Antibodies to heavy neurofilament subunit detect a subpopulation of damaged ganglion cells in retina

Neurofilaments ( NFs ) consist of three protein subunits with apparent molecular weights of 68,000 ( 68K ), 145K and 200K , which are found closely associated in most but not all locations in the nervous system. One of these exceptions is the inner retina of the mouse, where antibodies to 145K NFs label large ganglion cells throughout the extent of the cells, while antibodies to 200K NFs label only more distal portions of the optic axons but usually fail to label the ganglion cell somata and proximal axons. Very rarely, however, and more often in old mice, anti- 200K NF antibodies do label a ganglion cell completely. To determine whether these rare, completely labelled cells reflect a pathological alteration, we cut the optic axons, and report here that after a few days some of the axotomized cells could be labelled completely, in a Golgi-like fashion, by anti- 200K NF antibodies. These cells seem to represent the population that forms the projection to the bulk of the lateral geniculate nucleus, as suggested by their size, distribution and projection pattern. Hence, antibodies to the heavy NF subunit in combination with lesions may allow selective retrograde tracing of a subpopulation of ganglion cells, and such antibodies can be used to detect damage in NF-rich neurones at a very early stage, long before they eventually degenerate.     

Segregation of functionally distinct axons in the monkey's optic tract

The classical neuro-ophthalmologic literature describes the organization of the primate's optic tract as containing a single topographic representation of the complete contralateral visual hemifield. In contrast, cats have separate visual field representations for the optic axons of the functionally distinct retinal ganglion cell classes. As the line of decussation for each ganglion cell class in the cat occupies a different location on the retinal surface, whereas in primates they are all superimposed, such a species difference might be expected. We report that implants of horseradish peroxidase placed in either the deep or superficial extremes of the monkey's optic tract produce retrograde labelling of distinct retinal ganglion cell classes, and produce anterograde labelling confined to distinct laminae of the lateral geniculate nucleus. Hence, the optic tract of the primate cannot contain a single representation of the contralateral visual hemifield; rather, independent visual field representations for the functionally distinct optic axons must exist. Their anatomical segregation may account for the clinical observation of selective impairments of distinct visual abilities following partial interruption of the optic tract in man.     

Non-retinotopic arrangement of fibres in cat optic nerve

Fibres in the mammalian optic nerve are generally thought to be organised retinotopically. Recording electrophysiologically from the cat optic nerve, we found little evidence to support this notion, which led us to investigate the problem by anatomical methods. We made a localised injection of horseradish peroxidase into the lateral geniculate body of the cat, labelling a small clump of retinal ganglion cells and their axons in the optic nerve. These fibres, emanating from neighbouring cells in the retina, became widely scattered through the optic nerve, indicating that retinotopic order is essentially lacking.     

Cortical magnification factor and the ganglion cell density of the primate retina

It has long been contentious whether the large representation of the fovea in the primate visual cortex (V1) indicates a selective magnification of this part of the retina, or whether it merely reflects the density of retinal ganglion cells. The measurement of the retinal ganglion-cell density is complicated by lateral displacements of cells around the fovea and the presence of displaced amacrine cells in the ganglion cell layer. We have now identified displaced amacrine cells by GABA immunohistochemistry and by retrograde degeneration of ganglion cells. By reconstructing the fovea from serial sections, we were able to compare the densities of cones, cone pedicles and ganglion cells; in this way we found that there are more than three ganglion cells per foveal cone. Between the central and the peripheral retina, the ganglion cell density changes by a factor of 1,000-2,000, which is within the range of estimates of the cortical magnification factor. There is therefore no need to postulate a selective magnification of the fovea in the geniculate and/or the visual cortex.     

Disruption of retinogeniculate afferent segregation by antagonists to NMDA receptors.

Afferent activity has an important role in the formation of connections in the developing mammalian visual system. But the extent to which the activity of target neurons shapes patterns of afferent termination and synaptic contact is not known. In the ferret's visual pathway, retinal ganglion cell axons from each eye segregate early in development into eye-specific laminae in the lateral geniculate nucleus (LGN). The dorsal laminae (termed laminae A and A1) then segregate further into inner and outer sublaminae that retain input from on-centre and off-centre retinal axons, respectively. Thus, individual retinogeniculate axons form terminal arbors within laminae A and A1 that are restricted to one inner or outer sublamina. We report here that blockade of N-methyl-D-aspartate (NMDA) receptors on LGN cells with specific antagonists during the period of sublamina formation prevents retinal afferents from segregating into 'On' and 'Off' sublaminae. Retinogeniculate axons have arbors that are not restricted appropriately, or are restricted in size but inappropriately positioned within the eye-specific laminae. NMDA receptor antagonists may specifically disrupt a mechanism by which LGN neurons detect correlated afferent and target activity, and have been shown to reduce retinogeniculate transmission more generally, causing LGN cells to have markedly reduced levels of activity. These results therefore indicate that the activity of postsynaptic cells can significantly influence the patterning of inputs and the structure of presynaptic afferents during development.     

Ganglion cell dendrites are presynaptic in catfish retina

The retinal ganglion cells are third-order, spike-generating neurones whose axons transmit the output of the retina to the rest of the brain. It has long been believed that the dendrites of the retinal ganglion cells, like the dendrites of most other Golgi type I neurones, are only postsynaptic. Here we have studied the synapses made onto the ganglion cells in the catfish (Ictalurus punctatus), and we report that the distal dendrites of large-field ganglion cells make conventional chemical synapses onto other inner plexiform layer processes. We have also found that, more than 100 microns away from the cell perikaryon, the synapses made onto and by these ganglion cell dendrites are often tightly clustered. These synaptic clusters appear to be quite regularly spaced along the dendrites. Our results have important implications for the identification of ganglion cell dendrites within the inner plexiform layer as well as for the understanding of the ganglion cell response and receptive field generation.     

Functions of the ON and OFF channels of the visual system

In the mammalian eye, the ON-centre and OFF-centre retinal ganglion cells form two major pathways projecting to central visual structures from the retina. These two pathways originate at the bipolar cell level: one class of bipolar cells becomes hyperpolarized in response to light, as do all photoreceptor cells, and the other class becomes depolarized on exposure to light, thereby inverting the receptor signal. It has recently become possible to examine the functional role of the ON-pathway in vision by selectively blocking it at the bipolar cell level using the glutamate neurotransmitter analogue 2-amino-4-phosphonobutyrate (APB)1. APB application to monkey, cat and rabbit retinas abolishes ON responses in retinal ganglion cells, the lateral geniculate nucleus and the visual cortex but has no effect on the centre-surround antagonism of OFF cells or the orientation and direction selectivities in the cortex2-5. These and related findings6-11 suggest that the ON and OFF pathways remain largely separate through the lateral geniculate nucleus and that in the cortex, contrary to some hypotheses, they are not directly involved in mechanisms giving rise to orientation and direction selectivities. We have examined the roles of the ON and OFF channels in vision in rhesus monkeys trained to do visual detection and discrimination tasks. We report here that the ON channel is reversibly blocked by injection of APB into the vitreous. Detection of light increment but not of light decrement is severely impaired, and there is a pronounced loss in contrast sensitivity. The perception of shape, colour, flicker, movement and stereo images is only mildly impaired, but longer times are required for their discrimination. Our results suggest that two reasons that the mammalian visual system has both ON and OFF channels is to yield equal sensitivity and rapid information transfer for both incremental and decremental light stimuli and to facilitate high contrast sensitivity.     

Position-dependent properties of retinal axons and their growth cones

The formation of the very orderly neuronal projection from the retina to the optic tectum is not yet understood, but several mechanisms are thought to be involved in a coordinated fashion. These mechanisms may include mechanical or chemical guidance in channels, guidance by spatial gradients of positional markers, gradients of temporal (maturation) markers or specific inter-axon interactions (see ref. 1 for review). The last-mentioned mechanism could explain the fibre order found in optic nerve and tract. It requires that some or all growing retinal axons can distinguish between retinal axons of various origins and grow preferentially along retinal axons originating from the same area as themselves. The in vitro experiments described here show that growth cones from the temporal half of the chick retina grow preferentially along temporal axons, whereas growth cones from nasal retina do not distinguish between nasal and temporal axons.     

Synaptic connectivity of a local circuit neurone in lateral geniculate nucleus of the cat

Although receptive fields of relay cells in the lateral geniculate nucleus of the cat nearly match those of their retinal afferents, only 10-20% of the synapses on these cells derive from the retina and are excitatory. Many more (30-40%) are inhibitory and largely control the gating of retinogeniculate transmission. These inhibitory synapses derive chiefly from two cell types: intrinsic local circuit neurones and cells in the adjacent perigeniculate nucleus. It has been difficult to study the functional organization of these inhibitory pathways; most efforts have relied on indirect approaches. Here we describe the use of direct techniques to study a local circuit neurone by iontophoresing horseradish peroxidase (HRP) into it, which completely labels the soma and processes of cells for subsequent light- and electron microscopic analysis. Although the response properties of the labelled cell are virtually indistinguishable from those of many relay cells, its morphology is typical of 'class 3' neurones (see Fig. 1 legend), which are widely believed to be interneurones (but see ref. 12). Here, we refer to the cell as a 'local circuit neurone', which allows for the possibility of a projection axon, rather than as an 'interneurone', a term that commonly excludes a projection axon. We find that the labelled cell has a myelinated axon, but that the axon loses its myelin within 50 microns of the soma and has not yet been traced further. The dendrites of the labelled cell possess presynaptic terminals that act as intrinsic sources of inhibition on geniculate relay cells. We also characterize other morphological aspects of this inhibitory circuitry.     

Changes in geniculate cell size following brief monocular blockade of retinal activity in kittens

When a kitten is subjected to monocular lid suture early in life, cells in laminae of the lateral geniculate nucleus (LGN) connected to the sutured eye grow less than normal and cells in those laminae connected to the non-sutured eye grow more than normal. These changes are seen primarily in the binocular segment of the LGN, which corresponds to the central visual field, and are due to competition either between intracortical afferents originating from the different LGN laminae, or directly among cells within the LGN. The afferent deprivation induced by lid suture, however, is not complete, as retinal ganglion cells fire tonically both in darkness and in light. It is generally thought that this tonic retinal activity is necessary to maintain neuronal excitability at normal threshold in the central visual pathway. In the visual cortex of developing kittens, we previously showed a long-lasting change in ocular dominance of binocular cells by a brief blockade of retinal activity in one optic nerve. We report here that a complete blockade of retinal activity in one eye causes major changes in LGN cell size within 1 week. These changes occur throughout the LGN, including the monocular segment where binocular competition does not occur. The results indicate that tonic retinal activity may have an important role in the control of geniculate cell size.     

Induction of functional retinal projections to the somatosensory system

Optic axons can be induced to form permanent, retinotopic connections in the auditory (medial geniculate, MG) and somatosensory (ventrobasal, VB) nuclei of the Syrian hamster thalamus; this occurs when the principal targets of retinofugal axons are ablated in newborn hamsters and alternative terminal space is created by partial deafferentation of MG or VB. The experimentally induced retinal projection to the somatosensory nucleus occurs by the stabilization of an early, normally transient projection. The present study was undertaken to determine whether the anomalous, stabilized retino-VB projection is functional. Newborn hamsters were operated on to produce permanent retino-VB projections and when the animals were mature, neurophysiological recordings were made in the cortical targets of VB, the first and second somatosensory cortices (SI and SII, respectively). Visual stimulation within well-defined receptive fields reliably evoked multi-unit responses in SI and SII of operated, but not normal hamsters. The representations of the visual field in SI and SII showed a partially retinotopic organization. These results demonstrate that optic tract axons can form functional synapses in the thalamic somatosensory nucleus, and suggest that neural structures which normally process information specific to one sensory modality have the potential to mediate function for other modalities.     

Melanopsin-expressing ganglion cells in primate retina signal colour and irradiance and project to the LGN

Human vision starts with the activation of rod photoreceptors in dim light and short (S)-, medium (M)-, and long (L)- wavelength-sensitive cone photoreceptors in daylight. Recently a parallel, non-rod, non-cone photoreceptive pathway, arising from a population of retinal ganglion cells, was discovered in nocturnal rodents. These ganglion cells express the putative photopigment melanopsin and by signalling gross changes in light intensity serve the subconscious, 'non-image-forming' functions of circadian photoentrainment and pupil constriction. Here we show an anatomically distinct population of 'giant', melanopsin-expressing ganglion cells in the primate retina that, in addition to being intrinsically photosensitive, are strongly activated by rods and cones, and display a rare, S-Off, (L + M)-On type of colour-opponent receptive field. The intrinsic, rod and (L + M) cone-derived light responses combine in these giant cells to signal irradiance over the full dynamic range of human vision. In accordance with cone-based colour opponency, the giant cells project to the lateral geniculate nucleus, the thalamic relay to primary visual cortex. Thus, in the diurnal trichromatic primate, 'non-image-forming' and conventional 'image-forming' retinal pathways are merged, and the melanopsin-based signal might contribute to conscious visual perception.     

Wnt-Ryk signalling mediates medial-lateral retinotectal topographic mapping

Computational modelling has suggested that at least two counteracting forces are required for establishing topographic maps. Ephrin-family proteins are required for both anterior-posterior and medial-lateral topographic mapping, but the opposing forces have not been well characterized. Wnt-family proteins are recently discovered axon guidance cues. We find that Wnt3 is expressed in a medial-lateral decreasing gradient in chick optic tectum and mouse superior colliculus. Retinal ganglion cell (RGC) axons from different dorsal-ventral positions showed graded and biphasic response to Wnt3 in a concentration-dependent manner. Wnt3 repulsion is mediated by Ryk, expressed in a ventral-to-dorsal decreasing gradient, whereas attraction of dorsal axons at lower Wnt3 concentrations is mediated by Frizzled(s). Overexpression of Wnt3 in the lateral tectum repelled the termination zones of dorsal RGC axons in vivo. Expression of a dominant-negative Ryk in dorsal RGC axons caused a medial shift of the termination zones, promoting medially directed interstitial branches and eliminating laterally directed branches. Therefore, a classical morphogen, Wnt3, acting as an axon guidance molecule, plays a role in retinotectal mapping along the medial-lateral axis, counterbalancing the medial-directed EphrinB1-EphB activity.     

Rapid BDNF-induced retrograde synaptic modification in a developing retinotectal system

In cultures of hippocampal neurons, induction of long-term synaptic potentiation or depression by repetitive synaptic activity is accompanied by a retrograde spread of potentiation or depression, respectively, from the site of induction at the axonal outputs to the input synapses on the dendrites of the presynaptic neuron. We report here that rapid retrograde synaptic modification also exists in an intact developing retinotectal system. Local application of brain-derived neurotrophic factor (BDNF) to the Xenopus laevis optic tectum, which induced persistent potentiation of retinotectal synapses, led to a rapid modification of synaptic inputs at the dendrites of retinal ganglion cells (RGCs), as shown by a persistent enhancement of light-evoked excitatory synaptic currents and spiking activity of RGCs. This retrograde effect required TrkB receptor activation, phospholipase Cgamma activity and Ca2+ elevation in RGCs, and was accounted for by a selective increase in the number of postsynaptic AMPA-subtype glutamate receptors at RGC dendrites. Such retrograde information flow in the neuron allows rapid regulation of synaptic inputs at the dendrite in accordance to signals received at axon terminals, a process reminiscent of back-propagation algorithm for learning in neural networks.     

Mixed parvocellular and magnocellular geniculate signals in visual area V4.

Visual information from the retina is transmitted to the cerebral cortex by way of the lateral geniculate nucleus (LGN) in the thalamus. In primates, most of the retinal ganglion cells that project to the LGN belong to one of two classes, P and M, whose axons terminate in the parvocellular or magnocellular subdivisions of the LGN. These cell classes give rise to two channels that have been distinguished anatomically, physiologically and behaviourally. The visual cortex also can be subdivided into two pathways, one specialized for motion processing and the other for colour and form information. Several lines of indirect evidence have suggested a close correspondence between the subcortical and cortical pathways, such that the M channel provides input to the motion pathway and the P channel drives the colour/form pathway. This hypothesis was tested directly by selectively inactivating either the magnocellular or parvocellular subdivision of the LGN and recording the effects on visual responses in the cortex. We have previously reported that, in accordance with the hypothesis, responses in the motion pathway in the cortex depend primarily on magnocellular LGN. We now report that in the colour/form pathway, visual responses depend on both P and M input. These results argue against a simple correspondence between the subcortical and cortical pathways.     

Modification of retinal ganglion cell axon morphology by prenatal infusion of tetrodotoxin

The cellular mechanisms by which the axons of individual neurons achieve their precise terminal branching patterns are poorly understood. In the visual system of adult cats, retinal ganglion cell axons from each eye form narrow cylindrical terminal arborizations restricted to alternate non-overlapping layers within the lateral geniculate nucleus (LGN). During prenatal development, axon arborizations from the two eyes are initially simple in shape and are intermixed with each other; they then gradually segregate to form complex adult-like arborizations in separate eye-specific layers by birth. Here we report that ganglion cell axons exposed to tetrodotoxin (TTX) to block neuronal activity during fetal life fail to form the normal pattern of terminal arborization. Individual TTX-treated axon arborizations are not stunted in their growth, but instead produce abnormally widespread terminal arborizations which extend across the equivalent of approximately two eye-specific layers. These observations suggest that during fetal development of the central nervous system, the formation of morphologically appropriate and correctly located axon terminal arborizations within targets is brought about by an activity-dependent process.     

Acetylcholine inhibits identified interneurons in the cat lateral geniculate nucleus

The transmission of visual information from retina to cortex through the dorsal lateral geniculate nucleus (LGNd) is controlled by non-retinal inputs. Enhanced visually evoked responses in cat LGNd relay cells during periods of increased alertness have been attributed in large part to increased rate of acetylcholine (ACh) release by fibres ascending from the brainstem reticular formation. ACh can modulate geniculate visual responses in vivo, but comparatively little is known about the underlying ionic mechanisms of these cholinergic actions. Although direct excitation of LGNd relay neurons has been shown in vitro, the situation is complicated because cholinergic axons form numerous and complex synapses not only with relay cells, but also with inhibitory interneurons, and electrical activation of the brainstem cholinergic neurons reduces inhibitory postsynaptic potentials in the LGNd. We report here that morphologically characterized interneurons in the cat LGNd possess distinctive electrophysiological properties in comparison with those of relay cells and are inhibited by ACh through a muscarinic receptor-mediated increase in potassium conductance. Together the direct excitation of relay cells and inhibition of intrageniculate interneurons allow the ascending cholinergic system to exert a powerful facilitatory influence over the transfer of visual information to the cerebral cortex.     

Functions of the colour-opponent and broad-band channels of the visual system

The colour-opponent and broad-band channels of the primate visual system originate in the retina and remain segregated through several neural stations in the visual system. Until now inferences about their function in vision have been based primarily on studies examining single-cell receptive field properties which have shown that the colour-opponent retinal ganglion cells have small receptive fields, produce sustained responses and receive spatially segregated inputs from different cone types; the broad-band cells have large receptive fields, respond transiently and receive cone inputs that are not spatially separated. We have now examined the visual capacities of rhesus monkeys before and after interrupting either of these channels with small lesions at the lateral geniculate nucleus. Here we report that the colour-opponent channel is essential for the processing of colour, texture, fine pattern and fine stereopsis, whereas the broad-band channel is crucial for the perception of fast flicker and motion. Little or no deficits were found in brightness and coarse-shape discrimination, low spatial frequency stereopsis and contrast sensitivity after the disruption of either of the channels.     

Formation of target-specific neuronal projections in organotypic slice cultures from rat visual cortex.

A characteristic feature of the mammalian cortex is that projection neurons located in distinct cortical layers send their axons to different targets. In visual cortex, cells in layers 2 and 3 project to other cortical areas, whereas cells in layers 5 and 6 project to subcortical targets such as the lateral geniculate nucleus. The proper development of these projections is crucial for correct functioning of the visual system. Here we show that specific connections are established in an organotypic culture system in which rat visual cortex slices are co-cultured with another slice of the visual cortex or with a thalamic slice. The laminar origin and cellular morphology in vitro of cortical projections to other cortical regions or to subcortical targets are remarkably similar to those seen in vivo. In addition, axons of projecting cells are not restricted to particular pathways, but appear instead to grow directly towards their appropriate target. These observations raise the possibility that chemotropic attraction from the target areas may play an important part in the development of the cortical projection pattern.     

Ganglion cell death during development of ipsilateral retino-collicular projection in golden hamster

In rats and hamsters all parts of the superior colliculus (SC) receive a topographically organized projection from the retina of the contralateral eye, and the rostral part also has a direct input from the lower temporal crescent of the ipsilateral retina, which views the central, binocular portion of the visual field. Initially the uncrossed projection covers the entire SC, but over the first 2 weeks of postnatal life it becomes progressively restricted to its adult distribution. However, if the opposite eye is removed at birth there is a persistent widespread uncrossed projection to the SC. We have used short- and long-term retrogradely transported neuronal markers to examine the distribution and fate of the ganglion cells of origin of the uncrossed retino-collicular projection throughout postnatal development. We conclude that the withdrawal of the early exuberant projection to the caudal SC is associated with death of ganglion cells and their virtual elimination outside the temporal crescent of the ipsilateral retina. Early enucleation of the other eye rescues many of these cells.