Acetylcholine compartments in mouse diaphragm. Comparison of the effects of black widow spider venom, electrical stimulation, and high concentrations of potassium

Latrophilins updated

Latrophilins (LPHN) are part of a yet unexplored family of receptors comprising three isoforms, LPHN1-3, and belonging to a unique branch of G protein-coupled receptors (GPCR) named adhesion GPCR (aGPCR). LPHN are considered to be prototypical models for the study of aGPCR as they are one of the most evolutionary conserved members. Previously described as the target for a potent neurotoxin from the black widow spider venom, LPHN are now being studied under a whole new perspective. Indeed, recent advances have provided a better understanding of different aspects of this prototypical family of receptors: 1) elucidation of LPHN ectodomain organization by crystallography has unveiled a new functional domain with great repercussion on all the other members of the aGPCR family, 2) proteomic approaches have opened the gate to unsuspected functional characteristics of LPHN cellular role, and 3) genetic approaches have provided hints into the physiological functions of LPHN in specific systems and organisms. Moreover, genomic linkage studies screening human patients from diverse genetic backgrounds have involved LPHN gene defects in human disorders such as attention-deficit hyperactivity disorder and cancer. In this review, we will provide a historical perspective addressing experimental research on these receptors while highlighting the new advances and discoveries concerning LPHN functions. As GPCR still represent the most studied targets for the development of pharmacological approaches aiming at alleviating human disorders, the relevance of studying LPHN retains a high pertinence to better understand these receptors for the treatment of human diseases.

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.

Latrophilin, neurexin, and their signaling-deficient mutants facilitate alpha -latrotoxin insertion into membranes but are not involved in pore formation

Pure alpha-latrotoxin is very inefficient at forming channels/pores in artificial lipid bilayers or in the plasma membrane of non-secretory cells. However, the toxin induces pores efficiently in COS-7 cells transfected with the heptahelical receptor latrophilin or the monotopic receptor neurexin. Signaling-deficient (truncated) mutants of latrophilin and latrophilin-neurexin hybrids also facilitate pore induction, which correlates with toxin binding irrespective of receptor structure. This rules out the involvement of signaling in pore formation. With any receptor, the alpha-latrotoxin pores are permeable to Ca(2+) and small molecules including fluorescein isothiocyanate and norepinephrine. Bound alpha-latrotoxin remains on the cell surface without penetrating completely into the cytosol. Higher temperatures facilitate insertion of the toxin into the plasma membrane, where it co-localizes with latrophilin (under all conditions) and with neurexin (in the presence of Ca(2+)). Interestingly, on subsequent removal of Ca(2+), alpha-latrotoxin dissociates from neurexin but remains in the membrane and continues to form pores. These receptor-independent pores are inhibited by anti-alpha-latrotoxin antibodies. Our results indicate that (i) alpha-latrotoxin is a pore-forming toxin, (ii) receptors that bind alpha-latrotoxin facilitate its insertion into the membrane, (iii) the receptors are not physically involved in the pore structure, (iv) alpha-latrotoxin pores may be independent of the receptors, and (v) pore formation does not require alpha-latrotoxin interaction with other neuronal proteins.

Effects of tetrodotoxin, Ca2+ absence, d-tubocurarine and vesamicol on spontaneous acetylcholine release from rat muscle

1. Rat hemidiaphragms were incubated in a physiological low-K+ medium without stimulation and the amount of acetylcholine (ACh) released was measured radioenzymatically. Cholinesterases were inhibited by paraoxon. 2. In the presence of 1 microM tetrodotoxin (TTX), the amount of ACh released during a 2 h incubation was lowered by 40%. A similar decrease was observed in the absence of Ca2+ and in the presence of 10 microM-d-tubocurarine (dTC). The effects of TTX combined with Ca2+ removal, and of TTX combined with dTC were no greater than those of TTX, dTC or Ca2+ removal alone. TTX and dTC had no effect on the release of ACh from diaphragms 4 days after denervation. 3. The reduction of spontaneous ACh release observed in the presence of TTX or dTC or in the absence of Ca2+ is best interpreted on the assumption that about 40% of the ACh release was due to the impulse activity known to be generated in intramuscular motor nerve branches by the ACh which accumulates after the inhibition of cholinesterases. 4. In the presence of 1 and 10 microM vesamicol (AH5183, 2-(4-phenylpiperidino)-cyclohexanol), the release of ACh was also diminished by approximately 40%. Vesamicol did not augment the inhibition of release produced by TTX or by the omission of Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective presynaptic insectotoxin (alpha-latroinsectotoxin) isolated from black widow spider venom

A homogenous protein of 120,000 mol. wt isolated from black widow spider (Lactrodectus mactans tredecimguttatus) venom and referred to as alpha-latroinsectotoxin was highly potent (4 nM) in the induction of an increase of the frequency of miniature excitatory postsynaptic potentials in blowfly (Calliphora vicina) larvae neuromuscular preparations. In the frog nerve ending, however, even 50 nM alpha-latroinsectotoxin failed to affect transmitter release. Pretreatment of insect preparations with alpha-latrotoxin or frog preparations with alpha-latroinsectotoxin did not prevent the specific effect of consequent applications of alpha-latroinsectotoxin (insect) and alpha-latrotoxin (frog), respectively. The binding of labelled [125I]alpha-latroinsectotoxin to insect and [125I]alpha-latrotoxin to bovine membrane preparations was saturable and highly specific. The presynaptic effect, but not the binding of alpha-latroinsectotoxin, was dependent on the presence of divalent cations in the external medium. Mg2+ could readily substitute for Ca2+ and increase of transmitter release induced by alpha-latroinsectotoxin also occurred in Ca(2+)-free solutions. Pretreatment of preparations with 300 micrograms/ml concanavalin A completely abolished both the presynaptic effect of alpha-latroinsectotoxin and its binding to insect membrane preparations. Thus, the phenomenology of alpha-latroinsectotoxin action on insects resembles in general that described for the action of alpha-latrotoxin on vertebrates. The selectivity of alpha-latrotoxin and alpha-latroinsectotoxin seems to be due to differences in the structure of neurotoxin receptors in nerve endings of vertebrates and insects, although the mode of presynaptic action has a great deal in common.

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.

Effect of alpha-latrotoxin on the frog neuromuscular junction at low temperature

1. alpha-Latrotoxin (alpha-LTx) was applied to frog cutaneous pectoris muscles bathed at 1-3 degrees C in either Ringer solution, Ca2+-free Ringer solution with 1 mM-EGTA and 4 mM-Mg2+ or Ringer solution plus 4 mM-Mg2+, and its effects on miniature end-plate potential (MEPP) frequency, nerve terminal ultrastructure and uptake of horseradish peroxidase (HRP) were studied. 2. Large concentrations (2 micrograms/ml) of alpha-LTx increased MEPP rates to levels above 100/s at all junctions, but the time course of the increases depended upon the divalent cation content of the bathing solution. However, similar numbers of MEPPs (0.3-0.7 x 10(6] were recorded at all junctions during 2 h of secretion. 3. Nerve terminals exposed to alpha-LTx for 2 h lost 60-75% of their synaptic vesicles and were swollen; their presynaptic membranes were deeply infolded and they often contained many large vesicular structures. Terminals in Ringer solution retained the largest number of synaptic vesicles; terminals in Ringer solution plus Mg2+ swelled the least and contained the largest number of coated vesicles. The average number of synaptic vesicles lost was approximately equal to the average number of MEPPs recorded. 4. Few vesicles became loaded with HRP when this extracellular tracer was present in the bathing solution and the muscles were fixed near the peak of secretion. 5. When the terminals were warmed to 20 degrees C, those in the Ca2+-free solution with Mg2+ secreted additional quanta and lost almost all their residual vesicles; those in Ringer solution without Mg2+ secreted few additional quanta and retained most of their residual vesicles. 6. These results suggest that recycling was blocked at these terminals and that for each quantum secreted a vesicle became permanently incorporated into the axolemma.

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)

Acetylcholine mobilization in a sympathetic ganglion in the presence and absence of 2-(4-phenylpiperidino)cyclohexanol (AH5183)

The present experiments measured the release of acetylcholine (ACh) by the cat superior cervical ganglia in the presence of, and after exposure to, 2-(4-phenylpiperidino)cyclohexanol (AH5183), a compound known to block the uptake of ACh by cholinergic synaptic vesicles. We confirmed that AH5183 blocks evoked ACh release during preganglionic nerve stimulation when approximately 13-14% of the initial ganglial ACh stores had been released; periods of rest in the presence of the drug did not promote recovery from the block, but ACh release recovered following the washout of AH5183. ACh was synthesized in AH5183-treated ganglia, as determined by the synthesis of [3H]ACh from [3H]choline, and this [3H]ACh could be released by stimulation following drug washout. The specific activity of the released ACh matched that of the tissue's ACh, and thus we conclude that ACh synthesized in the presence of AH5183 is a releasable as pre-existing ACh stores once the drug is removed. We tested the relative releasability of ACh synthesized during AH5183 exposure (perfusion with [3H]choline) and that synthesized during recovery from the drug's effects (perfusion with [14C]choline: the ratio of [3H]ACh to [14C]ACh released by stimulation was similar to the ratio in the tissue. These results suggest that the mobilization of ACh for release by ganglia during recovery from an AH5183-induced block is independent of the conditions under which the ACh was synthesized. Unlike nerve impulses, black widow spider venom (BWSV) induced the release of ACh from AH5183-blocked ganglia, even in the drug's continued presence. Venom-induced release of ACh from AH5183-treated ganglia was not less than the venom-induced release from tissues not exposed to AH5183. This effect of BWSV was attributed to the action of the protein, alpha-latrotoxin, because an anti-alpha-latrotoxin antiserum blocked the venom's action. ACh synthesized during AH5183 exposure was labelled from [3H]choline, and subsequent treatment with BWSV released [3H]ACh with the same temporal pattern as the release of total ACh. To exclude a nonexocytotic origin for the [3H]ACh released by BWSV, ganglia were preloaded with [3H]diethylhomocholine to form [3H]acetyldiethylhomocholine, an ACh analogue excluded from vesicles; the venom did not increase the rate of [3H]acetyldiethylhomocholine efflux. It is concluded that a vesicular ACh pool insensitive to the inhibitory action of AH5183 might exist and that this vesicular pool is not mobilized by electrical stimulation to exocytose in the presence of AH5183, but it is by BWSV.

GABA release from Xenopus retina does not correlate with horizontal cell membrane potential

The relationship between horizontal cell membrane potential and the release of GABA was explored in the retina of Xenopus laevis. The intracellularly recorded membrane potential of horizontal cells was monitored while the retina was exposed to different concentrations of depolarizing agents. The dose-response curves obtained revealed a rise from 5 to 95% maximum depolarization in 0.5-1.5 log unit concentration change. The molar concentrations that elicited a 20 mV depolarization were 40 mM (potassium), 0.8 mM (glutamate), 0.8 mM (glycine), 5 microM (kainate) and 1.3 microM (quisqualate). Autoradiography revealed that radiolabel was accumulated almost exclusively by horizontal cells when isolated retinas were incubated in medium containing 1 microM [3H]GABA. Thus, retinal release of radioactivity was used as a measure of [3H]GABA release from horizontal cells. Endogenous GABA released from retinas was measured using high performance liquid chromatography and was taken to reflect both amacrine and horizontal cell GABA pools. The release of both [3H]GABA and endogenous GABA was stimulated by glutamate, kainate and potassium, but not by glycine or quisqualate. Similar dose-response curves for GABA release and for depolarization were obtained in the case of potassium and kainate but not for glutamate. Potassium-evoked release either of endogenous GABA or [3H]GABA was both calcium- and sodium-dependent, whereas kainate- or glutamate-evoked GABA release was sodium-dependent but calcium-independent. The results indicate that depolarization per se is not necessarily associated with transmitter release in Xenopus retinal horizontal cells. It is suggested that the action of a given neurotransmitter upon the efflux of GABA from horizontal cells may depend on the degree to which it modifies the sodium conductance of the horizontal cell.

Blockade and recovery of cholinergic transmission in rats treated with hemicholinium 3

Nerve-induced responses of parotid gland and gastrocnemius muscle were reduced by HC-3 (1 mg kg-1) in proportion to the number of stimuli. Contractions by somatic muscle at 100 Hz were abolished after 6.0 X 10(3) stimuli while 14 X 10(3) were applied at 20 Hz before secretion was blocked. As stimulus rate was decreased, blockade of secretion resulted from fewer stimuli but no difference in ACh content was found between stimulated and unstimulated glands. When stimuli were withheld for 1.5 h transmission recovered temporarily; initial secretory flow rate was only 50% of that in untreated controls when stimulation resumed. In both organs, the time during which responses were sustained, however, was much shorter than when the preparations were stimulated initially. After choline, recovery of transmission was dose-dependent: 150 mg kg-1 were required to restore responsiveness to the muscle and the gland comparable to that in HC-3-treated rats stimulated for the first time. Resting recovery, when stimuli are withheld, probably depends upon stored transmitter becoming mobilized rather than on de novo transmitter synthesis because the endogenous choline in plasma is only 1/1000 of that following exogenous choline.

Differential release of [3H]acetylcholine from the rat phrenic nerve-hemidiaphragm preparation by electrical nerve stimulation and by high potassium

Neuronal transmitter stores of the phrenic nerve were labelled under different conditions. Subsequently, transmitter release evoked by electrical nerve stimulation and by a high potassium-low sodium solution was studied. Incubation of the end-plate preparation with [3H]choline at rest led to the synthesis of [3H]acetylcholine which could not be released by electrical nerve stimulation but it was released by high potassium-low sodium solution, independent of the presence of extracellular calcium. When the end-plate preparation was labelled during stimulation at 1 Hz, prolonged periods of electrical nerve stimulation released 83% of the total releasable [3H]transmitter pool in a completely calcium-dependent manner. After exhaustion of the electrically releasable pool, high potassium-low sodium solution still caused a significant outflow. Without a preceding exhaustion of the [3H]acetylcholine pool, high potassium-low sodium solution released a similar amount in the absence of extracellular calcium or after pretreatment with the intracellular calcium chelating substance, Quin-2. When evoked transmitter release was studied at different temperatures (36, 26 and 16 degrees C) Q 10 values of 1.6 and 1.0 were found for the release caused by electrical nerve stimulation and high potassium-low sodium solution (calcium-independent effect), respectively. After labelling during a short interval (2 min) but at a high stimulation rate (50 Hz), only 72% of the releasable [3H]transmitter could be released by electrical nerve stimulation, whereas the outflow due to the calcium-independent effect of high potassium-low sodium solution increased from 17 (labelling during stimulation at 1 Hz) to 28%. It is suggested that the calcium-independent effect of high potassium-low sodium solution reflects the release of acetylcholine from the cytoplasmic compartment, as this outflow occurred after labelling at rest and increased when cytoplasmic synthesis was enhanced by a high loading stimulation. In contrast to high potassium-low sodium solution, propagated nerve activity cannot release acetylcholine synthesized at rest (presumed to be cytoplasmic), but only [3H]acetylcholine synthesized during quantal release (presumed to be vesicular). The absolute requirement of extracellular calcium for electrically stimulated release suggests an exocytotic release mechanism. The low Q 10 value of 1.6 does not fit into the concept of a carrier- or channel-operated release mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)

Surplus acetylcholine and acetylcholine release in the rat diaphragm

1. Skeletal muscles from rat, mouse and frog were incubated under different conditions and the amounts of acetylcholine (ACh) extractable from the tissue and released into the medium were determined by mass fragmentography. In some experiments measurements were made of the amounts of ACh ('bound' ACh) surviving in a muscle homogenate to which an excess of acetylcholinesterase had been added. In other experiments the membrane potentials, end-plate potentials (e.p.p.s), and miniature end-plate potentials (m.e.p.p.s) were studied. 2. During incubation in Ringer medium the ACh content of the rat hemidiaphragm usually did not change, but after inhibition of cholinesterase by soman the ACh content rose gradually from about 100 to 150 pmol to a plateau of about 400 pmol after 4 h. A similar formation of 'surplus ACh' after cholinesterase inhibition was found in the mouse diaphragm, but not in the frog sartorius muscle. 3. Surplus ACh accumulated predominantly in the end-plate region of the rat diaphragm. In muscles, 16-18 h after in vivo denervation, the capacity to form surplus ACh was decreased by more than 80%. 4. The amount of ACh diffusing from the resting hemidiaphragm into the incubation medium ('resting release') varied between 0.5 and 0.9 pmol min-1 in different experiments; it remained at the same level during accumulation of surplus ACh. It was reduced by more than 80% 16-18 h after denervation. 5. The amplitude of m.e.p.p.s and e.p.p.s did not increase while surplus ACh was accumulating. 6. Incubation of hemidiaphragms in Ringer solution containing [3H]choline caused the formation of [3H]ACh. Additional amounts of [3H]choline were incorporated into ACh when the nerve was stimulated for 60 min. However, incubation in the presence of soman (3,3-dimethyl-2-butylmethylphosphonofluoridate), in the absence of stimulation, did not cause an increase of the [3H]ACh content of the muscles. 7. From hemidiaphragms with active cholinesterase about 120 pmol ACh was lost after prolonged nerve stimulation or incubation with 50 mM-KCl in the presence of hemicholinium-3, and about 35 pmol remained in the tissue. In soman-treated muscles, containing surplus ACh, about as much ACh was released by nervous stimulation as from untreated hemidiaphragms, and much more ACh remained unreleased. 8. Transection of the muscle at both sides of the end-plate or incubation of intact muscles in the presence of 50 mM-KCl depolarized the muscle fibres to -35 and -31 mV, respectively. Surplus ACh was partially released by 50 mM-KCl, but not by muscle transection.(ABSTRACT TRUNCATED AT 400 WORDS)

Temporal coincidence between synaptic vesicle fusion and quantal secretion of acetylcholine

We applied the quick-freezing technique to investigate the precise temporal coincidence between the onset of quantal secretion and the appearance of fusions of synaptic vesicles with the prejunctional membrane. Frog cutaneous pectoris nerve-muscle preparations were soaked in modified Ringer's solution with 1 mM 4-aminopyridine, 10 mM Ca2+, and 10(-4) M d-Tubocurarine and quick-frozen 1-10 ms after a single supramaximal shock. The frozen muscles were then either freeze-fractured or cryosubstituted in acetone with 13% OsO4 and processed for thin section electron microscopy. Temporal resolution of less than 1 ms can be achieved using a quick-freeze device that increases the rate of freezing of the muscle after it strikes the chilled copper block (15 degrees K) and that minimizes the precooling of the muscle during its descent toward the block. We minimized variations in transmission time by examining thin sections taken only from the medial edge of the muscle, which was at a fixed distance from the point of stimulation of the nerve. The ultrastructure of the cryosubstituted preparations was well preserved to a depth of 5 - 10 micron, and within this narrow band vesicles were found fused with the axolemma after a minimum delay of 2.5 ms after stimulation of the nerve. Since the total transmission time to this edge of the muscle was approximately 3 ms, these results indicate that the vesicles fuse with the axolemma precisely at the same time the quanta are released. Freeze-fracture does not seem to be an adequate experimental technique for this work because in the well-preserved band of the muscle the fracture plane crosses, but does not cleave, the inner hydrophobic domain of the plasmalemma. Fracture faces may form in deeper regions of the muscle where tissue preservation is unsatisfactory and freezing is delayed.

Differential effects of alpha-latrotoxin on mouse nerve endings and fibers

The action of alpha-latrotoxin (alpha-LTx), the major toxic component from black widow spider venom, was studied on mammalian motor nerve endings and fibers. Electrophysiologic recordings of presynaptic and postsynaptic responses at early stages after the application of different doses of alpha-LTx showed massive transient increases in transmitter release and changes in the configuration of presynaptic currents. Later, a loss of invasion of terminals by nerve impulses occurred, whereas the impulse conduction in nerve fibers was unmodified. These results support the idea of a selective effect of alpha-LTx on presynaptic membrane excitability.

Veratridine-induced release of acetylcholine from mouse forebrain minces: dependence on the hydrolysis of cytoplasmic acetylcholine for a source of choline

The importance of depolarization induced hydrolysis of cytoplasmic acetylcholine (ACh) in providing choline for the veratridine-and high K+-induced release of acetylcholine was studied in mouse forebrain minces. Results indicated that a loss of hydrolyzable cytoplasmic ACh prior to depolarization reduced the amount of ACh released by veratridine but not the amount released by high K+. The reduction in the veratridine-induced release of ACh did not occur during the first 5 min of incubation. Loss of vesicular ACh prior to depolarization reduced both the veratridine- and K+-induced release of ACh during the first 5 min of incubation. Blockade of extra-cellular choline transport by hemicholinium (HC-3) did not affect the veratridine-induced release of ACh during a 10 min incubation period unless the cytoplasmic pool of ACh had first been depleted and was unavailable as a source of choline. In contrast, HC-3 reduced the K+-induced release of ACh from brain tissue with normal stores of cytoplasmic ACh. These results indicate that both depolarizing agents primarily stimulate the release of preformed ACh from a vesicular fraction during the first 5 min of mince incubation. Thereafter, they both stimulate the release of newly synthesized ACh, however, they differ in one important respect. The principal source of choline for the veratridine-induced release of newly synthesized ACh appears to be the cytoplasmic pool of ACh, whereas the major source of choline for the K+-induced release of newly synthesized ACh appears to be extracellular choline.

Presynaptic, facilitatory effects of the corticosteroid dexamethasone in rat diaphragm: modulation by beta-bungarotoxin

Low concentrations of dexamethasone (up to 200 nM) increase the accumulation of choline (Ch) and its incorporation into acetylcholine (ACh) in the endplate rich area (EPA) of stimulated and unstimulated diaphragms in the presence of 10 microM Ch. Tissue ACh is not significantly altered, even after 140 min incubation. The specific radioactivity of the ACh in the EPA is thus increased by dexamethasone (Dex). The corticosteroid has no effects on acetylcholinesterase or choline acetyltransferase in diaphragm extracts. In the same medium, the amplitudes of the MEPPs, MEPCs and EPCs are also increased by Dex. Neither the quantal content of the EPCs nor the MEPP frequency, nor the half decay time of the MEPCs are altered. Therefore Dex (200 nM) increases both the resting and evoked output, and turnover of ACh in rat diaphragm. Beta-bungarotoxin (beta-BuTx) antagonizes the Dex-induced increase in Ch accumulation and its incorporation into ACh, and abolishes the increases in MEPC- and EPC-amplitudes, providing further argument for a presynaptic effect of Dex. In continuously-stimulated diaphragms, beta-BuTx causes an accumulation of ACh which is much greater than in unstimulated tissue. This accumulation of ACh is less in the presence of Dex, provided that Dex is added before beta-BuTx. The interaction of Dex and beta-BuTx is discussed in terms of their possible presynaptic sites of action.

Acetylcholine turnover in an autoactive molluscan neuron

We have studied acetylcholine (ACh) turnover at the cholinergic synapse between an identified motoneuron, the salivary burster (SB), and the muscle cells of the salivary duct (SD) in the terrestrial mollusk Limax maximus. Electrophysiological recordings were made of the SB action potentials and the SB-elicited junction potentials (JPs) on the SD. The amplitude of the JP was used as a measure of ACh release by the SB. The SB is an autoactive neuron that discharges 1 to 12 bursts of action potentials per min. During sustained bursting activity, the SB is able to maintain transmitter release for 18 hr even in the absence of exogenous choline. The size of SB-elicited JPs does not vary during 18 hr of activity. If the choline uptake blocker, hemicholinium-3 (HC-3; 20 microM), is present in the saline, transmitter release and JP size are depressed by about 30% after 14 hr of activity. Thus, the SB is partially dependent upon choline reuptake for maintained ACh synthesis and release. In high (9.45 mM)-potassium (K+) saline, the SB fired tonically at twice its average spike frequency. JP amplitude initially increased, then declined to an amplitude which was 60% of the initial level. The addition of 20 microM HC-3 to the high-K+ saline caused a 75 to 100% decrease in JP size within 30 min. Thus, during high-frequency tonic firing, the SB was primarily dependent on choline reuptake for ACh synthesis and release. After JP size had been reduced in high-K+ saline containing HC-3, the SB-SD synapse was returned to normal choline-free saline. The SB resumed bursting activity. JP amplitude gradually increased over the next 30 min. Thus, high-frequency firing in HC-3 had not depleted the SB of its entire endogenous store of choline or ACh. If the synapse was fatigued in high-K+ saline containing HC-3 and then placed in saline enriched with 300 microM choline, JP size increased within minutes. Thus, uptake of choline for ACh synthesis and release may be a more rapid process than mobilization of an endogenous transmitter store. Finally, the SB-SD synapse was fatigued in high-K+ saline containing HC-3. HC-3 was then removed from the saline. The SB maintained high-frequency tonic activity. JP size did not increase unless choline was added to the saline.(ABSTRACT TRUNCATED AT 400 WORDS)

Depolarization of mouse forebrain minces with veratridine and high K+: failure to stimulate the Ca2+ independent, spontaneous release of acetylcholine from the cytoplasm due to hydrolysis of the acetylcholine stored there

Both high K+ and veratridine, depolarizing agents with different mechanisms of action, lowered the ACh content of the cytoplasmic (S3) fraction of mouse forebrain minces incubated in a Ca2+-free Krebs solution, without stimulating ACh release or altering the level of ACh in the vesicle-bound (P3) fraction. Veratridine increased the level of choline in the P3 fraction by the same amount as it reduced the level of ACh in the S3 fraction, and these changes did not occur in the presence of tetrodotoxin (TTX). Pretreatment of minces in normal Krebs increased the ACh but not the choline content of the S3 fraction. Following this expansion of the S3 ACh content, veratridine caused an even greater loss of S3 ACh, and increased the Ca2+-independent release of ACh slightly. Under these conditions, veratridine also stimulated the Ca2+ independent release of choline, and this increase exceeded that obtained for the Ca2+-independent release of ACh. Preincubation in normal Krebs with paraoxon did not alter the S3 ACh content after 5 min, but raised it by 78% after 30 min. Under the latter conditions of pretreatment, veratridine then stimulated the Ca2+-independent release of ACh even more, but did not stimulate the release of choline. These results suggest that depolarization of brain tissue does not facilitate the Ca2+-independent release of ACh from the cytoplasm because a portion of ACh stored there is hydrolyzed. When the cytoplasmic level of ACh is sufficiently elevated prior to depolarization, then some ACh escapes hydrolysis and is released independently of Ca2+. It is suggested that the depolarization-induced hydrolysis of cytoplasmic ACh may be mediated by an intraterminal form of AChE and may, in addition to the hydrolysis of extracellular ACh, provide substrate for the formation and release of ACh by the vesicle-bound fraction.

The effect of alpha-latrotoxin on the neurosecretory PC12 cell line: electron microscopy and cytotoxicity studies

Neurosecretory PC12 cells were exposed in a variety of experimental conditions to nanomolar concentrations of alpha-latrotoxin purified from the venom of the black widow spider. When applied in a modified Ringer medium containing millimolar Ca2+ the toxin rapidly elicited a marked stimulation of exocytosis, as indicated by the appearance of typical images of granule-plasmalemma interaction and by the decreased density (number/unit area) of secretion granules in the cytoplasm. Without Ca2+ in the medium this early toxin effect was delayed and evolved less rapidly, but was still clearly appreciable. These morphological results appear in good quantitative agreement with the biochemical data on dopamine release reported in the preceding article. The stimulation of exocytosis was followed after a short delay by a stimulation of endocytosis, as revealed by an increased accumulation of the extracellular tracer, [14C]sucrose, within the toxin-treated cells. At later times after the application of alpha-latrotoxin other effects appeared, but only in the presence of Ca2+: these included changes in cell shape; focal alterations of the mitochondrial matrix (clear discrete areas and dense precipitates) and frank signs of cytotoxicity (rupture of the plasmalemma, clearing of the cytoplasmatic matrix). The toxin-induced cell death was studied quantitatively by using trypan blue exclusion as well as the 51Cr test, and was found to be dependent on alpha-latrotoxin concentration, temperature of incubation and Ca2+ concentration in the medium. Ionic substitutions concerning anions as well as cations other than Ca2+ had minor or no consequences. Thus, the early effect of alpha-latrotoxin in PC12 cells (stimulation of exocytosis, at least partially Ca2+-independent) can be dissociated from the late 'toxic' effect (strictly Ca2+-dependent).

The effect of alpha-latrotoxin on the neurosecretory PC12 cell line: studies on toxin binding and stimulation of transmitter release

alpha-Latrotoxin of black widow spider venom was found to bind with high affinity (KA = 1.8 X 10(9)M-1) to specific sites present in discrete number (approximately 6300/cell, approximately 12/micron2) at the surface membrane of PC12 cells. This binding correlated with (and therefore, probably caused) the secretory response produced by the toxin. Binding was enhanced (approximately 2-fold) in the presence of mM concentrations of various divalent cations (Ca2+, Mn2+ and Co2+) while Ba2+ and Sr2+ had a smaller effect and Mg2+ was inactive. Hypertonicity, concanavalin A and trypsin pretreatment of the cells blocked the binding interaction. The alpha-latrotoxin-induced stimulation of 3H-dopamine release was massive and occurred very rapidly when cells were exposed to the toxin in a Ca2+-containing Krebs-Ringer medium, whereas it occurred at a much slower rate in a Ca2+-free, Mg2+-containing Ringer. Introduction of Ca2+ into the latter medium resulted in a shift of the release rate from slow to fast. In contrast, in divalent cation-free medium the response was abolished. The toxin-induced secretory response was unaffected by Na+ and Ca2+ channel blockers (tetrodotoxin and D600) as well as by calmodulin inhibitors (calmidazolium and trifluoperazine). The effects of Ca2+ and Mg2+ were found to be concentration-dependent, with half maximal responses occurring at approximately 0.3 and 1.5 mM for the two divalent cations, respectively. Other divalent cations could substitute for Ca2+ and Mg2+, the relative efficacy being Sr2+ greater than Ca2+ greater than Ba2+ much greater than Mn2+ greater than Mg2+ greater than Co2+. Moreover, the response occurring at suboptimal concentration of Ca2+ (0.4 mM) was potentiated by the concomitant addition of either Mg2+, Mn2+ or Co2+. The effect(s) of divalent cations in supporting the alpha-latrotoxin-induced release response seem(s) to occur primarily at step(s) beyond toxin binding because (a) the stimulatory effects of the various cations on release were not matched by parallel effects on binding, and (b) Ca2+ maintained its ability to stimulate fast release even when toxin binding had occurred in a Ca2+-free medium. Delays in the release responses were observed when cells were exposed to alpha LTx in Na+-free, glucosamine or methylamine-based media, or depolarized with high K+ (in the presence of D600) before toxin treatment. Moreover, in these two conditions the ability of Mg2+ to support the alpha LTx response was considerably decreased.(ABSTRACT TRUNCATED AT 400 WORDS)

Spontaneous release of acetylcholine and acetylhomocholine from mouse forebrain minces: cytoplasmic or vesicular origin

The objective of this study was to determine the subcellular origin of cholinergic transmitter released spontaneously from mouse forebrain minces. To accomplish this objective, minces were pretreated in ionic media and then loaded with [14C]homocholine, an analog of choline, to form the false transmitter [14C]acetylhomocholine [( 14C]AHCh). The ratio of the false transmitter [14C]AHCh to the true transmitter ACh was then used as an index of cholinergic transmitter contents for both the cytoplasmic (S3) and vesicle-bound (P3) fractions. Three different pretreatment procedures were used to cause the following changes in S3 and P3 false to true transmitter ratios prior to spontaneous release: 1) a small increase in the S3 ratio of [14C]AHCh to acetylcholine (ACh) and a large increase in the P3 ratio of [14C] AHCh to ACh; 2) a decrease in the S3 ratio of [14C]AHCh to ACh and an increase in the P3 ratio of [14C]AHCh to ACh; 3) an increase in the P3 ratio of [14C]AHCh to ACh without affecting the S3 ratio of [14C]AHCh to ACh. The influence of each pretreatment on these subcellular ratios was then compared with its influence on the spontaneous release ratio of [14C]AHCh to ACh. In all 3 instances, the influence of pretreatment on the ratio of spontaneously released false and true cholinergic transmitters from minces coincided with the effect of pretreatment on the pre-release ratio of false to true transmitter in the S3 fraction. These results suggest that much of the cholinergic transmitter which is spontaneously released from mouse forebrain occurs from the cytroplasmic fraction.

Decrease of the spontaneous non-quantal release of acetylcholine from the phrenic nerve in botulinum-poisoned rat diaphragm

Botulinum type A toxin (BoTx) has been found to diminish by 40% the spontaneous release of acetylcholine (ACh) from normal (acutely denervated) rat diaphragms incubated in the presence of 5 mM K+, while the release of ACh from chronically (4 days) denervated diaphragms was not affected during 2 h incubations. The toxin has been found to rapidly remove (within 10 min) the local depolarization of about 8 mV which developed in the end-plate zones of the diaphragms after the inhibition of cholinesterases; after the administration of BoTx, tubocurarine lost its ability to increase the resting membrane potential (H-response, Katz and Miledi 1977) in the end-plate area of anticholinesterase-treated muscles. It is concluded that BoTx inhibits the non-quantal release of ACh from the motor nerve fibres and that it probably acts directly on the nerve terminal surface membrane (without internalization). The H-response in the rat diaphragm reflects the non-quantal release of ACh from the nerve terminals and not from the muscle fibres.

The effects of alpha-latrotoxin of black widow spider venom on synaptosome ultrastructure. A morphometric analysis correlating its effects on transmitter release

The morphological effects of alpha-latrotoxin, the major component of black widow spider venom, were studied quantitatively in a crude synaptosome fraction (prepared from rat brain cortices) which was incubated at 37 degrees C for 10 min in Ringer solutions. Two toxin concentrations were employed, one causing a very large stimulation of transmitter release (approximately 65% and approximately 43% release of [3H]noradrenaline from preloaded synaptosomes, with and without Ca2+ in the incubation buffer), the other 50-60% as active. Incubated synaptosomes, fixed in suspension with aldehydes, were evenly dispersed in agarose before embedding, to assure randomized sampling in the subsequent morphometric analysis. In all the experimental conditions investigated, alpha-latrotoxin treatment caused a significant decrease in the density (number/unit area) of synaptic vesicles in synaptosome profiles. Such an effect was dose-dependent and partially Ca2+-dependent, in good agreement with the data on transmitter release. At high toxin concentration a moderate increase of synaptosome volume and surface area was observed, both with and without Ca2+. Mitochondrial swelling appeared only in synaptosomes treated in Ca2+ containing medium. These effects of alpha-latrotoxin are similar to those described previously at the neuromuscular junction. Thus, the toxin might be a tool of general use for studying vertebrate synapses.

Muscle reinnervation--I. Restoration of transmitter release mechanisms

Following sciatic nerve crush the restoration of neuromuscular transmission in the extensor digitorum longus muscle of rat proceeds in a well defined manner: (a) as soon as the nerve-muscle contact is reformed, a subthreshold end-plate potential is recorded; no 'non-transmitting stage' is observed; (b) 24 hours later muscle action potentials are induced by nerve stimulation; (c) miniature end-plate potentials are absent or very rare at the newly reinnervated end-plates; their frequency returns to normal in about 4 weeks; (d) the frequency is also very much reduced in 30 mM K+ and hypertonic solutions and recovers slowly, in 4 and 5 weeks, respectively, while black widow spider venom is from the beginning as powerful as in normal neuromuscular junctions; (e) at the early stages of reinnervation the Ca2+-dependent release mechanisms are much stronger than control cases, while the Ca2+-independent mechanisms are weaker and recover in 5 weeks. The gradual reassembly and restoration of neurotransmitter release mechanisms of the extensor digitorum longus nerve terminal indicate the complexity of pre-synaptic ending organization.

Changes in acetylcholine concentration, miniature end-plate potentials and synaptic vesicles in frog neuromuscular preparations during lanthanum treatment

ACh content and synaptic ultrastructure were compared in neuromuscular preparations (sartorius muscle of Rana esculenta) incubated in control saline and in saline containing 1 mM LaCl3. ACh concentrations remained constant for 6 hr in control preparations. La3+ caused a 38% depletion of ACh within the first 30 min with subsequent recovery to 120% of control values within 3-4 hr. Recovery was prevented by hemicholinium-3. At 23 degrees C La3+ caused complete loss of synaptic vesicles: no depletion was seen at 4 degrees C. Initially MEPP frequency increased 300- to 700-fold (23 degrees C), then declined. Mean vesicle diameter did not change, but SD increased. As the frequency of MEPPs declined, the percentage of s-MEPPs greatly increased. La3+ had a postsynaptic effect which increased the amplitudes of both s-MEPPs and bell-MEPPs within a few seconds. The s-MEPP mean did not change during the course of La3+ treatment although the bell-MEPP mean usually decreased. How the decrease in synaptic vesicles, decrease in MEPP frequencies, and changes in ACh levels relate to changes in the percentage of different classes of quanta is discussed.

Use of chick biventer cervicis muscle in the bioassay of alpha-latrotoxin from black widow spider venom

alpha-Latrotoxin purified from black widow spider venom caused a sustained contraction of the chick biventer cervicis muscle. Muscle response to exogenous acetylcholine was not impaired. The time to reach half maximal contracture height was reproducible with small variations and can be used to quantitate the activity of alpha-LTX preparations.

The synthesis and release of acetylcholine in normal and denervated rat diaphragms during incubation in vitro

1. Normal and denervated rat diaphragms and neural (central) and aneural (peripheral) parts of normal diaphragms were incubated under several different conditions likely to affect the metabolism of acetylcholine (ACh), with the aim of discovering specific features of the control of neural and aneural ACh in the muscle. The concentrations of ACh in the tissue and the medium were measured at the end of the incubations using a radioenzymatic assay, and the amount of ACh synthesized during the incubations was calculated by subtracting the initial amount of ACh present in the tissue from that found in the tissue plus the medium at the end of the incubations.2. Confirming earlier results obtained with bioassays, it was found that, in a medium with 5 mM-K(+) and 2.5 mM-Ca(2+), denervated diaphragms released ACh into the medium at a rate equal to 47% of that observed in normal diaphragms; the amount of ACh released from aneural parts of normal diaphragms was 51% of that released from their neural parts. The release from normal diaphragms was increased (83%) in a Ca(2+)-dependent manner by raising the concentration of K(+) to 30 mM. In the denervated diaphragms, 30 mM-K(+) brought about a Ca(2+)-independent increase (67%) in the rate of ACh release. The elevation of K(+) was without effect on the release of ACh from aneural parts of normal diaphragms.3. The results indicate that a Ca(2+)-dependent mechanism of ACh release, known to function in the nerve terminals, is not likely to participate in the efflux of ACh from the muscle fibres. The K(+)-induced but Ca(2+)-independent enhancement of ACh release from the denervated diaphragms probably occurs by diffusion of ACh along the altered electrochemical gradient. It is suggested that the surface membranes of the muscle fibres become more permeable to ACh after denervation.4. During incubations with 30 mM-K(+) and 10 muM-hemicholinium-3 (HC-3), an inhibitor of the carrier-mediated transport of choline, the rates of ACh release and synthesis in normal diaphragms were diminished to the levels found in the denervated diaphragms, in which the concentration, release and synthesis of ACh were not affected by HC-3. The synthesis of aneural ACh thus appears to be independent of the carrier-mediated supply of choline across cell membranes.5. The release of ACh from normal diaphragms incubated with 5 mM-K(+) was increased in the presence of 100 muM-ouabain, whereas the release from denervated diaphragms was not affected. This finding suggests that the mechanism of ACh release that is activated by ouabain in the nerve cells involves, in addition to the inhibition of Na(+)-K(+)-ATPase, some other steps which are not operative in the muscle fibres.6. The results corroborate earlier evidence indicating that aneural ACh is produced, stored and released in the diaphragms. They fit the view that the aneural ACh is located in the cytoplasm of the muscle fibres and that it leaves the muscle fibres by molecular ;leakage' rather than by a specialized release mechanism. The efflux of ACh from the muscle fibres is likely to constitute about 50% of the total resting efflux (release) of ACh from normal diaphragms.

alpha-latrotoxin of black widow spider venom depolarizes the plasma membrane, induces massive calcium influx, and stimulates transmitter release in guinea pig brain synaptosomes

The effect of alpha-latrotoxin from black widow spider venom upon guinea pig cerebral cortical synaptosomes is described. Plasma membrane potential (delta psi p), in situ mitochondrial membrane potential (delta psi m), Ca2+ transport, gamma-amino[3H]butyrate release, [3H]noradrenaline release, and synaptosomal ATP were monitored under parallel conditions. Potentials were determined both isotopically and with a tetraphenylphosphonium-selective electrode. alpha-Latrotoxin depolarizes delta psi p selectively, both in the presence and absence of Ca2+. A slight toxin-induced depolarization of delta psi m is a consequence of a massive Ca2+ uptake across the plasma membrane. Depolarization of delta psi p is insensitive to tetrodotoxin, and Ca2+ entry is only partially inhibited by verapamil. Release of [3H]noradrenaline and gamma-amino[3H]butyrate is markedly stimulated by the toxin in the presence of Ca2+, and this effect is only slightly reduced in Ca2+-free conditions.

A morphometric study of cultured rat cerebral synapses exposed to different cationic media

Quantitative techniques have been applied to compare the effects of high-K+, Mg2+ and Li+ media on the ultrastructure of cultured synapses alongside Na+-incubated controls. The explant cultures were prepared from 18-day-old embryonic rat cerebral cortices and maintained for 19 days in vitro. K+ -Stimulation for 25 min resulted in an increase in the mean perimeter and area of presynaptic terminals. Of these, the perimeter increase was the more pronounced, as indicated by a decrease in the value of the two-dimensional form factor. Reductions were also observed in the number of synaptic vesicles per presynaptic terminal, in the vesicle-terminal area ratio and in the synaptic vesicle density in an area adjacent to the presynaptic membrane, the latter two parameters being in positive linear correlation. The frequency of presynaptic cisternal/vacuolar profiles increased, and the synaptic curvature shifted in a positive direction. Synaptic length did not change following K+-exposure. Qualitative assessment indicated the presence of a network subjacent to the post-synaptic thickening and swelling of the postsynaptic ending after K+-stimulation. Incubation and fixation in Mg2+-media of two concentrations resulted in an increase in the number and area ratio of synaptic vesicles per terminal, and an elevation in the synaptic vesicle density in the higher Mg2+ concentration medium. Li+-treatment reduced the number of synaptic vesicles per terminal, the vesicle-terminal area ratio, and the vesicle density in the vicinity of the presynaptic membrane, while the synaptic curvature shifted in the positive direction. These changes were less pronounced than those characteristic of synapses in the K+ medium.

Studies on alpha-latrotoxin receptors in rat brain synaptosomes: correlation between toxin binding and stimulation of transmitter release

alpha-Latrotoxin (alpha-LT), the major component of black widow spider venom, is a high-molecular-weight protein that acts presynaptically by stimulating the release of stored neurotransmitters. The purified toxin was iodinated to high specific radioactivity by the Bolton-Hunter procedure, without appreciable loss of biological activity. By the use of the 125I-toxin, specific receptors were revealed in synaptosome fractions isolated from various regions of the rat brain, but not in nonneural tissues. The density of alpha-LT receptors [which are probably composed of, or include, membrane protein(s)] varies between 0.6 and 0.88 pmol/mg of synaptosome protein, their affinity is very high (KA of the order of 10(10) M-1), their association rate is fast, and their dissociation rate slow. They might belong to a single, homogeneous class. This last conclusion, however, is still uncertain, because results suggesting a possible heterogeneity were obtained by studying the dissociation of the toxin from synaptosomes incubated in high-salt buffer. Experiments in which the binding of alpha-LT and its dopamine release activity in striatal synaptosomes were investigated in parallel in a variety of experimental conditions support the hypothesis that occupation of the high-affinity receptors is the initial step in the alpha-LT activation of the presynaptic response.

Neural and non-neural acetylcholine in the rat diaphragm

The compartmentation of acetylcholine (ACh) and of choline acetyltransferase in the rat diaphragm was analysed by measuring their contents in muscle segments containing endplates (e.p.) and endplate-free segments (non-e.p.) at different times following section of the phrenic nerve. In addition ACh release was determined before and after denervation. Freshly dissected hemidiaphragms contained about 125 pmol of ACh; more than 90% of this was localized in the e.p. portion. Between 10 and 18 h after denervation the ACh content of the e.p. portion decreased by 80% and its ACh concentration became approximately equal to that in the non-e.p. region, whose ACh content did not change. Spontaneous release of ACh was reduced by denervation and ACh release evoked by 50 mM KC1 was practically abolished. Choline acetyltransferase activity in freshly dissected preparations was about 30 nmol of ACh per gram per hour, Km 0.5 mM. About 65% of the enzyme disappeared in the first 24 h and the remaining 35% between 24 and 50 h after denervation. A different enzyme capable of ACh synthesis was found in the muscle fibres; its activity did not decrease after denervation. It is concluded that about 70% of the ACh in the diaphragm is contained in the motor nerve terminals, about 10% in the intramuscular nerve fibres and the remainder in the muscle fibres, and that about 65% of choline acetyltransferase is in the motor terminals and 35% in the nerve fibres.

4-aminoquinoline-induced 'giant' miniature endplate potentials at mammalian neuromuscular junctions

4-Aminoquinoline (4-AQ) in concentrations around 200 micrometers induces, within minutes of its application to isolated mouse or rat neuromuscular junctions, the appearance of a population of miniature endplate potentials (m.e.p.ps) with a larger than normal amplitude, so-called giant m.e.p.ps (g.m.e.p.ps). With amplitudes 2-12 times the modal value of m.e.p.p. amplitude, the population of g.m.e.p.ps varied between 15 and 45% of the total population of m.e.p.ps. There was no increase in the frequency of m.e.p.ps but a positive correlation between the frequency of g.m.e.p.ps and the total frequency of m.e.p.ps. In many instances the rise time and decay time of g.m.e.p.ps were prolonged compared to normal. Elevated extracellular calcium concentrations increased the frequency of m.e.p.ps but had no effect on g.m.e.p.p. frequency. High extracellular potassium concentrations markedly increased m.e.p.p. frequency but failed to influence g.m.e.p.p. frequency. Similar observations were made with ethanol 0.1 M, ouabain 200 micrometers or black widow spider venom. Botulinum toxin type A markedly reduced total m.e.p.p. frequency but 4-AQ still induced g.m.e.p.ps. Nerve stimulation failed to release quanta corresponding to the g.m.e.p.ps. G.m.e.p.ps seemed to originate from quantal acetylcholine release from the nerve terminal since they were abolished by surgical denervation and by the addition of d-tubocurarine to the medium. Blockade of voltage-sensitive calcium or sodium channels by, respectively, manganese ions or tetrodotoxin failed to affect the appearance and the frequency of g.m.e.p.ps. The electrophysiological findings and a statistical analysis of the characteristics of the m.e.p.ps indicate that they belong to two populations. One population is accelerated by the depolarization-release coupling mechanism responsible for evoked transmitter release and is characterized by an amplitude distribution and a process in time that indicate that they correspond to releases occurring at 'active zones' in the nerve terminal. The second population of m.e.p.ps is uninfluenced by nerve terminal depolarization and transmembrane calcium fluxes. This population apparently originates from sites dispersed in the nerve terminal membrane and outside the 'active zones'. 4-AQ increases the frequency of this second m.e.p.p. population without affecting the first population.

On the quantal release of endogenous glutamate from the crayfish neuromuscular junction

1. The abdominal slow flexor muscle was isolated from the crayfish (Cambarus clarkii) and placed in 150 microliters. Harreveld solution. The concentrations of glutamate and aspartate in this solution were measured by mass fragmentography. 2. Application of black widow spider venom (BWSV) produced a marked increase in the frequency of miniature excitatory post-synaptic potentials (m.e.p.s.p.s). During the high frequency discharge of m.e.p.s.p.s, the glutamate content in the solution was significantly increased. There was an approximately linear relationship between the increase in the glutamate efflux produced by BWSV and the variance of the membrane potential fluctuation during high frequency discharge of m.e.p.s.p.s. 3. In most cases, the efflux of aspartate during control rest periods was smaller than that of glutamate. During the discharge of m.e.p.s.p.s produced by BWSV, the increase in the aspartate efflux was very small compared to glutamate. 4. Nerve stimulation caused a significant increase in the efflux of glutamate, but the change in the aspartate efflux was very small and not significant. 5. Application of methylene blue increased the frequency of m.e.p.s.p.s and glutamate efflux, but little, if any, increase was found in aspartate efflux. 6. It is concluded that glutamate is preferentially released from nerve terminals in a quantal fashion.

Free and bound acetylcholine in frog muscle

1. Frog sartorius muscles were divided into end-plate containing (e.p.) and end-plate-free (non-e.p.) segments or homogenized in Ringer solution at 0 degrees C in the presence or absence of added acetylcholinesterase from electric eel. ACh was extracted from the tissue or from the homogenates and measured by mass fragmentography. 2. The concentration of ACh in non-e.p. segments was about six times lower than that in e.p. segments. 3. Homogenization of muscles in Ringer caused the hydrolysis of a small fraction ('free-1') of total ACh; addition of extra acetylcholinesterase caused hydrolysis of another, greater, fraction ('free-2' ACh). The esterase-resistant ('bound') ACh was stable at 0 degrees C up to 15 min of incubation. 4. Denervation for 15 days, which caused the disappearance of the nerve terminals, did not influence ACh in non-e.p. segments, but reduced total and bound ACh by about 75%, and free-2 ACh by 90%. 5. Treatment with La3+ ions, which caused the disappearance of synaptic vesicles, did not influence total ACh, but reduced bound ACh by 75%, whereas free-1 and free-2 ACh were increased. 6. Electrical stimulation of the nerve at 5 sec-1 or incubation with 50 mM-KCl did not affect ACh in the non-e.p. segments, but reduced by roughly 60% total, bound, and free ACh. 7. It is concluded that about 75% of bound ACh derives from synaptic vesicles, corresponding to 11,000 molecules per vesicle, and 25% from non-neural ACh; that free-1 and free-2 ACh derive mainly from the nerve terminal cytoplasm, although they may be contaminated by vesicular ACh.