High dendritic expression of Ih in the proximity of the axon origin controls the integrative properties of nigral dopamine neurons.

Dendrites of most neurons express voltage-gated ion channels in their membrane. In
combination with passive properties, active currents confer to dendrites a high computational
potential. The hyperpolarization-activated cation current Ih present in the dendrites of some
pyramidal neurons affects their membrane and integration properties, synaptic plasticity and
higher functions such as memory. A gradient of increasing h-channel density towards distal
dendrites has been found to be responsible for the location independence of excitatory postsynaptic
potential (EPSP) waveform and temporal summation in cortical and hippocampal
pyramidal cells. However, reports on other cell types revealed that smoother gradients or even
linear distributions of Ih can achieve homogeneous temporal summation. Although the existence
of a robust, slowly activating Ih current has been repeatedly demonstrated in nigral dopamine
neurons, its subcellular distribution and precise role in synaptic integration are unknown. Using
cell-attached patch-clamp recordings, we find a higher Ih current density in the axon-bearing
dendrite than in the soma or in dendrites without axon in nigral dopamine neurons. Ih is mainly
concentrated in the dendritic membrane area surrounding the axon origin and decreases with
increasing distances from this site. Single EPSPs and temporal summation are similarly affected
by blockade of Ih in axon- and non-axon-bearing dendrites. The presence of Ih close to the axon
is pivotal to control the integrative functions and the output signal of dopamine neurons and
may consequently influence the downstream coding of movement.