A dense cluster of D₁ + cells in the mouse nucleus accumbens

The striatum is known to be largely composed of intermingled medium-sized projection neurons expressing either the D₁ or the D₂ dopamine receptors. In the present study, we took advantage of the double BAC Drd1a-TdTomato/Drd2-GFP (D₁ /D₂ ) transgenic mice to reveal the presence of a peculiar cluster of densely-packed D₁ + cells located in the shell compartment of the nucleus accumbens. This spherical cluster has a diameter of 110 µm and is exclusively composed by D₁ + cells, which are all immunoreactive for the neuronal nuclear marker (NeuN). However, in contrast to other D₁ + or D₂ + striatal cells, those that form the accumbens cluster are devoid of calbindin (CB) and DARPP-32, two faithful markers for striatal projection neurons. Using GAD-GFP transgenic mice, we confirm the GABAergic nature of the D₁ + clustered neurons. Intracellular injections from fixed brain slices indicate that these neurons are endowed with distinctive morphological features, including a small (5-6 µm), round cell body giving rise to a single primary dendrite that branches into two secondary processes. Single-neuronal injections combined to electron microscopy reveal the existence of GAP junctions linking these D₁ + cells. Based on their location, morphological characteristics and neurochemical phenotype, we conclude that the D₁ + accumbens cluster form a highly compact group of small neurons distinct from the larger and more diffusely distributed D₁ + or D₂ + striatal projection neurons that surround it. This remarkable nucleus might play a crucial role in the limbic function of the murine striatum.