In vivo optogenetic tracing of functional corticocortical connections between motor forelimb areas

Riichiro Hira,Haruo Kasai,Yasuhiro R Tanaka,Yoshito Masamizu,Fuki Ohkubo,George J Augustine,Masanori Matsuzaki

doi:10.3389/fncir.2013.00055

Abstract

Interactions between distinct motor cortical areas are essential for coordinated motor behaviors. In rodents, the motor cortical forelimb areas are divided into at least two distinct areas: the rostral forelimb area (RFA) and the caudal forelimb area (CFA). The RFA is thought to be an equivalent of the premotor cortex (PM) in primates, whereas the CFA is believed to be an equivalent of the primary motor cortex. Although reciprocal connections between the RFA and the CFA have been anatomically identified in rats, it is unknown whether there are functional connections between these areas that can induce postsynaptic spikes. In this study, we used an in vivo Channelrhodopsin-2 (ChR2) photostimulation method to trace the functional connections between the mouse RFA and CFA. Simultaneous electrical recordings were utilized to detect spiking activities induced by synaptic inputs originating from photostimulated areas. This method, in combination with anatomical tracing, demonstrated that the RFA receives strong functional projections from layer 2/3 and/or layer 5a, but not from layer 5b (L5b), of the CFA. Further, the CFA receives strong projections from L5b neurons of the RFA. The onset latency of electrical responses evoked in remote areas upon photostimulation of the other areas was approximately 10 ms, which is consistent with the synaptic connectivity between these areas. Our results suggest that neuronal activities in the RFA and the CFA during movements are formed through asymmetric reciprocal connections.

Highlights

The coordinated activities of motor cortex network are thought to be essential for elaborate movements
The rostral area was approximately 2.5 mm rostral and 0.8 mm lateral from the bregma (RFA), and the caudal area was approximately 0.2 mm rostral and 1.2 mm lateral from the bregma (CFA) (Figures 1A,B). This is consistent with our previous study, which used photostimulation mapping in ChR2 transgenic mice expressing a high level of ChR2 in pyramidal neurons in layer 5b (L5b) (ChR2 Tg mice; Figure 2A; Wang et al, 2007; Hira et al, 2013)
OPTOGENETIC TRACINGS OF FUNCTIONAL CONNECTIONS In this study, we used an optogenetic mapping method to investigate the connectivity between the rostral forelimb area (RFA) and the caudal forelimb area (CFA) in mice

Summary

Introduction

The coordinated activities of motor cortex network are thought to be essential for elaborate movements. Whether activation of the RFA induces the CFA activity, or vice versa, has not yet been examined Both we and another group reported that scanning of light beams across the surface of the cortex can induce cortical activity around the photostimulated points in transgenic mice that express ChR2 mainly in layer 5b (L5b) pyramidal neurons (Ayling et al, 2009; Hira et al, 2009). With this optogenetic approach, the motor areas where photostimulation induces limb movements can be rapidly and reproducibly mapped.

Methods

Results

Conclusion