title: Ectopic release of glutamate contributes to spillover at parallel fibre synapses in the cerebellum.
abstract: In the rat cerebellar molecular layer, spillover of glutamate between parallel fibre synapses can lead to activation of perisynaptic receptors that mediate short- and long-term plasticity. This effect is greatest when clusters of fibres are stimulated at high frequencies, suggesting that glutamate clearance mechanisms must be overwhelmed before spillover can occur. However, parallel fibres can also release transmitter directly into the extracellular space, from 'ectopic' release sites. Ectopic transmission activates AMPA receptors on the Bergmann glial cell processes that envelop parallel fibre synapses, but the possible contribution of this extrasynaptic release to intersynaptic communication has not been explored. We exploited long-term depression of ectopic transmission, and selective pharmacology, to investigate the impact of these release sites on the time course of Purkinje neuron excitatory postsynaptic currents (EPSCs). Depletion of ectopic release pools by activity-dependent long-term depression decreased EPSC decay time, revealing a 'late' current that is present when fibres are stimulated at low frequencies. This effect was enhanced when glutamate transporters were inhibited, and reduced when extracellular diffusion was impeded. Blockade of N-type Ca(2+) channels inhibited ectopic transmission to Bergmann glia and decreased EPSC decay time. Similarly, perfusion of the Ca(2+) chelator EGTA-AM into the slice progressively eliminated ectopic transmission to glia and decreased EPSC decay time with closely similar time courses. Collectively, this evidence suggests that ectopically released glutamate contributes to spillover transmission, and that ectopic release therefore degrades the spatial precision of synapses that fire infrequently, and may make them more prone to exhibit plasticity.
authors.affiliation: School of Life Sciences, University of Nottingham Medical School, Queen's Medical Centre, Nottingham NG7 2UH, UK. tomas.bellamy@nottingham.ac.uk.
authors.forename: Saju
authors.initials: S
authors.lastname: Balakrishnan
authors.forename: Katharine L
authors.initials: KL
authors.lastname: Dobson
authors.forename: Claire
authors.initials: C
authors.lastname: Jackson
authors.forename: Tomas C
authors.initials: TC
authors.lastname: Bellamy
mesh_terms: D027322:Amino Acid Transport System X-AG / Q000037:antagonists & inhibitors / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000494:pharmacology; D020864:Calcium Channels, N-Type / Q000187:drug effects / Q000378:metabolism; D002614:Chelating Agents / Q000494:pharmacology; D004558:Electric Stimulation; D019706:Excitatory Postsynaptic Potentials; D005260:Female; D018698:Glutamic Acid / Q000187:drug effects / Q000378:metabolism*; D036881:Long-Term Synaptic Depression; D008297:Male; D009457:Neuroglia / Q000378:metabolism; D011689:Purkinje Cells / Q000187:drug effects / Q000378:metabolism*; D017208:Rats, Wistar; D013569:Synapses / Q000187:drug effects / Q000378:metabolism*; D009435:Synaptic Transmission / Q000187:drug effects; D013997:Time Factors
chemical_list: D027322:Amino Acid Transport System X-AG; D002121:Calcium Channel Blockers; D020864:Calcium Channels, N-Type; D002614:Chelating Agents; D018698:Glutamic Acid; D002118:Calcium