A recent study found that the initial sensitivity of neurons to cocaine in a brain region known as the nucleus accumbens shell can predict future increases in cocaine intake. These individual differences in neuronal sensitivity may have implications for understanding addiction susceptibility and developing personalized treatments or preventive measures. The new findings have been published in the journal Addiction Neuroscience.
The researchers conducted this study to investigate why only a subset of individuals who try drugs like cocaine develop problematic use and addiction. They aimed to understand the underlying differences in neuronal processing that predict drug abuse vulnerability, which could potentially improve addiction prevention and treatment strategies.
“Substance use disorders continue to be a major public health problem, and we saw an even greater uptick in substance abuse and overdose deaths starting in 2020,” said study author David J. Barker, an assistant professor at Rutgers, The State University of New Jersey.
“While most studies in the field of substance abuse focus on differences across groups of individuals, far fewer focus on the factors that might predict whether a specific individual is likely to abuse drugs like cocaine. For this reason, we examined a part of the brain called the striatum which is a major target of dopamine neurons. As a part of this, we examined how the activity in different parts of the striatum changed as rats progressed from their first drug use to chronic drug use.”
Cocaine exerts its effects on the central nervous system primarily by interfering with the normal functioning of certain neurotransmitters. Cocaine primarily acts as a potent inhibitor of the dopamine transporter. Normally, this transporter clears dopamine from the synaptic cleft after its release. By inhibiting the dopamine transporter, cocaine prevents dopamine reuptake, leading to increased dopamine levels in the synapse.
To conduct the new study, the researchers used rats as animal models and implanted microwire arrays in specific regions of their brains, such as the dorsal and ventral striatum. The striatum is known to play a key role in motivational processing and is implicated in substance abuse. The rats were trained to self-administer cocaine, and their neural activity was recorded using electrophysiology techniques over a period of 22 sessions.
The researchers found that firing patterns of neurons in the nucleus accumbens, a region within the ventral striatum, became increasingly correlated with drug levels during the self-administration sessions. This means that as the rats consumed more cocaine, the firing rates of nucleus accumbens neurons changed accordingly.
The nucleus accumbens consists of two main subdivisions: the shell and the core. The shell of the nucleus accumbens is the outer region surrounding the core. It has connections with various brain regions, including the prefrontal cortex, amygdala, hippocampus, and thalamus. The core of the nucleus accumbens is located deep within the shell. It has dense connections with other brain regions involved in the reward circuit, such as the ventral tegmental area and the prefrontal cortex.
The researchers discovered that the early sensitivity of nucleus accumbens shell neurons to self-administered cocaine predicted future increases in drug intake. Rats with stronger negative correlations between firing rates and drug levels on the first self-administration session showed the steepest increases in cocaine consumption over time.
“What we discovered was that neurons in a part of the striatum called the nucleus accumbens show different levels of initial sensitivity to cocaine,” Barker told PsyPost. “Perhaps most importantly, it turns out that individuals that show the strongest initial response to cocaine are also those that are likely to have excessive intake later. If these kinds of differences can be linked to some kind of genetic difference or druggable target, it may be possible to use this information to screen individuals for addiction risk, or even develop preemptive therapies.”
These findings suggest that changes in the firing patterns of nucleus accumbens neurons, particularly in the shell region, are associated with the development of addiction-like behaviors and increased drug intake. The study provides insights into the neural mechanisms underlying individual differences in drug susceptibility and highlights the potential role of the nucleus accumbens in drug reinforcement and addiction.
“The striatum is a very large brain region with important roles in motivated decision-making,” Barker said. “We were therefore surprised to discover that drug-related changes, especially those that helped predict individual susceptibility to increased drug use over time, were confined to a small portion of the striatum called the nucleus accumbens shell.”
But the study, like all research, includes some limitations.
“One major caveat of this dataset is that it included only males,” Barker noted. “There has been a drastic shift towards including both sexes in biomedical research, but these data came from a longitudinal study run almost a decade ago. Longitudinal data are incredibly valuable, but also very difficult to gather, so it will be important at some point to verify that this effect is not sex-specific.”
“There are many valuable studies on the individual genetics of addiction, but also many preclinical studies — like ours here — which indicate that individual differences could be derived from differences in only one brain region or brain circuit,” the researcher added. “It seems important moving forward to find better ways to merge these two perspectives in order to expand our ability to design highly individualized care.”
The study, “Nucleus Accumbens Shell Neurons’ Early Sensitivity to Cocaine is Associated with Future increases in Drug Intake“, was authored by Ashley K. Crawley, Anirudh Sharma, Kevin R. Coffey, Mark O. West, and David J. Barker.