Effects of neurotrophins on cortical plasticity: same or different?

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.

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.

Leukemia inhibitory factor impairs structural and neurochemical development of rat visual cortex in vivo

Minipump infusions into visual cortex in vivo at the onset of the critical period have revealed that the proinflammatory cytokine leukemia inhibitory factor (LIF) delays the maturation of thalamocortical projection neurons of the lateral geniculate nucleus, and tecto-thalamic projection neurons of the superior colliculus, and cortical layer IV spiny stellates and layer VI pyramidal neurons. Here, we report that P12-20 LIF infusion inhibits somatic maturation of pyramidal neurons and of all interneuron types in vivo. Likewise, DIV 12-20 LIF treatment in organotypic cultures prevents somatic growth GABA-ergic neurons. Further, while NPY expression is increased in the LIF-infused hemispheres, the expression of parvalbumin mRNA and protein, Kv3.1 mRNA, calbindin D-28k protein, and GAD-65 mRNA, but not of GAD-67 mRNA or calretinin protein is substantially reduced. Also, LIF treatment decreases parvalbumin, Kv3.1, Kv3.2 and GAD-65, but not GAD-67 mRNA expression in OTC. Developing cortical neurons are known to depend on neurotrophins. Indeed, LIF alters neurotrophin mRNA expression, and prevents the growth promoting action of neurotophin-4 in GABA-ergic neurons. The results imply that LIF, by altering neurotrophin expression and/or signaling, could counteract neurotrophin-dependent growth and neurochemical differentiation of cortical neurons.

Differential effects of BDNF and neurotrophin 4 (NT4) on endocytic sorting of TrkB receptors

Neurotrophins are a family of growth factors playing key roles in the survival, development, and function of neurons. The neurotrophins brain-derived neurotrophic factor (BDNF) and NT4 both bind to and activate TrkB receptors, however, they mediate distinct neuronal functions. The molecular mechanism of how TrkB activation by BDNF and NT4 leads to diverse outcomes is unknown. Here, we report that BDNF and NT4 lead to differential endocytic sorting of TrkB receptors resulting in diverse biological functions in cultured cortical neurons. Fluorescent microscopy and surface biotinylation experiments showed that both neurotrophins stimulate internalization of TrkB with similar kinetics. Exposure to BDNF for 2-3 h reduced the surface pool of TrkB receptors to half, whereas a longer treatment (4-5 h) with NT4 was necessary to achieve a similar level of down-regulation. Although BDNF and NT4 induced TrkB phosphorylation with similar intensities, BDNF induced more rapid ubiquitination and degradation of TrkB than NT4. Interestingly, TrkB receptor ubiquitination by these ligands have substantially different pH sensitivities, resulting in varying degrees of receptor ubiquitination at lower pH levels. Consequently, NT4 was capable of maintaining longer sustained downstream signaling activation that correlated with reduced TrkB ubiquitination at endosomal pH. Thus, by leading to altered endocytic trafficking itineraries for TrkB receptors, BDNF and NT4 elicit differential TrkB signaling in terms of duration, intensity, and specificity, which may contribute to their functional differences in vivo. The neurotrophins, brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT4), both bind to and activate TrkB receptors, however, they mediate distinct neuronal functions. Here, we propose that BDNF and NT4 lead to differential endocytic sorting of TrkB receptors resulting in diverse biological functions. BDNF induces more rapid ubiquitination and degradation of TrkB than NT4. Consequently, NT4 is capable of maintaining more sustained signaling downstream of TrkB receptors.

Experimental and clinical evidence of neuroprotection by nerve growth factor eye drops: Implications for glaucoma

Elevated intraocular pressure (IOP) in glaucoma causes loss of retinal ganglion cells (RGCs) and damage to the optic nerve. Although IOP is controlled pharmacologically, no treatment is available to restore retinal and optic nerve function. We evaluated the effects of NGF eye drops in a rat model of glaucoma. We also treated 3 patients with progressive visual field defects despite IOP control. Glaucoma was induced in rats through injection of hypertonic saline into the episcleral vein. Initially, 2 doses of NGF (100 and 200 mug/mL) were tested on 24 rats, and the higher dose was found to be more effective. Glaucoma was then induced in an additional 36 rats: half untreated and half treated with 200 mug/mL NGF QID for 7 weeks. Apoptosis/survival of RGCs was evaluated by histological, biochemical, and molecular analysis. Three patients with advanced glaucoma underwent psychofunctional and electrofunctional tests at baseline, after 3 months of NGF eye drops, and after 3 months of follow-up. Seven weeks of elevated IOP caused RGC degeneration resulting in 40% cell death. Significantly less RGC loss was observed with NGF treatment (2,530 +/- 121 vs. 1,850 +/- 156 RGCs/mm(2)) associated with inhibition of cell death by apoptosis. Patients treated with NGF demonstrated long lasting improvements in visual field, optic nerve function, contrast sensitivity, and visual acuity. NGF exerted neuroprotective effects, inhibiting apoptosis of RGCs in animals with glaucoma. In 3 patients with advanced glaucoma, treatment with topical NGF improved all parameters of visual function. These results may open therapeutic perspectives for glaucoma and other neurodegenerative diseases.

Experience-dependent DNA methylation regulates plasticity in the developing visual cortex

DNA methylation is an epigenetic repressor mark for transcription dynamically regulated in neurons. We analyzed visual experience regulation of DNA methylation in mice and its involvement in ocular dominance plasticity of the developing visual cortex. Monocular deprivation modulated the expression of factors controlling DNA methylation and exerted opposite effects on DNA methylation and hydroxymethylation in specific plasticity genes. Inhibition of DNA methyltrasferase (DNMT) blocked molecular and functional effects of monocular deprivation, partially reversing the monocular deprivation transcriptional program.

Balancing plasticity/stability across brain development

The potency of the environment to shape brain function changes dramatically across the lifespan. Neural circuits exhibit profound plasticity during early life and are later stabilized. A focus on the cellular and molecular bases of these developmental trajectories has begun to unravel mechanisms, which control the onset and closure of such critical periods. Two important concepts have emerged from the study of critical periods in the visual cortex: (1) excitatory-inhibitory circuit balance is a trigger; and (2) molecular "brakes" limit adult plasticity. The onset of the critical period is determined by the maturation of specific GABA circuits. Targeting these circuits using pharmacological or genetic approaches can trigger premature onset or induce a delay. These manipulations are so powerful that animals of identical chronological age may be at the peak, before, or past their plastic window. Thus, critical period timing per se is plastic. Conversely, one of the outcomes of normal development is to stabilize the neural networks initially sculpted by experience. Rather than being passively lost, the brain's intrinsic potential for plasticity is actively dampened. This is demonstrated by the late expression of brake-like factors, which reversibly limit excessive circuit rewiring beyond a critical period. Interestingly, many of these plasticity regulators are found in the extracellular milieu. Understanding why so many regulators exist, how they interact and, ultimately, how to lift them in noninvasive ways may hold the key to novel therapies and lifelong learning.

Experience-dependent plasticity of visual cortical microcircuits

The recent decade testified a tremendous increase in our knowledge on how cell-type-specific microcircuits process sensory information in the neocortex and on how such circuitry reacts to manipulations of the sensory environment. Experience-dependent plasticity has now been investigated with techniques endowed with cell resolution during both postnatal development and in adult animals. This review recapitulates the main recent findings in the field using mainly the primary visual cortex as a model system to highlight the more important questions and physiological principles (such as the role of non-competitive mechanisms, the role of inhibition in excitatory cell plasticity, the functional importance of spine and axonal plasticity on a microscale level). I will also discuss on which scientific problems the debate and controversies are more pronounced. New technologies that allow to perturbate cell-type-specific subcircuits will certainly shine new light in the years to come at least on some of the still open questions.

The bacterial protein toxin, cytotoxic necrotizing factor 1 (CNF1) provides long-term survival in a murine glioma model

Background

Glioblastomas are largely unresponsive to all available treatments and there is therefore an urgent need for novel therapeutics. Here we have probed the antineoplastic effects of a bacterial protein toxin, the cytotoxic necrotizing factor 1 (CNF1), in the syngenic GL261 glioma cell model. CNF1 produces a long-lasting activation of Rho GTPases, with consequent blockade of cytodieresis in proliferating cells and promotion of neuron health and plasticity.

Methods

We have tested the antiproliferative effects of CNF1 on GL261 cells and human glioma cells obtained from surgical specimens. For the in vivo experiments, we injected GL261 cells into the adult mouse visual cortex, and five days later we administered either a single intracerebral dose of CNF1 or vehicle. To compare CNF1 with a canonical antitumoral drug, we infused temozolomide (TMZ) via minipumps for 1 week in an additional animal group.

Results

In culture, CNF1 was very effective in blocking proliferation of GL261 cells, leading them to multinucleation, senescence and death within 15 days. CNF1 had a similar cytotoxic effect in primary human glioma cells. CNF1 also inhibited motility of GL261 cells in a scratch-wound migration assay. Low dose (2 nM) CNF1 and continuous TMZ infusion significantly prolonged animal survival (median survival 35 days vs. 28 days in vehicle controls). Remarkably, increasing CNF1 concentration to 80 nM resulted in a dramatic enhancement of survival with no obvious toxicity. Indeed, 57% of the CNF1-treated animals survived up to 60 days following GL261 glioma cell transplant.

Conclusions

The activation of Rho GTPases by CNF1 represents a novel potential therapeutic strategy for the treatment of central nervous system tumors.

Manipulation of BDNF signaling modifies the experience-dependent plasticity induced by pure tone exposure during the critical period in the primary auditory cortex

Sensory experience powerfully shapes cortical sensory representations during an early developmental “critical period” of plasticity. In the rat primary auditory cortex (A1), the experience-dependent plasticity is exemplified by significant, long-lasting distortions in frequency representation after mere exposure to repetitive frequencies during the second week of life. In the visual system, the normal unfolding of critical period plasticity is strongly dependent on the elaboration of brain-derived neurotrophic factor (BDNF), which promotes the establishment of inhibition. Here, we tested the hypothesis that BDNF signaling plays a role in the experience-dependent plasticity induced by pure tone exposure during the critical period in the primary auditory cortex. Elvax resin implants filled with either a blocking antibody against BDNF or the BDNF protein were placed on the A1 of rat pups throughout the critical period window. These pups were then exposed to 7 kHz pure tone for 7 consecutive days and their frequency representations were mapped. BDNF blockade completely prevented the shaping of cortical tuning by experience and resulted in poor overall frequency tuning in A1. By contrast, BDNF infusion on the developing A1 amplified the effect of 7 kHz tone exposure compared to control. These results indicate that BDNF signaling participates in the experience-dependent plasticity induced by pure tone exposure during the critical period in A1.

IGF-1 restores visual cortex plasticity in adult life by reducing local GABA levels

The central nervous system architecture is markedly modified by sensory experience during early life, but a decline of plasticity occurs with age. Recent studies have challenged this dogma providing evidence that both pharmacological treatments and paradigms based on the manipulation of environmental stimulation levels can be successfully employed as strategies for enhancing plasticity in the adult nervous system. Insulin-like growth factor 1 (IGF-1) is a peptide implicated in prenatal and postnatal phases of brain development such as neurogenesis, neuronal differentiation, synaptogenesis, and experience-dependent plasticity. Here, using the visual system as a paradigmatic model, we report that IGF-1 reactivates neural plasticity in the adult brain. Exogenous administration of IGF-1 in the adult visual cortex, indeed, restores the susceptibility of cortical neurons to monocular deprivation and promotes the recovery of normal visual functions in adult amblyopic animals. These effects were accompanied by a marked reduction of intracortical GABA levels. Moreover, we show that a transitory increase of IGF-1 expression is associated to the plasticity reinstatement induced by environmental enrichment (EE) and that blocking IGF-1 action by means of the IGF-1 receptor antagonist JB1 prevents EE effects on plasticity processes.

Brain-derived neurotrophic factor (BDNF) reverses the effects of rapid eye movement sleep deprivation (REMSD) on developmentally regulated, long-term potentiation (LTP) in visual cortex slices

Work in this laboratory demonstrated a role for rapid eye movement sleep (REMS) in critical period (CP), postnatal days (P) 17-30, synaptic plasticity in visual cortex. Studies in adolescent rats showed that REMS deprivation (REMSD) reinitiates a developmentally regulated form of synaptic plasticity that otherwise is observed only in CP animals. Subsequent work added that REMSD affects inhibitory mechanisms that are thought to be involved in terminating the CP. Neurotrophins are implicated in the synaptic plasticity that underlies CP maturation and also final closure of the CP in visual cortex. Expression of brain-derived neurotrophic factor (BDNF) is dependent upon neuronal activity, and REMSD may block BDNF expression. We propose that REMS contributes to the maturation of visual cortex through regulation of BDNF expression and consequent, downstream increase in cortical inhibitory tone. In this study, osmotic minipumps delivered BDNF into visual cortex on one side of brain. The opposite hemisphere was not implanted and served as an internal control. We tested the hypothesis that BDNF is blocked by REMSD in late-adolescent rats and investigated whether replacing BDNF prevents induction of LTPWM-III by theta burst stimulation (TBS). We also assessed relative inhibitory tone in visual cortex with paired-pulse stimulation (PPS) in animals that were similarly REMSD- and BDNF-infused. After REMSD, both hemispheres were prepared in parallel for in vitro synaptic plasticity studies (LTPWM-III or PPS). In visual cortex of REMSD rats on the side receiving BDNF infusions (8 of 8 animals), TBS consistently failed to induce LTPWM-III. In contrast, LTPWM-III was obtained (5 of 5 animals) in the matched, non-infused hemisphere, as expected in rats of this age. REMSD animals that were unilaterally infused with saline produced LTPWM-III in both hemispheres. PPS studies in another group of REMSD animals that were unilaterally BDNF-infused displayed age-appropriate inhibition of the second response on the BDNF-infused side (5/5), whereas on the non-infused side facilitation was observed (3/3). Intracortical infusion of BDNF in REMSD adolescent rats appears to restore neurochemical processes necessary for termination of the CP for developmentally regulated synaptic plasticity in visual cortex. The results suggest that REMSD blocks BDNF expression and also maturation of inhibitory processes in adolescent visual cortex. These data support REMS' function in brain development.

Inhibition of matrix metalloproteinases prevents the potentiation of nondeprived-eye responses after monocular deprivation in juvenile rats

The ocular dominance (OD) shift induced by monocular deprivation (MD) during the critical period is mediated by an initial depression of deprived-eye responses followed by an increased responsiveness to the nondeprived eye. It is not fully clear to what extent these 2 events are correlated and which are their physiological and molecular mediators. The extracellular synaptic environment plays an important role in regulating visual cortical plasticity. Matrix metalloproteinases (MMPs) are a family of activity-dependent zinc-dependent extracellular endopeptidases mediating extracellular matrix remodeling. We investigated the effects of MMP inhibition on OD plasticity in juvenile monocularly deprived rats. By using electrophysiological recordings, we found that MMP inhibition selectively prevented the potentiation of neuronal responses to nondeprived-eye stimulation occurring after 7 days of MD and potentiation of deprived-eye responses occurring after eye reopening. Three days of MD only resulted in a depression of deprived-eye responses insensitive to MMP inhibition. MMP inhibition did not influence homeostatic plasticity tested in the monocular cortex but significantly prevented an increase in dendritic spine density present after 7 days MD in layer II-III pyramids.

Topical nerve growth factor as a visual rescue strategy in pediatric optic gliomas: a pilot study including electrophysiology

BACKGROUND

To date, no specific therapy is available for optic glioma (OG)-induced visual loss.

OBJECTIVE

To evaluate the effects on visual function of murine nerve growth factor (NGF) eye drop administration in children with severe visual impairment due to low-grade OGs.

METHODS

Five patients with OGs and advanced optic nerve atrophy were assessed before and after a single 10-day course of 1 mg murine NGF topical administration by clinical evaluation, visual evoked potentials (VEPs), and brain magnetic resonance imaging (MRI). VEPs, the main functional outcome measure, were recorded at baseline and 1, 30, 45, 90, and 180 days posttreatment. MRI examinations were performed at baseline and at 180 days after NGF treatment. Six untreated control patients with OGs also underwent serial VEPs, clinical testing, and MRI assessments.

RESULTS

After NGF treatment, median VEPs amplitude showed a progressive increase from the baseline values (P < .01). VEPs reached a maximum amplitude at 90 days (170% increase) and declined at 180 days, still remaining above the baseline level. Perception of spontaneous visual phosphenes was noted in all patients after NGF administration. MRI showed stable tumor size. In controls, clinical findings and VEPs did not show any significant change over the observation period.

CONCLUSIONS

The findings from the study show that NGF administration may be an effective and safe adjunct therapy in children with optic atrophy due to OGs. The beneficial effect on optic nerve function suggests a visual rescuing mechanism exerted by murine NGF on the residual viable optic pathways.

Callosal contribution to ocular dominance in rat primary visual cortex

Ocular dominance (OD) plasticity triggered by monocular eyelid suture is a classic paradigm for studying experience-dependent changes in neural connectivity. Recently, rodents have become the most popular model for studies of OD plasticity. It is therefore important to determine how OD is determined in the rodent primary visual cortex. In particular, cortical cells receive considerable inputs from the contralateral hemisphere via callosal axons, but the role of these connections in controlling eye preference remains controversial. Here we have examined the role of callosal connections in binocularity of the visual cortex in naïve young rats. We recorded cortical responses evoked by stimulation of each eye before and after acute silencing, via stereotaxic tetrodotoxin (TTX) injection, of the lateral geniculate nucleus ipsilateral to the recording site. This protocol allowed us to isolate visual responses transmitted via the corpus callosum. Cortical binocularity was assessed by visual evoked potential (VEP) and single-unit recordings. We found that acute silencing of afferent geniculocortical input produced a very significant reduction in the contralateral-to-ipsilateral (C/I) VEP ratio, and a marked shift towards the ipsilateral eye in the OD distribution of cortical cells. Analysis of absolute strength of each eye indicated a dramatic decrease in contralateral eye responses following TTX, while those of the ipsilateral eye were reduced but maintained a more evident input. We conclude that callosal connections contribute to normal OD mainly by carrying visual input from the ipsilateral eye. These data have important implications for the interpretation of OD plasticity following alterations of visual experience.

Nerve growth factor as a paradigm of neurotrophins related to Alzheimer's disease

Converging lines of evidence on the possible connection between NGF signaling and Alzheimer's diseases (AD) are unraveling new facets which could depict this neurotrophin (NTF) in a more central role. AD animal models have provided evidence that a shortage of NGF supply may induce an AD-like syndrome. In vitro experiments, moreover, are delineating a possible temporal and causal link between APP amiloydogenic processing and altered post-translational tau modifications. After NGF signaling interruption, the pivotal upstream players of the amyloid cascade (APP, beta-secretase, and active form of gamma-secretase) are up-regulated, leading to an increased production of amyloid beta peptide (Abeta) and to its intracellular aggregation in molecular species of different sizes. Contextually, the Abeta released pool generates an autocrine toxic loop in the same healthy neurons. At the same time tau protein undergoes anomalous, GSKbeta-mediated, phosphorylation at specific pathogenetic sites (Ser262 and Thr 231), caspase(s) and calpain- I- mediated truncation, detachment from microtubules with consequent cytoskeleton collapse and axonal transport impairment. All these events are inhibited when the amyloidogenic processing is reduced by beta and gamma secretase inhibitors or anti-Abeta antibodies and appear to be causally correlated to TrkA, p75CTF, Abeta, and PS1 molecular association in an Abeta-mediated fashion. In this scenario, the so-called trophic action exerted by NGF (and possibly also by other neurotrophins) in these targets neurons is actually the result of an anti-amyloidogenic activity.

Functional masking of deprived eye responses by callosal input during ocular dominance plasticity

Monocular deprivation (MD) is a well-known paradigm of experience-dependent plasticity in which cortical neurons exhibit a shift of ocular dominance (OD) toward the open eye. The mechanisms underlying this form of plasticity are incompletely understood. Here we demonstrate the involvement of callosal connections in the synaptic modifications occurring during MD. Rats at the peak of the critical period were deprived for 7 days, resulting in the expected OD shift toward the open eye. Acute microinjection of the activity blocker muscimol into the visual cortex contralateral to the recording site restored binocularity of cortical cells. Continuous silencing of callosal input throughout the period of MD also resulted in substantial attenuation of the OD shift. Blockade of interhemispheric communication selectively enhanced deprived eye responses with no effect on open eye-driven activity. We conclude that callosal inputs play a key role in functional weakening of less active connections during OD plasticity.

Synaptic mechanisms for plasticity in neocortex

Sensory experience and learning alter sensory representations in cerebral cortex. The synaptic mechanisms underlying sensory cortical plasticity have long been sought. Recent work indicates that long-term cortical plasticity is a complex, multicomponent process involving multiple synaptic and cellular mechanisms. Sensory use, disuse, and training drive long-term potentiation and depression (LTP and LTD), homeostatic synaptic plasticity and plasticity of intrinsic excitability, and structural changes including formation, removal, and morphological remodeling of cortical synapses and dendritic spines. Both excitatory and inhibitory circuits are strongly regulated by experience. This review summarizes these findings and proposes that these mechanisms map onto specific functional components of plasticity, which occur in common across the primary somatosensory, visual, and auditory cortices.

Visual deprivation decreases somatic GAD65 puncta number on layer 2/3 pyramidal neurons in mouse visual cortex

Proper functioning of the visual system depends on maturation of both excitatory and inhibitory synapses within the visual cortex. Considering that perisomatic inhibition is one of the key factors that control the critical period in visual cortex, it is pertinent to understand its regulation by visual experience. To do this, we developed an immunohistochemical method that allows three-dimensional (3D) analysis of the glutamic acid decarboxylase (GAD) 65-positive inhibitory terminals in the visual cortex. Using this method on transgenic mice expressing yellow fluorescence protein (YFP) in a subset of neurons, we found that the number of somatic GAD65-puncta on individual layer 2/3 pyramidal neurons is reduced when mice are dark-reared from birth and reverted to normal levels by re-exposure to light. There was no change in GAD65-puncta volume or intensity. These results support the reorganization of inhibitory circuitry within layer 2/3 of visual cortex in response to changes in visual experience.

Molecular mechanisms of experience-dependent plasticity in visual cortex

A remarkable amount of our current knowledge of mechanisms underlying experience-dependent plasticity during cortical development comes from study of the mammalian visual cortex. Recent advances in high-resolution cellular imaging, combined with genetic manipulations in mice, novel fluorescent recombinant probes, and large-scale screens of gene expression, have revealed multiple molecular mechanisms that underlie structural and functional plasticity in visual cortex. We situate these mechanisms in the context of a new conceptual framework of feed-forward and feedback regulation for understanding how neurons of the visual cortex reorganize their connections in response to changes in sensory inputs. Such conceptual advances have important implications for understanding not only normal development but also pathological conditions that afflict the central nervous system.

TrkB kinase is required for recovery, but not loss, of cortical responses following monocular deprivation

Changes in visual cortical responses that are induced by monocular visual deprivation are a widely studied example of competitive, experience-dependent neural plasticity. It has been thought that the deprived-eye pathway will fail to compete against the open-eye pathway for limited amounts of brain-derived neurotrophic factor, which acts on TrkB and is needed to sustain effective synaptic connections. We tested this model by using a chemical-genetic approach in mice to inhibit TrkB kinase activity rapidly and specifically during the induction of cortical plasticity in vivo. Contrary to the model, TrkB kinase activity was not required for any of the effects of monocular deprivation. When the deprived eye was re-opened during the critical period, cortical responses to it recovered. This recovery was blocked by TrkB inhibition. These findings suggest a more conventional trophic role for TrkB signaling in the enhancement of responses or growth of new connections, rather than a role in competition.

Focal Photothrombotic Lesion of the Rat Motor Cortex Increases BDNF Levels in Motor-Sensory Cortical Areas Not Accompanied by Recovery of Forelimb Motor Skills

Brain infarct triggers neurodegeneration that often shades spontaneous plasticity, occurring in the areas related anatomically and functionally to the infarcted structures. Neurotrophins which promote neuronal survival and plasticity, may protect neurons and enhance remodeling of the remaining circuits, leading to restoration of function. In particular, the crucial role of brain-derived neurotrophic factor (BDNF) in cortical function is well documented. Since BDNF was implicated in the mechanism of postinfarct recovery, we investigated whether focal photothrombosis in the motor cortex of adult rats modifies cortical BDNF protein levels in a time- and region-dependent fashion. In parallel, we aimed to establish, which cortical cells respond with altered BDNF expression and whether these alterations are reflected by forelimb motor skill impairment and recovery, evaluated up to 1 month postinfarct. The distribution of BDNF protein was visualized immunohistochemically and BDNF tissue levels were evaluated with enzyme-linked immunosorbent assay (ELISA). Ipsilateral to the infarct, an increase in BDNF levels occurred both in injured and neighboring regions already 24 h after photothrombosis. This increase was sustained up to postlesion day 7 in the motor cortex and reduced at 28 days. No BDNF changes were detected in homotopic regions of the contralateral cortex. The time-course of enhanced neurotrophic expression was paralleled by bilateral deficits in skilled reaching, which was the only clear and measurable motor impairment observed in the study. We conclude that the spontaneous increase of BDNF is not sufficient to protect neurons from degeneration in the lesion proximity whereas plasticity reported in the adjacent regions may be attributable to enhanced BDNF-related stimuli, which do not counteract the impairment of skilled reaching but might be, at least in part, responsible for the absence of deficits in other functional/behavioral tests.

Neural plasticity after peripheral nerve injury and regeneration

Injuries to the peripheral nerves result in partial or total loss of motor, sensory and autonomic functions conveyed by the lesioned nerves to the denervated segments of the body, due to the interruption of axons continuity, degeneration of nerve fibers distal to the lesion and eventual death of axotomized neurons. Injuries to the peripheral nervous system may thus result in considerable disability. After axotomy, neuronal phenotype switches from a transmitter to a regenerative state, inducing the down- and up-regulation of numerous cellular components as well as the synthesis de novo of some molecules normally not expressed in adult neurons. These changes in gene expression activate and regulate the pathways responsible for neuronal survival and axonal regeneration. Functional deficits caused by nerve injuries can be compensated by three neural mechanisms: the reinnervation of denervated targets by regeneration of injured axons, the reinnervation by collateral branching of undamaged axons, and the remodeling of nervous system circuitry related to the lost functions. Plasticity of central connections may compensate functionally for the lack of specificity in target reinnervation; plasticity in human has, however, limited effects on disturbed sensory localization or fine motor control after injuries, and may even result in maladaptive changes, such as neuropathic pain, hyperreflexia and dystonia. Recent research has uncovered that peripheral nerve injuries induce a concurrent cascade of events, at the systemic, cellular and molecular levels, initiated by the nerve injury and progressing throughout plastic changes at the spinal cord, brainstem relay nuclei, thalamus and brain cortex. Mechanisms for these changes are ubiquitous in central substrates and include neurochemical changes, functional alterations of excitatory and inhibitory connections, atrophy and degeneration of normal substrates, sprouting of new connections, and reorganization of somatosensory and motor maps. An important direction for ongoing research is the development of therapeutic strategies that enhance axonal regeneration, promote selective target reinnervation, but are also able to modulate central nervous system reorganization, amplifying those positive adaptive changes that help to improve functional recovery but also diminishing undesirable consequences.

Insulin-like growth factor 1 (IGF-1) mediates the effects of enriched environment (EE) on visual cortical development

Enriched environment (EE) has been recently shown to affect visual cortex development and plasticity, and to prevent dark rearing effects. The factors mediating EE effects on visual cortical development and plasticity are still unclear. We have investigated whether IGF-1 is involved in mediating EE effects on the developing visual cortex. We show that EE increases the number of IGF-1 positive neurons in the visual cortex at P18. Increasing IGF-1 in the visual cortex of non-EE rats by means of osmotic minipumps implanted at P18 mimics EE effects, accelerating visual acuity development, assessed with Visual Evoked Potentials (VEPs). Blocking IGF-1 action in the visual cortex of EE rats by means of the IGF-1 receptor antagonist JB1 from P18 completely blocks EE action on visual acuity development. These results show that IGF-1 is a key factor mediating EE effects on visual cortical development. We then show that IGF-1 affects GAD65 immunoreactivity in perisomatic innervation and the condensation of Chondroitin Sulphate Proteoglycans (CSPGs) in perineuronal nets (PNNs) in the visual cortex. This suggests that IGF-1 action in mediating EE effects could be exerted through the modulation of intracortical inhibitory circuitry and PNN development.

Differential effects of NT-4, NGF and BDNF on development of neurochemical architecture and cell size regulation in rat visual cortex during the critical period

Development of inhibition is a crucial determinant of the time course of visual cortical plasticity. BDNF strongly affects interneuron development and the onset and closure of the critical period for ocular dominance plasticity. Less is known on the effects of NT-4 despite a clear involvement in ocular dominance plasticity. We have investigated the effects of NT-4 on interneuron development by supplying NT-4 with osmotic minipumps during two time windows overlapping the onset (P12-20) and the peak (P20-28) of the critical period. We assessed the expression of interneuronal markers and soma size maturation either after the end of the infusion periods or at the end of the critical period (P45). We found that NT-4 was very effective in regulating interneuron development. NPY, SOM and PARV neuron somata grew faster during both infusion periods whereas CR neurons only responded during the early infusion period. The effects of soma size elicited during the earlier infusion period were still present at P45. In PARV neurons, NT-4 caused a long-lasting stabilization of CB and NPY expression. Furthermore, NT-4 accelerated the expression of GAD-65 mRNA in a subset of non-PARV neurons of layer V, which normally up-regulate GAD-65 towards the end of the critical period. Most of these effects were shared by NT-4 and BDNF. Some were unexpectedly also shared by NGF, which promoted growth of layer V PARV neurons, stabilized the CB expression and accelerated the GAD-65 expression. The results suggest that neurotrophins act on critical period plasticity by strengthening inhibition.

Developmental changes of neurotrophin mRNA expression in the layers of rat visual cortex

Neurotrophins are essential factors for the structural, neurochemical and functional maturation of the brain including developmental and adult plasticity. Northern blots and polymerase chain reaction revealed the expression of neurotrophin 4 (NT4), neurotrophin 3 (NT3), brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) in the cortex. The cellular producers of NT3 and BDNF have been characterized by anatomical methods as being mostly pyramidal, and the tyrosine kinase B (TrkB) receptor is expressed by many cortical neurons. However, these methods have so far failed to identify the cells producing NT4 and NGF mRNA. These factors are much lower in expression than, e.g. BDNF, and apparently remain below detection levels of in situ hybridization. Given their specific actions on cell types and afferent systems, knowledge about the producing cell types is highly desirable. To narrow down on the producing cell types, we quantified by reverse transcriptase-polymerase chain reaction (RT-PCR) the developmental changes of BDNF, NT3, NT4, NGF and TrkB mRNA expression in total visual cortex lysates, and in the cortical layers dissected by tangential cryostat sectioning. We found dramatic changes in laminar expression of NT3 and NGF, mild changes of NT4, and no changes of BDNF and TrkB mRNA. For instance, NT3 is important early on for thalamocortical axons, and we found transient peaks of NT3 mRNA expression first in layer VI, then in layer IV. NT4 mRNA was in layers IV and VI, suggesting NT4 protein production in thalamorecipient layers, but peak expression gradually shifted to upper layers as did NGF expression. The layer-specific developmental expression shifts of neurotrophin mRNAs correlate with morphogenetic processes.

Brain-derived neurotrophic factor (BDNF) is required for the enhancement of hippocampal neurogenesis following environmental enrichment

Neurogenesis continues to occur in the adult mammalian hippocampus and is regulated by both genetic and environmental factors. It is known that exposure to an enriched environment enhances the number of newly generated neurons in the dentate gyrus. However, the mechanisms by which enriched housing produces these effects are poorly understood. To test a role for neurotrophins, we used heterozygous knockout mice for brain-derived neurotrophic factor (BDNF+/-) and mice lacking neurotrophin-4 (NT-4-/-) together with their wild-type littermates. Mice were either reared in standard laboratory conditions or placed in an enriched environment for 8 weeks. Animals received injections of the mitotic marker bromodeoxyuridine (BrdU) to label newborn cells. Enriched wild-type and enriched NT-4-/- mice showed a two-fold increase in hippocampal neurogenesis as assessed by stereological counting of BrdU-positive cells in the dentate gyrus and double labelling for BrdU and the neuronal marker NeuN. Remarkably, this enhancement of hippocampal neurogenesis was not seen in enriched BDNF+/- mice. Failure to up-regulate BDNF accompanied the lack of a neurogenic response in enriched BDNF heterozygous mice. We conclude that BDNF but not NT-4 is required for the environmental induction of neurogenesis.

A role for retinal brain-derived neurotrophic factor in ocular dominance plasticity

Visual deprivation is a classical tool to study the plasticity of visual cortical connections. After eyelid closure in young animals (monocular deprivation, MD), visual cortical neurons become dominated by the open eye, a phenomenon known as ocular dominance (OD) plasticity . It is commonly held that the molecular mediators of OD plasticity are cortically derived and that the retina is immune to the effects of MD . Recently, it has been reported that visual deprivation induces neurochemical, structural, and functional changes in the retina , but whether these retinal changes contribute to the effects of MD in the cortex is unknown. Here, we provide evidence that brain-derived neurotrophic factor (BDNF) produced in the retina influences OD plasticity. We found a reduction of BDNF expression in the deprived retina of young rats. We compensated this BDNF imbalance between the two eyes by either injecting exogenous BDNF in the deprived eye or reducing endogenous BDNF expression in the nondeprived eye. Both treatments were effective in counteracting the OD shift induced by MD. Retinal BDNF could also influence OD distribution in normal animals. These results show for the first time that OD plasticity is modulated by BDNF produced in the retina.

Gene expression changes and molecular pathways mediating activity-dependent plasticity in visual cortex

Two key models for examining activity-dependent development of primary visual cortex (V1) involve either reduction of activity in both eyes via dark-rearing (DR) or imbalance of activity between the two eyes via monocular deprivation (MD). Combining DNA microarray analysis with computational approaches, RT-PCR, immunohistochemistry and physiological imaging, we find that DR leads to (i) upregulation of genes subserving synaptic transmission and electrical activity, consistent with a coordinated response of cortical neurons to reduction of visual drive, and (ii) downregulation of parvalbumin expression, implicating parvalbumin-expressing interneurons as underlying the delay in cortical maturation after DR. MD partially activates homeostatic mechanisms but differentially upregulates molecular pathways related to growth factors and neuronal degeneration, consistent with reorganization of connections after MD. Expression of a binding protein of insulin-like growth factor-1 (IGF1) is highly upregulated after MD, and exogenous application of IGF1 prevents the physiological effects of MD on ocular dominance plasticity examined in vivo.

Induction of brain-derived neurotrophic factor by leptin in the ventromedial hypothalamus

Leptin, an adipocyte-derived hormone, reduces food intake by regulating orexigenic and anorexigenic factors in the hypothalamus. Although brain-derived neurotrophic factor is an important anorexigenic factor in the hypothalamus, little is known about the regulation of brain-derived neurotrophic factor expression by leptin in the hypothalamus. In the present study, we examined the effect of leptin on the expression of brain-derived neurotrophic factor in the hypothalamus. I.V. administration of leptin (10 microg/g) led to the increase in the expression of brain-derived neurotrophic factor mRNA, which was observed in the dorsomedial part of the ventromedial hypothalamic nucleus. The increased expression of brain-derived neurotrophic factor mRNA was detected in phosphorylated signal transducer and activator of transcription 3-positive neurons, suggesting that leptin induced brain-derived neurotrophic factor expression in neurons of the dorsomedial part of the ventromedial hypothalamic nucleus. In addition, the expression of brain-derived neurotrophic factor was increased at the protein level in the ventromedial hypothalamic nucleus of leptin-injected mice. Interestingly, brain-derived neurotrophic factor-positive fibers also increased in the ventromedial hypothalamic nucleus and dorsomedial hypothalamic nucleus of leptin-injected mice, which were in close apposition to tyrosine kinase receptor B-immunoreactive neurons and colocalized with synaptophysin, a marker of presynaptic terminals. These results suggest that leptin induces brain-derived neurotrophic factor expression in the dorsomedial part of the ventromedial hypothalamic nucleus and brain-derived neurotrophic factor may exert as anorexigenic factors possibly through the activation of tyrosine kinase receptor B in the ventromedial hypothalamic nucleus and dorsomedial hypothalamic nucleus.

Transcellular induction of neuropeptide Y expression by NT4 and BDNF

The transcellular signaling of neurotrophins is postulated, but evidence is scarce. We now show that a small number of NT4- and BDNF-overexpressing neurons in the cortical explant of thalamocortical cocultures rapidly evoked a Trk receptor-dependent upregulation of neuropeptide Y (NPY) mRNA in interneurons. In contrast to BDNF, the action of NT4 was independent of calcium influx through NMDA receptors and L-type calcium channels. NPY neurons vastly outnumbered the neurotrophin-overexpressing neurons (mostly pyramidal cells), arguing for a spread of the neurotrophin signal via axonally connected neuronal populations. Furthermore, NT4 transfection of one explant of axonally connected corticocortical cocultures evoked significantly larger numbers of NPY neurons in both explants. Delivery of the signal was not by diffusion of neurotrophins via the medium. Moreover, cortical NPY neuron numbers increased after NT4 and BDNF transfection of a cocultured tectal explant innervated selectively by cortical layer V pyramidal neurons. The transcellular induction of NPY suggests a source-to-sink model for axonal transport and a local cortical redistribution of TrkB ligands to interneurons competent for NPY expression.

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 involvement in the visual system: implications in allergic and neurodegenerative diseases

The purpose of this review is to outline the main role of nerve growth factor (NGF) in the visual system and particularly in the ocular surface in physiological and pathological conditions. The present review of experimental and clinical studies will highlight old and recent strategies for treating ocular surface and tear disorders with NGF.

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.

Hyperexcitability-associated rapid plasticity after a focal cerebral ischemia

BACKGROUND AND PURPOSE

This article addresses how neuroplastic changes are initiated after an ischemic stroke.

METHODS

A focal cerebral ischemia was photochemically induced on the primary somatosensory cortex of rats, and in vivo electrophysiological recordings were performed on the peri-infarct cortex before and from 1 to 6 hours after the infarction.

RESULTS

Paired-pulse analysis of evoked field potentials to peripheral electrical stimuli showed statistically significant neuronal hyperexcitability that was associated with rapid expansion of receptive fields (146.1% at 1 hour and 553.6% at 6 hours) as early as 1 hour after the infarction (P<0.05). Current source density analysis revealed increased current sinks in cortical layer II/III.

CONCLUSIONS

Our electrophysiological results showed, for the first time to our knowledge, rapid plastic changes in the peri-infarct cortex during the hyperacute stage of an ischemic stroke. Manipulation of this rapid plasticity may affect subsequent plastic changes.

BDNF and NT-3 promote thalamocortical axon growth with distinct substrate and temporal dependency

The role of neurotrophins in thalamic axon growth was studied by culturing embryonic rat thalamus on collagen-coated substrate or fixed cortical slices in the presence of either brain-derived neurotrophic factor (BDNF) or neurotrophin-3 (NT-3). Both BDNF and NT-3 promoted axonal growth, but the axonal growth-promoting activity depended on culture substrates. Axonal growth on collagen-coated membrane was accelerated by BDNF, but not by NT-3. In contrast, axonal outgrowth on fixed cortex was significantly enhanced by NT-3, but not by BDNF. Semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) analysis of cultured thalamic cells demonstrated that culture substrates did not alter the expression of their receptors, trkB and trkC. Terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) staining further demonstrated that axonal growth promoted by neurotrophins was not due to reduction of cell death. Measurement of the developmental changes in BDNF and NT-3 levels revealed that, in contrast to the rapid elevation of BDNF after the arrival of thalamocortical axons to their target layer, the regulation of NT-3 protein accompanies the phase of their outgrowth in neocortex. These findings suggest that BDNF and NT-3 promote thalamic axon growth in different manners in terms of substrate dependency and developmental stage.

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.

Complexity in the modulation of neurotrophic factor mRNA expression by early visual experience

The expression of mRNA for brain-derived neurotrophic factor (BDNF) is regulated by early visual experience. In this study, we sought to determine whether other neurotrophic factor mRNAs are similarly regulated. We reared pigmented rats from birth to postnatal day 21 in a normal light cycle, constant light (LR) or constant darkness (DR). In the retina, superior colliculus (SC), primary visual cortex (V1), hippocampus (HIPP) and cerebellum (CBL), using a ribonuclease protection assay (RPA), we examined expression of the mRNAs for nerve growth factor (NGF), BDNF, NT3, NT4, ciliary neurotrophic factor (CNTF) and glial cell line-derived neurotrophic factor (GDNF). LR or DR alter the expression of the mRNAs for NGF, BDNF and NT3 and CNTF within the visual system. LR also upregulated BDNF mRNA expression within the cerebellum. In all of the structures examined, NT4 mRNA expression was unaltered by LR or DR and GDNF mRNA was undetectable. Notably, the same rearing condition could induce changes of opposite sign in the mRNA for a single factor in different structures or for different factors in the same structure. Thus, during developmental stages when sensory experience and neuroelectric activity are important in the shaping of visual circuitry, vision regulates the expression of multiple neurotrophic factor mRNAs and each mRNA has a unique profile with respect to the locus and sign of activity-induced changes.

Molecular basis of plasticity in the visual cortex

Sensory experience is known to shape the maturation of cortical circuits during development. A paradigmatic example is the effect of monocular deprivation on ocular dominance of visual cortical neurons. Although visual cortical plasticity has been widely studied since its initial discovery by Hubel and Wiesel >40 years ago, the description of the underlying molecular mechanisms has lagged behind. Several new findings are now beginning to close this gap. Recent data deepen our knowledge of the factors involved in the intercellular communication and intracellular signaling that mediate experience-dependent plasticity in the developing visual cortex. In addition, new findings suggest a role for the extracellular matrix in inhibition of ocular-dominance plasticity in the adult visual cortex.

Activity-dependent change in the protein level of brain-derived neurotrophic factor but no change in other neurotrophins in the visual cortex of young and adult ferrets

Neurotrophins are suggested to play a role in activity-dependent plasticity of visual cortex during the critical period of postnatal development. Thus, the concentration of neurotrophins in the cortex is expected to change with development and/or with alteration in neuronal activities. To test this, we measured protein levels of nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3 and neurotrophin-4/5 in visual cortex of young (postnatal day 38-46, at the peak of the critical period) and adult ferrets with two-site enzyme-immunoassay systems. Measurements were carried out also in somatosensory cortex, hippocampus and cerebellum as control. With development the level of brain-derived neurotrophic factor did not significantly change, while those of the other neurotrophins changed in the visual cortex. A blockade of visual inputs for 24 h by an injection of tetrodotoxin into both eyes significantly decreased brain-derived neurotrophic factor protein level in the visual cortex, but not in the other regions in both young and adult ferrets. On the other hand, no significant decrease was seen in the protein level of the other neurotrophins in the visual cortex of young and adult ferrets. A monocular injection of tetrodotoxin in young ferrets resulted in the reduction of brain-derived neurotrophic factor by approximately half that by binocular injection. The degree of the decrease in the contralateral cortex to the injected eye was significantly larger than that in the ipsilateral cortex, reflecting that the contralateral eye is dominantly represented in the cortex in ferrets. Blockade of cortical neuronal activities by a GABA(A) receptor agonist led to a remarkable reduction of brain-derived neurotrophic factor protein in the visual cortex. These results suggest that the level of brain-derived neurotrophic factor protein in visual cortex is regulated by activities of cortical neurons.

Long-term depression is not induced by low-frequency stimulation in rat visual cortex in vivo: a possible preventing role of endogenous brain-derived neurotrophic factor

Low-frequency stimulation (LFS) at 1 Hz for 15 min is an effective protocol to induce homosynaptic long-term depression (LTD) in visual cortical slices. It is reported that LFS becomes ineffective when brain-derived neurotrophic factor (BDNF) is applied to slices. It is not known, however, whether such a protocol induces LTD in visual cortex in vivo, and whether endogenous BDNF has the same or similar action. To address these questions, we recorded field potentials of rat visual cortex evoked by stimulation of lateral geniculate nucleus, white matter, or cortical layer IV. We found that LFS did not induce LTD of cortical responses in vivo. To test the possibility that spontaneous activity from retinas would interfere with the induction of LTD, both eyes were removed or inactivated by tetrodotoxin. LTD was not induced in these conditions either. To test whether the difference in temperature between the two preparations is a factor for the discrepancy, the temperature of slices was increased from 31 to 37 degrees C. LTD was induced in slices at either temperature. Then, we hypothesized that endogenous BNDF and its receptors, TrkB, prevent the induction of LTD. To test this, we infused the cortex with an inhibitor of Trk receptor tyrosine kinases, anti-TrkB IgG1, anti-BDNF, and anti-neurotrophin 4/5 antibodies. LTD was induced when the BDNF-TrkB system was blocked. In slices, the level of phosphorylation of Trks was found to decrease with time. These results indicate that activation of TrkB signal pathway prevents LFS from inducing synaptic depression in visual cortex in vivo.

Effects of neurotrophins on synaptic protein expression in the visual cortex of dark-reared rats

Total lack of visual experience [dark rearing (DR)] is known to prolong the critical period and delay development of sensory functions in mammalian visual cortex. Recent results show that neurotrophins (NTs) counteract the effects of DR on functional properties of visual cortical cells and exert a strong control on critical period duration. NTs are known to modulate the development and synaptic efficacy of neurotransmitter systems that are affected by DR. However, it is still unknown whether the actions of NTs in dark-reared animals involve interaction with neurotransmitter systems. We have studied the effects of DR on the expression of key molecules in the glutamatergic and GABAergic systems in control and NT-treated animals. We have found that DR reduced the expression of the NMDA receptor 2A subunit and its associated protein PSD-95 (postsynaptic density-95), of GRIP (AMPA glutamate receptor interacting protein), and of the biosynthetic enzyme GAD (glutamic acid decarboxylase). Returning dark-reared animals to light for 2 hr restored normal expression of the above-mentioned proteins almost completely. NT treatment specifically counteracts DR effects; NGF acts primarily on the NMDA system, whereas BDNF acts primarily on the GABAergic system. Finally, the action of NT4 seems to involve both excitatory and inhibitory systems. These data demonstrate that different NTs counteract DR effects by modulating the expression of key molecules of the excitatory and inhibitory neurotransmitter systems.

Neurotrophins and cortical development

The mammalian cerebral cortex requires the proper formation of exquisitely precise circuits to function correctly. These neuronal circuits are assembled during development by the formation of synaptic connections between hundreds of thousands of differentiating neurons. Although the development of the cerebral cortex has been well described anatomically, the cellular and molecular mechanisms that guide neuronal differentiation and formation of connections are just beginning to be understood. Moreover, despite evidence that coordinated patterns of activity underlie reorganization of brain circuits during critical periods of development, the molecular signals that translate activity into structural and functional changes in connections remain unknown. Recently, the neurotrophins have emerged as attractive candidates not only for regulating neuronal differentiation in the developing brain, but also for mediating activity-dependent synaptic plasticity. The neurotrophins meet many of the criteria required for molecular signals involved in neuronal differentiation and plasticity. They are present in the cerebral cortex during development and their expression is regulated by synaptic activity. In turn, the neurotrophins themselves strongly influence both short-term synaptic plasticity and long-term potentiation and depression. In addition to their functional effects, the neurotrophins also profoundly regulate the structural changes that underlie axonal and dendritic differentiation. Finally, the neurotrophins have been implicated in mediating synaptic competition required for activity-dependent plasticity during the critical period. This chapter presents and discusses the rapidly accumulating evidence that the neurotrophins are critical for neuronal differentiation and that they may be involved in activity-dependent synaptic refinement in the developing cerebral cortex.

Axonal regrowth of layer II-III visual-projecting cortical neurons in rats fails beyond eye opening

Fetal neurons (embryonic age E16) of occipital origin grafted in the visual cortex of albino rats at increasing postnatal stages (P0, P7, P15, P30, P60, P120) can be activated by photic stimulation. Inputs originate from five major areas of the brain ipsilateral to the graft, namely, the claustrum, the periallocortex/proisocortex, the isocortex, the visual thalamus, and some unspecific subthalamic and hypothalamic nuclei. All inputs decrease in number with the age at which grafting was performed. Isocortical afferents exhibit furthermore a progressive laminar shaping. In neonates, layer II-III and layer V-VI neurons contribute equally to the graft input. In adults, grafts receive prominent input (approximately 70-80%) from layer VI neurons whereas layer II-III neurons account for less than 10%. Proportions of layer IV (approximately 2-4%) and layer V (approximately 15-20%) neurons innervating the graft remain stable, irrespective of the age of the recipient. The adult pattern of connectivity between the host brain and the graft establishes in frontal and temporal areas 1 week earlier than in occipital areas. It is nearly completed in postnatal day 15 (P15) grafted recipients. Supragranular neurons would be thus unable to innervate and to make stable synapses at the graft level beyond P15, i.e., when eyes open. Some infragranular neurons (supposedly remnants of the earliest generated cortical cell population) still have this capacity in adults.