Formation of Dendritic Spines with GABAergic Synapses Induced by Whisker Stimulation in Adult Mice

AbstractDuring development, alterations in sensory experience modify the structure of cortical neurons, particularly at the level of the dendritic spine. Are similar adaptations involved in plasticity of the adult cortex? Here we show that a 24 hr period of single whisker stimulation in freely moving adult mice increases, by 36%, the total synaptic density in the corresponding cortical barrel. This is due to an increase in both excitatory and inhibitory synapses found on spines. Four days after stimulation, the inhibitory inputs to the spines remain despite total synaptic density returning to pre-stimulation levels. Functional analysis of layer IV cells demonstrated altered response properties, immediately after stimulation, as well as four days later. These results indicate activity-dependent alterations in synaptic circuitry in adulthood, modifying the flow of sensory information into the cerebral cortex

Glial Glutamate Transporters and Maturation of the Mouse Somatosensory Cortex

In the adult nervous system, glutamatergic neurotransmission is tightly controlled by neuron-glia interactions through glial glutamate reuptake by the specific transporters GLT-1 and GLAST. Here, we have explored the role of these transporters in the structural and functional maturation of the somatosensory cortex of the mouse. We provide evidence that GLT-1 and GLAST are early and selectively expressed in barrels from P5 to P10. Confocal and electron microscopy confirm that the expression is restricted to the astroglial membrane. By P12, and despite an increased global expression as observed by immunoblotting, the barrel pattern of GLAST and GLT-1 staining is no longer evident. In P10 GLT-1 −/− and GLAST −/− mice, the cytoarchitectural segregation of the barrels is preserved. However, at P9-10, the functional response to whisker stimulation, measured by deoxyglucose uptake, is markedly decreased in GLT-1 −/− and GLAST −/− mice. The role of GLAST is transient since the metabolic response is already restored at P11-12 in GLAST −/− mice and remains unchanged in adulthood. However, deletion of GLT-1 seems to impair the functional metabolic response until adulthood. Our data suggest that astrocyte-neuron interactions via the glial glutamate transporters are involved in the functional maturation of the whisker representation in the somatosensory corte

Large-Scale Networks for Auditory Sensory Gating in the Awake Mouse

The amplitude of the brain response to a repeated auditory stimulus is diminished as compared to the response to the first tone (T1) for interstimulus intervals (ISI) lasting up to hundreds of milliseconds. This adaptation process, called auditory sensory gating (ASG), is altered in various psychiatric diseases including schizophrenia and is classically studied by focusing on early evoked cortical responses to the second tone (T2) using 500-ms ISI. However, mechanisms underlying ASG are still not well-understood. We investigated ASG in awake mice from the brainstem to cortex at variable ISIs (125-2000 ms) using high-density EEG and intracerebral recordings. While ASG decreases at longer ISIs, it is still present at durations (500-2000 ms) far beyond the time during which brain responses to T1 could still be detected. T1 induces a sequence of specific stable scalp EEG topographies that correspond to the successive activation of distinct neural networks lasting about 350 ms. These brain states remain unaltered if T2 is presented during this period, although T2 is processed by the brain, suggesting that ongoing networks of brain activity are active for longer than early evoked-potentials and are not overwritten by an upcoming new stimulus. Intracerebral recordings demonstrate that ASG is already present at the level of ventral cochlear nucleus (vCN) and inferior colliculus and is amplified across the hierarchy in bottom-up direction. This study uncovers the extended stability of sensory-evoked brain states and long duration of ASG, and sheds light on generators of ASG and possible interactions between bottom-up and top-down mechanisms

Functional Deficit and Recovery of Developing Sensorimotor Networks following Neonatal Hypoxic-Ischemic Injury in the Rat

Neonatal hypoxia-ischemia (HI) is the most important cause of brain injury in the newborn. Here we studied structural alterations and functional perturbations of developing large-scale sensorimotor cortical networks in a rat model of moderate HI at postnatal day 3 (P3). At the morphological level, HI led to a disorganized barrel pattern in the somatosensory cortex without detectable histological changes in the motor cortex. Functional effects were addressed by means of epicranial mapping of somatosensory-evoked potentials (SEPs) during the postischemic recovery period. At P10, SEPs were immature and evoked activity was almost restricted to the somatosensory and motor cortices of the contralateral hemisphere. Peak and topographic analyses of epicranial potentials revealed that responses were profoundly depressed in both sensory and motor areas of HI-lesioned animals. At the end of the postnatal period at P21, responses involved networks in both hemispheres. SEP amplitude was still depressed in the injured sensory region, but it completely recovered in the motor area. These results suggest a process of large-scale network plasticity in sensorimotor circuits after perinatal ischemic injury. The model provides new perspectives for investigating the temporal and spatial characteristics of the recovery process following HI and eventually developing therapeutic intervention

Reproducible network changes occur in a mouse model of temporal lobe epilepsy but do not correlate with disease severity

Studying the development of brain network disruptions in epilepsy is challenged by the paucity of data before epilepsy onset. Here, we used the unilateral, kainate mouse model of hippocampal epilepsy to investigate brain network changes before and after epilepsy onset and their stability across time. Using 32 epicranial electrodes distributed over the mouse hemispheres, we analyzed EEG epochs free from epileptic activity in 15 animals before and 28 days after hippocampal injection (group 1) and in 20 animals on two consecutive days (d28 and d29, group 2). Statistical dependencies between electrodes were characterized with the debiased-weighted phase lag index. We analyzed: a) graph metric changes from baseline to chronic stage (d28) in group 1; b) their reliability across d28 and d29, in group 2; c) their correlation with epileptic activity (EA: seizure, spike and fast-ripple rates), averaged over d28 and d29, in group 2. During the chronic stage, intra-hemispheric connections of the non-injected hemisphere strengthened, yielding an asymmetrical network in low (4–8 Hz) and high theta (8–12 Hz) bands. The contralateral hemisphere also became more integrated and segregated within the high theta band. Both network topology and EEG markers of EA were stable over consecutive days but not correlated with each other. Altogether, we show reproducible large-scale network modifications after the development of focal epilepsy. These modifications are mostly specific to the non-injected hemisphere. The absence of correlation with epileptic activity does not allow to specifically ascribe these network changes to mechanisms supporting EA or rather compensatory inhibition but supports the notion that epilepsy extends beyond the sole repetition of EA and impacts network that might not be involved in EA generation

Plasticity of Astrocytic Coverage and Glutamate Transporter Expression in Adult Mouse Cortex

Astrocytes play a major role in the removal of glutamate from the extracellular compartment. This clearance limits the glutamate receptor activation and affects the synaptic response. This function of the astrocyte is dependent on its positioning around the synapse, as well as on the level of expression of its high-affinity glutamate transporters, GLT1 and GLAST. Using Western blot analysis and serial section electron microscopy, we studied how a change in sensory activity affected these parameters in the adult cortex. Using mice, we found that 24 h of whisker stimulation elicited a 2-fold increase in the expression of GLT1 and GLAST in the corresponding cortical column of the barrel cortex. This returns to basal levels 4 d after the stimulation was stopped, whereas the expression of the neuronal glutamate transporter EAAC1 remained unaltered throughout. Ultrastructural analysis from the same region showed that sensory stimulation also causes a significant increase in the astrocytic envelopment of excitatory synapses on dendritic spines. We conclude that a period of modified neuronal activity and synaptic release of glutamate leads to an increased astrocytic coverage of the bouton–spine interface and an increase in glutamate transporter expression in astrocytic processes

Whole-scalp EEG mapping of somatosensory evoked potentials in macaque monkeys

High-density scalp EEG recordings are widely used to study whole-brain neuronal networks in humans non-invasively. Here, we validate EEG mapping of somatosensory evoked potentials (SSEPs) in macaque monkeys (Macaca fascicularis) for the long-term investigation of large-scale neuronal networks and their reorganisation after lesions requiring a craniotomy. SSEPs were acquired from 33 scalp electrodes in five adult anaesthetized animals after electrical median or tibial nerve stimulation. SSEP scalp potential maps were identified by cluster analysis and identified in individual recordings. A distributed, linear inverse solution was used to estimate the intracortical sources of the scalp potentials. SSEPs were characterised by a sequence of components with unique scalp topographies. Source analysis confirmed that median nerve SSEP component maps were in accordance with the somatotopic organisation of the sensorimotor cortex. Most importantly, SSEP recordings were stable both intra- and interindividually. We aim to apply this method to the study of recovery and reorganisation of large-scale neuronal networks following a focal cortical lesion requiring a craniotomy. As a prerequisite, the present study demonstrated that a 300-mm2 unilateral craniotomy over the sensorimotor cortex necessary to induce a cortical lesion, followed by bone flap repositioning, suture and gap plugging with calcium phosphate cement, did not induce major distortions of the SSEPs. In conclusion, SSEPs can be successfully and reproducibly recorded from high-density EEG caps in macaque monkeys before and after a craniotomy, opening new possibilities for the long-term follow-up of the cortical reorganisation of large-scale networks in macaque monkeys after a cortical lesion