Effects of chronic iTBS‐rTMS and enriched environment on visual cortex early critical period and visual pattern discrimination in dark‐reared rats

Early cortical critical period resembles a state of enhanced neuronal plasticity enabling the establishment of specific neuronal connections during first sensory experience. Visual performance with regard to pattern discrimination is impaired if the cortex is deprived from visual input during the critical period. We wondered how unspecific activation of the visual cortex before closure of the critical period using repetitive transcranial magnetic stimulation (rTMS) could affect the critical period and the visual performance of the experimental animals. Would it cause premature closure of the plastic state and thus worsen experience‐dependent visual performance, or would it be able to preserve plasticity? Effects of intermittent theta‐burst stimulation (iTBS) were compared with those of an enriched environment (EE) during dark‐rearing (DR) from birth. Rats dark‐reared in a standard cage showed poor improvement in a visual pattern discrimination task, while rats housed in EE or treated with iTBS showed a performance indistinguishable from rats reared in normal light/dark cycle. The behavioral effects were accompanied by correlated changes in the expression of brain‐derived neurotrophic factor (BDNF) and atypical PKC (PKCζ/PKMζ), two factors controlling stabilization of synaptic potentiation. It appears that not only nonvisual sensory activity and exercise but also cortical activation induced by rTMS has the potential to alleviate the effects of DR on cortical development, most likely due to stimulation of BDNF synthesis and release. As we showed previously, iTBS reduced the expression of parvalbumin in inhibitory cortical interneurons, indicating that modulation of the activity of fast‐spiking interneurons contributes to the observed effects of iTBS. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 19–33, 2016     

BDNF regulates the maturation of inhibition and the critical period of plasticity in mouse visual cortex.

Maturation of the visual cortex is influenced by visual experience during an early postnatal period. The factors that regulate such a critical period remain unclear. We examined the maturation and plasticity of the visual cortex in transgenic mice in which the postnatal rise of brain-derived neurotrophic factor (BDNF) was accelerated. In these mice, the maturation of GABAergic innervation and inhibition was accelerated. Furthermore, the age-dependent decline of cortical long-term potentiation induced by white matter stimulation, a form of synaptic plasticity sensitive to cortical inhibition, occurred earlier. Finally, transgenic mice showed a precocious development of visual acuity and an earlier termination of the critical period for ocular dominance plasticity. We propose that BDNF promotes the maturation of cortical inhibition during early postnatal life, thereby regulating the critical period for visual cortical plasticity.     

Differential effects of cortical neurotrophic factors on development of lateral geniculate nucleus and superior colliculus neurons: anterograde and retrograde actions

Neurotrophins strongly affect visual system development and plasticity. However, the mode of delivery and targets of neurotrophin action are still under debate. For instance, cortical NT-4/5 (neurotrophin 4/5; Ntf4/5) was shown to rescue lateral geniculate nucleus (LGN) neurons from monocular deprivation-induced atrophy suggesting a retrograde action on thalamic afferents. It is still unclear whether LGN neurons respond to NT-4/5 and other neurotrophins during development in animals with normal vision. We now show that infusions of NT-4/5 and NGF (nerve growth factor) into visual cortex at the onset and the peak of the critical period accelerated LGN neuron growth. BDNF (brain-derived neurotrophic factor) was ineffective. The effects of neurotrophin on LGN development were clearly dissociated from the effects at cortical level because soma growth of cortical layer IV and VI neurons was strongly promoted by BDNF. NT-4/5 was only effective at the onset, but no longer at the peak of the critical period suggesting a switch in neurotrophin dependency for these cortical cell classes. To dissociate retrograde and anterograde effects of the TrkB ligands, we analyzed the stratum griseum superficiale (SGS) of the superior colliculus, a target of visual cortical efferents. Indeed, TrkB-expressing inhibitory SGS neurons responded to cortical NT-4/5 infusion with somatic growth. Strikingly, the TrkB-expressing excitatory tectothalamic calbindin neurons in the SGS did not respond. This demonstrated for the first time a selective cell type-specific anterograde action of NT-4/5 and suggested for the LGN that anterograde as well as retrograde effects contribute to soma size regulation. Strikingly, cortical infusion of the cytokine LIF, which affects development of visual cortex neurochemical architecture, transiently inhibited growth of neurons in LGN, cortical layer IV and VI and SGS. In summary, the study presents three important results. First, central neurons regulate soma size development in an age-and ligand-specific fashion. Second, NT-4/5 and NGF accelerate LGN development in rats with normal vision while LIF delays growth. Third, anterogradely transported NT-4/5 effectively promotes neuronal maturation. These differential actions on subcortical neurons may contribute to the different effects of neurotrophins on visual system development and plasticity.     

Hypoactivity Affects IGF-1 Level and PI3K/AKT Signaling Pathway in Cerebral Structures Implied in Motor Control

A chronic reduction in neuromuscular activity through prolonged body immobilization in human alters motor task performance through a combination of peripheral and central factors. Studies performed in a rat model of sensorimotor restriction have shown functional and biochemical changes in sensorimotor cortex. However, the underlying mechanisms are still unclear. Interest was turned towards a possible implication of Insulin-like Growth Factor 1 (IGF-1), a growth factor known to mediate neuronal excitability and synaptic plasticity by inducing phosphorylation cascades which include the PI3K–AKT pathway. In order to better understand the influence of IGF-1 in cortical plasticity in rats submitted to a sensorimotor restriction, we analyzed the effect of hindlimb unloading on IGF-1 and its main molecular pathway in structures implied in motor control (sensorimotor cortex, striatum, cerebellum). IGF-1 level was determined by ELISA, and phosphorylation of its receptor and proteins of the PI3K–AKT pathway by immunoblot. In the sensorimotor cortex, our results indicate that HU induces a decrease in IGF-1 level; this alteration is associated to a decrease in activation of PI3K-AKT pathway. The same effect was observed in the striatum, although to a lower extent. No variation was noticed in the cerebellum. These results suggest that IGF-1 might contribute to cortical and striatal plasticity induced by a chronic sensorimotor restriction.     

From visual experience to visual function: roles of neurotrophins.

Recently, a role for neurotrophins in regulating cortical developmental plasticity has clearly emerged. We present in this review a summary of the early data on the action of nerve growth factor (NGF) in visual cortical development and plasticity in the rat and of other neurotrophins in the visual cortex of other mammals. In addition, to clarify the differences in the results obtained with the various neurotrophins in different animal preparations, we also report new data on the action of NGF, brain-derived neurotrophic factor (BDNF), neurotrophin (NT)3, and NT4 in the same preparation-namely, the visual cortex of the rat. We discuss old and new results in a physiological model in which different neurotrophins play different roles in regulating visual cortical development and plasticity by acting on different neural targets, such as lateral geniculate nucleus (LGN) afferents, intracortical circuitry, and subcortical afferents, and propose a tentative scheme summarizing these actions.     

Environmental enrichment promotes plasticity and visual acuity recovery in adult monocular amblyopic rats

Loss of visual acuity caused by abnormal visual experience during development (amblyopia) is an untreatable pathology in adults. In some occasions, amblyopic patients loose vision in their better eye owing to accidents or illnesses. While this condition is relevant both for its clinical importance and because it represents a case in which binocular interactions in the visual cortex are suppressed, it has scarcely been studied in animal models. We investigated whether exposure to environmental enrichment (EE) is effective in triggering recovery of vision in adult amblyopic rats rendered monocular by optic nerve dissection in their normal eye. By employing both electrophysiological and behavioral assessments, we found a full recovery of visual acuity in enriched rats compared to controls reared in standard conditions. Moreover, we report that EE modulates the expression of GAD67 and BDNF. The non invasive nature of EE renders this paradigm promising for amblyopia therapy in adult monocular people.     

Plasticity in the visual system: role of neurotrophins and electrical activity

We report recent results concerning the action of neurotrophins on the development and plasticity of the visual system of mammals and in particular of their visual cortex. It has been demonstrated that NGF prevents all the effects of monocular deprivation during the critical period. BDNF, that in part also prevents the effects of monocular deprivation, has the interesting additional property of accelerating the development of inhibitory processes. In transgenic mice overexpressing BDNF only in the cortex, the critical period for plasticity initiates a week earlier and presents a precocious closure. Visual acuity also develops much before than in normal animals. These phenomenological observations are paralleled by a precocious increase of inhibitory synapses and inhibitory currents in pyramidal neurons. LTP, tested by stimulation of the white matter, recording in layers 2 and 3 of the visual cortex, presents modifications correlated with the alterations observed in the critical period. Last we report the finding from in vitro and in vivo experiments that MAPkase (Erg 1 and 2) is the molecular chain of events driven both by light and neurotrophins, likely at the bases of the phenomena of plasticity observed during the critical period.     

An antagomir to microRNA Let7f promotes neuroprotection in an ischemic stroke model

We previously showed that middle-aged female rats sustain a larger infarct following experimental stroke as compared to younger female rats, and paradoxically, estrogen treatment to the older group is neurotoxic. Plasma and brain insulin-like growth factor-1 (IGF-1) levels decrease with age. However, IGF-1 infusion following stroke, prevents estrogen neurotoxicity in middle-aged female rats. IGF1 is neuroprotective and well tolerated, but also has potentially undesirable side effects. We hypothesized that microRNAs (miRNAs) that target the IGF-1 signaling family for translation repression could be alternatively suppressed to promote IGF-1-like neuroprotection. Here, we report that two conserved IGF pathway regulatory microRNAs, Let7f and miR1, can be inhibited to mimic and even extend the neuroprotection afforded by IGF-1. Anti-mir1 treatment, as late as 4 hours following ischemia, significantly reduced cortical infarct volume in adult female rats, while anti-Let7 robustly reduced both cortical and striatal infarcts, and preserved sensorimotor function and interhemispheric neural integration. No neuroprotection was observed in animals treated with a brain specific miRNA unrelated to IGF-1 (anti-miR124). Remarkably, anti-Let7f was only effective in intact females but not males or ovariectomized females indicating that the gonadal steroid environment critically modifies miRNA action. Let7f is preferentially expressed in microglia in the ischemic hemisphere and confirmed in ex vivo cultures of microglia obtained from the cortex. While IGF-1 was undetectable in microglia harvested from the non-ischemic hemisphere, IGF-1 was expressed by microglia obtained from the ischemic cortex and was further elevated by anti-Let7f treatment. Collectively these data support a novel miRNA-based therapeutic strategy for neuroprotection following stroke.     

Visual deprivation suppresses L5 pyramidal neuron excitability by preventing the induction of intrinsic plasticity

In visual cortex monocular deprivation (MD) during a critical period (CP) reduces the ability of the deprived eye to activate cortex, but the underlying cellular plasticity mechanisms are incompletely understood. Here we show that MD reduces the intrinsic excitability of layer 5 (L5) pyramidal neurons and enhances long-term potentiation of intrinsic excitability (LTP-IE). Further, MD and LTP-IE induce reciprocal changes in K(v)2.1 current, and LTP-IE reverses the effects of MD on intrinsic excitability. Taken together these data suggest that MD reduces intrinsic excitability by preventing sensory-drive induced LTP-IE. The effects of MD on excitability were correlated with the classical visual system CP, and (like the functional effects of MD) could be rapidly reversed when vision was restored. These data establish LTP-IE as a candidate mechanism mediating loss of visual responsiveness within L5, and suggest that intrinsic plasticity plays an important role in experience-dependent refinement of visual cortical circuits.     

Developmental and Visual Input-Dependent Regulation of the CB1 Cannabinoid Receptor in the Mouse Visual Cortex

The mammalian visual system exhibits significant experience-induced plasticity in the early postnatal period. While physiological studies have revealed the contribution of the CB1 cannabinoid receptor (CB1) to developmental plasticity in the primary visual cortex (V1), it remains unknown whether the expression and localization of CB1 is regulated during development or by visual experience. To explore a possible role of the endocannabinoid system in visual cortical plasticity, we examined the expression of CB1 in the visual cortex of mice. We found intense CB1 immunoreactivity in layers II/III and VI. CB1 mainly localized at vesicular GABA transporter-positive inhibitory nerve terminals. The amount of CB1 protein increased throughout development, and the specific laminar pattern of CB1 appeared at P20 and remained until adulthood. Dark rearing from birth to P30 decreased the amount of CB1 protein in V1 and altered the synaptic localization of CB1 in the deep layer. Dark rearing until P50, however, did not influence the expression of CB1. Brief monocular deprivation for 2 days upregulated the localization of CB1 at inhibitory nerve terminals in the deep layer. Taken together, the expression and the localization of CB1 are developmentally regulated, and both parameters are influenced by visual experience.     

Failed Stabilization for Long-Term Potentiation in the Auditory Cortex of Fmr1 Knockout Mice

Fragile X syndrome is a developmental disorder that affects sensory systems. A null mutation of the Fragile X Mental Retardation protein 1 (Fmr1) gene in mice has varied effects on developmental plasticity in different sensory systems, including normal barrel cortical plasticity, altered ocular dominance plasticity and grossly impaired auditory frequency map plasticity. The mutation also has different effects on long-term synaptic plasticity in somatosensory and visual cortical neurons, providing insights on how it may differentially affect the sensory systems. Here we present evidence that long-term potentiation (LTP) is impaired in the developing auditory cortex of the Fmr1 knockout (KO) mice. This impairment of synaptic plasticity is consistent with impaired frequency map plasticity in the Fmr1 KO mouse. Together, these results suggest a potential role of LTP in sensory map plasticity during early sensory development.     

Insulin-like growth factor 1 receptor signaling regulates skin development and inhibits skin keratinocyte differentiation

The insulin-like growth factor 1 receptor (IGF-1R) is a multifunctional receptor that mediates signals for cell proliferation, differentiation, and survival. Genetic experiments showed that IGF-1R inactivation in skin results in a disrupted epidermis. However, because IGF-1R-null mice die at birth, it is difficult to study the effects of IGF-1R on skin. By using a combined approach of conditional gene ablation and a three-dimensional organotypic model, we demonstrate that IGF-1R-deficient skin cocultures show abnormal maturation and differentiation patterns. Furthermore, IGF-1R-null keratinocytes exhibit accelerated differentiation and decreased proliferation. Investigating the signaling pathway downstream of IGF-1R reveals that insulin receptor substrate 2 (IRS-2) overexpression compensates for the lack of IGF-1R, whereas IRS-1 overexpression does not. We also demonstrate that phosphatidylinositol 3-kinase and extracellular signal-regulated kinase 1 and 2 are involved in the regulation of skin keratinocyte differentiation and take some part in mediating the inhibitory signal of IGF-1R on differentiation. In addition, we show that mammalian target of rapamycin plays a specific role in mediating IGF-1R impedance of action on keratinocyte differentiation. In conclusion, these results reveal that IGF-1R plays an inhibitory role in the regulation of skin development and differentiation.     

Experience-dependent transfer of Otx2 homeoprotein into the visual cortex activates postnatal plasticity

Neural circuits are shaped by experience in early postnatal life. Distinct GABAergic connections within visual cortex determine the timing of the critical period for rewiring ocular dominance to establish visual acuity. We find that maturation of the parvalbumin (PV)-cell network that controls plasticity onset is regulated by a selective re-expression of the embryonic Otx2 homeoprotein. Visual experience promoted the accumulation of non-cell-autonomous Otx2 in PV-cells, and cortical infusion of exogenous Otx2 accelerated both PV-cell development and critical period timing. Conversely, conditional removal of Otx2 from non-PV cells or from the visual pathway abolished plasticity. Thus, the experience-dependent transfer of a homeoprotein may establish the physiological milieu for postnatal plasticity of a neural circuit.     

Environmental enrichment modulates cortico-cortical interactions in the mouse

Environmental enrichment (EE) is an experimental protocol based on a complex sensorimotor stimulation that dramatically affects brain development. While it is widely believed that the effects of EE result from the unique combination of different sensory and motor stimuli, it is not known whether and how cortico-cortical interactions are shaped by EE. Since the primary visual cortex (V1) is one of the best characterized targets of EE, we looked for direct cortico-cortical projections impinging on V1, and we identified a direct monosynaptic connection between motor cortex and V1 in the mouse brain. To measure the interactions between these areas under standard and EE rearing conditions, we used simultaneous recordings of local field potentials (LFPs) in awake, freely moving animals. LFP signals were analyzed by using different methods of linear and nonlinear analysis of time series (cross-correlation, mutual information, phase synchronization). We found that EE decreases the level of coupling between the electrical activities of the two cortical regions with respect to the control group. From a functional point of view, our results indicate, for the first time, that an enhanced sensorimotor experience impacts on the brain by affecting the functional crosstalk between different cortical areas.     

A Small Motor Cortex Lesion Abolished Ocular Dominance Plasticity in the Adult Mouse Primary Visual Cortex and Impaired Experience-Dependent Visual Improvements

It was previously shown that a small lesion in the primary somatosensory cortex (S1) prevented both cortical plasticity and sensory learning in the adult mouse visual system: While 3-month-old control mice continued to show ocular dominance (OD) plasticity in their primary visual cortex (V1) after monocular deprivation (MD), age-matched mice with a small photothrombotically induced (PT) stroke lesion in S1, positioned at least 1 mm anterior to the anterior border of V1, no longer expressed OD-plasticity. In addition, in the S1-lesioned mice, neither the experience-dependent increase of the spatial frequency threshold (“visual acuity”) nor of the contrast threshold (“contrast sensitivity”) of the optomotor reflex through the open eye was present. To assess whether these plasticity impairments can also occur if a lesion is placed more distant from V1, we tested the effect of a PT-lesion in the secondary motor cortex (M2). We observed that mice with a small M2-lesion restricted to the superficial cortical layers no longer expressed an OD-shift towards the open eye after 7 days of MD in V1 of the lesioned hemisphere. Consistent with previous findings about the consequences of an S1-lesion, OD-plasticity in V1 of the nonlesioned hemisphere of the M2-lesioned mice was still present. In addition, the experience-dependent improvements of both visual acuity and contrast sensitivity of the open eye were severely reduced. In contrast, sham-lesioned mice displayed both an OD-shift and improvements of visual capabilities of their open eye. To summarize, our data indicate that even a very small lesion restricted to the superficial cortical layers and more than 3mm anterior to the anterior border of V1 compromised V1-plasticity and impaired learning-induced visual improvements in adult mice. Thus both plasticity phenomena cannot only depend on modality-specific and local nerve cell networks but are clearly influenced by long-range interactions even from distant brain regions.     

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.     

Pairing-induced changes of orientation maps in cat visual cortex.

We have studied the precise temporal requirements for plasticity of orientation preference maps in kitten visual cortex. Pairing a brief visual stimulus with electrical stimulation in the cortex, we found that the relative timing determines the direction of plasticity: a shift in orientation preference toward the paired orientation occurs if the cortex is activated first visually and then electrically; the cortical response to the paired orientation is diminished if the sequence of visual and electrical activation is reversed. We furthermore show that pinwheel centers are less affected by the pairing than the pinwheel surround. Thus, plasticity is not uniformly distributed across the cortex, and, most importantly, the same spike time-dependent learning rules that have been found in single-cell in vitro studies are also valid on the level of cortical maps.     

Sleep enhances plasticity in the developing visual cortex

During a critical period of brain development, occluding the vision of one eye causes a rapid remodeling of the visual cortex and its inputs. Sleep has been linked to other processes thought to depend on synaptic remodeling, but a role for sleep in this form of cortical plasticity has not been demonstrated. We found that sleep enhanced the effects of a preceding period of monocular deprivation on visual cortical responses, but wakefulness in complete darkness did not do so. The enhancement of plasticity by sleep was at least as great as that produced by an equal amount of additional deprivation. These findings demonstrate that sleep and sleep loss modify experience-dependent cortical plasticity in vivo. They suggest that sleep in early life may play a crucial role in brain development.