Nogo Receptor 1 Confines a Disinhibitory Microcircuit to the Critical Period in Visual Cortex.

A characteristic of the developing mammalian visual system is a brief interval of plasticity, termed the "critical period," when the circuitry of primary visual cortex is most sensitive to perturbation of visual experience. Depriving one eye of vision (monocular deprivation [MD]) during the critical period alters ocular dominance (OD) by shifting the responsiveness of neurons in visual cortex to favor the nondeprived eye. A disinhibitory microcircuit involving parvalbumin-expressing (PV) interneurons initiates this OD plasticity. The gene encoding the neuronal nogo-66-receptor 1 (ngr1/rtn4r) is required to close the critical period. Here we combined mouse genetics, electrophysiology, and circuit mapping with laser-scanning photostimulation to investigate whether disinhibition is confined to the critical period by ngr1 We demonstrate that ngr1 mutant mice retain plasticity characteristic of the critical period as adults, and that ngr1 operates within PV interneurons to restrict the loss of intracortical excitatory synaptic input following MD in adult mice, and this disinhibition induces a "lower PV network configuration" in both critical-period wild-type mice and adult ngr1-/- mice. We propose that ngr1 limits disinhibition to close the critical period for OD plasticity and that a decrease in PV expression levels reports the diminished recent cumulative activity of these interneurons.Significance statementLife experience refines brain circuits throughout development during specified critical periods. Abnormal experience during these critical periods can yield enduring maladaptive changes in neural circuits that impair brain function. In the developing visual system, visual deprivation early in life can result in amblyopia (lazy-eye), a prevalent childhood disorder comprising permanent deficits in spatial vision. Here we identify that the nogo-66 receptor 1 gene restricts an early and essential step in OD plasticity to the critical period. These findings link the emerging circuit-level description of OD plasticity to the genetic regulation of the critical period. Understanding how plasticity is confined to critical periods may provide clues how to better treat amblyopia

Nogo Receptor 1 Confines a Disinhibitory Microcircuit to the Critical Period in Visual Cortex

A characteristic of the developing mammalian visual system is a brief interval of plasticity, termed the “critical period,” when the circuitry of primary visual cortex is most sensitive to perturbation of visual experience. Depriving one eye of vision (monocular deprivation [MD]) during the critical period alters ocular dominance (OD) by shifting the responsiveness of neurons in visual cortex to favor the nondeprived eye. A disinhibitory microcircuit involving parvalbumin-expressing (PV) interneurons initiates this OD plasticity. The gene encoding the neuronal nogo-66-receptor 1 (ngr1/rtn4r) is required to close the critical period. Here we combined mouse genetics, electrophysiology, and circuit mapping with laser-scanning photostimulation to investigate whether disinhibition is confined to the critical period by ngr1. We demonstrate that ngr1 mutant mice retain plasticity characteristic of the critical period as adults, and that ngr1 operates within PV interneurons to restrict the loss of intracortical excitatory synaptic input following MD in adult mice, and this disinhibition induces a “lower PV network configuration” in both critical-period wild-type mice and adult ngr1(−/−) mice. We propose that ngr1 limits disinhibition to close the critical period for OD plasticity and that a decrease in PV expression levels reports the diminished recent cumulative activity of these interneurons. SIGNIFICANCE STATEMENT Life experience refines brain circuits throughout development during specified critical periods. Abnormal experience during these critical periods can yield enduring maladaptive changes in neural circuits that impair brain function. In the developing visual system, visual deprivation early in life can result in amblyopia (lazy-eye), a prevalent childhood disorder comprising permanent deficits in spatial vision. Here we identify that the nogo-66 receptor 1 gene restricts an early and essential step in OD plasticity to the critical period. These findings link the emerging circuit-level description of OD plasticity to the genetic regulation of the critical period. Understanding how plasticity is confined to critical periods may provide clues how to better treat amblyopia

Layer 4 Gates Plasticity in Visual Cortex Independent of a Canonical Microcircuit.

Disrupting binocular vision during a developmental critical period can yield enduring changes to ocular dominance (OD) in primary visual cortex (V1). Here we investigated how this experience-dependent plasticity is coordinated within the laminar circuitry of V1 by deleting separately in each cortical layer (L) a gene required to close the critical period, nogo-66 receptor (ngr1). Deleting ngr1 in excitatory neurons in L4, but not in L2/3, L5, or L6, prevented closure of the critical period, and adult mice remained sensitive to brief monocular deprivation. Intracortical disinhibition, but not thalamocortical disinhibition, accompanied this OD plasticity. Both juvenile wild-type mice and adult mice lacking ngr1 in L4 displayed OD plasticity that advanced more rapidly L4 than L2/3 or L5. Interestingly, blocking OD plasticity in L2/3 with the drug AM-251 did not impair OD plasticity in L5. We propose that L4 restricts disinhibition and gates OD plasticity independent of a canonical cortical microcircuit

Distinct Circuits for Recovery of Eye Dominance and Acuity in Murine Amblyopia

Degrading vision by one eye during a developmental critical period yields enduring deficits in both eye dominance and visual acuity. A predominant model is that "reactivating'' ocular dominance (OD) plasticity after the critical period is required to improve acuity in amblyopic adults. However, here we demonstrate that plasticity of eye dominance and acuity are independent and restricted by the nogo-66 receptor (ngr1) in distinct neuronal populations. Ngr1 mutant mice display greater excitatory synaptic input onto both inhibitory and excitatory neurons with restoration of normal vision. Deleting ngr1 in excitatory cortical neurons permits recovery of eye dominance but not acuity. Reciprocally, deleting ngr1 in thalamus is insufficient to rectify eye dominance but yields improvement of acuity to normal. Abolishing ngr1 expression in adult mice also promotes recovery of acuity. Together, these findings challenge the notion that mechanisms for OD plasticity contribute to the alterations in circuitry that restore acuity in amblyopia.National Eye Institute [R01EY021580, R01EY027407]; Children's Hospital Los Angeles; Research to Prevent Blindness; Saban Research Institute; Burroughs Wellcome Fund12 month embargo; published online: 7 June 2018This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Nogo receptor 1 limits tactile task performance independent of basal anatomical plasticity.

The genes that govern how experience refines neural circuitry and alters synaptic structural plasticity are poorly understood. The nogo-66 receptor 1 gene (ngr1) is one candidate that may restrict the rate of learning as well as basal anatomical plasticity in adult cerebral cortex. To investigate if ngr1 limits the rate of learning we tested adult ngr1 null mice on a tactile learning task. Ngr1 mutants display greater overall performance despite a normal rate of improvement on the gap-cross assay, a whisker-dependent learning paradigm. To determine if ngr1 restricts basal anatomical plasticity in the associated sensory cortex, we repeatedly imaged dendritic spines and axonal varicosities of both constitutive and conditional adult ngr1 mutant mice in somatosensory barrel cortex for two weeks through cranial windows with two-photon chronic in vivo imaging. Neither constant nor acute deletion of ngr1 affected turnover or stability of dendritic spines or axonal boutons. The improved performance on the gap-cross task is not attributable to greater motor coordination, as ngr1 mutant mice possess a mild deficit in overall performance and a normal learning rate on the rotarod, a motor task. Mice lacking ngr1 also exhibit normal induction of tone-associated fear conditioning yet accelerated fear extinction and impaired consolidation. Thus, ngr1 alters tactile and motor task performance but does not appear to limit the rate of tactile or motor learning, nor determine the low set point for synaptic turnover in sensory cortex

Myelination is extensive in S1 barrel cortex and does not reflect the distribution of Nogo-A.

<p>(A) Immunostaining for myelin basic protein (αMBP) of coronal sections of S1 barrel cortex reveals extensive myelination in cortical layers IV–VI at P26 when cortical spine dynamics are elevated relative to P40. Myelination increases in S1 barrel cortex by P40 to extend into layer II/III. (B) Immunostaining of V1 reveals myelination in cortical layers V–VI at P26 that extends into layer IV by P40. The approximate positions of cortical layers II/III to VI are indicated at right. The scale bar corresponds to 200 µm (C) Quantification of the relative distribution of staining intensity for myelin basic protein in S1 and V1 at P24 and P40 at increasing depths from the pial surface to the underlying white matter. Pixel intensity is normalized by the intensity of white matter (grey box). Error bars represent the standard deviation between at least 5 sections and 3 mice per group. (D) Higher magnification image of the distribution of myelinated fibers in layer I of S1 and V1 at P40. Few myelinated fibers are present in layer I of S1 relative to V1. The scale bar corresponds to 50 µm. (E) Nogo-A intensely labels the soma of putative oligodendrocytes but is also evident in cortical neurons. The pattern of expression is comparable at P26 and P40. The approximate positions of cortical layers II/III to VI are indicated at right. The scale bar corresponds to 200 µm.</p

Cranial windows are properly positioned over S1 barrel cortex and are a stable preparation for imaging cortical spine dynamics.

<p>(A) An example of optical imaging of intrinsic signals reveals the cortical region responsive to stimulation of the C2 whisker. Scale bar = 0.5 mm (B) Apical dendrites of layer V neurons in the boxed region (yellow) are shown at higher magnification in panels C and D. Scale bar = 50 µm (C) Higher magnification images of the boxed region (yellow) in panel B at day 0 (D) Higher magnification images of the boxed region (yellow) in panel B at day 12 (E) Higher magnification image of the boxed region in panel C on day 0. (F) Higher magnification image of the boxed region in panel D on day 12.</p

Axonal bouton turnover and stability are normal in <i>ngr1</i>−/− mice.

<p>(A) Examples of axons imaged repeatedly by repeated <i>in vivo</i> two-photon microscopy through cranial windows. Solid arrowheads (yellow) are examples of new boutons. Outlined arrowheads (yellow) are examples of boutons lost. Scale bar = 10 µm (B) Higher magnification of the boxed region (yellow) in panel A. Scale bar = 5 µm. (C) The turnover of axonal boutons every four days in WT (n = 4, 424 boutons) and <i>ngr1</i>−/− mice (n = 5, 749 boutons) is similar across 4-day intervals in S1 barrel cortex (p>0.2). (D) The average percent of axonal boutons gained and lost is similar between WT and <i>ngr1−/−</i> mice (gained p>0.5; lost p>0.6). (E) The survival fraction of boutons present on day 0 is similar at days 4, 8, and 12. (F) The percent of persistent boutons (p>0.1) and new boutons (p>0.3) present on day 12 is comparable between WT and <i>ngr1−/−</i> mice.</p

Mice lacking <i>ngr1</i> perform better on the gap cross assay but display normal tactile learning across sessions.

<p>(A) A schematic of the gap cross assay. The movement of a mouse from the starting, or ‘home’, platform to the target platform across a given gap distance is detected with motion sensors positioned at the back and edge of each platform. (B) Activation of each sensor (grey box) indicates the position of the mouse. (C) Successful crosses are defined as the movement of the mouse from the starting platform to the target platform (green circles). Failures are defined as trials in which the mouse approaches the edge of the home or target platform and returns to the back of the home platform (red crosses). (D) <i>ngr1−/−</i> mice cross ‘whisker’ distances at a significantly higher success rate (WT, n = 19; <i>ngr1−/−</i>, n = 14; p>.01 for distances 5.5 and 6 cm; p>.32 for distances 3.5 and 4 cm, two-way ANOVA). This greater success rate is most significant at longer distances, 5.5 cm and 6 cm (**, p>.01 with Bonferroni correction for multiple comparisons). (E) Despite better overall performance, the percent improvement for a given gap distance from the first 4 sessions (left value for each distance) to the second 4 sessions (right value for each distance) is similar for WT and <i>ngr1−/−</i> mice for a given gap distance. (F) WT mice improve with experience at ‘whisker’ gap distances (WT, n = 19, *, p<.05, two-way repeated measures ANOVA) from the first 4 sessions (Early, grey line) to the second 4 sessions (Late, black line). (G) <i>Ngr1</i> mutant mice improve with experience at ‘whisker only’ gap distances (<i>ngr1−/−</i>, n = 14, *, p<.05, two-way repeated measures ANOVA) from the first 4 sessions (Early, pink line) to the second 4 sessions (Late, red line).</p

Motor learning is normal in NgR1 mutant mice but consolidation of fear extinction is impaired.

<p>(A) <i>Ngr1</i> mutant mice display a deficit in overall performance on the rotarod, but the rate of improvement is similar in WT and <i>ngr1−/−</i> mice (WT n = 8, <i>ngr1−/−</i> n = 9) (**, p<.01; *, p<.05). (B) <i>Ngr1</i> mutant mice also exhibit a mild deficit at a slower acceleration rate at the conclusion of training (p<.05, unpaired two-tailed t-test with Welch's correction). (C) The average percent improvement plotted as the percent difference in average latency to fall of the second two trials and last two trials. Improvement is similar between WT and <i>ngr1−/−</i> mice (p>.22, Kolmogorov-Smirnov test) (D) Schematic for fear conditioning and extinction protocol. On day 1, adult female mice were conditioned to an acoustic tone that co-terminated with a 1 second foot shock (0.6 mA). On days 2 and 3, mice were presented with 12 unpaired tones during a 30′ period. (E) Extinction of the fear response is plotted at percent time spent freezing averaged across two consecutive trials on day 2 and day 3 (n = 16 WT, n = 12 <i>ngr1−/−</i>). Extinction between the two genotypes differs across trials during day 2 but not day 3 by RM-ANOVA (bracket, day 2, p<.005; day 3, p>.42). (F) Acquisition of the freezing response is similar between WT and <i>ngr1−/−</i> mice on consecutive trials of the conditioned (tone) and unconditioned (0.6 mA shock) stimulus (n = 16 WT, n = 12 <i>ngr1−/−</i>). (G) Acquisition of the freezing response is similar between WT and <i>ngr1−/−</i> mice on consecutive trials of the conditioned (tone) and a milder unconditioned (0.3 mA shock) stimulus (WT, n = 8; <i>ngr1−/−</i>, n = 6).</p