Unipolar brush cells form a glutamatergic projection system within the mouse cerebellar cortex

Abstract

Unipolar brush cells (UBCs) of the mammalian vestibulocerebellum receive mossy fiber projections primarily from the vestibular ganglion and vestibular nuclei. Recently, the axons of UBCs have been shown to generate an extensive system of cortex-intrinsic mossy fibers, which resemble traditional cerebellar mossy fiber afferents and synapse with granule cell dendrites and other UBCs. However, the neurotransmitter used by the UBC axon is still unknown. In this study, we used long-term organotypic slice cultures of the isolated nodulus (lobule X) from postnatal day 8 mouse cerebella to identify the neurotransmitter and receptors at synapses of the UBC axon terminals, relying on the notion that, in these cultures, all of the cortex-extrinsic fibers had degenerated during the first few days in vitro. Quantification of glutamate immunogold labeling showed that the UBC axon terminals have the same high gold-particle density as the glutamatergic parallel fiber varicosities. Furthermore, UBCs identified by calretinin immunoreactivity expressed the glutamate receptor subunits GluR2/3, NMDAR1, and mGluR2/3, like they do in the mature mouse cerebellum in situ. Evoked excitatory postsynaptic currents (EPSCs), spontaneous EPSCs, and burst discharges were demonstrated in UBCs and granule cells by patch-clamp recording. Both the evoked and spontaneous EPSCs were blocked by ionotropic glutamate receptor antagonists CNQX and D-AP5. We conclude that neurotransmission at the UBC axon terminals is glutamatergic. Thus, UBCs provide a powerful network of feedforward excitation within the granular layer, which may amplify vestibular signals and synchronize activity in clusters of functionally related granule cells which project vertically to patches of Purkinje cells.

Mixed Inhibitory Synaptic Balance Correlates with Glutamatergic Synaptic Phenotype in Cerebellar Unipolar Brush Cells

“…Each UBC receives a giant excitatory synapse from a single MF, at which various ionotropic and metabotropic glutamate receptors are expressed (Rossi et al, 1995;Jaarsma et al, 1998;Nunzi et al, 2001Nunzi et al, , 2002. UBCs can be grouped into distinct subfamilies by examining the molecular constitution of the MF synapse.…”

Besides Purkinje cells and granule neurons: an appraisal of the cell biology of the interneurons of the cerebellar cortex

“…Unipolar brush cells are glutamatergic (Nunzi et al 2001), and given their wiring, they may contribute to a patterned spread of vestibular aVerent excitation within the granule cell layer that has been considered to be equivalent to a feed-forward excitation (Dino et al 2000). Their primary function seems to be to coordinate the synchronized activity of sets of granule cells, which in turn would regulate activity in spatially restricted subsets of Purkinje cells (cf Nunzi et al 2001 for further details).…”

VAMP-2, SNAP-25A/B and syntaxin-1 in glutamatergic and GABAergic synapses of the rat cerebellar cortex

“…In the Purkinje neuron layer, only GABAergic synapses are present, concentrated at the deep pole of the body of Purkinje neurons and formed by terminals of basket neuron axons [42-44]. In the granular layer, glutamatergic synapses and GABAergic synapses are both located in the neuropil regions scattered among granules (islands of Held), consisting of multiple synapses (glomeruli) established by glutamatergic terminals of mossy fibres and of axons of unipolar brush neurons [35-38,45] and GABAergic axon terminals of Golgi, candelabrum and Lugaro neurons [44,46,47], on granule dendrites.…”