Novel neuronal and astrocytic mechanisms in thalamocortical loop dynamics

In this review, we summarize three sets of findings that have recently been observed in thalamic astrocytes and neurons, and discuss their significance for thalamocortical loop dynamics. (i) A physiologically relevant 'window' component of the low-voltage-activated, T-type Ca(2+) current (I(Twindow)) plays an essential part in the slow (less than 1 Hz) sleep oscillation in adult thalamocortical (TC) neurons, indicating that the expression of this fundamental sleep rhythm in these neurons is not a simple reflection of cortical network activity. It is also likely that I(Twindow) underlies one of the cellular mechanisms enabling TC neurons to produce burst firing in response to novel sensory stimuli. (ii) Both electrophysiological and dye-injection experiments support the existence of gap junction-mediated coupling among young and adult TC neurons. This finding indicates that electrical coupling-mediated synchronization might be implicated in the high and low frequency oscillatory activities expressed by this type of thalamic neuron. (iii) Spontaneous intracellular Ca(2+) ([Ca(2+)](i)) waves propagating among thalamic astrocytes are able to elicit large and long-lasting N-methyl-D-aspartate-mediated currents in TC neurons. The peculiar developmental profile within the first two postnatal weeks of these astrocytic [Ca(2+)](i) transients and the selective activation of these glutamate receptors point to a role for this astrocyte-to-neuron signalling mechanism in the topographic wiring of the thalamocortical loop. As some of these novel cellular and intracellular properties are not restricted to thalamic astrocytes and neurons, their significance may well apply to (patho)physiological functions of glial and neuronal elements in other brain areas.     

Astroglial excitability and gliotransmission: an appraisal of Ca2+ as a signalling route

Astroglial cells, due to their passive electrical properties, were long considered subservient to neurons and to merely provide the framework and metabolic support of the brain. Although astrocytes do play such structural and housekeeping roles in the brain, these glial cells also contribute to the brain''s computational power and behavioural output. These more active functions are endowed by the Ca²⁺-based excitability displayed by astrocytes. An increase in cytosolic Ca²⁺ levels in astrocytes can lead to the release of signalling molecules, a process termed gliotransmission, via the process of regulated exocytosis. Dynamic components of astrocytic exocytosis include the vesicular-plasma membrane secretory machinery, as well as the vesicular traffic, which is governed not only by general cytoskeletal elements but also by astrocyte-specific IFs (intermediate filaments). Gliotransmitters released into the ECS (extracellular space) can exert their actions on neighbouring neurons, to modulate synaptic transmission and plasticity, and to affect behaviour by modulating the sleep homoeostat. Besides these novel physiological roles, astrocytic Ca²⁺ dynamics, Ca²⁺-dependent gliotransmission and astrocyte–neuron signalling have been also implicated in brain disorders, such as epilepsy. The aim of this review is to highlight the newer findings concerning Ca²⁺ signalling in astrocytes and exocytotic gliotransmission. For this we report on Ca²⁺ sources and sinks that are necessary and sufficient for regulating the exocytotic release of gliotransmitters and discuss secretory machinery, secretory vesicles and vesicle mobility regulation. Finally, we consider the exocytotic gliotransmission in the modulation of synaptic transmission and plasticity, as well as the astrocytic contribution to sleep behaviour and epilepsy.     

Immunophilin deficiency augments Ca2+-dependent glutamate release from mouse cortical astrocytes

Immunophilins are receptors for immunosuppressive drugs such as the macrolides cyclosporin A (CsA) and FK506; correspondingly these immunophilins are referred to as cyclophilins and FK506-binding proteins (FKBPs). In particular, CsA targets cyclophilin D (CypD), which can modulate mitochondrial Ca(2+) dynamics. Since mitochondria have been implicated in the regulation of astrocytic cytosolic Ca(2+) (Ca(cyt)(2+)) dynamics and consequential Ca(2+)-dependent exocytotic release of glutamate, we investigated the role of CypD in this process. Cortical astrocytes isolated from CypD deficient mice Ppif(-/-) displayed reduced mechanically induced Ca(cyt)(2+) increases, even though these cells showed augmented exocytotic release of glutamate, when compared to responses obtained from astrocytes isolated from wild-type mice. Furthermore, acute treatment with CsA to inhibit CypD modulation of mitochondrial Ca(2+) buffering, or with FK506 to inhibit FKBP12 interaction with inositol-trisphosphate receptor of the endoplasmic reticulum, led to similar reductive effects on astrocytic Ca(cyt)(2+) dynamics, but also to an enhanced Ca(2+)-dependent exocytotic release of glutamate in wild-type astrocytes. These findings point to a possible role of immunophilin signal transduction pathways in astrocytic modulation of neuronal activity at the tripartite synapse.     

The neurosteroid system: implication in the pathophysiology of hepatic encephalopathy

Hepatic encephalopathy (HE) is a serious cerebral complication of both acute and chronic liver failure. In acute liver failure, astrocytes undergo swelling which results in increased intracranial pressure and may lead to brain herniation and death. In chronic liver failure, Alzheimer-type II astrocytosis is the characteristic neuropathologic finding. Patients with liver failure manifest severe alterations of their quality of life including sleep disorders as well as memory, learning, and locomotor abnormalities. Neurosteroids (NS) are synthesized in the brain mainly by astrocytes independent of peripheral steroidal sources (adrenals and gonads) and are suggested to play a role in the pathogenesis of HE. NS bind and modulate different types of neural receptors; effects on the gamma amino butyric acid (GABA)-A receptor complex are the most extensively studied. For example, the NS tetrahydroprogesterone (allopregnanolone), and tetrahydrodeoxycorticosterone (THDOC) are potent positive allosteric modulators of the GABA-A receptor. As a consequence of modulation of these receptors, NS stimulate inhibitory neurotransmission in the CNS, and neuroinhibitory changes including "increased GABA-ergic tone" have been suggested as pathophysiological mechanisms in HE. Moreover, some NS bind to intracellular receptors through which they also regulate gene expression, and there is substantial evidence confirming that expression of genes coding for key astrocytic and neuronal proteins are altered in HE. This review summarizes findings consistent with the involvement of NS in human and experimental HE.     

Cellular mechanisms of the slow (<1 Hz) oscillation in thalamocortical neurons in vitro

The slow (<1 Hz) rhythm is a defining feature of the electroencephalogram during sleep. Since cortical circuits can generate this rhythm in isolation, it is assumed that the accompanying slow oscillation in thalamocortical (TC) neurons is largely a passive reflection of neocortical activity. Here we show, however, that by activating the metabotropic glutamate receptor (mGluR), mGluR1a, cortical inputs can recruit intricate cellular mechanisms that enable the generation of an intrinsic slow oscillation in TC neurons in vitro with identical properties to those observed in vivo. These mechanisms rely on the "window" component of the T-type Ca(2+) current and a Ca(2+)-activated, nonselective cation current. These results suggest an active role for the thalamus in shaping the slow (<1 Hz) sleep rhythm.     

The cognitive neuroscience of sleep: neuronal systems,consciousness and learning

Sleep can be addressed across the entire hierarchy of biological organization. We discuss neuronal-network and regional forebrain activity during sleep, and its consequences for consciousness and cognition. Complex interactions in thalamocortical circuits maintain the electroencephalographic oscillations of non-rapid eye movement (NREM) sleep. Functional neuroimaging affords views of the human brain in both NREM and REM sleep, and has informed new concepts of the neural basis of dreaming during REM sleep -- a state that is characterized by illogic, hallucinosis and emotionality compared with waking. Replay of waking neuronal activity during sleep in the rodent hippocampus and in functional images of human brains indicates possible roles for sleep in neuroplasticity. Different forms and stages of learning and memory might benefit from different stages of sleep and be subserved by different forebrain regions.     

Reciprocal interaction of sleep and synaptic plasticity

Synaptic plasticity underlying learning and memory has been proposed, on the basis of several experimental approaches, to be intimately related with sleep: 1) The idea that sleep contributes to stabilization of acquired memory arises from numerous studies depriving subjects or animals of sleep. 2) Evidence from developing technologies supports "offline" reprocessing of recent experiences during sleep. 3) Recent analysis of the thalamocortical system establishes the reciprocal observation that sleep itself is a plastic process affected by waking experience. This overview synthesizes these converging perspectives across a variety of brain regions and species. We propose the developing visual pathway as a fruitful model for comprehensive understanding of sleep and synaptic plasticity.     

Thalamic synaptic transmission of sensory information modulated by synergistic interaction of adenosine and serotonin

The thalamic synapses relay peripheral sensory information to the cortex, and constitute an important part of the thalamocortical network that generates oscillatory activities responsible for different vigilance (sleep and wakefulness) states. However, the modulation of thalamic synaptic transmission by potential sleep regulators, especially by combination of regulators in physiological scenarios, is not fully characterized. We found that somnogen adenosine itself acts similar to wake‐promoting serotonin, both decreasing synaptic strength as well as short‐term depression, at the retinothalamic synapse. We then combined the two modulators considering the coexistence of them in the hypnagogic (sleep‐onset) state. Adenosine plus serotonin results in robust synergistic inhibition of synaptic strength and dramatic transformation of short‐term synaptic depression to facilitation. These synaptic effects are not achievable with a single modulator, and are consistent with a high signal‐to‐noise ratio but a low level of signal transmission through the thalamus appropriate for slow‐wave sleep. This study for the first time demonstrates that the sleep‐regulatory modulators may work differently when present in combination than present singly in terms of shaping information flow in the thalamocortical network. The major synaptic characters such as the strength and short‐term plasticity can be profoundly altered by combination of modulators based on physiological considerations.

     

Exocytosis of ATP From Astrocytes Modulates Phasic and Tonic Inhibition in the Neocortex

Communication between neuronal and glial cells is important for many brain functions. Astrocytes can modulate synaptic strength via Ca²⁺-stimulated release of various gliotransmitters, including glutamate and ATP. A physiological role of ATP release from astrocytes was suggested by its contribution to glial Ca²⁺-waves and purinergic modulation of neuronal activity and sleep homeostasis. The mechanisms underlying release of gliotransmitters remain uncertain, and exocytosis is the most intriguing and debated pathway. We investigated release of ATP from acutely dissociated cortical astrocytes using “sniff-cell” approach and demonstrated that release is vesicular in nature and can be triggered by elevation of intracellular Ca²⁺ via metabotropic and ionotropic receptors or direct UV-uncaging. The exocytosis of ATP from neocortical astrocytes occurred in the millisecond time scale contrasting with much slower nonvesicular release of gliotransmitters via Best1 and TREK-1 channels, reported recently in hippocampus. Furthermore, we discovered that elevation of cytosolic Ca²⁺ in cortical astrocytes triggered the release of ATP that directly activated quantal purinergic currents in the pyramidal neurons. The glia-driven burst of purinergic currents in neurons was followed by significant attenuation of both synaptic and tonic inhibition. The Ca²⁺-entry through the neuronal P2X purinoreceptors led to phosphorylation-dependent down-regulation of GABAA receptors. The negative purinergic modulation of postsynaptic GABA receptors was accompanied by small presynaptic enhancement of GABA release. Glia-driven purinergic modulation of inhibitory transmission was not observed in neurons when astrocytes expressed dn-SNARE to impair exocytosis. The astrocyte-driven purinergic currents and glia-driven modulation of GABA receptors were significantly reduced in the P2X4 KO mice. Our data provide a key evidence to support the physiological importance of exocytosis of ATP from astrocytes in the neocortex.     

Dose-dependent tazepam modulation of amplitude-temporal characteristics of thalamocortical responses and the constant potential of the sensorimotor cortex in rabbits at eye opening

The pronounced benzodiazepine (antiphobic) modulation of the amplitude-temporal parameters of different components of the thalamocortical responses (TCR) of the sensorimotor cortex is observed in rabbits in their early postnatal ontogeny. This modulation is of a dose-dependent character and is registered not after the injection of tazepam in a concentration of the "therapeutic tranquilizing window" but also in the psychotoxic plasma range. A gradual increase in blood tazepam concentration in a young rabbit pup is accompanied by the wave-like and differential decrease in the amplitude of the second and third positive (P2 and P3) and third negative (N3) TCR components, while the second negative (N2) and fourth positive (P4) components tend to a wave-like increase. The dose-dependent dynamics of tazepam modulation of the P2, P3, and N3 latencies is characterized by a wave-like and differential increase. The latency of P4 decreases slightly and that of the N2 increases with a low degree of significance. The selective dynamics of benzodiazepine modulation appears to be related with peculiarities of the electrogenesis of each of the components. The dose-dependent modulation of the level of cortical DC potential is of the same character as the respective amplitude changes in P2, P3, and N3, but its fluctiatuons are more pronounced.     

Astrocyte–Neuron Communication: Functional Consequences

Astrocyte–neuron communication has recently been proposed as a potential mechanism participating to synaptic transmission. With the development of this concept and accumulating evidences in favor of a modulation of synaptic transmission by astrocytes, has emerged the term gliotransmission. It refers to the capacity of astrocytes to release various transmitters, such as ATP, glutamate, d-serine, and GABA in the vicinity of synapses. While the cellular mechanisms involved in gliotransmission still need to be better described and, for some, identified, the aim of more and more studies is to determine the role of astrocytes from a functional point of view. This review will summarize the principal studies that have investigated a potential role of astrocytes in the various functions regulated by the brain (sleep, breathing, perception, learning and memory…). This will allow us to highlight the similarities and discrepancies in the signaling pathways involved in the different areas of the brain related to these functions.     

Toward an integrative theory of sleep and dreaming

Non-rapid-eye-movement sleep (NREMS) is triggered by the accumulation of adenosine, as a result of the perceptual overload of the brain cortex. NREMS starts in the most burdened regions of the cortex first and then eventually, after the released adenosine has reached the ventrolateral pre-optic nucleus area of the hypothalamus, triggers the "general NREMS pattern". This is accompanied by the usual familiar changes in the thalamocortical system. When NREMS reaches the slow-wave sleep (SWS) phase, with its predominant delta activity, brain metabolism drops significantly with the brain temperature, and this is recognized by the alarm system in the pre-optic anterior hypothalamus and/or the other thermostat circuit in the brainstem as a life-threatening situation. This alarm system triggers a reaction similar to abortive or partial awakening called rapid-eye-movement sleep (REMS), which is aimed at restoring the optimal body-core temperature. As soon as this restoration is accomplished by the activation of the brainstem-to-cortex ascending pathways, NREMS may continue, as may the interchange of the two sleep phases during the entire sleep period. During both NREMS and REMS, the same essential pattern occurs in the cortex: the loops "used" during the previous waking period, now deprived of external input, replay their waking activity at a lower frequency, one which enables them to restore the membrane's potential (possibly by means of LTD). During REMS, however, the cholinergic flood originating in the LTD/PPT nuclei of the pons tegmentum, increases in the basal forebrain and, provoking theta activity in the medial septum is extended to the hippocampus, causing the circuits that are active at that particular moment in the cortex, to store the information they carry as memory. This is the explanation of both the memory improvement known to be related to REMS and of dreams. Both phenomena are clearly side effects of REMS.     

Synchrony in sensation

How neurons encode information has been a hotly debated issue. Ultimately, any code must be relevant to the senders, receivers, and connections between them. This review focuses on the transmission of sensory information through the circuit linking thalamus and cortex, two distant brain regions. Strong feedforward inhibition in the thalamocortical circuit renders cortex highly sensitive to the thalamic synchrony evoked by a sensory stimulus. Neuromodulators and feedback connections may modulate the temporal sensitivity of such circuits and gate the propagation of synchrony into other layers and cortical areas. The prevalence of strong feedforward inhibitory circuits throughout the central nervous system suggests that synchrony codes and timing-sensitive circuits may be widespread, occurring well beyond sensory thalamus and cortex.     

Modelling the modulation of cortical Up-Down state switching by astrocytes

Up-Down synchronization in neuronal networks refers to spontaneous switches between periods of high collective firing activity (Up state) and periods of silence (Down state). Recent experimental reports have shown that astrocytes can control the emergence of such Up-Down regimes in neural networks, although the molecular or cellular mechanisms that are involved are still uncertain. Here we propose neural network models made of three populations of cells: excitatory neurons, inhibitory neurons and astrocytes, interconnected by synaptic and gliotransmission events, to explore how astrocytes can control this phenomenon. The presence of astrocytes in the models is indeed observed to promote the emergence of Up-Down regimes with realistic characteristics. Our models show that the difference of signalling timescales between astrocytes and neurons (seconds versus milliseconds) can induce a regime where the frequency of gliotransmission events released by the astrocytes does not synchronize with the Up and Down phases of the neurons, but remains essentially stable. However, these gliotransmission events are found to change the localization of the bifurcations in the parameter space so that with the addition of astrocytes, the network enters a bistability region of the dynamics that corresponds to Up-Down synchronization. Taken together, our work provides a theoretical framework to test scenarios and hypotheses on the modulation of Up-Down dynamics by gliotransmission from astrocytes.     

Dream contents and failing memories

Mentation during sleep states is thought to originate in an activation of brain circuits that encode inherited and experiential memories. Spontaneous degradation of the strengths of synapses occurs in all brain circuits because of "turnover" of molecules essential for synaptic function. In circuits employed frequently during waking, synaptic strengths are refreshed and maintained in their dedicated or functional ranges largely through use, by virtue of activity-dependent synaptic plasticity. In circuits employed infrequently during waking, synaptic strengths are refreshed largely during sleep, by circuit activations induced by spontaneous, self-generated, largely low-frequency brain waves, also by virtue of activity-dependent synaptic plasticity. The outputs of circuits activated during sleep do not necessarily rise to the level of 'unconscious' awareness. Such an absence of awareness of the outputs of individual circuits, that is, an absence of dreaming, is proposed to be the primitive condition in animals that sleep. On the other hand, temporal binding of these outputs is accompanied by the thoughts and perceptions of dreams, which is proposed to be the advanced condition. Linking or serial ordering of otherwise 'static' thoughts and perceptions gives rise to continuous, often narrative and veridical, dreams. In all cases, dream contents are derived from the memories--not necessarily veridical--encoded in the reinforced circuitry. In the absence of synaptic strength refreshments during sleep, synaptic strengths in infrequently used circuits would weaken and the circuits would become incompetent, with their encoded memories degraded or lost. Maintenance of synaptic strengths in infrequently used circuitry during sleep apparently does not always achieve perfection. Weakened synapses begin to occur in circuits in appreciable numbers in children after the age of about 5 years. When these 'incompetent' circuits (with weakened synapses) are activated during sleep, there are minimal influences on dream contents, namely, distortions that make some objects, such as animals, faces, and scenes, unrecognizable. As weakened synapses increase in numbers with age, the numbers of distorted objects in dreams also increase. In adults, people in as many as 80% of dreams may be unrecognizable. Besides the normal weakening of synaptic strengths, some synapses become defective, in consequence of deleterious, adventitious, exogenous influences, for example, radiation. As these faulty synapses accumulate in old memories, activation of circuits incorporating them during sleep leads to dreams with incoherent, bizarre, or impossible contents. The infrequent activation of such old, incompetent circuits in some waking conditions leads to false memories, delusions, or hallucinations.     

Facts and reflections on thalamo cortical reverberation of impulses

Experiments on cats, either unanesthetized or anesthetized with various doses of pentobarbital, showed that the cortical rhythmic after-discharge ("slow after-activity"), which has been regarded as a manifestation of reverberation of impulses in thalamocortical circuits [17], consists of a burst of spontaneous "spindles" evoked by stimulation. This conclusion is supported by the following facts: Spontaneous "spindles" and the rhythmic after-discharge respond absolutely identically (disappear) to activation of the EEG and deepening of pentobarbital anesthesia. The absence of thalamocortical reverberation is also indicated by the preservation of a rhythmic after-discharge (to clicks), synchronous with the cortex, in the thalamic relay nucleus (the medial geniculate body) after cooling or after removal of its projection zone.     

Complementary encoding of spatial information in hippocampal astrocytes

Calcium dynamics into astrocytes influence the activity of nearby neuronal structures. However, because previous reports show that astrocytic calcium signals largely mirror neighboring neuronal activity, current information coding models neglect astrocytes. Using simultaneous two-photon calcium imaging of astrocytes and neurons in the hippocampus of mice navigating a virtual environment, we demonstrate that astrocytic calcium signals encode (i.e., statistically reflect) spatial information that could not be explained by visual cue information. Calcium events carrying spatial information occurred in topographically organized astrocytic subregions. Importantly, astrocytes encoded spatial information that was complementary and synergistic to that carried by neurons, improving spatial position decoding when astrocytic signals were considered alongside neuronal ones. These results suggest that the complementary place dependence of localized astrocytic calcium signals may regulate clusters of nearby synapses, enabling dynamic, context-dependent variations in population coding within brain circuits.

A combination of functional imaging of astrocytes and neurons in the mouse hippocampus with information theory analysis shows that calcium dynamics in topographically-organized subcellular regions of astrocytes encode information about an animal’s position that is complementary and synergistic to that encoded in the spike output of surrounding neurons.     

Neuron-astrocyte signaling in the development and plasticity of neural circuits

Emerging evidence indicates that signaling between perisynaptic astrocytes and neurons at the tripartite synapse plays an important role during the critical period when neural circuits are formed and refined. Cross-talk between astrocytes and neurons during development mediates synaptogenesis, synapse elimination and structural plasticity through a variety of secreted and contact-dependent signals. Recent live imaging studies reveal a dynamic and cooperative interplay between astrocytes and neurons at synapses that is guided by a variety of molecular cues. A unifying theme from these recent findings is that astrocytes can promote the development and plasticity of synaptic circuits. Insight into the molecular mechanisms by which astrocytes regulate the wiring of the brain during development could lead to new therapeutic strategies to promote repair and rewiring of neural circuits in the mature brain following CNS injury and neurodegenerative disease.     

Somatosensory integration controlled by dynamic thalamocortical feed-forward inhibition

The temporal features of tactile stimuli are faithfully represented by the activity of neurons in the somatosensory cortex. However, the cellular mechanisms that enable cortical neurons to report accurate temporal information are not known. Here, we show that in the rodent barrel cortex, the temporal window for integration of thalamic inputs is under the control of thalamocortical feed-forward inhibition and can vary from 1 to 10 ms. A single thalamic fiber can trigger feed-forward inhibition and contacts both excitatory and inhibitory cortical neurons. The dynamics of feed-forward inhibition exceed those of each individual synapse in the circuit and are captured by a simple disynaptic model of the thalamocortical projection. The variations in the integration window produce changes in the temporal precision of cortical responses to whisker stimulation. Hence, feed-forward inhibitory circuits, classically known to sharpen spatial contrast of tactile inputs, also increase the temporal resolution in the somatosensory cortex.     

Decreased Thalamocortical Functional Connectivity after 36 Hours of Total Sleep Deprivation: Evidence from Resting State fMRI

Objectives

The thalamus and cerebral cortex are connected via topographically organized, reciprocal connections, which hold a key function in segregating internally and externally directed awareness information. Previous task-related studies have revealed altered activities of the thalamus after total sleep deprivation (TSD). However, it is still unclear how TSD impacts on the communication between the thalamus and cerebral cortex. In this study, we examined changes of thalamocortical functional connectivity after 36 hours of total sleep deprivation by using resting state function MRI (fMRI).

Materials and Methods

Fourteen healthy volunteers were recruited and performed fMRI scans before and after 36 hours of TSD. Seed-based functional connectivity analysis was employed and differences of thalamocortical functional connectivity were tested between the rested wakefulness (RW) and TSD conditions.

Results

We found that the right thalamus showed decreased functional connectivity with the right parahippocampal gyrus, right middle temporal gyrus and right superior frontal gyrus in the resting brain after TSD when compared with that after normal sleep. As to the left thalamus, decreased connectivity was found with the right medial frontal gyrus, bilateral middle temporal gyri and left superior frontal gyrus.

Conclusion

These findings suggest disruptive changes of the thalamocortical functional connectivity after TSD, which may lead to the decline of the arousal level and information integration, and subsequently, influence the human cognitive functions.