Transneuronal degeneration of beta retinal ganglion cells in the cat

Glaucoma of the brain: a disease model for the study of transsynaptic neural degeneration

The identification of mechanisms precipitating neuronal death and injury is an intense area of investigation requiring reliable models to assess the effects of neuroprotective agents. Most are suboptimal since the effects of initial damage are diffuse and may not be reproducible or easily quantifiable. The ideal laboratory model should have the ability to (a) clearly detect evidence of neuronal injury and recovery, (b) accurately measure morphologically the extent of these changes, and (c) provide functional evidence for damage and recovery. Glaucoma is a disease of visual neurons in the eye and brain. In the visual system, neuroanatomical pathways and retinotopic organization are exquisitely defined, functional modalities are highly characterized and can be dissected physiologically, visual input parameters can be modified, visual functional output can be readily tested and measured, changes in the eye and the visual brain can be directly visualized and imaged, and pathological and compensatory changes in brain centers of vision can be examined and measured specifically. For these reasons, the glaucoma disease model is ideal for the study of response and recovery to injury in the central nervous system due to anterograde and retrograde degeneration from the eye to the brain and the brain to the eye, respectively. The study of this glaucoma model of transsynaptic brain injury may be relevant to understanding more complex pathways and point to new strategies to prevent disease progression in other neurodegenerative diseases.

Neuroplasticity after unilateral visual cortex damage in the newborn cat

Anatomical, electrophysiological, and behavioral studies implicate extrastriate cortex as a major contributor to the sparing of visually guided behaviors following lesions of primary visual cortex incurred early in life. Here we report considerable sparing of the ability to detect and localize stimuli in the hemifield contralateral to unilateral early lesions of all contiguous visually-responsive primary and extrastriate cortical regions (occipital, visuoparietal, and visuotemporal cortices). In the adult cat this same lesion induces a dense blindness and cats are unable to orient to any visual stimulus introduced into the contralesional hemifield. In the absence of cortical circuits, the neural sparing identified following the neonatal lesion is based on the superior colliculus and it occurs despite massive retrograde transynaptic degeneration of large numbers of retinal ganglion cells.

Greater sparing of visually guided orienting behavior after early unilateral occipital lesions: insights from a comparison with the impact of bilateral lesions

We know that cats with bilateral lesions of occipital visual cortical areas 17, 18 and 19 sustained during the first postnatal week exhibit a modest level of sparing of the ability to re-orient head and eyes to new stimuli relative to cats that incurred equivalent lesions in adulthood. We now report that cats with equivalent unilateral lesions sustained during the first postnatal week (P1-4), or at the end of the first postnatal month (P27-30), orient to stimuli presented in the contralesional field as proficiently as to stimuli introduced into the ipsilesional field. Moreover, levels of proficiency are indistinguishable from those exhibited by intact cats. Thus, the sparing is greater following unilateral lesions than following bilateral lesions, and the level of sparing approaches completeness. The difference between the bilateral and unilateral lesion results suggests types of pathway reorganizations that may emerge as a result of unilateral occipital lesions. We postulate that the greater sparing is based on modifications in both excitatory and inhibitory circuitry linked to the intact hemisphere, and we provide a framework for future investigations that should be relevant to the comprehension of the repercussions of early unilateral and bilateral lesions sustained by monkeys and humans, which also show more robust residual vision following early relative to later damage of occipital cortex.

Limit of spared pattern vision following lesions of the immature visual cortex

Lesions of primary visual cortex sustained early in life spare certain aspects of visual processing that can be linked to expansions of bypass pathways to extrastriate cortex. They also trigger, in an age-dependent way, partial or complete transneuronal retrograde degeneration of beta (X) retinal ganglion cells, which are implicated in visual processing under conditions of low contrast. We used two-dimensional geometric patterns whose saliency was reduced by gradually increasing levels of superimposed masking lines, and by reductions in spatial contrast. Normative data were collected from intact cats, and baseline lesion data were collected from cats with lesions sustained as young adults (postnatal day 180, P180). Experimental data were collected from cats that sustained lesions on P1-3 or P26-30. For high contrast patterns, the adult group was impaired at both acquisition (sequential progressive levels of masking) and concurrent (parallel high and low levels of masking) performance, whereas the early-lesioned groups were impaired only at concurrent performance. All lesion groups were equally impaired when contrast was reduced to modest or lower levels. These results show that sparing of masked-pattern learning is limited to the high end of the spatial contrast domain.

Plasticity of the visual cortex after injury: what's different about the young brain?

The repercussions of localized injury of the cerebral cortex in young brains differ from the repercussions triggered by equivalent damage of the mature brain. In the young brain, some distant neurons are more vulnerable to the lesion, whereas others survive and expand their projections to bypass damaged and degenerated structures. The net result is sparing of neural processing and behaviors. This article summarizes both the modifications in visual pathways resulting from visual cortex lesions sustained early in life and the neural and behavioral processes that are spared or permanently impaired. Experiments using reversible deactivation show that at least two highly localizable functions of normal cerebral cortex are remapped across the cortical surface as a result of an early lesion of the primary visual cortex. Moreover, the redistributions have spread the essential neural operations underlying orienting behavior from the visual parietal cortex to a normally functionally distinct type of cortex in the visual temporal system, and in the opposite direction for complex-pattern recognition. Similar functional reorganizations may underlie sparing of neural processes and behavior following early lesions in other cerebral systems, and these other systems may respond well to emerging therapeutic strategies designed to enhance the sparing of functions.

Graded sparing of visually-guided orienting following primary visual cortex ablations within the first postnatal month

We compared the abilities of intact cats and cats that incurred lesions of areas 17 and 18 in adulthood, at one month of age (P28), or on the day of birth (P1), to detect and orient towards visual stimuli either moved into or illuminated in the periphery of the visual field, and to detect and orient towards a stationary, broad-band white-noise auditory stimulus. For all groups of cats, movement of a stimulus into the visual field was a more potent stimulus for evoking visually-guided orienting movements than illumination of a static light-emitting diode (LED). The potency of the auditory stimulus was also extremely high. Proficiency on both visual tasks was graded according to the age at which areas 17 and 18 were ablated in the sequence: adult, P1, P28 and intact in the sequence worst-->best performance. The superior performance of the P1- and P28-groups provided evidence for sparing of visually-guided orienting, but the sparing was incomplete because it did not match performance of intact cats. Lesions of areas 17 and 18 incurred in adulthood had no significant impact on orienting to auditory white-noise stimuli. However, orienting performance to auditory stimuli presented in the peripheral quadrants was slightly superior in the P28 group and reduced in the P1 group. Thus, the visual sparing exhibited by the P1 group may be at the expense of highly proficient orienting to auditory cues. Overall, these results extend our knowledge by showing that in addition to P1-cats, cats that incur lesions of areas 17 and 18 at one month-of-age also exhibit sparing of visually-guided orienting, and that the sparing is not confined to a single stimulation paradigm. Finally, the covariation in the magnitude of pathway modifications with the scale of the orienting proficiency in P1- and P28 cats helps to solidify the linkage between rewired brain pathways and spared visually-guided behaviors.

Brainstem inputs to the ferret medial geniculate nucleus and the effect of early deafferentation on novel retinal projections to the auditory thalamus

Following specific neonatal brain lesions in rodents and ferrets, retinal axons have been induced to innervate the medial geniculate nucleus (MGN). Previous studies have suggested that reduction of normal retinal targets along with deafferentation of the MGN are two concurrent factors required for the induction of novel retino-MGN projections. We have examined, in ferrets, the relative influence of these two factors on the extent of the novel retinal projection. We first characterized the inputs to the normal MGN, and the most effective combination of neonatal lesions to deafferent this nucleus, by injecting retrograde tracers into the MGN of normal and neonatally operated adult ferrets, respectively. In a second group of experiments, newborn ferrets received different combinations of lesions of normal retinal targets and MGN afferents. The resulting extent of retino-MGN projections was estimated for each case at adulthood, by using intraocular injections of anterograde tracers. We found that the extent of retino-MGN projections correlates well with the extent of MGN deafferentation, but not with extent of removal of normal retinal targets. Indeed, the presence of at least some normal retinal targets seems necessary for the formation of retino-MGN connections. The diameters of retino-MGN axons suggest that more than one type of retinal ganglion cells innervate the MGN under a lesion paradigm that spares the visual cortex and lateral geniculate nucleus. We also found that, after extensive deafferentation of MGN, other axonal systems in addition to retinal axons project ectopically to the MGN. These data are consistent with the idea that ectopic retino-MGN projections develop by sprouting of axon collaterals in response to signals arising from the deafferented nucleus, and that these axons compete with other sets of axons for terminal space in the MGN.

Transneuronal retrograde degeneration of retinal ganglion cells following cerebral hemispherectomy in cats

We have assessed the extent of transneuronal retrograde degeneration of retinal ganglion cells (RGCs) following the removal of a whole cerebral hemisphere at postnatal age 16 and 25 days. In the P16 animal, the nasal retina contralateral to the lesion suffered a 41% cell loss, whereas cell loss in the temporal retina ipsilateral to the lesion was 33%. Cell loss was greater in nasal retina and mainly included medium sized cells (200-600 microns2). In the P25 animal overall there was no evidence for ganglion cell loss.

Functional impact of cerebral connections

Cerebral networks are complex sets of connections that resemble a ladder-like web of multiple parallel feedforward, lateral, and feedback connections. This static anatomical description has been pivotal in guiding our understanding of signal processing within cerebral networks. However, measures on both magnitude and functional significance of connections are extremely limited. Here, we compare the anatomically defined strengths of a set of cerebral pathways emerging from the visual middle suprasylvian (MS) cortex of the cat with measures of the functional impact the same region has over distant sites. These functional measures were obtained by analyzing the local and distant effects of MS cooling deactivation on deoxyglucose uptake. Relative to major efferent projections from MS cortex that have a strong influence, projections to early visual processing stages have weaker functional influences than predicted from the anatomy. For higher processing stages, the converse holds: projections from MS cortex have stronger functional influence than predicted from the anatomy. We conclude that these and future functional measures, obtained using the same combination of techniques, will furnish fundamental, new information that complements and extends current models of static cerebral networks, and lead to more realistic models of cerebral network function and component interactions.

Age dependent modification of cytochrome oxidase activity in the cat dorsal lateral geniculate nucleus following removal of primary visual cortex

The purpose of the present study was to assess changes in the levels of cytochrome oxidase (CO) activity in the dorsal lateral geniculate nucleus (dLGN) of the adult cat following removal of primary visual cortical areas 17 and 18 on the day of birth (PI), P28, or in adulthood (> or = 6 months). Cytochrome oxidase activity was measured in histological sections 9 or more months after the cortical ablation. Control measures obtained from intact cats show that CO activity is normally highest in the A-laminae of dLGN, and slightly lower in the C-complex. Following visual cortex ablations incurred at any age, CO activity levels are reduced in the A-laminae. This reduction is most profound following ablations incurred on P28 or in adulthood. In contrast, CO activity in the C-complex of dLGN is at nearly normal levels following ablations on P1 or P28, but not in adulthood. These findings contribute to our understanding of the role played by the dLGN in the transfer of visual signals along retino-geniculo-extrastriate pathways that expand following early removal of areas 17 and 18. Moreover, they have implications for our understanding of spared behavioral functions attributed to the extrastriate cortex in cats which incurred early damage of areas 17 and 18.

Axonal target choice and dendritic development of ferret beta retinal ganglion cells

We have investigated the relationship between axon targeting and dendritic morphology in beta retinal ganglion cells in the postnatal ferret. Axonal projections were assessed by making separate injections of different fluorescent retrograde tracers into either the superior colliculus or lateral geniculate nucleus in vivo. The dendritic morphology of retrogradely labelled cells was revealed by the in vitro intracellular injection of lucifer yellow in fixed retina. In this way, 405 retinal ganglion cells were triple- or double-labelled and characterized by their dendritic branching styles. Both the distinct dendritic morphology of beta cells and the characteristic restriction of their adult axonal terminals to the lateral geniculate nucleus emerge postnatally. Beta cell dendritic morphology is established between postnatal days 5 and 9. As in the cat (Ramoa et al., 1989), beta cells extend and then retract a projection to the superior colliculus as part of their normal development. Transient beta axonal collaterals to the superior colliculus persist beyond the period of cell death, but nearly all are withdrawn by postnatal day 15. No dendritically distinct beta cell projects to the superior colliculus alone, at any age. Heterochronic injections of different colours of retrograde tracer into the superior colliculus were used to study changes in the complement of the retinocollicular projection over time. A significant proportion of cells (58%) labelled at postnatal day 0 from the superior colliculus, which subsequently survived the period of cell death, were found to be beta cells that could no longer be demonstrated to have a retinocollicular axon.(ABSTRACT TRUNCATED AT 250 WORDS)

Expansion of suprasylvian cortex projection in the superficial layers of the superior colliculus following damage of areas 17 and 18 in developing cats

Tritiated proline and leucine were injected into areas 17 and 18 of intact cats and into the medial bank of the lateral suprasylvian (LS) cortex of intact cats and cats from which areas 17 and 18 had been removed on postnatal day 1 (P1), P28, or in adulthood (A). The density of label transported to the superior colliculus was quantified using image-analysis equipment. The results from the intact cats confirmed previous reports that areas 17 and 18 project most heavily to stratum zonale (SZ) and stratum griseum superficiale (SGS) and LS cortex projects most heavily to stratum opticum (SO) of the superior colliculus. However, in cats with lesions of areas 17 and 18, the projections from LS cortex showed an age-dependent reorganization. LS projections to SGS and SZ were enhanced following ablation of areas 17 and 18 on P1, and projections to SGS were enhanced following an ablation on P28. The pattern of LS-collicular projection following ablations incurred in adulthood was indistinguishable from the pattern presented by intact cats. This study demonstrates that the LS corticocollicular projection expands in SGS and possibly substitutes for inputs eliminated by the removal of areas 17 and 18 from the immature brain. This enhanced pathway may contribute to compensatory neuronal changes and to spared behaviors that accompany damage of immature cortex.

Capacity of the retinogeniculate pathway to reorganize following ablation of visual cortical areas in developing and mature cats

The purpose of the present study was to determine the pattern and density of retinal projections to the dorsal lateral geniculate nucleus (dLGN) following ablation of visual cortical areas in developing cats of different postnatal ages and in mature cats. The terminations of retinal projections to the dLGN were evaluated following the injection of tritiated amino acids into one eye. Regardless of age, a visual cortical ablation of areas 17 and 18 induces massive death of neurons within the regions of the dLGN that are linked topographically to the cortical areas removed. However, the pattern of retinal projections to these degenerated regions of the dLGN differs depending upon whether the cortical lesion is incurred early in postnatal life or in adulthood. Following ablation on the day of birth (P1), virtually all surviving cells were found in the C-complex of dLGN with only a token number in the A-laminae. Correspondingly, retinal projections were maintained to the C-complex of the nucleus and were barely detectable in the degenerated A-laminae. However, in cats in which areas 17 and 18 had been removed in adulthood (> or = 6 months of age) retinal projections were maintained to the A-laminae even though nearly all neurons in those laminae had degenerated. Moreover, a subgroup of animals that incurred area 17 and 18 ablations at P1 showed that the modification of retinal projections to the A-laminae occurs within the first postnatal month, and an additional subgroup showed that retinal projections become increasingly resistant to the degenerative events in the dLGN that follow ablation of areas 17 and 18 at progressively older ages during the first postnatal month. Furthermore, retinal inputs also respond, in an age-dependent way, to degeneration of neurons in the C-complex induced by extension of the cortical ablation to include extrastriate visual areas.

Experimentally induced visual projections to the auditory thalamus in ferrets: evidence for a W cell pathway

We have previously reported that following specific neonatal brain lesions in ferrets, a retinal projection is induced into the auditory thalamus (Sur et al., Science 242:1437, '88). In these "rewired" ferrets, a novel visual pathway is established through auditory thalamus [the medial geniculate nucleus (MGN)] and primary auditory cortex (A1); cells in both MGN and A1 are visually responsive and exhibit properties similar to those of visual cells in the normal visual pathway. In this paper, we use three approaches--physiological, anatomical, and developmental--to examine which of the retinal ganglion cells project to the MGN in these rewired ferrets. We find that: 1) physiological response properties of postsynaptic visual cells in the MGN are W-like; 2) retinal ganglion cells back-filled from the MGN are small and similar to soma sizes of subsets of the normal retinal W cell population; and 3) subpopulations of these small cells can be preferentially rerouted to the MGN in response to different surgical manipulations at birth, consistent with differential W cell projection patterns in normal animals. These data suggest that retinal W cells come to project to the MGN in rewired animals. These findings not only provide a basis on which to interpret functional properties of this novel visual pathway, but also provide important information about the developmental capabilities of specific retinal ganglion cell classes and the regulation of their projections by target structures in the brain during development.

Visual cortex damage-induced growth of retinal axons into the lateral posterior nucleus of the cat

Ablation of visual cortical areas 17 and 18 in neonatal and young adult cats induces novel retinal projections to terminate bilaterally in the lateral posterior nucleus (LP) at a position ventromedial from the medial interlaminar nucleus. Comparison with the visual-field maps of LP indicate that the terminations are focussed on the representation of the visual-field center.

Selective depletion of beta cells affects the development of alpha cells in cat retina

The results of previous studies suggest that class-specific interactions contribute to the development of the different classes of retinal ganglion cells. We tested this hypothesis by examining the morphologies and distributions of alpha (alpha) cells in regions of mature cat retina selectively depleted of beta (beta) cells as a result of visual cortex lesions at birth. We find that alpha cells in regions of central retina depleted of beta cells are abnormally large while alpha cells in regions of peripheral retina depleted of beta cells are abnormally small. The normal central-to-peripheral alpha cell soma-size gradient is absent in hemiretinae depleted of beta cells. The dendritic fields of alpha cells in the border of beta-cell-depleted hemiretina extend preferentially into the beta-cell-poor hemiretina. In spite of this, alpha cell bodies retain their normal retinal distribution and remain distributed in a nonrandom mosaic-like pattern. Thus, it appears that the development of alpha retinal ganglion cells is influenced by interactions both with other alpha cells (class-specific interactions) and with surrounding beta cells (nonclass-specific interactions).

Response of retinal terminals to loss of postsynaptic target neurons in the dorsal lateral geniculate nucleus of the adult cat

We have used the neurotoxin kainic acid to produce rapid degeneration of neurons in the dorsal lateral geniculate nucleus (dLGN) of the adult cat. This degeneration mimics the rapid loss of geniculate neurons seen after visual cortex ablation in the neonate. Subsequent anterograde transport of horseradish peroxidase injected into the eye was used to reveal the projection patterns of retinal ganglion cell axons at different survival periods after the kainic acid injection. The density of retinal projections to the degenerated regions of the geniculate was reduced considerably at 4 and 6 months survival, but at 2 months was not significantly different from normal. The laminar pattern of projections to degenerated regions of the geniculate did not change in any animals studied, even when an adjacent lamina contained surviving cells. Electron microscopic examination of degenerated dLGN revealed intact retinal (RLP) and RSD terminals at all survival times, although the density of terminals appeared much reduced when compared to controls. Some RLP terminals exhibited the "dark reaction" of degeneration and these degenerating terminals were most numerous at 2 months survival. These findings demonstrate that, in response to degeneration of their usual target cells, mature retinal ganglion cells with withdraw their axon terminals from these regions of degeneration. We conclude that mature retinal ganglion cells continue to be dependent on target integrity for the maintenance of a normal axonal arborization.

Transneuronal induction of muscle atrophy in grasshoppers

Autotomy is a process in grasshoppers whereby one or both hindlimbs can be shed to escape a predator or can be abandoned if damaged. It occurs between the trochanter and the femur (second and third leg segments) and once lost, the legs never regenerate. Autotomy severs branches of the leg nerve (N5) but damages no muscles since none span the autotomy plane. We find, however, that undamaged muscles intrinsic to the thorax of grasshoppers, Barytettix psolus, atrophy to less than 15% of their normal mass after autotomy of a hindlimb. These muscles operate the coxa and trochanter (first and second leg segments) and are innervated by branches of nerves 3 and 4; nerve branches that are not damaged by autotomy. Atrophy is localized to the side and body segment where autotomy occurs. Atrophy is evident 7-10 days after loss of a limb, is complete by about 30 days, and follows a similar time course whether induced in young adult, or sexually mature grasshoppers. During autotomy, leg nerve 5 is served distal to the trochanter, the thoracic muscles lose their normal static and dynamic load, and these muscles are subsequently no longer used to support the weight of the insect during posture and locomotion. Experimental loading and unloading of the affected muscles, and cutting of nerves indicated that it is the severing of leg nerve 5 during autotomy that transneuronally induces muscle atrophy.

Parameters affecting the loss of ganglion cells of the retina following ablations of striate cortex in primates

Partial lesions of striate cortex were made in newborn and adolescent or young adult macaque monkeys, one newborn squirrel monkey, and adult squirrel and owl monkeys. After survival times ranging from 3 1/2 weeks to 8 years, the retinas were examined for transneuronal retrograde ganglion cell loss and retinal projections to the dorsal lateral geniculate nucleus, and other targets were examined for changes. After lesions in infant macaque monkeys and long postoperative survivals, nearly 80% of the ganglion cells were lost in the altered portions of the retinas. The degeneration appeared to be exclusively of ganglion cells projecting to the parvocellular layers of the lateral geniculate nucleus, and the loss of this class of cell appeared to be complete or nearly complete for the affected portions of the retina. Cases with shorter survivals showed that nine-tenths of the potential loss occurred within 6 months, and about half of the potential loss took place within one month. In cases where lesions were placed in adolescent and young adult macaque monkeys, the loss also was of ganglion cells projecting to the parvocellular layers. However, the rate of cell loss was slower so that little or no cell loss was apparent after six months, and only one-third to three-fourths of the potential loss occurred within 12-14 months. A cell loss of 22% was measured in the altered portions of the retina of a squirrel monkey lesioned as an infant and surviving for 6 months, but no regions of ganglion cell loss were apparent in the retinas of owl and squirrel monkeys lesioned as adults and surviving as long as two or more years. We conclude that nearly 80% of the ganglion cells project to the parvocellular layers in macaque monkeys, and that the ultimate survival of these ganglion cells depends on the presence of target neurons in the parvocellular layers. Age is important in that the loss of ganglion cells proceeds rapidly in infant macaque monkeys, but slowly in older animals. Infant New World monkeys, judging from one squirrel monkey, are also susceptible to ganglion cell loss, although apparently at a rate comparable to older macaque monkeys. Finally, adult New World monkeys do not appear to be susceptible to ganglion cell loss. These age and species differences in rates of loss and susceptibility to loss challenge a "sustaining collateral" hypothesis proposed earlier (Weller et al., 1979), and suggest alternatives and modifications.

Morphology of single, physiologically identified retinogeniculate Y-cell axons in the cat following damage to visual cortex at birth

It has been reported previously that neurons in the dorsal lateral geniculate nucleus (LGN) of cats with neonatal damage to visual cortex (KVC cats) have receptive fields that are abnormally large and that the receptive fields of these neurons sometimes do not appear to conform to the normal retinotopic order in the LGN. A primary aim of this study was to determine if these physiological abnormalities are related to inappropriate patterns of retinogeniculate connections. We therefore have analyzed the terminal arbors of retinogeniculate axons in adult cats that had received a lesion of visual cortex (areas 17, 18, and 19) on the day of birth. Single retinogeniculate axons were characterized physiologically and injected intracellularly with horseradish peroxidase. Consistent with earlier reports that neonatal removal of visual cortex results in a retrograde loss of retinal X-cells, all of the retinogeniculate axons that we recorded were from Y-cells. While the visual responses of these Y-cell axons were normal, the morphology of their terminal arbors in the LGN was abnormal. Retinal Y-cell axons in KVC cats have terminal fields in the A laminae of the LGN that are as large or larger than those of normal Y-cells. However, since the LGN in KVC cats is severely degenerated, single Y-cell arbors occupy a proportional volume of the LGN that is 12 times greater than normal. Thus an early lesion of visual cortex produces a severe mismatch between retinogeniculate axon arbor size and target size. Also, despite the normal size of retinogeniculate axon arbors in KVC cats, the number and density of terminal boutons are greatly decreased. Thus our morphological results suggest that the unusually large receptive fields of LGN cells in KVC cats and the relative lack of retinotopic precision in the LGN are due, at least in part, to anomalies in the relative size and distribution of retinogeniculate axon arbors that develop after neonatal removal of visual cortex.

Amacrine cells in the ganglion cell layer of the cat retina

Following transection of the optic nerve, ganglion cells in the cat retina undergo retrograde degeneration. However, many small profiles (less than or equal to 10 micron) survive in the ganglion cell layer. Previously considered to be neuroglia, there is now substantial evidence that they are displaced amacrine cells. Their density increases from approximately 1,000 cells/mm2 in peripheral retina to 7,000 cells/mm2 in the central area. Their total number was found to be 850,000, which is five times the number of ganglion cells and also five times the number of astrocytes. Uptake of 3H-muscimol followed by autoradiography labelled 75% of the displaced amacrine cells; hence, the majority seem to be GABAergic. Immunocytochemistry with an antibody directed against choline-acetyl-transferase labelled approximately 10% of the displaced amacrines in the peripheral retina and 17% in the central area. Uptake of serotonin (5-HT) followed by immunocytochemistry was found in 25-30% of displaced amacrines. NADPH diaphorase histochemistry labelled approximately 5% of displaced amacrine cells. The sum of the various percentages make colocalization likely. Intracellular injection of Lucifer Yellow under microscopic control revealed that displaced amacrine cells constitute several morphological types.

Synaptic reorganization in the dorsal lateral geniculate nucleus following damage to visual cortex in newborn or adult cats

We have studied the effects of making large lesions of visual cortex on the synaptic organization of the dorsal lateral geniculate nucleus (LGN) in the cat. Visual cortex was removed at birth in one group of cats and during adulthood in a second group. Following survival periods of 6 months to 2 years, the organization of synapses made by afferents from the retina in the LGN was investigated quantitatively with the electron microscope. In single thin sections we determined the percentage of retinal axon terminals that made synapses in the LGN, the average number of synapses made by each retinal axon terminal, and the identity of each postsynaptic process. These measurements were made separately for retinogeniculate connections in the A and C laminae of the LGN. For comparison, similar sets of measurements were made in adult cats that had been reared normally. When single thin sections from the A or C laminae of the LGN in normal cats are examined, about 60% of the axon terminals from the retina are seen to make at least one synaptic contact. These contacts can be with dendrites or F profiles or both. On average, each retinogeniculate terminal makes approximately 1.4 synapses in the plane of a single section and contacts dendrites three times as often as F profiles. In the A laminae of the LGN in cats that received a visual cortex lesion at birth or in adulthood, the percentage of retinal terminals that make synapses is the same as in normal cats. Similarly, the average number of synaptic contacts made by each retinogeniculate terminal is not changed by a lesion of visual cortex. In contrast, the number of contacts made with dendrites is reduced markedly, by about 29% after a lesion at birth and 53% after a lesion as an adult. However, these reductions are offset by compensatory increases in the number of contacts made with F profiles, and thus the mean number of contacts made by each retinogeniculate terminal is stabilized at a normal value. In the C laminae of the LGN, retinogeniculate terminals also reapportion their synaptic contacts. In cats with a lesion during adulthood, the redistribution of synapses is compensatory, as in the A laminae. When a lesion is made at birth, however, the number of new retinal contacts made with F profiles exceeds the number of dendritic contacts that are lost. As a result, each retinogeniculate terminal makes about 26% more synapses, in total, than normal.(ABSTRACT TRUNCATED AT 400 WORDS)

Pre- and postsynaptic correlates of interocular competition and segregation in the frog

Segregated zones of termination between converging inputs that arise from different presynaptic populations are a common property of topographically organized zones within the vertebrate central nervous system. Increasing evidence suggests that such segregation is at least in part established on the basis of competitive interactions that depend upon the activity patterns within each afferent population. However, the cellular mechanisms of these interactions are poorly understood. We have used a preparation in which a stereotyped interdigitating pattern of retina-specific termination stripes are produced in frog tecta innervated by two retinas as a result of embryonic implantation of a third eye primordia. In these animals it has been possible to examine the relationship between the number of retinal ganglion cells in each of the retinas innervating a striped tectum, the volumetric changes in the tectum as a result of this double innervation, and the pattern of eye-specific segregation that is produced. Counts of retinal ganglion cells in the retinas of the three-eyed frogs with one completely striped tectal lobe revealed no significant differences between cell numbers in the doubly innervating retinas and the normal retinas of the same animals. The average increase in retinal ganglion cell innervation to the striped tecta of these animals was 100%. However the tecta only increased in total volume by 26%. This later increase consisted of a 25% increase in the volume of the deep lying and predominantly cellular tectal laminae and a 37% increase in the superficial retinotectal synaptic zone. In many of these same animals HRP and 3H-proline were used to differentially label the set of stripes from each retina and measurements of the extent of each projection were performed. We found that the volume of tectal neuropil occupied by a striped projection is relatively unrelated to the number of ganglion cells making up that projection. Observations of the striping pattern after HRP processing to visualize stripes in whole unsectioned tecta indicate that the periodicities and rostrocaudal orientation of stripes are robust over a wide range of relative innervation densities. When one projection is much smaller than the other, stripes appear to break down into a series of "puffs" or islands of retina-specific termination zones. Nevertheless, these puffs still have a rostrocaudal alignment and the spacing of fully formed stripes. These observations suggest that the formation of exclusive termination zones may be a threshold phenomenon: so after a certain innervation density is reached one input can take over a unit of target neuropil in an all-or-none manner.(ABSTRACT TRUNCATED AT 400 WORDS)

Retinal crowding alters the morphology of alpha ganglion cells

It has been hypothesized that the dendritic field size of individual retinal ganglion cells is regulated early in development by interactions among neighboring cells (Wässle and Reiman, Proc. R. Soc. Lond. B. 200:441-461, 1978). An opportunity to test this hypothesis is provided by the consequences of prenatal enucleation that results in an increased density of ganglion cells in the remaining retina (Chalupa et al., Neuroscience 12:1139-1146, 1984). In the present study we compared dendritic field diameters of alpha ganglion cells in normal retinas to those of adult cats that were monocularly enucleated before birth (on embryonic days 42 and 51) and 6 days after birth. In each animal ganglion cells were labeled by central injections of horseradish peroxidase. The retinas were incubated according to a modified Hanker-Yates procedure and whole-mounted. All cells were sampled from the temporal retina along a corridor established by a line drawn through the optic disk and the area centralis. Alpha cells were differentiated into ON and OFF subtypes on the basis of their level of stratification in the inner plexiform layer (Peichl and Wässle, Proc. R. Soc. Lond. B. 212:139-156, 1981). Dendritic field and soma diameters were determined from complete cell drawings by calculating the mean of the two longest orthogonal diameters. Our main findings are: Early monocular enucleation does not disrupt the mosaics of ON and OFF alpha ganglion cells in the remaining retina of adult animals. Within the retinal corridor sampled, the density of alpha cells was substantially greater than normal in the remaining retina of the prenatal enucleates at all eccentricities except the far periphery. Except at the far periphery, the dendritic field diameters of the prenatally enucleated animals were significantly smaller than normal when compared at equivalent eccentricities. However, when compared at equivalent cell densities, dendritic field dimensions in the prenatally enucleated and normal animals were found to be similar. In the prenatal enucleates the somas of these neurons were also significantly smaller than normal. If the monocular enucleation was postnatal, however, the density and the dendritic field diameters of alpha cells did not differ appreciably from normal. These results indicate that the morphology of retinal ganglion cells can be influenced by increasing the density of developing ganglion cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of target tissue in regulating the development of retinal ganglion cells in the albino rat: effects of kainate lesions in the superior colliculus

Kainic acid or ibotenic acid was injected unilaterally into the major target regions of the axons of retinal ganglion cells--the superior colliculus (SC) or dorsal lateral geniculate nucleus (DLG)--of rat pups ranging in age from postnatal day 0 to postnatal day 10 (P0 - P10). While the collicular or geniculate neurons within the injection site died within 48 hours of the injection, damage to axons and terminals of extrinsic origin within the injected region was not apparent. The neuronal degeneration induced by the neurotoxins, observed at both the light and electron microscopic levels, resembled the neuronal degeneration that occurs in the colliculus during normal development. Macrophages were identified in the regions containing degenerating cells. Two to three weeks after the injections of neurotoxin, massive injections of the enzyme, horseradish peroxidase (HRP), were made into the retinorecipient nuclei. After about 24-hour survival time the numbers of retinal ganglion cells were estimated by counting the number of neurons containing HRP reaction products in sample areas distributed in a regular rectangular array across the entire retinal surface. In the animals in which the neurotoxin was injected into the SC during the first 4 postnatal days, there was a substantial reduction (on average 41.5%; the range: 27.5-65.5%) in the normal number (mean value of 113,000--Potts et al.: Dev. Brain Res. 3:481-486, '82) of retinal ganglion cells surviving the period of "naturally occurring ganglion cell death" in the retinae contralateral to the injected SC. By contrast, injections of neurotoxins into the DLG and/or the optic tract of newborn rats did not result in a significant reduction in the numbers of retinal ganglion cells surviving the period of naturally occurring ganglion cell death. The period of sensitivity of retinal ganglion cells to the injection of neurotoxin into the colliculi extends from birth to about the end of the first postnatal week; the greatest sensitivity seems to be restricted to the first 3-4 postnatal days. In the retinae in which the total number (and density) of ganglion cells was substantially reduced by the selective destruction of their target cells, the centro-peripheral difference in the somal diameters of the ganglion cells (apparent in normal animals) was abolished, both amongst the whole population of ganglion cells and amongst the ganglion cells with the largest somata, relatively thick axons, and large-gauge primary dendrites (Class I cells). The number and distribution of the Class I cells in the depleted retinae were, however, unaltered.(ABSTRACT TRUNCATED AT 400 WORDS)