The gephyrin scaffold modulates cortical layer 2/3 pyramidal neuron responsiveness to single whisker stimulation

Gephyrin is the main scaffolding protein at inhibitory postsynaptic sites, and its clusters are the signaling hubs where several molecular pathways converge. Post-translational modifications (PTMs) of gephyrin alter GABAA receptor clustering at the synapse, but it is unclear how this affects neuronal activity at the circuit level. We assessed the contribution of gephyrin PTMs to microcircuit activity in the mouse barrel cortex by slice electrophysiology and in vivo two-photon calcium imaging of layer 2/3 (L2/3) pyramidal cells during single-whisker stimulation. Our results suggest that, depending on the type of gephyrin PTM, the neuronal activities of L2/3 pyramidal neurons can be differentially modulated, leading to changes in the size of the neuronal population responding to the single-whisker stimulation. Furthermore, we show that gephyrin PTMs have their preference for selecting synaptic GABAA receptor subunits. Our results identify an important role of gephyrin and GABAergic postsynaptic sites for cortical microcircuit function during sensory stimulation.

Cortex-restricted deletion of Foxp1 impairs barrel formation and induces aberrant tactile responses in a mouse model of autism

BACKGROUND

Many children and young people with autism spectrum disorder (ASD) display touch defensiveness or avoidance (hypersensitivity), or engage in sensory seeking by touching people or objects (hyposensitivity). Abnormal sensory responses have also been noticed in mice lacking ASD-associated genes. Tactile sensory information is normally processed by the somatosensory system that travels along the thalamus to the primary somatosensory cortex. The neurobiology behind tactile sensory abnormalities, however, is not fully understood.

METHODS

We employed cortex-specific Foxp1 knockout (Foxp1-cKO) mice as a model of autism in this study. Tactile sensory deficits were measured by the adhesive removal test. The mice's behavior and neural activity were further evaluated by the whisker nuisance test and c-Fos immunofluorescence, respectively. We also studied the dendritic spines and barrel formation in the primary somatosensory cortex by Golgi staining and immunofluorescence.

RESULTS

Foxp1-cKO mice had a deferred response to the tactile environment. However, the mice exhibited avoidance behavior and hyper-reaction following repeated whisker stimulation, similar to a fight-or-flight response. In contrast to the wild-type, c-Fos was activated in the basolateral amygdala but not in layer IV of the primary somatosensory cortex of the cKO mice. Moreover, Foxp1 deficiency in cortical neurons altered the dendrite development, reduced the number of dendritic spines, and disrupted barrel formation in the somatosensory cortex, suggesting impaired somatosensory processing may underlie the aberrant tactile responses.

LIMITATIONS

It is still unclear how the defective thalamocortical connection gives rise to the hyper-reactive response. Future experiments with electrophysiological recording are needed to analyze the role of thalamo-cortical-amygdala circuits in the disinhibiting amygdala and enhanced fearful responses in the mouse model of autism.

CONCLUSIONS

Foxp1-cKO mice have tactile sensory deficits while exhibit hyper-reactivity, which may represent fearful and emotional responses controlled by the amygdala. This study presents anatomical evidence for reduced thalamocortical connectivity in a genetic mouse model of ASD and demonstrates that the cerebral cortex can be the origin of atypical sensory behaviors.

A model of lateral interactions as the origin of multiwhisker receptive fields in rat barrel cortex

While cells within barrel cortex respond primarily to deflections of their principal whisker (PW), they also exhibit responses to non-principal, or adjacent, whiskers (AWs), albeit responses with diminished amplitudes and longer latencies. The origin of multiwhisker receptive fields of barrel cells remains a point of controversy within the experimental literature, with three contending possibilities: (i) barrel cells inherit their AW responses from the AW responses of thalamocortical (TC) cells within their aligned barreloid; (ii) the axons of TC cells within a barreloid ramify to innervate multiple barrels, rather than only terminating within their aligned barrel; (iii) lateral intracortical transmission between barrels conveys AW responsivity to barrel cells. In this work, we develop a detailed, biologically plausible model of multiple barrels in order to examine possibility (iii); in order to isolate the dynamics that possibility (iii) entails, we incorporate lateral connections between barrels while assuming that TC cells respond only to their PW and that TC cell axons are confined to their home barrel. We show that our model is capable of capturing a broad swath of experimental observations on multiwhisker receptive field dynamics within barrels, and we compare and contrast the dynamics of this model with model dynamics from prior work in which employ a similar general modeling strategy to examine possibility (i).

Learning-induced plasticity in the barrel cortex is disrupted by inhibition of layer 4 somatostatin-containing interneurons

Gaba-ergic neurons are a diverse cell class with extensive influence over cortical processing, but their role in experience-dependent plasticity is not completely understood. Here we addressed the role of cortical somatostatin- (SOM-INs) and vasoactive intestinal polypeptide- (VIP-INs) containing interneurons in a Pavlovian conditioning where stimulation of the vibrissae is used as a conditioned stimulus and tail shock as unconditioned one. This procedure induces a plastic change observed as an enlargement of the cortical functional representation of vibrissae activated during conditioning. Using layer-targeted, cell-selective DREADD transductions, we examined the involvement of SOM-INs and VIP-INs activity in learning-related plastic changes. Under optical recordings, we injected DREADD-expressing vectors into layer IV (L4) barrels or layer II/III (L2/3) areas corresponding to the activated vibrissae. The activity of the interneurons was modulated during all conditioning sessions, and functional 2-deoxyglucose (2DG) maps were obtained 24 h after the last session. In mice with L4 but not L2/3 SOM-INs suppressed during conditioning, the plastic change of whisker representation was absent. The behavioral effect of conditioning was disturbed. Both L4 SOM-INs excitation and L2/3 VIP-INs inhibition during conditioning did not affect the plasticity or the conditioned response. We found the activity of L4 SOM-INs is indispensable in the formation of learning-induced plastic change. We propose that L4 SOM-INs may provide disinhibition by blocking L4 parvalbumin interneurons, allowing a flow of information into upper cortical layers during learning.

Transplantation of rat dental pulp stem cells facilities post-lesion recovery in the somatosensory whisker cortex of male Wistar rats

Damage to somatosensory "barrel" cortex reduces the rats' behavioral sensitivity in discrimination of tactile stimuli. Here, we examined how transplantation of stem cells into the lesioned barrel cortex can help in recovery of sensory capacities. We induced mechanical lesions in the right barrel cortex area of male rats. Three days after lesioning, rats received one of three transplantation types: un-differentiated dental pulp stem cells (U-DPSCs) or differentiated dental pulp stem cells (D-DPSCs), or cell medium (vehicle). A fourth group of rats were control without any Surgery. For 4 consecutive weeks, starting one week after transplantation, we evaluated the rats' preference to explore novel textures as a measure of sensory discrimination ability, also measured the expression of glial fibrillary acidic protein (GFAP), Olig 2, nestin, neuronal nuclei (NeuN), brain-derived neurotrophic factor (BDNF) and neuroligin1 by immunohistochemistry and western blotting. Unilateral mechanical lesion decreased the rats' preferential exploration of novel textures compared to the control group across the 4-week behavioral tests. Following stem cell therapy, the rats' performance significantly improved at week 2-4 compared to the vehicle group. Compared to the control group, there was a significant decrease in the expression of nestin, NeuN, Olig 2, BDNF, neuroligin1 and a significant increase in the expression of GFAP in the vehicle group. The expression of the neural markers was significantly higher in DPSCs compared with the vehicle group whereas GFAP level was lower in DPSCs compared to vehicle. We found that DPSCs therapy affected a range of neuronal markers in the barrel cortex post lesion, and improved the rats' recovery for sensory discrimination.

Stem cell restores thalamocortical plasticity to rescue cognitive deficit in neonatal intraventricular hemorrhage

Severe neonatal intraventricular hemorrhage (IVH) patients incur long-term neurologic deficits such as cognitive disabilities. Recently, the intraventricular transplantation of allogeneic human umbilical cord blood-derived mesenchymal stem cells (MSCs) has drawn attention as a therapeutic potential to treat severe IVH. However, its pathological synaptic mechanism is still elusive. We here demonstrated that the integration of the somatosensory input was significantly distorted by suppressing feed-forward inhibition (FFI) at the thalamocortical (TC) inputs in the barrel cortices of neonatal rats with IVH by using BOLD-fMRI signal and brain slice patch-clamp technique. This is induced by the suppression of Hebbian plasticity via an increase in tumor necrosis factor-α expression during the critical period, which can be effectively reversed by the transplantation of MSCs. Furthermore, we showed that MSC transplantation successfully rescued IVH-induced learning deficits in the sensory-guided decision-making in correlation with TC FFI in the layer 4 barrel cortex.

Loss of Calretinin in L5a impairs the formation of the barrel cortex leading to abnormal whisker-mediated behaviors

The rodent whisker-barrel cortex system has been established as an ideal model for studying sensory information integration. The barrel cortex consists of barrel and septa columns that receive information input from the lemniscal and paralemniscal pathways, respectively. Layer 5a is involved in both barrel and septa circuits and play a key role in information integration. However, the role of layer 5a in the development of the barrel cortex remains unclear. Previously, we found that calretinin is dynamically expressed in layer 5a. In this study, we analyzed calretinin KO mice and found that the dendritic complexity and length of layer 5a pyramidal neurons were significantly decreased after calretinin ablation. The membrane excitability and excitatory synaptic transmission of layer 5a neurons were increased. Consequently, the organization of the barrels was impaired. Moreover, layer 4 spiny stellate cells were not able to properly gather, leading to abnormal formation of barrel walls as the ratio of barrel/septum size obviously decreased. Calretinin KO mice exhibited deficits in exploratory and whisker-associated tactile behaviors as well as social novelty preference. Our study expands our knowledge of layer 5a pyramidal neurons in the formation of barrel walls and deepens the understanding of the development of the whisker-barrel cortex system.

Sensing and processing whisker deflections in rodents

The classical view of sensory information mainly flowing into barrel cortex at layer IV, moving up for complex feature processing and lateral interactions in layers II and III, then down to layers V and VI for output and corticothalamic feedback is becoming increasingly undermined by new evidence. We review the neurophysiology of sensing and processing whisker deflections, emphasizing the general processing and organisational principles present along the entire sensory pathway—from the site of physical deflection at the whiskers to the encoding of deflections in the barrel cortex. Many of these principles support the classical view. However, we also highlight the growing number of exceptions to these general principles, which complexify the system and which investigators should be mindful of when interpreting their results. We identify gaps in the literature for experimentalists and theorists to investigate, not just to better understand whisker sensation but also to better understand sensory and cortical processing.

Multimodal Functional Analysis Platform: 4. Optogenetics-Induced Oscillatory Activation to Explore Neural Circuits

To elucidate neural mechanisms underlying oscillatory phenomena in brain function, we have developed optogenetic tools and statistical methods. Specifically, opto-current-clamp induced oscillation reveals intrinsic frequency preferences in the neural circuits by oscillatory resonance. Furthermore, resonance or entrainment to intrinsic frequency is state-dependent. When resonance phenomena go beyond a certain range, it could even induce epileptic seizure in highly reproducible manner. We are able to study how seizures start, develop, and stop in neural circuits. Therefore, the optogenetics-induced oscillatory activation is a powerful tool in neuroscience research.

Elucidating mechanisms of neuronal circuit formation in layer 4 of the somatosensory cortex via intravital imaging

The cerebral cortex has complex yet perfectly wired neuronal circuits that are important for high-level brain functions such as perception and cognition. The rodent's somatosensory system is widely used for understanding the mechanisms of circuit formation during early developmental periods. In this review, we summarize the developmental processes of circuit formation in layer 4 of the somatosensory cortex, and we describe the molecules involved in layer 4 circuit formation and neuronal activity-dependent mechanisms of circuit formation. We also introduce the dynamic mechanisms of circuit formation in layer 4 revealed by intravital two-photon imaging technologies, which include time-lapse imaging of neuronal morphology and calcium imaging of neuronal activity in newborn mice.

Increased intrinsic excitability and decreased synaptic inhibition in aged somatosensory cortex pyramidal neurons

Sensorimotor performance declines during advanced age, partially due to deficits in somatosensory acuity. Cortical receptive field expansion contributes to somatosensory deficits, suggesting increased excitability or decreased inhibition in primary somatosensory cortex (S1) pyramidal neurons. To ascertain changes in excitability and inhibition, we measured both properties in neurons from vibrissal S1 in brain slices from young and aged mice. Because adapting and non-adapting neurons-the principal pyramidal types in layer 5 (L5)-differ in intrinsic properties and inhibitory inputs, we determined age-dependent changes according to neuron type. We found an age-dependent increase in intrinsic excitability in adapting neurons, caused by a decrease in action potential threshold. Surprisingly, in non-adapting neurons we found both an increase in excitability caused by increased input resistance, and a decrease in synaptic inhibition. Spike frequency adaptation, already small in non-adapting neurons, was further reduced by aging, whereas sag, a manifestation of I_h, was increased. Therefore, aging caused both decreased inhibition and increased intrinsic excitability, but these effects were specific to pyramidal neuron type.

Neurovascular coupling preserved in a chronic mouse model of Alzheimer's disease: Methodology is critical

Impaired neurovascular coupling has been suggested as an early pathogenic factor in Alzheimer's disease (AD), which could serve as an early biomarker of cerebral pathology. We have established an anaesthetic regime to allow repeated measurements of neurovascular function over three months in the J20 mouse model of AD (J20-AD) and wild-type (WT) controls. Animals were 9-12 months old at the start of the experiment. Mice were chronically prepared with a cranial window through which 2-Dimensional optical imaging spectroscopy (2D-OIS) was used to generate functional maps of the cerebral blood volume and saturation changes evoked by whisker stimulation and vascular reactivity challenges. Unexpectedly, the hemodynamic responses were largely preserved in the J20-AD group. This result failed to confirm previous investigations using the J20-AD model. However, a final acute electrophysiology and 2D-OIS experiment was performed to measure both neural and hemodynamic responses concurrently. In this experiment, previously reported deficits in neurovascular coupling in the J20-AD model were observed. This suggests that J20-AD mice may be more susceptible to the physiologically stressing conditions of an acute experimental procedure compared to WT animals. These results therefore highlight the importance of experimental procedure when determining the characteristics of animal models of human disease.

Direct Activation of Cortical Neurons in the Primary Somatosensory Cortex of the Rat in Vivo Using Focused Ultrasound

We address the recent controversy over whether focused ultrasound (FUS) activates cortical neurons directly or indirectly by initially activating auditory pathways. We obtained two types of evidence that FUS can directly activate cortical neurons. The depth profile of the local field potential (LFP) in the barrel cortex of the rat in vivo indicated a generator was located within the cortical gray matter. The onset and peak latencies of the initial component p1 were 3.2 ± 0.25 ms (mean ± standard error of the mean) and 7.6 ± 0.12 ms, respectively, for the direct cortical response (DCR), 6.8 ± 0.40 and 14.3 ± 0.54 ms for the FUS-evoked LFP (4 MHz, 3.2 MPa, 50 or 300 µs/pulse, 1-20 pulses at 1 kHz) and 6.9 ± 0.51 and 15.8 ± 0.94 ms for the LFP evoked by 1-ms deflection of the C2 whisker projecting to the same area. The peak latency of the FUS p1 was statistically (t-test) longer than the DCR, but shorter than the whisker p1 at p < 0.005.

In vivo two-photon imaging of neuronal and brain vascular responses in mice chronically exposed to ethanol

The effects of ethanol on brain function have been extensively studied using a variety of in vitro and in vivo techniques. For example, electrophysiological studies using brain slices from rodents and non-human primates have demonstrated that acute and chronic exposure to ethanol alters the intrinsic excitability and synaptic signaling of neurons within cortical and sub-cortical areas of the brain. In humans, neuroimaging studies reveal alterations in measures of brain activation and connectivity in subjects with alcohol use disorder. While complementary, these methods are inherently limited due to issues related to either disruption of normal sensory input (in vitro slice studies) or resolution (whole brain imaging). In the present study, we used 2-photon laser scanning microscopy in intact animals to assess the impact of chronic ethanol exposure on sensory-evoked neuronal and vascular responses. Adult male C57BL/6J mice were exposed to four weekly cycles of chronic intermittent ethanol (CIE) exposure, while control mice were exposed to air. After withdrawal (≥72 h), a cranial window was placed over the primary visual cortex (V1), and sensory-evoked responses were monitored using the calcium indicator OGB-1. CIE exposure produced small but significant changes in response amplitude (decrease) and orientation selectivity of V1 neurons (increase). While arteriole diameter did not differ between control and CIE mice under baseline conditions, sensory-evoked dilation was enhanced in vessels from CIE-exposed mice as compared to controls. This was accompanied by a reduced latency in response to stimulation. In separate experiments, pial arteriole diameter was measured in the barrel cortex of control and CIE-exposed mice. Baseline diameter of barrel cortex arterioles was similar between control and CIE-exposed mice, but unlike vessels in V1, sensory-evoked dilation of barrel cortex arterioles was similar between the two groups. Together, the results of these studies suggest that chronic exposure to alcohol induces changes in neurovascular coupling that are region-dependent.

miRNA-324/-133a essential for recruiting new synapse innervations and associative memory cells in coactivated sensory cortices

After the integrative storage of associated signals, a signal induces the recollection of its associated signal, or the other way around. This associative memory is essential to associative thinking, logical reasoning, imagination and computation. In terms of cellular mechanisms underlying associative memory, new mutual synapse innervations are formed among those coactivated neurons, so that they are recruited to be associative memory cells or associative memory neurons. These associative memory cells receive new synapse innervations alongside innate synapse inputs and encode signals carried by these inputs. We proposed to examine microRNAs as initiative factors for recruiting new synapse innervations and associative memory cells. In a mouse model of associative memory characterized as the reciprocal retrieval of associated whisker and odor signals, barrel and piriform cortical neurons gain their ability to encode whisker and odorant signals based on the newly formed synapse innervations between these coactivated cortices besides innate synapse inputs. miRNA-324 and miRNA-133a are required for recruiting these new synapse innervations and associative memory cells as well as sufficient for facilitating their recruitments, but not for innate synapse inputs. Therefore, the coactivation of sensory cortices through microRNA as initiative factor to recruit new mutual synapse innervations and associative memory cells for associative memory.

Visualizing cortical response to optogenetic stimulation and sensory inputs using multispectral handheld optoacoustic imaging

To date, the vast majority of intra-vital neuroimaging systems applied in clinic and diagnostics is stationary with a rigid scanning element, requires specialized facilities and costly infrastructure. Here, we describe a simple yet radical approach for optoacoustic (photoacoustic) brain imaging in vivo using a light-weight handheld probe. It enables multispectral video-rate visualization of hemoglobin gradient changes in the cortex of adult rats induced by whisker and forelimb sensory inputs, as well as by optogenetic stimulation of intra-cortical connections. With superb penetration and molecular specificity, described here in method holds major promises for future applications in research, routine ambulatory neuroimaging, and diagnostics.

Recommendations for measuring whisker movements and locomotion in mice with sensory, motor and cognitive deficits

BACKGROUND

Previous studies have measured whisker movements and locomotion to characterise mouse models of neurodegenerative disease. However, these studies have always been completed in isolation, and do not involve standardized procedures for comparisons across multiple mouse models and background strains.

NEW METHOD

We present a standard method for conducting whisker movement and locomotion studies, by carrying out qualitative scoring and quantitative measurement of whisker movements from high-speed video footage of mouse models of Amyotrophic Lateral Sclerosis, Huntington's disease, Parkinson's disease, Alzheimer's disease, Cerebellar Ataxia, Somatosensory Cortex Development and Ischemic stroke.

RESULTS

Sex, background strain, source breeder and genotype all affected whisker movements. All mouse models, apart from Parkinson's disease, revealed differences in whisker movements during locomotion. R6/2 CAG250 Huntington's disease mice had the strongest behavioural phenotype. Robo3R^3-5-CKO and RIM-DKOSert mouse models have abnormal somatosensory cortex development and revealed significant changes in whisker movements during object exploration.

COMPARISON WITH EXISTING METHOD(S)

Our results have good agreement with past studies, which indicates the robustness and reliability of measuring whisking. We recommend that differences in whisker movements of mice with motor deficits can be captured in open field arenas, but that mice with impairments to sensory or cognitive functioning should also be filmed investigating objects. Scoring clips qualitatively before tracking will help to structure later analyses.

CONCLUSIONS

Studying whisker movements provides a quantitative measure of sensing, motor control and exploration. However, the effect of background strain, sex and age on whisker movements needs to be better understood.

Somatostatin receptors (SSTR1-5) on inhibitory interneurons in the barrel cortex

Inhibitory interneurons in the cerebral cortex contain specific proteins or peptides characteristic for a certain interneuron subtype. In mice, three biochemical markers constitute non-overlapping interneuron populations, which account for 80–90% of all inhibitory cells. These interneurons express parvalbumin (PV), somatostatin (SST), or vasoactive intestinal peptide (VIP). SST is not only a marker of a specific interneuron subtype, but also an important neuropeptide that participates in numerous biochemical and signalling pathways in the brain via somatostatin receptors (SSTR1-5). In the nervous system, SST acts as a neuromodulator and neurotransmitter affecting, among others, memory, learning, and mood. In the sensory cortex, the co-localisation of GABA and SST is found in approximately 30% of interneurons. Considering the importance of interactions between inhibitory interneurons in cortical plasticity and the possible GABA and SST co-release, it seems important to investigate the localisation of different SSTRs on cortical interneurons. Here, we examined the distribution of SSTR1-5 on barrel cortex interneurons containing PV, SST, or VIP. Immunofluorescent staining using specific antibodies was performed on brain sections from transgenic mice that expressed red fluorescence in one specific interneuron subtype (PV-Ai14, SST-Ai14, and VIP-Ai14 mice). SSTRs expression on PV, SST, and VIP interneurons varied among the cortical layers and we found two patterns of SSTRs distribution in L4 of barrel cortex. We also demonstrated that, in contrast to other interneurons, PV cells did not express SSTR2, but expressed other SSTRs. SST interneurons, which were not found to make chemical synapses among themselves, expressed all five SSTR subtypes.

Comparison of pre-pulse inhibition, tactile discrimination learning and barrel cortical neural response in adult male rats following chronic exposure to morphine, methadone and buprenorphine

Chronic exposure to opioids is the most common treatment plan to reduce the pain. In this study, the stereotyped behaviors and cognitive functions related to different types of tactile and auditory inputs were investigated in the rats following chronic exposure to the morphine, methadone, and buprenorphine. Here, three addicted groups received morphine, methadone, and buprenorphine while the control rats received saline for 21 days. Our results demonstrated that the opioid-treated groups showed stereotyped behaviors including grooming and rearing. In the behavioral level, prepulse inhibition and preference indices were not changed significantly in the opioids-treated groups compared to those of the saline group as two criteria for acoustic startle reflex and tactile discrimination, respectively. In the neuronal level, chronic morphine and methadone treatment changed the response properties of the barrel cortical neurons to the whisker deflections in the experimental groups compared to the saline group. Thus, it was concluded that the excitatory receptive fields of neurons in the barrel cortex can be changed as a result of chronic exposure to morphine and methadone.

Effects of enriched housing on the neuronal morphology of mice that lack zinc transporter 3 (ZnT3) and vesicular zinc

In the central nervous system, certain neurons store zinc within the synaptic vesicles of their axon terminals. This vesicular zinc can then be released in an activity-dependent fashion as an intercellular signal. The functions of vesicular zinc are not entirely understood, but evidence suggests that it is important for some forms of experience-dependent plasticity in the brain. The ability of neurons to store and release vesicular zinc is dependent on expression of the vesicular zinc transporter, ZnT3. Here, we examined the neuronal morphology of mice that lack ZnT3. Brains were collected from mice housed under standard laboratory conditions and from mice housed in enriched environments - large, multilevel enclosures with running wheels, numerous objects and tunnels, and a greater number of cage mates. Golgi-Cox staining was used to visualize neurons for analysis of dendritic length and dendritic spine density. Neurons were analyzed from the barrel cortex, striatum, basolateral amygdala, and hippocampus (CA1). ZnT3 knockout mice, relative to wild type mice, exhibited increased basal dendritic length in the layer 2/3 pyramidal neurons of barrel cortex, independently of housing condition. Environmental enrichment decreased apical dendritic length in these same neurons and increased dendritic spine density on striatal medium spiny neurons. Elimination of ZnT3 did not modulate any of the effects of enrichment. Our results provide no evidence that vesicular zinc is required for the experience-dependent changes that occur in response to environmental enrichment. They are consistent, however, with recent reports suggesting increased cortical volume in ZnT3 knockout mice.

Analysis of feedforward mechanisms of multiwhisker receptive field generation in a model of the rat barrel cortex

There is substantial anatomical segregation in the organization of the rodent barrel system - each whisker on the mystacial pad sends input to TC cells within a dedicated thalamic barreloid, which in turn innervates a corresponding cortical barrel, and RS cells within a barrel respond primarily to deflections of the corresponding whisker at the beginning of the dedicated transmission line (the principal whisker, PW). However, it is also well-established that barrel cells exhibit multiwhisker receptive fields (RFs), and display lower amplitude, longer latency responses to deflections of non-PWs (or adjacent whiskers, AWs). There is considerable controversy regarding the origin of such multiwhisker RFs; three possibilities include: (i) TC cells within a barreloid respond to multiple whiskers, and barrel RS cells simply inherit multiwhisker responses from their aligned barreloid; (ii) TC cells respond only to the PW, but individual barreloids innervate multiple barrels; (iii) multiwhisker responses of barrel cells arise from lateral corticocortical (barrel-to-barrel) synaptic transmission. Ablation studies attempting to pinpoint the source of RS cell AW responses are often contradictory (though experimental work tends to suggest possibilities (i) or (iii) to be most plausible), and hence it is important to carefully evaluate these hypotheses in terms of available physiological data on barreloid and barrel response dynamics. In this work, I employ a biologically detailed model of the rat barrel cortex to evaluate possibility (i), and I show that, within the model, hypothesis (i) is capable of explaining a broad range of the available physiological data on responses to single (PW or AW) deflections and paired whisker deflections (AW deflection followed by PW deflection), as well as the dependence of such responses on the angular direction of whisker deflection. In particular, the model shows that barrel RS cells can exhibit AW direction tuning despite the fact that barreloid to barrel wiring has no systematic dependence on the AW direction preference of TC cells. Future modeling work will examine the other possibilities for the generation of multiwhisker RS cell RFs, and compare and contrast the different possible mechanisms within the context of available experimental data.

Early recovery of neuronal functioning in the sensory cortex after nerve reconstruction surgery

BACKGROUND

Nerve reconstructive surgery induces a transient loss and a prolonged and a gradual return of sensory inputs to the brain. It is unknown whether, following this massive peripheral denervation, the brain will experience a prolonged period of severe, intrinsic dysfunction.

OBJECTIVE

We aim to investigate the mechanisms of return of processing function in cortical neurons.

METHODS

We used the whisker model in rats to evaluate the functional recovery in the somatosensory cortex after a nerve reconstruction surgery. Multi-unit recording in the barrel cortex was performed in lightly anesthetized rats while their whiskers were stimulated by a whisker stimulator.

RESULTS

We observed a loss of neuronal responses to whisker stimulation 1 week after surgery, which started to recover 2 weeks after surgery. Following the surgery, only 11.8% of units had principle whiskers (PWs) returned to their original status while 17.7% had PWs different from their original status, indicating the effect of aberrant reinnervation on the whisker response map.

CONCLUSIONS

Robust neuronal responses to sensory stimulation even when only sparse sensory inputs are available in the early recovery phase. During this phase, aberrant reinnervation induces disorganized whisker tuning, a finding that might be account for the hypoesthesia and paresthesia during early recovery after nerve reconstruction.

Coactivations of barrel and piriform cortices induce their mutual synapse innervations and recruit associative memory cells

After associative learning, a signal induces the recall of its associated signal, or the other way around. This reciprocal retrieval of associated signals is essential for associative thinking and logical reasoning. For the cellular mechanism underlying this associative memory, we hypothesized that the formation of synapse innervations among coactivated sensory cortices and the recruitment of associative memory cells were involved in the integrative storage and reciprocal retrieval of associated signals. Our study indicated that the paired whisker and olfaction stimulations led to an odorant-induced whisker motion and a whisker-induced olfaction response, a reciprocal form of associative memory retrieval. In mice that showed the reciprocal retrieval of associated signals, their barrel and piriform cortical neurons became mutually innervated through their axon projection and new synapse formation. These piriform and barrel cortical neurons gained the ability to encode both whisker and olfaction signals based on synapse innervations from the innate input and the newly formed input. Therefore, the associated activation of sensory cortices by pairing input signals initiates their mutual synapse innervations, and the neurons innervated by new and innate synapses are recruited to be associative memory cells that encode these associated signals. Mutual synapse innervations among sensory cortices to recruit associative memory cells may compose the primary foundation for the integrative storage and reciprocal retrieval of associated signals. Our study also reveals that new synapses onto the neurons enable these neurons to encode memories to new specific signals.

Barrels XXXI comes to the inland empire

The 31st annual Barrels meeting was held on the campus of the University of California, Riverside on the first two days of November, 2018. The meeting focuses on the whisker to cortical barrel pathway and the systems it impacts. This year's meeting focussed on the neural mechanisms of motor control, the functions of higher order thalamic nuclei and adaptable perception and decision-making.

Assessing the performance of Granger-Geweke causality: Benchmark dataset and simulation framework

Nonparametric methods based on spectral factorization offer well validated tools for estimating spectral measures of causality, called Granger–Geweke Causality (GGC). In Pagnotta et al. (2018) [1] we benchmarked nonparametric GGC methods using EEG data recorded during unilateral whisker stimulations in ten rats; here, we include detailed information about the benchmark dataset. In addition, we provide codes for estimating nonparametric GGC and a simulation framework to evaluate the effects on GGC analyses of potential problems, such as the common reference problem, signal-to-noise ratio (SNR) differences between channels, and the presence of additive noise. We focus on nonparametric methods here, but these issues also affect parametric methods, which can be tested in our framework as well. Our examples allow showing that time reversal testing for GGC (tr-GGC) mitigates the detrimental effects due to SNR imbalance and presence of mixed additive noise, and illustrate that, when using a common reference, tr-GGC unambiguously detects the causal influence׳s dominant spectral component, irrespective of the characteristics of the common reference signal. Finally, one of our simulations provides an example that nonparametric methods can overcome a pitfall associated with the implementation of conditional GGC in traditional parametric methods.

Alteration of the nucleus basalis of Meynert afferents to vibrissae-related sensory cortex in de-whiskered adolescent congenital hypothyroid rats

INTRODUCTION

Thyroid hypofunction during early development results in anatomical alterations in the cerebellum, cerebrum, hippocampus and other brain structures. The plastic organization of the nucleus basalis of Meynert (nBM) projections to the whiskers-related somatosensory (wS1) cortex in adolescent pups with maternal thyroid hypofunction and sensory deprivation was assessed through retrograde WGA-HRP labeling.

METHODS

Congenital hypothyroidism induced by adding PTU (25 ppm) to the drinking water from embryonic day 16 to postnatal day (PND) 60. Pregnant rats were divided to intact and congenital hypothyroid groups. In each group, the total whiskers of pups (4 of 8) were trimmed continuously from PND 0 to PND 60.

RESULTS

Following separately WGA-HRP injections into wS1, retrogradely labeled neurons were observed in nBM. The number of labeled neurons in nBM were higher in the congenital hypothyroid and whisker deprived groups compared to their controls (P < 0.05).

CONCLUSION

Based on our results both congenital hypothyroidism and sensory deprivation may disturb normal development of cortical circuits in of nBM afferents to the wS1 cortex.

CB1 Cannabinoid Receptor Expression in the Barrel Field Region Is Associated with Mouse Learning

We found previously that fear conditioning by combined stimulation of a row B facial vibrissae (conditioned stimulus, CS) with a tail shock (unconditioned stimulus, UCS) leads to expansion of the cortical representation of the "trained" row, labeled with 2-deoxyglucose (2DG), in the layer IIIb/IV of the adult mouse the primary somatosensory cortex (S1) 24 h later. We have observed that these learning-dependent plastic changes are manifested by increased expression of somatostatin, cholecystokinin (SST+, CCK+) but not parvalbumin (PV+) immunopositive interneurons We have expanded this research and quantified a numerical value of CB1-expressing and PV-expressing GABAergic axon terminals (CB1+ and PV+ immunopositive puncta) that innervate different segments of postsynaptic cells in the barrel hollows of S1 cortex. We used 3D microscopy to identify the CB+ and PV+ puncta in the barrel cortex "trained" and the control hemispheres CS+UCS group and in controls: Pseudoconditioned, CS-only, UCS-only, and naive animals. We have identified that (i) the association between whisker-shock "trained" barrel B hollows and CB1+, but not PV+ puncta expression remained significant after Bonferroni correction, (ii) CS+UCS has had a significant increasing effect on expression of CB1+ but not PV+ puncta in barrel cortex "trained" hemisphere, and (iii) the pseudoconditioning had a significant decreasing effect on expression of CB1+, but not on PV+ puncta in barrel cortex, both trained and untrained hemispheres. It is correlated to disturbing behaviors. The results suggest that CB1+ puncta regulation is specifically linked with mechanisms leading to learning-dependent plasticity in S1 cortex.

Barrels XXX meeting report: Barrels in Baltimore

The Barrels meeting annually brings together researchers focused on the rodent whisker to cortical barrel system prior to the Society for Neuroscience meeting. The 2017 meeting focused on the classification of cortical interneurons, the role interneurons have in shaping brain dynamics, and finally on the circuitry underlying oral sensations. The meeting highlighted the latest advancements in this rapidly advancing field.

A Fluorescent Dye Method Suitable for Visualization of One or More Rat Whiskers

Visualization and tracking of the facial whiskers is critical to many studies of rodent behavior. High-speed videography is the most robust methodology for characterizing whisker kinematics, but whisker visualization is challenging due to the low contrast of the whisker against its background. Recently, we showed that fluorescent dye(s) can be applied to enhance visualization and tracking of whisker(s) ( Rigosa et al., 2017 ), and this protocol provides additional details on the technique.

The TrkB agonist 7,8-dihydroxyflavone changes the structural dynamics of neocortical pyramidal neurons and improves object recognition in mice

BDNF and its receptor TrkB have important roles in neurodevelopment, neural plasticity, learning, and memory. Alterations in TrkB expression have been described in different CNS disorders. Therefore, drugs interacting with TrkB, specially agonists, are promising therapeutic tools. Among them, the recently described 7,8-dihydroxyflavone (DHF), an orally bioactive compound, has been successfully tested in animal models of these diseases. Recent studies have shown the influence of this drug on the structure of pyramidal neurons, specifically on dendritic spine density. However, there is no information yet on how DHF may alter the structural dynamics of these neurons (i.e., real-time study of the addition/elimination of dendritic spines and axonal boutons). To gain knowledge on these effects of DHF, we have performed a real-time analysis of spine and axonal dynamics in pyramidal neurons of barrel cortex, using cranial windows and 2-photon microscopy during a chronic oral treatment with this drug. After confirming TrkB expression in these neurons, we found that DHF increased the gain rates of spines and axonal boutons, as well as improved object recognition memory. These results help to understand how the activation of the BDNF-TrkB system can improve basic behavioral tasks through changes in the structural dynamics of pyramidal neurons. Moreover, they highlight DHF as a promising therapeutic vector for certain brain disorders in which this system is altered.

Insulin receptor sensitization restores neocortical excitation/inhibition balance in a mouse model of autism

Background

Met receptor tyrosine kinase regulates neurogenesis, differentiation, migration, connectivity, and synaptic plasticity. The human Met gene has been identified as a prominent risk factor for autism spectrum disorder (ASD). Met gene-altered mice serve as useful models for mechanistic studies of ASD. Inactivation of Met in excitatory cortical neurons in mice (Emx1^cre/Met^flox mice) yields a phenotype in which significantly decreased GABA_A receptor-mediated inhibition shifts the excitation/inhibition (E/I) balance toward excitation in the somatosensory cortex. Further, unlike that seen in wild-type mice, insulin does not increase inhibition in the mutant cortex, suggesting that one of the consequences of kinase inactive Met gene could be desensitization of insulin receptors. To test this hypothesis, we investigated the effects of insulin receptor sensitizer, pioglitazone, on inhibition in the somatosensory thalamocortical circuitry.

Methods

We used whole-cell patch clamp electrophysiology and analyzed excitatory and inhibitory responses of cortical layer IV excitatory cells following stimulation of their thalamic input in thalamocortical pathway intact brain slices. We applied insulin alone and insulin + a thiazolidinedione, pioglitazone (PIO), to test the effects of sensitizing insulin receptors on inhibitory responses mediated by GABA_A receptors in the somatosensory cortex of Emx1^cre/Met^flox mice.

Results

In WT brain slices, application of insulin together with PIO did not enhance the effect of insulin alone. In contrast, PIO application induced a much larger inhibition than that of insulin alone in Met-defective cortex. Thus, insulin resistance of GABA_A receptor-mediated response in Met mutant mice may result from desensitized insulin receptors.

Conclusions

Sporadic clinical studies reported improved behavioral symptoms in children with autism following PIO treatment. We show that PIO can aid in normalization of the E/I balance in the primary somatosensory cortex, a potential physiological mechanism underlying the positive effects of PIO treatment.

Cellular diversity of the somatosensory cortical map plasticity

Sensory maps are representations of the sensory epithelia in the brain. Despite the intuitive explanatory power behind sensory maps as being neuronal precursors to sensory perception, and sensory cortical plasticity as a neural correlate of perceptual learning, molecular mechanisms that regulate map plasticity are not well understood. Here we perform a meta-analysis of transcriptional and translational changes during altered whisker use to nominate the major molecular correlates of experience-dependent map plasticity in the barrel cortex. We argue that brain plasticity is a systems level response, involving all cell classes, from neuron and glia to non-neuronal cells including endothelia. Using molecular pathway analysis, we further propose a gene regulatory network that could couple activity dependent changes in neurons to adaptive changes in neurovasculature, and finally we show that transcriptional regulations observed in major brain disorders target genes that are modulated by altered sensory experience. Thus, understanding the molecular mechanisms of experience-dependent plasticity of sensory maps might help to unravel the cellular events that shape brain plasticity in health and disease.

The pattern of thalamocortical and brain stem projections to the vibrissae-related sensory and motor cortices in de-whiskered congenital hypothyroid rats

The present study is designed to investigate the plastic organization of the thalamo-cortical (TC) and brain stem afferents of whisker primary sensory (wS1) and motor (wM1) cortical areas in congenital hypothyroid (CH) pups following whisker deprivation (WD) from neonatal to adolescence period. Maternal hypothyroidism was induced by adding propylthiouracil (PTU) to the drinking water from early embryonic day 16 to postnatal day (PND) 60. Pregnant rats were divided into intact and CH groups (n = 8). In each group, the total whiskers of pups (4 of 8) were trimmed continuously from PND 1 to PND 60. Retrograde tracing technique with WGA-HRP was performed in the present study. Retrogradely labeled neurons were observed in the specific thalamic nuclei (VPM and VL) following separately WGA-HRP injections into wS1/M1 cortical areas. The number of labeled cells in the VPM, VL, VM and PO nuclei of the thalamus significantly decreased in CH offsprings rats (P < 0.05). Neonatal WD did not show any significant effects on the number of VPM, VL, VM and PO labeled projection neurons to wS1 and wM1 cortical areas. In addition, retrogradely labeled neurons in dorsal raphe (DR) and locus coeruleus (LC) nuclei were observed in all experimental groups. The number of DR and LC labeled neurons were higher in the CH and whisker deprived groups compared to their matching controls (P < 0.05). Upon our results, CH and WD had no synergic or additive effects on the TC and brain stem afferent patterns of barrel sensory and motor cortices.

Respiratory chain complex III deficiency due to mutated BCS1L: a novel phenotype with encephalomyopathy, partially phenocopied in a Bcs1l mutant mouse model

BACKGROUND

Mitochondrial diseases due to defective respiratory chain complex III (CIII) are relatively uncommon. The assembly of the eleven-subunit CIII is completed by the insertion of the Rieske iron-sulfur protein, a process for which BCS1L protein is indispensable. Mutations in the BCS1L gene constitute the most common diagnosed cause of CIII deficiency, and the phenotypic spectrum arising from mutations in this gene is wide.

RESULTS

A case of CIII deficiency was investigated in depth to assess respiratory chain function and assembly, and brain, skeletal muscle and liver histology. Exome sequencing was performed to search for the causative mutation(s). The patient's platelets and muscle mitochondria showed respiration defects and defective assembly of CIII was detected in fibroblast mitochondria. The patient was compound heterozygous for two novel mutations in BCS1L, c.306A > T and c.399delA. In the cerebral cortex a specific pattern of astrogliosis and widespread loss of microglia was observed. Further analysis showed loss of Kupffer cells in the liver. These changes were not found in infants suffering from GRACILE syndrome, the most severe BCS1L-related disorder causing early postnatal mortality, but were partially corroborated in a knock-in mouse model of BCS1L deficiency.

CONCLUSIONS

We describe two novel compound heterozygous mutations in BCS1L causing CIII deficiency. The pathogenicity of one of the mutations was unexpected and points to the importance of combining next generation sequencing with a biochemical approach when investigating these patients. We further show novel manifestations in brain, skeletal muscle and liver, including abnormality in specialized resident macrophages (microglia and Kupffer cells). These novel phenotypes forward our understanding of CIII deficiencies caused by BCS1L mutations.

Distinct mechanisms of Up state maintenance in the medial entorhinal cortex and neocortex

The medial entorhinal cortex (mEC) is a key structure which controls the communication between the hippocampus and the neocortex. During slow-wave sleep, it stands out from other cortical regions by exhibiting persistent activity that outlasts neocortical Up states, decoupling the entorhinal cortex-hippocampal interaction from the neocortex. Here, we compared the mechanisms involved in the maintenance of the Up state in the barrel cortex (BC) and mEC using whole cell recordings in acute mouse brain slices. Bath application of an NMDA receptor antagonist abolished Up states in the BC, and reduced the incidence but not the duration of Up states in the mEC. Conversely, blockade of kainate receptors decreased Up state duration in the mEC, but not in the BC. Voltage clamp recordings demonstrated the presence of a non-NMDA glutamate receptor-mediated slow excitatory postsynaptic current, sensitive to the selective kainate receptor antagonist UBP-302, in layer III neurons of the mEC, which was not observed in the BC. Moreover, we found that kainate receptor-mediated currents assist in recovery back to the Up state membrane potential following a current-induced hyperpolarisation of individual cells in the mEC. Finally, we were able to generate Up state activity in a network model of exponential integrate-and-fire neurons only supported by AMPA and kainate receptor-mediated currents. We propose that synaptic kainate receptors are responsible for the unique properties of mEC Up states.

Feedforward motor information enhances somatosensory responses and sharpens angular tuning of rat S1 barrel cortex neurons

The primary vibrissae motor cortex (vM1) is responsible for generating whisking movements. In parallel, vM1 also sends information directly to the sensory barrel cortex (vS1). In this study, we investigated the effects of vM1 activation on processing of vibrissae sensory information in vS1 of the rat. To dissociate the vibrissae sensory-motor loop, we optogenetically activated vM1 and independently passively stimulated principal vibrissae. Optogenetic activation of vM1 supra-linearly amplified the response of vS1 neurons to passive vibrissa stimulation in all cortical layers measured. Maximal amplification occurred when onset of vM1 optogenetic activation preceded vibrissa stimulation by 20 ms. In addition to amplification, vM1 activation also sharpened angular tuning of vS1 neurons in all cortical layers measured. Our findings indicated that in addition to output motor signals, vM1 also sends preparatory signals to vS1 that serve to amplify and sharpen the response of neurons in the barrel cortex to incoming sensory input signals.

DOI:http://dx.doi.org/10.7554/eLife.21843.001

Establishment of high reciprocal connectivity between clonal cortical neurons is regulated by the Dnmt3b DNA methyltransferase and clustered protocadherins

BACKGROUND

The specificity of synaptic connections is fundamental for proper neural circuit function. Specific neuronal connections that underlie information processing in the sensory cortex are initially established without sensory experiences to a considerable extent, and then the connections are individually refined through sensory experiences. Excitatory neurons arising from the same single progenitor cell are preferentially connected in the postnatal cortex, suggesting that cell lineage contributes to the initial wiring of neurons. However, the postnatal developmental process of lineage-dependent connection specificity is not known, nor how clonal neurons, which are derived from the same neural stem cell, are stamped with the identity of their common neural stem cell and guided to form synaptic connections.

RESULTS

We show that cortical excitatory neurons that arise from the same neural stem cell and reside within the same layer preferentially establish reciprocal synaptic connections in the mouse barrel cortex. We observed a transient increase in synaptic connections between clonal but not nonclonal neuron pairs during postnatal development, followed by selective stabilization of the reciprocal connections between clonal neuron pairs. Furthermore, we demonstrate that selective stabilization of the reciprocal connections between clonal neuron pairs is impaired by the deficiency of DNA methyltransferase 3b (Dnmt3b), which determines DNA-methylation patterns of genes in stem cells during early corticogenesis. Dnmt3b regulates the postnatal expression of clustered protocadherin (cPcdh) isoforms, a family of adhesion molecules. We found that cPcdh deficiency in clonal neuron pairs impairs the whole process of the formation and stabilization of connections to establish lineage-specific connection reciprocity.

CONCLUSIONS

Our results demonstrate that local, reciprocal neural connections are selectively formed and retained between clonal neurons in layer 4 of the barrel cortex during postnatal development, and that Dnmt3b and cPcdhs are required for the establishment of lineage-specific reciprocal connections. These findings indicate that lineage-specific connection reciprocity is predetermined by Dnmt3b during embryonic development, and that the cPcdhs contribute to postnatal cortical neuron identification to guide lineage-dependent synaptic connections in the neocortex.

Somatosensory map expansion and altered processing of tactile inputs in a mouse model of fragile X syndrome

Fragile X syndrome (FXS) is a common inherited form of intellectual disability caused by the absence or reduction of the fragile X mental retardation protein (FMRP) encoded by the FMR1 gene. In humans, one symptom of FXS is hypersensitivity to sensory stimuli, including touch. We used a mouse model of FXS (Fmr1 KO) to study sensory processing of tactile information conveyed via the whisker system. In vivo electrophysiological recordings in somatosensory barrel cortex showed layer-specific broadening of the receptive fields at the level of layer 2/3 but not layer 4, in response to whisker stimulation. Furthermore, the encoding of tactile stimuli at different frequencies was severely affected in layer 2/3. The behavioral effect of this broadening of the receptive fields was tested in the gap-crossing task, a whisker-dependent behavioral paradigm. In this task the Fmr1 KO mice showed differences in the number of whisker contacts with platforms, decrease in the whisker sampling duration and reduction in the whisker touch-time while performing the task. We propose that the increased excitability in the somatosensory barrel cortex upon whisker stimulation may contribute to changes in the whisking strategy as well as to other observed behavioral phenotypes related to tactile processing in Fmr1 KO mice.

Propagation of spontaneous slow-wave activity across columns and layers of the adult rat barrel cortex in vivo

During slow-wave sleep, neocortical networks exhibit self-organized activity switching between periods of concurrent spiking (up-states) and periods of network silence (down-states), a phenomenon also occurring under the effects of different anesthetics and in in vitro brain slice preparations. Although this type of ongoing activity has been implicated into important functions such as memory consolidation and learning, the manner in which it propagates across different cortical modules (i.e., columns and layers) has not been fully characterized. In the present study, we investigated this issue by measuring spontaneous activity at large scale in the adult rat barrel cortex under urethane anesthesia by means of voltage-sensitive dye imaging and 128-channel probe recordings. Up to 74 neurons located in all layers of up to four functionally identified barrel-related columns were recorded simultaneously. The spontaneous activity propagated isotropically across the cortical surface with a median speed of ~35 µm/ms. A concomitant radial spread of activation was present from deep to superficial cortical layers. Thus, spontaneous activity occurred rather globally in the barrel cortex, with ≥50 % of the up-states presenting spikes in ≥3 columns and layers. Temporally precise spike sequences, which occurred repeatedly (although sporadically) within the up-states, were typically led by putative excitatory neurons in the infragranular cortical layers. In summary, our data provide for the first time an overall view of the spontaneous slow-wave activity within the barrel cortex circuit, characterizing its propagation across columns and layers at high spatio-temporal resolution.

Mild systemic inflammation and moderate hypoxia transiently alter neuronal excitability in mouse somatosensory cortex

During the perinatal period, the brain is highly vulnerable to hypoxia and inflammation, which often cause white matter injury and long-term neuronal dysfunction such as motor and cognitive deficits or epileptic seizures. We studied the effects of moderate hypoxia (HYPO), mild systemic inflammation (INFL), or the combination of both (HYPO+INFL) in mouse somatosensory cortex induced during the first postnatal week on network activity and compared it to activity in SHAM control animals. By performing in vitro electrophysiological recordings with multi-electrode arrays from slices prepared directly after injury (P8-10), one week after injury (P13-16), or in young adults (P28-30), we investigated how the neocortical network developed following these insults. No significant difference was observed between the four groups in an extracellular solution close to physiological conditions. In extracellular 8mM potassium solution, slices from the HYPO, INFL, and HYPO+INFL group were more excitable than SHAM at P8-10 and P13-16. In these two age groups, the number and frequency of spontaneous epileptiform events were significantly increased compared to SHAM. The frequency of epileptiform events was significantly reduced by the NMDA antagonist D-APV in HYPO, INFL, and HYPO+INFL, but not in SHAM, indicating a contribution of NMDA receptors to this pathophysiological activity. In addition, the AMPA/kainate receptor antagonist CNQX suppressed the remaining epileptiform activity. Electrical stimulation evoked prominent epileptiform activity in slices from HYPO, INFL and HYPO+INFL animals. Stimulation threshold to elicit epileptiform events was lower in these groups than in SHAM. Evoked events spread over larger areas and lasted longer in treated animals than in SHAM. In addition, the evoked epileptiform activity was reduced in the older (P28-30) group indicating that cortical dysfunction induced by hypoxia and inflammation was transient and compensated during early development.

Long, intrinsic horizontal axons radiating through and beyond rat barrel cortex have spatial distributions similar to horizontal spreads of activity evoked by whisker stimulation

Stimulation of a single whisker evokes a peak of activity that is centered over the associated barrel in rat primary somatosensory cortex, and yet the evoked local field potential and the intrinsic signal optical imaging response spread symmetrically away from this barrel for over 3.5 mm to cross cytoarchitectonic borders into other "unimodal" sensory cortical areas. To determine whether long horizontal axons have the spatial distribution necessary to underlie this activity spread, we injected adeno-associated viral vectors into barrel cortex and characterized labeled axons extending from the injection site in transverse sections of flattened cortex. Combined qualitative and quantitative analyses revealed labeled axons radiating diffusely in all directions for over 3.5 mm from supragranular injection sites, with density declining over distance. The projection pattern was similar at four different cortical depths, including infragranular laminae. Infragranular vector injections produced patterns similar to the supragranular injections. Long horizontal axons were detected both using a vector with a permissive cytomegalovirus promoter to label all neuronal subtypes and using a calcium/calmodulin-dependent protein kinase II α vector to restrict labeling to excitatory cortical pyramidal neurons. Individual axons were successfully reconstructed from series of supragranular sections, indicating that they traversed gray matter only. Reconstructed axons extended from the injection site, left the barrel field, branched, and sometimes crossed into other sensory cortices identified by cytochrome oxidase staining. Thus, radiations of long horizontal axons indeed have the spatial characteristics necessary to explain horizontal activity spreads. These axons may contribute to multimodal cortical responses and various forms of cortical neural plasticity.

An automated workflow for the anatomo-functional mapping of the barrel cortex

BACKGROUND

The rodent barrel cortex is a widely used model to study the cortical processing of tactile sensory information. It is notable by the cytoarchitecture of its layer IV, which contains distinguishable structural units called barrels that can be considered as anatomical landmarks of the functional columnar organization of the cerebral cortex. To study sensory integration in the barrel cortex it is therefore essential to map recorded functional data onto the underlying barrel topography, which can be reconstructed from the post hoc alignment of tangential brain slices stained for cytochrome oxidase.

NEW METHOD

This article presents an automated workflow to perform the registration of histological slices of the barrel cortex followed by the 2-D reconstruction of the barrel map from the registered slices. The registration of two successive slices is obtained by computing a rigid transformation to align sets of detected blood vessel cross-sections. This is achieved by using a robust variant of the classical iterative closest point method. A single fused image of the barrel field is then generated by computing a nonlinear merging of the gradients from the registered images.

COMPARISON WITH EXISTING METHODS

This novel anatomo-functional mapping tool leads to a substantial gain in time and precision compared to conventional manual methods. It provides a flexible interface for the user with only a few parameters to tune.

CONCLUSIONS

We demonstrate here the usefulness of the method for voltage sensitive dye imaging of the mouse barrel cortex. The method could also benefit other experimental approaches and model species.

Experience-dependent regulation of tissue-type plasminogen activator in the mouse barrel cortex

It has been suggested that tissue-type plasminogen activator (tPA), a serine protease, plays a key role in regulating the extracellular matrix core proteins, thereby impacting the structural plasticity in the cerebral cortex. Much is known about its role in regulating plasticity in the visual cortex. However, its permissive role has not been demonstrated to generalize to other cerebral cortical areas. By utilizing a combination of immunofluorescent histochemistry and confocal microscopy, we demonstrate that endogenous tPA is indeed present in the somatosensory cortex, and its expression is experience-dependent. Chronic sensory deprivation induced by whisker trimming from birth for one month leads to increased tPA immunoreactivity in all layers of the barrel cortex. Furthermore, tPA immunoreactivity remains high even after sensation has been restored to the mystacial pad (by allowing whiskers to grow back to full length for one month). Our results suggest that tPA levels in the cerebral cortex are regulated by sensory experience, and play a key role in regulating structural remodeling in the cerebral cortex.

Quantitative activity-induced manganese-dependent MRI for characterizing cortical layers in the primary somatosensory cortex of the rat

The ability of Mn(2+) to follow Ca(2+) pathways upon stimulation transform them into remarkable surrogate markers of neuronal activity using activity-induced manganese-dependent MRI (AIM-MRI). In the present study, a precise follow-up of physiological parameters during MnCl2 and mannitol infusions improved the reproducibility of AIM-MRI allowing in-depth evaluation of the technique. Pixel-by-pixel T1 data were investigated using histogram distributions in the barrel cortex (BC) and the thalamus before and after Mn(2+) infusion, after blood brain barrier opening and after BC activation. Mean BC T1 values dropped significantly upon trigeminal nerve (TGN) stimulation (-38 %, P = 0.02) in accordance with previous literature findings. T1 histogram distributions showed that 34 % of T1s in the range 600-1500 ms after Mn(2+ )+ mannitol infusions shifted to 50-350 ms after TGN stimulation corresponding to a twofold increase of the percentage of pixels with the lowest T1s in BC. Moreover, T1 changes in response to stimulation increased significantly from superficial cortical layers (I-III) to deeper layers (V-VI). Cortical cytoarchitecture detection during a functional paradigm was performed extending the potential of AIM-MRI. Quantitative AIM-MRI could thus offer a means to interpret local neural activity across cortical layers while identification of the role of calcium dynamics in vivo during brain activation could play a key role in resolving neurovascular coupling mechanisms.

Unilateral whisker clipping exacerbates ethanol-induced social and somatosensory behavioral deficits in a sex- and age-dependent manner

Prenatal exposure to ethanol results in sensory deficits and altered social interactions in animal and clinical populations. Sensory stimuli serve as important cues and shape sensory development; developmental exposure to ethanol or sensory impoverishment can impair somatosensory development, but their combined effects on behavioral outcomes are unknown. We hypothesized 1) that chronic prenatal ethanol exposure would disrupt social interaction and somatosensory performance during adolescence, 2) that a mild sensory impoverishment (neonatal unilateral whisker clipping; WC) would have a mildly impairing to sub-threshold effect on these behavioral outcomes, and 3) that the effect of ethanol would be exacerbated by WC. Long-Evans dams were fed a liquid diet containing ethanol or pair-fed with a non-ethanol diet on gestational days (G) 6-G21. Chow-fed control animals were also included. One male and female pup per litter underwent WC on postnatal day (P)1, P3, and P5. Controls were unclipped. Offspring underwent social interaction on P28 or P42, and gap-crossing (GC) on P31 or P42. Ethanol-exposed pups played less and crossed shorter gaps than control pups regardless of age or sex. WC further exacerbated ethanol-induced play fighting and GC deficits in all males but only in 28-day-old females. WC alone reduced sniffing in all males and in younger females. Thus, prenatal ethanol exposure induced deficits in social interaction and somatosensory performance during adolescence. Sensory impoverishment exacerbates ethanol's effect in 28-day-old male and female animals and in 42-day-old males, suggesting sex- and age-dependent changes in outcomes in ethanol-exposed offspring.

Structural determinants underlying the high efficacy of synaptic transmission and plasticity at synaptic boutons in layer 4 of the adult rat 'barrel cortex'

Excitatory layer 4 (L4) neurons in the 'barrel field' of the rat somatosensory cortex represent an important component in thalamocortical information processing. However, no detailed information exists concerning the quantitative geometry of synaptic boutons terminating on these neurons. Thus, L4 synaptic boutons were investigated using serial ultrathin sections and subsequent quantitative 3D reconstructions. In particular, parameters representing structural correlates of synaptic transmission and plasticity such as the number, size and distribution of pre- and postsynaptic densities forming the active zone (AZ) and of the three functionally defined pools of synaptic vesicles were analyzed. L4 synaptic boutons varied substantially in shape and size; the majority had a single, but large AZ with opposing pre- and postsynaptic densities that matched perfectly in size and position. More than a third of the examined boutons showed perforations of the postsynaptic density. Synaptic boutons contained on average a total pool of 561 ± 108 vesicles, with ~5% constituting the putative readily releasable, ~23% the recycling, and the remainder the reserve pool. These pools are comparably larger than other characterized central synapses. Synaptic complexes were surrounded by a dense network of fine astrocytic processes that reached as far as the synaptic cleft, thus regulating the temporal and spatial glutamate concentration, and thereby shaping the unitary EPSP amplitude. In summary, the geometry and size of AZs, the comparably large readily releasable and recycling pools, together with the tight astrocytic ensheathment, may explain and contribute to the high release probability, efficacy and modulation of synaptic transmission at excitatory L4 synaptic boutons. Moreover, the structural variability as indicated by the geometry of L4 synaptic boutons, the presence of mitochondria and the size and shape of the AZs strongly suggest that synaptic reliability, strength and plasticity is governed and modulated individually at excitatory L4 synaptic boutons.

Synaptic biology of barrel cortex circuit assembly

Mature neuronal circuits arise from the coordinated interplay of cell-intrinsic differentiation programs, target-derived signals and activity-dependent processes. Typically, cell-intrinsic mechanisms predominate at early stages of differentiation, while input-dependent processes modulate circuit formation at later stages of development. The whisker barrel cortex of rodents is particularly well suited to study this latter phase. During the first few days after birth, thalamocortical axons (TCA) from the somatosensory ventral posteromedial nucleus (VPM) form synapses onto layer 4 (L4) neurons, which aggregate to form barrels, whose spatial organization corresponds to the distribution of the whiskers on the snout. Besides specific genetic programs, which control TCA and L4 neuron specification, the establishment of the barrel pattern also depends on the information resulting from whisker activation. The plasticity of this system during the first few days after birth is critical for barrel formation: damage to the sensory periphery impairs TCA patterning, while lesions after this period have less pronounced effects. Here, we will review the role and position of L4 neurons within cortical columnar circuits and synaptogenesis during barrel formation.

An ocean full of BARRELS: Barrels XXVI meeting report

The 26th annual Barrels meeting was convened on the campus of the University of California San Diego, not far from the shores of the Pacific Ocean. The meeting focused on three main themes: the structure and function of the thalamic reticular nucleus, the neurovasculature system and its role in brain metabolism, and the origins and functions of cortical GABAergic interneurons. In addition to the major themes, there were short talks, a data blitz, and a poster session which highlighted the diversity and quality of the research ongoing in the rodent whisker-to-barrel system.

Sensory and cognitive neurophysiology in rats, Part 1: Controlled tactile stimulation and micro-ECoG recordings in freely moving animals

BACKGROUND

We have developed a setup for rats that allows for controlled sensory input to an animal engaged in a task while recording both electrophysiological signals and behavioral output.

NEW METHOD

We record electrophysiological signals using a novel high-density micro-electrocorticography (micro-ECoG) grid that covers almost the whole somatosensory system. We dealt with the well-known difficulty that the rat uses its whisker system in an active (motor-controlled) way to explore its environment by designing a head-mounted device that stimulates the rat's snout in a way unaffected by whisker movements.

RESULTS

We replicate the spatial specificity of early evoked responses in somatosensory and auditory cortex. In a companion paper (Cognitive Neurophysiology in Rats, Part 2: Validation and Demonstration) we validate our setup and show for the first time that the ECoG can be used to record evoked responses in a signal that reflects neural output (spiking activity).

COMPARISON WITH EXISTING METHODS

Compared with high-density wire recordings, micro-ECoG offers a much more stable signal without readjustments, and a much better scalability. Compared with head-fixed preparations, our head-mounted stimulator allows to stay closer to the rat's natural way of collecting sensory information.

CONCLUSIONS

For perceptual and cognitive research, our setup provides a unique combination of possibilities that cannot be achieved in other setups for rodents.

Sensory and cognitive neurophysiology in rats. Part 2: Validation and demonstration

BACKGROUND

We have developed a novel setup for rats that allows for controlled sensory input to an animal engaged in a task while recording both electrophysiological signals and behavioral output.

NEW METHOD

Our setup is described in a companion paper.

RESULTS

We validate our setup by replicating (1) the functionally nonspecific spread of neural activity following tactile stimulation, and (2) the effects of anesthesia on the tactile evoked responses. We also demonstrate for the first time that the ECoG can be used to record evoked responses in a signal that reflects neural output (spiking activity), and illustrate the usefulness of our setup by demonstrating that these evoked responses are modulated by both the phase of pre-stimulus oscillations and by expectation.

COMPARISON WITH EXISTING METHODS

Compared with high-density wire recordings, micro-ECoG offers a much more stable signal without readjustments, and a much better scalability. Compared with extracranial and regular ECoG recordings, micro-ECoG allows us to measure signals that reflect both neural input and neural output.

CONCLUSIONS

For sensory and cognitive research, our setup provides a unique combination of possibilities that cannot be achieved in other setups for rodents.