Further observations on the action of phenytoin on the cerebellum in rats: involvement of the lateral reticular nucleus.

The action of phenytoin (PHT) on both the spontaneous and evoked activity of lateral reticular nucleus (LRN) neurons in rats was studied. An inhibition appeared at 90 min after drug administration, with a maximum peak at 135 min, followed by an excitation at 220 min. This biphasic action depended on plasma-tissue concentrations: At low concentrations, PHT exerted an inhibitory effect that became excitation at relatively high doses (7.5 micrograms/ml and 10.36 micrograms/g in plasma and brain, respectively). It was concluded that PHT modified cerebellar cortical activity not only by exerting a direct control on the Purkinje cells and an indirect modulation through the climbing fibers, but also through the mossy afferents originating from the LRN.