Nerve growth factor prevents the amblyopic effects of monocular deprivation

Neurotrophin NT-4/5 Promotes Structural Changes in Neurons of the Developing Visual Cortex

Current hypotheses on the mechanisms underlying the development and plasticity of the ocular dominance system through competitive interactions between pathways serving the two eyes strongly suggest the involvement of neurotrophins and their high affinity receptors. In the cat, infusion of the tyrosine kinase B ligand (trkB), neurotrophin-4/5 (NT-4/5), abolishes ocular dominance plasticity that follows monocular deprivation (Gillespie et al., 2000), while tyrosine kinase A and C ligands (trkA and trkC) do not have this effect. One interpretation of this finding is that NT-4/5 causes overgrowth and sprouting of thalamocortical and/or corticocortical terminals, leading to promiscuous neuronal connections which override the experience-dependent fine tuning of connections based on correlated activity. The present study tested whether neurons in cortical regions infused with NT-4/5 showed anatomical changes compatible with this hypothesis. Cats at the peak of the critical period received chronic infusion NT-4/5 into visual cortical areas 17/18 via an osmotic minipump. Visual cortical neurons were labeled in fixed slices using the DiOlistics methods (Gan et al., 2000) and analyzed in confocal microscopy. Infusion of NT-4/5 induced a significant increase of spine-like processes on primary dendrites and a distinctive sprouting of protuberances from neuronal somata in all layers. The increase of neuronal membrane was paralleled by an increase in density of the presynaptic marker synaptophysin in infused areas, suggesting an increase in the numbers of synapses. A contingent of these newly formed synapses may feed into inhibitory circuits, as suggested by an increase of GAD-65 immunostaining in NT-4/5 affected areas. These anatomical changes are consistent with the physiological changes in such animals, suggesting that excess trkB neurotrophin can stimulate the formation of promiscuous connections during the critical period.

Ocular dominance plasticity: Molecular mechanisms revisited

Ocular dominance plasticity (ODP) is a type of cortical plasticity operating in visual cortex of mammals that are endowed with binocular vision based on the competition-driven disparity. Earlier, a molecular mechanism was proposed that catecholamines play an important role in the maintenance of ODP in kittens. Having survived the initial test, the hypothesis was further advanced to identify noradrenaline (NA) as a key factor that regulates ODP in the immature cortex. Later, the ODP-promoting effect of NA is extended to the adult with age-related limitations. Following the enhanced NA availability, the chain events downstream lead to the β-adrenoreceptor-induced cAMP accumulation, which in turn activates the protein kinase A. Eventually, the protein kinase translocates to the cell nucleus to activate cAMP responsive element binding protein (CREB). CREB is a cellular transcription factor that controls the transcription of various genes, underpinning neuronal plasticity and long-term memory. In the advent of molecular genetics in that various types of new tools have become available with relative ease, ODP research has lightly adopted in the rodent model the original concepts and methodologies. Here, after briefly tracing the strategic maturation of our quest, the review moves to the later development of the field, with the emphasis placed around the following issues: (a) Are we testing ODP per se? (b) What does monocular deprivation deprive of the immature cortex? (c) The critical importance of binocular competition, (d) What is the adult plasticity? (e) Excitation-Inhibition balance in local circuits, and (f) Species differences in the animal models.

Targeted Imaging of VCAM-1 mRNA in a Mouse Model of Laser-Induced Choroidal Neovascularization Using Antisense Hairpin-DNA-Functionalized Gold-Nanoparticles

Mouse laser-induced choroidal neovascularization (mouse LCNV) recapitulates the "wet" form of human age-related macular degeneration (AMD). Vascular cell adhesion molecule-1 (VCAM-1) is a known inflammatory biomarker, and it increases in the choroidal neovascular tissues characteristic of this experimental model. We have designed and constructed gold nanoparticles (AuNPs) functionalized with hairpin-DNA that incorporates an antisense sequence complementary to VCAM-1 mRNA (AS-VCAM-1 hAuNPs) and tested them as optical imaging probes. The 3' end of the hairpin is coupled to a near-infrared fluorophore that is quenched by the AuNP surface via Förster resonance energy transfer (FRET). Hybridization of the antisense sequence to VCAM-1 mRNA displaces the fluorophore away from the AuNP surface, inducing fluorescent activity. In vitro testing showed that hAuNPs hybridize to an exogenous complementary oligonucleotide within a pH range of 4.5-7.4, and that they are stable at reduced pH. LCNV mice received tail-vein injections of AS-VCAM-1 hAuNPs. Hyperspectral imaging revealed the delivery of AS-VCAM-1 hAuNPs to excised choroidal tissues. Fluorescent images of CNV lesions were obtained, presumably in response to the hybridization of AS-hAuNPs to LCNV-induced VCAM-1 mRNA. This is the first demonstration of systemic delivery of hAuNPs to ocular tissues to facilitate mRNA imaging of any target.

Neuronal plasticity and neurotrophic factors in drug responses

Neurotrophic factors, particularly brain-derived neurotrophic factor (BDNF) and other members of the neurotrophin family, are central mediators of the activity-dependent plasticity through which environmental experiences, such as sensory information are translated into the structure and function of neuronal networks. Synthesis, release and action of BDNF is regulated by neuronal activity and BDNF in turn leads to trophic effects such as formation, stabilization and potentiation of synapses through its high-affinity TrkB receptors. Several clinically available drugs activate neurotrophin signaling and neuronal plasticity. In particular, antidepressant drugs rapidly activate TrkB signaling and gradually increase BDNF expression, and the behavioral effects of antidepressants are mediated by and dependent on BDNF signaling through TrkB at least in rodents. These findings indicate that antidepressants, widely used drugs, effectively act as TrkB activators. They further imply that neuronal plasticity is a central mechanism in the action of antidepressant drugs. Indeed, it was recently discovered that antidepressants reactivate a state of plasticity in the adult cerebral cortex that closely resembles the enhanced plasticity normally observed during postnatal critical periods. This state of induced plasticity, known as iPlasticity, allows environmental stimuli to beneficially reorganize networks abnormally wired during early life. iPlasticity has been observed in cortical as well as subcortical networks and is induced by several pharmacological and non-pharmacological treatments. iPlasticity is a new pharmacological principle where drug treatment and rehabilitation cooperate; the drug acts permissively to enhance plasticity and rehabilitation provides activity to guide the appropriate wiring of the plastic network. Optimization of iPlastic drug treatment with novel means of rehabilitation may help improve the efficacy of available drug treatments and expand the use of currently existing drugs into new indications.

Nerve Growth Factor: A Focus on Neuroscience and Therapy

Nerve growth factor (NGF) is the firstly discovered and best characterized neurotrophic factor, known to play a critical protective role in the development and survival of sympathetic, sensory and forebrain cholinergic neurons. NGF promotes neuritis outgrowth both in vivo and in vitro and nerve cell recovery after ischemic, surgical or chemical injuries. Recently, the therapeutic property of NGF has been demonstrated on human cutaneous and corneal ulcers, pressure ulcer, glaucoma, maculopathy and retinitis pigmentosa. NGF eye drops administration is well tolerated, with no detectable clinical evidence of systemic or local adverse effects. The aim of this review is to summarize these biological properties and the potential clinical development of NGF.

In Vivo Imaging of Retinal Hypoxia in a Model of Oxygen-Induced Retinopathy

Ischemia-induced hypoxia elicits retinal neovascularization and is a major component of several blinding retinopathies such as retinopathy of prematurity (ROP), diabetic retinopathy (DR) and retinal vein occlusion (RVO). Currently, noninvasive imaging techniques capable of detecting and monitoring retinal hypoxia in living systems do not exist. Such techniques would greatly clarify the role of hypoxia in experimental and human retinal neovascular pathogenesis. In this study, we developed and characterized HYPOX-4, a fluorescence-imaging probe capable of detecting retinal-hypoxia in living animals. HYPOX-4 dependent in vivo and ex vivo imaging of hypoxia was tested in a mouse model of oxygen-induced retinopathy (OIR). Predicted patterns of retinal hypoxia were imaged by HYPOX-4 dependent fluorescence activity in this animal model. In retinal cells and mouse retinal tissue, pimonidazole-adduct immunostaining confirmed the hypoxia selectivity of HYPOX-4. HYPOX-4 had no effect on retinal cell proliferation as indicated by BrdU assay and exhibited no acute toxicity in retinal tissue as indicated by TUNEL assay and electroretinography (ERG) analysis. Therefore, HYPOX-4 could potentially serve as the basis for in vivo fluorescence-based hypoxia-imaging techniques, providing a tool for investigators to understand the pathogenesis of ischemic retinopathies and for physicians to address unmet clinical needs.

Role of electrical activity of neurons for neuroprotection

Neurons of the central nervous system (CNS) of adult mammals can be damaged in a variety of ways. Most neurons rapidly die after injury. Even if the injured CNS neurons do not die in a short time, the neurons eventually die because they are not able to regenerate their axons to reconnect with their normal targets. In addition, neurons are normally not replaced. Therefore, much work has been directed toward understanding of the molecular regulation of the CNS degeneration following injury, and different experimental strategies are being used to try to protect the damaged neurons. Following axonal lesion, the neurons not only need to survive but also to reconnect to be functionally relevant, and efforts are directed toward not only survival but also axonal regeneration and proper rewiring of injured neurons. Recent experimental data suggest that electrical activity, endogenous or exogenous, can enhance neuronal survival and regeneration in vitro and in vivo. This chapter reviews the evidence that have been obtained on the role of neuronal electrical activity on neuroprotection. We will develop perspectives toward neuroprotection and regeneration of adult lesioned CNS neurons based on electrical activity-dependent cell survival that may be applicable to various diseases of the CNS.

Setting the pace for retinal development: environmental enrichment acts through insulin-like growth factor 1 and brain-derived neurotrophic factor

Environmental enrichment strongly affects visual system maturation both at retinal and cortical levels. Which molecular pathways are activated by an enriched environment (EE) to regulate visual system development has not been clarified. Here, we show that early [postnatal day 1 (P1) to P7] insulin-like growth factor 1 (IGF-1) injections in the eyes of non-EE rat pups mimic EE effects both in increasing BDNF levels in the retinal ganglion cell layer at P10 and in determining a more adult-like retinal acuity, assessed with pattern electroretinogram at P25. Blocking IGF-1 action in EE animals during the same early postnatal time window by injecting the IGF-1 receptor antagonist JB1 prevents EE effects both on BDNF expression and on retinal acuity maturation. Reducing BDNF expression in the retina of IGF-1-treated rats prevents IGF-1 effects on retinal acuity development. Finally, we show that tyrosine hydroxylase (TH) expression is increased in the retina of P10 EE and IGF-1-treated rats and that blocking TH expression in EE animals prevents EE from affecting retinal acuity development. Thus, early levels of IGF-1 play a key role in mediating EE effects on retinal development through an action that requires BDNF and involves dopaminergic amacrine cell network.

Glaucoma alters the expression of NGF and NGF receptors in visual cortex and geniculate nucleus of rats: effect of eye NGF application

We investigated the effect of glaucoma (GL) on nerve growth factor (NGF) presence in two brain visual areas. Rats with elevated intraocular pressure (EIOP), induced by hypertonic saline injection in the episcleral vein, were treated with eye topical application of saline or NGF. Rats were subsequently sacrificed, and brain tissues were used for immunohistochemical, biochemical, and molecular analyses. We found that GL alters the basal level of NGF and NGF receptors in brain visual centers and that NGF eye application normalized these deficits. These findings demonstrate that the reduced presence of NGF can arise due to degenerative events in retinal and brain visual areas.

Odorant deprivation reversibly modulates transsynaptic changes in the NR2B-mediated CREB pathway in mouse piriform cortex

The olfactory system is an outstanding model for understanding activity-dependent neuronal plasticity in mammals. Olfactory sensory neurons (OSNs) in the periphery project onto mitral/tufted cells in the olfactory bulb (OB) and these mitral/tufted cells in turn project to piriform cortex (PC). Numerous studies have examined changes in OB after a permanent OSN ablation, but little is known about "trans-transsynaptic" changes taking place in the PC. Permanent zinc sulfate lesion of the olfactory epithelium resulted in a selective loss of the NMDA receptor NR2B protein and mRNA expression in pyramidal cells in layer IIb of PC after 2-7 d. Regulatory elements affected by NR2B signaling, namely the phosphorylation of CREB, were also downregulated only in layer IIb neurons. These changes could be caused by OSN axon loss in the zinc sulfate lesion, or to a reduced activity. To test this hypothesis, we performed both permanent and reversible naris occlusion, which blocks odorant access to the nasal cavities and OSN activity. The expression of NR2B and phospho-CREB were downregulated 5 d after occlusion and this reduction was fully restored 10 d after reopening of the naris. Subsequently, we identified the subset of pyramidal cells in layer IIb that are especially sensitive to the loss of odor-evoked activity using double retrograde tracers. In summary, the present study provides an initial characterization of the molecular mechanisms associated with odor stimulation on second order neuronal plasticity and phenotype in the olfactory system.

Preferential silencing of a common dominant rhodopsin mutation does not inhibit retinal degeneration in a transgenic model

Autosomal dominant retinitis pigmentosa caused by the frequent rhodopsin P23H mutation is characterized by progressive photoreceptor cell death eventually leading to blindness and for which no therapies are available. Considering the gain-of-function effect exerted by the P23H mutation, strategies aimed at silencing the expression of the mutated allele, like RNA interference, are desirable. We have designed small interfering RNAs (siRNA) to silence specifically the P23H rhodopsin allele expressed by a transgenic rat model of the disease. We have selected in vitro one siRNA and generated an adeno-associated viral (AAV) vector expressing the short hairpin RNA (shRNA) based on the selected siRNA. In vitro the shRNA significantly inhibits the expression of the P23H but not the wild-type rhodopsin allele. Subretinal administration of the AAV2/5 vector encoding the shRNA in P23H transgenic rats results in inhibition of rhodopsin P23H expression that is not able to prevent or block photoreceptor degeneration. Since rhodopsin is the most abundant rod photoreceptor protein, systems resulting in more robust shRNA expression in the retina may be required to achieve therapeutic efficacy in vivo.

Medical treatment of amblyopia: present state and perspectives

A brief review is provided on the role of experimental modulation of the visual system during its plastic period. The importance of several substances has been demonstrated. Potential clinical applications of available studies on the treatment of amblyopia are summarized. The role of the dopaminergic system and particularly of dopamine in amblyopia are also discussed. The results of the use of cytidine-5'-diphosphocholine (citicoline) in amblyopia are evaluated in detail. The advantages of and indications for an association of citicoline with part-time occlusion are also discussed. Finally, new administration modalities of citicoline, more detailed evaluation of its efficacy, and the use of new substances are briefly analyzed.

Structural and functional recovery from early monocular deprivation in adult rats

Visual deficits caused by abnormal visual experience during development are hard to recover in adult animals. Removal of chondroitin sulfate proteoglycans from the mature extracellular matrix with chondroitinase ABC promotes plasticity in the adult visual cortex. We tested whether chondroitinase ABC treatment of adult rats facilitates anatomical, functional, and behavioral recovery from the effects of a period of monocular deprivation initiated during the critical period for monocular deprivation. We found that chondroitinase ABC treatment coupled with reverse lid-suturing causes a complete recovery of ocular dominance, visual acuity, and dendritic spine density in adult rats. Thus, manipulations of the extracellular matrix can be used to promote functional recovery in the adult cortex.

Systemic but not intraocular Epo gene transfer protects the retina from light-and genetic-induced degeneration

Molecules with neurotrophic activity are being evaluated for treatment of retinitis pigmentosa in animal models. In particular, great interest has been focused recently on erythropoietin (Epo). Evidence of its neurotrophic activity comes mainly from data demonstrating photoreceptor protection in a rodent light-damage model through systemic administration of a recombinant form of this hormone. Our goal was to test whether Epo retinal gene transfer can rescue or delay photoreceptor cell death. We delivered adeno-associated viral vectors encoding Epo intraocularly and, for comparison, intramuscularly to one light-induced and two genetic models of retinal degeneration. Intraocular Epo gene transfer resulted in sustained hormone expression in the eye, which was undetectable systemically. In contrast, Epo intramuscular gene transfer resulted in hormone secretion in the circulation, which was not detected in ocular fluids. The protein secreted from muscle and retina is of the same molecular weight as a commercial recombinant human Epo. Interestingly, following systemic but not intraocular Epo delivery, morphological photoreceptor protection was observed in the light-damage and rds/peripherin (Prph2) models of retinal degeneration. In the light-damage model, the morphological rescue was accompanied by a significant electrophysiological improvement of photoreceptor function. In contrast, no photoreceptor rescue was observed following Epo gene transfer in the rd10 model. This suggests that different apoptotic mechanisms, with varying sensitivities to Epo, occur in different retinal degeneration models. In conclusion, our data support Epo as a neuroprotective agent in some, but not all, retinal degenerations. Further, rescue is observed in specific models after systemic but not intraocular Epo gene transfer.

Molecular substrates of plasticity in the developing visual cortex

Ocular dominance plasticity may be the paradigmatic in vivo model of activity-dependent plasticity. More than four decades of intense research has delineated the network-level rules that govern synaptic change in this model. The recent characterization of a murine model for ocular dominance plasticity has facilitated rapid progress on a new front, extending our understanding of the molecular mechanisms underlying ocular dominance plasticity. In this review, we highlight recent advances in this research effort, focusing in particular on signaling pathways mediating shifts in ocular dominance, and mechanisms underlying the timing of the critical period.

Nerve growth factor favours long-term depression over long-term potentiation in layer II-III neurones of rat visual cortex

Nerve growth factor (NGF) has been shown to regulate plasticity in the visual cortex of monocularly deprived animals. However, to date, few attempts have been made to investigate the role of NGF in synaptic plasticity at the cellular level. In the study reported here we looked at the effects of exogenously applied NGF on synaptic plasticity of layer II-III regular spiking (RS) neurones in visual cortex of 16- to 18-day-old rats. We found that local application of NGF converted high frequency stimulation (HFS)-induced long-term potentiation (LTP) into long-term depression (LTD). We showed that this shift of synaptic plasticity was also obtained with bath application of NGF during HFS. Application of NGF subsequent to HFS left LTP unaffected, conferring temporal constraints on NGF efficacy. NGF effects on LTP were mediated by TrkA receptors. Indeed, blockade of TrkA by monoclonal antibody prevented NGF from inducing LTD following HFS. Low frequency stimulation (LFS) elicited LTD in RS cells. We found that NGF or blockade of NGF signalling by anti-TrkA antibody did not change the amplitude of the LTD induced by LFS. Thus, the NGF effect is selective for synaptic modifications induced by HFS in RS cells. The present results indicate that NGF may modulate the sign of long-term changes of synaptic efficacy in response to high frequency inputs.

Acute and long-term synaptic modulation by neurotrophins

While it has now been well accepted that neurotrophins play an important role in synapse development and plasticity, the specific effects of each neurotrophin on different populations of neurons at different developmental stages have just begun to be worked out. Moreover, the cellular and molecular mechanisms underlying the synaptic function of neurotrophins remain poorly understood. In general, synaptic effects of neurotrophins could be divided into two categories: acute effect on synaptic transmission and plasticity occurring within seconds or minutes after cells are exposed to a neurotrophin, and long-term effect on synaptic structures and function that takes days to accomplish. In this review I have considered the previous findings on neurotrophic regulation of synapses in view of these two categories. Acute and long-term effects of neurotrophins are reexamined in detail in three model systems: the neuromuscular junction, the hippocampus and the visual cortex. Potential molecular mechanisms that mediate the acute or long-term neurotrophic regulation are discussed. Efforts are made to understand the mechanistic differences between the two effects and their relationships. Further study of these mechanisms will help us better understand how neurotrophins can achieve diverse and synapse-specific modulation.

Postnatal changes in nerve growth factor and brain derived neurotrophic factor levels in the retina, visual cortex, and geniculate nucleus in rats with retinitis pigmentosa

Royal College of Surgeons (RCS) rats are a well established animal model of inherited retinitis pigmentosa (RP). Using RCS rats we examined the distribution of nerve growth factor (NGF) and brain derived neurotrophic factor (BDNF) in the visual cortex, geniculate nucleus and retina at three different postnatal ages. It was found that the retina of rats with RP expresses low amounts of NGF and BDNF in young and adult life. Altered levels of these factors were found in the visual cortex and in the geniculate nucleus. Our findings indicate that NGF and BDNF are differentially affected in the visual system of developing and adult RCS rats, suggesting that neurotrophins may be implicated in the pathogenesis of inherited RP.

Rat visual cortical neurones express TrkA NGF receptor

In this study we report the expression of TrkA receptor within the rat visual cortex during postnatal development and in adulthood, using a specific monoclonal antibody which recognizes the extracellular domain of TrkA receptor. TrkA was not detected by immunohistochemistry at postnatal day 13 (P13), i.e. before eye opening. At P22 TrkA was mostly localised in cortical fibre-like processes. At P39 and P90, TrkA-positive neuronal cell bodies in supragranular and infragranular layers were found. Using double immunohistochemistry, labelled cells were identified as intrinsic cholinergic neurones, and as interneurones expressing calbindin and neuropeptide Y. We conclude that TrkA is expressed in visual cortical neurones during postnatal development and in adulthood and that its pattern of expression is developmentally regulated.

Tool-use learning selectively induces expression of brain-derived neurotrophic factor, its receptor trkB, and neurotrophin 3 in the intraparietal multisensorycortex of monkeys

When humans repeatedly use a tool, our body image alters until the tool finally becomes a part or an extension of the body. This alteration of body image perhaps results from re-integration of somatosensory and visual signals. We trained Japanese monkeys to use a rake-shaped tool to retrieve a distant food pellet, then used a novel tissue-sampling method to suction brain tissue from the anterior bank of their intraparietal sulcus, where somatosensory and visual signals converge. Examination of the messenger RNA expression levels of neurotrophins and their receptors using real-time quantitative polymerase chain reaction revealed learning-selective induction in the expression of brain-derived neurotrophic factor, its receptor trkB, and NT-3 during, but not after, the learning. These results suggest that these factors are involved in the reorganization of the somatosensory and visual signals in the anterior bank of the intraparietal sulcus when monkeys are learning the use of the tool.

Neurotrophin secretion from hippocampal neurons evoked by long-term-potentiation-inducing electrical stimulation patterns

The neurotrophin (NT) brain-derived neurotrophic factor (BDNF) plays an essential role in the formation of long-term potentiation (LTP). Here, we address whether this modulation by BDNF requires its continuous presence, or whether a local increase in BDNF is necessary during a specific time period of LTP initiation. Using electrical field stimulation of primary cultures of hippocampal neurons, we demonstrate that short high-frequency bursts of stimuli that induce LTP evoke also an instantaneous secretion of BDNF. In contrast, stimuli at low frequencies, inducing long-term depression, do not enhance BDNF secretion, suggesting that BDNF is specifically present, and thus required, at the time of LTP induction. The field-stimulation-mediated BDNF secretion depends on the formation of action potentials and is induced by IP(3)-mediated Ca(2+) release from intracellular stores. Experiments, aimed at determining the sites of NT secretion that use NT6, showed similar patterns of surface labeling by field stimulation to those shown previously by high potassium.

Acute cholinergic rescue of synaptic plasticity in the neurodegenerating cortex of anti-nerve-growth-factor mice

Deficits in cholinergic systems innervating cerebral cortex are associated with cognitive impairment during senescence and in age-related neurodegenerative pathologies. However, little is known about the role of cholinergic pathways in modulating cortical plasticity. Basal forebrain cholinergic neurons are a major target for nerve-growth factor (NGF). In order to investigate the relationship between cholinergic innervation and cortical synaptic plasticity, we exploited a transgenic mouse model in which the activity of NGF in the adult nervous system is neutralized by the expression of blocking antibodies to NGF itself (anti-NGF mice) [Ruberti, F. et al. (2000). J. Neurosci. 20, 2589-2601]. In 6-month-old anti-NGF mice, we show that the reduction in cholinergic innervation of the cortex is associated with different forms of synaptic plasticity impairment. A local, acute increase in the availability of acetylcholine rescues these synaptic plasticity deficits, thus indicating that a cholinergic system mediates the impairment of cortical plasticity at this early stage of the neurodegenerative process triggered by NGF neutralization. Our results represent an important step in unveiling the pivotal role of cholinergic transmission in modulating adult cortical plasticity.

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.

Effects of early visual experience and diurnal rhythms on BDNF mRNA and protein levels in the visual system, hippocampus, and cerebellum

The expression of brain-derived neurotrophic factor (BDNF) mRNA and the secretion of BDNF protein are tightly regulated by neuronal activity. Thus, BDNF has been proposed as a mediator of activity-dependent neural plasticity. Previous studies showed that dark rearing (DR) reduces BDNF mRNA levels in the primary visual cortex (V1), but the effects of visual experience on BDNF protein levels are unknown. We report that rearing in constant light or DR alters BDNF mRNA and protein levels in the retina, superior colliculus (SC), V1, hippocampus (HIPP), and cerebellum (CBL), although the changes in mRNA and protein are not always correlated. Most notably, DR increases BDNF protein levels in V1 although BDNF mRNA is decreased. BDNF protein levels also undergo diurnal changes. In the retina, V1, and SC, BDNF protein levels are higher during the light phase of the circadian cycle than during the dark phase. By contrast, in HIPP and CBL, the tissue concentration of BDNF protein is higher during the dark phase. The discrepancies between the experience-dependent changes in BDNF mRNA and protein suggest that via its effects on neuronal activity, early sensory experience alters the trafficking, as well as the synthesis, of BDNF protein. The circadian changes in BDNF protein suggest that BDNF could cause the diurnal modulation of synaptic efficacy in some neural circuits. The fluctuations in BDNF levels in nonvisual structures suggest a potential role of BDNF in mediating plasticity induced by hormones or motor activity.

Effects of early visual experience and diurnal rhythms on BDNF mRNA and protein levels in the visual system, hippocampus, and cerebellum

The expression of brain-derived neurotrophic factor (BDNF) mRNA and the secretion of BDNF protein are tightly regulated by neuronal activity. Thus, BDNF has been proposed as a mediator of activity-dependent neural plasticity. Previous studies showed that dark rearing (DR) reduces BDNF mRNA levels in the primary visual cortex (V1), but the effects of visual experience on BDNF protein levels are unknown. We report that rearing in constant light or DR alters BDNF mRNA and protein levels in the retina, superior colliculus (SC), V1, hippocampus (HIPP), and cerebellum (CBL), although the changes in mRNA and protein are not always correlated. Most notably, DR increases BDNF protein levels in V1 although BDNF mRNA is decreased. BDNF protein levels also undergo diurnal changes. In the retina, V1, and SC, BDNF protein levels are higher during the light phase of the circadian cycle than during the dark phase. By contrast, in HIPP and CBL, the tissue concentration of BDNF protein is higher during the dark phase. The discrepancies between the experience-dependent changes in BDNF mRNA and protein suggest that via its effects on neuronal activity, early sensory experience alters the trafficking, as well as the synthesis, of BDNF protein. The circadian changes in BDNF protein suggest that BDNF could cause the diurnal modulation of synaptic efficacy in some neural circuits. The fluctuations in BDNF levels in nonvisual structures suggest a potential role of BDNF in mediating plasticity induced by hormones or motor activity.

Mismatch between BDNF mRNA and protein expression in the developing visual cortex: the role of visual experience

Brain-derived neurotrophic factor (BDNF) messenger RNA (mRNA) expression in the rat visual cortex of young and postnatal day 90 (P90) animals is developmentally regulated and influenced by visual experience. In the present paper we compared the expression of BDNF mRNA to the actual changes of BDNF protein occurring during postnatal development and verified whether BDNF protein distribution is controlled by visual activity. To achieve this aim we analysed BDNF mRNA and/or BDNF protein cellular distribution in the rat visual cortex at different postnatal ages by using immunohistochemistry and highly sensitive in situ hybridization. We found that before eye opening (P13), in all cortical layers a large number of visual cortical neurons contain BDNF mRNA with no detectable amount of BDNF protein. At later ages (P23 and P90), the number of BDNF-immunostained cells increases; most neurons are double labelled for BDNF mRNA and protein, and a small group of neurons is labelled only for BDNF protein. The cellular increase of BDNF immunolabelling is blocked in animals deprived of visual experience from birth (dark rearing), with a large population of neurons containing BDNF mRNA but not BDNF protein. This is similar to what is observed before eye opening. Exposure of dark-reared rats to a brief period (2 h) of light restores a good match between BDNF mRNA and BDNF protein cellular expression. We propose that visual experience controls the neuronal content of BDNF mRNA and BDNF protein in developing visual cortex.

Infusion of nerve growth factor (NGF) into kitten visual cortex increases immunoreactivity for NGF, NGF receptors, and choline acetyltransferase in basal forebrain without affecting ocular dominance plasticity or column development

Intracerebroventricular or intracortical administration of nerve growth factor (NGF) has been shown to block or attenuate visual cortical plasticity in the rat. In cats and ferrets, the effects of exogenous NGF on development and plasticity of visual cortex have been reported to be small or nonexistent. To determine whether locally delivered NGF affects ocular dominance column formation or the plasticity produced by monocular deprivation in cats at the height of the critical period, we infused recombinant human NGF into the primary visual cortex of kittens using an implanted cannula minipump. NGF had no effect on the normal developmental segregation of geniculocortical afferents into ocular dominance columns as determined both physiologically and anatomically. The plasticity of binocular visual cortical responses induced by monocular deprivation was also normal in regions of immunohistochemically detectable NGF infusion, as measured using intrinsic signal optical imaging and single-unit electrophysiology. Immunohistochemical analysis of the basal forebrain regions of the same animals demonstrated that the NGF infused into cortex was biologically active, producing an increase in the number of NGF-, TrkA-, p75(NTR)-, and choline acetyltransferase-positive neurons in basal forebrain nuclei in the hemisphere ipsilateral to the NGF minipump compared to the contralateral basal forebrain neurons. We conclude that NGF delivered locally to axon terminals of cholinergic basal forebrain neurons resulted in increases in protein expression at the cell body through retrograde signaling.

Nitric oxide, impulse activity, and neurotrophins in visual system development(1)

Topographic refinement of synaptic connections within the developing visual system involves a variety of molecules which interact with impulse activity in order to produce the precise retinotopic maps found in the adult brain. Nitric oxide (NO) has been implicated in this process, as have various growth factors. Within the subcortical visual system, we have recently shown that nitric oxide contributes to pathway refinement in the superior colliculus (SC). Long-term potentiation (LTP) and long-term depression (LTD) are also expressed in SC during the time that this pathway undergoes refinement. The role of NO has been demonstrated by showing that refinement of ipsilateral fibers in the retinocollicular pathway is significantly delayed in gene knockout mice in which both the endothelial and neuronal isoforms of nitric oxide synthase (NOS) have been disrupted. The effect also depends upon Ca(2+) channels because refinement of both the ipsilateral retinocollicular and retinogeniculate pathways is disrupted in genetic mutants in which the beta3 subunit of the Ca(2+) channel has been deleted. LTD may also be involved in this process, because the time course of its expression correlates with that of pathway refinement and LTD magnitude is depressed by nitrendipine, an L-type Ca(2+) channel blocker. LTP is also expressed during early postnatal development in the LGN and SC and may contribute to synaptic stabilization. The role of neurotrophins in pathway refinement in the visual system is also reviewed.

Differential effects of neurotrophins on ocular dominance plasticity in developing and adult cat visual cortex

In the present study we examine the influence of neurotrophins on experience-dependent synaptic rearrangement in developing and adult visual cortex. Brain-derived neurotrophic factor (BDNF) or nerve growth factor (NGF) was continuously infused into cortical area 18, and the functional architecture of the cortex was examined by use of optical and electrophysiological recording techniques. In kittens, BDNF infusion during monocular deprivation (MD) reversed the normally occurring ocular dominance (OD) shift towards the non-deprived eye so that the deprived eye dominated the BDNF-treated cortex after MD. Under conditions of equal activation of thalamocortical synapses, i.e. when animals were either subject to binocular deprivation (BD) or reared without deprivation, BDNF infusion did not disrupt binocularity of cortical units, but reversed the natural OD bias towards the contralateral eye in favour of the ipsilateral eye. In addition, BDNF treatment in kittens led to a loss of the orientation selectivity of cortical units irrespective of rearing conditions. In adult animals, BDNF influenced neither OD distributions nor orientation selectivity. The effect of NGF was markedly different. It was ineffective in kittens but in adult animals it caused a shift of OD towards the deprived eye when MD was combined with NGF infusion. However, in this case orientation selectivity was preserved. Thus, both neurotrophins have profound activity- and age-dependent effects on the functional architecture of the visual cortex. Moreover, our results indicate that simple substitution of neurotrophins in excess is unlikely to compensate for deprivation effects by preserving or restoring the normal functional architecture of the cortex.

Blocking the NGF-TrkA interaction rescues the developmental loss of LTP in the rat visual cortex: role of the cholinergic system

Although nerve growth factor (NGF) is a crucial factor in the activity-dependent development and plasticity of visual cortex, its role in synaptic efficacy changes is largely undefined. We demonstrate that the maintenance phase of long-term potentiation (LTP) is blocked by local application of exogenous NGF in rat visual cortex at an early stage of postnatal development. Long-term depression (LTD) and bidirectional plasticity are unaffected. At later postnatal ages, blockade of either endogenous NGF by immunoadhesin (TrkA-IgG) or TrkA receptors by monoclonal antibody rescues LTP. Muscarinic receptor activation/inhibition suggests that LTP dependence on NGF is mediated by the cholinergic system. These results indicate that NGF regulates synaptic strength in well-characterized cortical circuitries.

Postsynaptic target specificity of neurotrophin-induced presynaptic potentiation

The role of the target cell in neurotrophin-induced modifications of glutamatergic synaptic transmission was examined in cultured hippocampal neurons. Brain-derived neurotrophic factor (BDNF) induced rapid and persistent potentiation of evoked glutamate release when the postsynaptic neuron was glutamatergic, or excitatory (E-->E), but not when it was GABAergic, or inhibitory (E-->1). This target-specific action of BDNF was also found at divergent outputs of a single presynaptic neuron innervating both glutamatergic and GABAergic neurons, suggesting that individual terminals can be independently modified. Surprisingly, BDNF increased the frequency of miniature postsynaptic currents at both E-->E and E-->I, although it had no effect on evoked currents at E-->I. Finally, potentiation by neurotrophin-3 (NT-3) was also target specific. The selective effect at E-->E suggests that retrograde signaling by the postsynaptic target cell endows a localized presynaptic action of neurotrophins.

Differential regulation of brain-derived neurotrophic factor messenger RNA cellular expression in the adult rat visual cortex

In this study, we report a comparative analysis of the distribution of brain-derived neurotrophic factor messenger RNA in the binocular primary visual cortex of rats analysed at the end of the critical period for monocular deprivation (postnatal day 35) and during adulthood (postnatal day 90). High-resolution non-isotopic in situ hybridization coupled with Nissl staining allowed to determine the relative number of neurons expressing brain-derived neurotrophic factor messenger RNA. In postnatal day 90 rats, the relative number of neurons positive for brain-derived neurotrophic factor messenger RNA significantly decreases in layer II/III with respect to postnatal day 35 animals, being constant in all the other cortical layers. Moreover, we demonstrate that dark rearing for 22 days, starting from postnatal day 90, determines: (i) a decrease of the overall level of brain-derived neurotrophic factor messenger RNA with a consequent reduction of labelling intensity in all cells throughout cortical layers II-VI; (ii) an increase of cell numbers expressing brain-derived neurotrophic factor messenger RNA in layers IV and V; and (iii) a decreased intensity of staining for brain-derived neurotrophic factor messenger RNA in dendrites after dark rearing. A re-exposure to light for 2 h after the period of darkness almost restores the number of brain-derived neurotrophic factor RNA-positive neurons. We conclude that the maturation of brain-derived neurotrophic factor messenger RNA in neurons of layer II/III goes beyond postnatal days 35-40, which can be considered the end of the critical period [Fagiolini M. et al. (1994) Vis. Res., 34, 709-720]. Moreover, we show that the cellular expression of brain-derived neurotrophic factor messenger RNA is regulated by light in adult rats as well as during development.

Amblyopia decreases activation of the corticogeniculate pathway and visual thalamic reticularis in attentive rats: a 'focal attention' hypothesis

In rats which were rendered monocular amblyopic by lid suturing one eye during a critical period, the intensity of neuronal activation in parts of the monocular segments of the striate cortex (layers 4 and 6) and lateral geniculate nucleus, and in the visual segment of the thalamic reticular nucleus, was determined after exploration of a novel-complex environment. Quantitative analysis of the number of Fos-labelled neurons per unit area showed that, in comparison to the structures contralateral to the normal eye, in the side contralateral to the deprived amblyopic eye there is a gradient of diminished activation. The strongest activation asymmetry was observed in the visual reticular segment, while in layers 6 and 4 of the visual cortex the activation asymmetry was less strong and weakest, respectively. In the lateral geniculate there was no Fos-detectable activation asymmetry. Furthermore, there was a positive correlation between the time rats spent in exploration and the degree of activation asymmetry in the visual reticular segment. From these results it is concluded: (1) Activation of the visual segment of the thalamic reticular nucleus in the alert, attentive animal is predominantly under visual cortical control via the cortico-reticulo-geniculate pathway originating in layer 6, because this layer showed activation asymmetry while the other visual input to reticularis, the geniculate, did not show this asymmetry. (2) Activation of the visual reticularis is a function of attention to the environment because its activation asymmetry was correlated to the amount of exploratory attentional behaviour. (3) Diminished activity in the cortico-reticulo-geniculate pathway originating in layer 6, and of visual reticularis, caused by visual deprivation during the critical period should be considered as additional etiological factors of the resulting amblyopia. The functional significance of these results is explained by a 'focal attention' hypothesis postulating that the observed activation of visual reticularis in exploring animals is necessarily a reflection of activation of the corticogeniculate pathway, because these axons innervate both the geniculate and the visual reticular segment. Mechanistically, a focus of animal's attention is transmitted in a top-down fashion from the extrastriate cortex, and from upper cortical layers, into striate cortex layer 6. In turn, activation of layer 6 cells corresponding to attentional foci generates a core of excitation in the geniculate by the direct glutamatergic corticogeniculate axons, and a surround inhibition by the disynaptic cortico-reticulo-geniculate (ultimately GABAergic) pathway. In the temporal domain, in light of recent results, activation of thalamic reticular nucleus visual segment will contribute to the induction of gamma oscillations in geniculocortical pathways and in their cortical targets. All together, these interactions result in increased effectiveness of thalamocortical transmission of features from the focalized visual scene. The postulated attention-dependent spatiotemporal influences on thalamocortical transmission would be a main function of the corticothalamic pathways in the awake, attentive animal.

Considerations in the routine assessment and treatment of anisometropic amblyopia

Anisometropic amblyopia has traditionally been treated by direct occlusion with or without refractive correction of the anisometropia. This treatment has generally been considered so successful that practitioners prescribe occlusion for a time and consider treatment complete when visual acuity is no longer improving. The aetiology of anisometropic amblyopia, addressing the functional deficits of the amblyopic system and patient compliance issues, are important to the success of amblyopia treatment in anisometropia and are briefly reviewed in this paper. We propose that, while each case of anisometropic amblyopia may have slightly different attributes, initially they should all be treated with occlusion and full-time spectacle correction. Some presentations may be responsive to individual treatment strategies to correct binocular functions such as stereopsis, accommodative response and fusional vergences.

Co-expression of TrkB and the N-methyl-D-aspartate receptor subunits NR1-C1, NR2A and NR2B in the rat visual cortex

In the visual cortex, brain-derived neurotrophic factor expression is modulated through glutamate receptors, including the N-methyl-D-aspartate glutamate receptor. It has been proposed that the N-methyl-D-aspartate glutamate receptor subunit composition itself might be regulated by brain-derived neurotrophic factor. Here, we investigated the co-expression of the neurotrophin-4/brain-derived neurotrophic factor receptor TrkB with the N-methyl-D-aspartate glutamate receptor subunits NR1-C1, NR2A and NR2B, on postnatal days 10 and 22 and in the adult rat primary visual cortex. At both postnatal days 10 and 22, TrkB is co-expressed in all cortical layers with the studied N-methyl-D-aspartate glutamate receptor subunits. In the adult, in layers IV-V, co-expression is restricted to a subpopulation of neurons, while in layers II-III, VI nearly all neurons co-express TrkB with NR1-C1, NR2A and NR2B. We conclude that in layers IV-V, the co-expression of TrkB with subunits NR2B and NR2A is developmentally regulated.

Blockade of NR2B-Containing NMDA Receptors Prevents BDNF Enhancement of Glutamatergic Transmission in Hippocampal Neurons

Application of brain-derived neurotrophic factor (BDNF) to hippocampal neurons has profound effects on glutamatergic synaptic transmission. Both pre- and postsynaptic actions have been identified that depend on the age and type of preparation. To understand the nature of this diversity, we have begun to examine the mechanisms of BDNF action in cultured dissociated embryonic hippocampal neurons. Whole-cell patch-clamp recording during iontophoretic application of glutamate revealed that BDNF doubled the amplitude of induced inward current. Coexposure to BDNF and the NMDA receptor antagonist AP-5 markedly reduced, but did not entirely prevent, the increase in current. Coexposure to BDNF and ifenprodil, an NR2B subunit antagonist, reproduced the response observed with AP-5, suggesting BDNF primarily enhanced activity of NR2B-containing NMDA receptors with a lesser effect on non-NMDA receptors. Protein kinase involvement was confirmed with the broad spectrum inhibitor staurosporine, which prevented the response to BDNF. PKCI19-31 and H-89, selective antagonists of PKC and PKA, had no effect on the response to BDNF, whereas autocamtide-2-related inhibitory peptide, an antagonist of CaM kinase II, reduced response magnitude by 60%. These results demonstrate the predominant role of a specific NMDA receptor subtype in BDNF modulation of hippocampal synaptic transmission.

Blockade of NR2B-containing NMDA receptors prevents BDNF enhancement of glutamatergic transmission in hippocampal neurons

Application of brain-derived neurotrophic factor (BDNF) to hippocampal neurons has profound effects on glutamatergic synaptic transmission. Both pre- and postsynaptic actions have been identified that depend on the age and type of preparation. To understand the nature of this diversity, we have begun to examine the mechanisms of BDNF action in cultured dissociated embryonic hippocampal neurons. Whole-cell patch-clamp recording during iontophoretic application of glutamate revealed that BDNF doubled the amplitude of induced inward current. Coexposure to BDNF and the NMDA receptor antagonist AP-5 markedly reduced, but did not entirely prevent, the increase in current. Coexposure to BDNF and ifenprodil, an NR2B subunit antagonist, reproduced the response observed with AP-5, suggesting BDNF primarily enhanced activity of NR2B-containing NMDA receptors with a lesser effect on non-NMDA receptors. Protein kinase involvement was confirmed with the broad spectrum inhibitor staurosporine, which prevented the response to BDNF. PKCI19-31 and H-89, selective antagonists of PKC and PKA, had no effect on the response to BDNF, whereas autocamtide-2-related inhibitory peptide, an antagonist of CaM kinase II, reduced response magnitude by 60%. These results demonstrate the predominant role of a specific NMDA receptor subtype in BDNF modulation of hippocampal synaptic transmission.

Synaptic Reliability Correlates with Reduced Susceptibility to Synaptic Potentiation by Brain-Derived Neurotrophic Factor

Recent studies have implicated brain-derived neurotrophic factor (BDNF) in use-dependent modification of hippocampal synapses. BDNF can rapidly potentiate synaptic transmission at glutamatergic synapses by enhancing transmitter release. Using simultaneous perforated patch recording from pairs and triplets of glutamatergic hippocampal neurons, we have examined how the initial state of the glutamatergic synapse determines its susceptibility to synaptic modification by BDNF. We found that the degree of synaptic potentiation by BDNF depends on the initial reliability and strength of the synapse: Relatively weak connections were strongly potentiated, whereas the effect was markedly reduced at stronger synapses. The degree of BDNF-induced potentiation strongly correlated with the initial coefficient of variation (CV) of the amplitude of excitatory postsynaptic currents (EPSCs) and inversely correlated with the initial paired–pulse facilitation, suggesting that synapses with lower release probability (Pr) are more susceptible to the action of BDNF. To determine whether saturation of Pr could have masked the potentiation effect of BDNF in the stronger synapses, we lowered the initial Pr either by reducing the extracellular Ca2+ concentration ([Ca2+]o) or by bath application of adenosine. Synapses that were initially strong remained unaffected by BDNF under these conditions of reduced Pr. Thus, the lack of BDNF effect on synaptic efficacy cannot simply be accounted for by saturation of Pr, but rather may be due to intrinsic changes associated with synaptic maturation that might covary with Pr. Finally, the dependence on initial synaptic strength was also found for divergent outputs of the same presynaptic neuron, suggesting that synaptic terminals with different degrees of responsiveness to BDNF can coexist within in the same neuron.

Synaptic reliability correlates with reduced susceptibility to synaptic potentiation by brain-derived neurotrophic factor

Recent studies have implicated brain-derived neurotrophic factor (BDNF) in use-dependent modification of hippocampal synapses. BDNF can rapidly potentiate synaptic transmission at glutamatergic synapses by enhancing transmitter release. Using simultaneous perforated patch recording from pairs and triplets of glutamatergic hippocampal neurons, we have examined how the initial state of the glutamatergic synapse determines its susceptibility to synaptic modification by BDNF. We found that the degree of synaptic potentiation by BDNF depends on the initial reliability and strength of the synapse: Relatively weak connections were strongly potentiated, whereas the effect was markedly reduced at stronger synapses. The degree of BDNF-induced potentiation strongly correlated with the initial coefficient of variation (CV) of the amplitude of excitatory postsynaptic currents (EPSCs) and inversely correlated with the initial paired-pulse facilitation, suggesting that synapses with lower release probability (Pr) are more susceptible to the action of BDNF. To determine whether saturation of Pr could have masked the potentiation effect of BDNF in the stronger synapses, we lowered the initial Pr either by reducing the extracellular Ca2+ concentration ([Ca2+]o) or by bath application of adenosine. Synapses that were initially strong remained unaffected by BDNF under these conditions of reduced Pr. Thus, the lack of BDNF effect on synaptic efficacy cannot simply be accounted for by saturation of Pr, but rather may be due to intrinsic changes associated with synaptic maturation that might covary with Pr. Finally, the dependence on initial synaptic strength was also found for divergent outputs of the same presynaptic neuron, suggesting that synaptic terminals with different degrees of responsiveness to BDNF can coexist within in the same neuron.

Neurotrophins and synaptic plasticity

Despite considerable evidence that neuronal activity influences the organization and function of circuits in the developing and adult brain, the molecular signals that translate activity into structural and functional changes in connections remain largely obscure. This review discusses the evidence implicating neurotrophins as molecular mediators of synaptic and morphological plasticity. Neurotrophins are attractive candidates for these roles because they and their receptors are expressed in areas of the brain that undergo plasticity, activity can regulate their levels and secretion, and they regulate both synaptic transmission and neuronal growth. Although numerous experiments show demonstrable effects of neurotrophins on synaptic plasticity, the rules and mechanisms by which they exert their effects remain intriguingly elusive.

CRE-mediated gene transcription in neocortical neuronal plasticity during the developmental critical period

Neuronal activity-dependent processes are believed to mediate the formation of synaptic connections during neocortical development, but the underlying intracellular mechanisms are not known. In the visual system, altering the pattern of visually driven neuronal activity by monocular deprivation induces cortical synaptic rearrangement during a postnatal developmental window, the critical period. Here, using transgenic mice carrying a CRE-lacZ reporter, we demonstrate that a calcium- and cAMP-regulated signaling pathway is activated following monocular deprivation. We find that monocular deprivation leads to an induction of CRE-mediated lacZ expression in the visual cortex preceding the onset of physiologic plasticity, and this induction is dramatically downregulated following the end of the critical period. These results suggest that CRE-dependent coordinate regulation of a network of genes may control physiologic plasticity during postnatal neocortical development.

TrkA activation in the rat visual cortex by antirat trkA IgG prevents the effect of monocular deprivation

It has been recently shown that intraventricular injections of nerve growth factor (NGF) prevent the effects of monocular deprivation in the rat. We have tested the localization and the molecular nature of the NGF receptor(s) responsible for this effect by activating cortical trkA receptors in monocularly deprived rats by cortical infusion of a specific agonist of NGF on trkA, the bivalent antirat trkA IgG (RTA-IgG). TrkA protein was detected by immunoblot in the rat visual cortex during the critical period. Rats were monocularly deprived for 1 week (P21-28) and RTA-IgG or control rabbit IgG were delivered by osmotic minipumps. The effects of monocular deprivation on the ocular dominance of visual cortical neurons were assessed by extracellular single cell recordings. We found that the shift towards the ipsilateral, non-deprived eye was largely prevented by RTA-IgG. Infusion of RTA-IgG combined with antibody that blocks p75NTR (REX), slightly reduced RTA-IgG effectiveness in preventing monocular deprivation effects. These results suggest that NGF action in visual cortical plasticity is mediated by cortical TrkA receptors with p75NTR exerting a facilitatory role.

Dark rearing blocks the developmental down-regulation of brain-derived neurotrophic factor messenger RNA expression in layers IV and V of the rat visual cortex

In this study, we describe the distribution of brain-derived neurotrophic factor messenger RNA in the binocular primary visual cortex of the rat during postnatal development, starting at postnatal day (P) 13. High-resolution non-isotopic in situ hybridization combined with Nissl staining were used to quantify the number of cells expressing brain-derived neurotrophic factor messenger RNA. At P13, most of the cells express brain-derived neurotrophic factor messenger RNA. After eye opening (P14-P15), the relative number of brain-derived neurotrophic factor messenger RNA-positive cells decreases by a factor of two in layer IV, i.e. that receiving the visual input, and in layer V. To verify the hypothesis that light could trigger this decrease, pups were kept in complete darkness from birth. At P22, pups reared in the dark were killed and the visual cortex processed for in situ hybridization and northern blotting. The results obtained in dark-reared animals prove that light deprivation can: (i) decrease the general levels of brain-derived neurotrophic factor messenger RNA, and (ii) increase the relative number of brain-derived neurotrophic factor messenger RNA-positive cells in layers IV and V with respect to control rats. Exposure to light for five days after the period of darkness restored the number of brain-derived neurotrophic factor messenger RNA-positive cells. We conclude that the expression of brain-derived neurotrophic factor messenger RNA in the rat primary visual cortex is regulated during development and that this process is under the control of visual input.

Nerve growth factor preserves a critical motor period in rat striatum

We previously found the occurrence of a critical motor period during rat postnatal development where circling training starting the 7-day schedule at 30 days-but not before or after-induces a lifetime drop in the binding to cholinergic muscarinic receptors (mAChRs) in striatum. Here, we studied whether nerve growth factor (NGF) participates in this restricted period of muscarinic sensitivity. For this purpose, we administered mouse salival gland 2.5S NGF (1.4 or 0.4 microg/day, infused by means of ALZA minipumps) by intrastriatal unilateral route between days 25 and 39, and then trained rats starting at 40 days. Under these conditions, NGF induced a long-term reduction in the striatal [3H] quinuclidilbenzylate (QNB) binding sites despite the fact that motor training was carried out beyond the natural critical period. Thus, at day 70, measurement of specific QNB binding in infused striata of trained rats showed decreases of 42% (p < .0004) and 33% (p < .02) after administration of the higher and lower NGF doses, respectively, with respect to trained rats treated with cytochrome C, for control. Noncannulated striata of the NGF-treated rats also showed a decrease in QNB binding sites (44%; p < .0001) only at the higher infusion rate. This effect was not found in the respective control groups. Our observations show that NGF modulates the critical period in which activity-dependent mAChR setting takes place during rat striatal maturation.

Chronic exposure to either somatostatin (SS) or octreotide, a long-lasting SS analogue, affects SS expression in the postnatal visual cortex of the rat

The peptide somatostatin (SS) is widely distributed in the mammalian brain where it modulates neuronal activity through interactions with specific membrane-bound receptor subtypes (ssts). Five different ssts were characterized so far (sst1-5) and their selective agonists were developed on the basis of their binding specificity. SS and ssts are transiently expressed in the developing brain, suggesting a functional role of somatostatinergic systems in neuronal maturation. In the present study, we investigated the effects of chronic exposure to either the SS synthetic analogue, SS-14 or octreotide (a long-acting sst2-preferring analogue) on the maturation of SS-immunoreactivity (-ir) in the primary visual cortex of the rat. SS-ir maturation was investigated both by an evaluation of the number of SS-immunoreactive cells and by radioimmunoassay (RIA) to measure the levels of SS in the postnatal visual cortex. In the visual cortex of normal rats, the number of SS-positive cells markedly increased during the second postnatal week and then significantly decreased until the adult value was reached at the third week. Early and repeated intracerebroventricular (i.c.v.) injections of either SS-14 or octreotide prevented the increase in the number of SS-positive cells, with adult values reached at the end of the first postnatal week. Similarly, administration of either SS-14 or octreotide significantly decreased the SS content of the visual cortex, measured at the end of the second postnatal week. These results show that high local concentrations of either SS-14 or octreotide interfere with SS expression in developing cortical neurons in a restricted postnatal period.

The messenger RNA encoding VGF, a neuronal peptide precursor, is rapidly regulated in the rat central nervous system by neuronal activity, seizure and lesion

The VGF gene encodes a neuronal secretory-peptide precursor that is rapidly induced by neurotrophic growth factors and by depolarization in vitro. VGF expression in the animal peaks during critical periods in the developing peripheral and central nervous systems. To gain insight into the possible functions and regulation of VGF in vivo, we have used in situ hybridization to examine the regulation of VGF messenger RNA by experimental manipulations, and have found it to be regulated in the CNS by paradigms that affect electrical activity and by lesion. Inhibition of retinal electrical activity during the critical period of visual development rapidly repressed VGF messenger RNA in the dorsal lateral geniculate nucleus of the thalamus. In the adult, kainate-induced seizures transiently induced VGF messenger RNA in neurons of the dentate gyrus, hippocampus, and cerebral cortex within hours. Cortical lesion strongly induced VGF messenger RNA in ipsilateral cortex within hours, and strongly repressed expression in ipsilateral striatum. Ten days postlesion there was a delayed induction of VGF messenger RNA in a portion of deafferented striatum where compensatory cortical sprouting has been detected. Expression of the neuronal secretory-peptide precursor VGF is therefore modulated in vivo by monocular deprivation, seizure, and cortical lesion, paradigms which lead to neurotrophin induction, synaptic remodeling and axonal sprouting.

Blockade of endogenous ligands of trkB inhibits formation of ocular dominance columns

We have examined the hypothesis that the segregation of LGN axon terminals into ocular dominance (OD) patches in layer 4 of the visual cortex requires neurotrophins, acting as signals to modulate the pattern of synaptic connectivity. Neurotrophin receptor antagonists, composed of the extracellular domain of each member of the trk family of neurotrophin receptors fused to a human Fc domain, were infused directly into visual cortex during the peak phase of OD column formation. Infusion of trkB-IgG, which binds BDNF and NT-4/5, inhibited the formation of OD patches within layer 4, while trkA-IgG and trkC-IgG, which preferentially bind NGF and NT-3, respectively, had no effect. The autoradiographic labeling of LGN terminals in cortical layer 4 was reduced by trkB-IgG, in contrast with the increased labeling observed following NT-4/5 infusion. These data suggest that an endogenous ligand of trkB, normally present in limiting amounts within visual cortex, is necessary for the selective growth and remodeling of LGN axons into eye-specific patches.