The development of alcoholism may involve a shift from goal-directed to habitual drinking. These action control systems are distinct in the dorsal striatum, with the dorsomedial striatum (DMS) important for goal-directed behavior and the dorsolateral striatum (DLS) required for habit formation.
Goal-directed behavior can be modeled in rats with a fixed ratio (FR) reinforcement schedule, while a variable interval (VI) schedule promotes habitual behavior (e.g. insensitivity to contingency degradation).
Using extracellular recordings from chronically implanted electrodes, we investigated how DMS and DLS neurons encoded lever-press responses and conditioned cues during operant alcohol self-administration in these two models.
In rats self-administering 10% alcohol on an FR schedule, the DMS neuronal population showed increased firing at the onset of start-of-session stimuli. During self-administration, the most prominent phasic firing patterns in the DMS occurred at the time of reinforcement and reinforcement-associated cues, while the most prominent phasic activity in the DLS surrounded the lever response.
Neural recordings from an additional cohort of rats trained on a VI schedule revealed a similar pattern of results; however, phasic changes in firing were smaller and differences between the medial and lateral dorsal striatum were less marked.
In summary, the DMS and DLS exhibited overlapping but specialized phasic firing patterns: DMS excitations were typically time-locked to reinforcement, while DLS excitations were generally associated with lever responses.
Furthermore, the regional specificities and magnitudes of phasic firing differed between reinforcement schedules, which may reflect differences in behavioral flexibility, reward expectancy and the action sequences required to procure reinforcement.
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