Dendroarchitecture and lateral inhibition in thalamic barreloids

Synaptic Origin of Early Sensory-evoked Oscillations in the Immature Thalamus

During the critical period of postnatal development, brain maturation is extremely sensitive to external stimuli. Newborn rodents already have functional somatosensory pathways and the thalamus, but the cortex is still forming. Immature thalamic synapses may produce large postsynaptic potentials in immature neurons, while non-synaptic membrane currents remain relatively weak and slow. The thalamocortical system generates spontaneous and evoked early gamma and spindle-burst oscillations in newborn rodents. How relatively strong synapses and weak intrinsic currents interact with each other and how they contribute to early thalamic activities remains largely unknown. Here, we performed local field potential (LFP), juxtacellular, and patch-clamp recordings in the somatosensory thalamus of urethane-anesthetized rat pups at postnatal days 6-7 with one whisker stimulation. We removed the overlying cortex and hippocampus to reach the thalamus with electrodes. Deflection of only one (the principal) whisker induced spikes in a particular thalamic cell. Whisker deflection evoked a group of large-amplitude excitatory events, likely originating from lemniscal synapses and multiple inhibitory postsynaptic events in thalamocortical cells. Large-amplitude excitatory events produced a group of spike bursts and could evoke a depolarization block. Juxtacellular recordings confirmed the partial inactivation of spikes. Inhibitory events prevented inactivation of action potentials and gamma-modulated neuronal firing. We conclude that the interplay of strong excitatory and inhibitory synapses and relatively weak intrinsic currents produces sensory-evoked early gamma oscillations in thalamocortical cells. We also propose that sensory-evoked large-amplitude excitatory events contribute to evoked spindle-bursts.

Endogenous Electric Field-Coupled PD@BP Biomimetic Periosteum Promotes Bone Regeneration through Sensory Nerve via Fanconi Anemia Signaling Pathway

To treat bone defects, repairing the nerve-rich periosteum is critical for repairing the local electric field. In this study, an endogenous electric field is coupled with 2D black phosphorus electroactive periosteum to explore its role in promoting bone regeneration through nerves. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to characterize the electrically active biomimetic periosteum. Here, the in vitro effects exerted by the electrically active periosteum on the transformation of Schwann cells into the repair phenotype, axon initial segment (AIS) and dense core vesicle (DCV) of sensory neurons, and bone marrow mesenchymal stem cells are assessed using SEM, immunofluorescence, RNA-sequencing, and calcium ion probes. The electrically active periosteum stimulates Schwann cells into a neuroprotective phenotype via the Fanconi anemia pathway, enhances the AIS effect of sensory neurons, regulates DCV transport, and releases neurotransmitters, promoting the osteogenic transformation of bone marrow mesenchymal stem cells. Microcomputed tomography and other in vivo techniques are used to study the effects of the electrically active periosteum on bone regeneration. The results show that the electrically active periosteum promotes nerve-induced osteogenic repair, providing a potential clinical strategy for bone regeneration.

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.

Sensory Abnormalities in Autism Spectrum Disorders: A Focus on the Tactile Domain, From Genetic Mouse Models to the Clinic

Sensory abnormalities are commonly recognized as diagnostic criteria in autism spectrum disorder (ASD), as reported in the last edition of the Diagnostic and Statistical Manual of Mental Disorder (DSM-V). About 90% of ASD individuals have atypical sensory experiences, described as both hyper- and hypo-reactivity, with abnormal responses to tactile stimulation representing a very frequent finding. In this review, we will address the neurobiological bases of sensory processing in ASD, with a specific focus of tactile sensitivity. In the first part, we will review the most relevant sensory abnormalities detected in ASD, and then focus on tactile processing deficits through the discussion of recent clinical and experimental studies. In the search for the neurobiological bases of ASD, several mouse models have been generated with knockout and humanized knockin mutations in many ASD-associated genes. Here, we will therefore give a brief overview of the anatomical structure of the mouse somatosensory system, and describe the somatosensory abnormalities so far reported in different mouse models of ASD. Understanding the neurobiological bases of sensory processing in ASD mouse models may represent an opportunity for a better comprehension of the mechanisms underlying sensory abnormalities, and for the development of novel effective therapeutic strategies.

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.

Whisker-Mediated Touch System in Rodents: From Neuron to Behavior

A key question in systems neuroscience is to identify how sensory stimuli are represented in neuronal activity, and how the activity of sensory neurons in turn is “read out” by downstream neurons and give rise to behavior. The choice of a proper model system to address these questions, is therefore a crucial step. Over the past decade, the increasingly powerful array of experimental approaches that has become available in non-primate models (e.g., optogenetics and two-photon imaging) has spurred a renewed interest for the use of rodent models in systems neuroscience research. Here, I introduce the rodent whisker-mediated touch system as a structurally well-established and well-organized model system which, despite its simplicity, gives rise to complex behaviors. This system serves as a behaviorally efficient model system; known as nocturnal animals, along with their olfaction, rodents rely on their whisker-mediated touch system to collect information about their surrounding environment. Moreover, this system represents a well-studied circuitry with a somatotopic organization. At every stage of processing, one can identify anatomical and functional topographic maps of whiskers; “barrelettes” in the brainstem nuclei, “barreloids” in the sensory thalamus, and “barrels” in the cortex. This article provides a brief review on the basic anatomy and function of the whisker system in rodents.

Input-dependent regulation of excitability controls dendritic maturation in somatosensory thalamocortical neurons

Input from the sensory organs is required to pattern neurons into topographical maps during development. Dendritic complexity critically determines this patterning process; yet, how signals from the periphery act to control dendritic maturation is unclear. Here, using genetic and surgical manipulations of sensory input in mouse somatosensory thalamocortical neurons, we show that membrane excitability is a critical component of dendritic development. Using a combination of genetic approaches, we find that ablation of N-methyl-d-aspartate (NMDA) receptors during postnatal development leads to epigenetic repression of Kv1.1-type potassium channels, increased excitability, and impaired dendritic maturation. Lesions to whisker input pathways had similar effects. Overexpression of Kv1.1 was sufficient to enable dendritic maturation in the absence of sensory input. Thus, Kv1.1 acts to tune neuronal excitability and maintain it within a physiological range, allowing dendritic maturation to proceed. Together, these results reveal an input-dependent control over neuronal excitability and dendritic complexity in the development and plasticity of sensory pathways.

Sensory input and neuronal activity are crucial for proper morphological development of neurons. Here, Frangeul and colleagues show that membrane excitability is a critical component of dendritic development in mouse somatosensory thalamocortical neurons.

Receptive field properties of cat perigeniculate neurons correlate with excitatory and inhibitory connectivity to LGN relay neurons

The cat perigeniculate nucleus (PGN) is a visual sector of the thalamic reticular nucleus that consists of GABAergic neurons. It receives excitatory axon-collateral input from relay neurons of the dorsal lateral geniculate nucleus (LGN) to which it provides inhibitory input. Thus, it is usually argued that the PGN works as feedback inhibition to the LGN. At the single neuron level, however, this circuit can also provide lateral inhibition. Which inhibition dominates in the visual circuit of the thalamus has yet to be well characterized. In this study, we conducted cross-correlation analysis of single spike trains simultaneously recorded from PGN and LGN neurons in anesthetized cats. For 12 pairs of functionally connected PGN and LGN neurons with overlapped receptive fields (RF), we quantitatively compared RF properties including the spatial frequency (SF) and temporal frequency (TF) tunings of each neuron. We found the SF and TF tunings of PGN neurons and LGN neurons were similar when there was only excitatory input from the LGN neuron to the PGN neuron, but different when the PGN neuron returned inhibitory inputs back, suggesting the circuit between PGN and LGN neurons works as lateral inhibition for these properties.

Adaptation of Thalamic Neurons Provides Information about the Spatiotemporal Context of Stimulus History

Adaptation of neural responses due to the history of sensory input has been observed across all sensory modalities. However, the computational role of adaptation is not fully understood, especially when one considers neural coding problems in which adaptation increases the ambiguity of the neural responses to simple stimuli. To address this, we quantified the impact of adaptation on the information conveyed by thalamic neurons about paired whisker stimuli in male rat. At the single neuron level, although paired-pulse adaptation reduces the information about the present stimulus, the information per spike increases. Moreover, the adapted response can convey significant amounts of information about whether, when and where a previous stimulus occurred. At the population level, ambiguity of the adapted responses about the present stimulus can be compensated for by large numbers of neurons. Therefore, paired-pulse adaptation does not reduce the discriminability of simple stimuli. It provides information about the spatiotemporal context of stimulus history.SIGNIFICANCE STATEMENT The present work provides a computational framework that demonstrates how adaptation allows neurons to encode spatiotemporal dynamics of stimulus history.

What single-cell stimulation has told us about neural coding

In recent years, single-cell stimulation experiments have resulted in substantial progress towards directly linking single-cell activity to movement and sensation. Recent advances in electrical recording and stimulation techniques have enabled control of single neuron spiking in vivo and have contributed to our understanding of neuronal coding schemes in the brain. Here, we review single neuron stimulation effects in different brain structures and how they vary with artificially inserted spike patterns. We briefly compare single neuron stimulation with other brain stimulation techniques. A key advantage of single neuron stimulation is the precise control of the evoked spiking patterns. Systematically varying spike patterns and measuring evoked movements and sensations enables 'decoding' of the single-cell spike patterns and provides insights into the readout mechanisms of sensory and motor cortical spikes.

Population-wide bias of surround suppression in auditory spatial receptive fields of the owl's midbrain

The physical arrangement of receptive fields (RFs) within neural structures is important for local computations. Nonuniform distribution of tuning within populations of neurons can influence emergent tuning properties, causing bias in local processing. This issue was studied in the auditory system of barn owls. The owl's external nucleus of the inferior colliculus (ICx) contains a map of auditory space in which the frontal region is overrepresented. We measured spatiotemporal RFs of ICx neurons using spatial white noise. We found a population-wide bias in surround suppression such that suppression from frontal space was stronger. This asymmetry increased with laterality in spatial tuning. The bias could be explained by a model of lateral inhibition based on the overrepresentation of frontal space observed in ICx. The model predicted trends in surround suppression across ICx that matched the data. Thus, the uneven distribution of spatial tuning within the map could explain the topography of time-dependent tuning properties. This mechanism may have significant implications for the analysis of natural scenes by sensory systems.

Surround suppression and sparse coding in visual and barrel cortices

During natural vision the entire retina is stimulated. Likewise, during natural tactile behaviors, spatially extensive regions of the somatosensory surface are co-activated. The large spatial extent of naturalistic stimulation means that surround suppression, a phenomenon whose neural mechanisms remain a matter of debate, must arise during natural behavior. To identify common neural motifs that might instantiate surround suppression across modalities, we review models of surround suppression and compare the evidence supporting the competing ideas that surround suppression has either cortical or sub-cortical origins in visual and barrel cortex. In the visual system there is general agreement lateral inhibitory mechanisms contribute to surround suppression, but little direct experimental evidence that intracortical inhibition plays a major role. Two intracellular recording studies of V1, one using naturalistic stimuli (Haider et al., 2010), the other sinusoidal gratings (Ozeki et al., 2009), sought to identify the causes of reduced activity in V1 with increasing stimulus size, a hallmark of surround suppression. The former attributed this effect to increased inhibition, the latter to largely balanced withdrawal of excitation and inhibition. In rodent primary somatosensory barrel cortex, multi-whisker responses are generally weaker than single whisker responses, suggesting multi-whisker stimulation engages similar surround suppressive mechanisms. The origins of suppression in S1 remain elusive: studies have implicated brainstem lateral/internuclear interactions and both thalamic and cortical inhibition. Although the anatomical organization and instantiation of surround suppression in the visual and somatosensory systems differ, we consider the idea that one common function of surround suppression, in both modalities, is to remove the statistical redundancies associated with natural stimuli by increasing the sparseness or selectivity of sensory responses.

Actions of Xanthurenic acid, a putative endogenous Group II metabotropic glutamate receptor agonist, on sensory transmission in the thalamus

Xanthurenic acid (XA), a molecule arising from tryptophan metabolism by transamination of 3-hydroxykynurenine, has recently been identified as an endogenous Group II (mGlu2 and mGlu3) metabotropic glutamate (mGlu) receptor ligand in vitro. Impairments in Group II mGlu receptor expression and function have been implicated in the pathophysiology of schizophrenia, as have multiple steps in the kynurenine metabolism pathway. Therefore, we examined XA in vivo to further investigate its potential as a Group II mGlu receptor ligand using a preparation that has been previously demonstrated to efficiently reveal the action of other Group II mGlu receptor ligands in vivo. Extracellular single-neurone recordings were made in the rat ventrobasal thalamus (VB) in conjunction with iontophoresis of agonists, an antagonist and a positive allosteric modulator and/or intravenous (i.v.) injection of XA. We found the XA effect on sensory inhibition, when applied iontophoretically and i.v., was similar to that of other Group II mGlu receptor agonists in reducing inhibition evoked in the VB from the thalamic reticular nucleus upon physiological sensory stimulation. Furthermore, we postulate that XA may be the first potential endogenous allosteric agonist (termed 'endocoid') for the mGlu receptors. As the Group II receptors and kynurenine metabolism pathway have both been heavily implicated in the pathophysiology of schizophrenia, XA could play a pivotal role in antipsychotic research as this potential endocoid represents both a convergence within these two biological parameters and a novel class of Group II mGlu receptor ligand. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Coding of apparent motion in the thalamic nucleus of the rat vibrissal somatosensory system

While exploring objects, rats make multiple contacts using their whiskers, thereby generating complex patterns of sensory information. The cerebral structures that process this information in the somatosensory system show discrete patterns of anatomically distinct units, each corresponding to one whisker. Moreover, the feedforward and feedback connections are remarkably topographic, with little cross-whisker divergence before reaching the cortical network. Despite this parallel design, information processing from several whiskers has been reported in subcortical nuclei. Here, we explored whether sensory neurons in the ventral posterior medial nucleus (VPM) of the thalamus encode emergent properties of complex multiwhisker stimulations. Using a 24-whisker stimulator, we tested the responses of VPM neurons to sequences of caudal deflections that generated an apparent motion in eight different directions across the whiskerpad. Overall, 45% of neurons exhibited an evoked increase in firing rate significantly selective to the direction of apparent motion of the global stimulus. Periods of suppression of firing rate were often observed, but were generally not selective. Global motion selectivity of VPM neurons could occur regardless of the extent and spatial organization of their receptive fields, and of their selectivity for the direction of motion of their principal whisker. To investigate whether the global selectivity could be due to corticothalamic feedback connections, we inactivated the barrel cortex while repeating the stimulation protocol. For most VPM neurons, the direction selectivity decreased but was still present. These results suggest that nonlinear processing of stimuli from different whiskers emerges in subcortical nuclei and is amplified by the corticofugal feedback.

Positive allosteric modulation reveals a specific role for mGlu2 receptors in sensory processing in the thalamus

Group II metabotropic glutamate receptor (mGlu) modulation of sensory processing in the rat ventrobasal thalamic nucleus (VB) has been extensively studied in vivo. However, it is not yet known what the relative contributions are of the Group II mGlu receptor subtypes (mGlu2 and mGlu3) to this modulation, nor to what extent these receptors may be activated under physiological conditions during this process. Using single-neurone recording in the rat VB in vivo with local application of the selective Group II agonist LY354740 and the subtype selective mGlu2 positive allosteric modulator (PAM) LY487379, our findings were twofold. Firstly, we found that there is an mGlu2 component to the effects of LY354740 on sensory responses in the VB. Secondly, we have demonstrated that application of the PAM alone can modulate sensory responses of single neurones in vivo. This indicates that mGlu2 receptors can be activated by endogenous agonist following physiological sensory stimulation. We speculate that the mGlu2 subtype could be activated under physiological stimulus-evoked conditions by 'glutamate spillover' from synapses between excitatory sensory afferents and VB neurones that can lead to a reduction in sensory-evoked inhibition arising from the thalamic reticular nucleus (TRN). We propose that this potential mGlu2 receptor modulation of inhibition could play an important role in discerning relevant information from background activity upon physiological sensory stimulation. Furthermore, this could be a site of action for mGlu2 PAMs to modulate cognitive processes.

On-Center/Inhibitory-Surround Decorrelation via Intraglomerular Inhibition in the Olfactory Bulb Glomerular Layer

Classical lateral inhibition, which relies on spatially ordered neural representations of physical stimuli, cannot decorrelate sensory representations in which stimulus properties are represented non-topographically. Recent theoretical and experimental studies indicate that such a non-topographical representation of olfactory stimuli predominates in olfactory bulb, thereby refuting the classical view that olfactory decorrelation is mediated by lateral inhibition comparable to that in the retina. Questions persist, however, regarding how well non-topographical decorrelation models can replicate the inhibitory “surround” that has been observed experimentally (with respect to odor feature-similarity) in olfactory bulb principal neurons, analogous to the spatial inhibitory surround generated by lateral inhibition in retina. Using two contrasting scenarios of stimulus representation – one “retinotopically” organized and one in which receptive fields are unpredictably distributed as they are in olfactory bulb – we here show that intracolumnar inhibitory interactions between local interneurons and principal neurons successfully decorrelate similar sensory representations irrespective of the scenario of representation. In contrast, lateral inhibitory interactions between these same neurons in neighboring columns are only able to effectively decorrelate topographically organized representations. While anatomical substrates superficially consistent with both types of inhibition exist in olfactory bulb, of the two only local intraglomerular inhibition suffices to mediate olfactory decorrelation.

Evidence for thalamic involvement in the thermal grill illusion: an FMRI study

BACKGROUND

Perceptual illusions play an important role in untangling neural mechanisms underlying conscious phenomena. The thermal grill illusion (TGI) has been suggested as a promising model for exploring percepts involved in neuropathic pain, such as cold-allodynia (pain arising from contact with innocuous cold). The TGI is an unpleasant/painful sensation from touching juxtapositioned bars of cold and warm innocuous temperatures.

AIM

To develop an MRI-compatible TGI-unit and explore the supraspinal correlates of the illusion, using fMRI, in a group of healthy volunteers.

METHODS

We constructed a TGI-thermode allowing the rapid presentation of warm(41°C), cold(18°C) and interleaved(41°C+18°C = TGI) temperatures in an fMRI-environment. Twenty volunteers were tested. The affective-motivational ("unpleasantness") and sensory-disciminatory ("pain-intensity") dimensions of each respective stimulus were rated. Functional images were analyzed at a corrected α-level <0.05.

RESULTS

The TGI was rated as significantly more unpleasant and painful than stimulation with each of its constituent temperatures. Also, the TGI was rated as significantly more unpleasant than painful. Thermal stimulation versus neutral baseline revealed bilateral activations of the anterior insulae and fronto-parietal regions. Unlike its constituent temperatures the TGI displayed a strong activation of the right (contralateral) thalamus. Exploratory contrasts at a slightly more liberal threshold-level also revealed a TGI-activation of the right mid/anterior insula, correlating with ratings of unpleasantness (rho = 0.31).

CONCLUSION/SIGNIFICANCE

To the best of our knowledge, this is the first fMRI-study of the TGI. The activation of the anterior insula is consistent with this region's putative role in processing of homeostatically relevant feeling-states. Our results constitute the first neurophysiologic evidence of thalamic involvement in the TGI. Similar thalamic activity has previously been observed during evoked cold-allodynia in patients with central neuropathic pain. Our results further the understanding of the supraspinal correlates of the TGI-phenomenon and pave the way for future inquiries into if and how it may relate to neuropathic pain.

Nanostimulation: Manipulation of Single Neuron Activity by Juxtacellular Current Injection

In the mammalian brain, many thousands of single-neuron recording studies have been performed but less than 10 single-cell stimulation studies. This paucity of single-cell stimulation data reflects a lack of easily applicable single-cell stimulation techniques. We provide a detailed description of the procedures involved in nanostimulation, a single-cell stimulation method derived from the juxtacellular labeling technique. Nanostimulation is easy to apply and can be directed to a wide variety of identifiable neurons in anesthetized and awake animals. We describe the recording approach and the parameters of the electric configuration underlying nanostimulation. We use glass pipettes with a DC resistance of 4–7 MΩ. Obtaining the juxtacellular configuration requires a close contact between pipette tip and neuron and is associated with a several-fold increase in resistance to values ≥20 MΩ. The recorded action potential (AP) amplitude grows to ≥2 mV, and neurons can be activated with currents in the nanoampere range—hence the term nanostimulation. While exact AP timing has not been achieved, AP frequency and AP number can be parametrically controlled. We demonstrate that nanostimulation can also be used to selectively inhibit sensory responses in identifiable neurons. Nanostimulation is biophysically similar to electroporation, and based on this assumption, we argue that nanostimulation operates on membranes in the micrometer area directly below the pipette tip, where membrane pores are induced by high transmembrane voltage. There is strong evidence to suggest that nanostimulation selectively activates single neurons and that the evoked effects are cell-specific. Nanostimulation therefore holds great potential for elucidating how single neurons contribute to behavior.

Modeling the emergence of whisker direction maps in rat barrel cortex

Based on measuring responses to rat whiskers as they are mechanically stimulated, one recent study suggests that barrel-related areas in layer 2/3 rat primary somatosensory cortex (S1) contain a pinwheel map of whisker motion directions. Because this map is reminiscent of topographic organization for visual direction in primary visual cortex (V1) of higher mammals, we asked whether the S1 pinwheels could be explained by an input-driven developmental process as is often suggested for V1. We developed a computational model to capture how whisker stimuli are conveyed to supragranular S1, and simulate lateral cortical interactions using an established self-organizing algorithm. Inputs to the model each represent the deflection of a subset of 25 whiskers as they are contacted by a moving stimulus object. The subset of deflected whiskers corresponds with the shape of the stimulus, and the deflection direction corresponds with the movement direction of the stimulus. If these two features of the inputs are correlated during the training of the model, a somatotopically aligned map of direction emerges for each whisker in S1. Predictions of the model that are immediately testable include (1) that somatotopic pinwheel maps of whisker direction exist in adult layer 2/3 barrel cortex for every large whisker on the rat's face, even peripheral whiskers; and (2) in the adult, neurons with similar directional tuning are interconnected by a network of horizontal connections, spanning distances of many whisker representations. We also propose specific experiments for testing the predictions of the model by manipulating patterns of whisker inputs experienced during early development. The results suggest that similar intracortical mechanisms guide the development of primate V1 and rat S1.

Glycine-mediated changes of onset reliability at a mammalian central synapse

Glycine is an inhibitory neurotransmitter activating a chloride conductance in the mammalian CNS. In vitro studies from brain slices revealed a novel presynaptic site of glycine action in the medial nucleus of the trapezoid body (MNTB) which increases the release of the excitatory transmitter glutamate from the calyx of Held. Here, we investigate the action of glycine on action potential firing of single MNTB neurons from the gerbil under acoustic stimulation in vivo. Iontophoretic application of the glycine receptor antagonist strychnine caused a significant decrease in spontaneous and sound-evoked firing rates throughout the neurons' excitatory response areas, with the largest changes at the respective characteristic frequency (CF). The decreased firing rate was accompanied by longer and more variable onset latencies of sound-evoked responses. Outside the neurons' excitatory response areas, firing rates increased during the application of strychnine due to a reduction of inhibitory sidebands, causing a broadening of frequency tuning. These results indicate that glycine enhances the efficacy for on-CF stimuli, while simultaneously suppressing synaptic transmission for off-CF stimuli. These in vivo results provide evidence of multiple excitatory and inhibitory glycine effects on the same neuronal population in the mature mammalian CNS.

A new thalamic pathway of vibrissal information modulated by the motor cortex

Three ascending pathways of information processing have been identified so far in the vibrissal system of rodents. In the ventral posterior medial nucleus of the thalamus, two of these pathways convey information through the core and tail of barrel-associated structures, called barreloids. The other pathway transits through the posterior group nucleus. The present study provides anatomical and electrophysiological evidence for the existence of an additional pathway that passes through the head of the barreloids. This pathway arises from multiwhisker-responsive cells in the principal trigeminal nucleus and differs from the classic lemniscal pathway, in that constituent thalamic cells have multiwhisker receptive field and receive corticothalamic input from lamina 6 of the vibrissa motor cortex. It is suggested that this pathway might be involved in relaying signals encoding phase of whisker motion during free whisking.

Information processing streams in rodent barrel cortex: the differential functions of barrel and septal circuits

Rodent somatosensory cortex contains an isomorphic map of the mystacial whiskers in which each whisker is represented by neuronal populations, or barrels, that are separated from each other by intervening septa. Separate afferent pathways convey somatosensory information to the barrels and septa that represent the input stages for 2 partially segregated circuits that extend throughout the other layers of barrel cortex. Whereas the barrel-related circuits process spatiotemporal information generated by whisker contact with external objects, the septa-related circuits encode the frequency and other kinetic features of active whisker movements. The projection patterns from barrel cortex indicate that information processed by the septa-related circuits is used both separately and in combination with information from the barrel-related circuits to mediate specific functions. According to this theory, outputs from the septal processing stream modulate the brain regions that regulate whisking behavior, whereas both processing streams cooperate with each other to identify external stimuli encountered by passive or active whisker movements. This theoretical view prompts several testable hypotheses about the coordination of neuronal activity during whisking behavior. Foremost among these, motor brain regions that control whisker movements are more strongly coordinated with the septa-related circuits than with the barrel-related circuits.

Thalamic synchrony and dynamic regulation of global forebrain oscillations

The circuitry within the thalamus creates an intrinsic oscillatory unit whose function depends critically on reciprocal synaptic connectivity between excitatory thalamocortical relay neurons and inhibitory thalamic reticular neurons along with a robust post-inhibitory rebound mechanism in relay neurons. Feedforward and feedback connections between cortex and thalamus reinforce the thalamic oscillatory activity into larger thalamocortical networks to generate sleep spindles and spike-wave discharge of generalized absence epilepsy. The degree of synchrony within the thalamic network seems to be crucial in determining whether normal (spindle) or pathological (spike-wave) oscillations occur, and recent studies show that regulation of excitability in the reticular nucleus leads to dynamical modulation of the state of the thalamic circuit and provide a basis for explaining how a variety of unrelated genetic alterations might lead to the spike-wave phenotype. In addition, given the central role of the reticular nucleus in generating spike-wave discharge, these studies have suggested specific interventions that would prevent seizures while still allowing normal spindle generation to occur. This review is part of the INMED/TINS special issue Physiogenic and pathogenic oscillations: the beauty and the beast, based on presentations at the annual INMED/TINS symposium (http://inmednet.com).

Cortical modulation of spatial and angular tuning maps in the rat thalamus

The massive feedback projections from cortex to the thalamus modulate sensory information transmission in many ways. We investigated the role of corticothalamic feedback projections on the directional selectivity (angular tuning) of neurons in the rat ventral posterior medial (VPM) nucleus to stimulation of their principal whisker. The angular tuning properties of single VPM neurons were compared before and after epochs of electrical stimulation of layer VI feedback neurons in the ipsilateral cortex under urethane anesthesia. Microstimulation of layer VI in "matched" (homologous) barrel columns sharpens the angular tuning curves of single VPM neurons that are tuned to the same direction as the stimulation site in the cortex. Further, microstimulation rotates the angular preference of VPM neurons initially tuned to a different direction toward the direction that cortical neurons prefer. Stimulation in "mismatched" (nonhomologous) barrel columns suppresses responses without consistent effects on angular tuning. We conclude that the primary sensory cortex exerts a significant influence on both spatial and angular tuning maps in the relay nuclei that project to it. The results suggest that the tuning properties of VPM cells in the behaving animal are continually modified to optimize perception of the most salient incoming messages.

Somatosensory integration controlled by dynamic thalamocortical feed-forward inhibition

The temporal features of tactile stimuli are faithfully represented by the activity of neurons in the somatosensory cortex. However, the cellular mechanisms that enable cortical neurons to report accurate temporal information are not known. Here, we show that in the rodent barrel cortex, the temporal window for integration of thalamic inputs is under the control of thalamocortical feed-forward inhibition and can vary from 1 to 10 ms. A single thalamic fiber can trigger feed-forward inhibition and contacts both excitatory and inhibitory cortical neurons. The dynamics of feed-forward inhibition exceed those of each individual synapse in the circuit and are captured by a simple disynaptic model of the thalamocortical projection. The variations in the integration window produce changes in the temporal precision of cortical responses to whisker stimulation. Hence, feed-forward inhibitory circuits, classically known to sharpen spatial contrast of tactile inputs, also increase the temporal resolution in the somatosensory cortex.

Dynamics of sensory thalamocortical synaptic networks during information processing states

The thalamocortical network consists of the pathways that interconnect the thalamus and neocortex, including thalamic sensory afferents, corticothalamic and thalamocortical pathways. These pathways are essential to acquire, analyze, store and retrieve sensory information. However, sensory information processing mostly occurs during behavioral arousal, when activity in thalamus and neocortex consists of an electrographic sign of low amplitude fast activity, known as activation, which is caused by several neuromodulator systems that project to the thalamocortical network. Logically, in order to understand how the thalamocortical network processes sensory information it is essential to study its response properties during states of activation. This paper reviews the temporal and spatial response properties of synaptic pathways in the whisker thalamocortical network of rodents during activated states as compared to quiescent (non-activated) states. The evidence shows that these pathways are differentially regulated via the effects of neuromodulators as behavioral contingencies demand. Thus, during activated states, the temporal and spatial response properties of pathways in the thalamocortical network are transformed to allow the processing of sensory information.

The vibrissal system as a model of thalamic operations

The highly segregated organization of the vibrissal system of rodents offers a unique opportunity to address key issues about thalamic operations in primary sensory and second order thalamic nuclei. In this short review, evidence showing that reticular thalamic neurons and relay cells with receptive fields on the same vibrissa form topographically closed loop connections has been summarized. Within whisker-related thalamic modules, termed barreloids, reticular axons synapse onto the cell bodies and dendrites of residing neurons as well as onto the distal dendrites of neurons that are located in adjacent barreloids. This arrangement provides a substrate for a mechanism of lateral inhibition whereby the spread of dendritic trees among surrounding barreloids determines whisker-specific patterns of lateral inhibition. The relay of sensory inputs in the posterior group, a second order nucleus associated with the vibrissal system is also examined. It is shown that in lightly anesthetized rats posterior group cells are tonically inhibited by GABAergic neurons of the ventral division of zona incerta. These observations suggest that a mechanism of disinhibition controls transmission of sensory signals in the posterior group nucleus. We further propose that disinhibition operates in a top-down manner, via motor instructions sent by cortex to brainstem and spinal cord. In this way posterior group nucleus would forward to the cerebral cortex sensory information that is contingent upon its action.