Electrophysiological and morphological properties of interneurones in the rat dorsal lateral geniculate nucleus in vitro

Narrowband gamma oscillations propagate and synchronize throughout the mouse thalamocortical visual system

Oscillations of neural activity permeate sensory systems. In the visual system, broadband gamma oscillations (30-80 Hz) are thought to act as a communication mechanism underlying perception. However, these oscillations show widely varying frequency and phase, providing constraints for coordinating spike timing across areas. Here, we examined Allen Brain Observatory data and performed causal experiments to show that narrowband gamma (NBG) oscillations (50-70 Hz) propagate and synchronize throughout the awake mouse visual system. Lateral geniculate nucleus (LGN) neurons fired precisely relative to NBG phase in primary visual cortex (V1) and multiple higher visual areas (HVAs). NBG neurons across areas showed a higher likelihood of functional connectivity and stronger visual responses; remarkably, NBG neurons in LGN, preferring bright (ON) versus dark (OFF), fired at distinct NBG phases aligned across the cortical hierarchy. NBG oscillations may thus serve to coordinate spike timing across brain areas and facilitate communication of distinct visual features during perception.

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.

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.

Cross-modal modulation of cell activity by sound in first-order visual thalamic nucleus

Cross-modal auditory influence on cell activity in the primary visual cortex emerging at short latencies raises the possibility that the first-order visual thalamic nucleus, which is considered dedicated to unimodal visual processing, could contribute to cross-modal sensory processing, as has been indicated in the auditory and somatosensory systems. To test this hypothesis, the effects of sound stimulation on visual cell activity in the dorsal lateral geniculate nucleus were examined in anesthetized rats, using juxta-cellular recording and labeling techniques. Visual responses evoked by light (white LED) were modulated by sound (noise burst) given simultaneously or 50-400 ms after the light, even though sound stimuli alone did not evoke cell activity. Alterations of visual response were observed in 71% of cells (57/80) with regard to response magnitude, latency, and/or burst spiking. Suppression predominated in response magnitude modulation, but de novo responses were also induced by combined stimulation. Sound affected not only onset responses but also late responses. Late responses were modulated by sound given before or after onset responses. Further, visual responses evoked by the second light stimulation of a double flash with a 150-700 ms interval were also modulated by sound given together with the first light stimulation. In morphological analysis of labeled cells projection cells comparable to X-, Y-, and W-like cells and interneurons were all susceptible to auditory influence. These findings suggest that the first-order visual thalamic nucleus incorporates auditory influence into parallel and complex thalamic visual processing for cross-modal modulation of visual attention and perception.

An Individual Interneuron Participates in Many Kinds of Inhibition and Innervates Much of the Mouse Visual Thalamus

One way to assess a neuron's function is to describe all its inputs and outputs. With this goal in mind, we used serial section electron microscopy to map 899 synaptic inputs and 623 outputs in one inhibitory interneuron in a large volume of the mouse visual thalamus. This neuron innervated 256 thalamocortical cells spread across functionally distinct subregions of the visual thalamus. All but one of its neurites were bifunctional, innervating thalamocortical and local interneurons while also receiving synapses from the retina. We observed a wide variety of local synaptic motifs. While this neuron innervated many cells weakly, with single en passant synapses, it also deployed specialized branches that climbed along other dendrites to form strong multi-synaptic connections with a subset of partners. This neuron's diverse range of synaptic relationships allows it to participate in a mix of global and local processing but defies assigning it a single circuit function.

Propriospinal Neurons of L3-L4 Segments Involved in Control of the Rat External Urethral Sphincter

Coordination of activity of external urethral sphincter (EUS) striated muscle and bladder (BL) smooth muscle is essential for efficient voiding. In this study we examined the morphological and electrophysiological properties of neurons in the L3/L4 spinal cord (SC) that are likely to have an important role in EUS-BL coordination in rats. EUS-related SC neurons were identified by retrograde transsynaptic tracing following injection of pseudorabies virus (PRV) co-expressing fluorescent markers into the EUS of P18-P20 male rats. Tracing revealed not only EUS motoneurons in L6/S1 but also interneurons in lamina X of the L6/S1 and L3/L4 SC. Physiological properties of fluorescently labeled neurons were assessed during whole-cell recordings in SC slices followed by reconstruction of biocytin-filled neurons. Reconstructions of neuronal processes from transverse or longitudinal slices showed that some L3/L4 neurons have axons projecting toward and into the ventro-medial funiculus (VMf) where axons extended caudally. Other neurons had axons projecting within laminae X and VII. Dendrites of L3/L4 neurons were distributed within laminae X and VII. The majority of L3/L4 neurons exhibited tonic firing in response to depolarizing currents. In transverse slices focal electrical stimulation (FES) in the VMf or in laminae X and VII elicited antidromic axonal spikes and/or excitatory synaptic responses in L3/L4 neurons; while in longitudinal slices FES elicited excitatory synaptic inputs from sites up to 400 μm along the central canal. Inhibitory inputs were rarely observed. These data suggest that L3/L4 EUS-related circuitry consists of at least two neuronal populations: segmental interneurons and propriospinal neurons projecting to L6/S1.

Developmental Remodeling of Thalamic Interneurons Requires Retinal Signaling

The dorsal lateral geniculate nucleus (dLGN) of the mouse is a model system to study the development of thalamic circuitry. Most studies focus on relay neurons of dLGN, yet little is known about the development of the other principal cell type, intrinsic interneurons. Here we examined whether the structure and function of interneurons relies on retinal signaling. We took a loss-of-function approach and crossed GAD67-GFP mice, which express GFP in dLGN interneurons, with math5 nulls (math5^-/-), mutants that lack retinal ganglion cells and retinofugal projections. In vitro recordings and 3-D reconstructions of biocytin-filled interneurons at different postnatal ages showed their development is a multistaged process involving migration, arbor remodeling, and synapse formation. Arbor remodeling begins during the second postnatal week, after migration to and dispersion within dLGN is complete. This phase includes a period of exuberant branching where arbors grow in number, complexity, and field size. Such growth is followed by branch pruning and stabilization, as interneurons adopt a bipolar architecture. The absence of retinal signaling disrupts this process. The math5^-/- interneurons fail to branch and prune, and instead maintain a simple, sparse architecture. To test how such defects influence connectivity with dLGN relay neurons, we used DHPG [(RS)-3,5-dihydroxyphenylglycine], the mGluR_1,5 agonist that targets F2 terminals. This led to substantial increases in IPSC activity among WT relay neurons but had little impact in math5^-/- mice. Together, these data suggest that retinal signaling is needed to support the arbor elaboration and synaptic connectivity of dLGN interneurons.SIGNIFICANCE STATEMENT Presently, our understanding about the development of the dorsal lateral geniculate nucleus is limited to circuits involving excitatory thalamocortical relay neurons. Here we show that the other principal cell type, intrinsic interneurons, has a multistaged developmental plan that relies on retinal innervation. These findings indicate that signaling from the periphery guides the maturation of interneurons and the establishment of inhibitory thalamic circuits.

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.

Short-term Synaptic Depression in the Feedforward Inhibitory Circuit in the Dorsal Lateral Geniculate Nucleus

Synaptic short-term plasticity (STP) regulates synaptic transmission in an activity-dependent manner and thereby has important roles in the signal processing in the brain. In some synapses, a presynaptic train of action potentials elicits post-synaptic potentials that gradually increase during the train (facilitation), but in other synapses, these potentials gradually decrease (depression). We studied STP in neurons in the visual thalamic relay, the dorsal lateral geniculate nucleus (dLGN). The dLGN contains two types of neurons: excitatory thalamocortical (TC) neurons, which transfer signals from retinal afferents to visual cortex, and local inhibitory interneurons, which form an inhibitory feedforward loop that regulates the thalamocortical signal transmission. The overall STP in the retino-thalamic relay is short-term depression, but the distinct kind and characteristics of the plasticity at the different types of synapses are unknown. We studied STP in the excitatory responses of interneurons to stimulation of retinal afferents, in the inhibitory responses of TC neurons to stimulation of afferents from interneurons, and in the disynaptic inhibitory responses of TC neurons to stimulation of retinal afferents. Moreover, we studied STP at the direct excitatory input to TC neurons from retinal afferents. The STP at all types of the synapses showed short-term depression. This depression can accentuate rapid changes in the stream of signals and thereby promote detectability of significant features in the sensory input. In vision, detection of edges and contours is essential for object perception, and the synaptic short-term depression in the early visual pathway provides important contributions to this detection process.

Synaptic organization of the dorsal lateral geniculate nucleus

A half century after Ray Guillery's classic descriptions of cell types, axon types, and synaptic architecture of the dorsal lateral geniculate nucleus, the functional organization of this nucleus, as well as all other thalamic nuclei, is still of enormous interest. This review will focus on two classic papers written by Ray Guillery: 'A study of Golgi preparations from the dorsal lateral geniculate nucleus of the adult cat', and 'The organization of synaptic interconnections in the laminae of the dorsal lateral geniculate nucleus of the cat', as well as the studies that most directly followed from the insights these landmark manuscripts provided. It is hoped that this review will honor Ray Guillery by encouraging further investigations of the synaptic organization of the dorsal thalamus.

Development, form, and function of the mouse visual thalamus

The dorsal lateral geniculate nucleus (dLGN) of the thalamus is the exclusive relay of retinal information en route to the visual cortex. Although much of our understanding about dLGN comes from studies done in higher mammals, such as the cat and primate, the mouse as a model organism has moved to the forefront as a tractable experimental platform to examine cell type-specific relations. This review highlights our current knowledge about the development, structure, and function of the mouse dLGN.

The multifaceted role of inhibitory interneurons in the dorsal lateral geniculate nucleus

Intrinsic interneurons within the dorsal lateral geniculate nucleus (dLGN) provide a feed-forward inhibitory pathway for afferent visual information originating from the retina. These interneurons are unique because in addition to traditional axodendritic output onto thalamocortical neurons, these interneurons have presynaptic dendrites that form dendrodendritic synapses onto thalamocortical neurons as well. These presynaptic dendrites, termed F2 terminals, are tightly coupled to the retinogeniculate afferents that synapse onto thalamocortical relay neurons. Retinogeniculate stimulation of F2 terminals can occur through the activation of ionotropic and/or metabotropic glutamate receptors. The stimulation of ionotropic glutamate receptors can occur with single stimuli and produces a short-lasting inhibition of the thalamocortical neuron. By contrast, activation of metabotropic glutamate receptors requires tetanic activation and results in longer-lasting inhibition in the thalamocortical neuron. The F2 terminals are predominantly localized to the distal dendrites of interneurons, and the excitation and output of F2 terminals can occur independent of somatic activity within the interneuron thereby allowing these F2 terminals to serve as independent processors, giving rise to focal inhibition. By contrast, strong transient depolarizations at the soma can initiate a backpropagating calcium-mediated potential that invades the dendritic arbor activating F2 terminals and leading to a global form of inhibition. These distinct types of output, focal versus global, could play an important role in the temporal and spatial roles of inhibition that in turn impacts thalamocortical information processing.

Inflammatory demyelination alters subcortical visual circuits

Background

Multiple sclerosis (MS) is an inflammatory demyelinating disease classically associated with axonal damage and loss; more recently, however, synaptic changes have been recognized as additional contributing factors. An anatomical area commonly affected in MS is the visual pathway; yet, changes other than those associated with inflammatory demyelination of the optic nerve, i.e., optic neuritis, have not been described in detail.

Methods

Adult mice were subjected to a diet containing cuprizone to mimic certain aspects of inflammatory demyelination as seen in MS. Demyelination and inflammation were assessed by real-time polymerase chain reaction and immunohistochemistry. Synaptic changes associated with inflammatory demyelination in the dorsal lateral geniculate nucleus (dLGN) were determined by immunohistochemistry, Western blot analysis, and electrophysiological field potential recordings.

Results

In the cuprizone model, demyelination was observed in retinorecipient regions of the subcortical visual system, in particular the dLGN, where it was found accompanied by microglia activation and astrogliosis. In contrast, anterior parts of the pathway, i.e., the optic nerve and tract, appeared largely unaffected. Under the inflammatory demyelinating conditions, as seen in the dLGN of cuprizone-treated mice, there was an overall decrease in excitatory synaptic inputs from retinal ganglion cells. At the same time, the number of synaptic complexes arising from gamma-aminobutyric acid (GABA)-generating inhibitory neurons was found increased, as were the synapses that contain the N-methyl-d-aspartate receptor (NMDAR) subunit GluN2B and converge onto inhibitory neurons. These synaptic changes were functionally found associated with a shift toward an overall increase in network inhibition.

Conclusions

Using the cuprizone model of inflammatory demyelination, our data reveal a novel form of synaptic (mal)adaption in the CNS that is characterized by a shift of the excitation/inhibition balance toward inhibitory network activity associated with an increase in GABAergic inhibitory synapses and a possible increase in excitatory input onto inhibitory interneurons. In addition, our data recognize the cuprizone model as a suitable tool in which to assess the effects of inflammatory demyelination on subcortical retinorecipient regions of the visual system, such as the dLGN, in the absence of overt optic neuritis.

A Comparison of Visual Response Properties in the Lateral Geniculate Nucleus and Primary Visual Cortex of Awake and Anesthetized Mice

The cerebral cortex of the mouse has become one of the most important systems for studying information processing and the neural correlates of behavior. Multiple studies have examined the first stages of visual cortical processing: primary visual cortex (V1) and its thalamic inputs from the dorsal lateral geniculate nucleus (dLGN), but more rarely in the lateral posterior nucleus (LP) in mice. Multiple single-unit surveys of dLGN and V1, both with electrophysiology and two-photon calcium imaging, have described receptive fields in anesthetized animals. Increasingly, awake animals are being used in physiological studies, so it is important to compare neuronal responses between awake and anesthetized state. We have performed a comprehensive survey of spatial and temporal response properties in V1, dLGN, and lateral posterior nucleus of both anesthetized and awake animals, using a common set of stimuli: drifting sine-wave gratings spanning a broad range of spatial and temporal parameters, and sparse noise stimuli consisting of flashed light and dark squares. Most qualitative receptive field parameters were found to be unchanged between the two states, such as most aspects of spatial processing, but there were significant differences in several parameters, most notably in temporal processing. Compared with anesthetized animals, the temporal frequency that evoked the peak response was shifted toward higher values in the dLGN of awake mice and responses were more sustained. Further, the peak response to a flashed stimulus was earlier in all three areas. Overall, however, receptive field properties in the anesthetized animal remain a good model for those in the awake animal.

SIGNIFICANCE STATEMENT

The primary visual cortex (V1) of the mouse and its inputs from visual thalamus (dLGN), have become a dominant model for studying information processing in the brain. Early surveys of visual response properties (receptive fields) were performed in anesthetized animals. Although most recent studies of V1 have been performed in awake animals to examine links between vision and behavior, there have been few comprehensive studies of receptive field properties in the awake mouse, especially in dLGN and lateral posterior nucleus. We have performed a comparative survey of receptive fields in dLGN, lateral posterior nucleus, and V1 in anesthetized and awake mice. We found multiple differences in processing of time-varying stimuli, whereas the spatial aspects of receptive fields remain comparatively unchanged.

Synaptic Contributions to Receptive Field Structure and Response Properties in the Rodent Lateral Geniculate Nucleus of the Thalamus

Comparative physiological and anatomical studies have greatly advanced our understanding of sensory systems. Many lines of evidence show that the murine lateral geniculate nucleus (LGN) has unique attributes, compared with other species such as cat and monkey. For example, in rodent, thalamic receptive field structure is markedly diverse, and many cells are sensitive to stimulus orientation and direction. To explore shared and different strategies of synaptic integration across species, we made whole-cell recordings in vivo from the murine LGN during the presentation of visual stimuli, analyzed the results with different computational approaches, and compared our findings with those from cat. As for carnivores, murine cells with classical center-surround receptive fields had a "push-pull" structure of excitation and inhibition within a given On or Off subregion. These cells compose the largest single population in the murine LGN (∼40%), indicating that push-pull is key in the form vision pathway across species. For two cell types with overlapping On and Off responses, which recalled either W3 or suppressed-by-contrast ganglion cells in murine retina, inhibition took a different form and was most pronounced for spatially extensive stimuli. Other On-Off cells were selective for stimulus orientation and direction. In these cases, retinal inputs were tuned and, for oriented cells, the second-order subunit of the receptive field predicted the preferred angle. By contrast, suppression was not tuned and appeared to sharpen stimulus selectivity. Together, our results provide new perspectives on the role of excitation and inhibition in retinothalamic processing.

SIGNIFICANCE STATEMENT

We explored the murine lateral geniculate nucleus from a comparative physiological perspective. In cat, most retinal cells have center-surround receptive fields and push-pull excitation and inhibition, including neurons with the smallest (highest acuity) receptive fields. The same is true for thalamic relay cells. In mouse retina, the most numerous cell type has the smallest receptive fields but lacks push-pull. The most common receptive field in rodent thalamus, however, is center-surround with push-pull. Thus, receptive field structure supersedes size per se for form vision. Further, for many orientation-selective cells, the second-order component of the receptive field aligned with stimulus preference, whereas suppression was untuned. Thus, inhibition may improve spatial resolution and sharpen other forms of selectivity in rodent lateral geniculate nucleus.

Tectal-derived interneurons contribute to phasic and tonic inhibition in the visual thalamus

The release of GABA from local interneurons in the dorsal lateral geniculate nucleus (dLGN-INs) provides inhibitory control during visual processing within the thalamus. It is commonly assumed that this important class of interneurons originates from within the thalamic complex, but we now show that during early postnatal development Sox14/Otx2-expressing precursor cells migrate from the dorsal midbrain to generate dLGN-INs. The unexpected extra-diencephalic origin of dLGN-INs sets them apart from GABAergic neurons of the reticular thalamic nucleus. Using optogenetics we show that at increased firing rates tectal-derived dLGN-INs generate a powerful form of tonic inhibition that regulates the gain of thalamic relay neurons through recruitment of extrasynaptic high-affinity GABA_A receptors. Therefore, by revising the conventional view of thalamic interneuron ontogeny we demonstrate how a previously unappreciated mesencephalic population controls thalamic relay neuron excitability.

Two types of interneurons in the mouse lateral geniculate nucleus are characterized by different h-current density

Although hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels and the corresponding h-current (I_h) have been shown to fundamentally shape the activity pattern in the thalamocortical network, little is known about their function in local circuit GABAergic interneurons (IN) of the dorsal part of the lateral geniculate nucleus (dLGN). By combining electrophysiological, molecular biological, immunohistochemical and cluster analysis, we characterized the properties of I_h and the expression profile of HCN channels in IN. Passive and active electrophysiological properties of IN differed. Two subclasses of IN were resolved by unsupervised cluster analysis. Small cells were characterized by depolarized resting membrane potentials (RMP), stronger anomalous rectification, higher firing frequency of faster action potentials (APs), appearance of rebound bursting, and higher I_h current density compared to the large IN. The depolarization exerted by sustained HCN channel activity facilitated neuronal firing. In addition to cyclic nucleotides, I_h in IN was modulated by PIP₂ probably based on the abundant expression of the HCN3 isoform. Furthermore, only IN with larger cell diameters expressed neuronal nitric oxide synthase (nNOS). It is discussed that I_h in IN is modulated by neurotransmitters present in the thalamus and that the specific properties of I_h in these cells closely reflect their modulatory options.

Effects of the Concomitant Activation of ON and OFF Retinal Ganglion Cells on the Visual Thalamus: Evidence for an Enhanced Recruitment of GABAergic Cells

A fundamental question in vision neuroscience is how parallel processing of Retinal Ganglion Cell (RGC) signals is integrated at the level of the visual thalamus. It is well-known that parallel ON-OFF pathways generate output signals from the retina that are conveyed to the dorsal lateral geniculate nucleus (dLGN). However, it is unclear how these signals distribute onto thalamic cells and how these two pathways interact. Here, by electrophysiological recordings and c-Fos expression analysis, we characterized the effects of pharmacological manipulations of the retinal circuit aimed at inducing either a selective activation of a single pathway, OFF RGCs [intravitreal L-(+)-2-Amino-4-phosphonobutyric, L-AP4] or an unregulated activity of all classes of RGCs (intravitreal 4-Aminopyridine, 4-AP). In in vivo experiments, the analysis of c-Fos expression in the dLGN showed that these two manipulations recruited active cells from the same area, the lateral edge of the dLGN. Despite this similarity, the unregulated co-activation of both ON and OFF pathways by 4-AP yielded a much stronger recruitment of GABAergic interneurons in the dLGN when compared to L-AP4 pure OFF activation. The increased activation of an inhibitory thalamic network by a high level of unregulated discharge of ON and OFF RGCs might suggest that cross-inhibitory pathways between opposing visual channels are presumably replicated at multiple levels in the visual pathway, thus increasing the filtering ability for non-informative or noisy visual signals.

T-type calcium channels cause bursts of spikes in motor but not sensory thalamic neurons during mimicry of natural patterns of synaptic input

Although neurons within intact nervous systems can be classified as ‘sensory’ or ‘motor,’ it is not known whether there is any general distinction between sensory and motor neurons at the cellular or molecular levels. Here, we extend and test a theory according to which activation of certain subtypes of voltage-gated ion channel (VGC) generate patterns of spikes in neurons of motor systems, whereas VGC are proposed to counteract patterns in sensory neurons. We previously reported experimental evidence for the theory from visual thalamus, where we found that T-type calcium channels (TtCCs) did not cause bursts of spikes but instead served the function of ‘predictive homeostasis’ to maximize the causal and informational link between retinogeniculate excitation and spike output. Here, we have recorded neurons in brain slices from eight sensory and motor regions of rat thalamus while mimicking key features of natural excitatory and inhibitory post-synaptic potentials. As predicted by theory, TtCC did cause bursts of spikes in motor thalamus. TtCC-mediated responses in motor thalamus were activated at more hyperpolarized potentials and caused larger depolarizations with more spikes than in visual and auditory thalamus. Somatosensory thalamus is known to be more closely connected to motor regions relative to auditory and visual thalamus, and likewise the strength of its TtCC responses was intermediate between these regions and motor thalamus. We also observed lower input resistance, as well as limited evidence of stronger hyperpolarization-induced (‘H-type’) depolarization, in nuclei closer to motor output. These findings support our theory of a specific difference between sensory and motor neurons at the cellular level.

The subcellular distribution of T-type Ca2+ channels in interneurons of the lateral geniculate nucleus

Inhibitory interneurons (INs) in the lateral geniculate nucleus (LGN) provide both axonal and dendritic GABA output to thalamocortical relay cells (TCs). Distal parts of the IN dendrites often enter into complex arrangements known as triadic synapses, where the IN dendrite plays a dual role as postsynaptic to retinal input and presynaptic to TC dendrites. Dendritic GABA release can be triggered by retinal input, in a highly localized process that is functionally isolated from the soma, but can also be triggered by somatically elicited Ca²⁺-spikes and possibly by backpropagating action potentials. Ca²⁺-spikes in INs are predominantly mediated by T-type Ca²⁺-channels (T-channels). Due to the complex nature of the dendritic signalling, the function of the IN is likely to depend critically on how T-channels are distributed over the somatodendritic membrane (T-distribution). To study the relationship between the T-distribution and several IN response properties, we here run a series of simulations where we vary the T-distribution in a multicompartmental IN model with a realistic morphology. We find that the somatic response to somatic current injection is facilitated by a high T-channel density in the soma-region. Conversely, a high T-channel density in the distal dendritic region is found to facilitate dendritic signalling in both the outward direction (increases the response in distal dendrites to somatic input) and the inward direction (the soma responds stronger to distal synaptic input). The real T-distribution is likely to reflect a compromise between several neural functions, involving somatic response patterns and dendritic signalling.

Interleukin-1β and interleukin-6 affect electrophysiological properties of thalamic relay cells

By acknowledging the relation between brain and body in health and disease, inflammatory processes may play a key role in this reciprocal relation. Pro-inflammatory cytokines such as interleukin-1β (IL-1β) and interleukin-6 (IL-6) are some of the agents involved in those processes. What exactly is their role in the CNS however is not that clear so far. To address the question of how pro-inflammatory cytokines may affect information processing at the cellular and molecular levels, relay neurons in the thalamic dorsal lateral geniculate nucleus in mouse brain slices were exposed to those cytokines and studied with the patch-clamp technique. IL-1β promoted hyperpolarization of the resting membrane potential (Vrest), decrease of input resistance (Rin), decrease of Ih rectification, decrease in action potential (AP) threshold and decrease in the number of APs in low threshold calcium spike (LTS) bursts, while IL-6 promoted decrease of Rin and decrease in the number of APs in LTS bursts. Computer simulations provided candidates for ionic conductance affected by those cytokines. Collectively, these findings demonstrate that IL-1β and IL-6 have modulatory effects on electrophysiological properties of thalamic neurons, implying that the thalamic functions may be affected by systemic disorders that present with high levels of those cytokines.

Retinal input directs the recruitment of inhibitory interneurons into thalamic visual circuits

Inhibitory interneurons (INs) critically control the excitability and plasticity of neuronal networks, but whether activity can direct INs into specific circuits during development is unknown. Here, we report that in the dorsal lateral geniculate nucleus (dLGN), which relays retinal input to the cortex, circuit activity is required for the migration, molecular differentiation, and functional integration of INs. We first characterize the prenatal origin and molecular identity of dLGN INs, revealing their recruitment from an Otx2(+) neuronal pool located in the adjacent ventral LGN. Using time-lapse and electrophysiological recordings, together with genetic and pharmacological perturbation of retinal waves, we show that retinal activity directs the navigation and circuit incorporation of dLGN INs during the first postnatal week, thereby regulating the inhibition of thalamocortical circuits. These findings identify an input-dependent mechanism regulating IN migration and circuit inhibition, which may account for the progressive recruitment of INs into expanding excitatory circuits during evolution.

Interneurons in the mouse visual thalamus maintain a high degree of retinal convergence throughout postnatal development

BACKGROUND

The dorsal lateral geniculate nucleus (dLGN) of the mouse thalamus has emerged as a powerful experimental system for understanding the refinement of developing sensory connections. Interestingly, many of the basic tenets for such developmental remodeling (for example, pruning of connections to form precise sensory maps) fail to take into account a fundamental aspect of sensory organization, cell-type specific wiring. To date, studies have focused on thalamocortical relay neurons and little is known about the development of retinal connections onto the other principal cell type of dLGN, intrinsic interneurons. Here, we used a transgenic mouse line in which green fluorescent protein (GFP) is expressed within dLGN interneurons (GAD67-GFP), making it possible to visualize them in acutely prepared thalamic slices in order to examine their morphology and functional patterns of connectivity throughout postnatal life.

FINDINGS

GFP-expressing interneurons were evenly distributed throughout dLGN and had highly complex and widespread dendritic processes that often crossed eye-specific borders. Estimates of retinal convergence derived from excitatory postsynaptic potential (EPSP) amplitude by stimulus intensity plots revealed that unlike relay cells, interneurons recorded throughout the first 5 weeks of life, maintain a large number (approximately eight to ten) of retinal inputs.

CONCLUSIONS

The lack of pruning onto interneurons suggests that the activity-dependent refinement of retinal connections in dLGN is cell-type specific. The high degree of retinal convergence onto interneurons may be necessary for these cells to provide both widespread and local forms of inhibition in dLGN.

Thalamic microcircuits: presynaptic dendrites form two feedforward inhibitory pathways in thalamus

In the visual thalamus, retinal afferents activate both local interneurons and excitatory thalamocortical relay neurons, leading to robust feedforward inhibition that regulates the transmission of sensory information from retina to neocortex. Peculiarly, this feedforward inhibitory pathway is dominated by presynaptic dendrites. Previous work has shown that the output of dendritic terminals of interneurons, also known as F2 terminals, are regulated by both ionotropic and metabotropic glutamate receptors. However, it is unclear whether both classes of glutamate receptors regulate output from the same or distinct dendritic terminals. Here, we used focal glutamate uncaging and whole cell recordings to reveal two types of F2 responses in rat visual thalamus. The first response, which we are calling a Type-A response, was mediated exclusively by ionotropic glutamate receptors (i.e., AMPA and NMDA). In contrast, the second response, which we are calling a Type-B response, was mediated by a combination of ionotropic and type 5 metabotropic glutamate receptors (i.e., mGluR(5)). In addition, we demonstrate that both F2 responses are evoked in the same postsynaptic neurons, which are morphologically distinct from neurons in which no F2 responses are observed. Since photostimulation was relatively focal and small in magnitude, these results suggest distinct F2 terminals, or small clusters of terminals, could be responsible for generating the two inhibitory responses observed. Because of the nature of ionotropic and metabotropic glutamate receptors, we predict the efficacy by which the retina communicates with the thalamus would be strongly regulated by 1) the activity level of a given retinogeniculate axon, and 2) the specific type of F2 terminals activated.

Noise normalizes firing output of mouse lateral geniculate nucleus neurons

The output of individual neurons is dependent on both synaptic and intrinsic membrane properties. While it is clear that the response of an individual neuron can be facilitated or inhibited based on the summation of its constituent synaptic inputs, it is not clear whether subthreshold activity, (e.g. synaptic "noise"--fluctuations that do not change the mean membrane potential) also serve a function in the control of neuronal output. Here we studied this by making whole-cell patch-clamp recordings from 29 mouse thalamocortical relay (TC) neurons. For each neuron we measured neuronal gain in response to either injection of current noise, or activation of the metabotropic glutamate receptor-mediated cortical feedback network (synaptic noise). As expected, injection of current noise via the recording pipette induces shifts in neuronal gain that are dependent on the amplitude of current noise, such that larger shifts in gain are observed in response to larger amplitude noise injections. Importantly we show that shifts in neuronal gain are also dependent on the intrinsic sensitivity of the neuron tested, such that the gain of intrinsically sensitive neurons is attenuated divisively in response to current noise, while the gain of insensitive neurons is facilitated multiplicatively by injection of current noise- effectively normalizing the output of the dLGN as a whole. In contrast, when the cortical feedback network was activated, only multiplicative gain changes were observed. These network activation-dependent changes were associated with reductions in the slow afterhyperpolarization (sAHP), and were mediated at least in part, by T-type calcium channels. Together, this suggests that TC neurons have the machinery necessary to compute multiple output solutions to a given set of stimuli depending on the current level of network stimulation.

Resolving the detailed structure of cortical and thalamic neurons in the adult rat brain with refined biotinylated dextran amine labeling

Biotinylated dextran amine (BDA) has been used frequently for both anterograde and retrograde pathway tracing in the central nervous system. Typically, BDA labels axons and cell somas in sufficient detail to identify their topographical location accurately. However, BDA labeling often has proved to be inadequate to resolve the fine structural details of axon arbors or the dendrites of neurons at a distance from the site of BDA injection. To overcome this limitation, we varied several experimental parameters associated with the BDA labeling of neurons in the adult rat brain in order to improve the sensitivity of the method. Specifically, we compared the effect on labeling sensitivity of: (a) using 3,000 or 10,000 MW BDA; (b) injecting different volumes of BDA; (c) co-injecting BDA with NMDA; and (d) employing various post-injection survival times. Following the extracellular injection of BDA into the visual cortex, labeled cells and axons were observed in both cortical and thalamic areas of all animals studied. However, the detailed morphology of axon arbors and distal dendrites was evident only under optimal conditions for BDA labeling that take into account the: molecular weight of the BDA used, concentration and volume of BDA injected, post-injection survival time, and toning of the resolved BDA with gold and silver. In these instances, anterogradely labeled axons and retrogradely labeled dendrites were resolved in fine detail, approximating that which can be achieved with intracellularly injected compounds such as biocytin or fluorescent dyes.

Morphology, classification, and distribution of the projection neurons in the dorsal lateral geniculate nucleus of the rat

The morphology of confirmed projection neurons in the dorsal lateral geniculate nucleus (dLGN) of the rat was examined by filling these cells retrogradely with biotinylated dextran amine (BDA) injected into the visual cortex. BDA-labeled projection neurons varied widely in the shape and size of their cell somas, with mean cross-sectional areas ranging from 60–340 µm². Labeled projection neurons supported 7–55 dendrites that spanned up to 300 µm in length and formed dendritic arbors with cross-sectional areas of up to 7.0×10⁴ µm². Primary dendrites emerged from cell somas in three broad patterns. In some dLGN projection neurons, primary dendrites arise from the cell soma at two poles spaced approximately 180° apart. In other projection neurons, dendrites emerge principally from one side of the cell soma, while in a third group of projection neurons primary dendrites emerge from the entire perimeter of the cell soma. Based on these three distinct patterns in the distribution of primary dendrites from cell somas, we have grouped dLGN projection neurons into three classes: bipolar cells, basket cells and radial cells, respectively. The appendages seen on dendrites also can be grouped into three classes according to differences in their structure. Short “tufted” appendages arise mainly from the distal branches of dendrites; “spine-like” appendages, fine stalks with ovoid heads, typically are seen along the middle segments of dendrites; and “grape-like” appendages, short stalks that terminate in a cluster of ovoid bulbs, appear most often along the proximal segments of secondary dendrites of neurons with medium or large cell somas. While morphologically diverse dLGN projection neurons are intermingled uniformly throughout the nucleus, the caudal pole of the dLGN contains more small projection neurons of all classes than the rostral pole.

Optogenetic stimulation of the corticothalamic pathway affects relay cells and GABAergic neurons differently in the mouse visual thalamus

The dorsal lateral geniculate nucleus (dLGN) serves as the primary conduit of retinal information to visual cortex. In addition to retinal input, dLGN receives a large feedback projection from layer VI of visual cortex. Such input modulates thalamic signal transmission in different ways that range from gain control to synchronizing network activity in a stimulus-specific manner. However, the mechanisms underlying such modulation have been difficult to study, in part because of the complex circuitry and diverse cell types this pathway innervates. To address this and overcome some of the technical limitations inherent in studying the corticothalamic (CT) pathway, we adopted a slice preparation in which we were able to stimulate CT terminal arbors in the visual thalamus of the mouse with blue light by using an adeno-associated virus to express the light-gated ion channel, ChIEF, in layer VI neurons. To examine the postsynaptic responses evoked by repetitive CT stimulation, we recorded from identified relay cells in dLGN, as well as GFP expressing GABAergic neurons in the thalamic reticular nucleus (TRN) and intrinsic interneurons of dLGN. Relay neurons exhibited large glutamatergic responses that continued to increase in amplitude with each successive stimulus pulse. While excitatory responses were apparent at postnatal day 10, the strong facilitation noted in adult was not observed until postnatal day 21. GABAergic neurons in TRN exhibited large initial excitatory responses that quickly plateaued during repetitive stimulation, indicating that the degree of facilitation was much larger for relay cells than for TRN neurons. The responses of intrinsic interneurons were smaller and took the form of a slow depolarization. These differences in the pattern of excitation for different thalamic cell types should help provide a framework for understanding how CT feedback alters the activity of visual thalamic circuitry during sensory processing as well as different behavioral or pathophysiological states.

Local dendrodendritic inhibition regulates fast synaptic transmission in visual thalamus

Inhibition from thalamic interneurons plays a critical role in modulating information transfer between thalamus and neocortex. Interestingly, these neurons yield inhibition via two distinct outputs: presynaptic dendrites that innervate thalamocortical relay neurons and axonal outputs. Since the dendrites of thalamic interneurons are the primary targets of incoming synaptic information, it has been hypothesized that local synaptic input could produce highly focused dendritic output. To gain additional insight into the computational power of these presynaptic dendrites, we have combined two-photon laser scanning microscopy, glutamate uncaging, and whole-cell electrophysiological recordings to locally activate dendritic terminals and study their inhibitory contribution to rat thalamocortical relay neurons. Our findings demonstrate that local dendritic release from thalamic interneurons is controlled locally by AMPA/NMDA receptor-mediated recruitment of L-type calcium channels. Moreover, by mapping these connections with single dendrite resolution we not only found that presynaptic dendrites preferentially target proximal regions, but such actions differ significantly across branches. Furthermore, local stimulation of interneuron dendrites did not result in global excitation, supporting the notion that these interneurons can operate as multiplexors, containing numerous independently operating input-output devices.

Morphologically distinct classes of relay cells exhibit regional preferences in the dorsal lateral geniculate nucleus of the mouse

A fundamental feature of the mammalian visual system is the presence of separate channels that work in parallel to efficiently extract and analyze specific elements of a visual scene. Despite the extensive use of the mouse as a model system, it is not clear whether such parallel organization extends beyond the retina to subcortical structures, such as the dorsal lateral geniculate (dLGN) of thalamus. To begin to address this, we examined the morphology of biocytin-filled relay cells recorded in dLGN of mice. Based on a quantitative assessment of their dendritic architecture, we found that even at early postnatal ages relay cells could be readily classified as X-like (biconical), Y-like (symmetrical), or W-like (hemispheric) and that each cell type was regionally specified in dLGN. X-like cells were confined primarily to the monocular ventral region of dLGN. Y-like cells occupied a central core that also contained ipsilateral eye projections, whereas W-like cells were found along the perimeter of dLGN. Similar to cat, Y-like cells were more prevalent than X- and W-like cells, and X-like cells tended to be smaller than other cell types. However, the dendritic fields of X- and W-like cells did not exhibit an orientation bias with respect to optic tract or boundaries of dLGN. Although we found clear morphological differences among relay cells, an analysis of their electrophysiological properties did not reveal any additional distinguishing characteristics. Overall, these data coupled with recent observations in the retina suggest that the mouse has many of the hallmark features of a system-wide parallel organization.

A multi-compartment model for interneurons in the dorsal lateral geniculate nucleus

GABAergic interneurons (INs) in the dorsal lateral geniculate nucleus (dLGN) shape the information flow from retina to cortex, presumably by controlling the number of visually evoked spikes in geniculate thalamocortical (TC) neurons, and refining their receptive field. The INs exhibit a rich variety of firing patterns: Depolarizing current injections to the soma may induce tonic firing, periodic bursting or an initial burst followed by tonic spiking, sometimes with prominent spike-time adaptation. When released from hyperpolarization, some INs elicit rebound bursts, while others return more passively to the resting potential. A full mechanistic understanding that explains the function of the dLGN on the basis of neuronal morphology, physiology and circuitry is currently lacking. One way to approach such an understanding is by developing a detailed mathematical model of the involved cells and their interactions. Limitations of the previous models for the INs of the dLGN region prevent an accurate representation of the conceptual framework needed to understand the computational properties of this region. We here present a detailed compartmental model of INs using, for the first time, a morphological reconstruction and a set of active dendritic conductances constrained by experimental somatic recordings from INs under several different current-clamp conditions. The model makes a number of experimentally testable predictions about the role of specific mechanisms for the firing properties observed in these neurons. In addition to accounting for the significant features of all experimental traces, it quantitatively reproduces the experimental recordings of the action-potential- firing frequency as a function of injected current. We show how and why relative differences in conductance values, rather than differences in ion channel composition, could account for the distinct differences between the responses observed in two different neurons, suggesting that INs may be individually tuned to optimize network operation under different input conditions.

mGluR control of interneuron output regulates feedforward tonic GABAA inhibition in the visual thalamus

Metabotropic glutamate receptors (mGluRs) play a crucial role in regulation of phasic inhibition within the visual thalamus. Here we demonstrate that mGluR-dependent modulation of interneuron GABA release results in dynamic changes in extrasynaptic GABA(A) receptor (eGABA(A)R)-dependent tonic inhibition in thalamocortical (TC) neurons of the rat dorsal lateral geniculate nucleus (dLGN). Application of the group I selective mGluR agonist dihydroxyphenylglycine produces a concentration-dependent enhancement of both IPSC frequency and tonic GABA(A) current (I(GABA)tonic) that is due to activation of both mGluR1a and mGluR5 subtypes. In contrast, group II/III mGluR activation decreases both IPSC frequency and I(GABA)tonic amplitude. Using knock-out mice, we show that the mGluR-dependent modulation of I(GABA)tonic is dependent upon expression of δ-subunit containing eGABA(A)Rs. Furthermore, unlike the dLGN, no mGluR-dependent modulation of I(GABA)tonic is present in TC neurons of the somatosensory ventrobasal thalamus, which lacks GABAergic interneurons. In the dLGN, enhancement of IPSC frequency and I(GABA)tonic by group I mGluRs is not action potential dependent, being insensitive to TTX, but is abolished by the L-type Ca(2+) channel blocker nimodipine. These results indicate selective mGluR-dependent modulation of dendrodendritic GABA release from F2-type terminals on interneuron dendrites and demonstrate for the first time the presence of eGABA(A)Rs on TC neuron dendritic elements that participate in "triadic" circuitry within the dLGN. These findings present a plausible novel mechanism for visual contrast gain at the thalamic level and shed new light upon the potential role of glial ensheathment of synaptic triads within the dLGN.

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.

Ca2+-Dependent and Na+-Dependent K+ Conductances Contribute to a Slow AHP in Thalamic Paraventricular Nucleus Neurons: A Novel Target for Orexin Receptors

Thalamic paraventricular nucleus (PVT) neurons exhibit a postburst apamin-resistant slow afterhyperpolarization (sAHP) that is unique to midline thalamus, displays activity dependence, and is abolished in tetrodotoxin. Analysis of the underlying s IAHP confirmed a requirement for Ca2+ influx with contributions from P/Q-, N-, L-, and R subtype channels, a reversal potential near EK+ and a significant reduction by UCL-2077, barium or TEA, consistent with a role for KCa channels. s IAHP was significantly reduced by activation of either the cAMP or the protein kinase C (PKC) signaling pathway. Further analysis of the sAHP revealed an activity-dependent but Ca2+-independent component that was reduced in high [K+]o and blockable after Na+ substitution with Li+ or in the presence of quinidine, suggesting a role for KNa channels. The Ca2+-independent sAHP component was selectively reduced by activation of the PKC signaling pathway. The sAHP contributed to spike frequency adaptation, which was sensitive to activation of either cAMP or PKC signaling pathways and, near the peak of membrane hyperpolarization, was sufficient to cause de-inactivation of low threshold T-Type Ca2+ channels, thus promoting burst firing. PVT neurons are densely innervated by orexin-immunoreactive fibers, and depolarized by exogenously applied orexins. We now report that orexin A significantly reduced both Ca2+-dependent and -independent s IAHP, and spike frequency adaptation. Furthermore orexin A-induced s IAHP inhibition was mediated through activation of PKC but not PKA. Collectively, these observations suggest that KCa and KNa channels have a role in a sAHP that contributes to spike frequency adaptation and neuronal excitability in PVT neurons and that the sAHP is a novel target for modulation by the arousal- and feeding-promoting orexin neuropeptides.

Cholinergic activation of M2 receptors leads to context-dependent modulation of feedforward inhibition in the visual thalamus

The temporal dynamics of inhibition within a neural network is a crucial determinant of information processing. Here, the authors describe in the visual thalamus how neuromodulation governs the magnitude and time course of inhibition in an input-dependent way.

In many brain regions, inhibition is mediated by numerous classes of specialized interneurons, but within the rodent dorsal lateral geniculate nucleus (dLGN), a single class of interneuron is present. dLGN interneurons inhibit thalamocortical (TC) neurons and regulate the activity of TC neurons evoked by retinal ganglion cells (RGCs), thereby controlling the visually evoked signals reaching the cortex. It is not known whether neuromodulation can regulate interneuron firing mode and the resulting inhibition. Here, we examine this in brain slices. We find that cholinergic modulation regulates the output mode of these interneurons and controls the resulting inhibition in a manner that is dependent on the level of afferent activity. When few RGCs are activated, acetylcholine suppresses synaptically evoked interneuron spiking, and strongly reduces disynaptic inhibition. In contrast, when many RGCs are coincidently activated, single stimuli promote the generation of a calcium spike, and stimulation with a brief train evokes prolonged plateau potentials lasting for many seconds that in turn lead to sustained inhibition. These findings indicate that cholinergic modulation regulates feedforward inhibition in a context-dependent manner.

Within the visual thalamus, a single type of inhibitory interneuron regulates activity evoked by retinal ganglion cells and controls the visual signals that reach the cortex. Here, we find that neuromodulation, of the sort thought to occur when an animal is attending to a task, regulates the firing mode of these interneurons and controls the resulting inhibition in an input-dependent manner. When few ganglion cells are activated, neuromodulation greatly decreases the number of spikes in interneurons, and as a result, strongly reduces the inhibition of relay neurons. This favors the lossless transmission of weak visual signals to the cortex by virtually eliminating inhibition within the thalamus. In contrast, when many ganglion cells are activated, the same neuromodulator leads to strong and prolonged inhibition. This is accomplished by promoting the generation of calcium spikes and prolonged depolarizations in interneurons. In this way, a modulator can regulate the flow of visual information in a context-dependent manner.

Excitatory actions of substance P in the rat lateral posterior nucleus

The lateral posterior nucleus (LP) receives inputs from both neocortex and superior colliculus (SC), and is involved with integration and processing of higher-level visual information. Relay neurons in LP contain tachykinin receptors and are innervated by substance P (SP)-containing SC neurons and by layer V neurons of the visual cortex. In this study, we investigated the actions of SP on LP relay neurons using whole-cell recording techniques. SP produced a graded depolarizing response in LP neurons along the rostro-caudal extent of the lateral subdivision of LP nuclei (LPl), with a significantly larger response in rostral LPl neurons compared with caudal LPl neurons. In rostral LPl, SP (5-2000 nm) depolarized nearly all relay neurons tested (> 98%) in a concentration-dependent manner. Voltage-clamp experiments revealed that SP produced an inward current associated with a decreased conductance. The inward current was mediated primarily by neurokinin receptor (NK)(1) tachykinin receptors, although significantly smaller inward currents were produced by specific NK2 and NK3 receptor agonists. The selective NK(1) receptor antagonist RP67580 attenuated the SP-mediated response by 71.5% and was significantly larger than the attenuation of the SP response obtained by NK2 and NK3 receptor antagonists, GR159897 and SB222200, respectively. The SP-mediated response showed voltage characteristics consistent with a K+ conductance, and was attenuated by Cs+, a K+ channel blocker. Our data suggest that SP may modulate visual information that is being processed and integrated in the LPl with inputs from collicular sources.

Preserving information in neural transmission

Along most neural pathways, the spike trains transmitted from one neuron to the next are altered. In the process, neurons can either achieve a more efficient stimulus representation, or extract some biologically important stimulus parameter, or succeed at both. We recorded the inputs from single retinal ganglion cells and the outputs from connected lateral geniculate neurons in the macaque to examine how visual signals are relayed from retina to cortex. We found that geniculate neurons re-encoded multiple temporal stimulus features to yield output spikes that carried more information about stimuli than was available in each input spike. The coding transformation of some relay neurons occurred with no decrement in information rate, despite output spike rates that averaged half the input spike rates. This preservation of transmitted information was achieved by the short-term summation of inputs that geniculate neurons require to spike. A reduced model of the retinal and geniculate visual responses, based on two stimulus features and their associated nonlinearities, could account for >85% of the total information available in the spike trains and the preserved information transmission. These results apply to neurons operating on a single time-varying input, suggesting that synaptic temporal integration can alter the temporal receptive field properties to create a more efficient representation of visual signals in the thalamus than the retina.

Distinct roles of metabotropic glutamate receptor activation on inhibitory signaling in the ventral lateral geniculate nucleus

The ventral lateral geniculate nucleus (vLGN) has been implicated in numerous functions including circadian rhythms, brightness discrimination, pupillary light reflex, and other visuomotor functions. The contribution of inhibitory mechanisms in the regulation of vLGN neuron excitability remains unexplored. We examined the actions of metabotropic glutamate receptor (mGluR) activation on the intrinsic excitability and inhibitory synaptic transmission in different lamina of vLGN. Activation of mGluRs exerts distinct pre- and postsynaptic actions in vLGN neurons. In the lateral magnocellular subdivision of vLGN (vLGNl), the general mGluR agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) enhanced the frequency of GABA(A) receptor-mediated spontaneous inhibitory postsynaptic currents (sIPSC) that persisted in the presence of sodium channel blocker tetrodotoxin (TTX) in a subpopulation of neurons (TTX insensitive). This increase is attributed to the increased output of dendritic GABA release from vLGN interneurons. In contrast, in the medial subdivision of vLGN (vLGNm), the mGluR agonist-mediated increase in sIPSC frequency was completely blocked by TTX. The selective Group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) increased sIPSC frequency, whereas the selective Group II mGluR agonist (2R, 4R)-4-aminopyrrolidine-2,4-dicarboxylate (APDC) significantly decreased sIPSC frequency in vLGNl neurons. Optic tract stimulation also produced an mGluR-dependent increase in sIPSC frequency in vLGNl neurons. In contrast, we were unable to synaptically evoke alterations in sIPSC activity in vLGNm neurons. In addition to these presynaptic actions, DHPG depolarized both vLGNl and vLGNm neurons. In vLGN interneurons, mGluR activation produced opposing actions: APDC hyperpolarized the membrane potential, whereas DHPG produced a membrane depolarization. The present findings demonstrate diverse actions of mGluRs on vLGN neurons localized within different vLGN lamina. Considering these different lamina are coupled with distinct functional roles, thus these diverse actions may be involved in distinctive forms of visual and visuomotor information processing.

Excitatory and anti-oscillatory actions of nitric oxide in thalamus

The dorsal lateral geniculate nucleus (dLGN) not only serves as the obligatory pathway for visual information transfer from the retina to neocortex but can also generate intrathalamic rhythmic activities associated with different arousal states and certain pathological conditions. The gating activity of thalamocortical circuits is under neuromodulatory control by various brainstem nuclei as well as intrinsic thalamic neurons (e.g. thalamic reticular nucleus (TRN) neurons and dLGN interneurons). In this study, we examined the effect of the putative neuromodulator nitric oxide (NO) on thalamic neuron excitability. There are multiple potential sources of NO in thalamus: cholinergic terminals originating from brainstem nuclei, GABAergic TRN neurons, and local GABAergic interneurons. Using whole cell recording techniques in in vitro thalamic slices, we found that the NO donor SNAP produced a robust, long-lasting depolarization in TRN neurons, a weaker depolarization in thalamocortical relay neurons, and no effect in local interneurons. SNAP preferentially depolarized stereotypical TRN neurons that could produced strong burst discharge. In contrast, SNAP had little effect on atypical burst and non-burst TRN cells. The NO donor SIN-1 and the endogenous NO precursor, L-arginine, mimicked the SNAP-mediated actions. The NO-mediated depolarizations were blocked by the guanylyl cyclase inhibitor ODQ indicating involvement of the cGMP pathway. In addition, the phosphodiesterase (PDE) inhibitor zaprinast depolarized and occluded the NO-mediated depolarization in TRN neurons. At the circuit level, NO activation significantly attenuated intrathalamic rhythmic activities likely resulting from the shifting of the firing mode of thalamic neurons, perhaps both TRN and thalamocortical neurons, from burst- to tonic-discharge mode. These alterations in thalamic neuron excitability not only change rhythmic circuit activity, but could also influence sensory information processing through thalamocortical circuits.

Synaptic release generates a tonic GABA(A) receptor-mediated conductance that modulates burst precision in thalamic relay neurons

Tonic inhibition has emerged as a key regulator of neuronal excitability in the CNS. Thalamic relay neurons of the dorsal lateral geniculate nucleus (dLGN) exhibit a tonic GABA(A) receptor (GABA(A)R)-mediated conductance that is correlated with delta-subunit expression. Indeed, consistent with the absence of delta-subunit expression, no tonic conductance is found in the adjacent ventral LGN. We show that, in contrast to the situation in cerebellar granule cells, thalamic delta-subunit-containing GABA(A)Rs (delta-GABA(A)Rs) do not contribute to a spillover component of IPSCs in dLGN. However, tonic activation of thalamic delta-GABA(A)Rs is sensitive to the global level of inhibition, showing an absolute requirement on the synaptic release of GABA. Thus, the tonic conductance is abolished when transmitter release probability is reduced or action potential-evoked release is blocked. We further show that continuous activation of delta-GABA(A)Rs introduces variability into the timing of low-threshold rebound bursts. Hence, activation of delta-GABA(A)Rs could act to destabilize thalamocortical oscillations and therefore have an important impact on behavioral state.

Modulation of inhibitory activity by nitric oxide in the thalamus

The dorsal lateral geniculate nucleus (dLGN) is essential for the transfer of visual information from the retina to visual cortex, and inhibitory mechanisms can play a critical in regulating such information transfer. Nitric oxide (NO) is an atypical neuromodulator that is released in gaseous form and can alter neural activity without direct synaptic connections. Nitric oxide synthase (NOS), an essential enzyme for NO production, is localized in thalamic inhibitory neurons and cholinergic brain stem neurons that innervate the thalamus, although NO-mediated effects on thalamic inhibitory activity remain unknown. We investigated NO effects on inhibitory activity in dLGN using an in vitro slice preparation. The NO donor, SNAP, selectively potentiated the frequency, but not amplitude, of spontaneous inhibitory postsynaptic currents (sIPSCs) in thalamocortical relay neurons. This increase also persisted in tetrodotoxin (TTX), consistent with an increase in GABA release from presynaptic terminals. The SNAP-mediated actions were attenuated not only by the NO scavenger carboxy-PTIO but also by the guanylyl cyclase inhibitor ODQ. The endogenous NO precursor L-arginine produced actions similar to those of SNAP on sIPSC activity and these L-arginine-mediated actions were attenuated by the NOS inhibitor L-NMMA acetate. The SNAP-mediated increase in sIPSC activity was observed in both dLGN and ventrobasal thalamic nucleus (VB) neurons. Considering the lack of interneurons in rodent VB, the NO-mediated actions likely involve an increase in the output of axon terminals of thalamic reticular nucleus neurons. Our results indicate that NO upregulates thalamic inhibitory activity and thus these actions likely influence sensory information transfer through thalamocortical circuits.

Metabotropic glutamate receptors differentially regulate GABAergic inhibition in thalamus

Thalamic interneurons and thalamic reticular nucleus (TRN) neurons provide inhibitory innervation of thalamocortical cells that significantly influence thalamic gating. The local interneurons in the dorsal lateral geniculate nucleus (dLGN) give rise to two distinct synaptic outputs: classical axonal and dendrodendritic. Activation of metabotropic glutamate receptors (mGluRs) by agonists or optic tract stimulation increases the output of these presynaptic dendrites leading to increased inhibition of thalamocortical neurons. The present study was aimed to evaluate the actions of specific mGluRs on inhibitory GABA-mediated signaling. We found that the group I mGluR (mGluR(1,5)) agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) or optic tract stimulation produced a robust increase in spontaneous IPSCs (sIPSCs) in thalamocortical neurons that was attenuated by the selective mGluR(5) antagonist 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP). In contrast, the group II mGluR (mGluR(2,3)) agonists (2R, 4R)-4-aminopyrrolidine-2,4-dicarboxylate (APDC) or (2S,2'R,3'R)-2-(2'3'-dicarboxycyclopropyl)glycine (DCG-IV) suppressed the frequency of sIPSCs. In addition, mGluR(1,5) agonist DHPG produced depolarizations and mGluR(2/3) agonists APDC or L-CCG-I [(2S,1'S,2'S)-2-(carboxycyclopropyl)glycine] produced hyperpolarizations in dLGN interneurons. Furthermore, the enhanced sIPSC activity by optic tract stimulation was reduced when paired with corticothalamic fiber stimulation. The present data indicate that activation of specific mGluR subtypes differentially regulates inhibitory activity via different synaptic pathways. Our results suggest that activation of specific mGluR subtypes can upregulate or downregulate inhibitory activity in thalamic relay neurons, and these actions likely shape excitatory synaptic integration and thus regulate information transfer through thalamocortical circuits.

Contribution of morphology and membrane resistance to integration of fast synaptic signals in two thalamic cell types

Thalamocortical cells (TCs) and interneurons (INs) in the lateral geniculate nucleus process visual information from the retina. The TCs have many short dendrites, whereas the INs have fewer and longer dendrites. Because of these morphological differences, it has been suggested that transmission of synaptic signals from dendritic synapses to soma is more efficient in TCs than in INs. However, a higher membrane resistance (R(m)) for the INs could, in theory, compensate for the attenuating effect of their long dendrites and allow distal synaptic inputs to significantly depolarize the soma. Compartmental models were made from biocytin filled TCs (n = 15) and INs (n = 3) and adjusted to fit the current- and voltage-clamp recordings from the individual cells. The confidence limits for the passive electrical parameters were explored by simulating the influence of noise, morphometric errors and non-uniform and active conductances. One of the useful findings was that R(m) was accurately estimated despite realistic levels of active conductance. Simulations to explore the somatic influence of dendritic synapses showed that a small (0.5 nS) excitatory synapse placed at different dendritic positions gave similar somatic potentials in the individual TCs, within the TC population and also between TCs and INs. A linear increase in the conductance of the synapse gave increases in somatic potentials that were more sublinear in INs than TCs. However, when the total synaptic conductance was increased by simultaneously activating many small, spatially distributed synapses, the INs converted the synaptic signals to soma potentials almost as efficiently as the TCs. Thus, INs can transfer fast synaptic signals to soma as efficiently as TCs except when the focal conductance is large.

Thalamic T-type Ca2+ channels and NREM sleep

T-type Ca2+ channels play a number of different and pivotal roles in almost every type of neuronal oscillation expressed by thalamic neurones during non-rapid eye movement (NREM) sleep, including those underlying sleep theta waves, the K-complex and the slow (<1 Hz) sleep rhythm, sleep spindles and delta waves. In particular, the transient opening of T channels not only gives rise to the 'classical' low threshold Ca2+ potentials, and associated high frequency burst of action potentials, that are characteristically present during sleep spindles and delta waves, but also contributes to the high threshold bursts that underlie the thalamic generation of sleep theta rhythms. The persistent opening of a small fraction of T channels, i.e. I(Twindow), is responsible for the large amplitude and long lasting depolarization, or UP state, of the slow (<1 Hz) sleep oscillation in thalamic neurones. These cellular findings are in part matched by the wake-sleep phenotype of global and thalamic-selective CaV3.1 knockout mice that show a decreased amount of total NREM sleep time. T-type Ca2+ channels, therefore, constitute the single most crucial voltage-dependent conductance that permeates all activities of thalamic neurones during NREM sleep. Since I(Twindow) and high threshold bursts are not restricted to thalamic neurones, the cellular neurophysiology of T channels should now move away from the simplistic, though historically significant, view of these channels as being responsible only for low threshold Ca2+ potentials.

Nucleus- and species-specific properties of the slow (<1 Hz) sleep oscillation in thalamocortical neurons

The slow (<1 Hz) rhythm is an electroencephalogram hallmark of resting sleep. In thalamocortical neurons this rhythm correlates with a slow (<1 Hz) oscillation comprising recurring UP and DOWN membrane potential states. Recently, we showed that metabotropic glutamate receptor activation brings about an intrinsic slow oscillation in thalamocortical neurons of the cat dorsal lateral geniculate nucleus in vitro which is identical to that observed in vivo. The aim of this study was to further assess the properties of this oscillation and compare them with those observed in thalamocortical neurons of three other thalamic nuclei in the cat (ventrobasal complex, medial geniculate body; ventral lateral nucleus) and two thalamic nuclei in rats and mice (lateral geniculate nucleus and ventrobasal complex). Slow oscillations were evident in all of these additional structures and shared several basic properties including, i) the stereotypical, rhythmic alternation between distinct UP and DOWN states with the UP state always commencing with a low-threshold Ca2+ potential, and ii) an inverse relationship between frequency and injected current so that slow oscillations always increase in frequency with hyperpolarization, often culminating in delta (delta) activity at approximately 1-4 Hz. However, beyond these common properties there were important differences in expression between different nuclei. Most notably, 44% of slow oscillations in the cat lateral geniculate nucleus possessed UP states that comprised sustained tonic firing and/or high-threshold bursting. In contrast, slow oscillations in cat ventrobasal complex, medial geniculate body and ventral lateral nucleus thalamocortical neurons exhibited such UP states in only 16%, 11% and 10% of cases, respectively, whereas slow oscillations in the lateral geniculate nucleus and ventrobasal complex of rats and mice did so in <12% of cases. Thus, the slow oscillation is a common feature of thalamocortical neurons that displays clear species- and nuclei-related differences. The potential functional significance of these results is discussed.

GABA(B) receptors in the centromedian/parafascicular thalamic nuclear complex: an ultrastructural analysis of GABA(B)R1 and GABA(B)R2 in the monkey thalamus

Strong gamma-aminobutyric acid type B (GABA(B)) receptor binding has been shown throughout the thalamus, but the distribution of the two GABA(B) receptor subunits, GABA(B) receptor subunit 1 (GABA(B)R1) and GABA(B) receptor subunit 2 (GABA(B)R2), remains poorly characterized. In primates, the caudal intralaminar nuclei, centromedian and parafascicular (CM/PF), are an integral part of basal ganglia circuits and a main source of inputs to the striatum. In this study, we analyzed the subcellular and subsynaptic distribution of GABA(B) receptor subunits by using light and electron microscopic immunocytochemical techniques. Quantitative immunoperoxidase and immunogold analysis showed that both subunits display a similar pattern of distribution in CM/PF, being expressed largely at extrasynaptic and perisynaptic sites in neuronal cell bodies, dendrites, and axon-like processes and less abundantly in axon terminals. Postsynaptic GABA(B)R1 labeling was found mostly on the plasma membrane (70-80%), whereas GABA(B)R2 was more evenly distributed between the plasma membrane and intracellular compartments of CM/PF neurons. A few axon terminals forming symmetric and asymmetric synapses were also labeled for GABA(B)R1 and GABA(B)R2, but the bulk of presynaptic labeling was expressed in small axon-like processes. About 20% of presynaptic vesicle-containing dendrites of local circuit neurons displayed GABA(B)R1/R2 immunoreactivity. Vesicular glutamate transporters (vGluT1)-containing terminals forming asymmetric synapses expressed GABA(B)R1 and/or displayed postsynaptic GABA(B)R1 at the edges of their asymmetric specialization. Overall, these findings provide evidence for multiple sites where GABA(B) receptors could modulate GABAergic and glutamatergic transmission in the primate CM/PF complex.