The visual field representation of the rat ventral lateral geniculate nucleus

The caudal prethalamus: Inhibitory switchboard for behavioral control?

Prethalamic nuclei in the mammalian brain include the zona incerta, the ventral lateral geniculate nucleus, and the intergeniculate leaflet, which provide long-range inhibition to many targets in the midbrain, hindbrain, and thalamus. These nuclei in the caudal prethalamus can integrate sensory and non-sensory information, and together they exert powerful inhibitory control over a wide range of brain functions and behaviors that encompass most aspects of the behavioral repertoire of mammals, including sleep, circadian rhythms, feeding, drinking, predator avoidance, and exploration. In this perspective, we highlight the evidence for this wide-ranging control and lay out the hypothesis that one role of caudal prethalamic nuclei may be that of a behavioral switchboard that-depending on the sensory input, the behavioral context, and the state of the animal-can promote a behavioral strategy and suppress alternative, competing behaviors by modulating inhibitory drive onto diverse target areas.

Projections of ipRGCs and conventional RGCs to retinorecipient brain nuclei

Retinal ganglion cells (RGCs), the output neurons of the retina, allow us to perceive our visual environment. RGCs respond to rod/cone input through the retinal circuitry, however, a small population of RGCs are in addition intrinsically photosensitive (ipRGCs) and project to unique targets in the brain to modulate a broad range of subconscious visual behaviors such as pupil constriction and circadian photoentrainment. Despite the discovery of ipRGCs nearly two decades ago, there is still little information about how or if conventional RGCs (non-ipRGCs) target ipRGC-recipient nuclei to influence subconscious visual behavior. Using a dual recombinase fluorescent reporter strategy, we showed that conventional RGCs innervate many subconscious ipRGC-recipient nuclei, apart from the suprachiasmatic nucleus. We revealed previously unrecognized stratification patterns of retinal innervation from ipRGCs and conventional RGCs in the ventral portion of the lateral geniculate nucleus. Further, we found that the percent innervation of ipRGCs and conventional RGCs across ipsi- and contralateral nuclei differ. Our data provide a blueprint to understand how conventional RGCs and ipRGCs innervate different brain regions to influence subconscious visual behaviors.

Hippocampal coding of conspecific position

Many previous studies have shown that hippocampal place cells respond to the spatial position of the animal itself. Several recent studies have shown that place cells in an observer animal can also encode the location of a conspecific. The interpretation of these previous studies is, however, compromised by the fact that the observer animal was required to complete a movement that was either a duplication of the others trajectory, or a modification of it. This raises the possibility that the observed representation of the other, may have instead been a plan for the self. To test for a representation of a conspecific in a task where immediate behaviour was not immediately required of the observer, Sprague-Dawley rats were trained to run the length of a shuttle box for a food reward. They then observed a second animal (the runner) performing the same task. Positional data was obtained from the runner, while hippocampal single unit data was collected from the observer. Hippocampal single units were observed to have only limited, low resolution, firing rate-modulated representations of the runner animal. There was also evidence of a weak relationship between place cell spatial firing representations of the self and other. Some above-chance evidence of phase-coding of the runner's position was also observed in the observer animals, with an observer-centred reference frame. These results indicate that hippocampal place cells encode some limited spatial information about others when the observer's subsequent behaviour is not dependent on that of the observed.

Integration of Descending Command Systems for the Generation of Context-Specific Locomotor Behaviors

Over the past decade there has been a renaissance in our understanding of spinal cord circuits; new technologies are beginning to provide key insights into descending circuits which project onto spinal cord central pattern generators. By integrating work from both the locomotor and animal behavioral fields, we can now examine context-specific control of locomotion, with an emphasis on descending modulation arising from various regions of the brainstem. Here we examine approach and avoidance behaviors and the circuits that lead to the production and arrest of locomotion.

Segregated anatomical input to sub-regions of the rodent superior colliculus associated with approach and defense

The superior colliculus (SC) is responsible for sensorimotor transformations required to direct gaze toward or away from unexpected, biologically salient events. Significant changes in the external world are signaled to SC through primary multisensory afferents, spatially organized according to a retinotopic topography. For animals, where an unexpected event could indicate the presence of either predator or prey, early decisions to approach or avoid are particularly important. Rodents’ ecology dictates predators are most often detected initially as movements in upper visual field (mapped in medial SC), while appetitive stimuli are normally found in lower visual field (mapped in lateral SC). Our purpose was to exploit this functional segregation to reveal neural sites that can bias or modulate initial approach or avoidance responses. Small injections of Fluoro-Gold were made into medial or lateral sub-regions of intermediate and deep layers of SC (SCm/SCl). A remarkable segregation of input to these two functionally defined areas was found. (i) There were structures that projected only to SCm (e.g., specific cortical areas, lateral geniculate and suprageniculate thalamic nuclei, ventromedial and premammillary hypothalamic nuclei, and several brainstem areas) or SCl (e.g., primary somatosensory cortex representing upper body parts and vibrissae and parvicellular reticular nucleus in the brainstem). (ii) Other structures projected to both SCm and SCl but from topographically segregated populations of neurons (e.g., zona incerta and substantia nigra pars reticulata). (iii) There were a few brainstem areas in which retrogradely labeled neurons were spatially overlapping (e.g., pedunculopontine nucleus and locus coeruleus). These results indicate significantly more structures across the rat neuraxis are in a position to modulate defense responses evoked from SCm, and that neural mechanisms modulating SC-mediated defense or appetitive behavior are almost entirely segregated.

Differential expression of Eph receptors and ephrins correlates with the formation of topographic projections in primary and secondary visual circuits of the embryonic chick forebrain

Repulsion plays a fundamental role in the establishment of a topographic map of the chick retinotectal projections. This has been highlighted by studies demonstrating the role of opposing gradients of the EphA3 receptor tyrosine kinase on retinal axons and two of its ligands, ephrin-A2 and ephrin-A5, in the tectum. We have analyzed the distribution of these two ephrins in other retinorecipient structures in the chick diencephalon and mesencephalon during the period when visual connections are being established. We have found that both ephrin-A2 and ephrin-A5 and their receptors EphA4 and EphA7 are expressed in gradients whose orientation is consistent with the topography of the nasotemporal axis of the respective retinofugal projections. In addition, their distribution suggests that receptor-ligand interactions may be involved in the organization of connections between the different primary visual centers and, thus, in the topographic organization of secondary visual projections. Interestingly, where projections lack a clear topographic representation, a uniform expression of the Eph-ephrin molecules was observed. Finally, we also show that a similar patterning mechanism may be implicated in the transfer of visual information to the telencephalon. These results suggest a conserved function for EphA receptors and their ligands in the elaboration of topographic maps at multiple levels of the visual pathway.

The anatomical organization of the macaque pregeniculate complex

Saccade-related activity recorded in the primate pregeniculate nucleus, and its anatomical connections with the pretectal nucleus of the optic tract (NOT) and superior colliculus (SC), suggest that it plays a role in visual-ocular motor integration. To study this role, a clearer understanding of pregeniculate organization is required. Based on its connectivity and neurotransmitter immunocytochemistry, we demonstrate that this nucleus is composed of several subnuclei, suggesting the term, pregeniculate complex (PrGC). The PrGC includes a weakly developed dorsal lamina, rostrally, and a well-developed ventral lamina. The ventral lamina includes the retinorecipient and superior sublayers, rostrally, and the medial division, caudally. A thin lamina of cells lateral to the dorsal lateral geniculate nucleus is contiguous with the PrGC; we term this the lateral division. The PrGC and the lateral division each project to the SC/NOT; the superior sublayer and medial division of the PrGC are connected reciprocally to the SC/NOT. Immunocytochemistry for gamma-aminobutyric acid (GABA) and substance P (SP) further delineate the PrGC subnuclei. The retinorecipient sublayer stains most intensely for GABA and SP. The superior sublayer and medial division also stain strongly for GABA and SP. Essentially all neurons in the lateral division are GABA-positive. The combination of tract tracing and immunocytochemistry demonstrate differences in the connectivity of the PrGC subnuclei and the lateral division with the SC/NOT. This, combined with the differential localization of GABA in the PrGC, provides a basis for further study of its functional role.

Topographic guidance labels in a sensory projection to the forebrain

Visual connections to the mammalian forebrain are known to be patterned by neural activity, but it remains unknown whether the map topography of such higher sensory projections depends on axon guidance labels. Here, we show complementary expression and binding for the receptor EphA5 in mouse retina and its ligands ephrin-A2 and ephrin-A5 in multiple retinal targets, including the major forebrain target, the dorsal lateral geniculate nucleus (dLGN). These ligands can act in vitro as topographically specific repellents for mammalian retinal axons and are necessary for normal dLGN mapping in vivo. The results suggest a general and economic modular mechanism for brain mapping whereby a projecting field is mapped onto multiple targets by repeated use of the same labels. They also indicate the nature of a coordinate system for the mapping of sensory connections to the forebrain.

The ventral lateral geniculate nucleus and the intergeniculate leaflet: interrelated structures in the visual and circadian systems

The ventral lateral geniculate nucleus (vLGN) and the intergeniculate leaflet (IGL) are retinorecipient subcortical nuclei. This paper attempts a comprehensive summary of research on these thalamic areas, drawing on anatomical, electrophysiological, and behavioral studies. From the current perspective, the vLGN and IGL appear closely linked, in that they share many neurochemicals, projections, and physiological properties. Neurochemicals commonly reported in the vLGN and IGL are neuropeptide Y, GABA, enkephalin, and nitric oxide synthase (localized in cells) and serotonin, acetylcholine, histamine, dopamine and noradrenalin (localized in fibers). Afferent and efferent connections are also similar, with both areas commonly receiving input from the retina, locus coreuleus, and raphe, having reciprocal connections with superior colliculus, pretectum and hypothalamus, and also showing connections to zona incerta, accessory optic system, pons, the contralateral vLGN/IGL, and other thalamic nuclei. Physiological studies indicate species differences, with spectral-sensitive responses common in some species, and varying populations of motion-sensitive units or units linked to optokinetic stimulation. A high percentage of IGL neurons show light intensity-coding responses. Behavioral studies suggest that the vLGN and IGL play a major role in mediating non-photic phase shifts of circadian rhythms, largely via neuropeptide Y, but may also play a role in photic phase shifts and in photoperiodic responses. The vLGN and IGL may participate in two major functional systems, those controlling visuomotor responses and those controlling circadian rhythms. Future research should be directed toward further integration of these diverse findings.

Anterograde, transneuronal transport of herpes simplex virus type 1 strain H129 in the murine visual system

Herpes simplex virus (HSV) undergoes retrograde and anterograde axonal transport as it establishes latency and later intermittently reactivates. Most strains of HSV show preferential retrograde transport within the central nervous system (CNS), however. Previous experiments suggest that an exception to this is HSV type 1 (HSV-1) strain H129, since this virus appears to spread primarily in the CNS via anterograde, transneuronal movement. The objective of the present study was to test how specifically this virus spreads in the visual system, a system with well-described neuronal connections. In the present study, the pattern of viral spread was examined following inoculation into the murine vitreous body. Virus was initially detected in the retina and optic tract. Virus then appeared in all known primary targets of the retina, including those in the thalamus (e.g., lateral geniculate complex), hypothalamus (suprachiasmatic nucleus), and superior colliculus (superficial layers). In previous studies, many strains of HSV were shown to infect these structures, even though they spread predominantly in a retrograde direction. However, the H129 strain was unique in then spreading, via anterograde transport, to the primary visual cortex (layer 4 of area 17) via thalamocortical connections. At later times after infection, specific labeling was also detected in other cortical and subcortical areas known to receive projections from the visual cortex. No labeling was ever detected in the contralateral retina, which is consistent with a lack of retrograde spread of HSV-1 strain H129. These results demonstrate the specific anterograde movement of this virus from the retina to subcortical and cortical regions, with no clear evidence for retrograde spread. HSV-1 strain H129 should be generally useful for tracing sensory pathways and may provide the basis for designing a virus vector capable of delivering genetic material via anterograde pathways within the CNS.

Induction of c-fos protein by patterned visual stimulation in central visual pathways of the rat

Localized patterned visual stimulation was used in rats to investigate the feasibility of stimulus-dependent induction of the immediate early gene c-fos in neurons of cortical and subcortical visual centers of this mammal. Moving and stationary visual patterns, consisting of gratings and arrays of dark dots, induced Fos-like immunoreactivity in populations of neurons in retinotopically corresponding stimulated regions of the dorsal and ventral lateral geniculate nucleus (dLGN, vLGN), stratum griseum superficiale of the superior colliculus, nucleus of the optic tract, and primary (striate) visual cortex. Only moving stimuli induced Fos-like immunoreactive (FLI) neurons in extrastriate visual areas, particularly in the anterolateral (AL) visual area. This suggests that area AL is equivalent to the motion sensitive areas MT and PMLS of the monkey and cat. Stimulus-induced FLI neurons in the striate cortex were predominantly distributed in layers 4 and 6, while few labeled neurons were present in layers 2-3, and almost none in layer 5. The laminar distribution of stimulus-induced FLI cells in the extrastriate cortical area AL was similar to that of the striate cortex, with the exception that more FLI cells were present in layer 5. Statistical comparison of somata size of the stimulus-induced FLI neurons in dLGN with that of Cresyl violet stained neurons in the same sections revealed that the population of geniculate FLI neurons is composed of relay cells and interneurons.

An oriented framework of neuronal processes in the ventral lateral geniculate nucleus of the rat demonstrated by NADPH diaphorase histochemistry and GABA immunocytochemistry

This study investigated the morphology and quantitative distribution of neurons containing NADPH diaphorase activity in the ventral lateral geniculate nucleus of the rat. The pattern of diaphorase staining revealed a strongly reactive lateral subdivision and a weakly staining medial subdivision. A characteristic feature of the diaphorase staining in the lateral part was its "stripe-like" appearance. These "diaphorase stripes" resulted from regions of strong somatic and neuropil diaphorase activity lying between unstained fibre bundles coursing dorsoventrally through the nucleus. Two distinct populations of diaphorase reactive cell types were present--class A and class B neurons. The ratio of class A to class B diaphorase neurons was approximately 14:1 (A:B). Diaphorase reactive neurons made up 73% of the total neuron population in the lateral subdivision, and 31% in the medial subdivision. A third population of cells was found exclusively in the optic tract--class C neurons. Quantitative analyses in the coronal and sagittal planes indicated that the principal processes of both class A and class B neurons were oriented preferentially--either parallel with, or perpendicular to the outlying optic tract. Diaphorase enzyme histochemistry in combination with GABA immunocytochemistry demonstrated the co-localization of GABA immunoreactivity in the majority of class B neurons, whereas class A and class C neurons were GABA immunonegative. Furthermore a large population of GABA-immunoreactive neurons was present that were not stained for diaphorase activity. From this and previous studies, it can be concluded that a high proportion of the diaphorase reaction class A neurons are geniculotectal projection cells, while diaphorase reaction class B neurons represent a numerically small subpopulation of "local-circuit" inhibitory neurons. Since diaphorase activity co-localizes with nitric oxide synthase, the results indicate the likely involvement of nitric oxide in the neuronal operations of both subpopulations of geniculotectal projection neurons and "local-circuit" GABAergic neurons in the rat's ventral lateral geniculate nucleus.

Immunohistochemical localization of calretinin in the rat lateral geniculate nucleus and its retino-geniculate projection

In the present study, we examined the distribution of calretinin-immunoreactive neuronal cell bodies and fibers in the lateral geniculate nucleus of the rat. In normal rats, clusters of immunoreactive cell bodies were found in: (i) the rostral portion of the ventral lateral geniculate nucleus pars medialis (VLGM), (ii) the intergeniculate leaflet (IGL), (iii) the intermediate region between the VLGM and the ventral lateral geniculate nucleus pars lateralis (VLGL), (iv) the caudomedial portion of the VLGM, and (v) the caudolateral portion of the VLGM. In the dorsal lateral geniculate nucleus (DLG), immunoreactive cell bodies were rarely observed. After uni- or bilateral eye enucleation, no significant alteration in the morphological features or distribution of immunoreactive cell bodies was detected in the lateral geniculate nucleus. In normal rats, immunoreactive fibers formed dense plexuses in: (i) the DLG, (ii) the external layer of the VLGL, (iii) the internal layer of the VLGL, (iv) the IGL, (v) the caudomedial portion of the VLGM, and (vi) the optic tract. After unilateral eye enucleation, immunoreactive fibers in the external layer of the VLG and in the optic tract almost totally disappeared on the contralateral side to the lesion. Unilateral eye enucleation caused a significant decrease of immunoreactive fibers in the DLG and in the internal layer of the VLGL, but a substantial number of immunoreactive fibers still remained there. In the IGL and the caudomedial portion of the VLGM, no observable alteration in the distribution of immunoreactive fibers was detected after uni- or bilateral eye enucleation.(ABSTRACT TRUNCATED AT 250 WORDS)

Prior optic nerve transection reduces capsaicin-induced degeneration in rat subcortical visual structures

Capsaicin is a neurotoxin capable of causing degeneration in specific sites throughout the neuraxis, including the suprachiasmatic nucleus (SCh), the ventrolateral geniculate nucleus (VLG), the intergeniculate leaflet (IGL), and the olivary and medial pretectal nuclei (OPT and MPT). In this experiment, we tested the hypothesis that capsaicin-induced terminal degeneration in the SCh, VLG, IGL, OPT, and MPT results from destruction of retinal ganglion cells and their axonal projections to these sites. In the first experiment, silver stains were used to examine degeneration in the retina induced by systemic capsaicin treatment. Capsaicin caused degeneration of ganglion cells, bipolar cells, and nerve terminals in the retina, which could be observed between 2 and 24 hours after treatment. In the second experiment, 15-day-old rat pups were enucleated unilaterally. Five days or 2, 5, or 10 months later, they were injected systemically with capsaicin and killed 6 hours (pups) or 18 hours (adults) later for analysis with a cupric silver stain. In rats of all ages, prior monocular enucleation reduced or eliminated capsaicin-induced degeneration in the contralateral SCh, VLG, IGL, OPT, and MPT. In the third experiment, rat pups were treated systemically with capsaicin or vehicle solution at 12 days of age and given unilateral intravitreal injections of cholera toxin conjugated to horseradish peroxidase (CT-HRP) 3 days prior to sacrifice at 20 days of age. Transport of CT-HRP to the SCh, VLG, IGL, MPT, and OPT was attenuated but not abolished by capsaicin pretreatment. Results suggest that capsaicin causes degeneration in the SCh, VLG, IGL, MPT, and OPT by selective destruction of a subpopulation of retinal ganglion cells with axonal projections to these sites.

Ultrastructural study of the avian ventral lateral geniculate nucleus

The ultrastructure of the pigeon and quail ventral lateral geniculate nucleus was analyzed with standard electron microscopy and horseradish peroxidase tracing of its retinal and tectal afferents. Six types of neurons were distinguished: two large, two medium-sized, and two small types. The latter do not project to the optic tectum and appear to be interneurons. Large and medium-sized neurons project to the optic tectum and are thus relay neurons. Profiles with round, large synaptic vesicles were identified as retinal axon terminal afferents and those with pleomorphic, loosely grouped synaptic vesicles as tectal afferents. Gap junctions were seen between perikarya of small neurons and also with unidentified profiles.

Effects of kainic acid lesions in rat ventral lateral geniculate nucleus upon field potentials of the superior colliculus: correlation between morphological and physiological observations

Morphological and physiological effects of kainic acid (KA) lesions in rat ventral lateral geniculate nucleus (LGV) were studied 1.5 and 6 h after KA injection. Morphological changes were examined mainly by electron micrographs. At 1.5 h after KA injection dendrites were dilated and some vacuolations occurred in both dendrites and perikarya including geniculotectal relay neurons while axons were completely intact and cell organelles almost remained intact. Six h after KA injection dendrites and cell bodies were massively dilated with degeneration of cell organelles accompanied by sparse cytoplasm and deformed chromatin in the nucleus. However, almost all presynaptic axons, mainly retinogeniculate fibers, still remained intact. The electron micrographs demonstrate that destruction occurred first in dendrites, next in cell bodies and finally axons were likely to be affected. These morphological changes induced by KA are compatible with physiological effects which were assessed by the field response of the superior colliculus (SC) evoked by stimulation of the optic chiasm. During 1.5-2 h after KA injection all components of the SC response, the presynaptic and postsynaptic negative-positive waves were enhanced. The enhancement of the SC response may be correlated with morphological changes in terms of excitatory action of KA resulting in facilitation of geniculotectal transmission. Six h after KA injection postsynaptic negative-positive waves gradually declined in amplitude while the presynaptic wave returned to control level. The late suppression of postsynaptic components of the SC response may be attributable to a marked loss of geniculotectal transmission resulting from destruction of geniculotectal relay neurons by KA.

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.

Absence of complete internasal-interocular transfer of habituation of exploratory behavior in rats

Normal, blind, anosmic, and unilaterally blind and contralaterally anosmic albino rats were submitted to unilateral peripheral sensory (olfactory and/or visual) occlusion and observed in an initially unfamiliar arena. Next day, either the same sensory periphery (control subjects) or the contralateral one (experimental subjects) was occluded and a new observation in the arena was made. The duration of the exploratory behavior of control and experimental subjects, on each occasion, was compared. There was not a complete internasal-interocular transfer of long-term habituation of exploratory behavior, either when olfaction and vision were suppressed on opposite sides or when they were suppressed on the same side, but there was a complete internasal transfer of this habituation in blind animals and a complete interocular transfer of this habituation in anosmic animals. These results suggest that long-term habituation of the exploratory behavior elicited by one olfactory and one visual periphery activation and that of the exploratory behavior elicited by the other olfactory and the other visual periphery activation depend upon different representations of the stimulatory situation in the central nervous system. These representations would, however, have only a small number of elements which are not shared.

Visual response properties of ventral lateral geniculate nucleus cells projecting to the pretectum and superior colliculus in the cat

The visual properties of cells in the cat ventral lateral geniculate nucleus (LGV) identified antidromically from the pretectum and/or superior colliculus (projection cells) were studied in comparison with those of LGV neurons which could not be activated antidromically (non-projection cells). ON-phasic receptive fields (RFs) were relatively predominant in 27 projection cells, whereas ON-tonic RFs were found more commonly in the non-projection group. The distribution of the RF centers revealed a centroperipheral gradient of the visual field representation within the LGV that the central visual field was more densely organized.