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

The hyperpolarization-activated cation current Ih 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, Ih 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 Ih 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 Ih on temporal summation using cable theory and derive bounds for temporal summation for any spatial distribution of Ih. We show that the total number of Ih channels, not their distribution, governs the degree of temporal summation of EPSPs. Our findings explain the effect of Ih 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 Ih (oma) and the dendritic spatial distribution of Ih (regulating local dendritic processing).