Multimodal Distribution of Frog Miniature Endplate Potentials in Adult, Denervated, and Tadpole Leg Muscle

Abstract

Amplitude histograms of spontaneous miniature endplate potentials (MEPPs) from adult sartorius muscle cells show a definite bimodality with the mean amplitude of the larger mode five to seven times that of the smaller mode which accounted for 2-5 % of the total MEPPs. Histograms were plotted after high frequency MEPP generation induced by increasing temperature, increasing external calcium or nerve stimulation. These plots showed a reversible left-shift of the major mode as well as a reversible increase in the proportion of small mode MEPPs. Repeated challenges shifted almost all MEPPs into the small mode. An increase in the percentage of small mode MEPPs also occurred spontaneously during the course of denervation before the quiescent period and some of the histogram profiles showed multiple modes whose means were integer multiples of the small mode mean. In the early stages of hind leg development the greatest proportion of MEPPs were of the small mode size; as metamorphosis progressed, the histograms showed a definite multimodality with the mean of each mode being an integer multiple of the small mode mean and with the proportion of MEPPs in each mode about the same. During tail resorption the percentage of larger MEPPs increased until the adult histogram profile was reached. Thus, the changes in MEPP amplitude histograms over the course of metamorphosis are the reverse of those found with denervation.

Quantal transmitter release by glioma cells: quantification of intramembrane particle changes

“…1, the size of the quantal steps was close to 100 pA. Knowing that (i) opening of one Xenopus embryonic nicotinic receptor under the present conditions will generate a current of 1^2 pA (Kidokoro and Rohrbough, 1990), (ii) two ACh molecules are needed to activate one receptor, (iii) presumably more than half of the molecules are wasted in the process, the quanta recorded here should result from the synchronous release of at least 400 ACh molecules. This is much less than the 6000^10 000 molecules estimated for the full quantum in adult neuromuscular or nerve^electroplaque junctions (Ku¥er and Yoshikami, 1975;Girod et al, 1993), but compares well with the values reported in embryonic endplates (Evers et al, 1989), sympathetic ganglia (Bennett, 1995), or in the population of small miniature currents which is believed to be the substructure of the classical quantum (Kriebel and Gross, 1974;Girod et al, 1993). A slightly larger value (200 pA) was obtained using neuroblastoma N18-TG-2 cells after transfection with the mediatophore, a proteolipid present in the presynaptic membrane cholinergic nerve terminals (Falk-Vairant et al, 1996a).…”

“…The endplate potentials generated by Schwann cells occur at a low frequency: this frequency is rather reduced than increased by rising the Ca 2þ concentration. In addition, their amplitude and time course exhibit large variations, with a majority of small quanta displaying a skewed size distribution (Birks et al, 1960;Kriebel and Gross, 1974;Kriebel et al, 1980;Colme ¤us et al, 1982). In the present work, ACh was released in a clearly Ca 2þ -dependent manner from pre-¢lled C6 cells, as demonstrated by using either the chemiluminescent assay, or the electrical stimulation combined to Xenopus myocyte real-time recording.…”

Neostigmine increases the size of subunits composing the quantum of transmitter release at mouse neuromuscular junction.

“…Most importantly, the integral peaks start from zero on the histograms and, in all but one junction, the number of central peaks did not change after neostigmine treatment just as Wernig & Motelica-Heino (1978) reported for the frog neuromuscular junction. Bevan (1976) did not find multiple peaks and stated that the origin of the peaks previously published by Kriebel & Gross (1974) Magleby & Miller (1981) is composed of ca. 3750 m.e.p.p.s with a noise-to-signal ratio of 6 %; and Fig.1 D is composed of 3941 m.e.p.p.s with a noise-to-signal ratio of 3.5 %.…”

Transmitter release from normal and degenerating locust motor nerve terminals.

“…'Small' miniature potentials have also been described at normal (Bevan, 1976) and degenerating (Kriebel & Gross, 1974; see also Birks, Katz & Miledi, 1960) Dennis & Miledi (1974) to account for such a discrepancy was that their evoked unit responses might be composed of two or more quanta. In the present studies it was assumed that all response failures were true Poisson failures to release transmitter.…”

“…Perhaps, as has been suggested by Bevan (1976) and Katz (1977) 'small' transmitter quanta represent some labile stage in the packaging and release of transmitter. Another possibility is that 'small' transmitter quanta may be real quantal units, several of which are released synchronously to produce a miniature potential (Harris & Miledi, 1971;Kriebel & Gross, 1974;Kreibel, Llados & Matteson, 1976;Wernig & Stirner, 1977).…”