Production of brain-derived neurotrophic factor gates plasticity in developing visual cortex.

We have previously shown that recovery of visual responses to a deprived eye during the critical period in mouse primary visual cortex requires phosphorylation of the TrkB receptor for BDNF [M. Kaneko, J. L. Hanover, P. M. England, M. P. Stryker, Nat. Neurosci. 11, 497-504 (2008)]. We have now studied the temporal relationship between the production of mature BDNF and the recovery of visual responses under several different conditions. Visual cortical responses to an eye whose vision has been occluded for several days during the critical period and is then re-opened recover rapidly during binocular vision or much more slowly following reverse occlusion, when the previously intact fellow eye is occluded in a model of "patch therapy" for amblyopia. The time to recovery of visual responses differed by more than 18 h between these two procedures, but in each, the production of mature BDNF preceded the physiological recovery. These findings suggest that a spurt of BDNF production is permissive for the growth of connections serving the deprived eye to restore visual responses. Attenuation of recovery of deprived-eye responses by interference with TrkB receptor activation or reduction of BDNF production by interference with homeostatic synaptic scaling had effects consistent with this suggestion.

Visual Deprivation during Mouse Critical Period Reorganizes Network-Level Functional Connectivity.

A classic example of experience-dependent plasticity is ocular dominance (OD) shift, in which the responsiveness of neurons in the visual cortex is profoundly altered following monocular deprivation (MD). It has been postulated that OD shifts also modify global neural networks, but such effects have never been demonstrated. Here, we use wide-field fluorescence optical imaging (WFOI) to characterize calcium-based resting-state functional connectivity during acute (3 d) MD in female and male mice with genetically encoded calcium indicators (Thy1-GCaMP6f). We first establish the fundamental performance of WFOI by computing signal to noise properties throughout our data processing pipeline. Following MD, we found that Δ band (0.4-4 Hz) GCaMP6 activity in the deprived visual cortex decreased, suggesting that excitatory activity in this region was reduced by MD. In addition, interhemispheric visual homotopic functional connectivity decreased following MD, which was accompanied by a reduction in parietal and motor homotopic connectivity. Finally, we observed enhanced internetwork connectivity between the visual and parietal cortex that peaked 2 d after MD. Together, these findings support the hypothesis that early MD induces dynamic reorganization of disparate functional networks including the association cortices.

Neuromodulation in the developing visual cortex after long-term monocular deprivation.

Neural dynamics are altered in the primary visual cortex (V1) during critical period monocular deprivation (MD). Synchronization of neural oscillations is pertinent to physiological functioning of the brain. Previous studies have reported chronic disruption of V1 functional properties such as ocular dominance, spatial acuity, and binocular matching after long-term monocular deprivation (LTMD). However, the possible neuromodulation and neural synchrony has been less explored. Here, we investigated the difference between juvenile and adult experience-dependent plasticity in mice from intracellular calcium signals with fluorescent indicators. We also studied alterations in local field potentials power bands and phase-amplitude coupling (PAC) of specific brain oscillations. Our results showed that LTMD in juveniles causes higher neuromodulatory changes as seen by high-intensity fluorescent signals from the non-deprived eye (NDE). Meanwhile, adult mice showed a greater response from the deprived eye (DE). LTMD in juvenile mice triggered alterations in the power of delta, theta, and gamma oscillations, followed by enhancement of delta-gamma PAC in the NDE. However, LTMD in adult mice caused alterations in the power of delta oscillations and enhancement of delta-gamma PAC in the DE. These markers are intrinsic to cortical neuronal processing during LTMD and apply to a wide range of nested oscillatory markers.

Daily dose-response from short-term monocular deprivation in adult humans.

Short-term monocular deprivation (MD) shifts sensory eye balance in favour of the previously deprived eye. The effect of MD on eye balance is significant but brief in adult humans. Recently, researchers and clinicians have attempted to implement MD in clinical settings for adults with impaired binocular vision. Although the effect of MD has been studied in detail in single-session protocols, what is not known is whether the effect of MD on eye balance deteriorates after repeated periods of MD (termed 'perceptual deterioration'). An answer to this question is relevant for two reasons. Firstly, the effect of MD (i.e., dose-response) should not decrease with repeated use if MD is to be used therapeutically (e.g., daily for weeks). Second, it bears upon the question of whether the neural basis of the effects of MD and contrast adaptation, a closely related phenomenon, is the same. The sensory change from contrast adaptation depends on recent experience. If the observer has recently experienced the same adaptation multiple times for consecutive days, then the adaptation effect will be smaller because contrast adaptation exhibits perceptual deterioration, so it is of interest to know if the effects of MD follow suit. This study measured the effect of 2-h MD for seven consecutive days on binocular balance of 15 normally sighted adults. We found that the shift in eye balance from MD stayed consistent, showing no signs of deterioration after subjects experienced multiple periods of MD. This finding shows no loss of effectiveness of repeated daily doses of MD if used therapeutically to rebalance binocular vision in otherwise normal individuals. Furthermore, ocular dominance plasticity, which is the basis of the effects of short-term MD, does not seem to share the property of 'perceptual deterioration' with contrast adaptation, suggesting different neural bases for these two related phenomena.

Crossmodal plasticity following short-term monocular deprivation.

A brief period of monocular deprivation (MD) induces short-term plasticity of the adult visual system. Whether MD elicits neural changes beyond visual processing is yet unclear. Here, we assessed the specific impact of MD on neural correlates of multisensory processes. Neural oscillations associated with visual and audio-visual processing were measured for both the deprived and the non-deprived eye. Results revealed that MD changed neural activities associated with visual and multisensory processes in an eye-specific manner. Selectively for the deprived eye, alpha synchronization was reduced within the first 150 ms of visual processing. Conversely, gamma activity was enhanced in response to audio-visual events only for the non-deprived eye within 100-300 ms after stimulus onset. The analysis of gamma responses to unisensory auditory events revealed that MD elicited a crossmodal upweight for the non-deprived eye. Distributed source modeling suggested that the right parietal cortex played a major role in neural effects induced by MD. Finally, visual and audio-visual processing alterations emerged for the induced component of the neural oscillations, indicating a prominent role of feedback connectivity. Results reveal the causal impact of MD on both unisensory (visual and auditory) and multisensory (audio-visual) processes and, their frequency-specific profiles. These findings support a model in which MD increases excitability to visual events for the deprived eye and audio-visual and auditory input for the non-deprived eye.

Activation of p38 MAPK hinders the reactivation of visual cortical plasticity in adult amblyopic mice.

OBJECTIVE: To investigate the impact of p38 mitogen-activated protein kinase (MAPK) signaling on reactivating visual cortical plasticity in adult amblyopic mice. MATERIALS AND METHODS: Reverse suture (RS), environment enrichment (EE), and combined with left intracerebroventricular injection of p38 MAPK inhibitor (SB203580, SB) or p38 MAPK agonist (dehydrocorydaline hydrochloride, DHC) were utilized to treat adult amblyopic mice with monocular deprivation (MD). The visual water task, visual cliff test, and Flash visual-evoked potential were used to measure the visual function. Then, Golgi staining and transmission electron microscopy were used to assess the reactivation of structural plasticity in adult amblyopic mice. Western blot and immunohistochemistry detected the expression of ATF2, PSD-95, p38 MAPK, and phospho-p38 MAPK in the left visual cortex. RESULTS: No statistically significant difference was observed in the visual function in each pre-intervention group. Compared to pre-intervention, the visual acuity of deprived eyes was improved significantly, the impairment of visual depth perception was alleviated, and the P wave amplitude and C/I ratio were increased in the EE + RS, the EE + RS + SB, and the EE + RS + DMSO groups, but no significant difference was detected in the EE + RS + DHC group. Compared to EE + RS + DHC group, the density of dendritic spines was significantly higher, the synaptic density of the left visual cortex increased significantly, the length of the active synaptic zone increased, and the thickness of post-synaptic density (PSD) thickened in the left visual cortex of EE + RS, EE + RS + SB, and EE + RS + DMSO groups. And that, the protein expression of p-p38 MAPK increased while that of PSD-95 and ATF2 decreased significantly in the left visual cortex of the EE + RS + DHC group mice. CONCLUSION: RS and EE intervention improved the visual function and synaptic plasticity of the visual cortex in adult amblyopic mice. However, activating p38 MAPK hinders the recovery of visual function by upregulating the phosphorylation of p38 MAPK and decreasing the ATF2 protein expression.

Functional Differentiation of Mouse Visual Cortical Areas Depends upon Early Binocular Experience.

The mammalian visual cortex contains multiple retinotopically defined areas that process distinct features of the visual scene. Little is known about what guides the functional differentiation of visual cortical areas during development. Recent studies in mice have revealed that visual input from the two eyes provides spatiotemporally distinct signals to primary visual cortex (V1), such that contralateral eye-dominated V1 neurons respond to higher spatial frequencies than ipsilateral eye-dominated neurons. To test whether binocular visual input drives the differentiation of visual cortical areas, we used two-photon calcium imaging to characterize the effects of juvenile monocular deprivation (MD) on the responses of neurons in V1 and two higher visual areas, LM (lateromedial) and PM (posteromedial). In adult mice of either sex, we find that MD prevents the emergence of distinct spatiotemporal tuning in V1, LM, and PM. We also find that, within each of these areas, MD reorganizes the distinct spatiotemporal tuning properties driven by the two eyes. Moreover, we find a relationship between speed tuning and ocular dominance in all three areas that MD preferentially disrupts in V1, but not in LM or PM. Together, these results reveal that balanced binocular vision during development is essential for driving the functional differentiation of visual cortical areas. The higher visual areas of mouse visual cortex may provide a useful platform for investigating the experience-dependent mechanisms that set up the specialized processing within neocortical areas during postnatal development.SIGNIFICANCE STATEMENT Little is known about the factors guiding the emergence of functionally distinct areas in the brain. Using in vivo Ca2+ imaging, we recorded visually evoked activity from cells in V1 and higher visual areas LM (lateromedial) and PM (posteromedial) of mice. Neurons in these areas normally display distinct spatiotemporal tuning properties. We found that depriving one eye of normal input during development prevents the functional differentiation of visual areas. Deprivation did not disrupt the degree of speed tuning, a property thought to emerge in higher visual areas. Thus, some properties of visual cortical neurons are shaped by binocular experience, while others are resistant. Our study uncovers the fundamental role of binocular experience in the formation of distinct areas in visual cortex.

Chronic Monocular Deprivation Reveals MMP9-Dependent and -Independent Aspects of Murine Visual System Plasticity.

The deletion of matrix metalloproteinase MMP9 is combined here with chronic monocular deprivation (cMD) to identify the contributions of this proteinase to plasticity in the visual system. Calcium imaging of supragranular neurons of the binocular region of primary visual cortex (V1b) of wild-type mice revealed that cMD initiated at eye opening significantly decreased the strength of deprived-eye visual responses to all stimulus contrasts and spatial frequencies. cMD did not change the selectivity of V1b neurons for the spatial frequency, but orientation selectivity was higher in low spatial frequency-tuned neurons, and orientation and direction selectivity were lower in high spatial frequency-tuned neurons. Constitutive deletion of MMP9 did not impact the stimulus selectivity of V1b neurons, including ocular preference and tuning for spatial frequency, orientation, and direction. However, MMP9-/- mice were completely insensitive to plasticity engaged by cMD, such that the strength of the visual responses evoked by deprived-eye stimulation was maintained across all stimulus contrasts, orientations, directions, and spatial frequencies. Other forms of experience-dependent plasticity, including stimulus selective response potentiation, were normal in MMP9-/- mice. Thus, MMP9 activity is dispensable for many forms of activity-dependent plasticity in the mouse visual system, but is obligatory for the plasticity engaged by cMD.

Effect of regulation of the NRG1/ErbB4 signaling pathway on the visual cortex synaptic plasticity of amblyopic adult rats.

This study aimed to investigate the effect of the neuregulin-1/epidermal growth factor 4 (NRG1/ErbB4) signaling pathway on visual cortex synaptic plasticity in adult amblyopic rats with monocular deprivation (MD). Compared with the control group, the P wave latency and amplitude of the MD group were prolonged and low, respectively, with reduced synaptic plasticity-related protein expression, lower number of visual cortex neurons, and increased apoptosis of visual cortex neurons. Recombinant neuregulin-1 (rNRG1) administration activated the NRG1/ErbB4 signaling pathway and improved the visual cortex synaptic plasticity in MD amblyopic rats. However, the effects of rNRG1 were reversed by AG1478 (ErbB4 receptor blockers). The NRG1/ErbB4 signaling pathway in the parvalbumin neurons from MD rats was also inactivated. Amblyopic rats had significantly low cell activity and downregulated expression of synaptic plasticity-related proteins. Thus, exogenous administration of NRG1 can activate ErbB4 signal transduction and improve the damaged synaptic plasticity of the visual cortex among amblyopic rats. Further studies are warranted to explore the potential for clinical management of amblyopia.

Polysialylated - neural cell adhesion molecule (PSA-NCAM) promotes recovery of vision after the critical period.

Vision loss has long since been considered irreversible after a critical period; however, there is potential to restore limited vision, even in adulthood. This phenomenon is particularly pronounced following complete loss of vision in the dominant eye. Adult neural cell adhesion molecule (NCAM) knockout mice have an age-related impairment of visual acuity. The underlying cause of early deterioration in visual function remains unknown. Polysialylated (PSA) NCAM is involved in different forms of neural plasticity in the adult brain, raising the possibility that NCAM plays a role in the plasticity of the visual cortex, and therefore, in visual ability. Here, we examined whether PSA-NCAM is required for visual cortical plasticity in adult C57Bl/6J mice following deafferentation and long-term monocular deprivation. Our results show that elevated PSA in the contralateral visual cortex of the reopened eye is accompanied by changes in other markers of neural plasticity: increased brain-derived neurotrophic factor (BDNF) levels and degradation of perineuronal nets (PNNs). The removal of PSA-NCAM in the visual cortex of these mice reduced BDNF expression, decreased PNN degradation, and resulted in impaired recovery of visual acuity after optic nerve transection and chronic monocular deprivation. Collectively, our results demonstrate that PSA-NCAM is necessary for the reactivation of visual cortical plasticity and recovery of visual function in adult mice. It also offers a potential molecular target for the therapeutic treatment of cortically based visual impairments.

Inhibition of Cdk5 in PV Neurons Reactivates Experience-Dependent Plasticity in Adult Visual Cortex.

Cyclin-dependent kinase 5 (Cdk5) has been shown to play a critical role in brain development, learning, memory and neural processing in general. Cdk5 is widely distributed in many neuron types in the central nervous system, while its cell-specific role is largely unknown. Our previous study showed that Cdk5 inhibition restored ocular dominance (OD) plasticity in adulthood. In this study, we specifically knocked down Cdk5 in different types of neurons in the visual cortex and examined OD plasticity by optical imaging of intrinsic signals. Downregulation of Cdk5 in parvalbumin-expressing (PV) inhibitory neurons, but not other neurons, reactivated adult mouse visual cortical plasticity. Cdk5 knockdown in PV neurons reduced the evoked firing rate, which was accompanied by an increment in the threshold current for the generation of a single action potential (AP) and hyperpolarization of the resting membrane potential. Moreover, chemogenetic activation of PV neurons in the visual cortex can attenuate the restoration of OD plasticity by Cdk5 inhibition. Taken together, our results suggest that Cdk5 in PV interneurons may play a role in modulating the excitation and inhibition balance to control the plasticity of the visual cortex.

Homer1a Is Required for Establishment of Contralateral Bias and Maintenance of Ocular Dominance in Mouse Visual Cortex.

It is well established across many species that neurons in the primary visual cortex (V1) display preference for visual input from one eye or the other, which is termed ocular dominance (OD). In rodents, V1 neurons exhibit a strong bias toward the contralateral eye. Molecular mechanisms of how OD is established and later maintained by plastic changes are largely unknown. Here we report a novel role of an activity-dependent immediate early gene Homer1a (H1a) in these processes. Using both sexes of H1a knock-out (KO) mice, we found that there is basal reduction in the OD index of V1 neurons measured using intrinsic signal imaging. This was because of a reduction in the strength of inputs from the contralateral eye, which is normally dominant in mice. The abnormal basal OD index was not dependent on visual experience and is driven by postnatal expression of H1a. Despite this, H1a KOs still exhibited normal shifts in OD index following a short-term (2-3 d) monocular deprivation (MD) of the contralateral eye with lid suture. However, unlike wild-type counterparts, H1a KOs continued to shift OD index with a longer duration (5-6 d) of MD. The same phenotype was recapitulated in a mouse model that has reduced Homer1 binding to metabotropic glutamate receptor 5 (mGluR5). Our results suggest a novel role of H1a and its interaction with mGluR5 in strengthening contralateral eye inputs during postnatal development to establish normal contralateral bias in mouse V1 without much impact on OD shift with brief MD.SIGNIFICANCE STATEMENT Visual cortical neurons display varying degree of responsiveness to visual stimuli through each eye, which determines their ocular dominance (OD). Molecular mechanisms responsible for establishing normal OD are largely unknown. Development of OD has been shown to be largely independent of visual experience, but guided by molecular cues and spontaneous activity. We found that activity-dependent immediate early gene H1a is critical for establishing normal OD in V1 of mice, which show contralateral eye dominance. Despite the weaker contralateral bias, H1aKOs undergo largely normal OD plasticity. The basic phenotype of H1aKO was recapitulated by mGluR5 mutation that severely reduces H1a interaction. Our results suggest a novel role of mGluR5-H1a interaction in strengthening contralateral eye inputs to V1 during postnatal development.

Binocular Disparity Selectivity Weakened after Monocular Deprivation in Mouse V1.

Experiences during the critical period sculpt the circuitry within the neocortex, leading to changes in the functional responses of sensory neurons. Monocular deprivation (MD) during the visual critical period causes shifts in ocular preference, or dominance, toward the open eye in primary visual cortex (V1) and disrupts the normal development of acuity. In carnivores and primates, MD also disrupts the emergence of binocular disparity selectivity, a cue resulting from integrating ocular inputs. This disruption may be a result of the increase in neurons driven exclusively by the open eye that follows deprivation or a result of a mismatch in the convergence of ocular inputs. To distinguish between these possibilities, we measured the ocular dominance (OD) and disparity selectivity of neurons from male and female mouse V1 following MD. Normal mouse V1 neurons are dominated by contralateral eye input and contralateral eye deprivation shifts mouse V1 neurons toward more balanced responses between the eyes. This shift toward binocularity, as assayed by OD, decreased disparity sensitivity. MD did not alter the initial maturation of binocularity, as disparity selectivity before the MD was indistinguishable from normal mature animals. Decreased disparity tuning was most pronounced in binocular and ipsilaterally biased neurons, which are the populations that have undergone the largest shifts in OD. In concert with the decline in disparity selectivity, we observed a shift toward lower spatial frequency selectivity for the ipsilateral eye following MD. These results suggest an emergence of novel synaptic inputs during MD that disrupt the representation of disparity selectivity.SIGNIFICANCE STATEMENT We demonstrate that monocular deprivation during the developmental critical period impairs binocular integration in mouse primary visual cortex. This impairment occurs despite an increase in the degree to which neurons become more binocular. We further demonstrate that our deprivation did not impair the maturation of disparity selectivity. Disparity selectivity has already reached a matured level before the monocular deprivation. The loss of disparity tuning is primarily observed in neurons dominated by the open eye, suggesting a link between altered inputs and loss of disparity sensitivity. These results suggest that new inputs following deprivation may not maintain the precise spatial relationship between the two eye inputs required for disparity selectivity.

Uncovering a critical period of synaptic imbalance during postnatal development of the rat visual cortex: role of brain-derived neurotrophic factor.

KEY POINTS: With daily electrophysiological recordings and neurochemical analysis, we uncovered a transient period of synaptic imbalance between enhanced inhibition and suppressed excitation in rat visual cortical neurons from the end of the fourth toward the end of the fifth postnatal weeks. The expression of brain-derived neurotrophic factor (BDNF), which normally enhances excitation and suppresses inhibition, was down-regulated during that time, suggesting that this may contribute to the inhibition/excitation imbalance. An agonist of the BDNF receptor tropomyosin-related kinase B (TrkB) partially reversed the imbalance, whereas a TrkB antagonist accentuated the imbalance during the transient period. Monocular lid suture during the transient period is more detrimental to the function and neurochemical properties of visual cortical neurons than before or after this period. We regard the period of synaptic imbalance as the peak critical period of vulnerability, and its existence is necessary for neurons to transition from immaturity to a more mature state of functioning. ABSTRACT: The mammalian visual cortex is immature at birth and undergoes postnatal structural and functional adjustments. The exact timing of the vulnerable period in rodents remains unclear. The critical period is characterized by inhibitory GABAergic maturation reportedly dependent on brain-derived neurotrophic factor (BDNF). However, most of the studies were performed on experimental/transgenic animals, questioning the relationship in normal animals. The present study aimed to conduct in-depth analyses of the synaptic and neurochemical development of visual cortical neurons in normal and monocularly-deprived rats and to determine specific changes, if any, during the critical period. We found that (i) against a gradual increase in excitation and inhibition with age, a transient period of synaptic and neurochemical imbalance existed with suppressed excitation and enhanced inhibition at postnatal days 28 to 33/34; (ii) during this window, the expression of BDNF and tropomyosin-related kinase B (TrkB) receptors decreased, along with glutamatergic GluN1 and GluA1 receptors and the metabolic marker cytochrome oxidase, whereas that of GABAA Rα1 receptors continued to rise; (iii) monocular deprivation reduced both excitatory and inhibitory synaptic activity and neurochemicals mainly during this period; and (iv) in vivo TrkB agonist partially reversed the synaptic imbalance in normal and monocularly-deprived neurons during this time, whereas a TrkB antagonist accentuated the imbalance. Thus, our findings highlight a transitory period of synaptic imbalance with a negative relationship between BDNF and inhibitory GABA. This brief critical period may be necessary in transitioning from an immature to a more mature state of visual cortical functioning.

Cross-modal restoration of ocular dominance plasticity in adult mice.

The temporal closure of one eye in juvenile and young adult mice induces a shift of the ocular dominance (OD) of neurons in the binocular visual cortex. However, OD plasticity typically declines with age and is completely absent in matured mice beyond postnatal day (PD) 110. As it has been shown that the deprivation of one sensory input can induce neuronal alterations in non-deprived sensory cortices, we here investigated whether cross-modal interactions have the potential to reinstall OD plasticity in matured mice. Strikingly, using intrinsic signal imaging we could demonstrate that both whisker deprivation and auditory deprivation for only one week reinstated OD plasticity in fully adult mice. These OD shifts were always mediated by an increase of V1 responsiveness to visual stimulation of the open eye, a characteristic feature of OD plasticity normally only found in young adult mice. Moreover, systemic administration of the competitive NMDA receptor antagonist CPP completely abolished cross-modally induced OD plasticity. Taken together, we demonstrate here for the first time that the deprivation of non-visual senses has the potential to rejuvenate the adult visual cortex.

Amblyopia: New molecular/pharmacological and environmental approaches.

Emerging technologies are now giving us unprecedented access to manipulate brain circuits, shedding new light on treatments for amblyopia. This research is identifying key circuit elements that control brain plasticity and highlight potential therapeutic targets to promote rewiring in the visual system during and beyond early life. Here, we explore how such recent advancements may guide future pharmacological, genetic, and behavioral approaches to treat amblyopia. We will discuss how animal research, which allows us to probe and tap into the underlying circuit and synaptic mechanisms, should best be used to guide therapeutic strategies. Uncovering cellular and molecular pathways that can be safely targeted to promote recovery may pave the way for effective new amblyopia treatments across the lifespan.

Visual Deprivation During the Critical Period Enhances Layer 2/3 GABAergic Inhibition in Mouse V1.

UNLABELLED: The role of GABAergic signaling in establishing a critical period for experience in visual cortex is well understood. However, the effects of early experience on GABAergic synapses themselves are less clear. Here, we show that monocular deprivation (MD) during the adolescent critical period produces marked enhancement of GABAergic signaling in layer 2/3 of mouse monocular visual cortex. This enhancement coincides with a weakening of glutamatergic inputs, resulting in a significant reduction in the ratio of excitation to inhibition. The potentiation of GABAergic transmission arises from both an increased number of inhibitory synapses and an enhancement of presynaptic GABA release from parvalbumin- and somatostatin-expressing interneurons. Our results suggest that augmented GABAergic inhibition contributes to the experience-dependent regulation of visual function. SIGNIFICANCE STATEMENT: Visual experience shapes the synaptic organization of cortical circuits in the mouse brain. Here, we show that monocular visual deprivation enhances GABAergic synaptic inhibition in primary visual cortex. This enhancement is mediated by an increase in both the number of postsynaptic GABAergic synapses and the probability of presynaptic GABA release. Our results suggest a contributing mechanism to altered visual responses after deprivation.

Dynamic Modulation of Myelination in Response to Visual Stimuli Alters Optic Nerve Conduction Velocity.

UNLABELLED: Myelin controls the time required for an action potential to travel from the neuronal soma to the axon terminal, defining the temporal manner in which information is processed within the CNS. The presence of myelin, the internodal length, and the thickness of the myelin sheath are powerful structural factors that control the velocity and fidelity of action potential transmission. Emerging evidence indicates that myelination is sensitive to environmental experience and neuronal activity. Activity-dependent modulation of myelination can dynamically alter action potential conduction properties but direct functional in vivo evidence and characterization of the underlying myelin changes is lacking. We demonstrate that in mice long-term monocular deprivation increases oligodendrogenesis in the retinogeniculate pathway but shortens myelin internode lengths without affecting other structural properties of myelinated fibers. We also demonstrate that genetically attenuating synaptic glutamate neurotransmission from retinal ganglion cells phenocopies the changes observed after monocular deprivation, suggesting that glutamate may constitute a signal for myelin length regulation. Importantly, we demonstrate that visual deprivation and shortened internodes are associated with a significant reduction in nerve conduction velocity in the optic nerve. Our results reveal the importance of sensory input in the building of myelinated fibers and suggest that this activity-dependent alteration of myelination is important for modifying the conductive properties of brain circuits in response to environmental experience. SIGNIFICANCE STATEMENT: Oligodendrocyte precursor cells differentiate into mature oligodendrocytes and are capable of ensheathing axons with myelin without molecular cues from neurons. However, this default myelination process can be modulated by changes in neuronal activity. Here, we show, for the first time, that experience-dependent activity modifies the length of myelin internodes along axons altering action potential conduction velocity. Such a mechanism would allow for variations in conduction velocities that provide a degree of plasticity in accordance to environmental needs. It will be important in future work to investigate how these changes in myelination and conduction velocity contribute to signal integration in postsynaptic neurons and circuit function.

Nogo Receptor 1 Limits Ocular Dominance Plasticity but not Turnover of Axonal Boutons in a Model of Amblyopia.

The formation and stability of dendritic spines on excitatory cortical neurons are correlated with adult visual plasticity, yet how the formation, loss, and stability of postsynaptic spines register with that of presynaptic axonal varicosities is unknown. Monocular deprivation has been demonstrated to increase the rate of formation of dendritic spines in visual cortex. However, we find that monocular deprivation does not alter the dynamics of intracortical axonal boutons in visual cortex of either adult wild-type (WT) mice or adult NgR1 mutant (ngr1-/-) mice that retain critical period visual plasticity. Restoring normal vision for a week following long-term monocular deprivation (LTMD), a model of amblyopia, partially restores ocular dominance (OD) in WT and ngr1-/- mice but does not alter the formation or stability of axonal boutons. Both WT and ngr1-/- mice displayed a rapid return of normal OD within 8 days after LTMD as measured with optical imaging of intrinsic signals. In contrast, single-unit recordings revealed that ngr1-/- exhibited greater recovery of OD by 8 days post-LTMD. Our findings support a model of structural plasticity in which changes in synaptic connectivity are largely postsynaptic. In contrast, axonal boutons appear to be stable during changes in cortical circuit function.

Extracellular signal-regulated kinase (ERK) activity during sleep consolidates cortical plasticity in vivo.

Ocular dominance plasticity (ODP) in the cat primary visual cortex (V1) is induced during waking by monocular deprivation (MD) and consolidated during subsequent sleep. The mechanisms underlying this process are incompletely understood. Extracellular signal-regulated kinase (ERK) is activated in V1 during sleep after MD, but it is unknown whether ERK activation during sleep is necessary for ODP consolidation. We investigated the role of ERK in sleep-dependent ODP consolidation by inhibiting the ERK-activating enzyme MEK in V1 (via U0126) during post-MD sleep. ODP consolidation was then measured with extracellular microelectrode recordings. Western blot analysis was used to confirm the efficacy of U0126 and to examine proteins downstream of ERK. U0126 abolished ODP consolidation and reduced both phosphorylation of eukaryotic initiation factor 4E (eIF4E) and levels of the synaptic marker PSD-95. Furthermore, interfering with ERK-mediated translation by inhibiting MAP kinase-interacting kinase 1 (Mnk1) with CGP57380 mimicked the effects of U0126. These results demonstrate that ODP consolidation requires sleep-dependent activation of the ERK-Mnk1 pathway.