Fibroblast growth factors redirect retinal axons in vitro and in vivo

Growth factors have been shown previously to participate in the process of axon target recognition. We showed that fibroblast growth factor receptor (FGFR) signaling is required for Xenopus laevis retinal ganglion cell (RGC) axons to recognize their major midbrain target, the optic tectum [neuron 17 (1996), 245]. Therefore, we have hypothesized that a change in expression of a fibroblast growth factor (FGF) at the entrance of the optic tectum, the border between the diencephalon and mesencephalon, may serve as a signal to RGC axons that they have reached their target. To determine whether RGC axons can sense changes in FGF levels, we asked whether they altered their behavior upon encountering an ectopic source of FGF. We found that in vivo RGC growth cones avoided FGF-misexpressing cells along their path, and that FGF-2 directly repelled RGC growth cones in an in vitro growth cone turning assay. These data support the idea that RGC axons can sense changes in FGF levels, and as such provide a mechanism by which FGFR signaling is involved in RGC axon target recognition.     

Regenerated retinal ganglion cell axons form normal numbers of boutons but fail to expand their arbors in the superior colliculus

Regenerated retinal ganglion cell (RGC) axons can re-form functional synapses with target neurons in the superior colliculus (SC). Because preterminal axon branching determines the size, shape and density of innervation fields, we investigated the branching patterns and bouton formation of individual RGC axons that had regrown along peripheral nerve (PN) grafts to the SC. Within the superficial layers of the SC, the regenerated axons formed terminal arbors with average numbers of terminal boutons that were similar to the controls. However, axonal branches were shorter than normal so that the mean area of the regenerated arbors was nearly one-tenth that of control arbors and the resulting fields of innervation contained greater than normal numbers of synapses concentrated in small areas of the target. Our results have delineated a critical defect in the reconstitution of retino-collicular circuitry in adult mammals: the failure of terminal RGC branches to expand appropriately. Because recent studies have documented that brain-derived neurotrophic factor (BDNF) can specifically lengthen RGC axonal branches not only during development in the SC but also within the adult retina after axotomy, the present quantitative studies should facilitate experimental attempts to correct this deficit of the regenerative response. © 1998 Chapman and Hall     

Responses of visual single cells in the superior colliculus of the albino rat to bright bars.

We studied the responses of 57 visual cells of the superior colliculus of the albino rat to bright sweeping and stationary flashing bright bars to determine the properties of their receptive fields. We observed that 9% (8% in superficial and 11% in deep layers) of the studied cells presented orientation preference and 16% showed direction selectivity (13% in superficial and 22% in deep layers). According to their responses to a flashed bright bar they were classified in OFF-type (19%) and ON-OFF-type (81%). No ON-type cells were found. All cells were driven by the contralateral eye, and only in three cases was single cell activation from the ipsilateral eye possible. Twenty-one per cent (22% in superficial and 18% in deep layers) showed end-stopping when they were tested with bright bars of several lengths.     

Dynamic space codes in the superior colliculus.

Space coding in the superior colliculus has traditionally been viewed as a static representation by multiple, aligned, sensory and motor maps. Recent evidence has revealed that the maps are dynamic, shaped by sensory experience in developing animals, and by eye and head position signals in adults. The superior colliculus thus provides an ideal model for studying the neural mechanisms underlying developmental and real-time modifications of information representation in the brain.     

Responses to innocuous,but not noxious,somatosensory stimulation by neurons in the ferret superior colliculus

The intermediate and deep layers of the superior colliculus (SC) are known for their role in initiating orienting behaviors. To direct these orienting functions, the SC of some animals (e.g., primates, carnivores) is dominated by inputs from the distance senses (vision, audition). In contrast, the rodent SC relies more heavily on non-visual inputs, such as touch and nociception, possibly as an adaptive response to the proximity of dangers encountered during their somatosensory-dominant search behaviors. The ferret (a carnivore) seems to employ strategies of both groups: above ground they use visual/auditory cues, but during subterranean hunting ferrets must rely on non-visual signals to direct orienting. Therefore, the present experiments sought to determine whether the sensory inputs to the ferret SC reveal adaptations common to functioning in both environments. The results showed that the ferret SC is dominated (63%; 181/286) by visual/auditory inputs (like the cat), rather than by somatosensory inputs (as found in rodents). Furthermore, tactile responses were driven primarily from hair-receptors (like cats), not from the vibrissae (as in rodents). Additionally, while a majority of collicular neurons in rodents respond to brief noxious stimulation, no such neurons were encountered in the ferret SC. A small proportion (4%; 13/286) of the ferret SC neurons were responsive to long-duration (> 5s) noxious stimulation, but further tests could not establish these responses as nociceptive. Collectively, these data indicate that the ferret SC is best adapted for the animal's visual/acoustically guided activities and most closely resembles the SC of its phylogenetic relative, the cat.     

Responses to innocuous, but not noxious, somatosensory stimulation by neurons in the ferret superior colliculus

The intermediate and deep layers of the superior colliculus (SC) are known for their role in initiating orienting behaviors. To direct these orienting functions, the SC of some animals (e.g., primates, carnivores) is dominated by inputs from the distance senses (vision, audition). In contrast, the rodent SC relies more heavily on non-visual inputs, such as touch and nociception, possibly as an adaptive response to the proximity of dangers encountered during their somatosensory-dominant search behaviors. The ferret (a carnivore) seems to employ strategies of both groups: above ground they use visual/auditory cues, but during subterranean hunting ferrets must rely on non-visual signals to direct orienting. Therefore, the present experiments sought to determine whether the sensory inputs to the ferret SC reveal adaptations common to functioning in both environments. The results showed that the ferret SC is dominated (63%; 181/286) by visual/auditory inputs (like the cat), rather than by somatosensory inputs (as found in rodents). Furthermore, tactile responses were driven primarily from hair-receptors (like cats), not from the vibrissae (as in rodents). Additionally, while a majority of collicular neurons in rodents respond to brief noxious stimulation, no such neurons were encountered in the ferret SC. A small proportion (4%; 13/286) of the ferret SC neurons were responsive to long-duration (>5 s) noxious stimulation, but further tests could not establish these responses as nociceptive. Collectively, these data indicate that the ferret SC is best adapted for the animal's visuallacoustically guided activities and most closely resembles the SC of its phylogenetic relative, the cat.     

The synaptic distribution of the retinal input in the superficial layers of the guinea-pig superior colliculus

An evoked potential consisting of four postsynaptic components was recorded in the guinea-pig superior colliculus following electrical stimulation of the contralateral optic nerve. This potential was generated in response to the activation of four populations of optic nerve fibres with different conduction velocities. Current source-density analysis revealed that the two slower conducting fibre populations synapse in the upper third of the stratum griseum superficiale on dendrites whose cell bodies appear to be found in the lower part of this layer and in the stratum opticum. The two faster conducting populations synapse deeper, near the border of the stratum griseum superficiale and stratum opticum, on neurons with cell bodies that may lie towards the upper part of the stratum griseum superficiale. The locations of these postsynaptic sites correspond to the layers in which the optic nerve terminates as revealed by neuroanatomical tracing techniques. Furthermore, neurons of the shape and orientation predicted by the current source-density analysis were found in the superficial layers by using the Golgi-Cox technique.     

Goldfish retinal axons respond to position-specific properties of tectal cell membranes in vitro

Using a special in vitro assay, we tested whether retinal ganglion cell axons in an adult vertebrate, the goldfish (which can regenerate a retinotopic projection after optic nerve section), recognize position-specific differences in cell surface membranes of their target, the tectum opticum. On a surface consisting of alternating stripes of membranes from rostral and caudal tectum, temporal axons accumulate on membranes derived from their retinotopically related rostral tectal half. Nasal axons grow randomly over both types of membranes. Nasal and temporal axons can elongate on both rostral and caudal membranes. A quantitative growth test, however, revealed that caudal membranes are less permissive substrates for the outgrowth of temporal axons than rostral membranes, and than rostral or caudal membranes for nasal axons.     

Response of retinal ganglion cell axons to striped linear gradients of repellent guidance molecules

Although molecular gradients have long been postulated to play a role in the development of topographic projections in the nervous system, relatively little is known about how axons evaluate gradients. Do growth cones respond to concentration or to slope? Do they react suddenly or gradually? Is there adaptation? In the developing retinotectal system, temporal retinal ganglion cell axons have previously been shown to avoid repellent cell-surface activities distributed in gradients across the optic tectum. We confronted temporal retinal axons with precisely formed striped linear gradients of repellent tectal membranes and of two candidate repellent molecules, ephrin-A2 and -A5. Axons entered gradient stripes independently of their slope and extended unhindered in the uphill direction until they suddenly avoided an apparent threshold concentration of repellent material that was independent of slope. This critical concentration was similar in both linear and nonlinear gradients, and hence independent of gradient shape. When gradients of identical slope were formed on different basal levels of repellent material, axons grew uphill for a fixed increment of concentration, possibly measured from the lowest point of the gradient, rather than up to a fixed absolute concentration. The speed of growth cones was not affected by repellent unstriped gradients below the critical concentration level. Similar results were found with membranes from cell lines stably transfected with either ephrin-A5 or ephrin-A2, two previously identified growth cone repellent cell-surface proteins. These data suggest that growth cones or axons can integrate guidance information over large distances, probably by a combined memory and adaptation mechanism. © 1998 John Wiley & Sons, Inc. J Neurobiol 37: 541–562, 1998     

Projections from cortical visual areas of the superior temporal sulcus to the superior colliculus, in macaque monkeys.

1. The distribution of tectal projections of two visual areas of the superior temporal sulcus (MT and MST areas) has been studied, in five Macaca fascicularis, by means of the autoradiographic method tracing the anterograde transport of tritiated aminoacids intracortically injected. 2. In all cases the ipsilateral superior colliculi (SC) were found labelled, whereas the contralateral ones were devoid of label. 3. The three brains injected in the MT area resulted in SC labels that involved the superficial gray layer (SGS), the stratum opticum (SO) and the intermediate gray layer (SGI), sparing the layers below SGI. 4. The collicular labels found after injections within the MST area exhibited their distribution over the deep SC subdivision, whereas they spared all the superficial layers but the deep part of the SO. 5. In two animals with large uptake zones, one in MT and the other in MST, the labelling within the SGI showed a cluster-like pattern. 6. The distinct found bulk of projections of MT and MST respectively to the superficial and deep subdivisions of the SC, along with a number of peculiar connections of the MST area as mentioned in the text, contribute to depict an overall neural network in which MST appears to be more strongly involved than MT in linking sensory visual with oculomotor attentive functions.     

Responses of cultured neural retinal cells to substratum-bound laminin and other extracellular matrix molecules

The responses of cultured chick embryo retinal neurons to several extracellular matrix molecules are described. Retinal cell suspensions in serum-free medium containing the "N1" supplement (J. E. Bottenstein, S. D. Skaper, S. Varon, and J. Sato, 1980, Exp. Cell Res. 125, 183-190) were seeded on tissue culture plastic surfaces pretreated with polyornithine (PORN) and with one of the factors to be tested. Substantial cell survival could be observed after 72 hr in vitro on PORN pretreated with serum or laminin, whereas most cells appeared to be degenerating on untreated PORN, PORN-fibronectin, and PORN-chondronectin. Cell attachment, although quantitatively similar for all these substrata, was temperature-dependent on serum and laminin but not on fibronectin or untreated PORN. In a short-term bioassay, neurite development was abundant on laminin, scarce on serum and fibronectin, and absent on PORN. No positive correlation between cell spreading and neurite production could be seen: cell spreading was more extensive on PORN and fibronectin than on laminin or serum, while on laminin-treated dishes, spreading was similar for neurite-bearing and non-neurite-bearing cells. Laminin effects on retinal neurons were clearly substratum dependent. When bound to tissue culture plastic, laminin showed a dose-dependent inhibitory effect on cell attachment and did not stimulate neurite development. PORN-bound laminin, on the other hand, did not affect cell attachment but caused marked stimulation of neurite development, suggesting that laminin conformation and/or the spatial distribution of active sites play an important role in the neurite-promoting function of this extracellular matrix molecule. Investigation of the embryonic retina with ELISA and immunocytochemical methods showed that laminin is present in this organ during development. Therefore, in vivo and in vitro observations are consistent with the possibility that laminin might influence neuronal development in the retina.     

Intrinsic processing in the mammalian superior colliculus

The mammalian superior colliculus receives visual inputs from the retina and primary visual cortex in its superficial layers and sends descending motor commands from its deeper layers. It is now becoming clear that a connection exists between these layers, but the signal transmission through it is not robust. The induction of burst discharges in the deeper layer neurons by direct visual inputs from the superficial layers may lead to 'express' saccadic eye movements with extremely short reaction times in behaving animals.     

Cellular dispersion patterns and phenotypes in the developing mouse superior colliculus.

The mammalian superior colliculus is structurally and functionally divided into two entities: superficial visual and deep multimodal motor. To discover the role, if any, of developmental processes in establishing separate tectal compartments, we have used highly unbalanced mouse chimaeras to mark cell dispersion pathways and trace cell lineages. Two forms of cell dispersion were detected: radial and tangential. Neither radial nor tangential forms of cell dispersion were found to exist on their own in any group of labeled cells. Radial cell dispersion was the predominant form of cell movement from the germinal zones and primarily associated with the differentiation of glutamatergic neurons. In contrast, tangential cell dispersion involved a minority of tectal cells, concentrated chiefly in the superficial layers and often associated with the upper aspects of radial columns. More scattered cells expressed gamma-aminobutyric acid (GABA) compared to columnar cells. Taken together, these results indicate separate developmental constraints for the development of glutamatergic and GABAergic neurons in the superior colliculus.     

The outgrowth response of the axons of developing and regenerating rat retinal ganglion cells in vitro to neurotrophin treatment

BDNF and NT-4 (but not NT-3 or CNTF) significantly enhanced the outgrowth of early embryonic and adult regenerating RGC axons when provided with a supportive substrate in vitro. BDNF and NT-4 treatment transiently increased RGC axon outgrowth from E15 rat retinas but not from retinas at older embryonic ages. The transient effect of BDNF and NT-4 and the inability of the neurotrophins to promote outgrowth from older embryonic retinal explants suggests a time frame of neurotrophin action and that other chemical factors (target-derived or otherwise) may be necessary for the continued maintenance of developing RGC axons. BDNF and NT-4 also enhanced the outgrowth of regenerating axons from adult retinal explants, but appeared to have a more subtle effect on axon outgrowth, in that the growth-promoting effects of BDNF and NT-4 appeared continuous throughout the incubation period. The suppression of RGC axon outgrowth from embryonic and adult retinae cultured in trkB-IgG-containing medium suggests that the response of developing and regenerating axons, to BDNF and NT-4 are likely to occur through trkB signalling.     

Nonlinear visuoauditory integration in the mouse superior colliculus

Sensory information from different modalities is processed in parallel, and then integrated in associative brain areas to improve object identification and the interpretation of sensory experiences. The Superior Colliculus (SC) is a midbrain structure that plays a critical role in integrating visual, auditory, and somatosensory input to assess saliency and promote action. Although the response properties of the individual SC neurons to visuoauditory stimuli have been characterized, little is known about the spatial and temporal dynamics of the integration at the population level. Here we recorded the response properties of SC neurons to spatially restricted visual and auditory stimuli using large-scale electrophysiology. We then created a general, population-level model that explains the spatial, temporal, and intensity requirements of stimuli needed for sensory integration. We found that the mouse SC contains topographically organized visual and auditory neurons that exhibit nonlinear multisensory integration. We show that nonlinear integration depends on properties of auditory but not visual stimuli. We also find that a heuristically derived nonlinear modulation function reveals conditions required for sensory integration that are consistent with previously proposed models of sensory integration such as spatial matching and the principle of inverse effectiveness.     

Electrophysiological relations between the superior colliculus and the red nucleus in the cat.

The electrical activity of single units located in the parvicellular part of the red nucleus (pRP) was recorded extracellularly in nitrous oxide anesthetized and C1-transected adult cats. In this area, neurons were found to respond to electrical stimulation applied to intermediate and deep layers of the right superior colliculus (SC). Forty neurons located in the pRN of both sides were studied. Three neurons out of 18 (16.6%) located in the contralateral pRN and six neurons out of 22 (27.3%) located in the ipsilateral pRN were driven by the right SC stimulation. The pRN neurons were separated into four groups according to the latency response to the SC stimulation: 1) 0.6-1.9 ms, 2) 2-4 ms, 3) 4-6 ms, 4) variable latency responses. Each of these four groups of neurons showed a particular pattern of discharge, even though their discharge frequency showed a strong consistency. Four pRN neurons, which responded to SC stimulation, showed a significant correlation with spontaneous horizontal eye movements of saccadic type. It is known that the SC represents one of the main outputs of the striato-nigral motor system. The relation between the SC and the pRN described in the present study suggest that connections exist between the cortico-rubral and the striato-nigral systems, since both have the SC as a common output structure. It is likely, therefore, that the cortico-rubral-SC system is involved in the control of oculomotor functions, and that the SC may serve to establish interactions between systems concerned with eye movements.