The supernormal period of the cerebellar parallel fibers effects of [Ca2+]o and [K+]o.

The nonmyelinated parallel fibers (Pfs) of the cerebellar cortex exhibit a pronounced supernormal period following a single conditioning volley. In the present investigation a comparison is made between the effects of changes in extracellular calcium ([Ca2+]o) and potassium ([K+]o) on the supernormal period of the Pfs. [Ca2+]o was monitored directly using ion-sensitive microelectrodes while rat cerebellar Pfs were continuously superfused with solutions containing varying concentrations of K+ (5-30 mM) or Ca2+ (0-6 mM). Pf recovery properties were studied by monitoring control (unconditioned) and test (conditioned by a previous impulse) response latencies. [Ca2+]o did not affect the activity-dependent relative increase in Pf excitability observed following conditioning stimulation (i.e. the supernormal period) although both control and test Pf volley latencies were related to [Ca2+]o. Relatively small increases in superfusate K+ concentration elicited a decrease in the control Pf volley latency but had no effect on the test latency. This resulted in the reduction or obliteration of the latency shift elicited by a conditioning stimulus. Simultaneously decreasing [Ca2+]o and increasing [K+]o decreased control Pf volley latency further than when each ion was altered separately. The test Pf volley latency was unchanged. Therefore, under these conditions, there was no Pf volley latency change following conditioning stimulation. These results are consistent with the hypothesis that activity-dependent changes in extracellular ionic concentrations may, in part, be responsible for the supernormal period in cerebellar parallel fibers.