Development of the light response in neonatal mammalian rods

Clearance phagocytosis by the retinal pigment epithelial during photoreceptor outer segment renewal: Molecular mechanisms and relation to retinal inflammation

Mammalian photoreceptor outer segment renewal is a highly coordinated process that hinges on timed cell signaling between photoreceptor neurons and the adjacent retinal pigment epithelial (RPE). It is a strictly rhythmic, synchronized process that underlies in part circadian regulation. We highlight findings from recently developed methods that quantify distinct phases of outer segment renewal in retinal tissue. At light onset, outer segments expose the conserved "eat-me" signal phosphatidylserine exclusively at their distal, most aged tip. A coordinated two-receptor efferocytosis process follows, in which ligands bridge outer segment phosphatidylserine with the RPE receptors αvβ5 integrin, inducing cytosolic signaling toward Rac1 and focal adhesion kinase/MERTK, and with MERTK directly, additionally inhibiting RhoA/ROCK and thus enabling F-actin dynamics favoring outer segment fragment engulfment. Photoreceptors and RPE persist for life with each RPE cell in the eye servicing dozens of overlying photoreceptors. Thus, RPE cells phagocytose more often and process more material than any other cell type. Mutant mice with impaired outer segment renewal largely retain functional photoreceptors and retinal integrity. However, when anti-inflammatory signaling in the RPE via MERTK or the related TYRO3 is lacking, catastrophic inflammation leads to immune cell infiltration that swiftly destroys the retina causing blindness.

Functional and Structural Development of Mouse Cone Photoreceptor Ribbon Synapses

Purpose

Cone photoreceptors of the retina use a sophisticated ribbon-containing synapse to convert light-dependent changes in membrane potential into release of synaptic vesicles (SVs). We aimed to study the functional and structural maturation of mouse cone photoreceptor ribbon synapses during postnatal development and to investigate the role of the synaptic ribbon in SV release.

Methods

We performed patch-clamp recordings from cone photoreceptors and their postsynaptic partners, the horizontal cells during postnatal retinal development to reveal the functional parameters of the synapses. To investigate the occurring structural changes, we applied immunocytochemistry and electron microscopy.

Results

We found that immature cone photoreceptor terminals were smaller, they had fewer active zones (AZs) and AZ-anchored synaptic ribbons, and they produced a smaller Ca2+ current than mature photoreceptors. The number of postsynaptic horizontal cell contacts to synaptic terminals increased with age. However, tonic and spontaneous SV release at synaptic terminals stayed similar during postnatal development. Multiquantal SV release was present in all age groups, but mature synapses produced larger multiquantal events than immature ones. Remarkably, at single AZs, tonic SV release was attenuated during maturation and showed an inverse relationship with the appearance of anchored synaptic ribbons.

Conclusions

Our developmental study suggests that the presence of synaptic ribbons at the AZs attenuates tonic SV release and amplifies multiquantal SV release. However, spontaneous SV release may not depend on the presence of synaptic ribbons or voltage-sensitive Ca2+ channels at the AZs.

Inducible Pluripotent Stem Cells to Model and Treat Inherited Degenerative Diseases of the Outer Retina: 3D-Organoids Limitations and Bioengineering Solutions

Inherited retinal degenerations (IRD) affecting either photoreceptors or pigment epithelial cells cause progressive visual loss and severe disability, up to complete blindness. Retinal organoids (ROs) technologies opened up the development of human inducible pluripotent stem cells (hiPSC) for disease modeling and replacement therapies. However, hiPSC-derived ROs applications to IRD presently display limited maturation and functionality, with most photoreceptors lacking well-developed outer segments (OS) and light responsiveness comparable to their adult retinal counterparts. In this review, we address for the first time the microenvironment where OS mature, i.e., the subretinal space (SRS), and discuss SRS role in photoreceptors metabolic reprogramming required for OS generation. We also address bioengineering issues to improve culture systems proficiency to promote OS maturation in hiPSC-derived ROs. This issue is crucial, as satisfying the demanding metabolic needs of photoreceptors may unleash hiPSC-derived ROs full potential for disease modeling, drug development, and replacement therapies.

Comparative Neurology of Circadian Photoreception: The Retinohypothalamic Tract (RHT) in Sighted and Naturally Blind Mammals

The mammalian eye contains two systems for light perception: an image detecting system constituted primarily of the classical photoreceptors, rods and cones, and a non-image forming system (NIF) constituted of a small group of intrinsically photosensitive retinal ganglion cells driven by melanopsin (mRGCs). The mRGCs receive input from the outer retina and NIF mediates light entrainment of circadian rhythms, masking behavior, light induced inhibition of nocturnal melatonin secretion, pupillary reflex (PLR), and affect the sleep/wake cycle. This review focuses on the mammalian NIF and its anatomy in the eye as well as its neuronal projection to the brain. This pathway is known as the retinohypothalamic tract (RHT). The development and functions of the NIF as well as the knowledge gained from studying gene modified mice is highlighted. Furthermore, the similarities of the NIF between sighted (nocturnal and diurnal rodent species, monkeys, humans) and naturally blind mammals (blind mole rats Spalax ehrenbergi and the Iberian mole, Talpa occidentalis) are discussed in relation to a changing world where increasing exposure to artificial light at night (ALAN) is becoming a challenge for humans and animals in the modern society.

The Development of Mid-Wavelength Photoresponsivity in the Mouse Retina

PURPOSE

Photoreceptors in the mouse retina express much of the molecular machinery necessary for phototransduction and glutamatergic transmission prior to eye opening at postnatal day 13 (P13). Light responses have been observed collectively from rod and cone photoreceptors via electroretinogram recordings as early as P13 in mouse, and the responses are known to become more robust with maturation, reaching a mature state by P30. Photocurrents from single rod outer segments have been recorded at P12, but no earlier, and similar studies on cone photoreceptors have been done, but only in the adult mouse retina. In this study, we wanted to document the earliest time point in which outer retinal photoreceptors in the mouse retina begin to respond to mid-wavelength light.

METHODS

Ex-vivo electroretinogram recordings were made from isolated mouse retinae at P7, P8, P9, P10, and P30 at seven different flash energies (561 nm). The a-wave was pharmacologically isolated and measured at each developmental time point across all flash energies.

RESULTS

Outer-retinal photoreceptors generated a detectable response to mid-wavelength light as early as P8, but only at photopic flash energies. a-wave intensity response curves and kinetic response properties are similar to the mature retina as early as P10.

CONCLUSION

These data represent the earliest recorded outer retinal light responses in the rodent. Photoreceptors are electrically functional and photoresponsive prior to eye opening, and much earlier than previously thought. Prior to eye opening, critical developmental processes occur that have been thought to be independent of outer retinal photic modulation. However, these data suggest light acting through outer-retinal photoreceptors has the potential to shape these critical developmental processes.

Accelerated and Improved Differentiation of Retinal Organoids from Pluripotent Stem Cells in Rotating-Wall Vessel Bioreactors

Pluripotent stem cells can be differentiated into 3D retinal organoids, with major cell types self-patterning into a polarized, laminated architecture. In static cultures, organoid development may be hindered by limitations in diffusion of oxygen and nutrients. Herein, we report a bioprocess using rotating-wall vessel (RWV) bioreactors to culture retinal organoids derived from mouse pluripotent stem cells. Organoids in RWV demonstrate enhanced proliferation, with well-defined morphology and improved differentiation of neurons including ganglion cells and S-cone photoreceptors. Furthermore, RWV organoids at day 25 (D25) reveal similar maturation and transcriptome profile as those at D32 in static culture, closely recapitulating spatiotemporal development of postnatal day 6 mouse retina in vivo. Interestingly, however, retinal organoids do not differentiate further under any in vitro condition tested here, suggesting additional requirements for functional maturation. Our studies demonstrate that bioreactors can accelerate and improve organoid growth and differentiation for modeling retinal disease and evaluation of therapies.

Pathophysiology, screening and treatment of ROP: A multi-disciplinary perspective

The population of infants at risk for retinopathy of prematurity (ROP) varies by world region; in countries with well developed neonatal intensive care services, the highest risk infants are those born at less than 28 weeks gestational age (GA) and less than 1 kg at birth, while, in regions where many aspects of neonatal intensive and ophthalmological care are not routinely available, more mature infants up to 2000 g at birth and 37 weeks GA are also at risk for severe ROP. Treatment options for both groups of patients include standard retinal laser photocoagulation or, more recently, intravitreal anti-VEGF drugs. In addition to detection and treatment of ROP, this review highlights new opportunities created by telemedicine, where screening and diagnosis of ROP in remote locations can be undertaken by non-ophthalmologists using digital fundus cameras. The ophthalmological care of the ROP infant is undertaken in the wider context of neonatal care and general wellbeing of the infant. Because of this context, this review takes a multi-disciplinary perspective with contributions from retinal vascular biologists, pediatric ophthalmologists, an epidemiologist and a neonatologist. This review highlights the latest insights regarding cellular and molecular mechanisms in the formation of the retinal vasculature in the human infant, pathogenesis of ROP, detection and treatment of severe ROP, the risks and benefits of anti-VEGF therapy, the identification of new therapies over the horizon, and the optimal neonatal care regimen for best ROP outcomes, and the benefits and pitfalls of telemedicine in the remote screening and diagnosis of ROP, all of which have the potential to improve ROP outcomes.

Cell-based therapeutic strategies for replacement and preservation in retinal degenerative diseases

Cell-based therapeutics offer diverse options for treating retinal degenerative diseases, such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP). AMD is characterized by both genetic and environmental risks factors, whereas RP is mainly a monogenic disorder. Though treatments exist for some patients with neovascular AMD, a majority of retinal degenerative patients have no effective therapeutics, thus indicating a need for universal therapies to target diverse patient populations. Two main cell-based mechanistic approaches are being tested in clinical trials. Replacement therapies utilize cell-derived retinal pigment epithelial (RPE) cells to supplant lost or defective host RPE cells. These cells are similar in morphology and function to native RPE cells and can potentially supplant the responsibilities of RPE in vivo. Preservation therapies utilize supportive cells to aid in visual function and photoreceptor preservation partially by neurotrophic mechanisms. The goal of preservation strategies is to halt or slow the progression of disease and maintain remaining visual function. A number of clinical trials are testing the safety of replacement and preservation cell therapies in patients; however, measures of efficacy will need to be further evaluated. In addition, a number of prevailing concerns with regards to the immune-related response, longevity, and functionality of the grafted cells will need to be addressed in future trials. This review will summarize the current status of cell-based preclinical and clinical studies with a focus on replacement and preservation strategies and the obstacles that remain regarding these types of treatments.

Understanding photoreceptor outer segment phagocytosis: use and utility of RPE cells in culture

RPE cells are the most actively phagocytic cells in the human body. In the eye, RPE cells face rod and cone photoreceptor outer segments at all times but contribute to shedding and clearance phagocytosis of distal outer segment tips only once a day. Analysis of RPE phagocytosis in situ has succeeded in identifying key players of the RPE phagocytic mechanism. Phagocytic processes comprise three distinct phases, recognition/binding, internalization, and digestion, each of which is regulated separately by phagocytes. Studies of phagocytosis by RPE cells in culture allow specifically analyzing and manipulating these distinct phases to identify their molecular mechanisms. Here, we compare similarities and differences of primary, immortalized, and stem cell-derived RPE cells in culture to RPE cells in situ with respect to phagocytic function. We discuss in particular potential pitfalls of RPE cell culture phagocytosis assays. Finally, we point out considerations for phagocytosis assay development for future studies.

Functional and molecular characterization of rod-like cells from retinal stem cells derived from the adult ciliary epithelium

In vitro generation of photoreceptors from stem cells is of great interest for the development of regenerative medicine approaches for patients affected by retinal degeneration and for high throughput drug screens for these diseases. In this study, we show unprecedented high percentages of rod-fated cells from retinal stem cells of the adult ciliary epithelium. Molecular characterization of rod-like cells demonstrates that they lose ciliary epithelial characteristics but acquire photoreceptor features. Rod maturation was evaluated at two levels: gene expression and electrophysiological functionality. Here we present a strong correlation between phototransduction protein expression and functionality of the cells in vitro. We demonstrate that in vitro generated rod-like cells express cGMP-gated channels that are gated by endogenous cGMP. We also identified voltage-gated channels necessary for rod maturation and viability. This level of analysis for the first time provides evidence that adult retinal stem cells can generate highly homogeneous rod-fated cells.

Melanopsin and mechanisms of non-visual ocular photoreception

In addition to rods and cones, the mammalian eye contains a third class of photoreceptor, the intrinsically photosensitive retinal ganglion cell (ipRGC). ipRGCs are heterogeneous irradiance-encoding neurons that primarily project to non-visual areas of the brain. Characteristics of ipRGC light responses differ significantly from those of rod and cone responses, including depolarization to light, slow on- and off-latencies, and relatively low light sensitivity. All ipRGCs use melanopsin (Opn4) as their photopigment. Melanopsin resembles invertebrate rhabdomeric photopigments more than vertebrate ciliary pigments and uses a G(q) signaling pathway, in contrast to the G(t) pathway used by rods and cones. ipRGCs can recycle chromophore in the absence of the retinal pigment epithelium and are highly resistant to vitamin A depletion. This suggests that melanopsin employs a bistable sequential photon absorption mechanism typical of rhabdomeric opsins.

The rat temporal association cortical area 2 (Te2) comprises two subdivisions that are visually responsive and develop independently

In this study, we have used the expression of non-phosphorylated neurofilament (NNF), a protein that exhibits differential areal and laminar neuronal patterning, to assess the chemoarchitectural organization of the rat temporal association cortex (Te). Since expression of NNF is associated with the latter stages of neuronal development, this enabled us to profile the hierarchical development of this region of the cortex. We also examined the expression of the protein Fos, the product of the immediate-early gene cFos, as a neuronal activity marker to determine which areas within this region are visually responsive. Our findings reveal the existence of two previously undescribed subdivisions within the dorsal and ventral domains of the rat temporal association cortical area 2 (Te2) which we have termed Te2d and Te2v, respectively. We also demonstrated the early maturation of the caudal region of Te2d while preceding the primary visual cortex. Within this region of the cortex, the Fos protein indicates that both subdivisions are visually responsive.

Phototransduction in mouse rods and cones

Phototransduction is the process by which light triggers an electrical signal in a photoreceptor cell. Image-forming vision in vertebrates is mediated by two types of photoreceptors: the rods and the cones. In this review, we provide a summary of the success in which the mouse has served as a vertebrate model for studying rod phototransduction, with respect to both the activation and termination steps. Cones are still not as well-understood as rods partly because it is difficult to work with mouse cones due to their scarcity and fragility. The situation may change, however.

Melanopsin changes in neonatal albino rat independent of rods and cones

Intrinsically photosensitive retinal ganglion cells employ the photopigment melanopsin and provide light information to brain areas responsible for the regulation of circadian rhythms. The expression of melanopsin is regulated by environmental illumination, but it remains to be clarified whether the rods and cones are involved. Here, we examined the influence of 5 days of constant light and dark conditions on melanopsin mRNA and protein expression in newborn albino rats, in which functional rods and cones have not yet been developed. We found that the melanopsin mRNA level was unaffected, whereas the melanopsin protein level was more than two-fold higher in the darkness-adapted group than in pups raised in constant light. In pups raised during 12 : 12 h light/dark cycles, the melanopsin protein level was significantly higher during the day than at night. Our findings indicate that melanopsin protein changes are independent of input from the rods and cones.

Light-evoked FOS induction within the suprachiasmatic nuclei (SCN) of melanopsin knockout (Opn4-/-) mice: a developmental study

The aims of this study were to address three related questions: (1) Do the photosensitive ganglion cells of the mouse convey light information to the suprachiasmatic nuclei (SCN) at P0? (2) Do the differentiating rods and cones contribute to light-evoked FOS induction within the murine SCN at P4? (3) How does light-evoked FOS induction within the SCN of melanopsin knockout (Opn4-/-) mice differ at P4 and P14? Our approaches took advantage of the published descriptions of murine ocular development, melanopsin knockout (Opn4-/-) mouse, and light-induced expression of FOS (the phosphoprotein product of immediate early gene c-fos) within the SCN as a marker of retinohypothalamic tract competence. Collectively, our results show that photosensitive melanopsin-dependent retinal ganglion cells provide light information to the murine SCN on the day of birth, and possibly beforehand, and that developing rods and cones fail to provide light information to the SCN during early postnatal life. On the basis of previous publications and data presented here, we suggest that at ages around P14 the rods and cones might be capable of fully compensating for the loss of melanopsin-photosensitive ganglion cells if exposure to light is of sufficiently long duration. These results are related to the broader context of recent findings and the potential role(s) of a neonatal photoreceptor.

Physiologic diversity and development of intrinsically photosensitive retinal ganglion cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs) mediate numerous nonvisual phenomena, including entrainment of the circadian clock to light-dark cycles, pupillary light responsiveness, and light-regulated hormone release. We have applied multielectrode array recording to characterize murine ipRGCs. We find that all ipRGC photosensitivity is melanopsin dependent. At least three populations of ipRGCs are present in the postnatal day 8 (P8) murine retina: slow onset, sensitive, fast off (type I); slow onset, insensitive, slow off (type II); and rapid onset, sensitive, very slow off (type III). Recordings from adult rd/rd retinas reveal cells comparable to postnatal types II and III. Recordings from early postnatal retinas demonstrate intrinsic light responses from P0. Early light responses are transient and insensitive but by P6 show increased photosensitivity and persistence. These results demonstrate that ipRGCs are the first light-sensitive cells in the retina and suggest previously unappreciated diversity in this cell population.

Rod sensitivity of neonatal mouse and rat

We have measured the sensitivity of rod photoreceptors isolated from overnight dark-adapted mice of age P12 (neonate) through P45 (adult) with suction-pipette recording. During this age period, the dark current increased roughly in direct proportion to the length of the rod outer segment. In the same period, the flash sensitivity of rods (reciprocal of the half-saturating flash intensity) increased by approximately 1.5-fold. This slight developmental change in sensitivity was not accentuated by dark adapting the animal for just 1 h or by increasing the ambient luminance by sixfold during the prior light exposure. The same small, age-dependent change in rod sensitivity was found with rat. After preincubation of the isolated retina with 9-cis-retinal, neonatal mouse rods showed the same sensitivity as adult rods, suggesting the presence of a small amount of free opsin being responsible for their lower sensitivity. The sensitivity of neonate rods could also be increased to the adult level by dark adapting the animal continuously for several days. By comparing the sensitivity of neonate rods in darkness to that of adult rods after light bleaches, we estimated that approximately 1% of rod opsin in neonatal mouse was devoid of chromophore even after overnight dark adaptation. Overall, we were unable to confirm a previous report that a 50-fold difference in rod sensitivity existed between neonatal and adult rats.

Melanopsin-dependent photoreception provides earliest light detection in the mammalian retina

BACKGROUND

The visual system is now known to be composed of image-forming and non-image-forming pathways. Photoreception for the image-forming pathway begins at the rods and cones, whereas that for the non-image-forming pathway also involves intrinsically photosensitive retinal ganglion cells (ipRGCs), which express the photopigment melanopsin. In the mouse retina, the rod and cone photoreceptors become light responsive from postnatal day 10 (P10); however, the development of photosensitivity of the ipRGCs remains largely unexplored.

RESULTS

Here, we provide direct physiological evidence that the ipRGCs are light responsive from birth (P0) and that this photosensitivity requires melanopsin expression. Interestingly, the number of ipRGCs at P0 is over five times that in the adult retina, reflecting an initial overproduction of melanopsin-expressing cells during development. Even at P0, the ipRGCs form functional connections with the suprachiasmatic nucleus, as assessed by light-induced Fos expression.

CONCLUSIONS

The findings suggest that the non-image-forming pathway is functional long before the mainstream image-forming pathway during development.

Developmental regulation of calcium-dependent feedback in Xenopus rods

The kinetics of activation and inactivation in the phototransduction pathway of developing Xenopus rods were studied. The gain of the activation steps in transduction (amplification) increased and photoresponses became more rapid as the rods matured from the larval to the adult stage. The time to peak was significantly shorter in adults (1.3 s) than tadpoles (2 s). Moreover, adult rods recovered twice as fast from saturating flashes than did larval rods without changes of the dominant time constant (2.5 s). Guanylate cyclase (GC) activity, determined using IBMX steps, increased in adult rods from ∼1.1 s⁻¹ to 3.7 s⁻¹ 5 s after a saturating flash delivering 6,000 photoisomerizations. In larval rods, it increased from 1.8 s⁻¹ to 4.0 s⁻¹ 9 s after an equivalent flash. However, the ratio of amplification to the measured dark phosphodiesterase activity was constant. Guanylate cyclase–activating protein (GCAP1) levels and normalized Na⁺/Ca²⁺, K⁺ exchanger currents were increased in adults compared with tadpoles. Together, these results are consistent with the acceleration of the recovery phase in adult rods via developmental regulation of calcium homeostasis. Despite these large changes, the single photon response amplitude was ∼0.6 pA throughout development. Reduction of calcium feedback with BAPTA increased adult single photon response amplitudes threefold and reduced its cutoff frequency to that observed with tadpole rods. Linear mathematical modeling suggests that calcium-dependent feedback can account for the observed differences in the power spectra of larval and adult rods. We conclude that larval Xenopus maximize sensitivity at the expense of slower response kinetics while adults maximize response kinetics at the expense of sensitivity.

Melanopsin containing retinal ganglion cells are light responsive from birth

Photoentrainment of the biological clock located in the suprachiasmatic nucleus (SCN) begins shortly after birth. Here we show using c-FOS immunoreactivity as a marker for neuronal activity that the melanopsin/PACAP containing retinal ganglion cells (RGCs) which project to the SCN as the retinohypothalamic tract (RHT) are responsive to light from birth. After postnatal day 12 where the classical photoreceptors become functional other RGCs and cells of the inner nuclear cell layer also respond to light. Light also induces c-FOS immunoreactivity in the retinorecipient SCN from the first postnatal day and accordingly PACAP immunoreactive fibres are visible in the SCN. The results indicate that the retina is light responsive before functional rods and cones and that the RHT is functional from birth supporting that photoentrainment of the biological clock begins shortly after birth.

Does constitutive phosphorylation protect against photoreceptor degeneration in Rpe65-/- mice?

Despite the presence of this virgin opsin, Rpe65-/- rods are behaving like dark-adapted rods. These results argue that opsin which has not been exposed to 11-cis retinal and is constitutively phosphorylated, does not generate the activity generally associated with the bleached apoprotein. However, increased light-independent activation of transducin (due to bleached opsin) could be demonstrated after the addition of exogenous 11-cis retinal. We hypothesize that free opsin in the Rpe65-/- rods does not cause degeneration of rods by constitutive activation of the phototransduction cascade; but rather rods may die due to other causes such as the impairment of RPE function due to excess unprocessed retinyl-esters in the RPE.

Structural and functional impairment of endocytic pathways by retinitis pigmentosa mutant rhodopsin-arrestin complexes

Retinitis pigmentosa (RP) is a clinically and genetically heterogeneous degenerative eye disease. Mutations at Arg135 of rhodopsin are associated with a severe form of autosomal dominant RP. This report presents evidence that Arg135 mutant rhodopsins (e.g., R135L, R135G, and R135W) are hyperphosphorylated and bind with high affinity to visual arrestin. Mutant rhodopsin recruits the cytosolic arrestin to the plasma membrane, and the rhodopsin-arrestin complex is internalized into the endocytic pathway. Furthermore, the rhodopsin-arrestin complexes alter the morphology of endosomal compartments and severely damage receptor-mediated endocytic functions. The biochemical and cellular defects of Arg135 mutant rhodopsins are distinct from those previously described for class I and class II RP mutations, and, hence, we propose that they be named class III. Impaired endocytic activity may underlie the pathogenesis of RP caused by class III rhodopsin mutations.

Recovery of the rod photoresponse in infant rats

We tested the hypothesis that the kinetics of recovery of the rod photoresponse differ between mature and immature rods. A paired flash paradigm was used. The effect of a test flash on the ERG a-wave response to a probe flash presented 60 to 2 s after the test flash was studied. The functions summarizing the interaction between the test and probe flash did not differ significantly between infants and adults if the stimuli were equated for estimated proportion of rhodopsin isomerized/rod/flash. The kinetics of rod cell recovery are likely the same in infants and adults.

Postnatal expression of the plasticity-related nerve growth factor-induced gene A (NGFI-A) protein in the superficial layers of the rat superior colliculus: Relation to N-methyl-d-aspartate receptor function

Immediate early gene expression in the CNS is induced by sensory stimulation and seems to be involved in long-term synaptic plasticity. We have used an immunohistochemical method to detect the nerve growth factor-induced gene A (NGFI-A) protein expression in the superficial layers of the rat superior colliculus during postnatal development. Our goal was to correlate the expression of this candidate plasticity protein with developmental events, especially the activity-dependent refinement of the retinocollicular and corticocollicular pathways. We have also investigated the N-methyl-D-aspartate (NMDA)-receptor dependence of the NGFI-A expression. Animals of various postnatal ages were used. Postnatal day (P) 12 and older animals were submitted to a protocol of dark adaptation followed by light stimulation. NGFI-A expression was never observed during the first 2 postnatal weeks. The first stained cells were observed at P15, 2 days after eye opening (P13). The highest number of stained cells was observed at the end of the third postnatal week (P22). Adult-like level of expression was reached at P30, since at this age, the number of stained cells was comparable to that found in adult rats (P90). Both P22 animals submitted to an acute treatment with MK-801 (i.p. injection) and adult animals submitted to chronic intracranial infusion of a MK-801 presented a clear decrease in the NGFI-A expression in response to light stimulation. These results suggest that the NGFI-A expression is dependent on the NMDA receptor activation, and the observed pattern of expression is in close agreement with previous descriptions of the changes in the NMDA receptor-mediated visual activity in the developing rat superior colliculus (SC). Our results suggest that the plasticity-related NGFI-A protein might play a role in the developmental plasticity of the superficial layers of the rat SC after eye opening.

Sodium and calcium currents shape action potentials in immature mouse inner hair cells

Before the onset of hearing at postnatal day 12, mouse inner hair cells (IHCs) produce spontaneous and evoked action potentials. These spikes are likely to induce neurotransmitter release onto auditory nerve fibres. Since immature IHCs express both alpha1D (Cav1.3) Ca2+ and Na+ currents that activate near the resting potential, we examined whether these two conductances are involved in shaping the action potentials. Both had extremely rapid activation kinetics, followed by fast and complete voltage-dependent inactivation for the Na+ current, and slower, partially Ca2+-dependent inactivation for the Ca2+ current. Only the Ca2+ current is necessary for spontaneous and induced action potentials, and 29 % of cells lacked a Na+ current. The Na+ current does, however, shorten the time to reach the action-potential threshold, whereas the Ca2+ current is mainly involved, together with the K+ currents, in determining the speed and size of the spikes. Both currents increased in size up to the end of the first postnatal week. After this, the Ca2+ current reduced to about 30 % of its maximum size and persisted in mature IHCs. The Na+ current was downregulated around the onset of hearing, when the spiking is also known to disappear. Although the Na+ current was observed as early as embryonic day 16.5, its role in action-potential generation was only evident from just after birth, when the resting membrane potential became sufficiently negative to remove a sizeable fraction of the inactivation (half inactivation was at -71 mV). The size of both currents was positively correlated with the developmental change in action-potential frequency.

Developmental acquisition of sensory transduction in hair cells of the mouse inner ear

Sensory transduction in hair cells requires assembly of membrane-bound transduction channels, extracellular tip-links and intracellular adaptation motors with sufficient precision to confer nanometer displacement sensitivity. Here we present evidence based on FM1-43 fluorescence, scanning electron microscopy and RT-PCR that these three essential elements are acquired concurrently between embryonic day 16 and 17, several days after the appearance of hair bundles, and that their acquisition coincides with the onset of mechanotransduction.

Developmental changes in the expression of potassium currents of embryonic, neonatal and mature mouse inner hair cells

Developmental changes in electrophysiological membrane properties of mouse cochlear inner hair cells (IHCs) were studied from just after terminal differentiation up to functional maturity. As early as embryonic day 14.5 (E14.5) newly differentiated IHCs express a very small outward K+ current that is largely insensitive to 4-aminopyridine (4-AP). One day later the inward rectifier, IK1, is first observed. These immature cells initially exhibit only slow graded voltage responses under current clamp. From E17.5 spontaneous action potentials occur. During the first week of postnatal development, the outward K+ current steadily increases in size and a progressively larger fraction of the current is sensitive to 4-AP. During the second postnatal week, the activation of the 4-AP-sensitive current, by now contributing about half of the outward K+ current, shifts in the hyperpolarizing direction. Together with an increase in size of IK1, this hyperpolarizes the cell, thus inhibiting the spontaneous spike activity, although spikes could still be evoked upon depolarizing current injection. Starting at about the onset of hearing (postnatal day 12, P12) immature IHCs make the final steps towards fully functional sensory receptors with fast graded voltage responses. This is achieved mainly by the expression of the large-conductance Ca2+-activated K+ current IK,f, but also of a current indistinguishable from the negatively activating IK,n previously described in mature outer hair cells (OHCs). The 4-AP-sensitive current continues to increase after the onset of hearing to form the major part of the mature delayed rectifier, IK,s. By P20 IHCs appear mature in terms of their complement of K+ conductances.

Rod sensitivity during Xenopus development

We have measured the sensitivity of rod photoreceptors from overnight-dark-adapted Xenopus laevis through developmental stages 46-66 into adulthood by using suction-pipette recording. The dark current increased gradually from approximately 5 pA at stage 46 to approximately 20 pA at stage 57, compared with an adult (metamorphosed) current of approximately 35 pA. This increase in dark current largely paralleled the progressive increase in length and diameter of the rod outer segment (ROS). Throughout stages 46-66, the dark current increased approximately linearly with ROS surface area. At stage 53, there was a steep (approximately 10-fold) increase in the rod flash sensitivity, accompanied by a steep increase in the time-to-peak of the half-saturated flash response. This covariance of sensitivity and time-to-peak suggested a change in the state of adaptation of rods at stage 53 and thereafter. When the isolated retina was preincubated with 11-cis-retinal, the flash sensitivity and the response time-to-peak of rods before stage 53 became similar to those at or after stage 53, suggesting that the presence of free opsin (i.e., visual pigment without chromophore) in rods before stage 53 was responsible for the adapted state (low sensitivity and short time-to-peak). By comparing the response sensitivity before stage 53 to the sensitivity at/after stage 53 measured from rods that had been subjected to various known bleaches, we estimated that 22-28% of rod opsin in stage 50-52 tadpoles (i.e., before stage 53) was devoid of chromophore despite overnight dark-adaptation. When continuously dark adapted for 7 d or longer, however, even tadpoles before stage 53 yielded rods with similar flash sensitivity and response time-to-peak as those of later-stage animals. In conclusion, it appears that chromophore regeneration is very slow in tadpoles before stage 53, but this regeneration becomes much more efficient at stage 53. A similar delay in the maturity of chromophore regeneration may partially underlie the low sensitivity of rods observed in newborn mammals, including human infants.

11-cis-retinal reduces constitutive opsin phosphorylation and improves quantum catch in retinoid-deficient mouse rod photoreceptors

Rpe65(-/-) mice produce minimal amounts of 11-cis-retinal, the ligand necessary for the formation of photosensitive visual pigments. Therefore, the apoprotein opsin in these animals has not been exposed to its normal ligand. The Rpe65(-/-) mice contain less than 0.1% of wild type levels of rhodopsin. Mass spectrometric analysis of opsin from Rpe65(-/-) mice revealed unusually high levels of phosphorylation in dark-adapted mice but no other structural alterations. Single flash and flicker electroretinograms (ERGs) from 1-month-old animals showed trace rod function but no cone response. B-wave kinetics of the single-flash ERG are comparable with those of dark-adapted wild type mice containing a full compliment of rhodopsin. Application (intraperitoneal injection) of 11-cis-retinal to Rpe65(-/-) mice increased the rod ERG signal, increased levels of rhodopsin, and decreased opsin phosphorylation. Therefore, exogenous 11-cis-retinal improves photoreceptor function by regenerating rhodopsin and removes constitutive opsin phosphorylation. Our results indicate that opsin, which has not been exposed to 11-cis-retinal, does not generate the activity generally associated with the bleached apoprotein.

Development of Light-activated Pupilloconstriction in Rats as Mediated by Normal and Transplanted Retinae

The relationship between the development of the pupilloconstriction response to changes in light levels and retinal maturation was studied in normal rats and rats that had received intracranial retinal transplants at birth. A pupillary response to light was first observed between postnatal days 7 and 9 in normal rats, and was typically of small amplitude and sluggish. By the time the eyelids first open, 2 weeks after birth, the pupillary response had improved to near adult levels. The inception of the pupillary response correlates with the first appearance of conventional synaptic contacts in the inner and outer plexiform layers of the retina, while improved responses correlate with maturation of photoreceptor outer segments and formation of synaptic ribbons in the inner plexiform layer. When embryonic retinae were transplanted to intracranial locations in newborn hosts and the transplants later illuminated as the host matured, the onset of a pupillary response to transplant illumination was delayed in proportion to the developmental disparity between the transplant and the host. The pattern of anatomical development in transplanted retinae was also similar, but delayed in time, compared to normal retinae. This indicates that the limiting factors for expression of light-activated pupilloconstriction exist within the retina, rather than being intrinsic to the central nuclei or to the output pathway subserving the response.

Activation of mislocalized opsin kills rod cells: a novel mechanism for rod cell death in retinal disease

Rod photoreceptors are highly compartmentalized sensory neurons that maintain strict ultrastructural and molecular polarity. Structural subdivisions include the outer segment, inner segment, cell body, and synaptic terminal. The visual pigment rhodopsin is found predominantly in membranes of the rod cell outer segment but becomes mislocalized, appearing throughout the plasma membrane of the cell in many retinal diseases and injuries. Currently, there is no known link between rhodopsin redistribution and rod cell death. We propose that activation of mislocalized rhodopsin kills rod cells by stimulating normally inaccessible signaling pathways. This hypothesis was tested in primary retinal cell cultures, which contain photoreceptors. In rod photoreceptors, opsin immunofluorescence occurred throughout the rod cell plasma membrane. Activation of this mislocalized opsin by photostimulation after formation of isorhodopsin or by incubation with beta-ionone (opsin agonist) killed 19-30% of rod cells. Rod cell death was apoptotic, as indicated by marked chromatin condensation and the requirement for caspase-3 activation. Rod cell death could be induced by forskolin (adenylate cyclase agonist), and conversely, beta-ionone-induced cell death could be blocked by cotreatment with SQ22536 (an adenylate cyclase inhibitor). Pertussis toxin (a G protein inhibitor) also blocked beta-ionone-induced cell death. The data support a mechanism by which activation of mislocalized opsin initiates apoptotic rod cell death through G protein stimulation of adenylate cyclase.

Confronting complexity: the interlink of phototransduction and retinoid metabolism in the vertebrate retina

Absorption of light by rhodopsin or cone pigments in photoreceptors triggers photoisomerization of their universal chromophore, 11-cis-retinal, to all-trans-retinal. This photoreaction is the initial step in phototransduction that ultimately leads to the sensation of vision. Currently, a great deal of effort is directed toward elucidating mechanisms that return photoreceptors to the dark-adapted state, and processes that restore rhodopsin and counterbalance the bleaching of rhodopsin. Most notably, enzymatic isomerization of all-trans-retinal to 11-cis-retinal, called the visual cycle (or more properly the retinoid cycle), is required for regeneration of these visual pigments. Regeneration begins in rods and cones when all-trans-retinal is reduced to all-trans-retinol. The process continues in adjacent retinal pigment epithelial cells (RPE), where a complex set of reactions converts all-trans-retinol to 11-cis-retinal. Although remarkable progress has been made over the past decade in understanding the phototransduction cascade, our understanding of the retinoid cycle remains rudimentary. The aim of this review is to summarize recent developments in our current understanding of the retinoid cycle at the molecular level, and to examine the relevance of these reactions to phototransduction.

Cell differentiation, synaptogenesis, and influence of the retinal pigment epithelium in a rat neonatal organotypic retina culture

This study was focused on the analysis of cell type differentiation and synaptogenesis as well as outer segment formation in an organotypic culture of the neonatal rat retina during a 6-14 day period of in vitro development. Moreover, the effects of the retinal pigment epithelium (RPE) on these processes were investigated. The in vitro development resulted in a retinal architecture and lamination comparable to that of in vivo retinas. The RPE influences the proper alignment of photoreceptors as well as the formation of the outer limiting membrane (OLM), but not processes of cell differentiation, synaptogenesis and inner retinal lamination.

Development of receptoral responses in pigmented and albino guinea-pigs (Cavia porcellus)

We describe the postnatal development of the electroretinogram (ERG) receptoral response in the guinea pig. In addition, the time course and nature of maturation was compared between albino and pigmented strains to consider the role that melanogenesis might have in this process. Electroretinograms were collected on groups of albino and pigmented animals from postnatal day (PD) PD1 to PD60. A-wave amplitudes and implicit times were extracted from filtered data (0-75 Hz). Receptoral components were modelled using the delayed gaussian model of Hood and Birch [1] fitted as an ensemble to the raw data. Guinea pigs show saturated amplitudes (RmP3) that are 50% of adult values at birth, these mature by PD12. Receptoral delay (t(d)) also undergoes some postnatal maturation, while phototransduction gain (log S) is adult-like at birth. Albino animals had significantly (p<0.05) larger RmP3 and log S across all ages. Guinea pigs have significant postnatal development in their receptoral response. Maturation of RmP3 implies a postnatal increase in rod outer segment length. Whereas the adult values of log S implies a mature phototransduction process at birth. We argue that the likely cause for the larger log S of albino eyes is compatible with theories of increased levels of internal light. Whereas the larger RmP3, even after allowing for increased light effectiveness, may reflect a lower ocular resistance in albino eyes due to their lower levels of melanin. Furthermore, decreased RmP3 and log S with age is observed in the pigmented group only and is consistent with increased ocular resistance due to melanin development in this strain.

Role of neurotrophin receptor TrkB in the maturation of rod photoreceptors and establishment of synaptic transmission to the inner retina

Brain-derived neurotrophic factor (BDNF) acts through TrkB, a receptor with kinase activity, and mitigates light-induced apoptosis in adult mouse rod photoreceptors. To determine whether TrkB signaling is necessary for rod development and function, we examined the retinas of mice lacking all isoforms of the TrkB receptor. Rod migration and differentiation occur in the mutant retina, but proceed at slower rates than in wild-type mice. In postnatal day 16 (P16) mutants, rod outer segment dimensions and rhodopsin content are comparable with those of photoreceptors in P12 wild type (WT). Quantitative analyses of the photoreceptor component in the electroretinogram (ERG) indicate that the gain and kinetics of the rod phototransduction signal in dark-adapted P16 mutant and P12 WT retinas are similar. In contrast to P12 WT, however, the ERG in mutant mice entirely lacks a b-wave, indicating a failure of signal transmission in the retinal rod pathway. In the inner retina of mutant mice, although cells appear anatomically and immunohistochemically normal, they fail to respond to prolonged stroboscopic illumination with the normal expression of c-fos. Absence of the b-wave and failure of c-fos expression, in view of anatomically normal inner retinal cells, suggest that lack of TrkB signaling causes a defect in synaptic signaling between rods and inner retinal cells. Retinal pigment epithelial cells and cells in the inner retina, including Müller, amacrine, and retinal ganglion cells, express the TrkB receptor, but rod photoreceptors do not. Moreover, inner retinal cells respond to exogenous BDNF with c-fos expression and extracellular signal-regulated kinase phosphorylation. Thus, interactions of rods with TrkB-expressing cells must be required for normal rod development.

Rod photoreceptor maturation does not vary with retinal eccentricity in mammalian retina

PURPOSE

Test the hypothesis that the development of mammalian rod outer segments (ROS) varies with retinal eccentricity.

METHODS

During the period of photoreceptor cell development, ROS lengths, opsin mRNA and (rhod)opsin were measured in central and peripheral retina of cows and pigmented rats. Published ROS length and/or rhodopsin data from albino rats, cows and monkeys were re-analyzed. Logistic growth curves were fitted to the newly obtained and published data. Within a species, growth in central and peripheral regions was compared.

RESULTS

The logistic growth curves fit all the data well and provide an excellent view of the developmental increases in ROS length, opsin mRNA and (rhod)opsin in each retinal region. Within a species, the growth curves for ROS length, opsin mRNA and (rhod)opsin concentration are superimposable. The age at which ROS length reaches 50% of its adult value is invariant with eccentricity. An exception to this pattern is the simian parafoveal ROS, which appears to have a delayed course of development.

CONCLUSIONS

The hypothesis is disproved. Unlike rod photoreceptor cell genesis, ROS development is invariant with retinal eccentricity. Primate parafoveal ROS appear to have a different pattern of development.

Maturation of visual receptive field properties in the rat superior colliculus

Visually responsive neurons were recorded in the superficial layers of rat superior colliculus from postnatal day 12 to 28. Receptive field properties such as size, type (ON, OFF, ON-OFF and motion sensitive) and direction selectivity were analyzed to disclose changes during maturation. Although some aspects of sensory properties are modified during development (latency, receptive field sizes, and proportions of receptive field types), a high level of sophistication is also present in young animals even before eyelid opening. For instance, direction selective and direction biased cells, which require complex synaptic relations, are already observed when the first light evoked responses emerge in the superior colliculus (P13), strongly suggesting that this property develops without visual experience. Furthermore, direction selectivity is present in the colliculus prior to the appearance of visually evoked activity in the cortex. This indicates that direction selectivity can not be attributable to incoming cortical afferents. This study provides the first direct evidence that, unlike the cat, the rat's cortico-tectal pathway is only weakly involved in the establishment of direction selectivity in collicular neurons.

Morphogenesis of the retinal pigment epithelium: toward understanding retinal degenerative diseases

The phenotype of an epithelial cell is defined by a unique combination of morphology, gene and protein expression, and protein localization. Results indicate that the terminal differentiation of the RPE cell can be described in part by changes in the polarity of its surface proteins alpha v beta 5 integrin, Na,K-ATPase, N-CAM, and EMMPRIN. Changes in protein/gene expression and protein localization in late stages of RPE development identify alpha v beta 5 integrin as a key player in RPE phagocytosis, and N-CAM and EMMPRIN as potentially important molecules in other RPE functions necessary for photoreceptor survival. By studying the trafficking of the later two proteins it is shown that entry into an apical or basolateral pathway in RPE cells cannot be predicted by the distribution of a given protein in other epithelial cells, and that this distribution may change through the course of RPE development. The mechanisms used by RPE and other epithelia to establish and maintain their specific polarity properties are fundamental to the formation and maintenance of their specific epithelial phenotype. The ability to therapeutically direct molecules incorporated into RPE by gene therapy into apical or basal surfaces requires an understanding of protein localization and expression. Furthermore, evidence is provided that assays capitalizing on changes in gene/protein expression and protein localization during the late stages of RPE development can prove a productive way of identifying proteins used by RPE for photoreceptor support. This approach can continue to be exploited to identify other proteins essential for the mission of the RPE cell, that may thus be likely candidates for participation in retinal degenerative disease.

Apical polarity of N-CAM and EMMPRIN in retinal pigment epithelium resulting from suppression of basolateral signal recognition

Retinal pigment epithelial (RPE) cells apically polarize proteins that are basolateral in other epithelia. This reversal may be generated by the association of RPE with photoreceptors and the interphotoreceptor matrix, postnatal expansion of the RPE apical surface, and/or changes in RPE sorting machinery. We compared two proteins exhibiting reversed, apical polarities in RPE cells, neural cell adhesion molecule (N-CAM; 140-kD isoform) and extracellular matrix metalloproteinase inducer (EMMPRIN), with the cognate apical marker, p75-neurotrophin receptor (p75-NTR). N-CAM and p75-NTR were apically localized from birth to adulthood, contrasting with a basolateral to apical switch of EMMPRIN in developing postnatal rat RPE. Morphometric analysis demonstrated that this switch cannot be attributed to expansion of the apical surface of maturing RPE because the basolateral membrane expanded proportionally, maintaining a 3:1 apical/basolateral ratio. Kinetic analysis of polarized surface delivery in MDCK and RPE-J cells showed that EMMPRIN has a basolateral signal in its cytoplasmic tail recognized by both cell lines. In contrast, the basolateral signal of N-CAM is recognized by MDCK cells but not RPE-J cells. Deletion of N-CAM's basolateral signal did not prevent its apical localization in vivo. The data demonstrate that the apical polarity of EMMPRIN and N-CAM in mature RPE results from suppressed decoding of specific basolateral signals resulting in randomized delivery to the cell surface.

Rod photoresponses in 6-week and 4-month-old human infants

Rod-only electroretinograms (ERGs) were recorded from 6-week and 4-month-old normal human infants. The leading edge of the rod a-wave was fitted with a model of the activation phase of phototransduction to provide estimates of S (a sensitivity parameter) and RmP3 (the maximum saturated photoreceptor response) at each of the investigated ages. Both S and RmP3 increased over the first postnatal months but followed different developmental time courses with S approaching adult-like values sooner than RmP3. The changes in S and RmP3 can be interpreted within the context of a model incorporating the combined effects of increased levels of rhodopsin and the changing structure of the rod outer segment during development.

Functional development of intrinsic properties in ganglion cells of the mammalian retina

Senosory neurons manifest pronounced changes in excitability during maturation, but the factors contributing to this ubiquitous developmental phenomenon are not well understood. To assess the contribution of intrinsic membrane properties to such changes in excitability, in the present study whole cell patch-clamp recordings were made from developing ganglion cells in the intact retina of postnatal rats. During a relatively brief developmental period (postnatal days P7-P27) ganglion cells exhibited pronounced changes in the discharge patterns generated by depolarizing current injections. The youngest cells (P7-P17) typically responded to maintained depolarizations with only a single spike or a rapidly adapting discharge pattern. In contrast, the predominant response mode of more mature cells (P21-P27) was a series of repetitive discharges that lasted for the duration of the depolarization period, and by P25 all cells responded in this manner. These functional changes characterized all three morphologically defined cell classes identified by intracellular labeling with Lucifer yellow. To determine if expression of the potassium current (Ia) and the kinetics of the Na-channel related to the increased excitability of developing ganglion cells described above, current- and voltage-clamp recordings were made from individual neurons. The different firing patterns manifested by developing retinal ganglion cells did not reflect the presence or absence of the Ia conductance, although cells expressing Ia tended to generate spikes of shorter duration. With maturation the speed of recovery from inactivation of the Na current increased markedly and this related to the increased excitability of developing ganglion cells. Neurons yielding only a single spike to maintained depolarization were characterized by the slowest speed of recovery; cells with rapidly adapting discharges showed a faster recovery and those capable of repetitive firing recovered fastest from Na-channel inactivation. It is suggested that these changes in intrinsic membrane properties may relate to the different functional roles subserved by ganglion cells during development.

Phagocytosis of rod outer segments by retinal pigment epithelial cells requires alpha(v)beta5 integrin for binding but not for internalization

Phagocytosis of shed photoreceptor rod outer segments (ROS) by the retinal pigment epithelium (RPE) is essential for retinal function. Here, we demonstrate that this process requires alpha(v)beta5 integrin, rather than alpha(v)beta3 integrin utilized by systemic macrophages. Although adult rat RPE expressed both alpha(v)beta3 and alpha(v)beta5 integrins, only alpha(v)beta3 was expressed at birth, when the retina is immature and phagocytosis is absent. Expression of alpha(v)beta5 was first detected in RPE at PN7 and reached adult levels at PN11, just before onset of phagocytic activity. Interestingly, alpha(v)beta5 localized in vivo to the apical plasma membrane, facing the photoreceptors, and to intracellular vesicles, whereas alpha(v)beta3 was expressed basolaterally. Using quantitative fluorimaging to assess in vitro uptake of fluorescent particles by human (ARPE-19) and rat (RPE-J) cell lines, alpha(v)beta5 function-blocking antibodies were shown to reduce phagocytosis by drastically decreasing (85%) binding of ROS but not of latex beads. In agreement with a role for alpha(v)beta5 in phagocytosis, immunofluorescence experiments demonstrated codistribution of alpha(v)beta5 integrin with internalized ROS. Control experiments showed that blocking alpha(v)beta3 function with antibodies did not inhibit ROS phagocytosis and that alpha(v)beta3 did not colocalize with phagocytosed ROS. Taken together, our results indicate that the RPE requires the integrin receptor alpha(v)beta5 specifically for the binding of ROS and that phagocytosis involves internalization of a ROS-alpha(v)beta5 complex. Alpha(v)beta5 integrin does not participate in phagocytosis by other phagocytic cells and is the first of the RPE receptors involved in ROS phagocytosis that may be specific for this process.

Behavioral estimates of absolute threshold in rat

Dark-adapted thresholds of albino and pigmented rats were estimated using behavioral methods. Albino and pigmented rats who had been water deprived learned to bar press for water reinforcement when a light stimulus was presented. Absolute threshold was defined to be the light intensity at which bar pressing behavior was significantly modified by the presence of the light stimulus. Albino rats had an average threshold of -5.23 log cd/m2 and the pigmented rats had a threshold of -5.0 log cd/m2. These values are close to -5.3 log cd/m2, the psychophysical threshold of human observers in the same apparatus. Consistent with our earlier electrophysiology, these behavioral experiments provide no evidence for an albino/pigmented sensitivity difference. Comparisons are made between behavioral and electrophysiological determinations of absolute threshold in albino and pigmented rats. Thresholds determined behaviorally agree remarkably well with those derived from visual evoked potentials.

PDGF and its receptors in the developing rodent retina and optic nerve

We have used in situ hybridization to visualize cells in the developing rat retina and optic nerve that express mRNAs encoding the A and B chains of platelet-derived growth factor (PDGF-A and PDGF-B), and the alpha and beta subunits of the PDGF receptor (PDGF-alpha R and PDGF-beta R). We have also visualized PDGF-A protein in these tissues by immunohistochemistry. In the retina, PDGF-A mRNA is present in pigment epithelial cells, ganglion neurons and a subset of amacrine neurons. PDGF-A transcripts accumulate in ganglion neurons during target innervation and in amacrine neurons around the time of eye opening, suggesting that PDGF-A expression in these cells may be regulated by target-derived signals or by electrical activity. In the mouse retina, PDGF-A immunoreactivity is present in the cell bodies, dendrites and proximal axons of ganglion neurons, and throughout the inner nuclear layer. PDGF-alpha R mRNA is expressed in the retina by astrocytes in the optic fibre layer and by a subset of cells in the inner nuclear layer that might be Muller glia or bipolar neurons. Taken together, our data suggest short-range paracrine interactions between PDGF-A and PDGF-alpha R, the ligand and its receptor being expressed in neighbouring layers of cells in the retina. In the optic nerve, PDGF-A immunoreactivity is present in astrocytes but apparently not in the retinal ganglion cell axons. PDGF-alpha R+ cells in the optic nerve first appear near the optic chiasm and subsequently spread to the retinal end of the nerve; these PDGF-alpha R+ cells are probably oligodendrocyte precursors (Pringle et al., 1992). RNA transcripts encoding PDGF-B and PDGF-beta R are expressed by cells of the hyaloid and mature vascular systems in the eye and optic nerve.

Appearance of cGMP-phosphodiesterase immunoreactivity parallels the morphological differentiation of photoreceptor outer segments in the rat retina

We have investigated by immunofluorescence the appearance of immunoreactive guanosine 3'-5' cyclic monophosphate phosphodiesterase (cGMP-PDE) during the postnatal development of the retina of the pigmented rat. We show that a sudden increase in immunoreactivity takes place during postnatal day five (P5), when rod outer segments begin to form; immunoreactivity develops rapidly in the following days. Labeling is restricted to the developing photoreceptor outer segments, sparing other retinal cells, as confirmed by electron microscopy immunocytochemistry. In addition, cGMP-PDE immunoreactivity follows a center-to-periphery gradient paralleling photoreceptor differentiation. It appears that cGMP-PDE is expressed when the photoreceptor subcellular compartments are already formed, and represents a specific marker of late photoreceptor differentiation. The appearance of cGMP-PDE during development is temporally correlated with the appearance of other proteins of the phototransduction machinery.

Temperature dependence of the light response in rat rods

1. The effects of temperature on the light responses of rat rods have been investigated over the range 17-40 degrees C. 2. The amplitude of the light-sensitive current increased with temperature with a mean temperature coefficient (Q10) of 2.47. 3. The amplitude of the Na(+)-Ca2+, K+ exchange current decreased with temperature when expressed as a fraction of the light-sensitive current, showing that the light-sensitive channel becomes less permeable to calcium as the temperature is raised. The time constant of relaxation of the exchange current was little affected by temperature. 4. The flash intensity required to give a half-saturating response increased with temperature with a mean Q10 of 1.68. 5. The responses to single photoisomerizations were determined from amplitude histograms of the responses to dim-flash trains. The amplitude of the response to a single photoisomerization decreased with temperature when expressed as a fraction of the light-sensitive current, but the change was not sufficient to account for the overall decrease in sensitivity. 6. The fraction of dim flashes that produced a photoisomerization decreased with temperature. This decrease in photon capture efficiency together with the decrease in the relative size of the single photon event fully accounts for the observed change in sensitivity. 7. The speed of the falling phase of the dim-flash response was accelerated more by warming than the rising phase, and it was therefore not possible to superimpose light responses at different temperatures by a simple change in time scale.

The photo-biochemical basis of infant colic: pineal intracellular calcium concentrations controlled by light, melatonin, and serotonin

Infant crying during the first 3 months of life exhibits a circadian rhythm with peak crying in the evening hours. Intracellular calcium ion within the pineal gland may be influenced by alternating light and dark, melatonin concentrations, and serotonin concentrations which both exhibit circadian rhythmicity. Differences in light by latitude and differences in the ontogenic development of melatonin and serotonin rhythmicity could combine to effect the pineal intracellular concentrations of calcium and result in high levels of infant crying called colic.