Hyperpolarization-activated currents in pyramidal neurons of mice frontal cortex

POMATA, PE; WEDEMEYER , C; CALVO, DJ

Carlos Paz- Prov. de Cordoba

Congreso; XXXVII Reunión Nacional de la Sociedad Argentina de Investigación Bioquímica y Biología Molecular; 2001

Sociedad Argentina de Investigación Bioquímica y Biología Molecular

Hyperpolarization-activated currents have been involved in regulation of cellular excitability in a variety of neural and non-neural tissue. They are activated by hyperpolarizing the cellular membrane below resting potential, rising an inward current that repolarizes the membrane. This phenomenon called anomalous rectification (IAR) can be mediated through two types of ion channels, the potassium inward rectifier (Kir) and the hyperpolarization-activated cationic channels (Ih). In order to characterize hyperpolarization-activated currents in layer V pyramidal neurons of mice frontal cortex we performed whole cell recordings in acute brain slices. In most cells the hyperpolarizing current-voltage relationship (I-V) from –50 to –140 mV showed an inward current starting at voltage near –60mV. Under current clamp conditions an hyperpolarization pulse of –100pA shows a depolarizing sag towards resting potential due to IAR activation. This anomalous rectification is blocked by external Cs+ but it is insensitive to tetrodotoxin, the potassium channel blockers tetraetilamonium and 4-aminopyridine, and to Ca2+ replacement by Co2+. The maximum IAR at –150mV varies between –300 and –700pA and its half-activation is –105 mV. The current turns on with a delay of some milliseconds and relaxes to steady-state in 1 or 2 seconds. Its activation at –105mV is well described by two exponential with a fast time constant of about 40ms and a slow one about 550ms. The rate of activation have strong voltage dependence with faster activation at more negative potential. The amplitude and half-activation are also dependent of extracellular K+ concentration. High K+ concentration (10mM) increase IAR amplitude and shift activation to more positive potentials while reduced extracellular K+ has the opposite effect. These data shows that layer V pyramidal neurons of mice frontal cortex posses hyperpolarization-activated currents that may contribute to firing regulation. The fast component of the kinetics and the high K+ effect account for the Kir nature of the current but the activation range and initial delay are characteristic of Ih channels mediated current. So we conclude that in these cell co-exists the two types of hyperpolarization-activated currents, but further experiments are needed to find the contribution ef each one.