Optokinetic stimulation activates visual climbing fiber pathways that synapse upon contralateral floccular Purkinje cells. Long-term horizontal optokinetic stimulation causes a progressive decrease in gain of the optokinetic reflex and leads to the subsequent genesis of a prolonged negative optokinetic afternystagmus. Since the flocculus is involved in adaptation to optokinetic stimulation, we used the technique of differential display reverse transcription-polymerase chain reaction to explore transcriptional changes in the flocculus evoked by long-term optokinetically evoked climbing fiber discharge. Several differentially transcribed gene products were isolated and sequenced. One of these, calbindin mRNA, was expressed in relatively decreased abundance in the flocculus that received increased climbing fiber input. Decreased transcription of calbindin mRNA was confirmed by northern blots. Hybridization histochemistry was used to localize calbindin mRNA to Purkinje cells and confirmed decreased transcription of calbindin mRNA in Purkinje cells located in folium 1 of the flocculus. Western blots and immunohistochemistry localized the climbing fiber-evoked decreased expression of calbindin to Purkinje cells in folia 1 of the flocculus. The expression of four other calcium-binding proteins in the flocculus was not influenced by optokinetic stimulation. Changes in expression of calbindin could be evoked by decreases in intracellular calcium associated with climbing fiber-evoked decreases in Purkinje cell simple spike activity. The application of differential display reverse transcription-polymerase chain reaction has provided a positive screen for several molecules in addition to calbindin whose expression is affected by naturally evoked activity in a major synaptic pathway to the cerebellum. Further experiments will be required to specify the functional role of each of these molecules.
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