Subtypes of Striatal Neurons. The striatum is a network of numerous morphologically and chemically distinct neuron subtypes. Inputs to the striatum converge on MSN, which are also the sole projection neurons of the striatum. The MSNs have a 10-20 µm diameter soma from which extend 7-10 moderately branched dendrites that are densely populated with spines. They express GABA as their neurotransmitter and are the major striatal cell type making up about 96% of striatal neurons (Aihara et al., 2001; Xxxxxxxxx et al., 2000; Kawaguchi, 1997). The rest ~4% of the striatal cell population is composed of interneurons: parvalbumin (PV) positive leptodendritic GABAergic neurons (2%), large cholinergic interneurons (1%), GABAergic calretinin-positive (1%), and medium-sized neurons containing somatostatin (SS), neuropeptide Y (NPY), NADPH-diaphorase (or nitric oxide synthase, NOS) and possibly GABA (1%) (Xxxxx et al., 1989; Xxxxxxxxx et al., 1998; Xxxxxxxx et al., 2005). PV-containing interneurons are categorized as fast-spiking cells (FS-cells) that receive strong excitatory inputs from the cerebral cortex (Lapper et al., 1992; Xxxxxxxxxx et al., 2002) and thalamus (Sidibe and Xxxxx, 1999) and primarily innervate the somata and dendrites of MSNs, forming asymmetric synapses, and producing substantial inhibition of MSNs (Xxxxxxx and Bolam, 1994; Kita, 1993; Kubota and Kawaguchi, 2000; Xxxxxx and Xxxxx, 2004). Striatal cholinergic cells are large (20- 50 µm diameter) aspiny interneurons that produces acetylcholine (ACh) as their neurotransmitter (Xxxxxxxxx et al., 2000). Cholinergic cells are classified as long-lasting afterhyperpolarization cells or tonically active neurons (TANs); this property provides a long afterhyperpolarization that is followed by a firing of single spikes. In vivo, these neurones fire tonically at a frequency of 2–10Hz. The majority of excitatory inputs to cholinergic interneurons are derived from the thalamus, particularly the parafascicular nucleus (Xxxxxxx and Xxxxxx, 1998; Lapper et al., 1992; Xxxxxx et al., 1990). Activation of cholinergic interneurons by thalamic afferents results in a pause and rebound facilitation in tonic activity of TANs. Thalamic Pf afferents can also indirectly inhibit TANs by innervating striatal MSNs, which, in turn inhibit cholinergic interneurons xxx XXXX xxxxxxx (Xxxxx xx xx., 0000; Zackheim and Abercrombie, 2005). During Pf activation, striatal acetylcholine levels decrease via inhibition of interneurons by the inhibitory MSN. However if MSN...
