Biomedical subjects

Joshua H Singer

Publications and source records attributed to Joshua H Singer.

5 recordsLinked to original sources

PubMed2007

Multivesicular release and saturation of glutamatergic signalling at retinal ribbon synapses.

Pronounced multivesicular release (MVR) occurs at the ribbon synapses of sensory neurones that signal via graded potential changes. As MVR increases the likelihood of postsynaptic receptor saturation, it is of interest to consider how sensory synapses overcome this problem and use MVR to encode signals of widely varying intensities. Here, I discuss three postsynaptic mechanisms that permit three different retinal synapses to utilize MVR.

Amacrine Cells↗

PubMed2006

Fast neurotransmitter release triggered by Ca influx through AMPA-type glutamate receptors.

Feedback inhibition at reciprocal synapses between A17 amacrine cells and rod bipolar cells (RBCs) shapes light-evoked responses in the retina. Glutamate-mediated excitation of A17 cells elicits GABA (gamma-aminobutyric acid)-mediated inhibitory feedback onto RBCs, but the mechanisms that underlie GABA release from the dendrites of A17 cells are unknown. If, as observed at all other synapses studied, voltage-gated calcium channels (VGCCs) couple membrane depolarization to neurotransmitter release, feedforward excitatory postsynaptic potentials could spread through A17 dendrites to elicit 'surround' feedback inhibitory transmission at neighbouring synapses. Here we show, however, that GABA release from A17 cells in the rat retina does not depend on VGCCs or membrane depolarization. Instead, calcium-permeable AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors (AMPARs), activated by glutamate released from RBCs, provide the calcium influx necessary to trigger GABA release from A17 cells. The AMPAR-mediated calcium signal is amplified by calcium-induced calcium release (CICR) from intracellular calcium stores. These results describe a fast synapse that operates independently of VGCCs and membrane depolarization and reveal a previously unknown form of feedback inhibition within a neural circuit.

Amacrine Cells↗

PubMed2006

Vesicle depletion and synaptic depression at a mammalian ribbon synapse.

We estimated the size of the readily releasable pool (RRP) of vesicles at a ribbon synapse in the rat retina by making paired voltage-clamp recordings from presynaptic rod bipolar cells (RBCs) and postsynaptic AII amacrine cells in an in vitro retinal slice preparation. The RRP at each active zone was estimated to constitute seven vesicles, in the range of estimated RRP sizes at conventional synapses. During sustained presynaptic Ca(2+) entry, the RRP could be released with a time constant of about 4 ms. This ribbon synapse exhibited pronounced paired-pulse depression (PPD), which was attributable primarily to vesicle depletion. Recovery from PPD was slow (tau approximately 4 s) but could be accelerated by increasing the duration of the depressing stimulus. The small RRP and very high release probability likely contribute to the transient characteristics of neurotransmission at RBC synapses.

Amacrine Cells↗

PubMed2004

Coordinated multivesicular release at a mammalian ribbon synapse.

Traditional models of synaptic transmission hold that release sites within an active zone operate independently. Although the release of multiple vesicles (multivesicular release; MVR) from single active zones occurs at some central synapses, MVR is not thought to require coordination among release sites. Ribbon synapses seem to be optimized to release many vesicles over an extended period, but the dynamics of MVR at ribbon synapses is unknown. We examined MVR at a ribbon synapse in a retinal slice preparation using paired recordings from presynaptic rod bipolar and postsynaptic AII amacrine cells. When evoked release was highly desynchronized, discrete postsynaptic events were larger than quantal miniature excitatory postsynaptic currents (mEPSCs) but had the same time course. The amplitude of these multiquantal mEPSCs, which seem to arise from the essentially simultaneous release of multiple vesicles, was reduced by lowering release probability. The release synchrony reflected in these multivesicular events suggests that release within an active zone is coordinated during MVR.

Animals↗

PubMed2003

Sustained Ca2+ entry elicits transient postsynaptic currents at a retinal ribbon synapse.

Night (scotopic) vision is mediated by a distinct retinal circuit in which the light responses of rod-driven neurons are faster than those of the rods themselves. To investigate the dynamics of synaptic transmission at the second synapse in the rod pathway, we made paired voltage-clamp recordings from rod bipolar cells (RBCs) and postsynaptic AII and A17 amacrine cells in rat retinal slices. Depolarization of RBCs from -60 mV elicited sustained Ca2+ currents and evoked AMPA receptor (AMPAR)-mediated EPSCs in synaptically coupled amacrine cells that exhibited large, rapidly rising initial peaks that decayed rapidly to smaller, steady-state levels. The transient component persisted in the absence of feedback inhibition to the RBC terminal and when postsynaptic AMPA receptor desensitization was blocked with cyclothiazide, indicating that it reflects a time-dependent decrease in the rate of exocytosis from the presynaptic terminal. The EPSC waveform was similar when RBCs were recorded in perforated-patch or whole-cell configurations, but asynchronous release from RBCs was enhanced when the intraterminal Ca2+ buffer capacity was reduced. When RBCs were depolarized from -100 mV, inactivating, low voltage-activated (T-type channel-mediated) Ca2+ currents were evident. Although Ca2+ influx through T-type channels boosted vesicle release, as reflected by larger EPSCs, it did not make the EPSCs faster, indicating that activation of T-type channels is not necessary to generate a transient phase of exocytosis. We conclude that the time course of vesicle release from RBCs is inherently transient and, together with the fast kinetics of postsynaptic AMPARs, speeds transmission at this synapse.

Amacrine Cells↗