Accounting for the shapes and size distributions of miniature endplate currents

Sharp Ca²⁺ nanodomains beneath the ribbon promote highly synchronous multivesicular release at hair cell synapses

Hair cell ribbon synapses exhibit several distinguishing features. Structurally, a dense body, or ribbon, is anchored to the presynaptic membrane and tethers synaptic vesicles; functionally, neurotransmitter release is dominated by large EPSC events produced by seemingly synchronous multivesicular release. However, the specific role of the synaptic ribbon in promoting this form of release remains elusive. Using complete ultrastructural reconstructions and capacitance measurements of bullfrog amphibian papilla hair cells dialyzed with high concentrations of a slow Ca²⁺ buffer (10 mM EGTA), we found that the number of synaptic vesicles at the base of the ribbon correlated closely to those vesicles that released most rapidly and efficiently, while the rest of the ribbon-tethered vesicles correlated to a second, slower pool of vesicles. Combined with the persistence of multivesicular release in extreme Ca²⁺ buffering conditions (10 mM BAPTA), our data argue against the Ca²⁺-dependent compound fusion of ribbon-tethered vesicles at hair cell synapses. Moreover, during hair cell depolarization, our results suggest that elevated Ca²⁺ levels enhance vesicle pool replenishment rates. Finally, using Ca²⁺ diffusion simulations, we propose that the ribbon and its vesicles define a small cytoplasmic volume where Ca²⁺ buffer is saturated, despite 10 mM BAPTA conditions. This local buffer saturation permits fast and large Ca²⁺ rises near release sites beneath the synaptic ribbon that can trigger multiquantal EPSCs. We conclude that, by restricting the available presynaptic volume, the ribbon may be creating conditions for the synchronous release of a small cohort of docked vesicles.

Loading and recycling of synaptic vesicles in the Torpedo electric organ and the vertebrate neuromuscular junction

In vertebrate motor nerve terminals and in the electromotor nerve terminals of Torpedo there are two major pools of synaptic vesicles: readily releasable and reserve. The electromotor terminals differ in that the reserve vesicles are twice the diameter of the readily releasable vesicles. The vesicles contain high concentrations of ACh and ATP. Part of the ACh is brought into the vesicle by the vesicular ACh transporter, VAChT, which exchanges two protons for each ACh, but a fraction of the ACh seems to be accumulated by different, unexplored mechanisms. Most of the vesicles in the terminals do not exchange ACh or ATP with the axoplasm, although ACh and ATP are free in the vesicle interior. The VAChT is controlled by a multifaceted regulatory complex, which includes the proteoglycans that characterize the cholinergic vesicles. The drug (-)-vesamicol binds to a site on the complex and blocks ACh exchange. Only 10-20% of the vesicles are in the readily releasable pool, which therefore is turned over fairly rapidly by spontaneous quantal release. The turnover can be followed by the incorporation of false transmitters into the recycling vesicles, and by the rate of uptake of FM dyes, which have some selectivity for the two recycling pathways. The amount of ACh loaded into recycling vesicles in the readily releasable pool decreases during stimulation. The ACh content of the vesicles can be varied over eight-fold range without changing vesicle size.

Vesicle size and transmitter release at the frog neuromuscular junction when quantal acetylcholine content is increased or decreased

We investigated whether the synaptic vesicles at the neuromuscular junction change size when their acetylcholine (ACh) content is altered. The size of the miniature endplate potential (MEPP) increased 3- or 4-fold in preparations pre-treated in a hypertonic solution in which the anion was gluconate. We measured the dimensions of synaptic vesicles in such preparations and in controls. The size of the vesicles and size distribution were indistinguishable. Quanta contained about half of the usual amount of ACh in preparations stimulated in the presence of hemicholinium-3, an inhibitor of choline uptake, or in NH(4)(+), which diminishes the proton gradient for ACh uptake into the vesicles. Neither treatment changed the size of the synaptic vesicles. ACh content and vesicle size were both decreased in preparations stimulated in (-)-vesamicol, an inhibitor of ACh uptake in vesicles. Since the other inhibitors decreased ACh content by a similar amount without altering vesicle size, (-)-vesamicol may decrease vesicle size by acting on another target. We also found that a hypertonic solution in which the anion was aspartate increased quantal size similar to gluconate. Both anions have high hydration energy and a large volume. When these treatments increased quantal size the mean 20-80 % rise time of MEPPs recorded with an extracellular electrode was 170 micros. In the controls it was 97 micros. Perhaps some of the added ACh is bound within the vesicles, which slows the rise. Our major conclusion is that ACh content can change notably without any change in the size of the synaptic vesicles.

Repetitive nerve stimulation decreases the acetylcholine content of quanta at the frog neuromuscular junction

We investigated how elevated quantal release produced by motor nerve stimulation affects the size of the quanta. The motor nerve was stimulated at 10 Hz in preparations in which excitation-contraction coupling was disrupted. Two hundred stimuli reduced the size of the time integrals of the miniature endplate currents ([integral]MEPCs), measured at the same junction immediately after stimulation, by 16 %. Three thousand stimuli reduced size by 23 %. When the solution contained 10 microM neostigmine (NEO) 3000 stimuli reduced [integral]MEPCs by 60 %, because with acetylcholinesterase (AChE) inhibited, [integral]MEPC size is more sensitive to changes in acetylcholine (ACh) content. Similar decreases in miniature endplate potential size ([integral]MEPP) followed repetitive stimulation of contracting preparations. The depolarization produced by iontophoretic pulses of ACh was scarcely changed by 3000 nerve stimuli at 10 Hz, suggesting that the decreases in miniature sizes are largely due to less ACh released per quantum. Following 3000 stimuli at 10 Hz the sizes of the [integral]MEPCs increased back to pre-stimulus values with a half-time of 8-10 min. Recovery was blocked by (-)-vesamicol (VES), by hemicholinium-3 (HC3) and by nicotinic cholinergic agonists - all of which inhibit ACh loading into synaptic vesicles. The number of quanta in the total store was estimated by releasing them with carbonyl cyanide m-chlorophenylhydrazone (CCCP). CCCP releases fewer quanta after stimulation than from unstimulated controls. After resting for hours following stimulation, the releasable number increased, even when ACh loading inhibitors were present. We conclude that the inhibitors do not block a significant fraction of the ACh loading into reformed reserve vesicles and propose that ACh can be loaded in a series of steps.

Monoethylcholine as a false transmitter precursor at the frog and mouse neuromuscular junctions

Monoethylcholine (MECH) enters motor nerve terminals where it is made into acetylmonoethylcholine (AMECH). AMECH opens endplate channels for about half of the average duration observed where they are opened by acetylcholine (ACH). Therefore when AMECH is present in a quantum the endplate currents decay more rapidly. MECH has been used to measure quantal turnover in motor nerve terminals. We find that the incorporation of AMECH into quanta is blocked by vesamicol, an inhibitor of ACH transport into synaptic vesicles. AMECH is incorporated more rapidly when acetylcholinesterase is inhibited, when the choline uptake inhibitor, hemicholinium-3, is present or when extracellular Na+ (required for active CH uptake) is replaced with methylamine. This suggests that in the absence of these inhibitors CH obtained from released ACH is recycled. Therefore, experiments on the rate of incorporation of MECH are misleading unless CH recycling is prevented. Previous work also suggested that MECH is incorporated at a faster rate into those quanta which are released by stimulation than into those released spontaneously. We conclude that quanta released spontaneously and following nerve stimulation probably come from the same pool. The distribution of t1/2's during the incorporation of MECH can be accounted for in the framework of recent studies of the recycling of synaptic vesicles. We conclude that false transmitter is a valuable tool for studying the loading of quanta, but that there are several complications to be considered when trying to use it to measure the turnover of the population of quanta.

Bromoacetylcholine and acetylcholinesterase introduced via liposomes into motor nerve endings block increases in quantal size

We incorporated bromoacetylcholine (an inhibitor of choline acetyltransferase), acetylcholinesterase, or both into liposomes made of phosphatidylcholine. Frog sartorius muscles were exposed to these liposomes for 30-60 min. The liposome treatment itself did not decrease the size of the quanta compared to untreated controls. Then the preparations were exposed for 10-20 min to a hypertonic solution, which increases the rate of spontaneous quantal release and elicits an increase in the amount of acetylcholine released per quantum. Following the hypertonic treatment the quanta were significantly smaller in the liposome-treated preparations. Most of the difference occurred because in the preparations not exposed to the liposomes quantal size increased following the hypertonic treatment. This increase is thought to be due to the incorporation of more acetylcholine into each quantum. Our conclusions are that the treatments decreased the acetylcholine concentration in the cytoplasm, and that the increase in size occurs because additional acetylcholine is added to the vesicles containing the quanta from the cytoplasm.