Local and global effects of I_h distribution in dendrites of mammalian neurons

The hyperpolarization-activated cation current I_h exhibits a steep gradient of channel density in dendrites of pyramidal neurons, which is associated with location independence of temporal summation of EPSPs at the soma. In striking contrast, here we show by using dendritic patch-clamp recordings that in cerebellar Purkinje cells, the principal neurons of the cerebellar cortex, I_h exhibits a uniform dendritic density, while location independence of EPSP summation is observed. Using compartmental modeling in realistic and simplified dendritic geometries, we demonstrate that the dendritic distribution of I_h only weakly affects the degree of temporal summation at the soma, while having an impact at the dendritic input location. We further analyze the effect of I_h on temporal summation using cable theory and derive bounds for temporal summation for any spatial distribution of I_h. We show that the total number of I_h channels, not their distribution, governs the degree of temporal summation of EPSPs. Our findings explain the effect of I_h on EPSP shape and temporal summation, and suggest that neurons are provided with two independent degrees of freedom for different functions: the total amount of I_h (controlling the degree of temporal summation of dendritic inputs at the soma) and the dendritic spatial distribution of I_h (regulating local dendritic processing).