Changes in the numbers of optic nerve fibers during late prenatal and postnatal development in the albino rat

Age Related Response of Neonatal Rat Retinal Ganglion Cells to Reduced TrkB Signaling in vitro and in vivo

During development of retinofugal pathways there is naturally occurring cell death of at least 50% of retinal ganglion cells (RGCs). In rats, RGC death occurs over a protracted pre- and early postnatal period, the timing linked to the onset of axonal ingrowth into central visual targets. Gene expression studies suggest that developing RGCs switch from local to target-derived neurotrophic support during this innervation phase. Here we investigated, in vitro and in vivo, how RGC birthdate affects the timing of the transition from intra-retinal to target-derived neurotrophin dependence. RGCs were pre-labeled with 5-Bromo-2'-Deoxyuridine (BrdU) at embryonic (E) day 15 or 18. For in vitro studies, RGCs were purified from postnatal day 1 (P1) rat pups and cultured with or without: (i) brain derived neurotrophic factor (BDNF), (ii) blocking antibodies to BDNF and neurotrophin 4/5 (NT-4/5), or (iii) a tropomyosin receptor kinase B fusion protein (TrkB-Fc). RGC viability was quantified 24 and 48 h after plating. By 48 h, the survival of purified βIII-tubulin immunopositive E15 but not E18 RGCs was dependent on addition of BDNF to the culture medium. For E18 RGCs, in the absence of exogenous BDNF, addition of blocking antibodies or TrkB-Fc reduced RGC viability at both 24 and 48 h by 25-40%. While this decrease was not significant due to high variance, importantly, each blocking method also consistently reduced complex process expression in surviving RGCs. In vivo, survival of BrdU and Brn3a co-labeled E15 or E18 RGCs was quantified in rats 24 h after P1 or P5 injection into the eye or contralateral superior colliculus (SC) of BDNF and NT-4/5 antibodies, or serum vehicle. The density of E15 RGCs 24 h after P1 or P5 injection of blocking antibodies was reduced after SC but not intraretinal injection. Antibody injections into either site had little obvious impact on viability of the substantially smaller population of E18 RGCs. In summary, most early postnatal RGC death in the rat involves the elimination of early-born RGCs with their survival primarily dependent upon the availability of target derived BDNF during this time. In contrast, late-born RGC survival may be influenced by additional factors, suggesting an association between RGC birthdate and developmental death mechanisms.

Optic nerve head anatomy in myopia and glaucoma, including parapapillary zones alpha, beta, gamma and delta: Histology and clinical features

The optic nerve head can morphologically be differentiated into the optic disc with the lamina cribrosa as its basis, and the parapapillary region with zones alpha (irregular pigmentation due to irregularities of the retinal pigment epithelium (RPE) and peripheral location), beta zone (complete RPE loss while Bruch's membrane (BM) is present), gamma zone (absence of BM), and delta zone (elongated and thinned peripapillary scleral flange) within gamma zone and located at the peripapillary ring. Alpha zone is present in almost all eyes. Beta zone is associated with glaucoma and may develop due to a IOP rise-dependent parapapillary up-piling of RPE. Gamma zone may develop due to a shift of the non-enlarged BM opening (BMO) in moderate myopia, while in highly myopic eyes, the BMO enlarges and a circular gamma zone and delta zone develop. The ophthalmoscopic shape and size of the optic disc is markedly influenced by a myopic shift of BMO, usually into the temporal direction, leading to a BM overhanging into the intrapapillary compartment at the nasal disc border, a secondary lack of BM in the temporal parapapillary region (leading to gamma zone in non-highly myopic eyes), and an ocular optic nerve canal running obliquely from centrally posteriorly to nasally anteriorly. In highly myopic eyes (cut-off for high myopia at approximately -8 diopters or an axial length of 26.5 mm), the optic disc area enlarges, the lamina cribrosa thus enlarges in area and decreases in thickness, and the BMO increases, leading to a circular gamma zone and delta zone in highly myopic eyes.

Evolutionary concept of inflammatory response and stroke

The immune system plays an important role under both physiological and pathological conditions. Immune surveillance as well as defense and healing processes are crucial for the organism, but the immune system has a natural tendency to act aggressively when excessively stimulated. We may assume that the immune system is not designed to deal with severe conditions, such as polytrauma or severe stroke, because these are not compatible with life in the wilderness and evolution has no chance to act in such cases. These conditions are associated with exaggerated/deregulated inflammatory response, which may cause more damage than initial pathology. In this article, we would like to sketch a basic concept of the immune system-brain interactions from the evolutionary point of view and to discuss some implications related to stroke.

Complement Targets Newborn Retinal Ganglion Cells for Phagocytic Elimination by Microglia

Microglia play important roles in shaping the developing CNS, and at early stages they have been proposed to regulate progenitor proliferation, differentiation, and neuronal survival. However, these studies reveal contradictory outcomes, highlighting the complexity of these cell-cell interactions. Here, we investigate microglia function during embryonic mouse retina development, where only microglia, progenitors, and neurons are present. In both sexes, we determine that microglia primarily interact with retinal neurons and find that depletion of microglia via conditional KO of the Csf1 receptor results in increased density of retinal ganglion cells (RGCs). Pharmacological inhibition of microglia also results in an increase in RGCs, with no effect on retinal progenitor proliferation, RGC genesis, or apoptosis. We show that microglia in the embryonic retina are enriched for phagocytic markers and observe engulfment of nonapoptotic Brn3-labeled RGCs. We investigate the molecular pathways that can mediate cell engulfment by microglia and find selective downregulation of complement pathway components with microglia inhibition, and further show that C1q protein marks a subset of RGCs in the embryonic retina. KO of complement receptor 3 (CR3; Itgam), which is only expressed by microglia, results in increased RGC density, similar to what we observed after depletion or inhibition of microglia. Thus, our data suggest that microglia regulate neuron elimination in the embryonic mouse retina by complement-mediated phagocytosis of non-apoptotic newborn RGCs.SIGNIFICANCE STATEMENT Microglia are emerging as active and important participants in regulating neuron number in development, during adult neurogenesis, and following stem cell therapies. However, their role in these contexts and the mechanisms involved are not fully defined. Using a well-characterized in vivo system, we provide evidence that microglia regulate neuronal elimination by complement-mediated engulfment of nonapoptotic neurons. This work provides a significant advancement of the field by defining in vivo molecular mechanisms for microglia-mediated cell elimination. Our data add to a growing body of evidence that microglia are essential for proper nervous system development. In addition, we elucidate microglia function in the developing retina, which may shed light on microglia involvement in the context of retinal injury and disease.

Developmental retinal ganglion cell death and retinotopicity of the murine retinocollicular projection

During mammalian visual system development, retinal ganglion cells (RGCs) undergo extensive apoptotic death. In mouse retina, approximately 50% of RGCs present at birth (postnatal day 0; P0) die by P5, at a time when axons innervate central targets such as the superior colliculus (SC). We examined whether RGCs that make short-range axonal targeting errors within the contralateral SC are more likely to be eliminated during the peak period of RGC death (P1-P5), compared with RGCs initially making more accurate retinotopic connections. A small volume (2.3 nL) of the retrograde nucleophilic dye Hoechst 33342 was injected into the superficial left SC of anesthetized neonatal C57Bl/6J mice at P1 (n = 5) or P4 (n = 8), and the contralateral retina wholemounted 12 hr later. Retrogradely labelled healthy and dying (pyknotic) RGCs were identified by morphological criteria and counted. The percentage of pyknotic RGCs was analyzed in relation to distance from the area of highest density RGC labelling, presumed to represent the most topographically accurate population. As expected, pyknotic RGC density at P1 was significantly greater than P4 (p < 0.05). At P4, the density of healthy RGCs 500-750 µm away from the central region was significantly less, although this was not reflected in altered pyknotic rates. However, at P1 there was a trend (p = 0.08) for an increased proportion of pyknotic RGCs, specifically in temporal parts of the retina outside the densely labelled center. Overall, the lack of consistent association between short-range targeting errors and cell death suggests that most postnatal RGC loss is not directly related to topographic accuracy. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 51-60, 2018.

Understanding Glaucomatous Optic Neuropathy: The Synergy Between Clinical Observation and Investigation

Glaucoma is a complex disorder of aging defined by the death of retinal ganglion cells and remodeling of connective tissues at the optic nerve head. Intraocular pressure-induced axonal injury at the optic nerve head leads to apoptosis. Loss of retinal ganglion cells follows a slowly progressive sequence. Clinical features of the disease have suggested and corroborated pathological events. The death of retinal ganglion cells causes secondary loss of neurons in the brain, but only as a by-product of injury to the retinal ganglion cells. Although therapy to lower intraocular pressure is moderately effective, new treatments are being developed to alter the remodeling of ocular connective tissue, to interrupt the injury signal from axon to soma, and to upregulate a variety of survival mechanisms.

The Acquisition of Target Dependence by Developing Rat Retinal Ganglion Cells

Similar to neurons in the peripheral nervous system, immature CNS-derived RGCs become dependent on target-derived neurotrophic support as their axons reach termination sites in the brain. To study the factors that influence this developmental transition we took advantage of the fact that rat RGCs are born, and target innervation occurs, over a protracted period of time. Early-born RGCs have axons in the SC by birth (P0), whereas axons from late-born RGCs do not innervate the SC until P4-P5. Birth dating RGCs using EdU allowed us to identify RGCs (1) with axons still growing toward targets, (2) transitioning to target dependence, and (3) entirely dependent on target-derived support. Using laser-capture microdissection we isolated ∼34,000 EdU⁺ RGCs and analyzed transcript expression by custom qPCR array. Statistical analyses revealed a difference in gene expression profiles in actively growing RGCs compared with target-dependent RGCs, as well as in transitional versus target-dependent RGCs. Prior to innervation RGCs expressed high levels of BDNF and CNTFR α but lower levels of neurexin 1 mRNA. Analysis also revealed greater expression of transcripts for signaling molecules such as MAPK, Akt, CREB, and STAT. In a supporting in vitro study, purified birth-dated P1 RGCs were cultured for 24-48 h with or without BDNF; lack of BDNF resulted in significant loss of early-born but not late-born RGCs. In summary, we identified several important changes in RGC signaling that may form the basis for the switch from target independence to dependence.

Changes in expression of Class 3 Semaphorins and their receptors during development of the rat retina and superior colliculus

Background

Members of the Semaphorin 3 family (Sema3s) influence the development of the central nervous system, and some are implicated in regulating aspects of visual system development. However, we lack information about the timing of expression of the Sema3s with respect to different developmental epochs in the mammalian visual system. In this time-course study in the rat, we document for the first time changes in the expression of RNAs for the majority of Class 3 Semaphorins (Sema3s) and their receptor components during the development of the rat retina and superior colliculus (SC).

Results

During retinal development, transcript levels changed for all of the Sema3s examined, as well as Nrp2, Plxna2, Plxna3, and Plxna4a. In the SC there were also changes in transcript levels for all Sema3s tested, as well as Nrp1, Nrp2, Plxna1, Plxna2, Plxna3, and Plxna4a. These changes correlate with well-established epochs, and our data suggest that the Sema3s could influence retinal ganglion cell (RGC) apoptosis, patterning and connectivity in the maturing retina and SC, and perhaps guidance of RGC and cortical axons in the SC. Functionally we found that SEMA3A, SEMA3C, SEMA3E, and SEMA3F proteins collapsed purified postnatal day 1 RGC growth cones in vitro. Significantly this is a developmental stage when RGCs are growing into and within the SC and are exposed to Sema3 ligands.

Conclusion

These new data describing the overall temporal regulation of Sema3 expression in the rat retina and SC provide a platform for further work characterising the functional impact of these proteins on the development and maturation of mammalian visual pathways.

Modeling activity and target-dependent developmental cell death of mouse retinal ganglion cells ex vivo

Programmed cell death is widespread during the development of the central nervous system and serves multiple purposes including the establishment of neural connections. In the mouse retina a substantial reduction of retinal ganglion cells (RGCs) occurs during the first postnatal week, coinciding with the formation of retinotopic maps in the superior colliculus (SC). We previously established a retino-collicular culture preparation which recapitulates the progressive topographic ordering of RGC projections during early post-natal life. Here, we questioned whether this model could also be suitable to examine the mechanisms underlying developmental cell death of RGCs. Brn3a was used as a marker of the RGCs. A developmental decline in the number of Brn3a-immunolabelled neurons was found in the retinal explant with a timing that paralleled that observed in vivo. In contrast, the density of photoreceptors or of starburst amacrine cells increased, mimicking the evolution of these cell populations in vivo. Blockade of neural activity with tetrodotoxin increased the number of surviving Brn3a-labelled neurons in the retinal explant, as did the increase in target availability when one retinal explant was confronted with 2 or 4 collicular slices. Thus, this ex vivo model reproduces the developmental reduction of RGCs and recapitulates its regulation by neural activity and target availability. It therefore offers a simple way to analyze developmental cell death in this classic system. Using this model, we show that ephrin-A signaling does not participate to the regulation of the Brn3a population size in the retina, indicating that eprhin-A-mediated elimination of exuberant projections does not involve developmental cell death.

In vivo evaluation of retinal and callosal projections in early postnatal development and plasticity using manganese-enhanced MRI and diffusion tensor imaging

The rodents are an excellent model for understanding the development and plasticity of the visual system. In this study, we explored the feasibility of Mn-enhanced MRI (MEMRI) and diffusion tensor imaging (DTI) at 7 T for in vivo and longitudinal assessments of the retinal and callosal pathways in normal neonatal rodent brains and after early postnatal visual impairments. Along the retinal pathways, unilateral intravitreal Mn2+ injection resulted in Mn2+ uptake and transport in normal neonatal visual brains at postnatal days (P) 1, 5 and 10 with faster Mn2+ clearance than the adult brains at P60. The reorganization of retinocollicular projections was also detected by significant Mn2+ enhancement by 2%-10% in the ipsilateral superior colliculus (SC) of normal neonatal rats, normal adult mice and adult rats after neonatal monocular enucleation (ME) but not in normal adult rats or adult rats after monocular deprivation (MD). DTI showed a significantly higher fractional anisotropy (FA) by 21% in the optic nerve projected from the remaining eye of ME rats compared to normal rats at 6 weeks old, likely as a result of the retention of axons from the ipsilaterally uncrossed retinal ganglion cells, whereas the anterior and posterior retinal pathways projected from the enucleated or deprived eyes possessed lower FA after neonatal binocular enucleation (BE), ME and MD by 22%-56%, 18%-46% and 11%-15% respectively compared to normal rats, indicative of neurodegeneration or immaturity of white matter tracts. Along the visual callosal pathways, intracortical Mn2+ injection to the visual cortex of BE rats enhanced a larger projection volume by about 74% in the V1/V2 transition zone of the contralateral hemisphere compared to normal rats, without apparent DTI parametric changes in the splenium of corpus callosum. This suggested an adaptive change in interhemispheric connections and spatial specificity in the visual cortex upon early blindness. The results of this study may help determine the mechanisms of axonal uptake and transport, microstructural reorganization and functional activities in the living visual brains during development, diseases, plasticity and early interventions in a global and longitudinal setting.

Fatty acid amide hydrolase expression during retinal postnatal development in rats

The endocannabinoid (eCB) system is thought to participate in developmental processes in the CNS. The rodent retina represents a valuable model to study CNS development because it contains well-identified cell types with established developmental timelines. The distribution of cannabinoid receptor type 1 (CB1R) was recently revealed in the developing retina; however, the expression patterns of other elements of this system remain unknown. In this study, we investigated the expression pattern of the degradative enzyme fatty acid amide hydrolase (FAAH), a key regulator of the eCB system, in the rat retina during postnatal development. To identify the cells expressing the enzyme, co-stainings were carried out for FAAH and retinal cell type markers. FAAH was expressed at postnatal day (P) 1 in ganglion and cholinergic amacrine cells. In the course of development, it appeared in cones, horizontal, and bipolar cells. For most cell types (horizontal, cholinergic amacrine cells, and cone bipolar cells), FAAH was transiently expressed, suggesting an important redistribution of the enzyme during postnatal development and thus a potential role of the eCB system in developmental processes. Our results also indicated that, in the adult retina, FAAH is expressed in cones, rod bipolar cells, and some retinal ganglion cells. The presence of FAAH in adult animals supports the hypothesis that the eCB system is involved in retinal functions. Overall these results indicate that, as shown in other structures of the brain, the eCB system could play an instrumental role in the development and function of the retina.

Early c-Jun N-terminal kinase-dependent phosphorylation of activating transcription factor-2 is associated with degeneration of retinal ganglion cells

Neuron death due to deprivation of target-derived neurotrophic factors depends on protein synthesis regulated by transcription factor activity. We investigated the content and phosphorylation of activating transcription factor 2 (ATF-2) in axon-damaged retinal ganglion cells of neonatal rats. In the retina of neonatal rats, the ATF-2 protein is predominantly located in the nucleus of the ganglion cells. A gradual loss of the immunoreactivity for ATF-2 occurred after explantation. ATF-2 is phosphorylated early after explantation, with a peak within 3 hours, preceding the peak of cell death that occurs at 18 hours. Both the phosphorylation of ATF-2 and ganglion cell death were blocked by treatment with an inhibitor of c-Jun N-terminal kinase (JNK), whereas an inhibitor of p38 reduced only slightly the rate of ganglion cell death with no effect upon phosphorylation of ATF-2. Inhibitors of phosphatidyl inositol 3 kinase (PI-3K), protein kinase C (PKC) or extracellular regulated kinase (ERK) had no effect. Finally, the inhibitor of JNK blocked the upregulation of both c-Jun and Hrk in the GCL after retinal explantation. The data show that phosphorylation of ATF-2 by JNK is associated with retinal ganglion cell death after axon damage.

Protein kinases JAK and ERK mediate protective effect of interleukin-2 upon ganglion cells of the developing rat retina

Interleukin-2 (IL-2), a prototypical pro-inflammatory cytokine firstly related to T cells differentiation, exerts pleiotrophic functions in several areas of the central nervous system. Previously we had described the neurotrophic roles of this interleukin upon retinal neurons. Therefore, the aim of this work was to investigate the signaling pathways involved in the neuroprotective effect of IL-2 on axotomized RGC. Herein we demonstrated that at postnatal day 2 IL-2 receptor α subunit (IL-2Rα) is expressed in inner plexiform layer, retinal ganglion cells layer and retinal nerve fibers layer. Moreover, using a model of organotypic retinal explants and rhodamine dextran retrograde labeling for specifically quantify RGC, we showed that IL-2 increased the survival of axotomized RGC after 2 (85.43±5.43%) and 5 (50.23%±5.32) days in vitro. Western blot analysis demonstrated that IL-2 treatment increased the phosphorilation of both extracellular signal-regulated kinases (ERK)1/2 and AKT (~two fold). However, its neuroprotective effect upon RGC was dependent of Janus kinase (JAK) and ERK1/2 activity but not of AKT activity. Taken together our results showed that the IL-2 neuroprotective action upon RGC in vitro is mediated by JAK and ERK1/2 activation.

Trophic factors GDNF and BDNF improve function of retinal sheet transplants

The aim of this study was to compare glial-derived neurotrophic factor (GDNF) treatment with brain-derived neurotrophic factor (BDNF) treatment of retinal transplants on restoration of visual responses in the superior colliculus (SC) of the S334ter line 3 rat model of rapid retinal degeneration (RD). RD rats (age 4-6 weeks) received subretinal transplants of intact sheets of fetal retina expressing the marker human placental alkaline phosphatase (hPAP). Experimental groups included: (1) untreated retinal sheet transplants, (2) GDNF-treated transplants, (3) BDNF-treated transplants, (4) none surgical, age-matched RD rats, (5) sham surgery RD controls, (6) progenitor cortex transplant RD controls, and (7) normal pigmented rat controls. At 2-8 months after transplantation, multi-unit visual responses were recorded from the SC using a 40 ms full-field stimulus (-5.9 to +1 log cd/m(2)) after overnight dark-adaptation. Responses were analyzed for light thresholds, spike counts, response latencies, and location within the SC. Transplants were grouped into laminated or rosetted (more disorganized) transplants based on histological analysis. Visual stimulation of control RD rats evoked no responses. In RD rats with retinal transplants, a small area of the SC corresponding to the position of the transplant in the host retina, responded to light stimulation between -4.5 and -0.08 log cd/m(2), whereas the light threshold of normal rats was at or below -5 log cd/m(2) all over the SC. Overall, responses in the SC in rats with laminated transplants had lower response thresholds and were distributed over a wider area than rats with rosetted transplants. BDNF treatment improved responses (spike counts, light thresholds and responsive areas) of rats with laminated transplants whereas GDNF treatment improved responses from rats with both laminated and rosetted (more disorganized) transplants. In conclusion, treatment of retinal transplants with GDNF and BDNF improved the restoration of visual responses in RD rats; and GDNF appears to exert greater overall restoration than BDNF.

Immunotoxin-induced ablation of melanopsin retinal ganglion cells in a non-murine mammalian model

In mammals, non-image-forming visual functions, including circadian photoentrainment and the pupillary light reflex, are thought to be mediated by the combination of rods, cones, and the melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs). Although several genetic models have been developed to clarify the individual roles of the rod, cone, and ipRGC systems in mediating non-image visual function, assessing the in vivo role(s) of the ipRGCs has been complicated by the possibility of ontogenetic issues in these genetically modified animal models. In the present study, we describe the development and validation of an immunotoxin that specifically targets the ipRGC population in the mature mammalian retina. This ipRGC immunotoxin, consisting of saporin conjugated to a melanopsin polyclonal antibody, was evaluated with respect to its effectiveness and specificity in depleting the ipRGC population in the fully developed rat retina. The results showed that the ipRGC toxin rapidly and permanently depleted approximately 70% of the ipRGC population, without inducing appreciable changes in the cell number or morphology of any of the non-melanopsin-containing retinal cell populations investigated. These findings suggest that the newly developed ipRGC immunotoxin provides a potent method for achieving relatively rapid, permanent, and selective depletion of the ipRGC population in a non-murine model system. The development of this ipRGC-ablation method is the next step in elucidating the role of ipRGCs in mediating non-visual and circadian light-resetting responses in a wide range of non-murine mammalian models.

Identification of neural programmed cell death through the detection of DNA fragmentation in situ and by PCR

Programmed cell death is a fundamental process for the development and somatic maintenance of organisms. This unit describes methods for visualizing both dying cells in situ and for detection of nucleosomal ladders. A description of various current detection strategies is provided, as well as support protocols for preparing positive and negative controls and for preparing genomic DNA.

Kainic acid intraocular injections during the postnatal critical period induce plastic changes in the visual system

Changes in the retino-collicular projection and in the number of optic nerve (ON) axons in adult rats were analyzed after partial loss of retinal ganglion cells (RGCs), induced by intravitreal injections of kainic acid (KA) on postnatal days 2-3 (P2-P3) or 10-12 (P10-P12). KA injected at P2-P3 decreased the volume of the adult contralateral superior colliculus (SC) and the density of the retino-collicular contralateral projection, but maintained the neonatal pattern in the ipsilateral projection from the un-injected eye. ON axon number was significantly increased in the un-injected eye but decreased in the KA-injected eye. Thus, restriction of the ipsilateral retino-collicular projection and RGC death in the un-injected eye are modified by KA at P2-P3, during the postnatal critical period, but not at P10-P12, after it is over. We suggest that, in the SC contralateral to the KA-injected eye, the disappearance of axon terminals belonging to RGC killed by KA would decrease competition between ipsilateral and contralateral terminals, thus contributing to maintaining the neonatal pattern in the ipsilateral retino-collicular projection. The reduction in RGC death in the un-injected eye could also be related to the disappearance of RGC terminals in the contralateral SC, which would have increased neurotrophic factor availability.

Identification of neural programmed cell death through the detection of DNA fragmentation in situ and by PCR

A universal feature in the development of multicellular organisms is a physiological form of cell death called programmed cell death (PCD). A subset of PCD is apoptosis, which is defined by characteristic morphological changes and genomic DNA fragmentation producing what are referred to as nucleosomal ladders. To understand how PCD operates in a developing tissue or in a tissue following an experimental procedure, dying cells must be identified in relation to their surviving neighbors. One way to accomplish this is to visualize fragmented DNA in situ, in conjunction with gel electrophoresis of isolated DNA to visualize the nucleosomal ladders associated with apoptosis. Two approaches are presented in this unit: in situ end-labeling plus (ISEL+), a technique to identify dying cells in tissue sections or cell cultures of central nervous system (CNS) tissue (optimized for embryonic samples); and the use of ligation-mediated polymerase chain reaction (LMPCR) to identify nucleosomal ladders from intact tissues. Also included are procedures for preparing thymocyte cell cultures for use as controls in the ISEL+ procedure and for isolating genomic DNA for LMPCR.

Correlation of axon size and myelin occupancy in rats prenatally exposed to methamphetamine

The abuse of methamphetamine (MA) and other psychostimulants is a social and medical problem. In particular, the use of these drugs by pregnant women results in an increased number of children exposed prenatally to psychostimulants. Our previous work has demonstrated that prenatal exposure to MA affects the normal development of the rat visual system due to alterations of biochemical mechanisms and oxidative stress. It was also demonstrated that prenatal exposure to MA affects the dopaminergic system of the rat retina and optic nerve (ON) myelination. The present work was conducted to evaluate the effects of prenatal exposure to MA on the development of the ON in terms of axon growth and the myelin sheath. Pregnant female rats were given 5 mg/kg/day MA, subcutaneously (s.c.), in 0.9% saline from gestational day (GD) 8 to 22. The pair-fed control group was injected s.c. with an isovolumetric dose of 0.9% saline. Qualitative analysis was performed using representative electron ultramicrographs. Quantitative analysis was performed at an electron microscopic level on ON cross sections; parameters measured included myelinated/unmyelinated ratio, outer axon mean area, inner axon mean area, myelin mean area, myelin occupancy and distribution of axons by size. The ON of prenatally MA-exposed rats presented a higher rate of deformed axons and slighter lamellar separation. At PND 21, the average outer axon area of MA-treated males was significantly reduced. The average inner axon area only showed a significant difference between MA and control males for axons with an area of less than 0.3 microm(2). The average myelin area of MA-treated males was significantly reduced, and in MA-treated females was only significantly reduced in axons with an area of less than 0.3 microm(2). The percentage of myelin occupancy was significantly affected in MA-treated males, and in MA-treated females in the group of axons with an area of more than 0.3 microm(2). At PND 14 no significant differences were found between MA and control groups. The spectrum of ON myelinated axon size of MA-treated animals was shifted to the left at PND 14 and PND 21 for both genders. These results are in agreement with previous animal studies of prenatal and perinatal exposure to drugs of abuse. Taken together, these data indicate that the ON is vulnerable to early exposure to MA which causes developmental changes and may interfere with the functioning of the visual system.

Remodelling of early axonal projections through the selective elimination of neurons and long axon collaterals

Studies using neuroanatomical techniques have shown that the connections characteristic of the mature vertebrate brain are brought about by a considerable refinement of the projections initially established during development. The selective loss of neurons and long axon collaterals plays a major role in this remodeling process as illustrated in the development of the retina and cortex of the rat. In the retina, two-thirds of the initial population of ganglion cells (RGCs) die early. This loss serves to remove selectively RGCs that make erroneous axonal projections, including those which project to an incorrect target, to an inappropriate part of a correct target, or to the wrong side of the brain. Studies using the sodium channel blocker, tetrodotoxin, suggest that in rats the selective elimination of erroneously projecting RGCs is based, in part, on patterns of impulse activity. In the cortex a different mechanism is illustrated. All neocortical areas initially give rise to callosal and pyramidal tract axons but through a process of selective collateral elimination not involving cell death these projections assume the limited distributions seen in adult rats. Manipulations resulting in the maintenance of such long collaterals suggest that their removal is functionally and locally determined. In contrast to error elimination, this phenomenon of collateral elimination may be a developmental strategy for generating connectional diversity while limiting the amount of information required for the regional specification of the cortex.

Emergence of topography in the developing hamster retinocollicular projection: axial differences and the role of cell death

The precise ordering of the hamster retinocollicular projection is established over the first two postnatal weeks, coincident with developmental cell death. We have used quantitative retrograde labelling to define topographic precision in the early postnatal projection, to describe its refinement and to assess the contribution played by selective retinal ganglion cell death. The hamster's short gestation period allows the investigation of events occurring prenatally in other rodents. Discrete injections of fluorescent beads in the superior colliculus followed by isodensity contour analysis of labelled retinal cells reveals a dramatic decrease in the extent of retina labelled between postnatal days 2, 6 and 12 (P2, P6, P12): the 20% contour encloses 38.3%, 8.3% and 1.8% of the retina at these ages. Paired injections of two different tracers at variable rostrocaudal (R-C) separations at P2 produced complete overlap of label even when injections were separated by over 1 mm. This was not true for paired mediolateral injections at P2 that were separated by more than 500 microm. Analysis of the segregation of the two tracers ('nearest-neighbour analysis') shows topography improving with age so that by P12 injections separated rostrocaudally by more than 500 microm produced no overlap in the retina. To examine the contribution of selective ganglion cell death to topographic refinement, animals given paired R-C injections at P2 were allowed to survive until P12. Nearest-neighbour analysis reveals significantly more order in the P2-P12 retinae than after overnight survival. Thus, selective cell death plays a small but appreciable role in correction of topographical errors.

The postnatal development of the optic nerve of a reptile (Vipera aspis): A quantitative ultrastructural study

The number of axons in the optic nerve of the ovoviviparous reptile Vipera aspis was estimated from electron micrographs taken during the first 5 weeks of postnatal life. One to two days after birth, the optic nerve contains about 170,000 fibres, of which about 9% are myelinated. At the end of the fifth postnatal week, the number of optic fibres has fallen to about 100,000, of which about 42% are myelinated. This fibre loss continues after the fifth postnatal week, since in the adult viper the nerve contains about 60,000 fibres, of which 85% are myelinated; overall, about 65% of the optic nerve fibres present at birth disappear before the number of axons stabilises at the adult level. This study shows, for the first time, that the mode of development of the visual axons of reptiles is not that of anamniote vertebrates but similar to that of birds and mammals.

Neurogenesis and cell death in the ganglion cell layer of vertebrate retina

The correct formation of all central nervous system tissues depends on the proper balance of neurogenesis and developmental cell death. A model system for studying these programs is the ganglion cell layer (GCL) of the vertebrate retina because of its simple and well-described structure and amenability to experimental manipulations. The GCL contains approximately equal numbers of ganglion cells and displaced amacrine cells. Ganglion cells are the first or among the first cells born in the retina in all the studied vertebrates. Neurogenesis and cell death have been studied extensively in the GCL of various amniotes (rodents, chicks, and monkeys) and anamniotes (fish and frogs), and the two processes highlight developmental differences between the groups. In amniotes, neurogenesis occurs during a defined period prior to birth/hatch or the opening of the eyes, whereas in anamniotes, neurogenesis extends past hatching into adulthood-sometimes for years. Roughly half of GCL neurons die during development in amniotes, whereas developmental cell death does not occur in the GCL neurons of anamniotes. This review discusses the spatial and temporal patterns of neurogenesis, cell death, and possible explanation of cell death in the GCL. It also examines markers widely used to distinguish between ganglion cells and displaced amacrine cells, and methods employed to birth date neurons.

Developmental changes in the fibre population of the optic nerve follow an avian/mammalian-like pattern in the turtle Mauremys leprosa

The changes in the axon and growth cone numbers in the optic nerve of the freshwater turtle Mauremys leprosa were studied by electron microscopy from the embryonic day 14 (E14) to E80, when the animals normally hatch, and from the first postnatal day (P0) to adulthood (5 years on). At E16, the first axons appeared in the optic nerve and were added slowly until E21. From E21, the fibre number increased rapidly, peaking at E34 (570,000 fibres). Thereafter, the axon number decreased sharply, and from E47 declined steadily until reaching the mature number (about 330,000). These observations indicated that during development of the retina there was an overproduction and later elimination of retinal ganglion cells. Growth cones were first observed in the optic nerve at as early as E16. Their number increased rapidly until E21 and continued to be high through E23 and E26. After E26, the number declined steeply and by E40 the optic nerve was devoid of growth cones. These results indicated that differentiation of the retinal ganglion cells occurred during the first half of the embryonic life. To examine the correlation between the loss of the fibres from the optic nerve and loss of the parent retinal ganglion cells, retinal sections were processed with the TUNEL technique. Apoptotic nuclei were detected in the ganglion cell layer throughout the period of loss of the optic fibres. Our results showed that the time course of the numbers of the fibres in the developing turtle optic nerve was similar to those found in birds and mammals.

Myelination changes in the rat optic nerve after prenatal exposure to methamphetamine

The use of psychostimulants during adolescence and early adult life has increased in recent years. It is known that these substances affect the sensory systems, and the optic nerve has been shown to be a target tissue. This work was conducted to evaluate the effects of prenatal exposure to methamphetamine (MA) on the developmental pattern of the rat optic nerve. Pregnant female rats were given 5 mg/kg body weight/day MA, s.c., in 0.9% saline from gestational days 8 to 22. The control group was injected with an isovolumetric dose of 0.9% saline. Animal model parameters, such as gestational body weight evolution, food intake and pups parameters were registered. The offspring were sacrificed at postnatal days (PND) 7, 14 and 21. Morphometric analyses were performed at light and electron microscopic levels on optic nerve cross sections; parameters measured included optic nerve diameter and area, axonal density, total number of axons and myelin thickness. Myelin basic protein (MBP) was measured by western blotting in optic nerve samples at PND14 and PND21. The animal model parameters, such as maternal and pup weight, showed no significant differences between MA and control groups. Optic nerve diameter was smaller at PND7 in the male MA group and in both male and female MA groups at PND21. The mean cross-sectional area was smaller at PND14 in the male MA group and in both male and female groups at PND21. The total number of myelinated axons did not vary between groups at any of the studied ages. The myelin thickness of the axons in MA-treated females was thinner when compared with the respective control group at PND21. No other differences were found concerning myelin thickness. There was a reduction of MBP protein expression in MA-injected females at PND14 and PND21. The combined results suggest that prenatal exposure to MA affects the myelination process.

Cell birth and death in the mouse retinal ganglion cell layer

Here we describe quantitatively the birth and death of the two separate populations of neurons, ganglion cells and displaced amacrine cells, in the mouse retinal ganglion cell layer (GCL). The two cell types, which are roughly equally numerous, were distinguished pre- and postnatally by labeling the ganglion cells retrogradely with fluorescent dye. Embryos were labeled cumulatively with bromodeoxyuridine (BrdU) delivered by an osmotic minipump implanted in the mother; cell birth dates were established as having occurred before or after pump implantation. Early cohorts (GCL cells born before embryonic day [E] 11.8 and E12.8) were 98+/-1.1% and 99+/-0.2% ganglion cells (mean+/-SEM), respectively, and a late cohort (born after E15.8) was 97+/-1.2% displaced amacrines. Thus birth date was a strong predictor of a GCL cell's ultimate identity. Cell death in each cohort was estimated by counting cells at different time points (soon after the cohort was produced and later) and subtracting the later from the earlier number. This method avoids the problem of simultaneous birth and death that has plagued many of the earlier attempts to assess cell death. Negligible numbers died during the first week after a cell's birthday. The amount of cell death differed in the two cohorts; 48.5+/-15% and 29.0+/-12.4% in early and late, respectively, and most of it was postnatal. These findings disagree sharply with an earlier conclusion that ganglion cells die within 5 days of their birthdays or not at all.

Anterograde axonal transport of BDNF and NT-3 by retinal ganglion cells: roles of neurotrophin receptors

Retinal ganglion cells (RGCs) transport exogenous neurotrophins anterogradely to the midbrain tectum/superior colliculus with significant downstream effects. We determined contributions of neurotrophin receptors for anterograde transport of intraocularly injected radiolabeled neurotrophins. In adult rodents, anterograde transport of brain-derived neurotrophic factor (BDNF) was receptor-mediated, and transport of exogenous BDNF and neurotrophin-3 (NT-3) was more efficient, per RGC, in rodents than chicks. RT-PCR and Western blot analysis of purified murine RGCs showed that adult RGCs express the p75 receptor. Anterograde transport of BDNF or NT-3 was not diminished in p75 knock-out mice (with unaltered final numbers of RGCs), but BDNF transport was substantially reduced by co-injected trkB antibodies. In chick embryos, however, p75 antisense or co-injected p75 antibodies significantly attenuated anterograde transport of NT-3 by RGCs. Thus, neither BDNF nor NT-3 utilizes p75 for anterograde transport in adult rodent RGCs, while anterograde NT-3 transport requires the p75 receptor in embryonic chicken RGCs.

Development of the human fetal cochlear nerve: a morphometric study

Ontogenesis of the human peripheral auditory pathway is relatively less explored. While the distal part of the auditory perception apparatus (i.e. the cochlea) received attention, studies on the neural element carrying information to the brainstem (i.e. the cochlear nerve) are scarce. In the present study, axonal differentiation, maturation and myelination of the distal end of the human cochlear nerve (CN) were assessed using light and electron microscopy. Seven human fetuses of 12, 15, 18, 20, 22, 28 and 38 weeks' gestation (WG) were analyzed. Light microscopy revealed nerve fascicles as early as 12 WG, initially arranged loosely but later compacted by 18 WG. Myelinated fibers were clearly detected at 28 WG. Ultrastructurally, at 12 WG developing Schwann cells were present between the thin unmyelinated axons. At 15 WG, the fascicular arrangement was distinct with blood vessels in the perineurium. The maximum number of axons was found at 20 WG, which subsequently reduced to reach the adult level at 22 WG. The myelinated axons in the CN were first observed on the left side at 20 WG, following which the number and proportion of myelinated axons increased until term, incorporating both small and large axons. The right CN lagged behind in maturation. Axon size also increased with age. Thus, the maturation of the human CN commences during the mid-gestation period and produces exuberant axons that are eventually pruned at a time when axons start to myelinate. During this developmental period the human CN maintains maturational asymmetry, the functional consequences of which remain to be elucidated.

Activation of epidermal growth factor receptors directs astrocytes to organize in a network surrounding axons in the developing rat optic nerve

In postnatal developing optic nerves, astrocytes organize their processes in a cribriform network to group axons into bundles. In neonatal rat optic nerves in vivo, the active form of EGFR tyrosine kinase is abundantly present when the organization of astrocytes and axons is most actively occurring. Blocking activity of EGFR tyrosine kinase during the development of rat optic nerves in vivo inhibits astrocytes from extending fine processes to surround axons. In vitro, postnatal optic nerve astrocytes, stimulated by EGF, organize into cribriform structures which look remarkably like the in vivo structure of astrocytes in the optic nerve. In addition, when astrocytes are co-cultured with neonatal rat retinal explants in the presence of EGF, astrocytes that are adjacent to the retinal explants, re-organize to an astrocyte-free zone into which neurites grow out from the retinal tissue. We hypothesize that in the developing optic nerve, EGFR activity directs the formation of a histo-architectural structure of astrocytes which surrounds axons and provides a permissive environment for axon development.

Functional specialization of the axon initial segment by isoform-specific sodium channel targeting

Voltage-dependent sodium channels cluster at high density at axon initial segments, where propagating action potentials are thought to arise, and at nodes of Ranvier. Here, we show that the sodium channel Na(v)1.6 is precisely localized at initial segments of retinal ganglion cells (RGCs), whereas a different isoform, Na(v)1.2, is found in the neighboring unmyelinated axon. During development, initial segments first expressed Na(v)1.2, and Na(v)1.6 appeared later, approximately in parallel with the onset of repetitive RGC firing. In Shiverer mice, Na(v)1.6 localization at the initial segment was unaffected, although Na(v)1.6 expression was severely disrupted in the aberrantly myelinated optic nerve. Targeting or retention of Na(v)1.6 requires molecular interactions that normally occur only at initial segments and nodes of Ranvier. Expression at nodes but not initial segments exhibits an additional requirement for intact myelination. Because of their high density at the initial segment, Na(v)1.6 channels may be crucial in determining neuronal firing properties.

Growth hormone in the nervous system: autocrine or paracrine roles in retinal function?

Growth hormone (GH) is primarily produced in the pituitary gland, although GH gene expression also occurs in the central and autonomic nervous systems. GH-immunoreactive proteins are abundant in the brain, spinal cord, and peripheral nerves. The appearance of GH in these tissues occurs prior to the ontogenic differentiation of the pituitary gland and prior to the presence of GH in systemic circulation. Neural GH is also present in neonates, juveniles, and adults and is independent of changes in pituitary GH secretion. Neural GH is therefore likely to have local roles in neural development or neural function, especially as GH receptors (GHRs) are widespread in the nervous system. In recent studies, GH mRNA and GH immunoreactive proteins have been identified in the neural retina of embryonic chicks. GH immunoreactivity is present in the optic cup of chick embryos at embryonic day (ED) 3 of the 21-d incubation period. It is widespread in the neural retina by ED 7 but also present in the nonpigmented retina, choroid, sclera, and cornea. This immunoreactivity is associated with proteins in the neural retina comparable in size with those in the adult pituitary gland, although it is primarily associated with 15-16 kDa moieties rather than with the full-length molecule of approximately 22 kDa. These small GH moieties may reflect proteolytic fragments of "monomer" GH and (or) the presence of different GH gene transcripts, since full-length and truncated GH cDNAs are present in retinal tissue extracts. The GH immunoreactivity in the retina persists throughout embryonic development but is not present in juvenile birds (after 6 weeks of age). This immunoreactivity is also associated with the presence of GH receptor (GHR) immunoreactivity and GHR mRNA in ocular tissues of chick embryos. The retina is thus an extrapituitary site of GH gene expression during early development and is probably an autocrine or paracrine site of GH action. The marked ontogenic pattern of GH immunoreactivity in the retina suggests hitherto unsuspected roles for GH in neurogenesis or ocular development.

Cytoplasmic c-Jun N-terminal immunoreactivity: a hallmark of retinal apoptosis

1. We investigated the association of c-Jun with apoptosis within retinal tissue. Explants of the retina of neonatal rats were subject to a variety of procedures that cause apoptosis of specific classes of retinal cells at distinct stages of differentiation. The expression of c-Jun was detected by Western Blot, and immunohistochemistry was done with antibodies made for either N-terminal or C-terminal domains of c-Jun, and correlated with apoptosis detected either by chromatin condensation or by in situ nick end labeling of fragmented DNA. 2. c-Jun protein content was increased in retinal tissue subject to induction of both photoreceptor and ganglion cell death. 3. c-Jun N-terminal immunoreactivity was found mainly in the cytoplasm of apoptotic cells regardless of cell type, of the stage of differentiation, including proliferating cells, or of the means of induction of apoptosis. 4. The data are consistent with the hypothesis that c-Jun is involved in the control of cell death in retinal tissue, but other proteins that cross-react with c-Jun N-terminal antibodies may also be major markers of retinal apoptosis. 5. Antibodies directed to c-Jun N-terminal (aa 91-105) are useful tools to follow apoptotic changes in retinal tissue.

Effect of Bcl-2 overexpression on establishment of ipsilateral retinocollicular projection in mice

During perinatal development in rodents, ipsilateral retinofugal projection spreading over the superior colliculus is eventually restricted to the rostromedial region. Since this restriction is accompanied by the apoptotic death of more than half of the retinal ganglion cells (RGCs), cell death is believed to play a major role in the restriction of transient ipsilateral projection from the retina to the superior colliculus. To determine the role of RGC death in the establishment of ipsilateral retinofugal projection, we examined the projection pattern in the superior colliculus and the dorsal lateral geniculate nucleus of transgenic mice overexpressing the human bcl-2 gene, which protects against cell death in the CNS. Retrograde labeling of RGCs showed that the number of ipsilaterally projecting RGCs in adult transgenic mice was approximately twice that in adult wild-type mice, indicating that the naturally occurring death of RGCs was prevented in these mutant mice. However, anterograde labeling of ipsilateral retinofugal pathways revealed that the innervation of retinogeniculate and retinocollicular projections was as restricted in transgenic mice as in wild-type mice. From these results we suggest that restriction of ipsilateral retinofugal projection during development is due to retraction or elimination of excessive terminals rather than to naturally occurring RGC death.

Melanopsin-containing retinal ganglion cells: architecture, projections, and intrinsic photosensitivity

The primary circadian pacemaker, in the suprachiasmatic nucleus (SCN) of the mammalian brain, is photoentrained by light signals from the eyes through the retinohypothalamic tract. Retinal rod and cone cells are not required for photoentrainment. Recent evidence suggests that the entraining photoreceptors are retinal ganglion cells (RGCs) that project to the SCN. The visual pigment for this photoreceptor may be melanopsin, an opsin-like protein whose coding messenger RNA is found in a subset of mammalian RGCs. By cloning rat melanopsin and generating specific antibodies, we show that melanopsin is present in cell bodies, dendrites, and proximal axonal segments of a subset of rat RGCs. In mice heterozygous for tau-lacZ targeted to the melanopsin gene locus, beta-galactosidase-positive RGC axons projected to the SCN and other brain nuclei involved in circadian photoentrainment or the pupillary light reflex. Rat RGCs that exhibited intrinsic photosensitivity invariably expressed melanopsin. Hence, melanopsin is most likely the visual pigment of phototransducing RGCs that set the circadian clock and initiate other non-image-forming visual functions.

Quantification of histological changes after calibrated crush of the intraorbital optic nerve in rats

BACKGROUND

Traumatic optic nerve lesions (TONL) are probable but unpredictable consequence after severe midface or skull base trauma. Based on a previously described rat model, the authors developed a new model in order to simulate optic nerve crush during trauma on the optic canal.

METHODS

To achieve a calibrated TONL, a microinjuring device was designed that made it possible to assess the correlation between a defined trauma and the neuronal degeneration in the rat retinal ganglion cell (RGC) layer. This device is based on a small dynamometer mounted onto a conventional micromanipulator. The supraorbital approach was chosen to expose the extracranial optic nerve.

RESULTS

In this rat model (n=100, Wistar strain) the parameters of "force" and "time" could be precisely monitored during the experiment. The decrease in the mean number of retinal neurons (N) according to the pressure exerted (2-30 cN x mm(-2)) on the optic nerve was linear for 1, 6, and 15 minutes of injuring time; the decrease in N for varying injuring forces also appears to be nearly linear.

CONCLUSION

The results show that this model provides a reliable method for studying quantitatively the anatomical effects of TONL on the RGC layer and the optic nerve itself, and may allow the design of treatment strategies following TONL.

Neuronal cell death, nerve growth factor and neurotrophic models: 50 years on

Viktor Hamburger has just died at the age of 100. It is 50 years since he and Rita Levi-Montalcini laid the foundations for the study of naturally occurring cell death and of neurotrophic factors in the nervous system. In a period of less than 10 years, from 1949 to 1958, Hamburger and Levi-Montalcini made the following seminal discoveries: that neuron cell death occurs in dorsal root ganglia, sympathetic ganglia and the cervical column of motoneurons; that the predictions arising from this observation, namely that survival is dependent on the supply of a trophic factor, could be substantiated by studying the effects of a sarcoma on the proliferation of ganglionic processes both in vivo and in vitro; and that the proliferation of these processes could be used as an assay system to isolate the factor. This work provides a short review mostly of the early history of this subject in the context of the Hamburger/Levi-Montalcini paradigm. This acts as an introduction to a consideration of models that have been proposed to account for how the different sources of growth factors provide for the survival of neurons during development. It is suggested that what has been called the 'social-control' model provides the most parsimonious quantitative description of the contribution of trophic factors to neuronal survival, a concept for which we are in debt to Viktor Hamburger and Rita Levi-Montalcini.

BDNF/trkB signaling in the developmental sculpting of visual connections

Neurotrophins are a family of secreted molecules that have multiple, profound actions on the structure and function of both developing and mature neurons. Neurotrophins exert their influences by signaling through the trk family of receptor tyrosine kinases and the p75 low affinity neurotrophin receptor. Here we review the contributions of neurotrophins to the development of neural circuitry in the mammalian visual system. We emphasize: (1) the role of neurotrophins as components of the cellular mechanisms by which neuroelectric activity sculpts pattern of brain connectivity; and (2) the results of recent experiments suggesting that the trafficking of neurotrophin proteins may be activity dependent.

Glial cell line derived neurotrophic factor delays photoreceptor degeneration in a transgenic rat model of retinitis pigmentosa

We designed experiments to evaluate the therapeutic potential of glial cell line derived neurotrophic factor (GDNF) to rescue photoreceptors from genetically determined cell death. Gene transfer of the neurotrophic factor to the retina was achieved via a recombinant adeno-associated virus (rAAV) vector containing the chicken beta-actin promoter/immediate early cytomegalovirus enhancer (CBA) driving the human GDNF gene. We delivered AAV-CBA-GDNF to the retinas of an animal model of retinitis pigmentosa, the TgN S334ter-4 rhodopsin line of transgenic rats. Immunohistochemical studies localized AAV-CBA-GDNF-derived recombinant protein to cell bodies, inner segments, and outer segments of photoreceptor cells as well as to retinal pigment epithelial cells. We assessed the effect of viral delivery by morphometric and electroretinographic analysis. These experiments showed that GDNF vector treatment leads to increased rod photoreceptor survival as indicated by morphometric analysis of outer nuclear layer thickness. AAV-CBA-GDNF-treated retinas also demonstrated functional improvement by the substantially increased amplitude of electroretinograms. AAV-CBA-GDNF delivery had a significant rescue effect on photoreceptor degeneration in this animal model.

Developmental expression of DCC in the rat retina

The protein product of the deleted in colorectal cancer (DCC) gene possesses netrin-binding activity and may be involved in axonal guidance during retinal development. The temporal and spatial expression of DCC was analyzed in developing rat retina by means of immunoblotting and immunohistochemistry as well as by reverse transcription-polymerase chain reaction. Transient DCC protein expression is evident on ganglion cell axons in embryonic and neonatal retina. Double labeling experiments demonstrate DCC immunolabeling on processes that stratify in the inner plexiform layer and are derived from cholinergic amacrine cells. This pattern is maintained during the early postnatal period. DCC immunolabeling in the inner plexiform layer declines with age and is not observed in adult retina. The down-regulation of the DCC protein is confirmed by Western blot analysis. mRNA for DCC is expressed in embryonic, postnatal and adult retina and shows no correlation with the protein down-regulation. We suggest that DCC expression may be correlated with the functional segregation of the inner plexiform layer.

Transient expression of synaptic zinc during development of uncrossed retinogeniculate projections

The transition metal zinc is an essential dietary constituent that is believed to serve an important intercellular signaling role at certain excitatory synapses in the central nervous system. In the present study, we used histochemical techniques to investigate the distribution of synaptic zinc during postnatal development of retinogeniculate projections in rats. From postnatal day (P) 1 until P-21, the pattern of zinc histochemical staining in the dorsal lateral geniculate nucleus (LGNd) precisely matched the distribution of axon terminals from the ipsilateral eye that were labeled by anterograde transport of horseradish peroxidase. Regions of the LGNd that contained only crossed axons were devoid of zinc staining. Abnormalities in the distribution of uncrossed retinogeniculate projections in albino versus pigmented rats were paralleled by identical variations in localization of synaptic zinc. Unilateral enucleation on P-10 was followed within 5 days by loss of zinc staining in the LGNd ipsilateral to the removed eye without affecting staining in the contralateral nucleus. Finally, the ability to detect zinc histochemically in the LGNd ceased at approximately P-24. These findings provide evidence that zinc is sequestered within synaptic boutons of a subpopulation of retinal ganglion cells whose axons terminate on the ipsilateral side of the brain. The duration of zinc staining overlaps with the major period of axonal remodeling in the LGNd, suggesting that synaptically released zinc may play a role in postnatal refinement of retinogeniculate projections.

Chronic NMDA receptor blockade from birth increases the sprouting capacity of ipsilateral retinocollicular axons without disrupting their early segregation

We have investigated the role of the NMDA glutamate receptor (NMDAR) in the genesis and regulation of structural plasticity during synaptogenesis in the visual layers of the rat superior colliculus (sSC). In this neuropil, three projections compete for synaptic space during development. By fluorescently labeling the projections of both eyes and imaging them with confocal microscopy, we can quantify the sprouting of the ipsilateral retinal projection that follows removal of a portion of the contralateral retinal and/or corticocollicular projection. Using these techniques we have studied the effects of NMDAR blockade under different levels of competition. NMDARs were chronically blocked from birth [postnatal day 0 (P0)] by suspending the competitive antagonist 2-amino-5-phosphonopentanoic acid in the slow release plastic Elvax, a slab of which was implanted over the sSC. Such treatment alone does not impair the normal segregation of the retinal projections. However, if sprouting of the ipsilateral projection is initiated with a small contralateral retinal lesion at P6, this sprouting can be further increased by blocking NMDARs from birth. Sprouting of the ipsilateral retinal projection is also induced by retinal lesions made at P10/P11, but NMDAR blockade does not augment the sprouting induced by this later lesion. However, when combined with simultaneous ablation of the ipsilateral visual cortex, P10/P11 lesions show increased sprouting after NMDAR blockade. These data indicate that P0 NMDAR blockade does not eliminate synaptic competition in the sSC. Instead, early elimination of NMDAR function appears to facilitate sprouting that is gated in a stepwise manner by the other visual afferents.

Postnatal development of the retinal projection to the nucleus of the optic tract and accessory optic nuclei in the hooded rat

Retinal projections to the nucleus of the optic tract (NOT) and accessory optic nuclei (AON) were studied in the postnatal hooded rat after monocular injection of cholera toxin B subunit (CTB) into the vitreous chamber of the eye. At all postnatal ages, retinal axons were labeled sensitively; they revealed dense projections to the contralateral, and sparse but distinct projections to the ipsilateral, NOT and AON. The CTB labeling enabled the first delineation of the complete morphology of developing retinal axons in the ipsilateral NOT and AON. From postnatal day (P) 1 to P3, axons with complex growth cones were seen, and unbranched collaterals with simple growth cones increased and extended gradually. At P6, complex growth cones disappeared while branched collaterals with simple growth cones as well as small-sized varicosities increased. By P12 (two days before eye-opening) the adult-like pattern of terminal arbors appeared. The branched collaterals with tiny, small-sized varicosities present probably represented developing synaptic boutons. At P16 (after eye opening), the pattern of terminal arbors was well developed, almost to the same extent as in the adult. By contrast, a broadly distributed, transient retinal projection around NOT and AON was gradually eliminated; it started to disappear during the first few postnatal days, and was fully retracted by the time of eye-opening time to a pattern normal for the adult.

The normal optic nerve head

BACKGROUND

The normal optic nerve head varies from one person to another, and there is often intraindividual variation as well. Factors such as race and age play an important role in distinguishing what may be considered normal variations in optic nerve head appearance.

METHODS

A literature search and review of the latest studies on the optic nerve head was conducted.

RESULTS

Results of recent studies showed that variations in the average cup-to-disc ratio exist for different races, and with age there is a gradual loss of nerve fibers leading to an overall increase in the cup-to-disc ratio. There is also evidence that congenitally larger optic nerves have larger cup-to-disc ratios and more nerve fibers. Smaller optic nerves, in contrast, have smaller cup-to-disc ratios and fewer nerve fibers. These findings are presented along with sample photographs depicting the normal variations in optic nerve head appearance.

CONCLUSION

Over the past 30 years, technology has allowed for changing views about what may be considered normal in reference to the optic nerve head. This information is valuable to the eye care practitioner in helping to make appropriate patient care management decisions.

Localization of mRNAs for trkB isoforms and p75 in rat retinal ganglion cells

Brain-derived neurotrophic factor (BDNF) plays an important role in the survival of retinal ganglion cells (RGCs). To better understand the potential role of BDNF receptors in the survival of RGCs, we studied the expression and localization of transcripts for trkB isoforms and p75, using reverse transcriptase-polymerase chain reaction (RT-PCR) and in situ hybridization with digoxigenin-labeled RNA probes in the adult rat retina. We found that truncated trkB and p75 were expressed in RGCs, as well as full-length trkB, in the adult rat retina. The localization patterns of full-length and truncated trkB mRNAs suggest that a subpopulation of RGCs expresses both full-length and truncated trkB. The localization pattern of p75 mRNA suggests that it is expressed in a subpopulation of RGCs. Expression of both trkB isoforms in RGCs raises the possibility that truncated trkB lessens BDNF effect on RGCs by forming nonfunctional heterodimers with full-length trkB. This possibility was supported by our observation that apoptosis of RGCs detected by the TUNEL method followed close on the onset of truncated trkB mRNA expression in the ganglion cell layer of the developing rat retina.

BDNF injected into the superior colliculus reduces developmental retinal ganglion cell death

The role of neurotrophins as survival factors for developing CNS neurons, including retinal ganglion cells (RGCs), is uncertain. Null mutations for brain-derived neurotrophic factor (BDNF) or neurotrophin 4 (NT4), individually or together, are without apparent effect on the number of RGCs that survive beyond the period of normal, developmental RGC death. This contrasts with the BDNF dependence of RGCs in vitro and the effectiveness of BDNF in reducing RGC loss after axotomy. To investigate the effect of target-derived neurotrophins on the survival of developing RGCs, we injected BDNF into the superior colliculus (SC) of neonatal hamsters. At the age when the rate of developmental RGC death is greatest, BDNF produces, 20 hr after injection, a 13-15-fold reduction in the rate of RGC pyknosis compared with the rates in vehicle-injected and untreated hamsters. There is no effect 8 hr after injection. Electrochemiluminescence immunoassay measurements of BDNF protein in the retinae and SC of normal and BDNF-treated hamsters demonstrate that the time course of BDNF transport to RGCs supports a role for target-derived BDNF in promoting RGC survival. The effectiveness of pharmacological doses of BDNF in reducing developmental RGC death may be useful in further studies of the mechanisms of stabilization and elimination of immature central neurons.

Developmentally regulated release of intraretinal neurotrophic factors in vitro

The effects of conditioned media either from aggregates or from explants of embryonic chick retinae and of recombinant neurotrophins were tested upon the survival in vitro of ganglion cells in dissociated cell cultures from the retina of newborn rats. Ganglion cells were identified by the detection of retrogradely transported horseradish peroxidase injected bilaterally into the superior colliculus. Conditioned media increased significantly the survival of ganglion cells after 2 days in culture, at a wide range of plating densities, and had no effect upon adhesion of rat retinal cells. Media conditioned by cell ensembles from chick retinae from embryonic day 8 (E8) to E16 had neurotrophic effects. Release of neurotrophic activity peaked at E10 E12, irrespective of the numbers of cells or total concentration of protein in the conditioned media. The active molecules were non-dialyzable and were released either in the presence or in the absence of fetal calf serum. The neurotrophic activity was abolished by trypsinization, and recovered by salting-out with 25 75% ammonium sulfate. NT-4, BDNF and, to a lesser extent, NT-3, increased the survival of ganglion cells in our assay, while NGF had no effect. The data show that chick retinal cells release soluble trophic proteins according to a developmentally regulated pattern. These neurotrophic factors may be involved in local competitive interactions that help control naturally occurring neuron death among ganglion cells of the vertebrate retina.