Serotonergic interactions with rat cerebellar Purkinje cells.

Recent immunocytochemical and histofluorescent studies have established the vastness of the serotonergic fibers arising from the raphe complex and projecting to the cerebellar cortex. Ultrastructurally, the indoleaminergic fibers are known to establish synaptic contacts as well as diffuse meandering nonsynaptic terminations near Purkinje cells. The objective of this study was to define the changes elicited by serotonin on spontaneous and chemically- and electrically-evoked excitatory synaptic activities on cerebellar Purkinje cells. Serotonin (10-50 nA) applied for 30-60 sec elicited predominantly a decrease in firing rate followed by rebound excitation (25 cells out of a total of 39 cells). Studies in which serotonin was continuously iontophoresed for prolonged periods, this amine produced an increase in the spontaneous firing rate in 55% of the Purkinje cells tested. A comparison of evoked excitation and spontaneous activity revealed that when serotonin was applied continuously there was a net decrease in evoked excitation to spontaneous activity ratio. Based on the evoked excitation studies, glutamate (5-30 nA) was applied for 30-40 sec in the presence of serotonin (10-30 nA). Seventy-seven percent of the cells displayed a decrease in responsiveness to glutamate while the remaining 23% were potentiated. More definitive explanation of these results are given in the body of the test.