An intrinsic oscillation in interneurons of the rat lateral geniculate nucleus

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.

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.

Gamma and infra-slow oscillations shape neuronal firing in the rat subcortical visual system

KEY POINTS

Neuronal oscillations observed in sensory systems are physiological carriers of information about stimulus features. Rhythm in the infra-slow range, originating from the retina, was previously found in the firing of subcortical visual system nuclei involved in both image and non-image forming functions. The present study shows that the firing of neurons in the lateral geniculate nucleus is also governed by gamma oscillation (∼35 Hz) time-locked to high phase of infra-slow rhythm that codes the intensity of transient light stimulation. We show that both physiological rhythms are synchronized within and between ipsilateral nuclei of the subcortical visual system and are dependent on retinal activity. The present study shows that neurophysiological oscillations characterized by various frequencies not only coexist in the subcortical visual system, but also are subjected to complex interference and synchronization processes.

ABSTRACT

The physiological function of rhythmic firing in the neuronal networks of sensory systems has been linked with information coding. Also, neuronal oscillations in different frequency bands often change as a signature of brain state or sensory processing. Infra-slow oscillation (ISO) in the neuronal firing dependent on the retinal network has been described previously in the structures of the subcortical visual system. In the present study, we show for the first time that firing of ISO neurons in the lateral geniculate nucleus is also characterized by a harmonic discharge pattern (i.e. action potentials are separated by the intervals governed by fundamental frequency in the gamma range: ∼35 Hz). A similar phenomenon was recently described in the suprachiasmatic nuclei of the hypothalamus: the master biological clock. We found that both gamma and ISO rhythms were synchronized within and between ipsilateral nuclei of the subcortical visual system and were dependent on the retinal activity of the contralateral eye. These oscillatory patterns were differentially influenced by transient and prolonged light stimulation with respect to both frequency change direction and sustainability. The results of the present study show that the firing pattern of neurons in the subcortical visual system is shaped by oscillations from infra-slow and gamma frequency bands that are plausibly generated by the retinal network. Additionally, the results demonstrate that both rhythms are not a distinctive feature of image or non-image forming visual systems but, instead, they comprise two channels carrying distinctive properties of photic information.

Multiple excitatory actions of orexins upon thalamo-cortical neurons in dorsal lateral geniculate nucleus - implications for vision modulation by arousal

The orexinergic system of the lateral hypothalamus plays a crucial role in maintaining wakefulness and mediating arousal in a circadian time-dependent manner. Due to the extensive connections of orexinergic neurons, both orexins (OXA and OXB) exert mainly excitatory effects upon remote brain areas, including the thalamus. The dorsal lateral geniculate nucleus (DLG) is a relay thalamic centre for the visual system. Its thalamo-cortical (TC) neurons convey photic information from the retina to the primary visual cortex. The present study shows that orexins are powerful modulators of neuronal activity in the DLG. OXA directly depolarised the majority of neurons tested, acting predominately on postsynaptic OX2 receptors. Moreover, OXA was found to increase excitability and enhance neuronal responses to both glutamate and γ-aminobutyric acid (GABA). Mechanistic studies showed the involvement of voltage-gated calcium currents and GIRK channels in the observed depolarisations. Immunohistochemical staining showed sparse orexinergic innervation of the DLG during the light phase, with increased density at night. We hypothesise that the depolarising effects of orexins upon DLG neurons may facilitate signal transmission through the visual thalamo-cortical pathway during behavioural arousal. Thus, the action of orexin on DLG TC neurons may underlie the circadian/behavioural modulation of vision.

A Spiking Neural Network Model of the Lateral Geniculate Nucleus on the SpiNNaker Machine

We present a spiking neural network model of the thalamic Lateral Geniculate Nucleus (LGN) developed on SpiNNaker, which is a state-of-the-art digital neuromorphic hardware built with very-low-power ARM processors. The parallel, event-based data processing in SpiNNaker makes it viable for building massively parallel neuro-computational frameworks. The LGN model has 140 neurons representing a "basic building block" for larger modular architectures. The motivation of this work is to simulate biologically plausible LGN dynamics on SpiNNaker. Synaptic layout of the model is consistent with biology. The model response is validated with existing literature reporting entrainment in steady state visually evoked potentials (SSVEP)-brain oscillations corresponding to periodic visual stimuli recorded via electroencephalography (EEG). Periodic stimulus to the model is provided by: a synthetic spike-train with inter-spike-intervals in the range 10-50 Hz at a resolution of 1 Hz; and spike-train output from a state-of-the-art electronic retina subjected to a light emitting diode flashing at 10, 20, and 40 Hz, simulating real-world visual stimulus to the model. The resolution of simulation is 0.1 ms to ensure solution accuracy for the underlying differential equations defining Izhikevichs neuron model. Under this constraint, 1 s of model simulation time is executed in 10 s real time on SpiNNaker; this is because simulations on SpiNNaker work in real time for time-steps dt ⩾ 1 ms. The model output shows entrainment with both sets of input and contains harmonic components of the fundamental frequency. However, suppressing the feed-forward inhibition in the circuit produces subharmonics within the gamma band (>30 Hz) implying a reduced information transmission fidelity. These model predictions agree with recent lumped-parameter computational model-based predictions, using conventional computers. Scalability of the framework is demonstrated by a multi-node architecture consisting of three "nodes," where each node is the "basic building block" LGN model. This 420 neuron model is tested with synthetic periodic stimulus at 10 Hz to all the nodes. The model output is the average of the outputs from all nodes, and conforms to the above-mentioned predictions of each node. Power consumption for model simulation on SpiNNaker is ≪1 W.

Causal Role of Thalamic Interneurons in Brain State Transitions: A Study Using a Neural Mass Model Implementing Synaptic Kinetics

Experimental studies on the Lateral Geniculate Nucleus (LGN) of mammals and rodents show that the inhibitory interneurons (IN) receive around 47.1% of their afferents from the retinal spiking neurons, and constitute around 20–25% of the LGN cell population. However, there is a definite gap in knowledge about the role and impact of IN on thalamocortical dynamics in both experimental and model-based research. We use a neural mass computational model of the LGN with three neural populations viz. IN, thalamocortical relay (TCR), thalamic reticular nucleus (TRN), to study the causality of IN on LGN oscillations and state-transitions. The synaptic information transmission in the model is implemented with kinetic modeling, facilitating the linking of low-level cellular attributes with high-level population dynamics. The model is parameterized and tuned to simulate alpha (8–13 Hz) rhythm that is dominant in both Local Field Potential (LFP) of LGN and electroencephalogram (EEG) of visual cortex in an awake resting state with eyes closed. The results show that: First, the response of the TRN is suppressed in the presence of IN in the circuit; disconnecting the IN from the circuit effects a dramatic change in the model output, displaying high amplitude synchronous oscillations within the alpha band in both TCR and TRN. These observations conform to experimental reports implicating the IN as the primary inhibitory modulator of LGN dynamics in a cognitive state, and that reduced cognition is achieved by suppressing the TRN response. Second, the model validates steady state visually evoked potential response in humans corresponding to periodic input stimuli; however, when the IN is disconnected from the circuit, the output power spectra do not reflect the input frequency. This agrees with experimental reports underpinning the role of IN in efficient retino-geniculate information transmission. Third, a smooth transition from alpha to theta band is observed by progressive decrease of neurotransmitter concentrations in the synaptic clefts; however, the transition is abrupt with removal of the IN circuitry in the model. The results imply a role of IN toward maintaining homeostasis in the LGN by suppressing any instability that may arise due to anomalous synaptic attributes.

Biophysical Network Modelling of the dLGN Circuit: Different Effects of Triadic and Axonal Inhibition on Visual Responses of Relay Cells

Despite its prominent placement between the retina and primary visual cortex in the early visual pathway, the role of the dorsal lateral geniculate nucleus (dLGN) in molding and regulating the visual signals entering the brain is still poorly understood. A striking feature of the dLGN circuit is that relay cells (RCs) and interneurons (INs) form so-called triadic synapses, where an IN dendritic terminal can be simultaneously postsynaptic to a retinal ganglion cell (GC) input and presynaptic to an RC dendrite, allowing for so-called triadic inhibition. Taking advantage of a recently developed biophysically detailed multicompartmental model for an IN, we here investigate putative effects of these different inhibitory actions of INs, i.e., triadic inhibition and standard axonal inhibition, on the response properties of RCs. We compute and investigate so-called area-response curves, that is, trial-averaged visual spike responses vs. spot size, for circular flashing spots in a network of RCs and INs. The model parameters are grossly tuned to give results in qualitative accordance with previous in vivo data of responses to such stimuli for cat GCs and RCs. We particularly investigate how the model ingredients affect salient response properties such as the receptive-field center size of RCs and INs, maximal responses and center-surround antagonisms. For example, while triadic inhibition not involving firing of IN action potentials was found to provide only a non-linear gain control of the conversion of input spikes to output spikes by RCs, axonal inhibition was in contrast found to substantially affect the receptive-field center size: the larger the inhibition, the more the RC center size shrinks compared to the GC providing the feedforward excitation. Thus, a possible role of the different inhibitory actions from INs to RCs in the dLGN circuit is to provide separate mechanisms for overall gain control (direct triadic inhibition) and regulation of spatial resolution (axonal inhibition) of visual signals sent to cortex.

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.

The role of two-pore-domain background K⁺ (K₂p) channels in the thalamus

The thalamocortical system is characterized by two fundamentally different activity states, namely synchronized burst firing and tonic action potential generation, which mainly occur during the behavioral states of sleep and wakefulness, respectively. The switch between the two firing modes is crucially governed by the bidirectional modulation of members of the K2P channel family, namely tandem of P domains in a weakly inward rectifying K(+) (TWIK)-related acid-sensitive K(+) (TASK) and TWIK-related K(+) (TREK) channels, in thalamocortical relay (TC) neurons. Several physicochemical stimuli including neurotransmitters, protons, di- and multivalent cations as well as clinically used drugs have been shown to modulate K2P channels in these cells. With respect to modulation of these channels by G-protein-coupled receptors, PLCβ plays a unique role with both substrate breakdown and product synthesis exerting important functions. While the degradation of PIP2 leads to the closure of TREK channels, the production of DAG induces the inhibition of TASK channels. Therefore, TASK and TREK channels were found to be central elements in the control of thalamic activity modes. Since research has yet focused on identifying the muscarinic pathway underling the modulation of TASK and TREK channels in TC neurons, future studies should address other thalamic cell types and members of the K2P channel family.

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.

Reciprocal inhibition and slow calcium decay in perigeniculate interneurons explain changes of spontaneous firing of thalamic cells caused by cortical inactivation

The role of cortical feedback in the thalamocortical processing loop has been extensively investigated over the last decades. With an exception of several cases, these searches focused on the cortical feedback exerted onto thalamo-cortical relay (TC) cells of the dorsal lateral geniculate nucleus (LGN). In a previous, physiological study, we showed in the cat visual system that cessation of cortical input, despite decrease of spontaneous activity of TC cells, increased spontaneous firing of their recurrent inhibitory interneurons located in the perigeniculate nucleus (PGN). To identify mechanisms underlying such functional changes we conducted a modeling study in NEURON on several networks of point neurons with varied model parameters, such as membrane properties, synaptic weights and axonal delays. We considered six network topologies of the retino-geniculo-cortical system. All models were robust against changes of axonal delays except for the delay between the LGN feed-forward interneuron and the TC cell. The best representation of physiological results was obtained with models containing reciprocally connected PGN cells driven by the cortex and with relatively slow decay of intracellular calcium. This strongly indicates that the thalamic reticular nucleus plays an essential role in the cortical influence over thalamo-cortical relay cells while the thalamic feed-forward interneurons are not essential in this process. Further, we suggest that the dependence of the activity of PGN cells on the rate of calcium removal can be one of the key factors determining individual cell response to elimination of cortical input.

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.

Cortical feedback regulation of input to visual cortex: role of intrageniculate interneurons

Neurons in the dorsal lateral geniculate nucleus (dLGN) process and transmit visual signals from retina to visual cortex. The processing is dynamically regulated by cortical excitatory feedback to neurons in dLGN, and synaptic short-term plasticity (STP) has an important role in this regulation. It is known that corticogeniculate synapses on thalamocortical (TC) projection-neurons are facilitating, but type and characteristics of STP of synapses on inhibitory interneurons in dLGN are unknown. We studied STP at corticogeniculate synapses on interneurons and compared the results with STP-characteristics of corticogeniculate synapses on TC neurons to gain insights into the dynamics of cortical regulation of processing in dLGN. We studied neurons in thalamic slices from glutamate decarboxylase 67 (GAD67)–green fluorescent protein (GFP) knock-in mice and made whole-cell recordings of responses evoked by electrical paired-pulse and pulse train stimulation of cortical afferents. We found that cortical excitations of interneurons and TC neurons have distinctly different properties. A single pulse evoked larger EPSCs in interneurons than in TC neurons. However, repetitive stimulation induced frequency-dependent depression of interneurons in contrast to the facilitation of TC neurons. Thus, through these differences of STP mechanisms, the balance of cortical excitation of the two types of neurons could change during stimulation from strongest excitation of interneurons to strongest excitation of TC neurons depending on stimulus frequency and duration, and thereby contribute to activity-dependent cortical regulation of thalamocortical transmission between net depression and net facilitation. Studies of postsynaptic response patterns of interneurons to train stimulation demonstrated that cortical input can activate different types of neuronal integration mechanisms that in addition to the STP mechanisms may change the output from dLGN. Lower stimulus intensity, presumably activating few cortical afferents, or moderate frequencies, elicited summation of graded EPSPs reflecting synaptic depression. However, strong activation through higher intensity or frequency, elicited complex response patterns in interneurons caused at least partly by activation of calcium conductances.

Subcellular distribution of α1G subunit of T-type calcium channel in the mouse dorsal lateral geniculate nucleus

T-type calcium channels play a pivotal role in regulating neural membrane excitability in the nervous system. However, the precise subcellular distributions of T-type channel subunits and their implication for membrane excitability are not well understood. Here we investigated the subcellular distribution of the α1G subunit of the calcium channel which is expressed highly in the mouse dorsal lateral geniculate nucleus (dLGN). Light microscopic analysis demonstrated that dLGN exhibits intense immunoperoxidase reactivity for the α1G subunit. Electron microscopic observation showed that the labeling was present in both the relay cells and interneurons and was found in the somatodendritic, but not axonal, domains of these cells. Most of the immunogold particles for the α1G subunit were either associated with the plasma membrane or the intracellular membranes. Reconstruction analysis of serial electron microscopic images revealed that the intensity of the intracellular labeling exhibited a gradient such that the labeling density was higher in the proximal dendrite and progressively decreased towards the distal dendrite. In contrast, the plasma membrane-associated particles were distributed with a uniform density over the somatodendritic surface of dLGN cells. The labeling density in the relay cell plasma membrane was about 3-fold higher than that of the interneurons. These results provide ultrastructural evidence for cell-type-specific expression levels and for uniform expression density of the α1G subunit over the plasma membrane of dLGN cells.

Synaptic information transfer in computer models of neocortical columns

Understanding the direction and quantity of information flowing in neuronal networks is a fundamental problem in neuroscience. Brains and neuronal networks must at the same time store information about the world and react to information in the world. We sought to measure how the activity of the network alters information flow from inputs to output patterns. Using neocortical column neuronal network simulations, we demonstrated that networks with greater internal connectivity reduced input/output correlations from excitatory synapses and decreased negative correlations from inhibitory synapses, measured by Kendall's τ correlation. Both of these changes were associated with reduction in information flow, measured by normalized transfer entropy (nTE). Information handling by the network reflected the degree of internal connectivity. With no internal connectivity, the feedforward network transformed inputs through nonlinear summation and thresholding. With greater connectivity strength, the recurrent network translated activity and information due to contribution of activity from intrinsic network dynamics. This dynamic contribution amounts to added information drawn from that stored in the network. At still higher internal synaptic strength, the network corrupted the external information, producing a state where little external information came through. The association of increased information retrieved from the network with increased gamma power supports the notion of gamma oscillations playing a role in information processing.

Temporal framing of thalamic relay-mode firing by phasic inhibition during the alpha rhythm

Several aspects of perception, particularly those pertaining to vision, are closely linked to the occipital alpha (α) rhythm. However, how the α rhythm relates to the activity of neurons that convey primary visual information is unknown. Here we show that in behaving cats, thalamocortical neurons in the lateral geniculate nucleus (LGN) that operate in a conventional relay-mode form two groups where the cumulative firing is subject to a cyclic suppression that is centered on the negative α rhythm peak in one group and on the positive peak in the other. This leads to an effective temporal framing of relay-mode output and results from phasic inhibition from LGN interneurons, which in turn are rhythmically excited by thalamocortical neurons that exhibit high-threshold bursts. These results provide a potential cellular substrate for linking the α rhythm to perception and further underscore the central role of inhibition in controlling spike timing during cognitively relevant brain oscillations.

Coupling of mesoscopic brain oscillations: recent advances in analytical and theoretical perspectives

Oscillatory brain activities have been traditionally studied in the context of how oscillations at a single frequency recorded from a single area could reveal functional insights. Recent advances in methodology used in signal analysis have revealed that cross-frequency coupling, within or between functional related areas, is more informative in determining the possible roles played by brain oscillations. In this review, we begin by describing the cellular basis of oscillatory field potentials and its theorized as well as demonstrated role in brain function. The recent development of mathematical tools that allow the investigation of cross-frequency and cross-area oscillation coupling will be presented and discussed in the context of recent advances in oscillation research based on animal data. Particularly, some pitfalls and caveats of methods currently available are discussed. Data generated from the application of examined techniques are integrated back into the theoretical framework regarding the functional role of brain oscillations. We suggest that the coupling of oscillatory activities at different frequencies between brain regions is crucial for understanding the brain from a functional ensemble perspective. Effort should be directed to elucidate how cross-frequency and area coupling are modulated and controlled. To achieve this, only the correct application of analytical tools may shed light on the intricacies of information representation, generation, binding, encoding, storage and retrieval in the brain.

Stochastically gating ion channels enable patterned spike firing through activity-dependent modulation of spike probability

The transformation of synaptic input into patterns of spike output is a fundamental operation that is determined by the particular complement of ion channels that a neuron expresses. Although it is well established that individual ion channel proteins make stochastic transitions between conducting and non-conducting states, most models of synaptic integration are deterministic, and relatively little is known about the functional consequences of interactions between stochastically gating ion channels. Here, we show that a model of stellate neurons from layer II of the medial entorhinal cortex implemented with either stochastic or deterministically gating ion channels can reproduce the resting membrane properties of stellate neurons, but only the stochastic version of the model can fully account for perithreshold membrane potential fluctuations and clustered patterns of spike output that are recorded from stellate neurons during depolarized states. We demonstrate that the stochastic model implements an example of a general mechanism for patterning of neuronal output through activity-dependent changes in the probability of spike firing. Unlike deterministic mechanisms that generate spike patterns through slow changes in the state of model parameters, this general stochastic mechanism does not require retention of information beyond the duration of a single spike and its associated afterhyperpolarization. Instead, clustered patterns of spikes emerge in the stochastic model of stellate neurons as a result of a transient increase in firing probability driven by activation of HCN channels during recovery from the spike afterhyperpolarization. Using this model, we infer conditions in which stochastic ion channel gating may influence firing patterns in vivo and predict consequences of modifications of HCN channel function for in vivo firing patterns.

Data mining through simulation

Data integration is particularly difficult in neuroscience; we must organize vast amounts of data around only a few fragmentary functional hypotheses. It has often been noted that computer simulation, by providing explicit hypotheses for a particular system and bridging across different levels of organization, can provide an organizational focus, which can be leveraged to form substantive hypotheses. Simulations lend meaning to data and can be updated and adapted as further data come in. The use of simulation in this context suggests the need for simulator adjuncts to manage and evaluate data. We have developed a neural query system (NQS) within the NEURON simulator, providing a relational database system, a query function, and basic data-mining tools. NQS is used within the simulation context to manage, verify, and evaluate model parameterizations. More importantly, it is used for data mining of simulation data and comparison with neurophysiology.

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.

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.

Brainstem input modulates globally the transmission through the lateral geniculate nucleus

The transmission of visual information from the retina to the visual cortex through the lateral geniculate nucleus (LGN) is a complex process, which involves several neuronal mechanisms, elements, and circuits. The authors investigated this process in anesthetized, paralyzed cats by recording from LGN relay neurons, together with their retinal input, which appeared as slow (S) potentials. The major findings are: (1) The transfer ratio (LGN firing/retinal firing) fluctuated slowly and (2) these fluctuations in transfer ratio were synchronized across the nucleus, did not depend on visual stimulation, and were highly correlated with neural activity in the parabrachial nucleus of the brainstem (PBN). Electrical stimulation of the PBN increased transmission from retina to cortex through the LGN. It is concluded that the PBN, which is part of the Ascending Arousal System, can modulate globally the transmission of information through the thalamus.

Bursting of thalamic neurons and states of vigilance

This article addresses the functional significance of the electrophysiological properties of thalamic neurons. We propose that thalamocortical activity, is the product of the intrinsic electrical properties of the thalamocortical (TC) neurons and the connectivity their axons weave. We begin with an overview of the electrophysiological properties of single neurons in different functional states, followed by a review of the phylogeny of the electrical properties of thalamic neurons, in several vertebrate species. The similarity in electrophysiological properties unambiguously indicates that the thalamocortical system must be as ancient as the vertebrate branch itself. We address the view that rather than simply relays, thalamic neurons have sui generis intrinsic electrical properties that govern their specific functional dynamics and regulate natural functional states such as sleep and vigilance. In addition, thalamocortical activity has been shown to be involved in the genesis of several neuropsychiatric conditions collectively described as thalamocortical dysrhythmia syndrome.

Thalamic mechanisms of EEG alpha rhythms and their pathological implications

During relaxed wakefulness, the human brain exhibits pronounced rhythmic electrical activity in the alpha frequency band (8-13 Hz). This activity consists of 3 main components: the classic occipital alpha rhythm, the Rolandic mu rhythm, and the so-called third rhythm. In recent years, the long-held belief that alpha rhythms are strongly influenced by the thalamus has been confirmed in several animal models and, in humans, is well supported by numerous noninvasive imaging studies. Of specific importance is the emergence of 2 key cellular thalamic mechanisms, which come together to generate locally synchronized alpha activity. First, a novel form of rhythmic burst firing, termed high-threshold (HT) bursting, which occurs in a specialized subset of thalamocortical (TC) neurons, and second, the interconnection of this subset via gap junctions (GJs). Because repetitive HT bursting in TC neurons occurs in the range of 2 to 13 Hz, with the precise frequency increasing with increasing depolarization, the same cellular components that underlie thalamic alpha rhythms can also lead to theta (2-7 Hz) rhythms when the TC neuron population is less depolarized. As such, this scenario can explain both the deceleration of alpha rhythms that takes place during early sleep and the chronic slowing that characterizes a host of neurological and psychiatric disorders.

Dopaminergic control of local interneuron activity in the thalamus

The dorsal lateral geniculate nucleus (dLGN), the major thalamic station of the visual pathway, contains a fairly large number of dopaminergic terminals, and dopamine was found to reduce spontaneous and visually evoked activity in the dLGN in vivo. The cellular basis of this influence remained unknown. Here we have used whole cell patch-clamp techniques to analyse the effects of dopamine (DA) on GABAergic transmission in dLGN slices of juvenile postnatal day (P) 12-P24 Long-Evans rats or juvenile (P12-P22) GAD67-GFP (Deltaneo) mice. Spontaneous inhibitory postsynaptic currents (sIPSCs) were increased in frequency by the D(2)-like agonist quinpirole (QUIN) in rat (n = 6), as well as in mouse (n = 5) thalamic slices. This effect was blocked in the presence of the D(2)-like antagonist sulpiride (SULP, n = 5) and was absent in the ventrobasal complex (VB) of rat (n = 7) and mouse (n = 4) thalamus, which is devoid of GABAergic interneurons. Direct recordings from labelled GABAergic neurons in the dLGN of GAD67-GFP mice revealed a QUIN-mediated membrane depolarization (n = 12), which was attenuated by SULP (n = 6). These data demonstrate that DA through activation of D(2)-like receptors in GABAergic interneurons induces an increase in inhibitory interactions most likely at F2 dendrodendritic terminals, thereby providing a cellular correlate of the observation made in vivo that DA predominantly acts through inhibition of relay cell activity in the dLGN.

Bioinformation within the biofield: beyond bioelectromagnetics

This review article extends previous scientific definitions of the biofield (endogenous energy fields of the body) to include nonclassical and quantum energy fields. The biofield is defined further in terms of its functional property to act as a resonance target for external forms of energy used as treatment modalities in energy medicine. The functional role of the biofield in the body's innate self-healing mechanisms is hypothesized, based on the concept of bioinformation which, mediated by consciousness, functions globally at the quantum level to supply coherence, phase, spin, and pattern information to regulate and heal all physiologic processes. This model is used to explain a wide variety of anomalies reported in the scientific literature, which can not be explained by traditional biophysics and bioelectromagnetics.

Synchronized oscillations at alpha and theta frequencies in the lateral geniculate nucleus

In relaxed wakefulness, the EEG exhibits robust rhythms in the alpha band (8-13 Hz), which decelerate to theta (approximately 2-7 Hz) frequencies during early sleep. In animal models, these rhythms occur coherently with synchronized activity in the thalamus. However, the mechanisms of this thalamic activity are unknown. Here we show that, in slices of the lateral geniculate nucleus maintained in vitro, activation of the metabotropic glutamate receptor (mGluR) mGluR1a induces synchronized oscillations at alpha and theta frequencies that share similarities with thalamic alpha and theta rhythms recorded in vivo. These in vitro oscillations are driven by an unusual form of burst firing that is present in a subset of thalamocortical neurons and are synchronized by gap junctions. We propose that mGluR1a-induced oscillations are a potential mechanism whereby the thalamus promotes EEG alpha and theta rhythms in the intact brain.

Novel vistas of calcium-mediated signalling in the thalamus

Traditionally, the role of calcium ions (Ca(2+)) in thalamic neurons has been viewed as that of electrical charge carriers. Recent experimental findings in thalamic cells have only begun to unravel a highly complex Ca(2+) signalling network that exploits extra- and intracellular Ca(2+) sources. In thalamocortical relay neurons, interactions between T-type Ca(2+) channel activation, Ca(2+)-dependent regulation of adenylyl cyclase activity and the hyperpolarization-activated cation current ( I(h)) regulate oscillatory burst firing during periods of sleep and generalized epilepsy, while a functional triad between Ca(2+) influx through high-voltage-activated (most likely L-type) Ca(2+) channels, Ca(2+)-induced Ca(2+) release via ryanodine receptors (RyRs) and a repolarizing mechanism (possibly via K(+) channels of the BK(Ca) type) supports tonic spike firing as required during wakefulness. The mechanisms seem to be located mostly at dendritic and somatic sites, respectively. One functional compartment involving local GABAergic interneurons in certain thalamic relay nuclei is the glomerulus, in which the dendritic release of GABA is regulated by Ca(2+) influx via canonical transient receptor potential channels (TRPC), thereby presumably enabling transmitters of extrathalamic input systems that are coupled to phospholipase C (PLC)-activating receptors to control feed-forward inhibition in the thalamus. Functional interplay between T-type Ca(2+) channels in dendrites and the A-type K(+) current controls burst firing, contributing to the range of oscillatory activity observed in these interneurons. GABAergic neurons in the reticular thalamic (RT) nucleus recruit a specific set of Ca(2+)-dependent mechanisms for the generation of rhythmic burst firing, of which a particular T-type Ca(2+) channel in the dendritic membrane, the Ca(2+)-dependent activation of non-specific cation channels ( I(CAN)) and of K(+) channels (SK(Ca) type) are key players. Glial Ca(2+) signalling in the thalamus appears to be a basic mechanism of the dynamic and integrated exchange of information between glial cells and neurons. The conclusion from these observations is that a localized calcium signalling network exists in all neuronal and probably also glial cell types in the thalamus and that this network is dedicated to the precise regulation of the functional mode of the thalamus during various behavioural states.

Computer model of passive signal integration based on whole-cell in vitro studies of rat lateral geniculate nucleus

Computer models were used to investigate passive properties of lateral geniculate nucleus thalamocortical cells and thalamic interneurons based on in vitro whole-cell study. Two neurons of each type were characterized physiologically and morphologically. Thalamocortical cells transmitted 37% of steady-state signal orthodromically (distal dendrite to soma) and 93% antidromically (soma to distal dendrite); interneurons transmitted 18% orthodromically and 53% antidromically. Lowering membrane resistance caused a dramatic drop in steady-state signal transmission. Simulation of brief signals such as orthodromically transmitted postsynaptic potentials and antidromically transmitted action potentials showed relatively poor transmission due to the low-pass filtering property of dendrites. This attenuation was particularly pronounced in interneurons. By contrast, bursts of postsynaptic potentials or action potentials were relatively well transmitted as the temporal summation of these recurring signals gave prolonged depolarizations comparable to prolonged current injection. While synaptic clustering, active channels and reduction of membrane resistance by ongoing synaptic activity will have additional profound effects in vivo, the present in vitro modelling suggests that passive signal transmission in neurons will depend on type of signal conveyed, on directionality and on membrane state. This will be particularly important for thalamic interneurons, whose presynaptic dendrites may either work independently or function in concert with each other and with the soma. Our findings suggest that bursts may be particularly well transmitted along dendrites, allowing firing format to alter the functional anatomy of the cell.

The contribution of the amygdala to conditioned thalamic arousal

Previous research has demonstrated that thalamocortical neurons within the dorsal lateral geniculate nucleus (dLGN) are affected by an acoustic, fear-arousing, conditioned stimulus (Cain et al., 2000). This effect is reflected in an increase in activity and a tonic firing pattern, a pattern that assures the most accurate relay of information from the retina to the visual neocortex. Such an effect is considered to be indicative of a heightened state of arousal. The present research was designed to determine the extent to which the central nucleus of the amygdala (ACe) contributes to this effect. To this end, in experiment 1 extracellular recordings were made from single dLGN neurons in the awake rabbit during electrical stimulation of the ACe. Increased neuronal activity was observed in response to stimulation in the majority of neurons. Neurons that were in a burst firing pattern immediately before stimulation assumed a tonic firing pattern in response to stimulation. Experiment 2 was designed to determine whether inactivation of the ACe with muscimol would attenuate the response of dLGN neurons in the awake rabbit to the presentation of acoustic, fear-arousing, conditioned stimuli. Compared with vehicle injections, infusions of muscimol attenuated both the spontaneous activity and the response of dLGN neurons to the presentations of these stimuli. The results provide support for the hypothesis that the amygdala, and in particular the ACe, contributes to a heightened state of arousal during conditioned fear.

Bursting in leech heart interneurons: cell-autonomous and network-based mechanisms

Rhythmic activity within the heartbeat pattern generator of the medicinal leech is based on the alternating bursting of mutually inhibitory pairs of oscillator heart interneurons (half-center oscillators). Bicuculline methiodide has been shown to block mutual inhibition between these interneurons and to cause them to spike tonically while recorded intracellularly (Schmidt and Calabrese, 1992). Using extracellular recording techniques, we show here that oscillator and premotor heart interneurons continue to burst when pharmacologically isolated with bicuculline, although the bursting is not robust in some preparations. We propose that a nonspecific leak current introduced by the intracellular microelectrode suppresses endogenous bursting activity to account for the discrepancy with results using intracellular recording. A two-parameter bifurcation diagram (E(leak) vs g(leak)) of a mathematical model of a single heart interneuron shows a narrow stripe of parameter values where bursting occurs, separating large zones of tonic spiking and silence. A similar analysis performed for a half-center oscillator model outlined a much larger area of bursting. Bursting in the half-center oscillator model is also less sensitive to variation in the maximal conductances of voltage-gated currents than in the single-neuron model. Thus, in addition to ensuring appropriate bursting characteristics such as period, phase, and duty cycles, the half-center configuration enhances oscillation robustness, making them less susceptible to random or imposed changes in membrane parameters. Endogenous bursting, in turn, ensures appropriate bursting if the strength of mutual inhibition is weakened and limits the minimum period of the half-center oscillator to a period near that of the single neuron.

The 5-HT1A serotonin receptor is located on calbindin- and parvalbumin-containing neurons in the rat brain

The 5-HT(1A) receptor is a well-characterized serotonin receptor playing a role in many central nervous functions and known to be involved in depression and other mental disorders. In situ hybridization, immunocytochemical, and binding studies have shown that the 5-HT(1A) receptor is widely distributed in the rat brain, with a particularly high density in the limbic system. The receptor's localization in the different neuronal subtypes, which may be of importance for understanding its role in neuronal circuitries, is, however, unknown. In this study we show by immunocytochemical double-labeling techniques, that the 5-HT(1A) receptor is present on both pyramidal and principal cells, and calbindin- and parvalbumin-containing neurons, which generally define two different subtypes of interneurons. Moreover, semiquantitative analysis showed that the receptor's distribution in the different neuronal types varies between brain areas. In cortex, hippocampus, hypothalamus, and amygdala the receptor was located on both principal cells and calbindin- and parvalbumin-containing neurons. In septum and thalamus, the receptor was mostly present on calbindin- and parvalbumin-containing cells. Especially in the medial septum and thalamic reticular nucleus, the receptor highly colocalized with parvalbumin-positive neurons. These results suggest a diverse function of the 5-HT(1A) receptor in modulating neuronal circuitry in different brain areas, that may depend on the type of neuron the receptor is predominantly located on.

Postnatal development of GABAergic signalling in the rat lateral geniculate nucleus: presynaptic dendritic mechanisms

Diverse forms of GABAergic inhibition are found in the mature brain. To understand how this diversity develops, we studied the changes in morphology of inhibitory interneurons and changes in interneuron-mediated synaptic transmission in the rat dorsal lateral geniculate nucleus (dLGN). We found a steady expansion of the dendritic tree of interneurons over the first three postnatal weeks. During this period, the area around a thalamocortical cell from which GABA(A) inhibition could be elicited also expanded. Dendritic branching and burst firing in interneurons evolved more slowly. The distal dendrites of interneurons began to branch extensively after the third week, and at the same time burst firing appeared. The appearance of burst firing and an elaborated dendritic tree were accompanied by a pronounced GABA(B) inhibition of thalamocortical cells. Thus, GABA inhibition of thalamocortical cells developed from one type of GABA(A) inhibition (spatially restricted) in the young animal into two distinct types of GABA(A) inhibition (short- and long-range) and GABA(B) inhibition in the adult animal. The close temporal relationships between the development of the diverse forms of inhibition and the postnatal changes in morphology of local GABAergic interneurons in the dLGN suggest that postnatal dendritic maturation is an important presynaptic factor for the developmental time course of the various types of feedforward inhibition in thalamus.

Muscarinic regulation of dendritic and axonal outputs of rat thalamic interneurons: a new cellular mechanism for uncoupling distal dendrites

Inhibition is crucial for sharpening the sensory information relayed through the thalamus. To understand how the interneuron-mediated inhibition in the thalamus is regulated, we studied the muscarinic effects on interneurons in the lateral posterior nucleus and lateral geniculate nucleus of the thalamus. Here, we report that activation of muscarinic receptors switched the firing pattern in thalamic interneurons from bursting to tonic. Although neuromodulators switch the firing mode in several other types of neurons by altering their membrane potential, we found that activation of muscarinic subtype 2 receptors switched the fire mode in thalamic interneurons by selectively decreasing their input resistance. This is attributable to the muscarinic enhancement of a hyperpolarizing potassium conductance and two depolarizing cation conductances. The decrease in input resistance appeared to electrotonically uncouple the distal dendrites of thalamic interneurons, which effectively changed the inhibition pattern in thalamocortical cells. These results suggest a novel cellular mechanism for the cholinergic transformation of long-range, slow dendrite- and axon-originated inhibition into short-range, fast dendrite-originated inhibition in the thalamus observed in vivo. It is concluded that the electrotonic properties of the dendritic compartments of thalamic interneurons can be dynamically regulated by muscarinic activity.

NEURON: a tool for neuroscientists

NEURON is a simulation environment for models of individual neurons and networks of neurons that are closely linked to experimental data. NEURON provides tools for conveniently constructing, exercising, and managing models, so that special expertise in numerical methods or programming is not required for its productive use. This article describes two tools that address the problem of how to achieve computational efficiency and accuracy.

Corticothalamic resonance, states of vigilance and mentation

During various states of vigilance, brain oscillations are grouped together through reciprocal connections between the neocortex and thalamus. The coherent activity in corticothalamic networks, under the control of brainstem and forebrain modulatory systems, requires investigations in intact-brain animals. During behavioral states associated with brain disconnection from the external world, the large-scale synchronization of low-frequency oscillations is accompanied by the inhibition of synaptic transmission through thalamocortical neurons. Despite the coherent oscillatory activity, on the functional side there is dissociation between the thalamus and neocortex during slow-wave sleep. While dorsal thalamic neurons undergo inhibitory processes due to the prolonged spike-bursts of thalamic reticular neurons, the cortex displays, periodically, a rich spontaneous activity and preserves the capacity to process internally generated signals that dominate the state of sleep. In vivo experiments using simultaneous intracellular recordings from thalamic and cortical neurons show that short-term plasticity processes occur after prolonged and rhythmic spike-bursts fired by thalamic and cortical neurons during slow-wave sleep oscillations. This may serve to support resonant phenomena and reorganize corticothalamic circuitry, determine which synaptic modifications, formed during the waking state, are to be consolidated and generate a peculiar kind of dreaming mentation. In contrast to the long-range coherent oscillations that occur at low frequencies during slow-wave sleep, the sustained fast oscillations that characterize alert states are synchronized over restricted territories and are associated with discrete and differentiated patterns of conscious events.

Spiking-bursting activity in the thalamic reticular nucleus initiates sequences of spindle oscillations in thalamic networks

Recent intracellular and local field potential recordings from thalamic reticular (RE) neurons in vivo as well as computational modeling of the isolated RE nucleus suggest that, at relatively hyperpolarized levels of membrane potentials, the inhibitory postsynaptic potentials (IPSPs) between RE cells can be reversed and gamma-aminobutyric acid-A (GABA(A)) -mediated depolarization can generate persistent spatio-temporal patterns in the RE nucleus. Here we investigate how this activity affects the spatio-temporal properties of spindle oscillations with computer models of interacting RE and thalamocortical (TC) cells. In a one-dimensional network of RE and TC cells, sequences of spindle oscillations alternated with localized patterns of spike-burst activity propagating inside the RE network. New sequences of spindle oscillations were initiated after removal of I(h)-mediated depolarization of the TC cells. The length of the interspindle lulls depended on the intrinsic and synaptic properties of RE and TC cells and was in the range of 3-20 s. In a two-dimensional model, GABA(A)-mediated 2-3 Hz oscillations persisted in the RE nucleus during interspindle lulls and initiated spindle sequences at many foci within the RE-TC network simultaneously. This model predicts that the intrinsic properties of the reticular thalamus may contribute to the synchrony of spindle oscillations observed in vivo.

Three GABA receptor-mediated postsynaptic potentials in interneurons in the rat lateral geniculate nucleus

Inhibition is crucial for the thalamus to relay sensory information from the periphery to the cortex and to participate in thalamocortical oscillations. However, the properties of inhibitory synaptic events in interneurons are poorly defined because in part of the technical difficulty of obtaining stable recording from these small cells. With the whole-cell recording technique, we obtained stable recordings from local interneurons in the lateral geniculate nucleus and studied their inhibitory synaptic properties. We found that interneurons expressed three different types of GABA receptors: bicuculline-sensitive GABA(A) receptors, bicuculline-insensitive GABA(A) receptors, and GABA(B) receptors. The reversal potentials of GABA responses were estimated by polarizing the membrane potential. The GABA(A) receptor-mediated responses had a reversal potential of approximately -82 mV, consistent with mediation via Cl(-) channels. The reversal potential for the GABA(B) response was -97 mV, consistent with it being a K(+) conductance. The roles of these GABA receptors in postsynaptic responses were also examined in interneurons. Optic tract stimulation evoked a disynaptic IPSP that was mediated by all three types of GABA receptors and depended on activation of geniculate interneurons. Stimulation of the thalamic reticular nucleus evoked an IPSP, which appeared to be mediated exclusively by bicuculline-sensitive GABA(A) receptors and depended on the activation of reticular cells. The results indicate that geniculate interneurons form a complex neuronal circuitry with thalamocortical and reticular cells via feed-forward and feedback circuits, suggesting that they play a more important role in thalamic function than thought previously.

Properties of a hyperpolarization-activated cation current in interneurons in the rat lateral geniculate nucleus

A hyperpolarization-activated cation conductance contributes to the membrane properties of a variety of cell types. In the thalamus, a prominent hyperpolarization-activated cation conductance exists in thalamocortical cells, and this current is implicated in the neuromodulation of complex firing behaviors. In contrast, the GABAergic cells in the reticular nucleus in the thalamus appear to lack this conductance. The presence and role of this cation conductance in the other type of thalamic GABAergic cells, local interneurons, is still unclear. To resolve this issue, we studied 54 physiologically and morphologically identified local interneurons in the rat dorsal lateral geniculate nucleus using an in vitro whole-cell patch recording technique. We found that hyperpolarizing current injections induced depolarizing voltage sags in these geniculate interneurons. The I-V relationship revealed an inward rectification. Voltage-clamp study indicated that a slow, hyperpolarization-activated cation conductance was responsible for the inward rectification. We then confirmed that this slow conductance had properties of the hyperpolarization-activated cation conductance described in other cell types. The slow conductance was insensitive to 10 mM tetraethylammonium and 0.5 mM 4-aminopyridine, but was largely blocked by 1-1.5 mM Cs+. It was permeable to both K+ and Na+ ions and had a reversal potential of -44 mV. The voltage dependence of the hyperpolarization-activated cation conductance in interneurons was also studied: the activation threshold was about -55 mV, half-activation potential was about -80 mV and maximal conductance was about 1 nS. The activation and deactivation time constants of the conductance ranged from 100 to 1000 ms, depending on membrane potential. The depolarizing voltage sags and I-V relationship were further simulated in a model interneuron, using the parameters of the hyperpolarization-activated cation conductance obtained from the voltage-clamp study. The time-course and voltage dependence of the depolarizing voltage sags and I-V relationship in the model cell were very similar to those found in geniculate interneurons in current clamp. Taken together, the results of the present study suggest that thalamic local interneurons possess a prominent hyperpolarization-activated cation conductance, which may play important roles in determining basic membrane properties and in modulating firing patterns.