GABAergic projections from the hippocampus to the retrosplenial cortex in the rat

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Abstract
The retrosplenial cortex (RS) in rats has been implicated in a wide range of behaviors, including spatial navigation and memory. Relevant to this, the RS is closely interconnected with the hippocampus by multiple direct and indirect routes. Here, by injecting the retrograde tracer cholera toxin subunit B conjugated with Alexa488 (CTB‐Alexa488) in the granular retrosplenial cortex (GRS), we demonstrate a moderately dense non‐pyramidal projection from CA1. Neurons are in several layers, but mainly (about 65%) at the border of the stratum radiatum (SR) and stratum lacunosum moleculare (SLM). In particular, by double‐labeling with GAD67 or γ‐aminobutyric acid (GABA), we establish that these neurons are GABAergic. Further immunocytochemical screening for calcium‐binding proteins, somatostatin (SS) or cholecystokinin (CCK) failed to identify additional neurochemical subgroups; but a small subset (about 14%) is positive for the m2 muscarinic acetylcholine receptor (M2R). Terminations target layer 1 of the GRS, as shown by biotinylated dextran amine (BDA) injections into CA1 and confirmed by a very superficial injection of CTB‐Alexa488 in GRS. The superficial injection shows that there is a sparse GABAergic projection from the subiculum to layer 1 of the GRS, in addition to the dense excitatory connections to layer 3. The role of these dual inhibitory–excitatory pathways − within the subiculum, and in parallel from CA1 and the subiculum − remains to be determined, but may be related to synchronized oscillatory activity in the hippocampal complex and GRS, or to the generation of rhythmic activity within the GRS.