The action potential and its all-or-none nature is fundamental to neural communication. These binary spikes are used by neurons to signal that a decision has been made; that their membrane potential has crossed a threshold either due to synaptic inputs or intrinsic mechanisms. Canonically the action potential is initiated once voltage-gated Na+ (Nav) channels are activated, these channels underpin the rapid upstroke of the action potential and their rapid kinetics give rise to the all-or-none nature of the action potential. Here we show that cerebrospinal fluid contacting neurons (CSFcNs) surrounding the central canal of the mouse spinal cord employ a different strategy. Rather than using Nav channels to generate binary spikes, CSFcNs use two different types of voltage-gated Ca2+ channel, enabling spikes of different amplitude. T-type Ca2+ channels are required for spontaneous spiking and generate lower amplitude spikes which signal purinergic inputs. In contrast large amplitude spikes require high voltage activated Cd2+ sensitive Ca2+ channels and these larger spikes signal cholinergic inputs. Our results show that the neurotransmitter impinging upon CSFcNs can be differentially signalled by the spike amplitude and this is supported by different types of voltage-gated Ca2+ channel. CSFcNs have recently been shown to be integral to coordinating posture and motor output in zebrafish and lamprey, our results show that these neurons can employ novel mechanisms to integrate and signal the neurotransmitter systems in which they are embedded.

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Graded calcium spikes differentially signal neurotransmitter input in cerebrospinal fluid contacting neurons of mouse spinal cord

Homeostatic plasticity in the retina

The statistics of vesicle release determine how synapses transfer information, but the classical Poisson model of independent release does not always hold at the first stages of vision and hearing. There, ribbon synapses also encode sensory signals as events comprising two or more vesicles released simultaneously. The implications of such coordinated multivesicular release (MVR) for spike generation are not known. Here we investigate how MVR alters the transmission of sensory information compared with Poisson synapses using a pure rate-code. We used leaky integrate-and-fire models incorporating the statistics of release measured experimentally from glutamatergic synapses of retinal bipolar cells in zebrafish (both sexes) and compared these with models assuming Poisson inputs constrained to operate at the same average rates. We find that MVR can increase the number of spikes generated per vesicle while reducing interspike intervals and latency to first spike. The combined effect was to increase the efficiency of information transfer (bits per vesicle) over a range of conditions mimicking target neurons of different size. MVR was most advantageous in neurons with short time constants and reliable synaptic inputs, when less convergence was required to trigger spikes. In the special case of a single input driving a neuron, as occurs in the auditory system of mammals, MVR increased information transfer whenever spike generation required more than one vesicle. This study demonstrates how presynaptic integration of vesicles by MVR can increase the efficiency with which sensory information is transmitted compared with a rate-code described by Poisson statistics.SIGNIFICANCE STATEMENT Neurons communicate by the stochastic release of vesicles at the synapse and the statistics of this process will determine how information is represented by spikes. The classical model is that vesicles are released independently by a Poisson process, but this does not hold at ribbon-type synapses specialized to transmit the first electrical signals in vision and hearing, where two or more vesicles can fuse in a single event by a process termed coordinated multivesicular release. This study shows that multivesicular release can increase the number of spikes generated per vesicle and the efficiency of information transfer (bits per vesicle) over a range of conditions found in the retina and peripheral auditory system. 

/pdf/the-impact-of-multivesicular-release-on-the-transmission-of-1awe7v7p.pdf

The Impact of Multivesicular Release on the Transmission of Sensory Information by Ribbon Synapses

Graded spikes differentially signal neurotransmitter input in cerebrospinal fluid contacting neurons of the mouse spinal cord

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. 

/pdf/mechanisms-of-simultaneous-linear-and-nonlinear-computations-2cncamtp.pdf

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

Abstract Neuromodulators adapt sensory circuits to changes in the external world or the animal’s internal state and synapses are key control sites for such plasticity. Less clear is how neuromodulation alters the amount of information transmitted through the circuit. We investigated this question in the context of the diurnal regulation of visual processing in the retina of zebrafish, focusing on ribbon synapses of bipolar cells. We demonstrate that contrast-sensitivity peaks in the afternoon accompanied by a four-fold increase in the average Shannon information transmitted from an active zone. This increase reflects higher synaptic gain, lower spontaneous “noise” and reduced variability of evoked responses. Simultaneously, an increase in the probability of multivesicular events with larger information content increases the efficiency of transmission (bits per vesicle) by factors of 1.5-2.7. This study demonstrates the multiplicity of mechanisms by which a neuromodulator can adjust the synaptic transfer of sensory information. 

/pdf/diurnal-changes-in-the-efficiency-of-information-1u9u3c9c.pdf

Diurnal changes in the efficiency of information transmission at a sensory synapse

3D Orbital Tracking of Single Gold Nanoparticles: A New Approach to Study Vesicle Trafficking in Chromaffin Cells

The importance of the immediately releasable pool (IRP) of vesicles was proposed to reside in the maintenance of chromaffin cell secretion during the firing of action potentials at basal physiological frequencies. To accomplish this duty, IRP should be replenished as a function of time. We have previously reported that an action potential‐like stimulus (APls) triggers the release of ~50% IRP, followed by a fast dynamin‐dependent endocytosis and an associated rapid replenishment process. In this work, we investigated the endocytosis and IRP replenishment produced after the exocytosis of variable IRP fractions in mice primary chromaffin cell cultures. Exocytosis and endocytosis were estimated by membrane capacitance measurements obtained in patch‐clamped cells. In addition to the dynamin‐dependent fast endocytosis activated after the application of APls or 5 ms squared depolarizations, we found that depolarizations lasting 25–50 ms, which release >80% of IRP, are related with a fast dynamin‐independent, Ca2+‐ and protein kinase C (PKC)‐dependent endocytosis (time constant <1 s). PKC inhibitors, such as staurosporine, bisindolylmaleimide XI, PKC 19–31 peptide, and prolonged treatments with high concentrations of phorbol esters, reduced and decelerated this endocytosis. Additionally, we found that the inhibition of PKC also abolished a slow component of replenishment (time constant ~8 s) observed after total IRP exocytosis. Therefore, our results suggest that PKC contributes to the coordination of membrane retrieval and vesicle replenishment mechanisms that occur after the complete exocytosis of IRP.

Membrane retrieval after Immediately Releasable Pool (IRP) exocytosis is produced by dynamin‐dependent and dynamin‐independent/protein kinase C‐dependent mechanisms

The importance of the immediately releasable pool (IRP) of vesicles was proposed to reside in the maintenance of chromaffin cell secretion during the firing of action potentials at basal physiological frequencies. To accomplish this duty, IRP should be replenished as a function of time. We have previously reported that an action potential-like stimulus (APls) triggers the release of ∽50% IRP, followed by a fast dynamin-dependent endocytosis and an associated rapid replenishment process. In this work we investigated the endocytosis and IRP replenishment produced after the exocytosis of variable IRP fractions in mice primary chromaffin cell cultures. Exocytosis and endocytosis were estimated by membrane capacitance measurements obtained in patch-clamped cells. In addition to the dynamin-dependent fast endocytosis activated after the application of APls or 5 ms squared depolarizations, we found that depolarizations lasting 25-50 ms, which release >80% of IRP, are related with a fast dynamin-independent, Ca2+- and protein kinase C (PKC)-dependent endocytosis (time constant < 1 s). PKC inhibitors, such as staurosporine, bisindolylmaleimide XI and prolonged treatments with high concentrations of phorbol esters, reduced and decelerated this endocytosis. Additionally, we found that the inhibition of PKC also abolished a slow component of replenishment (time constant ∽8 s) observed after total IRP exocytosis. Therefore, our results suggest that PKC contributes to the coordination of membrane retrieval and vesicle replenishment mechanisms that occur after the complete exocytosis of IRP.

Jose Moya-Diaz

Papers

Diurnal changes in the efficiency of information transmission at a sensory synapse

3D Orbital Tracking of Single Gold Nanoparticles: A New Approach to Study Vesicle Trafficking in Chromaffin Cells

Membrane retrieval after Immediately Releasable Pool (IRP) exocytosis is produced by dynamin‐dependent and dynamin‐independent/protein kinase C‐dependent mechanisms

Membrane Retrieval after Immediately Releasable Pool (IRP) Exocytosis is produced by Dynamin-Dependent and Dynamin-Independent Mechanisms