Researchers have found that brain circuits are hijacked, leading to increasingly rigid and repetitive behavior. The brain circuit normally involved in selecting natural mouth and facial movements, but which is disproportionately taxed when behavior declines with repeated cocaine exposure.
The new study by researchers at the Hebrew University of Jerusalem found that the striatum is a brain region involved in selecting moment-to-moment actions.
The team recorded the behavior as observers
The team confirmed that disruption in the ability to define actions is associated with the development of repetitive and rigid/maladaptive behaviors seen in a range of neurological and psychiatric disorders, including Tourette syndrome, Parkinson’s disease and drug-induced movement disorders.
“We wanted to capture the behavior as an observer actually sees it: grooming, licking, exploring, but that was impossible to capture manually,” said Itay Shalom, a graduate student at the Hebrew University of Jerusalem. “STEREO allowed us to track the entire behavioral repertoire, which became increasingly narrowed until one type of action became dominant.”
Repeated exposure to cocaine
Using repeated cocaine exposure as an experimental model of behavioral rigidity, the researchers observed a striking change. Diverse and exploratory behavior gradually gave way to sustained repetition. On the fifth day of exposure, licking of the floor and walls of the enclosure accounted for more than 60% of the time observed. According to a press release, these behaviors have rarely been observed in the absence of cocaine, showing how a normally disparate set of actions can be narrowed down to a persistent pattern of behavior.
The team also attributed this change to the ventrolateral striatum (VLS), an area of the striatum that is particularly involved in movements of the mouth and tongue. Within this region there are two major neural pathways, known as direct and indirect pathways, which appear to have opposing influences on which actions are expressed.
When the researchers activated neurons in the indirect pathway, the cocaine-induced repetitive behavior was immediately interrupted and the behavior shifted toward alternative actions. Once the stimulation was stopped, the effect quickly reversed and repetitive behavior returned. Suppressing this pathway had the opposite result: it limited the ability to switch behavior and led to longer periods of repetitive behavior, the press release says.
The research team found that the direct route showed the opposite pattern. Reducing its activity attenuated drug-induced repetitive behavior, while activation in the absence of cocaine alone was sufficient to produce rigid patterns of repetitive actions similar to those observed after drug exposure.
“Cocaine doesn’t seem to create a whole new behavioral program,” said Ben Jerry Gonzales, a student at Hebrew University. “It takes control of a circuit that the brain already uses for natural actions and drives behavior toward constant repetition.”
The team also pointed out that the same circuit is responsible for the expression of normal, context-appropriate grooming and licking. Cocaine therefore appears to hijack a normal action selection system rather than recruiting a circuit focused on abnormal behavior.
The results may help researchers better understand how repetitive motor behaviors arise in various neurological and psychiatric diseases. Because different areas of the striatum correspond to different types of movement, the researchers suggest that similar imbalances in other striatal circuits could potentially contribute to other forms of behavioral rigidity, the press release says.
