Analysis of Neuronal Microcircuits in L2/3 of Mouse Barrel Cortex

Neocortical microcircuits are thought to contribute importantly to the sophisticated processing abilities of the mammalian brain. Neurons within the neocortex communicate with each other through releasing the excitatory neurotransmitter glutamate or the inhibitory neurotransmitter GABA. In order to begin to understand the functional operation of the neocortex, it will clearly be important to establish the basic organizing principles of synaptic connectivity between different classes of cortical neurons. Here I specifically investigate excitatory and GABAergic neuronal networks within layer 2/3 of mouse somatosensory cortex in vitro. In order to analyze synaptic connectivity I made multiple simultaneous whole-cell recordings from GFP expressing GABAergic neurons and from GFP-negative pyramidal neurons targeted by two-photon microscopy. Additionally, I express channelrhodopsin-2 (ChR2), a light-activated cation channel, in genetically defined populations of excitatory neurons which allows optically control of neuronal networks with millisecond precision. I show that optically evoked, near synchronous action potentials in excitatory neurons are sufficient to drive rapid and reliable disynaptic local inhibition onto neighboring pyramidal neurons. Local inhibition was mediated by parvalbumin expressing, fast-spiking (FS) GABAergic neurons, which responded to excitatory network input with significantly larger amplitude depolarizing PSPs compared to both non-fast-spiking (NFS) GABAergic and excitatory pyramidal neurons. The strong excitatory drive onto FS neurons can partially be explained with the finding that pyramidal neurons in cortical layer 2/3 synaptically connect to FS neurons with a higher probabilities and larger unitary EPSPs. Furthermore, FS neurons provided strong unitary inhibitory feedback onto pyramidal neurons and other FS neurons in layer 2/3. These results suggest an important role of FS GABAergic neurons in mediating strong local inhibition to control the extent of excitatory layer 2/3 network activity.