Two dynamically distinct circuits drive inhibition in the sensory thalamus

Most sensory information destined for the neocortex is relayed through the thalamus, where considerable transformation occurs^1,2. One means of transformation involves interactions between excitatory thalamocortical neurons that carry data to the cortex and inhibitory neurons of the thalamic reticular nucleus (TRN) that regulate the flow of those data^3-6. Although the importance of the TRN has long been recognised^7-9, understanding of its cell types, their organization and their functional properties has lagged behind that of the thalamocortical systems they control. Here we address this by investigating the somatosensory and visual circuits of the TRN in mice. In the somatosensory TRN we observed two groups of genetically defined neurons that are topographically segregated and physiologically distinct, and that connect reciprocally with independent thalamocortical nuclei through dynamically divergent synapses. Calbindin-expressing cells-located in the central core-connect with the ventral posterior nucleus, the primary somatosensory thalamocortical relay. By contrast, somatostatin-expressing cells-which reside along the surrounding edges of the TRN-synapse with the posterior medial thalamic nucleus, a higher-order structure that carries both top-down and bottom-up information^10-12. The two TRN cell groups process their inputs in pathway-specific ways. Synapses from the ventral posterior nucleus to central TRN cells transmit rapid excitatory currents that depress deeply during repetitive activity, driving phasic spike output. Synapses from the posterior medial thalamic nucleus to edge TRN cells evoke slower, less depressing excitatory currents that drive more persistent spiking. Differences in the intrinsic physiology of TRN cell types, including state-dependent bursting, contribute to these output dynamics. The processing specializations of these two somatosensory TRN subcircuits therefore appear to be tuned to the signals they carry-a primary central subcircuit tuned to discrete sensory events, and a higher-order edge subcircuit tuned to temporally distributed signals integrated from multiple sources. The structure and function of visual TRN subcircuits closely resemble those of the somatosensory TRN. These results provide insights into how subnetworks of TRN neurons may differentially process distinct classes of thalamic information.

Infrabarrels Are Layer 6 Circuit Modules in the Barrel Cortex that Link Long-Range Inputs and Outputs

The rodent somatosensory cortex includes well-defined examples of cortical columns-the barrel columns-that extend throughout the cortical depth and are defined by discrete clusters of neurons in layer 4 (L4) called barrels. Using the cell-type-specific Ntsr1-Cre mouse line, we found that L6 contains infrabarrels, readily identifiable units that align with the L4 barrels. Corticothalamic (CT) neurons and their local axons cluster within the infrabarrels, whereas corticocortical (CC) neurons are densest between infrabarrels. Optogenetic experiments showed that CC cells received robust input from somatosensory thalamic nuclei, whereas CT cells received much weaker thalamic inputs. We also found that CT neurons are intrinsically less excitable, revealing that both synaptic and intrinsic mechanisms contribute to the low firing rates of CT neurons often reported in vivo. In summary, infrabarrels are discrete cortical circuit modules containing two partially separated excitatory networks that link long-distance thalamic inputs with specific outputs.

A corticothalamic switch: controlling the thalamus with dynamic synapses

Corticothalamic neurons provide massive input to the thalamus. This top-down projection may allow the cortex to regulate sensory processing by modulating the excitability of thalamic cells. Layer 6 corticothalamic neurons monosynaptically excite thalamocortical cells, but also indirectly inhibit them by driving inhibitory cells of the thalamic reticular nucleus. Whether corticothalamic activity generally suppresses or excites the thalamus remains unclear. Here we show that the corticothalamic influence is dynamic, with the excitatory-inhibitory balance shifting in an activity-dependent fashion. During low-frequency activity, corticothalamic effects are mainly suppressive, whereas higher-frequency activity (even a short bout of gamma frequency oscillations) converts the corticothalamic influence to enhancement. The mechanism of this switching depends on distinct forms of short-term synaptic plasticity across multiple corticothalamic circuit components. Our results reveal an activity-dependent mechanism by which corticothalamic neurons can bidirectionally switch the excitability and sensory throughput of the thalamus, possibly to meet changing behavioral demands.

Electrical and chemical synapses between relay neurons in developing thalamus

Gap junction-mediated electrical synapses interconnect diverse types of neurons in the mammalian brain, and they may play important roles in the synchronization and development of neural circuits. Thalamic relay neurons are the major source of input to neocortex. Electrical synapses have not been directly observed between relay neurons in either developing or adult animals. We tested for electrical synapses by recording from pairs of relay neurons in acute slices of developing ventrobasal nucleus (VBN) of the thalamus from rats and mice. Electrical synapses were common between VBN relay neurons during the first postnatal week, and then declined sharply during the second week. Electrical coupling was reduced among cells of connexin36 (Cx36) knockout mice; however, some neuron pairs remained coupled. This implies that electrical synapses between the majority of coupled VBN neurons require Cx36 but that other gap junction proteins also contribute. The anatomical distribution of a beta-galactosidase reporter indicated that Cx36 was expressed in some VBN neurons during the first postnatal week and sharply declined over the second week, consistent with our physiological results. VBN relay neurons also communicated via chemical synapses. Rare pairs of relay neurons excited one another monosynaptically. Much more commonly, spikes in one relay neuron evoked disynaptic inhibition (via the thalamic reticular nucleus) in the same or a neighbouring relay neuron. Disynaptic inhibition between VBN cells emerged as electrical coupling was decreasing, during the second postnatal week. Our results demonstrate that thalamic relay neurons communicate primarily via electrical synapses during early postnatal development, and then lose their electrical coupling as a chemical synapse-mediated inhibitory circuit matures.

Pathway-specific feedforward circuits between thalamus and neocortex revealed by selective optical stimulation of axons

Thalamocortical and corticothalamic pathways mediate bidirectional communication between the thalamus and neocortex. These pathways are entwined, making their study challenging. Here we used lentiviruses to express channelrhodopsin-2 (ChR2), a light-sensitive cation channel, in either thalamocortical or corticothalamic projection cells. Infection occurred only locally, but efferent axons and their terminals expressed ChR2 strongly, allowing selective optical activation of each pathway. Laser stimulation of ChR2-expressing thalamocortical axons/terminals evoked robust synaptic responses in cortical excitatory cells and fast-spiking (FS) inhibitory interneurons, but only weak responses in somatostatin-containing interneurons. Strong FS cell activation led to feedforward inhibition in all cortical neuron types, including FS cells. Corticothalamic stimulation excited thalamic relay cells and inhibitory neurons of the thalamic reticular nucleus (TRN). TRN activation triggered inhibition in relay cells but not in TRN neurons. Thus, a major difference between thalamocortical and corticothalamic processing was the extent to which feedforward inhibitory neurons were themselves engaged by feedforward inhibition.

Synaptic basis for intense thalamocortical activation of feedforward inhibitory cells in neocortex

The thalamus provides fundamental input to the neocortex. This input activates inhibitory interneurons more strongly than excitatory neurons, triggering powerful feedforward inhibition. We studied the mechanisms of this selective neuronal activation using a mouse somatosensory thalamocortical preparation. Notably, the greater responsiveness of inhibitory interneurons was not caused by their distinctive intrinsic properties but was instead produced by synaptic mechanisms. Axons from the thalamus made stronger and more frequent excitatory connections onto inhibitory interneurons than onto excitatory cells. Furthermore, circuit dynamics allowed feedforward inhibition to suppress responses in excitatory cells more effectively than in interneurons. Thalamocortical excitatory currents rose quickly in interneurons, allowing them to fire action potentials before significant feedforward inhibition emerged. In contrast, thalamocortical excitatory currents rose slowly in excitatory cells, overlapping with feedforward inhibitory currents that suppress action potentials. These results demonstrate the importance of selective synaptic targeting and precise timing in the initial stages of neocortical processing.

Abrupt maturation of a spike-synchronizing mechanism in neocortex

Synchronous activity is common in the neocortex, although its significance, mechanisms, and development are poorly understood. Previous work showed that networks of electrically coupled inhibitory interneurons called low-threshold spiking (LTS) cells can fire synchronously when stimulated by metabotropic glutamate receptors. Here we found that the coordinated inhibition emerging from an activated LTS network could induce correlated spiking patterns among neighboring excitatory cells. Synchronous activity among LTS cells was absent at postnatal day 12 (P12) but appeared abruptly over the next few days. The rapid development of the LTS-synchronizing system coincided with the maturation of the inhibitory outputs and intrinsic membrane properties of the neurons. In contrast, the incidence and magnitude of electrical synapses remained constant between P8 and P15. The developmental transformation of LTS interneurons into a synchronous, oscillatory network overlaps with the onset of active somatosensory exploration, suggesting a potential role for this synchronizing system in sensory processing.

Connexon connexions in the thalamocortical system

Electrical synapses are composed of gap junction channels that interconnect neurons. They occur throughout the mammalian brain, although this has been appreciated only recently. Gap junction channels, which are made of proteins called connexins, allow ionic current and small organic molecules to pass directly between cells, usually with symmetrical ease. Here we review evidence that electrical synapses are a major feature of the inhibitory circuitry in the thalamocortical system. In the neocortex, pairs of neighboring inhibitory interneurons are often electrically coupled, and these electrical connections are remarkably specific. To date, there is evidence that five distinct subtypes of inhibitory interneurons in the cortex make electrical interconnections selectively with interneurons of the same subtype. Excitatory neurons (i.e., pyramidal and spiny stellate cells) of the mature cortex do not appear to make electrical synapses. Within the thalamus, electrical coupling is observed in the reticular nucleus, which is composed entirely of GABAergic neurons. Some pairs of inhibitory neurons in the cortex and reticular thalamus have mixed synaptic connections: chemical (GABAergic) inhibitory synapses operating in parallel with electrical synapses. Inhibitory neurons of the thalamus and cortex express the gap junction protein connexin 36 (C x 36), and knocking out its gene abolishes nearly all of their electrical synapses. The electrical synapses of the thalamocortical system are strong enough to mediate robust interactions between inhibitory neurons. When pairs or groups of electrically coupled cells are excited by synaptic input, receptor agonists, or injected current, they typically display strong synchrony of both subthreshold voltage fluctuations and spikes. For example, activating metabotropic glutamate receptors on coupled pairs of cortical interneurons or on thalamic reticular neurons can induce rhythmic action potentials that are synchronized with millisecond precision. Electrical synapses offer a uniquely fast, bidirectional mechanism for coordinating local neural activity. Their widespread distribution in the thalamocortical system suggests that they serve myriad functions. We are far from a complete understanding of those functions, but recent experiments suggest that electrical synapses help to coordinate the temporal and spatial features of various forms of neural activity.

Potent block of Cx36 and Cx50 gap junction channels by mefloquine

Recently, great interest has been shown in understanding the functional roles of specific gap junction proteins (connexins) in brain, lens, retina, and elsewhere. Some progress has been made by studying knockout mice with targeted connexin deletions. For example, such studies have implicated the gap junction protein Cx36 in synchronizing rhythmic activity of neurons in several brain regions. Although knockout strategies are informative, they can be problematic, because compensatory changes sometimes occur during development. Therefore, it would be extremely useful to have pharmacological agents that block specific connexins, without major effects on other gap junctions or membrane channels. We show that mefloquine, an antimalarial drug, is one such agent. It blocked Cx36 channels, expressed in transfected N2A neuroblastoma cells, at low concentrations (IC(50) approximately 300 nM). Mefloquine also blocked channels formed by the lens gap junction protein, Cx50 (IC(50) approximately 1.1 microM). However, other gap junctions (e.g., Cx43, Cx32, and Cx26) were only affected at concentrations 10- to 100-fold higher. To further examine the utility and specificity of this compound, we characterized its effects in acute brain slices. Mefloquine, at 25 microM, blocked gap junctional coupling between interneurons in neocortical slices, with minimal nonspecific actions. At this concentration, the only major side effect was an increase in spontaneous synaptic activity. Mefloquine (25 microM) caused no significant change in evoked excitatory or inhibitory postsynaptic potentials, and intrinsic cellular properties were also mostly unaffected. Thus, mefloquine is expected to be a useful tool to study the functional roles of Cx36 and Cx50.

Auditory thalamocortical synaptic transmission in vitro

To facilitate an understanding of auditory thalamocortical mechanisms, we have developed a mouse brain-slice preparation with a functional connection between the ventral division of the medial geniculate (MGv) and the primary auditory cortex (ACx). Here we present the basic characteristics of the slice in terms of physiology (intracellular and extracellular recordings, including current source density analysis), pharmacology (including glutamate receptor involvement), and anatomy (gross anatomy, Nissl, parvalbumin immunocytochemistry, and tract tracing with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate). Thalamocortical transmission in this preparation (the "primary" slice) involves both alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid/kainate and N-methyl-D-aspartate-type glutamate receptors that appear to mediate monosynaptic inputs to layers 3-4 of ACx. MGv stimulation also initiates disynaptic inhibitory postsynaptic potentials and longer-duration intracortical, polysynaptic activity. Important differences between responses elicited by MGv versus conventional columnar ("on-beam") stimulation emphasize the necessity of thalamic activation to infer thalamocortical mechanisms. We also introduce a second slice preparation, the "shell" slice, obtained from the brain region immediately ventral to the primary slice, that may contain a nonprimary thalamocortical pathway to temporal cortex. In the shell slice, stimulation of the thalamus or the region immediately ventral to it appears to produce fast activation of synapses in cortical layer 1 followed by robust intracortical polysynaptic activity. The layer 1 responses may result from orthodromic activation of nonprimary thalamocortical pathways; however, a plausible alternative could involve antidromic activation of corticotectal neurons and their layer 1 collaterals. The primary and shell slices will provide useful tools to investigate mechanisms of information processing in the ACx.

Parvalbumin and calbindin are differentially distributed within primary and secondary subregions of the mouse auditory forebrain

The calcium binding proteins parvalbumin and calbindin are thought to differentially regulate physiological functions and often show complementary distributions in the CNS. Our goal was to determine parvalbumin and calbindin distributions in the different subdivisions of mouse auditory thalamus and auditory cortex. Following fixation, FVB mouse brains (postnatal days 38-80) were sectioned along coronal and horizontal planes, then processed for parvalbumin and calbindin immunohistochemistry (antibodies: parvalbumin pa-235, calbindin-d-28k cl-300). Strong complementary differences in calcium binding protein distributions were found in mouse auditory thalamus. The ventral division of the medial geniculate, which is the principal relay to primary auditory cortex, exhibited dense parvalbumin but weak calbindin immunoreactivity. In contrast, most of the 'secondary' auditory thalamic regions surrounding the ventral division showed strong calbindin and lighter parvalbumin levels. Thus, the mouse auditory thalamus is composed of a parvalbumin positive 'core' surrounded by a calbindin positive 'shell'. Parvalbumin immunoreactivity was also more prominent in the primary auditory cortex than in the secondary belt auditory cortex. Calbindin immunoreactivity in auditory cortex was less clearly divided along primary/secondary lines, especially in supragranular layers. However, within infragranular layers, there was heavier staining in belt areas than in primary auditory cortex. In auditory thalamus, parvalbumin labeling was largely confined to the neuropil, whereas calbindin labeling involved somata and neuropil. In auditory cortex, somata and neuropil were positive for both proteins.In summary, the calcium binding proteins parvalbumin and calbindin were found to be differentially distributed within the primary and non-primary regions of mouse auditory forebrain. These differences in protein distribution may contribute to the distinct types of physiological responses that occur in the primary vs. non-primary areas.

In vivo Hebbian and basal forebrain stimulation treatment in morphologically identified auditory cortical cells

The present study concerns the interactions of local pre/postsynaptic covariance and activity of the cortically-projecting cholinergic basal forebrain, in physiological plasticity of auditory cortex. Specifically, a tone that activated presynaptic inputs to a recorded auditory cortical neuron was repeatedly paired with a combination of two stimuli: (1) local juxtacellular current that excited the recorded cell and (2) basal forebrain stimulation which desynchronized the cortical EEG. In addition, the recorded neurons were filled with biocytin for morphological examination. The hypothesis tested was that the combined treatment would cause increased potentiation of responses to the paired tone, relative to similar conditioning treatments involving either postsynaptic excitation alone or basal forebrain stimulation alone. In contrast, there was no net increase in plasticity and indeed the combined treatment appears to have decreased plasticity below that previously found for either treatment alone. Several alternate interpretations of these results are discussed.

Differential modulation of auditory thalamocortical and intracortical synaptic transmission by cholinergic agonist

To investigate synaptic mechanisms underlying information processing in auditory cortex, we examined cholinergic modulation of synaptic transmission in a novel slice preparation containing thalamocortical and intracortical inputs to mouse auditory cortex. Extracellular and intracellular recordings were made in cortical layer IV while alternately stimulating thalamocortical afferents (via medial geniculate or downstream subcortical stimulation) and intracortical afferents. Either subcortical or intracortical stimulation elicited a fast, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)-sensitive, monosynaptic EPSP followed by long-duration, polysynaptic activity. The cholinergic agonist carbachol suppressed each of the synaptic potentials to different degrees. At low concentrations (5 microM) carbachol strongly reduced (>60%) the polysynaptic slow potentials for both pathways but did not affect the monosynaptic fast potentials. At higher doses (10-50 microM), carbachol also reduced the fast potentials, but reduced the intracortically-elicited fast potential significantly more than the thalamocortically-elicited fast potential, which at times was actually enhanced. Atropine (0.5 microM) blocked the effects of carbachol, indicating muscarinic receptor involvement. We conclude that muscarinic modulation can strongly suppress intracortical synaptic activity while exerting less suppression, or actually enhancing, thalamocortical inputs. Such differential actions imply that auditory information processing may favor sensory information relayed through the thalamus over ongoing cortical activity during periods of increased acetylcholine (ACh) release.

Thalamocortical inputs trigger a propagating envelope of gamma-band activity in auditory cortex in vitro

To investigate how auditory cortex responds to thalamic inputs, we have used electrophysiological and anatomical techniques to characterize a brain slice containing functionally linked thalamocortical and intracortical pathways. In extracellular recordings, stimulation of thalamic afferents elicited a short-latency field potential and current sink in layer IV of the cortex, followed by 100-500 ms of polysynaptic activity containing rapid (gamma-band, 20-80 Hz) fluctuations. Paired intracellular and extracellular recordings showed that a short-latency excitatory postsynaptic potential (EPSP) corresponded to the fast extracellular potential, and that a slow intracellular depolarization with superimposed rapid fluctuations corresponded to the polysynaptic extracellular activity. Pharmacological manipulations demonstrated that glutamate receptors contributed to mono- and polysynaptic activity, and that the gamma-band fluctuations contained intermixed rapid depolarizations and Cl(-)-mediated inhibition. The spread of evoked activity through auditory cortex was determined by extracellular mapping away from the excitatory focus (the site of the largest amplitude fast response). The short-latency potential traversed auditory cortex at 1.25 m/s and decreased over 1-2 mm, likely reflecting sequential activation of cells contacted by thalamocortical arbors. In contrast, polysynaptic activity did not decrease but propagated as a spatially restricted wave at a 57-fold slower velocity (0.022 m/s). Thus, stimulation of the auditory thalamocortical pathway in vitro elicited a fast glutamatergic potential in layer IV, followed by polysynaptic activity, including gamma-band fluctuations, that propagated through the cortex. Propagating activity may form transient neural assemblies that contribute to auditory information processing.

Evidence for the Hebbian hypothesis in experience-dependent physiological plasticity of neocortex: a critical review

Over the past decade, the number of experimental papers reporting physiological plasticity in primary neocortical regions, following certain types of controlled sensory experience, have increased greatly. These reports have been characterized by specific changes in receptive fields of individual neurons and/or the distributions of receptive fields across cortical maps. There is a widespread belief these types of plasticities have underlying Hebbian/covariance induction mechanisms. This belief appears to be based mainly on: (a) indirect evidence, largely from experiments on the kitten visual cortex, indicating that Hebbian induction mechanisms could be involved in neocortical plasticity; (b) the observation that some types of plasticity in systems other than neocortex follow Hebbian rules of induction; and (c) the adaptability of Hebbian induction mechanisms to models of neural plasticity. In addition, some experiments have directly tested the role of Hebbian induction mechanisms in experience-dependent neocortical plasticity. The present review critically analyzes these (and related) experiments, in order to evaluate the evidence for the Hebbian Hypothesis in experience-dependent physiological plasticity of neocortex. First, we present a set of criteria to show the involvement of a Hebbian process in any form of plasticity. Next, we compare evidence from each primary neocortical region to these criteria. Finally, we examine unresolved issues. While selected developmental studies are included, emphasis is placed on plasticity in the adult neocortex. It is concluded that there is some evidence meeting the criteria for the Hebbian hypothesis in neocortical plasticity. However, this evidence is quite limited considering the popular belief in the validity of the Hebbian hypothesis.

Receptive-field plasticity in the adult auditory cortex induced by Hebbian covariance

The goal of this experiment was to evaluate the role of cellular interactions postulated by the Hebbian, or covariance, hypothesis in the induction of receptive-field (RF) plasticity in the adult auditory cortex (ACx). This was accomplished by determining whether a "covariance treatment" (see below) was sufficient to induce RF plasticity without behavioral experiences that normally induce such plasticity. During the covariance treatment (conducted in urethane-anesthetized adult guinea pigs), one tone was paired with excitatory juxtacellular current, applied to a single postsynaptic cell in the primary ACx. Excitatory current increased postsynaptic discharge, thereby increasing covariance between activity of the postsynaptic cell and its afferents that were activated by the tone. In alternation, within the same cell a second, different tone was paired with inhibitory juxtacellular current, decreasing covariance between the postsynaptic cell and afferents activated by the second tone. After treatment, responses to tones associated with increased covariance strengthened significantly relative to tones associated with decreased covariance, as predicted by the Hebbian hypothesis. This occurred in 7 of 22 (32%) cells undergoing 120 pairing trials, but in only 4 of 38 (11%) cells undergoing 60 trials. Fewer than 5% of cells showed significant effects opposite those predicted by the hypothesis. Significant plasticity lasted > or = 15 min. Probability of plasticity was significantly higher when the cortical electroencephalogram was nonsynchronized during treatment (5/9 cells) than when synchronized (2/13 cells). These findings support the role of presynaptic-postsynaptic covariance processes in the induction of adult neocortical RF plasticity and suggest that factors associated with cortical state "gate" such plasticity.

Stimulation at a site of auditory-somatosensory convergence in the medial geniculate nucleus is an effective unconditioned stimulus for fear conditioning

The medial division of the medial geniculate nucleus (MGm) and the posterior intralaminar nucleus (PIN) are necessary for fear conditioning to an auditory conditioned stimulus (CS), receive both auditory and somatosensory input, and project to the amygdala, which is involved in production of fear conditioned responses. If CS-unconditioned stimulus (US) convergence in the MGm-PIN is critical for fear conditioning, then microstimulation of this area should serve as an effective US during classical conditioning, in place of standard footshock. Guinea pigs underwent conditioning (40-60 trials) using a tone as the CS and medial geniculate complex microstimulation as the US. Conditioned bradycardia developed when the US electrodes were in the PIN. However, microstimulation was not an effective US for conditioning in other parts of the medial geniculate or for sensitization training in the PIN or elsewhere. Learning curves were similar to those found previously for footshock US. Thus, the PIN can be a locus of functional CS-US convergence for previously for footshock US. Thus, the PIN can be a locus of functional CS-US convergence for fear conditioning to acoustic stimuli.

Stimulation at a site of auditory-somatosensory convergence in the medial geniculate nucleus is an effective unconditioned stimulus for fear conditioning

The medial division of the medial geniculate nucleus (MGm) and the posterior intralaminar nucleus (PIN) are necessary for fear conditioning to an auditory conditioned stimulus (CS), receive both auditory and somatosensory input, and project to the amygdala, which is involved in production of fear conditioned responses. If CS-unconditioned stimulus (US) convergence in the MGm-PIN is critical for fear conditioning, then microstimulation of this area should serve as an effective US during classical conditioning, in place of standard footshock. Guinea pigs underwent conditioning (40-60 trials) using a tone as the CS and medial geniculate complex microstimulation as the US. Conditioned bradycardia developed when the US electrodes were in the PIN. However, microstimulation was not an effective US for conditioning in other parts of the medial geniculate or for sensitization training in the PIN or elsewhere. Learning curves were similar to those found previously for footshock US. Thus, the PIN can be a locus of functional CS-US convergence for previously for footshock US. Thus, the PIN can be a locus of functional CS-US convergence for fear conditioning to acoustic stimuli.