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

Synaptic vesicles control the time course of neurotransmitter secretion via a Ca²+/H+ antiport

We investigated the physiological role of the vesicular Ca2+/H+ antiport in rapid synaptic transmission using the Torpedo electric organ (a modified neuromuscular system). By inhibiting V-type H+-transporting ATPase (V-ATPase), bafilomycin A1 dissipates the H+ gradient of synaptic vesicles, thereby abolishing the Ca2+/H+ antiport driving force. In electrophysiology experiments, bafilomycin A1 significantly prolonged the duration of the evoked electroplaque potential. A biochemical assay for acetylcholine (ACh) release showed that the effect of bafilomycin A1 was presynaptic. Indeed, bafilomycin A1 increased the amount of radio-labelled ACh released in response to paired-pulse stimulation. Bafilomycin A1 also enhanced Ca2+-dependent ACh release from isolated nerve terminals (synaptosomes). The bafilomycin-induced electroplaque potential lengthening did not arise from cholinesterase inhibition, since eserine (which also prolonged the electroplaque potential) strongly decreased evoked ACh release. Bafilomycin A1 augmented the amount of calcium accumulating in nerve terminals following a short tetanic stimulation and delayed subsequent calcium extrusion. By reducing stimulation-dependent calcium accumulation in synaptic vesicles, bafilomycin A1 diminished the corresponding depletion of vesicular ACh, as tested using both intact tissue and isolated synaptic vesicles. Strontium ions inhibit the vesicular Ca2+/H+ antiport, while activating transmitter release at concentrations one order of magnitude higher than Ca2+ does. In the presence of Sr2+ the time course of the electroplaque potential was also prolonged but, unlike bafilomycin A1, Sr2+ enhanced facilitation in paired-pulse experiments. It is therefore proposed that the vesicular Ca2+/H+ antiport function is to shorten 'phasic' transmitter release, allowing the synapse to transmit briefer impulses and so to work at higher frequencies.

Aconitine-induced increase and decrease of acetylcholine release in the mouse phrenic nerve-hemidiaphragm muscle preparation

The effect of aconitine on acetylcholine (ACh) release from motor nerve terminals in the mouse phrenic nerve-diaphragm muscle preparation was studied by a radioisotope method. Both electrical stimulation-evoked release and spontaneous release of 3H-ACh from the preparation preloaded with 3H-choline were measured. The change in the muscle tension was simultaneously recorded in the same preparation. Aconitine (0.1 microM) increased electrically evoked 3H-ACh release, while at higher concentrations (0.3-3 microM) it decreased the evoked release and muscle tension. High concentrations of aconitine (3-30 microM) caused a concentration-dependent increase in spontaneous 3H-ACh release. All these effects were suppressed by tetrodotoxin. The aconitine-induced spontaneous release consisted of two different components: a Ca(2+)-dependent phasic release that was inactivated within a few minutes and a Ca(2+)-independent, long lasting release at a low level. The depression of the Ca(2+)-dependent quantal release seems attributable to the decline of Ca2+ influx into the nerve rather than inactivation of sodium channels. We conclude that aconitine increases and then decreases electrical stimulation-evoked ACh release from the motor nerve through prolonged activation of sodium channels. Further activation of the channels enhances spontaneous release and the subsequent complete inactivation of the quantal release may be due to block of Ca2+ influx.

Presynaptic muscarinic receptors and the release of acetylcholine from cerebrocortical prisms: roles of Ca2+ and K+ concentrations

The mechanism by which presynaptic muscarinic autoreceptors inhibit the release of acetylcholine (ACh) from cerebrocortical cholinergic fibres has not been clarified. To test the view that muscarinic autoreceptors act by decreasing Ca2+ influx, we performed experiments in which rat cerebrocortical prisms were preloaded with (14C)choline, washed, depolarized with 14-65 mM K+ in the absence of Ca2+ and then exposed (still under depolarization) to various concentrations of Ca2+ to evoke the release of (14C)ACh. The muscarinic agonist, oxotremorine, used at a 100 microM concentration, inhibited the release of (14C)ACh by 59-86% in experiments with 14 and 26.5 mM K+ but had no significant effect at 65.5 mM K+. No systematic changes in the inhibitory effects of oxotremorine could be found at any of the K+ concentrations used when the concentration of Ca2+ was varied in the range of 0.25-4.0 mM. At 2 mM Ca2+ and K+ concentrations above 14 mM, the inhibitory effect of oxotremorine was inversely related to the concentration of K+. The inhibitory effect of oxotremorine on (14C)ACh release was not blocked by 100 microM 4-amino-pyridine. The fact that the inhibitory effect of oxotremorine could not be overcome by an increase in the concentration of Ca2+ suggests that, under the conditions used, a restriction of the influx of Ca2+ did not play a major role in the muscarinic inhibition of ACh release; rather, oxotremorine appeared to act by decreasing membrane depolarization.2+ of the Ca(2+)-voltage hypothesis of neurotransmitter release, supposing

Intracellular calcium and vasopressin release of rat isolated neurohypophysial nerve endings

1. Monitoring of [Ca2+]i and vasopressin secretion in isolated nerve endings from the rat neurohypophysis were studied to determine the relationship between the time course of vasopressin secretion and depolarization-induced changes in [Ca2+]i. 2. Membrane depolarization by increasing the extracellular [K+] led to concentration-dependent, parallel increases in the amount of vasopressin release and in peak increases in [Ca2+]i. Half-maximal activation of a change in [Ca2+]i was attained at 40 mM extracellular K+. 3. The Ca2+ chelator dimethyl-BAPTA (1,2-bis(O-aminophenoxy)ethane-N,N,N'N'-tetraacetic acid), loaded into the nerve endings, reduced K+ depolarization-evoked vasopressin release and efficiently antagonized K(+)-induced changes in [Ca2+]i. Moreover, dimethyl-BAPTA dramatically reduced basal [Ca2+]i without a reduction in basal secretion. 4. The duration of the vasopressin secretory response was similar regardless of applied 50 mM K+ depolarizations longer than 30 s. The t1/2 of the secretory response was 45 s. Application of repetitive K+ depolarization pulses repetitive secretory responses of similar amplitude and duration. 5. The K(+)-induced changes in [Ca2+]i remained elevated throughout the duration of the depolarizing stimulus decreasing less than 30% over 3 min. The sustained increase in [Ca2+]i resulted largely from continued enhanced Ca2+ influx, demonstrated by susceptibility to the dihydropyridine, L-type calcium channel blocker, nicardipine. 6. Vasopressin secretion could be reinitiated following its decline to a step K+ depolarization by a further step increase in K+ or by removal and readdition of extracellular [Ca2+]. Alterations in [Ca2+]i paralleled periods of secretory activity. 7. Analysis of secretory responsiveness and change in [Ca2+]i to K+ depolarization in medium of altered extracellular [Ca2+] indicates that [Ca2+]i of 20 microM is sufficient to trigger vasopressin release. K(+)-induced alterations in [Ca2+]i could be observed at [Ca2+]o as low as 5 microM. Although smaller in amplitude to that observed at 2.2 mM [Ca2+]o the duration of the K(+)-induced secretory response increased at lower [Ca2+]o. 8. Transient vasopressin secretory responses were observed to sustained levels of [Ca2+] in digitonin and streptolysin-O-permeabilized nerve endings. Secretion could be re-evoked, following its decline, by a step increase in [Ca2+] or by removal and readdition of [Ca2+]o.(ABSTRACT TRUNCATED AT 400 WORDS)

Agelenopsis aperta venom and FTX, a purified toxin, inhibit acetylcholine release in Torpedo synaptosomes

The presence of P-type calcium channels in synaptosomes prepared from electric organ of Torpedo marmorata was investigated by using the venom of Agelenopsis aperta, a toxin purified from it, FTX, and its synthetic analog. We analysed the action of these agents on acetylcholine release which was continuously followed using a chemiluminescent assay. Agelenopsis aperta venom, FTX and synthetic FTX inhibit acetylcholine release from synaptosomes induced by a presynaptic membrane depolarization with 60 mM KCl. A stronger inhibition of acetylcholine release was observed with the venom than with FTX (70 and 50%, respectively). Another way of triggering acetylcholine release from Torpedo synaptosomes is to insert in the presynaptic membrane a calcium ionophore A23187 which allows the bypass of the natural calcium channels. The venom of Agelenopsis aperta inhibits A23187-evoked acetylcholine release. Purified and synthetic FTX does not possess this property, suggesting that this inhibition of acetylcholine release was due to other toxins of the venom. Another type of pharmacological sensitivity of Torpedo calcium channels was also demonstrated using omega-conotoxin GVIA. At a concentration of 20 microM, this toxin was able to inhibit about 35% of KCl-evoked acetylcholine release. When FTX + omega-conotoxin GVIA were applied together, the inhibitory effect on KCl-evoked acetylcholine release was not significantly increased in comparison with the one observed with FTX alone. In conclusion, we examined the effect of different agents on acetylcholine release from Torpedo marmorata electric organ synaptosomes; acetylcholine release was elicited with KCl depolarization and followed continuously with a chemiluminescent assay.(ABSTRACT TRUNCATED AT 250 WORDS)

Spontaneous and evoked quantal neurotransmitter release at the neuromuscular junction of the larval housefly, Musca domestica

The release of neurotransmitter was monitored at the neuromuscular junctions of larval housefly ventrolateral muscles 6a and 7a, using intracellular recording, and a loose patch clamp technique to isolate discrete release sites. Transmitter release occurred spontaneously and could also be evoked by neural stimuli. Spontaneous discharges consisted of events which were randomly distributed in time and of bursts of temporally ordered events. Evoked and spontaneous release occurred in a quantal manner. The quantal content of evoked excitatory postsynaptic currents (EPSCs) was dependent upon the extracellular calcium concentration, increasing with a 3.8 power dependency. The relationship between the quantal content of a response and extracellular calcium concentration was offset by the presence of magnesium in the bathing saline. The rates of decay of miniature EPSCs (mEPSCs) and EPSCs were also found to increase with extracellular calcium concentration, consistent with a non-diffusion limited block of the glutamate receptor-channel complex by calcium ions (KB 2.5 x 10(4) s-1 M-1, P less than 0.01). The frequency of random mEPSCs (0.26 +/- 0.32 Hz, n = 24 cells) was independent of the extracellular calcium concentration. Random mEPSCs were not inhibited by 1 microM tetrodotoxin which blocked mEPSC bursts and neurally evoked responses. EPSCs evoked during mEPSC bursts had a significantly lower quantal content than those EPSCs recorded from the same nerve terminal between bursting, indicating that both of these forms of release recruited quanta from a common pool of transmitter. Following a neurally evoked EPSC the mEPSC frequency was potentiated severalfold, this delayed release was influenced by EPSCs with large quantal contents evoked in saline containing elevated calcium concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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)

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.