Permeability changes induced by L‐glutamate at the crayfish neuromuscular junction.

SUMMARY1. In patch-clamp recordings from outer segments of dark-adapted rod photoreceptors, single-channel recordings were obtained from the light-sensitive conductance when divalent cations were omitted from the pipette solution bathing the extracellular face of the recorded patch of membrane.2. Activity of the light-sensitive channel was suppressed by light within the normal response range of the dark-adapted rod. During dim, steady illumination, the rate of opening of the channel fluctuated dramatically, as expected qualitatively from statistical fluctuations in the number of photoisomerizations occurring within the effective collecting area of the recorded patch.3. The light-sensitive channel flickered rapidly in the open state, so that individual events appeared as a burst of openings and closings. The average duration of a burst was 0-78+0-03 ms (mean+ s.E.). The average duration of an individual opening was 0 18 + 0008 ms. The average closed duration within a burst was 0-37 + 0-02 ms.4. Hyperpolarization of the recorded patch had no effect on average burst or open duration, although opening frequency increased slightly (+ 18-6 + 4-9%; n = 13; mean+ S.E.). Average single-channel current increased linearly with hyperpolarization, giving an estimated single-channel conductance of 20-5 + 1F1 pS. By extrapolation of the relation between channel current and hyperpolarization, the dark driving force was estimated to be about 48 mV.5. In addition to reducing the rate of channel events, dim non-saturating light also reduced the average duration of a burst of openings and the average duration of openings within a burst.6. About 50 % of cell-attached patches showed no channel activity in darkness.

Section: Discussionsupporting

confidence: 69%

Properties of ion channels closed by light and opened by guanosine 3',5'‐cyclic monophosphate in toad retinal rods.

Matthews

Watanabe

1987

“…An increase in, rather than a depression of the responses, was observed in other preparations following Con A action (Shinozaki & Ishida, 1979;Dekin, 1983). In fact, the potentiation of the response due to elimination of desensitization and increase in the channel lifetime may mask a simultaneous partial blockade.…”

Section: Discussionmentioning

confidence: 90%

“…This may suggest that Ca2+ removal also reduces glutamate uptake. Due to the very high apparent co-operativity of the response (Dudel, 1975b;Dekin, 1983), a small increase in effective agonist concentration could account for the observed potentiation. In untreated preparations, accumulations of agonist due to reduced uptake, especially around the innermost receptors of the bundle of electrically coupled fibres, could also increase the rate of desensitization.…”

Section: Discussionmentioning

confidence: 99%

“…It is now clear that the inward current generating the glutamate response is carried mainly by Na+ ions, although both Ca2+ and Mg2+ ions have been shown to be permeant (Dekin, 1983). Since Na+ is by far the major cation in crustacean physiological solutions, reducing the normal concentration of Ca2+ would not be expected to have a marked effect on the amplitude of the glutamate response.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

The effect of calcium ions on the glutamate response and its desensitization in crayfish muscle fibres.

Thieffry¹

1984

SUMMARY1. The responses of crayfish muscle fibres to bath application or long ionophoresis of L-glutamate were studied in normal and low Ca2+ solutions.2. The smaller responses recorded in low Ca2+ solutions have characteristics suggesting a faster desensitization.3. Desensitization and recovery have complex kinetics. Desensitization is faster and recovery slower when external Ca2+ concentration is reduced. Both components of the recovery phase, which can be fitted by the sum of two exponential, are affected by the external Ca2+ concentration.4. Recovery can be accelerated by external Ca2+ ionophoresis onto desensitized glutamate receptors.5. Responses to brief glutamate pulses of low intensity are not affected by Ca2+ reduction. For higher intensities, signs of desensitization are detectable early in the rising phase of the response.6. Concanavalin A (Con A) blocks both desensitization and Ca2+ dependence with similar time courses.7. Whether or not the preparation has been treated with Con A, the slowly rising responses recorded in isotonic Ca2+ do not show signs of desensitization.8. Con A causes a partial blockade of the glutamate response. 9. The Ca2+ dependence of the glutamate response can be explained by the Ca2+ dependence of the desensitization process, the cation acting at ectocellular sites of the muscle membrane.

“…This method had three drawbacks. First, the changes in ionic concentration in the crab saline could produce a small change in the size of the unit quantum (see Dudel, 1981), but since the glutamate-operated channel is highly selective for monovalent cations (Dekin, 1983) this effect is not a serious one. Second, in some preparations, the excitatory motor axon became inexcitable before the maximum evoked synaptic current had been reduced to include less than two quanta.…”

Section: Electrophysiological Recordingmentioning

confidence: 99%

Changes in binomial parameters of quantal release at crustacean motor axon terminals during presynaptic inhibition.

Atwood¹,

Tse²

1988

SUMMARY1. The effects of presynaptic inhibition on quantal release of transmitter were investigated at neuromuscular junctions of the motor axon supplying one of the limb muscles of a crab (Pachygrapsus crassipes).2. Binomial analysis of transmitter release recorded at selected neuromuscular junctions with an extracellular 'macro-patch' electrode indicated high probability of release (p) from a limited number of available sites (n). During presynaptic inhibition, both n and p were reduced.3. The binomial model provided a good description of results from non-inhibited junctions. During presynaptic inhibition, results from some junctions could be described by the binomial model, while those from other junctions could not. An interpretation of this finding is that presynaptic inhibition differentially affects the probability of release at various release sites of the neuromuscular junctional complex.4. A morphological study of the region of transmitter release under the macropatch electrode was made. Release-dependent uptake of horseradish peroxidase (HRP) into presynaptic terminals was restricted to the region under the recording electrode, by perfusing the preparation with calcium-free solution containing HRP. Transmitter release, and HRP uptake, occurred only at the site of the electrode, which was filled with a calcium-containing solution. Subsequently, serial sections were prepared for electron microscopy and the region of transmitter release was reconstructed.5. Numerous axo-axonal synapses were found in the HRP-labelled region. Thus, the morphological prerequisite for presynaptic inhibition exists at the site of transmitter release, and not exclusively at a more remote region.6. The number of morphologically identified excitatory neuromuscular synapses exceeded the 'release sites' estimated from the binomial model (n) by a wide margin. Morphological differences among synapses were observed. It is proposed that not all morphologically identified synapses participated in transmitter release under the experimental conditions employed. Thus, morphologically defined synapses are * Present address: