DURHAM, N.C. -- Researchers at the Howard Hughes Medical Institute at Duke
University Medical Center have deleted the gene for a crucial molecular
component of a mouse's nervous system and created an animal that, in essence,
mimics a person constantly high on illicit drugs.
Marc Caron, a Hughes investigator and professor of cell biology at Duke,
said initial studies with the mice already have yielded surprising insights
that challenge conventional theories about drug addiction and Parkinson's
disease and may provide the first realistic model for testing new treatments
for psychiatric disorders.
"We were astonished that a single genetic deletion would have such
a profound effect on both biology and behavior," Caron said. "We
believe this mouse will provide an ideal model to study addictive behavior
and psychiatric disease."
Caron and his colleagues report their findings in the Feb. 15 issue of the
journal Nature. Bruno Giros, the study's lead author, Mohamed Jaber, and
Sara Jones of the department of cell biology; and R. Mark Wightman, of the
department of neurobiology and chemistry at the University of North Carolina
at Chapel Hill, also contributed to the research. The study was funded in
part by the National Institutes of Health and an unrestricted neuroscience
award from Bristol Myers Squibb.
Caron said in an interview that while it may seem a stretch for one genetically
engineered mouse to help answer fundamental questions about the mechanism
of addictive drugs, Parkinson's disease, and psychiatric disorders, all
are linked by a common problem: a malfunction in the body's regulation of
dopamine, an essential messenger of the nervous system.
Neurotransmitters such as dopamine, serotonin and adrenaline are chemical
messengers that neurons release to their neighbors to signal them to fire
nerve impulses or initiate metabolic changes. Because neurotransmitters
are so critical to the smooth functioning of the nervous system, the body
has evolved a precise system for regulating them.
This neurotransmission system involves two main elements: receptors and
transporters. Receptors are the molecular "locks" on nerve cells
that receive a neurotransmitter "key," causing a neuron to fire
a nerve impulse. Transporters are "pumps" on the surface of the
transmitting neuron that recycle neurotransmitters back to the nerve cell,
to prepare for the next burst.
The neurotransmitter dopamine is stored in tiny hollow spheres, called vesicles,
within synaptic "bulbs," which are tiny buds on the nerve cell's
surface. When a nerve impulse reaches a bulb, the transmitting neuron releases
a flood of dopamine into the narrow space between neurons, called the synapse.
When the dopamine reaches the receiving neuron, it binds to specific dopamine
receptors, thereby triggering a response in that neuron. Almost immediately,
the dopamine transporter scavenges excess dopamine from the synapse to terminate
the signal.
It is this dopamine transporter protein that Caron and his colleagues knocked
out by disrupting the gene that encodes its production in mice. The resulting
mice have no functional dopamine transporter, which means dopamine signals
flood the brain and can't be shut off. Depending on where in the brain such
overstimulation occurs, it may cause the temporary "high" perceived
by cocaine or amphetamine addicts or the permanent and severe delusions
experienced by schizophrenics. On the other hand, the lack of dopamine in
the brain's motor areas produces the symptoms of Parkinson's disease.
"Virtually every addicting substance appears to modify the dopamine
system, implying it may have a central role in addiction," said Alan
I. Leshner, director of the National Institute on Drug Abuse (NIDA). "This
finding provides a fundamentally new scientific tool in the arsenal to understand
and ultimately break the addictive power of drugs."
Caron said the research "points to neutrotransmitter transporters as
the most important determinant of the strength and duration of cellular
communications in the nervous system."
Dr. Ralph Snyderman, chancellor for health affairs at Duke, said, "These
findings represent a superb example of the way in which basic research in
academic medical centers leads to practical benefit. This discovery may
well change our understanding of the nature of addiction and lead to new
treatments for psychiatric disorders and Parkinson's disease. It once again
shows how our nation's commitment to research is needed to provide practical
solutions for common health care problems."
Insights into the mechanism of amphetamine drugs derived from studying the
"knockout" mice should yield new strategies for treating amphetamine
addiction, Caron said.
"Much of what we know about dopamine's function comes from studies
of drugs that tamper with the dopamine system," he said. Cocaine, for
example, temporarily blocks the transporter from outside the cell, so the
dopamine remains at high concentrations in the synapse and continues to
stimulate adjacent neurons, producing cocaine's characteristic "high."
Amphetamines, by contrast, are believed to enter the neuron via both the
transporter and directly through the cell membrane, where they burst dopamine
vesicles inside the cell and promote the release of dopamine.
"What happens when amphetamines break dopamine vesicles has been a
matter of some speculation, since it has never been directly tested,"said
Jones, a post-doctoral fellow.
These studies provide the first direct demonstration that the dopamine transporter
is absolutely required for amphetamine's releasing action on dopamine. Amphetamine
appears to reverse the dopamine transporter pump so instead of pumping dopamine
into the cell, it pumps dopamine out of the cell, Caron said.
In the knockout mice with no dopamine transporters, amphetamines can still
get into the neuron through the cell membrane and disrupt dopamine vesicles.
However, without transporters, dopamine cannot get out of the cell into
the synapse.
"Our results show the transporter is absolutely essential for amphetamine
dependent transport of free dopamine into the synaptic space, to the exclusion
of any other mechanism," Caron said. "In essence, no functional
transporter would mean no amphetamine high. This information should provide
new understanding of the mechanism of amphetamine drugs and new strategies
for treating amphetamine addiction."
Besides dopamine's direct role in producing a drug-induced high, said Caron,
recent studies have shown that dopamine plays a central role in the brain's
reward centers. These dopamine-activated pleasure centers are key to the
addictive reinforcing pattern, even though addictive substances such as
alcohol, nicotine, and cocaine may exert their influence on many different
areas of the brain, Caron said.
Additional background: Insights into Parkinson's disease
The research with the knockout mice could also provide new insights into
the management of Parkinson's disease, said Caron. The mouse research may
point the way toward drugs that maintain the higher levels of dopamine in
the brains of Parkinson's sufferers.
In Parkinson's disease, dopamine-producing neurons in the brain's motor
control center, called the substantia nigra, slowly deteriorate and die.
Thus, Parkinson's disease begins with small tremors and progresses to a
total inability to initiate movement. Currently incurable, Parkinson's disease
affects almost a million Americans and 50,000 new cases are diagnosed each
year.
Drug therapies for Parkinson's disease have focused on replenishing the
body's diminished supply of dopamine.
"Nobody has ever considered the dopamine transporter as a target, in
part because its crucial role in maintaining dopamine levels had never been
fully appreciated," said Jaber, a post-doctoral fellow.
Indeed, research with the knockout mouse shows that conserving existing
dopamine may be more effective.
"When we measured the amount of dopamine being released in the knockout
mice, we were astounded to find they make only 5 percent to 10 percent the
normal amount of dopamine, about the same as Parkinson's patients,"
said Caron. "Yet we thought the dopamine levels would be much higher
than normal because without the transporter protein, there would be no way
to remove excess dopamine from the synapse."
To resolve this apparent paradox, Jones and Wightman measured how long dopamine
stays in the synapse after it is released. Their measurements showed that
in a normal mouse, the dopamine is scavenged by the transporter protein
in less than one second. However, in the knockout mice with no dopamine
transporter protein, the dopamine stays in the synapse at least 100 times
longer. Because dopamine is staying in the synapse longer, it prolongs the
neurotransmission like cocaine or amphetamine would, even though there is
very little of it.
The researchers realized this very dopamine-conservation strategy could
be used to treat Parkinson's patients. Because Parkinson's patients make
very little dopamine, the trick is to keep what little dopamine they have
in the synapse longer, significantly increasing neurotransmission, Caron
said. This could be achieved with drugs that selectively block the transporter
protein. Such a strategy may prove to be more potent than standard treatments.
Giving more dopamine to Parkinson's patients, as is done now, just keeps
the active transporters busy longer, he said.
"Blocking the transporter with therapeutic drugs should greatly benefit
Parkinson's patients," Caron said. "We believe these findings
could have profound implications for the treatment of Parkinson's disease."
Knockout mice and schizophrenia
The researchers believe the mice may accelerate the pace of research into
schizophrenia by providing the first realistic animal model to test new
treatments.
Researchers have known for many years that schizophrenics behave as if they
have too much dopamine in the brain's limbic system, which controls emotion
and behavior. Conversely, not enough dopamine contributes to clinical depression.
Although therapeutic drugs for schizophrenia block dopamine receptors, they
have limited effectiveness and produce side effects in many patients.
"Years of research by many investigators have helped determine the
cellular components that regulate dopamine levels in the synapse,"
said Giros, now a researcher at INSERM in Paris. "We are now in the
position to ask questions about what role each of the components play and
what happens when one crucial component is missing or malfunctioning."