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

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.

Mechanisms of simultaneous linear and nonlinear computations at the mammalian cone photoreceptor synapse

Neurons enhance their computational power by combining linear and nonlinear transformations in extended dendritic trees. Rich, spatially distributed processing is rarely associated with individual synapses, but the cone photoreceptor synapse may be an exception. Graded voltages temporally modulate vesicle fusion at a cone's ~20 ribbon active zones. Transmitter then flows into a common, glia-free volume where bipolar cell dendrites are organized by type in successive tiers. Using super-resolution microscopy and tracking vesicle fusion and postsynaptic responses at the quantal level in the thirteen-lined ground squirrel, Ictidomys tridecemlineatus, we show that certain bipolar cell types respond to individual fusion events in the vesicle stream while other types respond to degrees of locally coincident events, creating a gradient across tiers that are increasingly nonlinear. Nonlinearities emerge from a combination of factors specific to each bipolar cell type including diffusion distance, contact number, receptor affinity, and proximity to glutamate transporters. Complex computations related to feature detection begin within the first visual synapse.

Glycine Release Is Potentiated by cAMP via EPAC2 and Ca2+ Stores in a Retinal Interneuron

Neuromodulation via the intracellular second messenger cAMP is ubiquitous at presynaptic nerve terminals. This modulation of synaptic transmission allows exocytosis to adapt to stimulus levels and reliably encode information. The AII amacrine cell (AII-AC) is a central hub for signal processing in the mammalian retina. The main apical dendrite of the AII-AC is connected to several lobular appendages that release glycine onto OFF cone bipolar cells and ganglion cells. However, the influence of cAMP on glycine release is not well understood. Using membrane capacitance measurements from mouse AII-ACs to directly measure exocytosis, we observe that intracellular dialysis of 1 mm cAMP enhances exocytosis without affecting the L-type Ca²⁺ current. Responses to depolarizing pulses of various durations show that the size of the readily releasable pool of vesicles nearly doubles with cAMP, while paired-pulse depression experiments suggest that release probability does not change. Specific agonists and antagonists for exchange protein activated by cAMP 2 (EPAC2) revealed that the cAMP-induced enhancement of exocytosis requires EPAC2 activation. Furthermore, intact Ca²⁺ stores were also necessary for the cAMP potentiation of exocytosis. Postsynaptic recordings from OFF cone bipolar cells showed that increasing cAMP with forskolin potentiated the frequency of glycinergic spontaneous IPSCs. We propose that cAMP elevations in the AII-AC lead to a robust enhancement of glycine release through an EPAC2 and Ca²⁺ store signaling pathway. Our results thus contribute to a better understanding of how AII-AC crossover inhibitory circuits adapt to changes in ambient luminance.SIGNIFICANCE STATEMENT The mammalian retina operates over a wide dynamic range of light intensities and contrast levels. To optimize the signal-to-noise ratio of processed visual information, both excitatory and inhibitory synapses within the retina must modulate their gain in synaptic transmission to adapt to different levels of ambient light. Here we show that increases of cAMP concentration within AII amacrine cells produce enhanced exocytosis from these glycinergic interneurons. Therefore, we propose that light-sensitive neuromodulators may change the output of glycine release from AII amacrine cells. This novel mechanism may fine-tune the amount of tonic and phasic synaptic inhibition received by bipolar cell terminals and, consequently, the spiking patterns that ganglion cells send to the upstream visual areas of the brain.

The mammalian rod synaptic ribbon is essential for Cav channel facilitation and ultrafast synaptic vesicle fusion

Rod photoreceptors (PRs) use ribbon synapses to transmit visual information. To signal ‘no light detected’ they release glutamate continually to activate post-synaptic receptors. When light is detected glutamate release pauses. How a rod’s individual ribbon enables this process was studied here by recording evoked changes in whole-cell membrane capacitance from wild-type and ribbonless (Ribeye-ko) mice. Wild-type rods filled with high (10 mM) or low (0.5 mM) concentrations of the Ca²⁺-buffer EGTA created a readily releasable pool (RRP) of 87 synaptic vesicles (SVs) that emptied as a single kinetic phase with a τ<0.4 ms. The lower concentration of EGTA accelerated Ca_v channel opening and facilitated release kinetics. In contrast, ribbonless rods created a much smaller RRP of 22 SVs, and they lacked Ca_v channel facilitation; however, Ca²⁺ channel-release coupling remained tight. These release deficits caused a sharp attenuation of rod-driven scotopic light responses. We conclude that the synaptic ribbon facilitates Ca²⁺-influx and establishes a large RRP of SVs.

Quantifying neurotransmitter secretion at single-vesicle resolution using high-density complementary metal-oxide-semiconductor electrode array

Neuronal exocytosis facilitates the propagation of information through the nervous system pertaining to bodily function, memory, and emotions. Using amperometry, the sub-millisecond dynamics of exocytosis can be monitored and the modulation of exocytosis due to drug treatment or neurodegenerative diseases can be studied. Traditional single-cell amperometry is a powerful technique for studying the molecular mechanisms of exocytosis, but it is both costly and labor-intensive to accumulate statistically significant data. To surmount these limitations, we have developed a silicon-based electrode array with 1024 on-chip electrodes that measures oxidative signal in 0.1 millisecond intervals. Using the developed device, we are able to capture the modulation of exocytosis due to Parkinson's disease treatment (L-Dopa), with statistical significance, within 30 total minutes of recording. The validation study proves our device's capability to accelerate the study of many pharmaceutical treatments for various neurodegenerative disorders that affect neurotransmitter secretion to a matter of minutes.

Sensory Processing at Ribbon Synapses in the Retina and the Cochlea

In recent years, sensory neuroscientists have made major efforts to dissect the structure and function of ribbon synapses which process sensory information in the eye and ear. This review aims to summarize our current understanding of two key aspects of ribbon synapses: 1) their mechanisms of exocytosis and endocytosis and 2) their molecular anatomy and physiology. Our comparison of ribbon synapses in the cochlea and the retina reveals convergent signaling mechanisms, as well as divergent strategies in different sensory systems.

Ultrafast Glutamate Biosensor Recordings in Brain Slices Reveal Complex Single Exocytosis Transients

Neuronal communication relies on vesicular neurotransmitter release from signaling neurons and detection of these molecules by neighboring neurons. Glutamate, the main excitatory neurotransmitter in the mammalian brain, is involved in nearly all brain functions. However, glutamate has suffered from detection schemes that lack temporal and spatial resolution allowed by electrochemistry. Here we show an amperometric, novel, ultrafast enzyme-based nanoparticle modified sensor, measuring random bursts of hundreds to thousands of rapid spontaneous glutamate exocytotic release events at approximately 30 Hz frequency in the nucleus accumbens of rodent brain slices. Characterizing these single submillisecond exocytosis events revealed a great diversity in spike shape characteristics and size of quantal release, suggesting variability in fusion pore dynamics controlling the glutamate release by cells in this brain region. Hence, this novel biosensor allows recording of rapid single glutamate exocytosis events in the brain tissue and offers insight on regulatory aspects of exocytotic glutamate release, which is critical to understanding of brain glutamate function and dysfunction.

Ca2+ Regulates the Kinetics of Synaptic Vesicle Fusion at the Afferent Inner Hair Cell Synapse

The early auditory pathway processes information at high rates and with utmost temporal fidelity. Consequently, the synapses in the auditory pathway are highly specialized to meet the extraordinary requirements on signal transmission. The calyceal synapses in the auditory brainstem feature more than a hundred active zones (AZs) with thousands of releasable synaptic vesicles (SVs). In contrast, the first auditory synapse, the afferent synapse of inner hair cells (IHCs) and type I spiral ganglion neurons (SGNs), typically exhibits a single ribbon-type AZ tethering only tens of SVs resulting in a highly stochastic pattern of transmitter release. Spontaneous excitatory postsynaptic currents (sEPSCs), besides more conventional EPSCs with a single peak, fast rise and decay (compact), also include EPSCs with multiple peaks, variable rise and decay times (non-compact). The strong heterogeneity in size and shape of spontaneous EPSCs has led to the hypothesis of multivesicular release (MVR) that is more (compact) or less (non-compact) synchronized by coordination of release sites. Alternatively, univesicular release (UVR), potentially involving glutamate release through a flickering fusion pore for non-compact EPSCs, has been suggested to underlie IHC exocytosis. Here, we further investigated the mode of release by recording sEPSCs from SGNs of hearing rats while manipulating presynaptic IHC Ca²⁺ influx by changes in extracellular [Ca²⁺] ([Ca²⁺]_e) and by application of the Ca²⁺ channel antagonist, isradipine, or the Ca²⁺ channel agonist, BayK8644 (BayK). Our data reveal that Ca²⁺ influx manipulation leaves the distributions of sEPSC amplitude and charge largely unchanged. Regardless the type of manipulation, the rate of sEPSC decreased with the reduction in Ca²⁺ influx. The fraction of compact sEPSCs was increased in the presence of BayK, an effect that was abolished when combined with decreased [Ca²⁺]_e. In conclusion, we propose that UVR is the prevailing mode of exocytosis at cochlear IHCs of hearing rats, whereby the rate of exocytosis and the kinetics of SV fusion are regulated by Ca²⁺ influx.

The synaptic ribbon is critical for sound encoding at high rates and with temporal precision

We studied the role of the synaptic ribbon for sound encoding at the synapses between inner hair cells (IHCs) and spiral ganglion neurons (SGNs) in mice lacking RIBEYE (RBE^KO/KO). Electron and immunofluorescence microscopy revealed a lack of synaptic ribbons and an assembly of several small active zones (AZs) at each synaptic contact. Spontaneous and sound-evoked firing rates of SGNs and their compound action potential were reduced, indicating impaired transmission at ribbonless IHC-SGN synapses. The temporal precision of sound encoding was impaired and the recovery of SGN-firing from adaptation indicated slowed synaptic vesicle (SV) replenishment. Activation of Ca²⁺-channels was shifted to more depolarized potentials and exocytosis was reduced for weak depolarizations. Presynaptic Ca²⁺-signals showed a broader spread, compatible with the altered Ca²⁺-channel clustering observed by super-resolution immunofluorescence microscopy. We postulate that RIBEYE disruption is partially compensated by multi-AZ organization. The remaining synaptic deficit indicates ribbon function in SV-replenishment and Ca²⁺-channel regulation.

Our sense of hearing relies on our ears quickly and tirelessly processing information in a precise manner. Sounds cause vibrations in a part of the inner ear called the cochlea. Inside the cochlea, the vibrations move hair-like structures on sensory cells that translate these movements into electrical signals. These hair cells are connected to specialized nerve cells that relay the signals to the brain, which then interprets them as sounds.

Hair cells communicate with the specialized nerve cells via connections known as chemical synapses. This means that the electrical signals in the hair cell activate channel proteins that allow calcium ions to flow in. This in turn triggers membrane-bound packages called vesicles inside the hair cell to fuse with its surface membrane and release their contents to the outside. The contents, namely chemicals called neurotransmitters, then travels across the space between the cells, relaying the signal to the nerve cell.

The junctions between the hair cells and the nerve cells are more specifically known as ribbon synapses. This is because they have a ribbon-like structure that appears to tether a halo of vesicles close to the active zone where neurotransmitters are released. However, the exact role of this synaptic ribbon has remained mysterious despite decades of study.

The ribbon is mainly composed of a protein called Ribeye, and now Jean, Lopez de la Morena, Michanski, Jaime Tobón et al. show that mutant mice that lack this protein do not have any ribbons at their “ribbon synapses”. Hair cells without synaptic ribbons are less able to timely and reliably send signals to the nerve cells, most likely because they cannot replenish the vesicles at the synapse quickly enough. Further analysis showed that the synaptic ribbon also helps to regulate the calcium channels at the synapse, which is important for linking the electrical signals in the hair cell to the release of the neurotransmitters.

Jean et al. also saw that hair cells without ribbons reorganize their synapses to form multiple active zones that could transfer neurotransmitter to the nerve cells. This could partially compensate for the loss of the ribbons, meaning the impact of their loss may have been underestimated. Future studies could explore this by eliminating the Ribeye protein only after the ribbon synapses are fully formed.

These findings may help scientists to better understand deafness and other hearing disorders in humans. They will also be of interest to neuroscientists who research synapses, hearing and other sensory processes.

Mechanism of High-Frequency Signaling at a Depressing Ribbon Synapse

Ribbon synapses mediate continuous release in neurons that have graded voltage responses. While mammalian retinas can signal visual flicker at 80-100 Hz, the time constant, τ, for the refilling of a depleted vesicle release pool at cone photoreceptor ribbons is 0.7-1.1 s. Due to this prolonged depression, the mechanism for encoding high temporal frequencies is unclear. To determine the mechanism of high-frequency signaling, we focused on an "Off" cone bipolar cell type in the ground squirrel, the cb2, whose transient postsynaptic responses recovered following presynaptic depletion with a τ of ∼0.1 s, or 7- to 10-fold faster than the τ for presynaptic pool refilling. The difference in recovery time course is caused by AMPA receptor saturation, where partial refilling of the presynaptic pool is sufficient for a full postsynaptic response. By limiting the dynamic range of the synapse, receptor saturation counteracts ribbon depression to produce rapid recovery and facilitate high-frequency signaling.

RIM1/2-Mediated Facilitation of Cav1.4 Channel Opening Is Required for Ca2+-Stimulated Release in Mouse Rod Photoreceptors

Night blindness can result from impaired photoreceptor function and a subset of cases have been linked to dysfunction of Cav1.4 calcium channels and in turn compromised synaptic transmission. Here, we show that active zone proteins RIM1/2 are important regulators of Cav1.4 channel function in mouse rod photoreceptors and thus synaptic activity. The conditional double knock-out (cdko) of RIM1 and RIM2 from rods starting a few weeks after birth did not change Cav1.4 protein expression at rod ribbon synapses nor was the morphology of the ribbon altered. Heterologous overexpression of RIM2 with Cav1.4 had no significant influence on current density when examined with BaCl2 as the charge carrier. Nonetheless, whole-cell voltage-clamp recordings from cdko rods revealed a profound reduction in Ca(2+) currents. Concomitantly, we observed a 4-fold reduction in spontaneous miniature release events from the cdko rod terminals and an almost complete absence of evoked responses when monitoring changes in membrane incorporation after strong step depolarizations. Under control conditions, 49 and 83 vesicles were released with 0.2 and 1 s depolarizations, respectively, which is close to the maximal number of vesicles estimated to be docked at the base of the ribbon active zone, but without RIM1/2, only a few vesicles were stimulated for release after a 1 s stimulation. In conclusion, our study shows that RIM1/2 potently enhance the influx of Ca(2+) into rod terminals through Cav1.4 channels, which is vitally important for the release of vesicles from the rod ribbon. Significance statement: Active zone scaffolding proteins are thought to bring multiple components involved in Ca(2+)-dependent exocytosis into functional interactions. We show that removal of scaffolding proteins RIM1/2 from rod photoreceptor ribbon synapses causes a dramatic loss of Ca(2+) influx through Cav1.4 channels and a correlated reduction in evoked release, yet the channels remain localized to synaptic ribbons in a normal fashion. Our findings strongly argue that RIM1/2 facilitate Ca(2+) entry and in turn Ca(2+) evoked release by modulating Cav1.4 channel openings; however, RIM1/2 are not needed for the retention of Cav1.4 at the synapse. In summary, a key function of RIM1/2 at rod ribbons is to enhance Cav1.4 channel activity, possibly through direct or indirect modulation of the channel.

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.

Microelectronics-based biosensors dedicated to the detection of neurotransmitters: a review

Dysregulation of neurotransmitters (NTs) in the human body are related to diseases such as Parkinson's and Alzheimer's. The mechanisms of several neurological disorders, such as epilepsy, have been linked to NTs. Because the number of diagnosed cases is increasing, the diagnosis and treatment of such diseases are important. To detect biomolecules including NTs, microtechnology, micro and nanoelectronics have become popular in the form of the miniaturization of medical and clinical devices. They offer high-performance features in terms of sensitivity, as well as low-background noise. In this paper, we review various devices and circuit techniques used for monitoring NTs in vitro and in vivo and compare various methods described in recent publications.