Quantitative activity-induced manganese-dependent MRI for characterizing cortical layers in the primary somatosensory cortex of the rat

The ability of Mn2+ to follow Ca2+ pathways upon stimulation transform them into remarkable surrogate markers of neuronal activity using activity-induced manganese-dependent MRI (AIM–MRI). In the present study, a precise follow-up of physiological parameters during MnCl2 and mannitol infusions improved the reproducibility of AIM–MRI allowing in-depth evaluation of the technique. Pixel-by-pixel T1 data were investigated using histogram distributions in the barrel cortex (BC) and the thalamus before and after Mn2+ infusion, after blood brain barrier opening and after BC activation. Mean BC T1 values dropped significantly upon trigeminal nerve (TGN) stimulation (−38 %, P = 0.02) in accordance with previous literature findings. T1 histogram distributions showed that 34 % of T1s in the range 600–1500 ms after Mn2+ + mannitol infusions shifted to 50–350 ms after TGN stimulation corresponding to a twofold increase of the percentage of pixels with the lowest T1s in BC. Moreover, T1 changes in response to stimulation increased significantly from superficial cortical layers (I–III) to deeper layers (V–VI). Cortical cytoarchitecture detection during a functional paradigm was performed extending the potential of AIM–MRI. Quantitative AIM–MRI could thus offer a means to interpret local neural activity across cortical layers while identification of the role of calcium dynamics in vivo during brain activation could play a key role in resolving neurovascular coupling mechanisms.

The Excitatory Neuronal Network of the C2 Barrel Column in Mouse Primary Somatosensory Cortex

Sandrine Lefort, Carl Petersen, Juan-Carlos Sarria, Christian Tomm

Local microcircuits within neocortical columns form key determinants of sensory processing. Here, we investigate the excitatory synaptic neuronal network of an anatomically defined cortical column, the C2 barrel column of mouse primary somatosensory cortex. This cortical column is known to process tactile information related to the C2 whisker. Through multiple simultaneous whole-cell recordings, we quantify connectivity maps between individual excitatory neurons located across all cortical layers of the C2 barrel column. Synaptic connectivity depended strongly upon somatic laminar location of both presynaptic and postsynaptic neurons, providing definitive evidence for layer-specific signaling pathways. The strongest excitatory influence upon the cortical column was provided by presynaptic layer 4 neurons. In all layers we found rare large-amplitude synaptic connections, which are likely to contribute strongly to reliable information processing. Our data set provides the first functional description of the excitatory synaptic wiring diagram of a physiologically relevant and anatomically well-defined cortical column at single-cell resolution.

2009

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

BOLD responses to trigeminal nerve stimulation

Rolf Gruetter, Nathalie Just, Carl Petersen

The current study investigates a new model of barrel cortex activation using stimulation of the infraorbital branch of the trigeminal nerve. A robust and reproducible activation of the rat barrel cortex was obtained following trigeminal nerve stimulation. Blood oxygen level-dependent (BOLD) effects were obtained in the primary somatosensory barrel cortex (S1BF), the secondary somatosensory cortex (S2) and the motor cortex. These cortical areas were reached from afferent pathways from the trigeminal ganglion, the trigeminal nuclei and thalamic nuclei from which neurons project their axons upon whisker stimulation. The maximum BOLD responses were obtained for a stimulus frequency of 1 Hz, a stimulus pulse width of 100 μs and for current intensities between 1.5 and 3 mA. The BOLD response was nonlinear as a function of frequency and current intensity. Additionally, modeling BOLD responses in the rat barrel cortex from separate cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO(2)) measurements showed good agreement with the shape and amplitude of measured BOLD responses as a function of stimulus frequency and will potentially allow to identify the sources of BOLD nonlinearities. Activation of the rat barrel cortex using trigeminal nerve stimulation will contribute to the interpretation of the BOLD signals from functional magnetic resonance imaging studies.

2010

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

Michael Avermann

EPFL2011