Axonal and Dendritic Morphology of Excitatory Neurons in Layer 2/3 Mouse Barrel Cortex Imaged Through Whole-Brain Two-Photon Tomography and Registered to a Digital Brain Atlas

Communication between cortical areas contributes importantly to sensory perception and cognition. On the millisecond time scale, information is signaled from one brain area to another by action potentials propagating across long-range axonal arborizations. Here, we develop and test methodology for imaging and annotating the brain-wide axonal arborizations of individual excitatory layer 2/3 neurons in mouse barrel cortex through single-cell electroporation and two-photon serial section tomography followed by registration to a digital brain atlas. Each neuron had an extensive local axon within the barrel cortex. In addition, individual neurons innervated subsets of secondary somatosensory cortex; primary somatosensory cortex for upper limb, trunk, and lower limb; primary and secondary motor cortex; visual and auditory cortical regions; dorsolateral striatum; and various fiber bundles. In the future, it will be important to assess if the diversity of axonal projections across individual layer 2/3 mouse barrel cortex neurons is accompanied by functional differences in their activity patterns.

Membrane Potential Dynamics of Neocortical Projection Neurons Driving Target-Specific Signals

Yves Kremer, Aurélie Pala, Leticia Pedrido, Carl Petersen, Egbert Welker, Takayuki Yamashita

Primary sensory cortex discriminates incoming sensory information and generates multiple processing streams toward other cortical areas. However, the underlying cellular mechanisms remain unknown. Here, by making whole-cell recordings in primary somatosensory barrel cortex (Si) of behaving mice, we show that S1 neurons projecting to primary motor cortex (M1) and those projecting to secondary somatosensory cortex (S2) have distinct intrinsic membrane properties and exhibit markedly different membrane potential dynamics during behavior. Passive tactile stimulation evoked faster and larger postsynaptic potentials (PSPs) in M1-projecting neurons, rapidly driving phasic action potential firing, well-suited for stimulus detection. Repetitive active touch evoked strongly depressing PSPs and only transient firing in M1-projecting neurons. In contrast, PSP summation allowed S2-projecting neurons to robustly signal sensory information accumulated during repetitive touch, useful for encoding object features. Thus, target-specific transformation of sensory-evoked synaptic potentials by Si projection neurons generates functionally distinct output signals for sensorimotor coordination and sensory perception.

Cell Press2013

Target-specific membrane potential dynamics of neocortical projection neurons during goal-directed behavior

Carl Petersen, Takayuki Yamashita

Goal-directed behavior involves distributed neuronal circuits in the mammalian brain, including diverse regions of neocortex. However, the cellular basis of long-range cortico-cortical signaling during goal-directed behavior is poorly understood. Here, we recorded membrane potential of excitatory layer 2/3 pyramidal neurons in primary somatosensory barrel cortex (S1) projecting to either primary motor cortex (M1) or secondary somatosensory cortex (S2) during a whisker detection task, in which thirsty mice learn to lick for water reward in response to a whisker deflection. Whisker stimulation in 'Good performer' mice, but not 'Naive' mice, evoked long-lasting biphasic depolarization correlated with task performance in S2-projecting (S2-p) neurons, but not M1-projecting (M1-p) neurons. Furthermore, S2-p neurons, but not M1-p neurons, became excited during spontaneous unrewarded licking in 'Good performer' mice, but not in 'Naive' mice. Thus, a learning-induced, projection-specific signal from S1 to S2 may contribute to goal-directed sensorimotor transformation of whisker sensation into licking motor output.

Elife Sciences Publications Ltd2016

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

Michael Avermann

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.

EPFL2011

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

Michael Avermann

EPFL2011