WASHINGTON – A new finding from researchers at Georgetown University Medical Center shows that the learning process of associating cues with rewards can be altered by increased or decreased activity of a specific protein in the brain. Knowing when to respond positively to cues that result in beneficial outcomes or rewards vs. ignoring cues that result in bad habits, such as smoking addiction, is an essential part of learned behaviors.
"Our ability to link certain cues or stimuli with positive or rewarding experiences is a basic brain process, and it is disrupted in many conditions such as addiction, depression, and schizophrenia," says Alexey Ostroumov, PhD, assistant professor in the Department of Pharmacology & Physiology at Georgetown University School of Medicine and senior author of the study. "For example, drug abuse can cause changes in the KCC2 protein that is crucial for normal learning. By interfering with this mechanism, addictive substances can hijack the learning process."
The National Institutes of Health (NIH)-funded study appears December 9 in the journal Nature Communications.
The investigators found that changes in the learning process can happen because of changes in KCC2. In what is an inverse relationship, diminished KCC2 levels can cause increased dopamine neuron firing, which in turn leads to new reward associations being formed. Dopamine neurons are specialized nerve cells that produce and release the neurotransmitter dopamine that is involved in reward, motivation, and motor control.
The researchers looked at rodent brain tissue and also observed the behaviors of lab rats in classic Pavlovian cue-reward experiments, where a signal such as a short sound notifies the rats that they will receive a sugar cube. Beyond just determining how accelerated or diminished neuron firings are related to changes in KCC2, the investigators also found that when neurons act in a synchronized way, they can unexpectedly increase dopamine firing activity. These brief dopamine bursts appear to be key signals that can help the brain assign value to shared learning experiences.
"Our findings help explain why powerful and unwanted associations form so easily, like when a smoker who always pairs morning coffee with a cigarette later finds that just drinking coffee triggers a strong craving to smoke," notes Ostroumov. "Preventing even relatively benign drug-induced associations with situations or places, or restoring healthy learning mechanisms, can help develop better treatments for addiction and related disorders."
The researchers also wanted to figure out if certain drugs that act directly on cellular receptors, such as calming drugs called benzodiazepines, could influence learning mechanisms. In previous studies, scientists found that changes in KCC2 production, and hence neuron activity, can alter how diazepam, better known as valium, produces its calming, inhibitory effects in the brain. This study highlights a new dimension in neuronal function: neurons don't just change their activity, they can also coordinate it, and when the activity is coordinated, the neurons can transmit information more efficiently. The researchers found that this coordination can be aided by drugs such as diazepam when they tested it in their experiments.
"To reach our conclusions, we combined many experimental approaches, including electrophysiology, pharmacology, fiber photometry, behavior, computational modeling, and molecular analyses," says the study's first author Joyce Woo, a PhD candidate in Ostroumov's lab.
She also noted that while many experiments use mice as their animal models, the research team used rats in the behavioral part of their investigations because rats tend to perform more reliably than mice on longer or more complex tasks; they usually do better on many parts of reward-learning experiments, which helped provide the investigators with more stable and consistent data.
"We believe these discoveries extend beyond basic learning research," says Ostroumov. "They reveal new ways the brain regulates communication between neurons. And because this communication can go wrong in different brain disorders, our hope is that by preempting these disruptions, or by fixing normal communication when it's impaired, we can help develop better treatments for a wide range of brain disorders."
