Properties of cells responding to visual stimuli in the rat ventral lateral geniculate nucleus

Enhancement and contextual modulation of visuospatial processing by thalamocollicular projections from ventral lateral geniculate nucleus

In the mammalian visual system, the ventral lateral geniculate nucleus (vLGN) of the thalamus receives salient visual input from the retina and sends prominent GABAergic axons to the superior colliculus (SC). However, whether and how vLGN contributes to fundamental visual information processing remains largely unclear. Here, we report in mice that vLGN facilitates visually-guided approaching behavior mediated by the lateral SC and enhances the sensitivity of visual object detection. This can be attributed to the extremely broad spatial integration of vLGN neurons, as reflected in their much lower preferred spatial frequencies and broader spatial receptive fields than SC neurons. Through GABAergic thalamocollicular projections, vLGN specifically exerts prominent surround suppression of visuospatial processing in SC, leading to a fine tuning of SC preferences to higher spatial frequencies and smaller objects in a context-dependent manner. Thus, as an essential component of the central visual processing pathway, vLGN serves to refine and contextually modulate visuospatial processing in SC-mediated visuomotor behaviors via visually-driven long-range feedforward inhibition.

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.

Flexible inhibitory control of visually evoked defensive behavior by the ventral lateral geniculate nucleus

Animals can choose to act upon, or to ignore, sensory stimuli, depending on circumstance and prior knowledge. This flexibility is thought to depend on neural inhibition, through suppression of inappropriate and disinhibition of appropriate actions. Here, we identified the ventral lateral geniculate nucleus (vLGN), an inhibitory prethalamic area, as a critical node for control of visually evoked defensive responses in mice. The activity of vLGN projections to the medial superior colliculus (mSC) is modulated by previous experience of threatening stimuli, tracks the perceived threat level in the environment, and is low prior to escape from a visual threat. Optogenetic stimulation of the vLGN abolishes escape responses, and suppressing its activity lowers the threshold for escape and increases risk-avoidance behavior. The vLGN most strongly affects visual threat responses, potentially via modality-specific inhibition of mSC circuits. Thus, inhibitory vLGN circuits control defensive behavior, depending on an animal’s prior experience and its anticipation of danger in the environment.

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Activity of vLGN axons in the mSC reflects the previous experience of threat

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The vLGN bidirectionally controls escape from visual threat

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Activating the vLGN specifically reduces the activity of visual units in mSC

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Activating vLGN axons in the mSC specifically suppresses escape from visual threat

Visual Information Processing in the Ventral Division of the Mouse Lateral Geniculate Nucleus of the Thalamus

Even though the lateral geniculate nucleus of the thalamus (LGN) is associated with form vision, that is not its sole role. Only the dorsal portion of LGN (dLGN) projects to V1. The ventral division (vLGN) connects subcortically, sending inhibitory projections to sensorimotor structures, including the superior colliculus (SC) and regions associated with certain behavioral states, such as fear (Monavarfeshani et al., 2017; Salay et al., 2018). We combined computational, physiological, and anatomical approaches to explore visual processing in vLGN of mice of both sexes, making comparisons to dLGN and SC for perspective. Compatible with past, qualitative descriptions, the receptive fields we quantified in vLGN were larger than those in dLGN, and most cells preferred bright versus dark stimuli (Harrington, 1997). Dendritic arbors spanned the length and/or width of vLGN and were often asymmetric, positioned to collect input from large but discrete territories. By contrast, arbors in dLGN are compact (Krahe et al., 2011). Consistent with spatially coarse receptive fields in vLGN, visually evoked changes in spike timing were less precise than for dLGN and SC. Notably, however, the membrane currents and spikes of some cells in vLGN displayed gamma oscillations whose phase and strength varied with stimulus pattern, as for SC (Stitt et al., 2013). Thus, vLGN can engage its targets using oscillation-based and conventional rate codes. Finally, dark shadows activate SC and drive escape responses, whereas vLGN prefers bright stimuli. Thus, one function of long-range inhibitory projections from vLGN might be to enable movement by releasing motor targets, such as SC, from suppression.SIGNIFICANCE STATEMENT Only the dorsal lateral geniculate nucleus (dLGN) connects to cortex to serve form vision; the ventral division (vLGN) projects subcortically to sensorimotor nuclei, including the superior colliculus (SC), via long-range inhibitory connections. Here, we asked how vLGN processes visual information, making comparisons with dLGN and SC for perspective. Cells in vLGN versus dLGN had wider dendritic arbors, larger receptive fields, and fired with lower temporal precision, consistent with a modulatory role. Like SC, but not dLGN, visual stimuli entrained oscillations in vLGN, perhaps reflecting shared strategies for visuomotor processing. Finally, most neurons in vLGN preferred bright shapes, whereas dark stimuli activate SC and drive escape behaviors, suggesting that vLGN enables rapid movement by releasing target motor structures from inhibition.

Learning-related neuronal activity in the ventral lateral geniculate nucleus during associative cerebellar learning

During delay eyeblink conditioning, rats learn to produce an eyelid-closure conditioned response (CR) to a conditioned stimulus (CS), such as a light, which precedes and coterminates with an unconditioned stimulus (US). Previous studies have suggested that the ventral lateral geniculate nucleus (LGNv) might play an important role in visual eyeblink conditioning by supplying visual sensory input to the pontine nuclei (PN) and also receiving feedback from the cerebellum. No prior study has investigated LGNv neuronal activity during eyeblink conditioning. The present study used multiple tetrodes to monitor single-unit activity in the rat LGNv during pre-exposure (CS only), unpaired CS/US, and paired CS-US training conditions. This behavioral-training sequence was used to investigate nonassociative- and associative-driven neuronal activity in the LGNv during training. LGNv neuronal activity habituated during unpaired training and then recovered from habituation during subsequent paired training, which may indicate that the LGNv plays a role in attention to the CS. The amplitude of LGNv neuronal activity correlated with CR production during paired but not unpaired CS/US training. Cerebellar feedback to the LGNv may play a role in modulating LGNv activity and attention to the CS during paired training. Based on the present findings, we hypothesize that the role of LGNv in visual eyeblink conditioning goes beyond simply routing visual CS information to the PN and involves modulation of attention.

Spatial receptive field properties of rat retinal ganglion cells

The rat is a popular animal model for vision research, yet there is little quantitative information about the physiological properties of the cells that provide its brain with visual input, the retinal ganglion cells. It is not clear whether rats even possess the full complement of ganglion cell types found in other mammals. Since such information is important for evaluating rodent models of visual disease and elucidating the function of homologous and heterologous cells in different animals, we recorded from rat ganglion cells in vivo and systematically measured their spatial receptive field (RF) properties using spot, annulus, and grating patterns. Most of the recorded cells bore likeness to cat X and Y cells, exhibiting brisk responses, center-surround RFs, and linear or nonlinear spatial summation. The others resembled various types of mammalian W cell, including local-edge-detector cells, suppressed-by-contrast cells, and an unusual type with an ON-OFF surround. They generally exhibited sluggish responses, larger RFs, and lower responsiveness. The peak responsivity of brisk-nonlinear (Y-type) cells was around twice that of brisk-linear (X-type) cells and several fold that of sluggish cells. The RF size of brisk-linear and brisk-nonlinear cells was indistinguishable, with average center and surround diameters of 5.6 ± 1.3 and 26.4 ± 11.3 deg, respectively. In contrast, the center diameter of recorded sluggish cells averaged 12.8 ± 7.9 deg. The homogeneous RF size of rat brisk cells is unlike that of cat X and Y cells, and its implication regarding the putative roles of these two ganglion cell types in visual signaling is discussed.

Visual-ocular motor activity in the macaque pregeniculate complex

The anatomical connections of the pregeniculate complex (PrGC) with components of the visual-ocular motor system suggested its contribution to ocular motor behavior. Subsequent studies reported saccade-related activity in the primate PrGC. To determine its contribution, we characterized pregeniculate units (n = 128) in alert macaques during ocular motor tasks and visual stimulation. We found that 36/109 saccade-related units exhibited postsaccadic bursts or pauses in tonic discharge for saccades of any amplitude or direction. In contrast to previous results, 46/109 responses preceded or coincided with the saccade, while 47/109 responses were directionally tuned. Pregeniculate units were modulated not only in association with saccades (109/128) but also with smooth eye movements and visual motion (20/128) or eye position (23/128). Multiple ocular motor signals were recorded from 19% of the units, indicating signal convergence on individual neurons. Visual responses were demonstrated in 51% of PrGC units: visual field illumination modulated the resting discharge of 33 units; the responses of 37 saccade-related units and all 23 position-dependent units were modulated by visual stimulation. Early saccadic activity in the PrGC suggests that it contributes more to gaze than postsaccadic modulation of visual or ocular motor activity. The patterns of saccadic responses and the modulation of PrGC activity in association with a variety of visual-ocular motor behaviors suggest its potential role as a relay between the parietal cortex and elements of the brain stem ocular motor pathways, such as the superior colliculus and pretectal nucleus of the optic tract.

Quantitative age-related changes in NADPH-diaphorase-positive neurons in the ventral lateral geniculate nucleus

Age-related changes in nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) were examined in the rat ventral lateral geniculate nucleus (vLGN) using histochemical methods. Eighteen rats aged 3, 24, and 26 months were studied using quantitative methods to investigate the number of neurons per mm(2), the cross-sectional area, and the orientation of dendritic processes of NADPH-d-positive neurons. We have described three types of neurons: types A and B are both located in the lateral and medial vLGN (vLGN-l and vLGN-m, respectively), and type C neurons over the optic tract. The number of NADPH-d-positive neurons was significantly reduced in the old rats (-39%) when compared with controls (3-month-old rats). The quantitative analysis of cell areas revealed a significant decrease of somatic size in type B neurons, both in the lateral and medial vLGN, and in C neurons; however, type A neurons did not show significant changes. By quantifying the orientation of dendritic processes, we observed a predominant dorsolateral orientation in type A and B neurons. During aging, there are no changes in the dendritic orientation of neurons located in the vLGN-m; however, vLGN-l neurons show an increase in dendritic processes with dorsoventral orientation. In type C neurons, our results show that 87.4% of dendritic processes are lateromedially oriented at 26 months old. Therefore, the types A and B neurons behave differently during senescence. Type A neurons do not change in size, but those located in the vLGN-l modify the orientation of their dendritic processes; however, type B neurons, reduce their size and those located in the vLGN-l also modify their dendritic process orientation. Finally, the type C neurons modify their size and dendritic process.

Release from GABA(A) receptor-mediated inhibition unmasks interlaminar connection within superior colliculus in anesthetized adult rats

The superior colliculus (SC) contains two major subdivisions, the superficial layers (sSC) and the deeper layers (dSC). sSC receives visual information from the retina and visual cortex, while dSC sends descending projections to the brainstem and spinal cord. It has not been clear whether and how sSC directly activates dSC, however recent studies in slice preparations reported that electrical stimulation of sSC induces burst firing in dSC neurons after application of bicuculline. In the present study, we tested whether sSC directly activates dSC in vivo. In isoflurane-anesthetized rats, electrical stimulation of the optic nerve (ON) induced negative field responses mainly in sSC, but not as much in dSC, under control conditions. However, after injection of bicuculline into dSC, ON stimulation induced long-lasting negative field responses in dSC. dSC neurons including the tectofugal neurons exhibited burst firing during the long-lasting negative field responses. The burst responses remained after ablation of the cerebral cortex ipsilateral to the SC recordings. These results suggest that retinal inputs induce burst responses in tectofugal neurons in dSC via subcortical pathways when the SC circuit is released from GABA(A) receptor-mediated inhibition. We propose that sSC is the main candidate for the source of the excitatory synaptic inputs to dSC among the subcortical regions.

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.

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.

The circadian visual system

The retina transduces photic stimuli and transmits that information centrally for further processing. This review emphasizes the fact that the nervous system components governing circadian rhythmicity constitute a specialized subdivision of the vertebrate visual system. The brain houses different targets for retinal efferents parcellated according circadian or non-circadian function. Although the suprachiasmatic nucleus (SCN), being the site of the master circadian clock, is necessary for the generation of circadian rhythmicity, precise phase regulation of any rhythm is subject to modulation by SCN-afferent processes. Photic information necessary for entrainment arrives at the SCN via the retinohypothalamic tract. The geniculohypothalamic tract, originating in the intergeniculate leaflet (IGL), provides a secondary route by which photic information can reach the SCN. It also projects extensively to the contralateral IGL and receives reciprocal input from the SCN region. An interaction between the circadian and non-circadian visual systems may exist through connections of the superior colliculus with ventrolateral geniculate leaflet (VLG) and IGL. The SCN, IGL, VLG and superior colliculus are all innervated by serotonin-containing fibers. The following observations are likely to have an impact beyond the rhythm field itself: certain transneuronal tracers label only the circadian visual system; c-fos protein synthesis is induced in the circadian, but not non-circadian, visual system by a phasically active stimulus; blockade of SCN action potentials is unable to alter circadian rhythmicity; transplantation of dispersed fetal SCN cells to arrhythmic adults restores circadian periodicity, but not phase response to light; and the IGL is actually a very extensive part of the lateral geniculate complex.

Functional organization of the ventral lateral geniculate complex of the tree shrew (Tupaia belangeri): II. Connections with the cortex, thalamus, and brainstem

Connections of the ventral lateral geniculate complex (GLv) in the tree shrew were traced by anterograde and retrograde transport of WGA-HRP. The results buttress earlier findings that GLv in this species is composed of two main divisions, lateral and medial, each of which differs in its connections with the brainstem and cerebral cortex. The connections of the lateral division (GLv) suggest that it participates in visuosensory functions: it receives input from the retina, striate cortex, pretectum, and retino-recipient layers of the superior colliculus. These connections help clarify the identification of the internal and external subdivisions of GLv inasmuch as projections from both the superior colliculus and pretectum terminate in the external subdivision and each, in turn, receives a projection from the internal subdivision. Connections of the medial division suggest that this part of the nucleus is involved with visuomotor functions. Thus, the medio-caudal subdivision projects to the pontine nuclei, the prerubral field and the central lateral nucleus. The medio-caudal subdivision also receives projections from the lateral cerebellar nucleus, so that the GLv-ponto-cerebello-GLv loop involves mainly one subdivision of GLv. The medio-rostral subdivision receives projections from the pretectum and parietal cortex. Its output is directed primarily at the intermediate and deep layers of the superior colliculus. All of these targets of GLv, the pons, prerubral field, and deep layers of the superior colliculus, are known to play a role in the coordination of head and eye movements. Additional connections of GLv with the vestibular nuclei, intralaminar nuclei, hypothalamus, and facial motor nucleus are also described.

Modulation of visually evoked responses in units of the ventral lateral geniculate nucleus of the rat by somatic stimuli

Single unit activity was recorded from the ventral part of the lateral geniculate nucleus (vLGN) in rats anaesthetized with urethane. Most of the cells located laterally in the nucleus were excited by light. The studied vLGN neurones did not respond to electrical stimulation of the tail, but about half of them changed their response to light significantly when the light flash was paired with the electrical stimulation. When the tail stimulus preceded the light, the changes consisted in a pronounced facilitation of flash-evoked activity. When the electrical stimulus was applied after the flash in a forward conditioning paradigm, facilitations were less pronounced and responses of some neurones were suppressed. These results are in contrast to those of similar experiments on the dorsal LGN, neurones of which were mainly facilitated by the conditioning paradigm. Thus, light-evoked activity of ventral geniculate cells can be enhanced by arousal-related processes.

Luminance coding properties of intergeniculate leaflet neurons in the golden hamster and the effects of chronic clorgyline

Cells in the intergeniculate leaflet (IGL) project to the suprachiasmatic nuclei, a mammalian circadian pacemaker. Chronic treatment with clorgyline alters hamster circadian rhythms in ways similar to alterations seen after ablation of the IGL. Chronic clorgyline also alters the light intensity dependence of phase-shifting. In this study luminance coding properties of IGL cells were measured in control hamsters and in hamsters chronically treated with clorgyline. In control animals three patterns of response to increasing and decreasing luminance were observed. Type I cells showed a monotonic pattern. Type II cells were similar to Type I with additional increases in firing rate at several specific luminance levels. Type III cells only coded increases in luminance. Cells from clorgyline-treated animals did not differ from those from control animals in the pattern of luminance response but IGL cells from these animals showed decreased firing rate in both light and dark conditions. These results suggest that the effects of clorgyline on the photic sensitivity of circadian rhythms may be related to a clorgyline-induced decrease in firing rate of IGL cells. They also indicate that some IGL cells show complex patterns of response to luminance changes in addition to those showing simple monotonic responses.

Presumptive catecholaminergic ganglion cells in the pigeon retina

An antiserum directed against tyrosine hydroxylase (TH), the rate-limiting enzyme in the synthesis of dopamine, was used to study the pigeon retina. Labeled cells were observed in both the inner nuclear layer (INL) and ganglion cell layer (GCL). Two populations of TH-immunoreactive neurons were observed in the INL. Some of these cells were 7-10 microns in diameter and gave rise to processes that arborized in three layers of the inner plexiform layer (IPL). These cells appeared similar to the dopaminergic amacrine cells described previously (Marc, 1988). Other labeled cells in the INL were 12-20 microns in diameter and were recognizable as a previously described subpopulation of TH-immunoreactive displaced ganglion cells (Britto et al., 1988). A population of labeled cells was observed in the GCL. Counts of these cells in two retinae revealed 5000 and 7000 cells, respectively. They ranged in size from 8-15 microns in diameter in the central retina and from 8-20 microns in diameter in the peripheral retina. The density of labeled cells was highest in the central retina and red field and lowest in the retinal periphery. The difference in cell size and cell density as a function of eccentricity is characteristic of the total population of ganglion cells in the avian retina (Ehrlich, 1981; Hayes, 1982). Some of the TH-positive cells in the GCL could be classified as ganglion cells for two reasons: (1) The axons of many of the TH-positive cells in the GCL were TH-immunoreactive as well and could be followed to the optic nerve head. (2) The injection of rhodamine-labeled microspheres into the nucleus geniculatus lateralis, pars ventralis (GLv), resulted in the retrograde labeling of many of the TH-positive cells in the contralateral retina.

Chemically specific retinal ganglion cells collateralize to the pars ventralis of the lateral geniculate nucleus and optic tectum in the pigeon (Columba livia)

Immunohistochemical and retrograde tracing techniques were combined to study the retinal ganglion cells which project to the pars ventralis of the lateral geniculate nucleus (GLv) in the pigeon. Using two different fluorescent tracers, two histochemically-distinct populations of ganglion cells were found to project to both the GLv and the optic tectum. The first population of ganglion cells exhibited tyrosine hydroxylase-like immunoreactivity and represented about 20% of all ganglion cells which were retrogradely labeled from the GLv. The second population of ganglion cells showed substance P-like immunoreactivity and represented about 13% of all ganglion cells projecting to the GLv. These results confirm earlier suggestions that the retinal axons projecting to the GLv also project elsewhere and demonstrate that heterogeneity of retinal ganglion cells transmitters is evident even within a single retino-recipient nucleus such as the GLv.

Photic responses of geniculo-hypothalamic tract neurons in the Syrian hamster

The putative neural pacemaker controlling circadian rhythms in mammals is contained in the suprachiasmatic nuclei of the hypothalamus. These nuclei receive a projection, the geniculo-hypothalamic tract (GHT), from neurons in the intergeniculate leaflet (IGL) and portions of the ventral lateral geniculate nucleus (vLGN) of the thalamus. We examined the responses of putative GHT neurons to diffuse illumination using extracellular electrophysiological recordings. The great majority of IGL neurons showed sustained ON responses to diffuse retinal illumination; vLGN neurons showed more variation in their responses. Discharge rates of sustained ON neurons increased monotonically as light intensity was increased and saturated over 2-3 log units of intensity changes. Many IGL neurons had binocular input, and input from the ipsilateral eye was often inhibitory. These results indicate that GHT neurons may provide information about ambient light intensity to the suprachiasmatic nuclei.

Optic tract stimulation evokes GABAA but not GABAB IPSPs in the rat ventral lateral geniculate nucleus

The inhibitory postsynaptic potentials (IPSPs) evoked in neurons of the rat ventral geniculate nucleus (vLGN) by electrical stimulation of the optic tract and the action of GABA and baclofen on the same cells were studied using intracellular recording technique in an in vitro slice preparation. A short latency short duration IPSP always followed the monosynaptic excitatory postsynaptic potential (EPSP). This IPSP reversed in polarity at about -65 mV and was reversibly blocked by bicuculline (50 microM) thus indicating that it represents a GABAA receptor-mediated IPSP. No long-lasting IPSP was evoked in vLGN cells by stimulation of the optic tract, while in the same slice, long-lasting GABAB IPSPs were routinely recorded in the dorsal lateral geniculate nucleus. GABA applied by ionophoresis evoked a hyperpolarization that had a reversal potential close to -70 mV and was antagonized by bicuculline. Baclofen hyperpolarized vLGN neurons and its action was reversibly blocked by the selective GABAB antagonist phaclofen (1 mM). In the presence of bicuculline GABA also produced a hyperpolarization that had properties similar to that evoked by baclofen. These results indicate that, although functional GABAA and GABAB receptors are present on vLGN neurons, stimulation of the optic tract evokes only GABAA but not GABAB mediated IPSPs. The lack of long-lasting GABAB IPSPs could explain the absence of long-lasting inhibition observed in vLGN neurons in vivo following stimulation of the optic tract.

Effects of square-wave gratings and diffuse light on metabolic activity in the rat visual system

The 2-deoxyglucose (2-DG) technique was used to determine the effects of pattern and diffuse light stimulation on glucose metabolism with hooded rats. Rats placed in a stimulation chamber covered with horizontal and vertical square wave gratings while wearing goggles with one of three pairings of light-occluding, diffusing, or clear lenses, allowed the assessment of the effects of different but simultaneous visual conditions on two sides of the strongly crossed visual system. Eyes covered with occluding lenses were lid-sutured shut 24 h before 2-DG. In order to assess the possibly confounding effects of this lid suture a second group of rats had one eyelid sutured for 24 h, and the other covered with an occluding lens for 20 min, before 2-DG. To further assess the effects of diffuse light a third group of rats was tested in a featureless white box with one eye occluded and the other covered by a diffusing lens. Exposure to pattern stimulation significantly increased metabolic activity in the dorsal lateral geniculate nucleus (LGNd), lateral posterior nucleus (LPN), superior colliculus (SC), and in visual cortex (VC). In contrast, diffuse light only slightly elevated LGNd activity and appeared to have little or no effect in the LPN or VC. Diffuse light, however, was as effective as patterned light in increasing ventral lateral geniculate nucleus activity and strongly suppressed SC activity to a level well below that produced by darkness. Evidently diffuse light, not just patterned light, can significantly govern the operation of central nervous system visual structures.

The intergeniculate leaflet partially mediates effects of light on circadian rhythms

Photic signals affect circadian activity rhythms by both phasic and tonic mechanisms that modulate pacemaker phase and period. In mammals, the effects of light on circadian activity are mediated by the retina, which communicates with the suprahiasmatic nucleus (SCN) by two different anatomical routes: the retino-hypothalamic tract (RHT), originating in the retina, and the geniculo-hypothalamic tract (GHT), arising from a retino-recipient nucleus, the intergeniculate leaflet (IGL). We assessed the roles of these two afferent systems in mediating phasic and tonic effects of light on circadian activity in IGL-lesioned animals. Destruction of the IGL significantly affected phase shifts produced by brief light pulses (phasic effect) and modified the change in period (tau) of the free-running activity rhythm produced by changing the level of constant light (LL) (tonic effect). Phase advances produced by brief light pulses were decreased in amplitude while phase delays were increased in IGL-lesioned animals as compared to controls. The free-running period in constant dark (tau DD) of IGL-lesioned animals was greater than tau DD of controls, and the lengthening of tau normally produced by LL was not observed or was greatly reduced in IGL-lesioned animals. Entrainment to light-dark cycles was unaffected by the lesions, as were other aspects of the circadian activity rhythm that normally change in response to LL (e.g., activity-rest ratio, total activity, splitting). Our data support the interpretation that the IGL plays a significant role in relaying information regarding illumination intensity to the SCN.

On the excitatory post-synaptic potential evoked by stimulation of the optic tract in the rat lateral geniculate nucleus

1. The electrophysiological and pharmacological properties of the excitatory post-synaptic potentials (e.p.s.p.) evoked by electrical stimulation of the optic tract were studied in projection neurones of the ventral and dorsal lateral geniculate nucleus (l.g.n.) of the rat in vitro. 2. No difference was found in the rise time of e.p.s.p.s. recorded in the dorsal and ventral l.g.n. and in their threshold for action potentials. At membrane potentials more negative than -60 mV, e.p.s.p.s. in the dorsal l.g.n. were always followed by a Ca2+-dependent potential. Its amplitude could easily reach threshold for generating an action potential and thus evoke firing from an e.p.s.p. that was subthreshold at resting potential. No Ca2+ potential was observed to follow e.p.s.p.s. recorded in the ventral l.g.n. 3. At resting potential the excitability of dorsal and ventral cells was unaffected following an initial shock to the optic tract. However, in dorsal neurones, at potentials more negative than -60 mV, the presence of Ca2+ potentials evoked by the e.p.s.p.s. resulted in a period of decreased excitability. 4. Using intrasomatic injection of Cs+ the reversal potential (E) of the e.p.s.p. and of the depolarization produced by glutamate could be measured in the same l.g.n. neurone. They were: Eepsp, -0.9 mV; and Eglut, -3.9 mV. 5. gamma-D-glutamylglycine (DGG), an excitatory amino acid antagonist, reversibly inhibited the e.p.s.p. and depolarization produced by quisqualate and glutamate by a competitive action. The concentration of DGG that produced 50% inhibition (IC50) was 2.7 mM. 6. D-2-amino-5-phosphonovalerate (APV), the potent and selective N-methyl-D-aspartate (NMDA) antagonist, had no effect on the e.p.s.p. both in the presence and absence of Mg2+. The isomers of 2-amino-4-phosphonobutyrate (APB) were inactive or had a non-specific action on the e.p.s.p. 7. No difference could be detected in either the reversal potential or the action of the antagonists between neurones of the dorsal and the ventral l.g.n. 8. These results suggest that Ca2+-dependent potentials play an important role in modulating synaptic efficacy in principal neurones of the dorsal l.g.n. The quisqualate/kainate nature of the optic nerve receptors and the similarity of Eepsp and Eglut constitute strong support in favour of a glutamate-like substance as the transmitter of the optic nerve.

The ventral and dorsal lateral geniculate nucleus of the rat: intracellular recordings in vitro

1. The membrane properties and the electrotonic structure of neurones in the ventral and dorsal lateral geniculate nucleus (l.g.n.) of the rat were studied using an in vitro slice preparation. 2. Following electrophysiological characterization, horseradish peroxidase (HRP) was injected intrasomatically and the morphological features of impaled cells were characteristic of principal neurones of the rat ventral and dorsal l.g.n. 3. Neurones in the ventral l.g.n. had a higher input resistance but similar membrane time constants (tau o) and resting potentials than cells in the dorsal l.g.n. 4. Using a simple neuronal model, the electrotonic length (L) and the dendritic to somatic conductance ratio (rho) were calculated and found to be similar for cells in both divisions of the l.g.n. The mean value of L (0.7) and rho (1.5) suggest that both groups of neurones are electrotonically compact. 5. The width and after-hyperpolarization of directly evoked action potentials, but not their threshold or their amplitude, were different between cells of the ventral and dorsal l.g.n. 6. At potentials more negative than -55 mV, a slow rising and falling potential could be evoked in each neurone (n = 310) of the dorsal l.g.n. but only in three cells of the ventral l.g.n. (n = 94). The electrophysiological and pharmacological properties of this potential were identical with those of the low-threshold Ca2+-dependent potential observed in other thalamic nuclei. 7. These results indicate that some of the passive and active membrane properties of ventral and dorsal l.g.n. neurones are different. The implications of these findings for the control of the integrative capability and the response of l.g.n. neurones to visual stimulation are discussed.

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

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

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.

Identification of ventral lateral geniculate nucleus cells projecting to the pretectum and superior colliculus in the cat

Among 235 histologically identified cells of the ventral lateral geniculate nucleus (LGV) in the cat, 66 responded antidromically to electrical stimulation of the pretectum (PT) and/or superior colliculus (SC): 22 projected to PT, 22 to SC and 22 to both sites. The LGV cells were innervated by optic tract fibers corresponding to axons of X- as well as W-type retinal ganglion cells.

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).

The organization of retinal maps within the dorsal and ventral lateral geniculate nuclei of the rabbit

The cytoarchitectonic subdivisions in the rabbit's dorsal and ventral lateral geniculate nuclei have been related to the several retinal maps that can be defined in terms of the distribution of retinal axons within these nuclei. Destruction of different retinal sectors was combined with intravitreal injections of 3H-proline, so that the distribution of fiber degeneration and autoradiographic label in the geniculate nuclei could be used to define the retinal maps in each nucleus, and to compare the two nuclei with each other. The two nuclei show surprisingly similar patterns of organization. Each is made up of a laminated alpha sector that curves around a relatively cell-sparse beta sector. Two morphologically distinct layers of each alpha sector receive contralateral retinal afferents and between these there is a small region in receipt of ipsilateral afferents. In each nucleus, the lines of projection that represent single points in visual space pass perpendicular to the layers of the alpha sector and continue an almost straight course into the beta sector. Quantitative comparisons of the retinal maps show that the relative volumes devoted to the representation of segments of the visual field are approximately the same in the two nuclei.

Retinal synapses of the cat medial interlaminar nucleus and ventral lateral geniculate nucleus differ in size and synaptic organization

The retinal terminals of the medial interlaminar nucleus (MIN) and ventral lateral geniculate nucleus ( VLG ) have been examined quantitatively to determine if there are morphological differences in their synaptic ultrastructure which reflect their distinctive physiologies . The cross-sectional area and density (number per unit area) of synaptic contact zones with conventional and presynaptic dendrites (F2 profiles) were measured for each retinal terminal. The densities of F2 presynaptic dendrites and F1 flattened vesicle axon terminals were also measured. Retinal terminals in MIN were often large (mean size = 2.7 micron2 area) and had a high density of synaptic contacts (0.14 per micron surface area) with conventional dendrites, presynaptic dendrites, and dendritic spines. A high density of F2 presynaptic dendrites (0.08 per micron2 area) was found in MIN. F1 axon terminals were also found frequently (0.04 per micron2). MIN retinal terminals were often organized in glomeruli like those of the dorsal lateral geniculate nucleus. The retinal terminals in VLG were almost always small (mean size = 0.94 micron2 area), although they also had a high density of synaptic contacts (0.17 per micron surface area). They frequently synapsed on small dendrites and dendritic spines and less frequently on large dendrites. Unlike MIN, retinal terminals in VLG rarely contacted F2 presynaptic dendrites which were much less frequent in VLG (0.01 per micron2 area). Like MIN, VLG contained numerous F1 axon terminals (0.06 per micron2 area). No typical retinal glomeruli were found in VLG . These results show that MIN, which contains many Y cells, has a population of large retinal terminals and many F2 presynaptic dendrites. VLG , which apparently has only W cells, contains only small retinal terminals and has fewer F2 presynaptic dendrites. Both have a high density of F1 flat vesicle axon terminals.

Identification of geniculo-tectal relay neurons in the rat's ventral lateral geniculate nucleus

In the rat's ventral lateral geniculate nucleus (vLGN), geniculo-tectal relay neurons (GTR-neurons) could be identified by the retrograde transport of horseradish peroxidase (HRP) after injection in the superior colliculus (SC). GTR-neurons correspond to class III cells described by Brauer and Schober (1973) in Golgi preparations of the rat's LGN. The distribution of GTR-neurons is restricted to the lateral subnucleus of vLGN. According to Swanson et al. (1974), the axons of these cells terminate in lower Stratum griseum superficiale and in Stratum opticum, Stratum griseum intermedium and Stratum album intermedium of SC. The GTR-neurons are characterized by very thick and long proximal dendritic segments which have a smooth surface. Dendrites branch preponderantly in their distal regions and only in this part form many multiform protrusions. There is some evidence that retinal axons terminate on these dendritic surface structures. The supposed differences in the afferent patterns between GTR-neurons in the vLGN and geniculo-cortical relay neurons in the dorsal lateral geniculate nucleus are discussed.

Receptive field properties of rat ventral lateral geniculate cells projections to the superior colliculus

Cells of the ventral lateral geniculate nucleus (LGV) in rats sending their axons to the superior colliculus (SC), were identified electrophysiologically as the ones responding antidromically to electrical stimulation of SC. They were located in the external part of LGV. Visual receptive fields of these cells were mostly of ON-tonic types and some of the movement-sensitive ones. Evidence was presented supporting existence of the reciprocal fiber connection between the LGV and the SC.

Visual pathways and acuity hooded rats

Three experiments on the effects of lesions of the visual system on contrast-detection in hooded rats are described, in which the ability of rats to detect stationary high-contrast square-wave gratings of various fundamental frequencies presented in the central visual field was measured before and after operation. The results suggested the following conclusions: (i) The pathways from retina to striate cortex via dorsal lateral geniculate nucleus (dLGN) conveys information about high spatial frequencies sufficient for normal detection of these gratings, that is up to about 1 cycles/deg. It may be the only pathway to carry this information, and may thus play a unique role in the analysis of fine detail. The high-frequency information is probably relayed from striate cortex to extrastriate cortex, rather than to subcortical sites. (ii) The superior colliculus receives information from the retina up to at least 0.7 cycles/deg, which it then relays to extrastriate visual cortex, probably via the lateral posterior nucleus of the thalamus. (iii) Neither the projections from superior colliculus to other, non-thalamic sites nor the remaining pathways from the retina (e.g. to ventral LGN) appear to carry contrast information higher than 0.3 cycles/deg. These sets of projections therefore do not appear to be used for precise analysis of stationary scenes. These findings suggest that there are considerable similarities between the visual systems of rats and other mammals with respect to the routing of information about stationary spatial contrast, and may help to explain the results of some experiments that have used tasks besides contrast-detection to assess the visual capacities of rats after lesions.

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.