Giant axon

Electrophysiological protocols utilizing crayfish and squid giant axons revealed that external application of brevetoxin caused a concentration-dependent... 

In squid giant axons, PbTx causes a depolarization of the plasma membrane, repetitive discharges followed by depression of action potentials, and a complete blockade of excitability. This action is antagonized by TTX (83,84). PbTx depolarizes nerve terminals and induces neurotransmitter release (85,86) it depolarizes skeletal muscle cells (87) and increases the frequency of action potentials in crayfish nerve cord (88). PbTx also produces a contraction of the guinea pig ileum (89). All these effects are prevented by TTX. 

Much evidence supports this scheme. For example, neuronal depolarisation increases the amount of free synapsin in the cytosol and microinjection of CAM kinase II into the terminals of the squid giant axon or brain synaptosomes increases depolarisation-evoked transmitter release. By contrast, injection of dephosphorylated synapsin I into either the squid giant axon or goldfish Mauthner neurons inhibits transmitter release. 

Much evidence supports a role for these proteins in exocytosis. For instance, injection of recombinant SNAP into the squid giant axon increases vesicular exocytosis. Also, membrane SNAP-25 and syntaxin are both targets for botulinum toxin while the vesicule protein, synaptobrevin, is a target for tetanus and botulinum toxins both these toxins are well known for disrupting transmitter release. 

Hodgkin and Huxley [81] formulated a membrane model that accounts for K" ", Na" ", and ion leakage channels in squid giant axons [Fig. 22(a)]. The membrane resting potential for each ion species is treated like a battery and the degree to which the channel is open is modeled by a variable resistor. 

Ross WN, Salzberg BM, Cohen LB et al (1977) Changes in absorption, fluorescence, dichroism and birefringence in stained giant axons optical measurement of membrane potential. J Membr Biol 33 141-183... 

Video microscopy allows study of molecular mechanisms through direct observation of organelle movements while precise control of experimental conditions is maintained. Fast axonal transport continues unabated in isolated axoplasm from giant axons of the squid Loligo pealeii for hours [14]. Video microscopy applied to isolated axoplasm permits a more rigorous dissection of the molecular mechanisms for fast axonal transport... 

Brady, S. T., Lasek, R. J. and Allen, R. D. Fast axonal transport in extruded axoplasm from squid giant axon. Cell Mot. 3 (Video Supplement), 1983. 

Spencer PS, Schaumburg HH. 1977a. Ultrastructural studies of the dying-back process. III. The evolution of experimental peripheral giant axonal degeneration. J Neuropathol Exp Neurol 36(2) 276-299. 

Armstrong, C.M. (1971) Interaction of tetraethylammonium ion derivatives with the potassium channels of giant axons. The Journal of General Physiology, 58, 413-437. 

It has been reported4 that cholinesterase inhibitors (such as di-isopropyl phosphorofluoridate) increase the permeability of squid giant axons towards sodium and potassium. There is also an indication that the erythrocyte requires, among other factors, an adequate acetylcholine-cholinesterase system to prevent a gain of sodium or a loss of potassium.5 The conclusion that permeability is dependent on cholinesterase activity, however, seems to be contested by Strickland and Thompson.6... 

Haydon DA, Simon AJB. 1988. Excitation of the squid giant axon by general anaesthetics. J Physiol 402 375-389. 

The sulfamate saxitoxins have very low potencies relative to their carbamate hydrolysis products. This relationship has been observed in every assay system tried, including the standard mouse bioassay (Figure 3), squid giant axon (1 ), frog sciatic nerve (16), mammalian brain (1 ), and single rat sarcolemma sodium channels incorporated into lipid bilayers (15). It seems unlikely that human oral potencies are an exception to this trend. 

The neurotoxic actions of T17 on membrane excitability were examined in squid giant axon initially and in more detail using crayfish giant axon and intracellular microelectrode techniques(14). Detailed studies utilizing T34 are not available due to technical problems associated with its extreme hydrophobicity and resulting diffi-... 

The crayfish ventral nerve cords were excised, the medial and lateral giant axons partially desheathed. and the preparations placed in a chamber divided into 3 (1 cm ) sections. Each section was separated by petroleum jelly. Experiments were performed at room temperature (20-22 C). The crayfish Ringers was composed of (in mM/1) NaCl (146) KCl (4) CaCl2 Tris-... 

The relative recovery times after Ringer wash suggest that aphantoxins acted more like STX than TTX in crayfish lateral and medial giant axons (Ilf 1 ). 

Figure I. Action Potentials from crayfish giant axons. Control = Top Trace. Toxin (0.8 ug/ml) reduced rise rate and amplitude after 3, 10, 15, 20, and 25 min.

Preliminary testing on whole animal (bioassay) and isolated tissue preparations (voltage-clamped squid giant axons) showed effects similar to those measured for STX and neo-STX. Again the presence of anionic cryptic forms of the toxins was indicated. 

Davenport JG, Farrell DF, Sumi SM. 1976. Giant axonal neuropathy caused by industrial chemicals Neurofilamentous axonal masses in man. Neurology 26 919-923. 

Procaine and the other local anaesthetic drugs prevent the generation and the conduction of the nerve impulses. Their main site of action is the cell membrane, since conduction block can be demonstrated in giant axons from which the axoplasm has been removed [25]. 

Kanai Y, Katsuki H, Takasaki M Comparisons of the anesthetic potency and intracellular concentrations of S(-) and / (+) bupivacaine and ropivacaine in crayfish giant axon in vitro. Anesth Analg 2000 90 415. [PMID 10648331]... 

Haydon DA, Elliot JR, Hendry BM Effects of anesthetics on the squid giant axon in Kleinzeller A (ed) Current Topics in Membranes and Transports, vol 22. New York, Academic Press, 1984, pp 445-482. 

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