Estimating the Readily-Releasable Vesicle Pool Size at Synaptic Connections in the Neocortex

Previous studies based on the 'Quantal Model' for synaptic transmission suggest that neurotransmitter release is mediated by a single release site at individual synaptic contacts in the neocortex. However, recent studies seem to contradict this hypothesis and indicate that multi-vesicular release (MVR) could better explain the synaptic response variability observed in vitro. In this study we present a novel method to estimate the number of release sites per synapse, also known as the size of the readily releasable pool (N-RRP), from paired whole-cell recordings of connections between layer 5 thick tufted pyramidal cell (L5_TTPC) in the juvenile rat somatosensory cortex. Our approach extends the work of Loebel et al. (2009) by leveraging a recently published data-driven biophysical model of neocortical tissue. Using this approach, we estimated N-RRP to be between two to three for synaptic connections between L5_TTPCs. To constrain N-RRP values for other connections in the microcircuit, we developed and validated a generalization approach using published data on the coefficient of variation (CV) of the amplitudes of post-synaptic potentials (PSPs) from literature and comparing them against in silico experiments. Our study predicts that transmitter release at synaptic connections in the neocortex could be mediated by MVR and provides a data-driven approach to constrain the MVR model parameters in the microcircuit.

Cellular, Synaptic and Network Effects of Acetylcholine in the Neocortex

Cristina Colangelo, Daniel Keller, Henry Markram, Srikanth Ramaswamy, Polina Shichkova

The neocortex is densely innervated by basal forebrain (BF) cholinergic neurons. Long-range axons of cholinergic neurons regulate higher-order cognitive function and dysfunction in the neocortex by releasing acetylcholine (ACh). ACh release dynamically reconfigures neocortical microcircuitry through differential spatiotemporal actions on cell-types and their synaptic connections. At the cellular level, ACh release controls neuronal excitability and firing rate, by hyperpolarizing or depolarizing target neurons. At the synaptic level, ACh impacts transmission dynamics not only by altering the presynaptic probability of release, but also the magnitude of the postsynaptic response. Despite the crucial role of ACh release in physiology and pathophysiology, a comprehensive understanding of the way it regulates the activity of diverse neocortical cell-types and synaptic connections has remained elusive. This review aims to summarize the state-of-the-art anatomical and physiological data to develop a functional map of the cellular, synaptic and microcircuit effects of ACh in the neocortex of rodents and non-human primates, and to serve as a quantitative reference for those intending to build data-driven computational models on the role of ACh in governing brain states.

FRONTIERS MEDIA SA2019

Structural basis and biophysical properties of synaptic plasticity studied by atomic force microscopy

Alexandre Yersin

In this thesis, we studied two systems important for synaptic plasticity, one presynaptic and another postsynaptic. The protein complex composed of VAMP 2, SNAP-25 and syntaxin 1 (SNARE complex) is essential for docking and fusion of neurotransmitter-filled synaptic vesicles with the presynaptic membrane. In order to better understand the fusion mechanisms, we reconstituted the synaptic SNARE complex in the imaging chamber of an atomic force microscope (AFM) and measured the interaction forces between its components. Each protein was tested against the two others, taken either individually or as binary complexes. This approach allowed us to determine specific interaction forces, dissociation kinetics of the SNAREs and led us to propose a sequence of formation for the complex. A theoretical model based on our measurements suggests that a minimum of four complexes is probably necessary for fusion to occur. We also showed that the regulatory protein nSec1 injected into the AFM chamber prevented the SNARE complex formation. Finally we measured the effect of tetanus toxin protease (TeTx) on the SNARE complex and its activity by on-line registration during TeTx injection. These experiments reveal that AFM is a valuable tool to investigate complicate interactions among a protein complex containing up to four components. Trafficking of glutamate receptors AMPA is closely related to postsynaptic induction of long-term potentiation. In a second part of this thesis, we used AFM to explore the distribution of AMPA receptors on-line at the surface of living cells in correlation with cellular viscoelastic properties. First, we showed that AFM permits to detect specifically AMPA receptors at the surface of living neurons and we proved that the specificity of detection is stable during time. Moreover, we were able to visualize variations in AMPA receptor density on surface in response to different stimulations. Finally, AFM also allowed us to study local viscoelastic properties of the cell and their modifications occurring during AMPA receptor trafficking. Our measurements reveal that AMPA receptors are situated at the cell surface on local stiffness maxima. We have noticed that an excitatory stimulation removes these stiffness maxima before producing internalization of receptors. Our results suggest also that AMPA receptors may be constituted of two subpopulations depending on the cell stiffness. A "hard" subpopulation may be preferentially internalized after an excitatory stimulation whereas a "soft" one seems to remain at the cell surface.

EPFL2004

Generalized Bienenstock-Cooper-Munro rule for spiking neurons that maximizes information transmission

Wulfram Gerstner, Jean-Pascal Théodor Pfister, Taro Toyoizumi

Maximization of information transmission by a spiking-neuron model predicts changes of synaptic connections that depend on timing of pre- and postsynaptic spikes and on the postsynaptic membrane potential. Under the assumption of Poisson firing statistics, the synaptic update rule exhibits all of the features of the Bienenstock-Cooper-Munro rule, in particular, regimes of synaptic potentiation and depression separated by a sliding threshold. Moreover, the learning rule is also applicable to the more realistic case of neuron models with refractoriness, and is sensitive to correlations between input spikes, even in the absence of presynaptic rate modulation. The learning rule is found by maximizing the mutual information between presynaptic and postsynaptic spike trains under the constraint that the postsynaptic firing rate stays close to some target firing rate. An interpretation of the synaptic update rule in terms of homeostatic synaptic processes and spike-timing-dependent plasticity is discussed.

2005

Structural basis and biophysical properties of synaptic plasticity studied by atomic force microscopy

Alexandre Yersin

EPFL2004