Layer 4 of the rodent barrel cortex receives information of whisker stimulation. This area shows typical cortical column structure. Although previous studies have shown much evidence about the ascending circuits from whisker to barrel cortex, local circuits composed of GABAergic neurons remains to be investigated.
Parvalbumin immunopositive GABAergic neurons (PV neuron) constitute the major interneuron in the cerebral cortex and are involved in inhibitory regulation of excitatory projection neurons.
In the present study we observed detailed morphological properties of the PV neuron in layer 4 of the mouse barrel cortex using immunohistochemical methods.
Barrel was visualized by immunolabeling of vesicular glutamate transporter 2 (VGluT2), the thalamocortical input marker.
First we performed three-dimensional reconstructions of individual barrels and intervening septa based on serial 3.3 μm-thick optical slices in CLSM images by using Neurolucida software. Then dendrites of PV neurons were reconstructed and their positional relationship to barrels was analyzed. PV neurons were classified into four groups depending on their dendritic morphologies and somatic locations as follows:
Type1. All dendrites were within a single barrel
Type2. All dendrites were within the interbarrel septa
Type3. Dendrites crossed barrel/ septa border, soma was located in a barrel
Type4. Dendrites crossed barrel/ septa border, soma was located outside the barrels
Next we observed gap junctions between different types of PV neurons using triple immunolabeling of PV, VGluT2, and connexin36, the last being marker protein of the neuronal gap junctions. Type1 PV neurons formed dendrodendritic gap junctions with Type3 and Type4 neurons, but never formed with one another.