A study of two inward currents and their involvement in membrane potential oscillations of thalamocortical cells in vitro

In this study intracellular current clamp and single electrode voltage clamp experiments on single thalamocortical cells in brain slices of the cat dorsal lateral geniculate nucleus have revealed the presence of two voltagedependent inward currents that are activated at relatively hyperpolarized membrane potentials. These currents have been isolated, characterized physiologically and pharmacologically, and their involvement in the generation of spontaneous membrane potential oscillations has been assessed. The first current is a low-threshold, transient, inward calcium ion current. It is called low-threshold because it is activated at relatively hyperpolarized membrane potentials, upon depolarization. Since it is a transient conductance it has been termed If and indeed, throughout its development, has many kinetic characteristics in common with T-type calcium conductances found in other cells, but its pharmacology differs somewhat. It has been found that IT is the generator of low-threshold calcium potentials i.e. of the large depolarizations seen in thalmocortical cells during membrane potential oscillations in vitro and in vivo. The second current is a slowly developing, non-inactivating, inward, mixed sodium/potassium ion current. This is an inward rectifier current as it is responsible for causing an increase in membrane conductance upon hypeipolarization. Since it is activated when a cell is hyperpolarized it has been termed Ih. Many of its kinetic properties and pharmacology are similar to those of mixed sodium/potassium ion inward rectifier currents found in other cells. Ih is responsible for depolarizing the cells membrane potential upon hyperpolarization and, like IT, has been found to be essential for spontaneous membrane potential oscillations. Enhancing or inhibitng Ih, with noradrenaline or caesium ions respectively, can transform one kind of oscillation into another or, indeed, inhibit them altogether