Parvalbumin positive dendrites co-localize with apical dendritic bundles in rat retrosplenial cortex

The granular retrosplenial cortex in rats, involved in learning and memory, has a highly modular organization in layer 1. Apical dendrites of layer 2 pyramidal neurons form bundles, which correspond to patchy thalamic input. Here, we demonstrate that dendrites of parvalbumin-immunoreactive (PV-ir) GABAergic neurons in layers 2/3 also form bundles, and that these co-localize in layer 1 with the apical dendritic bundles, as verified by double immunofluorescence for PV and microtuble-associated protein. Deeper, at the border of layers 1/2, the PV-ir bundles merge into a honeycomb-like structure, with walls consisting of PV-ir neuropil. Compartmentalization in layers 1/2 is characteristic of other periallocortical structures. Further work is necessary to determine whether these specializations may be specifically related to learning and memory.