Electrical and chemical synapses between relay neurons in developing thalamus

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.

Loss of Ephaptic Contacts in the Murine Thalamus during Osmotic Demyelination Syndrome

BACKGROUND AND AIM

A murine model mimicking osmotic demyelination syndrome (ODS) revealed with histology in the relay posterolateral (VPL) and ventral posteromedial (VPM) thalamic nuclei adjoined nerve cell bodies in chronic hyponatremia, amongst the damaged 12 h and 48 h after reinstatement of osmolality. This report aims to verify and complement with ultrastructure other neurophysiology, immunohistochemistry, and molecular biochemistry data to assess the connexin-36 protein, as part of those hinted close contacts.This ODS investigation included four groups of mice: Sham (NN; n = 13), hyponatremic (HN; n = 11), those sacrificed 12 h after a fast restoration of normal natremia (ODS12h; n = 6) and mice sacrificed 48 h afterward, or ODS48 h (n = 9). Out of these, thalamic zones samples included NN (n = 2), HN (n = 2), ODS12h (n = 3) and ODS48h (n = 3).

RESULTS

Ultrastructure illustrated junctions between nerve cell bodies that were immunolabeled with connexin36 (Cx36) with light microscopy and Western blots. These cell's junctions were reminiscent of low resistance junctions characterized in other regions of the CNS with electrophysiology. Contiguous neurons showed neurolemma contacts in intact and damaged tissues according to their location in the ODS zones, at 12 h and 48 h post correction along with other demyelinating alterations. Neurons and ephaptic contact measurements indicated the highest alterations, including nerve cell necrosis in the ODS epicenter and damages decreased toward the outskirts of the demyelinated zone.

CONCLUSION

Ephapses contained C × 36between intact or ODS injured neurons in the thalamus appeared to be resilient beyond the core degraded tissue injuries. These could maintain intercellular ionic and metabolite exchanges between these lesser injured regions and, thus, would partake to some brain plasticity repairs.

Sleep spindles in primates: Modeling the effects of distinct laminar thalamocortical connectivity in core, matrix, and reticular thalamic circuits

Sleep spindles are associated with the beginning of deep sleep and memory consolidation and are disrupted in schizophrenia and autism. In primates, distinct core and matrix thalamocortical (TC) circuits regulate sleep spindle activity through communications that are filtered by the inhibitory thalamic reticular nucleus (TRN); however, little is known about typical TC network interactions and the mechanisms that are disrupted in brain disorders. We developed a primate-specific, circuit-based TC computational model with distinct core and matrix loops that can simulate sleep spindles. We implemented novel multilevel cortical and thalamic mixing, and included local thalamic inhibitory interneurons, and direct layer 5 projections of variable density to TRN and thalamus to investigate the functional consequences of different ratios of core and matrix node connectivity contribution to spindle dynamics. Our simulations showed that spindle power in primates can be modulated based on the level of cortical feedback, thalamic inhibition, and engagement of model core versus matrix, with the latter having a greater role in spindle dynamics. The study of the distinct spatial and temporal dynamics of core-, matrix-, and mix-generated sleep spindles establishes a framework to study disruption of TC circuit balance underlying deficits in sleep and attentional gating seen in autism and schizophrenia.

Role of auditory-somatosensory corticothalamic circuit integration in analgesia

Our sensory environment is permeated by a diverse array of auditory and somatosensory stimuli. The pairing of acoustic signals with concurrent or forthcoming tactile cues are abundant in everyday life and various survival contexts across species, thus deeming the ability to integrate sensory inputs arising from the combination of these stimuli as crucial. The corticothalamic system plays a critical role in orchestrating the construction, integration and distribution of the information extracted from these sensory modalities. In this mini-review, we provide a circuit-level description of the auditory corticothalamic pathway in conjunction with adjacent corticothalamic somatosensory projections. Although the extent of the functional interactions shared by these pathways is not entirely elucidated, activation of each of these systems appears to modulate sensory perception in the complementary domain. Several specific issues are reviewed. Under certain environmental noise conditions, the spectral information of a sound could induce modulations in nociception and even induce analgesia. We begin by discussing recent findings by Zhou et al. (2022) implicating the corticothalamic system in mediating sound-induced analgesia. Next, we describe relevant components of the corticothalamic pathway's functional organization. Additionally, we describe an emerging body of literature pointing to intrathalamic circuitry being optimal for controlling and selecting sensory signals across modalities, with the thalamic reticular nucleus being a candidate mechanism for directing cross-modal interactions. Finally, Ca2+ bursting in thalamic neurons evoked by the thalamic reticular nucleus is explored.

Thalamic control of sensory processing and spindles in a biophysical somatosensory thalamoreticular circuit model of wakefulness and sleep

Thalamoreticular circuitry plays a key role in arousal, attention, cognition, and sleep spindles, and is linked to several brain disorders. A detailed computational model of mouse somatosensory thalamus and thalamic reticular nucleus has been developed to capture the properties of over 14,000 neurons connected by 6 million synapses. The model recreates the biological connectivity of these neurons, and simulations of the model reproduce multiple experimental findings in different brain states. The model shows that inhibitory rebound produces frequency-selective enhancement of thalamic responses during wakefulness. We find that thalamic interactions are responsible for the characteristic waxing and waning of spindle oscillations. In addition, we find that changes in thalamic excitability control spindle frequency and their incidence. The model is made openly available to provide a new tool for studying the function and dysfunction of the thalamoreticular circuitry in various brain states.

Thalamus sends information about arousal but not valence to the amygdala

RATIONALE

The basolateral amygdala (BLA) and medial geniculate nucleus of the thalamus (MGN) have both been shown to be necessary for the formation of associative learning. While the role that the BLA plays in this process has long been emphasized, the MGN has been less well-studied and surrounded by debate regarding whether the relay of sensory information is active or passive.

OBJECTIVES

We seek to understand the role the MGN has within the thalamoamgydala circuit in the formation of associative learning.

METHODS

Here, we use optogenetics and in vivo electrophysiological recordings to dissect the MGN-BLA circuit and explore the specific subpopulations for evidence of learning and synthesis of information that could impact downstream BLA encoding. We employ various machine learning techniques to investigate function within neural subpopulations. We introduce a novel method to investigate tonic changes across trial-by-trial structure, which offers an alternative approach to traditional trial-averaging techniques.

RESULTS

We find that the MGN appears to encode arousal but not valence, unlike the BLA which encodes for both. We find that the MGN and the BLA appear to react differently to expected and unexpected outcomes; the BLA biased responses toward reward prediction error and the MGN focused on anticipated punishment. We uncover evidence of tonic changes by visualizing changes across trials during inter-trial intervals (baseline epochs) for a subset of cells.

CONCLUSION

We conclude that the MGN-BLA projector population acts as both filter and transferer of information by relaying information about the salience of cues to the amygdala, but these signals are not valence-specified.

Thalamic regulation of frontal interactions in human cognitive flexibility

Interactions across frontal cortex are critical for cognition. Animal studies suggest a role for mediodorsal thalamus (MD) in these interactions, but the computations performed and direct relevance to human decision making are unclear. Here, inspired by animal work, we extended a neural model of an executive frontal-MD network and trained it on a human decision-making task for which neuroimaging data were collected. Using a biologically-plausible learning rule, we found that the model MD thalamus compressed its cortical inputs (dorsolateral prefrontal cortex, dlPFC) underlying stimulus-response representations. Through direct feedback to dlPFC, this thalamic operation efficiently partitioned cortical activity patterns and enhanced task switching across different contingencies. To account for interactions with other frontal regions, we expanded the model to compute higher-order strategy signals outside dlPFC, and found that the MD offered a more efficient route for such signals to switch dlPFC activity patterns. Human fMRI data provided evidence that the MD engaged in feedback to dlPFC, and had a role in routing orbitofrontal cortex inputs when subjects switched behavioral strategy. Collectively, our findings contribute to the emerging evidence for thalamic regulation of frontal interactions in the human brain.

The expansion of frontal cortex during mammalian evolution suggested a prominent role in intelligent and adaptive behavior, overshadowing earlier parts of the brain. However, recent rodent studies have pointed to a role for the cognitive mediodorsal thalamus (MD) in sustaining and flexibly switching representations in the frontal cortex, but direct relevance to human decision-making are unclear. Here, inspired by animal work, we extended a neural model of an executive frontal-MD network and trained it on a human decision-making task for which human neuroimaging data were collected. We found that the model MD thalamus learned an abstract representation of its cortical inputs and provided direct feedback to frontal cortex leading to flexible computations and enhanced task switching. These abstract MD representations and ability to re-organize frontal computations created an efficient mechanism where MD can integrate input from other regions to select behavioral strategy dynamically. The model predicted an efficient route through the MD for frontal region interactions and we found consistent evidence in human neuroimaging data. Collectively, our findings contribute to the emerging evidence for thalamic regulation of frontal interactions in the human brain.

Morphology, physiology and synaptic connectivity of local interneurons in the mouse somatosensory thalamus

Abstract

The thalamic reticular nucleus (TRN) neurons, projecting across the external medullary lamina, have long been considered to be the only significant source of inhibition of the somatosensory ventral posterior (VP) nuclei of the thalamus. Here we report for the first time effective local inhibition and disinhibition in the VP. Inhibitory interneurons were found in GAD67–GFP‐expressing mice and studied using in vitro multiple patch clamp. Inhibitory interneurons have expansive bipolar or tripolar morphologies, reach across most of the VP nucleus and display low threshold bursting behaviour. They form triadic and non‐triadic synaptic connections onto thalamocortical relay neurons and other interneurons, mediating feedforward inhibition and disinhibition. Synaptic inputs arrive before those expected from the TRN neurons, suggesting that local inhibition plays an early and significant role in the functioning of the somatosensory thalamus.

Key points

The physiology and structure of local interneurons in the mouse somatosensory thalamus is described for the first time.

Inhibitory interneurons have extensive dendritic arborization providing significant local dendro‐dendritic inhibition in the somatosensory thalamus.

Triadic and non‐triadic synaptic connectivity onto thalamic relay neurons and other interneurons provides both local feedforward inhibition and disinhibition.

Interneurons of the somatosensory thalamus provide inhibition before the thalamic reticular nucleus, suggesting they play an important role in sensory perception.

Corticothalamic gating of population auditory thalamocortical transmission in mouse

The mechanisms that govern thalamocortical transmission are poorly understood. Recent data have shown that sensory stimuli elicit activity in ensembles of cortical neurons that recapitulate stereotyped spontaneous activity patterns. Here, we elucidate a possible mechanism by which gating of patterned population cortical activity occurs. In this study, sensory-evoked all-or-none cortical population responses were observed in the mouse auditory cortex in vivo and similar stochastic cortical responses were observed in a colliculo-thalamocortical brain slice preparation. Cortical responses were associated with decreases in auditory thalamic synaptic inhibition and increases in thalamic synchrony. Silencing of corticothalamic neurons in layer 6 (but not layer 5) or the thalamic reticular nucleus linearized the cortical responses, suggesting that layer 6 corticothalamic feedback via the thalamic reticular nucleus was responsible for gating stochastic cortical population responses. These data implicate a corticothalamic-thalamic reticular nucleus circuit that modifies thalamic neuronal synchronization to recruit populations of cortical neurons for sensory representations.

Rodent somatosensory thalamocortical circuitry: Neurons, synapses, and connectivity

As our understanding of the thalamocortical system deepens, the questions we face become more complex. Their investigation requires the adoption of novel experimental approaches complemented with increasingly sophisticated computational modeling. In this review, we take stock of current data and knowledge about the circuitry of the somatosensory thalamocortical loop in rodents, discussing common principles across modalities and species whenever appropriate. We review the different levels of organization, including the cells, synapses, neuroanatomy, and network connectivity. We provide a complete overview of this system that should be accessible for newcomers to this field while nevertheless being comprehensive enough to serve as a reference for seasoned neuroscientists and computational modelers studying the thalamocortical system. We further highlight key gaps in data and knowledge that constitute pressing targets for future experimental work. Filling these gaps would provide invaluable information for systematically unveiling how this system supports behavioral and cognitive processes.

Retinal ganglion cell interactions shape the developing mammalian visual system

Retinal ganglion cells (RGCs) serve as a crucial communication channel from the retina to the brain. In the adult, these cells receive input from defined sets of presynaptic partners and communicate with postsynaptic brain regions to convey features of the visual scene. However, in the developing visual system, RGC interactions extend beyond their synaptic partners such that they guide development before the onset of vision. In this Review, we summarize our current understanding of how interactions between RGCs and their environment influence cellular targeting, migration and circuit maturation during visual system development. We describe the roles of RGC subclasses in shaping unique developmental responses within the retina and at central targets. Finally, we highlight the utility of RNA sequencing and genetic tools in uncovering RGC type-specific roles during the development of the visual system.

Regulatory Roles of Metabotropic Glutamate Receptors on Synaptic Communication Mediated by Gap Junctions

Variations of synaptic strength are thought to underlie forms of learning and can functionally reshape neural circuits. Metabotropic glutamate receptors play key roles in regulating the strength of chemical synapses. However, information within neural circuits is also conveyed via a second modality of transmission: gap junction-mediated synapses. We review here evidence indicating that metabotropic glutamate receptors also play important roles in the regulation of synaptic communication mediated by neuronal gap junctions, also known as 'electrical synapses'. Activity-driven interactions between metabotropic glutamate receptors and neuronal gap junctions can lead to long-term changes in the strength of electrical synapses. Further, the regulatory action of metabotropic glutamate receptors on neuronal gap junctions is not restricted to adulthood but is also of critical relevance during brain development and contributes to the pathological mechanisms that follow brain injury.

Signal Propagation via Open-Loop Intrathalamic Architectures: A Computational Model

Propagation of signals across the cerebral cortex is a core component of many cognitive processes and is generally thought to be mediated by direct intracortical connectivity. The thalamus, by contrast, is considered to be devoid of internal connections and organized as a collection of parallel inputs to the cortex. Here, we provide evidence that "open-loop" intrathalamic pathways involving the thalamic reticular nucleus (TRN) can support propagation of oscillatory activity across the cortex. Recent studies support the existence of open-loop thalamo-reticulo-thalamic (TC-TRN-TC) synaptic motifs in addition to traditional closed-loop architectures. We hypothesized that open-loop structural modules, when connected in series, might underlie thalamic and, therefore cortical, signal propagation. Using a supercomputing platform to simulate thousands of permutations of a thalamocortical network based on physiological data collected in mice, rats, ferrets, and cats and in which select synapses were allowed to vary both by class and individually, we evaluated the relative capacities of closed-loop and open-loop TC-TRN-TC synaptic configurations to support both propagation and oscillation. We observed that (1) signal propagation was best supported in networks possessing strong open-loop TC-TRN-TC connectivity; (2) intrareticular synapses were neither primary substrates of propagation nor oscillation; and (3) heterogeneous synaptic networks supported more robust propagation of oscillation than their homogeneous counterparts. These findings suggest that open-loop, heterogeneous intrathalamic architectures might complement direct intracortical connectivity to facilitate cortical signal propagation.

Thalamic inhibitory circuits and network activity development

Inhibitory circuits in thalamus and cortex shape the major activity patterns observed by electroencephalogram (EEG) in the adult brain. Their delayed maturation and circuit integration, relative to excitatory neurons, suggest inhibitory neuronal development could be responsible for the onset of mature thalamocortical activity. Indeed, the immature brain lacks many inhibition-dependent activity patterns, such as slow-waves, delta oscillations and sleep-spindles, and instead expresses other unique oscillatory activities in multiple species including humans. Thalamus contributes significantly to the generation of these early oscillations. Compared to the abundance of studies on the development of inhibition in cortex, however, the maturation of thalamic inhibition is poorly understood. Here we review developmental changes in the neuronal and circuit properties of the thalamic relay and its interconnected inhibitory thalamic reticular nucleus (TRN) both in vitro and in vivo, and discuss their potential contribution to early network activity and its maturation. While much is unknown, we argue that weak inhibitory function in the developing thalamus allows for amplification of thalamocortical activity that supports the generation of early oscillations. The available evidence suggests that the developmental acquisition of critical thalamic oscillations such as slow-waves and sleep-spindles is driven by maturation of the TRN. Further studies to elucidate thalamic GABAergic circuit formation in relation to thalamocortical network function would help us better understand normal as well as pathological brain development.

Electrical synapses in mammalian CNS: Past eras, present focus and future directions

Gap junctions provide the basis for electrical synapses between neurons. Early studies in well-defined circuits in lower vertebrates laid the foundation for understanding various properties conferred by electrical synaptic transmission. Knowledge surrounding electrical synapses in mammalian systems unfolded first with evidence indicating the presence of gap junctions between neurons in various brain regions, but with little appreciation of their functional roles. Beginning at about the turn of this century, new approaches were applied to scrutinize electrical synapses, revealing the prevalence of neuronal gap junctions, the connexin protein composition of many of those junctions, and the myriad diverse neural systems in which they occur in the mammalian CNS. Subsequent progress indicated that electrical synapses constitute key elements in synaptic circuitry, govern the collective activity of ensembles of electrically coupled neurons, and in part orchestrate the synchronized neuronal network activity and rhythmic oscillations that underlie fundamental integrative processes. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Differential Excitation of Distally versus Proximally Targeting Cortical Interneurons by Unitary Thalamocortical Bursts

Thalamocortical neurons relay sensory and motor information to the neocortex using both single spikes and bursts; bursts prevail during low-vigilance states but also occur during awake behavior. Bursts are suggested to provide an alerting signal to the cortex and enhance stimulus detection, but the synaptic mechanisms underlying these effects are not clear, because the postsynaptic responses of different subtypes of cortical neurons to unitary thalamocortical bursts are mostly unknown. Using optogenetically guided recordings in mouse thalamocortical slices, we achieved the first reported paired intracellular recordings from nine monosynaptically connected thalamic and cortical neurons, including principal cells and two subtypes of inhibitory interneurons, and compared between cortical responses to single thalamocortical spikes and bursts. In 18 additional cortical neurons, we elicited unitary burst responses optogenetically. Short-term dynamics and temporal summation of burst-evoked EPSPs were cell-type dependent: in principal cells and somatostatin-containing (SOM), but not fast-spiking (FS), interneurons, peak response during a burst was on average more than twofold larger than the response to the first spike. Thus, firing a burst instead of a single spike would more than double the probability of firing in postsynaptic excitatory neurons and in SOM, but not FS, interneurons. Consistent with this prediction, FS interneurons held near firing threshold fired most often on the first burst component, whereas SOM interneurons fired only on the second or later components. By increasing excitation of principal cells together with SOM-mediated, distally directed inhibition, thalamocortical bursts could momentarily enhance the saliency of the ascending sensory stimulus over less urgent, top-down inputs.

SIGNIFICANCE STATEMENT

Thalamocortical neurons relay sensory and motor information to the cerebral cortex using both single spikes and high-frequency bursts, but the function of bursts is not fully understood. Using brain slices from mouse somatosensory thalamus and cortex, we achieved the first dual recordings of directly connected thalamic and cortical neurons and compared between cortical responses to single thalamic spikes and to bursts. We report that bursts enhanced the responses of excitatory neurons and of inhibitory interneurons that preferentially target dendrites. A potential consequence is that bursts will enhance the response to the immediate sensory event over responses to less urgent, modulatory inputs.

Electrical synapses and the development of inhibitory circuits in the thalamus

KEY POINTS

The thalamus is a structure critical for information processing and transfer to the cortex. Thalamic reticular neurons are inhibitory cells interconnected by electrical synapses, most of which require the gap junction protein connexin36 (Cx36). We investigated whether electrical synapses play a role in the maturation of thalamic networks by studying neurons in mice with and without Cx36. When Cx36 was deleted, inhibitory synapses were more numerous, although both divergent inhibitory connectivity and dendritic complexity were reduced. Surprisingly, we observed non-Cx36-dependent electrical synapses with unusual biophysical properties interconnecting some reticular neurons in mice lacking Cx36. The results of the present study suggest an important role for Cx36-dependent electrical synapses in the development of thalamic circuits.

ABSTRACT

Neurons within the mature thalamic reticular nucleus (TRN) powerfully inhibit ventrobasal (VB) thalamic relay neurons via GABAergic synapses. TRN neurons are also coupled to one another by electrical synapses that depend strongly on the gap junction protein connexin36 (Cx36). Electrical synapses in the TRN precede the postnatal development of TRN-to-VB inhibition. We investigated how the deletion of Cx36 affects the maturation of TRN and VB neurons, electrical coupling and GABAergic synapses by studying wild-type (WT) and Cx36 knockout (KO) mice. The incidence and strength of electrical coupling in TRN was sharply reduced, but not abolished, in KO mice. Surprisingly, electrical synapses between Cx36-KO neurons had faster voltage-dependent decay kinetics and conductance asymmetry (rectification) than did electrical synapses between WT neurons. The properties of TRN-mediated inhibition in VB also depended on the Cx36 genotype. Deletion of Cx36 increased the frequency and shifted the amplitude distributions of miniature IPSCs, whereas the paired-pulse ratio of evoked IPSCs was unaffected, suggesting that the absence of Cx36 led to an increase in GABAergic synaptic contacts. VB neurons from Cx36-KO mice also tended to have simpler dendritic trees and fewer divergent inputs from the TRN compared to WT cells. The findings obtained in the present study suggest that proper development of thalamic inhibitory circuitry, neuronal morphology, TRN cell function and electrical coupling requires Cx36. In the absence of Cx36, some TRN neurons express asymmetric electrical coupling mediated by other unidentified connexin subtypes.

Analysis of the role of the low threshold currents IT and Ih in intrinsic delta oscillations of thalamocortical neurons

Thalamocortical neurons are involved in the generation and maintenance of brain rhythms associated with global functional states. The repetitive burst firing of TC neurons at delta frequencies (1–4 Hz) has been linked to the oscillations recorded during deep sleep and during episodes of absence seizures. To get insight into the biophysical properties that are the basis for intrinsic delta oscillations in these neurons, we performed a bifurcation analysis of a minimal conductance-based thalamocortical neuron model including only the I_T channel and the sodium and potassium leak channels. This analysis unveils the dynamics of repetitive burst firing of TC neurons, and describes how the interplay between the amplifying variable m_T and the recovering variable h_T of the calcium channel I_T is sufficient to generate low threshold oscillations in the delta band. We also explored the role of the hyperpolarization activated cationic current I_h in this reduced model and determine that, albeit not required, I_h amplifies and stabilizes the oscillation.

Synaptic circuit organization of motor corticothalamic neurons

Corticothalamic (CT) neurons in layer 6 constitute a large but enigmatic class of cortical projection neurons. How they are integrated into intracortical and thalamo-cortico-thalamic circuits is incompletely understood, especially outside of sensory cortex. Here, we investigated CT circuits in mouse forelimb motor cortex (M1) using multiple circuit-analysis methods. Stimulating and recording from CT, intratelencephalic (IT), and pyramidal tract (PT) projection neurons, we found strong CT↔ CT and CT↔ IT connections; however, CT→IT connections were limited to IT neurons in layer 6, not 5B. There was strikingly little CT↔ PT excitatory connectivity. Disynaptic inhibition systematically accompanied excitation in these pathways, scaling with the amplitude of excitation according to both presynaptic (class-specific) and postsynaptic (cell-by-cell) factors. In particular, CT neurons evoked proportionally more inhibition relative to excitation (I/E ratio) than IT neurons. Furthermore, the amplitude of inhibition was tuned to match the amount of excitation at the level of individual neurons; in the extreme, neurons receiving no excitation received no inhibition either. Extending these studies to dissect the connectivity between cortex and thalamus, we found that M1-CT neurons and thalamocortical neurons in the ventrolateral (VL) nucleus were remarkably unconnected in either direction. Instead, VL axons in the cortex excited both IT and PT neurons, and CT axons in the thalamus excited other thalamic neurons, including those in the posterior nucleus, which additionally received PT excitation. These findings, which contrast in several ways with previous observations in sensory areas, illuminate the basic circuit organization of CT neurons within M1 and between M1 and thalamus.

Two functionally distinct networks of gap junction-coupled inhibitory neurons in the thalamic reticular nucleus

Gap junctions (GJs) electrically couple GABAergic neurons of the forebrain. The spatial organization of neuron clusters coupled by GJs is an important determinant of network function, yet it is poorly described for nearly all mammalian brain regions. Here we used a novel dye-coupling technique to show that GABAergic neurons in the thalamic reticular nucleus (TRN) of mice and rats form two types of GJ-coupled clusters with distinctive patterns and axonal projections. Most clusters are elongated narrowly along functional modules within the plane of the TRN, with axons that selectively inhibit local groups of relay neurons. However, some coupled clusters have neurons arrayed across the thickness of the TRN and target their axons to both first- and higher-order relay nuclei. Dye coupling was reduced, but not abolished, among cells of connexin36 knock-out mice. Our results suggest that GJs form two distinct types of inhibitory networks that correlate activity either within or across functional modules of the thalamus.

Electrical synapses and their functional interactions with chemical synapses

Brain function relies on the ability of neurons to communicate with each other. Interneuronal communication primarily takes place at synapses, where information from one neuron is rapidly conveyed to a second neuron. There are two main modalities of synaptic transmission: chemical and electrical. Far from functioning independently and serving unrelated functions, mounting evidence indicates that these two modalities of synaptic transmission closely interact, both during development and in the adult brain. Rather than conceiving synaptic transmission as either chemical or electrical, this article emphasizes the notion that synaptic transmission is both chemical and electrical, and that interactions between these two forms of interneuronal communication might be required for normal brain development and function.

Large-scale somatotopic refinement via functional synapse elimination in the sensory thalamus of developing mice

Functional synapse elimination and strengthening are crucial developmental processes in the formation of precise neuronal circuits in the somatosensory system, but the underlying alterations in topographical organization are not yet fully understood. To address this issue, we generated transgenic mice in which afferent fibers originating from the whisker-related brain region, called the maxillary principal trigeminal nucleus (PrV2), were selectively visualized with genetically expressed fluorescent protein. We found that functional synapse elimination drove and established large-scale somatotopic refinement even after the thalamic barreloid architecture was formed. Before functional synapse elimination, the whisker sensory thalamus was innervated by afferent fibers not only from the PrV2, but also from the brainstem nuclei representing other body parts. Most notably, only afferent fibers from PrV2 onto a whisker sensory thalamic neuron selectively survived and were strengthened, whereas other afferent fibers were preferentially eliminated via their functional synapse elimination. This large-scale somatotopic refinement was at least partially dependent on somatosensory experience. These novel results uncovered a previously unrecognized role of developmental synapse elimination in the large-scale, instead of the fine-scale, somatotopic refinement even after the initial segregation of the barreloid map.

Re-evaluation of connexins associated with motoneurons in rodent spinal cord, sexually dimorphic motor nuclei and trigeminal motor nucleus

Electrical synapses formed by neuronal gap junctions composed of connexin36 (Cx36) are a common feature in mammalian brain circuitry, but less is known about their deployment in spinal cord. It has been reported based on connexin mRNA and/or protein detection that developing and/or mature motoneurons express a variety of connexins, including Cx26, Cx32, Cx36 and Cx43 in trigeminal motoneurons, Cx36, Cx37, Cx40, Cx43 and Cx45 in spinal motoneurons, and Cx32 in sexually dimorphic motoneurons. We re-examined the localization of these connexins during postnatal development and in adult rat and mouse using immunofluorescence labeling for each connexin. We found Cx26 in association only with leptomeninges in the trigeminal motor nucleus (Mo5), Cx32 only with oligodendrocytes and myelinated fibers among motoneurons in this nucleus and in the spinal cord, and Cx37, Cx40 and Cx45 only with blood vessels in the ventral horn of spinal cord, including those among motoneurons. By freeze-fracture replica immunolabeling, > 100 astrocyte gap junctions but no neuronal gap junctions were found based on immunogold labeling for Cx43, whereas 16 neuronal gap junctions at postnatal day (P)4, P7 and P18 were detected based on Cx36 labeling. Punctate labeling for Cx36 was localized to the somatic and dendritic surfaces of peripherin-positive motoneurons in the Mo5, motoneurons throughout the spinal cord, and sexually dimorphic motoneurons at lower lumbar levels. In studies of electrical synapses and electrical transmission between developing and between adult motoneurons, our results serve to focus attention on mediation of this transmission by gap junctions composed of Cx36.

Neuronal gap junction coupling as the primary determinant of the extent of glutamate-mediated excitotoxicity

In the mammalian central nervous system (CNS), coupling of neurons by gap junctions (electrical synapses) increases during early postnatal development, then decreases, but increases in the mature CNS following neuronal injury, such as ischemia, traumatic brain injury and epilepsy. Glutamate-dependent neuronal death also occurs in the CNS during development and neuronal injury, i.e., at the time when neuronal gap junction coupling is increased. Here, we review our recent studies on regulation of neuronal gap junction coupling by glutamate in developing and injured neurons and on the role of gap junctions in neuronal cell death. A modified model of the mechanisms of glutamate-dependent neuronal death is discussed, which includes neuronal gap junction coupling as a critical part of these mechanisms.

Gap junctions in developing thalamic and neocortical neuronal networks

The presence of direct, cytoplasmatic, communication between neurons in the brain of vertebrates has been demonstrated a long time ago. These gap junctions have been characterized in many brain areas in terms of subunit composition, biophysical properties, neuronal connectivity patterns, and developmental regulation. Although interesting findings emerged, showing that different subunits are specifically regulated during development, or that excitatory and inhibitory neuronal networks exhibit various electrical connectivity patterns, gap junctions did not receive much further interest. Originally, it was believed that gap junctions represent simple passageways for electrical and biochemical coordination early in development. Today, we know that gap junction connectivity is tightly regulated, following independent developmental patterns for excitatory and inhibitory networks. Electrical connections are important for many specific functions of neurons, and are, for example, required for the development of neuronal stimulus tuning in the visual system. Here, we integrate the available data on neuronal connectivity and gap junction properties, as well as the most recent findings concerning the functional implications of electrical connections in the developing thalamus and neocortex.

Effect of phase response curve skewness on synchronization of electrically coupled neuronal oscillators

We investigate why electrically coupled neuronal oscillators synchronize or fail to synchronize using the theory of weakly coupled oscillators. Stability of synchrony and antisynchrony is predicted analytically and verified using numerical bifurcation diagrams. The shape of the phase response curve (PRC), the shape of the voltage time course, and the frequency of spiking are freely varied to map out regions of parameter spaces that hold stable solutions. We find that type 1 and type 2 PRCs can hold both synchronous and antisynchronous solutions, but the shape of the PRC and the voltage determine the extent of their stability. This is achieved by introducing a five-piecewise linear model to the PRC and a three-piecewise linear model to the voltage time course, and then analyzing the resultant eigenvalue equations that determine the stability of the phase-locked solutions. A single time parameter defines the skewness of the PRC, and another single time parameter defines the spike width and frequency. Our approach gives a comprehensive picture of the relation of the PRC shape, voltage time course, and stability of the resultant synchronous and antisynchronous solutions.

Neuronal gap junctions: making and breaking connections during development and injury

In the mammalian central nervous system (CNS), coupling of neurons by gap junctions (i.e., electrical synapses) and the expression of the neuronal gap junction protein, connexin 36 (Cx36), transiently increase during early postnatal development. The levels of both subsequently decline and remain low in the adult, confined to specific subsets of neurons. However, following neuronal injury [such as ischemia, traumatic brain injury (TBI), and epilepsy], the coupling and expression of Cx36 rise. Here we summarize new findings on the mechanisms of regulation of Cx36-containing gap junctions in the developing and mature CNS and following injury. We also review recent studies suggesting various roles for neuronal gap junctions and in particular their role in glutamate-mediated neuronal death.

Postnatal development of synaptic properties of the GABAergic projection from the inferior colliculus to the auditory thalamus

The development of auditory temporal processing is important for processing complex sounds as well as for acquiring reading and language skills. Neuronal properties and sound processing change dramatically in auditory cortex neurons after the onset of hearing. However, the development of the auditory thalamus or medial geniculate body (MGB) has not been well studied over this critical time window. Since synaptic inhibition has been shown to be crucial for auditory temporal processing, this study examined the development of a feedforward, GABAergic connection to the MGB from the inferior colliculus (IC), which is also the source of sensory glutamatergic inputs to the MGB. IC-MGB inhibition was studied using whole cell patch-clamp recordings from rat brain slices in current-clamp and voltage-clamp modes at three age groups: a prehearing group [postnatal day (P)7-P9], an immediate posthearing group (P15-P17), and a juvenile group (P22-P32) whose neuronal properties are largely mature. Membrane properties matured substantially across the ages studied. GABAA and GABAB inhibitory postsynaptic potentials were present at all ages and were similar in amplitude. Inhibitory postsynaptic potentials became faster to single shocks, showed less depression to train stimuli at 5 and 10 Hz, and were overall more efficacious in controlling excitability with age. Overall, IC-MGB inhibition becomes faster and more precise during a time period of rapid changes across the auditory system due to the codevelopment of membrane properties and synaptic properties.

Novel model for the mechanisms of glutamate-dependent excitotoxicity: role of neuronal gap junctions

In the mammalian central nervous system (CNS), coupling of neurons by gap junctions (electrical synapses) increases during early post-natal development, then decreases, but increases in the mature CNS following neuronal injury, such as ischemia, traumatic brain injury and epilepsy. Glutamate-dependent neuronal death also occurs in the CNS during development and neuronal injury, i.e., at the time when neuronal gap junction coupling is increased. Here, we review our recent studies on the regulation of neuronal gap junction coupling by glutamate during development and injury and on the role of gap junctions in neuronal cell death. A novel model of the mechanisms of glutamate-dependent neuronal death is discussed, which includes neuronal gap junction coupling as a critical part of these mechanisms.

Synergy between electrical coupling and membrane properties promotes strong synchronization of neurons of the mesencephalic trigeminal nucleus

Electrical synapses are known to form networks of extensively coupled neurons in various regions of the mammalian brain. The mesencephalic trigeminal (MesV) nucleus, formed by the somata of primary afferents originating in jaw-closing muscles, constitutes one of the first examples supporting the presence of electrical synapses in the mammalian CNS; however, the properties, functional organization, and developmental emergence of electrical coupling within this structure remain unknown. By combining electrophysiological, tracer coupling, and immunochemical analysis in brain slices of rat and mouse, we found that coupling is mostly restricted to pairs or small clusters of MesV neurons. Electrical transmission is supported by connexin36 (Cx36)-containing gap junctions at somato-somatic contacts where only a small proportion of channels appear to be open (∼0.1%). In marked contrast with most brain structures, coupling among MesV neurons increases with age, such that it is absent during early development and appears at postnatal day 8. Interestingly, the development of coupling parallels the development of intrinsic membrane properties responsible for repetitive firing in these neurons. We found that, acting together, sodium and potassium conductances enhance the transfer of signals with high-frequency content via electrical synapses, leading to strong spiking synchronization of the coupled neurons. Together, our data indicate that coupling in the MesV nucleus is restricted to mostly pairs of somata between which electrical transmission is supported by a surprisingly small fraction of the channels estimated to be present, and that coupling synergically interacts with specific membrane conductances to promote synchronization of these neurons.

An old hypothesis and new tools: Alfred Fessard's approach to the problem of consciousness

In 1954, a symposium was held in Canada on "Brain Mechanisms and Consciousness." It was a time for the promotion of international and interdisciplinary scientific cooperation, of new technological expectation, and of speculating about complex human behavior. Alfred Fessard's lecture on "Mechanisms of Nervous Integration and Conscious Experience" was one of the outstanding presentations, rich in critical analysis of the then available experimental data and in working hypothesis proposals. Reading the concept expressed by Fessard, it was found that several of his ideas had anticipated data obtained in modern research with new technologies.

Synaptogenesis of electrical and GABAergic synapses of fast-spiking inhibitory neurons in the neocortex

Parvalbumin-expressing fast-spiking (FS) cells are interconnected via GABAergic and electrical synapses and represent a major class of inhibitory interneurons in the neocortex. Synaptic connections among FS cells are critical for regulating network oscillations in the mature neocortex. However, it is unclear whether synaptic connections among FS interneurons also play a central role in the generation of patterned neuronal activity in the immature brain, which is thought to underlie the formation of neocortical circuits. Here, we investigated the developmental time course of synaptogenesis of FS cell in mouse visual cortex. In layer 5/6 (L5/6), we recorded from two or three FS and/or pyramidal (PYR) neurons to study the development of electrical and chemical synaptic interactions from postnatal day 3 (P3) to P18. We detected no evidence for functional connectivity for FS-FS or FS-PYR pairs at P3-P4. However, by P5-P6, we found that 20% of FS pairs were electrically coupled, and 24% of pairs were connected via GABAergic synapses; by P15-P18, 42% of FS pairs had established functional electrical synapses, and 47% of FS pairs were connected via GABAergic synapses. FS cell GABAergic inhibition of pyramidal cells showed a similar developmental time line, but no electrical coupling was detected for FS-PYR pairs. We found that synaptogenesis of electrical and GABAergic connections of FS cells takes place in the same period. Together, our results suggest that chemical and electrical connections among FS cells can contribute to patterned neocortical activity only by the end of the first postnatal week.

Thalamic Gap Junctions Control Local Neuronal Synchrony and Influence Macroscopic Oscillation Amplitude during EEG Alpha Rhythms

Although EEG alpha (α; 8-13 Hz) rhythms are often considered to reflect an "idling" brain state, numerous studies indicate that they are also related to many aspects of perception. Recently, we outlined a potential cellular substrate by which such aspects of perception might be linked to basic α rhythm mechanisms. This scheme relies on a specialized subset of rhythmically bursting thalamocortical (TC) neurons (high-threshold bursting cells) in the lateral geniculate nucleus (LGN) which are interconnected by gap junctions (GJs). By engaging GABAergic interneurons, that in turn inhibit conventional relay-mode TC neurons, these cells can lead to an effective temporal framing of thalamic relay-mode output. Although the role of GJs is pivotal in this scheme, evidence for their involvement in thalamic α rhythms has thus far mainly derived from experiments in in vitro slice preparations. In addition, direct anatomical evidence of neuronal GJs in the LGN is currently lacking. To address the first of these issues we tested the effects of the GJ inhibitors, carbenoxolone (CBX), and 18β-glycyrrhetinic acid (18β-GA), given directly to the LGN via reverse microdialysis, on spontaneous LGN and EEG α rhythms in behaving cats. We also examined the effect of CBX on α rhythm-related LGN unit activity. Indicative of a role for thalamic GJs in these activities, 18β-GA and CBX reversibly suppressed both LGN and EEG α rhythms, with CBX also decreasing neuronal synchrony. To address the second point, we used electron microscopy to obtain definitive ultrastructural evidence for the presence of GJs between neurons in the cat LGN. As interneurons show no phenotypic evidence of GJ coupling (i.e., dye-coupling and spikelets) we conclude that these GJs must belong to TC neurons. The potential significance of these findings for relating macroscopic changes in α rhythms to basic cellular processes is discussed.

The role of neuronal connexins 36 and 45 in shaping spontaneous firing patterns in the developing retina

Gap junction coupling synchronizes activity among neurons in adult neural circuits, but its role in coordinating activity during development is less known. The developing retina exhibits retinal waves--spontaneous depolarizations that propagate among retinal interneurons and drive retinal ganglion cells (RGCs) to fire correlated bursts of action potentials. During development, two connexin isoforms, connexin 36 (Cx36) and Cx45, are expressed in bipolar cells and RGCs, and therefore provide a potential substrate for coordinating network activity. To determine whether gap junctions contribute to retinal waves, we compared spontaneous activity patterns using calcium imaging, whole-cell recording, and multielectrode array recording in control, single-knock-out (ko) mice lacking Cx45 and double-knock-out (dko) mice lacking both isoforms. Wave frequency, propagation speed, and bias in propagation direction were similar in control, Cx36ko, Cx45ko, and Cx36/45dko retinas. However, the spontaneous firing rate of individual retinal ganglion cells was elevated in Cx45ko retinas, similar to Cx36ko retinas (Hansen et al., 2005; Torborg and Feller, 2005), a phenotype that was more pronounced in Cx36/45dko retinas. As a result, spatial correlations, as assayed by nearest-neighbor correlation and functional connectivity maps, were significantly altered. In addition, Cx36/45dko mice had reduced eye-specific segregation of retinogeniculate afferents. Together, these findings suggest that although Cx36 and Cx45 do not play a role in gross spatial and temporal propagation properties of retinal waves, they strongly modulate the firing pattern of individual RGCs, ensuring strongly correlated firing between nearby RGCs and normal patterning of retinogeniculate projections.

Increased GABAergic inhibition in the midline thalamus affects signaling and seizure spread in the hippocampus-prefrontal cortex pathway

PURPOSE

The midline thalamus is an important component of the circuitry in limbic seizures, but it is unclear how synaptic modulation of the thalamus affects that circuitry. In this study, we wished to understand how synaptic modulation of the thalamus can affect interregional signaling and seizure spread in the limbic network.

METHODS

We examined the effect of γ-aminobutyric acid (GABA) modulation of the mediodorsal (MD) region of the thalamus on responses in the prefrontal cortex (PFC) by stimulation of the subiculum (SB). Muscimol, a GABA(A) agonist, was injected into the MD, and the effect on local responses to subiculum stimulation was examined. Evoked potentials were induced in the MD and the PFC by low-frequency stimulation of the SB, and seizures were generated in the subiculum by repeated 20-Hz stimulations. The effect of muscimol in the MD on the evoked potentials and seizures was measured.

KEY FINDINGS

Thalamic responses to stimulation of the subiculum were reduced in the presence of muscimol. Reduction of the amplitudes of evoked potentials in the MD resulted in an attenuation of the late, thalamic components of the responses in the PFC, as well as of seizure durations.

SIGNIFICANCE

Activation of GABA(A) receptors in the midline thalamus not only causes changes within the thalamus, but it has broader effects on the limbic network. This work provides further evidence that synaptic modulation within the midline thalamus alters system excitability more broadly and reduces seizure activity.

Low-threshold Ca2+ current amplifies distal dendritic signaling in thalamic reticular neurons

The low-threshold transient calcium current (I(T)) plays a critical role in modulating the firing behavior of thalamic neurons; however, the role of I(T) in the integration of afferent information within the thalamus is virtually unknown. We have used two-photon laser scanning microscopy coupled with whole-cell recordings to examine calcium dynamics in the neurons of the strategically located thalamic reticular nucleus (TRN). We now report that a single somatic burst discharge evokes large-magnitude calcium responses, via I(T), in distal TRN dendrites. The magnitude of the burst-evoked calcium response was larger than those observed in thalamocortical projection neurons under the same conditions. We also demonstrate that direct stimulation of distal TRN dendrites, via focal glutamate application and synaptic activation, can locally activate distal I(T), producing a large distal calcium response independent of the soma/proximal dendrites. These findings strongly suggest that distally located I(T) may function to amplify afferent inputs. Boosting the magnitude ensures integration at the somatic level by compensating for attenuation that would normally occur attributable to passive cable properties. Considering the functional architecture of the TRN, elongated nature of their dendrites, and robust dendritic signaling, these distal dendrites could serve as sites of intense intra-modal/cross-modal integration and/or top-down modulation, leading to focused thalamocortical communication.