Redistribution of intramembrane particles related to acetylcholine release by cholinergic synaptosomes

Partial purification of ?-glycerotoxin, a presynaptic neurotoxin from the venom glands of the polychaete annelid glycera convoluta

The venom secreted from glands appended to the jaws of Glycera convoluta, a Polychaete Annelid, increases the spontaneous quantal release of transmitter from nerve terminals. The component that is biologically active on vertebrate cholinergic nerve terminals has recently been shown to be a high molecular weight protein. In the present work, the crude extract from the venom apparatus was shown to be toxic for mammals and crustaceans. It was fractionated by gel filtrations and ion exchange chromatographies. The biologically active component at frog neuromuscular junctions, ?-glycerotoxin, was purified more than 1,000-fold. It is distinct from the components that are toxic for crustaceans. Purified ?-glycerotoxin is a globular protein of 300,000 +/- 20,000 mol wt. It has a Stokes radius of 65 A and a sedimentation coefficient of 11 S. By its molecular properties, ?-glycerotoxin appears distinct from other neurotoxins such as ?-latrotoxin, which also trigger transmitter release.

Quantal transmitter release by glioma cells: quantification of intramembrane particle changes

Glial cells in situ are able to release neurotransmitters such as glutamate or acetylcholine (ACh). Glioma C6BU-1 cells were used to determine whether the mechanisms of ACh release by a glial cell line are similar or not to quantal release from neurones. Individual C6BU-1 cells, pre-filled with ACh, were moved into contact with a Xenopus myocyte that was used as a real-time ACh detector. Upon electrical stimulation, C6BU-1 cells generated evoked ACh impulses which were Ca(2+)-dependent and quantal (quantal steps of ca. 100 pA). Changes in plasma membrane ultrastructure were investigated by using a freeze-fracture technique designed for obtaining large and flat replicas from monolayer cell cultures. A transient increase in the density of medium and large size intramembrane particles--and a corresponding decrease of small particles--occurred in the plasma membrane of C6BU-1 cells stimulated for ACh release. Changes in interaction forces between adjacent medium and large particles were investigated by computing the radial distribution function and the interaction potential. In resting cells, the radial distribution function revealed a significant increase in the probability to find two particles separated by an interval of 24 nm; the interaction potential suggested repulsive forces for intervals shorter than 24 nm and attractive forces between 24 and 26 nm. In stimulated cells, this interaction was displaced to 21 nm and made weaker, despite of the fact that the overall particle density increased. The nature of this transient change in intramembrane particles is discussed, particularly with regard to the mediatophore proteolipid which is abundant in the membranes C6-BU-1 like in those of cholinergic neurones. In conclusion, evoked ACh release from pre-filled C6-BU-1 glioma cells is quantal and Ca(2+)-dependent. It is accompanied by a transient changes in the size distribution and the organisation of intramembrane particles in the plasma membrane. Thus, for the release characteristics, glioma cells do not differ fundamentally from neurones.

Neurotransmitter release at rapid synapses

The classical concept of the vesicular hypothesis for acetylcholine (ACh) release, one quantum resulting from exocytosis of one vesicle, is becoming more complicated than initially thought. 1) synaptic vesicles do contain ACh, but the cytoplasmic pool of ACh is the first to be used and renewed on stimulation. 2) The vesicles store not only ACh, but also ATP and Ca(2+) and they are critically involved in determining the local Ca(2+) microdomains which trigger and control release. 3) The number of exocytosis pits does increase in the membrane upon nerve stimulation, but in most cases exocytosis happens after the precise time of release, while it is a change affecting intramembrane particles which reflects more faithfully the release kinetics. 4) The SNARE proteins, which dock vesicles close to Ca(2+) channels, are essential for the excitation-release coupling, but quantal release persists when the SNAREs are inactivated or absent. 5) The quantum size is identical at the neuromuscular and nerve-electroplaque junctions, but the volume of a synaptic vesicle is eight times larger in electric organ; at this synapse there is enough ACh in a single vesicle to generate 15-25 large quanta, or 150-200 subquanta. These contradictions may be only apparent and can be resolved if one takes into account that an integral plasmalemmal protein can support the formation of ACh quanta. Such a protein has been isolated, characterised and called mediatophore. Mediatophore has been localised at the active zones of presynaptic nerve terminals. It is able to release ACh with the expected Ca(2+)-dependency and quantal character, as demonstrated using mediatophore-transfected cells and other reconstituted systems. Mediatophore is believed to work like a pore protein, the regulation of which is in turn likely to depend on the SNARE-vesicle docking apparatus.

Quantal acetylcholine release: vesicle fusion or intramembrane particles?

Images of vesicle openings in the presynaptic membrane have regularly been shown to increase in number after stimulation of cholinergic nerves. However, with a very few exceptions, the occurrence of vesicle openings is delayed in time with respect to the precise moment of transmitter release. In contrast, a transient change in the size and distribution of intramembrane particles (IMPs) has constantly been found as a characteristic change affecting the presynaptic membrane in a strict time coincidence with the release of acetylcholine quanta. This is illustrated here in a rapid-freezing experiment performed on small specimens of the Torpedo electric organ during transmission of a single nerve impulse. A marked change affected IMPs in the presynaptic membrane for 3-4 ms, i.e., a population of IMPs larger than 10 nm momentarily occurred in coincidence with the passage of the impulse. The nicotinic receptors, abundantly visible in the postsynaptic membranes, also underwent very fleeting structural changes during synaptic transmission. In conclusion, for rapidly operating neurotransmitters like acetylcholine, a characteristic IMP change was regularly found to coincide in the presynaptic membrane with the production of neurotransmitter quanta, whereas images of vesicles fusion were either delayed or even dissociated from the release process. This is discussed in connection to the different modes of release recently described for other secreting systems.

Reconstitution of mediatophore-supported quantal acetylcholine release

Synaptic transmission of a nerve impulse is an extremely rapid event relying on transfer of brief chemical impulses from one cell to another. This transmission is dependent upon Ca2+ and known to be quantal, which led to the widely accepted vesicular hypothesis of neurotransmitter release. However, at least in the case of rapid synaptic transmission the hypothesis has been found difficult to reconcile with a number of observations. In this article, we shall review data from experiments dealing with reconstitution of quantal and Ca2+-dependent acetylcholine release in: i) proteoliposomes, ii) Xenopus oocytes, and iii) release-deficient cell lines. In these three experimental models, release is dependent on the expression of the mediatophore, a protein isolated from the plasma membrane of cholinergic nerve terminals of the Torpedo electric organ. We shall discuss the role of mediatophore in quantal acetylcholine release, its possible involvement in morphological changes affecting presynaptic membrane during the release, and its interactions with others proteins of the cholinergic nerve terminal.

Morphological changes related to reconstituted acetylcholine release in a release-deficient cell line

The membrane changes accompanying Ca(2+)-dependent acetylcholine release were investigated by comparing release-competent and release-incompetent clones of mouse neuroblastoma N18TG-2 cells. No release could be elicited in native N18 cells or in a N18-choline acetyltransferase clone in which acetylcholine synthesis was induced by transfection with the gene for rat choline acetyltransferase. However, acetylcholine release was operative in a To/9 clone which was co-transfected with complementary DNAs from rat choline acetyltransferase and Torpedo mediatophore 16,000 mol. wt subunit. In thin sections, the aspect of resting N18 and To/9 cells was identical: a very dense cytoplasm with practically no vesicle-like organelles. Cells were chemically fixed at different times during a stimulation using A-23187 and Ca2+, and examined following both freeze-fracture and thin section. Stimulation of To/9 cells induced a marked change affecting the intramembrane particles. The number of medium-sized particles (9.9-12.38 nm) increased, while that of the small particles decreased. This change was not observed in control, release-incompetent cell lines. In the To/9 clone (but not in control clones), this was followed by occurrence of a large new population of pits which initially had a large diameter, but subsequently became smaller as their number decreased. Coated depressions and invaginations became abundant after stimulation, suggesting an endocytosis process. By considering the succession of events and by comparison with data from experiments performed on synapses in situ, it is proposed that a particle alteration was the counterpart of acetylcholine release in co-transfected To/9 cells; this was followed by a massive endocytosis.

An improved approach to freeze-fracture morphology of monolayer cell cultures

Much work is currently done on cell cultures to elucidate membrane processes associated with different cell functions. We describe here a modified freeze-fracture method to obtain systematically large fractured areas of the plasma membrane from monolayer cell culture in situ. Cells are grown until confluence on a Thermanox coverslip overlaid with poly-L-ornithine. After chemical fixation, the culture is flattened overnight by sandwiching it between the Thermanox coverslip, a Falcon membrane and a glass coverslip, under a 5 g weight. After freeze-fracture, vast pictures of the protoplasmic leaflets are obtained in a reproducible manner. Our approach was applied to cultures which were stimulated to release acetylcholine; it has been found very appropriate for studying modifications affecting intramembrane particles and vesicles openings in the plasmalemma. Accurate quantifications were performed and correlations were established between the membrane changes and the data revealed by thin sections. The present sandwich method can be applied to a variety of cell preparations, allowing for quantitative study of structure-function relationships.

In vitro reconstitution of neurotransmitter release

The vesicular hypothesis has stimulated fruitful investigations on many secreting systems. In the case of rapid synaptic transmission, however, the hypothesis has been found difficult to reconcile with a number of well established observations. Brief impulses of transmitter molecules (quanta) are emitted from nerve terminals at the arrival of an action potential by a mechanism which is under the control of multiple regulations. It is therefore not surprising that quantal release could be disrupted by experimental manipulation of a variety of cellular processes, such as a) transmitter uptake, synthesis, or transport, b) energy supply, c) calcium entry, sequestration and extrusion, d) exo- or endocytosis, e) expression of vesicular and plasmalemmal proteins, f) modulatory systems and second messengers, g) cytoskeleton integrity, etc. Hence, the approaches by "ablation strategy" do not provide unequivocal information on the final step of the release process since there are so many ways to stop the release. We propose an alternate approach: the "reconstitution strategy". To this end, we developed several preparations for determining the minimal system supporting Ca2+-dependent transmitter release. Release was reconstituted in proteoliposomes, Xenopus oocytes and transfected cell lines. Using these systems, it appears that a presynaptic plasmalemmal proteolipid, that we called mediatophore should be considered as a key molecule for the generation of transmitter quanta in natural synapses.

Acetylcholine release and the cholinergic genomic locus

Choline acetyltransferase and vesicular acetylcholine-transporter genes are adjacent and coregulated. They define a cholinergic locus that can be turned on under the control of several factors, including the neurotrophins and the cytokines. Hirschprung's disease, or congenital megacolon, is characterized by agenesis of intramural cholinergic ganglia in the colorectal region. It results from mutations of the RET (GDNF-activated) and the endothelin-receptor genes, causing a disregulation in the cholinergic locus. Using cultured cells, it was shown that the cholinergic locus and the proteins involved in acetylcholine (ACh) release can be expressed separately ACh release could be demonstrated by means of biochemical and electrophysiological assays even in noncholinergic cells following preloading with the transmitter. Some noncholinergic or even nonneuronal cell types were found to be capable of releasing ACh quanta. In contrast, other cells were incompetent for ACh release. Among them, neuroblastoma N18TG-2 cells were rendered release-competent by transfection with the mediatophore gene. Mediatophore is an ACh-translocating protein that has been purified from plasma membranes of Torpedo nerve terminal; it confers a specificity for ACh to the release process. The mediatophores are activated by Ca2+; but with a slower time course, they can be desensitized by Ca2+. A strictly regulated calcium microdomain controls the synchronized release of ACh quanta at the active zone. In addition to ACh and ATP, synaptic vesicles have an ATP-dependent Ca2+ uptake system; they transiently accumulate Ca2+ after a brief period of stimulation. Those vesicles that are docked close to Ca2+ channels are therefore in the best position to control the profile and dynamics of the Ca2+ microdomains. Thus, vesicles and their whole set of associated proteins (SNAREs and others) are essential for the regulation of the release mechanism in which the mediatophore seems to play a key role.

Mediatophore and other presynaptic proteins. A cybernetic linking at the active zone

In rapidly transmitting synapses, the mediatophore, a protein located in the presynaptic membrane, seems to play a key role in the last step of transmitter release. Reconstituted either in proteoliposomes or in Xenopus oocytes, or transfected in particular cell lines, the mediatophore is able to release acetylcholine with characteristics which meet several typical features of transmitter release in natural synapses. Good correspondence between the two conditions was found for: i) the dependency of release upon calcium concentration; ii) the desensitisation of release by persistence of internal calcium; iii) the effect of several drugs; iv) the fleeting formation of a population of large intramembrane particles during the precise time of release; and v) the pulsatile or quantal nature of transmitter release. All these features therefore could well be ascribed to intrinsic properties of the mediatophore molecule. How is the mediatophore integrated in the whole presynaptic apparatus? To what extent is its function regulated by the other proteins of the active zone? These questions are far from being solved. We want nevertheless to propose here a general view in which characteristic presynaptic functions such as transmitter release, calcium entry, sequestration and extrusion, regulation of short- and long-term changes in release efficiency, are supported by an ordered succession of molecular events involving the proteins of the active zone. It will be seen that some proteins compete for a common binding site. It is thus expected that they will occupy this site in a regulated succession, according to simple cybernetic rules.

Ultrastructure and biophysics of acetylcholine release: central role of the mediatophore

We would like to review here some of the acquisitions gained by recent work in our two laboratories. Our approaches and results were intermingled and complementary. Thus we found it appropriate, for clarity and intelligibility, to merge them into a single chapter.

Ultrastructural changes induced by 12-O-tetradecanoylphorbol 13-acetate in pure cholinergic synaptosomes of Torpedo electric organ

We have studied the morphological changes induced by the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) treatment on pure cholinergic synaptosomes from Torpedo electric organ. These changes were studied by both ultrathin sections and freeze-fracture techniques. We found that after a treatment with TPA, a redistribution of synaptic vesicles inside the nerve endings and exocytotic images could be observed. Also, TPA, under conditions that induced the acetylcholine release, did not change the density of intramembrane particles at the synaptosomal protoplasmic hemimembrane leaflet. Similar results were found when calcium was not present in the extrasynaptosomal medium, and our results suggest that acetylcholine release induced by phorbol ester is probably mediated by exocytosis of synaptic vesicles.

Tetanus toxin blocks potassium-induced transmitter release and rearrangement of intramembrane particles at pure cholinergic synaptosomes

We have studied the action of tetanus toxin on the release of acetylcholine from a subcellular fraction of cholinergic nerve terminals (synaptosomes) isolated from the Torpedo electric organ. We have also studied the morphological changes induced by chemical stimulation on the presynaptic plasma membrane of poisoned synaptosomes. These changes were studied by means of freeze-fracture techniques. We found that tetanus toxin blocks the release of acetylcholine from isolated nerve terminals in a dose-dependent manner. The maximal inhibition is achieved at a concentration of 12.5 nM in 10 min. This effect is prevented by tetanus toxin antiserum. Tetanus toxin also blocks the rearrangement of intramembrane particles at plasma membrane of poisoned synaptosomes, specifically the decrease of small (less than or equal to 9.5 nm diameter) intramembranous particles at the protoplasmic hemimembrane leaflet and the increase of large (greater than 9.5 nm diameter) intramembrane particles at the external hemimembrane leaflet induced by potassium stimulation. These results suggest that intramembrane particle rearrangement could be related to acetylcholine secretion.

Increase in reactive cholesterol in the presynaptic membrane of depolarized Torpedo synaptosomes: blockade by botulinum toxin type A

We have investigated the redistribution of filipin-cholesterol complexes at freeze-fractured presynaptic membrane of pure cholinergic synaptosomes isolated from Torpedo electric organ during acetylcholine release. After chemical depolarization, filipin-induced lesions increase at the presynaptic membrane. These changes do not take place when synaptosomes are stimulated in a calcium-free medium. Botulinum neurotoxin type A blocks both acetylcholine release and the rearrangement of filipin-induced lesions induced by depolarization. Since botulinum neurotoxin type A does not block either membrane depolarization or calcium entry into the nerve terminal, our results suggest that the redistribution of filipin-cholesterol complexes is linked to the acetylcholine release process.

Botulinum toxin type A blocks the morphological changes induced by chemical stimulation on the presynaptic membrane of Torpedo synaptosomes

The action of botulinum neurotoxin on acetylcholine release, and on the structural changes at the presynaptic membrane associated with the transmitter release, was studied by using a subcellular fraction of cholinergic nerve terminals (synaptosomes) isolated from the Torpedo electric organ. Acetylcholine and ATP release were continuously monitored by chemiluminescent methods. To catch the membrane morphological changes, the quick-freezing method was applied. Our results show that botulinum neurotoxin inhibits the release of acetylcholine from these isolated nerve terminals in a dose-dependent manner, whereas ATP release is not affected. The maximal inhibition (70%) is achieved at neurotoxin concentrations as low as 125 pM with an incubation time of 6 min. This effect is not linked to an alteration of the integrity of the synaptosomes since, after poisoning by botulinum neurotoxin type A, they show a nonmodified occluded lactate dehydrogenase activity. Moreover, membrane potential is not altered by the toxin with respect to the control, either in resting condition or after potassium depolarization. In addition to acetylcholine release inhibition, botulinum neurotoxin blocks the rearrangement of the presynaptic intramembrane particles induced by potassium stimulation. The action of botulinum neurotoxin suggests that the intramembrane particle rearrangement is related to the acetylcholine secretion induced by potassium stimulation in synaptosomes isolated from the electric organ of Torpedo marmorata.

Transmission and scanning electron microscopy and freeze-fracture replication of normal human melanocytes and human melanoma cells in tissue culture

Basic LM, TEM, SEM, and FFR appearances of a pure line of normal human melanocytes derived from foreskin, and a human melanoma line, in cell culture are described. Normal melanocyte cultures exhibit side by side, cells of widely different melanogenic activities--possible clones--and melanosomes of bizarre shape and internal structure are frequent. Aggregates of melanosomes, with or without associated amorphous material, and with no discernible limiting membrane are present within many cells, and occasional simple specialised contacts occur between apposed cells. On replicas of plasma membrane of normal melanocytes, particle densities and diameters on P and E fracture faces were within the ranges for cells in general, and equivalent data for the melanoma cells were not significantly different. Similarly, there was no difference in density of distribution or diameter of nuclear pores between the normal and the tumoural cells.

Synaptic vesicle relationships with the presynaptic membrane as shown by a new method of fast chemical fixation

Brief vascular perfusion of the rat brain with a mixture of concentrated aldehydes completely insolubilized the brain protein in less than 30 s and yielded excellent ultrastructural preservation. Abundant synaptic vesicles closely and specifically attached to the presynaptic membrane were constantly detected. These vesicles appeared to undergo progressive transformation into amorphous, electron-dense material. No evidence of vesicle exocytosis was detected in the brains perfused in vivo but fixations performed 1 h after death showed abundant exocytotic-like images. The results suggest that the vesicles may not be exocytotically released to the intersynaptic cleft but disintegrate intracytoplasmically in the immediate vicinity of the presynaptic membrane.

Co-release of acetylcholine, glutamate and taurine from synaptosomes of Torpedo electric organ

The amino acid content of Torpedo synaptosomes was investigated. It was found that glutamate and taurine were particularly enriched, their concentration exceeding that of acetylcholine by 50-60%. Other amino acids such as alanine, gamma-aminobutyric acid (GABA), glycine, leucine and phenylalanine were also present, but in smaller quantities. The neurotransmitter-releasing agents gramicidin, calcium ionophore A23187, High K+ and Glycera venom all released acetylcholine, as shown previously. However, both glutamate and taurine were found to be released by these agents together with acetylcholine. The efflux of all 3 compounds was found to be largely calcium dependent.

Chronic ethanol consumption affects filipin-cholesterol complexes and intramembranous particles of synaptosomes of rat brain cortex

To assess the effect of ethanol on the planar distribution of cholesterol as well as on the surface architecture of presynaptic terminals of rats, synaptosomes isolated from cerebral cortex of rats chronically exposed to alcohol were incubated with filipin, a cytochemical marker for beta-hydroxycholesterol, and analyzed using both conventional (qualitative and quantitative) and freeze-fracture electron microscopy. Synaptosomes incubated in the absence of filipin were used as cytochemical controls. Biochemical determination indicates a 12% increase of cholesterol in synaptosomal membranes from alcohol treated rats. This increase was confirmed by a significant increment in the number of filipin-cholesterol complexes. Synaptosomes of treated rats showed a reduction in the total number of synaptic vesicles (SV) as well as a decrease in the density and total number of intramembranous particles (IMP) per synaptosome. In control rats, most synaptosomal IMP were distributed in clusters whereas in those of rats exposed to alcohol they were distributed at random. These changes in distribution of IMP were also observed in presynaptic terminals analyzed "in situ." These findings indicate that ethanol acts on the presynaptic terminals. The variations in cholesterol content as well as in the density and distribution of IMP appear to be related to alcohol-induced changes in the physicochemical properties of components of the synaptosomal membrane.

Structural changes at pure cholinergic synaptosomes during the transmitter release induced by A-23187 in Torpedo marmorata. A freeze-fracture study

Pure cholinergic synaptosomes isolated from the electric organ of Torpedo marmorata were stimulated by calcium ionophore A-23187. The effect of time course of stimulation on the changes in intramembrane particles (IMPs) on presynaptic membranes was studied by quick-freezing and aldehyde-fixation freeze-fracture. We showed that the decrease of small-particle density at the P-face and the increase of large-particle density at the E-face was maximum after 30 sec of A-23187 stimulation. Later, the density of synaptic vesicles decreased. We suggest that the redistribution of IMPs on the presynaptic membrane and acetylcholine (ACh) release from pure cholinergic synaptosomes have a similar time course when triggered by A-23187.

Spontaneous quantal and subquantal transmitter release at the Torpedo nerve-electroplaque junction

Focal electrodes were used to record the spontaneous miniature potentials generated on delimited patches of innervated membrane in the Torpedo electric organ. The main population of miniature potentials followed a bell-shaped amplitude distribution. In addition, we observed a second class of spontaneous events that were smaller and whose amplitude distribution was skewed. These subminiatures formed an homogenous population together with the regular miniatures with respect to their time course versus amplitude relationship. They were thus probably generated at the same sites. The proportion of potentials that were subminiature was less than 10% in resting, freshly excised tissue, but it increased markedly: (i) when the tissue was kept for 24-28 h in vitro after excision; (ii) in the period following a brief heat challenger or (iii) stimulation to exhaustion; and (iv) in the presence of dinitrophenol or dinitrofluorobenzene. In all these conditions, we measured the acetylcholine, adenosine 5'-triphosphate and creatine phosphate content of the tissue and found a correlation between the relative number of subminiature potentials and the lack of energy rich molecules. It is concluded that subminiature potentials are present in the electric organ as in neuromuscular junctions. They are probably produced at the same sites as the regular miniature potentials and their relative occurrence seems to increase greatly when the nerve terminals are in a state of energy deficiency.

Brief occurrence of a population of presynaptic intramembrane particles coincides with transmission of a nerve impulse

Small pieces of Torpedo electric organ were cryofixed at 1-ms time intervals in a liquid medium at -190 degrees C before, during, and after the passage of a single nerve impulse. In contrast to studies using this or other preparations, these experiments were done without 4-aminopyridine or other drugs that potentiate transmitter release. Freeze-fracture replicas were made from the most superficial layers of the tissue, where the rate of cooling was rapid enough to retain ultrastructure in the absence of chemical fixation. We found that the transmission of an impulse was accompanied by the momentary appearance of a population of large intramembrane particles in both the protoplasmic (P) and the external (E) leaflets of the presynaptic plasma membrane. The change was very brief, appearing soon after the stimulus artifact. It lasted for 2-3 ms. Large pits denoting vesicle openings at the presynaptic membrane were found in a small proportion of nerve terminals; their number did not increase during transmission of the nerve impulse. Reducing the temperature from 16 to 5 degrees C slowed the time course of both the electrophysiological response and the change in intramembrane particles. The number of large particles did not increase when stimulation was applied in a low-Ca medium, a condition where the nerve terminals were still depolarized by the action potential but did not release the neurotransmitter. From these and other observations, we conclude that this transient change of intramembrane particles is closely linked to the mechanism of acetylcholine release at the nerve-electroplaque junction.

Quantal release of acetylcholine evoked by focal depolarization at the Torpedo nerve-electroplaque junction

To analyse evoked acetylcholine (ACh) release in the electric organ of Torpedo marmorata, a loose patch-clamp technique was used that allowed with a single extracellular electrode both focal depolarization of nerve endings and recording of the post-synaptic currents produced by the released transmitter. Two different types of post-synaptic response could be evoked by depolarizing pulses of increasing intensity: a graded response appearing with a delay of 0.6 ms (pulses of 0.2 ms duration), and an all-or-none response characterized by a mean delay of 1.4 ms. Both responses had a similar maximal amplitude and a similar rise time of 0.6 ms. The graded response was evoked in all places where spontaneous miniature electroplaque currents (m.e.e.s) could be recorded. It was not modified by 1 microM-tetrodotoxin (TTX), but was Ca2+ dependent and was abolished by Cd2+ (0.2 mM) or Mg2+ (10 mM). The all-or-none response could be evoked in only 30% of places where m.e.c.s. were recorded, it was highly TTX sensitive, Ca2+ dependent, and abolished by Cd2+ (0.2 mM) or Mg2+ (10 mM). K+ channel blocking agents, such as 4-aminopyridine (4-AP) or tetraethylammonium (TEA), which are known to prolong the duration of action potentials, prolonged the delay of the all-or-none response, but not that of the graded response. At low strength stimulation, the graded response was clearly evoked in a quantal way, with the quantum corresponding to the amplitude of spontaneous m.e.c.s. The amplitude distribution of the evoked responses closely followed a Poisson distribution. The maximum synchronous release of transmitter was found to be approximately 1.3 quanta/micron2 of presynaptic membrane and a mean quantal size of about 7000 ACh molecules was estimated from the charge transfer of m.e.c.s. The nerve terminal time constant was calculated from strength-duration curves obtained with depolarizing pulses just able to evoke either the all-or-none response or the first few quanta of the graded response. Respective mean values of 0.22 and 0.40 ms were found. Increasing the duration of the depolarizing pulse had two consequences: it differently affected the delay of the all-or-none response and that of the graded response; it increased the mean quantal content of the graded response. Both effects could not simply be accounted for by the influence of the nerve terminal time constant.(ABSTRACT TRUNCATED AT 400 WORDS)

Increase in the number of presynaptic large intramembrane particles during synaptic transmission at the Torpedo nerve-electroplaque junction

Small pieces of Torpedo electric organ were treated with 4-aminopyridine, a drug which greatly increases the duration of transmitter release in a single nerve impulse, transforming the normally brief electroplaque potential to a giant discharge. Specimens of tissue were cryofixed by rapid freezing using liquid coolants at precise time intervals during transmission of a single giant discharge, and then examined by freeze fracture. In each experiment, we monitored the electrical response of one specimen during the freezing run to check the physiological responsiveness of the tissue and to determine the precise time of contact with the cryogenic liquid. The general appearance of nerve terminals after cryofixation was similar to that of terminals from chemically fixed and cryoprotected tissue. The major morphological change observed during the time course of the giant discharge was a marked increase in the density of intramembrane particles larger than 10 nm on both the protoplasmic and external faces of the presynaptic membrane. This change appeared in specimens frozen within the first few milliseconds after the stimulus, that is, at a time corresponding to the onset of the rising phase of the potential (3 ms). At the end of the giant discharge, the particle density returned to control values with the same time course as the potential trace. Pits of 20 nm or larger, probably due to vesicle-membrane interaction, were found in a small proportion of nerve terminals. Their occurrence increased only at 120-150 ms after the stimulus, that is, a long time after the beginning of the giant potential and of the change in intramembrane particles. The size distribution of particles was also determined in the membrane of synaptic vesicles exposed by cross fracture of terminal boutons; it was found to be similar to that of the unstimulated presynaptic membrane and it did not change during the giant discharge. Stimulation experiments were also carried out in a modified solution containing no added calcium, 20 mM magnesium and 4-aminopyridine. The propagation of impulses along the nerves to the electric organ was not inhibited in the modified solution but acetylcholine release was prevented and no increase in particle density was found on the presynaptic membrane. These and previous biochemical experiments on this tissue suggest that the release of the neuro-transmitter acetylcholine is associated with a transient occurrence of large intramembrane particles on the two fracture faces of the presynaptic membrane.(ABSTRACT TRUNCATED AT 400 WORDS)

Freeze-fracture analysis of synaptogenesis in glomeruli of mouse olfactory bulb

The relationship between intramembranous particle (IMP) aggregates appearing on the extracellular leaflet (E-face) of the postsynaptic membrane and postsynaptic densities was examined by electron microscopy during mouse olfactory bulb development. During prenatal development the IMP aggregates first increased in size and then decreased in size to the adult level, while the length of the postsynaptic densities tended to increase to a plateau. Concomitant with the size change, the shape of the IMP aggregates appeared to change during development from small, round clusters to large, anastomotic aggregations. Some of the IMP aggregates appeared to have a particle-free area in their centre. As development proceeded, the overall IMP density increased. The density of particles measuring 7-11 nm remained unchanged throughout prenatal life and decreased in the adult. These particles may be involved in stabilization of initial contacts and maintenance of mature synapses, rather than representing receptors or ion channels which would be expected to increase during development. The density of particles smaller than 7 nm increased prenatally, decreased at birth, then increased in the adult. These particles may represent two or more different macromolecules, one important in synaptogenesis, the other important in adult synapses.

Processes controlling sperm-egg fusion

Sperm interaction with the egg envelopes triggers the acrosome reaction. Indeed, sperm-egg fusion is accomplished by the fusion of the acrosomal process (or of the exposed inner acrosomal membrane in mammals) with the egg plasma membrane. Fusion must be preceded by the establishment of molecular contact between the two membranes. It is suggested that, as in the case of artificial phospholipid membranes, the two major obstacles to the establishment of molecular contact are electrostatic repulsion and the hydration barrier. It is argued that morphology of the acrosome is such as to favour the overcoming of such barriers. By analogy with the conditions governing fusion of artificial phospholipid membranes and cell fusion, it is proposed that the following processes play a role in sperm-egg fusion. The large calcium uptake accompanying the acrosome reaction may help fusion either through the known effect of calcium on fusion of phospholipid membranes or by shielding the surface charges of the acrosomal process. Fusogenic proteins at the surface of the acrosomal process are likely to play a role in the fusion of the acrosomal process with the egg plasma membrane. The activation of phospholipases in conjunction with the acrosome reaction may also be instrumental in sperm-egg fusion through the transient production of lysophosphatides. Clearance or translocation of intramembraneous proteins in the egg plasma membrane at the site of contact with the acrosomal process may also be required for fusion. Lastly it is suggested that a translocation or a conformational change of some proteins of the egg plasma membrane, which is required for fusion, may be induced by the depolarization of the egg plasma membrane that follows molecular contact with the acrosomal process.

A model for de novo synthesis and assembly of tight intercellular junctions. Ultrastructural correlates and experimental verification of the model revealed by freeze-fracture

The structure and function of intercellular tight (occluding) junctions, which constitute the anatomical basis for highly regulated interfaces between tissue compartments such as the blood-testis and blood-brain barriers, are well known. Details of the synthesis and assembly of tight junctions, however, have been difficult to determine primarily because no model for study of these processes has been recognized. Primary cultures of brain capillary endothelial cells are proposed as a model in which events of the synthesis and assembly of tight junctions can be examined by monitoring morphological features of each step in freeze-fracture replicas of the endothelial cell plasma membrane. Examination of replicas of non-confluent monolayers of endothelial cells reveals the following intramembrane structures proposed as 'markers' for the sequential events of synthesis and assembly of zonulae occludentes: development of surface contours consisting of elongate terraces and furrows (valleys) orientated parallel to the axis of cytoplasmic extensions of spreading endothelial cells, appearance of small circular PF face depressions (or volcano-like protrusions on the EF face) that represent cytoplasmic vesicle-plasma membrane fusion sites, which are positioned in linear arrays along the contour furrows, appearance of 13-15 nm intramembrane particles at the perimeter of the vesicle fusion sites, and alignment of these intramembrane particles into the long, parallel, anastomosed strands characteristic of mature tight junctions. These structural features of brain endothelial cells in monolayer culture constitute the morphological expression of: reshaping the cell surface to align future junction-containing regions with those of adjacent cells, delivery and insertion of newly synthesized junctional intramembrane particles into regions of the plasma membrane where tight junctions will form, and aggregation and alignment of tight junction intramembrane particles into the complex interconnected strands of mature zonulae occludentes. The distribution of filipin-sterol complex-free regions on the PF intramembrane fracture face of junction-forming endothelial plasmalemmae corresponds precisely to the furrows, aligned vesicle fusion sites and anastomosed strands of tight junctional elements.(ABSTRACT TRUNCATED AT 400 WORDS)

Relationship between presynaptic membrane potential and acetylcholine release in synaptosomes from Torpedo electric organ

The membrane potential of purely cholinergic synaptosomes isolated from Torpedo electric organ was monitored with fluorescent carbocyanine dyes. An increased fluorescence was associated with depolarization and a quenching with hyperpolarization. Fluorescence data provided evidence that Torpedo synaptosomes have a membrane potential mainly driven by a K+ diffusion potential and a membrane potential of about -50 mV could be estimated after calibration of fluorescence signals with ionophore antibiotics. The release of acetylcholine (ACh) from Torpedo synaptosomes was monitored continuously by measuring the light emitted by a chemiluminescent method (Israël & Lesbats, 1981 a). Using fluorescence data, the release of ACh was expressed as a function of membrane potential. The relationship between presynaptic potential and transmitter release as determined by biochemical methods at cholinergic nerve endings showed striking similarities to that observed at the squid giant synapse. Several substances were also tested with regard to their depolarizing and releasing properties and it was found that the toxin isolated from the venom of the annelid Glycera convoluta, which induced a large increase in quantal release of transmitter (Manaranche, Thieffry, & Israël, 1980) promoted a depolarization of Torpedo synaptosomes in addition to ACh release.

Reconstitution of a functional synaptosomal membrane possessing the protein constituents involved in acetylcholine translocation

Reconstitution of a functional presynaptic membrane possessing calcium-dependent acetylcholine release properties has been achieved. The proteoliposomal membrane obtained gains its acetylcholine-releasing capabilities from presynaptic membrane proteins. At the peak of acetylcholine release, intramembrane particles became more numerous in one of the proteoliposomal membrane faces. This phenomenon resembles the intramembrane particle rearrangements found in stimulated synaptosomes. No visible structures capable of releasing acetylcholine as a result of the calcium influx were found inside the proteoliposomes. This supports the view that the release of free cytosolic acetylcholine from stimulated nerve terminals can be directly attributed to presynaptic membrane proteins. These proteins were extracted in a functional form from the synaptosomal membrane.

Binding of a Glycera convoluta neurotoxin to cholinergic nerve terminal plasma membranes

The crude extract of venom glands of the polychaete annelid Glycera convoluta triggers a large Ca2+-dependent acetylcholine release from both frog motor nerve terminals and Torpedo electric organ synaptosomes. This extract was partially purified by Concanavalin A affinity chromatography. The biological activity was correlated in both preparations to a 300,000-dalton band, as shown by gel electrophoresis. This confirmed previous determinations obtained with chromatographic methods. This glycoprotein binds to presynaptic but not postsynaptic plasma membranes isolated from Torpedo electric organ. Pretreatment of intact synaptosomes by pronase abolished both the binding and the venom-induced acetylcholine release without impairing the high K+-induced acetylcholine release. Pretreatment of nerve terminal membranes by Concanavalin A similarly prevented the binding and the biological response. Binding to Torpedo membranes was still observed in the presence of EGTA. An antiserum directed to venom glycoproteins inhibited the neurotoxin so we could directly follow its binding to the presynaptic membrane. Glycera convoluta neurotoxin has to bind to a ectocellularly oriented protein of the presynaptic terminal to induce transmitter release.

Acetylcholine release from proteoliposomes equipped with synaptosomal membrane constituents

A lyophilized presynaptic membrane powder prepared from Torpedo electric organ synaptosomes was incorporated into liposomes. These proteoliposomes had a large internal volume. The P and E faces of their membrane showed particles which were comparable to the presynaptic membrane ones. The synaptosomal ecto-esterase activity was also incorporated. A large amount of acetylcholine could be entrapped in the proteoliposome which became permeable to acetylcholine in the presence of calcium. Acetylcholine was released in preference to choline. The calcium-induced acetylcholine release depended on the incorporation of a presynaptic membrane constituent. Proteoliposomes prepared from postsynaptic membrane powders gave a much slower acetylcholine efflux. The protein pattern of presynaptic and postsynaptic membrane proteoliposomes were compared.

Acetylcholine changes underlying transmission of a single nerve impulse in the presence of 4-aminopyridine in Torpedo

1. Transmission of a single nerve impulse has been investigated at the nerve-electroplaque junction of Torpedo marmorata in the presence of 4-aminopyridine (4-AP), a drug which powerfully potentiates evoked transmitter release.2. Three methodological approaches were used conjointly. These were (i) electrophysiological recording of the compound electroplaque potential (e.p.p.), (ii) radiochemical measurement of evoked acetylcholine (ACh) release and (iii) analysis of the content of ACh and ATP in the tissue at brief time intervals during the course of the e.p.p. and soon after. The last was achieved by using a stimulator coupled to a rapid tissue freezer.3. In the response to a single stimulus, 4-AP enhanced in a dose-dependent manner the size of the e.p.p., increasing the duration much more than the amplitude. At 10(-4) M-4-AP, this resulted in the generation of a characteristic ;giant e.p.p.' whose area (in V x ms) was approximately 120 times greater than that of a normal e.p.p.4. The giant e.p.p. consisted of an initial peak, lasting for some 100 ms, a late rebound at about 300 ms, and finished between 500 and 1000 ms after the stimulus. Temperature changes greatly affected the shape of the giant e.p.p., modifying particularly the amplitude and time course of the late rebound.5. The amount of ACh released in response to a single stimulus was measured radiochemically and was found to greatly increase in the presence of 4-AP, explaining the potentiation of the e.p.p. With 4-AP concentrations ranging from 10(-6) M to 10(-4) M, the augmentation of ACh release showed a close correlation with increase of the e.p.p. area.6. The large potentiation of evoked transmitter release occurred in spite of a reduction of ACh stores. After treatment with 10(-4) M-4-AP, the total ACh content was reduced by 30-40% in the absence of any electrical stimulation. The reduction affected to a similar extent the vesicular and extravesicular compartments of ACh. This was accompanied by a general increase in the resting rate of ACh turnover.7. Synaptic vesicles were isolated from small fragments of electric organ, rapidly frozen with our device. Compartmental analysis was carried out by labelling the transmitter pools with a radioactive precursor and it was confirmed that vesicular ACh has a relatively low metabolic rate, whereas free ACh (most probably cytoplasmic ACh) turns over more rapidly. The same finding was obtained after treatment with 4-AP, but the starting levels of ACh and the yield of synaptic vesicles were lower.8. The total ACh content was measured at 30 and 100 ms intervals during the course of the giant e.p.p., and soon after. We found characteristic and significant changes which were (i) an initial fall of total ACh occurring within 100-150 ms, (ii) a transient ACh increase which occurred later and seemed to correspond to the late rebound of the giant e.p.p. and (iii) a steady 20% lowering of total ACh, observed from the end of the giant e.p.p. and lasting for more than 1 s.9. The ATP content of the tissue, during and after the giant e.p.p., followed a time course which was remarkably similar to that of total ACh. A significant ATP/ACh relationship was found in most experiments separately, and in the pooled results with a higher degree of significance.10. Vesicular ACh did not exhibit any significant change during and after the giant e.p.p. Neither the transient initial variations of total ACh nor its later lowering were reflected in similar changes of vesicular ACh. It was therefore the extravesicular pool of ACh which was concerned in the characteristic pattern of changes of total ACh.11. Compartmental analysis of transmitter stores was performed during the course of transmission, after labelling ACh in the tissue with a radioactive precursor. It was found that no detectable transfer of ACh occurred from cytoplasm to vesicles, either during the giant e.p.p., or within the following second.12. The following conclusions were reached. The effect of 4-AP is to cause a very strong and long-lasting potentiation of ACh release, resulting in a giant and complex electrical discharge. Transmitter release under these conditions was not only due to sudden liberation of the preformed, available ACh but also to a marked contribution of new ACh made during the giant e.p.p. These changes in ACh content were very significant and took place exclusively in the extravesicular pool of transmitter.