Enhanced parallel fiber frequency-following after reduction of postsynaptic activity.

The frequency-following capabilities of the parallel fibers (Pf) of the rat cerebellar cortex were studied in the presence and absence of surround synaptic activity. When synaptic potentials were reduced or eliminated by local superfusion of the cerebellum with calcium antagonists or when they were reduced following a stimulation train, the Pfs showed more reliable frequency-following. Enhancement of synaptic activity with 4-aminopyridine (4-AP) application leads to a decrease in the frequency-following capabilities of the Pfs. When calcium antagonists were added to the 4-AP superfusate the synaptic enhancing effect of 4-AP was obliterated, and the Pfs displayed more reliable frequency-following capabilities. In addition, an extracellular slow potential was monitored and described in relation to the amplitude of the parallel fiber volley. We interpret these results to indicate that changes associated with postsynaptic activity of molecular layer neuronal elements may influence the frequency-following properties of presynaptic elements, i.e. the parallel fibers, possibly via an extracellular pathway.