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Updated: Nov 16, 2025

Assessment of Long-term Depression Induction in Adult Cerebellar Slices
Published on: October 16, 2019
Incomplete removal of extracellular glutamate controls synaptic transmission and integration at a cerebellar synapse
Timothy S Balmer1, Carolina Borges-Merjane1,2, Laurence O Trussell1
1Vollum Institute and Oregon Hearing Research Center, Oregon Health & Science University, Portland, United States.
Abstract:
Synapses of glutamatergic mossy fibers (MFs) onto cerebellar unipolar brush cells (UBCs) generate slow excitatory (ON) or inhibitory (OFF) postsynaptic responses dependent on the complement of glutamate receptors expressed on the UBC's large dendritic brush. Using mouse brain slice recording and computational modeling of synaptic transmission, we found that substantial glutamate is maintained in the UBC synaptic cleft, sufficient to modify spontaneous firing in OFF UBCs and tonically desensitize AMPARs of ON UBCs. The source of this ambient glutamate was spontaneous, spike-independent exocytosis from the MF terminal, and its level was dependent on activity of glutamate transporters EAAT1-2. Increasing levels of ambient glutamate shifted the polarity of evoked synaptic responses in ON UBCs and altered the phase of responses to in vivo-like synaptic activity. Unlike classical fast synapses, receptors at the UBC synapse are virtually always exposed to a significant level of glutamate, which varies in a graded manner during transmission.
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