The pulvinar nucleus (PUL) has long been proposed as an integrative hub for sensory processing, but whether this integration occurs at the level of individual neurons or through parallel pathways remains an open question. In this study, we focused on regions of the PUL that receive input from widefield vertical (WFV) neurons of the superior colliculus to investigate synaptic properties across PUL subdivisions. Using a variety of anatomical, optogenetic, and in vitro physiological techniques in male and female mice, we show that WFV inputs to the PUL define two subregions: a caudal medial region (Pcm) that receives bilateral nontopographic WFV input and a lateral region (Pl) that receives ipsilateral topographic WFV input. Electron microscopy revealed that terminals arising from WFV neurons are similar in size across both PUL subdivisions; they are significantly larger than terminals originating from cortical Layer 6 (L6) and significantly smaller than terminals originating from cortical L5. On average, optogenetic activation of WFV terminals in the Pcm evoked responses that displayed short-term synaptic facilitation, like responses to photoactivation of L6 inputs, whereas responses in the Pl displayed short-term synaptic depression similar to responses to photoactivation of L5 inputs. Finally, employing dual-opsin optogenetics, we found a high degree of convergence of ipsilateral and contralateral WFV and L5 and L6 input on individual PUL neurons in both the Pcm and Pl. Taken together, our results reveal subregion-specific responses to WFV input and widespread integration of both cortical and subcortical inputs by individual PUL neurons.

To determine whether intrinsic circuits of the dorsal lateral geniculate nucleus (dLGN) mediate excitatory and/or inhibitory binocular interactions, in vitro dual color optogenetics was used to examine convergence of eye-specific retinal inputs to thalamocortical (relay) cells and GABAergic interneurons. Both relay cells and interneurons were found to receive direct binocular retinogeniculate input, but the incidence was higher for interneurons. Moreover, the distribution of binocular relay cells was limited to regions in and around the ipsilateral patch of retinogeniculate terminals, while binocular interneurons were distributed throughout the dLGN. Accordingly, retinogeniculate-evoked inhibitory interactions were common. Retinal inputs arising from either eye inhibited the majority of relay cells via the activation of intrinsic interneuron circuits. In addition, the majority of interneurons were found to be interconnected, receiving both monocular and binocular retinogeniculate-evoked inhibition. These results reveal that intrinsic interneurons provide the primary source of binocular interactions within the dLGN.

Across mammals, the dorsal lateral geniculate nucleus (dLGN) is innervated by the superior colliculus (SC). In this issue of Neuron, Fei et al.1 employ a colorful approach to probe this pathway and find that within the dLGN, retinal and SC inputs collaborate to code the direction of visual movement.

The transfer of retinal signals from the dorsal lateral geniculate nucleus (dLGN) to the primary visual cortex (V1) is modulated by a variety of extraretinal inputs, including extrinsic connections formed by GABAergic neurons in the pretectum (PT) and visual sector of the thalamic reticular nucleus (vTRN), as well as the intrinsic connections of GABAergic dLGN interneurons. In the current study, we determined how GABAergic PT projections to the dLGN and vTRN can influence retinogeniculate transfer using a variety of viral tracing techniques, electron microscopy, in vitro physiological recordings, and optogenetics in male and female mice. We found that the PT provides over 75% of the GABAergic and over 30% of the total synaptic input to the vTRN. Optogenetic activation of PT terminals reduced the firing frequency of vTRN neurons as well as the amplitudes of their postsynaptic responses to V1 input. In the dLGN, synaptic terminals originating from the PT targeted interneurons more frequently than thalamocortical (relay) cells, and optogenetic activation of PT input had a greater impact on interneuron firing frequency compared with relay cells. This cell type-specific impact of PT input to the dLGN resulted in the disinhibition of relay cells and an increase in the amplitude of their postsynaptic responses to retinal input. Taken together, our results indicate that GABAergic PT projections to the visual thalamus serve to boost retinogeniculate transfer via two types of disinhibition, potentially enhancing the flow of visual information to V1 following gaze shifts.

The superior colliculus (SC) is a critical hub for the generation of visually-evoked orienting and defensive behaviors. Among the SC’s myriad downstream targets is the parabigeminal nucleus (PBG), the mammalian homolog of the nucleus isthmi, which has been implicated in motion processing and the production of defensive behaviors. The inputs to the PBG are thought to arise exclusively from the SC but little is known regarding the precise synaptic relationships linking the SC to the PBG. In the current study, we use optogenetics as well as viral tracing and electron microscopy in mice to better characterize the anatomical and functional properties of the SC-PBG circuit, as well as the morphological and ultrastructural characteristics of neurons residing in the PBG. We characterized GABAergic SC-PBG projections (that do not contain parvalbumin) and glutamatergic SC-PBG projections (which include neurons that contain parvalbumin). These two terminal populations were found to converge on different morphological populations of PBG neurons and elicit opposing postsynaptic effects. Additionally, we identified a population of non-tectal GABAergic terminals in the PBG that partially arise from neurons in the surrounding tegmentum, as well as several organizing principles that divide the nucleus into anatomically distinct regions and preserve a coarse retinotopy inherited from its SC-derived inputs. These studies provide an essential first step toward understanding how PBG circuits contribute to the initiation of behavior in response to visual signals.

Cholinergic projections from the brainstem serve as important modulators of activity in visual thalamic nuclei such as the dorsal lateral geniculate nucleus (dLGN). While these projections have been studied in several mammals, a comprehensive examination of their organization in the mouse is lacking. We used the retrograde transport of viruses or cholera toxin subunit B (CTB) injected in the dLGN, immunocytochemical labeling with antibodies against choline acetyltransferase (ChAT), brain nitric oxide synthase (BNOS), and vesicular acetylcholine transporter (VAChT), ChAT-Cre mice crossed with a reporter line (Ai9), as well as brainstem virus injections in ChAT-Cre mice to examine the pattern of thalamic innervation from cholinergic neurons in the pedunculopontine tegmental nucleus (PPTg), laterodorsal tegmental nucleus (LDTg), and the parabigeminal nucleus (PBG). Retrograde tracing demonstrated that the dLGN receives input from the PPTg, LDTg, and PBG. Viral tracing in ChAT-Cre mice and retrograde tracing combined with immunocytochemistry revealed that many of these inputs originate from cholinergic neurons in the PBG and PPTg. Most notable was an extensive cholinergic projection from the PBG which innervated most of the contralateral dLGN, with an especially dense concentration in the dorsolateral shell, as well as a small region in the dorsomedial pole of the ipsilateral dLGN. The PPTg was found to provide a sparse somewhat diffuse innervation of the ipsilateral dLGN. Neurons in the PPTg co-expressed ChAT, BNOS, and VAChT, whereas PBG neurons expressed ChAT, but not BNOS or VAChT. These results highlight the presence of distinct cholinergic populations that innervate the mouse dLGN.

In the dorsal lateral geniculate nucleus (LGN) of mice that lack retinal input, a population of large terminals supplants the synaptic arrangements normally made by the missing retinogeniculate terminals. To identify potential sources of these "retinogeniculate replacement terminals," we used mutant mice (math5(-/-)) which lack retinofugal projections due to the failure of retinal ganglion cells to develop. In this line, we labeled LGN terminals that originate from the primary visual cortex (V1) or the parabigeminal nucleus (PBG), and compared their ultrastructure to retinogeniculate, V1 or PBG terminals in the dLGN of C57Blk6 (WT) mice (schematically depicted above graph). Corticogeniculate terminals labeled in WT and math5(-/-) mice were similar in size and both groups were significantly smaller than WT retinogeniculate terminals. In contrast, the PBG projection in math5(-/-) mice was extensive and there was considerable overlap in the sizes of retinogeniculate terminals in WT mice and PBG terminals in math5(-/-) mice (summarized in histogram). The data indicate that V1 is not a source of "retinogeniculate replacement terminals" and suggests that large PBG terminals expand their innervation territory to replace retinogeniculate terminals in their absence.

Detailed studies of thalamic circuits have revealed many features that are shared across nuclei. For example, glutamatergic inputs to the thalamus can be placed into three categories based on the size of the synaptic terminals they form, their synaptic arrangements, and the postsynaptic responses they elicit. Remarkably, these three categories can be identified in most sensory nuclei of the dorsal thalamus. Likewise, in most sensory thalamic nuclei, circuits that release the neurotransmitter gamma aminobutyric acid (GABA) can be placed into two general categories based on their dendritic or axonal origins. Finally, similar cholinergic circuits have been identified across thalamic nuclei. The ultimate goal of examining the shared versus diverse features of thalamic circuits is to identify fundamental modules, mechanisms, and/or conceptual frameworks, in order to decipher thalamic function.

The superior colliculus is a conserved sensorimotor structure that integrates visual and other sensory information to drive reflexive behaviors. Although the evidence for this is strong and compelling, a number of experiments reveal a role for the superior colliculus in behaviors usually associated with the cerebral cortex, such as attention and decision-making. Indeed, in addition to collicular outputs targeting brainstem regions controlling movements, the superior colliculus also has ascending projections linking it to forebrain structures including the basal ganglia and amygdala, highlighting the fact that the superior colliculus, with its vast inputs and outputs, can influence processing throughout the neuraxis. Today, modern molecular and genetic methods combined with sophisticated behavioral assessments have the potential to make significant breakthroughs in our understanding of the evolution and conservation of neuronal cell types and circuits in the superior colliculus that give rise to simple and complex behaviors.

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 μm 2 ) 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.

The thalamic reticular nucleus (TRN) is a shell-like structure comprised of GABAergic neurons that surrounds the dorsal thalamus. While playing a key role in modulating thalamocortical interactions, TRN inhibition of thalamic activity is often thought of as having an all-or-none impact. Although TRN neurons have a dynamic firing range, it remains unclear how variable rates of TRN activity gate thalamocortical transmission. To address this, we examined the ultrastructural features and functional synaptic properties of the feedback connections in the mouse thalamus between TRN and the dorsal lateral geniculate nucleus (dLGN), the principal relay of retinal signals to visual cortex. Using electron microscopy to identify TRN input to dLGN, we found that TRN terminals formed synapses with non-GABAergic postsynaptic profiles. Compared with other nonretinal terminals in dLGN, those from TRN were relatively large and tended to contact proximal regions of relay cell dendrites. To evoke TRN activity in dLGN, we adopted an optogenetic approach by expressing ChR2, or a variant (ChIEF) in TRN terminals. Both in vitro and in vivo recordings revealed that repetitive stimulation of TRN terminals led to a frequency-dependent inhibition of dLGN activity, with higher rates of stimulation resulting in increasing levels of membrane hyperpolarization and corresponding decreases in spike firing. This relationship suggests that alterations in TRN activity lead to graded changes in relay cell spike firing.NEW & NOTEWORTHY The thalamic reticular nucleus (TRN) modulates thalamocortical transmission through inhibition. In mouse, TRN terminals in the dorsal lateral geniculate nucleus (dLGN) form synapses with relay neurons but not interneurons. Stimulation of TRN terminals in dLGN leads to a frequency-dependent form of inhibition, with higher rates of stimulation leading to a greater suppression of spike firing. Thus, TRN inhibition appears more dynamic than previously recognized, having a graded rather than an all-or-none impact on thalamocortical transmission.

To begin to unravel the complexities of GABAergic circuits in the superior colliculus (SC), we utilized mouse lines that express green fluorescent protein (GFP) in cells that contain the 67 kDa isoform of glutamic acid decarboxylase (GAD67-GFP), or Cre-recombinase in cells that contain glutamic acid decarboxylase (GAD; GAD2-cre). We used Cre-dependent virus injections in GAD2-Cre mice and tracer injections in GAD67-GFP mice, as well as immunocytochemical staining for gamma amino butyric acid (GABA) and parvalbumin (PV) to characterize GABAergic cells that project to the pretectum (PT), ventral lateral geniculate nucleus (vLGN) or parabigeminal nucleus (PBG), and interneurons in the stratum griseum superficiale (SGS) that do not project outside the SC. We found that approximately 30% of SGS neurons in the mouse are GABAergic. Of these GABAergic neurons, we identified three categories of potential interneurons in the GAD67-GFP line (GABA+GFP 45%, GABA+GFP + PV 15%, and GABA+PV 10%). GABAergic cells that did not contain GFP or PV were identified as potential projection neurons (GABA only 30%). We found that GABAergic neurons that project to the PBG are primarily located in the SGS and exhibit narrow field vertical, stellate, and horizontal dendritic morphologies, while GABAergic neurons that project to the PT and vLGN are primarily located in layers ventral to the SGS. In addition, we examined GABA and GAD67-containing elements of the mouse SGS using electron microscopy to further delineate the relationship between GABAergic circuits and retinotectal input. Approximately 30% of retinotectal synaptic targets are the presynaptic dendrites of GABAergic interneurons, and GAD67-GFP interneurons are a source of these presynaptic dendrites.

This review provides a historical account of the discovery of secondary visual pathways ( from retina to the superior colliculus to the dorsal thalamus and extrastriate cortex), and Vivien Casagrande's pioneering studies of this system using the tree shrew as a model. Subsequent studies of visual pathways in the tree shrew are also reviewed, beginning with a description of the organization and central projections of the tree shrew retina. The organization and connectivity of second visual system components that include the retino-recipient superior colliculus, tecto-recipient pulvinar nucleus and its projections, and the tecto-recipient dorsal lateral geniculate nucleus and its projections are detailed. Potential functions of the second visual system are discussed in the context of this work and in the context of the behavioral studies that initially inspired the secondary visual system concept.

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.

The superior colliculus (SC) is a major site of sensorimotor integration in which sensory inputs are processed to initiate appropriate motor responses. Projections from the primary visual cortex (V1) to the SC have been shown to exert a substantial influence on visually induced behavior, including "freezing." However, it is unclear how V1 corticotectal terminals affect SC circuits to mediate these effects. To investigate this, we used anatomical and optogenetic techniques to examine the synaptic properties of V1 corticotectal terminals. Electron microscopy revealed that V1 corticotectal terminals labeled by anterograde transport primarily synapse (93%) on dendrites that do not contain gamma aminobutyric acid (GABA). This preference was confirmed using optogenetic techniques to photoactivate V1 corticotectal terminals in slices of the SC maintained in vitro. In a mouse line in which GABAergic SC interneurons express green fluorescent protein (GFP), few GFP-labeled cells (11%) responded to activation of corticotectal terminals. In contrast, 67% of non-GABAergic cells responded to activation of V1 corticotectal terminals. Biocytin-labeling of recorded neurons revealed that wide-field vertical (WFV) and non-WFV cells were activated by V1 corticotectal inputs. However, WFV cells were activated in the most uniform manner; 85% of these cells responded with excitatory postsynaptic potentials (EPSPs) that maintained stable amplitudes when activated with light trains at 1-20 Hz. In contrast, in the majority of non-WFV cells, the amplitude of evoked EPSPs varied across trials. Our results suggest that V1 corticotectal projections may initiate freezing behavior via uniform activation of the WFV cells, which project to the pulvinar nucleus.

Almost all areas of the neocortex are connected with the claustrum, a nucleus located between the neocortex and the striatum, yet the functions of corticoclaustral and claustrocortical connections remain largely obscure. As major efforts to model the neocortex are currently underway, it has become increasingly important to incorporate the corticoclaustral system into theories of cortical function. This Mini-Symposium was motivated by a series of recent studies which have sparked new hypotheses regarding the function of claustral circuits. Anatomical, ultrastructural, and functional studies indicate that the claustrum is most highly interconnected with prefrontal cortex, suggesting important roles in higher cognitive processing, and that the organization of the corticoclaustral system is distinct from the driver/modulator framework often used to describe the corticothalamic system. Recent findings supporting roles in detecting novel sensory stimuli, directing attention and setting behavioral states, were the subject of the Mini-Symposium at the 2017 Society for Neuroscience Annual Meeting.

The pulvinar nucleus is a large thalamic structure involved in the integration of visual and motor signals. The pulvinar forms extensive connections with striate and extrastriate cortical areas, but the impact of these connections on cortical circuits has not previously been directly tested. Using a variety of anatomical, optogenetic, and in vitro physiological techniques in male and female mice, we show that pulvinocortical terminals are densely distributed in the extrastriate cortex where they form synaptic connections with spines and small-diameter dendrites. Optogenetic activation of these synapses in vitro evoked large excitatory postsynaptic responses in the majority of pyramidal cells, spiny stellate cells, and interneurons within the extrastriate cortex. However, specificity in pulvinar targeting was revealed when recordings were targeted to projection neuron subtypes. The neurons most responsive to pulvinar input were those that project to the striatum and amygdala (76% responsive) or V1 (55%), whereas neurons that project to the superior colliculus were rarely responsive (6%). Because the pulvinar also projects directly to the striatum and amygdala, these results establish the pulvinar nucleus as a hub linking the visual cortex with subcortical regions involved in the initiation and control of movement. We suggest that these circuits may be particularly important for coordinating body movements and visual perception. SIGNIFICANCE STATEMENT We found that the pulvinar nucleus can strongly influence extrastriate cortical circuits and exerts a particularly strong impact on the activity of extrastriate neurons that project to the striatum and amygdala. Our results suggest that the conventional hierarchical view of visual cortical processing may not apply to the mouse visual cortex. Instead, our results establish the pulvinar nucleus as a hub linking the visual cortex with subcortical regions involved in the initiation and control of movement, and predict that the execution of visually guided movements relies on this network.

The striatum contains D1 and D2 neurons within the striosome and matrix regions. Theobjective of this study was to identify the distribution of D1 and D2 striatal neurons in tree shrews byfl uorescent immunohistochemistry using D1, D2 and substance P antibodies. Animal brain sectionswere immunolabeled with antibodies against the D1 receptor and substance P and then counterstained for nuclei with DAPI. It was revealed that 91% of D1 labeled neurons in the striatum werein close contact with substance P labeled terminals. Thus, substance P antibody was used as amarker for D1 neurons. Brain sections were incubated with antibodies against substance P andD2 receptors and then counterstained for nuclei with DAPI. The results showed that 88% of totalneurons were in cotse contact with substance P labeled terminals in the striosome and 30% were D2labeled neurons that were in contact with substance P labeld terminals. The matrix contained 98%of D2 labeled neurons, and 29% of D2 labeled neurons that were in close contact with substance Plabeled terminals. In conclusion, approximately 30 % of striatal neurons in either the striosome ormatrix expressed both D1 and D2 receptors. These co-expressing D1-D2 neurons probably senddifferent outputs from those of D1 or D2 neurons.

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.

How is the picture of the visual scene that the eye encodes represented by neural circuits in the brain? In this issue of Cell, Morgan et al. address this question by forming an ultrastructural "connectome" of the mouse's visual thalamus that depicts individual retinal afferents and every contact these form with target relay cells.