Changes of quantal transmitter release caused by gadolinium ions at the frog neuromuscular junction

Abstract

1 The actions of the trivalent cation, gadolinium (Gd3+), were studied on frog isolated neuromuscular preparations by conventional electrophysiological techniques. 2 Gd3+ (450pM) applied to normal or formamide-treated cutaneous pectoris nerve-muscle preparations induced, after a short delay, a complete block of neuromuscular transmission. The reversibility of the effect was dependent on the time of exposure. 3 Gd3 + (5-450gM) had no consistent effect on the resting membrane potential of the muscle fibres. 4 Gd3+ (5-40pM) applied to preparations equilibrated in solutions containing high Mg2+ and low Ca2 + reduced the mean quantal content of endplate potentials (e.p.ps) in a dose-dependent manner. Under those conditions, 3,4-diaminopyridine (10guM) consistently reversed the depression of evoked quantal release. 5 The calcium current entering motor nerve terminals, revealed after blocking presynaptic potassium currents with tetraethylammonium (10mM) in the presence of elevated extracellular Ca2+ (8 mM), was markedly reduced by Gd3 + (0.2-0.5 mM).6 Gd3+ (40-200pM) increased the frequency of spontaneous miniature endplate potentials (m.e.p.ps) in junctions bathed either in normal Ringer solution or in a nominally Ca2 +-free medium supplemented with 0.7.fM tetrodotoxin. This effect may be due to Gd3+ entry into the nerve endings since it is not reversed upon removal of extracellular Gd3+ with chelators (1 mm EGTA or EDTA). Gd3 + also enhanced the frequency of me.p.ps appearing after each nerve stimulus in junctions bathed in a medium containing high Mg2 + and low Ca2 +. 7 Gd3 , in concentrations higher than 100gM, decreased reversibly the amplitude of m.e.p.ps suggesting a postsynaptic action.8 It is concluded that the block of nerve-impulse evoked quantal release caused by Gd3 + is related to its ability to block the calcium current entering the nerve endings, supporting the view that Gd3 + blocks N-type Ca2+ channels; while the enhancement of spontaneous quantal release is probably the result of Gd3 + entry into motor nerve endings. Besides its dual prejunctional effects on quantal release it is suggested that Gd3 + exerts a postsynaptic action on the endplate acetylcholine receptor-channel complex.

Antagonism of calcium currents and neurotransmitter release by barium ions at frog motor nerve endings

“…Many studies of perineural Ca 2+ currents are performed in the presence of higher concentrations of K + channel blockers (high K + channel block Ringer–see e.g. Molgo et al ., 1991; Redman & Silinsky, 1995; Katz et al ., 1995). Whilst the prolonged Ca 2+ currents in this solution may or may not be relevant to the Ca 2+ current that promotes ACh release (Anderson et al ., 1988; Redman & Silinsky, 1995; Katz et al ., 1995), such high concentrations of K + channel blockers; (i) minimize the possibility of any secondary effects of Ba 2+ on unblocked K + channels and (ii) eliminate the confounding effects of Ba 2+ on residual outward current contaminants as such contaminants are rare in the presence of these high concentrations of K + channel blockers (Katz et al ., 1995–see also Silinsky & Solsona, 1992 and Discussion).…”

Calcium currents at motor nerve endings: absence of effects of adenosine receptor agonists in the frog.

“…Preliminary results suggest that the entry of Ca2+ through N-type Ca2+ channels elicits evoked ACh release in frog motor nerve endings (Kerr & Yoshikami, 1984;Arnon, David, Hevron & Yaari, 1988;Robitaille, Adler & Charlton, 1990;Molgo, Del Pozo, Banos & Angaut-Petit, 1991). It would thus appear of interest to confirm the presence of N-type Ca2+ currents in this species and to examine the effect of adenosine on such currents in the hope of providing further information on the mechanism by which this purine inhibits transmitter release.…”

“…The Ca2+ current may be contaminated to a limited degree (generally less than 25%) by Na+ currents and by the passive current flow associated with the repolarization of the nerve ending (see e.g. Hamilton & Smith, 1991 in rat;Molgo et al 1991 in frog). For example, the current in Fig.1B, w-conotoxin was not further reduced by adding Co2+ or removing extracellular Ca2+, suggesting that w-conotoxin was producing a maximal inhibition of Ca2+ currents and that the remainder of the upward tracing, in Fig.…”

“…In some experiments, repetitive firing was eliminated with 100 #M procaine (e.g. Molgo et al 1991 Shimoni, Alnaes & Rahamimoff, 1977). Addition of sucrose (50 mM) elevates the cytoplasmic Ca2+ concentration to the micromolar range and produces a reverse gradient for Ca2+ (Shimoni et al 1977).…”

Relative potencies of metal ions on transmitter release at mouse motor nerve terminals

“…have generally examined only the effects of a single metal ion (e.g. Balnave & Gage, 1973;Weakly, 1973;Forshaw, 1977;Molgo et al, 1991;Satoh et al, 1982) and have not, as in the present study, systematically compared a range of metal ions. However, there have been two other studies using a series of metal ions, but they have measured the effect of the ions on twitch tension (Lin-Shiau & Fu, 1980) and extracellularly recorded presynaptic ion channel currents (Hamilton & Smith, 1992).…”