Extracting functionally feed forward networks from a population of spiking neurons

Neuronal avalanches are a ubiquitous form of activity characterized by spontaneous bursts whose size distribution follows a power-law. Recent theoretical models have replicated power-law avalanches by assuming the presence of functionally feedforward connections (FFCs) in the underlying dynamics of the system. Accordingly, avalanches are generated by a feedforward chain of activation that persists despite being embedded in a larger, massively recurrent circuit. However, it is unclear to what extent networks of living neurons that exhibit power-law avalanches rely on FFCs. Here, we employed a computational approach to reconstruct the functional connectivity of cultured cortical neurons plated on multielectrode arrays, and investigated whether pharmacologically-induced alterations in avalanche dynamics are accompanied by changes in FFCs. This approach begins by extracting a functional network of directed links between pairs of neurons, and then evaluates the strength of FFCs using Schur decomposition. In a first step, we examined the ability of this approach to extract FFCs from simulated spiking neurons. The strength of FFCs obtained in strictly feedforward networks diminished monotonically as links were gradually rewired at random. Next, we estimated the FFCs of spontaneously active cortical neuron cultures in the presence of either a control medium, a GABAA receptor antagonist (PTX), or an AMPA receptor antagonist combined with an NMDA receptor antagonist (APV/DNQX). The distribution of avalanche sizes in these cultures was modulated by this pharmacology, with a shallower power-law under PTX (due to the prominence of larger avalanches) and a steeper power-law under APV/DNQX (due to avalanches recruiting fewer neurons) relative to control cultures. The strength of FFCs increased in networks after application of PTX, consistent with an amplification of feed forward activity during avalanches. Conversely, FFCs decreased after application of APV/DNQX, consistent with fading feed forward activation. The observed alterations in FFCs provide experimental support for recent theoretical work linking power-law avalanches to the feed forward organization of functional connections in local neuronal circuits. \ua9 2012 Vincent, Tauskela and Thivierge.Peer reviewed: YesNRC publication: Ye

Preconditioning with 4-aminopyridine protects cerebellar granule neurons against excitotoxicity

Preconditioning by excitatory stimuli such as <i>N</i>-methyl-D-aspartate (NMDA) offers good neuroprotection against excitotoxic insults, but is potentially limited by the risk of damage associated with the treatment. We report here on the potential of an alternative strategy, tested on rat neonatal cerebellar granule neurons, which involves a 48-hour preconditioning step using the potassium channel blocker 4-aminopyridine (4-AP), at a low (50 µM) and at a higher (2500 µM) concentration (in the presence or absence of the GABA<sub>A</sub> receptor antagonist, bicuculline). 4-Aminopyridine gave extensive protection against a number of stressors (glutamate, NMDA and 3-nitropropionic acid) applied 24 hours following the end of the preconditioning period. Blockade of neuronal depolarisation by tetrodotoxin during preconditioning attenuated but did not eliminate protection, whilst co-application with the NMDA receptor blocker MK-801 increased protection. Western blot analysis showed that CREB phosphorylation was significantly increased by the 4-AP preconditioning, although bcl-2 expression was not stimulated. Glutamate induced cell death without significant activation of caspase-3, suggesting that 4-AP preconditioning is effective primarily against necrotic excitotoxicity. Since 4-AP preconditioning affords extensive protection against a range of neurotoxic insults we propose that it could provide the basis for a novel neuroprotective therapy worthy of further investigatio

Altered network communication following a neuroprotective drug treatment

Preconditioning is defined as a range of stimuli that allow cells to withstand subsequent anaerobic and other deleterious conditions. While cell protection under preconditioning is well established, this paper investigates the influence of neuroprotective preconditioning drugs, 4-aminopyridine and bicuculline (4-AP/bic), on synaptic communication across a broad network of in vitro rat cortical neurons. Using a permutation test, we evaluated cross-correlations of extracellular spiking activity across all pairs of recording electrodes on a 64-channel multielectrode array. The resulting functional connectivity maps were analyzed in terms of their graph-theoretic properties. A small-world effect was found, characterized by a functional network with high clustering coefficient and short average path length. Twenty-four hours after exposure to 4-AP/bic, small-world properties were comparable to control cultures that were not treated with the drug. Four hours following drug washout, however, the density of functional connections increased, while path length decreased and clustering coefficient increased. These alterations in functional connectivity were maintained at four days post-washout, suggesting that 4-AP/bic preconditioning leads to long-term effects on functional networks of cortical neurons. Because of their influence on communication efficiency in neuronal networks, alterations in small-world properties hold implications for information processing in brain systems. The observed relationship between density, path length, and clustering coefficient is captured by a phenomenological model where connections are added randomly within a spatially-embedded network. Taken together, results provide information regarding functional consequences of drug therapies that are overlooked in traditional viability studies and present the first investigation of functional networks under neuroprotective preconditioning. \ua9 2013 Vincent et al.Peer reviewed: YesNRC publication: Ye