The Thalamostriatal Pathway and the Hierarchical Control of Action.

Sequential ordering of motor commands is required for the simplest of our daily activities. In this issue of Neuron, Díaz-Hernández et al. (2018) show that distinct thalamic inputs to different regions of the dorsal striatum critically modulate the initiation and execution of action sequences.

Aging-Related Dysfunction of Striatal Cholinergic Interneurons Produces Conflict in Action Selection.

For goal-directed action to remain adaptive, new strategies are required to accommodate environmental changes, a process for which parafascicular thalamic modulation of cholinergic interneurons in the striatum (PF-to-CIN) appears critical. In the elderly, however, previously acquired experience frequently interferes with new learning, yet the source of this effect has remained unexplored. Here, combining sophisticated behavioral designs, cell-specific manipulation, and extensive neuronal imaging, we investigated the involvement of the PF-to-CIN pathway in this process. We found functional alterations of this circuit in aged mice that were consistent with their incapacity to update initial goal-directed learning, resulting in faulty activation of projection neurons in the striatum. Toxicogenetic ablation of CINs in young mice reproduced these behavioral and neuronal defects, suggesting that age-related deficits in PF-to-CIN function reduce the ability of older individuals to resolve conflict between actions, likely contributing to impairments in adaptive goal-directed action and executive control in aging. VIDEO ABSTRACT.

The thalamostriatal pathway and cholinergic control of goal-directed action: interlacing new with existing learning in the striatum.

The capacity for goal-directed action depends on encoding specific action-outcome associations, a learning process mediated by the posterior dorsomedial striatum (pDMS). In a changing environment, plasticity has to remain flexible, requiring interference between new and existing learning to be minimized, yet it is not known how new and existing learning are interlaced in this way. Here we investigated the role of the thalamostriatal pathway linking the parafascicular thalamus (Pf) with cholinergic interneurons (CINs) in the pDMS in this process. Removing the excitatory input from Pf to the CINs was found to reduce the firing rate and intrinsic activity of these neurons and produced an enduring deficit in goal-directed learning after changes in the action-outcome contingency. Disconnection of the Pf-pDMS pathway produced similar behavioral effects. These data suggest that CINs reduce interference between new and existing learning, consistent with claims that the thalamostriatal pathway exerts state control over learning-related plasticity.