Characterisation of bipolar cell synaptic transmission in goldfish retina using paired recordings

The role of syntaxins in retinal function and health

The soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptor (SNARE) superfamily plays a pivotal role in cellular trafficking by facilitating membrane fusion events. These SNARE proteins, including syntaxins, assemble into complexes that actively facilitate specific membrane fusion events. Syntaxins, as integral components of the SNARE complex, play a crucial role in initiating and regulating these fusion activities. While specific syntaxins have been extensively studied in various cellular processes, including neurotransmitter release, autophagy and endoplasmic reticulum (ER)-to-Golgi protein transport, their roles in the retina remain less explored. This review aims to enhance our understanding of syntaxins' functions in the retina by shedding light on how syntaxins mediate membrane fusion events unique to the retina. Additionally, we seek to establish a connection between syntaxin mutations and retinal diseases. By exploring the intricate interplay of syntaxins in retinal function and health, we aim to contribute to the broader comprehension of cellular trafficking in the context of retinal physiology and pathology.

Functional maturation of the rod bipolar to AII-amacrine cell ribbon synapse in the mouse retina

Retinal ribbon synapses undergo functional changes after eye opening that remain uncharacterized. Using light-flash stimulation and paired patch-clamp recordings, we examined the maturation of the ribbon synapse between rod bipolar cells (RBCs) and AII-amacrine cells (AII-ACs) after eye opening (postnatal day 14) in the mouse retina at near physiological temperatures. We find that light-evoked excitatory postsynaptic currents (EPSCs) in AII-ACs exhibit a slow sustained component that increases in magnitude with advancing age, whereas a fast transient component remains unchanged. Similarly, paired recordings reveal a dual-component EPSC with a slower sustained component that increases during development, even though the miniature EPSC (mEPSC) amplitude and kinetics do not change significantly. We thus propose that the readily releasable pool of vesicles from RBCs increases after eye opening, and we estimate that a short light flash can evoke the release of ∼4,000 vesicles onto a single mature AII-AC.

A sign-inverted receptive field of inhibitory interneurons provides a pathway for ON-OFF interactions in the retina

A fundamental organizing plan of the retina is that visual information is divided into ON and OFF streams that are processed in separate layers. This functional dichotomy originates in the ON and OFF bipolar cells, which then make excitatory glutamatergic synapses onto amacrine and ganglion cells in the inner plexiform layer. We have identified an amacrine cell (AC), the sign-inverting (SI) AC, that challenges this fundamental plan. The glycinergic, ON-stratifying SI-AC has OFF light responses. In opposition to the classical wiring diagrams, it receives inhibitory inputs from glutamatergic ON bipolar cells at mGluR8 synapses, and excitatory inputs from an OFF wide-field AC at electrical synapses. This "inhibitory ON center - excitatory OFF surround" receptive-field of the SI-AC allows it to use monostratified dendrites to conduct crossover inhibition and push-pull activation to enhance light detection by ACs and RGCs in the dark and feature discrimination in the light.

Two Pools of Vesicles Associated with Synaptic Ribbons Are Molecularly Prepared for Release

Neurons that form ribbon-style synapses are specialized for continuous exocytosis. To this end, their synaptic terminals contain numerous synaptic vesicles, some of which are ribbon associated, that have difference susceptibilities for undergoing Ca²⁺-dependent exocytosis. In this study, we probed the relationship between previously defined vesicle populations and determined their fusion competency with respect to SNARE complex formation. We found that both the rapidly releasing vesicle pool and the releasable vesicle pool of the retinal bipolar cell are situated at the ribbon-style active zones, where they functionally interact. A peptide inhibitor of SNARE complex formation failed to block exocytosis from either pool, suggesting that these two vesicle pools have formed the SNARE complexes necessary for fusion. By contrast, a third, slower component of exocytosis was blocked by the peptide, as was the functional replenishment of vesicle pools, indicating that few vesicles outside of the ribbon-style active zones were initially fusion competent. In cone photoreceptors, similar to bipolar cells, fusion of the initial ribbon-associated synaptic vesicle cohort was not blocked by the SNARE complex-inhibiting peptide, whereas a later phase of exocytosis, attributable to the recruitment and subsequent fusion of vesicles newly arrived at the synaptic ribbons, was blocked. Together, our results support a model in which stimulus-evoked exocytosis in retinal ribbon synapses is SNARE-dependent; where vesicles higher up on the synaptic ribbon replenish the rapidly releasing vesicle pool; and at any given time, there are sufficient SNARE complexes to support the fusion of the entire ribbon-associated cohort of vesicles. An important implication of these results is that ribbon-associated vesicles can form intervesicular SNARE complexes, providing mechanistic insight into compound fusion at ribbon-style synapses.

Postsynaptic Plasticity Triggered by Ca²⁺-Permeable AMPA Receptor Activation in Retinal Amacrine Cells

Amacrine cells are thought to be a major locus for mechanisms of light adaptation and contrast enhancement in the retina. However, the potential for plasticity in their AMPA receptor currents remains largely unknown. Using paired patch-clamp recordings between bipolar cell terminals and amacrine cells, we have simultaneously measured presynaptic membrane capacitance changes and EPSCs. Repetitive bipolar cell depolarizations, designed to maintain the same amount of exocytosis, nevertheless significantly potentiated evoked EPSCs in a subpopulation of amacrine cells. Likewise, repetitive iontophoresis (or puffs) of glutamate (or AMPA) onto the dendrites of amacrine cells also significantly potentiated evoked currents and [Ca(2+)]i rises. However, strong postsynaptic Ca(2+) buffering with BAPTA abolished the potentiation and selective antagonists of Ca(2+)-permeable AMPA receptors also blocked the potentiation of AMPA-mediated currents. Together these results suggest that Ca(2+) influx via Ca(2+)-permeable AMPA receptors can elicit a rapid form of postsynaptic plasticity in a subgroup of amacrine cell dendrites.

Calcium spike-mediated digital signaling increases glutamate output at the visual threshold of retinal bipolar cells

Most retinal bipolar cells (BCs) transmit visual input from photoreceptors to ganglion cells using graded potentials, but some also generate calcium or sodium spikes. Sodium spikes are thought to increase temporal precision of light-evoked BC signaling; however, the role of calcium spikes in BCs is not fully understood. Here we studied how calcium spikes and graded responses mediate neurotransmitter release from Mb-type BCs, known to produce both. In dark-adapted goldfish retinal slices, light induced spikes in 40% of the axon terminals of intact Mbs; in the rest, light generated graded responses. These light-evoked membrane potentials were used to depolarize axotomized Mb terminals where depolarization-evoked calcium current (ICa) and consequent exocytosis-associated membrane capacitance increases (ΔCm) could be precisely measured. When evoked by identical dim light intensities, spiking responses transferred more calcium (Q(Ca)) and triggered larger exocytosis with higher efficiency (ΔCm/Q(Ca)) than graded potentials. Q(Ca) was translated into exocytosis linearly when transferred with spikes and supralinearly when transferred with graded responses. At the Mb output (ΔCm), spiking responses coded light intensity with numbers and amplitude whereas graded responses coded with amplitude, duration, and steepness. Importantly, spiking responses saturated exocytosis within scotopic range but graded potentials did not. We propose that calcium spikes in Mbs increase signal input-output ratio by boosting Mb glutamate release at threshold intensities. Therefore, spiking Mb responses are suitable to transfer low-light-intensity signals to ganglion cells with higher gain, whereas graded potentials signal for light over a wider range of intensities at the Mb output.

Nitric oxide mediates activity-dependent plasticity of retinal bipolar cell output via S-nitrosylation

Coding a wide range of light intensities in natural scenes poses a challenge for the retina: adaptation to bright light should not compromise sensitivity to dim light. Here we report a novel form of activity-dependent synaptic plasticity, specifically, a "weighted potentiation" that selectively increases output of Mb-type bipolar cells in the goldfish retina in response to weak inputs but leaves the input-output ratio for strong stimuli unaffected. In retinal slice preparation, strong depolarization of bipolar terminals significantly lowered the threshold for calcium spike initiation, which originated from a shift in activation of voltage-gated calcium currents (ICa) to more negative potentials. The process depended upon glutamate-evoked retrograde nitric oxide (NO) signaling as it was eliminated by pretreatment with an NO synthase blocker, TRIM. The NO-dependent ICa modulation was cGMP independent but could be blocked by N-ethylmaleimide (NEM), indicating that NO acted via an S-nitrosylation mechanism. Importantly, the NO action resulted in a weighted potentiation of Mb output in response to small (≤-30 mV) depolarizations. Coincidentally, light flashes with intensity ≥ 2.4 × 10(8) photons/cm(2)/s lowered the latency of scotopic (≤ 2.4 × 10(8) photons/cm(2)/s) light-evoked calcium spikes in Mb axon terminals in an NEM-sensitive manner, but light responses above cone threshold (≥ 3.5 × 10(9) photons/cm(2)/s) were unaltered. Under bright scotopic/mesopic conditions, this novel form of Mb output potentiation selectively amplifies dim retinal inputs at Mb → ganglion cell synapses. We propose that this process might counteract decreases in retinal sensitivity during light adaptation by preventing the loss of visual information carried by dim scotopic signals.

Amperometric resolution of a prespike stammer and evoked phases of fast release from retinal bipolar cells

The neurotransmitter glutamate is used by most neurons in the brain to activate a multitude of different types of glutamate receptors and transporters involved in fast and relatively slower signaling. Synaptic ribbons are large presynaptic structures found in neurons involved in vision, balance, and hearing, which use a large number of glutamate-filled synaptic vesicles to meet their signaling demands. To directly measure synaptic vesicle release events, the ribbon-type presynaptic terminals of goldfish retinal bipolar cells were coaxed to release a false transmitter that could be monitored with amperometry by placing the carbon fiber directly on the larger synaptic terminal. Spontaneous secretion events formed a unimodal charge distribution, but single spike properties were heterogeneous. Larger events rose exponentially without interruption (τ ∼ 30 μs), and smaller events exhibited a stammer in their rising phase that is interpreted as a brief pause in pore dilation, a characteristic commonly associated with large dense core granule fusion pores. These events were entirely Ca(2+)-dependent. Holding the cells at -60 mV halted spontaneous release; and when the voltage was stepped to >-40 mV, secretion ensued. When stepping the voltage to 0 mV, novel kinetic phases of vesicle recruitment were revealed. Approximately 14 vesicles were released per ribbon in two kinetic phases with time constants of 1.5 and 16 ms, which are proposed to represent different primed states within the population of docked vesicles.

Alert response to motion onset in the retina

Previous studies have shown that motion onset is very effective at capturing attention and is more salient than smooth motion. Here, we find that this salience ranking is present already in the firing rate of retinal ganglion cells. By stimulating the retina with a bar that appears, stays still, and then starts moving, we demonstrate that a subset of salamander retinal ganglion cells, fast OFF cells, responds significantly more strongly to motion onset than to smooth motion. We refer to this phenomenon as an alert response to motion onset. We develop a computational model that predicts the time-varying firing rate of ganglion cells responding to the appearance, onset, and smooth motion of a bar. This model, termed the adaptive cascade model, consists of a ganglion cell that receives input from a layer of bipolar cells, represented by individual rectified subunits. Additionally, both the bipolar and ganglion cells have separate contrast gain control mechanisms. This model captured the responses to our different motion stimuli over a wide range of contrasts, speeds, and locations. The alert response to motion onset, together with its computational model, introduces a new mechanism of sophisticated motion processing that occurs early in the visual system.

Light-evoked lateral GABAergic inhibition at single bipolar cell synaptic terminals is driven by distinct retinal microcircuits

Inhibitory amacrine cells (ACs) filter visual signals crossing the retina by modulating the excitatory, glutamatergic output of bipolar cells (BCs) on multiple temporal and spatial scales. Reciprocal feedback from ACs provides focal inhibition that is temporally locked to the activity of presynaptic BC activity, whereas lateral feedback originates from ACs excited by distant BCs. These distinct feedback mechanisms permit temporal and spatial computation at BC terminals. Here, we used a unique preparation to study light-evoked IPSCs recorded from axotomized terminals of ON-type mixed rod/cone BCs (Mb) in goldfish retinal slices. In this preparation, light-evoked IPSCs could only reach axotomized BC terminals via the lateral feedback pathway, allowing us to study lateral feedback in the absence of overlapping reciprocal feedback components. We found that light evokes ON and OFF lateral IPSCs (L-IPSCs) in Mb terminals having different temporal patterns and conveyed via distinct retinal pathways. The relative contribution of rods versus cones to ON and OFF L-IPSCs was light intensity dependent. ACs presynaptic to Mb BC terminals received inputs via AMPA/KA- and NMDA-type receptors in both the ON and OFF pathways, and used TTX-sensitive sodium channels to boost signal transfer along their processes. ON and OFF L-IPSCs, like reciprocal feedback IPSCs, were mediated by both GABA(A) and GABA(C) receptors. However, our results suggest that lateral and reciprocal feedback do not cross-depress each other, and are therefore mediated by distinct populations of ACs. These findings demonstrate that retinal inhibitory circuits are highly specialized to modulate BC output at different light intensities.

The diverse roles of ribbon synapses in sensory neurotransmission

Sensory synapses of the visual and auditory systems must faithfully encode a wide dynamic range of graded signals, and must be capable of sustained transmitter release over long periods of time. Functionally and morphologically, these sensory synapses are unique: their active zones are specialized in several ways for sustained, rapid vesicle exocytosis, but their most striking feature is an organelle called the synaptic ribbon, which is a proteinaceous structure that extends into the cytoplasm at the active zone and tethers a large pool of releasable vesicles. But precisely how does the ribbon function to support tonic release at these synapses? Recent genetic and biophysical advances have begun to open the 'black box' of the synaptic ribbon with some surprising findings and promise to resolve its function in vision and hearing.