Synaptology of retinal terminals in the dorsal lateral geniculate nucleus of the cat

We have made a fine structural investigation of the synaptic patterns made by axon terminals of retinal ganglion cells in the dorsal lateral geniculate nucleus of the cat. We compared the retinal input to dendritic processes that bear clusters of large appendages with the retinal input to relatively smooth dendritic segments that have only a few isolated spines. The study was restricted to the portion of laminae A and A1 that receive central visual field input. We were able to completely reconstruct 33 individual terminal boutons from long series of consecutive thin sections. Retinal terminals that were presynaptic to dendritic appendages tended to occupy the central position in the complex synaptic zones of geniculate fine structure called glomeruli. These terminals were surrounded by significantly more profiles than retinal terminals that were presynaptic to dendritic stems and averaged twice as many synaptic contacts per terminal bouton. The retinal input to dentritic appendages was heavily involved in a specific synaptic pattern called the triadic arrangement while retinal input to dendritic stems was only lightly involved in triads. Dendritic appendages in triads received greater synaptic input from profiles with flattened vesicles than did the dendritic stems that were found in triads.     

Y retinal terminals contact interneurons in the cat dorsal lateral geniculate nucleus

One of the largest influences on dorsal lateral geniculate nucleus (dLGN) activity comes from interneurons, which use the neurotransmitter gamma-aminobutyric acid (GABA). It is well established that X retinogeniculate terminals contact interneurons and thalamocortical cells in complex synaptic arrangements known as glomeruli. However, there is little anatomical evidence for the involvement of dLGN interneurons in the Y pathway. To determine whether Y retinogeniculate axons contact interneurons, we injected the superior colliculus (SC) with biotinylated dextran amine (BDA) to backfill retinal axons, which also project to the SC. Within the A lamina of the dLGN, this BDA labeling allowed us to distinguish Y retinogeniculate axons from X retinogeniculate axons, which do not project to the SC. In BDA-labeled tissue prepared for electron microscopic analysis, we subsequently used postembedding immunocytochemical staining for GABA to distinguish interneurons from thalamocortical cells. We found that the majority of profiles postsynaptic to Y retinal axons were GABA-negative dendrites of thalamocortical cells (117/200 or 58.5%). The remainder (83/200 or 41.5%) were GABA-positive dendrites, many of which contained vesicles (59/200 or 29.5%). Thus, Y retinogeniculate axons do contact interneurons. However, these contacts differed from X retinogeniculate axons, in that triadic arrangements were rare. This indicates that the X and Y pathways participate in unique circuitries but that interneurons are involved in the modulation of both pathways.     

Quantitative comparison of retinal synapses in the dorsal and ventral (parvicellular) C laminae of the cat dorsal lateral geniculate nucleus

Three physiological classes of retinal ganglion cell project to the cat dorsal lateral geniculate nucleus (DLGN). The dorsal laminae A, A1, and magnocellular C receive X and Y retinal input, whereas the ventral parvicellular laminae C1 and C2 receive predominantly W input. We have compared quantitatively the retinal synaptic terminals of the dorsal and ventral laminae to determine whether there are morphological differences in the terminals that correspond to their different response properties. Anterogradely labeled retinal synaptic terminals in all laminae contained pale mitochondria and large, round synaptic vesicles. However, retinal terminals with pale mitochondria varied in size and synaptic organization in different laminae. The terminals in the A laminae were, on average, quite large and made numerous contacts with conventional dendritic profiles and with profiles that themselves contained synaptic vesicles (F2 profiles). The terminals in lamina C that contained pale mitochondria had a smaller overall mean area. Terminals with pale mitochondria in C1 and C2 were almost all small and synapsed with F2 profiles less frequently than did terminals in the A laminae or in lamina C. These results provide quantitative evidence that visual areas receiving W-type retinal input contain smaller retinal terminals and have a different synaptic organization from that of laminae receiving X and Y input.     

Synaptic connections between corticogeniculate axons and interneurons in the dorsal lateral geniculate nucleus of the cat

Anatomical evidence is provided for direct synaptic connections by axons from visual cortex with interneurons in lamina A of the cat's dorsal lateral geniculate nucleus. Corticogeniculate axon terminals were labeled selectively with 3H-proline and identified by means of electron microscopic autoradiography. Interneurons in the lateral geniculate nucleus were stained with antibodies that had been raised against gamma aminobutyric acid (GABA). We found that corticogeniculate terminals synapsed with dendrites stained positively for GABA about three times as often as with unstained dendrites. Of the corticogeniculate terminals that contacted GABA-positive dendrites, 97% made synaptic connections with dendritic shafts. Only 3% synapsed with F profiles, the vesicle-filled dendritic appendages characteristic of lateral geniculate interneurons. These results suggest that the corticogeniculate pathway in the cat is directed primarily at interneurons and is organized synaptically to influence the integrated output of these cells, rather than the local interactions in which their dendritic specializations participate.     

Synaptic targets of cholinergic terminals in the pulvinar nucleus of the cat

We compared the cholinergic innervation of the pulvinar nucleus, a thalamic association nucleus, to previous studies of the cholinergic innervation of the dorsal lateral geniculate nucleus (dLGN), a thalamic relay nucleus. Both nuclei receive a dense innervation from cholinergic cells of the brainstem parabrachial region (PBR). In the dLGN, PBR terminals are located in close proximity to retinal terminals. Our goal was to determine whether PBR terminals in the pulvinar nucleus are located in close proximity to corticothalamic terminals. We identified PBR terminals with a monoclonal antibody directed against choline acetyltransferase (ChAT). Cholinergic terminals contacted dendrites (142 of 160, or 89%) or vesicle-filled profiles (18 of 160, or 11%). A subset of 55 terminals was stained for γ-aminobutyric acid (GABA) to determine whether profiles postsynaptic to cholinergic terminals originate from thalamocortical cells (GABA-) or interneurons (GABA+). The majority (44 of 55, or 80%) of postsynaptic profiles were GABA- dendrites. The minority (11 of 55, or 20%) were GABA+ dendrites with vesicles. This distribution of contacts is very similar to that seen in the dLGN. However, the most significant finding was that most cholinergic contacts (121 of 160, or 76%) were located within complex clusters identified as glomeruli. This is the primary site of contacts made by corticothalamic terminals originating from layer V cells. These results suggest that while the PBR enhances retinal signals in the dLGN, it may also enhance cortical signals in the pulvinar nucleus. Thus, activity in the PBR may stimulate both an increased flow of retinal information to visual cortex, as well as an increased flow of information between different visuomotor areas of cortex. J. Comp. Neurol. 387:266–278, 1997. © 1997 Wiley-Liss, Inc.     

Qualitative and quantitative analyses of the patterns of retinal input to neurons in the dorsal lateral geniculate nucleus of the cat

In the visual system of the cat the projection from the retina to the lateral geniculate nucleus has been studied extensively. However, the patterns of synaptic contacts made by individual axons onto individual cells have not been described. In this study these patterns have been examined for class 1 cells (Guillery: J. Comp. Neurol. 128:21, '66). Retinogeniculate axons and lateral geniculate neurons are labeled with horseradish peroxidase (HRP) via injections into the optic tracts and optic radiations, respectively. Sections are then processed for combined light and electron microscopic analysis. They are examined with the light microscope to identify labeled lateral geniculate neurons that appear to be contacted by labeled retinal axons. These cells and axons are then analyzed by a computerized microscope system, and sites of apparent synaptic contact are recorded. This light microscopic analysis indicates that individual class 1 cells are contacted by many retinogeniculate axons (> 10) and that each of these axons contacts many lateral geniculate neurons (> 20). Some axons make numerous contacts that are concentrated onto a few dendrites, while others make only a few contacts, which are spread over several dendrites. In all cases, the majority of contacts are on the dendritic shafts of relatively thick secondary and tertiary dendrites. Electron microscopic analysis confirms that most of the contacts identified with the light mciroscope are synaptic. It also reveals that labeled and unlabeled retinal axons can innervate the same dendritic segment. Finally, one cell was studied that had its soma and most of its dendrites in lamina A1 but some of its dendrites extended into lamina A. This cell received input from retinal axons in both layers, thus suggesting that it may have been binocularly excitable. © 1993 Wiley-Liss, Inc.     

GABAergic circuitry in the dorsal division of the cat medial geniculate nucleus

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter of the thalamus. We used postembedding immunocytochemistry to examine the synaptic organization of GABA-positive profiles in the dorsal superficial subdivision of the cat medial geniculate nucleus (MGN). Three groups of GABA-positive profiles participate in synapses: axon terminals, dendrites, and presynaptic dendrites. The presynaptic GABA-positive terminals target mainly GABA-negative dendrites. The GABA-positive postsynaptic profiles receive input primarily from GABA-negative axons. The results indicate that the synaptic organization of GABA-positive profiles in the dorsal superficial subdivision of the MGN nucleus is very similar to that in other thalamic nuclei.     

The serotoninergic fibers in the dorsal lateral geniculate nucleus of the cat: distribution and synaptic connections demonstrated with immunocytochemistry

The distribution, morphology, and synaptic contacts of serotoninergic fibers were studied with immunocytochemical methods in the lateral geniculate complex of the cat. The serotonin-immunoreactive fibers are diffusely distributed throughout the main laminae of the dorsal lateral geniculate nucleus (dLGN) and the perigeniculate nucleus (PGN) and reach a particular density in the ventral lateral geniculate nucleus (vLGN). The labeled fibers are in most cases very thin and sometimes varicose. There is no obvious order in their distribution pattern except that they sometimes partially encircle the unlabeled cell bodies of the dLGN. The synaptic connections of the serotoninergic fibers were investigated mainly in the A laminae of the dLGN. Few synaptic complexes were found, most of them with asymmetric morphology. The postsynaptic elements were small dendritic profiles. Perisomatic serotoninergic fibers were seen, but no convincing synaptic contacts were found between labeled fibers and cell somata. In the dLGN, serotoninergic profiles were almost exclusively confined to the extraglomerular neuropile. In the PGN serotoninergic fibers also contacted dendritic profiles and formed asymmetrical synapses, but as in the geniculate, synaptic specializations were very rare.     

Ultrastructure and synaptic targets of tectothalamic terminals in the cat lateral posterior nucleus

The recent appreciation of the fact that the pulvinar and lateral posterior (LP) nuclei receive two distinct types of cortical input has sparked renewed interest in this region of the thalamus. A key question is whether the primary or "driving" inputs to the pulvinar/LP complex originate in cortical or subcortical areas. To begin to address this issue, we examined the synaptic targets of tectothalamic terminals within the LP nucleus. Tectothalamic terminals were labeled using the anterograde transport of biotinylated dextran amine (BDA) or Phaselous leucoagglutinin placed in the superior colliculus or using immunocytochemical staining for substance P, a neurotransmitter found to be used by the tectothalamic pathway (Hutsler and Chalupa [ 1991] J. Comp. Neurol. 312:379-390). Our results suggest that most tectothalamic terminals are large and occupy a proximal position on the dendritic arbor of LP relay cells. In the medial LP, tectothalamic terminals labeled by the transport of neuronal tracers or substance P immunocytochemistry can form tubular clusters that surround the proximal dendrites of relay cells. In a rostral and lateral subdivision of the lateral LP nucleus (LPl-2), tectothalamic terminals form more typical glomerular arrangements. When compared with existing physiological data, these results suggest that a unique integration of tectal and cortical inputs may contribute to the response properties of LP neurons.     

Comparison of the ultrastructure of cortical and retinal terminals in the rat dorsal lateral geniculate and lateral posterior nuclei

We compared the ultrastructure and synaptic targets of terminals of cortical or retinal origin in the rat dorsal lateral geniculate nucleus (LGN) and lateral posterior nucleus (LPN). Following injections of biotinylated dextran amine (BDA) into cortical area 17, two types of corticothalamic terminals were labeled by anterograde transport. Type I terminals, found throughout the LGN and LPN, were small, drumstick-shaped terminals that extended from thin axons. At the ultrastructural level in both the LGN and LPN, labeled type I corticothalamic terminals were observed to be small profiles that contained densely packed round vesicles (RS profiles) and contacted small-caliber dendrites. In tissue stained for gamma amino butyric acid (GABA) using postembedding immunocytochemical techniques, most dendrites postsynaptic to type I corticothalamic terminals did not contain GABA (97%). Type II corticothalamic terminals, found only in the LPN, were large terminals that sometimes formed clusters. At the ultrastructural level, type II terminals were large profiles that contained round vesicles (RL profiles) and contacted large-caliber dendrites, most of which did not contain GABA (98%). Retinogeniculate terminals, identified by their distinctive pale mitochondria, were similar to type II corticothalamic terminals except that 26% of their postsynaptic targets were vesicle-containing profiles that contained GABA (F2 profiles). Our results suggest that type I corticothalamic terminals are very similar across nuclei but that the postsynaptic targets of RL profiles vary. Comparison of the responses to retinal inputs in the LGN and to layer V cortical inputs in the LPN may provide a unique opportunity to determine the function of interneurons in the modulation of retinal signals and, in addition, may provide insight into the signals relayed by cortical layer V.     

Genesis of interneurons in the dorsal lateral geniculate nucleus of the cat

Most neurons in the A-laminae of the cat's dorsal lateral geniculate nucleus (LGN) are born between embryonic days 22 and 32. Whereas approximately 78% of these cells are destined to become geniculocortical relay cells, the remaining 22% of LGN neurons do not appear to establish connections with visual cortex, and therefore can be considered interneurons. In the present study we have combined the 3H-thymidine method for labeling dividing neurons with the retrograde horseradish peroxidase (HRP) method for identifying LGN relay cells in order to study specifically the genesis of interneurons in the cat's LGN. Developing LGN interneurons in 12 kittens were labeled with 3H-thymidine by injecting the radioactive label into the allantoic cavity of their pregnant mothers on different embryonic days. Approximately 8-22 weeks after birth LGN relay cells in the A-laminae were labeled retrogradely by injecting large volumes of HRP into visual cortex areas 17 and 18. LGN cells that could not be labeled retrogradely with HRP were considered to be interneurons. Our results show that interneurons are born on each of the embryonic days studied, E24-E30. This period represents approximately the middle two-thirds of the entire period of LGN neurogenesis. Although the birth rate for interneurons is not uniform, there is no indication from our data that interneurons and relay cells in the cat's LGN are born at different times during LGN neurogenesis.     

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

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

Immunocytochemistry and distribution of parabrachial terminals in the lateral geniculate nucleus of the cat: A comparison with corticogeniculate terminals

We used immunohistochemistry in cats to demonstrate the presence of brain nitric oxide synthase (BNOS) in cholinergic fibers within the A-laminae of the lateral geniculate nucleus. We used a double labeling procedure with electron microscopy and found that all terminals labeled for choline acetyltransferase (ChAT) in the geniculate A-laminae were double labeled for BNOS. Also, some interneuron dendrites, identified by labeling for γ-aminobutyric acid (GABA), contained BNOS, but relay cell dendrites did not. We then compared parabrachial and corticogeniculate terminals, identifying the former by BNOS/ChAT labeling and the latter by orthograde transport of biocytin injected into cortical area 17, 18, or 19. All corticogeniculate terminals and most BNOS- or ChAT-positive brainstem terminals displayed RSD morphology, whereas some brainstem terminals exhibited RLD morphology. However, parabrachial terminals were larger, on average, then corticogeniculate terminals. We also found that parabrachial terminals were located both inside and outside of glomeruli, and they always contacted relay cell dendrites proximally among retinal terminals (the retinal recipient zone). In contrast, the cortical terminals were limited to peripheral dendrites (the cortical recipient zone). Thus, little if any overlap exists in the distribution of parabrachial and corticogeniculate terminals on the dendrites of relay cells. J. Comp. Neurol. 377:535–549, 1997. © 1997 Wiley-Liss, Inc.     

Cerebellar projections to the ventral lateral geniculate nucleus and the thalamic reticular nucleus in the cat

The ventral lateral geniculate nucleus (LGNv) is a retinorecipient part of the ventral thalamus and in cats, it consists of medial (M), medial intermediate (IM), lateral intermediate (IL), lateral (L), and dorsal (D) subdivisions. These subdivisions can be differentiated not only by their cytoarchitecture, but also by their connectivity and putative functions. The LGNv may play a role in visuomotor gating, in that there is evidence of cerebellar afferent projections to the intermediate subdivisions. The cerebellar posterior interpositus (IP) and lateral (LC) nuclei are known to project to IM and IL, but the specifics of these projections are unclear. We hypothesized that the IP and LC project differentially to IM and IL. To evaluate LGNv innervation by the deep cerebellar nuclei, we injected the tract‐tracer wheat germ agglutinin‐horseradish peroxidase (WGA‐HRP) into several different regions of the LGNv and cerebellar nuclei of adult cats in either sex. Small injections into the middle and posterior LGNv retrogradely labeled cells in the ventral part of the IP. However, injections in the anterior regions of the LGNv, with or without diffusion into the thalamic reticular nucleus (Re), retrogradely labeled cells in the ventral part of both the IP and the LC. Confirmatory injections into the IP and LC produced terminal‐like labeling distributed in IM, IL, and Re; injections mostly localized to the LC resulted in labeling mainly in IM and Re. We concluded that the IP projects to IL whereas the LC projects to IM and Re.     

Ventral lateral geniculate nucleus projects to the dorsal lateral geniculate nucleus in the cat

The lamina C3 of the dorsal lateral geniculate nucleus of the cat does not receive retinal projections but instead receives visual information from the small subpopulation of W-type ganglion cells via the upper substratum of the stratum griseum superficiale of the superior colliculus. We herein report a projection from the lateral division of the ventral lateral geniculate nucleus into the lamina C3 of the dorsal lateral geniculate nucleus. As the lateral division receives projections from the contralateral retina and the ipsilateral upper stratum griseum superficiale of the superior colliculus, we suggest that these regions make up a small cell type W-cell neuronal network that provides visual information to layer I of the striate cortex via the lamina C3.     

Inhibitory interaction between X and Y units in the cat lateral geniculate nucleus

Intracellular and extracellular recordings were obtained from X and Y type relay neurons in the cat lateral geniculate nucleus (LGN). In both cell populations, unit responses to visual and electrical stimuli were studied. Stimulation electrodes were placed in the optic chiasm (OX), the optic radiation (OR), the homolateral superior colliculus (SC) and the homolateral mesencephalic reticular formation (MRF).X neurons have a different response to certain visual stimuli than Y neurons; when the visual stimulus exceeds the receptive field center (RFC) X neurons show a delayed response more often than do Y neurons. This differential delay between X and Y responses is partly because of short latency postsynaptic inhibition in the LGN. For fast, moving visual stimuli, Y cells respond with a brisk burst at stimulus velocities of 300°/sec; X cells are inhibited by such stimuli.The EPSP threshold for OX stimuli is higher in X cells than in Y cells. At stimulus intensities close to the EPSP threshold, X cells show a near maximal IPSP whereas in Y cells, the IPSP is minimal. The IPSP latency to OX stimulation is between 1.8 and 2.5 msec in X and Y cells; because the EPSP latency in X cells is significantly longer than in Y cells, X cell EPSPs frequently start at the same time as the IPSP.75% of the Y cells could be orthodromically driven from both the OX and SC stimuli. Analysis of the EPSP shape and correlation of the latencies suggests that the activation from SC is mediated via bifurcating optic tract fibers which project to both SC and LGN. X cells were never activated from SC; however, there was an IPSP in X and Y cells after stimulation of the SC. IPSP amplitude increased with the eccentricity of the neuron's RFC and was comparable to the IPSP that followed an OX stimulus set at the estimated threshold of the Y fibers.These results suggest an inhibitory convergence of the X and Y system in the LGN. The analysis of EPSPs shows that quite frequently several OT fibers converge onto a single X or Y relay cell; often, however, one fiber appears to provide the dominant input. With a few exceptions, the converging fibers have similar conduction velocities.     

Distribution of morphologically different retinal axon terminals in the hamster dorsal lateral geniculate nucleus

Afferent terminal arbors in the hamster LGBd were labelled with horseradish peroxidase (HRP) implanted into the optic tract. Three morphologically distinct terminal types, each with a different regional distribution, were observed. Type R1 terminals are large, ovoid swellings and are predominantly distributed medially within the nucleus. Type R2 terminals are very small, clustered varicosities and are distributed laterally and ventrally. Type R3 terminals are medium in size and their distribution overlaps with that of Type R1 and R2 terminals.     

Synaptic development of the mouse dorsal lateral geniculate nucleus

The dorsal lateral geniculate nucleus (dLGN) of the mouse has emerged as a model system in the study of thalamic circuit development. However, there is still a lack of information regarding how and when various types of retinal and nonretinal synapses develop. We examined the synaptic organization of the developing mouse dLGN in the common pigmented C57/BL6 strain, by recording the synaptic responses evoked by electrical stimulation of optic tract axons, and by investigating the ultrastructure of identified synapses. At early postnatal ages (<P12), optic tract evoked responses were primarily excitatory. The full complement of inhibitory responses did not emerge until after eye opening (>P14), when optic tract stimulation routinely evoked an excitatory postsynaptic potential/inhibitory postsynaptic potential (EPSP/IPSP) sequence, with the latter having both a GABA_A and GABA_B component. Electrophysiological and ultrastructural observations were consistent. At P7, many synapses were present, but synaptic profiles lacked the ultrastructural features characteristic of the adult dLGN, and little γ‐aminobutyric acid (GABA) could be detected by using immunocytochemical techniques. In contrast, by P14, GABA staining was robust, mature synaptic profiles of retinal and nonretinal origin were easily distinguished, and the size and proportion of synaptic contacts were similar to those of the adult. The emergence of nonretinal synapses coincides with pruning of retinogeniculate connections, and the transition of retinal activity from spontaneous to visually driven. These results indicate that the synaptic architecture of the mouse dLGN is similar to that of other higher mammals, and thus provides further support for its use as a model system for visual system development. J. Comp. Neurol. 518:622–635, 2010. © 2009 Wiley‐Liss, Inc.