Inhibitory dendrite dynamics as a general feature of the adult cortical microcircuit

Abstract

The mammalian neocortex is functionally subdivided into architectonically distinct regions that process various types of information based on their source of afferent input. Yet, the modularity of neocortical organization in terms of cell type and intrinsic circuitry allows afferent drive to continuously reassign cortical map space. New aspects of cortical map plasticity include dynamic turnover of dendritic spines on pyramidal neurons and remodeling of interneuron dendritic arbors. While spine remodeling occurs in multiple cortical regions, it is not yet known whether interneuron dendrite remodeling is common across primary sensory and higher-level cortices. It is also unknown whether, like pyramidal dendrites, inhibitory dendrites respect functional domain boundaries. Given the importance of the inhibitory circuitry to adult cortical plasticity and the reorganization of cortical maps, we sought to address these questions by using two-photon microscopy to monitor interneuron dendritic arbors of thy1-GFP-S transgenic mice expressing GFP in neurons sparsely distributed across the superficial layers of the neocortex. We find that interneuron dendritic branch tip remodeling is a general feature of the adult cortical microcircuit, and that remodeling rates are similar across primary sensory regions of different modalities, but may differ in magnitude between primary sensory versus higher cortical areas. We also show that branch tip remodeling occurs in bursts and respects functional domain boundaries.

Figure 1.

Chronic two-photon in vivo imaging of dendritic branch tip dynamics in superficial L2/3 cortical interneurons. A, Maximum z-projection (MZP) near the cell body (above) along with two-dimensional projections of three-dimensional skeletal reconstructions (below) of a superficial L2/3 interneuron acquired over 2 weeks. Arrows indicate dynamic branch tips. B, High-magnification view of one branch tip extension (green box and arrow in A). Green arrow marks the approximate distal end of the branch tip at 14 d. C, High-magnification view of one branch tip retraction (red box and arrow in A). Red arrow marks the approximate distal end of the branch tip at 0 d. D, MZP of chronically imaged interneuron (green arrow) superimposed over blood vessel map with primary visual cortex (V1, red), secondary visual cortex (V2, yellow), and primary somatosensory cortex (S1, blue) outlined. E, Dynamic branch tip analysis of cell imaged in A. Diamonds indicate Fano factor values of individual monitored branch tips with filled diamonds indicating dynamic branch tips. Dotted line marks the experimentally determined threshold for dynamic branch tip. Scale bars: A, 200 μm; B, 5 μm; C, 0.5 mm.

Figure 2.

Interneuron dendritic branch tip dynamics across multiple cortical regions. A, Time course of dynamic branch tips by weekly imaging interval for individual cells in V1, S1, and V2. Filled points indicate maximal weekly branch tip change. Top panels represent cells imaged for 2 or 3 weeks. Bottom panels represent cells imaged for 4 or more weeks. B, Quantification of fractional rate of dynamic branch tips per week for individual cells in V1, S1, and V2. C, Comparison across cortical regions of fractional rate of dynamic branch tips (left) and maximal fractional branch tip change per week (right) (V1: n = 17 cells from 17 mice, 48 dynamic branch tips, 623 total branch tips; S1: n = 11 cells from 11 mice, 45 dynamic branch tips, 485 total branch tips; V2: n = 15 cells from 15 mice, 75 dynamic branch tips, 614 total branch tips) (***p < 0.01, **p < 0.02, *p < 0.05). Error bars, SEM.

Figure 3.

Dynamic branch tip bias in cells along functional cortical boundaries. A, Map of primary monocular (V1M) and binocular (V1B) visual cortex obtained by optical intrinsic signal imaging of contralateral (left) and ipsilateral (right) eye stimulation. Stereotaxic boundaries from the Franklin and Paxinos mouse brain atlas are overlaid in red (V1M) and green (V1B). B, Overlay of mapped imaged cells with cranial window (white region) and boundaries of respective cortical regions defined. Border cells (<130 μm from the nearest border) are denoted with black circles. Nonborder cells are denoted with magenta circles. C, Example of branch tip bias index calculation of a border cell (black arrow in B) and a nonborder cell (magenta arrow in B). Red dotted line indicates bisecting plane parallel to nearest boundary. Red arrows indicate dynamic branch tips. D, Quantification of branch tip bias for stable and dynamic branch tips in border and nonborder cells (border cells, n = 16; nonborder cells, n = 37) (**p < 0.002, *p < 0.005). Error bars, SEM.