Inhibitory circuitry involving Y cells and Y retinal terminals in the C laminae of the cat dorsal lateral geniculate nucleus

We previously established (Datskovskaia et al. [2001] J Comp Neurol 430:85-100) that roughly 40% of Y retinal terminals contact interneurons in the A lamina of the dorsal lateral geniculate nucleus (dLGN) of the cat. However, we did not establish whether the dendritic terminals of interneurons postsynaptic to Y retinal terminals subsequently contact Y thalamocortical cells. To begin to address this issue, we examined the synaptic targets of Y retinal terminals in the magnocellular C lamina of the dLGN, which is populated almost exclusively by Y thalamocortical cells and interneurons. We utilized material generated from our previous work, in which we injected the superior colliculus with biotinylated dextran amine to backfill the geniculate branches of Y retinogeniculate axons in the dLGN. Sections prepared for electron microscopy were stained for gamma aminobutyric acid (GABA) to distinguish interneurons from thalamocortical cells. We found that the majority of profiles postsynaptic to Y retinal axons were the GABA-negative dendrites of thalamocortical cells (116/200, 58%). The remainder were GABA-positive dendrites of interneurons (84/200, 42%), many of which contained vesicles (F2 profiles; 54/200, 27%). In addition, we examined the synaptic targets of F2 profiles and found that almost all contacts of F2 profiles in the magnocellular C lamina were made onto the GABA-negative dendrites of thalamocortical cells (199/200, 99.5%). Thus, Y retinogeniculate axons contact interneurons and interneurons contact Y thalamocortical cells in the magnocellular C lamina of the dLGN. This indicates that interneurons are involved in modulation of the Y pathway.     

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.     

A Golgi study of dendritic development in the dorsal lateral geniculate nucleus of normal ferrets.

The development of neurons in the dorsal lateral geniculate nucleus (dLGN) of pigmented ferrets was studied by using the Golgi-Hortega technique. In adult ferrets, four dLGN cell classes were defined on the basis of somatic and dendritic morphology. Classes 1 and 2 were divided into stellate and oriented subtypes. Class 1 and 4 cells are characterized by filiform appendages, class 2 cells by club-like appendages, and class 3 cells by stalked appendages. At birth, dLGN neurons have simple dendritic arbors. During the first postnatal week, dendritic length and proximal branching density increase markedly. By postnatal day 21 (P21), dendritic morphology begins to take on mature characteristics and by the time of eye opening (P30-P35), most neurons can be classified. Also by that time, dLGN cells are covered with abundant filiform appendages. Developmental changes in appendage density were quantified for class 1 stellate cells. These data reveal that appendage density reaches a peak at P56, decreases sharply until P90, and then gradually declines to mature levels by P180. Elaboration and elimination of transient appendages occurs centrifugally; at maturity appendage density remains greater distally.     

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 morphologically identified neurons in the dorsal lateral geniculate nucleus of the cat

Neurons in the dorsal lateral geniculate nucleus of the cat can be grouped into five morphological classes based on a variety of structural characteristics. These same structural characteristics can serve as morphological signatures for the three physiological classes (X, Y, W) of neurons found in this nucleus. The purpose of this study was to determine if a relationship exists between the birthdate of neurons within the dorsal lateral geniculate nucleus and the adult morphology of those neurons. Seven cats, each of which had received a single injection of 3H-thymidine, were studied. A total of 2,138 Golgi-impregnated neurons were identified in the dorsal lateral geniculate nuclei of these seven cats; 1,517 of these neurons were successfully resectioned and recovered, of which 385 (25%) were found to contain the 3H label. Neurons from each of the five morphological classes were labeled in each of the six animals that received a 3H-thymidine injection between embryonic day 24 (E24) and E28. Class 3 and class 5 neurons were labeled in a cat injected with 3H-thymidine on E30. These findings demonstrate that the development of the morphological class of a neuron in the dorsal lateral geniculate nucleus is independent of the time of its final cell division. Further, given the relationship that exists in the cat's dorsal lateral geniculate nucleus between neuronal structure and function, the present findings suggest that the different physiological classes of cells found in this nucleus undergo their final cell divisions throughout most of the period of neurogenesis except that the functional role of neurons born late in this period may be more restricted.     

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.     

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.     

Response of retinal terminals to loss of postsynaptic target neurons in the dorsal lateral geniculate nucleus of the adult cat.

We have used the neurotoxin kainic acid to produce rapid degeneration of neurons in the dorsal lateral geniculate nucleus (dLGN) of the adult cat. This degeneration mimics the rapid loss of geniculate neurons seen after visual cortex ablation in the neonate. Subsequent anterograde transport of horseradish peroxidase injected into the eye was used to reveal the projection patterns of retinal ganglion cell axons at different survival periods after the kainic acid injection. The density of retinal projections to the degenerated regions of the geniculate was reduced considerably at 4 and 6 months survival, but at 2 months was not significantly different from normal. The laminar pattern of projections to degenerated regions of the geniculate did not change in any animals studied, even when an adjacent lamina contained surviving cells. Electron microscopic examination of degenerated dLGN revealed intact retinal (RLP) and RSD terminals at all survival times, although the density of terminals appeared much reduced when compared to controls. Some RLP terminals exhibited the "dark reaction" of degeneration and these degenerating terminals were most numerous at 2 months survival. These findings demonstrate that, in response to degeneration of their usual target cells, mature retinal ganglion cells with withdraw their axon terminals from these regions of degeneration. We conclude that mature retinal ganglion cells continue to be dependent on target integrity for the maintenance of a normal axonal arborization.     

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.     

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

Neurons that will ultimately form the dorsal and ventral lateral geniculate nuclei, the medial interlaminar nucleus, the perigeniculate nucleus, and the nucleus reticularis of the cat undergo their final cell division beginning on, or slightly before, embryonic day 22 (E22) and ending on, or before, E32. Early in this period, neurogenesis proceeds for all of these geniculate nuclei, whereas only in the dorsal lateral geniculate nucleus does cell birth continue until E32. Distinct spatiotemporal gradients of cell birth are not obvious within any of the individual geniculate nuclei. For the dorsal lateral geniculate nucleus in particular, and for the other geniculate nuclei in general, neurons born early in this period exhibit a full range of adult soma sizes, including large and small neurons. Neurons born late in this period exhibit only small adult somas. The location and size of a neuron within the dorsal lateral geniculate nucleus provide clues to that cell's functional properties. On the basis of presently available information regarding the relationship between structure and function of neurons in the cat's dorsal lateral geniculate nucleus, the findings described here suggest that all functional classes of neurons in the dorsal lateral geniculate nucleus are born at the same time throughout most of this period.     

Abnormal axonal growth in the dorsal lateral geniculate nucleus of the cat

Retino-geniculate axons in the cat were induced to grow abnormally by cutting one optic nerve in kittens. Surviving optic tract axons that had grown into the denervated regions were then filled in the adults with horseradish peroxidase to reveal the terminal arbors of individual axons. Two types of abnormal axonal growth are described—translaminar growth and monocular segment growth. Translaminar growth is the most common and occurs between lamine in the binocular part to the nucleus. Axons giving rise to translaminar growth do not branch as they pass through the denervated regions of the nucleus. Instead, the abnormal branches originate from portions of the terminal arbor within the normal target lamina. These axons look like normal retino-geniculate axons in terms of their branching patterns, cytological features, and patterns of synaptic contacts except that parts of their terminal arbors have expanded to innervate inappropriate laminae. The distribution of translaminar branches overlaps the distribution of a restricted group of surviving large neurons that have not undergone denervation atrophy. Monocular segment growth invades the lateral pole of the nucleus directly from the optic tract. These branches arise from axons passing through or near the denervated region and appear to represent the formation of new terminal arbors. The synaptic swellings arising from these branches have cytological features like the synaptic swellings arising from translaminar branches and they from similar patterns of synaptic contacts. However, monocular segment branches degenerate more rapidly when damaged and they are not associated with surviving large neurons.     

Organization of the dorsal lateral geniculate nucleus in a cat with congenital microphthalmia

The effects of congenital monocular microphthalmia on the development of the lateral geniculate nucleus were examined in a 10-week-old cat. The left eye and optic nerve in this animal appear normal. The right eye is about 30% smaller in volume than the left and the optic nerve from this eye has a cross-sectional area that is only 15% that of the left. In addition, this nerve contains few, if any, large myelinated axons. Both lateral geniculate nuclei are abnormal and the abnormality differs rostrally and caudally. The caudal portion most closely resembles the normal nucleus. Retinal input from both eyes is segregated into cellular laminae that are separated from each other by cell sparse interlaminar zones. However, the input from the microphthalmic eye seems to be sparse and patchy and it does not support normal cell growth. All neurons, including glutamic acid decarboxylase-positive (GAD+) neurons, in laminae innervated by the small eye are reduced in size in a pattern similar to that seen following the removal of retinal input. In comparison, the rostral portion of the nucleus receives very little input from the microphthalmic eye. Instead the normal eye densely innervates nearly the entire nucleus. In this region, interlaminar zones fail to form but the input from the normal eye is able to support cell growth including the growth of GAD+ neurons.     

Structural correlates of functionally distinct X-cells in the lateral geniculate nucleus of the cat

In the companion paper (Humphrey and Weller, '88), we demonstrated 2 physiologically different groups of X-cells (XL and XN) in the A-laminae of the cat lateral geniculate nucleus. In order to investigate their possible morphological correlates, we iontophoresed horseradish peroxidase intracellularly into physiologically identified XL- and XN-cells and examined their light microscopic appearance. The 11 HRP-labeled XL-cells constituted the smallest relay neurons in the A-laminae, and were similar morphologically. All had small somata (mean soma size = 236 micron2), very thin (less than 1.0 micron) axons, few primary dendrites, and narrow, sinuous distal dendrites, which usually formed trees that were oriented perpendicular to laminar borders. The dendrites could be smooth or display beadlike varicosities, hairlike appendages, and/or occasional complex stalked appendages, but their most consistent feature was numerous clusters of grapelike dendritic appendages located at or near dendritic branch points. The 14 labeled XN-cells were structurally more heterogeneous, and they included relay neurons and interneurons. Eight of 11 XN-relay cells differed markedly from the XL-cells. These XN-cells were multipolar neurons with medium to large somata (mean soma size = 365 micron2), small to medium-size axons (1.0-2.0 micron), numerous primary dendrites, and straight distal dendrites that formed radially symmetric trees. The dendrites of the cells were largely smooth, except for occasional spines and/or hairs, and they were devoid of grapelike and other complex appendages. The three other XN-relay neurons had morphologies either similar to XL-cells or intermediate between XL-cells and more simple, multipolar XN-relay cells, but two of these cells had larger somata and axons than most XL-cells. Finally, three XN-cells were intrageniculate interneurons, which possessed small somata (mean soma size = 174 micron2), fine sinuous dendrites covered with beadlike varicosities on stalked appendages, and no obvious axon. These results reveal that, despite minor overlap, there are marked structural differences between XL- and XN-cells. Among the relay cells, these differences relate to soma and axon diameter, dendritic orientation, and the presence or absence of grapelike dendritic appendages. Our finding that interneurons were strongly excited at short latencies by spot onset supports the hypothesis (Mastronarde, '87a; Humphrey and Weller, '88) that such interneurons provide the major inhibitory input to XL-cells, and that this input is important in generating the spot-induced early dips in XL-cell discharge.     

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.     

The morphology of corticofugal axons to the dorsal lateral geniculate nucleus in the cat

The structural features of corticogeniculate axons were studied in adult cats after labeling them with horseradish peroxidase (HRP). Injections of HRP into the optic radiations near the dorsal lateral geniculate nucleus result in Golgi-like filling of both geniculate relay neurons and corticogeniculate axons. In the present material at least two main types of axons could be defined. The most common type is called the type I axon because it so closely resembles the type I axons described by Guillery ('66, '67) in Golgi preparations. These fine axons have smooth surfaces and consistent fiber diameter. Most terminal swellings are at the ends of short collateral branches and these swellings form asymmetric synaptic contacts onto small and medium-sized dendrites. Type I axons typically innervate more than one lamina as well as interlaminar zones and they clearly arise from the cerebral cortex. The second type of axon is called the beaded axon because of its numerous swellings, en passant. These swellings frequently are larger than those on type I axons and they differ from previously described corticogeniculate axon terminals in their ultrastructural features. That is, their synaptic contacts appear symmetrical and they form axosomatic contacts. Because of these differences, the possibility that beaded axons are of subcortical origin, particularly from the perigeniculate nucleus, is discussed. When type I axons and geniculate relay neurons are filled in the same region of the nucleus it is possible to identify probable sites of synaptic contact by using the light microscope. Such analyses indicate that corticogeniculate axons synapse directly onto relay cells, primarily on peripheral dendritic branches. Further, it appears that single axons contact many geniculate neurons and that single neurons are contacted by many axons.     

Visual pretectal neurons projecting to the dorsal lateral geniculate nucleus and pulvinar nucleus in the cat

We observed morphological subtypes of visual pretectal neurons ascending to the dorsal thalamus, following injections of wheat germ agglutinin conjugated to horseradish peroxidase into the dorsal lateral geniculate nucleus (LGNd) or the pulvinar nucleus. These neurons are composed of fusiform cells and small-sized multipolar cells in the olivary pretectal nucleus, superficial horizontal cells, fusiform cells, small-, medium- and large-sized multipolar cells in the optic tract nucleus, and small- and medium-sized multipolar cells in the posterior pretectal nucleus. When somal size of the neurons projecting to the LGNd was compared to the size of neurons projecting to the pulvinar, the neuronal groups were not identical.     

Amino acids in the dorsal lateral geniculate nucleus of the cat—collection in vivo

A dialysis sampling probe was used to collect amino acids from the dorsal lateral geniculate nucleus (LGN) in vivo. The sampling probe was equipped with an electrode to allow local stimulation and recording of nerve activity. The amino acids in the dialysates were determined fluorimetrically by precolumn derivation and hple-separation. Local electrical stimulation of the LGN caused a multifold increase in glutamate, aspartate and GABA levels. Smaller changes were observed for taurine, alanine and glycine. The results indicate that the dialysis sampling probe is rather atraumatic and can be used to detect stimulation-induced changes in extracellular amino acid concentrations.