Pavlov’s Drug Dogs: The Neural Mechanism Underlying Drug-Environment Associations

Guest post by Alixandria Trombley

Maybe we’re not all familiar with the term operant conditioning, but it would be hard to find someone who doesn’t know about the age-old phenomenon of Pavlov’s dogs; stimuli, like the ding of a bell, become linked with a previously unrelated behavioral responses through repetition, like salivation. Operant conditioning occurs in countless aspects of our daily lives, and our brains form associations between the cues we experience and the behaviors that follow them. When it comes to drug abuse, operant conditioning plays a vital role in creating associations that can induce drug craving and seeking. In drug use, learned behaviors like drug-seeking and drug-taking can go awry and lead to drug abuse. Studying the associations that form between the many drug-related cues (like the image of a bar) and the drug-taking behavior (consuming alcohol) can give us insight into why addiction occurs.

Linking context and behavior through neuronal ensembles

For these links to develop, the brain must effectively discriminate between environmental stimuli that are irrelevant to the behavior and those that become involved in drug cues. But how exactly do these associations form in our brains? Contradictory to the longstanding theory that entire brain regions produce individual behaviors, recent studies suggest that sparse and distributed groups of neurons encode the associations between learned behaviors and their related cues. These groups of neurons are called neuronal ensembles because, like a group of musicians, they work together to produce one integrated product. Through their coordinated activation, these ensembles encode the associations that underlie drug seeking and taking.

Let’s illustrate this concept: Todd regularly takes cocaine. Specifically, Todd takes cocaine with his friends, Amanda and John, on Saturday nights at Amanda’s house. There are countless environmental and other stimuli present at that place and time. Still, only some of them are biologically relevant to the fact that Todd will be taking cocaine. For example, Amanda and John are likely necessary, drug-related cues that might make Todd think of cocaine or experience cravings when at Amanda’s house on a Saturday. However, if Todd runs into John without Amanda at the grocery store on a Wednesday, he likely wouldn’t experience the same cravings. Additionally, all the relevant stimuli at Amanda’s house on Saturday nights (the look and smell of her home, the music they play, the day and time, etc.) are components of the overall context that becomes associated with cocaine-taking. A group of neurons in Todd’s nucleus accumbens (a brain area involved in reward) recognizes that, when Todd is in this context, cocaine often follows. This group of neurons works together as an ensemble to form a relationship between the context and the cocaine-taking behavior. Because of this, when Todd is in the context, he may experience craving for cocaine or even seek out the drug.

Using animal models to understand addiction in the brain

This concept can also be applied to animal models, like rats. After days of repeatedly placing a rat in a chamber to self-administer cocaine, the rat eventually associates the contextual and other stimuli with the drug-taking behavior. To test the association, we can return the rat to the same context without cocaine present. If an association has formed, then the ensemble encoding the association between the chamber and the cocaine-taking becomes activated when the rat is exposed to the context, and the rat exhibits cocaine-seeking behavior. We can use rat models of drug-seeking and drug-taking to further explore the role of ensembles in addiction-related behavior, both in post-mortem brain slices and in live animals, by identifying the specific neurons that make up the ensemble.

Ensemble activation can be measured using markers of strong neuronal activity. One of these markers is Fos, a protein produced in the small subset of ensemble neurons that are most strongly activated. When Fos is expressed, it can be stained and measured in brain slices under a microscope, allowing us to see which neurons are activated after certain stimuli (like those found in a self-administration chamber). To make inferences about the behavior that an ensemble causes, we can selectively kill only the highly active cells (the ensemble) using new inactivation techniques called chemogenics. In one of these techniques, we inject a drug that is inactive in most cells. However, a protein that is expressed in only the highly active ensemble neurons metabolized the drug into one that is toxic. In this way, we can specifically kill an activated ensemble, while leaving the rest of the brain region intact. Killing only the ensemble responsible for the association between the behavior and its related cues abolishes the link such that the cues no longer induce the behavior. 

Let’s return to our example of a rat in a self-administration chamber to better understand this. Once the rat has developed the association of the context to the cocaine-taking, placing the rat in the chamber will activate the ensemble that is responsible for the association. By inactivating the ensemble, the association between the chamber and the cocaine-taking is dissolved, and the presence of the stimuli in the chamber no longer induces drug-seeking. If the behavior (drug-seeking) does not occur in the previously associated context, it can be inferred that the ensembles that were ablated were responsible for the association between the behavior and the cues. By this logic, ensemble-specific inactivation provides a tool to make causal inferences about the role of ensembles in encoding operant associations and can be generalized to other areas of the brain that are involved in countless other learned behaviors.

Where do we go from here?

Though we’re not quite ready to start identifying ensembles in humans, we can garner some really useful information from our animal models. Understanding which parts of the brain are active when something goes awry helps to illuminate solutions to the problem, and in the realm of drug abuse, it might be the first of many steps to eliminating addiction.

Neuronal ensembles encode associations between contextual cues and learned behaviors.

With repeated experience, environmental and other cues act as a context that becomes associated with the behavior that follows. In this example, the context of a bar is associated with alcohol drinking. The relationship between the context and behavior is encoded by a group of highly active neurons called a neuronal ensemble.

Alixandria is in pursuit of a PhD in Translational Neuroscience at Wayne State University in Detroit where she uses a transgenic rat model to study the neural mechanism that underlies motivated cocaine taking. When not in the lab, she loves to curl up on the couch with her “Detroit Special” rescue dog, lift weights or play soccer!