The striatum (both dorsal and ventral) contains several cell types that are generally divided into two general groups: projection neurons and interneurons. Projection neurons are the most common cell type and are the medium spiny neurons (MSN) or the principal neurons: in nonprimate species they account for over 90% of the cells, and probably far less in the primates.28,29 These cells project to both segments of the globus pallidus, the VP, and the SN21,22,30-32 Their axon collaterals also terminate within the striatum, onto other MSNs and interneurons forming symmetrical GABAergic synaptic contacts.33-35 On the basis of this data and electrophysiological data, the projection cells of the striatum are considered inhibitory.36 There are two general types of MSNs: one that co-contains substance P and GABA; and one that co-contains enkephalin and GABA. Substance P-containing MSNs project primarily to the GPi and SN, while the enkephalin-containing cells project primarily to the GPe. 37-38
