Effect of calcium on excitatory neuromuscular transmission in the crayfish

103

SUMMARY1. A quantitative description of facilitation in the crayfish claw opener muscle is presented. The facilitation of a test response following one or more conditioning stimuli, and the growth of facilitation during a tetanus, are measured.2. In superficial central fibres facilitation following one or more impulses can be described as the sum of two components which are both maximum at the end of the conditioning train and decline simultaneously and exponentially with different time constants thereafter.3. During a tetanus, facilitation to successive stimuli grows more rapidly than is predicted by assuming that each impulse adds a constant facilitative effect to an accumulating total state of facilitation.4. Sufficiently large values of tetanic facilitation are predicted by a model which assumes that transmitter release is proportional to the nth power of a substance or factor accumulating in nerve terminals. But no single value of n predicts the correct rise of facilitation in a tetanus and the time course of its subsequent decline from the facilitation following a single spike. 5. A model which assumes that the facilitative effects of successive spikes multiply in a tetanus predicts responses that are larger than those observed.6. The effects of varying the calcium concentration ([Ca2+]) on transmitter release and facilitation were studied. When a magnesium-EDTA buffering system is used to vary [Ca2+], transmitter release is found to be nearly linearly related to [Ca2+] in the range 0 1-13-5 mM.

mentioning

confidence: 65%

Characteristics of crayfish neuromuscular facilitation and their calcium dependence

Zucker¹

1974

103

“…This was tested in three preparations by reducing the quantal content of the e.j.p.s by lowering the Ca concentration from the normal 13X5 mm to 3 mM (Bracho & Orkand, 1970) and then measuring facilitation at superficial distal synapses following two conditioning stimuli. If the lesser accumulation of facilitation at the distal synapses were due to insufficient available transmitter, one would expect facilitation to be increased above the linear prediction as the quantal content is lowered.…”

Section: Resultsmentioning

confidence: 99%

“…In the crayfish, where non-linear accumulation can be found, the relationship between external Ca and transmitter release is linear (Bracho & Orkand, 1970 …”

Section: Discussionmentioning

confidence: 99%

The accumulative properties of facilitation at crayfish neuromuscular synapses

Linder¹

1974

4. The linear addition of facilitation at superficial distal synapses is not altered when the quantal content is lowered by decreasing the external Ca concentration from 13-5 to 3 mm.5. The rate of decay of facilitation is the same following both one and three conditioning stimuli, even though the facilitation is nearly six times larger in the latter case. 6. The results are discussed in terms of mechanisms for synaptic facilitation.

“…At 23 'C the synaptic delay ranged between 0 4 and 2-9 msec and the peak value was 0 7 msec. At 12 'C the synaptic delay was between 1-2 and 5-3 msec with peak value of 2-3 msec (see also Bracho & Orkand, 1970). If L-glutamate were to release endogenous transmitter by depolarizing the presynaptic nerve terminal, the time delay between the application of glutamate and the start of depolarization should be longer than the synaptic delay.…”

Section: Glutamate Receptors In Cra Yfish Musclementioning

confidence: 99%

Distribution and pharmacological properties of synaptic and extrasynaptic glutamate receptors on crayfish muscle.

Onodera¹,

Takeuchi²

1980

SUMMARY1. The distribution of glutamate sensitivity was measured in the opener muscle in the walking legs of the crayfish (Cambarus clarkii). L-Glutamate was ionophoretically applied under visual control.2. Bundles of a few muscle fibres were isolated and viewed with Nomarski optics. Two axons, presumably excitatory and inhibitory, branched widely over the surface of individual muscle fibres, forming numerous clusters of boutons or varicosities.3. Glutamate sensitivity was measured from the slope of the dose-response curves obtained by ionophoretic application of L-glutamate and expressed as mV/nC. The highest sensitivities were about 100 mV/nC, obtained at the edge of synaptic boutons. The sensitivity declined to less than 5 % about 2 Am away from the synaptic terminal. The time course of glutamate potentials was approximately the same as that of spontaneous synaptic potentials.4. Glutamate depolarization started within 300 /tsec after ionophoretic release of glutamate. This time lag was shorter than the synaptic delay of the nerve-evoked synaptic potential measured with an extracellular micro-electrode. This indicates that glutamate depolarization results from a direct action on the post-synaptic receptor.5. Application of L-a-kainic acid decreased the amplitude of the glutamate potential produced at the synaptic region, whereas kainate increased the amplitude of the glutamate potential at the extrasynaptic region. It is suggested that the pharmacological properties of the extrasynaptic receptor differ from those of the synaptic receptor. Possible mechanisms for the different actions of kainate are discussed.