Synaptotagmin-2 Is a Reliable Marker for Parvalbumin Positive Inhibitory Boutons in the Mouse Visual Cortex

Background: Inhibitory innervation by parvalbumin (PV) expressing interneurons has been implicated in the onset of the sensitive period of visual plasticity. Immunohistochemical analysis of the development and plasticity of these inhibitory inputs is difficult because PV expression is low in young animals and strongly influenced by neuronal activity. Moreover, the synaptic boutons that PV neurons form onto each other cannot be distinguished from the innervated cell bodies by immunostaining for this protein because it is present throughout the cells. These problems call for the availability of a synaptic, activity-independent marker for PV+ inhibitory boutons that is expressed before sensitive period onset. We investigated whether synaptotagmin-2 (Syt2) fulfills these properties in the visual cortex. Syt2 is a synaptic vesicle protein involved in fast Ca 2+ dependent neurotransmitter release. Its mRNA expression follows a pattern similar to that of PV throughout the brain and is present in 30–40 % of hippocampal PV expressing basket cells. Up to now, no quantitative analyses of Syt2 expression in the visual cortex have been carried out. Methodology/Principal Findings: We used immunohistochemistry to analyze colocalization of Syt2 with multiple interneuron markers including vesicular GABA transporter VGAT, calbindin, calretinin, somatostatin and PV in the primary visual cortex of mice during development and after dark-rearing. Conclusions/Significance: We show that in the adult visual cortex Syt2 is only found in inhibitory, VGAT positive boutons

Syt2 in L2-3 is expressed in PV positive boutons.

<p>(A–C, arrowhead) Syt2 almost always colocalizes with VGAT in L2-3 of the visual cortex. The arrow indicates a VGAT positive bouton without Syt2 expression. (D–I) Syt2 puncta (arrowhead) are not calretinin (CR, arrow) or somatostatin (SOM, arrow) positive. (J–L) Syt2 shows strong co-expression with PV (arrowheads). A–C & G–I n = 3; D–F & J–L n = 4. Scale bars 10 µm.</p

Syt2 in L2-3 and L5 is not expressed in VGLUT1&2 positive boutons.

<p>(A–F) Syt2 puncta (arrowheads) in L2-3 and L5 are not VGLUT1 positive. Arrows indicate VGLUT1 boutons. VGLUT1 and Syt2 show an alternating perisomatic localization (F). Similarly VGLUT2 boutons, indicated with arrows do not show expression of Syt2. Syt2 puncta indicated with arrowheads (G–L). A–L n = 3. Scale bars 10 µm.</p

Scatter plot pixel analyses of Syt2 colocalization with cell type specific markers in various cortical areas.

<p>(A) Example images of neurons with perisomatic punctate innervation from different areas of the cortex. Adjacent are scatter plots for the same images showing distribution of fluorescence intensities for Syt2 (vertical green channel) and PV (horizontal red channel). As a negative control for colocalization (B) shows examples of Syt2 positive innervations with VGLUT2 or CR. Adjacent are scatter plots showing distribution of fluorescence intensities for Syt2 (vertical green channel) and VGLUT2 or CR (horizontal red channel). (C) The average Manders' coefficients of all the scatter plots. Coefficient values in different cortical areas are similar to V1 for colocalization of Syt2 and PV, and differ strongly from non-colocalization values for Syt2 with VGLUT2 or CR (n = 8; n = 7&8). M1, primary motor cortex (n = 6&9); S1, primary somatosensory cortex (n = 7&10); S1BF, barrel field cortex (n = 11&11); Au1, auditory cortex (n = 4&9). Data in C are given in mean coefficients ± SEM. Scale bar 10 µm.</p

Syt2 in L5 is expressed in PV positive boutons.

<p>(A–C, arrowheads) Syt2 almost always colocalizes with inhibitory VGAT positive puncta in L5 of the visual cortex. The arrow indicates a VGAT positive punctum without Syt2 expression. (D–L) These inhibitory Syt2 puncta (arrowheads) are not calbindin positive (D–F), CR positive (G–I) or SOM positive (J–L) (arrows). (M–O) As in L2-3, Syt2 shows a high colocalization with PV positive puncta in L5 of the visual cortex (arrowheads). Note the PV+ neurite (arrow) which is without expression of Syt2 (M–O). A–C & J–L n = 3; D–I & M–O n = 4. Scale bars 10 µm.</p

Percentages of colocalization of Syt2 with different markers.

<p>Values represent e.g. the mean percentages (± SEM) of Syt2 puncta that also expressed VGAT.</p

Syt2 and PV expression in the mouse brain.

<p>(A–C) Localization of Syt2 and PV mRNA in the mouse brain according to the Allen Brain Atlas <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0035323#pone.0035323-Allen1" target="_blank">[26]</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0035323#pone.0035323-Lein1" target="_blank">[27]</a>. Note the resemblance between the two patterns. Syt2 mRNA is detectable in the Caudate Putamen (CPu), reticular thalamic nucleus (Rt), zona incerta (ZI) and ventromedial hypothalamic nucleus (VMH). Expression is also seen in the hippocampal areas and the subiculum, as well as in the superior colliculus (SC) and the molecular layer of the cerebellum (ML). A sparse but layered signal is seen in the visual cortex (V1) (C). (D) Syt2 protein expression is visualized in a coronal section of the mouse brain using the i735 antibody. The white box highlights the part of the visual cortex from which data for <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0035323#pone-0035323-g002" target="_blank">figures 2</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0035323#pone-0035323-g004" target="_blank">4</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0035323#pone-0035323-g007" target="_blank">7</a>&<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0035323#pone-0035323-g008" target="_blank">8</a> are obtained. (E) Syt2 is expressed in all layers of the visual cortex, except for layer 1. It is localized in a perisomatic fashion around many cells in L2-3 – L6. Expression is strongest in the lower part of L5 where larger pyramidal neurons reside. Scale bar (C) 180 µm, (E) 50 µm.</p

Quantification of colocalization of Syt2 and markers for specific subsets of inhibitory and excitatory neurons.

<p>(A) Percentage of colocalization of VGAT in Syt2+ puncta in L2-3 and L5. (B) Percentage of calbindin colocalization in Syt2+ puncta (CB/Syt2), and Syt2 in calbindin+ puncta (Syt2/CB) in L5. (C) Percentage of colocalization of calretinin in Syt2+ puncta (CR/Syt2) and Syt2 in calretinin+ puncta (Syt2/CR). (D) Percentages colocalization of somatostatin in Syt2+ puncta (SOM/Syt2) and Syt2 in somatostatin+ puncta (Syt2/SOM). (E) Percentages colocalization of PV in Syt2+ puncta (PV/Syt2) and Syt2 in PV+ puncta (Syt2/PV). (F) Percentages colocalization of VGLUT1 or VGLUT2 in Syt2+ puncta (VGLUT1/Syt2 and VGLUT2/Syt2) and Syt2 in VGLUT1+ or VGLUT2+ puncta (Syt2/VGLUT1 and Syt2/VGLUT2). Data are given in mean percentage ± SEM.</p

Thalamic inhibition regulates critical-period plasticity in visual cortex and thalamus

During critical periods of development, experience shapes cortical circuits, resulting in the acquisition of functions used throughout life. The classic example of critical-period plasticity is ocular dominance (OD) plasticity, which optimizes binocular vision but can reduce the responsiveness of the primary visual cortex (V1) to an eye providing low-grade visual input. The onset of the critical period of OD plasticity involves the maturation of inhibitory synapses within V1, specifically those containing the GABAA receptor α1 subunit. Here we show that thalamic relay neurons in mouse dorsolateral geniculate nucleus (dLGN) also undergo OD plasticity. This process depends on thalamic α1-containing synapses and is required for consolidation of the OD shift in V1 during long-term deprivation. Our findings demonstrate that thalamic inhibitory circuits play a central role in the regulation of the critical period. This has far-reaching consequences for the interpretation of studies investigating the molecular and cellular mechanisms regulating critical periods of brain development