Recent results indicate that astrocytic β-adrenergic receptors (βAR) participate in noradrenergic modulation of synaptic activity. In this study, we sought to examine whether neural activity can, in turn, regulate astrocytic βAR. To address this question, an antiserum that recognizes β-adrenergic receptors (βAR) specifically in astrocytes was used to assess the distribution of the receptors across ocular dominance columns in VI of two monocular and four visually intact adult monkeys. Cytochrome oxidase histochemistry (CO) was used to identify the position of the cortical laminae and of the ocular dominance columns receiving visual inputs from the intact and enucleated eyes. This stain revealed the expected pattern within V1 of monocular monkeys–i.e. darker and lighter bands of equal widths (ca. 500μm) spanning laminae 4–6, each associated with larger and smaller blobs, respectively, in lamina 2/3;. Alignment of CO sections with adjacent sections stained for astrocytic βAR by the immunoperoxidase method revealed intense βAR-like immunoreactivity (βAR-li) in the superficial laminae, a slightly weaker staining in the infragranular laminae and weakest staining in lamina 4C. Within lamina 4C., a prominent striped pattern was evident. The darker bands of the stripe closely matched widths and positions of the lighter CO columns associated with the enucleated eye. On the other hand, immunocytochemical staining for the astrocytic intermediate filament protein, GFAP, within V1 of monocular monkeys revealed no inter-columnar difference in the density of astrocytic cell bodies or processes. Nissl stain also revealed no overt inter-columnar differences in cell density. V1 of visually intact monkeys exhibited a similar laminar distribution pattern of βAR-li and of CO. Within lamina 4C., βAR-li was uniformly faint and CO staining was uniformly intense. This suggests that the striped pattern of βAR-li seen in lamina 4C of monocular monkeys results from elevation of the βAR-antigen within the inactive columns. The results indicate that astrocytic βAR density is regulated by local neural activity. The mechanisms regulating βAR density are likely to be independent of those regulating glial cell proliferation or GFAP synthesis. In vitro experimental results of others suggest that elevation of astrocytic βAR may be a mechanism compensating for chronic neural inactivity, since the coincident release of noradrenaline with visual stimulation would elevate neuropil excitability via the astrocytic mechanism of (1) decreasing the uptake of neuronally released L-glutamate; (2) increasing GABA uptake; and (3) stimulating glycogenolysis. Alternatively, the changes in βAR-li may reflect an up-regulation of the receptors within inactive columns due to reduced levels of noradrenaline release.

In monolayer cultures prepared from immature early postnatal rabbit retina, small populations of neurons can be demonstrated to differentiate into apparently mature A- and B-type horizontal cells. Using wholecell, single-channel, patch-clamp recording techniques, we have analyzed the pattern of voltage-gated conductances expressed by mammalian horizontal cells under these conditions. A total of six different voltage-dependent ionic currents were recorded. Tetrodotoxin-sensitive fast sodium inward currents (INa) were found in 81% of the A-type and 90% of the B-type cells. Inward calcium currents could be demonstrated in all cells tested after blockade of other conductances. Two types of outward potassium currents with properties of the 4–aminopyridine-sensitive transient IA and the tetraethylammonium sensitive delayed rectifier IK, respectively, could be characterized in whole-cell recordings. An inward rectifying potassium current (Ianom) typical for horizontal cells was activated in response to hyperpolarizing voltage steps. These types of currents have also been described in dissociated adult horizontal cells from lower vertebrates and cat. With single-channel recordings on inside-out patches excised from B-type cells, an additional Ca2+-dependent current (IK(Ca)) was observed which, so far, has not been described in horizontal cells developing in situ. Our results demonstrate that cultured rabbit horizontal cells express a set of voltage-gated currents which largely, but not completely, corresponds to that described in situ for horizontal cells of other species. The culture system will allow further investigation of developmental and functional aspects of mammalian horizontal cells.

This study profiled the expression of the family of GABAA receptor β-subunits in the adult rat retina. Using a combination of reverse transcriptase reaction followed by polymerase chain reaction (RT-PCR) with gene-specific primers, the expression of mRNAs encoding the β1, β2, and β3 subunits was first examined in the intact retina and then in separated retinal nuclear layers. However, it was found that a critical analysis. had to be carried out at the level of the single cell in order to resolve the differential patterns of expression among the retinal cell types. When cells were isolated and identified following acute dissociation, RT-PCR revealed that individual rod photoreceptor cells expressed consistently the β1 and β2 messages while the bipolar cells expressed the β1 and β3 messages. Ganglion cells displayed considerable variability in β-subunit expression, perhaps reflecting their functional and morphological heterogeneity in the retina. In contrast, the nonneuronal Mueller cells did not express any of the β-subunit messages. These results indicate that the expression of GABAA receptor subunits is cell-type dependent. Furthermore, as the expression of other families of GABAA receptor subunits are profiled and the patterns of subunit assembly are better understood, our results raise the possibility that GABAA receptors with different subunit compositions can be expected to be coexpressed within a single retinal neuron.

The rhabdomeral microvilli in the eyes of invertebrates have a cytoskeletal core (Saibil, 1982; Blest et al., 1982a, b, 1983) of actin (de Couet et al., 1984; Arikawa et al., 1990; Williams, 1991; Caiman & Chamberlain, 1992) which might be involved in phototransduction. Tsukita et al. (1988) suggested that light stimulation triggers the breakdown of the microvillar actin filament complex and that this may play a role in phototransduction. To test their suggestion, we pressure-injected phalloidin into Limulus ventral photoreceptors to prevent actin depolymerization (Cooper, J.A., 1987) and tested to see if this blocked phototransduction. We have previously shown that injected rho-damine-phalloidin brightly labels the microvillar actin filaments in living Limulus ventral photoreceptors for several hours (Feng et al., 1994). In the experiments reported here, phalloidin unconjugated with fluorophore was used, and the final concentration in the cell after many small injections from a pipette containing 10 mg/ml phalloidin was estimated to be 1 mg/ml (1.2 mM). This concentration is 100 times higher than that used in an extracted cellular preparation to stabilize actin (Biegel & Pachter, 1992). Fig. 1 shows that the responses to steps of light at different intensities are normal 1 h after phalloidin injection. Similar results were obtained in two other cells.

One of the most striking properties of the mammalian visual system is that it is only the central part of the visual field, the fovea, where vision is most acute. The superiority of the fovea is particularly evident in tasks requiring accurate spatial localization. It is currently thought that peripheral spatial uncertainty is a simple consequence of the decreased sampling grain of the peripheral field. We show that the topological fidelity of the afferent projection declines with eccentricity away from the fovea and that it is this rather than the sampling grain that underlies the poorer performance of the periphery in tasks involving spatial localization. The combination of normal sampling and a disordered topology results in the periphery having good sensitivity for detection but poor sensitivity for object recognition.

Single isolated photoreceptors can be taken through a visual cycle of light adaptation by bleaching visual pigment, followed by dark adaptation when supplied with 11–cis retinal. Light adaptation after bleaching is manifested by faster response kinetics and a permanent reduction in sensitivity to light flashes, presumed to be due to the presence of bleached visual pigment. The recovery of flash sensitivity during dark adaptation is assumed to be due to regeneration of visual pigment to pre-bleach levels. In previous work, the outer segments of bleached, light-adapted cells were exposed to 11–cis retinal. In the present work, the cell bodies of bleached photoreceptors were exposed. We report a marked difference between rods and cones. Bleached cones recover sensitivity when their cell bodies are exposed to 11–cis retinal. Bleached rods do not. These results imply that retinal can move freely along the cone photoreceptor, but retinal either is not taken up by the rod cell body or retinal cannot move from the rod cell body to the rod outer segment. The free transfer of retinal along cone but not along rod photoreceptors could explain why, during dark adaptation in the retina, cones have access to a store of 11–cis retinal which is not available to rods. Additional experiments investigated the movement of retinal along bleached rod outer segments. The results indicate that retinal can move along the rod outer segment, but that this movement is slow, occurring at about the same rate as the regeneration of visual pigment.

Ocular following responses induced by brief movements of the visual scene were examined in monkeys for their dependence on disparity in the peripheral field. A random dot pattern was projected onto a tangent screen and partitioned into central and peripheral regions. Test stimuli were velocity steps applied in the central region, while stimuli in the periphery were stationary. The visual images in the central region were seen always in the plane of the screen, while stimuli in the periphery could be seen in front, behind, or in the plane of the screen (achieved by a system of polarizing filters). Initial ocular following responses were larger when the peripheral stimuli were presented with an uncrossed disparity than without disparity. On the other hand, responses were smaller when the peripheral stimuli were presented with crossed disparity (<5.0 deg) than without disparity. The result is consistent with the idea that ocular following responses are dependent on the perceived viewing distance.

On page 397 of Visual Neuroscience 11(2), the paragraph at the top of the right column erroneously has a line of copy repeated and one deleted. The paragraph below has the correct copy, which actually ran from the bottom of the left column to the top of the right column on page 397.

In birds, neurons of the isthmo-optic nucleus (ION), as well as “ectopic” neurons, send axons to the retina, where they synapse on cells in the inner nuclear layer (INL). Previous work has shown that centrifugal axons can be divided into two anatomically distinct types depending on their mode of termination: either “convergent” or “divergent” (Ramon y Cajal, 1889; Maturana & Frenk, 1965). We show that cytochrome-oxidase histochemistry specifically labels “convergent” centrifugal axons and target neurons which appear to be amacrine cells, as well as three “types” of ganglion cells: two types found in the INL (displaced ganglion cells) and one in the ganglion cell layer. Labeled target amacrine cells have distinct darkly labeled “nests” of boutons enveloping the somas, are associated with labeled centrifugal fibers, and are confined to central retina. Lesions of the isthmo-optic tract abolish the cytochrome-oxidase labeling in the centrifugal axons and in the target amacrine cells but not in the ganglion cells. Cytochrome-oxidase-labeled ganglion cells in the INL are large; one type is oval and similar to the classical displaced ganglion cells of Dogiel, which have been reported to receive centrifugal input; the other type is rounder. Rhodamine beads injected into the accessory optic system results in retrograde label in both types of cells, showing that two distinct types of displaced ganglion cells project to the accessory optic system in chickens. The ganglion cells in the ganglion cell layer that label for cytochrome oxidase also project to the accessory optic system. These have proximal dendrites that ramify in the outer inner plexiform layer. Neither the target amacrine cells nor either of the displaced ganglion cells are immunoreactive for the inhibitory transmitter gamma aminobutyric acid. At least some of the target amacrine cells may, however, be cholinoceptive: we found that the antibody to the alpha-7 subunit of the nicotinic ACh receptor labels a population of cells in the INL that are similar in location, size, and the presence of labeled bouton-like structures to those we find labeled with cytochrome oxidase. This antibody also labels neurons in the ION proper but not ectopic cells. In conclusion, it appears that cytochrome oxidase may be a marker for “convergent” centrifugal axons and at least one of their target cells in the INL.