Hot Topics in Imaging Technology–TOPIM TECH 2019
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Abstract Lecture by Yi Chen, Tübingen

Session chair: Buzug, Thorsten, (Lübeck, Germany)
 
Date: Thursday, 4 July, 2019, 10:15 a.m.
Room: Aristotle
Session type: Abstract Lecture

Contents

10:15 a.m. -01

Integration of thalamocortical and callosal inputs by optogenetic activation of the rat corpus callosum (CC) with MRI-guided robotic arm (MgRA) (#2)

Yi Chen1, Filip Sobczak1, Patricia Pais1, Cornelius Schwarz2, Alan P. Koretsky3, Xin Yu1, 4

1 Max Planck Institute for Biological Cybernetics, Tübingen, Baden-Württemberg, Germany
2 Werner Reichardt Center for Integrative Neuroscience, Tübingen, Baden-Württemberg, Germany
3 National Institute of Neurological Disorders and Stroke, Bethesda, Maryland, United States of America
4 Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital and Harvard Medical School, Charlestown, Massachusetts, United States of America

Introduction

The hypothesis that CC inhibits contralateral cortex can explain dampened neural responses in cortex in human and rodents(1-4), e.g., the first stimulus suppressed neural responses to the subsequent stimulus on the other eye within a certain time. Here, we optogenetically activated CC(8) and provided direct evidence for CC-mediated interhemispheric inhibition(II), showing that the direct callosal inputs suppressed evoked calcium and BOLD signals in barrel cortex(BC) by whisker stimulation. Our work links callosal circuit-specific regulation to the global brain dynamic changes based on II(5-7).

Methods

AAV.CaMKII.ChR2.mCherry was injected into the BC of rats, expressed in callosal projection neurons (CPN) and along their axonal fiber bundles projecting to the opposite BC (Fig.1a), where the GCaMP6f was expressed (Fig.1f). Optogenetic stimulation will be delivered on corpus callosum, followed by a whisker stimulus to the whisker pad with different intervals (0-200 ms) and the paired conditions of each trail were randomized (Fig. 2c). Whole brain BOLD signals were acquired with simultaneous calcium signal in the BC while an MRI-guided robotic arm was used to precisely target the callosal fiber bundle to deliver blue light pulses (473nm) at 2Hz, 10ms width for  the fMRI block design (8s on/52s off,13 epochs,Fig.2b,c). Whole brain 3D EPI: TR,1.5s, 400×400×400 μm3 spatial resolution.

Results/Discussion

Upon the optogenetic stim on CC, salient BOLD signal was detected due to the antidromic activity from the axonal fibers backward to the soma of callosal projection neurons in the ipsilateral BC (Fig.1c,d), further confirmed by LFP (Fig.1e). For the orthodromic activity, there was clear spike for each stimulus at 2Hz, while with higher frequencies, light flashes 2-16 induced responses were consistently weaker than the first response (Fig.1g), Moreover, there was a baseline drift during the whole 40Hz stimulation period (Fig.1g), therefore, confirming the CC-mediated interhemispheric inhibition. With two stimuli paradigm, the anti-dromic activity in the right cortex kept similar for 6 conditions, while the BOLD and calcium signals in the left cortex induced by paired whisker stimuli was the strongest for OW condition, kept suppressed for the O50W and O100W conditions (Fig.2d-h), almost recovered for the O200W condition.

Conclusion

By taking advantage of fMRI, the optogenetic stimuli on CC and cell-specific calcium signal recordings for layer 5 pyramidal excitatory neurons in the BC, we confirmed the CC-mediated interhemispheric inhibition, further provided direct evidence for the dampened neural responses to subsequent contralateral stimulus after an ipsilateral stimulus for a period of several hundred milliseconds in human and rodents at function level.

Acknowledgement

We thank Mr. Shanyi Yu for building up the first prototype of the robotic arm, Fine Mechanic and Electronic Workshop at MPI for Biological Cybernetics for MgRA system automation. The financial support of the Max-Planck-Society and the China Scholarship Council (Ph.D. fellowship to Y. Chen) are gratefully acknowledged.

References

1.            Schnitzler A, Kessler KR, & Benecke R (1996) Transcallosally mediated inhibition of interneurons within human primary motor cortex. Exp Brain Res 112(3):381-391.

2.            Bocci T, et al. (2011) Transcallosal inhibition dampens neural responses to high contrast stimuli in human visual cortex. Neuroscience 187:43-51.

3.            Ogawa S, et al. (2000) An approach to probe some neural systems interaction by functional MRI at neural time scale down to milliseconds. Proc Natl Acad Sci U S A 97(20):11026-11031.

4.            Nemoto M, et al. (2012) Diversity of neural-hemodynamic relationships associated with differences in cortical processing during bilateral somatosensory activation in rats. Neuroimage 59(4):3325-3338.

5.            Palmer LM, et al. (2012) The cellular basis of GABA(B)-mediated interhemispheric inhibition. Science 335(6071):989-993.

6.            Kawaguchi Y (1992) Receptor subtypes involved in callosally-induced postsynaptic potentials in rat frontal agranular cortex in vitro. Exp Brain Res 88(1):33-40.

7.            Kumar SS & Huguenard JR (2001) Properties of excitatory synaptic connections mediated by the corpus callosum in the developing rat neocortex. J Neurophysiol 86(6):2973-2985.

8.           Yu X, et al. (2013) Targeting projection fibers for optogentics and fMRI.

Anti-dromic and orthodromic activation by corpus callosum optogenetic stimulation.

a Schematic of experimental design and CaMKII.mCherry expression.

b Overview of the MgRA for optical fiber insertion inside 14.1T scanner.

c Averaged fMRI map of brain-wide activity upon optogenetic stimulation on CC from 8 rats.

D Average time courses of BOLD in right BC(n = 8) upon light stimulation. Error bars represent mean±SD.

e The representative local field potential for antidromic activation.

f Schematic of experimental design and CaMKII.mCherry expressed in the right BC while GCaMP6f in the left BC.

g Representative calcium signal changes for 8 s of the orthodromic activation responses.

Simultaneous BOLD and calcium signals upon CC opto stim and whisker stim with varying intervals

a Stimulation scheme. 6 conditions: W, O, O50W, O100W, O200W.

b Experimental setup.

c Typical calcium signals for condition W(blue dotted box) and O100W(red dotted box).

d fMRI map of brain-wide activity for 6 conditions(n=6).

e Averaged normalized calcium signal in left BC.

f Normalized calcium signal for individual rat.

g Averaged BOLD in the left BC (left) evoked by whisker stimulation and right BC (right) evoked by CC stimulation.

h Averaged normalized calcium signal changes across 6 rats for different conditions.

i Averaged normalized calcium signal changes across 4 rats for different conditions.

Keywords: Corpus Callosum, Interhemispheric Inhibition, Thalamocortical Pathway