In addition to a smaller dendritic tree, the D1/D2 MSNs also contain less dendritic spines than D1 and D2 MSNs. The head of dendritic spines is the preferential synaptic contact site of glutamatergic corticostriatal projections arising from the cerebral cortex and the intralaminar thalamic nuclei56. With their less profuse dendritic arborization, the D1/D2 MSNs might receive significantly less glutamatergic input and thus be less vulnerable to excitotoxicity involved in different neuropathological conditions such as Huntington’s disease. More importantly in the context of the present study is the spatial distribution of DA terminals in contact with different parts of the MSNs. Those terminals that contact the cell body and proximal dendritic shafts might produce a relatively non-specific effect mediated by the volumic release of DA57. In contrast, as suggested Freund and colleagues58, the major DA input that occurs on the necks of dendritic spines is likely to be much more selective since it could prevent the excitatory glutamatergic input to the same spines from reaching the dendritic shaft. One of the main functions of striatal DA release might be to alter the pattern of firing of striatal output neurons by regulating their input58.
