Development of the rat thalamus: II. Time and site of origin and settling pattern of neurons derived from the anterior lobule of the thalamic neuroepithelium

Short-survival, sequential, and long-survival thymidine radiograms of rat embryos, fetuses, and young pups were analyzed in order to examine the time of origin, settling pattern, and neuroepithelial site of origin of the anterior thalamic nuclei--the lateral dorsal (lateral anterior), anterodorsal, anteroventral and anteromedial nuclei--and of two rostral midline structures--the anterior paraventricular and paratenial nuclei. The neurons of the lateral dorsal nucleus are generated over a 3-day period between days E14-E16 and their settling pattern displays a combined lateral-to-medial and dorsal-to-ventral neurogenetic gradient. The bulk of the neurons of the anteroventral nucleus are generated over a 3-day period between days E15-E17 and settle with an oblique lateral-to-medial and ventral-to-dorsal neurogenetic gradient. The bulk of the neurons of the anteromedial nucleus are generated over a 2-day period between days E16-E17 and show the same settling pattern as the anteroventral nucleus. The neurons of the anterodorsal nucleus are generated over a 3-day period between days E15-E17 and show a lateral-to-medial neurogenetic gradient. The bulk of the neurons of the central part and lateral part of the paraventricular nucleus are generated over a 2-day period (E16-E17 and E17-E18, respectively) and each part displays a ventral-to-dorsal neurogenetic gradient. Finally, the bulk of the neurons of the paratenial nucleus are generated over a 4-day period between days E15-E18 and settle with a lateral-to-medial neurogenetic gradient. Observations are presented that the anterior thalamic nuclei, constituting the distinct "limbic thalamus," derive from a discrete neuroepithelial source. This is the crescent-shaped germinal matrix lining the diencephalic (medial) wall of the hitherto unrecognized anterior transitional promontory, which we call the anterior thalamic neuroepithelial lobule. On day E16 three migratory streams leave the anterior neuroepithelial lobule and, on the basis of their labeling pattern in relation to the neurogenetic gradients of the anterior thalamic nuclei, they are identified, from dorsal to ventral, as the putative migratory streams of the anterodorsal, anteroventral, and lateral dorsal nuclei. On day E17 the putative migratory stream of the anteromedial nucleus appears to leave the same neuroepithelial region that on the previous days was the source of the anteroventral nucleus. Dorsally, two neuroepithelial patches persist after day E17 and these are identified as the putative cell lines of the anterior paraventricular and paratenial nuclei.     

Development of the rat thalamus: IV. The intermediate lobule of the thalamic neuroepithelium, and the time and site of origin and settling pattern of neurons of the ventral nuclear complex

Short-survival, sequential, and long-survival thymidine radiograms of rat embryos, fetuses, and young pups were analyzed in order to examine the time of origin, settling pattern, migratory route, and site of origin of neurons of the ventral nuclear complex of the thalamus. Quantitative examination of long-survival radiograms established that the bulk of the neurons of the ventral nuclear complex are generated between days E14 and E16 but with statistically significant differences between its three nuclei. The ventrobasal nucleus is the oldest component (97% of the cells are generated on days E14 and E15); the ventrolateral nucleus is next (82% of the cells are generated on days E14 and E15); and the ventromedial nucleus is last (51% of the cells are generated on days E14 and E15). In addition to this caudal-to-rostral (from the ventrobasal nucleus to the ventrolateral nucleus) and lateral-to-medial (from the ventrobasal nucleus to the ventromedial nucleus) internuclear gradients, there are lateral-to-medial and ventral-to-dorsal intranuclear neurogenetic gradients within the ventrobasal and ventrolateral nuclei. Qualitative examination of short and sequential survival thymidine radiograms indicate that the neurons of the ventral nuclear complex originate in the unique intermediate thalamic neuroepithelial lobule, which is distinguished from the rest of the thalamic neuroepithelium by the presence of a mitotically active secondary neuroepithelial matrix. Two sublobules can be distinguished in the intermediate lobule during the early stages of thalamic development. On the basis of their location and chronological pattern of cell production and differentiation, it is inferred that the neurons of the ventrobasal nucleus originate in the earlier differentiating, posteroventrally situated inverted sublobule, and the neurons of the ventrolateral nucleus are produced in the later differentiating, anterodorsally situated everted sublobule. The neurons of the ventromedial nucleus appear to originate from the intermediate neuroepithelial lobule after its two sublobules are no longer distinguishable. The heavily labeled neurons generated soon after injection on day E15 form a wave front that translocates in a lateral direction at a steady rate of 215 microns/day. Examination of methacrylate-embedded materials showed that, in day E15 rats the actively migrating cells are spindle-shaped, with their long axis oriented horizontally. The far-laterally situated differentiating cells (the oldest neurons) become vertically oriented by day E16. Associated with this change in polarity, vertically oriented fibers appear among the cells. These fibers can be traced to the inte     

Development of the rat thalamus: III. Time and site of origin and settling pattern of neurons of the reticular nucleus

Short-survival, sequential, and long-survival thymidine radiograms of rat embryos, fetuses, and young pups were analyzed in order to examine the time of origin, settling pattern, migratory route, and site of origin of neurons of the reticular nuclear complex of the thalamus. On the basis of its chrono-architectonics, the reticular nucleus was divided into a central, medial, and lateral subnucleus. The central subnucleus is the earliest produced component of the entire thalamus with over 50% of its neurons being generated on day E13 and another 40% on day E14. Peak production of neurons of the lateral and medial subnuclei is on day E14. There is a lateral (earlier) to medial (later) neurogenetic gradient between these two components of the reticular complex: only about 12% of the lateral subnucleus neurons, but close to 30% of the medial subnucleus neurons, are generated on day E15. Because the lateral and medial subnuclei display the typical outside-in gradient found in the thalamus, they are considered to constitute a single cytogenetic sector; the early generated central subnucleus, which violates this order, is considered to constitute a separate cytogenetic sector. Observations are presented that neurons of the central reticular subnucleus originate in a unique neuroepithelial region, the reticular protuberance. The migration of heavily labeled cells was traced from this region in rats labeled with 3H-thymidine on day E13 and killed on the subsequent days. The neurons of the lateral and medial reticular subnuclei originate in the reticular lobule of the thalamic neuroepithelium. The migration of heavily labeled, spindle-shaped cells was traced from this region in rats labeled with 3H-thymidine on days E14 and E15 and killed at daily intervals thereafter. The neurogenetic gradient of the reticular thalamic complex seen in postnatal rats is established before birth.     

Development of the rat thalamus: I. Mosaic organization of the thalamic neuroepithelium

Short-survival, sequential, and long-survival thymidine radiograms of rat embryos, fetuses and young pups were analyzed in order to delineate the boundaries of the proliferative thalamic neuroepithelium, describe its early transformations, identify its regional divisions, and, finally, attempt to relate its distinct neuroepithelial components to specific thalamic nuclei that they supply with neurons. On day E13 the thalamic neuroepithelium consists of two divisions, the rostral lobe and the caudal lobe, and interposed between the two is a small transient structure, the reticular protuberance. By day E14 the rostral lobe has become partitioned into the anterior lobule and the reticular lobule, and the caudal lobe into the intermediate lobule and the posterior lobule. By day E15 these four lobules have become further partitioned into sublobules, characterized as regional eversions and inversions (concavities and convexities) of the thalamic neuroepithelium. Several of these sublobules are still recognizable on day E16 but progressively disappear thereafter. In this introductory paper, some evidence is presented in support of the hypothesis that the identified thalamic sublobules represent putative cell lines committed to produce neurons for specific, early-generated thalamic nuclei. Detailed documentation of the evidence on which the identifications are based is provided in subsequent papers of this series which deal with the early development of specific thalamic regions and nuclei. In our attempt to identify these putative cell lines, we sought to meet the following criteria: (1) a good match between the time course of mitotic activity in a neuroepithelial sublobule and the birth days of neurons in the nucleus that it is postulated to supply with neurons, (2) relative proximity between the putative neuroepithelial source and the thalamic target structure, and, where possible, (3) the tracing of migrating cells from the germinal source to its destination. Using these criteria we have made the following tentative identifications. The early derivatives of the anterior thalamic lobules are the sublobules (committed cell lines) of the anterior thalamic nuclei, and of the central lateral and mediodorsal nuclei. The early derivatives of the reticular lobule and reticular protuberance are the sublobules of the reticular nuclear complex. The early derivatives of the intermediate lobule are the sublobules of the ventrolateral and ventrobasal nuclei. Finally, the early derivatives of the posterior lobule are the sublobules of the dorsal geniculate, ventral geniculate, and medial geniculate nuclei.(ABSTRACT TRUNCATED AT 400 WORDS)     

Sensitivity to GABA of neurons of the dorsal and ventral lateral geniculate nuclei in the rat

GABA was applied iontophoretically to dorsal and ventral lateral geniculate (LGd and LGv) neurons in rats. Spontaneous discharges were readily suppressed in both species of neurons. While in LGd neurons, evoked discharges by optic nerve stimulation were suppressed as readily as were spontaneous discharges, LGv neurons were characterized in that evoked discharges were much more resistant than spontaneous discharges.     

Development of the rat thalamus: V. The posterior lobule of the thalamic neuroepithelium and the time and site of origin and settling pattern of neurons of the medial geniculate body

Long-survival, sequential, and short-survival thymidine radiograms of rat embryos, fetuses, and young pups were analyzed in order to examine the time of origin, site of origin, migratory route, and settling pattern of neurons of the medial geniculate body (MG). Quantitative evaluation of long-survival radiograms established that the bulk of MG neurons are generated between embryonic (E) days E13 and E15, with a pronounced peak on day E14. There is an overall lateral-to-medial and caudal-to-rostral chronological gradient in MG neurogenesis. On the basis of significant regional differences in the birth dates of neurons, the MG was divided into several chronoarchitectonic areas. The earliest-generated neurons (with close to 20% of the cells produced on day E13 and a negligible proportion on day E15) form the dorsal and ventral clusters far laterally. Next in sequential order are the neurons of the lateral shell, intermediate shell, and medial shell of the MG. The medial shell with it latest-generated neurons (with over 30% produced rostrally on day E15) corresponds to the medial (magnocellular) subnucleus of the MG. There were no neurogenetic differences between the traditional dorsal and ventral divisions of the MG. Examination of sequential radiograms in rats labeled with 3H-thymidine on day E14 or E15 and killed on successive days brought supportive evidence for our earlier identification, in short-survival radiograms, of a posteroventral thalamic neuroepithelial evagination as the putative source, or committed cell line, of MG neurons. Wave fronts of apparently migrating unlabeled and labeled cells could be traced from this sublobule in a posterolateral direction to the future site of the MG.     

Development of the precerebellar nuclei in the rat: I. The precerebellar neuroepithelium of the rhombencephalon

Short-survival thymidine radiograms from rat embryos aged 13-19 days were analyzed to delineate the precerebellar neuroepithelium of the rhombencephalon. The original definition of the term "rhombencephalon" was modified to refer only to the unique dorsal portion (surface plate) of the medulla and pons where the neural groove fails to fuse and, instead, the medullary velum covers the rhomboid lumen of the fourth ventricle. Initially, the neuroepithelial tissue of the rhombencephalon consists of a pair of rostral and caudal bridgeheads: the former the primary neuroepithelium of the cerebellum and the latter the primary neuroepithelium of the octavo-precerebellar system. The spatial relationship between the cerebellar and precerebellar neuroepithelia soon changes as a result of ongoing morphogenetic events, such that the cerebellar primordium assumes a dorsal position and the precerebellar primordium a ventral position, and the distance between the two decreases. Concurrently the tela choroidea invaginates into the fourth ventricle and a secondary precerebellar neuroepithelium develops. The rostral portion of the secondary precerebellar neuroepithelium grows forward along the choroid plexus and forms the medial recess of the anterior fourth ventricle, while its caudal portion grows in the opposite direction beneath the medullary velum and forms the rostral wall of the posterior fourth ventricle. Evidence will be presented in the succeeding papers that the primary precerebellar neuroepithelium first generates the neurons of the inferior olive that migrate by a circumferential intramural (parenchymal) route to their destination. Next, the neurons of the lateral reticular and external cuneate nuclei are generated. These migrate by a posterior extramural (superficial) route and settle contralaterally. Subsequently, the primary precerebellar neuroepithelium produces the neurons of the nucleus reticularis tegmenti pontis and these form the anterior extramural migratory stream and settle ipsilaterally. Finally, the secondary precerebellar neuroepithelium produces the latest generated neurons of the basal pontine gray that follow the anterior extramural stream and settle ipsilaterally.     

Serotonin-mediated inhibition from dorsal raphe nucleus of neurons in dorsal lateral geniculate and thalamic reticular nuclei

Electrophysiological studies using rats anesthetized with chloral hydrate were performed to determine whether or not serotonin originating in the dorsal raphe nucleus (DR) acts as an inhibitory transmitter or neuromodulator on neurons of the dorsal lateral geniculate nucleus (LGN) and neurons located in the thalamic reticular nucleus (TRN) immediately rostral to the dorsal LGN. In the LGN, conditioning stimuli applied to the DR preceding test stimulus to the optic tract and visual cortex inhibited orthodromic and antidromic spikes in about one-third of the relay neurons and in more than half of the intrageniculate interneurons. Conditioning stimulation of the DR also produced an inhibition of the spikes elicited by stimulation of the optic tract and visual cortex of at least three-quarters of the TRN neurons. Iontophoretic application of serotonin (25 nA) inhibited the orthodromic spikes of the LGN relay neuron and TRN neuron. A close correlation was observed between the effects of DR conditioning stimulation and iontophoretic serotonin in the same neurons. The inhibition with DR conditioning stimulation and iontophoretically applied serotonin was antagonized during iontophoretic application of methysergide (15-40 nA), a serotonin antagonist. These results strongly suggest that serotonin derived from the DR acts on the LGN and TRN neurons as an inhibitory transmitter or neuromodulator to inhibit transmission in these nuclei.     

Quantitative analysis of the lateral geniculate nucleus in the mutant microphthalmic rat

A quantitative analysis of the lateral geniculate nucleus was carried out in the mutant microphthalmic rat. In the dorsal lateral geniculate nucleus (LGNd) of the microphthalmic rat we found the total volume and neuronal population were reduced by 45 and 68% of normal values, respectively. The size of normal LGNd neurons was 8 to 20 μm and that of mutant LGNd cells from 6 to 16 μm. Neurons of the normal LGNd were medium-size and round or oval, and their cell bodies were filled with Nissl substance. Microphthalmic LGNd neurons, on the other hand, had narrow cytoplasmic spaces with few Nissl granules, and pale cell nuclei. In the microphthalmic rat, the lateral part of the ventral lateral geniculate nucleus (LGNvl) also showed a marked reduction in the total volume and neuronal population which were 42 and 76% of normal values, respectively. The size of normal LGNvl neurons was 8 to 20 μm and that of the microphthalmic neurons from 6 to 16 μm. These findings suggested that a marked reduction in the size of the LGNd and LGNvl in the mutant can be attributed to a decrease in neuronal population to a diminution of cell size.     

Development of the precerebellar nuclei in the rat: III. The posterior precerebellar extramural migratory stream and the lateral reticular and external cuneate nuclei

Sequential thymidine radiograms from rats injected on day E15 and killed thereafter at daily intervals up to day E22 were analyzed to trace the migratory routes and settling patterns of neurons of the lateral reticular nucleus and the external cuneate nucleus. The neurons of the lateral reticular and external cuneate nuclei originate in the primary precerebellar neuroepithelium at the same site as the inferior olivary neurons but follow a different migratory route. The labeled young neurons that are produced on day E15 (the last one-third of the total) join the posterior precerebellar extramural migratory stream. The cells move circumferentially over the wall of the medulla in a ventral direction and by day E17 reach the midline and cross it beneath the inferior olive. The crossing cells apparently continue to migrate circumferentially on the opposite side. One complement of these cells begins to form a ventrolateral extramural condensation on day E19. By day E20 some cells begin to penetrate the parenchyma and settle as neurons of the lateral reticular nucleus. The settling of the lateral reticular neurons continues on the following day, and by day E22 all the cells destined for the lateral reticular nucleus have penetrated the parenchyma. A dorsomedial-to-ventrolateral neurogenetic gradient is indicated for the settling lateral reticular neurons. Another complement of migrating cells continues dorsally and forms a condensation on day E19 that we interpret as the external cuneate component of the crossed stream. These cells begin to penetrate the parenchyma on day E20, and by days E21 and E22 two components of the external cuneate nucleus are identifiable-the dorsal and ventral external cuneate nuclei. The neurons of the lateral reticular and external cuneate nuclei differ from neurons of all the other precerebellar nuclei in that their cerebellar projection is predominantly ipsilateral. We speculate that the axons of all precerebellar neurons are genetically specified to cross the midline ventrally to provide a contralateral efferent projection, but this is modified in the case of the ipsilaterally projecting lateral reticular and external cuneate neurons by the cell bodies following their neurites to the opposite side.     

The organization of the lateral thalamus of the hooded rat

Analysis of cytoarchitecture and connectivity showed that the lateral thalamus of the hooded rat is composed of eight nuclei. An examination of the cytoarchitecture permitted the identification of seven cellular fields: nucleus suprageniculatus (sg), nucleus lateralis posterior pars caudomedialis (lpcm), nucleus lateralis posterior pars lateralis (lpl), nucleus lateralis posterior pars rostromedialis (lprm), intramedullary area (ima), nucleus lateralis dorsale pars ventrolateralis (ldvl), and nucleus lateralis dorsale pars dorsomedialis (lddm). An analysis of the connectivity showed that lpl is further divisible into a rostral (lplr) and a caudal (lplc) sector, bringing the total number of nuclei to eight. Nucleus suprageniculatus, the most caudal element of the lateral thalamus, is composed of medium to large, fusiform, and multipolar neurons. It contains a terminal field of the projection of the superficial layers of the ipsilateral superior colliculus. Nucleus lpcm, found rostrolateral to sg, is loosely packed with large multipolar neurons. A terminal field of the superficial layers of the superior colliculus of both sides fits precisely within its cytoarchitectural boundaries. Nucleus lpl, a long cellular territory found lateral to lpcm, extends from the caudal pole of the dorsal lateral geniculate nucleus to the caudal pole of ldvl and contains round cells which are smaller and more densely packed than those of lpcm. Its caudal portion (lplc) contains another terminal field of the ipsilateral superior colliculus while its rostral portion (lplr) contains a terminal field of the projection of Area 17. Area 18 also projects to lplr, whereas Area 18a projects to both lplr and lplc. The intramedullary area, which occupies the fibrous zone between lpl and the dorsal lateral geniculate nucleus, contains round and fusiform neurons and is innervated by Area 18a. Nucleus lprm, situated medial to lpl, is characterized by round neurons which are frequently found in clusters. It is innervated by Areas 17, 18, and 18a. Nucleus ldvl is evenly packed with moderately large, polygonal cells and contains the complete terminal fields of both Areas 17 and 18. It also receives inputs from Area 18a. Finally, lddm, tightly packed with small, round cells and lying medial to ldvl, receives inputs from Area 4.     

Visual function in the ventral lateral geniculate nucleus of the cat

The visual receptive fields of 293 single units in the ventral lateral geniculate nucleus of the cat were studied. In addition to the wide variety of types described by others, a group of units responding differentially to color was identified that included units responding particularly to blue and others with opponent color properties. Some units with spontaneous firing and without definite visual receptive fields were inhibited by stimulation of the optic chiasm (OX). A study of latency of firing to OX stimulation suggested that these cells were driven by retinal ganglion cells of the W type. One-third of all units studied were binocularly driven.     

Development of the projections from the dorsal lateral geniculate nucleus to the lateral suprasylvian visual area of cortex in the cat.

In the study reported in the preceding paper, we used retrograde labeling methods to show that the enhanced projection from the thalamus to the posteromedial lateral suprasylvian (PMLS) visual area of cortex that is present in adult cats following neonatal visual cortex damage arises at least partly from surviving neurons in the dorsal lateral geniculate nucleus (LGN). In the C layers of the LGN, many more cells than normal are retrogradely labeled by horseradish peroxidase (HRP) injected into PMLS cortex ipsilateral to a visual cortex lesion. In addition, retrogradely labeled cells are found in the A layers, which normally have no projection to PMLS cortex in adult cats. The purpose of the present study was to investigate the mechanisms of this enhanced projection by examining the normal development of projections from the thalamus, especially the LGN, to PMLS cortex. Injections of HRP were made into PMLS cortex on the day of birth or at 1, 2, 4, or 8 weeks of age. Retrogradely labeled neurons were present in the lateral posterior nucleus, posterior nucleus of Rioch, pulvinar, and medial interlaminar nucleus, as well as in the LGN, at all ages studied. Within the LGN of the youngest kittens, a small number of retrogradely labeled cells was present in the interlaminar zones and among the cells in the A layers that border these zones. Such labeled cells were virtually absent by 8 weeks of age, and they are not found in normal adult cats. Sparse retrograde labeling of C-layer neurons also was present in newborn kittens.(ABSTRACT TRUNCATED AT 250 WORDS)     

GABA-immunoreactive neurons in the rat dorsal lateral geniculate nucleus: light microscopical observations

The dorsal lateral geniculate nucleus (dLGN) of the rat was investigated immunocytochemically using an antiserum against the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). The appearance of GABA-immunopositive dendrites, dendritic appendages, and the size and shape of neuronal somata closely resembled the putative intrinsic neurons described previously in Golgi-impregnation studies of the rat dLGN.     

The ventral lateral geniculate nucleus in the cat: thalamic and commissural connections revealed by the use of WGA-HRP transport

The present investigation was carried out to clarify the topographical details of both the origin and terminal site of the thalamic projections and the commissural connections of the ventral lateral geniculate nucleus (LGNv) in the cat by using bidirectional transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Thalamic projections: Unilateral injections of WGA-HRP into the LGNv produced orthograde labeling in the intralaminar nuclei bilaterally and in the lateralis posterior (LP) and the pulvinar (Pul) nucleus ipsilaterally. In the intralaminar nuclei the rostral part of the nucleus centralis lateralis (CL) was most densely labeled by orthogradely transported material, particularly in its dorsal and lateral large-celled portion. Other intralaminar nuclei--such as the nucleus paracentralis, centralis medialis, and centralis dorsalis--also were labeled bilaterally with ipsilateral predominance, but no labeling was detected in the caudal portion of the CL and the centromedian and parafascicular nuclei. In the Pul, labeling of terminal ramifications was found to be concentrated in a region just medial to the so-called retinorecipient zone of the Pul as a slim band of labeling inclining dorsoventrally. In the LP, fine labeled fibers were located in the lateral portion of the LP. Commissural connections: Commissural fibers crossed in the dorsal part of the posterior commissure and reached the most caudal part of the contralateral LGNv. Labeling in the contralateral LGNv was concentrated in the dorsomedial part of the medial zone that extends medially to the middle portion of the cerebral peduncle. Origins of the commissural connections arose mostly from the medial zone that roughly corresponds to the commissural terminal zone and partly from aberrant cells dispersed among optic tract fibers. From these results, together with the previous studies, it is concluded that although the cat's LGNv has connections with diverse structures in the central nervous system, the origin and terminal site of the connections are partially segregated within the nucleus, which suggests that the LGNv may contain functional subsystems.     

The visual field representation of the rat ventral lateral geniculate nucleus

The representation of the visual field in the ventral lateral geniculate nucleus (LGNv) was studied in rats anesthetized with urethane by recording the response of single units to visual stimulation. Receptive fields of LGNv units were plotted on a campimeter, 60 cm in diameter, which was placed 30 cm from the contralateral eye. LGNv neurons responded mainly to stimulation of the contralateral eye with on-tonic characteristics. Few neurons responded only to stimulation of the ipsilateral eye and no binocular interaction was observed. Retinotopic organization was clearly seen in the LGNv; the nasal visual fields were represented dorsally, the temporal fields ventrally, and the upper to lower visual fields were in the rostrolateral to caudomedial parts of the LGNv. A given point in the visual field is represented along a line running through the LGNv in a rostrocaudal direction. Almost the entire horizontal extent of the contralateral visual field was represented in the LGNv, whereas vertically the visual field between 40 degrees above and 20 degrees below the distribution axis was represented. The major axis of the strip of the visual field containing all the RF centers, which is referred to as the distribution axis, inclined nasally up and temporally down at an angle of 10.4 degrees to the 0 degree horizontal meridian line. The representation of the distribution axis in the retina was in accordance with the major axis of retinal ganglion cell distribution (Fukuda, '77; Schober and Gruschka, '77).     

Mode of termination of afferents from the thalamic reticular nucleus in the dorsal lateral geniculate nucleus of the rat

The terminals of axons projecting to the dorsal lateral geniculate nucleus from the thalamic reticular nucleus were identified by electron microscopy 8–24 h after placing small lesions in the ipsilateral reticular nucleus. The terminals contained flattened synaptic vesicles and made Gray type II axo-dendritic synaptic contacts with geniculate neurons. Their identification as F-axons accords well with physiological evidence for a powerful monosynaptic inhibitory input to geniculocortical projection cells from reticular nucleus neurons.     

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. J. Comp. Neurol. 440:321–341, 2001. © 2001 Wiley‐Liss, Inc.     

Interconnections among nuclei of the subcortical visual shell: The intergeniculate leaflet is a major constituent of the hamster subcortical visual system

The intergeniculate leaflet (IGL), a major constituent of the circadian visual system, is one of 12 retinorecipient nuclei forming a “subcortical visual shell” overlying the diencephalic–mesencephalic border. The present investigation evaluated IGL connections with nuclei of the subcortical visual shell and determined the extent of interconnectivity between these nuclei. Male hamsters received stereotaxic, iontophoretic injections of the retrograde tracer, cholera toxin β fragment, or the anterograde tracer, Phaseolus vulgaris-leucoagglutin, into nuclei of the pretectum (medial, commissural, posterior, olivary, anterior, nucleus of the optic tract, posterior limitans), into the superior colliculus, or into the visual thalamic nuclei (lateral posterior, dorsal lateral geniculate, intergeniculate leaflet, ventral lateral geniculate). Retrogradely labeled cell bodies identified nuclei with afferents projecting to the site of injection, whereas the presence of anterogradely labeled fibers with terminals revealed brain nuclei targeted by neurons at the site of injection. The IGL projects bilaterally to all nuclei of the visual shell except the lateral posterior and dorsal lateral geniculate nuclei. The IGL also has afferents from the same set of nuclei, except the nucleus of the optic tract. The extensive bilateral efferent projections distinguish IGL from the ventral lateral geniculate nucleus. The superior colliculus, commissural pretectal, olivary pretectal, and posterior pretectal nuclei also project bilaterally to the majority of subcortical visual nuclei. The IGL has a well-established role in circadian rhythm regulation, but there is as yet no known function for it in the larger context of the subcortical visual system, much of which is involved in oculomotor control. J. Comp. Neurol. 396:288–309, 1998. © 1998 Wiley-Liss, Inc.     

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

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