Correlation between quantal secretion and vesicle loss at the frog neuromuscular junction

Biochemical and cytotoxic evaluation of latroeggtoxin-VI against PC12 cells

Latroeggtoxin-VI (LETX-VI) is a peptide neurotoxin discovered from Latrodectus tredecimguttatus eggs. In the current study, the action features of the neurotoxin on PC12 cells were systematically investigated. LETX-VI could promote dopamine release from PC12 cells in the absence and presence of Ca2+, involving an even more complex action mechanism in the presence of Ca2+ and when the treatment time was longer. Although LETX-VI enchanced the autophagy and secretion activity in PC 12 cells, it showed no remarkable influence on the proliferation, cell cycle, apoptosis and ultrastructure of the cells. Pulldown combined with CapLC-MS/MS analysis suggested that LETX-VI affected PC12 cells by interacting with multiple proteins involved in the metabolism, transport, and release of neurotransmitters, particularly dopamine. The low cytotoxicity and effective regulatory action of LETX-VI on PC12 cells suggest the potential of the active peptide in the development of drugs for the treatment of some dopamine-related psychotic diseases and cancers.

Penelope's web: using alpha-latrotoxin to untangle the mysteries of exocytosis

For more than three decades, the venom of the black widow spider and its principal active components, latrotoxins, have been used to induce release of neurotransmitters and hormones and to study the mechanisms of exocytosis. Given the complex nature of alpha--latrotoxin (alpha-LTX) actions, this research has been continuously overshadowed by many enigmas, misconceptions and perpetual changes of the underlying hypotheses. Some of the toxin's mechanisms of action are still not completely understood. Despite all these difficulties, the extensive work of several generations of neurobiologists has brought about a great deal of fascinating insights into pre-synaptic processes and has led to the discovery of several novel proteins and synaptic systems. For example, alpha-LTX studies have contributed to the widespread acceptance of the vesicular theory of transmitter release. Pre-synaptic receptors for alpha-LTX--neurexins, latrophilins and protein tyrosine phosphatase sigma--and their endogenous ligands have now become centrepieces of their own areas of research, with a potential of uncovering new mechanisms of synapse formation and regulation that may have medical implications. However, any future success of alpha-LTX research will require a better understanding of this unusual natural tool and a more precise dissection of its multiple mechanisms.

The origin of the skewed amplitude distribution of spontaneous excitatory junction potentials in poorly coupled smooth muscle cells

The skewed amplitude distribution of spontaneous excitatory junction potentials (sEJPs) in the mouse vas deferens and other electrically-coupled smooth muscle syncytia has been attributed to electrically-attenuated depolarizations resulting from the spontaneous release of quantized packets of ATP acting on remote smooth muscle cells (SMCs). However, in the present investigation surface SMCs of the mouse isolated vas deferens were poorly electrically coupled, with input resistances (176±18 MΩ, range: 141–221 MΩ, n=4) similar to those of dissociated cells. Furthermore, the amplitude of evoked EJPs was more variable in surface compared with deeper SMCs (F test, F=17.4, P<0.0001). Using simultaneous electrophysiology and confocal microscopy to investigate these poorly-coupled cells, it is shown that α-latrotoxin-stimulated sEJPs correlate, in timing (median delay ranged from −30 to −57 ms, P<0.05 in all experiments, n=5) and amplitude (Pearson product moment correlation, ρ>0.55 and P<0.001), with purinergic neuroeffector Ca²⁺ transients (NCTs) in SMCs. The temporal correlation between sEJPs of widely ranging amplitude with NCTs in the impaled SMC demonstrates that all sEJPs could arise from neurotransmitter action on the impaled cell and that the skewed distribution of sEJPs can be explained by the variable effect of packets of ATP on a single SMC. The amplitude correlation of sEJPs and NCTs argues against the attenuation of electrical signal amplitude along the length of a single SMC. The skewed sEJP amplitude distribution arising from neurotransmitter release on single SMCs is consistent with a broad neurotransmitter packet size distribution at sympathetic neuroeffector junctions.

Vesicular transmitter release from astrocytes

Astrocytes can release a variety of transmitters, including glutamate and ATP, in response to stimuli that induce increases in intracellular Ca(2+) levels. This release occurs via a regulated, exocytotic pathway. As evidence of this, astrocytes express protein components of the vesicular secretory apparatus, including synaptobrevin 2, syntaxin, and SNAP-23. Additionally, astrocytes possess vesicular organelles, the essential morphological elements required for regulated Ca(2+)-dependent transmitter release. The location of specific exocytotic sites on these cells, however, remains to be unequivocally determined.

The multiple actions of black widow spider toxins and their selective use in neurosecretion studies

The black widow spider venom contains several large protein toxins--latrotoxins--that are selectively targeted against different classes of animals: vertebrates, insects, and crustaceans. These toxins are synthesised as large precursors that undergo proteolytic processing and activation in the lumen of the venom gland. The mature latrotoxins demonstrate strong functional structure conservation and contain multiple ankyrin repeats, which mediate toxin oligomerisation. The three-dimensional structure has been determined for alpha-latrotoxin (alphaLTX), a representative venom component toxic to vertebrates. This reconstruction explains the mechanism of alphaLTX pore formation by showing that it forms tetrameric complexes, harbouring a central channel, and that it is able to insert into lipid membranes. All latrotoxins cause massive release of neurotransmitters from nerve terminals of respective animals after binding to specific neuronal receptors. A G protein-coupled receptor latrophilin and a single-transmembrane receptor neurexin have been identified as major high-affinity receptors for alphaLTX. Latrotoxins act by several Ca(2+)-dependent and -independent mechanisms based on pore formation and activation of receptors. Mutant recombinant alphaLTX that does not form pores has been used to dissect the multiple actions of this toxin. As a result, important insights have been gained into the receptor signalling and the role of intracellular Ca(2+) stores in the effect of alphaLTX.

Anti-GQ1b antibodies and evoked acetylcholine release at mouse motor endplates

Miller Fisher syndrome (MFS) is clinically characterized by ataxia, areflexia, and ophthalmoplegia, and is associated with serum anti-GQ1b-ganglioside antibodies. We have previously shown that anti-GQ1b antibodies induce complement-dependent, alpha-latrotoxin-like effects at mouse neuromuscular junctions (NMJs) in vitro. This effect comprises a massive increase in spontaneous quantal acetylcholine (ACh) release, accompanied by block of evoked release and muscle paralysis. This mechanism may contribute to the motor features of MFS. Whether the block of evoked ACh release is a primary effect of anti-GQ1b antibodies or occurs secondary to massive complement-dependent spontaneous release is unknown. Using conventional micro-electrode methods, we measured in detail ACh release evoked with low- and high-rate nerve stimulation, and studied the effect on it of a purified MFS IgG and a mouse monoclonal anti-GQ1b IgM (without added complement). We found that evoked transmitter release was unaffected. Control experiments proved binding of anti-GQ1b antibody at the NMJ. We conclude that the block of nerve-evoked ACh release at the NMJ is not a primary effect of anti-GQ1b antibodies, but is dependent on antibody-mediated complement activation. It remains to be determined whether the block of nerve-evoked ACh release is the consequence of massive spontaneous ACh release or occurs as a concomitant event.

Neurotoxins affecting neuroexocytosis

Nerve terminals are specific sites of action of a very large number of toxins produced by many different organisms. The mechanism of action of three groups of presynaptic neurotoxins that interfere directly with the process of neurotransmitter release is reviewed, whereas presynaptic neurotoxins acting on ion channels are not dealt with here. These neurotoxins can be grouped in three large families: 1) the clostridial neurotoxins that act inside nerves and block neurotransmitter release via their metalloproteolytic activity directed specifically on SNARE proteins; 2) the snake presynaptic neurotoxins with phospholipase A(2) activity, whose site of action is still undefined and which induce the release of acethylcholine followed by impairment of synaptic functions; and 3) the excitatory latrotoxin-like neurotoxins that induce a massive release of neurotransmitter at peripheral and central synapses. Their modes of binding, sites of action, and biochemical activities are discussed in relation to the symptoms of the diseases they cause. The use of these toxins in cell biology and neuroscience is considered as well as the therapeutic utilization of the botulinum neurotoxins in human diseases characterized by hyperfunction of cholinergic terminals.

Polypeptide neurotoxins from spider venoms

Spider venoms contain a variety of toxic components. The polypeptide toxins are divided into low and high molecular mass types. Small polypeptide toxins interacting with cation channels display spatial structure homology. They can affect the functioning of calcium, sodium, or potassium channels. A family of high molecular mass toxic proteins was found in the venom of the spider genus Latrodectus. These neurotoxins, latrotoxins, cause a massive transmitter release from a diversity of nerve endings. The latrotoxins are proteins of about 1000 amino acid residues and share a high level of structure identity. The structural and functional properties of spider polypeptide toxins are reviewed in this paper.

Multitude of ion channels in the regulation of transmitter release

The presynaptic nerve terminal is of key importance in communication in the nervous system. Its primary role is to release transmitter quanta on the arrival of an appropriate stimulus. The structural basis of these transmitter quanta are the synaptic vesicles that fuse with the surface membrane of the nerve terminal, to release their content of neurotransmitter molecules and other vesicular components. We subdivide the control of quantal release into two major classes: the processes that take place before the fusion of the synaptic vesicle with the surface membrane (the pre-fusion control) and the processes that occur after the fusion of the vesicle (the post-fusion control). The pre-fusion control is the main determinant of transmitter release. It is achieved by a wide variety of cellular components, among them the ion channels. There are reports of several hundred different ion channel molecules at the surface membrane of the nerve terminal, that for convenience can be grouped into eight major categories. They are the voltage-dependent calcium channels, the potassium channels, the calcium-gated potassium channels, the sodium channels, the chloride channels, the non-selective channels, the ligand gated channels and the stretch-activated channels. There are several categories of intracellular channels in the mitochondria, endoplasmic reticulum and the synaptic vesicles. We speculate that the vesicle channels may be of an importance in the post-fusion control of transmitter release.

Miller Fisher anti-GQ1b antibodies: alpha-latrotoxin-like effects on motor end plates

In the Miller Fisher syndrome (MFS) variant of the Guillain-Barré syndrome, weakness is restricted to extraocular muscles and occasionally other craniobulbar muscles. Most MFS patients have serum antibodies against ganglioside type GQ1b of which the pathophysiological relevance is unclear. We examined the in vitro effects of MFS sera, MFS IgG, and a human monoclonal anti-GQ1b IgM antibody on mouse neuromuscular junctions (NMJs). It was found that anti-GQ1b antibodies bind at NMJs where they induce massive quantal release of acetylcholine from nerve terminals and eventually block neuromuscular transmission. This effect closely resembled the effect of the paralytic neurotoxin alpha-latrotoxin at the mouse NMJs, implying possible involvement of alpha-latrotoxin receptors or associated downstream pathways. By using complement-deficient sera, the effect of anti-GQ1b antibodies on NMJs was shown to be entirely dependent on activation of complement components. However, neither classical pathway activation nor the formation of membrane attack complex was required, indicating the effects could be due to involvement of the alternative pathway and intermediate complement cascade products. Our findings strongly suggest that anti-GQ1b antibodies in conjunction with activated complement components are the principal pathophysiological mediators of motor symptoms in MFS and that the NMJ is an important site of their action.

Galanin receptor-mediated inhibition of glutamate release in the arcuate nucleus of the hypothalamus

It is thought that galanin, a 29 amino acid neuropeptide, is involved in various neuronal functions, including the regulation of food intake and hormone release. Consistent with this idea, galanin receptors have been demonstrated throughout the brain, with high levels being observed in the hypothalamus. However, little is known about the mechanisms by which galanin elicits its actions in the brain. Therefore, we studied the effects of galanin and its analogs on synaptic transmission using an in vitro slice preparation of rat hypothalamus. In arcuate nucleus neurons, application of galanin resulted in an inhibition of evoked glutamatergic EPSCs and a decrease in paired-pulse depression, indicating a presynaptic action. The fragments galanin 1-16 and 1-15 produced a robust depression of synaptic transmission, whereas the fragment 3-29 produced a lesser degree of depression. The chimeric peptides C7, M15, M32, and M40, which have been reported to antagonize some actions of galanin, all produced varying degrees of depression of evoked EPSCs. In a minority of cases, C7, M15, and M40 antagonized the actions of galanin. Analysis of mEPSCs in the presence of TTX and Cd2+, or after application of alpha-latrotoxin, indicated a site of action for galanin downstream of Ca2+ entry. Thus, our data suggest that galanin acts via several subtypes of presynaptic receptors to depress synaptic transmission in the rat arcuate nucleus.

Galanin receptor-mediated inhibition of glutamate release in the arcuate nucleus of the hypothalamus

It is thought that galanin, a 29 amino acid neuropeptide, is involved in various neuronal functions, including the regulation of food intake and hormone release. Consistent with this idea, galanin receptors have been demonstrated throughout the brain, with high levels being observed in the hypothalamus. However, little is known about the mechanisms by which galanin elicits its actions in the brain. Therefore, we studied the effects of galanin and its analogs on synaptic transmission using an in vitro slice preparation of rat hypothalamus. In arcuate nucleus neurons, application of galanin resulted in an inhibition of evoked glutamatergic EPSCs and a decrease in paired-pulse depression, indicating a presynaptic action. The fragments galanin 1-16 and 1-15 produced a robust depression of synaptic transmission, whereas the fragment 3-29 produced a lesser degree of depression. The chimeric peptides C7, M15, M32, and M40, which have been reported to antagonize some actions of galanin, all produced varying degrees of depression of evoked EPSCs. In a minority of cases, C7, M15, and M40 antagonized the actions of galanin. Analysis of mEPSCs in the presence of TTX and Cd2+, or after application of alpha-latrotoxin, indicated a site of action for galanin downstream of Ca2+ entry. Thus, our data suggest that galanin acts via several subtypes of presynaptic receptors to depress synaptic transmission in the rat arcuate nucleus.

Are some minis multiquantal?

The amplitude distribution of miniature postsynaptic currents (minis) in many central neurons has a large variance and positive skew, but the sources of this variance and skew are unresolved. Recently it has been proposed that spontaneous Ca2+ influx into a presynaptic bouton with multiple release sites could cause spontaneous multiquantal minis by synchronizing release at all sites in the bouton, accounting for both the large variance and skew of the mini distribution. We tested this hypothesis by evoking minis with internally perfused, buffered Ca2+ and the secretagogue alpha-latrotoxin, both in the absence of external Ca2+. With these manipulations, the synchronized release model predicts that the mini distribution should collapse to a Gaussian distribution with a reduced coefficient of variation. Contrary to this expectation, we find that mini amplitude distributions under these conditions retain a large variance and positive skew and are indistinguishable from amplitude distributions of depolarization-evoked minis, strongly suggesting that minis are uniquantal.

Neuromuscular transmission and its pharmacological blockade. Part 1: Neuromuscular transmission and general aspects of its blockade

Blockade of neuromuscular transmission is an important feature during anaesthesia and intensive care treatment of patients. The neuromuscular junction exists in a prejunctional part where acetylcholine is synthesized, stored and released in quanta via a complicated vesicular system. In this system a number of proteins is involved. Acetylcholine diffuses across the junctional cleft and binds to acetylcholinereceptors at the postjunctional part, and is thereafter metabolized by acetylcholinesterase in the junctional cleft. Binding of acetylcholine to its postjunctional receptor evokes muscle contraction. Normally a large margin of safety exists in the neuromuscular transmission. In various situations, apart from up-and-down regulation of acetylcholine receptors, adjustment of acetylcholine release can occur. Pharmacological interference can interrupt the neuromuscular transmission and causes muscle relaxation. For this reason both depolarizing and non-depolarizing muscle relaxants are clinically used. The characteristics of an ideal clinical muscle relaxant are defined. In the description of the pharmacology of the relaxants the importance of pharmacodynamic and pharmacokinetic parameters are defined. Stereoisomerism plays a role with the relaxants. Toxins and venoms also interfere with neuromuscular transmission, through both pre- and postjunctional mechanisms.

Alpha-latrotoxin-induced transmitter release in feline oesophageal smooth muscle: focus on nitric oxide and vasoactive intestinal peptide

1. The effects of alpha-latrotoxin (alpha LTX) on muscle tone, resting membrane potential, cyclic nucleotide content, and ultrastructure were examined in feline oesophageal smooth muscle, including the lower oesophageal sphincter (LOS). 2. In circular smooth muscle strips from LOS developing active tone alpha LTX (1 nM) induced a 94 +/- 3% (n = 16) relaxation. Intermittent treatment with alpha LTX for 4 h abolished the response. Pretreatment with NG-nitro-L-arginine (L-NOARG; 0.1 mM) attenuated the relaxation. 3. In carbachol-contracted circular smooth muscle strips from the LOS and oesophageal body (OB), alpha LTX induced a 95 +/- 5% (n = 6) and 73 +/- 9% (n = 8) relaxation, respectively. The relaxations were attenuated by L-NOARG, and in LOS strips, the relaxation was abolished by the combination of L-NOARG and vasoactive intestinal peptide (VIP)-antiserum (1:25). At resting tension in circular smooth muscle strips from the OB, alpha LTX induced a scopolamine sensitive contraction in the presence of L-NOARG. 4. In circular LOS and OB preparations, alpha LTX changed the resting membrane potential from -49 +/- 2mV to -59 +/- 3 mV (n = 4), and -62 +/- 2 mV to -71 +/- 3 mV (n = 4), respectively. 5. The alpha LTX-induced relaxation of LOS and OB muscle was associated with a 138% and 72% increase in cyclic GMP levels, respectively. No changes in cyclic AMP levels were observed. 6. Ultrastructural analysis of LOS and OB revealed a rich supply of nerve profiles containing small synaptic and large dense core vesicles. alpha LTX treatment resulted in a loss of both types of vesicle. 7. These results suggest that alpha LTX induces relaxation of oesophageal circular smooth muscle associated with NO-generation and transmitter release from synaptic vesicles. Beside NO, VIP seems to be involved in the relaxant effects of alpha LTX on the LOS. In addition, alpha LTX may have contractile effects by release of acetylcholine.

Isolation and biochemical characterization of a Ca2+-independent alpha-latrotoxin-binding protein

alpha-Latrotoxin, a black widow spider neurotoxin, can bind to high affinity receptors on the presynaptic plasma membrane and stimulate massive neurotransmitter release in the absence of Ca2+. Neurexins, previously isolated as alpha-latrotoxin receptors, require Ca2+ for their interaction with the toxin and, thus, may not participate in the Ca2+-independent alpha-latrotoxin activity. We now report the isolation of a novel protein that binds alpha-latrotoxin with high affinity in the presence of various divalent cations (Ca2+, Mg2+, Ba2+, and Sr2+) as well as in EDTA. This protein, termed here latrophilin, has been purified from detergent-solubilized bovine brain membranes by affinity chromatography on immobilized alpha-latrotoxin and concentrated on a wheat germ agglutinin affinity column. The single polypeptide chain of latrophilin is N-glycosylated and has an apparent molecular weight of 120,000. Sucrose gradient centrifugations demonstrated that latrophilin and alpha-latrotoxin form a stable equimolar complex. In the presence of the toxin, anti-alpha-latrotoxin antibodies precipitated iodinated latrophilin, whose binding to immobilized toxin was characterized by a dissociation constant of 0.5-0.7 nM. This presumably membrane-bound protein is localized to and differentially distributed among neuronal tissues, with about four times more latrophilin expressed in the cerebral cortex than in the cerebellum; subcellular fractionation showed that the protein is highly enriched in synaptosomal plasma membranes. Our data suggest that latrophilin may represent the Ca2+-independent receptor and/or molecular target for alpha-latrotoxin.

Hypertonic treatment reversibly increases the ratio of giant skew-miniature endplate potentials to bell-miniature endplate potentials

Miniature endplate potentials were recorded from single frog muscle fibers before, during and after treatment with hypertonic saline (200-500 mM NaCl or Na gluconate added to frog saline). Miniature endplate potential amplitude distributions were plotted from small muscle fibers so that the modes and ratios of the skew-miniature endplate potential to bell-miniature endplate potential classes could be defined. Muscle fibers were voltage clamped with two electrodes to determine the input resistance before, during and after treatment. Input resistance increased from two to 100 times during treatment and rapidly fell towards control values (no more than 30% greater) when preparations were returned to normal frog saline. Short duration treatments with 200-300 mM hypertonic salines immediately increased frequencies (100-fold) of both skew-miniature endplate potential and bell-miniature endplate potential classes. Preparations when returned to normal frog saline after a few minutes of treatment showed control miniature endplate potential distributions within minutes. One to two hour treatments left only the skew-miniature endplate potential class and with hour-long recovery periods bell-miniature endplate potentials reappeared and ratios of skew-miniature endplate potential to bell-miniature endplate potential classes returned to control values. Treatment with 500 mM NaCl added to frog saline immediately increased the percentage of skew-miniature endplate potentials (from 2 to 50%) with little or no increase in overall miniature endplate potential frequencies. The mode of the skew-miniature endplate potential class was unchanged after hypertonic treatment, whereas that of the bell-miniature end plate potential class either remained about the same size or decreased depending on the duration of treatment. The number and percentage of giant-miniature endplate potentials belonging to the skew-miniature endplate potential class increased as a function of the duration of 200-300 mM hypertonic saline treatments. Most giant-miniature endplate potentials had a slow rising phase with a foot and/or breaks demonstrating a composite structure. Sequentially recorded giant-miniature endplate potentials had similar initial slopes indicating either repetitive releases from single sites or releases from cooperative sites. After hypertonic treatment the bell-miniature endplate potential size was never more than that expected with the increase (under 30%) in input resistance. The results presented here are completely different from those of Yu and Van der Kloot [(1991) J. Physiol. 433, 677-704] who reported that the bell-miniature endplate potential amplitude was increased two- to four-fold after hypertonic treatment. The wide range of results in the ratio of skew-miniature endplate potential to bell-miniature endplate potential classes is discussed in regards to the quantal hypothesis which is based on a single class of immutable amounts of transmitter; and, a hypothesis based on a dynamical process that meters transmitter in subunit amounts to control miniature endplate potential size and class during release.

Effects of cytochalasin treatment on short-term synaptic plasticity at developing neuromuscular junctions in frogs

1. The role of actin microfilaments in synaptic transmission was tested by monitoring spontaneous and evoked transmitter release from developing neuromuscular synapses in Xenopus nerve-muscle cultures, using whole-cell recording of synaptic currents in the absence and presence of microfilament-disrupting agents cytochalasins B and D. 2. Treatment with cytochalasins resulted in disruption of microfilament networks in the growth cone and the presynaptic nerve terminal of spinal neurons in Xenopus nerve-muscle cultures, as revealed by rhodamine-phalloidin staining. 3. The same cytochalasin treatment did not significantly affect the spontaneous or evoked synaptic currents during low-frequency stimulation at 0.05 Hz in these Xenopus cultures. Synaptic depression induced by high-frequency (5 Hz) stimulation, however, was reduced by this treatment. Paired-pulse facilitation for short interpulse intervals was also increased by the treatment. 4. These results indicate that disruption of microfilaments alters short-term changes in transmitter release induced by repetitive activity, without affecting normal synaptic transmission at low frequency. 5. Our results support the notion that actin microfilaments impose a barrier for mobilization of synaptic vesicles from the reserve pool, but do not affect the exocytosis of immediately available synaptic vesicles at the active zone.

Population trends in the fine spatial re-organization of synaptic elements in forebrain regions of chicks 0.5 and 24 hours after passive avoidance training

Two regions in the forebrain of domestic chicks (Gallus domesticus), the intermediate and medial hyperstriatum ventrale and the lobus parolfactorius, have previously been shown to be important centres of biochemical, pharmacological and physiological change following one-trial passive avoidance training. The purpose of the present study was to examine, at the electron microscopic level, the fine spatial re-arrangement of synaptic structures in the intermediate and medial hyperstriatum ventrale (at 30 min), and in the lobus parolfactorius (at 24 h), post-training using comprehensive biometrical designs, image analysis and stochastic approaches. In intermediate and medial hyperstriatum ventrale, no significant differences in the numerical density of synapses either between control and trained chicks, or between hemispheres, were revealed using the disector method. However, after training, a nested-ANOVA demonstrated an increase in the thickness of pre- and post-synaptic electron densities (estimated via image analysis) only in the left intermediate and medial hyperstriatum ventrale, whereas synaptic apposition zone profiles increased in length bilaterally. In presynaptic terminals from the intermediate and medial hyperstriatum ventrale, stochastic analysis revealed that training resulted in the re-distribution of synaptic vesicles between two spatial pools relative to synaptic apposition zones, in both hemispheres producing a large number of synaptic vesicles closer to synaptic apposition zones; a nearest neighbour analysis of synaptic apposition zone profiles indicated that the lateral shape of the synaptic apposition zone after training is more complex in both hemispheres. In the lobus parolfactorius at 24 h post-training the main changes in synaptic fine structure involved a shift of synaptic vesicles away from synaptic apposition zones in the right hemisphere with the distance between synaptic apposition zones decreasing; in the left lobus parolfactorius, synaptic apposition zones became more regular/round in shape with a greater distance between them after training. These data suggest that the initial acquisition of memory involves population changes in the fine spatial organization of synaptic vesicles and synaptic apposition zones in synapses in the intermediate and medial hyperstriatum ventrale, which indicate a possible tendency towards greater synaptic efficacies. These changes are as dynamics as the molecular changes which have hitherto been considered the preserve of short-term correlates of memory formation.

alpha-latrotoxin is a potent inducer of neurotransmitter release in Torpedo electric organ--functional and morphological characterization

In this report we show that alpha-latrotoxin from black widow spider venom is a potent activator of neurotransmitter release in synaptosomes from the Torpedo electric organ. Binding of the purified toxin (5 nM) to the synaptosomal fraction occurs already at 4 degrees C and is dependent on the presence of divalent ions. However, neurotransmitter release commences only after temperature elevation (22 degrees C) and is completed within 2 min. The effect of alpha-latrotoxin on release is achieved at 1 nM and is already saturated at 5 nM. The release is stimulated by the presence of Ca2+ ions. Activation of release by alpha-latrotoxin is accompanied by morphological changes in electric organ synaptosomes. The synaptosomes swell, resulting in a 55% increase in section area. Moreover, the number of synaptic vesicles per unit area decreases about three-fold, and rows of docked synaptic vesicles are rarely detected as opposed to control synaptosomes. These morphological changes indicate that the massive release is mainly due to synaptic vesicle fusion. alpha-Latrotoxin binding sites are highly concentrated in the innervated face of the electrocytes. Immunoelectron microscopy on electric organ sections reveals alpha-latrotoxin binding sites over the entire plasma membrane at release sites and facing Schwann cells surrounding Torpedo nerve terminals. Surprisingly, a high concentration of binding sites is also found at structures surrounding branching unmyelinated axons. This staining is in close proximity to Schwann cell envelopes and to the basal lamina around axonal tips. The mode of action of alpha-latrotoxin in view of the localization of its binding sites is discussed.

Alpha-latrotoxin stimulates glutamate release from cortical astrocytes in cell culture

The mechanism responsible for the ability of bradykinin to cause calcium-dependent release of glutamate from astrocytes in vitro was investigated. The glutamate transport inhibitor, dihydrokainate, did not block bradykinin-induced glutamate release, and bradykinin did not cause cell swelling. These data exclude the involvement of glutamate transporters or swelling mechanisms as mediating glutamate release in response to bradykinin. alpha-Latrotoxin (3 nM), a component of black widow spider venom, stimulated calcium-independent glutamate release from astrocytes. Since alpha-latrotoxin induces vesicle fusion and calcium-independent neuronal neurotransmitter release, our data suggest that astrocytes may release neurotransmitter using a mechanism similar to the neuronal secretory process.

How can exocytosis account for the actual properties of miniature synaptic signals?

It is broadly accepted that a postsynaptic "miniature" is the most elementary chemically transmitted signal and results from the all-or-none release of transmitter packaged in a single presynaptic vesicle. Hitherto, it has not been possible to directly verify this renowned representation, although it is consistent with evidence of vesicle traffic and, following an intense period of release, vesicle depletion. However, vesicle traffic involving molecular components similar to those implicated in transmitter release has been attributed to other functions including membrane repair. Furthermore, as a number of investigators have recently proposed, miniature signals recorded at peripheral and central synapses may actually reflect several rather than a single discharge of transmitter. It is not clear whether such putative multiple-discharge miniatures represent near-synchronous exocytoses of several vesicles or a burst of openings in a pore that couples a vesicle with the outer membrane. In any case, despite the popularity of the vesicular hypothesis, the molecular mechanism involved in synchronizing fast elementary secretion has not yet been elucidated. Interdependencies among subminiature discharges composing a miniature have suggested that the underlying process is a regenerative signal restricted to a presynaptic terminal unit, confirming Fatt and Katz's first speculation on miniatures, which was not vesicular exocytosis [Fatt and Katz (1952), J. Physiol., 117:109-128]. Here we discuss the possibility that this regenerative signal might be a localized cytosolic Ca2+ transient and attempt to reconcile this hypothesis with the exocytotic models proposed to explain fast transmitter release.

alpha-Latrotoxin receptor. Implications in nerve terminal function

alpha-Latrotoxin is a potent stimulator of neurotransmitter release from nerve terminals. High affinity membrane alpha-latrotoxin receptor was purified in an active binding form. It is a membrane glycoprotein (M(r) 160,000-220,000) which may be complexed to a smaller polypeptide (M(r) 29,000). The structure of the receptor protein suggests that it may be a synapse-specific cell recognition molecule. Intracellularly, the alpha-latrotoxin receptor interacts with synaptotagmin, a calcium- and phospholipid-binding protein specifically localized in the synaptic vesicle membrane. This interaction may be important for targeting of synaptic vesicles to presynaptic release sites.

Spatial re-arrangement of the vesicle apparatus in forebrain synapses of chicks 30 min after passive avoidance training

A quantitative ultrastructural study of synapses was carried out in the forebrain IMHV (intermediate and medial hyperstriatum ventrale) of 1-day-old chicks 30 min after training to avoid pecking at a bead coated with methyl anthranilate. In 10 birds (5 control and 5 trained), the length, curvature and the number of the synaptic active zone profiles were measured, and the active zone profile length was observed to increase in trained chicks. The spatial arrangement of synaptic vesicles with respect to the active zone was examined using a statistical stereological approach. This showed that vesicles are not located uniformly but accumulate in two spatial pools which appear to rearrange following training, with a greater number of vesicles near the active zone. These data may reflect subtle changes in the functional efficacy of synapses in the IMHV in the initial phases of memory formation.

Storage and release of neurotransmitters

Because synaptic vesicles and secretory granules are simple in composition and easy to purify, many of their protein components have been identified and often sequenced. Attempts are underway to link the small number of membrane proteins to the small number of functions the vesicles perform. The discovery of sequence homologies has helped greatly with this. In addition, techniques that have begun to prove successful involve microinjection, identification of proteins that bind synaptic vesicle proteins, DNA transfection into cells and oocytes, and more recently, in vitro reconstitution of exocytosis, endocytosis, and vesicle biogenesis. Advances in the latter areas have been strongly influenced by the breakthroughs in our knowledge of membrane traffic in nonneuronal cells. The budding reactions involved in making synaptic vesicles and secretory granules resemble in many ways the generation of carrier vesicles from the ER and the Golgi complex. Finally, exocytosis in neurons may closely resemble fusion of carrier vesicles with target organelles in nonneuronal cells, using complexes of peripheral membrane proteins, GTP hydrolysis, and integral membrane proteins with fusogenic domains. The usefulness of in vitro reconstitution, reverse genetics, and the parallels with better understood systems compensates in part for a major weakness in the field, namely the difficulty in obtaining viable mutants that are defective in the storage and release of secretory vesicle content.

Frequency modulation of transmitter release

In 1952 Fatt and Katz recorded at a frog neuromuscular junction while stimulating the nerve and found "... that successive endplate potential responses varied in a step-like manner, corresponding to units of miniature endplate potentials" (J Physiol 117, 109-128). This led them to propose that fast neuromuscular transmission is 'quantal'. Quantal release is now commonly ascribed to a vesicular form of neurosecretion since vesicles have routinely been visualized in presynaptic terminals. The vesicular hypothesis (Del Castillo and Katz, 1955) assumes that quanta, or 'transmitter packets of standard size', are assembled and stored in the numerous vesicles routinely identified in micrographs of virtually all central and peripheral presynaptic nerve terminals. Simply stated, this model predicts that each one of the miniature synaptic signals (MSSs) follows from the exocytosis of one vesicle's contents. However, the time required for membrane fusion preceding exocytosis (Almers and Tse, 1990) and the variability in MSS amplitude and time course (Vautrin et al, 1992a,b) cannot readily be reconciled by a simple, exocytotic model of quantal release from preloaded vesicles. These difficulties with the original model have led us to re-evaluate MSSs generated at the classical peripheral synapse, the cholinergic neuromuscular junction of the mouse diaphragm, as well as at central synapses between embryonic hippocampal neurons mediated by gamma-aminobutyric acid (GABA). At these synapses, the release of GABA is also assumed to have classical quantal properties like peripheral acetylcholine release (Edwards et al, 1990). Our results show that at both synapses, progressive alterations in elementary signal properties can be induced in a remarkably rapid manner. The original report of preferred amplitudes and intervals in the spontaneous miniature signals (Fatt and Katz, 1952) has repeatedly been confirmed and is here incorporated into a dynamic model of fast synaptic transmission. Although MSSs exhibit variable rise-times and peak amplitudes, they can both be described in terms of synchronization of transmitter release. We have reviewed many experimental findings, which together strongly suggest that the original interpretation of Fatt and Katz (1952) regarding MSSs as reflecting the non-propagated 'neurogenic' activity of 'terminal spots' may be a useful concept to pursue since it may help to explain part of the underlying molecular basis of quantal release.(ABSTRACT TRUNCATED AT 400 WORDS)

The spatial pattern of the synaptic vesicular apparatus as a correlate of transmitter storage models

A statistical stereological approach which allows one to derive a three-dimensional pattern of synaptic vesicle accumulation in relation to the active zone from an analysis of electron micrograms (random sections) of synapses is described. This approach is illustrated with a study of presynaptic terminals from the dorsal horn of the cat spinal cord, based on the morphometrical treatment of 105 micrographs containing 5190 synaptic vesicles. The spatial pattern obtained was found to have a bimodal shape, which can be considered a possible structural correlate of the two-pool model of transmitter storage. The connection of similar quantitative estimates with physiological data is discussed.

Active zone organization and vesicle content scale with bouton size at a vertebrate central synapse

A common observation in studies of neuronal structure is that axons differ in the size of their synaptic boutons. The significance of this size variation is unclear, in part because we do not know how the size of synaptic boutons is related to their internal organization. The present study has addressed this issue by using three-dimensional reconstruction of serial thin sections to examine the ultrastructure of synaptic boutons that vary in size. Our observations are based on complete or near-complete reconstructions of 53 synaptic boutons contacting large neurons in the ventromedial gray matter of the upper cervical spinal cord (probable neck motor neurons). We characterized bouton size in terms of volume and total area of membrane apposed to the motor neuron surface (apposition area). Boutons vary in apposition area by a factor of 40, and there is a significant positive correlation between our two measures of bouton size. In addition, bouton size is systematically related to four ultrastructural variables: 1) total active zone area, 2) number of active zones, 3) individual active zone area, and 4) number of synaptic vesicles. The correlations between these variables and both of our measures of bouton size are positive and significant. These data suggest that bouton size may be an index of ultrastructural features that are thought to influence transmitter storage and release.