Anion blockers inhibit impulse-evoked quantal transmitter release at the crayfish neuromuscular junction (original) (raw)
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The Journal of physiology, 1982
1. The effects of a hypertonic bathing medium, containing either NaCl or melezitose, on the average number (m) and the time course of quantal release following an action potential were studied using focal extracellular recording methods at synaptic sites on the opener muscle of the crayfish leg.2. After the application of hypertonic saline, the rate of spontaneous quantal release increased but m decreased to a new level within 1 min; the extent of depression depended on the magnitude of the increase in tonicity until the osmolarity was 50% greater than the normal value of 0.43 osmol/l (Osm), but greater increases in tonicity exerted little further effect.3. The synaptic delays were increased and distributed over a longer range of time in hypertonic solutions; also, the latency between the first and second quantal releases in a multiple response to a single nerve impulse was also increased.4. Hypertonicity had no significant effect on the conduction velocity of the action potential, ...
The Journal of General Physiology
Membrane potential was recorded intracellularly near presynaptic terminals of the excitor axon of the crayfish opener neuromuscular junction (NMJ), while transmitter release was recorded postsynaptically. This study focused on the effects of a presynaptic calcium-activated potassium conductance, gK~c~, on the transmitter release evoked by single and paired depolarizing current pulses. Blocking gK~c~) by adding tetraethylammonium ion (TEA; 5-20 mM) to a solution containing tetrodotoxin and aminopyridines caused the relation between presynaptic potential and transmitter release to steepen and shift to less depolarized potentials. When two depolarizing current pulses were applied at 20-ms intervals with g~c~ not blocked, the presynaptic voltage change to the second (test) pulse was inversely related to the amplitude of the first (conditioning) pulse. This effect of the conditioning prepulse on the response to the test pulse was eliminated by 20 mM TEA and by solutions containing 0 mM Ca2+/1 mM EGTA, suggesting that the reduction in the amplitude of the test pulse was due to activation of gK~c~ by calcium remaining from the conditioning pulse. In the absence of TEA, facilitation of transmitter release evoked by a test pulse increased as the conditioning pulse grew from -40 to -20 mV, but then decreased with further increase in the conditioning depolarization. A similar nonmonotonic relationship between facilitation and the amplitude of the conditioning depolarization was reported in previous studies using extracellular recording, and interpreted as supporting an additional voltagedependent step in the activation of transmitter release. We suggest that this result was due instead to activation of a gKtc~ by the conditioning depolarization, since facilitation of transmitter release increased monotonically with the amplitude of the conditioning depolarization, and the early time course of the decay of facilitation was prolonged when gK<ca~ was blocked. The different time courses for decay of the presynaptic potential (20 ms) and facilitation (> 50 ms) suggest either that residual Zucker, R. S., and L. O. Lara-Estrella. 1983. Post°tetanic decay of evoked and spontaneous transmitter release and a residual calcium model of synaptic facilitation at crayfish neuromuscular junctions.
The Journal of Physiology, 1995
The effects of the calcium channel blockers, funnel-web spider toxin (FTX), w-agatoxin IVA (w-Aga IVA) and w-conotoxin GVIA (w-CgTX), were tested on transmitter release and presynaptic currents in frog motor nerve endings. 2. Evoked transmitter release was blocked by FTX (IC50= 0o02 #l ml-') and w-CgTX (1 #M) but was not affected by w-Aga IVA (0'5/M). When FTX (0 1 #l ml-') was assayed on spontaneous release either in normal Ringer solution or in low Ca2+-high Mg2+ solution, it was found not to affect miniature endplate potential (MEPP) amplitude but to increase MEPP frequency by-2-fold in both conditions. 3. Presynaptic calcium currents (Ica), measured by the perineurial technique in the presence of 10 mm tetraethylammonium chloride (TEA) and 200 juM BaCl2 to block K+ currents, were blocked by w-CgTX (5 ,UM), partially blocked by FTX (1 1d ml-') and not affected by w-Aga IVA (0 5,UM). 4. The presynaptic calcium-activated potassium current (IK(ca)) measured by the perineurial technique in the presence of 0 F5jM 3,4-aminopyridine (DAP) to block voltage-dependent K+ currents, was strongly affected by charybdotoxin (ChTX) (300 nM) and completely abolished by BaCl2 (200 juM). This current was also blocked by w-CgTX (5 FM) and by CdC12 (200 FM) but was not affected by FTX (1 Fl ml-'). The blockade by w-CgTX could not be reversed by elevating [Ca]o to 10 mM. 5. The results suggest that in frog synaptic terminals two w-CgTX-sensitive populations might coexist. The transmitter release process seems to be mediated by calcium influx through a w-CgTXand FTX-sensitive population.
Toxicon, 1986
J . MOLGO, M. LE~tetcNwty and F. TwaEFF-DEP~swtE. Enhancement by Anemonia suirnta toxin II of spontaneous quanta) transmitter release from mammalian motor nerve terminals. Toxicon 24, 441-450, 1986 . -The action of Anemonia svitata toxin II (ATX-II) on spontaneous quanta) transmitter release from motor nerve terminals was inveatigated by recording miniature end-plate potentials (MEPPs) from isolated mouse phrenic nerve-hemidiaphragm nerve-muscle preparations . ATX-II (3 .2~when applied for 30-40 min to junctions bathed in a normal ionic medium enhanced about one hundred fold the rate of apoataaeous MEPPs. Concomitantly, ATX-II depolarized the muscle fiber. The effect of the toxin on MEPP frequency was markedly reduced when junctions were exposed to Na-deficient solutions or pre-treated with dantrolene sodium (10~. ATX-II (0.24-3.2 FM) increased MEPP rate in junctions exposed to a Ca-free medium containing 2 mM EGTA and 2 mM Mg" in a dose-and time-dependent manner . Tetrodotoxin (0.2 -1 kM) prevented the effects of ATX-II oa MEPP frequency and on the rating membrane potential of muscle fibers . Tetrodotoxin also antagonized the aaeleration of MEPP induced by ATX-II . The experimental findings suggest that ATX-II acts to increase quanta) transmitter output from motor nerve terminals by enhancing Na' influx through tetrodotoxinsensitive presyaaptic channels, since ATX-II action does not appear to depend upon entry of Ca" from the extracellular medium . It is likely that ATX-II, by increasing intraterminal Na' concentration, may trigger calcium release from internal stores.
Changes of quantal transmitter release caused by gadolinium ions at the frog neuromuscular junction
British Journal of Pharmacology, 1991
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).
Neuroscience, 1989
The dihydropyridine, Bay K 8644, was applied in vitro to mouse phrenic nerve-diaphragm muscle preparations. The drug increased both spontaneous and evoked release of acetylcholine from the motor nerve terminal in a concentration- and time-dependent manner. The rise in miniature endplate potential frequency, however, was the result of an increased intraterminal mobilization of free calcium, rather than well-established activation of voltage-dependent calcium channels. This view is supported by the following observations: (1) an increase in frequency was apparent in Ca2+-free medium; (2) Bay K 8644 is known to require a moderate depolarization to affect Ca2+ channels, but no membrane depolarization was detected; and (3) exposure to low Ca2+ and high Mg2+ medium did not diminish the effect on miniature endplate potential frequency. In a medium containing low Ca2+ and high Mg2+, Bay K 8644 increased quantal content of the evoked endplate potentials to a greater degree and with a faster ...
Proceedings of the National Academy of Sciences, 1994
The toxin fraction (FIX) and peptide to-Aga-WA from the venom of the funnel-web spider Agelenopsis aperta, as well as a synthetic analogue ofFX, speciically block the P-type voltage-dependent Cam channel (VDCC). The effects of these toxins on synaptic transmission were studied in the neuromuscular synapses of the crayfish opener muscle, which has a single excitatory and a single inhibitory motoneuron. FIX selectively and reversibly blocked excitatory and inhibitory stsynaptic currents and potentials in a dosedependent manner. FIX had no effect on (resting and postaynaptic membrane conductance, (it) postsynaptic L-type VDCC, and (ii) both glutamateand-aminobutyric acidinduced postsynaptic responses. Mean amplitude and frequency ofminiatre postsynaptic potentials were unchanged by FIX. The postsynaptic VDCC was inhibited by nifedipine, a selective dihydropyridine antagonist of L-type VDCC, whereas synapti transmission was unaffected. Transmision was also undisturbed by o,-conotoxin, suggesting that N-type VDCCs are not involved. The peptide w-Aga-IVA blocked excitatory and inhibitory tsmission without affecting postsynaptic VDCC. Synaptic transmission was also blocked by synthetic FTX. We conclude that presynaptic P-type VDCCs are involved in both evoked excitatory and inhibitory transmitter release in crayfish neuromuscular synapses. Ca2+ influx from voltage-dependent Ca2+ channels (VDCCs) plays a key role in cellular physiology. Besides the electrical activity generated by Ca2+ influx, numerous cellular functions are triggered by the subsequent transient rise in intracellular Ca2+ concentration, which acts as an intracellular chemical messenger for Ca2+-sensitive mechanisms that control ion channel gating, enzyme activation, metabolism, gene expression, evoked transmitter release, etc. (1, 2). Four major types of VDCCs, named L, N, T, and P have been described (2, 3). While the L-type is selectively sensitive to 1,4-dihydropyridines (DHPs) (which act as agonists or antagonists), the high-threshold L and N types are blocked by w-conotoxin (w-CgTX) (4), although recent experiments suggest that this toxin blocks N-type but not L-type VDCCs (5-7). The intermediate-threshold P type is insensitive to both w.CgTX and DHPs but is selectively blocked by the low molecular weight polyamine funnel-web spider toxin (FTX) (3) and by the peptide w-agatoxin, fraction IVA (w.Aga-IVA)
The Journal of General Physiology
Membrane potential changes that typically evoke transmitter release were studied by recording intracellularly from the excitor axon near presynaptic terminals of the crayfish opener neuromuscular junction. Depolarization of the presynaptic terminal with intracellular current pulses activated a conductance that caused a decrease in depolarization during the constant current pulse. This conductance was identified as a calcium-activated potassium conductance, g~c~), by its disappearance in a zero-calcium/EGTA medium and its block by cadmium, barium, tetraethylammonium ions, and charybdotoxin. In addition to gK~c,), a delayed rectifier potassium conductance (gK) is present in or near the presynaptic terminal. Both these potassium conductances are involved in the repolarization of the membrane during a presynaptic action potential.
Neuroscience Letters, 1988
Recordings of synaptic currents from the crayfish opener muscle were made with a macro-patch recording technique, permitting clear detection of neurotransmitter quanta at individual nerve terminals before and after induction of long-term facilitation (LTF). Depolarization of the terminal by propagated action potentials or by local intracellular pulses induced LTF. The quantal content was increased on average by 93%. Binomial analysis indicated increased probability of release and also increased number of available quantal units. The increase occurred regardless of a blockade of sodium, calcium or potassium channels by appropriate pharmacological agents. Presynaptic recording with an intracellutar microelectrode showed no change in presynaptic electrical properties. Also, there were no changes in the synaptic delay. It is concluded that LTF results from a depolarization-dependent alteration of synaptic release sites.