Corticostriatal Plasticity, Learning and Choice.

Abstract

The striatum, the entry point for a wide range of information into the basal ganglia, is perhaps the most critical information processing region in this brain system. In addition to playing a role in the facilitation and selection of many kinds of movement, the connections from the cortex to the dorsal striatum may also be a critical site for the learning and storage of a wide range of behavioral routines that allow us to perform well-learned complex actions without having to think through every step. Synaptic plasticity, a prevalent neural phenomenon, has been reported in the corticostriatal pathway and may be a neural correlate of the learning that takes place in the striatum. However, corticostriatal plasticity is a complex process and one that is inherently difficult to investigate in an intact corticostriatal network because of the broad connectivity of the basal ganglia system. Here we investigate the basic rules of this plasticity in the corticostriatal system in awake, freely behaving animals, and report that long term depression is the normal form of synaptic change in response to repeated activation of excitatory corticostriatal inputs. However, the outcome of plasticity is reversed when conducted under conditions of anesthesia, indicating that the state of the basal ganglia network can affect normal plasticity rules. Dopamine is also a critical factor in the processing of information in the striatum. Altering striatal dopamine signaling can direct synaptic plasticity in the corticostriatal pathway and block or enhance some forms of learning that are dependent upon the striatum. Dopamine signaling in the striatum may also affect the performance of actions by altering motivation selectively, or generally, in addition to regulating some aspects of the decision making. We investigated the role of striatal dopamine signaling in learning and choice and found that enhanced dopamine tone in a region of the striatum that is critical for selecting a contralaterally directed choice can broadly alter the threshold for action and invigorate all chosen actions. In contrast, strongly decreasing dopamine signaling can selectively affect choice and reaction time in a coordinated fashion that reflects the storage of task related information.