The striatum is the largest nucleus of the basal ganglia. In primates the striatum comprises the caudate nucleus and the putamen, and in all mammals, the ventral striatum or nucleus accumbens (Gerfen and Wilson 1996, Voorn et al. 2004). It receives direct input from most regions of the cerebral cortex and limbic structures including the amygdala and hippocampus. Additional input from sensorimotor and motivational regions of the brainstem arrives indirectly via relays in the thalamus. Finally, important modulatory afferents come from substantia nigra pars compacta (dopamine) and the raphe nuclei (serotonin) in the midbrain. The striatum is subdivided into sectors along a ventromedial-dorsolateral continuum largely on the basis of external connectivity (Voorn et al. 2004). All regions of the striatum are divided further into regions of patch/striosomes and matrix, again on the basis of differential connectivity and distribution of neurochemical markers (Gerfen and Wilson 1996). The principal cell type (representing >90% of all neurones) in all striatal regions is the GABAergic medium spiny neuron. Spiny neurons have been separated into two further populations according to which neuroactive peptide they contain (Substance P and dynorphin or enkephalin) and the relative proportions of D1- and D2-type dopamine receptors they express. Striatal medium spiny neurones are GABAergic providing inhibitory inputs to adjacent spiny neurones via local axon collaterals, to the globus pallidus (external), and to both basal ganglia output nuclei. The remaining 5-10% of neurones in the striatum (fewer in rodents, more in primates) are either GABAergic or cholinergic interneurons, which can be distinguished according to neurochemical and in some cases morphological characteristics (for precise details of relative numbers see - Tepper and Bolam 2004).