Retinal ischemia, a major cause of visual loss, is believed to result from overexcitation of glutamate receptors. However, under euglycemic and normoxic conditions, exogenously applied glutamate is not neurotoxic in the retina. Under such conditions, exogenous glutamate typically causes glia swelling and requires very high concentrations to produce neurotoxicity. To determine whether ischemic conditions enhance the neurotoxicity of endogenous and exogenous glutamate, we examined the effects of simulated ischemia (deprivation of both glucose and oxygen) on retinal morphology and lactate dehydrogenase (LDH) release. In an ex vivo rat retinal preparation, glutamate was administered during simulated ischemia in the presence of riluzole, an inhibitor of glutamate release. Deprivation of both glucose and oxygen for 60 min at 30°C produced severe acute neurodegeneration. This neurodegeneration, characterized by bull's eye formation in the inner nuclear layer and spongy appearance in the inner plexiform layer, was prevented by the combination of MK-801 and DNQX, antagonists of N-methyl-D-aspartate (NMDA) and non-NMDA receptors, indicating that the damage results from activation of both glutamate receptors. We also found that administration of glutamate pyruvate transaminase (alanine aminotransaminase) with pyruvate diminished the neurodegeneration during simulated ischemia. Furthermore, riluzole, an inhibitor of glutamate release, attenuated the neurodegeneration, suggesting the importance of endogenous glutamate in ischemic damage. In the presence of riluzole and simulated ischemia, exogenously applied glutamate failed to cause Müller cell swelling but was extremely neurotoxic. These results suggest that simulated ischemia enhances glutamate-mediated neurotoxicity in part by depressing glutamate uptake. When glutamate transport is impaired, sub-millimolar glutamate concentrations become profoundly neurotoxic.

In the retina of rat, cones make up ∼0.85% of the photoreceptor population: 93% of these cones contain a midwave-sensitive pigment, the rest expresses a short-wave-sensitive pigment (Szel & Rohlich, 1992). We used normal adult Long Evans rats to determine the spectral sensitivity of the cone-driven electroretinogram (ERG) b-wave and its absolute sensitivity at λmax of the cone pigments. ERGs were recorded at the cornea of anesthetized animals under dark- and light-adapted conditions. Rod responses were suppressed by steady rod-saturating orange backgrounds and/or by a flashed “white” background. Cone-driven b-waves were evoked by “white” or narrowband full-field stimuli of varying intensity. The action spectrum for the cone b-wave indicates the presence of an absorbance peak at 510 nm; a second, twofold lower, peak was found at 360 nm (after correction for transmittance by the lens). Chromatic adaptation experiments strongly suggest that retinal responses to midwave and UV stimuli are mediated by a single cone type. On a background producing ∼17,000 R* rod−1 s−1, which completely suppressed the saturated a-wave, the absolute sensitivity of the cone b-wave was 18 nV photon−1 μm2 at 510 nm and 4 nV photon−1 μm2 at 360 nm which is 20–30 times higher than for the mouse. It is suggested that the relatively large number of on-cone bipolar cells in the retina of rat is responsible for the remarkable sensitivity of the cone b-wave.

The morphology and spectral absorption characteristics of the retinal photoreceptors in the southern hemisphere lamprey Geotria australis (Agnatha) were studied using light and electron microscopy and microspectrophotometry. The retinae of both downstream and upstream migrants of Geotria contained two types of cone photoreceptor and one type of rod photoreceptor. Visual pigments contained in the outer segments of these three photoreceptor types had absorbance spectra typical of porphyropsins and with wavelengths of maximum absorbance (downstream/ upstream) at 610/616 nm (long-wavelength-sensitive cone, LWS), 515/515 nm (medium-wavelength-sensitive cone, MWS), and 506/500 nm (medium-wavelength-sensitive rod). A “yellow” photostable pigment was present in the myoid region of all three types of photoreceptor in the downstream migrant. The same short-wavelength-absorbing pigment, which prevents photostimulation of the beta band of the visual pigment in the outer segment, was present in the rods and LWS cones of the upstream migrant, but was replaced by a large transparent ellipsosome in the MWS cones. Using microspectrophotometric and anatomical data, the quantal spectral sensitivity of each photoreceptor type was calculated. Our results provide the first evidence of a jawless vertebrate, represented today solely by the lampreys and hagfishes, with two morphologically and physiologically distinct types of cone photoreceptors, in addition to a rod-like photoreceptor containing a colored filter (a cone-like characteristic). In contrast, all other lampreys studied thus far have either (1) one type of cone and one type of rod, or (2) a single type of rod-like photoreceptor.

Patch-clamp recordings were used to investigate the properties of the regenerative activity in acutely isolated bipolar cells from the rat retina. Spontaneous, pacemaker-like membrane potential oscillations were observed in all rod bipolar cells and the majority of cone bipolar cells. The waveform of the regenerative potential was stereotypical but distinct among different bipolar cell groups, especially between rod and cone bipolar cells. The spontaneous activity was completely blocked by Co2+, suggesting that Ca2+ influx through voltage-dependent Ca2+ channels was required for initiating such activity. Ca2+-induced Ca2+ release, however, was not found to be involved. The spontaneous activity was also blocked by mibefradil, a T-type Ca2+ channel antagonist. In contrast, application of nimodipine, an L-type Ca2+ current antagonist, affected mainly the waveform of the regenerative potential. This study shows that mammalian retinal bipolar cells in isolation are also capable of generating Ca2+-dependent spontaneous regenerative potential. However, T-type Ca2+ channels appear to be essential for the initiation of the spontaneous activity in mammalian bipolar cells.

We have evaluated the spatial distribution of low-voltage-activated calcium currents in ganglion cells of the tiger salamander retina. Whole-cell recordings were obtained from ganglion cells in a retinal slice preparation and from acutely dissociated ganglion cells that were identified through retrograde dye injection. In single dissociated cells, we estimated the magnitude (pA) and current density (pA/pF) of LVA currents in ganglion cells, both with and without dendritic processes. Ganglion cells that retained a portion of their dendritic arbor had larger LVA calcium currents and higher LVA current densities than those which lacked processes. When cell capacitance measurements were used to derive the surface area of the soma and dendritic processes, we concluded that a higher LVA current density was present in the dendrites; we estimate that, on average, the current density in the dendrites is approximately five times that of the soma. The presence of a significant density of LVA calcium channels in the dendrites of ganglion cells suggests that they could be involved in a number of cellular functions, including dendritic integration of synaptic currents, impulse generation, and homeostatic functions related to changes in the intradendritic calcium concentration.

The expression of glutamate decarboxylase forms, GAD-65 and GAD-67, in GABAergic cells was studied by immunocytochemistry in the retina of the New World monkey, Cebus apella. In the innermost rows of the inner nuclear layer (INL), somata that express GABA correspond to about 45% of the total number of cells in the central retina and about 25% on the periphery. Three subsets of GABAergic amacrine cells were identified along the horizontal meridian: about 5% express only GAD-65 and 40% GAD-67, and approximately 50% contain both forms of GAD. In the INL, GAD-65 immunoreactivity was detected in broad bands around strata 1, 3, and 5. GAD-67 immunoreactivity was observed throughout all strata. Somata that expressed GAD-67 exclusively stratified only in narrow bands around strata 2 and 4 of the INL and colocalized with β2 and β3 subunits of GABA-A receptor. Interplexiform and amacrine cells that express GABA also express tyrosine hydroxylase (TH) or nitric oxide synthase (NOS). GAD-67 colocalized with TH or NOS in presumed amacrine cells of the inner plexiform layer (IPL) and ganglion cell layer (GCL). GAD-65 was expressed in the TH- and NOS-immunoreactive interplexiform and amacrine cells, respectively. Different from what has been described in other mammals, TH and NOS were coexpressed in some neurons, indicating a partial overlap in retinal cell populations containing dopamine or nitric oxide in this primate.

Acetylcholine is one of the primary excitatory neurotransmitters/neuromodulators in the retina, but little is known about the downstream signaling pathways it can activate. The present study immunocytochemically examines the potential sources of acetylcholine and the location of the nicotinic cholinergic receptors in the turtle retina. It also examines how activation of these receptors can influence the nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) signal-transduction pathways. Photoreceptors, amacrine cells, and potentially ganglion cells contain choline acetyltransferase-like immunoreactivity (LI). Nicotinic acetylcholine receptors are immunocytochemically localized on photoreceptors, horizontal, bipolar, and ganglion cells. Nitric oxide imaging indicates that stimulation with nicotine increases NO production primarily in photoreceptors, horizontal, Müller, bipolar, and ganglion cells. In turn, very select populations of amacrine cells respond to this NO with increased levels of cGMP-LI. Selective inhibitors reveal that nitric oxide synthase is involved in most, but not all, of these increases in cGMP-LI. These results show that acetylcholine can activate the NO/cGMP signal-transduction pathways in both the inner and outer retina. This indicates that both of the major excitatory retinal transmitters, glutamate and acetylcholine, can stimulate NO production that increases levels of cGMP-LI in overlapping populations of retinal cells.

Endogenous cannabinoids modulate neurotransmitter action and release in the brain. The effects are exerted on membrane permeability to Ca2+ and K+via protein kinase A (PKA). Cannabinoid CB1 receptors are present at the synaptic terminals of cones in goldfish retina. We investigated the effects of CB1 receptor agonist WIN 55212-2 on voltage-gated currents of goldfish cones. Whole-cell currents were recorded with conventional-patch-clamp methods in goldfish retinal slices. Depolarizing pulses elicited inward ICa and Ioutward that contained several components: IK, IA, and ICl. WIN 55212-2 (<1 μM) enhanced IK, ICl, and ICa, while at >1 μM, IK, ICl, and ICa were suppressed. The voltage-activation ranges of these currents were not affected. All effects of WIN 55212-2 were blocked by the CB1 receptor antagonist SR 141716A as well as the PKA inhibitor Wiptide. The enhancing effect of WIN 55212-2 was blocked selectively by 0.5 nM cholera toxin and the suppressive effect was blocked by pertussis toxin. The results obtained from long and short single cones and double cones were basically the same.

We examine the assumptions that the fovea contains equal numbers of inner (invaginating or ON) and outer (flat or OFF) midget bipolar cells and equal numbers of inner and outer diffuse bipolar cells. Based on reconstruction from electron photomicrographs of serial thin sections through the fovea of a macaque monkey, we reject both assumptions. First, every foveal L and M cone is presynaptic to one inner and one outer midget bipolar cell; however, S cones are presynaptic to one outer but no inner midget bipolar cell. Second, we measure the density of all foveal cells in the same patch of fovea, affording accurate cell density ratios. For each foveal cone pedicle, at a density of 26,500 mm−2, there is close to one (0.88) outer diffuse bipolar cell but only 0.40 inner diffuse bipolar cells. This asymmetry may be related to differences in resolution and sensitivity for light increments and decrements. We also find one (1.01) Müller cell, one (1.01) amacrine cell in the inner nuclear layer, and close to one (0.83) horizontal cell for each cone pedicle. In addition, for each S cone, there are two inner S-cone bipolar cells and two small bistratified ganglion cells. In total, there are 3.4 cone bipolar cells per cone but only 2.6 ganglion cells per cone. The latter ratio is enough to accommodate one midget ganglion cell for each midget bipolar cell.

To determine whether the photoreceptors change structurally and functionally during aging, and to analyze whether pigmentation in the retinal pigment epithelium might be a contributing factor. Young, adult, and aged C57BL/6 and Balb/c mice (1, 4, and 17 months of age) were housed under a 12-h light/12-h dark cycle, with an ambient light intensity at the eye level of the mice of 85 ± 18 lux. Scotopic single-flash and photopic-flicker electroretinograms (ERGs) after complete dark adaptation were used to assess rod and cone function, respectively. Numbers of rod photoreceptors were counted in plastic sections, and rhodopsin levels were measured using absorption difference spectrophotometry. Numbers and types of cones were determined using lectin staining in retinal flatmounts and cone-specific antibodies in radial frozen sections. Young pigmented C57BL/6 and nonpigmented Balb/c mice had similar numbers of rods. In both mouse strains, there was an overall decline in rod photoreceptor number during aging, which was more pronounced in albino mice. Rod cell numbers correlated with a drop in the overall amount of rhodopsin and a reduction in the maximum a-wave of the rod ERG. The number of short-wavelength cones was unaffected by age and pigmentation, whereas an age-related decline was observed in mid-wavelength (MWL) cones in albino, but not in pigmented mice. In contrast, MWL cone function was reduced during aging in both strains.