Biomedical subjects

L C Silveira

Publications and source records attributed to L C Silveira.

At least 19 recordsLinked to original sources

PubMed2001

M and P retinal ganglion cells of the owl monkey: morphology, size and photoreceptor convergence.

We have estimated photoreceptor convergence to M and P retinal ganglion cells of two closely related nocturnal (owl monkey, Aotus) and diurnal (capuchin monkey, Cebus) anthropoids. Rod convergence is higher in the owl monkey retina while cone convergence to both M and P cells are very similar in the retinas of the owl monkey and the capuchin monkey. These results indicate that during evolution, the owl monkey retina has undergone changes compatible with a more nocturnal lifestyle, but kept a cone to ganglion cell relation similar to that found in diurnal primates.

Animals↗

PubMed2001

Influence of contrast on the responses of marmoset lateral geniculate cells to drifting gratings.

The responses of lateral geniculate nucleus (LGN) cells in the common marmoset (Callithrix jacchus) to drifting luminance or cone isolating gratings of different spatial frequencies and contrasts were measured. The response noise, defined as the variability of the responses to single sweeps in the complex plane, was independent of stimulus contrast and spatial frequency but increased with increasing overall responsiveness of the cell. The signal-to-noise ratio of parvocellular (PC) cells was smaller than of magnocellular (MC) cells. At each contrast, the response amplitude as a function of spatial frequency could be described with a difference of Gaussians model. With this model, the sizes and the peak sensitivities of the receptive field centers and surrounds were estimated. It was found that receptive field center and surround sizes of LGN cells decrease slightly with increasing contrast. Further, the peak sensitivity decreases with increasing contrast. The two factors are involved in a decrease in responsivity (the response per unit contrast) with increasing contrast which is compatible to response saturation for low spatial frequency stimuli. PC cells did not saturate as much to luminance stimuli although some saturation was found with cone isolating gratings. We found that the response phase lag of both PC and MC cells decreased with increasing contrast, which cannot be explained on the basis of linear response behavior. Apparently the phase of LGN cell responses to drifting gratings is altered in comparison with the retinal inputs by additional nonlinearities.

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PubMed2000

Visual responses of ganglion cells of a New-World primate, the capuchin monkey, Cebus apella.

1. The genetic basis of colour vision in New-World primates differs from that in humans and other Old-World primates. Most New-World primate species show a polymorphism; all males are dichromats and most females trichromats. 2. In the retina of Old-World primates such as the macaque, the physiological correlates of trichromacy are well established. Comparison of the retinae in New- and Old-World species may help constrain hypotheses as to the evolution of colour vision and the pathways associated with it. 3. Ganglion cell behaviour was recorded from trichromatic and dichromatic members of a New-World species (the capuchin monkey, Cebus apella) and compared with macaque data. Despite some differences in quantitative detail (such as a temporal response extended to higher frequencies), results from trichromatic animals strongly resembled those from the macaque. 4. In particular, cells of the parvocellular (PC) pathway showed characteristic frequency-dependent changes in responsivity to luminance and chromatic modulation, cells of the magnocellular (MC) pathway showed frequency-doubled responses to chromatic modulation, and the surround of MC cells received a chromatic input revealed on changing the phase of heterochromatically modulated lights. 5. Ganglion cells of dichromats were colour-blind versions of those of trichromats. 6. This strong physiological homology is consistent with a common origin of trichromacy in New- and Old-World monkeys; in the New-World primate the presence of two pigments in the middle-to-long wavelength range permits full expression of the retinal mechanisms of trichromatic vision.

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PubMed2000

Conservation of absolute foveal area in New World monkeys. A constraint on eye size and conformation.

The foveal specializations of five New World monkeys, the marmoset, Callithrix jacchus; the golden-handed tamarin, Saguinus midas niger; the squirrel monkey, Saimiri ustius; the capuchin monkey, Cebus apella; and the howler monkey, Alouatta caraya were compared. Although retinal area varies by over a factor of two in these monkeys, the area of the fovea does not covary with retinal area and remains approximately the same absolute size, as measured by the dimensions of the high density region of cones, or the rod-free region. This constancy in foveal size also holds for rhesus monkeys and humans, bringing the variation in retinal area to a factor of five. Alouatta caraya is unusual, distinguished by a very high central cone density and a small rod-free zone. Physiological constraints that might limit foveal area over a wide range of eye sizes are considered.

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PubMed1998

Post-receptoral mechanisms of colour vision in New World primates.

Diurnal platyrrhines, both di- and trichromats, have magnocellular (M-) and parvocellular (P-) retinal ganglion cells which are morphologically very similar to those found in catarrhines. Catarrhine central P ganglion cells contact single midget bipolar cells, which contact single cones. Physiological recordings of retinal ganglion cells of dichromatic Cebus monkeys showed very similar cell properties to the catarrhine macaque, except that P ganglion cells lacked colour-opponency. We describe the presence of single-headed midget bipolar cells in the Cebus retina. These midget bipolar cells have axon terminal sizes in the same range as the dendritic tree sizes of P ganglion cells as far as 2 mm of retinal eccentricity. This result supports the view that, as in catarrhines, central P ganglion cells of platyrrhines receive input from single midget bipolar cells which in turn, receive input from single cones. This finding is consistent with the idea that a P pathway with one-to-one connectivity was present in the anthropoid ancestor before the divergence between catarrhines and platyrrhines.

Action Potentials↗

PubMed1998

Morphology of P and M retinal ganglion cells of the bush baby.

P/midget ganglion cells mediate red-green color opponency in anthropoids. It has been proposed that these cells evolved as a specialization to subserve color vision in primates. If that is correct, they must have evolved about the same time as the long-wavelength ('red') and medium-wavelength ('green') pigment genes diverged, thirty million years ago. Strepsirhines are another group of primates that diverged from the ancestor of the anthropoids at least 55 million years ago. If P/midget ganglion cells evolved to subserve color vision, they should be absent in strepsirhines. We tested this hypothesis in a nocturnal strepsirhine, the greater bush baby Otolemur. The retinal ganglion cells were labeled with the lipophilic tracer Dil and the results show that bush babies have P/midget and M/parasol cells similar to those found in the peripheral retinas of anthropoids. A number of studies have shown that the P and M pathways of bush babies share many similarities with those of anthropoids, and our results show that the same is true for their retinal ganglion cells. These results support the hypothesis that the P system evolved prior to the emergence of red-green color opponency.

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PubMed1996

Late development of Zif268 ocular dominance columns in primary visual cortex of primates.

Zif268 transcription factor is expressed throughout Cebus apella visual cortex at high basal levels. Monocular eyelid suture alters the levels of Zif268 on neurons connected to the deprived eye, revealing ocular dominance columns in the striate cortex of Cebus as previously demonstrated in Old World monkeys (Chaudhuri and Cynader, Brain Res., 605 (1993) 349-353). Zif268 ocular dominance columns are revealed in adult Cebus monkey after 24-48 h of monocular deprivation, but not in infant monkeys up to 3 months of age. In 6-month-old Cebus monkeys, Zif268 ocular dominance columns are still poorly defined. These results suggest that Zif268 ocular dominance columns establish late during normal primate visual system development, and that some degree of visual plasticity is still present at this age in the Cebus monkey.

Aging↗

PubMed1996

Distribution of M retinal ganglion cells in diurnal and nocturnal New World monkeys.

The topography of M ganglion cell distribution was studied in the retinae of two New World monkey species, the diurnal capuchin monkey Cebus apella and the nocturnal owl monkey Aotus azarae. Retinal whole mounts were stained by the neurofibrillar method of Gros-Schultze. As occurs with other diurnal primates, the Cebus M-ganglion cell density peaks in the foveal slope and declines towards the periphery. In the Aotus retina, the M ganglion cell density peaks in the area centralis and declines toward the periphery. In both species the cell density in the temporal, dorsal, and ventral meridians are similar for equivalent eccentricities. The cell density in the nasal meridian is higher than in the other meridians. The naso-temporal density ratio ranges between 1.2 and 4.3 in the Cebus and 1.6 and 2.2 in the Aotus. The total number of M-ganglion cells was 140,300 and 74,000 in the Cebus and Aotus retinae, respectively, corresponding to about 10% and 15.4% of the total retinal ganglion cell population in these species. The results indicate that M ganglion cells are similarly organized in both diurnal and nocturnal simians, but may be proportionally more important for the nocturnal species.

Animals↗

PubMed1996

Factors affecting the survival of cat retinal ganglion cells after optic nerve injury.

After partial transection of one optic nerve in adult cats the majority of beta retinal ganglion cells degenerate and die 1 week after axotomy, whilst other cell classes degenerate slowly and survive for a long period after the lesion. We have investigated the effects of intravitreal and intraperitoneal injections of MK-801, a NMDA-glutamate receptor antagonist, on the early degeneration of retinal ganglion cells after partial optic nerve section. Control animals received saline intravitreal injections. Retinal flat mounts were retrogradely labelled with horseradish peroxidase and counterstained with Cresyl Violet. We evaluated the ganglion cell loss in the three experimental groups 1 week after lesion and compared them with normal uninjured controls and injured untreated retinae. In untreated retinae 49% of ganglion cells die 1 week after the lesion. Systemic MK-801 or saline prolonged survival of 41% of retinal ganglion cells that would die without treatment. Intravitreal MK-801 or saline prolonged survival of 71% of retinal ganglion cells that would die without treatment, but the results of saline administration had a larger range of variability. In untreated retinae many pyknotic cells were observed. They decreased in number after systemic MK-801 treatment and in some retinae treated with intravitreal injections of saline solution. There were no pyknotic cells after local, intravitreal MK-801 treatment. These results support the hypothesis that NMDA-receptor mediated neurotoxicity plays an important role in the early retinal ganglion cell death after retrobulbar axotomy. They also support the existence of an endogenous source of neurotrophins whose release is triggered by eyeball injury. We conclude that the early death of beta retinal ganglion cells after axotomy occurs by a mechanism that can be controlled by neurotrophins and antagonists to NMDA-glutamate receptors.

Animals↗

PubMed1996

Effect of congenital hypothyroidism on cell density in the ganglion cell layer of the rat retina.

The effect of congenital hypothyroidism on the visual system of Wistar rats was studied by determining neuron density in the retinal ganglion cell layer. Retinae of adult rats from mothers treated with propylthiouracil, 50 mg/day, starting on the 15th day of pregnancy (PTU group), and of adult rats from untreated mothers (control group) were examined. Retinae were prepared, and the neurons in the nasotemporal region located above the optic disc were counted. Hypothyroid rats showed a significant reduction in the retinal area (about 6.8%), when compared to controls. The cell density in the retinal ganglion cell layer was significantly decreased in 6 PTU-treated compared to 5 control retinae in total (2,793 +/- 330 vs 3,704 +/- 662 neurons/mm2), nasal (3,031 +/- 580 vs 3,853 +/- 699 neurons/mm2) and temporal (2,555 +/- 155 vs 3,555 +/- 827 neurons/mm2) regions. These alterations in a region considered to be one of the most specialized in the visual process suggest a structural deficiency induced by congenital hypothyroidism, with a possible decrease in the visual acuity of the rat.

Animals↗

PubMed1996

Parallel pathways in the retina of Old and New World primates.

Old-world simians are all trichromats, but in most new-world primates there is a polymorphism; males are dichromats but most females are trichromats. In the old world simian, luminance and red-green chromatic channels defined by psychophysical experiments have as a basis parasol ganglion cells of the magnocellular (MC) pathway and midget ganglion cells of the parvocellular (PC) pathway respectively. Small bistratified ganglion cells provide a basis for a blue-yellow chromatic channel, which should probably be considered a separate entity. In both dichromatic and trichromatic new-world animals, the MC pathway and the small bistratified, blue-yellow system seem anatomically and physiologically similar to those in their old-world relatives. The midget ganglion cells of the parvocellular pathway in trichromats are anatomically and physiologically similar to the old-world pattern. In dichromatic animals, they are anatomically similar and physiologically resemble those of trichromatic animals, except for the lack of chromatic opponency. We conclude that these three systems may from a basic pattern for the visual pathway of primates. However, the results from dichromats indicate that the evolution of trichromacy may be found to be more complex than presently supposed.

Animals↗

PubMed1996

Joint entropy loci of M and P cells: a hypothesis for parallel processing in the primate visual system.

The M and P neurons connect the retinal inner plexiform layer to the magnicellular and parvicellular layers of the lateral geniculate nucleus and to layer 4 of the primary visual cortex. The M and P pathways transmit visual signals with different six-dimensional joint entropy for space, spatial frequency, time and temporal frequency. Beyond V1 layer 4, M and P influences mix in information processing streams which connect different visual cortical areas. Some visual cortical cells perform better than it is possible for linear filters, regarding to simultaneous spatial and spatial frequency localization of their response. This can be explained by nonlinear combination of M and P input on such cells.

Animals↗

PubMed1996

The retinal ganglion cell classes of New World primates.

In the primate retina there are distinct ganglion cell classes, exhibiting particular morphologies and central projections, each responsible for conveying particular types of visual information to the brain. The chief retinal inputs to the cortex arise from specific ganglion cell classes, M-ganglion cells, responsible for carrying the luminance signal, and P-ganglion cells, that convey the red-green color opponent signal, as well as high contrast luminance signal. There are other ganglion cell classes, such as small-field bistratified cells, exhibiting dendrites that stratify at two different levels in the inner plexiform layer, which convey the blue-yellow color opponent signal. Most published data concerning primate retinal ganglion cell anatomy and physiology have been obtained from Old World species. Studies on New World monkeys have recently become of interest since they differ from the Old World monkeys with respect to the color vision inheritance pattern. On reviewing retinal ganglion cell layer organization in New World monkeys, it seems that there are more similarities than differences in relation to the Old World monkeys. Diurnal genera of New World monkeys exhibit a well-developed fovea centralis and ganglion cell density peak, as well as peripheral density values which are in the range reported for Old World monkeys and human. Moreover, all the major ganglion cell classes identified in Old World monkeys are also present in New World primates. Up to now, no obvious anatomical differences between dichromats and trichromats have been reported. The only genus that is significantly different from the others is the Aotus. It exhibits lower ganglion cell density in the central retina, and apparently lacks the small-field bistratified cells.

Animals↗

PubMed1994

M and P retinal ganglion cells of diurnal and nocturnal New-World monkeys.

M and P retinal ganglion cell morphology revealed by biocytin retrograde labelling was compared in two closely related New-World monkeys, Cebus and Aotus, to investigate whether nocturnal and diurnal species of primates have similar cell classes. Monkey and cat ganglion cells from regions of matching cell class densities were also compared. Cat alpha, cat beta, Aotus M, and Cebus M cells were similar in many aspects, but Cebus M cells had higher branching density. Cebus and Aotus P cells formed a distinct group and represent a primate specialization common to diurnal and nocturnal simians.

Animals↗

PubMed1994

The ganglion cell response to optic nerve injury in the cat: differential responses revealed by neurofibrillar staining.

The early responses of cat retinal ganglion cells to axotomy have been examined using neurofibrillar and Nissl-stained wholemounts. We were interested to learn whether the enhanced neurofilament expression, seen in a number of neuronal systems, was also present in different neuronal populations of the cat retina and could be used to study the distribution of these cells. We found that beta ganglion cells degenerate very rapidly after axotomy with the nuclei becoming pyknotic within a few days. Few beta cells showed increased neurofibrillar staining of the dendrites. The cell body degenerated prior to any visible degenerative changes in the axon. A proportion of the alpha and gamma ganglion cells degenerated in the first two to three weeks after axotomy. The alpha cells underwent markedly enhanced neurofibrillar staining of their dendrites prior to degeneration. The Nissl material of the cell bodies diminished as the cells degenerated but we have not observed pyknotic nuclei. The dendritic trees of some axotomised gamma cells were also revealed by the neurofibrillar stain three weeks after axotomy. These results show that retinal ganglion cells do not degenerate by a dying back process. We suggest that the rapid degeneration of the beta ganglion cell population comes about by excitotoxic cell death, a consequence of their large glutamatergic input from bipolar cells. The degenerating beta ganglion cells have the morphological appearance of cells undergoing apoptosis.

Animals↗

PubMed1993

The M-ganglion cell density gradient in New World monkeys.

The M-ganglion cell distribution was studied in the retinae of New World monkeys, the diurnal Cebus and the nocturnal Aotus. Retinal whole mounts were stained by the neurofibrillar method of Gros-Schultze. The M-ganglion cell distribution in the Cebus and Aotus is similar to that reported for Old World primates, such as the Macaca. The M-ganglion cell density peak occurs in the foveal slope and declines towards the periphery. There is a nasotemporal asymmetry: in the Cebus retina the M-ganglion cell density is 1.2-4.3 times higher in the nasal when compared to the temporal region at the equivalent eccentricities; in the Aotus, the nasotemporal asymmetry is smaller, 1.6-2.2 higher in the nasal region. We compared the M-ganglion cell density with previously published values for the density of the total population of ganglion cells and observed a larger proportion of M-ganglion cells in the Aotus, when compared with Cebus or Macaca.

Animals↗

PubMed1992

Visual effects of damage to P ganglion cells in macaques.

Four indices of visual performance were measured in control macaques and in macaques that had been exposed to monomeric acrylamide, a neurotoxicant that preferentially damages P retinal ganglion cells. Morphological examination of the retina and visual pathways of these monkeys showed virtually complete loss of P ganglion cells over a region extending to at least 40 deg from the fovea, and relative sparing of M ganglion cells. The four tests examined visual functions for which the visual pathway from P ganglion cells might be of great importance: visual acuity, contrast discrimination, hyperacuity, and shape discrimination. In the acrylamide-dosed monkeys, visual acuity was reduced slightly more than fourfold, a somewhat larger reduction than that seen previously after ibotenic-acid lesions of the P pathway in the geniculate. The residual acuity was in good agreement with the Nyquist frequency calculated from the density of ON or OFF M ganglion cells. Contrast increment thresholds were elevated for the dosed monkeys only in one of the two conditions tested. The elevation was found only under those spatiotemporal conditions for which we have previously shown that contrast thresholds are increased by acrylamide exposure, and was most marked at low background contrasts. Vernier acuity was elevated in one dosed monkey, but not affected in a second monkey that also had severe loss of P ganglion cells. Finally, we found no effect of acrylamide exposure on the number of training trials required to learn simple or complex shape discriminations. These results support previous findings in showing that the P pathway mediates visual acuity, and they show that several other important aspects of visual perception are not exclusively dependent on the P pathway.

Acrylamide↗

PubMed1992

Biocytin as a retrograde tracer in the mammalian visual system.

We have successfully used biocytin as a retrograde tracer in the mammalian visual system. Retinal ganglion cells, pyramidal and stellate cortical neurons were labelled. Both pressure injections and gel implants were used successfully for retrograde labelling. Biocytin was detected using avidin conjugates and horseradish peroxidase histochemistry. Retrograde filling with biocytin proved to be more reliable and to allow better morphological resolution than other commonly used neurotracers such as horseradish peroxidase. The fine details of cell morphology observable by this method are comparable in many cases to the results obtained with intracellular tracer injections. The morphological resolution obtained with this method allows the study of brain microcircuits using extracellular deposits of biocytin.

Animals↗