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T A Reh

Publications and source records attributed to T A Reh.

64 records · Page 4Linked to original sources
PubMed1987

Axons of the pyramidal tract do not increase their transport of growth-associated proteins after axotomy.

We compared the effects of axotomy on the composition of fast axonally transported proteins in rat sciatic sensory axons and pyramidal tract axons, 7 days after axotomy. L-[35S]methionine was administered to dorsal root ganglia or sensorimotor cortex and after 6 or 4 h, respectively, labelled proteins were obtained from sciatic nerve or pyramidal tract. Transported proteins were characterized by one- and two-dimensional electrophoresis, followed by fluorography. As previously reported, sciatic axotomy produced changes in the labelling of fast-transported proteins, notably, increased labelling of a spot designated GAP43. In contrast axotomy of pyramidal axons produced no increased labelling of GAP43, although it could be identified in both normal and axotomized samples. Autoradiography of spinal cord distal to the lesion confirmed that our procedures did indeed label axons and that the axotomy did interrupt these axons. Our finding that axotomy does not stimulate changes in composition of fast-transported proteins in the corticospinal tract agrees with previous studies on mammalian retinal ganglion cell axons and leads us to conclude that this failure to respond to axotomy is a general feature of non-regenerating CNS axons.

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PubMed1987

Cell-specific regulation of neuronal production in the larval frog retina.

We have previously postulated the existence of a feedback mechanism from differentiated neurons that regulates the production of new neurons. Evidence for such regulatory feedback comes from experiments in which dopamine-containing amacrine cells, ablated in the developing retina by 6-hydroxydopamine (6-OHDA), were up-regulated in their production. To determine whether this is a general phenomenon of the developing retina, the neurotoxin kainic acid (KA) was injected intraocularly in midlarval-stage Rana pipiens tadpoles to produce selective lesions of certain retinal cell types. After periods of 1-21 d, the animals received intraperitoneal injections of 3H-thymidine. Animals were then allowed to survive for periods of up to 3 weeks and were then fixed, the eyes embedded in plastic, sectioned at 3 micron, and processed for autoradiography by standard methods. At the dosage used, the KA produced a 52% decline in the cell density of the inner nuclear layer (INL), a 37% decline in the retinal ganglion cell layer (RGC), and no significant change in the density of cells in the outer nuclear layer (ONL). The 3H-thymidine allowed us to detect any changes in the number of new cells added to the retina after the KA lesion. Within the first week after the KA injection, there was a decrease in the number of 3H-thymidine (3H-Thy)-labeled cells in the lesioned eye as compared to in the control retina; however, KA treatment of slice cultures demonstrated that the toxin does not affect proliferating neuroblasts directly.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1986

Regulation of tyrosine hydroxylase-containing amacrine cell number in larval frog retina.

To determine whether production of new neurons of a particular type is regulated by the presence of previously differentiated neurons of the same type, we ablated all tyrosine hydroxylase immunoreactive (THIR) cells from larval frog retina with the neurotoxin 6-hydroxydopamine, and examined the retinas in subsequent weeks for newly generated THIR neurons. Three weeks after neurotoxin administration, new THIR cells appeared near the zone of neural proliferation at the ciliary margin at a higher density than that of normal retina, while the densities of other amacrine cell types, serotonin (t-HT) immunoreactive and substance P immunoreactive (SPIR), remained the same as controls. Thus the production of new retinal TRIR cells is selectively up-regulated following ablation of previously differentiated cells of this type.

Animals↗
PubMed1985

Growth cone-target interactions in the frog retinotectal pathway.

The growth cones of retinal ganglion cell axons were studied in the optic tract and tectum with horseradish peroxidase (HRP) histochemistry and electron microscopy. The ganglion cell growth cones has many morphological features similar to those described in vitro and in other in vivo systems. However, we found that some processes formed highly differentiated terminal arborizations, while retaining growth cones on many of their branches. In addition, ultrastructural examination of the tectal neuropil revealed that many ganglion cell axonal processes had characteristics of both growth cones and presynaptic endings. These findings are discussed in the context of the hypothesis of shifting connections and the evidence that retinotectal map formation involves several mechanisms, including a process that depends on the action potential activity in the optic fibers.

Animals↗
PubMed1985

Eye-specific segregation requires neural activity in three-eyed Rana pipiens.

The addition of a third eye primordium to the forebrain region of a Rana pipiens embryo invariably results in the development of a series of periodic, mutually exclusive eye-specific bands in tectal lobes dually innervated by the host and supernumerary fibers. A number of investigators have proposed that such source-specific segregation arises as a compromise between two mechanisms that are normally involved in retinotectal map formation: one which is dependent on cell surface affinities to align the map and produce a rough retinotopy and a second that "fine tunes" the map by stabilizing adjacent terminals from neighboring retinal ganglion cell bodies at the expense of terminals from non-neighboring cells. In this study we have tested the idea that this second "fine-tuning" mechanism is dependent on neural activity by blocking impulse activity in the optic nerves of three-eyed tadpoles. To assess the requirement for activity on the formation of bands, both normal optic nerves of 17 three-eyed tadpoles were crushed intraorbitally. Two weeks after this operation, the supernumerary retinal projection had debanded and spread to cover the entire tectum in a continuous fashion. By 4 weeks, however, the host optic fibers regenerated back to the tecta and began to form segregated stripes with the fibers from the third eye. Six to 7 weeks after the optic nerve crush the periodic pattern of eye-specific segregation characteristic of dually innervated tecta was again pronounced. When activity in all three optic nerves was eliminated with tetrodotoxin (TTX; embedded in a slow release plastic) during the last 3 weeks of this process, the fibers from the two competing eyes failed to segregate and, instead, formed two completely overlapping, continuous projections across the tectal surface. To test for the requirement of activity in the maintenance of segregation, we also subjected three-eyed tadpoles without optic nerve crush to TTX blockade for 2, 3, and 4 weeks. Animals sacrificed at 2 weeks show overlap of the projections in the rostral tectum but distinct interdigitating stripes in other regions of these lobes. After 3 weeks of blockade, segregation of the projections was less distinct in the central tectum as well. After 4 weeks of TTX blockade the terminals from both eyes spread to form continuous overlapping projections throughout the tectum. Examination of well isolated, individual retinal ganglion cell terminal arbors during this period reveals that they occupy a significantly greater area of tectum following the TTX treatment.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1984

Retinal ganglion cell terminals change their projection sites during larval development of Rana pipiens.

Interconnecting neuronal populations in the vertebrate CNS are typically not well matched in their overall topographic patterns of histogenesis and differentiation during development. One striking example of this mismatch is the retinotectal system of the frog, where the retina grows in concentric annuli, while the optic tectum, a major retinal target, adds new neurons at only the caudo-medial border. The retinal ganglion cell (RGC) terminals nevertheless form an organized map in the tectum during the period when the two structures are undergoing such disparate modes of growth. This led Gaze et al. (Gaze, R. M., M. J. Keating, and S. H. Chung (1974) Proc. R. Soc. Lond. (Biol.) 185: 301-330) to propose that the terminals must shift caudally during development. In the present study, we have directly tested the hypothesis of "shifting connections" by selectively labeling an identified population of RGC terminals, those at the optic nerve head (ONH), and determining their tectal projection site relative to a particular group of [3H]thymidine-labeled tectal neurons. With this double-label technique, we have found that RGC terminals from cells at the ONH move from a position rostral to the [3H]thymidine-labeled tectal cells to a position caudal to these same cells during the latter half of larval development. This represents a movement of approximately 1.4 mm across the tectal surface between stages T&K XII and T&K XXV. In addition, we have used electron microscopy and electrophysiology to demonstrate that the RGC terminals make functional synaptic connections during this period. This indicates that RGC terminals continually change the tectal neurons with which they form functional synapses during the development of the retinotectal system. We propose that such moving, but highly ordered connections can best be explained by a two stage mechanism for map formation, in which graded selective adhesions between cells in appropriate regions of retina and tectum provide the overall gross retinotopy of the projection, while competitive interactions between RGC terminals are responsible for the refinement of the precision in this system.

Animals↗
PubMed1983

The organization of the fibers in the optic nerve of normal and tectum-less Rana pipiens.

We have examined the detailed order of retinal ganglion cell (RGC) axons in the optic nerve and tract of the frog, Rana pipiens. By using horseradish peroxidase (HRP) injections into small regions of the retina, the tectum, and at various points along the visual pathway, it has been possible to follow labelled fibers throughout their course in the nerve and tract. Several surprising features in the order of fibers in the visual pathway were discovered in our investigation. The fascicular pattern of RGC axons in the retina is similar to that described in other vertebrates; however, immediately central to their entry into the optic nerve head, approximately half of the fibers from the nasal or temporal retina cross over to the opposite side of the nerve. Although the axons from the dorsal and ventral regions of the retina generally remain in the dorsal and ventral regions of the nerve, some fiber crossing occurs in those axons as well. The result of this seemingly complex rearrangement is that the optic nerve of Rana pipiens contains mirror symmetric representations of the retinal surface on either side of the dorsal ventral midline of the nerve. The fibers in each of these representations are arranged as semicircles representing the full circumference of the retina. This precise fiber order is preserved in the nerve until immediately peripheral to the optic chiasm, at which point age-related axons from both sides of the nerve bundle together. Consequently, when a small pellet of HRP is placed in the chiasmic region of the nerve, an annulus of retinal ganglion cells and a corresponding annulus of RGC terminals in the tectum are labelled. As the age-related bundles of fibers emerge from the chiasm they split to form a medial bundle and a lateral bundle, which grow in the medial and lateral branches of the optic tract, respectively. Although the course followed by RGC axons in the visual pathway is complex, we propose a model in which the organization of fibers in the nerve and tract can arise from a few rules of axon guidance. To determine whether the optic tecta, the primary retinal targets, play a role in the development and organization of the optic nerve and tract, we removed the tectal primordia in Rana embryos and examined the order in the nerve when the animals had reached larval stages. We found that the order in the nerve and tract was well preserved in tectumless frogs. Therefore, we propose that guidance factors independent of the target direct axon growth in the frog visual system.

Animals↗
PubMed1983

Qualitative and quantitative measures of plasticity during the normal development of the Rana pipiens retinotectal projection.

We have examined the following aspects of retinal development in the frog, Rana pipiens: (1) the overall pattern of cell addition to the retina; (2) the relative rate of retinal ganglion cell (RGC) accretion; and (3) the changes in RGC density during larval development. In addition, we have studied the development of the retinal projection onto the tectum by means of the anterograde transport of horseradish peroxidase (HRP) and measurements of the volume of tectal neuropil at several larval stages and in postmetamorphic frogs. We find that the addition of new cells to the retina of Rana pipiens larva is restricted to the ciliary margin and that this addition is concentric at all larval stages. Additionally, the morphometric measures of retinal and tectal growth, along with the HRP histochemistry, indicate that the retinal projection exhibits considerable plasticity during normal development. The plasticity we observe in normal development may explain why the retinotectal projection can compensate its area and volume in experimental paradigms that effect drastic changes in innervation density.

Animals↗
PubMedDate not supplied

Retinal pigmented epithelial cells induced to transdifferentiate to neurons by laminin.

Although the regeneration of nervous tissue in the vertebrate is very limited, there are a few remarkable examples of this process. Understanding the factors that regulate CNS regeneration in those areas of the nervous system where it occurs, will doubtless provide generally applicable, essential information about the process. It has been known for some time that the amphibian retina regenerates following its destruction. Transplant studies, confirmed later by in vitro experiments, have shown that one source of new neurons in regenerating retina is the retinal pigmented epithelium (RPE). RPE cells can transdifferentiate to either neurons or lens cells in culture, but little is known about the factors that regulate this process. A recent study in vivo of retinal regeneration provided evidence that the association of RPE cells with the retinal vascular membrane is an important step in transdifferentiation. We report here that transdifferentiation in vitro is profoundly influenced by the substrate on which the cells are cultured; RPE cells plated on laminin-containing substrates frequently transdifferentiate into neurons. In addition, we have found a high concentration of laminin in the Rana retinal vascular membrane. Therefore, we propose that retinal regeneration is initiated by changes in the composition of the extracellular matrix that RPE cells contact early in the process.

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