Synaptic organization of the tectorecipient zone of the rat lateral posterior nucleus

Excitatory Projections of Wide Field Collicular Neurons to the Nucleus of the Optic Tract in the Rat

The superficial layers of the mammalian superior colliculus (SC) contain neurons that are generally responsive to visual stimuli but can differ considerably in morphology and response properties. To elucidate the structure and function of these neurons, we combined extracellular recording and juxtacellular labeling, detailed anatomical reconstruction, and ultrastructural analysis of the synaptic contacts of labeled neurons, using transmission electron microscopy. Our labeled neurons project to different brainstem nuclei. Of particular importance are neurons that fit the morphological criteria of the wide field (WF) neurons and whose dendrites are horizontally oriented. They display a rather characteristic axonal projection pattern to the nucleus of optic tract (NOT); thus, we call them superior collicular WF projecting to the NOT (SCWFNOT) neurons. We corroborated the morphological characterization of this neuronal type as a distinct neuronal class with the help of unsupervised hierarchical cluster analysis. Our ultrastructural data demonstrate that SCWFNOT neurons establish excitatory connections with their targets in the NOT. Although, in rodents, the literature about the WF neurons has focused on their extensive projection to the lateral posterior nucleus of the thalamus, as a conduit for information to reach the visual association areas of the cortex, our data suggest that this subclass of WF neurons may participate in the optokinetic nystagmus.

Distinct "driving" versus "modulatory" influences of different visual corticothalamic pathways

Higher-order (HO) thalamic nuclei interact extensively and reciprocally with the cerebral cortex. These corticothalamic (CT) interactions are thought to be important for sensation and perception, attention, and many other important brain functions. CT projections to HO thalamic nuclei, such as the visual pulvinar, originate from two different excitatory populations in cortical layers 5 and 6, whereas first-order nuclei (such as the dorsolateral geniculate nucleus; dLGN) only receive layer 6 CT input. It has been proposed that these layer 5 and layer 6 CT pathways have different functional influences on the HO thalamus, but this has never been directly tested. By optogenetically inactivating different CT populations in the primary visual cortex (V1) and recording single-unit activity from V1, dLGN, and pulvinar of awake mice, we demonstrate that layer 5, but not layer 6, CT projections drive visual responses in the pulvinar, even while both pathways provide retinotopic, baseline excitation to their thalamic targets. Inactivating the superior colliculus also suppressed visual responses in the same subregion of the pulvinar, demonstrating that cortical layer 5 and subcortical inputs both contribute to HO visual thalamic activity-even at the level of putative single neurons. Altogether, these results indicate a functional division of "driver" and "modulator" CT pathways from V1 to the visual thalamus in vivo.

Ultrastructure of ipsilateral and contralateral tectopulvinar projections in the mouse

Visual pathways of the brain are organized into parallel channels that code different features of the external environment. In the current study, we investigated the anatomical organization of parallel pathways from the superior colliculus (SC) to the pulvinar nucleus in the mouse. Virus injections placed in the ipsilateral and contralateral SC to induce the expression of different fluorescent proteins define two pulvinar zones. The lateral pulvinar (Pl) receives ipsilateral SC input and the caudal medial pulvinar (Pcm) receives bilateral SC input. To examine the ultrastructure of these projections using transmission electron microscopy, we injected the SC with viruses to induce peroxidase expression within synaptic vesicles or mitochondria. We quantitatively compared the sizes of ipsilateral and contralateral tectopulvinar terminals and their postsynaptic dendrites, as well as the sizes of the overall population of synaptic terminals and their postsynaptic dendrites in the Pl and Pcm. Our ultrastructural analysis revealed that ipsilateral tectopulvinar terminals are significantly larger than contralateral tectopulvinar terminals. In particular, the ipsilateral tectopulvinar projection includes a subset of large terminals (≥ 1 μm² ) that envelop dendritic protrusions of postsynaptic dendrites. We also found that both ipsilateral and contralateral tectopulvinar terminals are significantly larger than the overall population of synaptic terminals in both the Pl and Pcm. Thus, the ipsilateral tectopulvinar projection is structurally distinct from the bilateral tectopulvinar pathway, but both tectopulvinar channels may be considered the primary or "driving" input to the Pl and Pcm.

Structural assessment of thalamus morphology in brain disorders: A review and recommendation of thalamic nucleus segmentation and shape analysis

The thalamus is a central brain structure crucially involved in cognitive, emotional, sensory, and motor functions and is often reported to be involved in the pathophysiology of neurological and psychiatric disorders. The functional subdivision of the thalamus warrants morphological investigation on the level of individual subnuclei. In addition to volumetric measures, the investigation of other morphological features may give additional insights into thalamic morphology. For instance, shape features offer a higher spatial resolution by revealing small, regional differences that are left undetected in volumetric analyses. In this review, we discuss the benefits and limitations of recent advances in neuroimaging techniques to investigate thalamic morphology in vivo, leading to our proposed methodology. This methodology consists of available pipelines for volume and shape analysis, focussing on the morphological features of volume, thickness, and surface area. We demonstrate this combined approach in a Parkinson's disease cohort to illustrate their complementarity. Considering our findings, we recommend a combined methodology as it allows for more sensitive investigation of thalamic morphology in clinical populations.

Corticothalamic Pathways From Layer 5: Emerging Roles in Computation and Pathology

Large portions of the thalamus receive strong driving input from cortical layer 5 (L5) neurons but the role of this important pathway in cortical and thalamic computations is not well understood. L5-recipient "higher-order" thalamic regions participate in cortico-thalamo-cortical (CTC) circuits that are increasingly recognized to be (1) anatomically and functionally distinct from better-studied "first-order" CTC networks, and (2) integral to cortical activity related to learning and perception. Additionally, studies are beginning to elucidate the clinical relevance of these networks, as dysfunction across these pathways have been implicated in several pathological states. In this review, we highlight recent advances in understanding L5 CTC networks across sensory modalities and brain regions, particularly studies leveraging cell-type-specific tools that allow precise experimental access to L5 CTC circuits. We aim to provide a focused and accessible summary of the anatomical, physiological, and computational properties of L5-originating CTC networks, and outline their underappreciated contribution in pathology. We particularly seek to connect single-neuron and synaptic properties to network (dys)function and emerging theories of cortical computation, and highlight information processing in L5 CTC networks as a promising focus for computational studies.

Visual Response Characteristics in Lateral and Medial Subdivisions of the Rat Pulvinar

The pulvinar is a higher-order thalamic relay and a central component of the extrageniculate visual pathway, with input from the superior colliculus and visual cortex and output to all of visual cortex. Rodent pulvinar, more commonly called the lateral posterior nucleus (LP), consists of three highly-conserved subdivisions, and offers the advantage of simplicity in its study compared to more subdivided primate pulvinar. Little is known about receptive field properties of LP, let alone whether functional differences exist between different LP subdivisions, making it difficult to understand what visual information is relayed and what kinds of computations the pulvinar might support. Here, we characterized single-cell response properties in two V1 recipient subdivisions of rat pulvinar, the rostromedial (LPrm) and lateral (LPl), and found that a fourth of the cells were selective for orientation, compared to half in V1, and that LP tuning widths were significantly broader. Response latencies were also significantly longer and preferred size more than three times larger on average than in V1; the latter suggesting pulvinar as a source of spatial context to V1. Between subdivisons, LPl cells preferred higher temporal frequencies, whereas LPrm showed a greater degree of direction selectivity and pattern motion detection. Taken together with known differences in connectivity patterns, these results suggest two separate visual feature processing channels in the pulvinar, one in LPl related to higher speed processing which likely derives from superior colliculus input, and the other in LPrm for motion processing derived through input from visual cortex. SIGNIFICANCE STATEMENT: The pulvinar has a perplexing role in visual cognition as no clear link has been found between the functional properties of its neurons and behavioral deficits that arise when it is damaged. The pulvinar, called the lateral posterior nucleus (LP) in rats, is a higher order thalamic relay with input from the superior colliculus and visual cortex and output to all of visual cortex. By characterizing single-cell response properties in anatomically distinct subdivisions we found two separate visual feature processing channels in the pulvinar, one in lateral LP related to higher speed processing which likely derives from superior colliculus input, and the other in rostromedial LP for motion processing derived through input from visual cortex.

Projections between visual cortex and pulvinar in the rat

The extrageniculate visual pathway, which carries visual information from the retina through the superficial layers of the superior colliculus and the pulvinar, is poorly understood. The pulvinar is thought to modulate information flow between cortical areas, and has been implicated in cognitive tasks like directing visually guided actions. In order to better understand the underlying circuitry, we performed retrograde injections of modified rabies virus in the visual cortex and pulvinar of the Long-Evans rat. We found a relatively small population of cells projecting to primary visual cortex (V1), compared to a much larger population projecting to higher visual cortex. Reciprocal corticothalamic projections showed a similar result, implying that pulvinar does not play as big a role in directly modulating rodent V1 activity as previously thought.

The Evolution of the Pulvinar Complex in Primates and Its Role in the Dorsal and Ventral Streams of Cortical Processing

Current evidence supports the view that the visual pulvinar of primates consists of at least five nuclei, with two large nuclei, lateral pulvinar ventrolateral (PLvl) and central lateral nucleus of the inferior pulvinar (PIcl), contributing mainly to the ventral stream of cortical processing for perception, and three smaller nuclei, posterior nucleus of the inferior pulvinar (PIp), medial nucleus of the inferior pulvinar (PIm), and central medial nucleus of the inferior pulvinar (PIcm), projecting to dorsal stream visual areas for visually directed actions. In primates, both cortical streams are highly dependent on visual information distributed from primary visual cortex (V1). This area is so vital to vision that patients with V1 lesions are considered "cortically blind". When the V1 inputs to dorsal stream area middle temporal visual area (MT) are absent, other dorsal stream areas receive visual information relayed from the superior colliculus via PIp and PIcm, thereby preserving some dorsal stream functions, a phenomenon called "blind sight". Non-primate mammals do not have a dorsal stream area MT with V1 inputs, but superior colliculus inputs to temporal cortex can be more significant and more visual functions are preserved when V1 input is disrupted. The current review will discuss how the different visual streams, especially the dorsal stream, have changed during primate evolution and we propose which features are retained from the common ancestor of primates and their close relatives.

A Differential Circuit via Retino-Colliculo-Pulvinar Pathway Enhances Feature Selectivity in Visual Cortex through Surround Suppression

In the mammalian visual system, information from the retina streams into parallel bottom-up pathways. It remains unclear how these pathways interact to contribute to contextual modulation of visual cortical processing. By optogenetic inactivation and activation of mouse lateral posterior nucleus (LP) of thalamus, a homolog of pulvinar, or its projection to primary visual cortex (V1), we found that LP contributes to surround suppression of layer (L) 2/3 responses in V1 by driving L1 inhibitory neurons. This results in subtractive suppression of visual responses and an overall enhancement of orientation, direction, spatial, and size selectivity. Neurons in V1-projecting LP regions receive bottom-up input from the superior colliculus (SC) and respond preferably to non-patterned visual noise. The noise-dependent LP activity allows V1 to "cancel" noise effects and maintain its orientation selectivity under varying noise background. Thus, the retina-SC-LP-V1 pathway forms a differential circuit with the canonical retino-geniculate pathway to achieve context-dependent sharpening of visual representations.

Differentiating Cerebellar Impact on Thalamic Nuclei

The cerebellum plays a role in coordination of movements and non-motor functions. Cerebellar nuclei (CN) axons connect to various parts of the thalamo-cortical network, but detailed information on the characteristics of cerebello-thalamic connections is lacking. Here, we assessed the cerebellar input to the ventrolateral (VL), ventromedial (VM), and centrolateral (CL) thalamus. Confocal and electron microscopy showed an increased density and size of CN axon terminals in VL compared to VM or CL. Electrophysiological recordings in vitro revealed that optogenetic CN stimulation resulted in enhanced charge transfer and action potential firing in VL neurons compared to VM or CL neurons, despite that the paired-pulse ratio was not significantly different. Together, these findings indicate that the impact of CN input onto neurons of different thalamic nuclei varies substantially, which highlights the possibility that cerebellar output differentially controls various parts of the thalamo-cortical network.

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Cerebello-thalamic axons form terminals of varying size in distinct thalamic nuclei

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Cerebello-thalamic responses vary in amplitude in distinct thalamic nuclei

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Repetitive stimuli depress cerebello-thalamic responses in all thalamic nuclei

The medial pulvinar: function, origin and association with neurodevelopmental disorders

The pulvinar is primarily referred to for its role in visual processing. However, the 'visual pulvinar' only encompasses the inferior and lateral regions of this complex thalamic nucleus. The remaining medial portion (medial pulvinar, PM) establishes distinct cortical connectivity and has been associated with directed attention, executive functions and working memory. These functions are particularly impaired in neurodevelopmental disorders, including schizophrenia and attention deficit and hyperactivity disorder (ADHD), both of which have been associated with abnormal PM architecture and connectivity. With these disorders becoming more prevalent in modern societies, we review the literature to better understand how the PM can participate in the pathophysiology of cognitive disorders and how a better understanding of the development and function of this thalamic nucleus, which is most likely exclusive to the primate brain, can advance clinical research and treatments.

Distinct cell types in the superficial superior colliculus project to the dorsal lateral geniculate and lateral posterior thalamic nuclei

The superficial layers of the superior colliculus (sSC) receive retinal input and project to thalamic regions, the dorsal lateral geniculate (dLGN) and lateral posterior (LP; or pulvinar) nuclei, that convey visual information to cortex. A critical step toward understanding the functional impact of sSC neurons on these parallel thalamo-cortical pathways is determining whether different classes of sSC neurons, which are known to respond to different features of visual stimuli, innervate overlapping or distinct thalamic targets. Here, we identified a transgenic mouse line that labels sSC neurons that project to dLGN but not LP. We utilized selective expression of fluorophores and channelrhodopsin in this and previously characterized mouse lines to demonstrate that distinct cell types give rise to sSC projections to dLGN and LP. We further show that the glutamatergic sSC cell type that projects to dLGN also provides input to the sSC cell type that projects to LP. These results clarify the cellular origin of parallel sSC-thalamo-cortical pathways and reveal an interaction between these pathways via local connections within the sSC.

NEW & NOTEWORTHY The superficial layers of the superior colliculus (sSC) project to two visual thalamic targets: the dorsal lateral geniculate (dLGN) and lateral posterior (LP) nuclei. We show that distinct excitatory sSC cell types give rise to these projections; stellate cells project to dLGN and wide-field (WF) cells project to LP. Moreover, these pathways interact via a connection within the sSC from stellate to WF cells.

Thalamocortical Projection Neuron and Interneuron Numbers in the Visual Thalamic Nuclei of the Adult C57BL/6 Mouse

A key parameter to constrain predictive, bottom-up circuit models of a given brain domain is the number and position of the neuronal populations involved. These include not only the neurons whose bodies reside within the domain, but also the neurons in distant regions that innervate the domain. The mouse visual cortex receives its main subcortical input from the dorsal lateral geniculate nucleus (dLGN) and the lateral posterior (LP) complex of the thalamus. The latter consists of three different nuclei: lateral posterior lateral (LPL), lateral posterior medial rostral (LPMR), and lateral posterior medial caudal (LPMC), each exhibiting specific patterns of connections with the various visual cortical areas. Here, we have determined the number of thalamocortical projection neurons and interneurons in the LP complex and dLGN of the adult C57BL/6 male mouse. We combined Nissl staining and histochemical and immunolabeling methods for consistently delineating nuclei borders, and applied unbiased stereological cell counting methods. Thalamic interneurons were identified using GABA immunolabeling. The C57BL/6 dLGN contains ∼21,200 neurons, while LP complex contains ∼31,000 total neurons. The dLGN and LP are the only nuclei of the mouse dorsal thalamus containing substantial numbers GABA-immunoreactive interneurons. These interneurons, however, are scarcer than previously estimated; they are 5.6% of dLGN neurons and just 1.9% of the LP neurons. It can be thus inferred that the dLGN contains ∼20,000 and the LP complex ∼30,400 thalamocortical projection neurons (∼12,000 in LPL, 15,200 in LPMR, and 4,200 in LPMC). The present dataset is relevant for constraining models of mouse visual thalamocortical circuits, as well as for quantitative comparisons between genetically modified mouse strains, or across species.

The mouse pulvinar nucleus: Organization of the tectorecipient zones

Comparative studies have greatly contributed to our understanding of the organization and function of visual pathways of the brain, including that of humans. This comparative approach is a particularly useful tactic for studying the pulvinar nucleus, an enigmatic structure which comprises the largest territory of the human thalamus. This review focuses on the regions of the mouse pulvinar that receive input from the superior colliculus, and highlights similarities of the tectorecipient pulvinar identified across species. Open questions are discussed, as well as the potential contributions of the mouse model for endeavors to elucidate the function of the pulvinar nucleus.

Atlas of the Striatum and Globus Pallidus in the Tree Shrew: Comparison with Rat and Mouse

The striatum and globus pallidus are principal nuclei of the basal ganglia. Nissl- and acetylcholinesterase-stained sections of the tree shrew brain showed the neuroanatomical features of the caudate nucleus (Cd), internal capsule (ic), putamen (Pu), accumbens, internal globus pallidus, and external globus pallidus. The ic separated the dorsal striatum into the Cd and Pu in the tree shrew, but not in rats and mice. In addition, computer-based 3D images allowed a better understanding of the position and orientation of these structures. These data provided a large-scale atlas of the striatum and globus pallidus in the coronal, sagittal, and horizontal planes, the first detailed distribution of parvalbumin-immunoreactive cells in the tree shrew, and the differences in morphological characteristics and density of parvalbumin-immunoreactive neurons between tree shrew and rat. Our findings support the tree shrew as a potential model for human striatal disorders.

Secondary visual cortex is critical to the expression of surprise-induced enhancements in cue associability in rats

Considerable evidence indicates that reinforcement prediction error, the difference between the obtained and expected reinforcer values, modulates attention to potential cues for reinforcement. The surprising delivery or omission of a reinforcer enhances the associability of the stimuli that were present when the error was induced, so that they more readily enter into new associations in the future. Previous research from our laboratory identified brain circuit elements critical to the enhancement of stimulus associability by omission of an expected event and to the subsequent expression of that altered associability in more rapid learning. A key finding was that the rat posterior parietal cortex was essential during the encoding, consolidation and retrieval of associability memories that were altered by the surprising omission of an expected event in a serial prediction task. Here, we found that the function of adjacent secondary visual cortex was critical only to the expression of altered cue associability in that same task. This specialization of function is discussed in the context of broader cortical and subcortical networks for modulation of attention in associative learning, as well as recent anatomical investigations that suggest that the rodent posterior parietal cortex overlaps with and may subsume secondary visual cortex.

Thalamic Circuit Diversity: Modulation of the Driver/Modulator Framework

The idea that dorsal thalamic inputs can be divided into “drivers”, which provide the primary excitatory drive for the relay of information to cortex, and “modulators”, which alter the gain of signal transmission, has provided a valuable organizing principle for the study of thalamic function. This view further promoted the identification of “first order” and “higher order” thalamic nuclei, based on the origin of their driving inputs. Since the introduction of this influential terminology, a number of studies have revealed the existence of a wide variety of thalamic organizational schemes. For example, some thalamic nuclei are not innervated by typical driver inputs, but instead receive input from terminals which exhibit features distinct from those of either classic drivers or modulators. In addition, many thalamic nuclei contain unique combinations of convergent first order, higher order, and/or other “driver-like” inputs that do not conform with the driver/modulator framework. The assortment of synaptic arrangements identified in the thalamus are reviewed and discussed from the perspective that this organizational diversity can dramatically increase the computational capabilities of the thalamus, reflecting its essential roles in sensory, motor, and sensory-motor circuits.

Retinal and Tectal "Driver-Like" Inputs Converge in the Shell of the Mouse Dorsal Lateral Geniculate Nucleus

The dorsal lateral geniculate nucleus (dLGN) is a model system for understanding thalamic organization and the classification of inputs as "drivers" or "modulators." Retinogeniculate terminals provide the primary excitatory drive for the relay of information to visual cortex (V1), while nonretinal inputs act in concert to modulate the gain of retinogeniculate signal transmission. How do inputs from the superior colliculus, a visuomotor structure, fit into this schema? Using a variety of anatomical, optogenetic, and in vitro physiological techniques in mice, we show that dLGN inputs from the superior colliculus (tectogeniculate) possess many of the ultrastructural and synaptic properties that define drivers. Tectogeniculate and retinogeniculate terminals converge to innervate one class of dLGN neurons within the dorsolateral shell, the primary terminal domain of direction-selective retinal ganglion cells. These dLGN neurons project to layer I of V1 to form synaptic contacts with dendrites of deeper-layer neurons. We suggest that tectogeniculate inputs act as "backseat drivers," which may alert shell neurons to movement commands generated by the superior colliculus. Significance statement: The conventional view of the dorsal lateral geniculate nucleus (dLGN) is that of a simple relay of visual information between the retina and cortex. Here we show that the dLGN receives strong excitatory input from both the retina and the superior colliculus. Thus, the dLGN is part of a specialized visual channel that provides cortex with convergent information about stimulus motion and eye movement and positioning.

Distributions of vesicular glutamate transporters 1 and 2 in the visual system of tree shrews (Tupaia belangeri)

Vesicular glutamate transporter (VGLUT) proteins regulate the storage and release of glutamate from synapses of excitatory neurons. Two isoforms, VGLUT1 and VGLUT2, are found in most glutamatergic projections across the mammalian visual system, and appear to differentially identify subsets of excitatory projections between visual structures. To expand current knowledge on the distribution of VGLUT isoforms in highly visual mammals, we examined the mRNA and protein expression patterns of VGLUT1 and VGLUT2 in the lateral geniculate nucleus (LGN), superior colliculus, pulvinar complex, and primary visual cortex (V1) in tree shrews (Tupaia belangeri), which are closely related to primates but classified as a separate order (Scandentia). We found that VGLUT1 was distributed in intrinsic and corticothalamic connections, whereas VGLUT2 was predominantly distributed in subcortical and thalamocortical connections. VGLUT1 and VGLUT2 were coexpressed in the LGN and in the pulvinar complex, as well as in restricted layers of V1, suggesting a greater heterogeneity in the range of efferent glutamatergic projections from these structures. These findings provide further evidence that VGLUT1 and VGLUT2 identify distinct populations of excitatory neurons in visual brain structures across mammals. Observed variations in individual projections may highlight the evolution of these connections through the mammalian lineage.

Thalamic neuromodulation and its implications for executive networks

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

Diverse subthreshold cross-modal sensory interactions in the thalamic reticular nucleus: implications for new pathways of cross-modal attentional gating function

Our attention to a sensory cue of a given modality interferes with attention to a sensory cue of another modality. However, an object emitting various sensory cues attracts attention more effectively. The thalamic reticular nucleus (TRN) could play a pivotal role in such cross-modal modulation of attention given that cross-modal sensory interaction takes place in the TRN, because the TRN occupies a highly strategic position to function in the control of gain and/or gating of sensory processing in the thalamocortical loop. In the present study cross-modal interactions between visual and auditory inputs were examined in single TRN cells of anesthetised rats using juxta-cellular recording and labeling techniques. Visual or auditory responses were modulated by subthreshold sound or light stimuli, respectively, in the majority of recordings (46 of 54 visual and 60 of 73 auditory cells). However, few bimodal sensory cells were found. Cells showing modulation of the sensory response were distributed in the whole visual and auditory sectors of the TRN. Modulated cells sent axonal projections to first-order or higher-order thalamic nuclei. Suppression predominated in modulation that took place not only in primary responses but also in late responses repeatedly evoked after sensory stimulation. Combined sensory stimulation also evoked de-novo responses, and modulated response latency and burst spiking. These results indicate that the TRN incorporates sensory inputs of different modalities into single cell activity to function in sensory processing in the lemniscal and non-lemniscal systems. This raises the possibility that the TRN constitutes neural pathways involved in cross-modal attentional gating.

Drivers of the primate thalamus

The activity of thalamocortical neurons is primarily determined by giant excitatory terminals, called drivers. These afferents may arise from neocortex or from subcortical centers; however, their exact distribution, segregation, or putative absence in given thalamic nuclei are unknown. To unravel the nucleus-specific composition of drivers, we mapped the entire macaque thalamus using vesicular glutamate transporters 1 and 2 to label cortical and subcortical afferents, respectively. Large thalamic territories were innervated exclusively by either giant vGLUT2- or vGLUT1-positive boutons. Codistribution of drivers with different origin was not abundant. In several thalamic regions, no giant terminals of any type could be detected at light microscopic level. Electron microscopic observation of these territories revealed either the complete absence of large multisynaptic excitatory terminals (basal ganglia-recipient nuclei) or the presence of both vGLUT1- and vGLUT2-positive terminals, which were significantly smaller than their giant counterparts (intralaminar nuclei, medial pulvinar). In the basal ganglia-recipient thalamus, giant inhibitory terminals replaced the excitatory driver inputs. The pulvinar and the mediodorsal nucleus displayed subnuclear heterogeneity in their driver assemblies. These results show that distinct thalamic territories can be under pure subcortical or cortical control; however, there is significant variability in the composition of major excitatory inputs in several thalamic regions. Because thalamic information transfer depends on the origin and complexity of the excitatory inputs, this suggests that the computations performed by individual thalamic regions display considerable variability. Finally, the map of driver distribution may help to resolve the morphological basis of human diseases involving different parts of the thalamus.

Differential expression of vesicular glutamate transporters 1 and 2 may identify distinct modes of glutamatergic transmission in the macaque visual system

Glutamate is the primary neurotransmitter utilized by the mammalian visual system for excitatory neurotransmission. The sequestration of glutamate into synaptic vesicles, and the subsequent transport of filled vesicles to the presynaptic terminal membrane, is regulated by a family of proteins known as vesicular glutamate transporters (VGLUTs). Two VGLUT proteins, VGLUT1 and VGLUT2, characterize distinct sets of glutamatergic projections between visual structures in rodents and prosimian primates, yet little is known about their distributions in the visual system of anthropoid primates. We have examined the mRNA and protein expression patterns of VGLUT1 and VGLUT2 in the visual system of macaque monkeys, an Old World anthropoid primate, in order to determine their relative distributions in the superior colliculus, lateral geniculate nucleus, pulvinar complex, V1 and V2. Distinct expression patterns for both VGLUT1 and VGLUT2 identified architectonic boundaries in all structures, as well as anatomical subdivisions of the superior colliculus, pulvinar complex, and V1. These results suggest that VGLUT1 and VGLUT2 clearly identify regions of glutamatergic input in visual structures, and may identify common architectonic features of visual areas and nuclei across the primate radiation. Additionally, we find that VGLUT1 and VGLUT2 characterize distinct subsets of glutamatergic projections in the macaque visual system; VGLUT2 predominates in driving or feedforward projections from lower order to higher order visual structures while VGLUT1 predominates in modulatory or feedback projections from higher order to lower order visual structures. The distribution of these two proteins suggests that VGLUT1 and VGLUT2 may identify class 1 and class 2 type glutamatergic projections within the primate visual system (Sherman and Guillery, 2006).

The dorsal tectal longitudinal column (TLCd): a second longitudinal column in the paramedian region of the midbrain tectum

The tectal longitudinal column (TLC) is a longitudinally oriented, long and narrow nucleus that spans the paramedian region of the midbrain tectum of a large variety of mammals (Saldaña et al. in J Neurosci 27:13108–13116, 2007). Recent analysis of the organization of this region revealed another novel nucleus located immediately dorsal, and parallel, to the TLC. Because the name “tectal longitudinal column” also seems appropriate for this novel nucleus, we suggest the TLC described in 2007 be renamed the “ventral tectal longitudinal column (TLCv)”, and the newly discovered nucleus termed the “dorsal tectal longitudinal column (TLCd)”. This work represents the first characterization of the rat TLCd. A constellation of anatomical techniques was used to demonstrate that the TLCd differs from its surrounding structures (TLCv and superior colliculus) cytoarchitecturally, myeloarchitecturally, neurochemically and hodologically. The distinct expression of vesicular amino acid transporters suggests that TLCd neurons are GABAergic. The TLCd receives major projections from various areas of the cerebral cortex (secondary visual mediomedial area, and granular and dysgranular retrosplenial cortices) and from the medial pretectal nucleus. It densely innervates the ipsilateral lateral posterior and laterodorsal nuclei of the thalamus. Thus, the TLCd is connected with vision-related neural centers. The TLCd may be unique as it constitutes the only known nucleus made of GABAergic neurons dedicated to providing massive inhibition to higher order thalamic nuclei of a specific sensory modality.

Efferent projections of C3 adrenergic neurons in the rat central nervous system

C3 neurons constitute one of three known adrenergic nuclei in the rat central nervous system (CNS). While the adrenergic C1 cell group has been extensively characterized both physiologically and anatomically, the C3 nucleus has remained relatively obscure. This study employed a lentiviral tracing technique that expresses green fluorescent protein behind a promoter selective to noradrenergic and adrenergic neurons. Microinjection of this virus into the C3 nucleus enabled the selective tracing of C3 efferents throughout the rat CNS, thus revealing the anatomical framework of C3 projections. C3 terminal fields were observed in over 40 different CNS nuclei, spanning all levels of the spinal cord, as well as various medullary, mesencephalic, hypothalamic, thalamic, and telencephalic nuclei. The highest densities of C3 axon varicosities were observed in Lamina X and the intermediolateral cell column of the thoracic spinal cord, as well as the dorsomedial medulla (both commissural and medial nuclei of the solitary tract, area postrema, and the dorsal motor nucleus of the vagus), ventrolateral periaqueductal gray, dorsal parabrachial nucleus, periventricular and rhomboid nuclei of the thalamus, and paraventricular and periventricular nuclei of the hypothalamus. In addition, moderate and sparse projections were observed in many catecholaminergic and serotonergic nuclei, as well as the area anterior and ventral to the third ventricle, Lamina X of the cervical, lumbar, and sacral spinal cord, and various hypothalamic and telencephalic nuclei. The anatomical map of C3 projections detailed in this survey hopes to lay the first steps toward developing a functional framework for this nucleus.

Distinctions in burst spiking between thalamic reticular nucleus cells projecting to the dorsal lateral geniculate and lateral posterior nuclei in the anesthetized rat

Thalamic cell activity is under a significant influence of inhibition from the thalamic reticular nucleus (TRN) that is composed of domains connected with first and higher order thalamic nuclei, which are thought to subserve transmission of sensory inputs to the cortex and cortico-thalamo-cortical transmission of cortical outputs, respectively. Provided that TRN cells have distinct activities along with their projections to first and higher order thalamic nuclei, TRN cells could shape cell activities of the two thalamic nuclei in different manners for the distinct functions. In anesthetized rats, visual response and spontaneous activity were recorded from TRN cells projecting to the dorsal lateral geniculate (first order) and lateral posterior (higher order) nuclei (TRN-DLG and TRN-LP cells), using juxta-cellular recording and labeling techniques. TRN-DLG cells had a higher propensity for burst spiking and exhibited bursts of larger numbers of spikes with shorter inter-spike intervals as compared to TRN-LP cells in both visual response and spontaneous activity. Sustained effects of visual input on burst spiking were recognized in recurrent activation of TRN-DLG but not of TRN-LP cells. Further, the features of burst spiking were related with the locations of topographically connected cell bodies and terminal fields. The difference in burst spiking contrasts with the difference between thalamic cells in the DLG and LP, which show low and high levels of burst spiking, respectively. The synergy between thalamic and TRN cell activities with their contrasting features of burst spiking may compose distinctive sensory processing and attentional gating functions of geniculate and extra-geniculate systems.

Bilateral and ipsilateral ascending tectopulvinar pathways in mammals: a study in the squirrel (Spermophilus beecheyi)

The mammalian pulvinar complex is a collection of dorsal thalamic nuclei related to several visual and integrative processes. Previous studies have shown that the superficial layers of the superior colliculus project to multiple divisions of the pulvinar complex. Although most of these works agree about the existence of an ipsilateral tectopulvinar projection arising from the stratum griseum superficialis, some others report a bilateral projection originating from this same tectal layer. We investigated the organization of the tectopulvinar projections in the Californian ground squirrel using cholera toxin B (CTb). We confirmed previous studies showing that the caudal pulvinar of the squirrel receives a massive bilateral projection originating from a specific cell population located in the superficial collicular layers (SGS3, also called the "lower SGS" or "SGSL"). We found that this projection shares striking structural similarities with the tectorotundal pathway of birds and reptiles. Morphology of the collicular cells originating this projection closely corresponds to that of the bottlebrush tectal cells described previously for chickens and squirrels. In addition, we found that the rostral pulvinar receives an exclusively ipsilateral projection from a spatially separate population of collicular cells located at the base of the stratum opticum, deeper than the cells projecting to the caudal pulvinar. These results strongly support, at a structural level, the homology of the pathway originating in the SGS3 collicular cells upon the caudal pulvinar with the tectorotundal pathway of nonmammalian amniotes and contribute to clarifying the general organization of the tectopulvinar pathways in mammals.

A reassessment of whether cortical motor neurons die following spinal cord injury

Over the past century, the question of whether the cells of origin of the corticospinal tract (CST) die following spinal cord injury (SCI) has been debated. A recent study reported an approximately 20% loss of retrogradely labeled cortical motoneurons following damage to their axons resulting from SCI at T9 (Hains et al. [2003] J. Comp. Neurol. 462:328-341). In follow-up studies, however, we failed to find any evidence of loss of CST axons in the medullary pyramid, which must occur if CST neurons die. Here, we seek to resolve the discrepancy by re-evaluating possible loss of CST neurons using the same techniques as Hains et al. (quantitative analysis of retrograde labeling and staining for cell death markers including TUNEL and Hoechst labeling of the nuclei). Following either dorsal funiculus lesions at thoracic level 9 (T9) or lateral hemisection at cervical level 5 (C5), our results reveal no evidence for a loss of retrogradely labeled neurons and no evidence for TUNEL staining of axotomized cortical motoneurons. These results indicate that CST cell bodies do not undergo retrograde cell death following SCI, and therefore targeting such cell death is not a valid therapeutic target.

Diffuse and specific tectopulvinar terminals in the tree shrew: synapses, synapsins, and synaptic potentials

The pulvinar nucleus of the tree shrew receives both topographic (specific) and nontopographic (diffuse) projections from superior colliculus (SC), which form distinct synaptic arrangements. We characterized the physiological properties of these synapses and describe two distinct types of excitatory postsynaptic potentials (EPSPs) that correlate with structural properties of the specific and diffuse terminals. Synapses formed by specific terminals were found to be significantly longer than those formed by diffuse terminals. Stimulation of these two terminal types elicited two types of EPSPs that differed in their latency and threshold amplitudes. In addition, in response to repetitive stimulation (0.5-20 Hz) one type of EPSP displayed frequency-dependent depression whereas the amplitudes of the second type of EPSP were not changed by repetitive stimulation of up to 20 Hz. To relate these features to vesicle release, we compared the synapsin content of terminals in the pulvinar nucleus and the dorsal lateral geniculate (dLGN) by combining immunohistochemical staining for synapsin I or II with staining for the type 1 or type 2 vesicular glutamate transporters (markers for corticothalamic and tectothalamic/retinogeniculate terminals, respectively). We found that retinogeniculate terminals do not contain either synapsin I or synapsin II, corticothalamic terminals in the dLGN and pulvinar contain synapsin I, but not synapsin II, whereas tectopulvinar terminals contain both synapsin I and synapsin II. Finally, both types of EPSPs showed a graded increase in amplitude with increasing stimulation intensity, suggesting convergence; this was confirmed using a combination of anterograde tract tracing and immunocytochemistry. We suggest that the convergent synaptic arrangements, as well as the unique synapsin content of tectopulvinar terminals, allow them to relay a dynamic range of visual signals from the SC.

Superior colliculus connections with visual thalamus in gray squirrels (Sciurus carolinensis): evidence for four subdivisions within the pulvinar complex

As diurnal rodents with a well-developed visual system, squirrels provide a useful comparison of visual system organization with other highly visual mammals such as tree shrews and primates. Here, we describe the projection pattern of gray squirrel superior colliculus (SC) with the large and well-differentiated pulvinar complex. Our anatomical results support the conclusion that the pulvinar complex of squirrels consists of four distinct nuclei. The caudal (C) nucleus, distinct in cytochrome oxidase (CO), acetylcholinesterase (AChE), and vesicular glutamate transporter-2 (VGluT2) preparations, received widespread projections from the ipsilateral SC, although a crude retinotopic organization was suggested. The caudal nucleus also received weaker projections from the contralateral SC. The caudal nucleus also projects back to the ipsilateral SC. Lateral (RLl) and medial (RLm) parts of the previously defined rostral lateral pulvinar (RL) were architectonically distinct, and each nucleus received its own retinotopic pattern of focused ipsilateral SC projections. The SC did not project to the rostral medial (RM) nucleus of the pulvinar. SC injections also revealed ipsilateral connections with the dorsal and ventral lateral geniculate nuclei, nuclei of the pretectum, and nucleus of the brachium of the inferior colliculus and bilateral connections with the parabigeminal nuclei. Comparisons with other rodents suggest that a variously named caudal nucleus, which relays visual inputs from the SC to temporal visual cortex, is common to all rodents and possibly most mammals. RM and RL divisions of the pulvinar complex also appear to have homologues in other rodents.

Gateways of ventral and dorsal streams in mouse visual cortex

It is widely held that the spatial processing functions underlying rodent navigation are similar to those encoding human episodic memory (Doeller et al., 2010). Spatial and nonspatial information are provided by all senses including vision. It has been suggested that visual inputs are fed to the navigational network in cortex and hippocampus through dorsal and ventral intracortical streams (Whitlock et al., 2008), but this has not been shown directly in rodents. We have used cytoarchitectonic and chemoarchitectonic markers, topographic mapping of receptive fields, and pathway tracing to determine in mouse visual cortex whether the lateromedial field (LM) and the anterolateral field (AL), which are the principal targets of primary visual cortex (V1) (Wang and Burkhalter, 2007) specialized for processing nonspatial and spatial visual information (Gao et al., 2006), are distinct areas with diverse connections. We have found that the LM/AL border coincides with a change in type 2 muscarinic acetylcholine receptor expression in layer 4 and with the representation of the lower visual field periphery. Our quantitative analyses also show that LM strongly projects to temporal cortex as well as the lateral entorhinal cortex, which has weak spatial selectivity (Hargreaves et al., 2005). In contrast, AL has stronger connections with posterior parietal cortex, motor cortex, and the spatially selective medial entorhinal cortex (Haftig et al., 2005). These results support the notion that LM and AL are architecturally, topographically, and connectionally distinct areas of extrastriate visual cortex and that they are gateways for ventral and dorsal streams.

Evolution of the amniote pallium and the origins of mammalian neocortex

Karten's neocortex hypothesis holds that many component cell populations of the sauropsid dorsal ventricular ridge (DVR) are homologous to particular cell populations in layers of auditory and visual tectofugal-recipient neocortex of mammals (i.e., temporal neocortex), as well as to some amygdaloid populations. The claustroamygdalar hypothesis, based on gene expression domains, proposes that mammalian homologues of DVR are found in the claustrum, endopiriform nuclei, and/or pallial amygdala. Because hypotheses of homology need to account for the totality of the evidence, the available data on multiple forebrain features of sauropsids and mammals are reviewed here. While some genetic data are compatible with the claustroamygdalar hypothesis, and developmental (epigenetic) data are indecisive, hodological, morphological, and topographical data favor the neocortex hypothesis and are inconsistent with the claustroamygdalar hypothesis. Detailed studies of gene signaling cascades that establish neuronal cell-type identity in DVR, tectofugal-recipient neocortex, and claustroamygdala will be needed to resolve this debate about the evolution of neocortex.

Pulvinar projections to the striatum and amygdala in the tree shrew

Visually guided movement is possible in the absence of conscious visual perception, a phenomenon referred to as “blindsight.” Similarly, fearful images can elicit emotional responses in the absence of their conscious perception. Both capabilities are thought to be mediated by pathways from the retina through the superior colliculus (SC) and pulvinar nucleus. To define potential pathways that underlie behavioral responses to unperceived visual stimuli, we examined the projections from the pulvinar nucleus to the striatum and amygdala in the tree shrew (Tupaia belangeri), a species considered to be a prototypical primate. The tree shrew brain has a large pulvinar nucleus that contains two SC-recipient subdivisions; the dorsal (Pd) and central (Pc) pulvinar both receive topographic (“specific”) projections from SC, and Pd receives an additional non-topographic (“diffuse”) projection from SC (Chomsung et al., 2008). Anterograde and retrograde tract tracing revealed that both Pd and Pc project to the caudate and putamen, and Pd, but not Pc, additionally projects to the lateral amygdala. Using immunocytochemical staining for substance P (SP) and parvalbumin (PV) to reveal the patch/matrix organization of tree shrew striatum, we found that SP-rich/PV-poor patches interlock with a PV-rich/SP-poor matrix. Confocal microscopy revealed that tracer-labeled pulvino-striatal terminals preferentially innervate the matrix. Electron microscopy revealed that the postsynaptic targets of tracer-labeled pulvino-striatal and pulvino-amygdala terminals are spines, demonstrating that the pulvinar nucleus projects to the spiny output cells of the striatum matrix and the lateral amygdala, potentially relaying: (1) topographic visual information from SC to striatum to aid in guiding precise movements, and (2) non-topographic visual information from SC to the amygdala alerting the animal to potentially dangerous visual images.

Frequency-dependent release of substance P mediates heterosynaptic potentiation of glutamatergic synaptic responses in the rat visual thalamus

To investigate the interaction between peptides and glutamatergic synapses in the dorsal thalamus, we compared the frequency-dependent plasticity of excitatory postsynaptic potentials (EPSPs) in the tectorecipient zone of rodent lateral posterior nucleus (LPN), which is densely innervated by axons that contain the neuromodulator substance P (SP). Immunocytochemistry and confocal and electron microscopy revealed that neurokinin 1 (NK1) receptors are distributed on the dendrites of LPN cells, whereas SP is contained in axons originating from the superior colliculus (SC) and is reduced following SC lesions. In vitro whole cell recordings in parasagittal slices revealed that stimulation of the SC or optic radiations (corticothalamic axons [CTXs]) evoked LPN EPSPs that increased in amplitude with increasing stimulation intensity, suggesting convergence. With 0.5- to 10-Hz stimulus trains, CTX EPSP amplitudes displayed frequency-dependent facilitation, whereas SC EPSP amplitudes were unchanged. High-frequency SC stimulation (100 Hz for 0.5 s), or bath application of SP, resulted in gradual increases in both SC and CTX EPSP amplitudes to twofold or greater above baseline within 15-20 min poststimulation/application. This enhancement correlated with increases in input resistance and both the potentiation and resistance change were abolished in the presence of the NK1 antagonist L-703,606. These results indicate that SP is released when SC-LPN neurons fire at high frequency and SP acts postsynaptically via NK1 receptors to potentiate subsequent LPN responses to both cortical and tectal inputs. We suggest that the SP-mediated potentiation of synaptic responses may serve to amplify responses to threatening objects that move across large regions of the visual field.

Synaptic organization of connections between the temporal cortex and pulvinar nucleus of the tree shrew

We examined the synaptic organization of reciprocal connections between the temporal cortex and the dorsal (Pd) and central (Pc) subdivisions of the tree shrew pulvinar nucleus, regions innervated by the medial and lateral superior colliculus, respectively. Both Pd and Pc subdivisions project topographically to 2 separate regions of the temporal cortex; small injections of anterograde tracers placed in either Pd or Pc labeled 2 foci of terminals in the temporal cortex. Pulvinocortical pathways innervated layers I-IV, with beaded axons oriented perpendicular to the cortical surface, where they synapsed with spines that did not contain gamma amino butyric acid (GABA), likely located on the apical dendrites of pyramidal cells. Projections from the temporal cortex to the Pd and Pc originate from layer VI cells, and form small terminals that contact small caliber non-GABAergic dendrites. These results suggest that cortical terminals are located distal to tectopulvinar terminals on the dendritic arbors of Pd and Pc projection cells, which subsequently contact pyramidal cells in the temporal cortex. This circuitry could provide a mechanism for the pulvinar nucleus to activate subcortical visuomotor circuits and modulate the activity of other visual cortical areas. The potential relation to primate tecto-pulvino-cortical pathways is discussed.