Cell death in the lateral geniculate nucleus, and its possible relationship with nicotinic receptors and sudden infant death syndrome (SIDS).

The role of the lateral geniculate nucleus (LGN) in vision has been extensively studied, yet its extraretinal capacities are still being investigated, including its role in arousal from sleep. The ß2 nicotinic acetylcholine receptor (nAChR) subunit is involved in the laminal organisation of the LGN with magnocellular (MC) and parvocellular (PC) neurons. Sudden infant death syndrome (SIDS) occurs during a sleep period and, neuropathologically, is associated with increased neuronal cell death and altered nAChRs. A recent qualitative pilot study from our group implicates the possibility of increased neuronal death/apoptosis in the SIDS LGN. The present study used quantitative analysis to report the baseline expression of apoptotic and nAChR subunits α7 and ß2 in the PC and MC layers of the LGN, to determine correlations amongst these markers within layers and across layers, and to evaluate changes in the expression of these markers in the LGN of SIDS infants, along with associations with SIDS risk factors, such as age, sex, cigarette smoke exposure, bed-sharing, and presence of an upper respiratory tract infection (URTI). Tissue was immunohistochemically stained for cell death markers of active caspase-3 (Casp-3) and TUNEL, and for the α7 and ß2 nAChR subunits. Amongst 43 cases of sudden and unexpected deaths in infancy (SUDI), classifications included explained deaths (eSUDI, n = 9), SIDS I (n = 5) and SIDS II (n = 29). Results indicated a strong correlation of the apoptotic markers and ß2 nAChR subunit between the LGN layers, but not across the markers within the layers. Amongst the diagnostic groups, compared to eSUDI, the SIDS II cases had decreased Casp-3 expression while ß2 nAChR expression was increased in both PC and MC layers. Amongst the SIDS risk factors, URTI and bed-sharing were associated with changes in neuronal death but not in the α7 and ß2 markers. In conclusion, our findings do not support a role for the α7 and ß2 nAChRs in apoptotic regulation of the LGN layers during infancy. However, for SIDS victims, an inverse correlation between the changes for markers of apoptosis and the ß2 nAChR subunit expression suggests altered LGN function.

SYNPLA, a method to identify synapses displaying plasticity after learning.

Which neural circuits undergo synaptic changes when an animal learns? Although it is widely accepted that changes in synaptic strength underlie many forms of learning and memory, it remains challenging to connect changes in synaptic strength at specific neural pathways to specific behaviors and memories. Here we introduce SYNPLA (synaptic proximity ligation assay), a synapse-specific, high-throughput, and potentially brain-wide method capable of detecting circuit-specific learning-induced synaptic plasticity.

Medial geniculate body and primary auditory cortex differentially contribute to striatal sound representations.

The dorsal striatum has emerged as a key region in sensory-guided, reward-driven decision making. A posterior sub-region of the dorsal striatum, the auditory striatum, receives convergent projections from both auditory thalamus and auditory cortex. How these pathways contribute to auditory striatal activity and function remains largely unknown. Here we show that chemogenetic inhibition of the projections from either the medial geniculate body (MGB) or primary auditory cortex (ACx) to auditory striatum in mice impairs performance in an auditory frequency discrimination task. While recording striatal sound responses, we find that transiently silencing the MGB projection reduced sound responses across a wide-range of frequencies in striatal medium spiny neurons. In contrast, transiently silencing the primary ACx projection diminish sound responses preferentially at the best frequencies in striatal medium spiny neurons. Together, our findings reveal that the MGB projection mainly functions as a gain controller, whereas the primary ACx projection provides tuning information for striatal sound representations.

Cell type specific tracing of the subcortical input to primary visual cortex from the basal forebrain.

The basal forebrain provides cholinergic inputs to primary visual cortex (V1) that play a key modulatory role on visual function. While basal forebrain afferents terminate in the infragranular layers of V1, acetylcholine is delivered to more superficial layers through volume transmission. Nevertheless, direct synaptic contact in deep layers 5 and 6 may provide a more immediate effect on V1 modulation. Using helper viruses with cell type specific promoters to target retrograde infection of pseudotyped and genetically modified rabies virus evidence was found for direct synaptic input onto V1 inhibitory neurons. These inputs were similar in number to geniculocortical inputs and, therefore, considered robust. In contrast, while clear evidence for dorsal lateral geniculate nucleus input to V1 excitatory neurons was found, there was no evidence of direct synaptic input from the basal forebrain. These results suggest a direct and more immediate influence of the basal forebrain on local V1 inhibition.

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.

Ultrastructure of geniculocortical synaptic connections in the tree shrew striate cortex.

To determine whether thalamocortical synaptic circuits differ across cortical areas, we examined the ultrastructure of geniculocortical terminals in the tree shrew striate cortex to compare directly the characteristics of these terminals with those of pulvinocortical terminals (examined previously in the temporal cortex of the same species; Chomsung et al. [] Cereb Cortex 20:997-1011). Tree shrews are considered to represent a prototype of early prosimian primates but are unique in that sublaminae of striate cortex layer IV respond preferentially to light onset (IVa) or offset (IVb). We examined geniculocortical inputs to these two sublayers labeled by tracer or virus injections or an antibody against the type 2 vesicular glutamate antibody (vGLUT2). We found that layer IV geniculocortical terminals, as well as their postsynaptic targets, were significantly larger than pulvinocortical terminals and their postsynaptic targets. In addition, we found that 9-10% of geniculocortical terminals in each sublamina contacted GABAergic interneurons, whereas pulvinocortical terminals were not found to contact any interneurons. Moreover, we found that the majority of geniculocortical terminals in both IVa and IVb contained dendritic protrusions, whereas pulvinocortical terminals do not contain these structures. Finally, we found that synaptopodin, a protein uniquely associated with the spine apparatus, and telencephalin (TLCN, or intercellular adhesion molecule type 5), a protein associated with maturation of dendritic spines, are largely excluded from geniculocortical recipient layers of the striate cortex. Together our results suggest major differences in the synaptic organization of thalamocortical pathways in striate and extrastriate areas.

Spatiotemporal Profile of Voltage-Sensitive Dye Responses in the Visual Cortex of Tree Shrews Evoked by Electric Microstimulation of the Dorsal Lateral Geniculate and Pulvinar Nuclei.

The primary visual cortex (V1) receives its main thalamic drive from the dorsal lateral geniculate nucleus (dLGN) through synaptic contacts terminating primarily in cortical layer IV. In contrast, the projections from the pulvinar nucleus to the cortex are less clearly defined. The pulvinar projects predominantly to layer I in V1, and layer IV in extrastriate areas. These projection patterns suggest that the pulvinar nucleus most strongly influences (drives) activity in cortical areas beyond V1. Should this hypothesis be true, one would expect the spatiotemporal responses evoked by pulvinar activation to be different in V1 and extrastriate areas, reflecting the different connectivity patterns. We investigated this issue by analyzing the spatiotemporal dynamics of cortical visual areas' activity following thalamic electrical microstimulation in tree shrews, using optical imaging and voltage-sensitive dyes. As expected, electrical stimulation of the dLGN induced fast and local responses in V1, as well as in extrastriate and contralateral cortical areas. In contrast, electrical stimulation of the pulvinar induced fast and local responses in extrastriate areas, followed by weak and diffuse activation in V1 and contralateral cortical areas. This study highlights spatiotemporal cortical activation characteristics induced by stimulation of first (dLGN) and high-order (pulvinar) thalamic nuclei. SIGNIFICANCE STATEMENT: The pulvinar nucleus represents the main extrageniculate thalamic visual structure in higher-order mammals, but its exact role remains enigmatic. The pulvinar receive prominent inputs from virtually all visual cortical areas. Cortico-thalamo-cortical pathways through the pulvinar nuclei may then provide a complementary route for corticocortical information flow. One step toward the understanding of the role of transthalamic corticocortical pathways is to determine the nature of the signals transmitted between the cortex and the thalamus. By performing, for the first time, high spatiotemporal mesoscopic imaging on tree shrews (the primate's closest relative) through the combination of voltage-sensitive dye recordings and brain stimulation, we revealed clear evidence of distinct thalamocortical functional connectivity pattern originating from the geniculate nucleus and the pulvinar nuclei.

Relationship between monocularly deprivation and amblyopia rats and visual system development.

OBJECTIVE: To explore the changes of lateral geniculate body and visual cortex in monocular strabismus and form deprived amblyopic rat, and visual development plastic stage and visual plasticity in adult rats. METHODS: A total of 60 SD rats ages 13 d were randomly divided into A, B, C three groups with 20 in each group, group A was set as the normal control group without any processing, group B was strabismus amblyopic group, using the unilateral extraocular rectus resection to establish the strabismus amblyopia model, group C was monocular form deprivation amblyopia group using unilateral eyelid edge resection + lid suture. At visual developmental early phase (P25), meta phase (P35), late phase (P45) and adult phase (P120), the lateral geniculate body and visual cortex area 17 of five rats in each group were exacted for C-fos Immunocytochemistry. Neuron morphological changes in lateral geniculate body and visual cortex was observed, the positive neurons differences of C-fos expression induced by light stimulation was measured in each group, and the condition of radiation development of P120 amblyopic adult rats was observed. RESULTS: In groups B and C, C-fos positive cells were significantly lower than the control group at P25 (P<0.05), there was no statistical difference of C-fos protein positive cells between group B and group A (P>0.05), C-fos protein positive cells level of group B was significantly lower than that of group A (P<0.05). The binoculus C-fos protein positive cells level of groups B and C were significantly higher than that of control group at P35, P45 and P120 with statistically significant differences (P<0.05). CONCLUSIONS: The increasing of C-fos expression in geniculate body and visual cortex neurons of adult amblyopia suggests the visual cortex neurons exist a certain degree of visual plasticity.

Evidence for ape and human specializations in geniculostriate projections from VGLUT2 immunohistochemistry.

Vesicular glutamate transporters (VGLUTs) reuptake glutamate into synaptic vesicles at excitatory synapses. VGLUT2 is localized in the cortical terminals of neuronal somas located in the main sensory nuclei of the thalamus. Thus, immunolabeling of cortex with antibodies to VGLUT2 can reveal geniculostriate terminal distributions in species in which connectivity cannot be studied with tract-tracing techniques, permitting broader comparative studies of cortical specializations. Here, we used VGLUT2 immunohistochemistry to compare the organization of geniculostriate afferents in primary visual cortex in hominid primates (humans, chimpanzees, and an orangutan), Old World monkeys (rhesus macaques and vervets), and New World monkeys (squirrel monkeys). The New and Old World monkeys had a broad, dense band of terminal-like labeling in cortical layer 4C, a narrow band of labeling in layer 4A, and additional labeling in layers 2/3 and 6, consistent with results from conventional tract-tracing studies in these species. By contrast, although the hominid primates had a prominent layer 4C band, labeling of layer 4A was sparse or absent. Labeling was also present in layers 2/3 and 6, although labeling of layer 6 was weaker in hominids and possibly more individually variable than in Old and New World monkeys. These findings are consistent with previous observations from cytochrome oxidase histochemistry and a very small number of connectivity studies, suggesting that the projections from the parvocellular layers of the lateral geniculate nucleus to layer 4A were strongly reduced or eliminated in humans and apes following their evolutionary divergence from the other anthropoid primates.

Diffusion tensor imaging reveals normal geniculocalcarine-tract integrity in acquired blindness.

We investigated optic nerve and geniculocalcarine tract (GCT) in acquired blindness (AB) using routine cranium magnetic resonance imaging and diffusion tensor imaging. Twenty individuals with AB were compared with 20 normally sighted (NS) individuals. The transverse diameters of optic nerves in NS were significantly bigger than the AB participants in T(1)WI maps. AB participants had higher mean diffusivity and transverse diffusivity and lower fractional anisotropy and primary diffusivity in the optic nerve. This pattern of diffusion change suggests axonal degeneration or atrophy of nerve fibers. No diffusion-index alterations in the GCT were found between AB participants and NS controls. White matter integrity remained normal in the GCT. Thus, the GCT may not rely on visual afferent input to maintain integrity after development.

Broad characterization of endogenous peptides in the tree shrew visual system.

Endogenous neuropeptides, acting as neurotransmitters or hormones in the brain, carry out important functions including neural plasticity, metabolism and angiogenesis. Previous neuropeptide studies have focused on peptide-rich brain regions such as the striatum or hypothalamus. Here we present an investigation of peptides in the visual system, composed of brain regions that are generally less rich in peptides, with the aim of providing the first broad overview of peptides involved in mammalian visual functions. We target three important parts of the visual system: the primary visual cortex (V1), lateral geniculate nucleus (LGN) and superior colliculus (SC). Our study is performed in the tree shrew, a close relative of primates. Using a combination of data dependent acquisition and targeted LC-MS/MS based neuropeptidomics; we identified a total of 52 peptides from the tree shrew visual system. A total of 26 peptides, for example GAV and neuropeptide K were identified in the visual system for the first time. Out of the total 52 peptides, 27 peptides with high signal-to-noise-ratio (>10) in extracted ion chromatograms (EIC) were subjected to label-free quantitation. We observed generally lower abundance of peptides in the LGN compared to V1 and SC. Consistently, a number of individual peptides showed high abundance in V1 (such as neuropeptide Y or somatostatin 28) and in SC (such as somatostatin 28 AA1-12). This study provides the first in-depth characterization of peptides in the mammalian visual system. These findings now permit the investigation of neuropeptide-regulated mechanisms of visual perception.

Perisynaptic aggrecan-based extracellular matrix coats in the human lateral geniculate body devoid of perineuronal nets.

The extracellular matrix surrounds different neuronal compartments in the mature nervous system. In a variety of vertebrates, most brain regions are loaded with a distinct type of extracellular matrix around the somatodendritic part of neurons, termed perineuronal nets. The present study reports that chondrotin sulfate proteoglycan-based matrix is structured differently in the human lateral geniculate body. Using various chondrotin sulfate proteoglycan-based extracellular matrix antibodies, we show that perisomatic matrix labeling is rather weak or absent, whereas dendrites are contacted by axonal coats appearing as small, oval structures. Confocal laser scanning microscopy and electron microscopy demonstrated that these typical structures are associated with synaptic loci on dendrites. Using multiple labelings, we show that different chondrotin sulfate proteoglycan components of the extracellular matrix do not associate exclusively with neuronal structures but possibly associate with glial structures as well. Finally, we confirm and extend previous findings in primates that intensity differences of various extracellular matrix markers between magno- and parvocellular layers reflect functional segregation between these layers in the human lateral geniculate body.

Histological determination of the areas enriched in cholinergic terminals and M2 and M3 muscarinic receptors in the mouse central auditory system.

Cholinergic projections to auditory system are vital for coupling arousal with sound processing. Systematic search with in situ hybridization and immunohistochemistry indicated that the ventral nucleus of the medial geniculate body and the nucleus of the brachium of the inferior colliculus constituted cholinergic synaptic sites in the brainstem auditory system, containing a significant number of cholinergic axon terminals and m2 receptor-expressing cell bodies.

Different distributions of calbindin and calretinin immunostaining across the medial and dorsal divisions of the mouse medial geniculate body.

We studied the distributions of calretinin and calbindin immunoreactivity in subdivisions of the mouse medial geniculate body and the adjacent paralaminar nuclei. We found that the vast majority of labeled cells in the dorsal division of the medial geniculate body were immunoreactive for calbindin-only, whereas most of the remaining labeled cells were double-labeled. Very few calretinin+ only cells were observed. By contrast, we observed significant proportions of calbindin+ only, calretinin+ only and double-labeled cells in the medial division of the medial geniculate body. Further, the distributions of calbindin-only, calretinin-only and double-labeled cells did not differ between the medial division of the medial geniculate body, the suprageniculate nucleus, the peripeduncular nucleus and the posterior intralaminar nucleus. We found essentially no somatic staining for either calbindin or calretinin in the ventral division of the medial geniculate body. These data suggest that there are distinct neurochemical differences between the two non-lemniscal auditory thalamic nuclei. In addition, these data extend previous observations that the medial division of the medial geniculate body shares many properties with the paralaminar group of nuclei.

[Distribution of met-enkephalin expression in the visual pathway. Experimental study by inmunocytochemistry]. / Distribución de la metencefalina en la vía óptica. Estudio experimental inmunocitoquímico.

PURPOSE: The localization and distribution of neuropeptide expression in the cat visual pathway can provide information about the function of that pathway. METHOD: Study of optic pathway in eight cats. Following extraction of the brain, slices were prepared using a microkeratome. The slices were examined by indirect immunocytochemistry using anti-metenkephalin as antibody to determine the presence or absence of this pentapeptide in the visual pathway. RESULTS: Met-enkephalin receptors in both cortical and subcortical regions of the brain were detected. This suggests that met-enkephalin could be involved in the visual mechanism. CONCLUSIONS: The presence of met-enkephalin receptors in both cortical and subcortical regions of the brain suggests that this pentapeptide could be involved in the visual mechanism.

Glaucoma alters the expression of NGF and NGF receptors in visual cortex and geniculate nucleus of rats: effect of eye NGF application.

We investigated the effect of glaucoma (GL) on nerve growth factor (NGF) presence in two brain visual areas. Rats with elevated intraocular pressure (EIOP), induced by hypertonic saline injection in the episcleral vein, were treated with eye topical application of saline or NGF. Rats were subsequently sacrificed, and brain tissues were used for immunohistochemical, biochemical, and molecular analyses. We found that GL alters the basal level of NGF and NGF receptors in brain visual centers and that NGF eye application normalized these deficits. These findings demonstrate that the reduced presence of NGF can arise due to degenerative events in retinal and brain visual areas.

Distribution of neurofilament proteins in the lateral geniculate nucleus, primary visual cortex, and area MT of adult Cebus monkeys.

We investigated the distribution pattern of SMI-32-immunopositive cells in the lateral geniculate nucleus (LGN) and in the primary (V1) and middle temporal (MT) cortical visual areas of the adult New World monkey Cebus apella. In the LGN, the reaction for SMI-32 labeled cells in both the magnocellular (M) and parvocellular (P) layers. However, the cellular label was heavier in M layers, which also showed a more intense labeling in the neuropil. In V1, the reaction showed a lamination pattern, with the heaviest labeling occurring in layer 4B and upper layer 6 (layers that project to area MT). Area MT shows a dense band of labeled neuropil and large pyramidal neurons in layer 3, large darkly labeled but less densely packed neurons in layer 5, and a population of small, lightly labeled cells in layer 6. These results resemble those found in other New and Old World monkeys, which suggest that the preferential labeling of projection neurons associated with fast-conducting pathways to the extrastriate dorsal stream is a common characteristic of simian primates. In the superficial layers of V1 in Cebus monkeys, however, SMI-32-labeled neurons are found in both cytochrome oxidase blobs and interblob regions. In this aspect, our results in Cebus are similar to those found in the Old World monkey Macaca and different from those described for squirrel monkey, a smaller New World Monkey. In Cebus, as well as in Macaca, there is no correlation between SMI-32 distribution and the blob pattern.

Molecular organization of the ferret visual thalamus.

The visual system encodes and deciphers information using parallel, anatomically segregated, processing streams. To reveal patterns of gene expression in the visual thalamus correlated with physiological processing streams, we designed a custom ferret cDNA microarray. By isolating specific subregions and layers of the thalamus, we identified a set of transcription factors, including Zic2, Islet1, and Six3, the unique distribution profiles of which differentiated the lateral geniculate nucleus (LGN) from the associated perigeniculate nucleus. Within the LGN, odd homeobox1 differentiated the A layers, which contain X cells and Y cells, from the C layers. One neuron-specific protein, Purkinje cell protein 4 (PCP4), was strongly expressed in Y cells in the ferret LGN and in the magnocellular layers of the primate LGN. In the ferret LGN, PCP4 expression began as early as postnatal day 7 (P7), suggesting that Y cells are already specified by P7. These results reveal a rich molecular repertoire that correlates with functional divisions of the LGN.

Increased c-fos-like immunoreactivity in the superior colliculus and lateral geniculate nucleus of the rd mouse.

In most subcortical visual centers in normal mice maintained for a period in the dark, very few neurons express fos-like immunoreactivity (FLI), most likely reflecting c-fos expression, but if an animal is exposed to a flashing light, there is transient increase in the number of FLI-expressing cells. In dark-maintained retinal degeneration (rd) mice, with an inherited photoreceptor defect, numbers of FLI-positive cells, identified immunohistochemically, are anomalously elevated in the superior colliculus (SC) and lateral geniculate nucleus (LGN). Eye removal largely prevents the elevated counts. The difference in number of FLI-positive cells in the SC of rd mice and nondystrophic controls is highly significant (p<0.001). Because we have previously found a similar phenomenon in Royal College of Surgeons (RCS) rats, in which photoreceptor loss is caused by a retinal pigment cell defect, it argues for an effect related to photoreceptor loss rather than its cause.

Pretectal and tectal afferents to the dorsal lateral geniculate nucleus of the turtle: an electron microscopic axon tracing and gamma-aminobutyric acid immunocytochemical study.

The pretectal and tectal projections to the dorsal lateral geniculate nucleus (GLd) of two species of turtle (Emys orbicularis and Testudo horsfieldi) were examined under the electron microscope by using axonal tracing techniques (horseradish peroxidase or biotinylated dextran amine) and postembedding gamma-aminobutyric acid (GABA) immunocytochemistry. After injection of tracer into the pretectum, two types of axon terminals were identified as those of pretectogeniculate pathways. Both contained pleomorphic synaptic vesicles and were more numerous in the inner part of the nucleus. They could be distinguished on the bases of size and shape of their synaptic vesicles, type of synaptic contact, and level of GABA immunoreactivity. One type had a higher density of immunolabeling and established symmetric synaptic contacts, whereas the other, less densely immunolabeled, made asymmetric synaptic contacts. In both cases, synaptic contacts were mainly with relay cells and occasionally with interneurons. We suggest that these two types of pretectogeniculate terminals originate in two separate pretectal nuclei. After injection of tracer into the optic tectum, a single population of GABA-immunonegative tracer-labeled terminals was identified as belonging to the tectogeniculate pathway. These were small, had smooth contours, contained very small round synaptic vesicles, and established asymmetric synaptic contacts with long active zones, predominantly with relay cells and less frequently with interneurons, in the inner part of the nucleus. In addition, a population of GABA-negative and occasionally GABA-positive terminals, labeled by tracer injected into either the pretectum or the tectum, was identified as retinal terminals; these were presumably labeled by the retrograde transport of tracer in collateral branches of visual fibers innervating both the GLd and the pretectum or tectum. Comparison of the present ultrastructural findings in turtles with those previously reported in mammals shows that the cytological features, synaptic morphology, and immunochemical properties of the pretectogeniculate and tectogeniculate terminals of both groups share many similarities. Nevertheless, the postsynaptic targets of these two categories of terminals display some pronounced differences between the two groups, which are discussed in terms of their possible functional significance.