Dorsal Retina Research Articles

FMRFamide-immunoreactive retinopetal fibers in the frog, Rana pipiens: demonstration by lesion and immunocytochemical techniques

Retinopetal fibers, immunoreactive to a molluscan cardioexcitatory-like peptide (FMRFamide-ir), were examined in Rana pipiens with the use of immunocytochemical and lesion techniques. In intact frogs, FMRFamide-ir retinopetal fibers were found in the optic nerve, optic nerve head, and nerve fiber, ganglion cell and inner plexiform layers of the retina. Presumptive monostratified amacrine cells were also labeled. As observed in flat-mounted retinas, the retinopetal fibers radiated from the optic disc toward the peripheral retina, branched many times along their course and were more prevalent in the dorsal retina. Crushing the optic nerve eliminated retinopetal fibers from all regions except the cerebral stump of the optic nerve, indicating that this projection was of central origin. Bilateral prechiasmatic lesions completely eliminated retinopetal fibers from both retinas, indicating that the fibers arose from the rostral forebrain. Within the rostral brain, FMRFamide-ir perikarya were found in olfactory bulb, diagonal band, medial septum, anterior commissure area, and two regions of the posterior preoptic area. Olfactory bulbectomy and midforebrain lesions equally reduced the numners of these fibers in the retina, implicating the nervus terminalis as a possible source for some of the retinopetal projection. These data will serve as a foundation for future studies on the function of retinopetal fibers in the frog retina.

Development of ganglion cell topography in ferret retina

The adult ferret has approximately 90,000 retinal ganglion cells, arranged in a prominent area centralis and visual streak. The role of differential cell generation, cell death, and retinal growth in the control of adult retinal ganglion cell number and distribution was evaluated by examining basic aspects of retinogenesis, including growth in retinal area, developmental changes in the number, size, and distribution of retinal ganglion cells (identification aided by retrograde transport of HRP), and the incidence of degenerating cells in the ganglion cell layer. Retinal development in the ferret was also compared to retinal development in the cat (which has an even more differentiated area centralis) to determine what alterations of developmental parameters are most closely associated with this species difference in adult morphology. The area of the retina increases linearly from birth (12 mm2) to postnatal day 24 (54 mm2), reaching an eventual adult value of 64 mm2. Ganglion cell numbers peak at 155,000 (approximately twice the adult number) on postnatal day 3, and fall to adult numbers by postnatal day 6. The remaining cells of the ganglion cell layer, principally displaced amacrine cells, reach their peak number on postnatal day 10 (approximately 280,000), falling to 200,000 by adulthood. Degenerating cells are abundant in the ganglion cell layer in the immediate postnatal period. A difference in the incidence of degenerating cells in the presumptive area centralis versus that in the retinal periphery was not observed postnatally, though there were other striking spatial nonuniformities, suggesting that differential cell loss might contribute to other features of retinal topographic organization. Ganglion cell density is virtually uniform across the retina at birth. Cell density is first reduced in the dorsal retina, resulting in a dorsal-to-ventral gradient in cell density that persists until day 10, when ganglion cell number has stabilized. By postnatal day 24, an area centralis and visual streak has emerged, but not of adult magnitude. Because ganglion cell number has stabilized long before the area centralis and visual streak emerge, we conclude that differential retinal growth is the principal mechanism producing this feature of retinal topography. Comparison with the cat suggests that the proportionately greater nonuniform growth of the cat's eye accounts for the greater differentiation of its area centralis.

Visual system of the channel catfish (Ictalurus punctatus): III. Fiber order in the optic nerve and optic tract.

In channel catfish the ganglion cell axons leave the retina via a ring of approximately 13 separate optic papillae. Each papilla serves an area of retina extending from the central zone of the retina to the periphery. Papillae located at a dorsal position in the ring serve exclusively dorsal retina. Ventrally located papillae, however, have an exaggerated peripheral retinal representation, so that they serve mostly ventral retina but also some areas of peripheral retina dorsal to the nasal and temporal poles. The ganglion cell axon bundles departing from the retina via individual papillae were labelled with horseradish peroxidase, and sections of the optic pathway were examined to reveal the topographic organization of the fibers. The topographic order of the optic nerve was dissimilar to that of cichlids and goldfish. Fibers from individual papillae remained together throughout the optic nerve. Close to the optic nerve head, the papillae were arranged as a continuum around the U-shaped optic nerve, without the discontinuity in the representation of the ventral retina seen in other fish. Fibers associated with the dorsal papillae were located at the tip of the caudolateral arm of the U, and fibers from ventral papillae were on the rostromedial arm. Fibers from nasally and temporally located papillae were found on the base of the U. By the level of the optic chiasm the U shape had flattened out but retained the relative ordering of the papillae. Rotation of the nerve as it became the optic tract brought the representation of the ventral papillae to the dorsal pole of the tract, and the dorsal papillae to the ventral tract. It was only in the optic tract that rearrangement of fibers became apparent. As described above, the axons of some ganglion cells in dorsal, peripheral retina left the retina and travelled through the optic nerve with axons from extreme ventral retina. In the optic tract, these dorsal fibers joined the main body of fibers from the dorsal retina. The significance of these observations for theories of fiber rearrangement is discussed.

Initial disorder and secondary retinotopic refinement of regenerating axons in the optic tract of the goldfish: signs of a new role for axon collateral loss

ABSTRACT The optic tract of the goldfish splits into two brachia just before it reaches the tectum, normal optic axons being distributed systematically between the two according to their retinal origins. The orderliness of this division, like that of the retinotectal projection itself, is conventionally attributed to a system of specific axonal guidance cues. However, the brachial distribution of regenerated axons is much less orderly; and, since there is evidence that these axons have many collateral branches in the nerve and tract, the gross order that remains after regeneration could potentially arise secondarily, in parallel with refinement of the retinotectal map, by a preferential loss of collaterals from the inappropriate brachium. The brachial paths of normal axons, and axons regenerated after optic nerve cut for periods ranging from 19 days to 5 years, were therefore studied by anterograde labelling with horseradish peroxidase from discrete retinal lesions or retrograde labelling of ganglion cells from a cut brachium. From 19 to 28 days, regenerating axons showed little or no preference for their normal brachium. During this period (which includes the first week of tectal synaptogenesis) an average of 46·3 % of cells retrogradely labelled from a cut medial brachium were in dorsal retina, compared with only 1· 45% in normal fish. Some preference for the normal brachium was evident at 35 days and significant order had returned by 42–70 days, when the average proportion of labelled cells in dorsal retina had fallen to 25·4 % though the average number in the whole retina was unchanged. Thus a brachial refinement had occurred in parallel with refinement of the retinotectal map. These results support the idea of a selective loss of axon collaterals from the inappropriate brachium, though they do not exclude the possibility of some concurrent gain in the appropriate one. We suggest that refinement may depend on a process we term ‘sibling rivalry’: competition between different collaterals of the same axon to form a critical number of stable tectal synapses, in which the most-normally-routed branches have the best chance of succeeding and surviving. Developing normal axons might also make use of collateral formation and ‘sibling rivalry’ to generate and refine the complex interwoven patterns of the normal optic tract.

Corresponding spatial gradients of TOP molecules in the developing retina and optic tectum.

The topographic map of cell position in the avian retina is inverted in its projection to the optic tectum. Dorsal retinal ganglion cell axons project to ventral tectum, and ventral retinal ganglion cells project to dorsal tectum. Topographic gradients of toponymic (TOP) cell surface molecules along the dorsoventral axes of retina and tectum also are inverted. TOP molecules are most abundant in dorsal retina and ventral tectum and least abundant in ventral retina and dorsal tectum during the period of initial retinal-tectal interaction. Thus, TOP molecules may be involved in orienting the retinotectal map.

Dependence of cytochrome oxidase activity in the rat lateral geniculate nucleus on retinal innervation

Patterns of cytochrome oxidase (CO) activity were examined histochemically in the dorsal lateral geniculate nucleus (LGNd) and retina of pigmented rats. CO staining was not uniform and was distributed in a pattern similar to that of retinal afferents. Portions of the LGNd receiving an exclusively crossed projection were moderately reactive whereas regions receiving an uncrossed or overlapping crossed and uncrossed projection were darkly reactive. The dependence of oxidative metabolic activity in the LGNd on retinal innervation was verified in animals with unilateral enucleation. In adults, chronic monocular enucleation led to a decrease in CO staining in portions of the LGNd deprived of retinal input; in animals enucleated at birth, normal patterns of CO reactivity failed to develop and both LGNds had a more uniform pattern of moderate CO staining. Most neurons in the ganglion cell layer of the retina were moderately reactive for CO. However, there were approximately 3,000 darkly reactive cells, most of which appear to be ganglion cells. The darkly reactive cells were more numerous in the peripheral temporal retina. The laminar pattern of CO staining in the retina was similar to that described previously for carnivores and primates. The most reactive laminae were the inner and outer plexiform layers and the photoreceptor inner segments. Within the inner plexiform layer, sublamina a was more darkly stained than sublamina b. These results suggest that the physiological properties of crossed and uncrossed visual pathways in rats are functionally dissimilar at the level of both the retina and the LGNd.

Preferential loss of collaterals from goldfish retinal axons in the optic tract is delayed by tetrodotoxin

Following optic nerve section (ONS) in goldfish, the right eye was repeatedly injected with tetrodotoxin (TTX) and the left eye with Ringer solution. At various survival periods after ONS, horseradish peroxidase (HRP) was applied to small groups of axons in dorsal or ventral retina in both eyes. Counts of labeled regenerating retinal axons show that 43 ± 4.6% of regenerating axons course through the inappropriate brachium of the optic tract between 20 and 65 days after ONS. The amount of misrouted axons declines to 30 ± 6.1% between 70 and 80 days after ONS. Under TTX blockade the reduction of misrouted axons is delayed but reaches 27 ± 3.6% at 150 days after ONS.

Ganglion cell dendritic structure and retinal topography in the rat.

The dendritic field size, the distribution of the dendrites relative to the cell body, and the overall shape of the dendritic field of type I ganglion cells in the rat retina were analyzed. These features of neuronal structure were related to the topography of the rat retina. As in the cat, the cell bodies of type I ganglion cells are arranged in a nonrandom mosaic. Previous work has demonstrated that the density of type I cells in the rat retina does not covary with the density of all ganglion cells. Type I dendritic field size varies over the retina; the increase in dendritic field size is accounted for better by the decrease in type I density than by the decrease in overall ganglion cell density. The center of the dendritic field of most type I cells is displaced in the plane of the retina from the cell body. Unlike in carnivore retina (Schall and Leventhal: J. Comp. Neurol. 257:149-159, '87), the dendritic fields in the rat are not displaced down the ganglion cell density gradient. Rather, there is a tendency for the dendritic trees, especially in temporal retina, to be displaced toward dorsal retina. Most of the dendritic fields are elongated, but the degree of elongation is less than that observed in carnivore or primate retina. Unlike in carnivore and primate retina (Leventhal and Schall: J. Comp. Neurol. 220:465-475, '83; Schall et al.: Brain Res. 368:18-23, '86), there is no relationship between dendritic tree orientation and position relative to any point on the retina in the rat.(ABSTRACT TRUNCATED AT 250 WORDS)

The photopic spectral sensitivity of the dorsal and ventral retinae of the chicken

The relative spectral sensitivities of the dorsal and ventral chicken (Gallus gallus) retinae were determined under photopic conditions by means of electroretinography and compared with data from the pigeon (Columba livid). Differences in spectral sensitivity between the dorsal and ventral chicken retinae appear only in the short wavelength range. In the chicken the dorsal retina is more sensitive to near UV light than the ventral retina relative to long wavelengths (beyond 470 nm), but both retinal areas are less sensitive to near UV than in the pigeon. The variation in relative near UV sensitivity is discussed in relation to recent data on the retinal distribution of different types of oil droplets in birds. The adaptive significance of near UV sensitivity is also discussed.

Course of retinogeniculate projection fibers in the cat optic nerve.

The fiber courses of the cat optic nerve were studied by using wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP), which was iontophoretically applied to electrophysiologically defined positions in the lateral geniculate nucleus (LGN). Grossly, there were two distinct bends along the length of the optic nerve. The ventrally flexing anterior bend was located approximately 2 mm from the eyeball, while the dorsally flexing posterior bend was found at some 6 mm distant. The optic nerve fibers showed a tendency to scatter toward the chiasm. At the optic nerve head, the fibers from the different retinal areas maintained the retinal topography in a simplified form according to the trajectory of optic fibers surrounding the optic disc. Between the anterior and posterior bends, the fibers from the pericentral, middle-temporal, and most upper areas of the dorsal retina migrated ventrally and were arranged in the middle of the lateral, middle, and medial parts of the optic nerve, respectively, while fibers from the middle-temporal area of the ventral retina migrated dorsally and scattered into the lateral half. The fibers from the temporal and nasal horizontal meridian areas tended to hold their respective positions in the lateral and medial halves of the optic nerve. As a result, in this level they displayed a complex retinotopy in that the fibers from each part of the retina were mixed. Passing the posterior bend, as the optic nerve proceeded toward the chiasm, the characteristic pattern became less defined. Near the chiasm, the retinotopy became very scattered, showing a partial dorsoventral inversion of the retinal topography with substantial overlapping. It was noted from the present findings that the fibers from ventral retina scattered more quickly than the fibers from the dorsal retina, which tended to hold their grouping until the anterior bend, but the central or pericentral retinal fibers proceeded without significant scatter as far as the posterior bend. The analysis of labelled ganglion cells suggested that even if fibers that arise from one certain "mode" of ganglion cells are selected, it is unlikely that they maintain their initial fiber topography along the entire length of the optic nerve.

Purification of a membrane protein distributed in a topographic gradient in chicken retina.

Antigenic molecules termed TOP, which are distributed in a dorsal greater than ventral concentration gradient in chicken retina, are expressed early in development (by 48 hr after fertilization) in the optic cup of chicken embryos and continue to be expressed in retina thereafter. 35S-labeled-TOP-antibody complexes were purified by protein A-Sepharose column chromatography and subjected to NaDodSO4/polyacrylamide gel electrophoresis and autoradiography. TOP also was purified from dorsal retina by anti-TOP IgG-Affi-Gel 10 affinity column chromatography. In both cases, one major band of protein at Mr approximately 47,000 was obtained. A protein of Mr approximately 47,000 also was purified from chicken embryo brain. Cultured cells dissociated from 8-day chicken embryo retinas accumulated the amount of TOP expected of cells in the intact retina, depending on the position of the cells in the retina. TOP accumulations by cells dissociated from dorsal or ventral retina, mixed in different proportions, and cocultured were additive. These results show that TOP is a protein, that the gradient of TOP is established early in development, and that perpetuation of the gradient does not depend on the continuous presence of an extracellular gradient of diffusable molecules or on maintenance of interactions between cells.

Terminations of photoreceptor axons from different regions of the compound eye of the desert ant Cataglyphis bicolor

The morphology of the photoreceptors from different regions of the desert an t’s compound eye is investigated by Golgi-impregnations and anterograde or retrograde haem peptide filling, combined with electron microscopy. A new type of receptor axon is described. This receptor is of the short (svf) type but terminates in the proximal layer of the lamina (epl-C), in contrast to the other short visual fibres, which terminate in the distal layer (epl-A) : Golgi-EM investigations show that this receptor axon belongs to the small photoreceptor R9. The receptor R9 appears to be pre- and postsynaptic to other receptor axons and to second order neurons. In each ommatidium, the axons of the two long photoreceptors (lvf) R1 and R5 (probably uv-receptors) remain paired down to their terminations in the distal layer of the medulla. The regional specialization of the retina (dorsal rim area (dra), dorsal retina (dr) and ventral retina (vr)) is reflected by the morphology of the receptor terminals and the second order neurons. The lamina underlying the dra consists of only one layer, an extended epl-A; in the remainder of the eye, the lamina is trilateral. In the dra all short visual fibres (svf) are equal in length. The extension of the monopolar cell dendrites is restricted to one cartridge only. In the medulla, the terminals of the lvfs deriving from the dra (R1 and R5) have more extensive arborizations than elsewhere in the eye.

The visual system of the channel catfish (ictalurus punctatus). I. Retinal ganglion cell morphology

Horseradish peroxidase was applied to lesions in the optic nerve of catfish (Ictalurus punctatus). The retinae were processed to reveal HRP-labelled ganglion cells. The histochemical techniques employed allowed fine details of the dendritic arbor to be resolved. Flat-mounted retinae were examined and the following characteristics were noted in individual ganglion cells: Soma area, shape, and depth; number and diameter of major dendrites; shape, area, and depth(s) within the inner plexiform layer (ipl) of the dendritic arbor; origin of the axon (from the soma or a dendrite). On the basis of these characteristics, eleven classes of ganglion cells were delineated: four classes of giant cells (G1-G4) and seven classes of smaller cells (S1-S7). G1 cells had dendrites arborizing in the most distal sublamina of the ipl. G1 cells in the dorsal retina had nasotemporally elongated dendritic arbors. G2 cells had dendrites in the proximal portion of the ipl. G3 cells were almost completely confined to a band running between the nasal and temporal retinal poles, through the center of the retina. In this location, the cells had dorsoventrally elongated dendritic arbors, which were bistratified in the ipl. G4 cells were displaced into the inner nuclear layer. S1 and S4 cells had axons arising from their somata, and dendrites arborizing in the distal and the proximal ipl, respectively. S2 cells were typified by their unstratified dendritic arbors. Similarly, S3 cells were characterised by their bistratified arbors. S5 cells arborized in the most proximal ipl sublamina. S6 cells were small ganglion cells with their somata lying in the inner nuclear layer. S7 cells tended to have complex dendritic arbors, and their axons arose from dendrites.

Simple and complex retinal ganglion cell axonal rearrangements at the optic chiasm

The rearrangements that retinal ganglion cell (RGC) axons undergo near the optic chiasm were determined by ablating either the nasal, temporal, dorsal, ventral, or peripheral retina. The axons of the remaining intact RGCs were then labelled with cobaltous lysine. RGC axons change their relationship with respect to the axes of the brain and with respect to one another. Toward the caudal end of the optic chiasm, the optic tract begins to rotate axially such that its rostral edge ultimately becomes located medially. Thereby, the column of ventronasal RGC axons shifts from a rostral to a medical position. In addition, columns of axons from other retinal sectors move with respect to one another. Ventrotemporal RGC axons, located initially at the caudal edge of the tract, move toward and come to be positioned laterally to, the column of ventronasal RGC axons. The column of dorsonasal RGC axons moves from the rostral to the lateral side of the column of dorsotemporal RGC axons. Concurrently, the axons within each column reorganize internally. Each chronological lamina of axons within a column twists such that the nasal and temporal axons within each column invert their positions with respect to the edges of the column. All of these reorganizations take place between the caudal end of the optic chiasm and the division of the main optic tract into the optic brachia. Furthermore, the rearrangements that occur do not involve any alterations in the positions of central and peripheral RGC axons with respect to the surface of the diencephalon. The results are discussed with respect to mechanisms that might influence the organization of the visual pathways.

Relationship of ocular pigmentation to the boundaries of dorsal and ventral retina in a nonmammalian vertebrate

The goldfish eye and retina are partitioned traditionally into dorsal and ventral sectors by a horizontal meridian that passes through the optic disc and is perpendicular to a vertical meridian that extends from the remnant of the choroid fissure through the optic disc. Axons of retinal ganglion cells (RGCs) situated above the horizontal meridian are thought to reach the optic tectum via the ventrolateral optic tract and axons of RGCs situated below the horizontal meridian are thought to reach the optic tectum via the dorsomedial optic tract. When cobaltous-lysine was applied to small temporal retinal slits that were centered on the traditional horizontal meridian, filled fibers were found in the dorsomedial, but not in the ventrolateral, optic tract (Springer and Mednick, '83). Since cobalt-filled axons should have been found in both optic tracts, the traditional horizontal meridian does not indicate the actual boundary between dorsal and ventral retina. We report here that the goldfish iris contains nasal and temporal pigmentation lines (darts) that are each located approximately 21 degrees above the traditional horizontal retinal meridian. Cobalt applied to retinal slits located just above the darts filled RGC axons in the ventrolateral optic tract and cobalt applied to retinal slits just below the darts filled RGC axons in the dorsomedial optic tract. Converging evidence for the reliability of the darts as indicators of the boundary between dorsal and ventral retina was obtained by applying cobalt to severed RGC axons along the dorsomedial edge of the tectum. Cobalt-filled RGCs were found below the nasal dart.(ABSTRACT TRUNCATED AT 250 WORDS)

A quantitative study of the relative contribution of different retinal sectors to the innervation of various thalamic and pretectal nuclei in goldfish

The contribution of retinal ganglion cells situated in different retinal quadrants to the innervation of eight nontectal, retinorecipient targets was examined in goldfish. In some fish, cobaltous-lysine was used to selectively fill severed intraretinal ganglion cell axons and the number of filled axons within each nucleus was determined. In other fish, either the dorsal or ventral or nasal or temporal retina was ablated and the remaining axons from the intact retina were filled with cobalt. The density of the cobalt-filled axons within the retinorecipient targets was quantified with a microdensitometer. All of the eight targets received different degrees of innervation when the contributions from dorsal and ventral retina were compared. The suprachiasmatic nucleus received axons from ventral, but not from dorsal, retinal ganglion cells (RGCs), while the nucleus opticus dorsolateralis, nucleus opticus commissurae posterior, and nucleus opticus pretectalis dorsalis received more axons from ventral than from dorsal RGCs. The tuberal region, nucleus corticalis, and the accessory optic nucleus received axons from dorsal, but not from ventral, RGCs. The nucleus opticus pretectalis ventralis received more axons from dorsal then from ventral RGCs. Only one target, nucleus corticalis, appeared to receive more axons from nasal than from temporal RGCs. In general, those nuclei that were closest to the dorsal optic tract were innervated exclusively or predominantly by ventral RGC axons, whereas those nuclei that were closest to the ventral optic tract were innervated exclusively or predominantly by dorsal RGC axons. These data indicate that in this particular vertebrate, the dorsal and ventral retinal regions are not homogeneous with respect to their projections to nontectal nuclei. The possible role that the nontectal nuclei play in determining the course of optic axons is discussed.

Topography of regenerating optic fibers in goldfish traced with local wheat germ injections into retina: evidence for discontinuous microtopography in the retinotectal projection.

A small injection of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) was injected into dorsal or ventral peripheral retina in normal goldfish and in goldfish with prior optic nerve crush. Serial sections of tectum were subsequently taken for horseradish peroxidase (HRP) histochemistry 18 hours after injection and studied with light microscopy and densitometric reconstructions. In normals, a small, sharply delineated patch of product 200-300 microns wide was observed at the appropriate medial or lateral periphery of tectum. This product filled the entire SFGS, the main optic termination layer, and fell off abruptly at its edges. No labelling was detected in the optic pathways. In regenerates at about 20 days after nerve crush, these retinal injections yielded product that was dispersed across 1,000 microns or more of tectum but not in a uniform fashion. The densest product was biased toward the appropriate tectal position while product of intermediate density was mainly distributed along a path from the anterior end of tectum to this region. Product in the inappropriate half of tectum was much lighter and typically fiberlike in appearance. By about 40 days, product had condensed considerably at roughly the correct region of tectum but it was not as sharply delimited as in normals. Dense label occupied a single area about twice that of normals and exhibited flanking regions of light label extending for several hundred micrometers. At 59-148 days, a further condensation was observed but into more than one patch of product. The patches were of variable size and consisted of sharply delimited dense product which filled the entire SFGS at each position. Morphologically, these patches bore a remarkable resemblance to the ocular dominance columns previously seen in this system.

Topography of the retinal projection to the superficial pretectal parvicellular nucleus of goldfish: A cobaltous‐lysine study

The retinal projection to the superficial pretectal parvicellular nucleus (SPp) of goldfish was examined by filling select groups of optic axons with cobaltous-lysine. The tracer was applied intraocularly to peripheral retinal slits in some fish. In other fish, it was applied to optic axons from an intact hemiretina after one-half of the retina was ablated and the corresponding optic axons had degenerated. The results indicated that SPp is a folded structure, having a dorsal surface innervated by axons from temporal retinal ganglion cells and a ventral surface innervated by axons from nasal retinal ganglion cells. Peripheral retina innervates the anterodorsal and anteroventral edges of SPp, while central retina innervates the posterior genu. Dorsal retina innervates lateral SPp and ventral retina innervates medial SPp. Thus, although SPp is a folded nucleus, the topography of the retino-SPp projection is similar to the topography of the retinotectal projection. That is, the relative position of optic axons within SPp mirrors the retinal location of the ganglion cells that project to SPp. Retino-SPp axons occupy the center of the main optic tract before it divides into the two optic brachia. These axons are topographically arranged, with temporal retino-SPp axons being flanked on both sides by nasal retino-SPp axons. Retino-SPp axons arborize within SPp and then continue to enter the superficial tectal retino-recipient lamina. Thus, these axons innervate both SPp and the optic tectum. These findings are discussed with respect to chemospecific and morphogenetic views of visual system topography.

Spatial distribution of visual pigment and dopamine in the bullfrog retina

Visual pigments and a neurotransmitter, dopamine, were quantitatively investigated in the retina of adult bullfrog, Rana catesbeiana. The adult bullfrog (body length 15–16 cm, body weight 375±52 g, n = 10) had 21·4±4·2 nmol visual pigment and 209±28 pmol dopamine in retinal areas of 266±27 mm 2. Greater pigment densities were recorded in a semicircular band around the optic disc, extending to the nasal and temporal peripheries of the ventral retina. The area with the highest concentration of visual pigment was found in the middle of the dorsal retina, 3–4 mm dorsal to the optic disc. A high concentration of vitamin A 2-based pigment was found in the dorsal quarter of the retina (porphyropsin zone); the zone extended up to the most ventral part along peripheral regions of the retina. There was also a band with higher dopamine concentrations although it was not so prominent as that of the visual pigment; the highest concentration of dopamine was found in the area immediately dorsotemporal to the optic disc. Fluorescence micrography indicated that the distribution pattern of catecholamine-containing amacrine cells paralleled that of the dopamine content. The topographic map of dopamine was slightly different from that of visual pigment in the bullfrog retina.

Starburst amacrine cells: morphological constancy and systematic variation in the anisotropic field of rabbit retinal neurons

Starburst amacrine cells of rabbit retina have been characterized previously in terms of their highly distinctive and regular dendritic geometry. They have been identified as probable cholinergic neurons of the retina and have been shown to direct output solely to ganglion cells. The objectives of this paper are to chart the variation of starburst amacrine cells across the retina, to register the morphological features which are held constant for individual cells, and to examine factors which may remain invariant for the population with change in retinal position. Starburst amacrine cells occur as two completely segregated mirror-symmetrical populations, type a and type b cells, separately serving OFF and ON pathways, respectively. They are treated here as two distinct subpopulations with very similar features. A characteristic morphological feature of both types, related to branching pattern and best seen in flat view, is the location of boutons in the distal annular zone. This is the effective zone of synaptic output, which is constant at 50 to 60% of dendritic field area, regardless of the cell's retinal location. Both type a and type b cells exhibit systematic increase in cell body size and dendritic field diameter, and systematic decrease in frequency of branching and of synaptic boutons with perpendicular distance from the visual streak. These rates of increase or decrease fall off considerably at distances greater than about 1.5 mm dorsal and ventral to the visual streak, but at this distance, the dendritic field diameters of cells in dorsal retina are about 65% larger than the diameters of cells in ventral retina. When type a and type b cells are closely compared, they are seen to differ in several respects. Branching patterns of type a and type b cells differ slightly, the latter being more highly branched, and the normalized branching frequency histograms, characteristic for each type, remain constant with changing retinal position. At the same retinal location type a cells always have larger dendritic field diameters than type b cells. This difference is significant in ventral retina, out to a distance of at least 4.5 mm from the streak. The maximum percentage difference in size occurs not at mid-visual streak, but about 1.5 mm ventral to the streak. The population statistics of dendritic field overlap and areal dendritic coverage have been calculated using published data on cell densities. It is concluded that overlap is extraordinarily high (k greater than 25), more than 10 times that calculated for retinal ganglion cells.(ABSTRACT TRUNCATED AT 400 WORDS)