PubMed · 6997562
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Y Watanabe, S Yamada. 1980. [Arrhythmia].. https://pubmed.ncbi.nlm.nih.gov/6997562/
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Spontaneous and light-evoked excitatory postsynaptic currents (sEPSCs and leEPSCs) in retinal ganglion cells of the larval tiger salamander were recorded under voltage clamp conditions from living retinal slices. sEPSCs were isolated from the spontaneous inhibitory postsynaptic currents (sIPSCs) by application of 100 M picrotoxin+1 microM strychnine. In addition to the previously reported sEPSCs [K. Matsui, N. Hosoi, M. Tachibana, Excitatory synaptic transmission in the inner retina: pair recordings of bipolar cells and neurons of the ganglion cell layer, J. Neurosci. 18 (1998) 4500-4510; W.R. Taylor, E. Chen, D.R. Copenhagen, Characterization of spontaneous excitatory synaptic currents in salamander retinal ganglion cells, J. Physiol. 486 (1995) 207-221] [which are equivalent to our fast AMPA receptor-mediated sEPSCs (fAMPAsEPSCs)], we found another type of AMPA receptor-mediated sEPSC with slower rise and decay time courses and larger peak amplitudes (sAMPAsEPSCs), and the NMDA receptor-mediated sEPSCs (NMDAsEPSCs) in ON-OFF ganglion cells. The frequency of all three types of sEPSCs is greatly reduced by cobalt (with zero calcium) and increased by hyperosmotic solution, suggesting that these events are mediated by calcium-dependent exocytosis of glutamatergic synaptic vesicles. The amplitude histograms of sEPSCs do not show multiple peaks, suggesting that larger events are not discrete multiples of elementary events, or quanta, of similar neurotransmitter contents, as in the neuromuscular junction [P. Fatt, B. Katz, Spontaneous subthreshold activity at motor nerve endings, J. Physiol. 117 (1952) 109-128]. The average I-V relations of the fAMPAsEPSCs and sAMPAsEPSCs were outward rectified with reversal potentials at -12.2 mV and -10.8 mV, and that of the NMDAsEPSCs was N-shaped with a reversal potential at -5.8 mV. The average conductance increase associated with a single fAMPAsEPSC, a single sAMPAsEPSC, and a single NMDAsEPSC were 163. 26+/-51.02 pS, 233.33+/-163.64 pS, and 37.5+/-50.0 pS at -110 mV; 241.67+/-22.92 pS, 444.90+/-469.94 pS, and 25.93+/-70.37 pS at -60 mV; and 440.48+/-183.33 pS, 1,192.68+/-651.22 pS, and 517.71+/-238. 24 pS at +30 mV, respectively. The average frequency of the three sEPSCs at +30 mV were 15 Hz, 3.7 Hz and 3.6 Hz, respectively. The rise time (time to peak) of fAMPAsEPSCs was 1.5+/-1.05 ms and the decay time could be fitted with a single exponential with an average time constant of 3.4+/-4.1 ms. The rise and decay time course of the sAMPAsEPSCs and NMDAsEPSCs were much slower and sawtooth-shaped, and each 'sawtooth' had time course and amplitude similar to those of individual fAMPAsEPSCs. We propose that each fAMPAsEPSC is mediated by single or synchronized multiples of glutamatergic synaptic vesicles from bipolar cells, and each sAMPAsEPSC or NMDAsEPSC is mediated by larger clusters of synaptic vesicles triggered by spontaneous calcium spikes in bipolar cell axon terminals [J. Burrone, L. Lagnado, Electrical resonance and calcium influx in the synaptic terminal of depolarizing bipolar cells from the goldfish retina, J. Physiol. 505 (1997) 571-584; D. Zenisek, G. Matthews, Calcium action potentials in retinal bipolar neurons, Vis. Neurosci. 15 (1998) 69-75].
X-ray scattering and electrophysiological experiments performed on toad sciatic nerves as a function of the exposure to either low temperature or tetracaine yielded the following results: (i) the main structural effect is to thicken the individual membranes, thus to stiffen the acyl chains and increase the repeat distance of the one-dimensional lattice, phenomena that are typical of lipid-containing systems with disordered chains; (ii) the electrophysiological effect is to decrease the amplitude and velocity of the compound action potential; (iii) the structural and physiological effects of the two agents are practically identical. Since the structural and the electrophysiological parameters have different origins in the nerves (the structure regards the myelin sheath, the electrical signals originate at the nodes of Ranvier) it is inferred that tetracaine and low temperature exert similar effects on the membranes of both the myelin sheath and the nodes of Ranvier. Also, since local anesthetics act by inhibiting the Na+ channels, these observations suggest that the acyl chain conformation modulates the channel function and thus the generation of action potential.
Acutely dissociated cell bodies of mouse Purkinje neurons spontaneously fired action potentials at approximately 50 Hz (25 degrees C). To directly measure the ionic currents underlying spontaneous activity, we voltage-clamped the cells using prerecorded spontaneous action potentials (spike trains) as voltage commands and used ionic substitution and selective blockers to isolate individual currents. The largest current flowing during the interspike interval was tetrodotoxin-sensitive sodium current (approximately -50 pA between -65 and -60 mV). Although the neurons had large voltage-dependent calcium currents, the net current blocked by cobalt substitution for calcium was outward at all times during spike trains. Thus, the electrical effect of calcium current is apparently dominated by rapidly activated calcium-dependent potassium currents. Under current clamp, all cells continued firing spontaneously (though approximately 30% more slowly) after block of T-type calcium current by mibefradil, and most cells continued to fire after block of all calcium current by cobalt substitution. Although the neurons possessed hyperpolarization-activated cation current (Ih), little current flowed during spike trains, and block by 1 mM cesium had no effect on firing frequency. The outward potassium currents underlying the repolarization of the spikes were completely blocked by 1 mM TEA. These currents deactivated quickly (<1 msec) after each spike. We conclude that the spontaneous firing of Purkinje neuron cell bodies depends mainly on tetrodotoxin-sensitive sodium current flowing between spikes. The high firing rate is promoted by large potassium currents that repolarize the cell rapidly and deactivate quickly, thus preventing strong hyperpolarization and restoring a high input resistance for subsequent depolarization.