Previous studies show that glycine transporter-1 (glyt-1) is a consistent membrane marker of adult retinal neurons that are likely to release glycine at their synaptic terminals (Pow, 1998; Vaney et al., 1998; Pow & Hendrickson, 1999). The current study investigated when glyt-1 immunoreactivity appeared in the postnatal rat retina, and whether all glycine-containing neurons also labelled for glyt-1. Ganglion cells, horizontal cells, and photoreceptors showed transient labelling. Many cells in the ganglion cell layer are immunoreactive for both glycine and glyt-1 at postnatal day (Pd) 1 but both are minimal by Pd5. Transient immunoreactivity for both glyt-1 and glycine was observed in presumptive horizontal cells between Pd5 and Pd10. At Pd1 many cells in the outer part of the retina which resembled immature photoreceptors were heavily labelled for glycine, but did not express glyt-1; these disappeared at older ages. These findings suggest diverse mechanisms and transient roles for glycine in the developing rat retina. In the adult rat retina, a subpopulation of amacrine cells are prominently immunoreactive for both glycine and glyt-1. These cells labelled for glycine at Pd1, but did not express significant levels of glyt-1 until Pd5. Processes from these amacrine cells did not reach the inner half of the inner plexiform layer until Pd10–14. Bipolar cells became glycine-IR between Pd10 and Pd14, but consistently lacked any glyt-1 immunoreactivity. This temporal pattern of labelling strongly indicates that bipolar cells label for glycine when gap junctions become functional between glycine/glyt-1 immunoreactive amacrine cells and cone bipolar cells.

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the retina and also appears to act as a trophic factor regulating photoreceptor development and regeneration. Although the tiger salamander is a major model system for the study of retinal circuitry and regeneration, our understanding of GABA receptors in this species is almost exclusively based on the results of physiological studies. Therefore, we have examined the pharmacological binding properties of GABAA receptors and their anatomical localization in the tiger salamander retina. Radioligand-binding studies showed that specific 3H-GABA binding to GABAA receptors was dominated by a single high-affinity binding site (Kd = 15.6 ± 6.9 nM). Specific binding of 3H-GABA was almost completely eliminated by muscimol (Ki = 105 ± 62 nM) and bicuculline (Ki = 14.3 ± 2.2 μM); however, SR-95531 only displaced about 40% of specific 3H-GABA binding (Ki = 35.0 ± 3.8 nM). These data indicate that there are at least two subtypes of GABAA receptors present in the salamander retina that can be distinguished by their antagonist binding properties: one sensitive to both bicuculline and SR-95531, and one sensitive to bicuculline but insensitive to SR-95531. Because localization of GABA receptors in the salamander retina by immunocytochemistry is problematic, GABAA receptors were localized by fluorescent ligand binding combined with immunocytochemical labeling for cell specific markers. Binding of fluorescently labeled muscimol to GABAA receptors was present in both plexiform layers and on photoreceptor cell bodies. GABAA receptors in the outer plexiform layer were localized to both photoreceptor terminals and horizontal cell processes.

There were three objectives to the work undertaken for this paper: (1) to provide a comprehensive characterization of the statistical properties of arrays of β-cell somata; (2) to develop a model that simulates cellular arrays with the same properties; and (3) to use this model to examine whether the array of β-cells should be viewed as one array or as two arrays, one each for its OFF- and ON-center cells. β-cells are morphological correlates of the electrophysiological X-cells and those β-cells whose dendrites stratify within the outer and inner sublamina of the retina's inner plexiform layer correspond, respectively, to OFF- and ON-center X-cells. Arrays of peripheral β-cell somata from two retinas were studied. A Delaunay triangulation and a Voronoi tessellation were generated for each array and measures derived from these constructs used to analyze the arrays' spatial organization. As others have shown previously with a less complete statistical characterization, we found that the arrays of OFF- and ON-center β-cells have similar spatial properties and are more regular than the array of all β-cells. We developed a model to simulate cellular arrays with spatial properties like those of arrays of β-cells. A good fit between model and real arrays was found when the model assumed an explicit spatial dependence between the placement of OFF- and ON-center cells. We propose therefore that a single array of β-cells formed of both OFF- and ON-center cells is consistent with the data currently available for β-cell somatic arrays.

The N-methyl-D-aspartate receptor (NMDAR) is an ionotropic glutamate receptor that is important in neurotransmission as well as in processes of synaptic plasticity in the mammalian superior colliculus (SC). Despite the importance of this receptor in synaptic transmission, there is as yet no evidence that demonstrates directly the synaptic localization of the NMDAR receptor in SC. We have used electron-microscope (EM) immunocytochemistry to localize the NMDAR1 subunit of this receptor protein and its association with sensory afferents in the cat SC. Retinal synaptic terminals were identified by normal morphology and cortical synaptic terminals by degeneration after lesions of areas 17–18 of the visual cortex. At the light-microscope level, label was densest within the superficial gray and upper optic layers, but also present in all other layers. Label was contained within cell bodies, dendrites, and a few putative axons. At the EM level, antibody labeling was found along postsynaptic densifications and internalized within the cytoplasm of a variety of dendrites and some cell bodies. Postsynaptic profiles labeled by NMDAR1 included conventional dendrites and presynaptic dendrites which contained pleomorphic synaptic vesicles and are known to be GABAergic. Many of the labeled postsynaptic densifications of both of these profile types received synaptic inputs from retinal or cortical terminals. Virtually no NMDAR1 immunoreactivity was found on thin dendritic thorns or putative spines, even when these were postsynaptic to retinal or cortical terminals. In summary, these results show that the NMDAR1 subunit is postsynaptic to both retinal and cortical afferents, which are known to be glutamatergic, and are consistent with physiological evidence showing that stimulation of either pathway can activate the NMDA receptor.

Thalamic relay cells fire in two distinct modes, burst or tonic, and the operative mode is dictated by the inactivation state of low-threshold, voltage-gated, transient (or T-type) Ca2+ channels. Tonic firing is seen when the T channels are inactivated via membrane depolarization, and burst firing is seen when the T channels are activated from a hyperpolarized state. These response modes have very different effects on the relay of information to the cortex. It had been thought that only tonic firing is seen in the awake, alert animal, but recent evidence from several species suggests that bursting may also occur. We have begun to explore this issue in macaque monkeys by recording from thalamic relay cells of unanesthetized, behaving animals. In the lateral geniculate nucleus, the thalamic relay for retinal information, we found that tonic mode dominated responses both during alert behavior as well as during sleep. We nonetheless found burst firing present during the vigilant, waking state. There was, however, considerably more burst mode firing during sleep than wakefulness. Surprisingly, we did not find the bursting during sleep to be rhythmic. We also recorded from relay cells of the somatosensory thalamus. Interestingly, not only did these somatosensory neurons exhibit much more burst mode activity than did geniculate cells, but bursting during sleep was highly rhythmic. It thus appears that the level and nature of relay cell bursting may not be constant across all thalamic nuclei.

Distribution of the mt1 melatonin receptor in the guinea pig retina was immunocytochemically investigated using peptide-specific anti-mt1 receptor antibody. Western blots of the guinea pig retina showed a single band at approximately 37 kilodalton (kD) immunoreactive to the anti-mt1 antibody. The most intense immunoreactivity for the mt1 receptor was detected in the cell bodies of ganglion cells. Their dendrites and axons were also immunolabeled. Subpopulations of amacrine cells, the inner plexiform layer, and the outer plexiform layer also exhibited moderate to weak immunolabeling. The mt1-positive amacrine cells were located either at the vitreal border of the inner nuclear layer or displaced in the ganglion cell layer. Double immunolabeling using antibodies to the mt1 receptor and tyrosine hydroxylase revealed that the majority of dopaminergic amacrine cells showed mt1 immunoreactivity. Almost all the 1CA type dopaminergic cells were mt1 positive while the 2CA type cells less frequently exhibited mt1 immunoreaction. By double immunolabeling for the mt1 receptor and GABA, more than 50% of the mt1-immunoreactive amacrine cells were shown to be GABAergic neurons. Approximately one-third of the GABAergic amacrine cells were immunolabeled for the mt1 receptor. The present results demonstrate expression of the mt1 receptor in diverse neuronal cell types in the guinea pig retina and provide the first evidence for the direct effect of melatonin on dopaminergic and GABAergic amacrine cells via the mt1 receptor.

How neurons in the primary visual cortex (V1) of primates process parallel inputs from the magnocellular (M) and parvocellular (P) layers of the lateral geniculate nucleus (LGN) is not completely understood. To investigate whether signals from the two pathways are integrated in the cortex, we recorded contrast-response functions (CRFs) from 20 bush baby V1 neurons before, during, and after pharmacologically inactivating neural activity in either the contralateral LGN M or P layers. Inactivating the M layer reduced the responses of V1 neurons (n = 10) to all stimulus contrasts and significantly elevated (t = 8.15, P < 0.01) their average contrast threshold from 8.04 (± 4.1)% contrast to 22.46 (± 6.28)% contrast. M layer inactivation also significantly reduced (t = 4.06, P < 0.01) the average peak response amplitude. Inactivating the P layer did not elevate the average contrast threshold of V1 neurons (n = 10), but significantly reduced (t = 4.34, P < 0.01) their average peak response amplitude. These data demonstrate that input from the M pathway can account for the responses of V1 neurons to low stimulus contrasts and also contributes to responses to high stimulus contrasts. The P pathway appears to influence mainly the responses of V1 neurons to high stimulus contrasts. None of the cells in our sample, which included cells in all output layers of V1, appeared to receive input from only one pathway. These findings support the view that many V1 neurons integrate information about stimulus contrast carried by the LGN M and P pathways.

Lateral inhibition is one of the first and most important stages of visual processing. There are at least four theories related to information theory in the literature for the role of early retinal lateral inhibition. They are based on the spatial redundancy in natural images and the advantage of removing this redundancy from the visual code. Here, we contrast these theories with data from the retina's outer plexiform layer. The horizontal cells' lateral-inhibition extent displays a bell-shape behavior as function of background luminance, whereas all the theories show a fall as luminance increases. It is remarkable that different theories predict the same luminance behavior, explaining “half” of the biological data. We argue that the main reason is how these theories deal with photon-absorption noise. At dim light levels, for which this noise is relatively large, large receptive fields would increase the signal-to-noise ratio through averaging. Unfortunately, such an increase at low luminance levels may smooth out basic visual information of natural images. To explain the biological behavior, we describe an alternate hypothesis, which proposes that the role of early visual lateral inhibition is to deal with noise without missing relevant clues from the visual world, most prominently, the occlusion boundaries between objects.

To date, two cannabinoid receptors, CB1 and CB2, have been cloned. The CB1 receptor has been found in a variety of tissues, particularly in the brain. CB2 receptor mRNA is mainly expressed in the immune system, though one group has found it in mouse cerebellum. Previous immunostaining studies in our lab demonstrated the presence of CB1 receptors in the retina though little evidence exists for the presence of CB2. The putative endogenous ligand for CB2 has been found in retina, however, suggesting that further study of CB2 in retina is warranted. Because glutamate is toxic to retinal ganglion cells in glaucoma and activation of CB2 receptors may be able to protect neurons from glutamate-induced death, we examined the expression of CB2 mRNA in adult rat retina in order to better understand possible neuroprotective mechanisms relevant to glaucoma. Using in situ hybridization, we demonstrated that CB2 cannabinoid receptor messenger RNA was clearly expressed in the adult rat retina, including the somas of retinal ganglion cells. Antisense cRNA probe detected strong signals in the retinal ganglion cell layer, the inner nuclear layer, and the inner segments of photoreceptor cells. Using reverse transcription polymerase chain reaction (RT-PCR) in both rat and mouse tissue, we obtained an RT-PCR product with the same sequence as that reported for CB2 in the GenBank database, thus confirming the presence of CB2 mRNA in retina. The presence of CB2 in retina provides new evidence for the presence of CB2 in the central nervous system (CNS) and an excellent model for its study.

In the vertebrate retina, vision is initiated and maintained by the photolysis and regeneration, respectively, of light-sensitive pigments in the disk membranes of the photoreceptor outer segments. This cyclical process depends on an exchange of retinoids between the photoreceptors and the retinal pigment epithelium (RPE). There is a great deal of indirect evidence that the transport of retinoids between these cellular compartments is mediated by the interphotoreceptor retinoid-binding protein (IRBP), a large glycoprotein synthesized in the photoreceptors and extruded into the interphotoreceptor matrix (IPM) that fills the subretinal space. Nevertheless, a number of in vitro experiments have demonstrated that an intermembranous transfer of retinoids can occur through an aqueous medium independent of any retinoid-binding protein. This led to the suggestion that IRBP may play the more passive role of an extracellular buffer, serving to prevent the degradation and potentially cytotoxic effects of free retinoids when large amounts are released into the IPM. We have studied the structural and functional properties of transgenic mice in which homologous recombination was used to delete the IRBP gene. Light- and electron-microscopic examination of the retinas of “knockout” (IRBP−/−) mice revealed a significant loss of photoreceptor nuclei, and profound changes in the structure and organization of the receptor outer segments. Consistent with these observations, electroretinographic recordings showed a marked reduction in response amplitude for both rod- and cone-mediated potentials. However, despite the histological and electrophysiological changes, there was no evidence of gross abnormalities in the visual cycle. After bleaching a significant fraction of the available rhodopsin, electroretinogram amplitude and rhodopsin density gradually increased toward their pre-bleach levels, and the rates of recovery were even more rapid than those seen in wild-type (IRBP+/+) mice.

The thalamus is the major gate to the cortex and its control over cortical responses is well established. Cortical feedback to the thalamus is, in turn, the anatomically dominant input to relay cells, yet its influence on thalamic processing has been difficult to interpret. For an understanding of complex sensory processing, detailed concepts of the corticothalamic interplay need yet to be established. Drawing on various physiological and anatomical data, we elaborate the novel hypothesis that the visual cortex controls the spatiotemporal structure of cortical receptive fields via feedback to the lateral geniculate nucleus. Furthermore, we present and analyze a model of corticogeniculate loops that implements this control, and exhibit its ability of object segmentation by statistical motion analysis in the visual field.

Psychophysical (behavioral) detection thresholds and color-discrimination thresholds were determined in a macaque using a two-alternative forced-choice procedure. On a white background, detection thresholds were determined for a white increment and three spectral increments: 618, 516, and 456 nm. Intermixed with detection threshold determinations, color-discrimination thresholds were determined by presenting the white increment, and one of the spectral increments, at 1.0 log units above their respective detection thresholds and dimming both until discrimination performance fell to threshold. The monkey could discriminate the color of the increments at detection threshold because the average color-discrimination threshold was 0.98 ± 0.14 log attenuation. Because the monkey was much more sensitive to the spectral increments than the white increment, we performed an unconventional experiment. We determined the monkey's detection threshold for the white increment alone, and with broadband color filters in the white light path without adjusting the light's intensity. Insertion of several color filters in the light path lowered detection thresholds of both the macaque and six human trichromats. We believe that this improvement in detection thresholds produced by simply inserting color filters in a white light path is a threshold manifestation of the Helmholtz-Kohlrausch effect and suggests that one of color vision's important evolutionary advantages may be improved detection sensitivity.

Cephalopod retinas exhibit several responses to light and dark adaptation, including rhabdom size changes, photopigment movements, and pigment granule migration. Light- and dark-directed rearrangements of microfilament and microtubule cytoskeletal transport pathways could drive these changes. Recently, we localized actin-binding proteins in light-/dark-adapted octopus rhabdoms and suggested that actin cytoskeletal rearrangements bring about the formation and degradation of rhabdomere microvilli subsets. To determine if the microtubule cytoskeleton and associated motor proteins control the other light/dark changes, we used immunoblotting and immunocytochemical procedures to map the distribution of tubulin, kinesin, and dynein in dorsal and ventral halves of light- and dark-adapted octopus retinas. Immunoblots detected α- and β-tubulin, dynein intermediate chain, and kinesin heavy chain in extracts of whole retinas. Epifluorescence and confocal microscopy showed that the tubulin proteins were distributed throughout the retina with more immunoreactivity in retinas exposed to light. Kinesin localization was heavy in the pigment layer of light- and dark-adapted ventral retinas but was less prominent in the dorsal region. Dynein distribution also varied in dorsal and ventral retinas with more immunoreactivity in light- and dark-adapted ventral retinas and confocal microscopy emphasized the granular nature of this labeling. We suggest that light may regulate the distribution of microtubule cytoskeletal proteins in the octopus retina and that position, dorsal versus ventral, also influences the distribution of motor proteins. The microtubule cytoskeleton is most likely involved in pigment granule migration in the light and dark and with the movement of transport vesicles from the photoreceptor inner segments to the rhabdoms.

Pursuit and perception both require accurate information about the motion of objects. Recovering the motion of objects by integrating the motion of their components is a difficult visual task. Successful integration produces coherent global object motion, while a failure to integrate leaves the incoherent local motions of the components unlinked. We compared the ability of perception and pursuit to perform motion integration by measuring direction judgments and the concomitant eye-movement responses to line-figure parallelograms moving behind stationary rectangular apertures. The apertures were constructed such that only the line segments corresponding to the parallelogram's sides were visible; thus, recovering global motion required the integration of the local segment motion. We investigated several potential motion-integration rules by using stimuli with different object, vector-average, and line-segment terminator-motion directions. We used an oculometric decision rule to directly compare direction discrimination for pursuit and perception. For visible apertures, the percept was a coherent object, and both the pursuit and perceptual performance were close to the object-motion prediction. For invisible apertures, the percept was incoherently moving segments, and both the pursuit and perceptual performance were close to the terminator-motion prediction. Furthermore, both psychometric and oculometric direction thresholds were much higher for invisible apertures than for visible apertures. We constructed a model in which both perception and pursuit are driven by a shared motion-processing stage, with perception having an additional input from an independent static-processing stage. Model simulations were consistent with our perceptual and oculomotor data. Based on these results, we propose the use of pursuit as an objective and continuous measure of perceptual coherence. Our results support the view that pursuit and perception share a common motion-integration stage, perhaps within areas MT or MST.

(article appeared in Visual Neuroscience (1999), 16, 1105–1114.)

Due to an oversight, Table 2 that appeared on p. 1106 of Visual Neuroscience 16:6 had errors in the body of the Table. A corrected version of Table 2 is printed below.

Cambridge University Press and the authors regret any inconvenience that this inadvertent error may have caused.

(article appeared in Visual Neuroscience (1999), 16, 1145–1156.)

Due to an unfortunate misunderstanding, Figure 3 that appeared on p. 1149 of Visual Neuroscience 16:6 was not sized to the full two-column measure. A corrected version of the article, with Figure 3 reshot to full size, is reproduced on pp. 1145R–1156R, which follows. Cambridge University Press regrets any inconvenience that this inadvertent error may have caused.