Acquired neuromyotonia is characterized by hyperexcitability of motor nerves leading to muscle twitching, cramps, and weakness. The symptoms may improve following plasma exchange, and injection of immunoglobulin G (IgG) from 1 neuromyotonia patient into mice increased the resistance of neuromuscular transmission to d-tubocurarine. Here we examine nerves and muscle in vitro from mice injected with plasma or purified IgG from 6 neuromyotonia patients or pooled control subjects, and cultured dorsal root ganglion cells after treatment with IgG. Three of the patients had antibodies against human voltage-gated potassium channels labeled with 125I-alpha-dendrotoxin. The quantal release of acetylcholine (quantal content) at end-plates in diaphragms from mice treated with neuromyotonia IgG preparations was increased by 21% relative to control values (p = 0.0053). With one IgG preparation, the duration of the superficial peroneal nerve compound action currents was increased by 93%. The dorsal root ganglion cells treated with this IgG showed a marked increase in repetitive firing of action potentials. All effects were similar to those obtained with aminopyridines. We conclude that at least some patients with acquired neuromyotonia have antibodies directed against aminopyridine- or alpha-dendrotoxin-sensitive K+ channels in motor and sensory neurons, and they are likely to be implicated in the disease process.

SUMMARY1. Rats were injected once every 48 h with a-bungarotoxin (aBTX) for periods up to 6 weeks. Injections caused weakness of facial muscles which lasted about 8 h. Hemidiaphragms were dissected for biochemical and electrophysiological measurements.2. In muscles from animals treated for 2-3 weeks with toxin, the binding of 125-aBTX was reduced to 58 %, and the ACh content to 81 % of control values. Choline acetyltransferase activity was unchanged. ACh release evoked by 3 Hz nerve stimulation was increased to 175 % of control values.3. The use of ,u-conotoxin, which specifically blocks muscle action potentials, enabled the recording of full-sized endplate potentials (EPPs) and miniature endplate potentials (MEPPs) at normal muscle membrane potentials ( -70 to -80 mV). The amplitude of MEPPs was decreased to 57 % in muscles from animals treated for 3 weeks with aBTX. The mean of the quantal contents, calculated from the ratio of the corrected EPPs and the MEPPs, was increased to 154 %.4. Within individual muscles of both acBTX-treated and control rats, there was an inverse relationship between the quantal content of an endplate and its MEPP amplitude.5. The MEPP frequency of endplates from control muscles was positively correlated with the quantal content. However, this correlation was not found in aBTX-affected muscles.6. Three hours after a single injection of aBTX the amplitude of the MEPPs was reduced to about 60 % of control values but no increase of the quantal content was found. During the first few days of aBTX treatment the quantal content gradually increased; it reached a plateau between 20 and 30 days.7. The results suggest the existence of an adaptive mechanism, operating at individual endplates, in which retrograde signals at the motor nerve terminals modulate ACh release when neuromuscular transmission is endangered by block of acetylcholine receptors. MS 1036 J. J. PLOMP AND OTHERS

Neurotransmission requires Ca(2+)-dependent release of secretory products through fusion pores that open and reclose (partial membrane distention) or open irreversibly (complete membrane distention). It has been challenging to distinguish between these release modes; however, in the work presented here, we were able to deduce different modes of depolarization-evoked exocytosis in neuroendocrine chromaffin and PC12 cells solely by analyzing amperometric recordings. After we determined the quantal size (Q), event half-width (t(50)), event amplitude (I(peak)), and event decay time constant (τ(decay)), we fitted scatter plots of log-transformed data with a mixture of one- and two-dimensional Gaussian distributions. Our analysis revealed three distinct and differently shaped clusters of secretory events, likely corresponding to different modes of exocytosis. Complete membrane distention, through fusion pores of widely varying conductances, accounted for 70% of the total amount of released catecholamine. Two different kinds of partial membrane distention (kiss-and-run and kiss-and-stay exocytosis), characterized by mode-specific fusion pores with unitary conductances, accounted for 20% and 10%, respectively. These results show that our novel one- and two-dimensional analysis of amperometric data reveals new release properties and enables one to distinguish at least three different modes of exocytosis solely by analyzing amperometric recordings.

1. The presynaptic component of an adaptive feedback mechanism leading to increased acetylcholine (ACh) release was studied in endplates of diaphragms from rats treated chronically with a-bungarotoxin (aBTX). 2. Quantal contents were calculated 'directly' from the amplitude of miniature endplate potentials (MEPPs) and endplate potentials (EPPs) which were recorded after juconotoxin treatment to prevent muscle action potentials.3. In vitro application of the Ca2" channel blockers nifedipine (10 /M) or c-conotoxin (40 nM) had no significant effect on the increased quantal content of endplates from aBTXtreated rats. 4. At control endplates, in vitro block of presynaptic K+ channels by 5/SM 3,4-diaminopyridine did increase the quantal content to a level which was similar to that found in endplates ofc aBTX-treated rats but also induced a broadening of EPPs, which was not found at endplates after aBTX treatment. 5. The difference between quantal contents of aBTX-treated and control rats was highly the phrenic nerve at 30-50 Hz was somewhat larger at endplates from aBTX-treated rats than at control endplates. At low [Ca2+]0, the potentiation of EPP amplitudes during a stimulus train was much larger at endplates from azBTX-treated rats than from controls. 7. The results do not support the idea that the increased release of ACh is caused via regulatory effects on the presynaptic Ca2+ or K+ channels. Instead, the anomalous dependency of ACh release on Ca2+ in muscles of aBTX-treated rats suggests that a cytoplasmic, Ca2+-dependent, component is involved in the adaptive change of transmitter release.

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