Development of postreceptoral function in pigmented and albino guinea pigs

Retinal neurons are generated in overlapping growth spurts with ganglion cell and cone populations peaking sooner than rod and bipolar cell numbers. As such the functional development of the inner and outer retinal components and elements within these strata (rods vs. cones) may differ. We considered the postnatal development of the postreceptoral components of the ERG (P2, oscillatory potentials) in the guinea pig. ERGs were also evaluated across albino and pigmented strains in order to consider the role that pigmentation has for functional development. Electroretinograms were collected on postnatal days PDI to PD60 (n = 4-7 per time point). The postreceptoral P2 amplitude and implicit time was extracted (digital subtraction of modelled P3 and filtering, 0.5-49 Hz). Intensity-response relationships were described using Naka-Rushton functions whose parameters were compared using a nonparametric bootstrap. Oscillatory potentials (OPs) were extracted following signal conditioning and filtering to remove the a- and b-waves and were described using a Gabor function. OP response parameters were compared using repeated measures ANOVA. Postreceptoral P2 amplitudes mature soon after birth (PD10-PD12). Oscillatory potentials show a similar postnatal amplitude development (PD10-PD12) but a later maturation in timing (PD20) compared with the postreceptoral waveform. All components (P3, P2, and OPs) declined at the same relative rate with age after PD12. Albino animals gave larger, faster, and more sensitive waveforms at all ages but showed the same age-related trends as did pigmented animals. Early development of inner retinal synapses in guinea pigs may underlie the rapid postnatal maturation of their postreceptoral response. These appear to be constrained by the development of receptoral responses. All components declined at the same rate suggesting either a change in the photoreceptoral response or changes to ocular impedance with age.     

Comparison of visual function in pigmented and albino rats by electroretinography and visual evoked potentials

Purpose  To check differences in visual function between Wistar (albino) and Long-Evans (pigmented) rats. Methods  The animals were born in our facilities and reared under identical light conditions avoiding bright light. Visual electrophysiology was performed at the ages of 1.5, 4, 7 and 10 months (electroretinography, ERG) and at 1.5 and 7 months (visual evoked potentials, VEP). Results  ERG measurements showed that: 1) The amplitudes of both scotopic and photopic b-waves were markedly larger in Long-Evans rats than in Wistar rats, and also the amplitudes of scotopic oscillatory potentials and photopic 30 Hz Flicker amplitudes, 2) scotopic a-wave amplitudes were larger in Wistar rats at low light intensities, whereas they were smaller in bright light, 3) both a-wave and b-wave latencies were shorter in Wistar rats, 4) the maximum response Rm_P3 was larger in Long-Evans rats, 5) the sensitivity parameter S was larger in Wistar rats, and 6) the post-receptoral response of cones was smaller in Wistar rats. In the VEP measurements, amplitudes of both photopic and scotopic visual evoked potentials of Long-Evans rats were only slightly larger than those of Wistar rats. Conclusions  ERG b-wave amplitudes are markedly decreased in Wistar rats, which requires further investigation. As the b/a and OP/a ratios were also decreased in Wistar rats, it can be suggested that post-receptoral processing, in particular, is impaired in albino animals. Presented in part at the 105th congress of the DOG, 20–23 September 2007 in Berlin. The authors have full control of all primary data and agree to allow Graefe’s Archive for Clinical and Experimental Ophthalmology to review their data upon request.     

Rod and Cone a-Waves in Central Retinal Vein Occlusion

Purpose

To evaluate rod and cone a-waves in cases with unilateral central retinal vein occlusion (CRVO).

Methods

Scotopic and photopic flash electroretinograms (ERGs) were recorded in seven patients aged 54–84 with unilateral hemorrhagic CRVO. Rod and cone a-waves were analyzed using photoreceptor models, and Rm _p3 (maximum a-wave amplitude) and S (sensitivity) were calculated.

Results

Decreased rod log?S was found in all seven cases, and decreased cone log?S was found in five cases. In only one case, rod log?S in the fellow eye was decreased. The alterations in rod and cone log Rm _p3 were smaller than those in rod and cone log?S. Of three cases in which ERGs could be recorded again after a certain follow-up period, rod log?S and cone logS became larger in two cases and smaller in one case.

Conclusions

The change in the phototransduction cascade was confirmed not only in rods but also in cones in five of our seven cases of CRVO. The ERG findings might reflect the functional change in the photoreceptor layer after the onset of CRVO. Jpn J Ophthalmol 2005;49:402–410 © Japanese Ophthalmological Society 2005     

Altered retinal function and structure after chronic placental insufficiency

PURPOSE: To consider whether growth restriction secondary to chronic placental insufficiency results in postnatal deficits in retinal structure and function. METHODS: Chronic placental insufficiency was induced just before midgestation in guinea pigs through unilateral ligation of the uterine artery. Eight weeks after birth, electroretinograms were recorded from prenatally compromised (PC, n = 6) and control (n = 15) animals. Data were collected for b-wave amplitude and implicit time, also the modeled receptoral (P3) response and oscillatory potentials were extracted. After electroretinography, retinas were prepared for structural analysis (PC, n = 6; control, n = 7). A separate cohort of PC (n = 8) and control (n = 9) animals underwent tyrosine hydroxylase immunoreactivity (TH-IR, dopaminergic neurons) and nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry (neuronal nitric oxide synthase, nNOS)--these being markers of amacrine cell subpopulations. RESULTS: Electroretinography revealed two PC guinea pigs with marked changes to saturated receptoral amplitude (Rm(P3)), sensitivity (log S) and postreceptoral waveforms. Grouped PC data revealed significantly reduced Rm(P3), whereas log S was not affected. The b-wave amplitudes were normal, but b-wave implicit times were delayed (P < 0.05) in PC animals. Amplitudes and peak times of oscillatory potentials were also significantly reduced and delayed (P < 0.05). Morphologic analysis revealed significant reductions in all cellular and plexiform (synaptic) layers in both the central (P < 0.05) and peripheral (P < 0.05) retina in PC animals. The outer retina, which contains the photoreceptors and the outer plexiform layer was particularly affected. The reduced growth of plexiform layers suggests a reduction in the growth of the neuropile in PC animals compared with control animals. The total number (P < 0.03) and density (P < 0.05) of TH-IR neurons was reduced, whereas the total number and density of nNOS-positive amacrine cells was not significantly different between PC and control animals. CONCLUSIONS: Chronic placental insufficiency results in morphologic and functional alterations to the retina. Electroretinogram deficits in PC animals indicated both inner and outer retinal anomalies. Such affects could contribute to the visual impairments reported in very-low-birth-weight children, some of whom are growth restricted.     

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.     

Interaction of antiglaucomatous drugs and melanin granule

The author investigated the effects of several antiglaucomatous drugs and prostaglandins (E2 and F2 alpha) on intraocular pressure responses in pigmented and albino rabbits, and spectrophotometrically assessed the binding of these drugs to synthetic melanin. Topical application of 0.5% timolol, 1% epinephrine and 3% pilocarpine had greater ocular hypotensive effects in albino rabbits than in pigmented rabbits. However no such differences were seen with application of prostaglandins. Melanin binding of drugs was higher in the order of befunolol, carteolol, timolol, epinephrine and pilocarpine. At higher concentrations of the drugs, the degree of binding increased. Prostaglandins had no binding ability. Thus drugs with lower hypotensive effects in pigmented rabbits than in albino rabbits had high melanin binding ability, whereas drugs with similar effects in pigmented and albino rabbits had no melanin binding ability. It is speculated that some antiglaucomatous drugs bind to melanin, resulting in decreased pharmacological action.     

Ocular pigmentation and intraocular pressure response to forskolin

We studied the effects of a forskolin suspension on intraocular pressure (IOP) in normal albino and pigmented rabbits and in alpha-chymotrypsin induced ocular hypertensive rabbits. Experimental-induced ocular hypertensive rabbits were produced by injecting of alpha-chymotrypsin (167 units) into the posterior chamber of the eye of albino rabbits. Ocular hypertensive rabbits were classified into 3 groups according to the IOP (Group A; 15-19 mmHg, B; 20-24 mmHg, C; 25-29 mmHg). Topical application of 1% forskolin caused a significant decrease in IOP in Groups B and C, as well as in normal albino and pigmented rabbits. The hypotensive effects were lower in pigmented rabbits than in albino rabbits, although the duration was longer. Subconjunctival injection of 1% forskolin reduced IOP 1 to 5 hrs after treatment in albino rabbits. However, in pigmented rabbits, a slight increase in IOP was observed at 30 min, followed by a significant decrease 5 to 10 hrs after injection. Furthermore, the binding ability of forskolin to melanin granules was determined in vitro. Forskolin exhibited specific affinity towards melanin granules obtained from bovine eyes, with the binding reaching a plateau after 5 min of incubation.     

Correlation between rod photoreceptor numbers and levels of ocular pigmentation

PURPOSE: Ocular melanin synthesis modulates rod photoreceptor production, because in albino eyes, rod numbers are reduced by approximately 30%. In this study, rod numbers and ocular rhodopsin concentrations were measured in intermediate pigmentation phenotypes to determine whether proportional reductions in melanin are correlated with proportional changes in rod numbers. Further, patterns of cell production and death were examined around the time of birth, when rod production peaks, to determine whether there are abnormalities in these features associated with hypopigmentation. METHODS: Four mouse pigmentation phenotypes were used: fully pigmented, albino, Beige, and Himalayan. The latter two are intermediate-pigmentation phenotypes, with Beige having markedly more pigment than Himalayan. Ocular melanin concentrations were measured during development and at maturity. Rods were counted at maturity and measurements of ocular rhodopsin undertaken. Mitotic and pyknotic cells were also counted in neonates. RESULTS: Rods and ocular rhodopsin were reduced in both Beige and Himalayan mice below levels found in fully pigmented mice, but not to levels found in albino animals. This was more marked in Himalayan than Beige mice, reflecting the lower concentration of melanin found in the former compared with the latter, both in development and at maturity. Although patterns of cell production were elevated in the hypopigmented animals, such patterns varied. CONCLUSIONS: Rod numbers are modulated within a range between that in fully pigmented and albino phenotypes by the concentration of ocular melanin. However, in these animals, there is no obvious correlation between these events and patterns of cell production and death in neonates.     

The dynamics of melanin-affinitive and non-affinitive antibacterial agents in the iris-ciliary body of rabbit eyes--comparative studies in pigmented and albino rabbits]

The differences between the drug penetration levels in the iris-ciliary bodies of sparfloxacin (SPFX) and cefmenoxime (CMX), which respectively have high and low affinity to melanin, were examined using pigmented and albino rabbit eyes. Each drug was mixed with a homogenate of the iris-ciliary bodies of pigmented and albino rabbit eye, respectively. All CMX was distributed in a water soluble protein of the above homogenate of both pigmented and albino eyes, while all SPFX was detected from the water soluble protein of the tissue homogenate of the albino eye. However, in homogenate of the pigmented eyes, 60% of the drug was detected from water soluble protein and 20% of that was detected from water non-soluble protein. In the in vivo study, each drug was topically administered to pigmented and albino rabbit eyes. The SPFX concentration in the iris-ciliary body was significantly higher in the pigmented than in the albino eyes. The results indicated that the intraocular dynamics of the drug which has a high affinity to melanin showed significant differences between pigmented and albino rabbit eyes. This should be considered in studies of ocular pharmacology as an important factor which influences intraocular drug dynamics.     

Effect of topical 8-hydroxy carteolol on intraocular pressure and melanin granules

It was reported that 8-Hydroxycarteolol (8-OH CA), the major metabolite of carteolol hydroxychloride (CA), has a slightly different pharmacological effect from CA. We studied the reduction of intraocular pressure (IOP) on a single eyedrop application of 8-OH CA in albino and pigmented rabbit eyes. To determine the characteristic of 8-OH CA and CA, we investigated the binding ability of these drugs to synthetic melanin. In the present study, topically applied 2% CA did not significantly decrease IOP in albino and pigmented rabbit eyes. Topically applied 0.01, 0.05, 0.1 and 1.0% 8-OH CA significantly decreased the IOP of albino rabbit as did 0.1 and 1.0% 8-OH to pigmented rabbit eyes. The maximum reduction of IOP was 3.5 +/- 0.33 mmHg (mean +/- SEM) in albino rabbit and 3.0 +/- 0.45 mmHg (mean +/- SEM) in pigmented rabbit. Maximum IOP reduction was obtained after 30 min. from topical application in albino rabbit, but in pigmented rabbit after 1 hour or later. Our binding studies to melanin show that the melanin binding ability is less for 8-OH CA than for CA at any concentrations. These results may indicate that lower concentrations of topically applied 8-OH CA profoundly reduce IOP compared to CA, and 8-OH CA has less effect on melanin than CA.     

Background adaptation in a rat model of retinopathy of prematurity

Low dark-adapted, scotopic retinal and visual sensitivity in retinopathy of prematurity (ROP) could be due to disease of the inner retina, or the recently described rod photoreceptor abnormalities. Receptoral disease decreases catch of quanta from both test flashes and steady background lights; increment threshold functions are shifted up and right. In diseases with normal receptors but low retinal sensitivity due to abnormal post receptoral processing, the increment threshold functions are shifted up with no horizontal translation. Herein we test the hypothesis that the rod photoreceptors are the site of ROP disease which causes low dark adapted b-wave sensitivity. The effect of steady background light on the ERG b-wave in a rat model of ROP is studied. ERG stimulus/response functions were obtained using full-field stimuli in the dark-adapted state, and in the presence of a steady background light. In each adaptation condition, log , the test flash intensity that produced a half-maximum b-wave amplitude, was calculated. In pilot experiments, the background light selected had raised log about a log unit in controls. In dark-adapted ROP rats log was significantly higher, 0.35 log unit, than in controls. In the presence of the background light, log in ROP and control rats did not differ significantly indicating a relative shift, up and right, of the increment sensitivity function for the less sensitive ROP rats. The effect of the background light is consistent with receptoral disease causing low dark adapted b-wave sensitivity in ROP rats.     

Ultrastructural localization of lipid peroxides as benzidine-reactive substances in the albino mouse eye

· Background: Lipid peroxidation is considered to be a prominent feature of retinal degeneration and has also been proposed to be involved in the pathogenesis of age-related macular degeneration. Melanin protects against lipid peroxidation and takes part in the detoxification of lipid peroxides (LP). LP can be ultrastructurally detected as benzidine-reactive substances (BRS) using tetramethylbenzidine (TMB). Albino mice lack melanin. In the present study, LP were localized as BRS in the eyes of albino and pigmented mice. · Methods: Eye cups of an albino mouse lineage and of wild-type mice were fixed with 2% glutaraldehyde, incubated with 0.5 mg/ml TMB and embedded for electron microscopy. · Results: BRS were detected in the eyes of albino mice, but no reaction product was seen in pigmented eyes. BRS located in the retinal pigment epithelium (RPE) and in the choroid of the albino mouse; no BRS were found in intact rod outer segments (ROS). · Conclusion: The lack of melanin in albino mice is associated with a higher level of lipid peroxidation in RPE and choroid. Melanin seems to protect against LP in RPE and choroid. A lack of melanin is not associated with lipid peroxidation in intact ROS. The present investigation demonstrates the significance of melanin in protection against LP in RPE and choroid. Received: 16 November 1998 Revised version received: 18 January 1999 Accepted: 19 January 1999     

Effect of eye pigmentation on transscleral drug delivery

PURPOSE: To determine the influence of eye pigmentation on transscleral retinal delivery of celecoxib. METHODS: Melanin content in ocular tissues of both the strains was determined by sodium hydroxide solubilization METHOD: The affinity of celecoxib to synthetic and natural melanin was estimated by co-incubating celecoxib and melanin in isotonic phosphate-buffered saline. The binding affinity (k) and the maximum binding (r(max)) for celecoxib to both natural and synthetic melanin were estimated. Suspension of celecoxib (3 mg/rat) was injected periocularly into one eye of Sprague-Dawley (SD, albino) and Brown Norway (BN, pigmented) rats. The animals were euthanatized at the end of 0.25, 0.5, 1, 2, 3, 4, 8, or 12 hours after the drug was administered, and celecoxib levels in ocular tissues (sclera, choroid-RPE, retina, vitreous, lens, and cornea) were estimated with an HPLC assay. In addition, celecoxib-poly(lactide) microparticles (750 microg drug/rat) were administered periocularly in SD and BN rats, and celecoxib levels in these eye tissues were assessed on day 8, to determine the effectiveness of the sustained release system. RESULTS: The r(max) and k for celecoxib's binding to natural melanin were (3.92 +/- 0.06) x 10(-7) moles/mg of melanin and (0.08 +/- 0.01) x 10(6) M(-1), respectively. The affinity and the extent of celecoxib's binding to natural melanin were not significantly different from those observed with synthetic melanin. The concentrations of melanin in choroid-RPE, sclera, and retina of BN rats were 200 +/- 30, 12 +/- 4, and 3 +/- 0.2 mug/mg tissue, respectively. Melanin was not detectable in the vitreous, lens, and cornea of BN rats. In SD rats, melanin was not detected in all tissues assessed except in the choroid-RPE, wherein melanin-like activity was 100-fold less than in BN rats. The area under the curve (AUC) for tissue concentration versus time profiles for animals administered with celecoxib suspension was not significantly different between the two strains for sclera, cornea, and lens. However, the retinal (P = 0.001) and vitreal (P = 0.001) AUCs of celecoxib in the treated eyes were approximately 1.5-fold higher in SD rats than in BN rats. Further, the choroid-RPE AUC in the treated and untreated eyes, respectively, were 1.5-fold (P = 0.001) and 2-fold (P = 0.0001) higher in BN rats than in SD rats. With celecoxib-poly(lactide) microparticles, choroid-RPE, retina, and vitreous concentrations on day 8 exhibited similar trends in differences between the two strains, with the differences being greater than those recorded for the celecoxib suspension. CONCLUSIONS: Transscleral retinal and vitreal drug delivery of lipophilic celecoxib is significantly lower in pigmented rats than in albino rats. This difference may be attributable to significant binding of celecoxib to melanin and its accumulation/retention in the melanin-rich choroid-RPE of pigmented rats. The hindrance of retinal and vitreal drug delivery by the choroid-RPE in pigmented rats is also true of sustained-release microparticle systems.     

The effect of melanin and hemoglobin on the dye laser photocoagulation in pigmented and albino rabbits

Photocoagulation was performed in pigmented and albino rabbits using four different wave-lengths of the dye laser (577 nm, 590 nm, 610 nm and 630 nm), and each photocoagulated lesion was examined histologically. The 577 nm dye laser produced chorioretinal coagulation in albino rabbits, which was almost the same as that in pigmented rabbits. This result confirmed that the 577 nm dye laser was well absorbed by hemoglobin in the choriocapillaris, and could produce the sufficient coagulation even if there was no melanin pigment. The 590 nm dye laser produced chorioretinal coagulation in albino rabbits, but it was weaker than that in pigmented rabbits. It was considered that this was because the absorption by hemoglobin of the 590 nm dye laser was less than that of the 577 nm dye laser. The 610 and 630 nm dye laser could not yield appropriate coagulation in albino rabbits under the same conditions as in the 577 nm dye laser. More intense coagulation, that is longer duration and higher power, was required to make chorioretinal coagulation in albino rabbits using the 610 and 630 nm dye laser. In photocoagulation of normal fundus, melanin is the main substance of laser absorption, but in various disorders lacking melanin, the differences of the hemoglobin absorption rate becomes important.     

Changes in ocular hypotensive effect of griseolic acid with isoproterenol, timolol and melanin

Griseolic acid-ester (GA-ester), one of the strongest cAMP phosphodiesterase inhibitors, was combined with isoproterenol and timolol in this study to evaluate the effect on intraocular pressure (IOP). Furthermore, the ocular hypotensive effect of GA-ester in pigmented and albino rabbits was compared, and the binding ability of GA-ester to synthetic melanin was examined. GA-ester markedly enhanced the hypotensive effect of isoproterenol, and the combination of GA-ester with timolol resulted in an additional fall in IOP. No differences in the hypotensive effect of GA-ester between pigmented and albino rabbits were observed. GA-ester did not bind to synthetic melanin. GA-ester has unique characteristics as an ocular hypotensive agent.     

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)     

Dark adaptation is faster in pigmented than albino rats

Purpose: Previous reports have raised the possibility that, compared to pigmented rats, albino rats might be night blind. The purpose of this study was to reinvestigate this issue by comparing the dark-adaptation process of the pigmented Long-Evans (LE) and albino Sprague Dawley (SD) rats. Methods: Scotopic ERGs obtained from LE and SD rats were recorded following periods of dark adaptation 0.5, 3 and 12 h. Intensity response functions were generated with flashes of white light spanning over a 7 log-unit range with a maximal intensity of 8 cd.s.m^–2 in energy. Results: SD rats showed a gradual increase in the amplitude of the scotopic b-wave Vmax (maximal `saturated' rod b-wave amplitude) and retinal sensitivity (k) as the duration of the dark-adaptation period increased. In contrast, LE rats did not demonstrate any further significant gain in retinal function (Vmax or k) beyond 30 min of dark-adaptation. Thus for periods of dark-adaptation of 30 min or less, the rod function of the LE rats is superior to that of the SD rats while both strains have comparable retinal functions following 3 h or more of dark-adaptation. Conclusions: Our results indicate that LE and SD rats differ in their rapidity to dark-adapt, a finding that could explain the previous claim that SD rats were night blind. The reduced bio-availability of calcium ions in eyes lacking melanin could explain this difference. Calcium was previously shown to play a key role in retinal adaptation processes.     

Visual thresholds in albino and pigmented rats.

Albino rats have recently been reported to have increment thresholds against dim backgrounds that are two log units higher than those of pigmented rats. We, on the other hand, have failed to confirm these differences using electroretinogram b-waves and pupillary light reflexes. This paper reports on experiments using evoked potentials from cortex and colliculus and single-unit recordings from colliculus. We recorded visual-evoked potentials from cortex and superior colliculus in the strains of albino (CD) and pigmented (Long-Evans) rats used in the earlier studies. Thresholds were determined on eight fully dark-adapted animals by extrapolating intensity-response curves to the point at which there was zero evoked potential. The average dark-adapted threshold for the visual-evoked cortical potential was -5.6 log cd/m2 in pigmented and -5.80 log cd/m2 in albino animals. The average dark-adapted threshold for the superior colliculus evoked response was -5.54 log cd/m2 in pigmented and -5.84 log cd/m2 in albinos. The differences were not statistically significant. On the same apparatus, the average absolute threshold for three human observers was -5.3 log cd/m2, a value close to the rat dark-adapted thresholds. Thus, visual-evoked cortical potentials and superior collicular evoked potentials failed to confirm the report of higher dark-adapted thresholds for albinos. In addition, we find that single units in superior colliculus in the albino rat respond to very dim flashes.     

Tapetum lucidum in the pigmented and albino ferret

Light and electron microscopy showed that the tapetum lucidum in the pigmented ferret is morphologically indistinguishable from that in the albino ferret. The matrix of the rods of the tapetal cells was strongly osmiophilic, but glutaraldehyde fixation before osmium tetroxide treatment caused a dissolution of the matrix material. It has been proposed that the tapetal cells are modified melanocytes and that the tapetal rods are composed of melanin, but it can be concluded from our data that the matrix of the tapetal rods is not melanin. Further studies by plasma-atomic emission spectrometry showed that the tapetal cells are very rich in zinc, with similar levels in pigmented and albino ferrets. Excessive concentrations of other metals were not observed. Histochemical demonstration of heavy metal showed that the zinc is present in the tapetal rods and indicated a localization mainly in the rod membranes.     

Structural and functional maturation of the retina of the albino Hartley guinea pig

PURPOSE: Altricial animals, such as rats and mice, are born with their eyes closed, compared to precocial animals, such as guinea pigs and humans, which have their eyes opened at birth. The purpose of this study was to investigate if the retina of guinea pigs (precocial animal) is subjected to a postnatal maturation process similar to that previously reported for rodents. METHODS: Photopic and scotopic electroretinograms (ERG) and retinal histology were obtained from albino guinea pigs aged P1 to P75. RESULTS: Photopic ERG responses reached maximal amplitudes at P5 (a-and b-waves), that is 5 days (b-wave) to 10 days (a-wave) earlier than scotopic responses. However, the postnatal gain in b-wave amplitude was significantly (P < 0.05) more important for the cone (73.38 +/- 4.4%) signal than for the rod (15.23 +/- 3.96%), suggesting that the rod function is more mature at birth. Similarly, the short latency photopic oscillatory potential (ie: OP2) reached its maximal value 5 days (P10) earlier than its scotopic equivalent (P15), while the long latency OPs (ie: OP3, OP4), reached their maximal values nearly 20 days sooner in scotopic condition. Finally retinal histology revealed a thinning of the retina with age, the latter being most pronounced at the level of the ganglion cell layer (GCL). CONCLUSION: Our results thus confirm that despite its relative maturity at birth (compared to rodents), the retina of newborn albino guinea pigs undergoes significant postnatal maturation modifying its structure as well as its function, albeit not as extensive as that previously documented for altricial animals.