Dynamic clamp study of I h modulation of burst firing and δ oscillations in thalamocortical neurons in vitro

Abstract

The dynamic clamp technique was used in thalamocortical neurons of the rat and cat dorsal lateral geniculate nucleus in vitro to investigate the effects of the hyperpolarization-activated cation current, I h, and of its neuromodulation on burst firing and δ oscillations. Specific block of endogenous I h using 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino)pyridinium chloride (ZD7288) (300 μM) abolished the depolarizing “sag” response to negative current steps, markedly increased the latency and shortened the duration of the low-threshold Ca 2+ potentials, and decreased the number of action potentials in the burst evoked by the low-threshold Ca 2+ potential. Subsequent introduction of artificial I h using the dynamic clamp re-instated the “sag” and all the original properties of the low-threshold Ca 2+ potential. In the absence of ZD7288, introduction of artificial outward I h with the intention of abolishing endogenous I h removed the depolarizing “sag” and produced similar effects on the low-threshold Ca 2+ potentials as those observed during the pharmacological block of I h. Application of ZD7288 to thalamocortical neurons displaying δ oscillations led to a reduction in the voltage range of their existence or to a complete cessation of this behaviour. A subsequent introduction of artificial I h re-enabled the generation of δ oscillations. In the presence of ZD7288, physiologically relevant positive shifts in the voltage-dependence of artificial I h increased the amplitude and duration of the low-threshold Ca 2+ potential and increased the likelihood of δ oscillations while negative shifts had opposite effects. These results highlight the important difference between the dependence of burst firing and oscillations on membrane potential and their dependence on the properties of I h, and demonstrate that the modulation by I h of low-threshold Ca 2+ potentials and burst firing in thalamocortical neurons, as well as the ability of these neurons to generate δ oscillations, is more elaborate than previously described.