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Prolonged post tetanic de polarization of frog rana pipiens dorsal root fibers






Neuroscience 9(1): 235-243

Prolonged post tetanic de polarization of frog rana pipiens dorsal root fibers

In the isolated frog spinal cord, tetanic stimulation of a dorsal root resulted in a sustained negative potential recorded from that root. This negative potential was followed by a large (5-12 mV) and slow (5-60 s) negative after-potential which persisted after the train of impulses and which was succeeded by a long-lasting positive potential. The negative after-potential was postulated to be largely a consequence of excess extracellular K+ accumulated during the tetanus, because the negative afterpotential and the extracellular [K+] measured with K+-sensitive microelectrodes were a function of the stimulating current, the stimulation frequency and the duration of the tetanus. Both measures were augmented by procedures which interfered with the clearance of K+ from the extracellular space (addition of dinitrophenol or ouabain, partial substitution of Li+ for Na+, elimination of K+ from the superfusate and cooling). The negative after-potential and the liberation of much of the excess extracellular K+ were reduced by increasing the superfusing divalent ion concentration with 4 mM Ca2+ or 50-100 .mu.M Mn2+. Both measures were augmented in low Ca2+-containing Ringer's solution. The results with low concentrations of Ca2+, Na+ and K+ and with high concentrations of Ca2+ indicate that the negative after-potential was not caused by a conductance increase for these cations. reduction of extracellular [Cl-] did not change the negative after-potential making it unlikely that this anion is a factor in its production. The negative after-potential was not mediated by GABA because its amplitude and duration were increased by the GABA antagonist picrotoxin and were reduced by pentobarbital which enhances the actions of GABA. Application of the excitatory amino acid antagonists DL-.alpha.-aminoadipic acid and glutamic acid diethylester eliminated the negative after-potential and markedly reduced the amount of K+ liberated by a tetanus. These results suggest that the K+ release that produces the negative after-potential may result from the action of an excitatory amino acid such as L-glutamate or L-aspartate either synaptically released or released from nonsynaptic regions of afferent fibers.

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Accession: 006200222



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