Relationship between presynaptic calcium current and postsynaptic potential in squid giant synapse

1. The electric properties of the giant synapse in the stellate ganglion of the squid have been further investigated.2. During tetrodotoxin (TTX) paralysis, a local response can be elicited from the terminal parts of the presynaptic axons after intracellular injection of tetraethyl ammonium ions (TEA).3. The response is characterized by an action potential of variable size and duration, whose fall is often preceded by a prolonged plateau. The response, especially the duration of the plateau, is subject to ;fatigue' during repetitive stimulation.4. The TTX-resistant form of activity is localized in the region of the synaptic contacts, and shows a marked electrotonic decrement even within less than 1 mm from the synapse. It is found only on the afferent, not on the efferent, side of the synapse.5. During the plateau of the response, the membrane resistance is greatly reduced below its resting value.6. The response depends on presence of external calcium and increases in size and duration with the calcium concentration. Strontium and barium substitute effectively for calcium. Manganese and, to a lesser extent, magnesium, counteract calcium and reduce the response. The response also declines, and ultimately disappears, if sodium is withdrawn for long periods.7. The relation of the local TTX-resistant response to the influx of calcium ions and to the release of the synaptic transmitter is discussed.

Tetrodotoxin-resistant electric activity in presynaptic terminals

By subjecting isolated adrenal medullary cells to intense electric fields of brief duration it is possible to gain access to the cell interior without impairing the ability of the cell to undergo exocytosis. After a single exposure to a field of 2 kV/cm, τ=200 μsec, adrenal medullary cells behave as if their plasma membrane contains two pores of effective radius 2 nm. At 37°C these ‘equivalent pores’ remain patent for up to 1 hr. The formation and stability of these ‘pores’ is not affected by the Ca content of the bathing solution. The ‘pores’ permit externally applied catecholamine and Ca-EGTA to equilibrate rapidly with the cell water.

Calcium-dependence of catecholamine release from bovine adrenal medullary cells after exposure to intense electric fields.

Rapid conduction and the evolution of giant axons and myelinated fibers.

Criteria for distinguishing between monosynaptic and polysynaptic transmission.

Depolarization of the presynaptic terminal by current produced a postsynaptic potential (PSP) which increased with increasing presynaptic polarization and then reached a plateau. Iontophoretic injection of tetraethylammonium ions (TEA) into the presynaptic axon near the terminal produced a prolonged presynaptic spike. The resulting PSP is increased in size and its time course closely followed that of the presynaptic spike. The presynaptic fiber no longer exhibited rectification and strong depolarizations revealed that the PSP reached a maximum with about 110 mv depolarization. Further depolarization produced a decrease in PSP amplitude and finally transmission was blocked. However, a PSP then always appeared on withdrawal of the depolarizing current. Under the conditions of these experiments, the PSP could be considered a direct measure of transmitter release. Bathing the TEA-injected synapse with concentrations of tetrodotoxin (TTX) sufficient to block spike activity in both pre- and postsynaptic axons did not greatly modify postsynaptic electrogenesis. However, doubling TTX concentration reversibly blocked PSP. Thus the permeability changes to Na and K accompanying the spike do not appear necessary for transmitter release. Some other processes related to the level of presynaptic polarization must be involved to explain the data. The inhibition of transmitter release by strong depolarizations appears to be related to Ca action. A membrane Ca current may also be necessary for normal transmitter release.

https://rupress.org/jgp/article-pdf/50/11/2579/1243656/2579.pdf

Correlation of Transmitter Release with Membrane Properties of the Presynaptic Fiber of the Squid Giant Synapse

Morphology and recordings of electrical activity of Kuruma shrimp (Penaeus japonicus) giant medullated nerve fibers were carried out. A pair of giant fibers with external diameter of about 120 μ and 10 μ in myelin thickness were found in the ventral nerve cord. The diameter of the axon is about 10 μ. Thus there is a wide gap between the axon and the external myelin sheath. Each axon is doubly coated directly by Schwann cells and indirectly by the myelin sheath layer which is produced by those Schwann cells.



Impulse conduction velocities of these giant fibers showed a range between 90–210 m/sec at about 22°C. Large action potentials (up to 113 mV, rise time of 0.16–0.3 msec, maximum rate of rise of 650–1250 V/sec, half decay time of 0.2–0.3 msec, maximum rate of fall of 250–450 V/sec and total duration of less than 1.5 msec) could be obtained by inserting microelectrodes or by longitudinal insertion of 25 μ diameter capillary electrodes into the gap but no DC-potential difference was observed across the myelin sheath. Transmyelin electrical parameters were very favorable for fast impulse conduction: myelin resistance of 3 × 104 Ω cm2; time constant of 0.38 msec; myelin capacitance of 1.35 × 10−8 F/cm2; gap fluid resistivity of 23 Ω cm. The existence of nodes of Ranvier could not be demonstrated morphologically, but electrophysiological evidence suggests that a type of saltatory conduction occurs in these giant fibers.

Electrical activity and structural correlates of giant nerve fibers in Kuruma shrimp (Penaeus japonicus)

The external potential field along the functionally isolated medullated giant fiber of the shrimp (Penaeus setiferus) was recorded during impulse conduction using 1 M NaCl-filled glass capillary electrodes. The giant fiber displays no nodes of Ranvier, yet shows a conduction velocity greater than 90m/second at 20°C. Recordings from most parts of the myelin-covered giant fiber surface showed only small monophasic positive potentials. However, at the areas of giant fiber-motor giant fiber synapses and where giant fiber axons branch into abdominal ganglia, a small positivity was followed by a negativity (sink of the action current). The local application of action potential depressants to these areas abolished the negativity and converted it into positivity, and consequently impulse conduction was blocked. Application of the action potential depressants to other areas of the giant fiber did not block impulse conduction. The sink of the action current was not found from the axolemma in the myelin cylinder; this part of the axolemma together with the low resistance gap fluid simply serves as a passive current conductor. Light and electron microscopic studies of the two functionally excitable areas revealed the absence of a myelin sheath in some small areas of both regions. Thus, impulse conduction appears to be saltatory, and these excitable areas function as nodes of Ranvier.

Impulse conduction in the shrimp medullated giant fiber with special reference to the structure of functionally excitable areas

The minimal presynaptic depolarization (MPD) for producing a detectable postsynaptic potential (PSP) was lower than 25 mv in normal or tetrodotoxin (TTX)-containing seawater. The MPD was about 10 mv when a small amount of tetraethylammonium ions (TEA) was injected into the presynaptic terminal. Application of linearly increasing depolarizing current to the normal presynaptic terminal at times produced a PSP before a presynaptic spike was evoked; the rate of rise of the resulting PSP was much slower than that of a PSP triggered by the normal presynaptic spike. A brief depolarizing pulse that preceded the presynaptic spike in normal seawater or the initial transient presynaptic depolarization in TTX decreased the PSP. It increased the PSP when it was applied during the spike or initial transient depolarization. Hyperpolarizing pulses had the reverse effect. The Off-PSP was also modified by inserting pulses at an initial part of the recovery phase of the strong presynaptic depolarization. These results indicate further that increases in Na+ and K+ conductance during presynaptic spike activity are not a requirement for transmitter release; the rate of release of transmitter can be controlled by electrical manipulation of the presynaptic terminal; there is a superficial correspondence between the time courses of presynaptic depolarization and the resulting PSP. Thus presynaptic depolarization appears to be only the first step in the series of events constituting excitation-transmitter release coupling. It may not be a necessary step for the release mechanism.

/pdf/further-study-of-the-relationship-between-pre-and-2gj9skjkb6.pdf

Further study of the relationship between pre- and postsynaptic potentials in the squid giant synapse.

Tetraethylammonium ions were injected into the presynaptic axon of the squid giant synapse. Injection of these ions caused prolongation of the action potential with decreased out ward current. The prolonged spike was associated with increased release and prolonged activity of the transmitter substance. Although the amplitude of the postsynaptic potential increased with presynaptic depolarization, strong depolarization blocked transmitter re lease. In the injected presynaptic axon, transmitter release was blocked by 10(-6) gram of tetrodotoxin per milliliter. Transmitter release appears to be under control of presynaptic potential levels.

https://science.sciencemag.org/content/sci/155/3767/1257.full.pdf

Kiyoshi Kusano

Papers

Correlation of Transmitter Release with Membrane Properties of the Presynaptic Fiber of the Squid Giant Synapse

Electrical activity and structural correlates of giant nerve fibers in Kuruma shrimp (Penaeus japonicus)

Impulse conduction in the shrimp medullated giant fiber with special reference to the structure of functionally excitable areas

Further study of the relationship between pre- and postsynaptic potentials in the squid giant synapse.

Tetraethylammonium ions: effect of presynaptic injection on synaptic transmission.