Abstract
Layer 4 of the sensory neocortex receives widespread convergent inputs from thalamic, intracortical, and corticocortical sources. Yet, the relative information bearing roles for most of these pathways remain largely undefined. Here we show that the intracortical projections from layer 6 to layer 4 exhibit a physiological property that is consistent with a modulator role. Using in vitro slice preparations of the auditory and somatosensory cortices, we found that electrical stimulation or photostimulation of layer 6 elicits a prolonged depolarizing response that is attributable to the activation of group 1 metabotropic glutamate receptors. These results complement the known physiological properties of the layer 6 to layer 4 pathway, and further suggest that this pathway is not a principle conduit for information flow, but rather acts as a modulator of cortical activity.
Highlights
The sensory neocortex is composed of six layers whose distinct patterns of connectivity underlie their functional properties
The group 1 metabotropic glutamate receptor (mGluR) have been found to correlate strongly with modulator synapses in the thalamus (Bartlett and Smith, 2002; Reichova and Sherman, 2004). It is this property that is of particular interest, since group 1 mGluRs have been reported on layer 4 neurons in the sensory neocortex (LópezBendito et al, 2002; Ryo et al, 1993; Stinehelfer et al, 2000), and may be activated postsynaptically by the intracortical pathway from layer 6
It is known that the thalamic inputs to layer 4, which exhibit driving synaptic properties, do not elicit a group 1 mGluR response, while the intracortical layer 6 inputs exhibit all of the other synaptic properties associated with modulator pathways (Lee and Sherman, 2008)
Summary
The sensory neocortex is composed of six layers whose distinct patterns of connectivity underlie their functional properties. A major challenge for understanding information processing in the sensory neocortex is to determine how information is transferred and transformed from the principal cortical input layers to the principal cortical output layers In this regard, multiple inputs converge onto layer 4 neurons of the sensory neocortex from thalamic (Sherman and Guillery, 2002), intracortical (Hirsch and Martinez, 2006) and corticocortical (Rockland and Pandya, 1979) sources. The group 1 mGluRs have been found to correlate strongly with modulator synapses in the thalamus (Bartlett and Smith, 2002; Reichova and Sherman, 2004) It is this property that is of particular interest, since group 1 mGluRs have been reported on layer 4 neurons in the sensory neocortex (LópezBendito et al, 2002; Ryo et al, 1993; Stinehelfer et al, 2000), and may be activated postsynaptically by the intracortical pathway from layer 6. Determining whether the layer 6 inputs to layer 4 elicit a group 1 mGluR response would help further to characterize its role in information processing
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