Role of tonically-active neurons in the control of striatal function: Cellular mechanisms and behavioral correlates

1. The striatum is primarily involved in motor planning and motor learning. Human diseases involving its complex circuitry lead to movement disorders such as Parkinson's disease (PD) and Huntington's disease (HD). Moreover th e striatum has been involved in processes linked to reward, cognition and d rug addiction. 2. The high content of acetylcholine (ACh) found in the striatum is due to the presence of cholinergic interneurons. The intrinsic electrical and syna ptic properties of these interneurons have been recently characterized. How ever, their functional significance is far from being fully elucidated. 3. In vivo electrophysiological experiments from behaving monkeys have iden tified these cholinergic interneurons as "Tonically Active Neurons" (TANs). They are activated by presentation of sensory stimuli of behavioral signif icance or linked to reward. 4. Experimental evidence showed that integrity of the nigrostriatal dopamin ergic system is essential for TANs to express learned activity. 5. PD is known to be due to the loss of the nigrostriatal dopaminergic path way and the ensuing imbalance between the content of dopamine and acetylcho line in the striatum. This evidence supports the hypothesis that cholinergi c interneurons, or TANs, play a key role in the modulation of striatal func tion.