The thalamic reticular nucleus of the adult rat: experimental anatomical studies

Functionally Distinct Circuits Are Linked by Heterocellular Electrical Synapses in the Thalamic Reticular Nucleus

The thalamic reticular nucleus (TRN) inhibits sensory thalamocortical relay neurons and is a key regulator of sensory attention as well as sleep and wake states. Recent developments have identified two distinct genetic subtypes of TRN neurons, calbindin-expressing (CB) and somatostatin-expressing (SOM) neurons. These subtypes differ in localization within the TRN, electrophysiological properties, and importantly, targeting of thalamocortical relay channels. CB neurons send inhibition to and receive excitation from first-order thalamic relay nuclei, while SOM neurons send inhibition to and receive excitation from higher-order thalamic areas. These differences create distinct channels of information flow. It is unknown whether TRN neurons form electrical synapses between SOM and CB neurons and consequently bridge first-order and higher-order thalamic channels. Here, we use GFP reporter mice to label and record from CB-expressing and SOM-expressing TRN neurons. We confirm that GFP expression properly differentiates TRN subtypes based on electrophysiological differences, and we identified electrical synapses between pairs of neurons with and without common GFP expression for both CB and SOM types. That is, electrical synapses link both within and across subtypes of neurons in the TRN, forming either homocellular or heterocellular synapses. Therefore, we conclude that electrical synapses within the TRN provide a substrate for functionally linking thalamocortical first-order and higher-order channels within the TRN.

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

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

On the Diverse Functions of Electrical Synapses

Electrical synapses are the neurophysiological product of gap junctional pores between neurons that allow bidirectional flow of current between neurons. They are expressed throughout the mammalian nervous system, including cortex, hippocampus, thalamus, retina, cerebellum, and inferior olive. Classically, the function of electrical synapses has been associated with synchrony, logically following that continuous conductance provided by gap junctions facilitates the reduction of voltage differences between coupled neurons. Indeed, electrical synapses promote synchrony at many anatomical and frequency ranges across the brain. However, a growing body of literature shows there is greater complexity to the computational function of electrical synapses. The paired membranes that embed electrical synapses act as low-pass filters, and as such, electrical synapses can preferentially transfer spike after hyperpolarizations, effectively providing spike-dependent inhibition. Other functions include driving asynchronous firing, improving signal to noise ratio, aiding in discrimination of dissimilar inputs, or dampening signals by shunting current. The diverse ways by which electrical synapses contribute to neuronal integration merits furthers study. Here we review how functions of electrical synapses vary across circuits and brain regions and depend critically on the context of the neurons and brain circuits involved. Computational modeling of electrical synapses embedded in multi-cellular models and experiments utilizing optical control and measurement of cellular activity will be essential in determining the specific roles performed by electrical synapses in varying contexts.

Cre driver mouse lines for thalamocortical circuit mapping

Subpopulations of neurons and associated neural circuits can be targeted in mice with genetic tools in a highly selective manner for visualization and manipulation. However, there are not well-defined Cre "driver" lines that target the expression of Cre recombinase to thalamocortical (TC) neurons. Here, we characterize three Cre driver lines for the nuclei of the dorsal thalamus: Oligodendrocyte transcription factor 3 (Olig3)-Cre, histidine decarboxylase (HDC)-Cre, and corticotropin-releasing hormone (CRH)-Cre. We examined the postnatal distribution of Cre expression for each of these lines with the Cre-dependent reporter CAG-tdTomato (Ai9). Cre-dependent expression of tdTomato reveals that Olig3-Cre expresses broadly within the thalamus, including TC neurons and interneurons, while HDC-Cre and CRH-Cre each have unique patterns of expression restricted to TC neurons within and across the sensory relay nuclei of the dorsal thalamus. Cre expression is present by the time of natural birth in all three lines, underscoring their utility for developmental studies. To demonstrate the utility of these Cre drivers for studying sensory TC circuitry, we targeted the expression of channelrhodopsin-2 to thalamus from the CAG-COP4*H134R/EYFP (Ai32) allele with either HDC-Cre or CRH-Cre. Optogenetic activation of TC afferents in primary visual cortex was sufficient to measure frequency-dependent depression. Thus, these Cre drivers provide selective Cre-dependent gene expression in thalamus suitable for both anatomical and functional studies.

GABABR Modulation of Electrical Synapses and Plasticity in the Thalamic Reticular Nucleus

Two distinct types of neuronal activity result in long-term depression (LTD) of electrical synapses, with overlapping biochemical intracellular signaling pathways that link activity to synaptic strength, in electrically coupled neurons of the thalamic reticular nucleus (TRN). Because components of both signaling pathways can also be modulated by GABA_B receptor activity, here we examined the impact of GABA_B receptor activation on the two established inductors of LTD in electrical synapses. Recording from patched pairs of coupled rat neurons in vitro, we show that GABA_B receptor inactivation itself induces a modest depression of electrical synapses and occludes LTD induction by either paired bursting or metabotropic glutamate receptor (mGluR) activation. GABA_B activation also occludes LTD from either paired bursting or mGluR activation. Together, these results indicate that afferent sources of GABA, such as those from the forebrain or substantia nigra to the reticular nucleus, gate the induction of LTD from either neuronal activity or afferent glutamatergic receptor activation. These results add to a growing body of evidence that the regulation of thalamocortical transmission and sensory attention by TRN is modulated and controlled by other brain regions. Significance: We show that electrical synapse plasticity is gated by GABA_B receptors in the thalamic reticular nucleus. This effect is a novel way for afferent GABAergic input from the basal ganglia to modulate thalamocortical relay and is a possible mediator of intra-TRN inhibitory effects.

Thalamic reticular nucleus in Alligator mississippiensis: Soma and dendritic morphology

The thalamic reticular nucleus (TRN) is a critical structure influencing information transfer to the forebrain. In crocodilians, the TRN shares many features with its mammalian counterpart. One area that has not been explored is how individual neurons in the crocodilian TRN compare with those found in mammals. In mammals, TRN neurons are aligned parallel to the external border of the dorsal thalamus, have their dendrites oriented perpendicular to the fibers in the internal capsule, have fine, filamentous dendritic appendages, are either bipolar or multipolar, and are commonly considered to be a homogeneous morphological population of cells. To investigate the cellular morphology of the TRN complex, a Golgi analysis was undertaken in Alligator mississippiensis. This study examined features that have been used in mammals. In Alligator, the four TRN divisions are the dorsal peduncular nucleus, the perireticular nucleus, the interstitial nucleus, and the neurons in the medial forebrain bundle associated with the interstitial nucleus. In crocodilians, the dorsal peduncular nucleus is homologous to the TRN of mammals. From the 1787 drawn neuron profiles in the traditional three planes of section, the following were concluded. First, neurons in each part of the TRN complex in Alligator were similar in morphology. Second, each part of the TRN complex of Alligator contained a heterogenous population of cells. These variations between the cellular morphology of the dorsal peduncular nucleus of crocodilians and the TRN of mammals are speculated to partly result from differences in forebrain organization.

Adjudicating Between Local and Global Architectures of Predictive Processing in the Subcortical Auditory Pathway

Predictive processing, a leading theoretical framework for sensory processing, suggests that the brain constantly generates predictions on the sensory world and that perception emerges from the comparison between these predictions and the actual sensory input. This requires two distinct neural elements: generative units, which encode the model of the sensory world; and prediction error units, which compare these predictions against the sensory input. Although predictive processing is generally portrayed as a theory of cerebral cortex function, animal and human studies over the last decade have robustly shown the ubiquitous presence of prediction error responses in several nuclei of the auditory, somatosensory, and visual subcortical pathways. In the auditory modality, prediction error is typically elicited using so-called oddball paradigms, where sequences of repeated pure tones with the same pitch are at unpredictable intervals substituted by a tone of deviant frequency. Repeated sounds become predictable promptly and elicit decreasing prediction error; deviant tones break these predictions and elicit large prediction errors. The simplicity of the rules inducing predictability make oddball paradigms agnostic about the origin of the predictions. Here, we introduce two possible models of the organizational topology of the predictive processing auditory network: (1) the global view, that assumes that predictions on the sensory input are generated at high-order levels of the cerebral cortex and transmitted in a cascade of generative models to the subcortical sensory pathways; and (2) the local view, that assumes that independent local models, computed using local information, are used to perform predictions at each processing stage. In the global view information encoding is optimized globally but biases sensory representations along the entire brain according to the subjective views of the observer. The local view results in a diminished coding efficiency, but guarantees in return a robust encoding of the features of sensory input at each processing stage. Although most experimental results to-date are ambiguous in this respect, recent evidence favors the global model.

Advances in Studies on Stroke-Induced Secondary Neurodegeneration (SND) and Its Treatment

Background:

The occurrence of secondary neurodegeneration has exclusively been observed after the first incidence of stroke. In humans and rodents, post-stroke secondary neurodegeneration (SND) is an inevitable event that can lead to progressive neuronal loss at a region distant to initial infarct. SND can lead to cognitive and motor function impairment, finally causing dementia. The exact pathophysiology of the event is yet to be explored. It is seen that the thalami, in particular, are susceptible to cause SND. The reason behind this is because the thalamus functioning as the relay center and is positioned as an interlocked structure with direct synaptic signaling connection with the cortex. As SND proceeds, accumulation of misfolded proteins and microglial activation are seen in the thalamus. This leads to increased neuronal loss and worsening of functional and cognitive impairment.

Objective:

There is a necessity of specific interventions to prevent post-stroke SND, which are not properly investigated to date owing to sparsely reproducible pre-clinical and clinical data. The basis of this review is to investigate about post-stroke SND and its updated treatment approaches carefully.

Methods:

Our article presents a detailed survey of advances in studies on stroke-induced secondary neurodegeneration (SND) and its treatment.

Results:

This article aims to put forward the pathophysiology of SND. We have also tabulated the latest treatment approaches along with different neuroimaging systems that will be helpful for future reference to explore.

Conclusion:

In this article, we have reviewed the available reports on SND pathophysiology, detection techniques, and possible treatment modalities that have not been attempted to date.

Early lesion of the reticular thalamic nucleus disrupts the structure and function of the mediodorsal thalamus and prefrontal cortex

The thalamic reticular nucleus (TRN), part of the thalamus, is a thin GABAergic cell layer adjacent to the relay nuclei of the dorsal thalamus. It receives input from the cortex and other thalamic nuclei and provides major inhibitory input to each thalamic nucleus, particularly the mediodorsal nucleus (MD). As the MD is important for supporting optimal cortico-thalamo-cortical interactions during brain maturation, we hypothesized that that early damage to the TRN will cause major disturbances to the development and the functioning of the prefrontal cortex (PFC) and the MD. Rat pups at P4 were randomized in three groups: electrolytic lesion of TRN, TRN-sham-lesion group, and the classical control group. Seven weeks later, all rats were tested with several behavioral and cognitive paradigms, and then perfused for histological and immunohistochemical studies. Results showed that TRN lesion rats exhibited reduced spontaneous activity, high level of anxiety, learning and recognition memory impairments. Besides the behavioral effects observed after early TRN lesions, our study showed significant cytoarchitectural and functional changes in the cingulate cortex, the dorsolateral and prelimbic subdivisions of the PFC, as well as in the MD. The assessment of the basal levels of neuronal activity revealed a significant reduction of the basal expression of C-Fos levels in the PFC. These experiments, which are the first to highlight the effects of early TRN lesions, provided evidence that early damage of the anterior part of the TRN leads to alterations that may control the development of the thalamocortical-corticothalamic pathways.

Sleep Spindles: Mechanisms and Functions

Sleep spindles are burstlike signals in the electroencephalogram (EEG) of the sleeping mammalian brain and electrical surface correlates of neuronal oscillations in thalamus. As one of the most inheritable sleep EEG signatures, sleep spindles probably reflect the strength and malleability of thalamocortical circuits that underlie individual cognitive profiles. We review the characteristics, organization, regulation, and origins of sleep spindles and their implication in non-rapid-eye-movement sleep (NREMS) and its functions, focusing on human and rodent. Spatially, sleep spindle-related neuronal activity appears on scales ranging from small thalamic circuits to functional cortical areas, and generates a cortical state favoring intracortical plasticity while limiting cortical output. Temporally, sleep spindles are discrete events, part of a continuous power band, and elements grouped on an infraslow time scale over which NREMS alternates between continuity and fragility. We synthesize diverse and seemingly unlinked functions of sleep spindles for sleep architecture, sensory processing, synaptic plasticity, memory formation, and cognitive abilities into a unifying sleep spindle concept, according to which sleep spindles 1) generate neural conditions of large-scale functional connectivity and plasticity that outlast their appearance as discrete EEG events, 2) appear preferentially in thalamic circuits engaged in learning and attention-based experience during wakefulness, and 3) enable a selective reactivation and routing of wake-instated neuronal traces between brain areas such as hippocampus and cortex. Their fine spatiotemporal organization reflects NREMS as a physiological state coordinated over brain and body and may indicate, if not anticipate and ultimately differentiate, pathologies in sleep and neurodevelopmental, -degenerative, and -psychiatric conditions.

Postnatal development of cholinergic input to the thalamic reticular nucleus of the mouse

The thalamic reticular nucleus (TRN), a shell-like structure comprised of GABAergic neurons, gates signal transmission between thalamus and cortex. While TRN is innervated by axon collaterals of thalamocortical and corticothalamic neurons, other ascending projections modulate activity during different behavioral states such as attention, arousal, and sleep-wake cycles. One of the largest arise from cholinergic neurons of the basal forebrain and brainstem. Despite its integral role, little is known about how or when cholinergic innervation and synapse formation occurs. We utilized genetically modified mice, which selectively express fluorescent protein and/or channelrhodopsin-2 in cholinergic neurons, to visualize and stimulate cholinergic afferents in the developing TRN. Cholinergic innervation of TRN follows a ventral-to-dorsal progression, with nonvisual sensory sectors receiving input during week 1, and the visual sector during week 2. By week 3, the density of cholinergic fibers increases throughout TRN and forms a reticular profile. Functional patterns of connectivity between cholinergic fibers and TRN neurons progress in a similar manner, with weak excitatory nicotinic responses appearing in nonvisual sectors near the end of week 1. By week 2, excitatory responses become more prevalent and arise in the visual sector. Between weeks 3-4, inhibitory muscarinic responses emerge, and responses become biphasic, exhibiting a fast excitatory, and a long-lasting inhibitory component. Overall, the development of cholinergic projections in TRN follows a similar plan as the rest of sensory thalamus, with innervation of nonvisual structures preceding visual ones, and well after the establishment of circuits conveying sensory information from the periphery to the cortex.

Neuropeptide Y affects thalamic reticular nucleus neuronal firing and network synchronization associated with suppression of spike-wave discharges

OBJECTIVES

Neuropeptide Y (NPY) potently suppresses spike-wave discharges (SWDs) in a genetic rat model of absence epilepsy (GAERS), but the underlying neurophysiologic mechanisms are not clear. We therefore sought to determine the in vivo effects of NPY on neuronal firing in the cortico-thalamo-cortical network activity, known to play a critical role in the generation of SWDs in these rats.

METHODS

NPY was administered intracerebroventricularly (ICV) or in separate experiments locally on the neurons of caudal thalamic reticular nucleus (NRT) by use of juxtacellular iontophoresis in triple-barrel electrodes in male GAERS aged 12-15 weeks, in vivo under neuroleptic anesthesia. Drug infusions and electroencephalography (EEG) monitoring were performed simultaneously with juxtacellular single neuronal recordings. Effect of NPY on electrically induced SWD induction threshold were also measured.

RESULTS

NPY administration ICV led to a decrease in the total length of SWDs in EEG recordings. Both ICV administration and iontophoresis of NPY on NRT neurons led to an increase in interictal neuronal firing of NRT neurons. During ictal periods, ICV NPY administration reduced the number of thalamic action potentials per SWDs, as well as reduced waveform correlations between field potentials within the NRT and the cortical EEG. NPY administration ICV did not significantly alter the firing patterns of relay thalamic neurons interictally and cortical neurons during ictal and interictal periods. In addition, SWD induction threshold in the S2 region of the cortex was significantly increased after NPY administration.

SIGNIFICANCE

Our results show alterations in cortico-thalamo-cortical local and network properties following ICV administration of NPY, suggesting mechanisms of SWD suppression in GAERS. Cellular and network alteration of NRT activity, resulting from a direct action of NPY, may be a contributor to this effect.

In Vivo Visualization of Active Polysynaptic Circuits With Longitudinal Manganese-Enhanced MRI (MEMRI)

Manganese-enhanced magnetic resonance imaging (MEMRI) is a powerful tool for in vivo non-invasive whole-brain mapping of neuronal activity. Mn²⁺ enters active neurons via voltage-gated calcium channels and increases local contrast in T₁-weighted images. Given the property of Mn²⁺ of axonal transport, this technique can also be used for tract tracing after local administration of the contrast agent. However, MEMRI is still not widely employed in basic research due to the lack of a complete description of the Mn²⁺ dynamics in the brain. Here, we sought to investigate how the activity state of neurons modulates interneuronal Mn²⁺ transport. To this end, we injected mice with low dose MnCl₂ 2. (i.p., 20 mg/kg; repeatedly for 8 days) followed by two MEMRI scans at an interval of 1 week without further MnCl₂ injections. We assessed changes in T₁ contrast intensity before (scan 1) and after (scan 2) partial sensory deprivation (unilateral whisker trimming), while keeping the animals in a sensory enriched environment. After correcting for the general decay in Mn²⁺ content, whole brain analysis revealed a single cluster with higher signal in scan 1 compared to scan 2: the left barrel cortex corresponding to the right untrimmed whiskers. In the inverse contrast (scan 2 > scan 1), a number of brain structures, including many efferents of the left barrel cortex were observed. These results suggest that continuous neuronal activity elicited by ongoing sensory stimulation accelerates Mn²⁺ transport from the uptake site to its projection terminals, while the blockage of sensory-input and the resulting decrease in neuronal activity attenuates Mn²⁺ transport. The description of this critical property of Mn²⁺ dynamics in the brain allows a better understanding of MEMRI functional mechanisms, which will lead to more carefully designed experiments and clearer interpretation of the results.

Electrical synapses between inhibitory neurons shape the responses of principal neurons to transient inputs in the thalamus: a modeling study

As multimodal sensory information proceeds to the cortex, it is intercepted and processed by the nuclei of the thalamus. The main source of inhibition within thalamus is the reticular nucleus (TRN), which collects signals both from thalamocortical relay neurons and from thalamocortical feedback. Within the reticular nucleus, neurons are densely interconnected by connexin36-based gap junctions, known as electrical synapses. Electrical synapses have been shown to coordinate neuronal rhythms, including thalamocortical spindle rhythms, but their role in shaping or modulating transient activity is less understood. We constructed a four-cell model of thalamic relay and TRN neurons, and used it to investigate the impact of electrical synapses on closely timed inputs delivered to thalamic relay cells. We show that the electrical synapses of the TRN assist cortical discrimination of these inputs through effects of truncation, delay or inhibition of thalamic spike trains. We expect that these are principles whereby electrical synapses play similar roles in regulating the processing of transient activity in excitatory neurons across the brain.

The Potential Role of Gap Junctional Plasticity in the Regulation of State

Electrical synapses are the functional correlate of gap junctions and allow transmission of small molecules and electrical current between coupled neurons. Instead of static pores, electrical synapses are actually plastic, similar to chemical synapses. In the thalamocortical system, gap junctions couple inhibitory neurons that are similar in their biochemical profile, morphology, and electrophysiological properties. We postulate that electrical synaptic plasticity among inhibitory neurons directly interacts with the switching between different firing patterns in a state-dependent and type-dependent manner. In neuronal networks, electrical synapses may function as a modifiable resonance feedback system that enables stable oscillations. Furthermore, the plasticity of electrical synapses may play an important role in regulation of state, synchrony, and rhythmogenesis in the mammalian thalamocortical system, similar to chemical synaptic plasticity. Based on their plasticity, rich diversity, and specificity, electrical synapses are thus likely to participate in the control of consciousness and attention.

A calcium-dependent pathway underlies activity-dependent plasticity of electrical synapses in the thalamic reticular nucleus

KEY POINTS

Electrical synapses are modified by various forms of activity, including paired activity in coupled neurons and tetanization of the input to coupled neurons. We show that plasticity of electrical synapses that results from paired spiking activity in coupled neurons depends on calcium influx and calcium-initiated signalling pathways. Plasticity that results from tetanization of input fibres does not depend on calcium influx or dynamics. These results imply that electrically coupled neurons have distinct sets of mechanisms for adjusting coupling according to the specific type of activity they experience.

ABSTRACT

Recent results have demonstrated modification of electrical synapse strength by varied forms of neuronal activity. However, the mechanisms underlying plasticity induction in central mammalian neurons are unclear. Here we show that the two established inductors of plasticity at electrical synapses in the thalamic reticular nucleus - paired burst spiking in coupled neurons, and mGluR-dependent tetanization of synaptic input - are separate pathways that converge at a common downstream endpoint. Using occlusion experiments and pharmacology in patched pairs of coupled neurons in vitro, we show that burst-induced depression depends on calcium entry via voltage-gated channels, is blocked by BAPTA chelation, and recruits intracellular calcium release on its way to activation of phosphatase activity. In contrast, mGluR-dependent plasticity is independent of calcium entry or calcium dynamics. Together, these results show that the spiking-initiated mechanisms underlying electrical synapse plasticity are similar to those that induce plasticity at chemical synapses, and offer the possibility that calcium-regulated mechanisms may also lead to alternate outcomes, such as potentiation. Because these mechanistic elements are widely found in mature neurons, we expect them to apply broadly to electrical synapses across the brain, acting as the crucial link between neuronal activity and electrical synapse strength.

Astrocytes modulate thalamic sensory processing via mGlu2 receptor activation

Astrocytes possess many of the same signalling molecules as neurons. However, the role of astrocytes in information processing, if any, is unknown. Using electrophysiological and imaging methods, we report the first evidence that astrocytes modulate neuronal sensory inhibition in the rodent thalamus.

We found that mGlu2 receptor activity reduces inhibitory transmission from the thalamic reticular nucleus to the somatosensory ventrobasal thalamus (VB): mIPSC frequencies in VB slices were reduced by the Group II mGlu receptor agonist LY354740, an effect potentiated by mGlu2 positive allosteric modulator (PAM) LY487379 co-application (30 nM LY354740: 10.0 ± 1.6% reduction; 30 nM LY354740 & 30 μM LY487379: 34.6 ± 5.2% reduction).

We then showed activation of mGlu2 receptors on astrocytes: astrocytic intracellular calcium levels were elevated by the Group II agonist, which were further potentiated upon mGlu2 PAM co-application (300 nM LY354740: ratio amplitude 0.016 ± 0.002; 300 nM LY354740 & 30 μM LY487379: ratio amplitude 0.035 ± 0.003).

We then demonstrated mGlu2-dependent astrocytic disinhibition of VB neurons in vivo: VB neuronal responses to vibrissae stimulation trains were disinhibited by the Group II agonist and the mGlu2 PAM (LY354740: 156 ± 12% of control; LY487379: 144 ± 10% of control). Presence of the glial inhibitor fluorocitrate abolished the mGlu2 PAM effect (91 ± 5% of control), suggesting the mGlu2 component to the Group II effect can be attributed to activation of mGlu2 receptors localised on astrocytic processes within the VB.

Gating of thalamocortical function via astrocyte activation represents a novel sensory processing mechanism. As this thalamocortical circuitry is important in discriminative processes, this demonstrates the importance of astrocytes in synaptic processes underlying attention and cognition.

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Thalamic inhibition is mediated by both neuronal and astrocytic mechanisms.

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Group II mGlu receptor (mGlu2/3) activation can modulate this thalamic inhibition.

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Thalamic astrocytes can be activated upon mGlu2 receptor stimulation.

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This process may enable relevant activity to be discerned from background noise.

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Targeting astrocytic mGlu2 receptors may therefore affect attention and cognition.

Morphology of visual sector thalamic reticular neurons in the macaque monkey suggests retinotopically specialized, parallel stream-mixed input to the lateral geniculate nucleus

The thalamic reticular nucleus (TRN) is a unique brain structure at the interface between the thalamus and the cortex. Because the TRN receives bottom-up sensory input and top-down cortical input, it could serve as an integration hub for sensory and cognitive signals. Functional evidence supports broad roles for the TRN in arousal, attention, and sensory selection. How specific circuits connecting the TRN with sensory thalamic structures implement these functions is not known. The structural organization and function of the TRN is particularly interesting in the context of highly organized sensory systems, such as the primate visual system, where neurons in the retina and dorsal lateral geniculate nucleus of the thalamus (dLGN) are morphologically and physiologically distinct and also specialized for processing particular features of the visual environment. To gain insight into the functional relationship between the visual sector of the TRN and the dLGN, we reconstructed a large number of TRN neurons that were retrogradely labeled following injections of rabies virus expressing enhanced green fluorescent protein (EGFP) into the dLGN. An independent cluster analysis, based on 10 morphological metrics measured for each reconstructed neuron, revealed three clusters of TRN neurons that differed in cell body shape and size, dendritic arborization patterns, and medial-lateral position within the TRN. TRN dendritic and axonal morphologies are inconsistent with visual stream-specific projections to the dLGN. Instead, TRN neuronal organization could facilitate transmission of global arousal and/or cognitive signals to the dLGN with retinotopic precision that preserves specialized processing of foveal versus peripheral visual information. J. Comp. Neurol. 525:1273-1290, 2017. © 2016 Wiley Periodicals, Inc.

Abolished perineuronal nets and altered parvalbumin-immunoreactivity in the nucleus reticularis thalami of wildtype and 3xTg mice after experimental stroke

Treatment strategies for ischemic stroke are still limited, since numerous attempts were successful only in preclinical research but failed under clinical condition. To overcome this translational roadblock, clinical relevant stroke models should consider co-morbidities, age-related effects and the complex neurovascular unit (NVU) concept. The NVU includes neurons, vessels and glial cells with astrocytic endfeet in close relation to the extracellular matrix (ECM). However, the role of the ECM after stroke-related tissue damage is poorly understood and mostly neglected for treatment strategies. This study is focused on alterations of perineuronal nets (PNs) as ECM constituents and parvalbumin-containing GABAergic neurons in mice with emphasis on the nucleus reticularis thalami (NRT) in close proximity to the ischemic lesion as induced by a filament-based stroke model. One day after ischemia onset, immunofluorescence-based quantitative analyses revealed drastically declined PNs in the ischemia-affected NRT from 3- and 12-month-old wildtype and co-morbid triple-transgenic (3xTg) mice with Alzheimer-like alterations. Parvalbumin-positive cells decreased numerically in the ischemia-affected NRT, while staining intensity did not differ between the affected and non-affected hemisphere. Additional qualitative analyses demonstrated ischemia-induced loss of PNs and allocated neuropil ECM immunoreactive for aggrecan and neurocan, and impaired immunoreactivity for calbindin, the potassium channel subunit Kv3.1b and the glutamate decarboxylase isoforms GAD65 and GAD67 in the NRT. In conclusion, these data confirm PNs as highly sensitive constituents of the ECM along with impaired neuronal integrity of GABAergic neurons. Therefore, specific targeting of ECM components might appear as a promising strategy for future treatment strategies in stroke.

Two classes of excitatory synaptic responses in rat thalamic reticular neurons

The thalamic reticular nucleus (nRt), composed of GABAergic cells providing inhibition of relay neurons in the dorsal thalamus, receives excitation from the neocortex and thalamus. The two excitatory pathways promoting feedback or feedforward inhibition of thalamocortical neurons contribute to sensory processing and rhythm generation. While synaptic inhibition within the nRt has been carefully characterized, little is known regarding the biophysics of synaptic excitation. To characterize the functional properties of thalamocortical and corticothalamic connections to the nRt, we recorded minimal electrically evoked excitatory postsynaptic currents from nRt cells in vitro. A hierarchical clustering algorithm distinguished two types of events. Type 1 events had larger amplitudes and faster kinetics, largely mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, whereas type 2 responses had more prominent N-methyl-d-aspartate (NMDA) receptor contribution. Type 1 responses showed subnormal axonal propagation and paired pulse depression, consistent with thalamocortical inputs. Furthermore, responses kinetically similar to type 1 events were evoked by glutamate-mediated activation of thalamic neurons. Type 2 responses, in contrast, likely arise from corticothalamic inputs, with larger NMDA conductance and weak Mg(2+)-dependent block, suggesting that NMDA receptors are critical for the cortical excitation of reticular neurons. The long-lasting action of NMDA receptors would promote reticular cell burst firing and produce powerful inhibitory output to relay neurons proposed to be important in triggering epilepsy. This work provides the first complete voltage-clamp analysis of the kinetics and voltage dependence of AMPA and NMDA responses of thalamocortical and corticothalamic synapses in the nRt and will be critical in optimizing biologically realistic neural network models of thalamocortical circuits relevant to sensory processing and thalamocortical oscillations.

Activity-dependent plasticity of electrical synapses: increasing evidence for its presence and functional roles in the mammalian brain

Gap junctions mediate electrical synaptic transmission between neurons. While the actions of neurotransmitter modulators on the conductance of gap junctions have been extensively documented, increasing evidence indicates they can also be influenced by the ongoing activity of neural networks, in most cases via local interactions with nearby glutamatergic synapses. We review here early evidence for the existence of activity-dependent regulatory mechanisms as well recent examples reported in mammalian brain. The ubiquitous distribution of both neuronal connexins and the molecules involved suggest this phenomenon is widespread and represents a property of electrical transmission in general.

mGluR-mediated calcium signalling in the thalamic reticular nucleus

The thalamic reticular nucleus (TRN) plays a major role in modulating the transfer of information from the thalamus to the cortex. GABAergic inhibition via the TRN is differentially regulated by metabotropic glutamate receptors (mGluRs) and the effect of mGluRs on the membrane potential, on ion channels, and on the plasticity of electrical coupling of TRN neurons has been studied previously. Although mGluRs are generally known to trigger Ca(2+) transients, mGluR-mediated Ca(2+)-transients in TRN neurons have not yet been investigated. In this study, we show that mGluRs can trigger Ca(2+)-transients in TRN neurons, that these transients depend on intracellular Ca(2+)-stores, and are mediated by IP3 receptors. Ca(2+) transients caused by the group I mGluR agonist DHPG elicit a current that is sensitive to flufenamic acid and has a reversal potential around -40mV. Our results add mGluR-mediated Ca(2+)-signalling in the TRN to the state-dependent modulators of the thalamocortical system.

Selectively driving cholinergic fibers optically in the thalamic reticular nucleus promotes sleep

Cholinergic projections from the basal forebrain and brainstem are thought to play important roles in rapid eye movement (REM) sleep and arousal. Using transgenic mice in which channelrhdopsin-2 is selectively expressed in cholinergic neurons, we show that optical stimulation of cholinergic inputs to the thalamic reticular nucleus (TRN) activates local GABAergic neurons to promote sleep and protect non-rapid eye movement (NREM) sleep. It does not affect REM sleep. Instead, direct activation of cholinergic input to the TRN shortens the time to sleep onset and generates spindle oscillations that correlate with NREM sleep. It does so by evoking excitatory postsynaptic currents via α7-containing nicotinic acetylcholine receptors and inducing bursts of action potentials in local GABAergic neurons. These findings stand in sharp contrast to previous reports of cholinergic activity driving arousal. Our results provide new insight into the mechanisms controlling sleep.

DOI:http://dx.doi.org/10.7554/eLife.10382.001

Sleep is one of the most familiar activities in our lives and yet there are still many unanswered questions related to how it is regulated. The cholinergic system (or the part of the nervous system that sends signals using a chemical called acetylcholine) is thought to be important for the phase of sleep that is most similar to being awake, so-called REM sleep. This collection of nerve cells has also been implicated in the process of waking up from sleep. However, it remains unclear how the cholinergic system acts on sleep.

Ni, Hou et al. have now used a technique called optogenetics to use light to stimulate the cholinergic system in specific areas in the brains of mice. These experiments found that the activation of the cholinergic system caused awake mice to fall asleep, and promoted more non-REM sleep in sleeping mice. As such, this discovery challenges the previously held view that cholinergic activity was linked to waking up.

Acetylcholine affects cells in a similar way to nicotine from cigarettes. In the future, Ni, Hou et al. would like to explore how many nicotine-like substances are released by the cholinergic system in specific brain areas, and to further investigate when and how sleep is promoted.

DOI:http://dx.doi.org/10.7554/eLife.10382.002

A subset of thalamocortical projections to the retrosplenial cortex possesses two vesicular glutamate transporter isoforms, VGluT1 and VGluT2, in axon terminals and somata

Vesicular glutamate transporter isoforms, VGluT1-VGluT3, accumulate glutamate into synaptic vesicles and are considered to be important molecules in glutamatergic transmission. Among them, VGluT2 mRNA is expressed predominantly throughout the dorsal thalamus, whereas VGluT1 mRNA is expressed in a few thalamic nuclei. In the thalamic nuclei that project to the retrosplenial cortex (RSC), VGluT1 mRNA is expressed strongly in the anterodorsal thalamic nucleus (AD), is expressed moderately in the anteroventral and laterodorsal thalamic nuclei, and is not expressed in the anteromedial thalamic nucleus. Thus, it has been strongly suggested that a subset of thalamocortical projections to RSC possesses both VGluT1 and VGluT2. In this study, double-labeled neuronal somata showing both VGluT1 and VGluT2 immunolabelings were found exclusively in the ventral region of AD (vAD). Many double-labeled axon terminals were also found in two major targets of vAD, the rostral part of the reticular thalamic nucleus and layers Ia and III-IV of the retrosplenial granular b cortex (RSGb). Some were also found in layer Ia of the retrosplenial granular a cortex (RSGa). These axon terminals contain significant amounts of both VGluTs. Because the subset of thalamocortical projections to RSC has a unique molecular basis in the glutamatergic transmission system, it might play an important role in the higher cognitive functions processed in the RSC. Furthermore, double-labeled axon terminals of a different type were distributed in RSGb and RSGa. Because they are small and the immunoreactivity of VGluT2 is significantly weaker than that of VGluT1, they seemed to be a subset of corticocortical terminals.

Modulation of short-term plasticity in the corticothalamic circuit by group III metabotropic glutamate receptors

Recurrent connections in the corticothalamic circuit underlie oscillatory behavior in this network and range from normal sleep rhythms to the abnormal spike-wave discharges seen in absence epilepsy. The propensity of thalamic neurons to fire postinhibitory rebound bursts mediated by low-threshold calcium spikes renders the circuit vulnerable to both increased excitation and increased inhibition, such as excessive excitatory cortical drive to thalamic reticular (RT) neurons or heightened inhibition of thalamocortical relay (TC) neurons by RT. In this context, a protective role may be played by group III metabotropic receptors (mGluRs), which are uniquely located in the presynaptic active zone and typically act as autoreceptors or heteroceptors to depress synaptic release. Here, we report that these receptors regulate short-term plasticity at two loci in the corticothalamic circuit in rats: glutamatergic cortical synapses onto RT neurons and GABAergic synapses onto TC neurons in somatosensory ventrobasal thalamus. The net effect of group III mGluR activation at these synapses is to suppress thalamic oscillations as assayed in vitro. These findings suggest a functional role of these receptors to modulate corticothalamic transmission and protect against prolonged activity in the network.

Dynamics of intrinsic dendritic calcium signaling during tonic firing of thalamic reticular neurons

The GABAergic neurons of the nucleus reticularis thalami that control the communication between thalamus and cortex are interconnected not only through axo-dendritic synapses but also through gap junctions and dendro-dendritic synapses. It is still unknown whether these dendritic communication processes may be triggered both by the tonic and the T-type Ca²⁺ channel-dependent high frequency burst firing of action potentials displayed by nucleus reticularis neurons during wakefulness and sleep, respectively. Indeed, while it is known that activation of T-type Ca²⁺ channels actively propagates throughout the dendritic tree, it is still unclear whether tonic action potential firing can also invade the dendritic arborization. Here, using two-photon microscopy, we demonstrated that dendritic Ca²⁺ responses following somatically evoked action potentials that mimic wake-related tonic firing are detected throughout the dendritic arborization. Calcium influx temporally summates to produce dendritic Ca²⁺ accumulations that are linearly related to the duration of the action potential trains. Increasing the firing frequency facilitates Ca²⁺ influx in the proximal but not in the distal dendritic compartments suggesting that the dendritic arborization acts as a low-pass filter in respect to the back-propagating action potentials. In the more distal compartment of the dendritic tree, T-type Ca²⁺ channels play a crucial role in the action potential triggered Ca²⁺ influx suggesting that this Ca²⁺ influx may be controlled by slight changes in the local dendritic membrane potential that determine the T-type channels’ availability. We conclude that by mediating Ca²⁺ dynamic in the whole dendritic arborization, both tonic and burst firing of the nucleus reticularis thalami neurons might control their dendro-dendritic and electrical communications.

Comparing GABAergic cell populations in the thalamic reticular nucleus of normal and genetic absence epilepsy rats from Strasbourg (GAERS)

The GABAergic neurons of the thalamic reticular nucleus (TRN) play a critical role in the generation and control of spike-and-wave discharges (SWDs) in absence epilepsy. We have used the disector method to count the GABA+ve and GABA-ve neurons in the intermediate TRN sector of genetic absence epilepsy rats from Strasbourg (GAERS) and of Wistar rats during postnatal (P) development at P10, P20, P30, and P60 days. The same part of TRN was removed from each animal, the GABAergic neurons were labelled using light-microscopical GABA immunohistochemistry and the data were statistically analysed. Both the GAERS and Wistar animals showed an increase in the density of GABA+ve and GABA-ve cells from P10 to P20. From P20 to P60, Wistar animals showed no significant differences for either cell type, but in the GAERS a progressive decrease from P20 to P60 was observed in both GABA+ve and GABA-ve cells. The decrease of the GABA-ve cells was more pronounced than that of the GABA+ve cells. There were no significant differences between cell sizes for GAERS and Wistar rats at any developmental age. The lower density GABA+ve and GABA-ve neurons at P30 and P60 of GAERS compared to Wistar animals may contribute to the generation of SWDs in absence epilepsy.

Bursts modify electrical synaptic strength

Changes in synaptic strength resulting from neuronal activity have been described in great detail for chemical synapses, but the relationship between natural forms of activity and the strength of electrical synapses had previously not been investigated. The thalamic reticular nucleus (TRN), a brain area rich in gap junctional (electrical) synapses, regulates cortical attention, initiates sleep spindles, and participates in shifts between states of arousal. Plasticity of electrical synapses in the TRN may be a key mechanism underlying these processes. Recently, we demonstrated a novel activity-dependent form of long-term depression of electrical synapses in the TRN (Haas et al., 2011). Here we provide an overview of those findings and discuss them in broader context. Because gap junctional proteins are widely expressed in the mammalian brain, modification of synaptic strength is likely to be a widespread and powerful mechanism at electrical synapses throughout the brain.

Peptidergic Edinger-Westphal neurons and the energy-dependent stress response

The continuously changing environment demands for adequate stress responses to maintain the internal dynamic equilibrium of body and mind. A successful stress response requires energy, in an amount matching the severity of the stressor and the type of response ('fight, flight or freeze'). The stress response is generated by the central nervous system, which needs to be informed about both the threatening stressor and the availability of energy. In this review, evidence is considered for a role of the midbrain Edinger-Westphal centrally projecting neuron population (EWcp; synonym: non-preganglionic Edinger-Westphal nucleus) in the energy-dependent stress adaptation response. It deals with studies on the neurochemical organization of the EWcp with particular reference to the neuropeptides urocortin-1 and cocaine- and amphetamine-regulated transcript peptide, on the EWcp responses to different types of stressor (e.g., acute and chronic) and a changed energy state (e.g., fasting and leptin change), and on the sex-specificity of these responses. Finally, a model is presented for the way the EWcp might contribute to the coordination of the energy-dependent stress adaptation response.

The importance of combinatorial gene expression in early Mammalian thalamic patterning and thalamocortical axonal guidance

The thalamus is essential for sensory perception. In mammals, work on the mouse has taught us most of what we know about how it develops and connects to the cortex. The mature thalamus of all mammalian species comprises numerous anatomically distinct collections of neurons called nuclei that differ in function, connectivity, and molecular constitution. At the time of its initial appearance as a distinct structure following neural tube closure, the thalamus is already patterned by the regional expression of numerous regulatory genes. This patterning, which lays down the blueprint for later development of thalamic nuclei, predates the development of thalamocortical projections. In this review we apply novel analytical methods to gene expression data available in the Allen Developing Mouse Brain Atlas to highlight the complex organized molecular heterogeneity already present among cells in the thalamus from the earliest stages at which it contains differentiating neurons. This early patterning is likely to invest in axons growing from different parts of the thalamus the ability to navigate in an ordered way to their appropriate area in the cerebral cortex. We review the mechanisms and cues that thalamic axons use, encounter, and interpret to attain the cortex. Mechanisms include guidance by previously generated guidepost cells, such as those in the subpallium that maintain thalamic axonal order and direction, and axons such as those of reciprocal projections from intermediate structures or from the cortex itself back toward the thalamus. We show how thalamocortical pathfinding involves numerous guidance cues operating at a series of steps along their route. We stress the importance of the combinatorial actions of multiple genes for the development of the numerous specific identities and functions of cells in this exquisitely complex system and their orderly innervation of the cortex.

Activity-dependent long-term depression of electrical synapses

Use-dependent forms of synaptic plasticity have been extensively characterized at chemical synapses, but a relationship between natural activity and strength at electrical synapses remains elusive. The thalamic reticular nucleus (TRN), a brain area rich in gap-junctional (electrical) synapses, regulates cortical attention to the sensory surround and participates in shifts between arousal states; plasticity of electrical synapses may be a key mechanism underlying these processes. We observed long-term depression resulting from coordinated burst firing in pairs of coupled TRN neurons. Changes in gap-junctional communication were asymmetrical, indicating that regulation of connectivity depends on the direction of use. Modification of electrical synapses resulting from activity in coupled neurons is likely to be a widespread and powerful mechanism for dynamic reorganization of electrically coupled neuronal networks.

The sleep relay--the role of the thalamus in central and decentral sleep regulation

Surprisingly, the concept of sleep, its necessity and function, the mechanisms of action, and its elicitors are far from being completely understood. A key to sleep function is to determine how and when sleep is induced. The aim of this review is to merge the classical concepts of central sleep regulation by the brainstem and hypothalamus with the recent findings on decentral sleep regulation in local neuronal assemblies and sleep regulatory substances that create a scenario in which sleep is both local and use dependent. The interface between these concepts is provided by thalamic cellular and network mechanisms that support rhythmogenesis of sleep-related activity. The brainstem and the hypothalamus centrally set the pace for sleep-related activity throughout the brain. Decentral regulation of the sleep-wake cycle was shown in the cortex, and the homeostat of non-rapid-eye-movement sleep is made up by molecular networks of sleep regulatory substances, allowing individual neurons or small neuronal assemblies to enter sleep-like states. Thalamic neurons provide state-dependent gating of sensory information via their ability to produce different patterns of electrogenic activity during wakefulness and sleep. Many mechanisms of sleep homeostasis or sleep-like states of neuronal assemblies, e.g. by the action of adenosine, can also be found in thalamic neurons, and we summarize cellular and network mechanisms of the thalamus that may elicit non-REM sleep. It is argued that both central and decentral regulators ultimately target the thalamus to induce global sleep-related oscillatory activity. We propose that future studies should integrate ideas of central, decentral, and thalamic sleep generation.

A new mode of corticothalamic transmission revealed in the Gria4(-/-) model of absence epilepsy

Cortico-thalamo-cortical circuits mediate sensation and generate neural network oscillations associated with slow-wave sleep and various epilepsies. Cortical input to sensory thalamus is thought to mainly evoke feed-forward synaptic inhibition of thalamocortical (TC) cells via reticular thalamic nucleus (nRT) neurons, especially during oscillations. This relies on a stronger synaptic strength in the cortico-nRT pathway than in the cortico-TC pathway, allowing the feed-forward inhibition of TC cells to overcome direct cortico-TC excitation. We found a systemic and specific reduction in strength in GluA4-deficient (Gria4(-/-)) mice of one excitatory synapse of the rhythmogenic cortico-thalamo-cortical system, the cortico-nRT projection, and observed that the oscillations could still be initiated by cortical inputs via the cortico-TC-nRT-TC pathway. These results reveal a previously unknown mode of cortico-thalamo-cortical transmission, bypassing direct cortico-nRT excitation, and describe a mechanism for pathological oscillation generation. This mode could be active under other circumstances, representing a previously unknown channel of cortico-thalamo-cortical information processing.

The gray area between synapse structure and function-Gray's synapse types I and II revisited

On the basis of ultrastructural parameters, the concept was formulated that asymmetric Type I and symmetric Type II synapses are excitatory and inhibitory, respectively. This "functional Gray synapses concept" received strong support from the demonstration of the excitatory neurotransmitter glutamate in Type I synapses and of the inhibitory neurotransmitter γ-aminobutyric acid in Type II synapses, and is still frequently used in modern literature. However, morphological and functional evidence has accumulated that the concept is less tenable. Typical features of synapses like shape and size of presynaptic vesicles and synaptic cleft and presence of a postsynaptic density (PsD) do not always fit the postulated (excitatory/inhibitory) function of Gray's synapses. Furthermore, synapse function depends on postsynaptic receptors and associated signal transduction mechanisms rather than on presynaptic morphology and neurotransmitter type. Moreover, the notion that many synapses are difficult to classify as either asymmetric or symmetric has cast doubt on the assumption that the presence of a PsD is a sign of excitatory synaptic transmission. In view of the morphological similarities of the PsD in asymmetric synapses with membrane junctional structures such as the zonula adherens and the desmosome, asymmetric synapses may play a role as links between the postsynaptic and presynaptic membrane, thus ensuring long-term maintenance of interneuronal communication. Symmetric synapses, on the other hand, might be sites of transient communication as takes place during development, learning, memory formation, and pathogenesis of brain disorders. Confirmation of this idea might help to return the functional Gray synapse concept its central place in neuroscience.

A computational model of thalamocortical dysrhythmia

Functional stereotactic lesions in the central lateral nucleus of the medial thalamus have proved to be an effective treatment of neurogenic pain and other neurological disorders associated with thalamocortical dysrhythmia. The mechanisms underlying patient recovery after surgery are currently being explored using quantitative electroencephalography. Here we test the hypothesis that the particular role played by the non-specific medial thalamic nuclei in thalamocortical dysrhythmia is based on the divergent connectivity between these non-specific and reticular nuclei. We built a spiking computer model of the human thalamocortical system consisting of specific, non-specific and reticular thalamic nuclei. In our simulations of the thalamocortical system, deafferentation of peripheral thalamic afferents leads to hyperpolarization and subsequent bursting in the reticular nucleus. This provides strong inhibitory feedback to both the specific and the non-specific thalamic nuclei and initiates a feedback cycle of thalamic bursts in the theta frequency range. The divergent connections between the reticular and non-specific thalamic nuclei provide synchronization of the oscillating circuits. Functional silencing of the non-specific model nucleus limits reverberation and rescues the system from these oscillations. The same effect could be achieved by increasing the input to the non-specific nucleus from cortical areas. The model predicts that the invasiveness of functional neurosurgery can be reduced by targeting only deafferented areas in the medial nuclei as these are the key areas for generation and maintenance of pathological rhythms.

MeCP2 is required for normal development of GABAergic circuits in the thalamus

Methyl-CpG binding protein 2 (MeCP2) is highly expressed in neurons in the vertebrate brain, and mutations of the gene encoding MeCP2 cause the neurodevelopmental disorder Rett syndrome. This study examines the role of MeCP2 in the development and function of thalamic GABAergic circuits. Whole cell recordings were carried out in excitatory neurons of the ventrobasal complex (VB) of the thalamus and in inhibitory neurons of the reticular thalamic nucleus (RTN) in acute brain slices from mice aged P6 through P23. At P14-P16, the number of quantal GABAergic events was decreased in VB neurons but increased in RTN neurons of Mecp2-null mice, without any change in the amplitude or kinetics of quantal events. There was no difference between mutant and wild-type mice in paired-pulse ratios of evoked GABAergic responses in the VB or the RTN. On the other hand, unitary responses evoked by minimal stimulation were decreased in the VB but increased in the RTN of mutants. Similar changes in the frequency of quantal events were observed at P21-P23 in both the VB and RTN. At P6, however, quantal GABAergic transmission was altered only in the VB not the RTN. Immunostaining of vesicular GABA transporter showed opposite changes in the number of GABAergic synaptic terminals in the VB and RTN of Mecp2-null mice at P18-P20. The loss of MeCP2 had no significant effect on intrinsic properties of RTN neurons recorded at P15-P17. Our findings suggest that MeCP2 differentially regulates the development of GABAergic synapses in excitatory and inhibitory neurons in the thalamus.

Sustained sensorimotor impairments after endothelin-1 induced focal cerebral ischemia (stroke) in aged rats

Despite recent advances, stroke remains a leading cause of neurological disability with the vast majority of victims being the elderly, who exhibit more severe neurological deficits and a reduced capacity to recover from these disabilities in comparison to young stroke survivors. The objective of the present study was to develop a model of focal ischemic stroke in aged rats using endothelin-1 (ET-1) to produce low mortality rates as well as reliable, robust sensorimotor deficits that resemble functional impairments associated with stroke in humans. Here, we studied the functional and histological outcome following unilateral ET-1 infusions into the sensorimotor cortex of aged rats (20-23 months old). This procedure resulted in low mortality rates (13.3%) and no loss in body weight one week following surgery. Functional assessment was performed using a number of reliable behavioural tests: staircase test (fine motor function), horizontal ladder (skilled locomotion), bilateral tactile stimulation test (somatosensory function) and cylinder test (postural weight support). Following ET-1 induced stroke, all tests demonstrated large and sustained sensorimotor deficits in both forelimb and hindlimb function that failed to improve over the 28-day testing period. In addition, histological assessment revealed a substantial loss of retrogradely labelled corticospinal neurons in the ipsilesional hemisphere following stroke. Our results establish a model for the use of aged rats in future preclinical studies, which will enhance assessment of the long-term benefit of potential neural repair and regenerative strategies.

Ultrastructural and immunocytochemical characterization of the rat non-preganglionic Edinger-Westphal nucleus

The rodent non-preganglionic Edinger-Westphal nucleus (npEW) is involved in the stress adaptation response. Here we describe the ultrastructural organization of this nucleus in the unchallenged rat, using different tissue fixation and embedding methods, and postembedding immunogold labeling. In this way we have (1) identified Ucn1-immunopositive neurons, (2) described the ultrastructure of these neurons with focus on cell organelles involved in secretion (rough endoplasmic reticulum, Golgi apparatus, secretory granules), (3) demonstrated the subcellular coexistence of Ucn1 with cocaine- and amphetamine-related transcript peptide, and (4) classified various morphological types and configurations of synaptic contact present in the npEW and, specifically, on the npEW-Ucn1 neurons. The data obtained provide the morphological basis for future studies on the plastic effects of acute and chronic stressors as well as feeding conditions specifically affecting the secretory activity of npEW-Ucn1 neurons.

Contrary roles of kainate receptors in transmitter release at corticothalamic synapses onto thalamic relay and reticular neurons

Corticothalamic fibres, which originate from layer VI pyramidal neurons in the cerebral cortex, provide excitatory synaptic inputs to both thalamic relay neurons and reticular neurons; reticular neurons in turn supply inhibitory inputs to thalamic relay neurons. Pyramidal cells in layer VI in the mouse somatosensory cortex highly express mRNA encoding kainate receptors, which facilitate or depress transmitter release at several synapses in the central nervous system. We report here that contrary modulation of transmitter release from corticothalamic fibres onto thalamic relay and reticular neurons is mediated by activation of kainate receptors in mouse thalamic ventrobasal complex and thalamic reticular nucleus. Exogenous kainate presynaptically depresses the synaptic transmission at corticothalamic synapses onto thalamic relay neurons, but facilitates it at corticothalamic synapses onto reticular neurons. Meanwhile, the lemniscal synaptic transmission, which sends primary somatosensory inputs to relay neurons, is not affected by kainate. In addition, GluR5-containing kainate receptors are involved in the depression of corticothalamic synaptic transmission onto relay neurons, but not onto reticular neurons. Furthermore, synaptically activated kainate receptors mimic these effects; high-frequency stimulation of corticothalamic fibres depresses synaptic transmission onto relay neurons, but facilitates it onto reticular neurons. Our results suggest that the opposite sensitivity of kainate receptors at the two corticothalamic synapses is governed by cortical activity and regulates the balance of excitatory and inhibitory inputs to thalamic relay neurons and therefore their excitability.

Two differential frequency-dependent mechanisms regulating tonic firing of thalamic reticular neurons

Transmission through the thalamus activates circuits involving the GABAergic neurons of the thalamic reticular nucleus (TRN). TRN cells receive excitatory inputs from thalamocortical and corticothalamic cells and send inhibitory projections to thalamocortical cells. The inhibitory output of TRN neurons largely depends on the level of excitatory drive to these cells but may also be partly under the control of mechanisms intrinsic to the TRN. We examined two such possible mechanisms, short-term plasticity at glutamatergic synapses in the TRN and intra-TRN inhibition. In rat brain slices, responses of TRN neurons to brief trains of stimuli applied to glutamatergic inputs were recorded in voltage- or current-clamp mode. In voltage clamp, TRN cells showed no change in alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor-mediated excitatory postsynaptic current amplitudes to stimulation at non-gamma frequencies (< 30 Hz), simulating background activity, but exhibited short-term depression in these amplitudes to stimulation at gamma frequencies (> 30 Hz), simulating sensory transmission. In current clamp, TRN cells increased their spike outputs in burst and tonic firing modes to increasing stimulus-train frequencies. These increases in spike output were most likely due to temporal summation of excitatory postsynaptic potentials. However, the frequency-dependent increase in tonic firing was attenuated at gamma stimulus frequencies, indicating that the synaptic depression selectively observed in this frequency range acts to suppress TRN cell output. In contrast, intra-TRN inhibition reduced spike output selectively at non-gamma stimulus frequencies. Thus, our data indicate that two intrinsic mechanisms play a role in controlling the tonic spike output of TRN neurons and these mechanisms are differentially related to two physiologically meaningful stimulus frequency ranges.

Heterogeneous expression of SNAP-25 in rat and human brain

Synaptosomal associated protein of 25 kDa (SNAP-25) is a SNARE component of the exocytotic apparatus involved in the release of neurotransmitter. We used multiple-labeling immunofluorescence, confocal microscopy, and ultrastructural immunocytochemistry to examine the expression of SNAP-25 in excitatory and inhibitory terminals from different rat and human brain areas. Glutamatergic and GABAergic terminals were identified by staining for the vesicular glutamate transporter (vGLUT1), glutamic acid decarboxylase (GAD67), or the vesicular GABA transporter (vGAT). In all examined areas GABAergic terminals did not display detectable levels of SNAP-25, whereas glutamatergic terminals expressed the protein to a variable extent. Codistribution analysis revealed a high colocalization between pixels detecting SNAP-25 labeling and pixels detecting vGLUT1 immunoreactivity. On the contrary, a low degree of pixel colocalization, comparable to that between two unrelated antigens, was detected between SNAP-25 and vGAT, thus suggesting a random overlap of immunofluorescence signals. Our immunofluorescence evidence was supported by ultrastructural data, which clearly confirmed that SNAP-25 was undetectable in GABAergic terminals identified by both their typical morphology and specific staining for GABA. Interestingly, our ultrastructural results confirmed that a subset of glutamatergic synapses do not contain detectable levels of SNAP-25. The present study extends our previous findings obtained in rodent hippocampus and provides evidence that SNAP-25 expression is highly variable between different axon terminals both in rat and human brain. The heterogeneous distribution of SNAP-25 may have important implications not only in relation to the function of the protein as a SNARE but also in the control of network excitability.

The pathways connecting the hippocampal formation, the thalamic reuniens nucleus and the thalamic reticular nucleus in the rat

Most dorsal thalamic nuclei send axons to specific areas of the neocortex and to specific sectors of the thalamic reticular nucleus; the neocortex then sends reciprocal connections back to the same thalamic nucleus, directly as well indirectly through a relay in the thalamic reticular nucleus. This can be regarded as a 'canonical' circuit of the sensory thalamus. For the pathways that link the thalamus and the hippocampal formation, only a few comparable connections have been described. The reuniens nucleus of the thalamus sends some of its major cortical efferents to the hippocampal formation. The present study shows that cells of the hippocampal formation as well as cells in the reuniens nucleus are retrogradely labelled following injections of horseradish peroxidase or fluoro-gold into the rostral part of the thalamic reticular nucleus in the rat. Within the hippocampal formation, labelled neurons were localized in the subiculum, predominantly on the ipsilateral side, with fewer neurons labelled contralaterally. Labelled neurons were seen in the hippocampal formation and nucleus reuniens only after injections made in the rostral thalamic reticular nucleus (1.6-1.8 mm caudal to bregma). In addition, the present study confirmed the presence of afferent connections to the rostral thalamic reticular nucleus from cortical (cingulate, orbital and infralimbic, retrosplenial and frontal), midline thalamic (paraventricular, anteromedial, centromedial and mediodorsal thalamic nuclei) and brainstem structures (substantia nigra pars reticularis, ventral tegmental area, periaqueductal grey, superior vestibular and pontine reticular nuclei). These results demonstrate a potential for the thalamo-hippocampal circuitry to influence the functional roles of the thalamic reticular nucleus, and show that thalamo-hippocampal connections resemble the circuitry that links the sensory thalamus and neocortex.

Positive allosteric modulation of metabotropic glutamate 4 (mGlu4) receptors enhances spontaneous and evoked absence seizures

Individual metabotropic glutamate (mGlu) receptor subtypes have been implicated in the pathophysiology of epileptic seizures, and are potential targets for novel antiepileptic drugs. Here, we examined the role of the mGlu4 receptor subtype in absence seizures using as models: (i) WAG/Rij rats, which develop spontaneous absence seizures after 2-3months of age; and (ii) mice treated with pentylentetrazole (PTZ, 30mg/kg, s.c.). Expression of mGlu4 receptors was enhanced in the reticular thalamic nucleus (RTN) of symptomatic WAG/Rij rats as compared with age-matched controls, as assessed by immunoblotting and immunohistochemistry. No changes were found in other regions of WAG/Rij rats including ventrobasal thalamic nuclei, somatosensory cortex, and hippocampus. Electron microscopy and in situ hybridization data suggested that mGlu4 receptors in the RTN are localized on excitatory cortical afferents. Systemic injection of the selective mGlu4 receptor positive allosteric modulator, N-phenyl-7-(hydroxyimino)cyclopropa[b]chromen1a-carboxamide (PHCCC, 10mg/kg, s.c.), substantially enhanced the number of spike-and-wave discharges (SWDs) in WAG/Rij rats. Injection of PHCCC also enhanced absence-like seizures in PTZ-treated mice, whereas it was totally inactive in mGlu4 receptor knockout mice, which were intrinsically resistant to PTZ-induced seizures, as expected. This data supports the hypothesis that activation of mGlu4 receptors participates in the generation of absence seizures which can be exacerbated with the use of a positive allosteric modulator.

Immunolocalization of muscarinic receptor subtypes in the reticular thalamic nucleus of rats

In this study, to identify the precise localization of the muscarinic receptor subtypes m2, m3 and m4 in the rostral part of the rat reticular thalamic nucleus (rRt), namely, the limbic sector, we used receptor-subtype-specific antibodies and characterized the immunolabeled structures by light, confocal laser scanning, and electron microscopies. The m2-immunolabeling was preferentially distributed in the distal dendrite region where cholinergic afferent fibers tend to terminate and in the peripheral region of somata, whereas the m3-immunolabeling was more preferentially distributed in a large part of somata and in proximal dendrite shafts than in the distal dendrite region. Dual-immunofluorescence experiments demonstrated that majority of rRt neurons with parvalbumin immunoreactivity contain both m2 and m3. Neither m2 nor m3 was detected in presynaptic terminals or axonal elements. No m4-immunolabeling was detected in the rostral part of the thalamus including rRt. These results show the different distributions of m2 and m3 in rRt neurons, and strongly suggest that m2 is more closely associated with cholinergic afferents than m3.

Functional mapping of GABA(B)-receptor subtypes in the thalamus

The thalamus plays an important role in attention mechanisms and the generation of brain rhythms. gamma-Aminobutyric acid type B (GABA(B)) receptors are known to regulate the main output neurons of the thalamus, the thalamocortical relay (TCR) cells. However, the contributions of the two predominant GABA(B)-receptor subtypes, GABA(B(1a,2)) and GABA(B(1b,2)), to the control of TCR cell activity are unknown. Here, we used genetic and electrophysiological methods to investigate subtype-specific GABA(B) effects at the inputs to TCR cells. We found that mainly GABA(B(1a,2)) receptors inhibit the release of glutamate from corticothalamic fibers impinging onto TCR cells. In contrast, both GABA(B(1a,2)) and GABA(B(1b,2)) receptors efficiently inhibit the release of GABA from thalamic reticular nucleus (TRN) neurons onto TCR neurons. Likewise, both GABA(B(1a,2)) and GABA(B(1b,2)) receptors efficiently activate somatodendritic K(+) currents in TCR cells. In summary, our data show that GABA(B(1b,2)) receptors cannot compensate for the absence of GABA(B(1a,2)) receptors at glutamatergic inputs to TCR cells. This shows that the predominant association of GABA(B(1a,2)) receptors with glutamatergic terminals is a feature that is preserved at several brain synapses. Furthermore, our data indicate that the cognitive deficits observed with mice lacking GABA(B(1a,2)) receptors could to some extent relate to attention deficits caused by disinhibited release of glutamate onto TCR neurons.

Do first order and higher order regions of the thalamic reticular nucleus have different developmental timetables?

The thalamic reticular nucleus (TRN) can been subdivided into sectors based on thalamic and cortical input. Additionally, in carnivores the visual sector of the TRN can be subdivided into first order (perigeniculate nucleus: PGN) and higher order (TRN) regions. This report examines whether TRN development reflects the nature of its higher order visual connections. 170 cells from 12 kittens aged between postnatal day 0 (P0) and P125 were fully analysed after single cell injections in 400-500 microm fixed brain slices. TRN cells have a period of exuberant dendritic branching that peaks between P3 and P12, around the time of eye opening (P7), followed by branch pruning until P68. Similarly, most dendritic appendages are added between P12 and P22 followed by pruning, which is also largely complete by P68. Most branch points occur within the first 10-30% of the dendritic arbor, peaking between 10 and 20% (roughly equivalent to 100 mum from the soma), while appendages were concentrated between 20 and 30% of the arbour; appendages tend to be distributed over a larger proportion of the arbor up to P14 compared to later ages. TRN and PGN maturation were not significantly different. The present data suggest that clear distinctions cannot be made between the maturation of first and higher order pathways and indicate that GABAergic cells of the ventral thalamus may mature earlier than relay cells of the dorsal thalamus. Furthermore, dendritic development in the TRN may be less dependent on extrinsic factors than an intrinsic growth pattern or factors other than a functional hierarchy within the visual pathway.

Influence of norepinephrine on somatosensory neuronal responses in the rat thalamus: a combined modeling and in vivo multi-channel, multi-neuron recording study

Norepinephrine released within primary sensory circuits from locus coeruleus afferent fibers can produce a spectrum of modulatory actions on spontaneous or sensory-evoked activity of individual neurons. Within the ventral posterior medial thalamus, membrane currents modulated by norepinephrine have been identified. However, the relationship between the cellular effects of norepinephrine and the impact of norepinephrine release on populations of neurons encoding sensory signals is still open to question. To address this lacuna in understanding the net impact of the noradrenergic system on sensory signal processing, a computational model of the rat trigeminal somatosensory thalamus was generated. The effects of independent manipulation of different cellular actions of norepinephrine on simulated afferent input to the computational model were then examined. The results of these simulations aided in the design of in vivo neural ensemble recording experiments where sensory-driven responses of thalamic neurons were measured before and during locus coeruleus activation in waking animals. Together the simulated and experimental results reveal several key insights regarding the regulation of neural network operation by norepinephrine including: 1) cell-specific modulatory actions of norepinephrine, 2) mechanisms of norepinephrine action that can improve the tuning of the network and increase the signal-to-noise ratio of cellular responses in order to enhance network representation of salient stimulus features and 3) identification of the dynamic range of thalamic neuron function through which norepinephrine operates.