A quantitative study of synaptic contacts on interneurons and relay cells of the cat lateral geniculate nucleus

Sleep and wake in a model of the thalamocortical system with Martinotti cells

The mechanisms leading to the alternation between active (UP) and silent (DOWN) states during sleep slow waves (SWs) remain poorly understood. Previous models have explained the transition to the DOWN state by a progressive failure of excitation because of the build-up of adaptation currents or synaptic depression. However, these models are at odds with recent studies suggesting a role for presynaptic inhibition by Martinotti cells (MaCs) in generating SWs. Here, we update a classical large-scale model of sleep SWs to include MaCs and propose a different mechanism for the generation of SWs. In the wake mode, the network exhibits irregular and selective activity with low firing rates (FRs). Following an increase in the strength of background inputs and a modulation of synaptic strength and potassium leak potential mimicking the reduced effect of acetylcholine during sleep, the network enters a sleep-like regime in which local increases of network activity trigger bursts of MaC activity, resulting in strong disfacilitation of the local network via presynaptic GABAB1a -type inhibition. This model replicates findings on slow wave activity (SWA) during sleep that challenge previous models, including low and skewed FRs that are comparable between the wake and sleep modes, higher synchrony of transitions to DOWN states than to UP states, the possibility of triggering SWs by optogenetic stimulation of MaCs, and the local dependence of SWA on synaptic strength. Overall, this work points to a role for presynaptic inhibition by MaCs in the generation of DOWN states during sleep.

Local interneurons in the murine visual thalamus have diverse receptive fields and can provide feature selective inhibition to relay cells

By influencing the type and quality of information that relay cells transmit, local interneurons in thalamus have a powerful impact on cortex. To define the sensory features that these inhibitory neurons encode, we mapped receptive fields of optogenetically identified cells in the murine dorsolateral geniculate nucleus. Although few in number, local interneurons had diverse types of receptive fields, like their counterpart relay cells. This result differs markedly from visual cortex, where inhibitory cells are typically less selective than excitatory cells. To explore how thalamic interneurons might converge on relay cells, we took a computational approach. Using an evolutionary algorithm to search through a library of interneuron models generated from our results, we show that aggregated output from different groups of local interneurons can simulate the inhibitory component of the relay cell's receptive field. Thus, our work provides proof-of-concept that groups of diverse interneurons can supply feature-specific inhibition to relay cells.

3D electron microscopy and volume-based bouton sorting reveal the selectivity of inputs onto geniculate relay cell and interneuron dendrite segments

Introduction

The visual signals evoked at the retinal ganglion cells are modified and modulated by various synaptic inputs that impinge on lateral geniculate nucleus cells before they are sent to the cortex. The selectivity of geniculate inputs for clustering or forming microcircuits on discrete dendritic segments of geniculate cell types may provide the structural basis for network properties of the geniculate circuitry and differential signal processing through the parallel pathways of vision. In our study, we aimed to reveal the patterns of input selectivity on morphologically discernable relay cell types and interneurons in the mouse lateral geniculate nucleus.

Methods

We used two sets of Scanning Blockface Electron Microscopy (SBEM) image stacks and Reconstruct software to manually reconstruct of terminal boutons and dendrite segments. First, using an unbiased terminal sampling (UTS) approach and statistical modeling, we identified the criteria for volume-based sorting of geniculate boutons into their putative origins. Geniculate terminal boutons that were sorted in retinal and non-retinal categories based on previously described mitochondrial morphology, could further be sorted into multiple subpopulations based on their bouton volume distributions. Terminals deemed non-retinal based on the morphological criteria consisted of five distinct subpopulations, including small-sized putative corticothalamic and cholinergic boutons, two medium-sized putative GABAergic inputs, and a large-sized bouton type that contains dark mitochondria. Retinal terminals also consisted of four distinct subpopulations. The cutoff criteria for these subpopulations were then applied to datasets of terminals that synapse on reconstructed dendrite segments of relay cells or interneurons.

Results

Using a network analysis approach, we found an almost complete segregation of retinal and cortical terminals on putative X-type cell dendrite segments characterized by grape-like appendages and triads. On these cells, interneuron appendages intermingle with retinal and other medium size terminals to form triads within glomeruli. In contrast, a second, presumed Y-type cell displayed dendrodendritic puncta adherentia and received all terminal types without a selectivity for synapse location; these were not engaged in triads. Furthermore, the contribution of retinal and cortical synapses received by X-, Y- and interneuron dendrites differed such that over 60% of inputs to interneuron dendrites were from the retina, as opposed to 20% and 7% to X- and Y-type cells, respectively.

Conclusion

The results underlie differences in network properties of synaptic inputs from distinct origins on geniculate cell types.

Cortical feedback modulates distinct critical period development in mouse visual thalamus

In primary visual cortex (V1), critical period for ocular dominance (OD) plasticity is a well-defined developmental stage to shape neuronal circuits based on visual experience. Recent studies showed that V1-like OD plasticity existed in mouse dorsal lateral geniculate nucleus (dLGN). It is still unclear what the exact time window is and how neural circuits contribute to OD plasticity in dLGN. Using in vivo electrophysiology, we defined a critical period for OD plasticity in dLGN from eye opening to puberty. There also existed an innate process of OD formation from contralateral to equal bias in dLGN binocular neurons. Instant V1 inactivation with muscimol had no effect on OD bias or plasticity. Short-term V1 inactivation with N-methyl-d-aspartate reversed the formation of equal OD bias, while long-term V1 inactivation retained dLGN development to an immature stage.

Selective corticofugal modulation on sound processing in auditory thalamus of awake marmosets

Cortical feedback has long been considered crucial for the modulation of sensory perception and recognition. However, previous studies have shown varying modulatory effects of the primary auditory cortex (A1) on the auditory response of subcortical neurons, which complicate interpretations regarding the function of A1 in sound perception and recognition. This has been further complicated by studies conducted under different brain states. In the current study, we used cryo-inactivation in A1 to examine the role of corticothalamic feedback on medial geniculate body (MGB) neurons in awake marmosets. The primary effects of A1 inactivation were a frequency-specific decrease in the auditory response of most MGB neurons coupled with an increased spontaneous firing rate, which together resulted in a decrease in the signal-to-noise ratio. In addition, we report for the first time that A1 robustly modulated the long-lasting sustained response of MGB neurons, which changed the frequency tuning after A1 inactivation, e.g. some neurons are sharper with corticofugal feedback and some get broader. Taken together, our results demonstrate that corticothalamic modulation in awake marmosets serves to enhance sensory processing in a manner similar to center-surround models proposed in visual and somatosensory systems, a finding which supports common principles of corticothalamic processing across sensory systems.

Immunocytochemical and ultrastructural organization of the taste thalamus of the tree shrew (Tupaia belangeri)

Ventroposterior medialis parvocellularis (VPMP) nucleus of the primate thalamus receives direct input from the nucleus of the solitary tract, whereas the homologous thalamic structure in the rodent does not. To reveal whether the synaptic circuitries in these nuclei lend evidence for conservation of design principles in the taste thalamus across species or across sensory thalamus in general, we characterized the ultrastructural and molecular properties of the VPMP in a close relative of primates, the tree shrew (Tupaia belangeri), and compared these to known properties of the taste thalamus in rodent, and the visual thalamus in mammals. Electron microscopy analysis to categorize the synaptic inputs in the VPMP revealed that the largest-size terminals contained many vesicles and formed large synaptic zones with thick postsynaptic density on multiple, medium-caliber dendrite segments. Some formed triads within glomerular arrangements. Smaller-sized terminals contained dark mitochondria; most formed a single asymmetric or symmetric synapse on small-diameter dendrites. Immuno-EM experiments revealed that the large-size terminals contained VGLUT2, whereas the small-size terminal populations contained VGLUT1 or ChAT. These findings provide evidence that the morphological and molecular characteristics of synaptic circuitry in the tree shrew VPMP are similar to that in nonchemical sensory thalamic nuclei. Furthermore, the results indicate that all primary sensory nuclei of the thalamus in higher mammals share a structural template for processing thalamocortical sensory information. In contrast, substantial morphological and molecular differences in rodent versus tree shrew taste nuclei suggest a fundamental divergence in cellular processing mechanisms of taste input in these two species.

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

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

Visual hallucinations, thalamocortical physiology and Lewy body disease: A review

One of the core diagnostic criteria for Dementia with Lewy Bodies (DLB) is the presence of visual hallucinations. The presence of hallucinations, along with fluctuations in the level of arousal and sleep disturbance, point to potential pathological mechanisms at the level of the thalamus. However, the potential role of thalamic dysfunction in DLB, particularly as it relates to the presence of formed visual hallucinations is not known. Here, we review the literature on the pathophysiology of DLB with respect to modern theories of thalamocortical function and attempt to derive an understanding of how such hallucinations arise. Based on the available literature, we propose that combined thalamic-thalamic reticular nucleus and thalamocortical pathology may explain the phenomenology of visual hallucinations in DLB. In particular, diminished α7 cholinergic activity in the thalamic reticular nucleus may critically disinhibit thalamocortical activity. Further, concentrated pathological changes within the posterior regions of the thalamus may explain the predilection for the hallucinations to be visual in nature.

Dominant Patterns of Information Flow in the Propagation of the Neuromagnetic Somatosensory Steady-State Response

Methods of functional connectivity are applied ubiquitously in studies involving non-invasive whole-brain signals, but may be not optimal for exploring the propagation of the steady-state responses, which are strong oscillatory patterns of neurodynamics evoked by periodic stimulation. In our study, we explore a functional network underlying the somatosensory steady-state response using methods of effective connectivity. Human magnetoencephalographic (MEG) data were collected in 10 young healthy adults during 23-Hz vibro-tactile stimulation of the right hand index finger. The whole-brain dynamics of MEG source activity was reconstructed with a linearly-constrained minimum variance beamformer. We applied information-theoretic tools to quantify asymmetries in information flows between primary somatosensory area SI and the rest of the brain. Our analysis identified a pattern of coupling, leading from area SI to a source in the secondary somato-sensory area SII, thalamus, and motor cortex all contralateral to stimuli as well as to a source in the cerebellum ipsilateral to the stimuli. Our results support previously reported empirical evidence collected both in in vitro and in vivo, indicating critical areas of activation of the somatosensory system at the level of systems neuroscience.

Synaptic organization of the dorsal lateral geniculate nucleus

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

Biophysical network modeling of the dLGN circuit: Effects of cortical feedback on spatial response properties of relay cells

Despite half-a-century of research since the seminal work of Hubel and Wiesel, the role of the dorsal lateral geniculate nucleus (dLGN) in shaping the visual signals is not properly understood. Placed on route from retina to primary visual cortex in the early visual pathway, a striking feature of the dLGN circuit is that both the relay cells (RCs) and interneurons (INs) not only receive feedforward input from retinal ganglion cells, but also a prominent feedback from cells in layer 6 of visual cortex. This feedback has been proposed to affect synchronicity and other temporal properties of the RC firing. It has also been seen to affect spatial properties such as the center-surround antagonism of thalamic receptive fields, i.e., the suppression of the response to very large stimuli compared to smaller, more optimal stimuli. Here we explore the spatial effects of cortical feedback on the RC response by means of a a comprehensive network model with biophysically detailed, single-compartment and multicompartment neuron models of RCs, INs and a population of orientation-selective layer 6 simple cells, consisting of pyramidal cells (PY). We have considered two different arrangements of synaptic feedback from the ON and OFF zones in the visual cortex to the dLGN: phase-reversed (‘push-pull’) and phase-matched (‘push-push’), as well as different spatial extents of the corticothalamic projection pattern. Our simulation results support that a phase-reversed arrangement provides a more effective way for cortical feedback to provide the increased center-surround antagonism seen in experiments both for flashing spots and, even more prominently, for patch gratings. This implies that ON-center RCs receive direct excitation from OFF-dominated cortical cells and indirect inhibitory feedback from ON-dominated cortical cells. The increased center-surround antagonism in the model is accompanied by spatial focusing, i.e., the maximum RC response occurs for smaller stimuli when feedback is present.

The functional role of the dorsal lateral geniculate nucleus (dLGN), placed on route from retina to primary visual cortex in the early visual pathway, is still poorly understood. A striking feature of the dLGN circuit is that dLGN cells not only receive feedforward input from the retina, but also a prominent feedback from cells in the visual cortex. It has been seen in experiments that cortical feedback modifies the spatial properties of dLGN cells in response to visual stimuli. In particular, it has been shown to increase the center-surround antagonism for flashing-spot and patch-grating visual stimuli, i.e., the suppression of responses to very large stimuli compared to smaller stimuli. Here we investigate the putative mechanisms behind this feature by means of a comprehensive network model of biophysically detailed neuron models for RCs and INs in the dLGN and orientation-selective cortical cells providing the feedback. Our results support that the experimentally observed feedback effects may be due to a phase-reversed (‘push-pull’) arrangement of the cortical feedback where ON-symmetry RCs receive (indirect) inhibitory feedback from ON-dominated cortical cell and excitation from OFF-dominated cortical cells, and vice versa for OFF-symmetry RCs.

Image Sharpness and Contrast Tuning in the Early Visual Pathway

The center-surround organization of the receptive fields (RFs) of retinal ganglion cells highlights the presence of local contrast in visual stimuli. As RF of thalamic relay cells follow the same basic functional organization, it is often assumed that they contribute very little to alter the retinal output. However, in many species, thalamic relay cells largely outnumber their retinal inputs, which diverge to contact simultaneously several units at thalamic level. This gain in cell population as well as retinothalamic convergence opens the door to question how information about contrast is transformed at the thalamic stage. Here, we address this question using a realistic dynamic model of the retinothalamic circuit. Our results show that different components of the thalamic RF might implement filters that are analogous to two types of well-known image processing techniques to preserve the quality of a higher resolution version of the image on its way to the primary visual cortex.

The importance of constructive feedback: Implications of top-down regulation in the development of neural circuits

Neural circuits in sensory pathways develop through a general strategy of overproduction of synapses followed by activity-driven pruning to fine-tune connectivity for optimal function. The early visual pathway, consisting of the retina → visual thalamus → primary visual cortex, has served for decades as a powerful model system for probing the mechanisms and logic of this process. In addition to these feedforward projections, the early visual pathway also includes a substantial feedback component in the form of corticothalamic projections from the deepest layer of primary visual cortex. The role of this feedback in visual processing has been studied extensively in mature animals, yet historically, its role in development has received comparatively little attention. Recent technological advances allowing for selective manipulation of neural activity in development led to the uncovering of a role for feedback in guiding the refinement of the forward projection from retina to visual thalamus. Here we discuss the implications of feedback exerting influence on the development of sensory pathways. We propose several possible advantages to constructing neural circuits with top-down regulation, and discuss the potential significance of this finding for certain neurologic disorders.

Optic nerve, superior colliculus, visual thalamus, and primary visual cortex of the northern elephant seal (Mirounga angustirostris) and California sea lion (Zalophus californianus)

The northern elephant seal (Mirounga angustirostris) and California sea lion (Zalophus californianus) are members of a diverse clade of carnivorous mammals known as pinnipeds. Pinnipeds are notable for their large, ape-sized brains, yet little is known about their central nervous system. Both the northern elephant seal and California sea lion spend most of their lives at sea, but each also spends time on land to breed and give birth. These unique coastal niches may be reflected in specific evolutionary adaptations to their sensory systems. Here, we report on components of the visual pathway in these two species. We found evidence for two classes of myelinated fibers within the pinniped optic nerve, those with thick myelin sheaths (elephant seal: 9%, sea lion: 7%) and thin myelin sheaths (elephant seal: 91%, sea lion: 93%). In order to investigate the architecture of the lateral geniculate nucleus, superior colliculus, and primary visual cortex, we processed brain sections from seal and sea lion pups for Nissl substance, cytochrome oxidase, and vesicular glutamate transporters. As in other carnivores, the dorsal lateral geniculate nucleus consisted of three main layers, A, A1, and C, while each superior colliculus similarly consisted of seven distinct layers. The sea lion visual cortex is located at the posterior side of cortex between the upper and lower banks of the postlateral sulcus, while the elephant seal visual cortex extends far more anteriorly along the dorsal surface and medial wall. These results are relevant to comparative studies related to the evolution of large brains.

An evolving view of retinogeniculate transmission

The thalamocortical (TC) relay neuron of the dorsoLateral Geniculate Nucleus (dLGN) has borne its imprecise label for many decades in spite of strong evidence that its role in visual processing transcends the implied simplicity of the term "relay". The retinogeniculate synapse is the site of communication between a retinal ganglion cell and a TC neuron of the dLGN. Activation of retinal fibers in the optic tract causes reliable, rapid, and robust postsynaptic potentials that drive postsynaptics spikes in a TC neuron. Cortical and subcortical modulatory systems have been known for decades to regulate retinogeniculate transmission. The dynamic properties that the retinogeniculate synapse itself exhibits during and after developmental refinement further enrich the role of the dLGN in the transmission of the retinal signal. Here we consider the structural and functional substrates for retinogeniculate synaptic transmission and plasticity, and reflect on how the complexity of the retinogeniculate synapse imparts a novel dynamic and influential capacity to subcortical processing of visual information.

Cortical Feedback Regulates Feedforward Retinogeniculate Refinement

According to the prevailing view of neural development, sensory pathways develop sequentially in a feedforward manner, whereby each local microcircuit refines and stabilizes before directing the wiring of its downstream target. In the visual system, retinal circuits are thought to mature first and direct refinement in the thalamus, after which cortical circuits refine with experience-dependent plasticity. In contrast, we now show that feedback from cortex to thalamus critically regulates refinement of the retinogeniculate projection during a discrete window in development, beginning at postnatal day 20 in mice. Disrupting cortical activity during this window, pharmacologically or chemogenetically, increases the number of retinal ganglion cells innervating each thalamic relay neuron. These results suggest that primary sensory structures develop through the concurrent and interdependent remodeling of subcortical and cortical circuits in response to sensory experience, rather than through a simple feedforward process. Our findings also highlight an unexpected function for the corticothalamic projection.

Statistical wiring of thalamic receptive fields optimizes spatial sampling of the retinal image

It is widely assumed that mosaics of retinal ganglion cells establish the optimal representation of visual space. However, relay cells in the visual thalamus often receive convergent input from several retinal afferents and, in cat, outnumber ganglion cells. To explore how the thalamus transforms the retinal image, we built a model of the retinothalamic circuit using experimental data and simple wiring rules. The model shows how the thalamus might form a resampled map of visual space with the potential to facilitate detection of stimulus position in the presence of sensor noise. Bayesian decoding conducted with the model provides support for this scenario. Despite its benefits, however, resampling introduces image blur, thus impairing edge perception. Whole-cell recordings obtained in vivo suggest that this problem is mitigated by arrangements of excitation and inhibition within the receptive field that effectively boost contrast borders, much like strategies used in digital image processing.

Comparison of numbers of interneurons in three thalamic nuclei of normal and epileptic rats

The inhibitory sources in the thalamic nuclei are local interneurons and neurons of the thalamic reticular nucleus. Studies of models of absence epilepsy have shown that the seizures are associated with an excess of inhibitory neurotransmission in the thalamus. In the present study, we used light-microscopic gamma-aminobutyric acid (GABA) immunocytochemistry to quantify the interneurons in the lateral geniculate (LGN), ventral posteromedial (VPM), and ventral posterolateral (VPL) thalamic nuclei, and compared the values from normal Wistar rats and genetic absence epilepsy rats from Strasbourg (GAERS). We found that in both Wistar rats and GAERS, the proportion of interneurons was significantly higher in the LGN than in the VPM and VPL. In the LGN of Wistar rats, 16.4% of the neurons were interneurons and in the GAERS, the value was 15.1%. In the VPM, the proportion of interneurons was 4.2% in Wistar and 14.9% in GAERS; in the VPL the values were 3.7% for Wistar and 11.1% for the GAERS. There was no significant difference between Wistar rats and the GAERS regarding the counts of interneurons in the LGN, whereas the VPM and VPL showed significantly higher counts in GAERS. Comparison of the mean areas of both relay cells and interneuronal profiles showed no significant differences between Wistar rats and GAERS. These findings show that in the VPL and the VPM there are relatively more GABAergic interneurons in GAERS than in Wistar rats. This may represent a compensatory response of the thalamocortical circuitry to the absence seizures or may be related to the production of absence seizures.

Intrinsic properties of and thalamocortical inputs onto identified corticothalamic-VPM neurons

Corticothalamic (CT) feedback plays an important role in regulating the sensory information that the cortex receives. Within the somatosensory cortex layer VI originates the feedback to the ventral posterior medial (VPM) nucleus of the thalamus, which in turn receives sensory information from the contralateral whiskers. We examined the physiology and morphology of CT neurons in rat somatosensory cortex, focusing on the physiological characteristics of the monosynaptic inputs that they receive from the thalamus. To identify CT neurons, rhodamine microspheres were injected into VPM and allowed to retrogradely transport to the soma of CT neurons. Thalamocortical slices were prepared at least 3 days post injection. Whole-cell recordings from labeled CT cells in layer VI demonstrated that they are regular spiking neurons and exhibit little spike frequency adaption. Two anatomical classes were identified based on their apical dendrites that either terminated by layer V (compact cells) or layer IV (elaborate cells). Thalamic inputs onto identified CT-VPM neurons demonstrated paired pulse depression over a wide frequency range (2-20 Hz). Stimulus trains also resulted in significant synaptic depression above 10 Hz. Our results suggest that thalamic inputs differentially impact CT-VPM neurons in layer VI. This characteristic may allow them to differentiate a wide range of stimulation frequencies which in turn further tune the feedback signals to the thalamus.

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

BACKGROUND

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

FINDINGS

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

CONCLUSIONS

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

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

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

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

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

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

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

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

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

The functional roles of feedback projections in the visual system

Neurons in the nervous system make connections with ascending feedforward projections and descending feedback projections, as well as projections from neural structures at the identical hierarchical level. These neurons form extremely complicated neural networks and pathways. Compared with the role of the feedforward projection, much less is known concerning the functional roles of the feedback projection. Visual cortex is a good model for studying functional roles of cortical feedback projections which involve many high functions, such as attention, searching and cognition. The present review mainly focused on the functional roles of feedback projections in the visual system.

Inhibitory circuits for visual processing in thalamus

Synapses made by local interneurons dominate the intrinsic circuitry of the mammalian visual thalamus and influence all signals traveling from the eye to cortex. Here we draw on physiological and computational analyses of receptive fields in the cat's lateral geniculate nucleus to describe how inhibition helps to enhance selectivity for stimulus features in space and time and to improve the efficiency of the neural code. Further, we explore specialized synaptic attributes of relay cells and interneurons and discuss how these might be adapted to preserve the temporal precision of retinal spike trains and thereby maximize the rate of information transmitted downstream.

Contrast adaptation in cat lateral geniculate nucleus and influence of corticothalamic feedback

Contrast adaptation is a basic property of visual information processing. However, important questions about contrast adaptation in the lateral geniculate nucleus (LGN) remain. For example, it is unclear whether the different information channels have the same or distinct contrast adaptation properties and mechanisms. It has been recognized that the visual system is not a one-way ascending pathway, but also contains descending feedback projections. Although studies have explored the role of this feedback system, it is unclear whether corticothalamic feedback contributes to adaptation in the LGN. To investigate these questions, we studied contrast adaptation of LGN neurons in anesthetized and paralysed cats by measuring electrophysiological responses to visual test stimuli before and after adaptation induced by prolonged visual stimulation. After adaptation, contrast response functions were usually shifted towards higher contrasts, indicating decreased contrast gain, and the maximum response decreased. Also, contrast adaptation effects were stronger in Y-cells than in X-cells. Furthermore, adaptation effects were still observed in the LGN when the corticothalamic feedback was inactivated. Changes in the contrast gain of Y-cells were diminished in the absence of feedback, while contrast gain was largely unchanged in X-cells. Our observations confirm that contrast adaptation occurs in LGN neurons and furthermore demonstrate that Y-cells show stronger adaptation effects than X-cells. These results also provide an example of how corticothalamic feedback modulates contrast information processing distinctly in different information channels.

Do the quantitative relationships of synaptic junctions and terminals in the thalamus of genetic absence epilepsy rats from Strasbourg (GAERS) differ from those in normal control Wistar rats

Abnormal functional properties of the thalamocortical connections were reported in the absence of epilepsy. The present study compares the ratios of terminals ('RL'-round vesicles, large terminals, 'RS'-round vesicles, small terminals and 'F'-flattened vesicles) and synapse in three first-order (ventrobasal, lateral geniculate and anteroventral) and in three higher-order (posterior, lateral posterior and mediodorsal) thalamic nuclei of genetic absence epilepsy rats from Strasbourg (GAERS) with our earlier quantitative studies of normal Wistar rats to show whether quantitative differences were present in GAERS as compared to Wistar rat. Rats were perfused transcardially, the brains were removed and cut as 300 μm coronal sections. Parts of the six thalamic nuclei were removed for routine electron microscopy and GABA immunocytochemistry. Twenty photographs from each section at 20,000× magnification were taken, and the terminals were identified as RL, RS or F. (1) In normal Wistar rats (as in cats), the proportion of driver terminals (RL) and synapses is lower in higher-order than in first-order thalamic nuclei, but this difference is not present in GAERS animals. (2) The proportions of RS terminals and synapses for each thalamic nucleus showed no significant differences between GAERS and Wistar rats for any of the thalamic nuclei. (3) In GAERS, the proportion of inhibitory F terminals and synapses was significantly high in the VB and low in the LP thalamic nucleus. These abnormal ratios in the GAERS may be the cause of the spike-and-wave discharges of absence seizures or may represent a compensatory response of the thalamocortical circuitry to the absence seizures.

Thalamic interneurons and relay cells use complementary synaptic mechanisms for visual processing

Synapses made by local interneurons dominate the thalamic circuits that process signals traveling from the eye downstream. The anatomical and physiological differences between interneurons and the (relay) cells that project to cortex are vast. To explore how these differences might influence visual processing, we made intracellular recordings from both classes of cells in vivo. Macroscopically, all receptive fields were similar, made of two concentrically arranged subregions in which dark and bright stimuli elicited responses of the reverse sign. Microscopically, however, the responses of the two types of cells had opposite profiles. Excitatory stimuli drove trains of single EPSPs in relay cells but graded depolarizations in interneurons. By contrast, suppressive stimuli evoked smooth hyperpolarizations in relay cells but unitary IPSPs in interneurons. Computational analyses suggested that these complementary patterns of response help preserve information encoded within the fine timing of retinal spikes and increase the amount of information transmitted to cortex.

Dynamics of stimulus-evoked spike timing correlations in the cat lateral geniculate nucleus

Precisely synchronized neuronal activity has been commonly observed in the mammalian visual pathway. Spike timing correlations in the lateral geniculate nucleus (LGN) often take the form of phase synchronized oscillations in the high gamma frequency range. To study the relations between oscillatory activity, synchrony, and their time-dependent properties, we recorded activity from multiple single units in the cat LGN under stimulation by stationary spots of light. Autocorrelation analysis showed that approximately one third of the cells exhibited oscillatory firing with a mean frequency ∼80 Hz. Cross-correlation analysis showed that 30% of unit pairs showed significant synchronization, and 61% of these pairs consisted of synchronous oscillations. Cross-correlation analysis assumes that synchronous firing is stationary and maintained throughout the period of stimulation. We tested this assumption by applying unitary events analysis (UEA). We found that UEA was more sensitive to weak and transient synchrony than cross-correlation analysis and detected a higher incidence (49% of cell pairs) of significant synchrony (unitary events). In many unit pairs, the unitary events were optimally characterized at a bin width of 1 ms, indicating that neural synchrony has a high degree of temporal precision. We also found that approximately one half of the unit pairs showed nonstationary changes in synchrony that could not be predicted by the modulation of firing rates. Population statistics showed that the onset of synchrony between LGN cells occurred significantly later than that observed between retinal afferents and LGN cells. The synchrony detected among unit pairs recorded on separate tetrodes tended to be more transient and have a later onset than that observed between adjacent units. These findings show that stimulus-evoked synchronous activity within the LGN is often rhythmic, highly nonstationary, and modulated by endogenous processes that are not tightly correlated with firing rate.

Electrical and chemical synapses between relay neurons in developing thalamus

Gap junction-mediated electrical synapses interconnect diverse types of neurons in the mammalian brain, and they may play important roles in the synchronization and development of neural circuits. Thalamic relay neurons are the major source of input to neocortex. Electrical synapses have not been directly observed between relay neurons in either developing or adult animals. We tested for electrical synapses by recording from pairs of relay neurons in acute slices of developing ventrobasal nucleus (VBN) of the thalamus from rats and mice. Electrical synapses were common between VBN relay neurons during the first postnatal week, and then declined sharply during the second week. Electrical coupling was reduced among cells of connexin36 (Cx36) knockout mice; however, some neuron pairs remained coupled. This implies that electrical synapses between the majority of coupled VBN neurons require Cx36 but that other gap junction proteins also contribute. The anatomical distribution of a beta-galactosidase reporter indicated that Cx36 was expressed in some VBN neurons during the first postnatal week and sharply declined over the second week, consistent with our physiological results. VBN relay neurons also communicated via chemical synapses. Rare pairs of relay neurons excited one another monosynaptically. Much more commonly, spikes in one relay neuron evoked disynaptic inhibition (via the thalamic reticular nucleus) in the same or a neighbouring relay neuron. Disynaptic inhibition between VBN cells emerged as electrical coupling was decreasing, during the second postnatal week. Our results demonstrate that thalamic relay neurons communicate primarily via electrical synapses during early postnatal development, and then lose their electrical coupling as a chemical synapse-mediated inhibitory circuit matures.

Synaptic development of the mouse dorsal lateral geniculate nucleus

The dorsal lateral geniculate nucleus (dLGN) of the mouse has emerged as a model system in the study of thalamic circuit development. However, there is still a lack of information regarding how and when various types of retinal and nonretinal synapses develop. We examined the synaptic organization of the developing mouse dLGN in the common pigmented C57/BL6 strain, by recording the synaptic responses evoked by electrical stimulation of optic tract axons, and by investigating the ultrastructure of identified synapses. At early postnatal ages (<P12), optic tract evoked responses were primarily excitatory. The full complement of inhibitory responses did not emerge until after eye opening (>P14), when optic tract stimulation routinely evoked an excitatory postsynaptic potential/inhibitory postsynaptic potential (EPSP/IPSP) sequence, with the latter having both a GABA(A) and GABA(B) component. Electrophysiological and ultrastructural observations were consistent. At P7, many synapses were present, but synaptic profiles lacked the ultrastructural features characteristic of the adult dLGN, and little gamma-aminobutyric acid (GABA) could be detected by using immunocytochemical techniques. In contrast, by P14, GABA staining was robust, mature synaptic profiles of retinal and nonretinal origin were easily distinguished, and the size and proportion of synaptic contacts were similar to those of the adult. The emergence of nonretinal synapses coincides with pruning of retinogeniculate connections, and the transition of retinal activity from spontaneous to visually driven. These results indicate that the synaptic architecture of the mouse dLGN is similar to that of other higher mammals, and thus provides further support for its use as a model system for visual system development.

Time course of cortically induced fos expression in auditory thalamus and midbrain after bilateral cochlear ablation

Expression of c-fos in the medial geniculate body (MGB) and the inferior colliculus (IC) in response to bicuculline-induced corticofugal activation was examined in rats at different time points after bilateral cochlear ablation (4 h-30 days). Corticofugal activation was crucial in eliciting Fos expression in the MGB after cochlear ablation. The pars ovoidea (OV) of the medial geniculate body ventral division (MGv) showed dense Fos expression 4 h after cochlear ablation; the expression declined to very low levels at 24 h and thereafter. In turn, strong Fos expression was found in the pars lateralis (LV) of the MGv 24 h after cochlear ablation and dropped dramatically at 14 days. The dorsal division of the MGB (MGd) showed high Fos expression 7 days after cochlear ablation, which persisted for a period of time. Using multi-electrode recordings, neuronal activity of different MGB subnuclei was found to correlate well with Fos expressions. The temporal changes in cortically activated Fos expression in different MGB subnuclei after bilateral cochlear ablation indicate differential denervation hypersensitivities of these MGB neurons and likely point to differential dependence of these nuclei on both auditory ascending and corticofugal descending inputs. After bilateral cochlear ablation, significant increases in Fos-positive neurons were detected unilaterally in all IC subnuclei, ipsilateral to the bicuculline injection.

The role of feedback in visual masking and visual processing

This paper reviews the potential role of feedback in visual masking, for and against. Our analysis reveals constraints for feedback mecha- nisms that limit their potential role in visual masking, and in all other general brain functions. We propose a feedforward model of visual masking, and provide a hypothesis to explain the role of feedback in visual masking and visual processing in general. We review the anato-my and physiology of feedback mechanisms, and propose that the massive ratio of feedback versus feedforward connections in the visual system may be explained solely by the critical need for top-down attentional modulation. We discuss the merits of visual masking as a tool to discover the neural correlates of consciousness, especially as compared to other popular illusions, such as binocular rivalry. Finally, we propose a new set of neurophysiological standards needed to establish whether any given neuron or brain circuit may be the neural substrate of awareness.

Corticofugal Projection Inhibits the Auditory Thalamus Through the Thalamic Reticular Nucleus

Electrical stimulation of the auditory cortex (AC) causes both facilitatory and inhibitory effects on the medial geniculate body (MGB). The purpose of this study was to identify the corticofugal inhibitory pathway to the MGB. We assessed two potential circuits: 1) the cortico-colliculo-thalamic circuit and 2) cortico-reticulo-thalamic one. We compared intracellular responses of MGB neurons to electrical stimulation of the AC following bilateral ablation of the inferior colliculi (IC) or thalamic reticular nucleus (TRN) in anesthetized guinea pigs. Cortical stimulation with intact TRN could cause strong inhibitory effects on the MGB neurons. The corticofugal inhibition remained effective after bilateral IC ablation, but it was minimized after the TRN was lesioned with kainic acid. Synchronized TRN neuronal activity and MGB inhibitory postsynaptic potentials (IPSPs) were observed with multiple recordings. The results suggest that corticofugal inhibition traverses the corticoreticulothalamic pathway, indicating that the colliculi-geniculate inhibitory pathway is probably only for feedforward inhibition.

Synaptic organization of thalamocortical axon collaterals in the perigeniculate nucleus and dorsal lateral geniculate nucleus

We examined the synaptic targets of large non-gamma-aminobutyric acid (GABA)-ergic profiles that contain round vesicles and dark mitochondria (RLD profiles) in the perigeniculate nucleus (PGN) and the dorsal lateral geniculate nucleus (dLGN). RLD profiles can provisionally be identified as the collaterals of thalamocortical axons, because their ultrastrucure is distinct from all other previously described dLGN inputs. We also found that RLD profiles are larger than cholinergic terminals and contain the type 2 vesicular glutamate transporter. RLD profiles are distributed throughout the PGN and are concentrated within the interlaminar zones (IZs) of the dLGN, regions distinguished by dense binding of Wisteria floribunda agglutinin (WFA). To determine the synaptic targets of thalamocortical axon collaterals, we examined RLD profiles in the PGN and dLGN in tissue stained for GABA. For the PGN, we found that all RLD profiles make synaptic contacts with GABAergic PGN somata, dendrites, and spines. In the dLGN, RLD profiles primarily synapse with GABAergic dendrites that contain vesicles (F2 profiles) and non-GABAergic dendrites in glomerular arrangements that include triads. Occasional synapses on GABAergic somata and proximal dendrites were also observed in the dLGN. These results suggest that correlated dLGN activity may be enhanced via direct synaptic contacts between thalamocortical cells, whereas noncorrelated activity (such as that occurring during binocular rivalry) could be suppressed via thalamocortical collateral input to PGN cells and dLGN interneurons.

Origin and dynamics of extraclassical suppression in the lateral geniculate nucleus of the macaque monkey

In addition to the classical, center/surround receptive field of neurons in the lateral geniculate nucleus (LGN), there is an extraclassical, nonlinear surround that can strongly suppress LGN responses. This form of suppression likely plays an important role in adjusting the gain of LGN responses to visual stimuli. We performed experiments in alert and anesthetized macaque monkies to quantify extraclassical suppression in the LGN and determine the roles of feedforward and feedback pathways in the generation of LGN suppression. Results show that suppression is significantly stronger among magnocellular neurons than parvocellular neurons and that suppression arises too quickly for involvement from cortical feedback. Furthermore, the amount of suppression supplied by the retina is not significantly different from that in the LGN. These results indicate that extraclassical suppression in the macaque LGN relies on feedforward mechanisms and suggest that suppression in the cortex likely includes a component established in the retina.

The role of cortical feedback in the generation of the temporal receptive field responses of lateral geniculate nucleus neurons: a computational modelling study

The influence of cortical feedback on thalamic visual responses has been a source of much discussion in recent years. In this study we examine the possible role of cortical feedback in shaping the spatiotemporal receptive field (STRF) responses of thalamocortical (TC) cells in the lateral geniculate nucleus (LGN) of the thalamus. A population-based computational model of the thalamocortical network is used to generate a representation of the STRF response of LGN TC cells within the corticothalamic feedback circuit. The cortical feedback is shown to have little influence on the spatial response properties of the STRF organization. However, the model suggests that cortical feedback may play a key role in modifying the experimentally observed biphasic temporal response property of the STRF, that is, the reversal over time of the polarity of ON and OFF responses of the centre and surround of the receptive field, in particular accounting for the experimentally observed mismatch between retinal cells and TC cells in respect of the magnitude of the second (rebound) phase of the temporal response. The model results also show that this mismatch may result from an anti-phase corticothalamic feedback mechanism.

Parallel driving and modulatory pathways link the prefrontal cortex and thalamus

Pathways linking the thalamus and cortex mediate our daily shifts from states of attention to quiet rest, or sleep, yet little is known about their architecture in high-order neural systems associated with cognition, emotion and action. We provide novel evidence for neurochemical and synaptic specificity of two complementary circuits linking one such system, the prefrontal cortex with the ventral anterior thalamic nucleus in primates. One circuit originated from the neurochemical group of parvalbumin-positive thalamic neurons and projected focally through large terminals to the middle cortical layers, resembling ‘drivers’ in sensory pathways. Parvalbumin thalamic neurons, in turn, were innervated by small ‘modulatory’ type cortical terminals, forming asymmetric (presumed excitatory) synapses at thalamic sites enriched with the specialized metabotropic glutamate receptors. A second circuit had a complementary organization: it originated from the neurochemical group of calbindin-positive thalamic neurons and terminated through small ‘modulatory’ terminals over long distances in the superficial prefrontal layers. Calbindin thalamic neurons, in turn, were innervated by prefrontal axons through small and large terminals that formed asymmetric synapses preferentially at sites with ionotropic glutamate receptors, consistent with a driving pathway. The largely parallel thalamo-cortical pathways terminated among distinct and laminar-specific neurochemical classes of inhibitory neurons that differ markedly in inhibitory control. The balance of activation of these parallel circuits that link a high-order association cortex with the thalamus may allow shifts to different states of consciousness, in processes that are disrupted in psychiatric diseases.

Corticofugal modulation of acoustically induced Fos expression in the rat auditory pathway

To investigate the corticofugal modulation of acoustic information ascending through the auditory pathway of the rat, immunohistochemical techniques were used to study the functional expression of Fos protein in neurons. With auditory stimulation at different frequencies, Fos expression in the medial geniculate body (MGB), inferior colliculus (IC), superior olivary complex, and cochlear nucleus was examined, and the extent of Fos expression on the two sides was compared. Strikingly, we found densely Fos-labeled neurons in all divisions of the MGB after both presentation of an auditory stimulus and administration of a gamma-aminobutyric acid type A (GABA(A)) antagonist (bicuculline methobromide; BIM) to the auditory cortex. The location of Fos-labeled neurons in the ventral division (MGv) after acoustic stimulation at different frequencies was in agreement with the known tonotopic organization. That no Fos-labeled neurons were found in the MGv with acoustic stimuli alone suggests that the transmission of ascending thalamocortical information is critically governed by corticofugal modulation. The dorsal (DCIC) and external cortices (ECIC) of the IC ipsilateral to the BIM-injected cortex showed a significantly higher number of Fos-labeled neurons than the contralateral IC. However, no difference in the number of Fos-labeled neurons was found between the central nucleus of the IC on either side, indicating that direct corticofugal modulation occurs only in the ECIC and DCIC. Further investigations are needed to assess the functional implications of the morphological differences observed between the descending corticofugal projections to the thalamus and the IC.

Synapsin utilization differs among functional classes of synapses on thalamocortical cells

Several proteins in nerve terminals participate in synaptic transmission between neurons. The synapsins, which are synaptic vesicle-associated proteins, have widespread distribution in the brain and are assumed essential for sustained recruitment of vesicles during high rates of synaptic transmission. We compared the role of synapsins in two types of glutamatergic synapses on thalamocortical cells in the dorsal lateral geniculate nucleus of mice: retinogeniculate synapses, which transmit primary afferent input at high frequencies and show synaptic depression, and corticogeniculate synapses, which provide modulatory feedback at lower frequencies and show synaptic facilitation. We used electrophysiological methods to determine effects of gene knock-out of synapsin I and II on short-term synaptic plasticity in paired-pulse, pulse-train, and posttetanic potentiation paradigms. The gene inactivation changed the plasticity properties in corticogeniculate, but not in retinogeniculate, synapses. Immunostaining with antibodies against synapsins in wild-type mice demonstrated that neither synapsin I nor II occurred in retinogeniculate terminals, whereas both occurred in corticogeniculate terminals. In GABAergic terminals, only synapsin I occurred. In corticogeniculate terminals of knock-out mice, the density of synaptic vesicles was reduced because of increased terminal size rather than reduced number of vesicles and the intervesicle distance was increased compared with wild-type mice. In the retinogeniculate terminals, no significant morphometric differences occurred between knock-out and wild-type mice. Together, this indicates that synapsin I and II are not present in the retinogeniculate terminals and therefore are not essential for sustained, high-rate synaptic transmission.

Timing and specificity of feed-forward inhibition within the LGN

Local interneurons provide feed-forward inhibition from retinal ganglion cells (RGCs) to thalamocortical (TC) neurons, but questions remain regarding the timing, magnitude, and functions of this inhibition. Here, we identify two types of inhibition that are suited to play distinctive roles. We recorded excitatory and inhibitory postsynaptic currents (EPSCs/IPSCs) in TC neurons in mouse brain slices and activated individual RGC inputs. In 34% of TC neurons, we identified EPSCs and IPSCs with identical thresholds that were tightly correlated, indicating activation by the same RGC. Such "locked" IPSCs occurred 1 ms after EPSC onset. The remaining neurons had only "nonlocked" inhibition, in which EPSCs and IPSCs had different thresholds, indicating activation by different RGCs. Nonlocked inhibition may refine receptive fields within the LGN by providing surround inhibition. In contrast, dynamic-clamp recordings suggest that locked inhibition improves the precision of synaptically evoked responses in individual TC neurons by eliminating secondary spikes.

Modeling Sleep and Wakefulness in the Thalamocortical System

When the brain goes from wakefulness to sleep, cortical neurons begin to undergo slow oscillations in their membrane potential that are synchronized by thalamocortical circuits and reflected in EEG slow waves. To provide a self-consistent account of the transition from wakefulness to sleep and of the generation of sleep slow waves, we have constructed a large-scale computer model that encompasses portions of two visual areas and associated thalamic and reticular thalamic nuclei. Thousands of model neurons, incorporating several intrinsic currents, are interconnected with millions of thalamocortical, corticothalamic, and both intra- and interareal corticocortical connections. In the waking mode, the model exhibits irregular spontaneous firing and selective responses to visual stimuli. In the sleep mode, neuromodulatory changes lead to slow oscillations that closely resemble those observed in vivo and in vitro. A systematic exploration of the effects of intrinsic currents and network parameters on the initiation, maintenance, and termination of slow oscillations shows the following. 1) An increase in potassium leak conductances is sufficient to trigger the transition from wakefulness to sleep. 2) The activation of persistent sodium currents is sufficient to initiate the up-state of the slow oscillation. 3) A combination of intrinsic and synaptic currents is sufficient to maintain the up-state. 4) Depolarization-activated potassium currents and synaptic depression terminate the up-state. 5) Corticocortical connections synchronize the slow oscillation. The model is the first to integrate intrinsic neuronal properties with detailed thalamocortical anatomy and reproduce neural activity patterns in both wakefulness and sleep, thereby providing a powerful tool to investigate the role of sleep in information transmission and plasticity.

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

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