Modulation of cerebellar granule cell activity by iontophoretic application of serotonergic agents

Serotonergic fibers have been identified within the granule cell layer of the cerebellar cortex; however, their functional significance has not been identified. In this study the effect of serotonin on granule cell spontaneous activity was determined in the rat cerebellum. Of the 136 granule cells tested, 44.8% displayed a decrease in firing rate, 21.3% increased firing rate and 33.8% were not affected. The serotonin-induced changes in activity were not blocked by bicuculline or methysergide. The serotonin agonist 1,3 (trifluoromethylphenyl) piperazine mimicked the serotonin-induced suppressive response. Iontophoretically applied serotonin was also found to modulate GABA-induced suppression of granule cell activity. The variable effects of serotonin on spontaneous activity suggests the presence of more than one type of serotonergic receptor in the cerebellar granule cell layer.