Subthreshold inward membrane currents in guinea-pig frontal cortex neurons.
Current-clamp and single-electrode voltage-clamp recordings were used to study the inward currents activated in the subthreshold membrane potential range of cortical pyramidal neurons. The experiments were done on slices from guinea-pig frontal cortex and all recordings were obtained at a distance of 600-900 microm from the pial surface. In current-clamp recordings and from membrane potentials hyperpolarized to about -70 mV, the depolarization leading to spike firing was partially blocked by 1 microM tetrodotoxin, but not by calcium-free extracellular solution. The calcium-free solution only affected this depolarization when the membrane potential was held at a level more negative than -75 mV. Under voltage-clamp, an inward current was recorded between the resting membrane potential and the level of spike firing. This current was activated at about -60 mV and part of it was blocked by 1 microM tetrodotoxin; the remaining current was blocked by calcium-free extracellular solution. In five neurons both components were recorded and isolated in the same cell. The tetrodotoxin-sensitive component activated at close to -60 mV, was similar to the persistent sodium current (I(Na-p)). The Ca2+-sensitive component activated at close to -60 or -65 mV, was less voltage-dependent than I(Na-p). This component was similar to the low threshold calcium current (I(T)). These results suggest that the subthreshold depolarization which led to spike firing was dependent on I(Na-p) and I(T), I(Na-p) being the most important factor up to resting membrane potentials of -70 or -75 mV. A physiological role of this finding is revealed by the action of dopamine, which (at 10 microM) prevented the firing of action potentials from -60 mV, but not from -80 mV due to the inhibition of I(Na-p) and the lack of effect on I(T).
Geijo-Barrientos
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Instituto de Neurociencias-CSIC, and Departamento de Fisiología, Universidad Miguel Hernández, Alicante, Spain.
D000818:Animals; D002118:Calcium / Q000494:pharmacology; D004056:Differential Threshold; D004298:Dopamine / Q000494:pharmacology; D004553:Electric Conductivity; D005625:Frontal Lobe / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D006168:Guinea Pigs; D007477:Ions; D008564:Membrane Potentials / Q000187:drug effects; D008566:Membranes / Q000502:physiology; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D017966:Pyramidal Cells / Q000187:drug effects / Q000502:physiology; D013779:Tetrodotoxin / Q000494:pharmacology
D007477:Ions; D013779:Tetrodotoxin; D002118:Calcium; D004298:Dopamine