Exploring cortical information processing using optogenetics

Abstract

Optogenetics provides a powerful new tool for studying the way neurons integrate the inputs they receive, and how this shapes information processing in the brain. In this thesis we apply this new tool to investigate the mechanisms governing information processing in the somatosensory cortex of the mouse. The first aspect of this study characterised the expression and activation of channelrhodopsin-2 (ChR2) in somatosensory barrel cortex of a widely used transgenic mouse line where ChR2 expression is driven by the Thy-1 promoter. Within somatosensory barrel cortex ChR2 expression was found in 96% of layer V and 72% of layer II/III pyramidal neurons. Expression of ChR2 was also found in the majority of fast-spiking interneurons in both layer II/III and V (78% and 86% respectively). This study shows that ChR2 expression is not limited to layer V pyramidal neurons in this mouse model as is commonly assumed. The second aspect of this study developed a spatiotemporally precise method for optogenetic stimulation based on a projector-based optical interface. As proof-of-principle, three studies were conducted to assess the application of this method. Firstly, subcellular dendritic photo-stimulation was used to investigate the somatic impact of distal dendritic inputs in cortical pyramidal neurons. Secondly, this method was applied to mapping the spatial distribution of synaptic inputs to neurons in layer II/III and IV within somatosensory barrel cortex. We demonstrate that layer II/III pyramidal cells receive the majority of their excitatory synaptic input from layer IV and II/III, with fewer and weaker inputs arising from layer V within the same barrel. Interneurons in layer II/III also received extensive excitatory synaptic input from layer IV in addition to surrounding layer II/III cells. The vertical profile of excitatory input to layer IV neurons indicated stronger input from layer IV and layer V than from layer II/III within the same barrel. Similar intra-columnar input profiles were also seen in interneurons residing in layer IV, where inputs were more numerous and stronger from layer IV than from layer II/III or layer V. Thirdly, using a thalamocortical slice we show that thalamocortical inputs arising from the thalamus have significantly higher connection probability and input strength in layer IV compared layer II/III and V. The final aspect of this study investigated how inhibition targeted to somatic versus dendritic compartments shapes the firing output of layer V pyramidal neurons, and how the location of excitatory input influences this process. The findings presented illustrate that when excitatory drive is close to the soma both somatic and dendritic GABAA-mediated inhibition has a purely subtractive effect on input-output relationships, whereas subtractive and divisive effects are observed during dendritic excitatory input. Somatic inhibition produces significantly more subtractive effects when excitation is driven somatically, whereas more divisive effects are observed during dendritic excitatory input. Together these findings suggest that both somatic and dendritic inhibition can generate divisive gain control provided excitatory drive is concentrated in dendritic compartments.