drd2-cre:ribotag mouse line unravels the possible diversity of dopamine d2 receptor-expressing cells of the dorsal mouse hippocampus.

Increasing evidences suggest that dopamine facilitates the encoding of novel memories by the hippocampus. However, the role of dopamine D2 receptors (D2R) in such regulations remains elusive due to the lack of the precise identification of hippocampal D2R-expressing cells. To address this issue, mice expressing the ribosomal protein Rpl22 tagged with the hemagglutinin (HA) epitope were crossed with Drd2-Cre mice allowing the selective expression of HA in D2R-containing cells (Drd2-Cre:RiboTag mice). This new transgenic model revealed a more widespread pattern of D2R-expressing cells identified by HA immunoreactivity than the one initially reported in Drd2-EGFP mice, in which the hilar mossy cells were the main neuronal population detectable. In Drd2-Cre:RiboTag mice, scattered HA/GAD67-positive neurons were detected throughout the CA1/CA3 subfields, being preferentially localized in stratum oriens and stratum lacunosum-moleculare. At the cellular level, HA-labeled cells located in CA1/CA3 subfields co-localized with calcium-binding proteins (parvalbumin, calbindin, and calretinin), neuropeptides (neuropeptide Y, somatostatin), and other markers (neuronal nitric oxide synthase, mGluR1α, reelin, coupTFII, and potassium channel-interacting protein 1). These results suggest that in addition to the glutamatergic hilar mossy cells, D2R-expressing cells constitute a subpopulation of GABAergic hippocampal interneurons.