The dorsal striatum is located in the subcortical region of the forebrain. In primates and other mammals it is divided by the anterior limb of a white matter tract called the internal capsule, into two parts: the caudate nucleus and the putamen.  In rodents, the internal capsule is poorly developed such that the caudate and putamen are not separated but form one large entity called the caudate putamen (CPu). The majority (about 95%) of cells in the dorsal striatum are GABAergic medium spiny neurons (MSNs) also known as spiny projection neurons (SPNs). Approximately half of these MSNs contain dopamine D1 receptors and project directly to the substantia nigra to form the direct pathway of the basal ganglia, whereas the other half express dopamine D2 receptors that project indirectly to the substantia nigra via the globus pallidus and subthalamic nucleus to form the indirect pathway of the basal ganglia. The remaining 5% of cells are interneurons that are either cholinergic neurons, or one of several types of GABAergic neurons. The axons and dendrites of these interneurons stay within the striatum.