Abstract
The in vitro thalamocortical slice preparation of mouse barrel cortex allows for stimulation of the cortex through its natural afferent thalamocortical pathway. This preparation was used here to investigate the first stage of cortical processing in the large postsynaptic dendritic networks as revealed by voltage sensitive dye imaging (VSDI). We identified the precise location and dimensions of two clearly distinguishable dendritic networks, one in the granular layer (GL) IV and one in the infragranular layer (IGL) V and VI and showed that they have different physiological properties. DiI fluorescent staining further revealed that thalamocortical axons project on to these two networks in the typical barrel like form, not only in the granular but also in the IGL. Finally we investigated the short-term dynamics of both the VSDI signal and the local field potential (LFP) in response to a train of eight-pulses at various frequencies in both these layers. We found evidence of differences in the plasticity between the first two response peaks compared to the remaining six peaks as well as differences in short-term plasticity between the VSDI response and the LFP. Our findings suggest, that at least early cortical processing takes place in two separate dendritic networks that may stand at the beginning of further parallel computation. The detailed characterization of the parameters of these networks may provide tools for further research into the complex dynamics of large dendritic networks and their role in cortical computation.
Highlights
In vitro, stimulation of the thalamocortical afferents, even at high stimulus amplitudes, leads to voltage sensitive dye imaging (VSDI) activity confined to two layers only in the rodent primary somatosensory whisker cortex (S1, Higashi et al, 1999; Laaris et al, 2000; Itami et al, 2001; Laaris and Keller, 2002; Llinás et al, 2002; Urbano et al, 2007)
A brighter and longer band at the border of layer V/VI and a weaker stained, shorter band in layer IV, resembling the two layers of thalamocortical synapses that reach from the VPM into S1
These barrels were more clearly delineated in the granular layer (GL), but could be discriminated in the infragranular layer (IGL) (n = 3, Figures 1A,B)
Summary
Stimulation of the thalamocortical afferents, even at high stimulus amplitudes, leads to voltage sensitive dye imaging (VSDI) activity confined to two layers only in the rodent primary somatosensory whisker cortex (S1, Higashi et al, 1999; Laaris et al, 2000; Itami et al, 2001; Laaris and Keller, 2002; Llinás et al, 2002; Urbano et al, 2007) This spatial confinement is in part due to the fact that in brain slice recordings in vitro, polysynaptic propagation and general background neuronal activity is attenuated because of the large number of severed neuronal projections and the absence of neuromodulators (Hájos and Mody, 2009). Groups of these inhibitory neurons can inhibit each other and are interconnected by gap junctions, resulting in highly synchronized, amplified and locally confined feedforward and feedback inhibitory networks (Gibson et al, 1999; Porter et al, 2001; Beierlein et al, 2003; Tan et al, 2008)
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