Fluid brain circuits drive the formation of cocaine habits

As animals learn to self-administer cocaine, a specific network of brain cells rapidly expands to acquire the habit and then shrinks as the behavior becomes automatic. The composition of this network constantly changes, revealing how the brain flexibly manages addictive behaviors. The study detailing these changing brain dynamics was published in bioRxiv.

Substance use disorders often begin with an initial learning period that later morphs into a deeply ingrained habit. Transitioning between these phases requires distinct mental efforts, yet the physical brain changes that support this shift remain somewhat mysterious. University of Pittsburgh researchers Linjie Jin, Xiguang Qi, and Yan Dong wanted to understand how brain networks adapt during this process.

They focused on the nucleus accumbens, a region deep in the forebrain that processes rewards, pleasure, and motivation. The main cells in this area are called medium spiny neurons. These neurons fire electrical signals in response to things like food or drugs, forming an active group called a neuronal ensemble. The researchers suspected this ensemble might change as an animal progresses from acquiring a drug habit to maintaining it.

In a small study, the researchers trained male mice to self-administer cocaine. The animals were placed in operant conditioning chambers, which are specialized testing environments where animals learn to associate a specific action with an outcome. These boxes contained two levers, with one active lever programmed to deliver the drug.

When a mouse pressed the active lever, it received an infusion of cocaine along with a flash of light and a sound cue. Over an eleven-day period, the team recorded the animals’ behavior during daily two-hour sessions. The researchers wanted to track the exact physical paths the animals took as they learned the task.

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Using a machine learning algorithm called DeepLabCut to track video recordings, they mapped the coordinates of the mice as they moved around the testing chamber. During the early days of training, the mice wandered randomly before pressing the lever. By the end of the eleven days, their behavior stabilized into a highly repetitive routine. The animals developed stereotyped, circular walking patterns immediately before and after taking the drug.

Their entries and exits from the lever area followed a highly consistent path. This circular movement pattern was not seen in a control group of mice trained to seek sugar. The specific physical routine suggested that the cocaine habit was becoming an automatic behavioral response over time. The total distance the mice traveled also increased across the training days, matching a known phenomenon where repeated cocaine use sensitizes motor activity.

To see what was happening inside the brain as this habit formed, the team used a technique called in vivo calcium imaging. They injected a specialized virus into the mice’s brains that caused the medium spiny neurons to produce a fluorescent protein. This protein was designed to react to changes in internal cellular activity.

When a neuron fires an electrical signal, calcium ions flood into the cell. The engineered protein binds to this calcium and emits a tiny flash of light. A microscopic lens implanted directly into the brain captured these flashes, allowing researchers to watch individual neurons turn on and off in real time while the mice were awake and moving.

The researchers observed that a specific set of neurons reliably lit up in the five seconds immediately after a mouse pressed the cocaine lever. During the first three days of training, the sheer number of these active neurons rapidly increased. The brain seemed to recruit a massive amount of cellular resources to learn the new drug-taking rule.

As the training progressed and the physical movements of the mice became automatic routines, the size of this active network began to shrink. By the ninth and eleventh days, the number of responding neurons dropped back down to the lower levels seen on day one.

The researchers noted that the intensity of each individual neuron’s signal stayed exactly the same across the entire experiment. The brain did not dial down the volume of the cells. Instead, fewer cells were needed to execute the established habit.

This pattern of broad recruitment followed by pruning is not unique to biological brains. The authors noted that artificial neural networks learn in a similar way. During initial training, reducing the size of an artificial network impairs its performance, showing that abundant computational resources are necessary to learn a task. Once the network is trained, many connections can be stripped away without affecting the final output.

The team also tracked individual neurons over consecutive days to see if the exact same cells made up this network over time. They used imaging software to match the shape, spatial position, and activity patterns of specific neurons from one testing session to the next.

They found that the network was highly fluid. Only about one quarter of the neurons that responded to a lever press on one day would respond again two days later. Individual cells constantly dropped into and out of the active group. The overall behavioral output remained consistent, but the physical makeup of the cellular network driving it was entirely dynamic.

The experimental design involved a few limitations. The imaging process focused exclusively on male mice. The researchers noted that the heavy head-mounted microscope equipment caused less behavioral disruption in the larger males, helping them achieve a more stable response rate.

The study also did not distinguish between different subtypes of medium spiny neurons. The nucleus accumbens contains cells with distinct receptors that respond differently to the chemical messenger dopamine. Some neurons possess D1 receptors, while others possess D2 receptors. These different subtypes are thought to play distinct and sometimes opposing roles in reward processing and movement.

The five-second window following a lever press includes the physical act of pressing, the onset of a cue light, and the physical sensation of the drug entering the bloodstream. The identified network of neurons is likely a composite of several smaller groups processing each of these separate stimuli. Tracking a larger number of individual cells in future experiments could help separate these overlapping signals.

The ever-changing nature of this cellular network challenges traditional ideas about how habits are stored in the brain. A fluid membership might allow the brain to constantly update learned information while maintaining a steady behavioral output. The flexibility of these cells ensures that the addiction remains firmly rooted even as individual neurons tag out.

The study, “Refinement of Nucleus Accumbens Neuronal Dynamics During Cocaine Self-Administration Training,” was authored by Linjie Jin, Xiguang Qi, Jianwei Liu, William J. Wright, Terra A. Schall, King-Lun Li, Bo Zeng, Charles Wang, Lirong Wang, and Yan Dong.