Isolation of pure cholinergic nerve endings from Torpedo electric organ. Evaluation of their metabolic properties

Effect of N,N'-dicyclohexylcarbodiimide on acetylcholine release from Torpedo synaptosomes and proteoliposomes reconstituted with the proteolipid mediatophore

The mediatophore is a presynaptic membrane protein that has been shown to translocate acetylcholine (ACh) under calcium stimulation when reconstituted into artificial membranes. The mediatophore subunit, a 15-kDa proteolipid, presents a very high sequence homology with the N,N'-dicyclohexylcarbodiimide (DCCD)-binding proteolipid subunit of the vacuolar-type H(+)-ATPase. This prompted us to study the effect of DCCD, a potent blocker of proton translocation, on calcium-dependent ACh release. The present work shows that DCCD has no effect on ACh translocation either from Torpedo synaptosomes or from proteoliposomes reconstituted with purified mediatophore. However, using [14C]DCCD, we were able to demonstrate that the drug does bind to the 15-kDa proteolipid subunit of the mediatophore. These results suggest that although the 15-kDa proteolipid subunits of the mediatophore and the vacuolar H(+)-ATPase may be identical, different domains of these proteins are involved in proton translocation and calcium-dependent ACh release and that the two proteins have a different membrane organization.

Binding of botulinum neurotoxin to pure cholinergic nerve terminals isolated from the electric organ of Torpedo

Torpedo electric organ has been used to study the binding of botulinum neurotoxin type A to pure cholinergic synaptosomes and presynaptic plasma membrane. 125I-labeled botulinum neurotoxin type A exhibits specific binding to cholinergic fractions. Two binding sites have been determined according to data analysis: a high affinity binding site (synaptosomes: Kd = 0.11 +/- 0.03 nM, Bmax = 50 +/- 10 fmol.mg prot-1; presynaptic plasma membrane: Kd = 0.2 +/- 0.05 nM, Bmax = 150 +/- 15 fmol.mg prot-1) and a low affinity binding site (synaptosomes: Kd approximately 26 nM, Bmax approximately 7.5 pmol.mg prot-1; presynaptic plasma membrane: Kd approximately 30 nM, Bmax approximately 52 pmol.mg prot-1). The binding of 125I-botulinum neurotoxin type A is decreased by previous treatment of synaptosomes by neuraminidase and trypsin, and by a preincubation with bovine brain gangliosides or antiserum raised against Torpedo presynaptic plasma membrane. When presynaptic plasma membranes are blotted to nitrocellulose sheet, either 125I-botulinum neurotoxin or botulinum toxin-gold complexes bind to a M(r) approximately 140,000 protein. Botulinum toxin-gold complexes have also been used to study the toxin internalization process into Torpedo synaptosomes. The images fit the three step sequence model in the pathway of botulinum neurotoxin poisoning.

Pulsatile release of acetylcholine by nerve terminals (synaptosomes) isolated from Torpedo electric organ

1. Electrophysiological detection of acetylcholine (ACh) release by synaptosomes from the electric organ of Torpedo was searched for by laying the isolated nerve terminals on a culture of Xenopus embryonic muscle cells (myocytes), and by recording the ACh-induced inward currents in the myocytes. 2. Whole-cell recording in one of the myocytes revealed rapid inward currents that where generated soon after synaptosome application. These pulsatile events strongly resembled those occurring normally during the early phase of synaptogenesis after nerve-muscle contact in Xenopus cell cultures. They were called spontaneous synaptic currents (SSCs). 3. The SSCs produced by the synaptosomes had a rapid time course, with mean time-to-peak and half-decay times of 2.6 +/- 0.4 ms and 6.0 +/- 1.1 ms, respectively. Most events had a falling phase that could be fitted with a single exponential. The mean time constant of decay was 6.2 +/- 1.1 ms. More than half of the SSCs (approximately 60%) constituted a rather homogenous population in which the time-to-peak versus amplitude showed a positive relationship, the smallest events displaying a shorter time course. The rest of the SSCs had a more variable and slower time course. Such events are also observed in young and mature junctions in situ. 4. The amplitudes of SSCs had a wide distribution which was skewed towards the smallest values. The mean amplitude was 65.2 +/- 16.1 pA. 5. During the minutes following an application of synaptosomes, the frequency of the SSCs tended to decrease, but their mean amplitude remained constant. Such behaviour could be reproduced during several successive additions of synaptosomes while recording in the same myocyte. 6. Just after synaptosome application, the SSCs were superposed to a noisy inward current that lasted for 20-60 s. Noise analysis of this current gave the values of 0.7 +/- 0.1 pA for the mean amplitude of the elementary event, and 4.7 +/- 0.2 ms for its mean duration, values that compare well with those reported for the activation of frog embryonic nicotinic receptor. This suggests that the noisy current was due to ACh molecules set free by synaptosomes which were either damaged or which released ACh at some distance. This view was strengthened by biochemical analysis of ACh release by synaptosomes in vitro. 7. Tubocurarine reversibly abolished the appearance of both the noise and the synaptosome-generated SSCs, showing that these currents were due to the action of ACh.(ABSTRACT TRUNCATED AT 400 WORDS)

Omega-conotoxin differentially blocks acetylcholine and adenosine triphosphate releases from Torpedo synaptosomes

We have examined the effect of several blockers of voltage-sensitive calcium channels on the release of acetylcholine and ATP from synaptosomes isolated from Torpedo marmorata electric organ. Depolarization of these nerve terminals with high K(+)-containing solutions resulted in a calcium-dependent release of both molecules. Cadmium ions (10(-6) to 10(-3) M) inhibited similarly both releases whereas nickel ions (10(-4) M) in the external medium did not affect either neurotransmitter or nucleotide release. Both releases were completely resistant to the effect of 1,4-dihydropyridines (antagonists nimodipine, nifedipine and agonist Bay K 8644) and of a related compound (diltiazem) at concentrations up to 10(-5) M. These drugs failed to cause any effect even when synaptosomes were submaximally depolarized during incubation. Omega-conotoxin (10(-8) to 5 x 10(-5) M) showed a differential effect on acetylcholine and ATP releases. Nucleotide release was inhibited 90% at the highest concentration tested (50 microns) while acetylcholine release was only moderately decreased (30%). EC50 values for acetylcholine and ATP were of 167 and 2 microM respectively. The results suggest the implication of different types of calcium channels in the release of these molecules.

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.

Effects of potassium depolarization on intracellular compartmentalization of ATP in cholinergic synaptosomes isolated from Torpedo electric organ

It is well known that acetylcholine (ACh) and ATP are co-stored and co-released in nerve terminals of the electric organ of Torpedo. Cholinergic synaptosomes were subjected to a cycle of freezing and thawing showing that ATP is distributed in two operational pools like those described for ACh. The bound pool is resistant to freezing and thawing, and it is presumably protected by membranes. When metabolically active ATP was prelabelled with [3H]adenosine, 76% of the radioactivity was associated with the free pool of ATP. When the preparation was depolarized in a calcium containing medium, there was a decrease in the specific radioactivity of ATP in the free pool and an increase in the bound pool. These results reflect that the patterns of distribution of ACh and ATP, in this synaptosomal preparation, are similar in resting conditions and during K+ depolarization.

Characterization of a synaptosomal ATP diphosphohydrolase from the electric organ of Torpedo marmorata

A true ecto-apyrase (ATP diphosphohydrolase, EC 3.6.1.5) enzyme was found in the synaptosomal fraction from the electric organ of the electric ray Torpedo marmorata. The activity could not be attributed to the combined action of different enzymes. The pH requirement and calcium dependence were the same for hydrolysis of both substrates ADP and ATP. The enzyme had an apparent Km value of 117 microM for ATP and of 123 microM for ADP. The involvement of nonspecific phosphatases in the hydrolysis of both substrates was excluded. The enzyme hydrolyses almost equally well different nucleoside di- and triphosphates. ATP and ADP hydrolysis was not inhibited by seven ATPase inhibitors, i.e., sodium azide, dinitrophenol, ruthenium red, oligomycin, ouabain, sodium orthovanadate and lanthanum.

The metabolic fate of [3H-methyl]choline in cultured human neuroblastoma cell lines, LA-N-1 and LA-N-2

The conversion of choline in cultures of the human neuroblastoma cell lines, LA-N-1 and LA-N-2 cells, was investigated in order to identify potential precursors in acetylcholine (AcCho) synthesis. LA-N-1, a catecholaminergic and LA-N-2, a cholinergic, cell line were incubated with [3H-methyl]choline (Cho) for varying periods of time up to 72 h. The radioactivity present in lipids and water-soluble metabolites increased linearly up to 24 h in both cell lines. Approximately 20% of the radioactivity associated with the water-soluble metabolites in both control (untreated) and retinoic acid-induced differentiated (RA-treated cells) LA-N-2 cells was present as Cho and AcCho. There was no detectable AcCho in the catecholaminergic cell line, LA-N-1. The untreated and RA-treated LA-N-1 and LA-N-2 cells were labeled for 24 h with [3H-methyl]Cho, followed by a chase in growth medium containing 100 microM unlabeled choline. The distribution of radioactivity in the LA-N-2 cells was 6-10% of AcCho, 84-89% as phosphocholine (PCho), 1-3% as glycerophosphocholine (GroPCho), and 2-4% as Cho. The distribution of radioactivity in the LA-N-1 cells was similar except for the absence of AcCho. The distribution of radioactivity in the culture medium of LA-N-1 cells was 70-80% as Cho, 20-30% as PCho, and 1-3% as GroPCho. In contrast, the radioactivity was equally distributed between Cho (50%) and PCho (50%), with only 1-3% as GroPCho in the medium of LA-N-2 cells.

Phospholipid transverse diffusion in synaptosomes: evidence for the involvement of the aminophospholipid translocase

We have studied in Torpedo marmorata electric organ synaptosomes the equilibration kinetics of spin-labeled phospholipid analogues initially incorporated into the outer plasma membrane monolayer. As assayed by evoked releases of both ATP and acetylcholine, the nerve endings were closed vesicles containing an energy source. The aminophospholipids (phosphatidylethanolamine and phosphatidylserine) were translocated toward the inner membrane leaflet faster and to a higher extent than their choline-containing counterparts (phosphatidylcholine and sphingomyelin). This difference was abolished by incubation of synaptosomal membranes with N-ethylmaleimide, suggesting that the accumulation of aminophospholipids in the inner layer was driven by a protein. This phenomenon is comparable with what was described in plasma membranes of other eucaryotic cells (erythrocyte, lymphocyte, platelet, fibroblast), and thus we would suggest that an aminophospholipid translocase, capable of moving the aminophospholipids from the outer to the inner layer at the expense of ATP, is also present in the synaptosomal plasma membrane.

Phorbol esters induce neurotransmitter release in cholinergic synaptosomes from Torpedo electric organ

The effect of phorbol esters and so the involvement of Ca2+/phospholipid-dependent protein kinase (protein kinase C;PKC) in the release of acetylcholine (ACh) was studied using Torpedo electric organ synaptosomes. 12-O-Tetradecanoylphorbol 13-acetate (TPA), a known activator of PKC, induced neurotransmitter release in a concentration-dependent manner and increased the potassium-evoked release of ACh. The effect of TPA was shown to be independent of the extrasynaptosomal calcium concentration. TPA-induced ACh release was reversed by H-7, an inhibitor of PKC activity. This drug showed no effect on potassium-evoked ACh release. Botulinum toxin, a strong blocker of potassium-induced ACh release in that synaptosomal preparation, showed no inhibitory effect on the TPA-induced ACh release. Our results suggest that activation of PKC potentiates the release of an ACh pool that is not releasable by potassium depolarization, independently of the extracellular calcium concentration.

Acetylcholine and ATP are coreleased from the electromotor nerve terminals of Narcine brasiliensis by an exocytotic mechanism

Although the exocytotic mechanism for quantal acetylcholine (ACh) release has been widely accepted for many years, it has repeatedly been challenged by reports that ACh released upon stimulation originates from the cytosol rather than synaptic vesicles. In this report, two independent experimental approaches were taken to establish the source of ACh released from the electromotor system of Narcine brasiliensis. Since ATP is colocalized with ACh in the cholinergic vesicle, the exocytotic theory predicts the corelease of these two components with a stoichiometry identical to that of the vesicle contents. The stimulated release of ATP from isolated synaptosomes could be accurately quantitated in the presence of the ATPase inhibitor adenosine 5'-[alpha, beta-methylene]triphosphate (500 microM), which prevented degradation of the released ATP. Various concentrations of elevated extracellular potassium (25-75 mM), veratridine (100 microM), and the calcium ionophore ionomycin (5 microM) all induced the corelease of ACh and ATP in a constant molar ratio of 5-6:1 (ACh/ATP), a stoichiometry consistent with that established for the vesicle content. In parallel to these stoichiometry studies, the compound 2-(4-phenylpiperidino)cyclohexanol (AH5183) was used to inhibit specifically the vesicular accumulation of newly synthesized (radiolabeled) ACh without affecting cytosolic levels of newly synthesized ACh in cholinergic nerve terminals. Treatment with AH5183 (10 microM) was shown to inhibit the release of newly synthesized ACh without markedly affecting total ACh release; thus, the entry of newly synthesized ACh into the synaptic vesicle is essential for its release. We conclude that ACh released upon stimulation originates exclusively from the vesicular pool and is coreleased stoichiometrically with other soluble vesicle contents.

The action of notexin from tiger snake venom (Notechis scutatus scutatus) on acetylcholine release and compartmentation in synaptosomes from electric organ of Torpedo marmorata

At rest, in the presence of calcium, notexin induced a rapid and concentration-dependent leakage of acetylcholine from nerve endings. In the presence of 20 nM notexin (5 min), synaptosomes were well-preserved structurally and they responded to addition of A23187 ionophore by a normal calcium-dependent acetylcholine release. When stimulated by high-K+ depolarization, evoked acetylcholine release was increased when notexin was present. These findings demonstrate that notexin (up to 100 nM) does not inhibit the acetylcholine release process itself. Further studies on intracellular acetylcholine compartmentation showed that, in the presence of calcium, nm concentrations of notexin were able to mobilize vesicular acetylcholine, the amount of which strongly decreased and fed the cytoplasmic compartment leading to an important redistribution of the neurotransmitter. Other metabolic studies under notexin confirmed the inhibition of the synaptosomal membrane choline transport, but failed to elicit changes in the choline acetyltransferase activity. In order to distinguish between the phospholipase A2 activity of notexin and its neurotoxic effects, we compared effects of notexin to those obtained with a non-neurotoxic pancreatic phospholipase A2. The latter exhibits similar effects but at a higher range of concentration than notexin.

The release of adenosine at the electric organ of Torpedo. A study using a continuous chemiluminescent method

Acetylcholine and ATP are costored and coreleased during synaptic activity at the electric organ of Torpedo. It has been suggested that released ATP is converted to adenosine at the synaptic cleft, and in turn this nucleoside would depress the evoked release of acetylcholine. In the present communication we have used a chemiluminescent reaction that let us to monitor continuously the presence of adenosine in this preparation. The chemiluminescent reaction is based on the conversion of adenosine into uric acid and H2O2 by adenosine deaminase, nucleoside phosphorylase, and xanthine oxidase enzymes. The hydrogen peroxide has been detected by peroxidase-luminol mixture. The reaction has a sensitivity on the picomol range and discerned between Adenosine, AMP, ADP, and ATP. We have developed this technique in the hope of understanding whether adenosine is released during synaptic activity or it comes from the released ATP. We have studied the release or formation of adenosine in fragments of the electric organ and in isolated cholinergic nerve terminals obtained from it. In both conditions we have followed the effect of potassium stimulation upon the detection of adenosine. Potassium stimulation increased the extracellular adenosine either in slices or the synaptosomal fraction of Torpedo electric organ. The presence of alpha, beta-methylene ADP, an inhibitor of 5'-nucleotidase, inhibits the detection of adenosine, suggesting that extracellular adenosine is a consequence of ectocellular dephosphorylation of released ATP.

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.

A monoclonal antibody raised against plasma membrane of cholinergic nerve terminals of the Torpedo inhibits choline acetyltransferase activity and acetylcholine release

Monoclonal antibodies were raised against the synaptosomal plasma membranes (SPMs) purified from the electric organ of the Torpedo. One antibody that reacts preferentially with SPMs rather than with other membrane fractions isolated from this tissue was previously found to inhibit hydrophilic and amphiphilic choline-O-acetyltransferase (ChAT) activity. On immunoblots of SPMs, this antibody recognizes two polypeptides of 135 and 66 kilodaltons that are related; the 66-kilodalton polypeptide appears to exist as a monomer and as a dimer in SPMs. The antibody was also able to inhibit the calcium-dependent release of acetylcholine in Torpedo synaptosomes without affecting the total neurotransmitter content. This inhibition was dependent on the antibody concentration and was observed when the release was elicited by either KCl depolarization or the calcium ionophore A23187; this suggests that inhibition was not mediated by a blockage of the depolarization-activated calcium influx. The inhibition could not be prevented by atropine, a result indicating that the antibody does not block release by mimicking the action of acetylcholine on presynaptic muscarinic autoreceptors. Thus, the antigen recognized by this antibody appeared to be involved in acetylcholine release; this antigen could be membrane-bound ChAT, another protein of the SPMs, or both.

Effect of cetiedil analogs on acetylcholine and choline fluxes in synaptosomes and vesicles

Cetiedil is a potent blocker of acetylcholine and choline fluxes. Several analogs of this compound have been synthesized and their effect on acetylcholine (ACh) and choline fluxes in synaptosomes and on ACh uptake in synaptic vesicles have been studied. The effects of these analogs were compared to those of other drugs acting on cholinergic functions. All these compounds were also studied in relation to the ACh translocating properties of the mediatophore, a protein recently purified from cholinergic nerve membranes and probably involved in the final step of release. We thus obtained a pattern of drug action for the three functions under study. The patterns of drug action on ACh release from synaptosomes or proteoliposomes were similar and clearly different from those for either synaptosomal choline transport or vesicular ACh uptake. In addition, the main outlines of the structure-function relationship for each of the functions studied, are described for cetiedil analogs.

Compared effects of two vesicular acetylcholine uptake blockers, AH5183 and cetiedil, on cholinergic functions in Torpedo synaptosomes: acetylcholine synthesis, choline transport, vesicular uptake, and evoked acetylcholine release

We examined the effects of two drugs, AH5183 and cetiedil, demonstrated to be potent inhibitors of acetylcholine (ACh) transport by isolated synaptic vesicles on cholinergic functions in Torpedo synaptosomes. AH5183 exhibited a high specificity toward vesicular ACh transport, whereas cetiedil was shown to inhibit both high-affinity choline uptake and vesicular ACh transport. Choline acetyltransferase was not affected by either drug. High external choline concentrations permitted us to overcome cetiedil inhibition of high-affinity choline transport, and thus synthesis of [14C]ACh in treated preparations was similar to that in controls. We then tested evoked ACh release in drug-treated synaptosomes under conditions where ACh translocation into the vesicles was blocked. We observed that ACh release was impaired only in cetiedil-treated preparations; synaptosomes treated with AH5183 behaved like the controls. Thus, this comparative study on isolated nerve endings allowed us to dissociate two steps in drug action: upstream, where both AH5183 and cetiedil are efficient blockers of the vesicular ACh translocation, and downstream, where only cetiedil is able to block the ACh release process.

Acetylcholine synthesis and secretion by LA-N-2 human neuroblastoma cells

We have investigated the rates of acetylcholine (ACh) synthesis and release in LA-N-2 cells in order to characterize them as a potential model of cholinergic neurons. When grown in a serum-containing medium the cells extend few neurites. In the absence of serum most cells develop processes. ACh content of the cells (determined by a radioenzymatic assay) varies with extracellular choline concentration in a saturable fashion, reaching a maximum of approximately 25 nmol/mg protein. Radiolabeled choline is taken up by the cell and converted to ACh or phosphocholine, as determined by purification from cell extracts by HPLC, in a saturable manner which is described by a single rectangular hyperbola. Hemicholinium-3 (100 microM) inhibits this uptake. The cells release ACh spontaneously and this release is enhanced upon depolarization with potassium or veratridine (the latter effect is blocked by tetrodotoxin). The data demonstrate that LA-N-2 cells exhibit some properties similar to cholinergic neurons and may therefore be useful for studies of ACh synthesis and release.

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.

GABAergic modulation of acetylcholine release in cholinergic synaptosomes from Torpedo marmorata electric organ

GABAA- and GABAB-receptor-specific agonists inhibit the depolarization-evoked release of acetylcholine in cholinergic synaptosomes from Torpedo electric organ. Over 60% of the release is inhibited by a 10(-4) M concentration of GABA itself. IC50s for muscimol and baclofen are 1.3 x 10(-4) and 2.2 x 10(-6) M, respectively. The effect of muscimol is totally blocked by the direct antagonist bicuculline methiodide, and also by the allosteric antagonists methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate, picrotoxinin and tert-butylbicyclo-orthobenzoate; the effect of baclofen is blocked by delta-aminovalerate. Furthermore, the inhibitory action of muscimol on acetylcholine release is substantially enhanced by flunitrazepam and pentobarbital. These results suggest the existence of typical GABAA and GABAB receptors in the presynaptic nerve terminals of the Torpedo electric organ regulating the liberation of acetylcholine and therefore the discharge of the electroplaques.

Binding of [3H]hemicholinium-3 to the high-affinity choline transporter in electric organ synaptosomal membranes

Sodium-dependent binding of [3H]hemicholinium-3 was observed to be 10-fold higher with presynaptic membranes from the electric organ than with electroplaque membranes and this binding site copurified with synaptosomal membranes. The KD for specific [3H]hemicholinium-3 binding was found to be 31 +/- 4 nM and the Bmax, 5.0 +/- 0.2 pmol/mg protein; a Ki of 16 nM was estimated for hemicholinium-3 as a competitive inhibitor of high-affinity choline transport in electric organ synaptosomes. Choline and choline analogues were equally potent as inhibitors of [3H]choline uptake and [3H]hemicholinium-3 binding. Tubocurarine and oxotremorine also inhibited uptake and binding, but carbachol was without effect in both tests. These findings suggest that [3H]hemicholinium binds to the high-affinity choline transporter present at the cholinergic nerve terminal membrane. A comparison of maximal velocities for choline transport and the maximal number of hemicholinium-3 binding sites indicated that the high-affinity choline transporter has an apparent turnover number of about 3s-1 at 20 degrees C under resting conditions. The high transport rates observed in electric organ synaptosomes are likely due to the high density of high-affinity choline transporters in this tissue, estimated on the basis of [3H]hemicholinium-3 binding to be of the order of 100/micron2 of synaptosomal membrane.

Ouabain induces acetylcholine release from pure cholinergic synaptosomes independently of extracellular calcium concentration

We have studied the correlation between [3H]ouabain binding sites, (Na+ + K+)ATPase (EC 3.6.1.3) activity and acetylcholine (ACh) release in different subcellular fractions of Torpedo marmorata electric organ (homogenate, synaptosomes, presynaptic plasma membranes). Presynaptic plasma membranes contained the greater number of [3H]ouabain binding sites, in good agreement with the high (Na+ + K+)ATPase activity found in this fraction. Blockade of this enzymatic activity by ouabain dose-dependently induced ACh release from pure cholinergic synaptosomes, either in the presence or absence of extracellular calcium ions. We suggest that one of the mechanisms involved in the ouabain-induced ACh release in the absence of Ca2+o may be an increase in Na+i that could (a) evoke Ca2+ release from internal stores and (b) inhibit ATP-dependent Ca2+ uptake by synaptic vesicles.

Structural and functional correlates of synaptic transmission in the vertebrate neuromuscular junction

Because vertebrate neuromuscular junctions are readily accessible for experimental manipulation, they have provided a superb model in which to examine and test functional correlates of chemical synaptic transmission. In the neuromuscular synapse, acetylcholine receptors have been localized to the crests of the junctional folds and visualized by a variety of ultrastructural techniques. By using ultrarapid freezing techniques with a temporal resolution of less than 1 msec, quantal transmitter release has been correlated with synaptic vesicle exocytosis at discrete sites called "active zones." Mechanisms for synaptic vesicle membrane retrieval and recycling have been identified by using immunological approaches and correlated with endocytosis via coated pits and coated vesicles. In this review, available ultrastructural, physiological, immunological, and biochemical data have been used to construct an ultrastructural model of neuromuscular synaptic transmission that correlates structure and function at the molecular level.

Selective distinction at equilibrium between the two alpha-neurotoxin binding sites of Torpedo acetylcholine receptor by microtitration

The binding of the monoiodinated alpha-neurotoxin I from Naja mossambica mossambica to the membrane-bound acetylcholine receptor from Torpedo marmorata was investigated using a new picomolar-sensitive microtitration assay. From equilibrium binding studies a non-linear Scatchard plot demonstrated two populations of binding sites characterized by the two dissociation constants Kd1 = 7 +/- 4 pM and Kd2 = 51 +/- 16 pM and having equal binding capacities. These two populations differed in their rate of dissociation (k-1.1 = 25 x 10(-6) s-1 and k-1.2 = 623 x 10(-6) s-1 respectively), but not in their rate of formation of the toxin-receptor complex (k + 1 = 11.7 x 10(6) M-1 s-1). From these rate constants the same two values of dissociation constant were deduced (Kd1 = 2 pM and Kd2 = 53 pM). All the specific binding was prevented by the cholinergic antagonists alpha-bungarotoxin and d-tubocurarine. In addition, a biphasic competition phenomenon allowed us to differentiate between two d-tubocurarine sites (Kda = 103 nM and Kdb = 13.7 microM respectively). Evidence is provided indicating that these two sites are shared by d-tubocurarine and alpha-neurotoxin I, with inverse affinities. Fairly conclusive agreement between our equilibrium, kinetic and competition data demonstrates that the two high-affinity binding sites for this short alpha-neurotoxin are selectively distinguishable.

Calcium-independent release of acetylcholine from electric organ synaptosomes and its changes by depolarization and cholinergic drugs

Chemiluminescent detection was applied to measure the continuous spontaneous Ca2+-independent liberation of acetylcholine (ACh) from Torpedo electric organ synaptosomes. Differentiation between the release of ACh and choline was achieved by inhibiting cholinesterases with phospholine, and a way to quantify the continuous release was devised. The method permitted measurements during short time intervals from minute amounts of tissue and without an accumulation of ACh in the medium. Synaptosomes continuously liberated small amounts of ACh during incubations in the presence of 3 mM K+ and in the absence of Ca2+. The spontaneous liberation of ACh was similar both quantitatively and qualitatively at pH values of 8.6 and 7.8. It was unaltered by MgCl2 (10.4 mM), 2-(4-phenylpiperidino)cyclohexanol (10 microM), ouabain (104 microM), atropine (10 microM), and valinomycin (102 nM). Carbamoylcholine brought about a decrease, which could be partially reversed by atropine. The Ca2+-independent output of ACh was increased considerably when the concentration of K+ ions was raised (eightfold at 103 and 35-fold at 203 mM K+). Carbamoylcholine (104 microM) blocked the increase in ACh release produced by high K+; this effect of carbamoylcholine was not reversed by atropine (10 microM). When Ca2+ was added to synaptosomes depolarized by a high concentration of K+, the amount of ACh released during the first 1-3 min after the addition of Ca2+ was at least 20 times higher than in the absence of Ca2+, but the release returned rapidly to predepolarization values. Similarly high values of ACh release could be achieved by adding Ca2+ plus the ionophore A23187 and even higher values by adding Ca2+ plus gramicidin.(ABSTRACT TRUNCATED AT 250 WORDS)

Energy metabolism and quantal acetylcholine release: effects of botulinum toxin, 1-fluoro-2,4-dinitrobenzene, and diamide in the Torpedo electric organ

In the Torpedo electric organ, a modified nerve-muscle system, type A botulinum toxin blocked the release of acetylcholine (ACh) quanta, both neurally evoked and spontaneous. At the same time, the toxin increased the release of a class of small miniature potentials (the subminiature potentials), reduced the ATP and more the creatine phosphate content of the tissue, and impaired the activity of creatine kinase (CK). Thus, we compared this pattern of changes with those provoked by 1-fluoro-2,4-dinitrobenzene (FDNB), an efficient inhibitor of CK. As expected, FDNB rapidly inactivated CK, which resulted in a profound depletion of ATP whereas the stores of creatine phosphate were preserved. In addition, FDNB caused conspicuous morphological alterations of nerve endings and ACh depletion. This agent also suppressed evoked and spontaneous quantal release whereas the occurrence of subminature potentials was markedly increased. Diamide, a penetrating thiol oxidizing substance, provoked first a transient rise in quantal ACh release and then blockade of transmission with, again, production of a large number of subminiature potentials. Creatine phosphate was depleted in the tissue by diamide, the ATP content reduced, and CK activity partly inhibited. The morphology of nerve terminals did not show obvious changes with either diamide or botulinum toxin at the stage of transmission failure. Although the three poisons acted by different mechanisms, this resulted in a rather similar pattern of physiological changes: failure of quantal release and enhancement of subquantal release. These results and experiments on synaptosomes indicated that CK inhibition was probably a crucial mechanism for FDNB but not for diamide or botulinum intoxication.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Calcium-induced desensitization of acetylcholine release from synaptosomes or proteoliposomes equipped with mediatophore, a presynaptic membrane protein

A "fatigue" of acetylcholine (ACh) release is described in cholinergic synaptosomes stimulated with the calcium ionophore A23187 or gramicidin. A small conditioning calcium entry, which did not trigger a large ACh release, led to a decrease of transmitter release elicited by a second large calcium influx. This fatigue was half-maximal at approximately 30 microM external calcium and developed in a few minutes. In contrast, activation of release by calcium was very rapid and was half-maximal at approximately 0.5 mM external calcium. Activation and desensitization of release could be attributed to the recently identified presynaptic membrane protein, the "mediatophore." Proteoliposomes equipped with purified mediatophore showed a calcium-dependent activation and "fatigue" of ACh release similar to that of synaptosomes. It was found that the ionophore A23187 rapidly equilibrated internal and external calcium concentrations in proteoliposomes. Thus, the external calcium concentration gave the internal concentration required for activation or desensitization of proteoliposomal ACh release. The mediatophore showed remarkable calcium binding properties (20 sites/molecule) with a KD of 25 microM. The physiological implications of desensitization on the organization of release sites are discussed.

Cetiedil, a drug that inhibits acetylcholine release in Torpedo electric organ

The effects of cetiedil, a vasodilatator substance with reported anticholinergic properties, were examined on cholinergic presynaptic functions at the nerve electroplaque junction of Torpedo marmorata using either synaptosomes or slices of intact tissue. Cetiedil abolished the calcium-dependent release of acetylcholine (ACh) triggered by depolarization or by addition of A23187 ionophore, a finding localizing the site of action downstream from the calcium entry step. In addition, a direct effect on the release process itself was indicated by the observation that cetiedil blocks the release of ACh mediated by a recently isolated presynaptic membrane protein, the mediatophore, reconstituted into ACh-containing proteoliposomes. In all three preparations, ACh release was inhibited by cetiedil with a Ki of 5-8 microM. Under the conditions used in these release experiments, the synthesis of ACh and its compartmentation within the nerve terminals were not modified. However, the drug was able to reduce high-affinity choline uptake and vesicular ACh incorporation when it was given together with the radioactive precursor, a result showing that cetiedil has a broad inhibitory action on cholinergic uptake processes.

Botulinum neurotoxin inhibits depolarization-stimulated protein phosphorylation in pure cholinergic synaptosomes

Botulinum neurotoxin, a strong blocker of acetylcholine release at peripheral cholinergic synapses, inhibits depolarization-stimulated protein phosphorylation in pure cholinergic synaptosomes isolated from the electric organ of Torpedo marmorata. Moreover, tetrodotoxin has the same effect on protein phosphorylation when cholinergic synaptosomes are depolarized by veratridine. Correlation between presynaptic protein phosphorylation and acetylcholine release is suggested by the fact that botulinum neurotoxin blocks specifically neurotransmitter release without affecting membrane depolarization and calcium fluxes in our synaptosomal preparation.

Monoclonal antibody Tor 23 recognizes a determinant of a presynaptic acetylcholinesterase

A significant proportion of the acetylcholinesterase that is present in the electric organ of Torpedo californica exists as a presynaptic membrane molecule. The monoclonal antibody Tor 23 binds the Torpedo presynaptic nerve membrane where it recognizes a polypeptide of 68,000 daltons. Our present studies indicate that Tor 23 identifies acetylcholinesterase. From the homogenates of Torpedo nerve terminals, Tor 23 immunoprecipitates measurable esterase activity. Esterase precipitation was not observed with no Tor 23 added; nor was it observed with any other test antibodies, including other Tor antibodies, in particular, Tor 70, which binds, as does Tor 23, to the presynaptic nerve membrane. The esterase activity was specific for acetylcholinesterase. Our studies indicate the molecule defined by Tor 23 has the solubility properties described for that of presynaptic acetylcholinesterase: it is soluble in detergent-treated electroplax homogenates and insoluble in high-salt extractions. In sections of Torpedo back muscle, both nerve and endplate acetylcholinesterase can be detected histochemically. Tor 23 localizes to the nerve and is not clustered at the endplate. The utility of the antibody Tor 23 thus includes biochemical and histological analyses of the multiple forms of acetylcholinesterase.

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.

ATP release from pure cholinergic synaptosomes is not blocked by tetanus toxin

Tetanus toxin (TeTx) is a neurotransmission impairing toxin that acts on several neurotransmitter systems. TeTx also inhibits the K+-induced release of acetylcholine (ACh) from synaptosomes isolated from the electric organ of Torpedo. Neither the membrane potential and depolarization, nor the depolarization-induced calcium uptake into cholinergic nerve terminals is modified after TeTx poisoning. On the other hand, it is known that, when cholinergic nerve terminals are stimulated, there is a release of ATP associated with the release of ACh. We have explored the action of TeTx on this co-release, and have found that there is no action of TeTx on the nucleotide release. Thus, TeTx blocks ACh release without modifying ATP release.

Characterization of elapidae snake venom components using optimized reverse-phase high-performance liquid chromatographic conditions and screening assays for alpha-neurotoxin and phospholipase A2 activities

The vast majority of Elapidae snake venoms, genus Naja, includes three classes of toxic polypeptides: alpha-neurotoxins, phospholipases A2, and cardiotoxins. A new experimental approach using reverse-phase high-performance liquid chromatography in particular has been developed, allowing their respective resolution, identification, and quantitation from milligram quantities of venom. First, definition of optimal chromatographic conditions for Naja mossambica mossambica toxins has been ascertained. Different column packing and solvent systems were compared for their efficiency, with particular attention to the ionic strength of the aqueous solvent. A medium-chain alkyl support (octyl) in conjunction with a volatile ammonium formate (0.15 M, pH 2.70)/acetonitrile solvent system was found to be particularly effective. All the components known until now from this venom could be resolved in a single step, and the elution order was alpha-neurotoxins, phospholipases A2, and cardiotoxins with a total recovery of absorbance and toxicity. Then, with these suitable conditions, we describe a new major cardiotoxin molecule in this venom by hydrophobic and not ionic-charge discrimination. Second, specific assays were designed to detect alpha-neurotoxin and phospholipase A2 activities in chromatographic fractions: alpha-neurotoxin activity was determined by competition for the binding of a radiolabeled alpha-neurotoxin to the acetylcholine receptor of the ray electric organ, and phospholipase A2 activity was defined by the enzymatic activity of these toxins with a fluorescent phospholipid as substrate. Finally, the applicability of these new methods to study other Naja snake venoms was demonstrated.

The effect of lactate on acetylcholine release evoked by various stimuli from Torpedo synaptosomes

The effect of external lactate on acetylcholine (ACh) release was examined at the nerve electroplaque junction of Torpedo marmorata using cholinergic synaptosomes prepared from the electric organ. Lactate reduced the release of ACh triggered by depolarization of synaptosomes with potassium. However, the release mechanism itself was not affected by lactate since, in its presence, the ACh release induced by different agents such as the calcium ionophore, A23187, or gramicidin D was equal to the release by control synaptosomes (without lactate). A possible site of action for lactate would be the voltage-dependent Ca2+ influx mediated by the natural calcium channel which is thought to couple depolarization and release and which would be bypassed during ionophore-induced Ca2+ entry. An intracellular target was indicated because decreasing the extracellular pH made L(+)lactate a slightly more potent inhibitor. The involvement of a membrane transporter for lactate was suggested by the observation that the D(-) isomer of lactate was less potent than the natural L(+) isomer. The release of endogenous lactate by electric organ prisms was also determined and depolarization with high potassium strongly stimulated L(+)lactate release from prisms. These results suggest that lactate production by stimulated postsynaptic electroplaques may inhibit acetylcholine release from presynaptic nerve terminals, constituting an example of negative feedback.

Solubilization and partial purification of a presynaptic membrane protein ensuring calcium-dependent acetylcholine release from proteoliposomes

In previous work, it was shown that cytoplasmic acetylcholine decreased on stimulation of Torpedo electric organ or synaptosomes in a strictly calcium-dependent manner. This led to the hypothesis that the presynaptic membrane contained an element translocating acetylcholine when activated by calcium. To test this hypothesis, the presynaptic membrane constituents were incorporated into the membranes of liposomes filled with acetylcholine. The proteoliposomes thus obtained released the transmitter in response to a calcium influx. The kinetics and calcium dependency of acetylcholine release were comparable for proteoliposomes and synaptosomes. The presynaptic membrane element ensuring calcium-dependent acetylcholine release is most probably a protein, since it was susceptible to Pronase, but only when the protease had access to the intracellular face of the presynaptic membrane. Postsynaptic membrane fractions contained very low amounts of this protein. It was extracted from the presynaptic membrane under alkaline conditions in the form of a protein-lipid complex of large size and low density which was partially purified. The specificity of the calcium-dependent release for acetylcholine was tested with proteoliposomes filled with equal amounts of acetylcholine and choline or acetylcholine and ATP. In both cases, acetylcholine was released preferentially. After cholate solubilization and gel filtration, the protein ensuring the calcium-dependent acetylcholine release was recovered at a high apparent molecular weight (between 600,000 and 200,000 daltons), its apparent sedimentation coefficient being 17S after cholate elimination. This protein is probably an essential coin of the transmitter release mechanism. We propose to name it mediatophore.

Incorporation of acetate into acetylcholine, acetylcarnitine, and amino acids in the Torpedo electric organ

The metabolism of acetate was investigated in the nerve-electroplaque system of Torpedo marmorata. In intact fragments of electric organ, radiolabeled acetate was incorporated into acetylcholine (ACh), acetylcarnitine (ACar), and three amino acids: aspartate, glutamate, and glutamine. These compounds were identified by TLC, high-voltage electrophoresis, column chromatography, and enzymic tests. The system responsible for acetate transport and incorporation into ACh displayed a higher affinity but a lower Vmax than that involved in the synthesis of ACar and amino acids. Choline, when added to the medium, increased the rate of acetate incorporation into ACh but decreased (at concentrations greater than 10(-5) M) that into ACar and amino acids. Monofluoroacetate slightly depressed ACh and ACar synthesis from external acetate but inhibited much more the synthesis of amino acids. During repetitive nerve stimulation, the level of the newly synthetized [14C]ACh was found to oscillate together with that of endogenous ACh, but the level of neither [14C]ACar nor the 14C-labeled amino acids exhibited any significant change as a function of time. This means that there is probably no periodic transfer of acetyl groups between ACh and the investigated metabolites in the course of activity. Acetate metabolism was also tested in the electric lobe (which contains the cell bodies of the neurons innervating the electric organ) and in Torpedo synaptosomes (which are nerve terminals isolated from the same neurons). Radioactive pyruvate and glutamine were also assayed in some experiments for comparison with acetate. These observations are discussed in connection with ACh metabolism under resting and active conditions in tissues where acetate is the preferred precursor of the neurotransmitter.

Large-scale purification of presynaptic plasma membranes from Torpedo marmorata electric organ

The presynaptic plasma membrane (PSPM) of cholinergic nerve terminals was purified from Torpedo electric organ using a large-scale procedure. Up to 500 g of frozen electric organ were fractioned in a single run, leading to the isolation of greater than 100 mg of PSPM proteins. The purity of the fraction is similar to that of the synaptosomal plasma membrane obtained after subfractionation of Torpedo synaptosomes as judged by its membrane-bound acetylcholinesterase activity, the number of Glycera convoluta neurotoxin binding sites, and the binding of two monoclonal antibodies directed against PSPM. The specificity of these antibodies for the PSPM is demonstrated by immunofluorescence microscopy.

Inactivation of acetylcholine release from Torpedo synaptosomes in response to prolonged depolarizations

The release of acetylcholine (ACh) from purely cholinergic Torpedo synaptosomes was monitored continuously using a chemiluminescent assay (Israël & Lesbats, 1981 a, b). Upon prolonged K+ depolarization in the presence of Ca2+, the release of ACh was transient and returned to a steady low level in about 3 min. Addition of the Ca2+ ionophore A23187 triggered the release again, suggesting that neither depletion of the transmitter store nor an inhibition of the release mechanism itself were involved in this phasic response, but rather an inactivation of the Ca2+ entry. The release response evoked by adding Ca2+ back after exposure of the synaptosomes to high K+ (70 mM) and low Ca2+ (0.57 mM) solution inactivates as a function of the duration of the pre-depolarization with a two-component time course with rapid (tau = 5.5 s) and slow phases (tau = 143 s). This response to Ca2+ addition was more strikingly reduced as the level of depolarization during pre-treatment was increased. The inactivation was found to be dose dependent with respect to the amount of Ca2+ present during the pre-depolarization period (conditioning Ca2+). Moreover, the presence of EGTA during pre-treatment with high-K+ solutions increased the response to applied Ca2+. These observations suggest that Ca2+ entry itself was responsible for this inactivation. No inactivation was found when ACh release was induced by the depolarizing agent Gramicidin D, except when external Na+ was replaced by Li+. This result indicates that part of the Ca2+ influx promoted by Gramicidin D depends on a Na+ entry, and may be mediated by the Na-Ca exchange mechanism.

Ultracytochemical localization of acetylcholine-like cations in excited motor end-plates by means of ionic fixation

Using rapid ionic fixation with molybdic or tungstic heteropolyanions (strong precipitating agents of quaternary ammonium cations such as choline and acetylcholine), acetylcholine-like cations were localized as point-like precipitates in the synaptic vesicles of resting (electrically nonstimulated) motor nerve terminals. When performed at low temperature, the same procedure revealed spot-like precipitates (presumed to be exocytotically released acetylcholine-like cations) in the synaptic cleft in the vicinity of the active zone. These precipitates were often seen in paired forms. Unlike resting motor-nerve terminals, excited terminals (electrical stimulation with occasional 4-aminopyridine pretreatment) after ionic fixation exhibited, at first, laminar precipitates both in the vicinity of the active zone inside the nerve terminals and in the synaptic space. In the vicinity of the active zone, the laminar precipitates were directed towards the synaptic membrane, while those in the synaptic space showed no orientation. Ionic fixation also revealed diffused precipitates both around the synaptic vesicles and on the axoplasmic side of the presynaptic membrane. Finally, the same fixation procedure demonstrated the presence of empty synaptic vesicles (without point-like precipitates) in close contact with the presynaptic membrane. The laminar and diffused precipitates are presumed to be two different forms of the same salts of acetylcholine-like cations that are insolubilized by ionic fixation in both the nerve terminals and the synaptic space of excited motor end-plates.

Reconstitution of carrier-mediated choline transport in proteoliposomes prepared from presynaptic membranes of Torpedo electric organ, and its internal and external ionic requirements

Proteoliposomes made by a butanol-sonication technique from electric organ presynaptic membranes showed choline transport activity. In contrast to intact nerve terminals, the uptake of choline was dissociated from its conversion to acetylcholine in this preparation. The kinetics of choline uptake by proteoliposomes was best described by two Michaelis-Menten components. At a low concentration of choline, uptake was inhibited by hemicholinium-3 and required external Na+ and, thus, closely resembled high-affinity choline uptake by intact cholinergic nerve terminals. Choline transport could be driven by the Na+ gradient and by the transmembrane potential (inside negative) but did not directly require ATP. External Cl-, but not a Cl- gradient, was needed for choline transport activity. It is suggested that internal K+ plays a role in the retention of choline inside the proteoliposome. Proteoliposomes should prove a useful tool for both biochemical and functional studies of the high-affinity choline carrier.

Membrane-bound choline acetyltransferase in Torpedo electric organ: a marker for synaptosomal plasma membranes?

The enzyme choline-O-acetyltransferase catalyses the biosynthesis of acetylcholine from acetyl coenzyme A and choline and is considered as one of the best markers for cholinergic nerve endings. The distribution of this enzymatic activity was analysed during the purification of plasma membranes of purely cholinergic nerve endings isolated from the electric organ of the fish Torpedo marmorata. This tissue, which receives a profuse and purely cholinergic innervation, can be considered as being a "giant" neuromuscular synapse. The isolated nerve endings (synaptosomes) were first osmotically disrupted and their plasma membranes isolated by equilibrium density centrifugation (discontinuous followed by continuous sucrose gradients). Choline acetyltransferase activity was found to exist in three forms: (1) a soluble form (the major one) present in the cytoplasm of the nerve endings, (2) a form which is ionically associated with membranes and which can be solubilized by washing exhaustively the membrane fraction with solutions of high ionic strength (0.5 M NaCl) and (iii) a form which is non-ionically bound to membranes and cannot be solubilized with high salt solution. The soluble and the non-ionically bound activities exhibited very similar affinities for choline (1.34 and 1.64 mM, respectively). The non-ionically membrane-associated form of choline acetyltransferase was found to "copurify" with the cholinergic synaptosomal plasma membranes of Torpedo, its specific activity being increased from 122 (crude fraction) to 475 (purified membrane fraction) nmol/h/mg protein. An enrichment was also observed for another cholinergic marker, the enzyme acetylcholinesterase, but not for the nicotinic receptor to acetylcholine, a marker for postsynaptic membranes. No choline acetyltransferase activity could be detected in preparations of synaptic vesicles that were highly purified from the electric organ. Also, the non-ionically associated form of choline acetyltransferase activity was hardly detectable (2.4 nmol/h/mg protein) in fractions enriched in axonal membranes prepared from the cholinergic electric nerves innervating the electric organ. The partition into soluble and membrane-bound activity was also analysed for choline acetyltransferase present in human placenta, a rich source for the enzyme but a non-innervated tissue. In this case the great majority of the enzyme appeared as soluble activity. Very low levels of non-ionically membrane-bound activity were found to be present in a crude membrane fraction from human placenta (2.8 nmol/h/mg protein).(ABSTRACT TRUNCATED AT 400 WORDS)

Presynaptic neurones may contribute a unique glycoprotein to the extracellular matrix at the synapse

As the extracellular matrix at the original site of a neuromuscular junction seems to play a major part in the specificity of synaptic regeneration, considerable attention has been paid to unique molecules localized to this region. Here we describe an extracellular matrix glycoprotein of the elasmobranch electric organ that is localized near the nerve endings. By immunological criteria, it is synthesized in the cell bodies, transported down the axons and is related to a glycoprotein in the synaptic vesicles of the neurones that innervate the electric organ. It is apparently specific for these neurones, as it cannot be detected elsewhere in the nervous system of the fish. Therefore, neurones seem to contribute unique extracellular matrix glycoproteins to the synaptic region. Synaptic vesicles could be involved in transporting these glycoproteins to or from the nerve terminal surface.

Large-scale purification of Torpedo electric organ synaptosomes

A procedure for the large-scale purification of Torpedo electric organ synaptosomes is described. The synaptosomal fraction obtained is very pure as judged from biochemical and morphological data. In addition, acetylcholine (ACh) release was demonstrated after KCl depolarization of synaptosomes in the presence of calcium. Two hundred grams of electric organ can be fractionated in a single run, allowing biochemical studies on presynaptic membrane constituents.