Functional roles of neurotransmitters and neuromodulators in the dorsal striatum

Abstract

The dorsal striatum, with its functional microcircuits galore, serves as the primary gateway of the basal ganglia and is known to play a key role in implicit learning. Initially, excitatory inputs from the cortex and thalamus arrive on the direct and indirect pathways, where the precise flow of information is then regulated by local GABAergic interneurons. The balance of excitatory and inhibitory transmission in the dorsal striatum is modulated by neuromodulators such as dopamine and acetylcholine. Under pathophysiological states in the dorsal striatum, an alteration in excitatory and inhibitory transmission may underlie dysfunctional motor control. Here, we review the cellular connections and modulation of striatal microcircuits and propose that modulating the excitatory and inhibitory balance in synaptic transmission of the dorsal striatum is important for regulating locomotion.The dorsal striatum is best known for its role in decision-making, especially in action selection and initiation through the convergence of sensorimotor, cognitive, and motivational information (DeLong 1990;Smith et al. 1998;Balleine et al. 2007). As the primary input of the basal ganglia, the striatum receives glutamatergic inputs from the cortex and thalamus and in turn projects GABAergic outputs to the globus pallidus and substantia nigra pars reticulata (SNr). Inputs from the cortex and thalamus both form excitatory synaptic connections on medium spiny neurons (MSN) in which cortical afferents are from the sensory, motor, and associational cortex (Bolam et al. 2000), and thalamic afferents are from the intralaminar thalamic nuclei (Doig et al. 2010). These glutamatergic inputs are then processed in the dorsal striatum where numerous connections between various types of neurons exist. Thus, the complexity of neuronal circuits has made it difficult to elucidate the functional roles of the striatum. Recently, studies focused on interneurons that reside in the dorsal striatum have characterized the physiological features and functional connections. For example, parvalbumin-expressing fastspiking interneurons (PV-FSI) and neuropeptide-Y positive lowthreshold spiking interneurons (NPY-LTS) form synaptic connections with MSNs and regulate the firing activity of the principal neuron MSNs (Koos and Tepper 1999;Gittis et al. 2010;Chuhma et al. 2011). These interneurons were shown to have distinct firing patterns and connections, and thus they may exert different effects on MSNs. Other crucial connections are the cholinergic, dopaminergic, and serotonergic axons that strongly innervate the dorsal striatum. These projections are essential for modulating striatal circuits and disruption of such signaling can result in movement impairments and neurological disorders such as Huntington's disease (Lovinger 2010). This review summarizes recent reports of the microcircuits present in the dorsal striatum, although serotonergic signaling is excluded, and suggests a putative role for striatal microcircuits in motor dysfunction and/or hyperactivity that ...

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