The output cells of the striatum are the medium spiny neurons (MSNs). Their firing is affected by the three major groups of interneurons: (1) fast spiking, parvalbumin (PV) expressing GABAergic interneurons, (2) burst firing, somatostatin/neuropeptide-Y (NPY)- releasing GABAergic interneurons and (3) slow tonically firing large aspiny cholinergic interneurons [1,7]. While the PV and NPY-expressing GABAergic interneurons exert a powerful inhibitory influence on MSNs [8,9], the function of cholinergic neurons is primarily modulatory and cannot be simply characterized as excitatory or inhibitory. The cholinergic interneurons in the striatum help regulate the duration, strength, and spatial pattern of striatal MSNs output.
