Effects of nerve growth factor on visual cortical plasticity require afferent electrical activity

Spontaneous Afferent Activity Carves Olfactory Circuits

Electrical activity has a key role in shaping neuronal circuits during development. In most sensory modalities, early in development, internally generated spontaneous activity sculpts the initial layout of neuronal wiring. With the maturation of the sense organs, the system relies more on sensory-evoked electrical activity. Stimuli-driven neuronal discharge is required for the transformation of immature circuits in the specific patterns of neuronal connectivity that subserve normal brain function. The olfactory system (OS) differs from this organizational plan. Despite the important role of odorant receptors (ORs) in shaping olfactory topography, odor-evoked activity does not have a prominent role in refining neuronal wiring. On the contrary, afferent spontaneous discharge is required to achieve and maintain the specific diagram of connectivity that defines the topography of the olfactory bulb (OB). Here, we provide an overview of the development of olfactory topography, with a focus on the role of afferent spontaneous discharge in the formation and maintenance of the specific synaptic contacts that result in the topographic organization of the OB.

Rapid plasticity of visually evoked responses in rat monocular visual cortex

Sensory cortical circuits are shaped by experience during sensitive periods in development. In the primary visual cortex (V1) altered visual experience results in changes in visual responsiveness of cortical neurons. The experience-dependent refinement of the circuit in V1 is thought to rely on competitive interactions between feedforward circuits driven by the two eyes. However, recent data have provided evidence for an additional role of cortico-cortical circuits in this process. Indeed, experience-dependent changes in intracortical circuits can be induced rapidly and may result in rapid-onset functional changes. Unilateral occlusion of vision rapidly alters visual responsiveness, synaptic strength and connectivity of local circuits in the binocular region of V1 (V1b), where the inputs from the two eyes converge. In the monocular region of rodent V1 (V1m), where feedforward inputs from the ipsilateral eye are virtually absent, visual deprivation induces rapid plasticity in local circuits; however, functional changes seem to occur only after long periods of deprivation. In V1m there is currently no evidence for functional changes occurring within a time window compatible with that of local circuit plasticity. Here, we probed the visual responsiveness of neurons in rat V1m and assessed the effect of one day unilateral eye lid suture on single neuron visual responses. We report a novel form of plasticity within V1m that occurs on a timescale consistent with the earliest known changes in synaptic strength. Our data provide new insights into how sensory experience can rapidly modulate neuronal responses, even in the absence of direct competition between feedforward thalamocortical inputs.

Circuit formation and function in the olfactory bulb of mice with reduced spontaneous afferent activity

The type of neuronal activity required for circuit development is a matter of significant debate. We addressed this issue by analyzing the topographic organization of the olfactory bulb in transgenic mice engineered to have very little afferent spontaneous activity due to the overexpression of the inwardly rectifying potassium channel Kir2.1 in the olfactory sensory neurons (Kir2.1 mice). In these conditions, the topography of the olfactory bulb was unrefined. Odor-evoked responses were readily recorded in glomeruli with reduced spontaneous afferent activity, although the functional maps were coarser than in controls and contributed to altered olfactory discrimination behavior. In addition, overexpression of Kir2.1 in adults induced a regression of the already refined connectivity to an immature (i.e., coarser) status. Our data suggest that spontaneous activity plays a critical role not only in the development but also in the maintenance of the topography of the olfactory bulb and in sensory information processing.

A switch from inter-ocular to inter-hemispheric suppression following monocular deprivation in the rat visual cortex

Binocularity is a key property of primary visual cortex (V1) neurons that is widely used to study synaptic integration in the brain and plastic mechanisms following an altered visual experience. However, it is not clear how the inputs from the two eyes converge onto binocular neurons, and how their interaction is modified by an unbalanced visual drive. Here, using visual evoked potentials recorded in the juvenile rat V1, we report evidence for a suppressive mechanism by which contralateral eye activity inhibits responses from the ipsilateral eye. Accordingly, we found a lack of additivity of the responses evoked independently by the two eyes in the V1, and acute silencing of the contralateral eye resulted in the enhancement of ipsilateral eye responses in cortical neurons. We reverted the relative cortical strength of the two eyes by suturing the contralateral eye shut [monocular deprivation (MD)]. After 7 days of MD, there was a loss of interocular suppression mediated by the contralateral, deprived eye, and weak inputs from the closed eye were functionally inhibited by interhemispheric callosal pathways. We conclude that interocular suppressive mechanisms play a crucial role in shaping normal binocularity in visual cortical neurons, and a switch from interocular to interhemispheric suppression represents a key step in the ocular dominance changes induced by MD. These data have important implications for a deeper understanding of the key mechanisms that underlie activity-dependent rearrangements of cortical circuits following alteration of sensory experience.

Environmental enrichment extends ocular dominance plasticity into adulthood and protects from stroke-induced impairments of plasticity

Ocular dominance (OD) plasticity in mouse primary visual cortex (V1) declines during postnatal development and is absent beyond postnatal day 110 if mice are raised in standard cages (SCs). An enriched environment (EE) promotes OD plasticity in adult rats. Here, we explored cellular mechanisms of EE in mouse V1 and the therapeutic potential of EE to prevent impairments of plasticity after a cortical stroke. Using in vivo optical imaging, we observed that monocular deprivation in adult EE mice (i) caused a very strong OD plasticity previously only observed in 4-wk-old animals, (ii) restored already lost OD plasticity in adult SC-raised mice, and (iii) preserved OD plasticity after a stroke in the primary somatosensory cortex. Using patch-clamp electrophysiology in vitro, we also show that (iv) local inhibition was significantly reduced in V1 slices of adult EE mice and (v) the GABA/AMPA ratio was like that in 4-wk-old SC-raised animals. These observations were corroborated by in vivo analyses showing that diazepam treatment significantly reduced the OD shift of EE mice after monocular deprivation. Taken together, EE extended the sensitive phase for OD plasticity into late adulthood, rejuvenated V1 after 4 mo of SC-rearing, and protected adult mice from stroke-induced impairments of cortical plasticity. The EE effect was mediated most likely by preserving low juvenile levels of inhibition into adulthood, which potentially promoted adaptive changes in cortical circuits.

Odorant receptors in the formation of the olfactory bulb circuitry

In mammals, smell is mediated by odorant receptors expressed by sensory neurons in the nose. These specialized receptors are found both on olfactory sensory neurons' cilia and axon terminals. Although the primary function of ciliary odorant receptors is to detect odorants, their axonal role remains unclear but is thought to involve axon guidance. This review discusses findings that show axonal odorant receptors are indeed functional and capable of modulating neural connectivity.

Visual cortex plasticity: a complex interplay of genetic and environmental influences

The central nervous system architecture is highly dynamic and continuously modified by sensory experience through processes of neuronal plasticity. Plasticity is achieved by a complex interplay of environmental influences and physiological mechanisms that ultimately activate intracellular signal transduction pathways regulating gene expression. In addition to the remarkable variety of transcription factors and their combinatorial interaction at specific gene promoters, epigenetic mechanisms that regulate transcription have emerged as conserved processes by which the nervous system accomplishes the induction of plasticity. Experience-dependent changes of DNA methylation patterns and histone posttranslational modifications are, in fact, recruited as targets of plasticity-associated signal transduction mechanisms. Here, we shall concentrate on structural and functional consequences of early sensory deprivation in the visual system and discuss how intracellular signal transduction pathways associated with experience regulate changes of chromatin structure and gene expression patterns that underlie these plastic phenomena. Recent experimental evidence for mechanisms of cross-modal plasticity following congenital or acquired sensory deprivation both in human and animal models will be considered as well. We shall also review different experimental strategies that can be used to achieve the recovery of sensory functions after long-term deprivation in humans.

Evidence for anterograde transport and transcytosis of botulinum neurotoxin A (BoNT/A)

Botulinum neurotoxin type A (BoNT/A) is a metalloprotease that blocks synaptic transmission via the cleavage of SNAP-25 (synaptosomal-associated protein of 25 kDa). BoNT/A is successfully used in clinical neurology for the treatment of several neuromuscular pathologies and pain syndromes. Despite its widespread use, relatively little is known on BoNT/A intracellular trafficking in neurons. Using the visual pathway as a model system, here we show that catalytically active BoNT/A is capable of undergoing anterograde axonal transport and transcytosis. Following BoNT/A injection into the rat eye, significant levels of BoNT/A-cleaved SNAP-25 appeared in the retinorecipient layers of the superior colliculus (SC). Anterograde propagation of BoNT/A effects required axonal transport, ruling out a systemic spread of the toxin. Cleaved SNAP-25 was present in presynaptic structures of the tectum, but retinal terminals were devoid of the immunoreactivity, indicative of transcytosis. Experiments based on sequential administration of BoNT/A and BoNT/E showed a persistent catalytic activity of BoNT/A in tectal cells following its injection into the retina. Our findings demonstrate that catalytically active BoNT/A is anterogradely transported from the eye to the SC and transcytosed to tectal synapses. These data are important for a more complete understanding of the mechanisms of action of BoNT/A.

Loss of visually driven synaptic responses in layer 4 regular-spiking neurons of rat visual cortex in absence of competing inputs

Monocular deprivation (MD) during development shifts the ocular preference of primary visual cortex (V1) neurons by depressing closed-eye responses and potentiating open-eye responses. As these 2 processes are temporally and mechanistically distinct, we tested whether loss of responsiveness occurs also in absence of competing inputs. We thus compared the effects of long-term MD in layer 4 regular-spiking pyramidal neurons (L4Ns) of binocular and monocular V1 (bV1 and mV1) with whole-cell recordings. In bV1, input depression was larger than potentiation, and the ocular dominance shift was larger for spike outputs. MD-but not retinal inactivation with tetrodotoxin-caused a comparable loss of synaptic and spike responsiveness in mV1, which is innervated only by the deprived eye. Conversely, brief MD depressed synaptic responses only in bV1. MD-driven depression in mV1 was accompanied by a proportional reduction of visual thalamic inputs, as assessed upon pharmacological silencing of intracortical transmission. Finally, sub- and suprathreshold responsiveness was similarly degraded in L4Ns of bV1 upon complete deprivation of patterned vision through a binocular deprivation period of comparable length. Thus, loss of synaptic inputs from the deprived eye occurs also in absence of competition in the main thalamorecipient lamina, albeit at a slower pace.

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.

Thalamic activity that drives visual cortical plasticity

Manipulations of activity in one retina can profoundly affect binocular connections in the visual cortex. Retinal activity is relayed to the cortex by the dorsal lateral geniculate nucleus (dLGN). We compared the qualities and amount of activity in the dLGN following monocular eyelid closure and monocular retinal inactivation in awake mice. Our findings substantially alter the interpretation of previous studies and define the afferent activity patterns that trigger cortical plasticity.

Long-distance retrograde effects of botulinum neurotoxin A

Botulinum neurotoxins (designated BoNT/A-BoNT/G) are bacterial enzymes that block neurotransmitter release by cleaving essential components of the vesicle fusion machinery. BoNT/A, which cleaves SNAP-25 (synaptosomal-associated protein of 25 kDa), is extensively exploited in clinical medicine to treat neuromuscular pathologies, facial wrinkles, and various types of pain. It is widely assumed that BoNT/A remains at the synaptic terminal and its effects are confined to the injection site. Here we demonstrate that catalytically active BoNT/A is retrogradely transported by central neurons and motoneurons and is then transcytosed to afferent synapses, in which it cleaves SNAP-25. SNAP-25 cleavage by BoNT/A was observed in the contralateral hemisphere after unilateral BoNT/A delivery to the hippocampus. Appearance of cleaved SNAP-25 resulted in blockade of hippocampal activity in the untreated hemisphere. Injections of BoNT/A into the optic tectum led to the appearance of BoNT/A-truncated SNAP-25 in synaptic terminals within the retina. Cleaved SNAP-25 also appeared in the facial nucleus after injection of the toxin into rat whisker muscles. Experiments excluded passive spread of the toxin and demonstrated axonal migration and neuronal transcytosis of BoNT/A. These findings reveal a novel pathway of BoNT/A trafficking in neurons and have important implications for the clinical uses of this neurotoxin.

Experience and activity-dependent maturation of perisomatic GABAergic innervation in primary visual cortex during a postnatal critical period

The neocortical GABAergic network consists of diverse interneuron cell types that display distinct physiological properties and target their innervations to subcellular compartments of principal neurons. Inhibition directed toward the soma and proximal dendrites is crucial in regulating the output of pyramidal neurons, but the development of perisomatic innervation is poorly understood because of the lack of specific synaptic markers. In the primary visual cortex, for example, it is unknown whether, and to what extent, the formation and maturation of perisomatic synapses are intrinsic to cortical circuits or are regulated by sensory experience. Using bacterial artificial chromosome transgenic mice that label a defined class of perisomatic synapses with green fluorescent protein, here we show that perisomatic innervation developed during a protracted postnatal period after eye opening. Maturation of perisomatic innervation was significantly retarded by visual deprivation during the third, but not the fifth, postnatal week, implicating an important role for sensory input. To examine the role of cortical intrinsic mechanisms, we developed a method to visualize perisomatic synapses from single basket interneurons in cortical organotypic cultures. Characteristic perisomatic synapses formed through a stereotyped process, involving the extension of distinct terminal branches and proliferation of perisomatic boutons. Neuronal spiking in organotypic cultures was necessary for the proliferation of boutons and the extension, but not the maintenance, of terminal branches. Together, our results suggest that although the formation of perisomatic synapses is intrinsic to the cortex, visual experience can influence the maturation and pattern of perisomatic innervation during a postnatal critical period by modulating the level of neural activity within cortical circuits.

Neuronal activity regulates the developmental expression and subcellular localization of cortical BDNF mRNA isoforms in vivo

Activity-dependent changes in BDNF expression have been implicated in developmental plasticity. Although its expression is widespread in visual cortex, developmental regulation of its different transcripts by visual experience has not been investigated. Here, we investigated the cellular expression of different BDNF transcripts in rat visual cortex during postnatal development. We found that transcripts I and II are expressed only in adults but III and IV are expressed from early postnatal stage. Total BDNF mRNA is expressed throughout the age groups. Transcripts III and IV show a differential intracellular localization, while former was detected only in cell bodies, latter is present both in cell bodies and dendritic processes. Inhibition of visual activity decreases the levels of exons, with exon IV transcript almost disappearing from dendrites. In vitro experiments also confirmed the above results, indicating activity-dependent regulation of different BDNF promoters with specific temporal and cellular patterns of expression in developing visual cortex.

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.

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.

Critical periods for experience-dependent synaptic scaling in visual cortex

The mechanisms underlying experience-dependent plasticity and refinement of central circuits are not yet fully understood. A non-Hebbian form of synaptic plasticity, which scales synaptic strengths up or down to stabilize firing rates, has recently been discovered in cultured neuronal networks. Here we demonstrate the existence of a similar mechanism in the intact rodent visual cortex. The frequency of miniature excitatory postsynaptic currents (mEPSCs) in principal neurons increased steeply between post-natal days 12 and 23. There was a concomitant decrease in mEPSC amplitude, which was prevented by rearing rats in complete darkness from 12 days of age. In addition, as little as two days of monocular deprivation scaled up mEPSC amplitude in a layer- and age-dependent manner. These data indicate that mEPSC amplitudes can be globally scaled up or down as a function of development and sensory experience, and suggest that synaptic scaling may be involved in the activity-dependent refinement of cortical connectivity.

Selegiline treatment facilitates recovery after stroke

OBJECTIVE

Selegiline (L-deprenyl) is a selective monoamine oxidase B (MAO-B) inhibitor used in the treatment of Parkinson's disease. In addition, it is thought to rescue neurons with a loss of target-derived trophic support. Several mechanisms have been proposed to explain these phenomena, such as the production of neurotrophic actions through astrocyte activation, reduction of free radical production, or the presence of antiapoptotic properties. The aim of this study was to investigate whether the systemic administration of selegiline facilitates recovery after a cerebral infarction in humans.

METHODS

This phase II study was randomized, double-blind, and placebo controlled. Selegiline, 5 mg, or matched placebo was given twice a day for 3 months. The drug therapy was started within 48 h after a hemispheric infarction in the territory of middle cerebral artery. There were 24 patients recruited. Twenty patients were followed up to 3 months or until their death, and they represent the efficacy analysis group. The primary efficacy parameters were Scandinavian Stroke Scale (SSS), Barthel Index (BI), and Fugl-Meyer Scale (FMS). Secondary parameters were Zung Self-Rating Depression Scale (ZDS) and 15-Dimensional Measure of Health Related Quality of Life test (15-D).

RESULTS

SSS improved statistically significantly from the baseline when compared with placebo (p = 0.019). The results were parallel among the other two primary efficacy variables (BI and FMS), showing a positive trend for selegiline, although they did not reach statistical significance. Similarly, in the analysis of the secondary efficacy variables, both the 15-D test and ZDS supported this positive trend in favor of selegiline, although no statistically significant differences between groups were found (p = 0.06 in 15-D test).

CONCLUSIONS

Selegiline seems to be beneficial after a cerebral infarction. This benefit may be due to the enhancement of the recovery process.

Neurotrophins and plasticity in the visual cortex

The visual cortex is one of the favorite models for the study of experience-dependent changes in neuronal structure and function. A number of recent investigations indicate that the neurotrophic factors of the nerve growth factor family (neurotrophins) play a pivotal role in visual cortical plasticity. Neurotrophins and their receptors are present in the cortex during the critical period for plasticity, and neurotrophin levels are regulated by electrical activity. Neurotrophins modulate synaptic transmission and patterns of neuronal connectivity in the cortex. This review summarizes the in vivo and in vitro data that demonstrate the involvement of neurotrophins in visual cortical plasticity and discusses the possible mechanisms of their action.

Selegiline combined with enriched-environment housing attenuates spatial learning deficits following focal cerebral ischemia in rats

Selegiline (l-deprenyl) is an irreversible monoamine oxidase B (MAO-B) inhibitor that is suggested to have neuroprotective and neuronal rescuing properties. The present study investigated whether systemic administration of selegiline facilitates behavioral recovery after transient focal cerebral ischemia in rats using a combination of different behavioral tests (limb placing, foot slip, water maze, and Montoya's staircase test) to measure the outcome of recovery. Selegiline (0.5 mg/kg, SC) or 0.9% NaCl was administered once a day, beginning on the second day after induction of ischemia and continuing for 30 days. Selegiline administration combined with enriched-environment housing attenuated ischemia-induced spatial learning deficits in a water-maze task and enhanced performance of both the contralateral affected and ipsilateral nonaffected forelimbs in a staircase test. Selegiline administration alone was not beneficial in any of the tests. Subsequent histologic examination revealed that the infarct volumes were not different between the experimental ischemic groups. Thus, these results suggest that selegiline combined with enriched-environment housing reduces behavioral and cognitive deficits without affecting infarct size.

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.

Effects of neurotrophins on cortical plasticity: same or different?

Neurotrophins are important regulators of visual cortical plasticity. It is still unclear, however, whether they play similar or different roles and which are their effects on the electrical activity of cortical neurons in vivo. We therefore compared the effects of all neurotrophins, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-4 (NT-4), and neurotrophin-3 (NT-3) on visual cortical plasticity and on cell spontaneous and visually evoked activity. Rats were monocularly deprived for 1 week at the peak of the critical period, and neurotrophins were infused intracortically. The main finding is that, with the exception of NT-3, all neurotrophins affect the outcome of monocular deprivation, but there are clear differences in their mechanisms of action. In particular, NT-4 and NGF counteract monocular deprivation effects without causing detectable alterations either in spontaneous or visually evoked neuronal activity. BDNF is less effective on ocular dominance plasticity and, in addition, strongly affects spontaneous and visually evoked activity in cortical neurons.

Critical periods during sensory development

Recent studies have made progress in characterizing the determinants of critical periods for experience-dependent plasticity. They highlight the role of neurotrophins, NMDA receptors and GABAergic inhibition. In particular, genetic manipulation of a single molecule, brain-derived neurotrophic factor (BDNF), has been shown to alter the timing of the critical period of plasticity in mouse visual cortex, establishing a causal relation between neurotrophin action, the development of visual function, and the duration of the critical period.