Computer simulations were carried out to evaluate the influence of varying the membrane resistance (Rm) on the dendritic integration capacity of three classes of ganglion cells in the mudpuppy (Necturus maculosus) retina. Three broadly different morphological classes of ganglion cells were selected for this study and represent the range of dendritic tree sizes found in the ganglion cell population of this species. Simulations were conducted on anatomical data obtained from cells stained with horseradish peroxidase; each cell was traced, using a computer as an entry device and later converted to a compartmental (electrical) representation of the cell. Computer-simulation analysis used a time-variant conductance change which was similar in waveform to light-activated bipolar cell input. The simulated membrane resistance for each cell varied between 5000 and 100,000 Ω cm2, and conductance changes were introduced into different regions of the soma-dendritic tree to evaluate dendritic integration efficiency. When higher values of Rm are used, even the largest cells become electrotonically compact and attenuation of voltage responses is minimized from distal to soma regions. Responses were less attenuated from proximal to distal regions of the cell because of the favorable impedance matching, and because less current is required to polarize small “sealed” dendritic terminations. Steady-state responses integrate more effectively than transient responses, particularly when Rm is high, since transient responses were more attenuated by the membrane capacitance. The possibility that Rm is a dynamic property of retinal ganglion cells is discussed in view of the functional organization of dendritic integration efficiency as Rm fluctuates from low to high values.

This study examines the spatiotemporal relationships between retinal pigment epithelium (RPE) and photoreceptors (PR) during development of Macaca nemestrina retina. Our aim was to learn more about the developmental dynamics of these two important cell populations, particularly whether developmental changes in RPE cell densities mimic those of PR at selected retinal points. Twelve eyes ranging in age from 100 fetal days (Fd) to adulthood were flatmounted; the retinal perimeters were traced; and then sample punches were taken of the RPE and neural retina at the fovea, optic disc, mid- and far-nasal periphery, and far temporal, inferior and superior periphery. The two tissues were gently separated and the RPE cells and photoreceptors from the same region of the punch were counted using Nomarski contrast interference optics. We found that the total number of cones remains stable around 4 million between Fd100 and adulthood, but RPE number increases from 1.6 million at Fd100 to 2.56 million in adulthood. At the fovea, the core:RPE ratio increases from 5.4:1 at Fd100 to 28:1 by adulthood. In the temporal periphery by contrast, the cone:RPE ratio declines from 2.2:1 at Fd100–110 to less than 1:1 in the adult. In the vicinity of the optic disc, the ratio of (cones + rods): RPE remains around 35:1 throughout development, but in the retinal periphery it decreases to the adult value of 22:1. These changing ratios indicate that photoreceptors and RPE cells are redistributed independently during development, and that these two cellular sheets slide over one another to achieve their final distribution. This situation suggests that the forces or factors causing foveation are intrinsic to the neural retina.

The outer plexiform layer of the retina contains a neural circuit in which cone synaptic terminals are electrically coupled and release glutamate onto wide-field and narrow-field horizontal cells. These are also electrically coupled and feed back through a GABAergic synapse to cones. In cat this circuit's structure is known in some detail, and much of the chemical architecture and neural responses are also known, yet there has been no attempt to synthesize this knowledge. We constructed a large-scale compartmental model (up to 50,000 compartments) to incorporate the known anatomical and biophysical facts. The goal was to discover how the various circuit components interact to form the cone receptive field, and thereby what possible function is implied. The simulation reproduced many features known from intracellular recordings: (1) linear response of cone and horizontal cell to intensity, (2) some aspects of temporal responses of cone and horizontal cell, (3) broad receptive field of the wide-field horizontal cell, and (4) center-surround cone receptive field (derived from a “deconvolution model"). With the network calibrated in this manner, we determined which of its features are necessary to give the cone receptive field a Gaussian center-surround shape. A Gaussian-like center that matches the center derived from the ganglion cell requires both optical blur and cone coupling: blur alone is too narrow, coupling alone gives an exponential shape without a central dome-shaped peak. A Gaussian-like surround requires both types of horizontal cell: the narrow-field type for the deep, proximal region and the wide-field type for the shallow, distal region. These results suggest that the function of the cone-horizontal cell circuit is to reduce the influence of noise by spatio-temporally filtering the cone signal before it passes through the first chemical synapse on the pathway to the brain.

Starburst amacrine cells in the rabbit retina were labeled following an intraocular injection of the fluorescent dye, 4, 6, diamidino-2-phenylindole (DAPI). From each eye a strip of retina was removed, mounted on a platform beneath an epifluorescence microscope, and superfused with a physiological solution. The tip of a tungsten microelectrode (for extracellular recording) was visually positioned near the cell body of a DAPI-labeled starburst amacrine cell that was located in the ganglion cell layer. Light-evoked responses from the displaced starburst amacrine cells were studied in normal retinas and in retinas that had received a small electrolytic lesion near the optic disk 5–9 months beforehand. In normal retinas, a small spot of light centered over the receptive field of a displaced starburst amacrine cell in nearly all cases evoked a brief burst of spikes only at light onset. When stimulated with a large spot or an annulus of light, many cells gave a small burst of spikes at light offset. In lesioned retinas, the light responses of displaced starburst amacrine cells were recorded in areas of the retina where ganglion cells had degenerated. All cells responded with a large burst of spikes at the onset and offset of a small, centered spot of light. Large spots and annuli of light also evoked robust ON/OFF responses from these cells. The results from this study show that the receptive-field properties of displaced starburst amacrine cells change following axotomy-induced degeneration of ganglion cells. This finding indicates that changes in either synaptic transmission or the membrane properties of neurons occur in the retina following degeneration of ganglion cells.

In the human rod visual system, self-cancellation of flicker signals is observed at high rod intensity levels near 15 Hz, both perceptually and in the electroretinogram (ERG). This and other evidence suggests that two rod signals are transmitted through the human retina with different speeds of transmission. Here we report a series of flicker ERG recordings from a normal observer and an observer who lacks cone vision. From these results, we propose a quantitative model of the two rod signals, which assumes (1) that the amplitude of the slow signal grows linearly with log intensity but then saturates at ~1 scot, td; (2) that the amplitude of the fast signal grows linearly with intensity; (3) that there is a difference in time delay of ~33 ms between two rod signals of the same polarity (or of ~67 ms if the signals are of inverted polarity); and (4) that the time delay of both signals declines linearly with log intensity (by ~10 ms per log scot. td). These simple assumptions provide a remarkably good account of the experimental data. Our results and model are relevant to current anatomical theories of the mammalian rod visual system. We speculate that the slower signal in the human ERG may reflect the transmission of the rod response via the rod bipolars and the An amacrine cells, while the faster signal may reflect its transmission via the rod-cone gap junctions and the cone bipolars. There are, however, several objections to this simple correspondence.

In visual neurones of the superficial layers of the superior colliculus (SSC), repetitive stimulation causes a progressive decline in the size of the response to the stimulus, usually known as response habituation or response adaptation. A mechanism has been proposed in which habituation results from coactivation of excitatory and inhibitory neurones, and the responses of the inhibitory neurones block the response to subsequent stimulus presentations. Excitatory amino acid (EAA) neurotransmitters mediate visual responses via NMDA and non-NMDA receptors in cat SSC. We have investigated the role of these receptors in the generation of response habituation. Following the iontophoretic application of the EAA antagonists CNQX, APS or CPP, repetitive visual stimulation paradigms which normally produce response habituation no longer do so. Indeed the response to each presentation of the stimulus is similar. Intravenous administration of the dissociative anesthetic ketamine (2–10 mg/kg) had similar actions to iontophoretically applied NMDA antagonists. The data imply that intracollicular mechanisms activated by NMDA and non-NMDA receptors contribute to the generation of the inhibitory responses in SCC which lead to response habituation. Furthermore, the effects seen with ketamine anesthesia suggest that the use of ketamine in studies of sensory systems may result in the lack of habituation.

Microelectrodes were used to record from red and green cones of the turtle Pseudemys scripta elegans. The purpose of this study was to determine the action spectra of the red and green cone photopigments, and to look closely for direct interactions between the two cone classes. An isolated retina preparation was employed so that cones could be stimulated from the outer segment side, thereby avoiding the oil droplets that reside in the inner segments of many cones and normally filter incident light. In agreement with some previous electrophysiological studies, we found little evidence for significant direct connections between red and green cones. Exceptions to this rule are noted and discussed. Measurements indicate that this result does not appear to be due to a general loss of cone connectivity in the isolated retina preparation. Action spectra of the cone photopigments differed markedly from action spectra reported for cones in the eyecup preparation. In contrast to cones in the eyecup, cones in the isolated retina showed higher short-wavelength sensitivity and had action spectra that were adequately described by photopigment nomograms. A model of cone optical properties suggests that in the eyecup up to about 40% of the light that reaches a cone outer segment may do so without first passing through an oil droplet.

Neonatal subcutaneous administration of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) to hamsters results in a marked depletion of serotonin (5-HT) in cortex and an increase in the concentration of this amine in the superior colliculus (SC). To determine whether this increase was associated with an alteration in the synaptic organization of 5-HT-containing axons in the superficial gray layer of the SC, immunocytochemistry was combined with electron microscopy. In normal adult hamsters, only 4.0% of 500 5-HT-immunoreactive profiles make synaptic contacts in the superficial gray layer of the hamster's SC. In 5,7-DHT-treated animals, examination of 400 individual profiles indicated that 25.5% of 5-HT-positive profiles made synaptic contacts (P < 0.05). Given the recently demonstrated effect of 5-HT on retinotectal transmission in this species, the present results suggest that the functional organization of the SC may also be markedly altered in animals that sustain neonatal 5,7-DHT administration.

We have examined the effects of neonatal monocular enucleation on the volume of the dorsal lateral geniculate nucleus (dLGN), the area of area 17, and the size and numbers of geniculate relay neurons identified by retrograde transport of HRP from cortex. Compared to values for normal animals, the only significant change contralateral to the remaining eye was an increase in relay cell radius. The effects ipsilateral to the remaining eye were more widespread: we found significant reductions in the volume of the dLGN (27% reduction), the area of striate cortex (22%), and the number (16%) and average soma radius (6%) of geniculate relay neurons. The relay neurons were also more densely packed, suggesting that other geniculate cell types were affected similarly, although this was not explicitly examined. These changes were not uniform throughout the nucleus, and as such, reflected the changes in retinal input. The greatest reduction in cell size occurred in the region of the ipsilateral dLGN receiving the most sparse retinal input subsequent to enucleation. Nor was the shrinkage of the dLGN uniform, being most apparent in the coronal plane especially along the axis orthogonal to the pia; there appeared to be little change in the anteroposterior extent. Shrinkage in area 17 ipsilateral to the remaining eye was the same (about 22%) whether it was defined by myelin staining or transneuronal transport of WGA-HRP. These results show that the transneuronal changes seen in the organization of visual cortex after early monocular enucleation in rodents are associated with only a moderate loss of geniculate relay cells.

Neurons containing catecholamine, indoleamine, and gamma-aminobutyric acid (GABA) were identified by immunohistochemistry in the chameleon retina. Tyrosine hydroxylase (TH) and serotonin (5HT) were observed mostly in two subtypes of orthotopic amacrine cells differing in their soma size and process distribution within the IPL. Some labelled cells were displaced either to the IPL (5HT) or to the GCL (TH and 5HT). A multiplicity of retinal cell types contained GABA including cones, horizontal, amacrine, and ganglion cells. Our results confirmed those obtained in the retinas of other lizards except for the presence of interstitial and displaced amacrine cells containing TH or 5HT of which this is the first report.

Direction-selective (DS) ganglion cells of the mammalian retina have their dendrites in the inner plexiform layer (IPL) confined to two narrow strata. The same strata are also occupied by the dendrites of cholinergic amacrine cells which are probably presynaptic to the DS ganglion cells. GABA is known to play a crucial role in creating DS responses. We examined the types of GABAA receptors expressed by the cholinergic amacrine cells and also those expressed by their presynaptic and postsynaptic neurons, by applying immunocytochemical markers to vertical sections of rat retinas. Double-labelling experiments with antibodies against choline acetyltransferase (ChAT) and specific antibodies against different GABAA receptor subunits were performed. Cholinergic amacrine cells seem to express an unusual combination of GABAA receptor subunits consisting of α2-, β1-, β2/3-, γ2-, and δ-subunits. Bipolar cells, which could provide synaptic input to the DS circuitry, were stained with antibodies against the glutamate transporter GLT-1. The axon terminals of these bipolar cells are narrowly stratified in close proximity to the dendritic plexus of displaced cholinergic amacrine cells. The retinal distribution of synaptoporin, a synaptic vesicle associated protein, was studied. Strong reduction of immunolabelling was observed in the two cholinergic strata. The anatomical findings are discussed in the context of models of the DS circuitry of the mammalian retina.

Enkephalin peptides are present in the retina of several vertebrate species. In the avian retina, enkephalin immunoreactivity is primarily localized to a population of amacrine cells. In the present study, we determined the localization of cells expressing preproenkephalin (PPE) mRNA, which encodes the precursor of enkephalin peptides, in adult as well as in embryonic chicken retinas. The localization of PPE mRNA-expressing cells to the proximal inner nuclear layer (INL) in the adult chicken retina is similar to that of enkephalin-immunoreactive cells observed in previous studies, indicating that amacrine cells expressing PPE mRNA synthesize Met5- and Leu5-enkephalin peptides and related extended forms. Specific hybridization signal is absent in retinas at embryonic day (E) 11, but it is detected in retinas at E 15 and at hatching. PPE mRNA-expressing cells at these ages are located in the proximal INL, and they can be classified as amacrine cells on the basis of their soma size and laminar position. These findings confirm and extend previous observations on the presence of opioid peptides in amacrine cells of the chicken retina. The presence of PPE mRNA at embryonic ages, together with the evidence that enkephalins influence developmental processes, suggests that these peptides modulate retinal maturation in birds.

This study of the tree shrew, Tupaia belangeri, provides evidence for an intracollicular pathway that arises in the superficial gray layer and terminates in the optic layer. As a first step, Nissl, myelin, and cytochrome oxidase stains were used to identify the layers of the superior colliculus in the tree shrew. Second, anterograde and retrograde axonal transport methods were used to determine relationships between laminar borders and patterns of connections. Intraocular injections of wheat germ agglutinin conjugated to horseradish peroxidase showed that the border between the superficial gray and optic layers in the tree shrew is marked by a sharp decrease in the density of retinotectal projections. The optic layer also could be distinguished from the subjacent intermediate gray layer by differences in connections. Of the two layers, only the intermediate gray layer received projections following injections of wheat germ agglutinin conjugated to horseradish peroxidase within substantia nigra pars reticulata. Similarly, following injections of horseradish peroxidase or biocytin in the paramedian pons, the intermediate gray but not the optic layer contained labeled cells of origin for the main premotor pathway from the tectum, the predorsal bundle. Next, cells in the superficial gray layer were intracellularly injected with biocytin in living brain slices. Axons were traced from narrow and wide field vertical cells in the deep part of the superficial gray layer to the gray matter surrounding the fiber fascicles of the optic layer. Small extracellular injections of biocytin in brain slices showed that the optic layer gray matter contains a population of stellate cells that are in position to receive the input from the superficial layer. Finally, small extracellular injections of biocytin in the intermediate gray layer filled cells that sent prominent apical dendrites into the optic layer, where they may be directly contacted by the superficial gray layer cells. Taken together, the results support the hypothesis that the optic layer is functionally distinct from its adjacent layers, and may provide a link in the transfer of information from the superficial, retinal recipient, to the intermediate, premotor, layer of the superior colliculus.

The large receptive fields of retinal horizontal cells are thought to reflect extensive electrical coupling via gap junctions. It was shown recently that the biotinylated tracers, biocytin and Neurobiotin, provide remarkable images of coupling between many types of retinal neuron, including horizontal cells. Further, these demonstrations of tracer coupling between horizontal cells rivaled the size of their receptive fields, suggesting that the pattern of tracer coupling may provide some index of the extent of electrical coupling. We studied this question by comparing the receptive field and tracer coupling size of dark-adapted horizontal cells recorded in the superfused, isolated retina-eyecup of the rabbit. Both the edge-to-edge receptive field and space constants (λ) were computed for each cell using a long, narrow slit of light displaced across the retinal surface. Cells were subsequently labeled by iontophoretic injection of Neurobiotin. The axonless A-type horizontal cells showed extensive, homologous tracer coupling in groups greater than 1000 covering distances averaging about 2 mm. The axon-bearing B-type horizontal cells were less extensively tracer coupled, showing homologous coupling of the somatic endings in groups of about 100 cells spanning approximately 400 μm and a separate homologous coupling of the axon terminal endings covering only about 275 μm. Moreover, we observed a remarkable, linear relationship between the size of the receptive fields of each of the three horizontal cell endings and the magnitude of their tracer coupling. Our findings suggest that the extent of tracer coupling provides a strong, linear index of the magnitude of electrical current flow, as derived from receptive-field measures, across groups of coupled horizontal cells. These data thus provide the first direct evidence that the receptive-field size of horizontal cells is related to the extent of their coupling via gap junctions.

Previous work has shown that small, stimulus-dependent changes in light absorption can be used to monitor cortical activity, and to provide detailed maps of ocular dominance and optimal stimulus orientation in the striate cortex of adult macaque monkeys (Blasdel & Salama, 1986; Ts'o et al., 1990). We now extend this approach to infant animals, in which we find many of the organizational features described previously in adults, including patch-like linear zones, singularities, and fractures (Blasdel, 1992b), in animals as young as 3/12 weeks of age. Indeed, the similarities between infant and adult patterns are more compelling than expected. Patterns of ocular dominance and orientation, for example, show many of the correlations described previously in adults, including a tendency for orientation specificity to decrease in the centers of ocular dominance columns, and for iso-orientation contours to cross the borders of ocular dominance columns at angles of 90 deg. In spite of these similarities, there are differences, one of which entails the strength of ocular dominance signals, which appear weaker in the younger animals and which increase steadily with age. Another, more striking, difference concerns the widths of ocular dominance columns, which increase by 20% during the first 3 months of life. Since the cortical surface area increases by a comparable amount, during the same time, this 20% expansion implies that growth occurs anisotropically, perpendicular to the ocular dominance columns, as the cortical surface expands. Since the observed patterns of orientation preference expand more slowly, at approximately half this rate, these results also imply that ocular dominance and orientation patterns change their relationship, and may even drift past one another, as young animals mature.