A researcher explains: dopamine deprivation

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10/06/2011

ESPCI ParisTech researchers publish on average one scientific article a day. We plan to discuss a number of key papers with their authors, through interviews to provide background on their work and the implications of their research. The first interview is with Serge Birman, the director of the Neurobiology Laboratory. Keywords are neurosciences, Parkinson’s disease, and genetics. Here he discusses a study on the behavioral consequences of dopamine deficiency in the Drosophila central nervous system.

Serge Birman (director), Thomas Riemensperger (postdoctoral research associate) and Marlène Cassar (PhD student). Credit: ESPCI ParisTech

Question: Serge Birman, you recently published in the Proceedings of the National Academy of Sciences (PNAS) an important article on the behavioral consequences of dopamine deficiency in the central nervous system of the fruit fly. When did the idea of this experiment arise, and how did you carry it out?

Serge Birman: "The idea of this project goes back some fifteen years, but at the time I didn’t have the means required to carry it out. First of all, dopamine is an essential neurotransmitter for the brain. It is being investigated intensively throughout the world, particularly because people affected by Parkinson’s disease begin by losing neurons capable of synthesizing dopamine—the neurotransmitter indispensable for regulating physiological functions, and for controlling behavior. Dopamine is involved in brain functions like learning, sleep, and pleasure."

Why choose Drosophila?

"Because for experimental purposes, the fruit fly offers innumerable genetic possibilities and is very interesting to study. The fruit fly also needs dopamine. And just like us, the insect has its own brain activity. Its brain doesn’t simply react to stimuli; it can function autonomously. In short, there are analogies between the human brain and the fruit fly’s. Its brain is small, but it allows us to understand in a simplified form what happen with humans."

What problems did you encounter?

"Studying dopamine-deficient fruit flies raises a number of initial problems, because dopamine also plays a vital role in the production of the insects’ cuticle, in other words, their . Dopamine allows the to harden and become pigmented. Only in the past five years or so have we been able to create genetically modified fruit flies with dopamine deficiency in their brain, but also with a normal cuticle—and therefore capable of staying alive."

Genetics

How to prevent the of neuronal dopamine? By selective deactivation of a gene, which is possible by taking advantage of a structural difference between the messenger RNA that codes for the responsible for of dopamine, namely tyrosine hydroxylase, depending on whether it is used for forming the cuticle or for forming neuronal dopamine.

"Finally, we had to be certain there was absolutely no dopamine. For the past two years, in collaboration with American researchers, we have had protocols for dopamine measurement accurate enough to allow us to affirm that this is the case. That’s when the experiment was really able to begin."

Serge Birman, 5 dates:

1986: earns degrees from ENS Ulm (École Normale Supérieure) and UPMC (Université Pierre et Marie Curie).

1992: begins four years of research at the University of Virginia (USA).

1996: returns to France and creates an ATIPE CNRS research group in Marseille.

2001: he and his team study the development and the physiology of neurotransmission in Drosophila, and develop new genetic tools like the TH-GAL4 driver.

2009: directs the Neurobiology Laboratory at ESPCI ParisTech (CNRS UMR, Mixed Research Unit, 7637)

You conducted numerous behavioral tests. What did they reveal?

"The first surprise was that these flies live as long as the others, or 70 days, and that was totally unexpected in light of earlier studies.
The second surprise was that they are often incapable of making a choice, even after experiencing the consequences. For example, during a learning process in which we associate an odor with an electrical discharge (in collaboration with the Genes and dynamics of memory systems, GDSM, laboratory at ESPCI ParisTech), we were amazed to see they turned out to be more attracted to the aversively conditioned odor than to a neutral odor. The memory of something appears to be related to the aversively conditioned odor, but they are incapable of attaching a negative value to it. With the loss of the ability to associate, the fly’s survival behavior is therefore totally destabilized.
Dopamine-deficient flies appear to lack "preferences," even in the presence of food like sugar, which they may possibly eat, but which has little appeal to them. As a matter of fact, their feeding habits are quite poor; they nourish themselves fairly poorly.
A few years ago, dopamine-deficient mice were obtained but they died very quickly because they failed to eat. This similarity and other observations suggest that essential functions of dopamine in the central nervous system have been preserved by evolution in insects and mammals.
Another surprise is that dopamine-deficient Drosophila are nevertheless capable of accomplishing complex tasks. Spatial memory, for instance, is nearly intact and remains efficient. Locomotion is indeed less assured, the trajectory is less precise, but if need be, their orientation in space is very good."

How was your paper received?

"Before publication, the paper was naturally subjected to peer review by anonymous reviewers; that’s the principle behind scientific journals. One of the reviewers was particularly severe, criticizing the entire experiment and arguing that what we observed was not a normal brain deprived of dopamine, but rather the consequences of a cerebral disorder resulting from dopamine deprivation during the fly’s development—and that our conclusions were therefore unfounded.
That’s when we decided to conduct a decisive experiment and administer L-DOPA to our adult fruit flies. The L-DOPA molecule is a precursor to dopamine, and is specifically used in the treatment of patients suffering from Parkinson’s disease, who are dopamine deficient. The very simple, but crucial, idea was to add a dye to the flies’ food, so we could be sure they had actually ingested the drug.
The experiment turned out to be positive. All the degraded behaviors that we tested were significantly restored. We proved that the central nervous system of the mutant fly was in working order, with normally developed dopaminergic connections which had been deactivated by the absence of the neurotransmitter. Our experiment was therefore valid. The criticisms improved and consolidated our paper."

These findings open new prospects. What are the next stages in your research?

"The next step is to attempt to understand how the fly’s brain adapts to the absence of dopamine. What happens has analogies with quantum physics, where the observed system changes when observed by the experimenter. The brain has tremendous plasticity, with considerable capacity for adaptation and regeneration. It’s probable that the brain of the fly, which we deliberately deprived of dopamine, compensated one way or another, so that the observed defects are attenuated to some extent. We want to know how this occurs.
This will surely provide an opportunity to improve our understanding of the function of the other neurotransmitters system, e.g., octopamine, serotonin, and glutamate, and their interactions. We also wish to use Drosophila to identify the specific functions of the dopaminergic neurons in the brain, which are still unknown."

Reference:

Riemensperger et al. (2011) Behavioral consequences of dopamine deficiency in the Drosophila central nervous system. Proc Natl Acad Sci USA 108(2):834-839

To learn more:

Parkinson’s disease
(Dossier prepared by Inserm, the French National Institute for Health and Medical Research)



Glossary

exoskeleton
The exoskeleton is an external skeleton that supports and protects an animal. Many invertebrates have an exoskeleton, also called the cuticle.
biosynthesis
Biosynthesis is the production of organic compounds by living organisms.
enzyme
An enzyme is a protein capable of catalyzing a chemical reaction.



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