Elevated free calcium levels in the subretinal space elevate the absolute dark-adapted threshold in hypopigmented mice

Abundant evidence spanning 25 years demonstrates that hypopigmentation is associated with sensory abnormalities manifested most clearly as elevated absolute dark-adapted thresholds in hypopigmented mice. Here we show that when ocular melanin is increased in the himalayan mouse via alpha-melanocyte stimulating hormone (alpha-MSH) injections, dark-adapted thresholds drop in proportion to the change in ocular melanin. We further measured free calcium concentration with calcium-sensitive microelectrodes in both albino and black mouse retinal eyecups in living subjects. The recordings were done in anesthetized animals as the defect is not present in isolated retinas or in the superfused eye preparation. A double-barreled electrode--pCa and Vref--was used to simultaneously record the calcium concentration and the electroretinogram (ERG) at each of many depths as the electrode was driven through the retina. The position of the electrode was confirmed with ERG and 1,1'-dioctadecyl-3, 3,3',3'-tetramethylindocarbocyanine perchlorate electrode tract reconstruction. Dark-adapted albinos (n = 6) had 1.4 +/- 0.015 mM calcium in the subretinal space compared with 0.80 +/- 0.025 mM in black mice (n = 6). The results of these experiments are consistent with the hypothesis that ocular hypopigmentation causes elevated calcium levels in the subretinal space that in turn mimic light adaptation in hypopigmented mice.

Diurnal variation in synaptic ribbon length and visual threshold

Previous work suggests that photoreceptor synaptic ribbon length and absolute dark-adapted threshold may vary during a 24-h diurnal cycle. To test this hypothesis, we examined the length of photoreceptor synaptic ribbons and the dark-adapted threshold in black (+/+) and albino (c2J/c2J) C57BL/6J mice at six times over a 24-h period. Testing began 2 h after light onset (ZT 2:00) and continued at successive 4-h intervals (12 h:12 h light:dark). We determined the length of the synaptic ribbons in frozen sections by labeling them with an antibody specific for synaptic ribbons. Synaptic ribbons vary in length at different points in the diurnal cycle in both types of mice, but the synaptic ribbons in black mice are longer than those in albino mice by an average of 0.33 microm. The synaptic ribbons of black mice also have a larger response to changes in the light cycle. Ribbon length in black mice ranges from 1.66 microm to 1.4 microm, whereas ribbon length in albino mice ranges from 1.32 microm to 1.25 microm. The shortest ribbons are evident 6 h after light onset in both types of mice, whereas the longest ribbons appear within 2 h after light onset. These changes in synaptic ribbon length support the idea that photoreceptor synaptic ribbons are dynamic structures whose length changes over a 24-h diurnal cycle. Examining black and albino mice with a water-maze behavioral assay showed that visual thresholds in black and albino mice vary over the 24-h diurnal cycle. The visual thresholds of albino mice are elevated compared with black mice at all times tested. This is consistent with previous findings of visual thresholds in hypopigmented mice. The lowest threshold (greatest sensitivity) is present 2 h after light onset (ZT 2:00) and corresponds to the time when synaptic ribbons are the longest. The highest threshold is observed 6 h after light onset, the time when synaptic ribbons are shortest. These results show that synaptic ribbon length and visual sensitivity vary together in relation to the time.

Disruption of laminin beta2 chain production causes alterations in morphology and function in the CNS

From the elegant studies of Ramon y Cajal (1909) to the current advances in molecular cloning (e.g., Farber and Danciger, 1997), the retina has served as an ideal model for the entire CNS. We have taken advantage of the well described anatomy, physiology, and molecular biology of the retina to begin to examine the role of the laminins, one component of the extracellular matrix, on the processes of neuronal differentiation and synapse formation in the CNS. We have examined the effect of the deletion of one laminin chain, the beta2 chain, on retinal development. The gross development of retinas from laminin beta2 chain-deficient animals appears normal, and photoreceptors are formed. However, these retinas exhibit several pathologies: laminin beta2 chain-deficient mice display abnormal outer segment elongation, abnormal electroretinograms, and abnormal rod photoreceptor synapses. Morphologically, the outer segments are reduced by 50% in length; the outer plexiform layer of mutant animals is disrupted specifically, because only 7% of observed rod invaginating synapses appear normal, whereas the inner plexiform layer is undisturbed; finally, the rate of apoptosis in the mutant photoreceptor layer is twice that of control mice. Physiologically, the electroretinogram is altered; the amplitude of the b-wave and the slope of the b-wave intensity-response function are both decreased, consistent with synaptic disruption in the outer retina. Together, these results emphasize the prominence of the extracellular matrix and, in particular, the laminins in the development and maintenance of synaptic function and morphogenesis in the CNS.

Antigenic epitopes of the photoreceptor synaptic ribbon

The purpose of this study was twofold: 1) to purify and identify a protein containing an epitope recognized by an anti-synaptic ribbon antibody B16 and 2) to identify and sequence the epitope. B16 recognizes several unrelated proteins in retina immunoblots. Purification and microsequencing of the strongest band (88 kDa) demonstrate 94% identity to aconitase over 111 amino acids. Polyclonal antibodies against aconitase recognize aconitase on Western blots, but not synaptic ribbons in sections. We conclude that although aconitase contains the epitope, aconitase is not the synaptic ribbon protein. The B16 epitope was identified to be 542DTYQHPPKDS551. A synthetic peptide to this sequence absorbs B16 activity in both Western blots and immunohistochemistry studies, whereas partial peptides fail to absorb activity. Additional antibodies against this peptide label synaptic ribbons. When mouse retina were double labeled with B16 and anti-alpha-actinin, B16 was found to label synaptic ribbons in the outer plexiform layer that partially enclosed the alpha-actinin label. We have determined the amino acid sequence of the B16 epitope and found that the B16 labeling colocalizes with alpha-actinin at the photoreceptor synapse.

Increased absolute light sensitivity in Himalayan mice with cold-induced ocular pigmentation

Controversy over the relationship between ocular pigmentation and absolute dark-adapted light sensitivity has persisted for over two decades. Previous electrophysiological experiments in hypopigmented mammals (mice, rats, rabbits) show increased thresholds in the dark-adapted state proportional to the deficit in ocular melanin. Animals with the least amount of ocular melanin have the most elevated thresholds. Dark-adapted thresholds in hypopigmented mice show similar threshold elevations in behavioral tests. The present study extends these findings to show that a specific increase in ocular pigmentation results in the converse effect, lowered absolute dark-adapted thresholds. The increase in ocular melanin was accomplished by keeping Himalayan mice in the cold (4 degrees C) for 6 weeks. Himalayan mice (C57BL/6J cH/cH) were compared to black mice (C57BL/6J (+/+)) and albino mice (C57BL/6J c2J/c2J) after 6 weeks at either 4 degrees C or 20 degrees C in 12-h cycling light (<1 cd/m2). The Himalayan mice that were kept in the cold exhibited a 44% increase in ocular melanin compared to Himalayan mice kept at room temperature. Cold rearing did not effect ocular melanin or visual thresholds in control animals (black mice = 10(-5.9) cd/m2 and albino mice = 10(-4.4) cd/m2). In contrast, the Himalayan mice maintained at 4 degrees C had thresholds of 10(-5.7) cd/m2 compared to 10(-5.1) cd/m2 for Himalayan mice kept at 20 degrees C. This represents compelling evidence of a direct relationship between ocular melanin concentration and absolute dark-adapted light sensitivity.

Ultrastructural localization of a synaptic ribbon protein recognized by antibody B16

This study examines the immunolabelled structures in the mouse retina following incubation with a monoclonal antibody (B16) that recognizes a highly conserved antigen found in retinas from lizards, frogs, fish, birds, mice, rats, rabbits, cats, and monkeys. This paper focuses on observations in the murine retina. The B16 labelling pattern in the retina was compared with that of two synaptic vesicle antigens: SV2 and anti-synaptophysin in the outer plexiform layer were more diffuse and apparently filled the entire presynaptic terminal whereas B16 labelling was more restricted and labelled a discrete structure resembling a semi-ellipse or an arc with the ends pointing to the inner nuclear layer and the middle curve facing the outer nuclear layer (1-2 microns long by < 0.05 micron in width). The structure labelled in the inner plexiform layer resembles a short bar (0.8 micron long by < 0.05 micron in width) that is confined to the inner half the inner plexiform layer. Cryo-ultra microtomy was used to examine the ultrastructural distribution of the labelling, because the B16 epitope is sensitive to fixation and plastic embedding. The tissue was incubated with the B16 antibody and visualized with goat-anti-mouse 10 nm gold particles. In all cases label was found to be confined to the ribbon structure in the photoreceptor terminal. This is consistent with previous evidence linking B16 to an epitope associated with the synaptic ribbon. The labelling is confined to the ribbon structure and does not appear to be associated with synaptic vesicles.

Water escape performance of adult RCS dystrophic and congenic rats: a functional and histomorphometric study

The dystrophic Royal College of Surgeons (RCS) rat undergoes photoreceptor degeneration due to a hereditary defect in the retinal pigment epithelium. The congenic rat remains unaffected. Although the retinal degeneration is well characterized, few functional studies of this strain are available. We wished to compare the visual function of congenic and dystrophic RCS rats using a water escape paradigm that tested their ability to find a submersed, randomly placed platform using a light source as a clue. Three different behavioral experiments were sequentially performed on all animals: Experiment 1--The Light and Platform test measured the time to swim from the center of the pool to a platform located under a light clue. Each animal was given 10 trials. Experiment 2--The No Light with Platform test was performed as above except no light clue was used. Experiment 3--The Light and No Platform test was performed with a light clue but without a platform. The animal was allowed to swim for 2 min. All trials were videotaped. After the final experiment, the animals were sacrificed and a histomorphometric analysis of their retinas were performed. Expt. 1--The time to find the platform using light as a clue was greater for the dystrophic than for the congenic rats. Expt. 2--In the absence of light clue, there was no significant difference in performance between the congenic and dystrophic animals. Expt. 3--In the absence of a platform, a significantly greater amount of time was spent in the area indicated by the light clue by the congenic animals as compared to the dystrophic. Morphometric analysis revealed a mean number of 133 photoreceptor nuclear profiles/90 microns of retina for congenic rats as compared to 0.14 for dystrophic rats. This simplified version of the Morris water maze permits quantitative evaluation of visually-guided behavior in an important model of retinal degeneration.

Visual thresholds in mice: comparison of retinal light damage and hypopigmentation

In previous electrophysiological experiments from hypopigmented animals (mice, rats, rabbits), single-unit recordings from both retinal ganglion axons and cells in the superior colliculus have demonstrated an increase in threshold in the dark-adapted state which is roughly proportional to the animal's ocular melanin concentration. We have examined the thresholds in hypopigmented mice by using a behavioral water maze screening test and found similar threshold elevations to the electrophysiology. In the present study, we investigated the contribution of retinal light damage to the threshold elevation in an albino mouse strain which is relatively resistant to light damage (C57BL/6J c2J/c2J) and mice with profound retinal degeneration (C57BL/6J rd/rd). Black or albino littermates (C57BL/6J +/c2J or c2J/c2J) were placed in either constant light (350 cd/m2) or dim cycling light (0.001 cd/m2) for 21 days before testing. The normally pigmented animals had thresholds of 1.00 x 10(-5) cd/m2 regardless of their light history. The albino mice (c2J/c2J) maintained in constant light had a slight 0.30 log unit elevation compared to their controls that were maintained in dim cycling light 6.3 x 10(-4) cd/m2 (similar to previously published reports). We examined the retinal morphology of representative animals in semi-thin plastic sections. We could not detect any light damage (overall morphology or cell counts in the outer-nuclear layer) in either the normally pigmented animals or the albino mice (c2J/c2J) maintained in dim cycling light. We found extensive light damage in the albino mice (c2J/c2J) maintained in constant light (virtual absence of photoreceptor outersegments) that corresponded to the slight elevation in threshold.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental expression of a synaptic ribbon antigen (B16) in mouse retina

We investigated the developmental expression of the synaptic antigen B16 in the outer and inner plexiform layers in the retina. We compared the expression of this protein with published accounts of retinal ribbon synaptogenesis. The time course of B16 expression was found to be similar, if not identical, to the time course of synaptogenesis in the photoreceptor terminal, supporting our idea of the synaptic nature of the B16 protein. This study provides further evidence which suggests that the B16 antigen is a ribbon associated protein. The morphology, location, and developmental expression of this antigen is coincident with the structure and development of the ribbon synaptic elements located in both the photoreceptor and bipolar terminals. The emergence of the B16 protein was also examined in retina from mice homozygous for the retinal degeneration (rd) mutation which affects photoreceptor synaptogenesis. The B16 expression in the rd retina parallels that in the wild-type retina at early stages of development. However, significant changes in the morphology and staining pattern of the B16 antigen in the rd retina occurring at later postnatal stages are consistent with the degeneration observed in the photoreceptor terminals.

Elevated dark-adapted thresholds in hypopigmented mice measured with a water maze screening apparatus

In previous electrophysiological experiments from hypopigmented animals (mice, rats, rabbits), single-unit recordings from both retinal ganglion axons and cells in the superior colliculus have demonstrated an increase in threshold in the dark-adapted state which is roughly proportional to the ocular melanin concentration. In the present study we compared an albino mouse strain which is relatively resistant to light damage and the beige mouse mutant to their wild-type controls in a situation that involved unanesthetized, unrestrained mice as a control to the electrophysiological single unit experiments. We used a six-chambered water maze. Animals were trained to swim to an illuminated ramp until their performances leveled off (about 10 days). The animals were then dark-adapted for 24 h and tested after reducing the luminance level of the water maze. We found that the albino mice failed to find the ramp when the luminance fell to 1.58 x 10(-3) cd/m2 (p < or = .0001), the beige mice failed at 2.00 x 10(-4) cd/m2 (p < or = .0001), and the normally pigmented controls performed to 5.00 x 10(-5) cd/m2 (p < or = .0001). These results support our previous findings that the sensitivity defect in hypopigmented animals is proportional to the degree of ocular hypopigmentation.

Biochemical correlates of epilepsy in the E1 mouse: analysis of glial fibrillary acidic protein and gangliosides

The E1 (epileptic) mouse is considered a model for complex partial seizures in humans. Seizures in E1 mice begin around 7-8 weeks of age and persist throughout life. To determine if astrocytic gliosis was present in adult seizing E1 mice, the distribution of glial fibrillary acidic protein (GFAP) was studied in the hippocampus using an antibody to GFAP. The mean number of GFAP-positive cells per square millimeter of hippocampus was approximately 15- to 40-fold higher in adult E1 mice than in nonseizing control C57BL/6J (B6) mice or in young nonseizing E1 mice. Relative GFAP concentration (expressed per milligram of total tissue protein) in hippocampus and cerebellum was estimated by densitometric scanning of peroxidase-stained western blots. GFAP concentration was 2.7-fold greater in hippocampus of adult seizing E1 mice than in the control B6 mice. No differences in GFAP content were detected between the strains in the cerebellum. Because gangliosides can serve as cell surface markers for changes in neuronal cytoarchitecture, they were analyzed to determine if the gliotic response in E1 mice was associated with changes in neural composition. Although the total ganglioside concentration of hippocampus, cerebral cortex, and cerebellum was similar in adult E1 and control B6 mice, a synaptic membrane enriched ganglioside, GD1a, was elevated in the adult E1 cerebral cortex and hippocampus. The findings indicate that E1 mice express a type of gliosis that is not accompanied by obvious neuronal loss.

A synaptic antigen (B16) is localized in retinal synaptic ribbons

This morphological and biochemical study examines the cytoplasmic synaptic determinant recognized by a monoclonal antibody (B16). This antibody was generated by using an immunosuppression protocol that generates antibodies to relatively rare antigens. The B16 antibody labels structures in the brain that are dot-shaped and in the retina that resemble synaptic ribbons in their location, size, developmental emergence, and biochemical composition. The antigen is apparently conserved across species as it is found in retinas from lizards, frogs, fish, birds, mice, rats, rabbits, cats, and monkeys. This paper focuses on observations in the murine retina. Labeling in the outer plexiform layer of the retina is confined to the margin between the outer plexiform layer (OPL) and the outer nuclear layer. The labeled structure resembles a semiellipse or an arc with the open end facing the OPL and the top facing the outer nuclear layer. Overall, the arc is approximately 1 micron in length and less than 0.5 micron thick. Approximately 10% of the labeled arcs occur in a proximal stratum of the OPL and form a planar cluster that resembles a flat plaque parallel to the OPL. Five to ten arcs are found in each plaque. The arcs found within the plaques are approximately 50% smaller than the larger isolated arcs. Counterstaining with peanut agglutinin (PNA), a lectin that recognizes cone photoreceptors and their associated processes, demonstrates that the plaques are associated with the cone pedicles. Animals that have a higher ratio of cones/rods than mice demonstrate a much higher ratio of plaques/isolated arcs in the OPL. The structure labeled in the inner plexiform layer resembles a short bar (0.8 micron long by less than 0.5 micron wide) that is confined to the inner half of the inner plexiform layer in mice. The relative mobility (Mr) of the B16 antigen obtained from mouse retinal and brain tissue is 88 kD, as determined by SDS-PAGE followed by Western blotting. The mouse 88 kD protein is relatively soluble (precipitates at 70% ammonium sulphate) and elutes at a pH of 7.3 from an isoelectric focusing column. It appears that the determinant recognized by the B16 antibody is a previously undescribed synaptic protein that is associated with the synaptic ribbons in photoreceptor and bipolar terminals of most vertebrate retinas.

Impaired visual thresholds in hypopigmented animals

Ocular hypopigmentation is associated with neurological defects in structure and function. This paper investigates the absolute visual thresholds in dark-adapted hypopigmented animals compared to their normally pigmented controls. Here we asked (1) whether the threshold elevation found in hypopigmented animals is a general consequence of the reduction in melanin content; (2) if so, which melanin components in the eye are likely to influence visual thresholds; and (3) whether similar threshold defects can be detected in orders other than rodents. By single-unit recordings from the superior colliculus, we compared incremental thresholds of normal black mice of the C57BL/6J strain to hypopigmented mutants: beige (bg/bg), pale ear (ep/ep), and albino (c2J/c2J) mice, three mutants in which melanin pigment throughout the body is affected; and Steel (Sl/Sld) and dominant-spotting/W-mice (W/Wv), two mutants with normal pigmentation in the retinal pigment epithelium (RPE) but without any melanin in the choroid or the rest of the body. We found that all mutants had elevated thresholds that varied with the reduction in melanin. The albinos were 25 times less sensitive than black mice, pale ear mice 20 times, beige mice 11 times, and Steel and W-mice 5 times. The mean thresholds of dark-adapted black mice were 0.008 cd/m2. Recordings from rabbits showed a similar impairment of visual sensitivity; incremental thresholds were elevated 40 times in New Zealand-White albino rabbits (0.0008 cd/m2) compared to Dutch-Belted pigmented controls (0.00002 cd/m2).(ABSTRACT TRUNCATED AT 250 WORDS)

Origins of uncrossed retinofugal projections in normal and hypopigmented mice

In albinos, the retinofugal projections to the ipsilateral side of the brain are reduced (e.g., see Guillery, 1969; La Vail et al., 1978; Lund, 1965). Although all ganglion cell types are affected, in mice the displaced ganglion cell population is the main target of the albino mutation (Dräger & Olsen, 1980). Here we tested whether this preferential effect on displaced ganglion cells is a general consequence of the melanin reduction or a pleiotropic effect unique to the albino locus, by retrogradely tracing retinal ganglion cells in normal C57BL/6J mice and in several non-allelic hypopigmentation mutants on the same background: albino (C57BL/6J-c2J), beige (C57BL/6J-bg), pale ear (C57BL/6J-ep), ruby-eye/haze (C57BL/6J-ru-2hz), and pearl (C57BL/6J-pe). All mutants have lower overall cell counts in the ipsilateral projection, but the displaced population is disproportionately affected: the albinos contain 42% of the normal number of displaced ganglion cells, and the other mutants have an average 57% of normal counts. The reduction in uncrossed retinofugal projections in albinos affects the inputs to the lateral geniculate nucleus and the superior colliculus, but not to the suprachiasmatic nucleus (Dräger, 1974). To address the question in which way the susceptible uncrossed projections differ from the nonsusceptible one, we compared ganglion cells backfilled from the suprachiasmatic nucleus to ganglion cells backfilled from the optic tract at geniculate level. Whereas the uncrossed optic tract projection originates from the binocular region in the ventro-temporal retina and contains a high fraction of large and displaced ganglion cells (Dräger & Olsen, 1980), both the crossed and uncrossed inputs to the suprachiasmatic nucleus originate from the entire retina with a relative preference for the lower nasal region that corresponds to part of the monocular visual field; all ganglion cells projecting to the suprachiasmatic nucleus are of medium size, and they are located in the ganglion cell layer. These observations allow the following conclusions: (1) All genetic mutants which cause a reduction in ocular melanin, regardless of the molecular or cell-biological mechanism underlying the pigment reduction, result in decreased uncrossed projections; this confirms previous reports (La Vail et al., 1978, Sanderson et al., 1974). (2) The decrease affects only projections involved in binocular vision. (3) In mice, the ganglion cells displaced to the inner nuclear layer, and hence located closer to the retinal pigment epithelium, are disproportionately affected by the melanin reductions. These observations may provide cues to the spatio-temporal mechanism of the

Elevated dark-adapted thresholds in albino rodents

Albino mice and rats have elevated dark-adapted thresholds compared to normally pigmented animals. The absolute dark-adapted incremental threshold for black mice is about 1.5 log units lower than the threshold for albino mice when measured by single-unit recordings from the superior colliculus. Cell counts from the outer nuclear layer in albino mice are not significantly different from those in black mice, indicating that the elevated dark-adapted thresholds are not due to light damage of photoreceptor cells. No photoreceptor outer segment damage was found in these albino animals at the light or electron microscopic level. These experiments have been repeated in hooded and albino rats. The thresholds from albino rats were about 2 log units higher than the thresholds from pigmented rats in the dark-adapted state. The proximity of the retinal pigment epithelium (RPE) and the pigmented choroid to the photoreceptors in these animals suggests that a reduction in ocular melanin in hypopigmented animals may be causal to their elevated thresholds.

Visually evoked eye movements in mouse mutants and inbred strains. A screening report

The authors screened various inbred strains and mutants of the mouse, Mus musculus, for qualitatively abnormal or reduced numbers of optokinetic nystagmus (OKN) eye movements. Thirteen hypopigmentation mutants and ten neuromuscular mutants were found to have abnormal or markedly reduced OKN, but none of these mutants had retinal degeneration. Ten other hypopigmentation mutants and nine other neuromuscular mutants had relatively normal OKN, thus showing that neither hypopigmentation nor neuromuscular abnormality per se are well-correlated with abnormal OKN. These findings show that many mutants with visual defects are available from existing holdings. These visual mutants may serve as a resource for the study of the function and development of the mammalian visual system.

Discrete visual defects in pearl mutant mice

The mutant mouse pearl, characterized by its hypopigmentation, has a specific functional defect in a sensory system--the retina. The intact pearl mouse has reduced sensitivity in the dark-adapted condition. Normal sensitivity is restored by isolation and superfusion of the retina with bicarbonate-buffered Ringer solution, suggesting that the retinal expression of the pearl mutation depends on a diffusible substance. The pearl phenotype is described as a possible model for human congenital stationary night blindness.

Electrophysiology of retinal ganglion cells in the mouse: a study of a normally pigmented mouse and a congenic hypopigmentation mutant, pearl

1. The organization of the receptive fields of retinal ganglion cells in te normal mouse was studied qualitatively in recordings from 43 single axons in the optic nerve and optic tract, and the light sensitivity was studied quantitatively in 26 of these cells by measuring incremental sensitivity. 2. The receptive fields of normal animals were elliptical, had concentric center and peripheral subdivisions, and had an antagonistic center/surround organization; the receptive-field centers ranged from 1.95 to 83 degrees in diameter, with a median of 7 degrees. 3. The incremental sensitivity to white light was measured using a criterion response of 10 extra spikes; the most sensitive dark-adapted cell required a stimulus luminance of 3.5 x 10(-3) cd/m2 to generate a criterion response. 4. The action spectrum measured at seven different wavelengths (433-619 nm) from ganglion cells in the normally pigmented mouse resembled the CIE (International Commission on Illumination, CIE 1957 (11)) relative scotopic luminous efficiency function (41) and is consistent with a curve having a peak around 500 nm. 5. On light adaptation with blue light (less than 460 nm), the sensitivity to longer wavelength stimuli increased by 0.2-0.5 log units relative to the sensitivity to the shorter wavelengths; these results are compatible with the presence of a photoreceptor sensitive to long wavelengths in the normally pigmented mouse (C57BL/6J+/+). 6. The organization of the receptive fields of 48 retinal ganglion cells from the hypopigmentation mutant pearl (C57BL/6J-pe) was also studied qualitatively; the receptive field organization was similar to that of the normally pigmented mouse. 7. In 25 cells from dark-adapted pearl mice, the incremental sensitivity to white light was, on the average, 1.6 log units less than that for normal mice. 8. The dark-adapted action spectrum of pearl mice was similar to that of normally pigmented mice. However, a shift in sensitivity to longer wavelengths did not occur on selective light adaptation with the most luminous blue light (less than 460 nm) background that we could produce. 9. We conclude that pearl is one of the mammalian genes that codes for functions that affect dark-adapted retinal sensitivity. The results of this study and past studies suggest that the pearl gene's action on light sensitivity is predominantly within the retina and before (distal to) the ganglion cells.

Cone outer segment shedding in the goldfish retina characterized with the 3H-fucose technique

After an intravitreal injection of 3H-fucose, red- and blue-sensitive cone outer segments (OSs) in the goldfish retina became heavily labeled, green-sensitive cone OSs showed light labeling, and rod OSs showed virtually no labeling. Fish were maintained in white light (light/dark: 12 hr/12 hr; 6 to 10 weeks) and were injected with 3H-fucose 24 hr before sacrifice. After light onset, only phagosomes with no label were found in the retinal pigment epithelium (RPE); after light offset, phagosomes with heavy, light, or no label were found in the RPE. A broad peak of cone OS shedding derived from all cone types was found beginning 2 hr after light offset and returning to baseline levels after 12 hr, with a maximum at 4 to 6 hr. When the white light was replaced with red light during the final 24 hr (irradiance matched to the white light at 625 nm), the green cones showed a reduction in shedding by 62%, the rods showed a 48% reduction in shedding, and the number of heavily labeled phagosomes was reduced by 24% (a value that may reflect normal and red cone shedding and a reduction in blue cone shedding). The results suggest that chromatic stimulation during the light period may influence the shedding response of a given class of cone OS. Finally, the 3H-fucose technique is useful for determination of the photoreceptor OS from which a given phagosome in the RPE originates in this species.

Characterization of abnormalities in the visual system of the mutant mouse pearl

Mice of the mutant strain pearl (pe/pe) differ from the wild strain by a single gene mutation, which leads to a lightening of the coat color. We tested this strain to see if this mutant gene also expressed itself in one or more visual abnormalities. Pearl mice were found to lack totally the optokinetic nystagmus reflex that was present in every normal mouse that we examined. This lack of optokinetic nystagmus was not due to oculomotor defects, since postrotatory nystagmus was normal. As described for other pigmentation mutants, we found that pearl mutants had a reduced ipsilateral projection to the lateral geniculate nucleus, superior colliculus, and visual cortex. We recorded from single cells in the superior colliculus and found response properties and light sensitivities to be normal over the luminance range at which optokinetic nystagmus was tested. However, at very dim backgrounds (scotopic levels), the incremental sensitivities of these cells in pearl mice were about 100 times lower than those of normal mice. This reduction in sensitivity was restricted to scotopic backgrounds and was not due to abnormalities in either the time course of dark adaptation or the receptive field sizes of single cells. In recordings of the electroretinographic response, both the waveforms and the normalized magnitudes of the A and B waves of pearl were indistinguishable from those of normal mice, which seems to indicate that the cause of pearl's sensitivity defect is located central to the main electrical events in the photoreceptors. The normality of many aspects of the visual system of pearl mice contrasts sharply with the complete absence of optokinetic nystagmus, with the reduced ipsilateral projection, and with the reduced dark sensitivity of the cells in the superior colliculus.

Retrograde labeling and dissociation of mouse retinal ganglion cells

Mouse retinal ganglion cells were labeled by retrograde axonal transport of the fluorescent dyes, DAPI and Primulin. The labeled retinae were dispersed by papain treatment and mechanical dissociation. Among the cells in the suspension, the ganglion cells could be unequivocally identified by their intense DAPI/Primulin fluorescence. These results establish that it is possible to label specific neurons in the central nervous system by retrograde transport of fluorescent dyes and to identify the labeled cells after proteolytic dissociation.