Zinc blocks acetylcholine release but not vesicle fusion at the Torpedo nerve-electroplate junction

Cadmium desynchronizes neurotransmitter release in the neuromuscular junction: Key role of ROS

Cd²⁺ is one of the most widespread environmental pollutants and its accumulation in central and peripheral nervous systems leads to neurotoxicity as well as aggravation of common neurodegenerative diseases. Mechanism of the Cd²⁺ toxicity is far from being resolved. Here, using microelectrode recordings of postsynaptic responses and fluorescent redox indicators we studied the effect of Cd²⁺ in the submicromolar range on timing of neurotransmitter release and oxidative status in two functionally different compartments of the same frog motor nerve terminal. Cd²⁺ (0.1-1 μM) acting as typical voltage-gated Ca²⁺channel (VGCC) antagonist decreased neurotransmitter release in both distal and proximal parts of the nerve terminal, but in contrast to the VGCC blockers Cd²⁺(0.1-0.5 μM) desynchronized the release selectively in the distal region. The latter action of Cd²⁺ was completely prevented by inhibitor of NADPH-oxidase and antioxidants, including mitochondrial specific, as well as redox-sensitive TRPV1 channel blocker. Cd²⁺ markedly increased levels of mitochondrial reactive oxygen species (ROS) in both the distal and proximal compartments of the nerve terminal, which was associated with lipid peroxidation mainly in the distal region. Zn²⁺, whose transport systems translocate Cd²⁺, markedly enhanced the effects of Cd²⁺ on both the mitochondrial ROS levels and timing of neurotransmitter release. Furthermore, in the presence of Zn²⁺ ions, Cd²⁺ also desynchronized the neurotransmitter release in the proximal region. Thus, in synapses Cd²⁺ at very low concentrations can increase mitochondrial ROS, lipid peroxidation and disturb the timing of neurotransmitter release via a ROS/TRPV-dependent mechanism. Desynchronization of neurotransmitter release and synaptic oxidative stress could be early events in Cd²⁺ neurotoxicity.

Quantal acetylcholine release: vesicle fusion or intramembrane particles?

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

Ionic selectivity of the Ca2+/H+ antiport in synaptic vesicles of sheep brain cortex

As we previously reported, synaptic vesicles isolated from sheep brain cortex contain a Ca2+/H+ antiport that permits Ca2+ accumulation inside the vesicles ( approximately 5 nmol/mg protein) at expenses of the pH gradient generated by the H+-pumping ATPase. We observed that the system associates Ca2+ influx to H+ release and operates with low affinity for Ca2+. In the present work, we found that Ca2+/H+ antiport mediates exchange of protons with other cations such as Zn2+ and Cd2+, suggesting that these cations and Ca2+ share the same transporter molecules to enter the intravesicular space. Zn2+ and Cd2+ induce H+ release in a concentration-dependent manner (fluorimetrically evaluated) and they inhibit the antiport-mediated Ca2+ uptake by the vesicles (isotopically measured). In contrast, large cations such as Ba2+ and Cs+ do not alter Ca2+ influx and they are unable to induce proton release from the vesicles. With respect to Sr2+, which has an intermediary size relatively to the other groups of cations, we found that it does not induce H+ liberation from the vesicles, but it has a concentration-dependent inhibitory effect on the Ca2+-induced H+ release and Ca2+ uptake by the vesicles. These results indicate that the cation selectivity of the synaptic vesicles Ca2+/H+ antiport is essentially determined by the size of the cation transported into the vesicles.

In vitro reconstitution of neurotransmitter release

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

Acetylcholine release and the cholinergic genomic locus

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