Adjacent Whiskers Research Articles

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).

Cross-Whisker Adaptation of Neurons in Layer 2/3 of the Rat Barrel Cortex.

Neurons in the barrel cortex respond preferentially to stimulation of one principal whisker and weakly to several adjacent whiskers. Such integration exists already in layer 4, the pivotal recipient layer of thalamic inputs. Previous studies show that cortical neurons gradually adapt to repeated whisker stimulations and that layer 4 neurons exhibit whisker specific adaptation and no apparent interactions with other whiskers. This study aimed to study the specificity of adaptation of layer 2/3 cortical cells. Towards this aim, we compared the synaptic response of neurons to either repetitive stimulation of one of two responsive whiskers or when repetitive stimulation of the two whiskers was interleaved. We found that in most layer 2/3 cells adaptation is whisker-specific. These findings indicate that despite the multi-whisker receptive fields in the cortex, the adaptation process for each whisker-pathway is mostly independent of other whiskers. A mechanism allowing high responsiveness in complex environments.

Environmental Enrichment Sharpens Sensory Acuity by Enhancing Information Coding in Barrel Cortex and Premotor Cortex

Environmental enrichment (EE) is beneficial to sensory functions. Thus, elucidating the neural mechanism underlying improvement of sensory stimulus discrimination is important for developing therapeutic strategies. We aim to advance the understanding of such neural mechanism. We found that tactile enrichment improved tactile stimulus feature discrimination. The neural correlate of such improvement was revealed by analyzing single-cell information coding in both the primary somatosensory cortex and the premotor cortex of awake behaving animals. Our results show that EE enhances the decision-information coding capacity of cells that are tuned to adjacent whiskers, and of premotor cortical cells.

Real-Time Closed-Loop Feedback in Behavioral Time Scales Using DeepLabCut.

Computer vision approaches have made significant inroads into offline tracking of behavior and estimating animal poses. In particular, because of their versatility, deep-learning approaches have been gaining attention in behavioral tracking without any markers. Here, we developed an approach using DeepLabCut for real-time estimation of movement. We trained a deep-neural network (DNN) offline with high-speed video data of a mouse whisking, then transferred the trained network to work with the same mouse, whisking in real-time. With this approach, we tracked the tips of three whiskers in an arc and converted positions into a TTL output within behavioral time scales, i.e., 10.5 ms. With this approach, it is possible to trigger output based on movement of individual whiskers, or on the distance between adjacent whiskers. Flexible closed-loop systems like the one we have deployed here can complement optogenetic approaches and can be used to directly manipulate the relationship between movement and neural activity.

Modulatory Effects of Memantine on Neuronal Response Properties in Rat Barrel Cortex.

Introduction:Memantine as N-Methyl-D-Aspartic Acid (NMDA) receptor antagonist is used in some neurological disorders. Moreover, memantine presents modulatory effects on the somatosensory information processing in healthy subjects. This study investigated the effects of memantine on electrophysiological properties of barrel cortex neurons in male rats.Methods:Single unit recording was used to evaluate the electrophysiological properties of barrel cortex neurons. The neural responses to the Principal Whisker (PW), Adjacent Whisker (AW), and combined displacement of two whiskers [20 ms Inter-Stimulus Intervals (ISIs)] were recorded before and 2 hours after memantine gavage (10 mg/kg). A Condition Test Ratio (CTR) was calculated for assessing inhibitory interactions.Results:Two hours after memantine gavage, neuronal ON and OFF responses to PW deflection were decreased. Furthermore, CTR for both ON and OFF responses was decreased following memantine administration.Conclusion:The current study demonstrated that memantine modified neural response properties in the rat barrel cortex.HighlightsMemantine modulated excitatory receptive fields in the rat somatosensory cortex.Memantine decreased integrative receptive fields in rat somatosensory cortex.Plain Language SummaryAs an NMDA receptor antagonist, memantine is used to treat moderate to severe Alzheimer’s disease. Memantine has beneficial effects on cognition, mood, and perform daily activities. However, the current study results suggested that memantine may affect information processing in the somatosensory system. This should be considered for future research in the clinic.

Tuning microstructure and mechanical properties of squeeze-cast whisker-reinforced 2024Al composites by hot forging process design

In order to minimize defects and obtain a uniform and consistent microstructure in Al-matrix composites, the hot forging process design, microstructure, and mechanical properties evolution in squeeze-cast 20 vol% Al18B4O33w/2024Al composites were investigated. Uniaxial cushioned forging was more effective than uniaxial free forging and uniaxial canned forging in increasing hot deformability and improving homogeneity of strain and microstructure in composites. Bi-directional forging accumulated larger strain in the microstructure, which produced alignment of whiskers and associated anisotropy in mechanical response. Both the strength factor and the whisker length affected composite strength. When loading parallel to whisker alignment, composites exhibited increased strength factor and greater distance between adjacent whiskers along the microcrack propagation direction, both of which resulted in higher strength and elongation. When loading normal to the whisker alignment, composite strength and ductility decreased due to a smaller strength factor, shorter microcrack propagation distance, and increased likelihood of interface debonding.

Nucleus basalis of Meynert modulates signal processing in rat layer 5 somatosensory cortex but leads to memory impairment and tactile discrimination deficits following lesion

In rodents, exploring through continuous whisking is a process resulted from sensorimotor networking among different layers of somatosensory cortex (SC) such as layer 5 (L5) or barrel field, and regions like the nucleus basalis of Meynert (NBM). NBM is densely packed with cholinergic fibers and its dysfunction leads to diminished acetylcholine release within SC, tactile deficits and Alzheimer’s disease (AD)-like memory impairment. Using extracellular single-unit recording, we investigated mechanisms underlying changes in response characteristics of L5b neurons to single or paired deflection of selected principle and adjacent whiskers (PW and AW), following NBM electrical stimulation in normal rats or ibotenic acid-induced NBM lesion leading to potential tactile deficiency and memory loss during passive avoidance learning (PAL) in AD-like neuropathology. Our results indicated that NBM electrical stimulation decreased ON and OFF response magnitude in nearly half of the units upon vibrissal deflection. The larger the response was evoked to whisker deflection before NBM stimulation, the smaller it gets after stimulation. Neuronal spontaneous activity was not changed with NBM stimulation or lesion. Leading to more sublinear response summation and decreased condition-test ratio, NBM lesion decreased ON response magnitude and facilitation, increased AW surround inhibition in paired whisker deflection, increased excitatory and decreased inhibitory receptive fields, weakened information processing during whisking, and resulted in AD-like declined PAL performance. These findings provide further understandings to develop translational approaches in precision therapeutics to target highly specific regions such as NBM or SC, and pathways like cholinergic system involved in tactile and memory deficits in AD.

The effect of Wi-Fi electromagnetic waves on neuronal response properties in rat barrel cortex

There is a growing number of studies on the possible biological effects of Wi-Fi radiations on nervous system. In this study we investigated the effect of Wi-Fi exposure on single neuron responses to natural stimuli by using whisker to barrel pathway. This study was done on 29 male Wistar rats. Neuronal spontaneous activity and ON and OFF responses to displacement of principal whisker (PW), adjacent whisker (AW) and combination of PW-AW stimulation (as natural stimuli) were recorded in barrel cortex of anaesthetised rats. A D-link Wi-Fi device was used for 1 h exposure to 2.4 GHz microwaves in data mode (18.2 dBm and 44% for power and duty cycle). A condition test ratio (CTR) was calculated for assessing neuronal integrative properties. Wi-Fi radiations decreased CTR for ON responses. However, neuronal spontaneous activity and ON and OFF responses were not significantly changed following exposure to Wi-Fi signals. The results of this study demonstrated that exposure to Wi-Fi radiation could modulate integrative responses to natural stimuli in barrel cortex.

Mechanical coupling through the skin affects whisker movements and tactile information encoding.

Rats use their whiskers to extract sensory information from their environment. While exploring, they analyze peripheral stimuli distributed over several whiskers. Previous studies have reported cross-whisker integration of information at several levels of the neuronal pathways from whisker follicles to the somatosensory cortex. In the present study, we investigated the possible coupling between whiskers at a preneuronal level, transmitted by the skin and muscles between follicles. First, we quantified the movement induced on one whisker by deflecting another whisker. Our results show significant mechanical coupling, predominantly when a given whisker's caudal neighbor in the same row is deflected. The magnitude of the effect was correlated with the diameter of the deflected whisker. In addition to changes in whisker angle, we observed curvature changes when the whisker shaft was constrained distally from the base. Second, we found that trigeminal ganglion neurons innervating a given whisker follicle fire action potentials in response to high-magnitude deflections of an adjacent whisker. This functional coupling also shows a bias toward the caudal neighbor located in the same row. Finally, we designed a two-whisker biomechanical model to investigate transmission of forces across follicles. Analysis of the whisker-follicle contact forces suggests that activation of mechanoreceptors in the ring sinus region could account for our electrophysiological results. The model can fully explain the observed caudal bias by the gradient in whisker diameter, with possible contribution of the intrinsic muscles connecting follicles. Overall, our study demonstrates the functional relevance of mechanical coupling on early information processing in the whisker system.NEW & NOTEWORTHY Rodents explore their environment actively by touching objects with their whiskers. A major challenge is to understand how sensory inputs from different whiskers are merged together to form a coherent tactile percept. We demonstrate that external sensory events on one whisker can influence the position of another whisker and, importantly, that they can trigger the activity of mechanoreceptors at its base. This cross-whisker interaction occurs pre-neuronally, through mechanical transmission of forces in the skin.

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.

Heterogeneous effects of cholecystokinin on neuronal response properties in deep layers of rat barrel cortex

Cholecystokinin (CCK) is one of the most studied neuropeptides in the brain. In this study, we investigated the effects of CCK-8s and LY225910 (CCK2 receptor antagonist) on properties of neuronal response to natural stimuli (whisker deflection) in deep layers of rat barrel cortex. This study was done on 20 male Wistar rats, weighing 230–260 g. CCK-8s (300 nmol/rat) and LY225910 (1 µmol/rat) were administered intracerebroventricularly (ICV). Neuronal responses to deflection of principal (PW) and adjacent (AW) whiskers were recorded in the barrel cortex using tungsten microelectrodes. Computer controlled mechanical displacement was used to deflect whiskers individually or in combination at 30 ms inter-stimulus intervals. ON and OFF responses for PW and AW deflections were measured. A condition-test ratio (CTR) was computed to quantify neuronal responses to whisker interaction. ICV administration of CCK-8s and LY225910 had heterogeneous effects on neuronal spontaneous activity, ON and OFF responses to PW and/or AW deflections, and CTR for both ON and OFF responses. The results of this study demonstrated that CCK-8s can modulate neuronal response properties in deep layers of rat barrel cortex probably via CCK2 receptors.

Muscular Basis of Whisker Torsion in Mice and Rats.

Whisking mammals move their whiskers in the rostrocaudal and dorsoventral directions with simultaneous rolling about their long axes (torsion). Whereas muscular control of the first two types of whisker movement was already established, the anatomic muscular substrate of the whisker torsion remains unclear. Specifically, it was not clear whether torsion is induced by asymmetrical operation of known muscles or by other largely unknown muscles. Here, we report that mystacial pads of newborn and adult rats and mice contain oblique intrinsic muscles (OMs) that connect diagonally adjacent vibrissa follicles. Each of the OMs is supplied by a cluster of motor end plates. In rows A and B, OMs connect the ventral part of the rostral follicle with the dorsal part of the caudal follicle. In rows C-E, in contrast, OMs connect the dorsal part of the rostral follicle to the ventral part of the caudal follicle. This inverse architecture is consistent with previous behavioral observations [Knutsen et al.: Neuron 59 (2008) 35-42]. In newborn mice, torsion occurred in irregular single twitches. In adult anesthetized rats, microelectrode mediated electrical stimulation of an individual OM that is coupled with two adjacent whiskers was sufficient to induce a unidirectional torsion of both whiskers. Torsional movement was associated with protracting movement, indicating that in the vibrissal system, like in the ocular system, torsional movement is mechanically coupled to horizontal and vertical movements. This study shows that torsional whisker rotation is mediated by specific OMs whose morphology and attachment sites determine rotation direction and mechanical coupling, and motor innervation determines rotation dynamics. Anat Rec, 300:1643-1653, 2017. © 2017 Wiley Periodicals, Inc.

Long-Term Deficits in Cortical Circuit Function after Asphyxial Cardiac Arrest and Resuscitation in Developing Rats.

Cardiac arrest is a common cause of global hypoxic-ischemic brain injury. Poor neurologic outcome among cardiac arrest survivors results not only from direct cellular injury but also from subsequent long-term dysfunction of neuronal circuits. Here, we investigated the long-term impact of cardiac arrest during development on the function of cortical layer IV (L4) barrel circuits in the rat primary somatosensory cortex. We used multielectrode single-neuron recordings to examine responses of presumed excitatory L4 barrel neurons to controlled whisker stimuli in adult (8 ± 2-mo-old) rats that had undergone 9 min of asphyxial cardiac arrest and resuscitation during the third postnatal week. Results indicate that responses to deflections of the topographically appropriate principal whisker (PW) are smaller in magnitude in cardiac arrest survivors than in control rats. Responses to adjacent whisker (AW) deflections are similar in magnitude between the two groups. Because of a disproportionate decrease in PW-evoked responses, receptive fields of L4 barrel neurons are less spatially focused in cardiac arrest survivors than in control rats. In addition, spiking activity among L4 barrel neurons is more correlated in cardiac arrest survivors than in controls. Computational modeling demonstrates that experimentally observed disruptions in barrel circuit function after cardiac arrest can emerge from a balanced increase in background excitatory and inhibitory conductances in L4 neurons. Experimental and modeling data together suggest that after a hypoxic-ischemic insult, cortical sensory circuits are less responsive and less spatially tuned. Modulation of these deficits may represent a therapeutic approach to improving neurologic outcome after cardiac arrest.

Effect of TPMPA (GABAC receptor antagonist) on neuronal response properties in rat barrel cortex

GABAC receptors are ligand-gated chloride channels and have important roles in some neurological functions like vision. Recent investigations demonstrated that these receptors are also expressed in the somatosensory cortex. In this study, we investigated the effect of (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA) (GABAC receptor antagonist) on the properties of the neuronal response to natural stimuli (whisker deflection) in deep layers of rat barrel cortex. Twenty-eight male Wistar rats, weighing 230–260 g, were used in this study. TPMPA (100 μmol/rat) was administered intracerebroventricularly (ICV). Neuronal responses to deflection of principal (PW) and adjacent (AW) whiskers were recorded in barrel cortex using tungsten microelectrodes. A computer-controlled mechanical displacement was used to deflect whiskers individually or in combination at 30 ms inter-stimulus intervals. ON and OFF responses for PW and AW deflections were measured. A condition-test ratio (CTR) was computed to quantify neuronal responses to whisker interactions. Our data suggest that ICV administration of TPMPA increased neuronal spontaneous activity, ON and OFF responses to PW, and/or AW deflections. However, CTR for neither ON nor OFF responses changed following TPMPA administration. The results of this study demonstrated that inhibition of GABAC receptors by TPMPA can modulate neuronal response properties in rat barrel cortex.

Effects of Achillea millefolium Aqueous Extract on Electrophysiological Properties of Rat Barrel Cortex Neurons

Background: Yarrow or Achillea millefolium (A. millefolium) is often prescribed for the treatment of nervous system-related diseases. Objectives: This study investigated the effects of acute oral administration of A. millefolium on the responses of barrel cortex neurons in the rat. Methods: In this experimental study, 21 male Wistar rats (body weight, 200 g - 250 g) were divided into three groups, with seven animals in each group. Aqueous extract of A. millefolium (1.4 and 2.8 mg/kg/mL) was administered by gavage, and the responses of barrel cortex neurons (i.e., the deflection of principal whiskers [PWs] and adjacent whiskers [AWs]) were recorded using tungsten microelectrodes. Computer-controlled mechanical displacement was used to deflect whiskers individually or in combination at 0, 10, 20, 30, 50, and 100 ms inter-stimulus intervals (ISIs). The ON and OFF responses of the PWs and AWs to deflection were measured. A condition-test ratio (CTR) was computed to quantify neuronal responses to whisker interactions. Results: The evoked ON and OFF responses of PWs and AWs did not change following the treatment with the A. millefolium aqueous extract (0.5, 1, and 2 hours postapplication). The stimulation of AWs before PWs decreased ON and OFF neuronal activities. The decrement in the level of response was related to the ISI. The administration of the A. millefolium aqueous extract did not change the level of ON and OFF responses at any of the ISIs. Conclusions: The results demonstrated that acute oral administration of A. millefolium aqueous extract did not modulate electrophysiological responses of neurons in the barrel cortex of rats.

Effects of Dimethyl Sulfoxide on Neuronal Response Characteristics in Deep Layers of Rat Barrel Cortex

Introduction:Dimethyl sulfoxide (DMSO) is a chemical often used as a solvent for water-insoluble drugs. In this study, we evaluated the effect of intracerebroventricular (ICV) administration of DMSO on neural response characteristics (in 1200–1500 μm depth) of the rat barrel cortex.Methods:DMSO solution was prepared in 10% v/v concentration and injected into the lateral ventricle of rats. Neuronal spontaneous activity and neuronal responses to deflection of the principal whisker (PW) and adjacent whisker (AW) were recorded in barrel cortex. A condition test ratio (CTR) was used to measure inhibitory receptive fields in barrel cortex.Results:The results showed that both PW and AW evoked ON and OFF responses, neuronal spontaneous activity and inhibitory receptive fields did not change following ICV administration of DMSO.Conclusion:Results of this study suggest that acute ICV administration of 10% DMSO did not modulate the electrophysiological characteristics of neurons in the l deep ayers of rat barrel cortex.

Whisking whiskers tell rats about surroundings

Watch a rat's twitching nose and you may think that it's taking a good sniff. However, Mitra Hartmann from Northwestern University, USA, explains that much of the time the animals are deftly exploring their surroundings by whisking their whiskers to and fro. Hartmann is fascinated by how animals perceive objects and how rats interpret deflections as their whiskers brush past obstacles. She explains that measuring the interactions between a rat's whiskers and a structure could shed light on how the rodents build an understanding of their surroundings. However, directly measuring the whisker interaction patterns for all of the possible poses that a rat's head could strike would be almost impossible in real life. So, having previously built a cyber-rat simulation – complete with 62 cyber-whiskers laid out in a grid – to learn more about the rodent's sixth sense, Hartmann teamed up with Jennifer Hobbs and Blythe Towal to calculate the contact patterns as the simulated rat's whiskers explored objects that it encountered.Explaining that the cyber-rat's head can be positioned in any orientation relative to an object, Hartmann and Hobbs placed a wall and floor in the simulation and embarked on the Herculean challenge of calculating the chance that the rat's whiskers would encounter the surfaces after positioning the head in 83,509 different positions (at distances up to 60 mm from the wall, looking over a range of angles from vertically up to vertically down and looking from 90 deg right to 90 deg left). Next, Hartmann and Hobbs positioned the rat side-on to the wall, explaining that this is the orientation that rats tend to assume naturally in a burrow, and simulated a tunnel while calculating the probability of the whiskers contacting the wall and floor while whisking.Analysing the calculations, Hartmann and Hobbs were delighted to see trends emerging in the contact patterns. When the rat faced the wall head on, the adjacent whiskers in the same row or column were most likely to contact the object. However, when the rat turned side-on to the wall, the duo could see that clusters of whiskers laid out in a triangle were most likely to touch the wall. And when the rat was placed in a burrow, the triangular arrangement became even more pronounced.But how many of the head positions that Hobbs and Hartmann had simulated would a flesh-and-blood rat use when thrown into the complete unknown? This time, Towal placed a rat on a perch near to a wall in the dark and filmed the first few seconds as the animal explored the new location with its whiskers. Measuring the position of the animal's head from the movies, the trio was pleased that the rat's behaviour strongly reflected their simulations. ‘We found that the rat tends to put its head at the angles where it is most likely to maximise the number of whiskers in contact with the ground and with a structure in front of it in an uncertain environment’ says Hartmann.Adding that motion is the key to an animal building an understanding of its surroundings, Hartmann explains that sensory systems have been shaped by the way that an animal moves through the environment. She stresses that we can only understand how the brain constructs a map of the objects in the vicinity by studying sensory signals in the context of the movements that generated them. ‘If you are going to understand what the brain does, you have to study it in awake animals that are free to explore the world the way they evolved,’ she says.

Multiple blocks of intermittent and continuous theta-burst stimulation applied via transcranial magnetic stimulation differently affect sensory responses in rat barrel cortex.

Cortical sensory processing varies with cortical state and the balance of inhibition to excitation. Repetitive transcranial magnetic stimulation (rTMS) has been shown to modulate human cortical excitability. In a rat model, we recently showed that intermittent theta-burst stimulation (iTBS) applied to the corpus callosum, to activate primarily supragranular cortical pyramidal cells but fewer subcortical neurons, strongly reduced the cortical expression of parvalbumin (PV), indicating reduced activity of fast-spiking interneurons. Here, we used the well-studied rodent barrel cortex system to test how iTBS and continuous TBS (cTBS) modulate sensory responses evoked by either single or double stimuli applied to the principal (PW) and/or adjacent whisker (AW) in urethane-anaesthetized rats. Compared to sham stimulation, iTBS but not cTBS particularly enhanced late (>18 ms) response components of multi-unit spiking and local field potential responses in layer 4 but not the very early response (<18 ms). Similarly, only iTBS diminished the suppression of the second response evoked by paired PW or AW–PW stimulation at 20 ms intervals. The effects increased with each of the five iTBS blocks applied. With cTBS a mild effect similar to that of iTBS was first evident after 4–5 stimulation blocks. Enhanced cortical c-Fos and zif268 expression but reduced PV and GAD67 expression was found only after iTBS, indicating increased cortical activity due to lowered inhibition. We conclude that iTBS but less cTBS may primarily weaken a late recurrent-type cortical inhibition mediated via a subset of PV+ interneurons, enabling stronger late response components believed to contribute to the perception of sensory events.

Imprecise Whisker Map in the Neonatal Rat Barrel Cortex.

The somatosensory barrel cortex in rodents contains a topographic map of the facial whiskers where each cortical barrel is tuned to a corresponding whisker. However, exactly when this correspondence is established during development and how precise the functional topography of the whisker protomap is at birth, before the anatomical formation of barrels, are questions that remain unresolved. Here, using extracellular and whole-cell recordings from the barrel cortex of 0- to 7-day-old (P0-7; P0 = day of birth) rat pups in vivo, we report a low level of tuning to the principal whisker at P0-1, with multiple adjacent whiskers evoking large multi- and single-unit responses and excitatory postsynaptic currents in cortical neurons. Additionally, we found broad and largely overlapping projection fields (PFs) for neighboring whiskers in the barrel cortex at P0-1. Starting from P2-3, a segregated whisker map emerged, characterized by preferential single whisker tuning and segregated whisker PFs. These results indicate that the functional whisker protomap in the somatosensory cortex is imprecise at birth, that for 2-3 days after birth, whiskers compete for the cortical target territories, and that formation of a segregated functional whisker map coincides with emergence of the anatomical barrel map.

Isoflurane suppresses early cortical activity.

ObjectiveIsoflurane and other volatile anesthetics are widely used in children to induce deep and reversible coma, but they may also exert neurotoxic actions. The effects of volatile anesthetics on the immature brain activity remain elusive, however.MethodsThe effects of isoflurane on spontaneous and sensory-evoked activity were explored using intracortical extracellular field potential and multiple unit recordings in the rat barrel cortex from birth to adulthood.ResultsDuring the first postnatal week, isoflurane suppressed cortical activity in a concentration-dependent manner. At surgical anesthesia levels (1.5–2%), isoflurane completely suppressed the electroencephalogram and silenced cortical neurons. Although sensory potentials evoked by the principal whisker deflection persisted, sensory-evoked early gamma and spindle-burst oscillations were completely suppressed by isoflurane. Isoflurane-induced burst-suppression pattern emerged during the second postnatal week and matured through the first postnatal month. Bursts in adolescent and adult rats were characterized by activation of entire cortical columns with a leading firing of infragranular neurons, and were triggered by principal and adjacent whiskers stimulation, and by auditory and visual stimuli, indicating an involvement of horizontal connections in their generation and horizontal spread.InterpretationThe effects of isoflurane on cortical activity shift from total suppression of activity to burst-suppression pattern at the end of the first postnatal week. Developmental emergence of bursts likely involves a development of the intracortical short-and long-range connections. We hypothesize that complete suppression of cortical activity under isoflurane anesthesia during the first postnatal week may explain neuronal apoptosis stimulated by volatile anesthetics in the neonatal rats.