Titles and abstracts

Tomas Bellamy (School of Biomedical Sciences, University of Nottingham)

"Crosstalk between nitric oxide and calcium signalling pathways in astrocytes"

Astrocytes are non-excitable cells of the central nervous system that express a wide range of neurotransmitter and hormone receptors linked to Ca²⁺ signalling pathways. These signals can range in scale from local transients to waves that spread through the whole cell, and even through networks of connected cells. The transition from local to global signalling events depends on the coordinated opening of clustered InsP₃ and ryanodine receptors, and therefore depends on the underlying stochasticity of channel openings. Consequently, astrocytes exposed to neurotransmitter receptor agonists have a given probability of exhibiting a global Ca²⁺ signal, and this probability varies between agonists.

We recently found that pre-incubation of astrocytes with nitric oxide (NO) donors increased the probability of Ca²⁺-mobilizing agonists triggering global signalling events. The NO effect depends on cGMP accumulation and activation of cGMP-dependent protein kinase, suggesting that phosphorylation of some aspect of the Ca²⁺ signalling machinery in astrocytes augments the local-to-global transition. Further investigation found evidence of complex crosstalk between NO and Ca²⁺ signalling pathways, but confusingly, NO-cGMP was found to decrease Ca²⁺ entry into the cytosol through InsP₃ receptors, ryanodine receptors, and store-operated Ca²⁺ channels, and had no effect on phospholipase C activity or Ca²⁺ clearance mechanisms. Looked at in isolation, these effects would be predicted to decrease Ca²⁺ signal probability, and yet overall, Ca²⁺ signals are enhanced. This suggests that a potentiatory mechanism remains undiscovered, and emphasizes the need to consider signalling networks as a whole, rather than extrapolating cell wide behaviour from the responses of underlying components

Eshel Ben-Jacob (School of Physics and Astronomy, Tel Aviv University)

"Astrocyte regulation of synaptic information transfer and calcium waves in networks of astrocytes"

Nigel Emptage (Department of Pharmacology, University of Oxford)

"Activity-regulated Ca²⁺ release from lysosomal stores in neurons"

We have identified an intracellular Ca²⁺ release mechanism in hippocampal pyramidal neurons that is triggered by depolarization of the dendritic membrane. The source of the Ca²⁺ rise appears not to be the endoplasmic reticulum but the lysosome. It has been known for some time that lysosomes are able to sequester Ca²⁺ however, only recently have Ca²⁺ efflux mechanisms across the lysosomal membrane been reported (Calcraft et al., 2009; Zeevi et al. 2007). Of these our work suggests that depolarization-induced release occurs via a NAADP-sensitive twin pore channel (TPC). In this study we seek to understand the role for depolarization-triggered lysosomal Ca²⁺ release by targeting egress of Ca²⁺ from lysosomes via TPCs. We achieve this in in two ways: The first is to pharmacologically inhibit TPC-mediated Ca²⁺ release; the second is to examine neuronal function following genetic deletion of TPCs in a mouse model. The consequences of these manipulations are not lethal to the neurons, but do profoundly alter the architecture of dendritic spines at excitatory synapses. Importantly, the morphological changes are reversible thus on relief of pharmacological inhibition of the TPCs dendritic spines are re-formed. This suggests that lysosomal Ca²⁺ release contributes to a process critical for the maintenance of dendritic spines. Remarkably, synaptic transmission, in both genetic KOs and pharmacologically manipulated tissue, is little changed. This result raises significant questions about the precise role of dendritic spines at excitatory synapses. What is, however, clear is that spine morphology can be dissociated from synaptic transmission.

Victor Kazantsev (Institute of Applied Physics, Nizhny Novgorod, Russia)

"Calcium signaling in astrocytes induced by the IP₃ diffusion: instability, synchronization and chaos"

Calcium oscillations in astrocytes and their role in cellular signaling functions are the subjects of growing interest in experimental and theoretical neuroscience. Astrocytes generate calcium signals that can regulate extracellular dynamics of neurotransmitters (e.g. glutamate, D-serine, ATP), hence affecting neuronal excitability and the efficiency of synaptic transmission (e.g. forming tripartite synapse). In basic kinetic models the intracellular calcium is mediated by IP₃ produced with PLC activation and regulates the calcium influx from endoplasmic reticulum (ER) through IP₃ sensitive receptors. Another source of IP₃ changes is the diffusion of IP₃ molecules through the gap junctions. Astrocyte gap junctions are formed by specific connexins (Cx43) permeable selectively to IP₃. So that, there is a network of astrocytes that may communicate by the IP₃ diffusion.

In the frame of very basic kinetics model of the local astrocyte dynamics we investigate theoretical consequences of the IP₃ diffusion in the astrocyte network. Surprisingly, the IP₃ diffusion does not sustain propagation of intercellular calcium waves as solitary chemical excitations. However, there may be another mechanism of communication based on subthreshold oscillations and synchronization. We found that there is a critical value of the IP₃ diffusion rate above which the homogeneous calcium concentration becomes unstable similarly to Turing instability models. In the result low-frequency spontaneous subthreshold oscillations (~0.1 Hz) appear. The oscillations are synchronized with certain modes of phase-locking what provides well-defined timing properties in the astrocyte network signaling controlled by the IP₃ diffusion rate. Biophysical mechanisms of the subthreshold oscillations are based on a balance between locally produced IP₃ through positive PLC-δ feedback and the intercellular IP₃ diffusion. Looking at the details of bifurcation transitions at the edge of the instability we also found a subcritical mechanism of calcium spikes generation when calcium oscillations coexist with the locally stable rest state. Moreover, with further increase of IP₃ diffusion rate spontaneous calcium spikes emerged from the network interaction satisfy the statistics of on-off intermittency and display chaotic dynamics similarly to population bursts events in neurons.

The research is supported by Russian Federal Programs (№ 14.740.11.0075, 16.512.11.2136), MCB RAS and by the Russian President grant МD-5096.2011.2.

Mark van Rossum (School of Informatics, University of Edinburgh)

"Weight dependent synaptic learning rules"

The strength of the synapses in the brain are presumably continuously subject to increases and decreases as the result of ongoing learning processes. This realization allows one to approximate the synaptic weight evolution as a stochastic process. This has been used to find fundamental limits of storage.

Recently we introduced a synaptic information capacity measure based on Shannon information (Barrett and van Rossum '08). We use this to find the optimal weight dependent learning rules. We find that soft-bound learning rules are better than hard bound rules, although the improvement is quite small. Furthermore, we show how feedforward inhibition further increases storage.

Kirill Volynski (Institute of Neurology, University College London)

"Variable presynaptic Ca²⁺ influx determines heterogeneity of release probability among small central synapses"

The probability that a synapse will release neurotransmitter in response to an action potential varies widely, even among synapses supplied by a single axon. The molecular mechanisms underlying this heterogeneity could be the key to target-specific regulation of synaptic strength in central circuits, yet they remain poorly understood. By imaging vesicular exocytosis and Ca²⁺ dynamics at individual presynaptic boutons in cultured hippocampal neurons, we show that the overall neurotransmitter release probability varies among synapses with the magnitude of spike-evoked presynaptic Ca²⁺ entry. We further demonstrate that this is a consequence of a steep dependence of the average fusion probability of release-ready vesicles on the size of the Ca²⁺ concentration transient. Heterogeneity of basal neurotransmitter release at small central synapses supplied by the same axon is thus accounted for not only by the number of release-ready vesicles, but also by their fusion probabilities, which are determined by the variable spike-evoked presynaptic Ca²⁺ influx.