Dynamic remodeling of dendritic arbors in GABAergic interneurons of adult visual cortex

Abstract

Despite decades of evidence for functional plasticity in the adult brain, the role of structural plasticity in its manifestation remains unclear. To examine the extent of neuronal remodeling that occurs in the brain on a day-to-day basis, we used a multiphoton-based microscopy system for chronic in vivo imaging and reconstruction of entire neurons in the superficial layers of the rodent cerebral cortex. Here we show the first unambiguous evidence (to our knowledge) of dendrite growth and remodeling in adult neurons. Over a period of months, neurons could be seen extending and retracting existing branches, and in rare cases adding new branch tips. Neurons exhibiting dynamic arbor rearrangements were GABA-positive non-pyramidal interneurons, while pyramidal cells remained stable. These results are consistent with the idea that dendritic structural remodeling is a substrate for adult plasticity and they suggest that circuit rearrangement in the adult cortex is restricted by cell type-specific rules.

Figure 1. Summary of Imaging Sessions Displayed by Age

Each neuron was named by an arbitrary three-letter code. Empty circles represent pyramidal neuron-imaging sessions, and filled circles represent non-pyramidal neuron-imaging sessions.

Figure 2. Dendritic Arbors of Pyramidal Neurons Are Stable

(A) MZPs near the cell body of the pyramidal cell “dow” acquired over 9 wk. (B) Two-dimensional projections of three-dimensional skeletal reconstructions of “dow.” (C) High-magnification view of branch tip (green arrow) in region outlined by green box in (B). Scale bars: (A and B), 50 μm; (C), 10 μm.

Figure 3. Dendritic Growth in Multiple Branches of a Non-Pyramidal Neuron

(A) MZPs near the cell body of the (∼118-μm deep) non-pyramidal cell “nmr” acquired over 4 wk. Two examples of dendritic branch growth are indicated by red arrowheads. (B) Two-dimensional projections of three-dimensional skeletal reconstructions of the non-pyramidal neuron “nmr.” (C) High-magnification view of one growing branch tip (#20) (red box in [A and B]). Red arrowhead marks the approximate distal end of the branch tip at 11 wk. (D) Three-dimensional isosurface reconstructions of branch tips in (C). (E) High-magnification view of branch tip #15 (green box in [A and B]). (F) Three-dimensional isosurface reconstructions of branch tips in (E). (G) Plot of change in BTL of dynamic branch tips as a function of age. Number to the right denotes branch tip number. Scale bars: (A and B), 25 μm; (B–F), 5 μm.

Figure 4. Large-Scale Dendritic Branch Growth in a Non-Pyramidal Neuron

(A) Three-dimensional skeletal reconstructions of the non-pyramidal neuron “paz” from images acquired over 7 wk. Note that a growing branch tip exceeded the imaging volume after 11 wk, and black arrowheads denote its postulated continuation. (B) MZPs of region outlined by red box in (A). The branch tip (#15) elongates radially in the x–y-axis and away from the pial surface in the z-axis. Non-specific labeling in the MZP is exacerbated by the summation of additional z stacks due to elongation on the z-axis. The traced branch of interest is highlighted by a green overlay. Red arrowheads mark the approximate distal end of the branch tip at 11 wk. (C) Three-dimensional isosurface reconstructions of the region of the branch tip outlined by a green box in (B). (D) Plot of change in BTL of dynamic branch tips as a function of age. Triangles denote the minimum length of the branch tip as it exceeds the border of the imaging volume. Number to the right denotes branch tip number. Scale bars: (A), 25 μm; (B and C), 10 μm.

Figure 5. Branch Extensions, Retractions, and De Novo Branch Tip Addition in a Non-Pyramidal Neuron

(A) Three-dimensional skeletal reconstructions of the sparsely spinous non-pyramidal neuron “zen” from images acquired over 5 wk. (B) High-magnification MZP view of region outlined by purple box in (A). Red arrowheads indicate examples of structural remodeling. Three-dimensional isosurface reconstructions show (C), the elongation and retraction of a spine toward an axon (yellow overlay). (D) Structural change in a cluster of branch tips (#50, far right) (red box in [B]). (E) Higher-magnification MZP view of region outlined by green box in (A). Examples of process retraction and branch-tip (#3) addition are labeled with yellow and green arrowheads, respectively. (F) Three-dimensional isosurface reconstructions of (E) with axon in yellow overlay. (G) MZP view of region outlined by cyan box in (A) shows branch-tip (#16) addition on a different dendrite. (H) Three-dimensional isosurface reconstructions of (G). (I) Plot of change in BTL of dynamic branch tips as a function of age. Number to the right denotes branch tip number. Scale bars: (A), 50 μm; (B–H), 5 μm.

Figure 6. Remodeling Adult Non-Pyramidal Neurons in the Superficial Layers of the Visual Cortex Are Inhibitory GABAergic Interneurons

(A) Confocal image stack of a coronal section containing the chronically imaged non-pyramidal neuron “ttr” is visualized by GFP staining (green, filled arrows) and is immunopositive for GABA (visualized in red) (B), overlay of GFP and GABA shown in (C). (D) Confocal image stack of a coronal section containing the chronically imaged pyramidal neuron “dow” is immunonegative for GABA (visualized in red) (E), overlay of GFP and GABA shown in (F). Scale bar: (A–F), 100 μm.

Figure 7. The Change in BTL is Plotted for Each Individual Monitored Branch Tip of Every Imaged Cell

Three-letter code, top right. (A–F) pyramidal cells; (G–N) non-pyramidal cells. Triangles and dashed lines denote the minimum length of the branch tip as it exceeds the border of the imaging volume.