Response inhibition and the cortico-striatal circuit
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The ability to flexibly control or inhibit unwanted actions is critical for everyday behavior. Lack of this capacity is characteristic of numerous psychiatric diseases including attention deficit hyperactivity disorder (ADHD). My project is designed to study the neural underpinnings of response inhibition and to what extent these mechanisms are disrupted in animals with impaired impulse control. I therefore recorded single neurons from dorsal striatum, orbitofrontal cortex, and medial prefrontal cortex from rats performing a novel rodent variant of the classic "stop signal" task used in clinical settings. This task asks motivated rats to repeatedly produce simple actions to obtain rewards while needing to semi-occasionally inhibit an already initiated response. To take this a step further, I compared normal rats to rats prenatally exposed to nicotine in order to better understand the mechanism underlying inhibitory control. Rats exposed to nicotine before birth show abnormal attention, poor inhibitory control, and brain deficits consistent with impairments seen in humans prenatally exposed to nicotine and those with ADHD.
I found that dorsal striatum neurons tend to encode the direction of a response and the motor refinement necessary to guide behaviors within the task rather than playing a causal role in response inhibition. However the orbitofrontal cortex, a direct afferent of dorsal striatum, possesses the capacity to inform the striatum of the correct action during response inhibition within the critical time window required to flexibly alter an initiated movement. On the other hand, medial prefrontal cortex functions as a conflict “monitor” to broadly increase preparedness for flexible response inhibition by aggregating current and past conflict history. Lastly, rat pups exposed to nicotine during gestation exhibit faster movement speeds and reduced capacity for inhibitory behavior. Physiologically, prenatal nicotine exposure manifests in a hypoactive prefrontal cortex, diminished encoding of task parameters, and reduced capacity to maintain conflict information.