Kinetic study and numerical reconstruction of A-type current in granule cells of rat cerebellar slices.
1. Whole-cell voltage-clamp techniques were used to study voltage-activated transient potassium currents in a large sample (n = 143) of granule cells (GrC) from rat cerebellar slices. Tetrodotoxin (TTX; 0.1 microM) was used to block sodium currents, while calcium current was too small to be seen under ordinary conditions. 2. Depolarizing pulses from -50 mV evoked a slow, sustained outward current, developing with a time constant of 10 ms, inactivating over a time scale of seconds and which could be suppressed by 20 mM tetraethylammonium (TEA). By preventing the Ca2+ inflow, this slow outward current could be further separated into a Ca(2+)-dependent and a Ca(2+)-independent component. 3. After conditioning hyperpolarizations to potentials negative to -60 mV, depolarizations elicited transient outward current, peaking in 1-2 ms and inactivating rapidly (approximately 10 ms at 20 degrees C), showing the overall kinetic characteristics of the A-current (IA). The current activated following third-order kinetics and showed a maximal conductance of 12 nS per cell, corresponding to a normalized conductance of 3.8 nS/pF. 4. IA was insensitive to TEA and to the Ca(2+)-channel blockers. 4-Aminopyridine (4-AP) reduced the A-current amplitude by approximately 20%, and the delayed outward currents by > 80%. 5. Voltage-dependent steady-state inactivation of peak IA was described by a Boltzmann function with a slope factor of 8.4 mV and half-inactivation occurring at -78.8 mV. Activation of IA was characterized by a Boltzmann curve with the midpoint at -46.7 mV and with a slope factor of 19.8 mV. 6. IA activation and inactivation was best fitted by the Hodgkin-Huxley m3h formalism. The rate of activation, tau a, was voltage-dependent, and had values ranging from 0.55 ms at -40 mV to 0.2 ms at +50 mV. Double-pulse experiment showed that development and removal of inactivation followed a single-exponential time course; the inactivation time constant, tau ha, was markedly voltage-dependent and ranged from approximately 10 ms at -40 and -100 mV and 70 ms at -70 mV. 7. A set of continuous equations has been developed describing the voltage-dependence of both the steady-state and time constant of activation and inactivation processes, allowing a satisfactory numerical reconstruction of the A-current over the physiologically significant membrane voltage range.(ABSTRACT TRUNCATED AT 400 WORDS)
Istituto di Fisiologia Generale dell'Università, Ferrara, Italy.
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Bardoni
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O
Belluzzi
D015761:4-Aminopyridine / Q000494:pharmacology; D000818:Animals; D001464:Barium / Q000494:pharmacology; D002531:Cerebellum / Q000166:cytology / Q000378:metabolism*; D002586:Cesium / Q000494:pharmacology; D003594:Cytoplasmic Granules / Q000378:metabolism; D004594:Electrophysiology; D066298:In Vitro Techniques; D007700:Kinetics; D008959:Models, Neurological; D009474:Neurons / Q000378:metabolism; D011188:Potassium / Q000378:metabolism; D015221:Potassium Channels / Q000378:metabolism*; D051381:Rats; D015222:Sodium Channels / Q000187:drug effects / Q000378:metabolism; D013779:Tetrodotoxin / Q000494:pharmacology
D015221:Potassium Channels; D015222:Sodium Channels; D002586:Cesium; D001464:Barium; D013779:Tetrodotoxin; D015761:4-Aminopyridine; D011188:Potassium