Increasing quantal size at the mouse neuromuscular junction and the role of choline.

Abstract

SUMMARY1. In frog a variety of treatments have been shown to increase quantal size at the neuromuscular junction (NMJ), apparently by releasing more acetylcholine (ACh) per quantum. The present experiments were undertaken to see whether similar changes occur at the mouse NMJ.2. None of the hormones tested, adrenaline, insulin or corticosteroids, significantly increased quantal size at the mouse NMJ.3. Soaking diaphragms in hypertonic solution (about 475 mosmol/kg) for 15-30 min roughly doubled the size of miniature endplate potentials (MEPPs), miniature endplate currents (MEPCs), and uniquantal endplate potentials (EPPs). We will refer to these as 'large quanta'. Note that all of the measurements were made after returning the preparation to normal (Tyrode) solution.4. In frog hypertonic solution made with sodium gluconate replacing NaCl increases quantal size four-rather than two-fold. In mouse there was little difference in the effects of solutions made with the two anions. In Cl--free hypertonic solution, made with sodium gluconate and SO42-solutions, quantal size increase is somewhat less, so there may be a role for Cl-in enlarging quantal size.5. After hypertonic treatment, quantal size remained elevated for at least 1 h and then gradually declined back to usual levels. The data suggest a gradual decrease in mean quantal size, rather than the appearance of a new subpopulation of smaller quanta.6. Hypertonic treatment did not change the endplate depolarization in response to ionophoretically applied ACh. This suggests that the increased quantal size is not due to a postsynaptic change. Large MEPP's disappear in the presence of tubocurarine and reappear when the drug is washed away.7. Vesamicol is an inhibitor of active ACh uptake into isolated synaptic vesicles.5 #m-vesamicol has no detectable postjunctional effect. However, when vesamicol was included in the Tyrode and the hypertonic solutions the increase in quantal size was blocked. This is further evidence that the large quanta are produced by the release of more ACh per quantum. 8. Even when added after the hypertonic treatment, vesamicol soon decreased quantal size back to the normal level. Two other inhibitors of active ACh uptake into vesicles, tetraphenylboron (TPB) and hexanitrodiphenylamine (HNPA) also de-MS 8417 S. P. YU AND W. VAN DER KLOOT creased quantal size after hypertonic treatment, apparently by a presynaptic action. This suggests that the additional ACh that produces large miniatures may be incorporated into the quanta shortly before release.9. Experiments were undertaken to test the hypothesis that the additional ACh added to the quanta is newly synthesized transmitter, produced by the acetylation of choline brought into the terminal by the high-affinity choline transport system (HAChT). The formation of large quanta was blocked when the solutions contained hemicholinium-3 (HC-3), an inhibitor of HAChT. At the concentrations used, HC-3 had no notable effect on the postjunctional acetylcholine receptor (AChR). After quantal size was incre...

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

“…However, following exposure to 300 mM NaCl the quantal sizes in the controls increased, while those in the liposome-treated preparations either decreased somewhat or were unchanged. Therefore the results are completely consistent with the hypothesis that quantal size is increased by the transport of additional ACh from the cytoplasm into the vesicles [21,23,30]. The similar effects of (-)-vesamicol further support this interpretation.…”

“…Previous work has identified a number of treatments that increase quantal size at the neuromuscular junction by increasing the amount of ACh in the quanta [21,23]. The increase in size is prevented by (-)-vesamicol, an inhibitor of active ACh transport into synaptic vesicles [1,20,29,30]. Our interpretation was that quantal size increases because additional ACh is transported from the cytoplasm into the vesicles.…”

“…It is a signal that raises the adenosine 3',5'-cyclic monophosphate (cAMP) concentration in the motor nerve terminal which, in turn, leads to an increase in quantal size [24]. Much of the increase in quantal size appears to be due to ACh pumped from the nerve terminal cytoplasm into the vesicles awaiting release at the active zone [26,30]. This process is known as second-stage loading.…”

Experimental muscle pain does not cause long-lasting increases in resting electromyographic activity

“…The endplate noise is believed to be extracellularly recorded MEPs. 3 Treatment with hypertonic saline has previously been shown to cause increased frequencies and sizes of MEPs in single muscle fiber preparations, 19,20,38,39 which could suggest a direct excitatory effect of the hypertonic saline injections on the muscle fibers. On the other hand, potassium released by the injection of hypertonic saline may also cause inhibitory processes.…”

Role of Acetylcholinesterase on the Structure and Function of Cholinergic Synapses: Insights Gained from Studies on Knockout Mice

“…MEPPs recorded from hemidiaphragm muscle of AChE ?/? mice exhibited an amplitude of 1.18 ± 0.11 mV, a decay time of 2.85 ± 0.16 ms and a frequency of 2.18 ± 0.21/s, which are typical values for control MEPPs in mammalian skeletal muscle(Del Castillo and Katz 1954;Yu and Van der Kloot 1991). MEPPs recorded in hemidiaphragm muscle of AChE -/-mice had an amplitude similar to control (1.09 ± 0.10 mV), but a lower frequency (1.38 ± 0.15/s)…”