The H-current secures action potential transmission at high frequencies in rat cerebellar parallel fibers

Most axons in the mammalian brain are unmyelinated and thin with pre-synaptic specializations (boutons) along their entire paths. The parallel fibers in the cerebellum are examples of such axons. Unlike most thin axons they have only one branch point. The granule cell soma, where they originate, can fire bursts of action potentials with spike intervals of about 2 ms. An important question is whether the axons are able to propagate spikes with similarly short intervals. By using extracellular single-unit and population-recording methods we showed that parallel fibers faithfully conduct spikes at high frequencies over long distances. However, when adding 20 mu m ZD7288 or 1 mm Cs+, or reducing the temperature from 35 to 24 degrees C, the action potentials often failed even when successfully initiated. Ba2+(1 mm), which blocks Kir channels, did not reproduce these effects. The conduction velocity was reduced by ZD7288 but not by Ba2+. This suggests that the parallel fibers have an H-current that is active at rest and that is important for their frequency-following properties. Interestingly, failures occurred only when the action potential had to traverse the axonal branch point, suggesting that the branch point is the weakest point in these axons.