Connectional studies of the primate lateral geniculate nucleus: distribution of axons arising from the thalamic reticular nucleus of Galago crassicaudatus

Cerebellar contributions to a brainwide network for flexible behavior in mice

The cerebellum regulates nonmotor behavior, but the routes of influence are not well characterized. Here we report a necessary role for the posterior cerebellum in guiding a reversal learning task through a network of diencephalic and neocortical structures, and in flexibility of free behavior. After chemogenetic inhibition of lobule VI vermis or hemispheric crus I Purkinje cells, mice could learn a water Y-maze but were impaired in ability to reverse their initial choice. To map targets of perturbation, we imaged c-Fos activation in cleared whole brains using light-sheet microscopy. Reversal learning activated diencephalic and associative neocortical regions. Distinctive subsets of structures were altered by perturbation of lobule VI (including thalamus and habenula) and crus I (including hypothalamus and prelimbic/orbital cortex), and both perturbations influenced anterior cingulate and infralimbic cortex. To identify functional networks, we used correlated variation in c-Fos activation within each group. Lobule VI inactivation weakened within-thalamus correlations, while crus I inactivation divided neocortical activity into sensorimotor and associative subnetworks. In both groups, high-throughput automated analysis of whole-body movement revealed deficiencies in across-day behavioral habituation to an open-field environment. Taken together, these experiments reveal brainwide systems for cerebellar influence that affect multiple flexible responses.

Structural connectivity of autonomic, pain, limbic, and sensory brainstem nuclei in living humans based on 7 Tesla and 3 Tesla MRI

Autonomic, pain, limbic, and sensory processes are mainly governed by the central nervous system, with brainstem nuclei as relay centers for these crucial functions. Yet, the structural connectivity of brainstem nuclei in living humans remains understudied. These tiny structures are difficult to locate using conventional in vivo MRI, and ex vivo brainstem nuclei atlases lack precise and automatic transformability to in vivo images. To fill this gap, we mapped our recently developed probabilistic brainstem nuclei atlas developed in living humans to high‐spatial resolution (1.7 mm isotropic) and diffusion weighted imaging (DWI) at 7 Tesla in 20 healthy participants. To demonstrate clinical translatability, we also acquired 3 Tesla DWI with conventional resolution (2.5 mm isotropic) in the same participants. Results showed the structural connectome of 15 autonomic, pain, limbic, and sensory (including vestibular) brainstem nuclei/nuclei complex (superior/inferior colliculi, ventral tegmental area‐parabrachial pigmented, microcellular tegmental–parabigeminal, lateral/medial parabrachial, vestibular, superior olivary, superior/inferior medullary reticular formation, viscerosensory motor, raphe magnus/pallidus/obscurus, parvicellular reticular nucleus‐alpha part), derived from probabilistic tractography computation. Through graph measure analysis, we identified network hubs and demonstrated high intercommunity communication in these nuclei. We found good (r = .5) translational capability of the 7 Tesla connectome to clinical (i.e., 3 Tesla) datasets. Furthermore, we validated the structural connectome by building diagrams of autonomic/pain/limbic connectivity, vestibular connectivity, and their interactions, and by inspecting the presence of specific links based on human and animal literature. These findings offer a baseline for studies of these brainstem nuclei and their functions in health and disease, including autonomic dysfunction, chronic pain, psychiatric, and vestibular disorders.

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

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

Population Receptive Field Estimation Reveals New Retinotopic Maps in Human Subcortex

The human subcortex contains multiple nuclei that govern the transmission of information to and among cortical areas. In the visual domain, these nuclei are organized into retinotopic maps. Because of their small size, these maps have been difficult to precisely measure using phase-encoded functional magnetic resonance imaging, particularly in the eccentricity dimension. Using instead the population receptive field model to estimate the response properties of individual voxels, we were able to resolve two previously unreported retinotopic maps in the thalamic reticular nucleus and the substantia nigra. We measured both the polar angle and eccentricity components, receptive field size and hemodynamic response function delay, in the these nuclei and in the lateral geniculate nucleus, the superior colliculus, and the lateral and intergeniculate pulvinars. The anatomical boundaries of these nuclei were delineated using multiple averaged proton density-weighted images and were used to constrain and confirm the functional activations. Deriving the retinotopic organization of these small, subcortical nuclei is the first step in exploring their response properties and their roles in neural dynamics.

Guarding the gateway to cortex with attention in visual thalamus

The massive visual input from the eye to the brain requires selective processing of some visual information at the expense of other information, a process referred to as visual attention. Increases in the responses of visual neurons with attention have been extensively studied along the visual processing streams in monkey cerebral cortex, from primary visual areas to parietal and frontal cortex1–4. Here we show, by recording neurons in attending monkeys, that attention modulates visual signals before they even reach cortex by increasing responses of both parvocellular and magnocellular neurons in the first relay between retina and cortex, the lateral geniculate nucleus (LGN), at the same time it decreases neuronal responses in the adjacent thalamic reticular nucleus (TRN). Francis Crick5, argued for such modulation of the LGN by observing that it is inhibited by the TRN, and suggested that “if the thalamus is the gateway to the cortex, the reticular complex might be described as the guardian of the gateway”, a reciprocal relationship we now show to be more than just hypothesis. The reciprocal modulation in LGN and TRN appears only during the initial visual response, but the modulation of LGN reappears later in the response, suggesting separate early and late sources of attentional modulation in LGN.

First order connections of the visual sector of the thalamic reticular nucleus in marmoset monkeys (Callithrix jacchus)

The thalamic reticular nucleus (TRN) supplies an important inhibitory input to the dorsal thalamus. Previous studies in non-primate mammals have suggested that the visual sector of the TRN has a lateral division, which has connections with first-order (primary) sensory thalamic and cortical areas, and a medial division, which has connections with higher-order (association) thalamic and cortical areas. However, the question whether the primate TRN is segregated in the same manner is controversial. Here, we investigated the connections of the TRN in a New World primate, the marmoset (Callithrix jacchus). The topography of labeled cells and terminals was analyzed following iontophoretic injections of tracers into the primary visual cortex (V1) or the dorsal lateral geniculate nucleus (LGNd). The results show that rostroventral TRN, adjacent to the LGNd, is primarily connected with primary visual areas, while the most caudal parts of the TRN are associated with higher order visual thalamic areas. A small region of the TRN near the caudal pole of the LGNd (foveal representation) contains connections where first (lateral TRN) and higher order visual areas (medial TRN) overlap. Reciprocal connections between LGNd and TRN are topographically organized, so that a series of rostrocaudal injections within the LGNd labeled cells and terminals in the TRN in a pattern shaped like rostrocaudal overlapping "fish scales." We propose that the dorsal areas of the TRN, adjacent to the top of the LGNd, represent the lower visual field (connected with medial LGNd), and the more ventral parts of the TRN contain a map representing the upper visual field (connected with lateral LGNd).

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

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

Attentional modulation of thalamic reticular neurons

The major pathway for visual information reaching cerebral cortex is through the lateral geniculate nucleus (LGN) of the thalamus. Acting on this vital relay is another thalamic nucleus, the thalamic reticular nucleus (TRN). This nucleus receives topographically organized collaterals from both thalamus and cortex and sends similarly organized projections back to thalamus. The inputs to the TRN are excitatory, but the output back to the thalamic relay is inhibitory, providing an ideal organization for modulating visual activity during early processing. This functional architecture led Crick in 1984 to hypothesize that TRN serves to direct a searchlight of attention to different regions of the topographic map; however, despite the substantial influence of this hypothesis, the activity of TRN neurons has never been determined during an attention task. We have determined the nature of the response of visual TRN neurons in awake monkeys, and the modulation of that response as the monkeys shifted attention between visual and auditory stimuli. Visual TRN neurons had a strong (194 spikes/s) and fast (25 ms latency) transient increase of activity to spots of light falling in their receptive fields, as well as high background firing rate (45 spikes/s). When attention shifted to the spots of light, the amplitude of the transient visual response typically increased, whereas other neuronal response characteristics remained unchanged. Thus, as predicted previously, TRN activity is modified by shifts of visual attention, and these attentional changes could influence visual processing in LGN via the inhibitory connections back to the thalamus.

The turtle thalamic anterior entopeduncular nucleus shares connectional and neurochemical characteristics with the mammalian thalamic reticular nucleus

Neurochemical and key connectional characteristics of the anterior entopeduncular nucleus (Enta) of the turtle (Testudo horsfieldi) were studied by axonal tracing techniques and immunohistochemistry of parvalbumin, gamma-aminobutyric acid (GABA) and glutamic acid decarboxylase (GAD). We showed that the Enta, which is located within the dorsal peduncle of the lateral forebrain bundle (Pedd), has roughly topographically organized reciprocal connections with the dorsal thalamic visual nuclei, the nucleus rotundus (Rot) and dorsal lateral geniculate nucleus (GLd). The Enta receives projections from visual telencephalic areas, the anterior dorsal ventricular ridge and dorsolateral cortex/pallial thickening. Most Enta neurons contained GABA and parvalbumin, and some of them were retrogradely labeled when the tracer was injected into the visual dorsal thalamic nuclei. Further experiments using double immunofluorescence revealed colocalization of GAD and parvalbumin in the vast majority of Enta neurons, and many of these cells showed retrograde labeling with Fluoro-gold injected into the Rot and/or GLd. According to these data, the Enta may be considered as a structural substrate for recurrent inhibition of the visual thalamic nuclei. Based on morphological and neurochemical similarity of the turtle Enta, caiman Pedd nucleus, the superior reticular nucleus in birds, and the thalamic reticular nucleus in mammals, we suggest that these structures represent a characteristic component which is common to the thalamic organization in amniotes.

The thalamic reticular nucleus: structure, function and concept

On the basis of theoretical, anatomical, psychological and physiological considerations, Francis Crick (1984) proposed that, during selective attention, the thalamic reticular nucleus (TRN) controls the internal attentional searchlight that simultaneously highlights all the neural circuits called on by the object of attention. In other words, he submitted that during either perception, or the preparation and execution of any cognitive and/or motor task, the TRN sets all the corresponding thalamocortical (TC) circuits in motion. Over the last two decades, behavioural, electrophysiological, anatomical and neurochemical findings have been accumulating, supporting the complex nature of the TRN and raising questions about the validity of this speculative hypothesis. Indeed, our knowledge of the actual functioning of the TRN is still sprinkled with unresolved questions. Therefore, the time has come to join forces and discuss some recent cellular and network findings concerning this diencephalic GABAergic structure, which plays important roles during various states of consciousness. On the whole, the present critical survey emphasizes the TRN's complexity, and provides arguments combining anatomy, physiology and cognitive psychology.

Structure and connections of the thalamic reticular nucleus: Advancing views over half a century

The advance of knowledge of the thalamic reticular nucleus and its connections has been reviewed and Max Cowan's contributions to this knowledge and to the methods used for studying the nucleus have been summarized. Whereas 50 years ago the nucleus was seen as a diffusely organized cell group closely related to the brain stem reticular formation, it can now be seen as a complex, tightly organized entity that has a significant inhibitory, modulatory action on the thalamic relay to cortex. The nucleus is under the control, on the one hand, of topographically organized afferents from the cerebral cortex and the thalamus, and on the other of more diffuse afferents from brain stem, basal forebrain, and other regions. Whereas the second group of afferents can be expected to have global actions on thalamocortical transmission, relevant for overall attentive state, the former group will have local actions, modulating transmission through the thalamus to cortex with highly specific local effects. Since it appears that all areas of cortex and all parts of the thalamus are linked directly to the reticular nucleus, it now becomes important to define how the several pathways that pass through the thalamus relate to each other in their reticular connections.

Laminar and cellular targets of individual thalamic reticular nucleus axons in the lateral geniculate nucleus in the prosimian primate Galago

The visual sector of the thalamic reticular nucleus is the source of the primary inhibitory projection to the visual thalamic relay nucleus, the dorsal lateral geniculate nucleus. The purpose of this study was to investigate laminar and cellular targets of individual thalamic reticular nucleus axons in the highly laminated lateral geniculate nucleus of the prosimian primate Galago to better understand the nature and function of this projection. Thalamic reticular axons labeled anterogradely by means of biotinylated dextran amine were examined by using light microscopic serial reconstruction and electron microscopic analysis in combination with postembedding immunohistochemical labeling for the neurotransmitter gamma-aminobutyric acid (GABA). The synaptic targets of labeled reticular terminal profiles were primarily GABA-negative dendrites (79-84%) of thalamocortical cells, whereas up to 16% were GABA-positive dendritic shafts or F2 terminals of interneurons. Reconstructed thalamic reticular nucleus axons were narrowly aligned along a single axis perpendicular to the geniculate laminar plane, exhibiting a high degree of visuotopic precision. Individual reticular axons targeted multiple or all geniculate laminae, with little laminar selectivity in the distribution of swellings with regard to the eye of origin or to the parvocellular, koniocellular, or magnocellular type neurons contained in the separate layers of the Galago lateral geniculate nucleus. These results suggest that cells in the visual thalamic reticular nucleus influence the lateral geniculate nucleus retinotopically, with little regard to visual functional streams.

Neurophysiology of Rett syndrome

Neurophysiological studies on Rett syndrome (RTT) are reviewed, and pathophysiology of RTT is discussed. The electroencephalography (EEG), sensory evoked potentials (SEP), sleep-wake rhythm study and polysomnography (PSG) study showed age-dependent characteristics. PSG revealed the brainstem and midbrain monoaminergic systems are deranged from early developmental stage, that is serotonin and noradrenaline systems seem to be hypoactive and dopaminergic system is also hypoactive associated with receptor supersensitivity. These monoaminergic systems are known to influence the maturation of the higher neuronal systems at specific areas and at specific ages. Particularly the synaptogenesis of the cerebral cortex is modulated by region or layer specifically from an early stage of the development. The observations made in EEG and SEP studies also suggested specific subcortical and cortical involvements taking place during the development. The age-dependent appearance of characteristic clinical features of RTT, and the variation of the clinical severities, e.g. classical, variant, form fruste, etc., can also be explained by the specific features of these monoaminergic systems. Furthermore, analysis of the components of rapid eye movement sleep suggested the onset of RTT lies between 36 gestational weeks to 3-4 months postnatally. The discovery of the mutations of methyl-CpG-binding protein 2 (MECP2) gene as the causative gene of RTT is an epoch helping not only to understand the pathophysiology of RTT but also various neurodevelopmental disorders.

Synaptic targets of thalamic reticular nucleus terminals in the visual thalamus of the cat

A major inhibitory input to the dorsal thalamus arises from neurons in the thalamic reticular nucleus (TRN), which use gamma-aminobutyric acid (GABA) as a neurotransmitter. We examined the synaptic targets of TRN terminals in the visual thalamus, including the A lamina of the dorsal lateral geniculate nucleus (LGN), the medial interlaminar nucleus (MIN), the lateral posterior nucleus (LP), and the pulvinar nucleus (PUL). To identify TRN terminals, we injected biocytin into the visual sector of the TRN to label terminals by anterograde transport. We then used postembedding immunocytochemical staining for GABA to distinguish TRN terminals as biocytin-labeled GABA-positive terminals and to distinguish the postsynaptic targets of TRN terminals as GABA-negative thalamocortical cells or GABA-positive interneurons. We found that, in all nuclei, the TRN provides GABAergic input primarily to thalamocortical relay cells (93-100%). Most of this input seems targeted to peripheral dendrites outside of glomeruli. The TRN does not appear to be a significant source of GABAergic input to interneurons in the visual thalamus. We also examined the synaptic targets of the overall population of GABAergic axon terminals (F1 profiles) within these same regions of the visual thalamus and found that the TRN contacts cannot account for all F1 profiles. In addition to F1 contacts on the dendrites of thalamocortical cells, which presumably include TRN terminals, another population of F1 profiles, most likely interneuron axons, provides input to GABAergic interneuron dendrites. Our results suggest that the TRN terminals are ideally situated to modulate thalamocortical transmission by controlling the response mode of thalamocortical cells.

Comparison of the fine structure of cortical and collicular terminals in the rat medial geniculate body

Neurons throughout the rat medial geniculate body, including the dorsal and ventral divisions, display a variety of responses to auditory stimuli. To investigate possible structural determinants of this variability, measurements of axon terminal profile area and postsynaptic dendrite diameter were made on inferior colliculus and corticothalamic terminal profiles in the medial geniculate body identified by anterograde tracer labeling following injections into the inferior colliculus or cortex. Over 90% of the synapses observed were axodendritic, with few axosomatic synapses. Small (<0.5 microm(2)) and large (>1.0 microm(2)) collicular profiles were found throughout the medial geniculate, but were smaller on average in the dorsal division (0.49+/-0.49 microm(2)) than in the ventral division (0.70+/-0.64 microm(2)). Almost all corticothalamic profiles were small and ended on small-caliber dendrites (0.57+/-0.25 microm diameter) throughout the medial geniculate. A few very large (>2.0 microm(2)) corticothalamic profiles were found in the dorsal division and in the marginal zone of the medial geniculate. GABA immunostaining demonstrated the presence of GABAergic profiles arising from cells in the inferior colliculus. These profiles were compared with GABAergic profiles not labeled with anterograde tracer, which were presumed to be unlabeled inferior colliculus profiles or thalamic reticular nucleus profiles. The distributions of dendritic diameters postsynaptic to collicular, cortical and unlabeled GABAergic profiles were compared with dendritic diameters of intracellularly labeled medial geniculate neurons from rat brain slices. Our results demonstrate a corticothalamic projection to medial geniculate body that is similar to other sensory corticothalamic projections. However, the heterogeneous distributions of excitatory inferior collicular terminal sizes and postsynaptic dendritic diameters, along with the presence of a GABAergic inferior collicular projection to dendrites in the medial geniculate body, suggest a colliculogeniculate projection that is more complex than the ascending projections to other sensory thalamic nuclei. These findings may be useful in understanding some of the differences in the response characteristics of medial geniculate neurons in vivo.

The koniocellular pathway in primate vision

A neurochemically distinct population of koniocellular (K) neurons makes up a third functional channel in primate lateral geniculate nucleus. As part of a general pattern, K neurons form robust layers through the full representation of the visual hemifield. Similar in physiology and connectivity to W cells in cat lateral geniculate nucleus, K cells form three pairs of layers in macaques. The middle pair relays input from short-wavelength cones to the cytochrome-oxidase blobs of primay visual cortex (V1), the dorsal-most pair relays low-acuity visual information to layer I of V1, and the ventral-most pair appears closely tied to the function of the superior colliculus. Throughout each K layer are neurons that innervate extrastriate cortex and that are likely to sustain some visual behaviors in the absence of V1. These data show that several pathways exist from retina to V1 that are likely to process different aspects of the visual scene along lines that may remain parallel well into V1.

Neurotransmitters contained in the subcortical extraretinal inputs to the monkey lateral geniculate nucleus

The lateral geniculate nucleus (LGN) is the thalamic relay of retinal information to cortex. An extensive complement of nonretinal inputs to the LGN combine to modulate the responsiveness of relay cells to their retinal inputs, and thus control the transfer of visual information to cortex. These inputs have been studied in the most detail in the cat. The goal of the present study was to determine whether the neurotransmitters used by nonretinal afferents to the monkey LGN are similar to those identified in the cat. By combining the retrograde transport of tracers injected into the monkey LGN with immunocytochemical labeling for choline acetyl transferase, brain nitric oxide synthase, glutamic acid decarboxylase, tyrosine hydroxylase, or the histochemical nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase reaction, we determined that the organization of neurotransmitter inputs to the monkey LGN is strikingly similar to the patterns occurring in the cat. In particular, we found that the monkey LGN receives a significant cholinergic/nitrergic projection from the pedunculopontine tegmentum, gamma-aminobutyric acid (GABA)ergic projections from the thalamic reticular nucleus and pretectum, and a cholinergic projection from the parabigeminal nucleus. The major difference between the innervation of the LGN in the cat and the monkey is the absence of a noradrenergic projection to the monkey LGN. The segregation of the noradrenergic cells and cholinergic cells in the monkey brainstem also differs from the intermingled arrangement found in the cat brainstem. Our findings suggest that studies of basic mechanisms underlying the control of visual information flow through the LGN of the cat may relate directly to similar issues in primates, and ultimately, humans.

Cortical projections from the suprasylvian gyrus to the reticular thalamic nucleus in the cat

The cat's suprasylvian gyrus was injected iontophoretically with either 4% wheat germ agglutinin-horseradish peroxidase, 4% dextran-fluororuby or 4% dextran-biotin. The locations of labelled fibres, presumed terminals and cell bodies were determined with the aid of a camera lucida attachment and computer aided stereometry. Cells from the crown of the suprasylvian gyrus project to the dorsal-most portion of the rostral half of the reticular nucleus. The region or 'sector' is distinct, albeit with some overlap, from the visual sector of the reticular nucleus defined by projections from adjacent extrastriate visual cortices. The projection from the suprasylvian gyrus to the reticular nucleus has a rough topography such that the caudal areas project to the more caudal aspects of the sector and rostral areas project to the more rostral areas of the reticular nucleus. There is a large degree of overlap of rostrocaudal projections from the suprasylvian gyrus within the sector, however, the projections originating from rostral sites are situated in a more ventral location compared to the projection originating from the caudal suprasylvian gyrus. Analysis of the distribution of biotin labelled presumptive terminals did not support the notion of 'slabs' or regional variation in terminal density across the mediolateral thickness of the reticular nucleus. In addition, a number of presumptive terminals were found within the internal capsule which coincided with the position of retrogradely labelled cells in the internal capsule following thalamic injections and appears to be part of the perireticular nucleus. The results suggest that the reticular nucleus may be segregated into sectors connected with modality specific cortical areas (e.g. striate and extrastriate visual areas) and nonspecific sectors connected with polymodal (e.g. area 7) cortical regions. The reticular nucleus and its connections with the suprasylvian gyrus may form an important link in binding eye movements to sensory integrative process through visuomotor and auditory thalamic connections.

Anatomical evidence for a mechanism of lateral inhibition in the rat thalamus

The aim of this study was to determine whether or not thalamic reticular nucleus (Rt) neurons form synaptic connections with the thalamocortical (TC) neurons from which they receive synaptic contacts. Therefore, we examined, in adult rats, the relationships between single TC and Rt neurons, which had been marked simultaneously with an anterograde/retrograde tracer (biocytin or Neurobiotin), using the extracellular or juxtacellular technique. (i) From 30 successful extracellular microapplications of marker into the Rt, 22 gave retrogradely marked TC somatodendritic arbors at the fringe of or clear outside the anterogradely darkly stained Rt axon terminal fields. Following biocytin application into the thalamus, few cells were retrogradely stained in the Rt at the periphery of the anterogradely labelled axon terminal field. (ii) The juxtacellular filling of a single Rt cell was accompanied by the back-filling of a single TC neuron (n = 4 pairs), which presumably formed synaptic contacts with the former cell. The somatodendritic complex of the back-filled TC neuron was located outside the Rt cell's axonal arbor. These anatomical data provide clear evidence that Rt and thalamic neurons predominantly form between themselves open rather than closed loop connections. Because TC neurons make glutamatergic synapses onto Rt cells, which are GABAergic, and are the first elements synaptically activated by prethalamic afferents into the TC-Rt network, the present results strongly support the hypothesis that Rt neurons principally generate a mechanism of lateral inhibition in the thalamus.

A new intrathalamic pathway linking modality-related nuclei in the dorsal thalamus

Transmission of sensory information through the dorsal thalamus involves two types of modality-related nuclei, first order and higher order, between which there are thought to be no intrathalamic interactions. We now show that within the somatosensory thalamus, cells in one nucleus, the ventrobasal complex, can influence activity in another nucleus, the medial division of the posterior complex. Stimulation of ventrobasal complex cells evoked inhibitory postsynaptic currents in cells of the medial division of the posterior complex. These currents exhibited the reversal potential and pharmacology of a GABAA receptor-mediated chloride conductance, indicating that they result from the activation of a disynaptic pathway involving the GABAergic cells of the thalamic reticular nucleus. These findings provide the first direct evidence for intrathalamic interactions between dorsal thalamic nuclei.

Corticocortical communication via the thalamus: ultrastructural studies of corticothalamic projections from area 17 to the lateral posterior nucleus of the cat and inferior pulvinar nucleus of the owl monkey

Electron microscopic anterograde autoradiography has been used to analyze the morphology and postsynaptic relationships of area 17 cortical terminals in the lateral division of the lateral posterior nucleus (LPl) of the cat and medial division of the inferior pulvinar nucleus (IPm) of the owl monkey. Such terminals are thought to arise exclusively from layer 5 in the cat and primate (Lund et al. [1975] J. Comp. Neurol. 164:287-304; Abramson and Chalupa [1985] Neuroscience 15:81-95). All labeled terminals in both nuclei exhibited the morphology of ascending "lemniscal" afferents. That is, they contained round vesicles, were large, made asymmetrical synaptic and filamentous nonsynaptic contacts, and were classified as RLs. These cortical RLs also exhibited the postsynaptic relationships of lemniscal afferents. Thus, they were presynaptic to large dendrites within glial encapsulated glomeruli, where a majority was involved in complex synaptic arrangements called triads. They also were found adjacent to terminal profiles with pleomorphic vesicles but never adjacent to small terminals containing round vesicles. Our results suggest that the layer 5 projection from area 17 provides a functional "drive" for some LPl and IPm neurons. Information carried over this "re-entrant" pathway (Guillery [1995] J. Anat. 187:583-592) could be modified within the LPl and IPm by both cortical and subcortical pathways and subsequently conveyed to higher visual cortical areas, where it could be integrated with messages carried through the well-documented corticocortical pathways (Casagrande and Kaas [1994] Cerebral cortex New York: Plenum Press).

Projection and innervation patterns of individual thalamic reticular axons in the thalamus of the adult rat: a three-dimensional, graphic, and morphometric analysis

The gamma-aminobutyric acid-ergic thalamic reticular nucleus (Rt), which carries matching topographical maps of both the thalamus and cortex and in which constituent cells can synaptically communicate between each other, is the major extrinsic source of thalamic inhibitions and disinhibitions. Whether all the Rt axonal projections into the thalamus are similarly organized and have common projection and innervation patterns are questions of great interest to further our knowledge of the functioning of the Rt. The present study provides architectural and morphometric data of individual, anterogradely labeled axonal arbors that arose from distinct parts of the Rt. One hundred twenty-seven Rt neurons from all regions of Rt were marked juxtacellularly with biocytin or Neurobiotin in urethane-anesthetized adult rats. Eighteen two-dimensional and 14 three-dimensional reconstructions of single tracer-filled Rt neurons were made from serial, frontal, horizontal, or sagittal sections. Both the somatodendritic and axonal fields of tracer-filled Rt cells were mapped in three dimensions and illustrated to provide a complementary stereotaxic reference for future studies. Most marked units projected to a single nucleus of the anterior, dorsal, intralaminar, posterior, or ventral thalamus. Axons emerging from cells in distinct sectors of the Rt projected to distinct nuclei. Within a sector, neurons with separate dendritic fields innervated separate regions either in a single nucleus or into different but functionally related thalamic nuclei. Neurons with an overlap of their dendritic fields gave rise either to overlapping axonal arborizations or, more rarely, to distinct axonal arbors within two different thalamic nuclei implicated in the same function. In rare instances, an Rt axon could project within these two nuclei. Thalamic reticular axons commonly displayed a single well-circumscribed arbor containing a total of about 4,000 +/- 1,000 boutons. Every arbor was composed of a dense central core, which encompassed a thalamic volume of 5-63 x 10(6) microm3 and was made up of patches of maximal innervation density (10 +/- 4 boutons/tissue cube of 25 microm each side), surrounded by a sparse component. The metric relationships between the Rt axonal arbors and the dendrites of their target thalamocortical neurons were determined. Both the size and maximal innervation density of the axonal patches were found to fit in with the somatodendritic architecture of the target cells. The Rt axonal projections of adult rats are thus characterized by their (1) well-focused terminal field with a patchy distribution of boutons and (2) parallel organization with a certain degree of divergence. The role of the Rt-mediated thalamic inhibition and disinhibition may be to contrast significant with nonrelevant ongoing thalamocortical information.

Paying attention to the thalamic reticular nucleus

The thalamic reticular nucleus can be divided into a number of sectors, each concerned with a different function (sight, touch, hearing, movement or 'limbic' functions). Each sector is connected to more than one thalamic nucleus and to more than one cortical area, and each sector has topographically mapped connections with the thalamus and the cortex. We consider the known details of these connections and show: (1) that they are not the same for each sector; (2) that the reticular nucleus serves as a nexus, where several functionally related cortical areas and thalamic nuclei can interact, modifying thalamocortical transmission through the inhibitory connections that go from the reticular cells to thalamic relay cells; and (3) that we need much more detailed information about these highly organized connections before we can understand exactly how the thalamic reticular nucleus might be influencing thalamocortical pathways in attentional mechanisms or in other, as yet undefined, roles.

Serial electron microscopic reconstruction of axon terminals on physiologically identified thalamocortical neurons in the cat ventral lateral nucleus

The distribution of different types of terminals on different portions of single thalamocortical neurons (TCNs) was quantitatively investigated in the cat ventral lateral nucleus (VL) by the application of computer-assisted three-dimensional reconstruction from serial ultrathin sections. Single neurons in the VL were intracellularly penetrated with a glass micropipette filled with horseradish peroxidase (HRP), and were electrophysiologically identified as TCNs by their antidromic responses to stimulation of the motor cortex. These TCNs received monosynaptic excitation from the contralateral cerebellum. After electrophysiological identification, they were injected with HRP iontophoretically. The spatial distribution of terminals of different types on two identified TCNs was analyzed on serial ultrathin sections, some of which were stained by a postembedding immunogold technique by using a gamma-aminobutyric acid (GABA) antibody. Terminals that synapsed on the injected cells were categorized as LR terminals (GABA-negative large axon terminals containing round vesicles), SR terminals (GABA-negative small axon terminals containing round vesicles), P terminals (GABA-positive axon terminals of various sizes containing pleomorphic vesicles), or PSDs (presynaptic dendrites). The order of dendritic branches of labeled TCNs was determined by computer-assisted reconstruction from serial sections. LR terminals made contacts mainly with proximal dendrites of TCNs. SR terminals made contacts predominantly with distal dendrites, and were never found on somata or primary dendrites. P terminals were observed on somata and on every portion of the dendritic trees. Synapses formed by PSDs were concentrated on the proximal dendrites and sometimes formed synaptic triads with LR terminals. Only a few terminals were found on somata, all of which were P type. Therefore, terminals belonging to different classes were not uniformly distributed on the somata and dendrites of single TCNs. These results suggest that terminals originating from different sources may preferentially contact specific regions of TCNs in the VL, and their topographical locations reflect the electrophysiological response properties of the TCNs.

Cortical somatosensory and trigeminal inputs to the cat superior colliculus: light and electron microscopic analyses

Two different axonal transport tracers were used in single animals to test the hypothesis that the expansive intermediate gray layer of the cat superior colliculus (stratum griseum intermediale, SGI) is composed of sensorimotor domains. The results show that two sensory pathways, the trigeminotectal and the corticotectal arising from the fourth somatosensory area, commingle in patches across the middle tier of the SGI. Furthermore, the data reveal that tectospinal cells are distributed within these patches. Taken together, these results show a commingling of functionally related afferents and a consistent spatial relationship between these afferents and tectospinal neurons. These relationships indicate that the SGI consists of domains that can be distinguished by their unique combinations of afferent and efferent connections. The ultrastructural characteristics and synaptic relationships of these somatosensory afferent pathways suggest that they have distinct roles within the sensorimotor domain of the SGI. The trigeminotectal terminals are relatively small, contain round vesicles and make asymmetrical synapses on small, presumably distal, dendrites. We submit that these trigeminal terminals bestow the basic receptive field properties upon SGI neurons. In contrast, the somatosensory corticotectal terminals are relatively large, contain round vesicles, make asymmetrical synapses, participate in triads, and are presynaptic to proximal dendrites. We suggest that these cortical terminals bestow integrative abilities on SGI neurons.

Quantitative analyses of synaptic contacts of interneurons in the dorsal lateral geniculate nucleus of the squirrel monkey

Three interneurons were recorded from and then injected with horseradish peroxidase in the parvocellular laminae of the squirrel monkey's (Saimiri sciureus) dorsal lateral geniculate nucleus. They were then examined using the electron microscope for their synaptic contacts, both the afferent contacts onto their dendrites and their presynaptic dendritic contacts onto presumptive projection (relay) neuron dendrites. The somata of these interneurons were small (mean = 178 microns 2), but the dendritic trees were large compared with those of projection neurons. All three interneurons had similar synaptic patterns onto their dendrites with about equal numbers of retinal, cortical, and GABAergic contacts. The distribution of these contacts was more uniform compared with the same types of contacts made onto projection neurons. The presynaptic dendrites were observed to contact only the dendrites of presumptive projection neurons, and these contacts were nearly all in the form of geniculate triads. None of the three interneurons displayed an axon. The receptive fields of these interneurons were similar to those of projection cells, but were larger and had center-response signs that were the opposite of the projection neurons around them (e.g. OFF center for the dorsal part of the parvocellular mass where ON-center projection neurons reside). The squirrel monkey data provides additional evidence that one aspect of the laminar pattern observed in the parvocellular pathway of the primate's dLGN might be related to a segregation of projection neurons of one center-response sign with interneurons of the opposite center-response sign.

Organization in the somatosensory sector of the cat's thalamic reticular nucleus

This study describes the organization of cells in the thalamic reticular nucleus (TRN) that project to the somatosensory part of the dorsal thalamus in the cat. Injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) and fluorescent dyes were made into the ventrobasal complex (VB) and the medial division of the posterior complex (POm) of the thalamus. The resultant retrograde labelling in TRN was analyzed. Large injections of a tracer in VB label many reticular cells that are restricted to a centroventral, or somatosensory, sector of TRN. Small injections of a tracer in VB produce narrow zones of labelled cells in this sector. In reconstructions these zones resemble thin "slabs," which lie parallel to the plane of TRN along its oblique rostrocaudal dimension and occupy only a fraction of its thickness. In comparisons of the zones of labelled cells in TRN resulting from tracer injections in different nuclei of VB, inner cells, intermediate cells, and outer cells across the thickness of TRN project to the ventral posteromedial, the medial division of the ventral posterolateral, and the lateral division of the ventral posterolateral nuclei, respectively. Furthermore, shifts in injected areas along the dorsoventral dimension of VB produce similar shifts in zones of labelled cells in TRN. Thus, reticular cells form an accurate map on the basis of their connections with VB. Large injections of a tracer in the ventral subdivision of POm label many reticular cells that are also restricted to the centroventral sector of TRN. Small injections of a tracer in ventral POm produce broad zones of labelled cells in this sector. In comparisons of the zones of labelled cells in TRN resulting from tracer injections in different regions of ventral POm, cells that project to these regions are scattered across the thickness of TRN and occupy overlapping territories. Large injections of a tracer in either VB or ventral POm also label cells in a restricted centroventral region of the perireticular nucleus. Double injections of different tracers in VB and ventral POm produce many cells in TRN that are labelled from both of these dorsal thalamic structures and fewer cells that are labelled from only one or the other of these structures. These results indicate that there is a dual organization in the projections of cells in the somatosensory sector of TRN to dorsal thalamus: Projections to VB are topographically organized whereas those to ventral POm lack a topographical organization. Furthermore, both of these mapped and nonmapped projections can arise from single reticular cells in the somatosensory sector.

Organization of the visual reticular thalamic nucleus of the rat

The visual sector of the reticular thalamic nucleus has come under some intense scrutiny over recent years, principally because of the key role that the nucleus plays in the processing of visual information. Despite this scrutiny, we know very little of how the connections between the reticular nucleus and the different areas of visual cortex and the different visual dorsal thalamic nuclei are organized. This study examines the patterns of reticular connections with the visual cortex and the dorsal thalamus in the rat, a species where the visual pathways have been well documented. Biotinylated dextran, an anterograde and retrograde tracer, was injected into different visual cortical areas [17; rostral 18a: presumed area AL: (anterolateral); caudal 18a: presumed area LM (lateromedial); rostral 18b: presumed area AM (anteromedial); caudal 18b: presumed area PM (posteromedial)] and into different visual dorsal thalamic nuclei (posterior thalamic, lateral geniculate nuclei), and the patterns of anterograde and retrograde labelling in the reticular nucleus were examined. From the cortical injections, we find that the visual sector of the reticular nucleus is divided into subsectors that each receive an input from a distinct visual cortical area, with little or no overlap. Further, the resulting pattern of cortical terminations in the reticular nucleus reflects largely the patterns of termination in the dorsal thalamus. That is, each cortical area projects to a largely distinct subsector of the reticular nucleus, as it does to a largely distinct dorsal thalamic nucleus. As with each of the visual cortical areas, each of the visual dorsal thalamic (lateral geniculate, lateral posterior, posterior thalamic) nuclei relate to a separate territory of the reticular nucleus, with little or no overlap. Each of these dorsal thalamic territories within the reticular nucleus receives inputs from one or more of the visual cortical areas. For instance, the region to the reticular nucleus that is labelled after an injection into the lateral geniculate nucleus encompasses the reticular regions which receive afferents from cortical areas 17, rostral 18b and caudal 18b. These results suggest that individual cortical areas may influence the activity of different dorsal thalamic nuclei through their reticular connections.

Sources of subcortical afferents to the macaque's dorsal lateral geniculate nucleus

BACKGROUND

The dorsal lateral geniculate nucleus (dLGN) is the thalamic region responsible for transmitting retina signals to cortex. Brainstem pathways to this nucleus have been described in several species and are believed to control the retinocortical pathway depending on the state of the animal (awake, asleep, drowsy, etc.). The purpose of this study was to determine all of the subcortical sources of afferents to the dLGN in a higher primate, the macaque monkey, whose visual system is similar to that of humans.

METHODS

Injections of horseradish peroxidase (HRP), with or without conjugation to wheat germ agglutinin, were made into the dLGNs of seven macaque monkeys, followed by perfusion, brain sectioning, and analyses of neurons in the brainstem, thalamus, and hypothalamus that contained the retrogradely transported marker.

RESULTS

The reticular nucleus of the thalamus, pedunculopontine nucleus, parabigeminal nucleus, pretectal nucleus of the optic tract, superior colliculus, dorsal raphe nucleus, and tuberomammillary region of the hypothalamus contained many retrogradely labeled neurons ipsilateral to the injections. In the contralateral brainstem, HRP-labeled cells were found only in the pedunculopontine nucleus, nucleus of the optic tract, and dorsal raphe nucleus. The number of labeled neurons on the contralateral side was about one-half of that in corresponding ipsilateral nuclei. The locus coeruleus contained no labeled neurons in four of the macaques that had injections limited to the dLGN.

CONCLUSION

There are seven subcortical regions that send afferents to the dLGNs of macaque monkeys. Except for the locus coeruleus, these are the same as observed for other species, such as the cat and rat, and indicate the possible sources of subcortical control over the dLGNs of humans.

Organization of connections between the thalamic reticular and the anterior thalamic nuclei in the rat

The thalamic reticular nucleus (TRN) receives topographically organized input from specific sensory nuclei such as the lateral geniculate nucleus. The present study shows this in the rat. However, the pattern of thalamic connections to the limbic reticular sector is unknown. Injecting biocytin into the ventral parts of anteroventral and anteromedial nuclei labeled neurons and axons in the rostral TRN. Filled axon collaterals and their terminals occupied a rectangular sheet in a plane close to the horizontal, and were confined to the inner zone (the medial portion) of the limbic TRN. Retrogradely filled cells were in the middle of the rostral pole in the same horizontal plane, receiving synapses from surrounding labeled boutons. In electron micrographs, thalamic terminals were found to contain round, densely packed synaptic vesicles and formed asymmetrical synapses onto reticular somata and dendritic profiles. Displacing the injection site along the dorso-ventral and rostro-caudal axis in the anterior nuclei produced corresponding shifts of antero- and retrograde labeling within the inner reticular zone. Projections from the dorsal portions of the anterior nuclei did not follow this pattern. Axons from the anterodorsal nucleus occupied the rostralmost tip of both inner and outer zones of the dorsal limbic sector. In accordance with earlier reports, the limbic sector was found to represent several dorsal thalamic nuclei parallel to each other medio-laterally. A topography is described for the limbic reticulo-thalamic connections, suggesting that the rostral TRN is able to influence circumscribed areas of the limbic thalamus.

Corticothalamic projections from the primary visual cortex in rats: a single fiber study using biocytin as an anterograde tracer

This study investigates the pattern of axonal projections of single corticothalamic neurons from the rat primary visual cortex. Microiontophoretic injections of biocytin were made in cortical laminae V and VI to label small pools of corticothalamic cells and their intrathalamic axonal projections. After a survival period of 48 h, the animals were perfused and the tissue was processed for biocytin histochemistry. On the basis of the intrathalamic distribution of axonal fields and the types of terminations found in the thalamus, three types of corticothalamic projections were identified. (1) Cells of the upper part of lamina VI projected to the dorsal lateral geniculate nucleus where they arborized in rostrocaudally oriented bands or "rods" parallel to the lines of projection of retinal afferents. (2) Cells of the lower part of lamina VI projected to the lateral part of the lateral posterior nucleus and they also sent collaterals to the dorsal lateral geniculate nucleus where they participated in the formation of rods. (3) The corticothalamic projection of lamina V cells originated from collaterals of corticofugal cells whose main axons reached the tectum and/or the pontine nuclei. These collaterals never terminated within the dorsal lateral geniculate nucleus; they arborized in the lateral posterior, lateral dorsal and ventral lateral geniculate nuclei. All corticothalamic cells from lamina VI displayed the same type of axonal network made of long branches decorated by terminal boutons emitted "en passant" at the tip of fine stalks. Corticothalamic fibers arising from lamina V, however, generated varicose endings in restricted regions of their target nuclei. All corticothalamic axons derived from lamina VI cells, but not those derived from lamina V cells, gave off collaterals as they traversed the thalamic reticular complex. These results demonstrate that corticothalamic fibers arising from the rat primary visual cortex display a lamina-dependent projection pattern. In the light of previous studies on the topographical organization of corticothalamic projections, it is proposed that a similar organizational plan characterizes corticothalamic relationships in other sensory systems in the rat and in other species.

Nucleus reticularis thalami connections with the mediodorsal thalamic nucleus: a light and electron microscopic study in the monkey

Wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) and biotinylated dextran amine (BDA) were used as tracers to study nucleus reticularis (NRT) connections with the mediodorsal nucleus (MD). Injections of WGA-HRP in the MD resulted in retrograde labeling of cells in the anteromedial segment of the NRT, the so-called rostral NRT pole. Injections of WGA-HRP and BDA in this NRT region resulted in dense anterograde labeling in the MD. Labeled NRT fibers gave off several collaterals to different MD regions ending with terminal plexuses of thin varicose fibers. In the neuropil, the varicosities were distributed at random, and no tendency to form pericellular baskets was noted. Postembedding immunocytochemistry for GABA was performed on the tissue containing anterograde WGA-HRP label for identification of NRT boutons under electron microscope. The double-labeled boutons were of small to medium size, contained a large number of pleomorphic vesicles, few mitochondria, and formed multiple symmetric synaptic contacts. The number of contacts established by one bouton ranged from 1 to 4 with an average of 1.8 per bouton. About 60% of these boutons made synapses on distal dendrites of GABAergic local circuit neurons; 33% of synaptic contacts were on distal dendrites of thalamocortical neurons, and the rest on their proximal dendrites and soma. NRT boutons were also found in serial synapses and triads. The results demonstrate that the NRT input to the MD is organized so that a single fiber innervates; different MD regions and its terminals form numerous synaptic contacts mostly on the distal dendrites of a large number of local circuit neurons and projection neurons.

Development of the pathway from the reticular and perireticular nuclei to the thalamus in ferrets: a Dil study

This study examines the connections of the thalamic reticular and perireticular cell groups in developing ferrets. Small crystals of Dil (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) were implanted into either the dorsal thalamus or the cerebral cortex of aldehyde-fixed prenatal and postnatal ferret brains. A small implant of Dil into the presumptive lateral geniculate nucleus during early prenatal development [between embryonic day 23 (E23) and E25] reveals many retrogradely labelled cells in the reticular nucleus. At E40, just before birth, the number of cells retrogradely labelled in the reticular nucleus has become reduced compared to earlier prenatal implants, whether from small or large implants of Dil into the lateral geniculate nucleus. By postnatal day 7, an adult-like pattern of retrograde labelling is seen in the reticular nucleus; at this age, a small implant of Dil limited to the lateral geniculate nucleus retrogradely labels a discrete group of cells located in the caudal regions of the reticular nucleus. In the internal capsule, adjacent to the reticular nucleus, there are two distinct groups of neurons. One group, called the large-celled perireticular zone (LPR), enters the internal capsule very early in development (from E25; Mitrofanis, J., Eur. J. Neurosci., 6, 253-263, 1994) and is not labelled from the lateral geniculate nucleus at any developmental stage. Small implants of Dil into presumptive visual and somatosensory cortices shows that the LPR lies in a distinct region of the primordial internal capsule. Corticothalamic and thalamocortical axons turn sharply in the region of the LPR, whilst corticospinal and corticobulbar axons pass straight through the LPR on towards their more caudal targets. Later, after both sets of axons have reached their targets, the LPR is not seen in the internal capsule. The other group of cells in the internal capsule, called the small-celled perireticular zone (SPR), forms a distinct band of cells lying midway between the reticular nucleus and the globus pallidus. These cells enter the internal capsule much later in development, at about E40. Unlike the cells in the LPR, cells in the SPR are retrogradely labelled after an implant of Dil into the lateral geniculate nucleus, and there are many which remain in the adult (Clemence, A. E. and Mitrofanis, J., J. Comp. Neurol., 322, 167-181, 1992).

GABAergic projection from the basal forebrain to the visual sector of the thalamic reticular nucleus in the cat

We examined the projection from the basal forebrain to thalamic and cortical regions of the visual system in cats, with particular reference to the visual sector of the thalamic reticular nucleus, the lateral geniculate nucleus, and the striate cortex. First, we made injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the visual sector of the thalamic reticular nucleus and found cells labeled by retrograde transport in the lateral nucleus basalis magnocellularis. Injection of biocytin into the basal forebrain resulted in the anterograde labeling of a dense band of fibers and terminals within the entire thalamic reticular nucleus; this labeling extended through the visual sector including the perigeniculate nucleus. No orthograde labeling was found in the lateral geniculate nucleus. Next, we addressed the issue of putative neurotransmitters used by this pathway using a variety of immunocytochemical and histochemical markers. In this fashion, we identified two populations of cells in the nucleus basalis magnocellularis of the cat; large cholinergic cells that contain choline acetyltransferase, NADPH-diaphorase, and calbindin and that project to striate cortex and smaller cells that contain gamma-aminobutyric acid (GABA), glutamic acid decarboxylase, and parvalbumin and that project to the visual sector of the thalamic reticular nucleus. We also examined at the electron microscopic level terminals in the visual sector of the thalamic reticular nucleus that were labeled from a biocytin injection in the basal forebrain. Most of these terminals form symmetric contacts onto dendrites and were revealed by postembedding immunocytochemical staining to be positive for GABA.

GABAergic and pallidal terminals in the thalamic reticular nucleus of squirrel monkeys

The ultrastructure of synaptic terminals from the external segment of the globus pallidus and of other synaptic terminals positive for gamma-aminobutyric acid (GABA) was examined in the thalamic reticular nucleus (TRN) of squirrel monkeys. Two GABA-positive terminals types were commonly encountered within the TRN neuropil. The most common type of GABAergic terminals (F terminals) are filled with dispersed pleomorphic synaptic vesicles and clusters of mitochondria. These terminals establish multiple symmetric synapses upon the somata and dendrites of TRN neurons. The external pallidal terminals, labeled with WGA-HRP, arise from thinly myelinated axons and correspond to the medium to large F terminals. A less prevalent population of smaller GABAergic synaptic profiles was also identified. The synaptic profiles in this second group contain considerably fewer pleomorphic synaptic vesicles in small irregular clusters and fewer mitochondria, establish symmetric synapses, are postsynaptic to other axonal terminals, are presynaptic to dendrites and soma, and are unlabeled following pallidal injections of WGA-HRP.

Ultrastructural studies of the primate lateral geniculate nucleus: morphology and spatial relationships of axon terminals arising from the retina, visual cortex (area 17), superior colliculus, parabigeminal nucleus, and pretectum of Galago crassicaudatus

The electron microscopic autoradiographic tracing method has been used to examine the morphology and postsynaptic relationships of five projections (retina, cortical area 17, superior colliculus (tectal), parabigeminal nucleus, and pretectum) to the dorsal lateral geniculate nucleus of the greater bush baby Galago crassicaudatus. Retinal terminals have been examined in the contralaterally innervated layer of each of the three matched pairs [parvi- (X-cell), magno- (Y-cell), and koniocellular (small, W-cell)] of geniculate layers. These terminals are large and contain pale mitochondria and round vesicles (RLPs). RLPs are presynaptic to juxtasomatic regions of parvi- and magnocellular neurons. In contrast, RLPs innervate more distal regions of koniocellular neurons. Labeled cortical, tectal, and parabigeminal terminals are relatively small and contain round vesicles and dark mitochondria. Cortical terminals in each of the three representative layers are presynaptic to small diameter dendrites. No convergence of cortical and retinal terminals has been seen in any layer. Labeled tectal and parabigeminal terminals are found primarily in the koniocellular layers, but the latter are also seen in all other layers. Tectal and parabigeminal terminals have been observed converging with retinal terminals on dendrites of some koniocellular neurons. Labeled pretectogeniculate terminals contain densely packed pleomorphic vesicles, dark mitochondria, and a dark cytoplasmic matrix. These terminals, which are present in each of the representative layers, are presynaptic to conventional dendrites and profiles containing loosely dispersed pleomorphic vesicles and a pale cytoplasmic matrix.

Organization of cortical afferents to the rostral, limbic sector of the rat thalamic reticular nucleus

The organization of limbic cortical afferents to the thalamic reticular nucleus (TRN) is described. Wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP), biocytin, neurobiotin, or fluorescent dextrans was delivered into the rat cingulate, retrosplenial, and, for comparison, somatosensory cortices. In other species a slab-like arrangement of cortical terminals has been described for sensory TRN sectors. Here this is seen in the rat somatosensory sector. Terminals from limbic cortices did not cluster into slabs but were found to fill the entire thickness of distinct rostral TRN regions. The cingulate and retrosplenial recipient TRN regions overlap, as do the projections from these cortical areas to anterior thalamic nuclei. Retrosplenial fibres contacted the dorsal and rostral TRN, which is known to be connected to the retrosplenial-recipient anteroventral, anterodorsal, and laterodorsal thalamic nuclei. Cingulate terminals occupied more ventral regions of the rostral TRN. This area is connected to thalamic nuclei also innervated by the cingulate cortex: the mediodorsal and anteromedial nuclei. A loose, but clear, topography could be defined for the cingulate-reticular pathway: rostrocaudal and mediolateral directions in the cortex are represented by ventrodorsal and rostrocaudal directions in the TRN, respectively. This organization of limbic corticoreticular pathway corresponds to the arrangement of limbic corticothalamic connections. The ultrastructure of the limbic cortical axon terminals was similar to that of the cortical boutons (D-type) described previously. The labelled terminals formed asymmetrical synapses onto dendritic profiles of reticular neurons. These findings, together with data in the literature, show significant morphological and connectional differences within the TRN that imply functional heterogeneities.

New views of the thalamic reticular nucleus in the adult and the developing brain

The thalamic reticular nucleus plays a crucial role in modifying the patterns of activity that can reach the cerebral cortex from the thalamus. Although the nucleus is generally viewed as a cell group with widespread and nonspecific thalamic and cortical connections, recent evidence has begun to stress the extent to which at least some of the reticular pathways transmit well-defined maps with a clear local sign from the cortex and the thalamus. Further, evidence from the adult structure of the nucleus and ongoing developmental studies suggest that the reticular nucleus plays an important part in organizing the earliest connections between cortex and thalamus and that the developmental sequence may explain the complex connections formed in the adult.

Ultrastructural studies of the primate parabigeminal nucleus: electron microscopic autoradiographic analysis of the tectoparabigeminal projection in Galago crassicaudatus

The normal ultrastructure of the parabigeminal nucleus and the morphology and synaptic relationships of tectoparabigeminal terminals have been examined. Five different morphological types of terminals have been observed within the parabigeminal nucleus. Three of these profiles contain round vesicles and make asymmetrical synapses, while two contain pleomorphic vesicles and make symmetrical synapses. Electron microscopic autoradiographic data indicate that labeled tectoparabigeminal terminals represent only one of the three profiles containing round vesicles. Such terminals are primarily presynaptic to dendritic shafts, and several labeled profiles have been observed presynaptic to the same dendrite.

Cytoarchitectonic heterogeneities in the thalamic reticular nucleus of cats and ferrets

The thalamic reticular nucleus has been classically defined as a group of cells surrounding most of the rostral and lateral surfaces of the dorsal thalamus, lateral to the fibres of the external medullary lamina and medial to those of the internal capsule. With the use of Nissl staining and antibodies to gamma-aminobutyric acid (GABA), somatostatin, and parvalbumin, this study describes the cytoarchitecture of the thalamic reticular nucleus of cats and ferrets. In cats, three subdivisions of the nucleus are distinguished, two of which are distinct in ferrets also. First, the main body of the reticular nucleus lies lateral to the fibres of the external medullary lamina (except ventrally) and medial to those of the internal capsule. In both cats and ferrets, this structure is heterogeneous, consisting of distinct layers, the details of which vary along the dorsoventral axis. A prominent rostroventral portion of comparatively small rounded cells is also apparent within the main body. Most reticular cells in all areas of the main body are labelled with all of the above mentioned antibodies. Second, the inner small-celled region is a group of small cells located between the external medullary lamina (ventrally) and the medial margin of the ventral regions of the main body of the reticular nucleus: the inner small-celled region is clearly differentiated in cats only. Previous studies have referred to this area as being part of the main body of the reticular nucleus, but we suggest that it may form a separate subnucleus. For example, the inner small-celled region stands in striking contrast to the main body of the reticular nucleus in that none of its cells are GABA immunoreactive and only a small caudal subpopulation are parvalbumin immunoreactive. A very similar pattern of immunostaining is apparent for the cells in the zona incerta, although the latter contains a small rostral subpopulation of GABA immunoreactive cells. Furthermore, although morphologically distinct from the zona incerta, the inner small-celled region fuses with it ventrocaudally. We suggest that the inner small-celled region may constitute a previously undescribed dorsal extension of the zona incerta, rather than a subdivision of the reticular nucleus. Third, the perireticular nucleus, hitherto unidentified, is a discrete group of small cells lateral to the main body of the reticular nucleus and medial to the corpus striatum (globus pallidus and caudate-putamen). It is apparent throughout most of the dorsoventral extent of the main body of the reticular nucleus of cats and ferrets.(ABSTRACT TRUNCATED AT 400 WORDS)

Calbindin immunoreactivity in a subset of cat thalamic reticular neurons

Recent studies have shown that the thalamic reticular nucleus of cats is made up of several cytoarchitectonically distinct subdivisions and that the nucleus contains accurate topographical maps of the cortical sheet and of the dorsal thalamus. The present study describes immunocytochemically demonstrable heterogeneity in the reticular nucleus of cats, with an antibody to calbindin D28k. The striking feature of calbindin immunoreactivity in the reticular nucleus of cats is that the immunoreactive neurones are located in the caudal half of the nucleus only. In these regions, labelled cells form a small proportion of the total population of reticular cells only and are not distinct in somal size or shape from neighbouring non-labelled reticular cells. Double labelling shows that the calbindin-immunoreactive cells are also immunoreactive to parvalbumin and GABA. There is a distinct tendency for the calbindin-immunoreactive cells to be more numerous ventrally than dorsally in the caudal half of the nucleus, which receives afferents from the somatosensory and auditory systems.

Ultrastructural studies of retinal, visual cortical (area 17), and parabigeminal terminals within the superior colliculus of Galago crassicaudatus

The morphology and synaptic relationships of anterogradely labeled retinal, visual cortical (area 17), and parabigeminal terminals have been analyzed within the superficial gray (stratum griseum superficiale) of Galago crassicaudatus. Our data regarding the retinocollicular projection reveal two populations of terminals based upon size. The population of smaller terminals are found in clusters, while the larger occur in isolation. Both populations of retinocollicular terminals form synapses primarily with dendritic spines, but synapses upon pale vesicle filled (PVF) profiles and dendritic shafts also occur. Corticotectal terminals contain round vesicles and make asymmetrical synapses, primarily onto dendritic spines; few form synapses with PVF profiles. Our findings suggest the possibility that there are two populations of corticotectal terminals based upon differences in size and morphology. Parabigeminotectal profiles contain densely packed round vesicles and make asymmetrical synapses. These terminals, which are exclusively cholinergic in Galago, are presynaptic to dendrites of various sizes. Convergence of retinal and cortical terminals has been observed. This convergence occurs on distinctly separate regions of the postsynaptic membrane. In contrast, convergence of retinal and parabigeminal terminals occurs on the same region of the postsynaptic cell(s).

Electron-microscopic analysis of synaptic input from the perigeniculate nucleus to the A-laminae of the lateral geniculate nucleus in cats

The perigeniculate nucleus of carnivores is thought to be a part of the thalamic reticular nucleus related to visual centers of the thalamus. Physiological studies show that perigeniculate neurons, which are primarily GABAergic, provide feedback inhibition onto neurons in the lateral geniculate nucleus. However, little is known about the anatomical organization of this feedback pathway. To address this, we used two complementary tracing methods to label perigeniculate axons for electron microscopic study in the geniculate A-laminae: intracellular injection of horseradish peroxidase (HRP) to fill an individual perigeniculate cell and its axon; and anterograde transport of Phaseolus vulgaris leucoagglutinin to label a population of perigeniculate axons. Labeled perigeniculate terminals display features of F1 terminals in the geniculate neuropil: they are small, contain dark mitochondria, and form symmetric synaptic contacts. We found that most of the perigeniculate terminals (greater than 90%) contact geniculate cell dendrites in regions that also receive a rich innervation from terminals deriving from visual cortex (e.g., "cortico-recipient" dendrites). The remainder of the perigeniculate synapses (10%) contacted dendrites in regions that also received direct retinal input (e.g., "retino-recipient" dendrites). Serial reconstruction of segments of dendrites postsynaptic to perigeniculate terminals suggests that these terminals contact both classes of relay cell in the A-laminae (X and Y), although our preliminary conclusion is that an individual perigeniculate cell contacts only one class. Finally, our quantitative comparison between labeled perigeniculate terminals and unlabeled F1 terminals indicates that these perigeniculate terminals form a distinct subset of F1 terminals. We quantitatively compared the labeled perigeniculate terminals to unlabeled F1 terminals. Although the parameters of the perigeniculate terminals fell entirely within the range of those for the unlabeled F1 terminals, as populations, we found consistent differences between these two groups. We thus conclude that, as populations, other sources of F1 terminals are morphologically distinct from perigeniculate terminals and innervate different targets.