title: Contribution of postsynaptic T-type calcium channels to parallel fibre-Purkinje cell synaptic responses.
abstract: CONCLUSIONS
  At the parallel fibre-Purkinje cell glutamatergic synapse, little or no Ca(2+) entry takes place through postsynaptic neurotransmitter receptors, although postsynaptic calcium increases are clearly involved in the synaptic plasticity. Postsynaptic voltage-gated Ca(2+) channels therefore constitute the sole rapid postsynaptic Ca(2+) signalling mechanism, making it essential to understand how they contribute to the synaptic signalling. Using a selective T-type calcium channel antagonist, we describe a T-type component of the EPSC that is activated by the AMPA receptor-mediated depolarization of the spine and thus will contribute to the local calcium dynamics. This component can amount up to 20% of the EPSC, and this fraction is maintained even at the high frequencies sometimes encountered in sensory processing. Modelling based on our biophysical characterization of T-type calcium channels in Purkinje cells suggests that the brief spine EPSCs cause the activated T-type channels to deactivate rather than inactivate, enabling repetitive activation.
  In the cerebellum, sensory information is conveyed to Purkinje cells (PC) via the granule cell/parallel fibre (PF) pathway. Plasticity at the PF-PC synapse is considered to be a mechanism of information storage in motor learning. The induction of synaptic plasticity in the cerebellum and elsewhere usually involves intracellular Ca(2+) signals. Unusually, postsynaptic Ca(2+) signalling in PF-PC spines does not involve ionotropic glutamatergic receptors because postsynaptic NMDA receptors are absent and the AMPA receptors are Ca(2+) -impermeable; postsynaptic voltage-gated Ca(2+) channels therefore constitute the sole rapid Ca(2+) signalling mechanism. Low-threshold activated T-type calcium channels are present at the synapse, although their contribution to PF-PC synaptic responses is unknown. Taking advantage of 3,5-dichloro-N-[1-(2,2-dimethyl-tetrahydro-pyran-4-ylmethyl)-4-fluoro-piperidin-4-ylmethyl]-benzamide, a selective T-type channel antagonist, we show in the mouse that inhibition of these channels reduces PF-PC excitatory postsynaptic currents and excitatory postsynaptic potentials by 15-20%. This contribution was preserved during sparse input and repetitive activity. We characterized the biophysical properties of native T-type channels in young animals and modelled their activation during simulated dendritic excitatory postsynaptic potential waveforms. The comparison of modelled and observed synaptic responses suggests that T-type channels only activate in spines that are strongly depolarized by their synaptic input, a process requiring a high spine neck resistance. This brief and local activation ensures that T-type channels rapidly deactivate, thereby limiting inactivation during repetitive synaptic activity. T-type channels are therefore ideally situated to provide synaptic Ca(2+) entry at PF-PC spines.
authors.affiliation: Ecole Normale Supérieure, Institut de Biologie de l'ENS (IBENS), CNRS UMR 8197 and INSERM U1024, Paris, France.
authors.forename: Romain
authors.initials: R
authors.lastname: Ly
authors.affiliation: Ecole Normale Supérieure, Institut de Biologie de l'ENS (IBENS), CNRS UMR 8197 and INSERM U1024, Paris, France.
authors.forename: Guy
authors.initials: G
authors.lastname: Bouvier
authors.affiliation: Ecole Normale Supérieure, Institut de Biologie de l'ENS (IBENS), CNRS UMR 8197 and INSERM U1024, Paris, France.
authors.forename: German
authors.initials: G
authors.lastname: Szapiro
authors.affiliation: GlaxoSmithKline Pharmaceuticals, New Frontiers Science Park, Third Avenue, Harlow, UK.| Wellcome Trust Sanger Institute, Hinxton, Cambridge, UK.
authors.forename: Haydn M
authors.initials: HM
authors.lastname: Prosser
authors.affiliation: GlaxoSmithKline Pharmaceuticals, New Frontiers Science Park, Third Avenue, Harlow, UK.|School of Physiology and Pharmacology, Medical Sciences Building, University of Bristol, Bristol, UK.
authors.forename: Andrew D
authors.initials: AD
authors.lastname: Randall
authors.affiliation: Department of Neurophysiology, Graduate School of Medicine, University of Tokyo, Tokyo, Japan.
authors.forename: Masanobu
authors.initials: M
authors.lastname: Kano
authors.affiliation: Department of Cellular Neurobiology, Brain Research Institute, Niigata University, Niigata, Japan.
authors.forename: Kenji
authors.initials: K
authors.lastname: Sakimura
authors.affiliation: INCI, CNRS UPR 3212, Centre de Neurochimie, Strasbourg, France.
authors.forename: Philippe
authors.initials: P
authors.lastname: Isope
authors.affiliation: Ecole Normale Supérieure, Institut de Biologie de l'ENS (IBENS), CNRS UMR 8197 and INSERM U1024, Paris, France.
authors.forename: Boris
authors.initials: B
authors.lastname: Barbour
authors.affiliation: Ecole Normale Supérieure, Institut de Biologie de l'ENS (IBENS), CNRS UMR 8197 and INSERM U1024, Paris, France.
authors.forename: Anne
authors.initials: A
authors.lastname: Feltz
mesh_terms: D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology; D020747:Calcium Channels, T-Type / Q000378:metabolism*; D020013:Calcium Signaling; D019706:Excitatory Postsynaptic Potentials; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D011689:Purkinje Cells / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D013569:Synapses / Q000378:metabolism* / Q000502:physiology
chemical_list: D002121:Calcium Channel Blockers; D020747:Calcium Channels, T-Type