Nerve frog sciatic, action

Tamplin et. al. (54) observed that V. cholerae and A. hydrophila cell extracts contained substances with TTX-like biological activity in tissue culture assay, counteracting the lethal effect of veratridine on ouabain-treated mouse neuroblastoma cells. Concentrations of TTX-like activity ranged from 5 to 100 ng/L of culture when compared to standard TTX. The same bacterial extracts also displaced radiolabelled STX from rat brain membrane sodium channel receptors and inhibited the compound action potential of frog sciatic nerve. However, the same extracts did not show TTX-like blocking events of sodium current when applied to rat sarcolemmal sodium channels in planar lipid bilayers. 

Sodium fluoroacetate (but not methyl fluoroacetate) is practically without action on frog nerve or brain in vitro. The ester decreases the action potential of frog sciatic nerve and reduces the conduction velocity.5 The inactivity of the salt may be related partly to its inability to penetrate cells. 

The neurotoxins isolated from Aph. flos-aquae were shown to have similar chemical and biological properties to paralytic shellfish poisons (PSP) (25,29,38) Sawyer et al. in 1968 (25) were the first to demonstrate that the crude preparation of aphantoxins behave like saxitoxin, the major paralytic shellfish poison. They showed that the toxins had no effect on the resting membrane potential of frog sartorius muscle blocked action potential on de-sheathed frog sciatic nerve and also abolished spontaneous contractions in frog heart. Sasner et al. (1981) (29) using the lab cultured strain reported similar results. 

Potassium acts as a powerful synergist for local anesthetics under some conditions (59-62). The anesthetic effect of cocaine and /3-eucaine on the sensory and motor endings of the frog sciatic and on the sensory nerve of frog and guinea pig skin and the action of novocaine on the sensory nerve is considerably enhanced by potassium. The skin anesthesia produced by potassium chloride is intensified by minute quantities of cocaine, d-eucaine,... 

The histrionicotoxins show only weak interactions with voltage-dependent sodium channels 245). Histrionicotoxins do reduce the rate of rise of action potentials in muscle. The profile of activity for seven histrionicotoxins in this regard is as follows isotetrahydrohistrionicotoxin > tetrahydrohistrionicotoxin > perhydrohistrionicotoxin > isodihydro-histrionicotoxin > neodihydrohistrionicotoxin > octahydrohistrionico-toxin > histrionicotoxin. Thus histrionicotoxin appeared the least potent alkaloid of this class with respect to classical local anesthetic activity. In frog sciatic nerve, histrionicotoxin at 500 pM causes only a slight blockade of action potentials (774). 

Taylor, K.S., G.D. Waller, and L.A. Crowder. 1987. Impairment of a classical conditioned response of the honey bee (Apis mellifera L.) by sublethal doses of synthetic pyrethroid insecticides. Apidologie 18 243-252. Theophilidis, G., M. Benaki, and E. Papadopoulu-Mourkidou. 1997. Neurotoxic action of six pyrethroid insecticides on the isolated sciatic nerve of a frog (Rana ridibunda). Comp. Biochem. Physiol. 118C 97-103. Tippe, A. 1987. Evidence for different mechanisms of action of the three pyrethroids, deltamethrin, cypermethrin, and fenvalerate, on the excitation threshold of myelinated nerve. Pestic. Biochem. Physiol. 28 67-74. 

Both central and peripheral sites of action have been implicated in these alterations of motor function. In the isolated sciatic nerve-sartorius muscle preparation of the frog, A -THC blocked neuromuscular transmission by depressing sartorius presynaptic acetylcholine release [197]. However, the precise mechanisms of cannabinoid action at cellular and neurobehavioural levels are as yet unknown. 

When the sciatic nerve is removed from a frog, placed in an isotonic salt solution, and stimulated electrically, it will generate action potentials that can be measured by an electrode placed at some distance from the site of stimulation. When metabolic poisons are added to the preparation, the nerve retains the capability of generating action potentials even though the supply of ATP to drive its Na -K pump has been depleted and it is thus incapable of carrying out active transport. Explain how this can be the case. 

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