Mechanisms of ocular dominance plasticity in the juvenile and adult mouse visual cortex

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Brain and Cognitive Sciences, 2011.Cataloged from PDF version of thesis. Vita.Includes bibliographical references (p. 171-185).Ocular dominance (OD) plasticity is a classic example of bidirectional experience-dependent plasticity in the primary visual cortex. This form of plasticity is most robust during early postnatal development (termed the "critical period"), when monocular deprivation (MD) leads to a rapid weakening of responses evoked through the deprived eye followed by a delayed strengthening of non-deprived eye inputs. It has been proposed that these bidirectional changes occur as a three-stage process: first, degradation of patterned visual input weakens deprived-eye responses via homosynaptic long-term depression (LTD); this is accompanied by a shift in the plasticity modification threshold (0m) that determines the direction of synaptic plasticity, such that synaptic strengthening is favored over synaptic weakening; finally, weak open-eye responses are strengthened via the mechanisms of homosynaptic long-term potentiation (LTP). Despite the growing evidence supporting this model of experience-dependent synaptic modification, the exact molecular and synaptic mechanisms that are responsible for these processes remain controversial. In my thesis work, I address three questions. First, I attempt to parse the relative contribution of excitatory and inhibitory processes to expression of the OD shift in order to understand how deprived-eye depression is expressed in the cortex. To address this, I first induce a shift in OD with 3 days of MD and then use several pharmacological methods to shut off cortical inhibitory synaptic transmission. I demonstrate that rapid deprived-eye depression is strongly expressed at excitatory thalamocortical synapses without any influences of polysynaptic intracortical inhibition. In the second part of my work, I try to resolve the nature/identity of the molecular mechanism that underlies the regulation of [theta]m. Using a transgenic mouse model, I find that a reduction in the NR2A/B subunit ratio of the N-methyl-d-aspartate (NMDA) receptor during MD alters the qualities of OD plasticity by impairing weakening of deprived-eye inputs and enhancing strengthening of open-eye inputs. These findings suggest that NMDAR subunit composition may specify the value and the rate of adjustment of synaptic 0m, which in turn determines the bidirectional cortical response to MD. The final portion of my thesis addresses the factors that limit OD plasticity beyond the critical period. I test the hypothesis that the developmental increase in intracortical GABAergic inhibitory synaptic transmission is a fundamental restricting factor for adult cortical plasticity and demonstrate that parvalbumin-expressing fast-spiking basket cells are specifically implicated in the absence of juvenile-like deprived-eye depression in adult mice.by Lena A. Khibnik.Ph.D

A Direct Projection from Mouse Primary Visual Cortex to Dorsomedial Striatum

The mammalian striatum receives inputs from many cortical areas, but the existence of a direct axonal projection from the primary visual cortex (V1) is controversial. In this study we use anterograde and retrograde tracing techniques to demonstrate that V1 directly innervates a topographically defined longitudinal strip of dorsomedial striatum in mice. We find that this projection forms functional excitatory synapses with direct and indirect pathway striatal projection neurons (SPNs) and engages feed-forward inhibition onto these cells. Importantly, stimulation of V1 afferents is sufficient to evoke phasic firing in SPNs. These findings therefore identify a striatal region that is functionally innervated by V1 and suggest that early visual processing may play an important role in striatal-based behaviors

Layer 5 neurons in primary visual cortex project to dorsomedial striatum.

<p><b>A–F</b>. Representative serial coronal sections from the brain of a <i>Rbp4</i>-Cre mouse virally injected in the primary visual cortex (V1) with an AAV encoding Cre-dependent EGFP to label the cell bodies and axonal processes of neurons in layer 5 (<b>A</b>). EGFP-labeled axons leave V1 in through the external capsule (<b>B</b>) and innervate various structures including several layers of the superior colliculus (SC; <b>B</b>) and dorsal lateral geniculate nucleus (DLG; <b>C</b>). V1 axons enter the posterior tail of the striatum (caudate/putamen, CPu; <b>D</b>) and course throughout the length of the striatum (<b>E</b>) before innervating the anterior dorsomedial quadrant (<b>F</b>). <i>Left</i>, entire hemisphere stained with DAPI to highlight different brain structures overlaid with the EGFP fluorescence. <i>Middle</i>, detailed view of regions outlined in white in the corresponding left panel. <i>Right</i>, corresponding images from Paxinos Mouse Brain Atlas highlighting the putative structures where EGFP fluorescence is detected. Similar results were observed eight different mouse brains. M1; primary motor cortex. S1BF; primary somatosensory cortex barrel field.</p

Retrograde labeling of V1 neurons innervating the DMS.

<p><b>A</b>. Coronal striatal section showing the site of red retrobead (RRB) injection into the dorsomedial striatum (DMS). The section is counterstained with DAPI. <b>B</b>. Coronal brain section of V1 containing RRBs retrogradedly transported from the dorsomedial striatum. Secondary visual cortex (V2ML and V2L) and secondary auditory cortex (AuD), but not Au1 (primary auditory cortex) were also labeled. As expected, retrobeads were also detected in putative dopaminergic neurons within the substantia nigra pars compacta (SNc). <b>C</b>. Detailed view of V1 showing the relative distribution of retrobead-labeled cells bodies across cortical layers. <b>D</b>. Distribution of retrobead-labeled neurons in V1 across cortical layers. The majority (81%) of labeled cells are situated within layer 5, while only 19% distribute to layers 2/3. <b>E</b>. Representative coronal section from a <i>Rbp4</i>-Cre;ZsGreen1 brain showing EGFP fluorescence concentrated in the cell bodies of Cre-containing cortical neurons in layer 5. The section is counterstained with DAPI. <b>F</b>. Detailed view of V1 in a <i>Rbp4</i>-Cre;ZsGreen1 brain injected with RRBs in the DMS illustrating the extensive overlap between retrobead- and Cre-containing (EGFP-positive) cells in upper layer 5. <b>G</b>. Percentage of all retrobead-labeled layer 5 neurons that also contain Cre (RRB/EGFP⁺; 65%, shown in yellow) vs. cells that are only RRB⁺ (35%, red). <b>H</b>. Coronal brain section showing red retrobead labeled cells in thalamus. ILT: Intralaminar thalamic nuclei. Scale: 1 mm for panels A, B, E and H; 100 µm for panels C and F.</p

Stimulation of V1 axons engages glutamatergic and GABAergic synaptic transmission onto SPNs.

<p><b>A</b>. Representative high magnification view of the dorsomedial striatum in a <i>Rbp4</i>-Cre; <i>Drd2-</i>EGFP mouse virally injected in V1 with an AAV encoding Cre-dependent ChR2-mCherry. Axonal projections from V1 (red) are clearly seen within the DMS. The section is stained with DAPI (blue) to label cell bodies. iSPNs are easily identified using EGFP fluorescence (green). A presumptive dSPN (DAPI⁺; EGFP⁻) is also shown. <b>B</b>. Cell-attached recording from a SPN showing that a 1 ms 473 nm light flash (blue bar) reliably evokes a single action potential. Three consecutive extracellular waveforms are overlaid. <b>C</b>. As in <b>B</b> for another SPN recorded in the whole-cell current-clamp configuration. <b>D</b>. Whole-cell voltage-clamp traces from a SPN upon optogenetic stimulation (1 ms, blue bar) of ChR2-expressing V1 axons. EPSCs (black) were recorded at <i>E</i>_Cl = −70 mV, while IPSCs (red) were recorded at 0 mV (the reversal potential for ionotropic glutamate receptor mediated currents). Dashed gray lines mark the onset of both currents and highlight the delayed onset of IPSCs relative to EPSCs. Both EPSC and IPSC were eliminated after bath application of the glutamate receptor antagonists NBQX and CPP (both at 10 µM; gray and pink lines, respectively), confirming the disynaptic origin of IPSCs. <b>E</b>. Mean (± s.e.m) peak EPSC amplitude in dSPNs (blue) and iSPNs (green). <b>F</b>. Mean (± s.e.m) latency from flash onset to current onset of EPSCs (black) and IPSCs (red). <b>G</b>. Mean (± s.e.m) EPSC (black) and IPSC (red) amplitudes. Asterisk represents statistical significance, <i>P</i><0.05 vs. baseline amplitude. Data in E–G represent mean ± s.e.m. Number of recordings are indicated in parentheses.</p

Relative contribution of feedforward excitatory connections to ocular dominance plasticity in layer 4 of visual cortex

Brief monocular deprivation (MD) shifts ocular dominance (OD) in primary visual cortex by causing depression of responses to the deprived eye. Here we address the extent to which the shift is expressed by a modification of excitatory synaptic transmission. An OD shift was first induced with 3 days of MD, and then the influences of intracortical polysynaptic inhibitory and excitatory synapses were pharmacologically removed, leaving only “feedforward” thalamocortical synaptic currents. The results show that the rapid OD shift following MD is strongly expressed at the level of thalamocortical synaptic transmission.National Institutes of Health (U.S.