Cortical plasticity induced by inhibitory neuron transplantation

Abstract

Critical periods are times of pronounced brain plasticity. During a critical period in the postnatal development of the visual cortex, the occlusion of one eye triggers a rapid reorganization of neuronal responses, a process known as ocular dominance plasticity. We have shown that the transplantation of inhibitory neurons induces ocular dominance plasticity after the critical period. Transplanted inhibitory neurons receive excitatory synapses, make inhibitory synapses onto host cortical neurons, and promote plasticity when they reach a cellular age equivalent to that of endogenous inhibitory neurons during the normal critical period. These findings suggest that ocular dominance plasticity is regulated by the execution of a maturational program intrinsic to inhibitory neurons. By inducing plasticity, inhibitory neuron transplantation may facilitate brain repair.

Fig. 1

Ocular dominance plasticity induced by transplantation of inhibitory neuron precursors. (A) Experimental design. Cortical inhibitory neurons are produced from E12 to 16. Mouse ocular dominance plasticity peaks at P26 to 28, when inhibitory neurons are 33 to 35 days old. Inhibitory neuron precursors were transplanted from E13.5 to 14.5 donor embryos into host animals of two ages: P0 to 2 (diamonds) and P9 to 11 (squares). Ocular dominance plasticity was assessed 17 (purple), 25 to 27 (blue), 33 to 35 (green), and 43 to 46 (red) DAT by measuring visual responses to the two eyes before (open symbols) and after (solid symbols) 4 days of monocular deprivation (MD). Visual responses were quantified using an ODI. (B) Results of plasticity studies. During the critical period (P28) in untreated mice, monocular deprivation shifted responses toward the nondeprived eye (open versus filled black circles). In P42 to 46 host animals, 14 to 18 days after the critical period, monocular deprivation produced a strong shift in responses 33 to 35 DAT (solid green squares). However, 43 to 46 DAT, monocular deprivation produced a weaker shift in responses (solid red diamonds). In P33 to 37 host animals, 5 to 9 days after the critical period, monocular deprivation produced a stronger effect 33 to 35 DAT (solid green diamonds) than at 25 to 27 DAT (solid blue squares). Transplantation did not alter the effects of monocular deprivation during the critical period (solid purple squares), as compared with untreated controls (solid black circles; Mann-Whitney test, U = 1, P = 0.057). Monocular deprivation produced an insignificant effect at 33 to 35 DAT using cells from the LGE (U = 4, P = 0.343, solid green squares with red crosses). (C) An ocular dominance shift quantified the change in ODI produced by monocular deprivation. Thirty-three to 35 DAT (green) the shift was 2.5 times as high as at 25 to 27 DAT (blue; P < 0.05) and 2.2 times as high as at 43 to 46 DAT (red; P < 0.05, Kruskal-Wallis test; H = 8.6 with Dunn’s post test). The shift observed 33 to 35 DAT (green) was 77% of that observed in untreated animals during the critical period (black). Error bars represent SEM.

Fig. 2

Transplanted MGE cells migrate into binocular visual cortex and develop properties of mature inhibitory neurons. (A) Host cortex labeled for transplanted DsRed (red) and host GAD67-GFP (green) inhibitory neurons. DiI injections (red; dotted yellow line for emphasis) labeled the boundaries of binocular visual cortex, as identified by intrinsic signal imaging. Images are oriented with dorsal at top and lateral at left. (B) Detailed view of transplanted DsRed cell morphology. (C) To characterize the expression of inhibitory neuron molecular markers in the transplanted population, E13.5 to 14.5 MGE cells expressing GFP under the beta-actin promoter were transplanted into P9 to 11 wild-type hosts. Transplanted cells were immunostained for GFP (green), calbindin (CB), calretinin (CR), neuropeptide Y (NPY), parvalbumin (PV), or somatostatin (SOM). White chevrons identify colabeled cells. (D) Quantification of marker expression (N = 6 animals). Error bars represent SEM. Scale bars: 250 μm (A), 50 μm [(B) and (C)].

Fig. 3

Transplanted inhibitory neurons make and receive numerous, weak synaptic connections. Whole-cell recordings made from fluorescent transplanted inhibitory cells (I_trans, red), host inhibitory cells (I_host, green), and nonfluorescent putative pre- and postsynaptic pyramidal cell partners (E_host). Loose-patch stimulation was sometimes used to trigger presynaptic cells. (A) Whole-cell recording from a transplanted inhibitory cell. Loose-patch stimulation of a presynaptic pyramidal neuron evoked EPSPs in the transplanted cell. Transient depolarization of the transplanted neuron evoked IPSPs in a different postsynaptic pyramidal cell. Scale bars: 25 ms, 90 mV (presynaptic); 25 ms, 0.125 mV (postsynaptic). (B) Recordings from pairs of host pyramidal and host inhibitory cells. (Top) Dual whole-cell recording of a presynaptic pyramidal cell and a postsynaptic I_host. (Bottom) Loose-patch stimulation of a presynaptic host inhibitory neuron and whole-cell recording of a postsynaptic pyramidal cell. Scale bars: 25 ms, 40 mV (presynaptic); 25 ms, 0.25 mV (postsynaptic). (C) Connection probabilities for host-transplanted (red) and host-host (green) cell pairs. Transplanted inhibitory neurons made and received about three times as many connections as host inhibitory neurons (P < 0.05; two-tailed Fisher’s exact test). (D) Connection strengths for all connected host-transplant and host-host cell pairs. Inhibitory and excitatory synapses made and received by transplanted cells were about one-third as strong as synapses of host inhibitory cells (P < 0.05).