of microtubules (MTs), has the potency of forming a variety of other assembly products in vitro: rings, ring-crystals, C- and S-shaped ribbons, 10 nm fibres, hoops, sheets, heaped sheets, MT doublets, MT triplets, double-wall MTs, microtubules, curled ribbons, and paracrystals. The supramolecular subunits of all of them are the protofilaments which might be arranged either parallel to the axis (e.g., in MTs, ribbons) or curved (e.g., in hoops, microtubules). There is strong evidence that in the second case the protofilaments have an inside-out orientation compared to MTs. All assembly products mentioned are described structurally and their relevance to the in vivo situation is considered. Moreover, MTs and the other assemblies undergo permanent changes. These dynamics occurring in both individual assemblies and assembly populations are discussed from the structural point of view.

Ultrastructure of metaplastic ciliated cells in human stomach

Intestinal metaplasia of the gastric mucosa occurs commonly in aged Japanese patients and has been discussed in relation to the high incidence of gastric cancer in Japanese. Ciliated cells in the gastric mucosa have frequently been found in association with intestinal metaplasia in the pyloric gland and rarely in the cardiac gland in many Japanese patients, and exceptionally in one Chinese and in one Swedish patient. Electron microscopic examination of 12 Japanese patients has revealed that these structures are not metaplastic stereocilia, but true cilia. Ciliated cells have been found in the basal part of the gastric glands and never in the surface epithelium. The fine structure of the gastric cilia was almost the same as that of normal respiratory cilia. However, in the gastric cilia, most dynein arms were inconspicuous even after tannic acid fixation, indicating that ciliary beating of the gastric cilia is problematic. Abnormal cilia and basal bodies also were found. Ciliated cells have always occurred in association with intestinal metaplasia, therefore this phenomenon might be a type of metaplasia and is named "ciliated metaplasia" of the gastric mucosa.

Paracrystalline inclusions in metaplastic ciliated cells of the human gastric mucosa. An ultrastructural study

Unusual electron-dense paracrystalline inclusions were found in metaplastic ciliated cells in the stomachs of three Japanese male patients with gastric carcinoma. These patients had not been given antitumour drugs before surgery and ethrane (enflurane) was used as the anaesthetic. Ciliated cells in the gastric mucosa are found not infrequently in the pyloric glands in association with intestinal metaplasia in elderly Japanese patients. Paracrystalline inclusions were found only in the ciliated cells and never in any other types of gastric mucosal cell. These inclusions were located in the apical portion of the ciliated cells in intimate association with the basal bodies. They consisted of twisted strings about 27 nm wide with a regularly repeated spacing of about 30 nm. On highly magnified electron micrographs, granules about 4 nm in diameter were detected. These paracrystalline inclusions have never been reported previously, although their location in ciliated cells and their morphological characteristics suggest an intimate relationship with the ciliogenesis of metaplastic ciliated cells in the human stomach.

Further characterization of a fodrin-containing transmembrane complex from mouse T-lymphoma cells

A transmembrane complex containing fodrin (an actin-binding protein) and a major surface glycoprotein (GP 180) was previously isolated from mouse T-lymphoma cells by the complementary techniques of non-ionic detergent extraction and sucrose gradient centrifugation (Bourguignon et al. (1985) J. Cell Biol. 101, 477-487). The analysis of this complex has been extended to verify the structural association and further define the interaction between fodrin and GP 180. The association between fodrin and GP 180 has been confirmed by the following evidence: co-sedimentation of fodrin and GP 180 in a single peak on a sucrose gradient with a sedimentation coefficient of 20 S; a constant ratio of fodrin and GP 180 across the 20 S peak; the specific co-precipitation of GP 180 with fodrin from the 20 S peak using anti-fodrin antibody; and the colocalization of fodrin and GP 180 from the 20 S peak on actin filaments using an immuno-electron microscopic technique. Furthermore, this fodrin-GP 180 complex can be readily dissociated and reassembled in the presence and absence of 0.6 M NaCl, respectively. The fact that this fodrin-GP 180 complex displays actin-binding ability indicates that this transmembrane complex may play an important role in the linking event between receptors and the cytoskeleton during lymphocyte patching and capping.

Microtubules and macrotubules in fish meiosis

During meiosis in the male of a cyprinodontid fish, Aphyosemion splendopleure, and during the organization of the spindle of division, the spindle is made of two types of tubules: microtubules (20-25 nm) and macrotubules (30-50 nm). The macrotubules are associated only with the polar region of the meiotic apparatus and are located outside the spindle of microtubules. At the end of meiosis, the spindle microtubules depolymerize whereas the macrotubules remain. One can find them throughout the entire process of spermiogenesis; later, they disappear only at the end of spermatid maturation. We have studied four populations from Cameroon, three of them with macrotubules.

Isolation of microvillar microfilaments and associated transmembrane complex from ascites tumor cell microvilli

The association of microvillar microfilaments with the microvillar membrane actin-containing transmembrane complex of MAT-C1 13762 ascites tumor cell microvilli has been investigated by differential centrifugation, gel electrophoresis and electron microscopy of detergent extracts of the isolated microvilli. Several methods have been used to reduce breakdown and solubilization of the microfilament core actin during the detergent extractions for preparation of microvillar core microfilaments. Gel electrophoresis of differential centrifugation fractions demonstrated that over 70% of the total microvillus actin could be pelleted with microfilament cores at 10 000 g under extraction conditions which reduce filament breakdown. Transmission electron microscopy (TEM) of all of the core preparations showed arrays of microfilaments and small microfilament bundles. The major protein components of the microfilament cores, observed by sodium dodecyl sulfate (SDS) electrophoresis, were actin and alpha-actinin. Among the less prominent polypeptide components was a 58 000 Dalton polypeptide (58 K), previously identified as a member of the MAT-Cl transmembrane complex. This three-component complex contains, in addition to 58 K, actin associated directly and stably with a cell surface glycoprotein (Carraway, CAC, Jung, G & Carraway, K L, Proc. natl acad. sci. US 80 (1983) 430). Evidence that the apparent association of complex with the microfilament core was not due simply to co-sedimentation was provided by myosin affinity precipitation. These results provide further evidence that the transmembrane complex is a site for the interaction of microfilaments with the microvillar plasma membrane.

The molecular mechanism of interaction of Et3Pb+ with tubulin

Triethyllead ion (Et3Pb+) was found to interact with 2 out of 18 thiol groups present in tubulin dimers. Specificity of the interaction was shown by the high affinity of Et3Pb+ to tubulin, by the fact that the 16 residual thiol groups in tubulin remained unaffected, and by the observation that other proteins with exposed thiol groups, e.g., actin, did not react with Et3Pb+. After complexation of the two thiol groups, tubulin in vitro had lost its capability for microtubule assembly. Likewise, polymerized tubulin disassembled on addition of the lead compound.

Purification and characterization of oocyte cytoplasmic tubulin and meiotic spindle tubulin of the surf clam Spisula solidissima

Assembly-competent tubulin was purified from the cytoplasm of unfertilized and parthogenetically activated oocytes, and from isolated meiotic spindles of the surf clam, Spisula solidissima. At 22 degrees C or 37 degrees C, Spisula tubulin assembled into 48-51-nm macrotubules during the first cycle of polymerization and 25-nm microtubules during the third and subsequent cycles of assembly. Macrotubules were formed from sheets of 26-27 protofilaments helically arranged at a 36 degree angle relative to the long axis of the polymer and were composed of alpha and beta tubulins and several other proteins ranging in molecular weight from 30,000 to 270,000. Third cycle microtubules contained 14-15 protofilaments in cross-section and were composed of greater than 95% alpha and beta tubulins. After three cycles of polymerization at 37 degrees C, unfertilized and activated oocyte tubulin self-assembled into microtubules at a critical concentration (Ccr) of 0.09 mg/ml. At the physiological temperature of 22 degrees C, unfertilized oocyte tubulin assembled into microtubules at a Ccr of 0.36 mg/ml, activated oocyte tubulin assembled at a Ccr of 0.42 mg/ml, and isolated meiotic spindle tubulin assembled at a Ccr of 0.33 mg/ml. The isoelectric points of tubulin from both unfertilized oocytes and isolated meiotic spindles were 5.8 for alpha tubulin and 5.6 for beta tubulin. In addition, one dimensional peptide maps of oocyte and spindle alpha and beta tubulins were very similar, if not identical. These results indicate that unfertilized oocyte tubulin and tubulin isolated from the first meiotic spindle are indistinguishable on the basis of assembly properties, isoelectric focusing, and one dimensional peptide mapping. These results suggest that the transition of tubulin from the quiescent oocyte state to that competent to form spindle microtubules in vivo does not require special modification of tubulin but may involve changes in the availability of microtubule organizing centers or assembly-promoting microtubule-associated proteins.

Central nervous system complications of coronary artery bypass graft surgery: prospective analysis of 421 patients

A prospective analysis of 421 patients undergoing coronary artery bypass graft (CABG) surgery as the sole cardiac procedure was carried out to assess the frequency of central nervous system (CNS) complications. In all, 451 variables were assessed in each patient. Stroke occurred in 5.2% but was severe in only 2%. Prolonged encephalopathy occurred in 11.6% but usually resolved before discharge. No statistically significant pre- or intraoperative risk variables for CNS complications were found; specifically, age or pump times in excess of two hours were not significant factors. Postoperative use of an intraaortic balloon pump and pressor agents were significantly correlated with prolonged encephalopathy. The frequency of CNS injury in CABG surgery is similar to that in other forms of open heart surgery, and there has been little change in the frequency of CNS complications over the past seven years. Possible mechanisms of CNS damage during CABG surgery are discussed.

The effect of caffeine, different fixation regimes and low temperature on microtubules in the cells of higher plants : Evidence for diversity in their response to chemical and physical treatments

Caffeine, (1:3:7-tri-methyl-xanthine), either as a prefixation treatment or included with glutaralde-hyde as the primary fixative, destroys or disorganises the microtubules associated with the formation of secondary walls in fibres from the flowering stem of the grass Lolium temulentum L. There is no observable effect of caffeine treatment on the microtubules associated with primary wall formation in collenchyma and young fibres from L. temulentum or in root cap cells of Zea mays L. and Phaseolus vulgaris L. The microtubules associated with primary wall formation are destroyed by cold treatment but not those associated with secondary wall formation. Tannic acid included in the fixative shows the microtubules associated with secondary wall formation in fibres of L. temulentum to be composed of 13 subunits. Treatment with lanthanum hydroxide does not stain the core or the halo of the microtubules.

Extracellular microtubule-like structures in the retina of the rainbow trout: development, intercellular connectivity and reaction to vincristine

Extracellular microtubule-like structures (MLS) are described in the retina of the rainbow trout. They appear about 30 to 40 days after hatching, when the yolk-sac is consumed and the animal begins to swim and to nourish actively. They fill a widely branched system of extracellular clefts and spaces, and connect different cells and cell types, especially outer horizontal cells and bipolar cells. The MLS are not affected by the vinca alcaloid vincristine, although this drug penetrates into the MLS-filled space, as has been shown by the formation of intracellular, vincristine-induced tubulin paracrystals. The MLS are compared with other extracellular tubular structures described in other animal tissues. Their functional significance remains unclear.

Microtubules associated with postsynaptic 'thickenings'

Using a new albumin technique, microtubules can be seen closely related to or associated with the postsynaptic 'thickening' of mature and immature central nervous synapses. Thus in conventionally fixed synapses (without albumin pretreatment) where microtubules cannot usually be observed running into the postsynaptic dense material, this material could perhaps, in part, consist of the debris of in vivo microtubules. Smooth endoplasmic reticulum (ER) is often seen associated with the microtubules near the postsynaptic 'thickening'. Microtubules, and possibly smooth ER, may have an important role in the initiation of synapse formation and in the maintenance of mature synapses.