Altered neocortical microcircuitry in the valproic acid rat model of autism

Autism is a wide-spectrum disorder with early childhood onset, affecting general cognitive capabilities and, in particular, complex information processing. Although interesting directions of research start to appear, the precise causes of the disorder and the resulting brain alterations have not been elucidated yet. The goal of this research project is to get a better understanding of the neocortical microcircuitry alterations in autism, using the valproic acid (VPA) rat model of autism. Exposure to VPA can cause several teratogenic effects, including autism in human if exposure occurs during the third week of gestation. Previous studies explored the results of prenatal VPA exposure in rats and found similar gross abnormalities to those in autism, such as diminished number of Purkinje cells. Behavioural studies further showed that a number of core symptoms, such as impairment in social interactions and higher sensitivity to sensory stimulation, are also present in the VPA-treated rats. We examined the postnatal effects of embryonic exposure to VPA on the microcircuitry properties of juvenile rat neocortex using in vitro electrophysiology. We found that prenatal VPA injection causes a significant increase of more than 50% of the local connectivity formed by pyramidal neurons of the somatosensory cortex, and that the neocortical network becomes hyper-reactive to external stimuli. We identify two possible compensatory mechanisms that may follow the observed hyperconnectivity: reduced excitability of pyramidal neurons and a decreased magnitude for individual synaptic connections. We also illustrate how NMDA receptors are significantly enhanced in the rat model and how this enhancement is associated with an increased plasticity in the neocortex. Both the somatosensorial and prefrontal cortex exhibit increased plasticity, suggesting a general enhanced plasticity in the brains of VPA-exposed rats. In addition, we show that multi-electrode array stimulation of the lateral amygdala reveals a hyper-reactive amygdala with increased synaptic plasticity. These alterations may account for some of the core symptoms in autism spectrum disorders. For example, hyperconnectivity could underlie symptoms such as aberrant reactions to sensory stimulation and altered attention. Also, the enhancement of NMDA mediated transmission and increased plasticity could explain the unusual learning and memory capabilities of some autistic children. Finally, we propose that the hyper-reactive and hyper-plastic amygdala underlies abnormal fear processing in this animal model of autism. We further speculate that abnormal fear processing could be a core pathology in autism which may give rise to behavioural symptoms, such as impairments in social interactions and resistance to rehabilitation.

Hyper-connectivity and hyper-plasticity in the medial prefrontal cortex in the valproic Acid animal model of autism

Henry Markram, Catherine Perrodin, Tania Rinaldi

The prefrontal cortex has been extensively implicated in autism to explain deficits in executive and other higher-order functions related to cognition, language, sociability and emotion. The possible changes at the level of the neuronal microcircuit are however not known. We studied microcircuit alterations in the prefrontal cortex in the valproic acid rat model of autism and found that the layer 5 pyramidal neurons are connected to significantly more neighbouring neurons than in controls. These excitatory connections are more plastic displaying enhanced long-term potentiation of the strength of synapses. The microcircuit alterations found in the prefrontal cortex are therefore similar to the alterations previously found in the somatosensory cortex. Hyper-connectivity and hyper-plasticity in the prefrontal cortex implies hyper-functionality of one of the highest order processing regions in the brain, and stands in contrast to the hypo-functionality that is normally proposed in this region to explain some of the autistic symptoms. We propose that a number of deficits in autism such as sociability, attention, multi-tasking and repetitive behaviours, should be re-interpreted in the light of a hyper-functional prefrontal cortex.

2008

Network Activity and Plasticity

Vincent Delattre

The cortex must maintain balanced levels of neural activity to correctly integrate inputs and to provide contextually meaningful outputs. Neuronal excitation is counterbalanced by various forms of inhibition such as spike frequency adaptation, short- and long-term depression at excitatory synapses, and inhibitory cell coupling. When excitatory activity is low, homeostatic mechanisms enhance network excitability. When excitation is stronger than inhibition, network bursts of activity arise that also need to be dampened lest they run out of control. A disruption of the fine balance between excitatory and inhibitory systems is observed in an increasing numbers of brain disorders such as epilepsy, mental retardation and autism. This thesis aims to study aspects of the regulation of excitation in cortical networks at a molecular, cellular, network and disease level of investigation. We present in the first chapter an investigation of the impact of network bursts on standard spike-timing dependent plasticity (STDP) events in acute cortical brain slices. We discovered that these two kinds of events, which are the most well-established ways to induce synaptic plasticity in vitro, can block each other as well as cooperate to change the amplitude and direction of plasticity. Because the outcome of the interaction depends on the precise timing of network bursts relative to a STDP event in a single synaptic pathway, the observed network-timing dependent plasticity provides a novel mechanism for the modulation of local synaptic plasticity by the network level of activity, a form of global control of local synaptic plasticity. We hypothesized that network bursts consume extracellular resources needed to potentiate, restricting potentiation to active synapses at the burst onset while yielding depression of synapses activated at later times. As we demonstrate with simulation, this simple mechanism supports paradigm-shifting implications for models of learning, solving a persistent and fundamental problem since BCM (Bienenstock Cooper Munro, 1981) to incorporate synaptic learning into realistic neuronal networks while simultaneously establishing and maintaining excitatory-inhibitory balance. The proposed mechanism also links experimental evidence for interactions between STDP and network activity to observations of self-organized criticality in neural systems, a regime known to be optimal for information coding. In the second chapter, we investigated the effect of the loss of NLGN4 protein on synapse function and on network response to electrical stimulation. The neuroligin family of genes encodes for trans-synaptic proteins involved in synaptic differentiation and maturation. Former studies have reported that both NLGN1-KO and NLGN2-KO give rise to pathologic deregulations of the balance between excitation and inhibition in neural networks, and the presence of loss-of-function mutations in NLGN4 in a small number of autistic patients indicates a possible link between Neuroligin-4 gene and autism. Using extra-cellular stimulation and intra-cellular recordings in acute brain slices, we observed a decreased response of both excitatory and inhibitory response in NLGN4-KO animals, suggesting that NLGN4 is expressed at both excitatory and inhibitory synapses. Furthermore, we observed that the NLGN4-KO resulted in a significantly larger decrease of the excitatory response than of the inhibitory response, thus yielding a hypo-excitable network. The final chapter consists of the characterization of excitatory pathways in acute slices of the somato-sensory cortex. In the first study, we used chemical stimulation to promote the emergence of spontaneous recurrent activity and observed waves of activity originating from the cortical layer five. In the second study, we performed whole-cell patch-clamping of layer five pyramidal cells along with recording local field potentials (LFP). With the systematic layer-wise application of electrical stimulation at various frequencies coupled with LFP recordings, we obtained a frequency-dependent mapping of cortical inputs at all cortical layers at both the cellular and the network levels. Using this data, we developed a virtual framework for performing an analogous mapping in the virtual cortical column of the Blue Brain Project (BBP). This helped us understand the contribution of each cell population in the network response to stimulation. The framework also served as a validation test for the BBP cortical models.

EPFL2013

High Bandwidth Synaptic Communication and Frequency Tracking in Human Neocortex

Michele Giugliano

Neuronal firing, synaptic transmission, and its plasticity form the building blocks for processing and storage of information in the brain. It is unknown whether adult human synapses are more efficient in transferring information between neurons than rodent synapses. To test this, we recorded from connected pairs of pyramidal neurons in acute brain slices of adult human and mouse temporal cortex and probed the dynamical properties of use-dependent plasticity. We found that human synaptic connections were purely depressing and that they recovered three to four times more swiftly from depression than synapses in rodent neocortex. Thereby, during realistic spike trains, the temporal resolution of synaptic information exchange in human synapses substantially surpasses that in mice. Using information theory, we calculate that information transfer between human pyramidal neurons exceeds that of mouse pyramidal neurons by four to nine times, well into the beta and gamma frequency range. In addition, we found that human principal cells tracked fine temporal features, conveyed in received synaptic inputs, at a wider bandwidth than for rodents. Action potential firing probability was reliably phase-locked to input transients up to 1,000 cycles/s because of a steep onset of action potentials in human pyramidal neurons during spike trains, unlike in rodent neurons. Our data show that, in contrast to the widely held views of limited information transfer in rodent depressing synapses, fast recovering synapses of human neurons can actually transfer substantial amounts of information during spike trains. In addition, human pyramidal neurons are equipped to encode high synaptic information content. Thus, adult human cortical microcircuits relay information at a wider bandwidth than rodent microcircuits.

Public Library Science2014

Network Activity and Plasticity

Vincent Delattre

EPFL2013