Excess target-derived brain-derived neurotrophic factor preserves the transient uncrossed retinal projection to the superior colliculus

Role of BDNF/TrkB pathway in the visual system: Therapeutic implications for glaucoma

INTRODUCTION

Neuroprotective therapeutics are needed to treat glaucoma, an optic neuropathy that results in death of retinal ganglion cells (RGCs).

AREAS COVERED

The BDNF/TrkB pathway is important for RGC survival. Temporal and spatial alterations in the BDNF/TrkB pathway occur in development and in response to acute optic nerve injury and to glaucoma. In animal models, BDNF supplementation is successful at slowing RGC death after acute optic nerve injury and in glaucoma, however, the BDNF/TrkB signaling is not the only pathway supporting long term RGC survival.

EXPERT COMMENTARY

Much remains to be discovered about the interaction between retrograde, anterograde, and retinal BDNF/TrkB signaling pathways in both neurons and glia. An ideal therapeutic agent for glaucoma likely has several modes of action that target multiple mechanisms of neurodegeneration including the BDNF/TrkB pathway.

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.

Intraocular BDNF promotes ectopic branching, alters motility and stimulates abnormal collaterals in regenerating optic fibers

A great deal of effort has been invested in using trophic factors and other bioactive molecules to promote cell survival and axonal regeneration in the adult central nervous system. Far less attention has been paid to investigating potential effects that trophic factors may have that might interfere with recovery. In the visual system, BDNF has been previously reported to prevent regeneration. To test if BDNF is inherently incompatible with regeneration, BDNF was given intraocularly during optic nerve regeneration in the adult goldfish. In vivo imaging and anatomical analysis of selectively labeled axons were used as a sensitive assay for effects on regeneration within the tectum. BDNF had no detectable inhibitory effect on the ability of axons to regenerate. Normal numbers of axons regenerated into the tectum, exhibited dynamic growth and retractions similar to controls, and were able to navigate to their correct target zone in the tectum. However, BDNF was found to have additional effects that adversely affected the quality of regeneration. It promoted premature branching at ectopic locations, diminished the growth rate of axons through the tectum, and resulted in the formation of ectopic collaterals. Thus, although BDNF has robust effects on axonal behavior, it is, nevertheless, compatible with axonal regeneration, axon navigation and the formation of terminal arbors.

Activity dependent mechanisms of visual map formation--from retinal waves to molecular regulators

The refinement of neural connections requires activity-dependent mechanisms in addition to the genetic program initially establishing wiring diagrams. The well-understood organization of the visual system makes it an accessible model for analyzing the contribution of activity in the formation of connectivity. Prior to visual experience, patterned spontaneous activity in the form of retinal waves has an important role for the establishment of eye-specific and retinotopic maps by acting on the refinement of axon arborization. In the present review, which focuses on experimental data obtained in mice and ferrets, we highlight the features of retinal activity that are important for visual map formation and question whether synaptic release and Hebbian based competition rules apply to this system. Recent evidence using genetic tools that allowed the manipulation of different features of neural activity have clarified the controversy on whether activity is instructive or permissive for visual map formation. Furthermore, current evidence strongly suggests that different mechanisms are at play for different types of axons (ipsilateral vs. contralateral), maps (eye-specific vs. retinotopic) or targets. Many molecules that either modulate activity or are modulated by activity are important in the formation of the visual map, such as adenylate cyclase 1, serotonin, or molecules from the immune system. Finally, new players in the game include retrograde messengers signaling from the target cell to the retinal axons as well as microglia that could help to eliminate inappropriate synapses.

Analysis of the astray/robo2 zebrafish mutant reveals that degenerating tracts do not provide strong guidance cues for regenerating optic axons

During formation of the optic projection in astray/robo2 mutant zebrafish, optic axons exhibit rostrocaudal pathfinding errors, ectopic midline crossing and increased terminal arbor size. Here we show that these errors persist into adulthood, even when robo2 function is conditionally reduced only during initial formation of the optic projection. Adult errors include massive ectopic optic tracts in the telencephalon. During optic nerve regeneration in astray/robo2 animals, these tracts are not repopulated and ectopic midline crossing is reduced compared with unlesioned mutants. This is despite a comparable macrophage/microglial response and upregulation of contactin1a in oligodendrocytes of entopic and ectopic tracts. However, other errors, such as expanded termination areas and ectopic growth into the tectum, were frequently recommitted by regenerating optic axons. Retinal ganglion cells with regenerating axons reexpress robo2 and expression of slit ligands is maintained in some areas of the adult optic pathway. However, slit expression is reduced rostral and caudal to the chiasm, compared with development and ubiquitous overexpression of Slit2 did not elicit major pathfinding phenotypes. This shows that (1) there is not an efficient correction mechanism for large-scale pathfinding errors of optic axons during development; (2) degenerating tracts do not provide a strong guidance cue for regenerating optic axons in the adult CNS, unlike the PNS; and (3) robo2 is less important for pathfinding of optic axons during regeneration than during development.

Effects of trkB knockout on topography and ocular segregation of uncrossed retinal projections

TrkB is an important receptor for brain-derived neurotrophic factor and NT4, members of the neurotrophin family. TrkB signaling is crucial in many activity-dependent and activity-independent processes of neural development. Here, we investigate the role of trkB signaling in the development of two distinct, organizational features of retinal projections--the segregation of crossed and uncrossed retinal inputs along the "lines of projection" that represent a single point in the visual field and the "retinotopic" mapping of retinofugal axons within their cerebral targets. Using anterograde tracing, we obtained quantitative measures of the distribution of retinal projections in the dorsal nucleus of the lateral geniculate body (LGd) and superior colliculus (SC) of wild-type mice and mice homozygous for constitutive null mutation (knockout) of the full-length trkB receptor (trkB(FL)(-/-)). In trkB(FL)(-/-) mice, uncrossed retinal projections cluster normally but there is a topographic expansion in the distribution of these clusters across the SC. By contrast, the absence of trkB signaling has no significant effect on the segregation of crossed and uncrossed retinal projections along the lines of projection in LGd or SC. We conclude that the normal topographic organization of uncrossed retinal projections depends upon trkB signaling, whereas the segregation of crossed and uncrossed retinal projections is trkB-independent. We also found that in trkB(FL)(-/-) mice, neuronal number was reduced in the LGd and SC and in the caudate-putamen. Previous studies by ourselves and others have shown that the number of retinal ganglion cells (RGCs) is unchanged in trkB(FL)(-/-) mice. Together, these results demonstrate that there is no matching of the numbers of RGCs with neuronal numbers in the LGd or SC.

Breaking the balance: ocular BDNF-injections induce visual asymmetry in pigeons

In pigeons, asymmetric photic stimulation around hatch induces functional visual asymmetries that are accompanied by left-right differences in tectal cell sizes. Different aspects of light-dependent neuronal differentiation are known to be mediated by the brain-derived neurotrophic factor (BDNF). Therefore, we investigated by means of single or triple BDNF- or saline-injections into the right eye of dark-incubated pigeon hatchlings if ocular BDNF enrichment mimics the effects of biased visual input. As adults, the birds were tested in a grit-grain discrimination task to estimate the degree and direction of visual lateralization followed by a morphometric analysis of retinal and tectal cells. The grit-grain discrimination task demonstrated that triple BDNF-injections enhanced visuoperceptual and visuomotor functioning of the left eye system. Morphometric analysis showed bilateral cell-type dependent effects within the optic tectum. While single-BDNF injections increased cell body sizes of calbindin-positive efferent neurons, triple-injections decreased cell sizes of parvalbumin-positive cells. Moreover, single BDNF-injections increased retinal cell sizes within the contralateral eye. Analysis of BDNF-induced intracellular signaling demonstrated enhanced downstream Ras activation for at least 24 h within both tectal halves whereas activity changes within the contralateral retina could not be detected. This points to primarily tectal effects of ocular BDNF. In sum, exogenous BDNF modulates the differentiation of retinotectal circuitries and dose-dependently shifts lateralized visuomotor processing towards the noninjected side. Since these effects are opposite to embryonic light stimulation, it is unlikely that the impact of light onto asymmetry formation is mediated by retinal BDNF.

Cell-autonomous TrkB signaling in presynaptic retinal ganglion cells mediates axon arbor growth and synapse maturation during the establishment of retinotectal synaptic connectivity

BDNF contributes to the activity-dependent establishment and refinement of visual connectivity. In Xenopus, BDNF applications in the optic tectum influence retinal ganglion cell (RGC) axon branching and promote synapse formation and stabilization. The expression patterns of BDNF and TrkB suggest that BDNF specifically regulates the maturation of RGC axons at the target. It is possible, however, that BDNF modulates retinotectal synaptic connectivity by differentially influencing presynaptic RGC axons and postsynaptic tectal cells. Here, we combined single-cell expression of a dominant-negative TrkB-enhanced green fluorescent protein (GFP) fusion protein with confocal microscopy imaging in live Xenopus tadpoles to differentiate between presynaptic and postsynaptic actions of BDNF. Disruption of TrkB signaling in individual RGCs influenced the branching and synaptic maturation of presynaptic axon arbors. Specifically, GFP-TrkB.T1 overexpression increased the proportion of axons with immature, growth cone-like morphology, decreased axon branch stability, and increased axon arbor degeneration. In addition, GFP-TrkB.T1 overexpression reduced the number of red fluorescent protein-synaptobrevin-labeled presynaptic specializations per axon terminal. In contrast, overexpression of GFP-TrkB.T1 in tectal neurons did not alter synaptic number or the morphology or dynamic behavior of their dendritic arbors. Electron microscopy analysis revealed a significant decrease in the number of mature synaptic profiles and in the number of docked synaptic vesicles at retinotectal synapses made by RGC axons expressing GFP-TrkB.T1. Together, our results demonstrate that presynaptic TrkB signaling in RGCs is a key determinant in the establishment of visual connectivity and indicate that changes in tectal neuron synaptic connectivity are secondary to the BDNF-elicited enhanced stability and growth of presynaptic RGCs.

Specific alterations of tyrosine hydroxylase immunopositive cells in the retina of NT-4 knock out mice

To assess the effect of NT-4 deprivation on maturation of retinal circuitry, we investigated a mouse with targeted deletion of the gene encoding nt-4 (nt-4(-/-)). In particular, we studied neurons immunostained by an antibody recognizing tyrosine hydroxylase (TH), the rate limiting enzyme for dopamine (DA) synthesis. We found that TH immunopositive processes were altered in the retina of nt-4(-/-). Alteration of TH immunopositive processes in nt-4(-/-) mice resulted in changes of DA turnover, as assessed by high-pressure liquid chromatography measurements. These findings suggest that retinal NT-4 plays a role in the morphological maturation of dopaminergic retinal cells.

Regulation of GDNF and its receptor components GFR-alpha1, -alpha2 and Ret during development and in the mature retino-collicular pathway

The development of the retino-tectal projection as part of the central visual pathway is accomplished around postnatal day (P) 10-14 in rodents, and trophic factors are important for topographic refinement of this projection. Emerging data indicate that GDNF may influence synaptic plasticity of this projection. To date, maturation-dependent kinetics of GDNF release and expression and biological function of single GDNF receptors along the retino-collicular pathway are ill-defined. Here, we examined mRNA and protein expression of GDNF and its multicomponent receptor complex in the retina and superior colliculus (SC) during postnatal development of the rat visual system, and after optic nerve (ON) injury by RT-PCR, immunoblotting and immunofluorescence. Stable mRNA transcription of GDNF and its receptors GFR-alpha1, -alpha2 and Ret was found in retina and SC throughout development into adulthood and after ON transection. Expression of GDNF protein increased during retinal development, declined in adulthood and was further reduced in injured retina. In the SC, GDNF peaked at P0, continuously declined with maturation, and was undetectable in the deafferentiated SC. GFR-alpha1 was abundant in retina and SC throughout, while GFR-alpha2 was not expressed. Since Ret was localized primarily to the vascular compartment, the receptor tyrosine kinase may play a minor role in neuronal GDNF signaling. In summary, we provide evidence for GDNF as survival and guidance factor during development of the retino-tectal projection with differential regulation in early and premature retina and SC. Postlesionally, midbrain targets do not induce GDNF, suggesting that retrograde GDNF is not essential for rescue of adult injured retinal ganglion cells (RGCs).

Normal eye-specific patterning of retinal inputs to murine subcortical visual nuclei in the absence of brain-derived neurotrophic factor

Brain-derived neurotrophic factor (BDNF) is a preferred ligand for a member of the tropomyosin-related receptor family, trkB. Activation of trkB is implicated in various activity-independent as well as activity-dependent growth processes in many developing and mature neural systems. In the subcortical visual system, where electrical activity has been implicated in normal development, both differential survival, as well as remodeling of axonal arbors, have been suggested to contribute to eye-specific segregation of retinal ganglion cell inputs. Here, we tested whether BDNF is required for eye-specific segregation of visual inputs to the lateral geniculate nucleus and the superior colliculus, and two other major subcortical target fields in mice. We report that eye-specific patterning is normal in two mutants that lack BDNF expression during the segregation period: a germ-line knockout for BDNF, and a conditional mutant in which BDNF expression is absent or greatly reduced in the central nervous system. We conclude that the availability of BDNF is not necessary for eye-specific segregation in subcortical visual nuclei.

Brain-derived neurotrophic factor is expressed in a gradient in the superior colliculus during development of the retinocollicular projection

Abstract Theoretical models of topographic map formation have postulated a gradient of attractant in addition to a gradient of repulsion in the target. In species where many axons grow past their correct positions initially, it has also been argued that a parallel gradient of attractant or branching signal is required to ensure collateral formation at the correct position (O'Leary et al., 1999). Brain-derived neurotrophic factor (BDNF) is a known attractant and promotes branching of retinal axons. We have examined its distribution in the superior colliculus and that of its receptor, trkB, in the retina, using immunohistochemistry and in situ hybridization, respectively, during the development of the topographic retinocollicular projection in the wallaby, a marsupial mammal. The number of glial endfeet expressing BDNF at the surface of the colliculus was found to be in a high caudal-to-low rostral gradient during the time when the retinocollicular projection was developing. When the projection was mature the rostrocaudal gradient had disappeared and the number of detectable endfeet expressing BDNF was very low. Messenger RNA for TrkB was expressed in the retinal ganglion cell layer throughout the time when the retinocollicular projection was developing, with no difference in expression across the nasotemporal axis of the retina. The low rostral to high caudal distribution of BDNF in glial endfeet supports the idea that it is providing a parallel gradient of attractant or branching signal in the colliculus.

Activity-driven sharpening of the retinotectal projection: the search for retrograde synaptic signaling pathways

Patterned visual activity, acting via NMDA receptors, refines developing retinotectal maps by shaping individual retinal arbors. Because NMDA receptors are postsynaptic but the retinal arbors are presynaptic, there must be retrograde signals generated downstream of Ca(++) entry through NMDA receptors that direct the presynaptic retinal terminals to stabilize and grow or to withdraw. This review defines criteria for retrograde synaptic messengers, and then applies them to the leading candidates: nitric oxide (NO), brain-derived neurotrophic factor (BDNF), and arachidonic acid (AA). NO is not likely to be a general mechanism, as it operates only in selected projections of warm blooded vertebrates to speed up synaptic refinement, but is not essential. BDNF is a neurotrophin with strong growth promoting properties and complex interactions with activity both in its release and receptor signaling, but may modulate rather than mediate the retrograde signaling. AA promotes growth and stabilization of synaptic terminals by tapping into a pre-existing axonal growth-promoting pathway that is utilized by L1, NCAM, N-cadherin, and FGF and acts via PKC, GAP43, and F-actin stabilization, and it shares some overlap with BDNF pathways. The actions of both are consistent with recent demonstrations that activity-driven stabilization includes directed growth of new synaptic contacts. Certain nondiffusible factors (synapse-specific CAMs, ephrins, neurexin/neuroligin, and matrix molecules) may also play a role in activity-driven synapse stabilization. Interactions between these pathways are discussed.

Target-derived and locally derived neurotrophins support retinal ganglion cell survival in the neonatal rat retina

Brain-derived neurotrophic factor (BDNF) and neurotrophin-4/5 (NT-4/5) protein and mRNA are found in the neonatal rat retina and also in target sites such as the superficial layers of the superior colliculus. Both neurotrophins support neonatal retinal ganglion cell survival in vitro. In vivo, injections of recombinant BDNF and NT-4/5 reduce naturally occurring cell death as well as death induced by removal of the contralateral superior colliculus. In the latter case, the peak of retinal ganglion cell death occurs about 24 h postlesion. We wished to determine: whether a similar time-course of degeneration occurs after selective removal of target cells or depletion of target-derived trophic factors, and whether ganglion cell viability also depends on intraretinally derived neurotrophins. Retinal ganglion cell death was measured 24 and 48 h following injections of kainic acid or a mixture of BDNF and NT-4/5 blocking antibodies into the superior colliculus and 24 h after intraocular injection of the same antibodies. Retinotectally projecting ganglion cells were identified by retrograde labeling with the nucleophilic dye diamidino yellow. We show that collicular injections of either kainic acid or BDNF and NT-4/5 blocking antibodies significantly increased retinal ganglion cell death in the neonatal rat 24 h postinjection, death rates returning to normal by 48 h. This increase in death was greatest following collicular injections; however, death was also significantly increased 24 h following intravitreal antibody injection. Thus retinal ganglion cell survival during postnatal development is not only dependent upon trophic factors produced by central targets but may also be influenced by local intraretinal neurotrophin release.

Brain-derived neurotrophic factor regulates expression of vasoactive intestinal polypeptide in retinal amacrine cells

Brain-derived neurotrophic-factor (BDNF) is expressed in the retina and controls the development of subtypes of amacrine cells. In the present study we investigated the effects of BDNF on amacrine cells expressing vasoactive intestinal polypeptide (VIP). Rats received three intraocular injections of BDNF on postnatal days (P) 16, 18, and 20. The animals were sacrificed on P22, P40, P60, P80, and P120, and VIP expression in their retinas was detected by immunohistochemistry (P22, P40) and by radioimmunoassay (RIA; P22, P40, P60, P80, P120) to assess the time course of BDNF effects on VIP. A significant increase in the density of VIP-positive amacrine cells was detected in BDNF-treated retinas, and VIP concentration was up-regulated by 150% both at P22 and at P40 with respect to untreated controls. VIP concentration then slowly declined in the treated retinas over a period of 3 months; however, a statistically significant increase of 50% was still detectable on P120. The impact of endogenous BDNF on the regulation of VIP expression in the retina was analyzed in mice homozygous for a targeted deletion of the BDNF gene locus (bdnf-/-). VIP immunohistochemistry revealed a marked reduction of VIP-positive amacrine cells and of VIP-immunopositive processes in the inner plexiform layer of the BDNF knockout mice. Mice lacking BDNF expressed only 5% of the VIP protein in their retinas compared with the retinas of wild-type mice as measured by RIA. Our data show that BDNF is a major regulator of VIP expression in retinal amacrine cells and exerts long-lasting effects on VIP content.

Neurotrophic cross-talk between the nervous and immune systems: implications for neurological diseases

Inflammatory reactions in the central nervous system usually are considered detrimental, but recent evidence suggests that they also can be beneficial and even have neuroprotective effects. Intriguingly, immune cells can produce various neurotrophic factors of various molecular families. The concept of "neuroprotective immunity" will have profound consequences for the pathogenesis and treatment of neuroinflammatory diseases such as multiple sclerosis. It also will prove important for neurodegenerative disorders, in which inflammatory reactions often occur. This review focuses on recent findings that immune cells produce brain-derived neurotrophic factor in multiple sclerosis lesions, whereas neurons and astrocytes express the appropriate tyrosine kinase receptor TrkB. Together with functional evidence for the neuroprotective effects of immune cells, these observations support the concept of "neuroprotective immunity." We next examine current and future therapeutic strategies for multiple sclerosis and experimental autoimmune encephalomyelitis in light of neuroprotective immunity and finally address the broader implications of this new concept for other neuroinflammatory and neurodegenerative diseases.

Intraocular elevation of cyclic AMP potentiates ciliary neurotrophic factor-induced regeneration of adult rat retinal ganglion cell axons

In vitro, cyclic AMP (cAMP) elevation alters neuronal responsiveness to diffusible growth factors and myelin-associated inhibitory molecules. Here we used an established in vivo model of adult central nervous system injury to investigate the effects of elevated cAMP on neuronal survival and axonal regeneration. We studied the effects of intraocular injections of neurotrophic factors and/or a cAMP analogue (CPT-cAMP) on the regeneration of axotomized rat retinal ganglion cell (RGC) axons into peripheral nerve autografts. Elevation of cAMP alone did not significantly increase RGC survival or the number of regenerating RGCs. Ciliary neurotrophic factor increased RGC viability and axonal regrowth, the latter effect substantially enhanced by coapplication with CPT-cAMP. Under these conditions over 60% of surviving RGCs regenerated their axons. Neurotrophin-4/5 injections also increased RGC viability, but there was reduced long-distance axonal regrowth into grafts, an effect partially ameliorated by cAMP elevation. Thus, cAMP can act cooperatively with appropriate neurotrophic factors to promote axonal regeneration in the injured adult mammalian central nervous system.

Lack of 5-HT(1B) receptor and of serotonin transporter have different effects on the segregation of retinal axons in the lateral geniculate nucleus compared to the superior colliculus

We have shown previously that raised levels of serotonin (5-hydroxytryptamine or 5-HT) during development prevent retinal ganglion cell axons from segregating into eye-specific regions in their principal targets: the superior colliculus and the dorsal lateral geniculate nucleus. Possible mediators of 5-HT in this system include its plasma membrane transporter, which is transiently expressed by a sub-population of retinal ganglion cells, and the presynaptic 5-HT(1B) receptor carried on retinal ganglion cell axons. We analysed the retinal projections of 5-HT(1B) knockout (n=15), serotonin transporter knockout (n=14), serotonin transporter/5-HT(1B) double knockout (n=4) and monoamine oxidase A/5-HT(1B) double knockout (n=3) mice. In all four different knockout mice, the ipsilateral retinal projection to the superior colliculus was more diffuse and lost its characteristic patchy distribution. The alterations were most severe in the serotonin transporter knockout mice, where the ipsilateral retinal fibres covered the entire rostrocaudal and mediolateral extent of the superior colliculus, whereas in the 5-HT(1B) and double knockout mice, fibres retracted from the caudal and lateral superior colliculus. Abnormalities in the 5-HT(1B) knockout mice appeared only after postnatal day (P) 4. Treatment with parachlorophenylalanine (at P1-P12) to decrease serotonin levels caused an exuberance of the ipsilateral retinal fibres throughout the superior colliculus (n=9). In the dorsal lateral geniculate nucleus in contrast, the distribution and size of the ipsilateral retinal projection was normal in all four knockout mice. In the serotonin transporter knockout mice however, the contralateral retinal fibres failed to retract from the mediodorsal dorsal lateral geniculate nucleus, an abnormality that was reversed by early treatment with parachlorophenylalanine and in the serotonin transporter/5-HT(1B) double knockout. OUR OBSERVATIONS INDICATE: (1) that the lack of 5-HT transporter and the associated changes in 5-HT levels impair the segregation of retinal axons in both the superior colliculus and the dorsal lateral geniculate nucleus; (2) that 5-HT and 5-HT(1B) receptors are necessary for the normal refinement of the ipsilateral retinal fibres in the superior colliculus, but are not essential for the establishment of eye-specific segregation in the thalamus. Thus, both an excess and a lack of 5-HT affect the refinement of the superior colliculus retinal projection, while the establishment of eye-specific patterns in the dorsal lateral geniculate nucleus appears not to be sensitive to the lack of 5-HT or 5-HT(1B) receptors.

Pharmacodynamics of citicoline relevant to the treatment of glaucoma

Citicoline (exogenous CDP-choline) is a nontoxic and well-tolerated drug used in pharmacotherapy of brain insufficiency and some other neurological disorders, such as stroke, brain trauma, and Parkinson's disease. A few reports indicate that citicoline treatment may also be beneficial in glaucoma. Currently glaucoma is considered a neurodegenerative disease in which retinal ganglion cells (RGC) slowly die, likely in the apoptotic mechanism. Endogenous CDP-choline is a natural precursor of cellular synthesis of phospholipids, mainly phosphatydylcholine (PtdCho). Enhancement of PtdCho synthesis may counteract neuronal apoptosis and provide neuroprotection. Citicoline, when administered, undergoes a quick transformation to cytidine and choline, which are believed to enter brain cells separately and provide neuroprotection by enhancing PtdCho synthesis; similar effect may be expected to occur in glaucomatous RGC. Furthermore, citicoline stimulates some brain neurotransmitter systems, including the dopaminergic system, and dopamine is known as a major neurotransmitter in retina and postretinal visual pathways. In a double-blind, placebo-controlled study, treatment of glaucoma resulted in functional improvement in the visual system noted with electrophysiological methods. Development of citicoline as a treatment for glaucoma is indicated.

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.

Developmental changes in BDNF protein levels in the hamster retina and superior colliculus

Quantitative studies of ontogenetic changes in the levels of brain-derived neurotrophic factor (BDNF) mRNA and its effector, BDNF protein, are not available for the retinal projection system. We used an electrochemiluminescence immunoassay to measure developmental changes in the tissue concentration of BDNF within the hamster retina and superior colliculus (SC). In the SC, we first detected BDNF (about 9 pg/mg tissue) on embryonic day 14 (E14). BDNF protein concentration in the SC rises about fourfold between (E14) and postnatal day 4 (P4), remains at a plateau through P15, then declines by about one-third to attain its adult level by P18. By contrast, BDNF protein concentration in the retina remains low (about 1 pg/mg tissue) through P12, then increases 4.5-fold to attain its adult level on P18. The developmental changes in retinal and collicular BDNF protein concentrations are temporally correlated with multiple events in the structural and functional maturation of the hamster retinal projection system. Our data suggest roles for BDNF in the cellular mechanisms underlying some of these events and are crucial to the design of experiments to examine those roles.

Absence of hippocampal mossy fiber sprouting in transgenic mice overexpressing brain-derived neurotrophic factor

Excess neuronal activity upregulates the expression of two neurotrophins, nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) in adult hippocampus. Nerve growth factor has been shown to contribute the induction of aberrant hippocampal mossy fiber sprouting in the inner molecular layer of the dentate gyrus, however the role of prolonged brain-derived neurotrophic factor exposure is uncertain. We examined the distribution and plasticity of mossy fibers in transgenic mice with developmental overexpression of brain-derived neurotrophic factor. Despite 2--3-fold elevated BDNF levels in the hippocampus sufficient to increase the intensity of neuropeptide Y immunoreactivity in interneurons, no visible changes in mossy fiber Timm staining patterns were observed in the inner molecular layer of adult mutant hippocampus compared to wild-type mice. In addition, no changes of the mRNA expression of two growth-associated proteins, GAP-43 and SCG-10 were found. These data suggest that early and persistent elevations of brain-derived neurotrophic factor in granule cells are not sufficient to elicit this pattern of axonal plasticity in the hippocampus.

Intravitreal adenoviral gene transfer evokes an immune response in the retina that is directed against the heterologous lacZ transgene product but does not limit transgene expression

Recombinant E1-deleted adenoviral vectors (DeltaE1-Ad) are promising tools for in vivo gene transfer into the mammalian CNS including the retina. However, the duration of transgene expression is limited, and this limitation has partly been attributed to an immune response directed against vector-derived proteins. Here, we employed immunocytochemistry to assess the immune response to intravitreously injected DeltaE1-Ad encoding the lacZ gene or various neurotrophins (NTs). beta-Galactosidase was expressed by retinal cells for up to 4 weeks. Following intravitreal inoculation of AdCMV-lacZ, microglial and T cells were detected with a panel of antibodies in the retinal cell layers after 2 days (D2). The inflammatory response reached a maximum between D7 and D14. In contrast, no immune response was seen following injection of Ad encoding NTs. Yet, like with Ad-CMV-lacZ, their expression was also limited to approximately 4 weeks. Thus, beta-galactosidase seems to trigger a host immune response following intravitreal adenoviral lacZ gene transfer, but immune responses are not the cause of limited NT transgene expression from the CMV promoter in the inner retina.

Effects of methamphetamine-induced neurotoxicity on the development of neural circuitry: a hypothesis

Exposure of the developing brain to methamphetamine has well-studied biochemical and behavioral consequences. We review: (1) the effects of methamphetamine on mature serotonergic and dopaminergic pathways; (2) the mechanisms of methamphetamine neurotoxicity and (3) the role of serotonergic and dopaminergic signaling in sculpting developing neural circuitry. Consideration of these data suggest the types of neural circuit alterations that may result from exposure of the developing brain to methamphetamine and that may underlie functional defects.

Neuroprotective effects of brain-derived neurotrophic factor in eyes with NMDA-induced neuronal death

PURPOSE

To determine if brain-derived neurotrophic factor (BDNF) has a neuroprotective effect against N-methyl-D-aspartate (NMDA)-induced cell death in retina.

METHODS

NMDA was injected into the vitreous of rat eyes. NMDA-induced neuronal death was measured by morphometric analyses on cell counts of ganglion cell layer cells and thickness of retinal layers. Also, we conducted additional experiment using retrograde labeling with a fluorescent tracer (Fluoro-Gold) for exact counting of retinal ganglion cells (RGCs). In addition, intravitreal glutamate levels were measured with the use of a high-performance liquid chromatography (HPLC) system.

RESULTS

Morphometric analysis of retinal damage in NMDA-injected eyes showed that BDNF could protect inner retinal cells from glutamate receptor-mediated neuronal death. Also, counts of RGCs labeled with a fluorescent tracer showed that BDNF could protect RGCs from glutamate receptor-mediated neuronal death. Furthermore, measurements of intravitreal glutamate levels indicated an increase in this excitatory amino acid in the vitreous after NMDA injection.

CONCLUSIONS

Exogenous BDNF can protect inner retinal cells (possible RGCs and amacrine cells) from NMDA-induced neuronal death. However, increased intravitreal glutamate levels in response to NMDA-mediated neurotoxicity may augment retinal degeneration.

Neurotrophins and the dynamic regulation of the neuronal cytoskeleton

The morphology of neuronal axons and dendrites is dependent on the dynamics of the cytoskeleton. An understanding of neurodevelopment and adult neuroplasticity must therefore include a detailed description of the intrinsic and extrinsic mechanisms that regulate the organization and dynamics of actin filaments and microtubules. In this paper we review recent advances in the understanding of the dynamic regulation of neuronal morphology by interactions among cytoskeletal components and the regulation of the cytoskeleton by neurotrophins.

Alteration of the retinotectal projection map by the graft of mesencephalic floor plate or sonic hedgehog

The floor plate plays crucial roles in the specification and differentiation of neurons along the dorsal-ventral (DV) axis of the neural tube. The transplantation of the mesecephalic floor plate (mfp) into the dorsal mesencephalon in chick embryos alters the fate of the mesencephalon adjacent to the transplant from the tectum to the tegmentum, a ventral tissue of the mesencephalon. In this study, to test whether the mfp is involved in the specification of the DV polarity of the tectum and affects the projection patterns of retinal fibers to the tectum along the DV axis, we transplanted quail mfp into the dorsal mesencephalon of chick embryos, and analyzed projection patterns of dorsal and ventral retinal fibers to the tectum. In the embryos with the mfp graft, dorsal retinal fibers grew into the dorsal part of the tectum which is the original target for ventral but not dorsal retinal fibers and formed tight focuses there. In contrast, ventral retinal fibers did not terminate at any part of the tectum. Transplantation of Sonic hedgehog (Shh)-secreting quail fibroblasts into the dorsal mesencephalon also induced the ectopic tegmentum and altered the retinotectal projection along the DV axis, as the mfp graft did. These results suggest that some factors from the mesencephalic floor plate or the tegmentum, or Shh itself, play a crucial role in the establishment of the DV polarity of the tectum and the retinotectal projection map along the DV axis.

Stabilization of growing retinal axons by the combined signaling of nitric oxide and brain-derived neurotrophic factor

The pattern of axonal projections early in the development of the nervous system lacks the precision present in the adult. During a developmental process of refinement, mistargeted projections are eliminated while correct projections are retained. Previous studies suggest that during development nitric oxide (NO) is involved in the elimination of mistargeted retinal axons, whereas brain-derived neurotrophic factor (BDNF) may stabilize retinal axon arbors. It is unclear whether these neuromodulators interact. This study showed that NO induced growth cone collapse and retraction of developing retinal axons. This effect was not attributable to NO-induced neurotoxicity. BDNF protected growth cones and axons from the effects of NO. This effect was specific to BDNF, because neither nerve growth factor (NGF) nor neurotrophin-3 (NT-3) prevented NO-induced growth cone collapse and axon retraction. Exposure to both BDNF and NO, but not either factor alone, stabilized growth cones and axons. Stabilized axons exhibited minimal retraction or extension. This response appears to be a new axon "state" and not simply a partial amelioration of the effect of NO, because lower doses of BDNF or NO allowed axon extension. Furthermore, BDNF/NO-induced growth cone stabilization correlated with the appearance of a cytochalasin D-resistant population of actin filaments. BDNF protection from NO likely was mediated locally at the level of the growth cone, because growth cones or individual filopodia in contact with BDNF-coated beads were protected from NO-induced collapse. These findings suggest a cellular mechanism by which some axonal connections are stabilized and some are eliminated during development.