Properties of transmission at a giant glutamatergic synapse in cerebellum: the mossy fiber-unipolar brush cell synapse

Abstract

1. The synaptic activation by mossy fibers (MFs) of unipolar brush cells (UBCs) in the vestibular cerebellum (nodulus and uvula) was examined using patch-clamp recording methods in thin, rat cerebellar slices with Lucifer yellow-filled pipettes for subsequent fluorescence microscopic verification of the cell morphology. 2. UBCs were distinguished from adjacent granule cells in thin cerebellar slices in the uvula and nodulus regions by their larger soma diameters and short dendritic brush, greater whole-cell capacitance, and a prolonged, biphasic excitatory postsynaptic current (EPSC) to stimulation of MFs. 3. Thin-section transmission electron micrographs of the MF-UBC synapse displayed an unusually extensive area of synaptic apposition estimated to measure 12-40 microns2. The majority of UBCs was innervated by a single MF. At high magnification, individual clusters of presynaptic vesicles could be discerned, separated by regions of presynaptic membrane lacking vesicles, but apposed to continuous regions of postsynaptic density. Thus, after release, transmitter diffusion from the synaptic cleft must traverse considerable stretches of postsynaptic membrane before escape into extracellular space. In contrast, MF-granule cell synapses in these cerebellar regions resembled glutamate synapses in other brain regions in that the total synaptic area measured < or = 4 microns2. These synaptic junctions were flanked by short stretches of unspecialized plasma membrane, providing a short (0.5 micron) diffusional path from the site of neurotransmitter release to a branch point of the extracellular space. 4. The MF-evoked EPSC in UBCs was composed of a fast (10-90% rise time: 0.70 ms) and slow (10-90% rise time: 395 ms; 10-90% decay time: 3.1 s) component. The fast component was blocked by the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate (AMPA/KA) antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM) and displayed linear current-voltage (I-V) relations in the presence or absence of external magnesium. 5. The slow EPSC was also mediated by glutamate receptors, but in most neurons both AMPA/KA and N-methyl-D-aspartate (NMDA) receptors contributed to the slow EPSC, with the contribution of NMDA receptors predominating in the majority of cells. Consequently, although all cells displayed linear I-V relations in Mg(2+)-free saline, cells in which the slow EPSC was predominently mediated by NMDA receptors exhibited voltage-dependent rectification in the presence of external Mg2+ (1 mM). 6. With increasing postnatal age (10-30 d), the contribution made to the slow EPSC by NMDA receptors declined, with a reciprocal increase in the contribution being made by AMPA/KA receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

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

“…Consequently, they appear as electronically compact neurons. UBCs are innervated by a single mossy Wber (MF), providing vestibular aVerents, which makes contact with the entire dendritic brush and forms an unusually large synapse comprising multiple presynaptic release sites apposed to continuous regions of postsynaptic densities Rossi et al 1995). This synapse is endowed with ionotropic, but lacks metabotropic glutamate receptors (Jaarsma et al 1995).…”

“…In the latter, UBC axons are surrounded by granule cell dendrites and putative Golgi cell dendrites. Structurally, the synapses in these rosettes appear asymmetric (Berthie and Axelrad 1994;Rossi et al 1995). 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).…”

Variable timing of synaptic transmission in cerebellar unipolar brush cells

“…Such slow resurgent EPSCs are mediated by AMPARs and are believed to arise from interplay between receptor desensitization and dose-response properties, due to long-lasting exposure to ambient glutamate (9). Earlier studies have generally considered the UBC EPSC as a stereotyped waveform (8)(9)(10)(11). It is the purpose of the current work to reexamine this assertion and characterize the temporal characteristics of slow UBC EPSCs.…”

“…At most central synapses, the lifetime of neurotransmitter in the cleft is short, limited by rapid diffusional escape to the surrounding medium (7). However, the glomerular structure and extensive synaptic apposition of UBC synapses have been hypothesized to promote prolonged entrapment of glutamate in the synaptic cleft (8,9).…”

“…The slow tail is believed to be due to the prolonged presence of glutamate in the cleft and is mediated by NMDA receptors (NMDARs) or AMPARs, or a combination of both (8). In most UBCs, the biphasic appearance of EPSCs is particularly striking as the slow current first rises to peak in several hundreds of milliseconds, before decaying to baseline in seconds.…”