N-methyl-D-aspartate receptors contribute to excitatory postsynaptic potentials of cat lateral geniculate neurons recorded in thalamic slices

Motor Control of Distinct Layer 6 Corticothalamic Feedback Circuits

Layer 6 corticothalamic (L6 CT) neurons provide massive input to the thalamus, and these feedback connections enable the cortex to influence its own sensory input by modulating thalamic excitability. However, the functional role(s) feedback serves during sensory processing is unclear. One hypothesis is that CT feedback is under the control of extra-sensory signals originating from higher-order cortical areas, yet we know nothing about the mechanisms of such control. It is also unclear whether such regulation is specific to CT neurons with distinct thalamic connectivity. Using mice (either sex) combined with in vitro electrophysiology techniques, optogenetics, and retrograde labeling, we describe studies of vibrissal primary motor cortex (vM1) influences on different CT neurons in the vibrissal primary somatosensory cortex (vS1) with distinct intrathalamic axonal projections. We found that vM1 inputs are highly selective, evoking stronger postsynaptic responses in Dual ventral posterior medial nucleus (VPm) and posterior medial nucleus (POm) projecting CT neurons located in lower L6a than VPm-only projecting CT cells in upper L6a. A targeted analysis of the specific cells and synapses involved revealed that the greater responsiveness of Dual CT neurons was due to their distinctive intrinsic membrane properties and synaptic mechanisms. These data demonstrate that vS1 has at least two discrete L6 CT subcircuits distinguished by their thalamic projection patterns, intrinsic physiology, and functional connectivity with vM1. Our results also provide insights into how a distinct CT subcircuit may serve specialized roles specific to contextual modulation of tactile-related sensory signals in the somatosensory thalamus during active vibrissa movements.

SIGNIFICANCE STATEMENT

Layer 6 corticothalamic (L6 CT) feedback circuits are ubiquitous across mammalian species and modalities, and their activities have a strong influence on thalamic excitability and information throughput to the neocortex. Despite clear evidence of CT effects on the thalamus, we know relatively little about how CT cells themselves are regulated. Our results show that input from the primary motor cortex strongly excites a subclass of CT neurons in the primary somatosensory cortex that innervate both core and higher-order somatosensory nuclei rather than those exclusively targeting core somatosensory thalamus. The cortico-cortico-thalamic pathway formed by these connections establishes a circuit-level substrate for supporting CT influence operating under the guidance of ongoing motor activities.

State-Dependent Modulation of Activity in Distinct Layer 6 Corticothalamic Neurons in Barrel Cortex of Awake Mice

Layer 6 corticothalamic (L6 CT) neurons are in a strategic position to control sensory input to the neocortex, yet we understand very little about their functions. Apart from studying their anatomic, physiological, and synaptic properties, most recent efforts have focused on the activity-dependent influences CT cells can exert on thalamic and cortical neurons through causal optogenetic manipulations. However, few studies have attempted to study them during behavior. To address this gap, we performed juxtacellular recordings from optogenetically identified CT neurons in whisker-related primary somatosensory cortex (wS1) of awake, head-fixed mice (either sex) free to rest quietly or self-initiate bouts of whisking and locomotion. We found a rich diversity of response profiles exhibited by CT cells. Their spiking patterns were either modulated by whisking-related behavior (∼28%) or not (∼72%). Whisking-responsive neurons exhibited both increases (activated-type) and decreases in firing rates (suppressed-type) that aligned with whisking onset better than locomotion. We also encountered responsive neurons with preceding modulations in firing rate before whisking onset. Overall, whisking better explained these changes in rates than overall changes in arousal. Whisking-unresponsive CT cells were generally quiet, with many having low spontaneous firing rates (sparse-type) and others being completely silent (silent-type). Remarkably, the sparse firing CT population preferentially spiked at the state transition point when pupil diameter constricted, and the mouse entered quiet wakefulness. Thus, our results demonstrate that L6 CT cells in wS1 show diverse spiking patterns, perhaps subserving distinct functional roles related to precisely timed responses during complex behaviors and transitions between discrete waking states.SIGNIFICANCE STATEMENT Layer 6 corticothalamic neurons provide a massive input to the sensory thalamus and local connectivity within cortex, but their role in thalamocortical processing remains unclear because of difficulty accessing and isolating their activity. Although several recent optogenetic studies reveal that the net influence of corticothalamic actions, suppression versus enhancement, depends critically on the rate these neurons fire, the factors that influence their spiking are poorly understood, particularly during wakefulness. Using the well-established Ntsr1-Cre line to target this elusive population in the whisker somatosensory cortex of awake mice, we found that corticothalamic neurons show diverse state-related responses and modulations in firing rate. These results suggest separate corticothalamic populations can differentially influence thalamocortical excitability during rapid state transitions in awake, behaving animals.

Cortical Stimulation Induces Excitatory Postsynaptic Potentials of Inferior Colliculus Neurons in a Frequency-Specific Manner

Corticofugal modulation of auditory responses in subcortical nuclei has been extensively studied whereas corticofugal synaptic transmission must still be characterized. This study examined postsynaptic potentials of the corticocollicular system, i.e., the projections from the primary auditory cortex (AI) to the central nucleus of the inferior colliculus (ICc) of the midbrain, in anesthetized C57 mice. We used focal electrical stimulation at the microampere level to activate the AI (ES_AI) and in vivo whole-cell current-clamp to record the membrane potentials of ICc neurons. Following the whole-cell patch-clamp recording of 88 ICc neurons, 42 ICc neurons showed ES_AI-evoked changes in the membrane potentials. We found that the ES_AI induced inhibitory postsynaptic potentials in 6 out of 42 ICc neurons but only when the stimulus current was 96 μA or higher. In the remaining 36 ICc neurons, excitatory postsynaptic potentials (EPSPs) were induced at a much lower stimulus current. The 36 ICc neurons exhibiting EPSPs were categorized into physiologically matched neurons (n = 12) when the characteristic frequencies of the stimulated AI and recorded ICc neurons were similar (≤1 kHz) and unmatched neurons (n = 24) when they were different (>1 kHz). Compared to unmatched neurons, matched neurons exhibited a significantly lower threshold of evoking noticeable EPSP, greater EPSP amplitude, and shorter EPSP latency. Our data allow us to propose that corticocollicular synaptic transmission is primarily excitatory and that synaptic efficacy is dependent on the relationship of the frequency tunings between AI and ICc neurons.

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.

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.

Restoration of visual performance by d-serine in models of inner and outer retinal dysfunction assessed using sweep VEP measurements in the conscious rat and rabbit

The NMDA subtype of glutamate receptor and its co-agonist d-serine play a key role in synaptic function in the central nervous system (CNS), including visual cortex and retina. In retinal diseases such as glaucoma and macular degeneration, a loss of vision arises from malfunction of retinal cells, resulting in a glutamate hypofunctional state along the visual pathway in the affected parts of the visual field. An effective strategy to remedy this loss of function might be to increase extracellular levels of d-serine and thereby boost synaptic NMDA receptor-mediated visual transmission and/or plasticity to compensate for the impairment. We tested this idea in brain slices of visual cortex exhibiting long-term potentiation, and in rodent models of visual dysfunction caused by retinal insults at a time when the injury had stabilized to look for neuroenhancement effects. An essential aspect of the in vivo studies involved adapting sweep VEP technology to conscious rats and rabbits and combining it with intracortical recording while the animals were actively attending to visual information. Using this technology allowed us to establish complete contrast sensitivity function curves. We found that systemic d-serine dose-dependently rescued the contrast sensitivity impairment in rats with blue light-induced visual dysfunction. In rabbits with inner retinal dysfunction, both systemic and intravitreal routes of d-serine provided a rescue of visual function. In sum, we show that co-agonist stimulation of the NMDA receptor via administration of exogenous d-serine might be an effective therapeutic strategy to enhance visual performance and compensate for the loss of vision resulting from retinal disease.

The discovery of the sub-threshold currents M and Q/H in central neurons

The history, content and consequences of the highly-cited 1982 Brain Research paper by Halliwell and Adams are summarized. The paper pioneered the use of the single-electrode voltage clamp in mammalian brain slices, described 2 novel sub-threshold voltage-dependent ionic currents, IM and IQ/H, and suggested that cholinergic inputs "enabled" pyramidal cell firing in response to conventional synaptic input, the first example of central neuromodulation. The paper, published in Brain Research to give the first author appropriate importance, heralded an ongoing tidal wave of quantitative electrophysiology in mammalian central neurons.

ORIGINAL ARTICLE ABSTRACT

Voltage-clamp analysis of muscarinic excitation in hippocampal neurons Pyramidal cells in the CA1 field of guinea pig hippocampal slices were voltage-clamped using a single microelectrode, at 23-30°C. Small inwardly relaxing currents triggered by step hyperpolarizations from holding potentials of -80 to -40mV were investigated. Inward relaxations occurring for negative steps between -40mV and -70mV resembled M-currents of sympathetic ganglion cells: they were abolished by addition of carbachol, muscarine or bethanechol, as well as by 1mM barium; the relaxations appeared to invert at around -80mV; they became faster at more negative potentials; and the inversion potential was shifted positively by raising external K(+) concentration. Inward relaxations triggered by steps negative to -80mV, in contrast, appeared to reflect passage of another current species, which has been labeled IQ.Thus IQ did not invert negative to -80mV, it was insensitive to muscarinic agonizts or to barium, and it was blocked by 0.5-3mM cesium (which does not block IM). Turn-on of IQ causes the well known droop in the hyperpolarizing electrotonic potential in these cells. The combined effects of IQ and IM make the steady-state current-voltage relation of CA1 cells slightly sigmoidal around rest potential. It is suggested that activation of cholinergic septal inputs to the hippocampus facilitates repetitive firing off pyramidal cells by turning off the M-conductance, without much change in the resting potential of the cell. © 1982. This article is part of a Special Issue entitled SI:50th Anniversary Issue.

Energy-Efficient Information Transfer by Visual Pathway Synapses

The architecture of computational devices is shaped by their energy consumption. Energetic constraints are used to design silicon-based computers but are poorly understood for neural computation. In the brain, most energy is used to reverse ion influxes generating excitatory postsynaptic currents (EPSCs) and action potentials. Thus, EPSCs should be small to minimize energy use, but not so small as to impair information transmission. We quantified information flow through the retinothalamic synapse in the visual pathway in brain slices, with cortical and inhibitory input to the postsynaptic cell blocked. Altering EPSC size with dynamic clamp, we found that a larger-than-normal EPSC increased information flow through the synapse. Thus, the evolutionarily selected EPSC size does not maximize retinal information flow to the cortex. By assessing the energy used on postsynaptic ion pumping and action potentials, we show that, instead, the EPSC size optimizes the ratio of retinal information transmitted to energy consumed. These data suggest maximization of information transmission per energy used as a synaptic design principle.

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.

Thalamic neuromodulation and its implications for executive networks

The thalamus is a key structure that controls the routing of information in the brain. Understanding modulation at the thalamic level is critical to understanding the flow of information to brain regions involved in cognitive functions, such as the neocortex, the hippocampus, and the basal ganglia. Modulators contribute the majority of synapses that thalamic cells receive, and the highest fraction of modulator synapses is found in thalamic nuclei interconnected with higher order cortical regions. In addition, disruption of modulators often translates into disabling disorders of executive behavior. However, modulation in thalamic nuclei such as the midline and intralaminar groups, which are interconnected with forebrain executive regions, has received little attention compared to sensory nuclei. Thalamic modulators are heterogeneous in regards to their origin, the neurotransmitter they use, and the effect on thalamic cells. Modulators also share some features, such as having small terminal boutons and activating metabotropic receptors on the cells they contact. I will review anatomical and physiological data on thalamic modulators with these goals: first, determine to what extent the evidence supports similar modulator functions across thalamic nuclei; and second, discuss the current evidence on modulation in the midline and intralaminar nuclei in relation to their role in executive function.

Temporally advanced dynamic change of receptive field of lateral geniculate neurons during brief visual stimulation: Effects of brainstem peribrachial stimulation

Processing of visual information in the brain seems to proceed from initial fast but coarse to subsequent detailed processing. Such coarse-to-fine changes appear also in the response of single neurons in the visual pathway. In the dorsal lateral geniculate nucleus (dLGN), there is a dynamic change in the receptive field (RF) properties of neurons during visual stimulation. During a stimulus flash centered on the RF, the width of the RF-center, presumably related to spatial resolution, changes rapidly from large to small in an initial transient response component. In a subsequent sustained component, the RF-center width is rather stable apart from an initial slight widening. Several brainstem nuclei modulate the geniculocortical transmission in a state-dependent manner. Thus, modulatory input from cholinergic neurons in the peribrachial brainstem region (PBR) enhances the geniculocortical transmission during arousal. We studied whether such input also influences the dynamic RF-changes during visual stimulation. We compared dynamic changes of RF-center width of dLGN neurons during brief stimulus presentation in a control condition, with changes during combined presentation of the visual stimulus and electrical PBR-stimulation. The major finding was that PBR-stimulation gave an advancement of the dynamic change of the RF-center width such that the different response components occurred earlier. Consistent with previous studies, we also found that PBR-stimulation increased the gain of firing rate during the sustained response component. However, this increase of gain was particularly strong in the transition from the transient to the sustained component at the time when the center width was minimal. The results suggest that increased modulatory PBR-input not only increase the gain of the geniculocortical transmission, but also contributes to faster dynamics of transmission. We discuss implications for possible effects on visual spatial resolution.

Activity-dependent regulation of retinogeniculate signaling by metabotropic glutamate receptors

Thalamocortical neurons in dorsal lateral geniculate nucleus (dLGN) dynamically convey visual information from retina to the neocortex. Activation of metabotropic glutamate receptors (mGluRs) exerts multiple effects on neural integration in dLGN; however, their direct influence on the primary sensory input, namely retinogeniculate afferents, is unknown. In the present study, we found that pharmacological or synaptic activation of type 1 mGluRs (mGluR(1)s) significantly depresses glutamatergic retinogeniculate excitation in rat thalamocortical neurons. Pharmacological activation of mGluR(1)s attenuates excitatory synaptic responses in thalamocortical neurons at a magnitude sufficient to decrease suprathreshold output of these neurons. The reduction in both NMDA and AMPA receptor-dependent synaptic responses results from a presynaptic reduction in glutamate release from retinogeniculate terminals. The suppression of retinogeniculate synaptic transmission and dampening of thalamocortical output was mimicked by tetanic activation of retinogeniculate afferent in a frequency-dependent manner that activated mGluR(1)s. Retinogeniculate excitatory synaptic transmission was also suppressed by the glutamate transport blocker TBOA (dl-threo-β-benzyloxyaspartic acid), suggesting that mGluR(1)s were activated by glutamate spillover. The data indicate that presynaptic mGluR(1) contributes to an activity-dependent mechanism that regulates retinogeniculate excitation and therefore plays a significant role in the thalamic gating of visual information.

Spatially distinct actions of metabotropic glutamate receptor activation in dorsal lateral geniculate nucleus

Thalamocortical neurons in the dorsal lateral geniculate nucleus (dLGN) dynamically communicate visual information from the retina to the neocortex, and this process can be modulated via activation of metabotropic glutamate receptors (mGluRs). Neurons within dLGN express different mGluR subtypes associated with distinct afferent synaptic pathways; however, the physiological function of this organization is unclear. We report that the activation of mGluR(5), which are located on presynaptic dendrites of local interneurons, increases GABA output that in turn produces an increased inhibitory activity on proximal but not distal dendrites of dLGN thalamocortical neurons. In contrast, mGluR(1) activation produces strong membrane depolarization in thalamocortical neurons regardless of distal or proximal dendritic locations. These findings provide physiological evidence that mGluR(1) appear to be distributed along the thalamocortical neuron dendrites, whereas mGluR(5)-dependent action occurs on the proximal dendrites/soma of thalamocortical neurons. The differential distribution and activation of mGluR subtypes on interneurons and thalamocortical neurons may serve to shape excitatory synaptic integration and thereby regulate information gating through the thalamus.

Cognitive and perceptual functions of the visual thalamus

The thalamus is classically viewed as passively relaying information to the cortex. However, there is growing evidence that the thalamus actively regulates information transmission to the cortex and between cortical areas using a variety of mechanisms, including the modulation of response magnitude, firing mode, and synchrony of neurons according to behavioral demands. We discuss how the visual thalamus contributes to attention, awareness, and visually guided actions, to present a general role for the thalamus in perception and cognition.

Receptor saturation controls short-term synaptic plasticity at corticothalamic synapses

Glutamatergic synapses of layer 6 corticothalamic (CT) neurons form a major excitatory input onto thalamic relay cells, allowing neocortex to continuously control sensory information processing in thalamic circuits. CT synapses display both short- and long-term forms of use-dependent synaptic enhancement, mediated at least in part by increases in the probability of transmitter release. At some synapses, such increases in release probability are accompanied by a higher degree of multivesicular release (MVR) and larger glutamate transients at individual release sites, resulting in the saturation of postsynaptic receptors. The extent to which MVR and postsynaptic saturation interact and control short-term plasticity at CT synapses is not known. Here we examined two distinct presynaptic forms of short-term enhancement, facilitation and augmentation, at CT synapses contacting relay neurons in the ventrobasal nucleus of the mouse thalamus. We found that, in the presence of the low-affinity antagonist γ-d-glutamylglycine, to relieve postsynaptic dl-α-amino-3-hydroxy-5-methylisox azole-propionic acid (AMPA) receptor saturation, the magnitude of facilitation and augmentation increased. Whereas receptor saturation was prominent for both AMPA and N-methyl-d-aspartate receptors, desensitization of AMPA receptors did not significantly alter short-term plasticity. Our results suggest that at CT synapses the activity-dependent increase in synaptic strength is controlled by postsynaptic receptor saturation.

Involvement of endoplasmic reticulum stress on neuronal cell death in the lateral geniculate nucleus in the monkey glaucoma model

We investigated whether endoplasmic reticulum (ER) stress was involved in the pathophysiological mechanisms underlying neuronal death of the lateral geniculate nucleus (LGN) after intraocular pressure (IOP) elevation. Five cynomolgus monkeys, four with a glaucomatous left eye after laser photocoagulation treatment and one normal monkey, were studied. At 4, 11, 15 and 24 weeks after the laser photocoagulation treatment, the numbers of LGN neurons and atrophy were immunohistochemically evaluated using anti-parvalbumin-antibody, which was used to specifically label relay neurons connecting to the visual cortex. In addition, terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL)-positive cells, polyubiquitin, and production of ER stress-related proteins, such as the phosphorylation of eukaryotic initiation factor 2α (p-eIF2α) and C/EBP-homologous protein (CHOP), were also measured using in situ hybridization and immunostaining. Loss of neurons and/or neuronal atrophy in layers 1, 4 and 6 of the LGN on the contralateral side were observed at 4-24 weeks after the laser photocoagulation treatment. Furthermore, the retinal input from the high IOP eye projected to layers 2 (magnocellular layer), 3 and 5 (parvocellular layer) on the ipsilateral side. Neuronal damage was also confirmed in these layers. In the LGN region, TUNEL-positive cells, polyubiquitin, p-eIF2α and CHOP were also detected at 11-24 weeks after the laser photocoagulation treatment. These findings indicate that ER stress may play a pivotal role in neuronal death of the LGN after IOP elevation.

Requirements for synaptically evoked plateau potentials in relay cells of the dorsal lateral geniculate nucleus of the mouse

In developing cells of the mouse dorsal lateral geniculate nucleus (dLGN), synaptic responses evoked by optic tract (OT) stimulation give rise to long-lasting, high-amplitude depolarizations known as plateau potentials. These events are mediated by L-type Ca2+ channels and occur during early postnatal life, a time when retinogeniculate connections are remodelling. To better understand the relationship between L-type activity and dLGN development we used an in vitro thalamic slice preparation which preserves the retinal connections and intrinsic circuitry in dLGN and examined how synaptic responses evoked by OT stimulation lead to the activation of plateau potentials. By varying the strength and temporal frequency of OT stimulation we identified at least three factors that contribute to the developmental regulation of plateau activity: the degree of retinal convergence, the temporal pattern of retinal stimulation and the emergence of feed-forward inhibition. Before natural eye opening (postnatal day 14), the excitatory synaptic responses of relay cells receiving multiple retinal inputs summated in both the spatial and temporal domains to produce depolarizations sufficient to activate L-type activity. After eye opening, when inhibitory responses are fully developed, plateau activity was rarely evoked even with high temporal rates of OT stimulation. When the bulk of this inhibition was blocked by bath application of bicuculline, the incidence of plateau activity increased significantly. We also made use of a transgenic mouse that lacks the β3 subunit of the L-type Ca2+ channel. These mutants have far fewer membrane-bound Ca2+ channels and attenuated L-type activity. In β3 nulls, L-type plateau activity was rarely observed even at young ages when plateau activity prevails. Thus, in addition to the changing patterns of synaptic connectivity and retinal activity, the expression of L-type Ca2+ channels is a requisite component in the manifestation of plateau activity.

Corticofugal modulation of initial neural processing of sound information from the ipsilateral ear in the mouse

Background

Cortical neurons implement a high frequency-specific modulation of subcortical nuclei that includes the cochlear nucleus. Anatomical studies show that corticofugal fibers terminating in the auditory thalamus and midbrain are mostly ipsilateral. Differently, corticofugal fibers terminating in the cochlear nucleus are bilateral, which fits to the needs of binaural hearing that improves hearing quality. This leads to our hypothesis that corticofugal modulation of initial neural processing of sound information from the contralateral and ipsilateral ears could be equivalent or coordinated at the first sound processing level.

Methodology/Principal Findings

With the focal electrical stimulation of the auditory cortex and single unit recording, this study examined corticofugal modulation of the ipsilateral cochlear nucleus. The same methods and procedures as described in our previous study of corticofugal modulation of contralateral cochlear nucleus were employed simply for comparison. We found that focal electrical stimulation of cortical neurons induced substantial changes in the response magnitude, response latency and receptive field of ipsilateral cochlear nucleus neurons. Cortical stimulation facilitated auditory response and shortened the response latency of physiologically matched neurons whereas it inhibited auditory response and lengthened the response latency of unmatched neurons. Finally, cortical stimulation shifted the best frequencies of cochlear neurons towards those of stimulated cortical neurons.

Conclusion

Our data suggest that cortical neurons enable a high frequency-specific remodelling of sound information processing in the ipsilateral cochlear nucleus in the same manner as that in the contralateral cochlear nucleus.

REVIEW: An approach for neuroprotective therapies of secondary brain damage after excitotoxic retinal injury in mice

BACKGROUND

Many current therapeutic strategies for several eye diseases, such as glaucoma, retinal ischemia, and optic neuropathy, are focused on protection of the retinal ganglion cells (RGCs). In fact, loss of visual field, including irreversible blindness, is caused by RGC damage in these diseases. However, recent evidence suggests that the RGC damage extends to visual center in brain: the visual impairment induced by these diseases may result not only from RGC loss, but also from neuronal degeneration within the visual center in brain.

OBJECTIVE

To protect neurons within the visual center in the brain, as well as retinal treatment, for the prevention of visual disorder in these diseases.

METHODS

Once considered difficult to study the visual center in brain following RGCs loss, because obtaining the human samples that are suitable for the study may be difficult. In addition, the monkey, mainly used as glaucomatous model, is relatively high cost and needs to long experiment-span. Here, we focused on mice, because of their high degree of availability, relatively low cost, and amenability to experimental and genetic manipulation.

CONCLUSION

In this review, we describe time-dependent alterations in the visual center in brain following RGCs loss, and whether some drugs prevent the neuronal damage of the visual center in the brain.

Retinogeniculate EPSPs recorded intracellularly in the ferret lateral geniculate nucleus in vitro: Role of NMDA receptors

We used an in vitro preparation of the ferret lateral geniculate nucleus (LGN) to examine the role of the NMDA class of excitatory amino acid (EAA) receptors in retinogeniculate transmission. Intracellular recordings revealed that blockade of NMDA receptors both shortened the time course and reduced the amplitude of fast and slow components of excitatory postsynaptic potentials (EPSPs) evoked by optic tract stimulation. The amplitude and width of the EPSPs mediated by NMDA receptors increased as membrane potential was depolarized towards spike threshold. Individual LGN cells were influenced to varying extents by blockade of NMDA receptors; NMDA and non-NMDA receptor blockade together attenuated severely the entire retinogeniculate EPSP. The dependence of all components of retinogeniculate EPSPs (and action potentials) on NMDA receptor activation supports the hypothesis that the NMDA receptor participates in fast (<10 ms) synaptic events underlying conventional retinogeniculate transmission. The voltage dependence of the NMDA receptor-gated conductance suggests strongly that the transmission of retinal information through the LGN is subject to modulation by extraretinal inputs that affect the membrane potential of LGN neurons.

Corticothalamic synchronization leads to c-fos expression in the auditory thalamus

In this study, we investigated the relationship between c-fos expression in the auditory thalamus and corticofugal activation. The contribution of neurotransmitters and related receptors, the involvement of thalamic reticular nucleus (TRN), and the role of neuronal firing patterns in this process were also examined. The principal nuclei of the medial geniculate body (MGB) showed c-fos expression when the auditory cortex (AC) was activated by direct injection of bicuculline methobromide. However, no expression was detectable with acoustic stimuli alone. This indicated that c-fos expression in the principal nuclei of the MGB was triggered by the corticofugal projection. c-fos expression could be elicited in the MGB by direct injection of glutamate. Direct administration of acetylcholine, alternatively, had no effect. Bicuculline methobromide injection in the AC also triggered synchronized oscillatory activities sequentially in the AC and MGB. Cortically induced c-fos expression in the MGB was not mediated by a pathway involving the TRN because it remained intact after a TRN lesion with kainic acid. The present results also conclude that c-fos expression is not simply associated with firing rate, but also with neuronal firing pattern. Burst firings that are synchronized with the cortical oscillations are proposed to lead to c-fos expression in the principal nuclei of the MGB.

A fast, reciprocal pathway between the lateral geniculate nucleus and visual cortex in the macaque monkey

Neurons in the lateral geniculate nucleus (LGN) not only provide feedforward input to primary visual cortex (V1), but also receive robust feedback from the cortex. Accordingly, visual processing in the LGN is continuously influenced by previous patterns of activity. This study examines the temporal properties of feedforward and feedback pathways between the LGN and V1 in the macaque monkey to provide a lower bound on how quickly the cortex can influence the LGN. In so doing, we identified a subclass of corticogeniculate neurons that receives direct, suprathreshold input from the LGN that is similar in latency to that directed to other recipient neurons (4.2 +/- 0.4 vs 4.0 +/- 0.2 ms). These neurons also provide feedback to the LGN that is significantly shorter in latency than that supplied by corticogeniculate neurons lacking LGN input (5.1 +/- 1.3 vs 11.1 +/- 2.3 ms, respectively). Across our sample of corticogeniculate neurons, the shortest combined visual response latency and feedback latency was 37 ms (mean, 52.5 +/- 3.8 ms), indicating that visual signals can rapidly travel from the periphery to the cortex and back to the LGN.

Changes in firing pattern of lateral geniculate neurons caused by membrane potential dependent modulation of retinal input through NMDA receptors

An optimal visual stimulus flashed on the receptive field of a retinal ganglion cell typically evokes a strong transient response followed by weaker sustained firing. Thalamocortical (TC) neurons in the dorsal lateral geniculate nucleus, which receive their sensory input from retina, respond similarly except that the gain, in particular of the sustained component, changes with level of arousal. Several lines of evidence suggest that retinal input to TC neurons through NMDA receptors plays a key role in generation of the sustained response, but the mechanisms for the state-dependent variation in this component are unclear. We used a slice preparation to study responses of TC neurons evoked by trains of electrical pulses to the retinal afferents at frequencies in the range of visual responses in vivo. Despite synaptic depression, the pharmacologically isolated NMDA component gave a pronounced build-up of depolarization through temporal summation of the NMDA receptor mediated EPSPs. This depolarization could provide sustained firing, the frequency of which depended on the holding potential. We suggest that the variation of sustained response in vivo is caused mainly by the state-dependent modulation of the membrane potential of TC neurons which shifts the NMDA receptor mediated depolarization closer to or further away from the firing threshold. The pharmacologically isolated AMPA receptor EPSPs were rather ineffective in spike generation. However, together with the depolarization evoked by the NMDA component, the AMPA component contributed significantly to spike generation, and was necessary for the precise timing of the generated spikes.

VPM and PoM nuclei of the rat somatosensory thalamus: intrinsic neuronal properties and corticothalamic feedback

Sensory information originating in individual whisker follicles ascends through focused projections to the brainstem, then to the ventral posteromedial nucleus (VPM) of the thalamus, and finally into barrels of the primary somatosensory cortex (S1). By contrast, the posteromedial complex (PoM) of the thalamus receives more diffuse sensory projections from the brainstem and projects to the interbarrel septa of S1. Both VPM and PoM receive abundant corticothalamic projections from S1. Using a thalamocortical slice preparation, we characterized differences in intrinsic neuronal properties and in responses to corticothalamic feedback in neurons of VPM and PoM. Due to the plane of the slice, the majority of our observed responses came from activation of layer VI because most or all of the layer V axons terminating in PoM are cut. We found that VPM neurons exhibit higher firing rates than PoM neurons when stimulated with injected current. Stimulation of corticothalamic fibers evoked monosynaptic excitation, disynaptic inhibition, or a combination of the two in both nuclei. A few differences in the feedback responses emerged: purely excitatory postsynaptic potentials (EPSPs) in VPM were smaller and facilitated more than those in PoM, and only the EPSPs in VPM had a strong NMDA component. For both nuclei, some of the feedback responses were purely disynaptic inhibitory postsynaptic potentials (IPSPs) from the thalamic reticular nucleus (TRN). This was due to EPSP failures within VPM and PoM combined with greater reliability of S1-originating synapses onto TRN. These findings suggest that despite the exclusively excitatory nature of corticothalamic fibers, activation of cortex can trigger excitation or inhibition in thalamic relay neurons.

Dynamic encoding of natural luminance sequences by LGN bursts

In the lateral geniculate nucleus (LGN) of the thalamus, visual stimulation produces two distinct types of responses known as tonic and burst. Due to the dynamics of the T-type Ca ²⁺ channels involved in burst generation, the type of response evoked by a particular stimulus depends on the resting membrane potential, which is controlled by a network of modulatory connections from other brain areas. In this study, we use simulated responses to natural scene movies to describe how modulatory and stimulus-driven changes in LGN membrane potential interact to determine the luminance sequences that trigger burst responses. We find that at low resting potentials, when the T channels are de-inactivated and bursts are relatively frequent, an excitatory stimulus transient alone is sufficient to evoke a burst. However, to evoke a burst at high resting potentials, when the T channels are inactivated and bursts are relatively rare, prolonged inhibitory stimulation followed by an excitatory transient is required. We also observe evidence of these effects in vivo, where analysis of experimental recordings demonstrates that the luminance sequences that trigger bursts can vary dramatically with the overall burst percentage of the response. To characterize the functional consequences of the effects of resting potential on burst generation, we simulate LGN responses to different luminance sequences at a range of resting potentials with and without a mechanism for generating bursts. Using analysis based on signal detection theory, we show that bursts enhance detection of specific luminance sequences, ranging from the onset of excitatory sequences at low resting potentials to the offset of inhibitory sequences at high resting potentials. These results suggest a dynamic role for burst responses during visual processing that may change according to behavioral state.

This visual neuroscience paper simulates how resting potential and stimulus driven modulations in membrane potential interact to determine the response mode of LGN neurons to natural images.

Cortical feedback to the thalamus is selectively enhanced by nitric oxide

The brain somehow merges visual information with the behavioral context in which it is being processed, a task that is often attributed to the cerebral cortex. We have identified a new role of the gaseous neurotransmitter, nitric oxide (NO), in the early selective enhancement of corticogeniculate communication that may participate in this process at the level of the thalamus. Visual information is dynamically gated through the thalamus by brainstem neurons that release acetylcholine and NO. Using in vitro electrophysiology, we characterized NO effects on excitatory postsynaptic potentials and currents (EPSCs) elicited from retinal and cortical pathways in the lateral geniculate nucleus of the ferret. NO selectively and reversibly increased cortically-evoked postsynaptic responses, and this effect was mimicked by cyclic guanosine 3',5'-monophosphate (cGMP). Conversely, NO inhibited retinally-evoked responses independently of cGMP. We demonstrated that these differential effects were specific to postsynaptic N-methyl-d-aspartate (NMDA) receptors by studying treatment effects on pharmacologically isolated EPSCs from each pathway. We propose that when brainstem activity is increased during behavioral arousal or rapid eye movement sleep, NO may increase the relative sensitivity of relay neurons to corticogeniculate feedback. The net effect of these changes in synaptic processing may be to selectively suppress peripheral information while unifying data carried by reentrant corticogeniculate loops with the behavioral context in which the visual information is processed.

Different composition of glutamate receptors in corticothalamic and lemniscal synaptic responses and their roles in the firing responses of ventrobasal thalamic neurons in juvenile mice

Thalamic ventrobasal (VB) relay neurons receive information via two major types of glutamatergic synapses, that is, from the medial lemniscus (lemniscal synapses) and primary somatosensory cortex (corticothalamic synapses). These two synapses influence and coordinate firing responses of VB neurons, but their precise operational mechanisms are not yet well understood. In this study, we compared the composition of glutamate receptors and synaptic properties of corticothalamic and lemniscal synapses. We found that the relative contribution of NMDA receptor-mediated excitatory postsynaptic currents (EPSCs) to non-NMDA receptor-mediated EPSCs was significantly greater in corticothalamic synapses than in lemniscal synapses. Furthermore, NMDA receptor 2B-containing NMDA receptor- and kainate receptor-mediated currents were observed only in corticothalamic synapses, but not in lemniscal synapses. EPSCs in corticothalamic synapses displayed the postsynaptic summation in a frequency-dependent manner, in which the summation of the NMDA receptor-mediated component was largely involved. The summation of kainate receptor-mediated currents also partially contributed to the postsynaptic summation in corticothalamic synapses. In contrast, the contribution of NMDA receptor-mediated currents to the postsynaptic summation of lemniscal EPSCs was relatively minor. Furthermore, our results indicated that the prominent NMDA receptor-mediated component in corticothalamic synapses was the key determinant for the late-persistent firing of VB neurons in response to corticothalamic stimuli. In lemniscal synapses, in contrast, the onset-transient firing in response to lemniscal stimuli was regulated mainly by AMPA receptors.

Diurnal gene expression patterns of T-type calcium channels and their modulation by ethanol

The transient (T-type) calcium channel participates in the generation of normal brain rhythms as well as abnormal rhythms associated with a range of neurological disorders. There are three different isoforms of T-type channels and all are particularly enriched in the thalamus, which is involved in generating many of these rhythms. We report a novel means of T-type channel regulation in the thalamus that involves diurnal regulation of gene expression. Using real time polymerase chain reaction we detected a diurnal pattern of gene expression for all T-type channel transcripts. The peak of gene expression for the CaV3.1 transcript occurred close to the transition from active to inactive (sleep) states, while expression for both CaV3.2 and CaV3.3 peaked near the transition of inactive to active phase. We assessed the effect of chronic consumption of ethanol on these gene expression patterns by examining thalamic tissues of ethanol-consuming cohorts that were housed with the controls, but which received ethanol in the form of a liquid diet. Ethanol consumption resulted in a significant shift of peak gene expression of approximately 5 h for CaV3.2 toward the normally active phase of the mice, as well as increasing the overall gene expression levels by approximately 1.7-fold. Peak gene expression was significantly increased for both CaV3.2 and CaV3.3. Measurements of CaV3.3 protein expression reflected increases in gene expression due to ethanol. Our results illustrate a novel regulatory mechanism for T-type calcium channels that is consistent with their important role in generating thalamocortical sleep rhythms, and suggests that alterations in the pattern of gene expression of these channels could contribute to the disruption of normal sleep by ethanol.

Looking back: corticothalamic feedback and early visual processing

Although once regarded as a simple sensory relay on the way to the cortex, it is increasingly apparent that the thalamus has a role in the ongoing moment-by-moment processing of sensory input and in cognition. This involves extensive corticofugal feedback connections and the interplay of these with the local thalamic circuitry and the other converging inputs. Here, using the feline visual system as the primary model, some of the latest developments in this field are reviewed and placed in the perspective of an integrated view of system function. Cortical feedback mediated by ionotropic and metabotropic glutamate receptors, and effects mediated by the neuromodulator nitric oxide, all have a role in integrating the thalamic mechanism into the cortical circuit. The essential point is that the perspective of higher-level sensory mechanisms shifts and modulates the thalamic circuitry in ways that optimize abstraction of a meaningful representation of the external world. This review is part of the TINS special issue on The Neural Substrates of Cognition.

The native T-type calcium current in relay neurons of the primate thalamus

The generation of thalamic bursts depends upon calcium currents that flow through transiently open (T)-type calcium channels. In this study, we characterized the native T-type calcium current underlying thalamic burst responses in the macaque monkey. Current clamp recordings from lateral geniculate nucleus (LGN) slices showed characteristic burst responses when relay cells were depolarized from relatively hyperpolarized membrane potentials. These bursts could also be elicited by stimulation of excitatory synaptic inputs to LGN cells. Under voltage clamp conditions, the inactivation kinetics of native currents recorded from primate LGN neurons showed consistency with T-type currents recorded in other mammals and in expression systems. Real-time reverse transcriptase PCR performed on RNA isolated from the LGN (including tissues isolated from magnocellular and parvocellular laminae) detected voltage-dependent calcium channel (Ca(v)) 3.1, Ca(v) 3.2, and Ca(v) 3.3 channel transcripts. Ca(v) 3.1 occurred at relatively higher expression than other isoforms, consistent with in situ hybridization studies in rats, indicating that the molecular basis for burst firing in thalamocortical systems is an important conserved property of primate physiology. Since thalamic bursts have been observed during visual processing as well as in a number of CNS disorders, studies of the expression and modulation of these currents at multiple levels are critical for understanding their role in vision and for the discovery of new treatments for disruptions of thalamic rhythms.

Chronic ethanol drinking reduces native T-type calcium current in the thalamus of nonhuman primates

BACKGROUND

Chronic ethanol use is known to disrupt normal sleep rhythms, but the cellular basis for this disruption is unknown. An important contributor to normal sleep patterns is a low-threshold calcium current mediated by T-type calcium channels. The T-type calcium current underlies burst responses in thalamic nuclei that are important to spindle propagation, and we recently observed that this current is sensitive to acute low doses of ethanol.

METHODS

We used a combination of current clamp and voltage clamp recordings in an in vitro brain slice preparation of the dorsal lateral geniculate nucleus (LGN) of macaque monkeys that have chronically self-administered ethanol to determine whether chronic ethanol exposure may affect T-type currents.

RESULTS

Current clamp recordings from the LGN of ethanol naive macaques showed characteristic burst responses. However, recordings from the LGN in macaques that self-administered ethanol revealed a significant attenuation of bursts across a range of voltages (n=5). Voltage clamp recordings from control LGN neurons (n=16) and neurons (n=29) from brain slices from chronically drinking macaques showed no significant differences (P>0.05) in T-type current kinetics or in the membrane resistance of the thalamic cells between the two cohorts. However, mean T-type current amplitude measured in the chronically drinking animals was reduced by 31% (P<0.01).

CONCLUSIONS

We conclude that chronic ethanol self-administration reduces calcium currents in thalamic relay cells without altering underlying current kinetics, which may provide a mechanistic framework for the well-documented disruptions in sleep/wake behavior in subjects with chronic ethanol exposure.

Low-threshold Ca2+-associated bursts are rare events in the LGN of the awake behaving monkey

It has been proposed that low-threshold Ca2+ (LT)-associated bursts in the lateral geniculate nucleus (LGN) of awake animals communicate significant or unexpected visual events to cortex. The present study investigated this hypothesis by examining the incidence of LT bursts in 146 cells recorded from the LGN of three macaque monkeys. Bursts were defined as clusters of two or more action potentials separated by not more than 4 ms and preceded by a > or = 100-ms quiescent interval. The incidence of bursts was examined in several intensive-training Go-NoGo and target selection tasks as well as in training-free tasks where natural scenes with both familiar and novel contents were shown. Our chief findings were as follows. 1) Bursts occur in the majority of cells under every condition tested, 2) burst incidence is very low (<1 burst every 10 s), 3) bursts occur in association with a receptive field stimulus on average only once every 23 times in 65% of cells tested, 4) cells responding with bursts to the stimulus also tended to exhibit higher levels of spontaneous bursting, 5) the presence of bursts did not depend on the novelty of the stimulus or its behavioral relevance. When the monkeys explored static natural scenes, 6) bursts were not correlated with short-term changes in the image sampled by the cell's receptive field during saccades. Burst incidence 7) did not increase when images were novel or when they evoked an emotional reaction, and 8) bursts did not decrease when images were familiar. 9) Bursts were not correlated with saccades in the dark, but 10) more spikes participated in bursts in the dark. Although these results confirm the occurrence of LT bursts in LGN cells of awake monkeys, they do not support the hypothesis that these bursts are a privileged means of transferring sensory information, that they signal unexpected or significant visual events, or that they are involved uniquely in the coding of natural scenes.

Three channels of corticothalamic communication during locomotion

We studied the flow of corticothalamic (CT) information from the motor cortex of the cat during two types of locomotion: visually guided (cortex dependent) and unguided. Spike trains of CT neurons in layers V (CT5s) and VI (CT6s) were examined. All CT5s had fast-conducting axons (<2 ms conduction time), and nearly all showed step-phase-related activity (94%), sensory receptive fields (100%), and spontaneous activity (100%). In contrast, conduction times along CT6 axons were much slower, with bimodal peaks occurring at 6 and 32 ms. Remarkably, almost none of the slowest conducting CT6s showed step-related activity, sensory receptive fields, or spontaneous activity. As a group, these enigmatic neurons were all but silent. Some of the CT6s with moderately conducting axons showed step-related behavior (35%), and this response was more precisely timed than that of the CT5s. We propose distinct functional roles for these diverse corticothalamic populations.

Depression of retinogeniculate synaptic transmission by presynaptic D2-like dopamine receptors in rat lateral geniculate nucleus

Extraretinal projections onto neurons in the dorsal lateral geniculate nucleus (dLGN) play an important role in modifying sensory information as it is relayed from the visual thalamus to neocortex. The dLGN receives dopaminergic innervation from the ventral tegmental area; however, the role of dopamine in synaptic transmission in dLGN has not been explored. In the present study, whole cell recordings were obtained to examine the actions of dopamine on glutamatergic synaptic transmission. Dopamine (2-100 microm) strongly suppressed excitatory synaptic transmission in dLGN relay neurons that was evoked by optic tract stimulation and mediated by both N-methyl-d-aspartate and non-N-methyl-d-aspartate glutamate receptors. In contrast, dopamine did not alter inhibitory synaptic transmission arising from either dLGN interneurons or thalamic reticular nucleus neurons. The suppressive action of dopamine on excitatory synaptic transmission was mimicked by the D(2)-like dopamine receptor agonist bromocriptine (2-25 microm) but not by the D(1)-like receptor agonist SKF38393 (10-25 microm). In addition, the dopamine-mediated suppression was antagonized by the D(2)-like receptor antagonist sulpiride (10-20 microm) but not by the D(1)-like receptor antagonist SCH23390 (5-25 microm). The dopamine-mediated decrease in evoked excitatory postsynaptic current amplitude was accompanied by an increase in the magnitude of paired-pulse depression. Furthermore, dopamine also reduced the frequency but not the amplitude of miniature excitatory postsynaptic currents. Taken together, these data suggest that dopamine may act presynaptically to regulate the release of glutamate at the retinogeniculate synapse and modify transmission of visual information in the dLGN.

Excitatory actions of synaptically released catecholamines in the rat lateral geniculate nucleus

The gating properties of thalamic relay neurons are influenced by the actions of a variety of neuromodulators in concert with the intrinsic properties of these relay neurons. In this study, we have investigated the consequences of synaptically released catecholamines on the excitability of neurons in the rat dorsal lateral geniculate nucleus. Tetanic stimulation of the optic tract, in which catecholamine fibers also course near or through, produced a strong depolarization that consisted of a fast and slow component. The fast excitatory postsynaptic potential was attenuated by ionotropic glutamate receptor antagonists and further unmasked the slow excitatory postsynaptic potential. The amplitude of the slow excitatory postsynaptic potential was dependent on the frequency and intensity of the tetanic stimulation. The alpha1-adrenergic receptor antagonist, prazosin, and the D1-like dopamine receptor antagonist, SCH23390, attenuated the slow excitatory postsynaptic potential; however, the slow excitatory postsynaptic potential was unaltered by metabotropic glutamate, cholinergic, alpha2-adrenergic, and beta-adrenergic receptor antagonists. On the other hand, tetanic stimulation of the optic radiations (corticothalamic axons) evoked a slow excitatory postsynaptic potential that was completely attenuated by metabotropic glutamate receptor antagonists. Our results suggest that tetanic stimulation of catecholamine fibers within the optic tract produces synaptic release of norepinephrine and dopamine that in turn activates both alpha(1)-adrenergic and D1-like dopamine receptors leading to a robust membrane depolarization. By altering the excitability of relay neurons, ascending activating systems may modulate the efficacy of information transfer through the thalamus.

Exploring spike transfer through the thalamus using hybrid artificial-biological neuronal networks

We use dynamic clamp to construct "hybrid" thalamic circuits by connecting a biological neuron in situ to silicon- or software-generated "neurons" through artificial synapses. The purpose is to explore cellular sensory gating mechanisms that regulate the transfer efficiency of signals during different sleep-wake states. Hybrid technology is applied in vitro to different paradigms such as: (1) simulating interactions between biological thalamocortical neurons, artificial reticular thalamic inhibitory interneurons and a simulated sensory input, (2) grafting an artificial sensory input to a wholly biological thalamic network that generates spontaneous sleep-like oscillations, (3) injecting in thalamocortical neurons a background synaptic bombardment mimicking the activity of corticothalamic inputs. We show that the graded control of the strength of intrathalamic inhibition, combined with the membrane polarization and the fluctuating synaptic noise in thalamocortical neurons, is able to govern functional shifts between different input/output transmission states of the thalamic gate.

Dynamics of sensory thalamocortical synaptic networks during information processing states

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

Active zones for presynaptic plasticity in the brain

Some of the most abundant synapses in the brain such as the synapses formed by the hippocampal mossy fibers, cerebellar parallel fibers and several types of cortical afferents express presynaptic forms of long-term potentiation (LTP), a putative cellular model for spatial, motor and fear learning. Those synapses often display presynaptic mechanisms of LTP induction, which are either NMDA receptor independent of dependent of presynaptic NMDA receptors. Recent investigations on the molecular mechanisms of neurotransmitter release modulation in short- and long-term synaptic plasticity in central synapses give a preponderant role to active zone proteins as Munc-13 and RIM1-alpha, and point toward the maturation process of synaptic vesicles prior to Ca(2+)-dependent fusion as a key regulatory step of presynaptic plasticity.

Encoding of natural scene movies by tonic and burst spikes in the lateral geniculate nucleus

The role of the lateral geniculate nucleus (LGN) of the thalamus in visual encoding remains an open question. Here, we characterize the function of tonic and burst spikes in cat LGN X-cells in signaling features of natural stimuli. A significant increase in bursting was observed during natural stimulation (relative to white noise stimulation) and was linked to the strong correlation structure of the natural scene movies. Burst responses were triggered by specific stimulus events consisting of a prolonged inhibitory stimulus, followed by an excitatory stimulus, such as the movement of an object into the receptive field. LGN responses to natural scene movies were predicted using an integrate-and-fire (IF) framework and compared with experimentally observed responses. The standard IF model successfully predicted LGN responses to natural scene movies during tonic firing, indicating a linear relationship between stimulus and response. However, the IF model typically underpredicted the LGN response during periods of bursting, indicating a nonlinear amplification of the stimulus in the actual response. The addition of a burst mechanism to the IF model was necessary to accurately predict the entire LGN response. These results suggest that LGN bursts are an important part of the neural code, providing a nonlinear amplification of stimulus features that are typical of the natural environment.

How much feedback from visual cortex to lateral geniculate nucleus in cat: A perspective

Corticothalamic feedback is believed to play an important role in selectively regulating the flow of sensory information from thalamus to cortex. But despite its importance, the size and nature of corticothalamic pathway connectivity is not fully understood. In light of recent empirical data, the aim of this paper was to quantify the contribution of area 17 axon connectivity to the synaptic organization of A-laminae in dorsal lateral geniculate nucleus (dLGN) in cat, the best studied corticothalamic pathway. Numerical constraints indicate that most corticogeniculate synapses are not formed with inhibitory interneurons. However, the main finding is that there was an order of magnitude difference between estimates of the mean number of cortical synapses per A-laminae neuron based on individual corticogeniculate axon data (12,000–16,000 cortical synapses per cell) than that previously derived from partial reconstructions of the synaptic input to two physiologically identified relay cells (1200–1500 cortical synapses per cell). In an attempt to reconcile these different estimates, parameter variation and comparative analyses suggest that previous work may have overestimated the density of corticogeniculate efferent neurons and underestimated the total number of synapses per geniculate neuron. But as this analysis did not include area 18 corticogeniculate axons innervating A-laminae, the discrepancy between different estimates may be greater and require further explanation. Thus, the analysis presented here suggests geniculate neurons receive on average a greater number of cortical synapses per cell but from far fewer corticogeniculate axons than previously thought.

Corticofugal gating of auditory information in the thalamus: an in vivo intracellular recording study

In the present study, we investigated the auditory responses of the medial geniculate (MGB) neurons, through in vivo intracellular recordings of anesthetized guinea pigs, while the auditory cortex was electrically activated. Of the 63 neurons that received corticofugal modulation of the membrane potential, 30 received potentiation and 33 received hyperpolarization. The corticofugal potentiation of the membrane potential (amplitude, mean +/- SD, 8.6 +/- 5.5 mV; duration, 125.5 +/- 75.4 msec) facilitated the auditory responses and spontaneous firing of the MGB neurons. The hyperpolarization of -11.3 +/- 4.9 mV in amplitude and 210.0 +/- 210.1 msec in duration suppressed the auditory responses and spontaneous firing of the MGB neurons. Four of the five neurons that were histologically confirmed to be located in the lemniscal MGB received corticofugal facilitatory modulation, and all of the four neurons that were confirmed to be located in the non-lemniscal MGB received corticofugal inhibitory modulation. The present intracellular recording provides novel results on how the corticofugal projection gates the sensory information in the thalamus: via the spatially selective depolarization of lemniscal MGB neurons and hyperpolarization of non-lemniscal MGB neurons. It is speculated that the systematic selectivity of facilitation and inhibition over the lemniscal and non-lemniscal MGB is related to the attention shift within the auditory modality and across the sensory modalities.

Synaptic Activation of Metabotropic Glutamate Receptors Regulates Dendritic Outputs of Thalamic Interneurons

Information gating through the thalamus is dependent on the output of thalamic relay neurons. These relay neurons receive convergent innervation from a number of sources, including GABA-containing interneurons that provide feed-forward inhibition. These interneurons are unique in that they have two distinct outputs: axonal and dendritic. In addition to conventional axonal outputs, these interneurons have presynaptic dendrites that may provide localized inhibitory influences. Our study indicates that synaptic activation of metabotropic glutamate receptors (mGluRs) increases inhibitory activity in relay neurons by increasing output of presynaptic dendrites of interneurons. Optic tract stimulation increases inhibitory activity in thalamic relay neurons in a frequency- and intensity-dependent manner and is attenuated by mGluR antagonists. Our data suggest that synaptic activation of mGluRs selectively alters dendritic output but not axonal output of thalamic interneurons. This mechanism could serve an important role in focal, feed-forward information processing in addition to dynamic information processing in thalamocortical circuits.

Two distinct types of corticothalamic EPSPs and their contribution to short-term synaptic plasticity

The lateral posterior nucleus (LPN) is innervated by two different morphological types of cortical terminals that originate from cortical layers V and VI. Here we describe two distinct types of excitatory postsynaptic potentials (EPSPs) that were recorded in the LPN after stimulation of corticothalamic fibers. These types of EPSPs differed in amplitude, latency, rise time, and response to increasing levels of stimulus intensity. The most frequently encountered EPSP, type I, displayed a longer latency and slower rise time than the less frequently encountered type II EPSP. Type I EPSPs also showed a graded increase in amplitude with increasing levels of stimulation, whereas type II EPSPs showed an all-or-none response. In response to repetitive stimulation (0.5-20 Hz), type I EPSPs displayed frequency-dependent facilitation, whereas type II EPSPs displayed frequency-dependent depression. Further details of these distinct forms of short-term synaptic plasticity were explored using paired-pulse stimuli. Pharmacology experiments revealed that both N-methyl-d-aspartate (NMDA) and non-NMDA glutamate receptors are involved in corticothalamic synaptic transmission in the LPN and contribute to both synaptic facilitation and depression. Taken together with the results of our previous anatomical studies, these results suggest that type I EPSPs arise from stimulation of layer VI afferents, whereas type II EPSPs arise from stimulation of layer V inputs. Moreover, type I and II EPSPs in the LPN may be functionally similar to corticogeniculate and retinogeniculate EPSPs, respectively.

Changes of immunocytochemical localization of vesicular glutamate transporters in the rat visual system after the retinofugal denervation

To clarify which vesicular glutamate transporter (VGluT) is used by excitatory axon terminals of the retinofugal system, we examined immunoreactivities and mRNA signals for VGluT1 and VGluT2 in the rat retina and compared immunoreactivities for VGluT1 and VGluT2 in the retinorecipient regions using double immunofluorescence method, anterograde tracing, and immunoelectron microscopy. Furthermore, the changes of VGluT1 and VGluT2 immunoreactivities were studied after eyeball enucleation. Intense immunoreactivity and mRNA signal for VGluT2, but not for VGluT1 immunoreactivity, were observed in most perikarya of ganglion cells in the retina. Immunoelectron microscopy revealed that VGluT1- and VGluT2-immunolabeled terminals made asymmetrical synapses, suggesting that they were excitatory synapses, and that VGluT1-immunolabeled terminals were smaller than VGluT2-labeled ones in many retinorecipient regions, such as the dorsal lateral geniculate nucleus (LGd) and superior colliculus (SC). Double immunofluorescence study further revealed that almost no VGluT2 immunoreactivity was colocalized with VGluT1 in the retinorecipient regions. After wheat germ agglutinin (WGA) injection into the eyeballs, WGA immunoreactivity was colocalized in the single axon terminals of LGd and SC with VGluT2 but not VGluT1 immunoreactivity. After unilateral enucleation, VGluT2 immunoreactivity in the LGd, SC, nucleus of the optic tract, and nuclei of the accessory optic tract in the contralateral side of the enucleated eye was clearly decreased. Although only a small change of VGluT2 immunoreactivity was observed in the contra- and ipsilateral suprachiasmatic nuclei, olivary pretectal nucleus, anterior pretectal nucleus, and posterior pretectal nucleus, moderate reduction of VGluT2 was found in these regions after bilateral enucleation. On the other hand, almost no change in VGluT1 immunoreactivity was found in the structures examined in the present enucleation study. Thus, the present results support the notion that the retinofugal pathways are glutamatergic, and indicate that VGluT2, but not VGluT1, is employed for accumulating glutamate into synaptic vesicles of retinofugal axons.

Retinogeniculate Synaptic Properties Controlling Spike Number and Timing in Relay Neurons

Retinal ganglion cells (RGC) transmit visual signals to thalamocortical relay neurons in the lateral geniculate nucleus via retinogeniculate synapses. Relay neuron spike patterns do not simply reflect those of RGCs, but the mechanisms controlling this transformation are not well understood. We therefore examined synaptic properties controlling the strength and precision of relay neuron firing in mouse (p28–33) brain slices using physiological stimulation patterns and a combination of current clamp and dynamic clamp. In tonic mode (-55 mV), activation of single RGC inputs elicited stereotyped responses in a given neuron. In contrast, responses in different neurons varied from unreliable, to faithfully following, to a gain in the number of spikes. Dynamic clamp experiments indicated these different responses primarily reflected variability in the amplitudes of the N-methyl-d-aspartate (NMDA) and AMPA components. Each of these components played a distinct role in transmission. The AMPA component evoked a single precisely timed, short-latency spike per stimulus, but efficacy decreased during repetitive stimulation due to desensitization and depression. The NMDA component elicited longer-latency spikes and multiple spikes per stimulus and became more effective during repetitive stimuli that led to NMDA current summation. We found that in burst mode (–75 mV), where low-threshold calcium spikes are activated, AMPA and NMDA components and synaptic plasticity influenced spike number, but no combination enabled relay cells to faithfully follow the stimulus. Thus the characteristics of AMPA and NMDA currents, the ratio of these currents and use-dependent plasticity interact to shape how RGC activity is conveyed to relay neurons.

Regulation of cell survival in the developing thalamus: an in vitro analysis

There is evidence that developing thalamic cells become dependent for their survival on the integrity of their afferent and/or efferent connections, which may provide required levels of neural activity and/or essential neurotrophic factors. These connections develop in the second half of gestation in mice and, during this time (embryonic days 17-19), isolated thalamic cells either grown as explants or dissociated from each other lose their ability to survive. Here we show that the loss of viability of explants, but not of dissociated cells, is delayed if the cultures are treated with depolarizing stimuli. The survival of dissociated thalamic cells is promoted by culture medium conditioned by thalamic explants grown with depolarizing stimuli, indicating that the effect of depolarization involves trophic factors released by thalamic cells. This survival promoting effect is found prenatally, but not postnatally, and is prevented by the neurotrophin blocker K252a. Culture medium conditioned by cortex also promotes the survival of thalamic cells and this effect does occur postnatally. These findings suggest that diffusible factors, possibly members of the neurotrophin family, and depolarizing stimuli regulate thalamic cell survival before birth, but trophic support from cortex becomes crucial after birth. This culture model may provide a means of investigating the mechanisms of thalamic cell survival during development.

The influence of the corticothalamic projection on responses in thalamus and cortex

We review results on the in vivo properties of neurons in the dorsal lateral geniculate nucleus (dLGN) that receives its afferent input from the retina and projects to the visual cortex. In addition, the dLGN receives input from the brain stem and from a rather strong corticothalamic back-projection, which originates in layer 6 of the visual cortex. We compare the behaviour of dLGN cells during spontaneous changes of the frequency contents of the electroencephalograph (EEG) (which are mainly related to a changing brain stem influence), with those that are obtained when experimentally silencing the corticothalamic feedback. The spatial and temporal response properties of dLGN cells are compared during these two conditions, and we report that the neurons behave similarly during a synchronized EEG state and during inactive corticothalamic feedback. In both situations, dLGN cells are rather phasic and their remaining tonic activity is temporally dispersed, indicating a hyperpolarizing effect. By means of a novel method, we were able to chronically eliminate a large proportion of the corticothalamic projection neurons from the otherwise intact cortex. In this condition, we found that cortical cells also lose their EEG specific response differences but, in this instance, probably due to a facilitatory (depolarizing) plasticity reaction of the remaining network.