Microcircuits in the Striatum Striatal Cell Types and Their Interaction

The neostriatum is strategically located in the forebrain and receives inputs from all cortical areas. The complexity of the corticostriatal pathways suggest that striatal neurons are in a unique position to process convergent inputs from cortex and through basal ganglia output nuclei control subcortical nuclei and/or contribute to cortical dynamics via the thalamus. The most abundant neuron in the striatum, the GABAergic spiny projection neuron, has been at the center of research on how cortical inputs are evaluated at the striatal level. Because of the spiny neuron's numerous axon collaterals, the striatum has historically been viewed as a lateral inhibitory network, where cortical inputs compete at the striatal level for control of basal ganglia output. However, electrophysiological evidence for the existence of the recurrent feedback inhibition between spiny projection neurons has only been forthcoming during the last 2 years, and does not fit the relatively simple previously suggested role of a selection circuit. Similarly, striatal interneurons (e.g., the fast-spiking interneuron), despite their small numbers, have been increasingly recognized to play a dominant role for certain microcircuit aspects such as feed-forward inhibition and neuromodulation. Finally, the striatum, which receives the densest dopaminergic input in the brain, reveals a highly heterogeneous dopamine receptor composition suggesting that the striatal microcircuitry is modulated by these dopaminergic inputs in several different sways. In this chapter we review the basic neurocytology, neurophysiology and connections of the components of the neostriatum and how they interact to generate the neostriatal microcircuitry.