Brazilian neuroscientists examine relapse among drug users

The objective was to identify the structural alterations of neurons that can be a target for new more efficient medicines against dependence.

By Karina Toledo

Agência FAPESP – In the quest for more efficient treatments against drug dependence, scientists from the National Institute on Drug Abuse (NIDA/NIH) in the United States have focused on creating methods to identify and study small groups of neurons related to drug cravings.

The group coordinated by Bruce Hope included Brazil’s own Fábio Cardoso Cruz, a former FAPESP master’s and post-doctoral fellow, who just published an article on the subject in Nature Reviews Neuroscience.

“Our line of research is based on the premise that dependence is a learned associative behavior. When an individual begins to use a given substance, his or her brain associates the effect of the drug with the site where it is being consumed, the people around and the paraphernalia involved, such as syringes. With repeated use, this association grows stronger until the mere exposure to the environment, the people or the objects awakens the craving for the drug in the addict,” stated Cruz.

Evidence in the scientific literature suggests that this associative memory linking drug use with environmental elements would be stored in small groups of interconnected neurons located in different regions, known as neuronal ensembles.

“When the addict comes in contact with something that makes him or her recall the drug, these small groups of neurons are simultaneously activated. In this manner, the memory of the drug’s effect on the organism emerges, making the individual feel a compulsive desire for that drug, which is capable of controlling his or her behavior and making an abstinent addict have a relapse, even though he or she is conscious of the negative consequences, including the loss of a job and family or health problems,” said Cruz.

Through experiments with animals, researchers at NIDA showed that only 4% of the neurons in the mesocorticolimbic system are activated in cases of an environment-induced relapse. “There are several small groups located in the regions of the brain related to pleasure sensations, such as the pre-frontal cortex, the nucleus accumbens, hippocampus, the amygdala and the thalamus,” Cruz explained.

According to Cruz, the majority of the studies that seek to examine the neurobiology of chemical dependency and to discover the possible behavior-induced molecular alterations leading to relapse have evaluated the entire set of neurons found in brain tissue samples instead of focusing on these small groups.
 
“We believe that a really significant alteration could be masked by a change in the other 96% of neurons not related to relapse. For this reason, we are seeking methodologies to specifically study the 4%,” he explained.

One of the strategies described in the article published in Nature Reviews Neuroscience uses a line of transgenic mice known as lacZ. The animals are modified to express the β-galactosidase enzyme only in active neurons.

“We placed the animal in a box and taught it beat on a bar to receive cocaine. After a time, we moved the animal to a different box where it did not receive the drug when it beat on a bar. Eventually, the animal stopped beating on the bar. It is as if the animal was abstinent. But when we put the animal back in the first box or, rather, the environment where it was trained to receive the drug, it immediately began to beat on the bar to get the drug,” related Cruz.

At that moment, these small neuronal groups were activated by elements of the environment. The researcher then administered a substance called Daun02; this substance interacts with β-galactosidase, which transforms it into an active drug called daunorubicin that causes the death of these active neurons.

“We waited almost two days for the drug to conclude its effect, and when we put the animal in the environment associated with the drug, it no longer exhibited the same behavior of pursuing the substance. It is as if the craving was removed after the death of this small group of neurons related to the relapse behavior,” explained Cruz.

Another technique described in the article also uses genetically modified animals that express a fluorescent protein only in activated cells. “With the aid of flow cytometry, we managed to isolate the cells that are fluorescent and then looked for possible molecular changes. Such changes can be structural alterations, including an increase in the number of dendritic spines, which increases the synaptic interaction and makes the neuron more sensitive; they can be intracellular proteins that also increase the activity of these neurons,” he described.

Once these changes are identified, adds Cruz, they will become targets for the development of drugs capable of efficiently treating dependence. “There is no medicine that is really effective, so much so that roughly 70% of cocaine users relapse after a period of abstinence. In the case of alcohol, the number is higher than 80%,” he stated.

The NIDA group also includes other Brazilian researchers among them Rodrigo Molini Leão, who has just concluded his doctorate as an FAPESP fellow at Universidade Estadual Paulista’s School of Pharmaceutical Sciences in Araraquara (FCFAr-Unesp). Doctoral student Paulo Eduardo Carneiro de Oliveira of FCFAr-Unesp also participated in the study.