December 13, 2002
Researchers Discover Gene that Controls Learned Fear
Researchers have discovered the first genetic component of a
biochemical pathway in the brain that governs the indelible imprinting
of fear-related experiences in memory.
The gene identified by researchers at the Howard Hughes Medical
Institute at Columbia University encodes a protein that inhibits the
action of the fear-learning circuitry in the brain. Understanding how
this protein quells fear may lead to the design of new drugs to treat
depression, panic and generalized anxiety disorders.
“These findings reveal a biological basis for what had only been previously inferred from psychological studies — that instinctive fear, chronic anxiety, is different from acquired fear.”
Eric R. Kandel
The findings were reported in the December 13, 2002 issue of the
journal Cell, by a research team that included Howard Hughes
Medical Institute (HHMI) investigators Eric
Kandel at Columbia University and Catherine
Dulac at Harvard University. Lead author of the paper was Gleb
Shumyatsky, a postdoctoral fellow in Kandel’s laboratory at
Columbia University. Other members of the research team are at the
National Institutes of Health and Harvard Medical School.
According to Kandel, earlier studies indicated that a specific
signaling pathway controls fear-related learning, which takes place in
a region of the brain called the amygdala. "Given these preliminary
analyses, we wanted to take a more systematic approach to obtain a
genetic perspective on learned fear," said Kandel.
One of the keys to doing these genetic analyses, Kandel said, was
the development of a technique for isolating and comparing the genes of
individual cells, which was developed at Columbia by Dulac with HHMI
investigator Richard
Axel.
Shumyatsky applied that technique, called differential screening of
single-cell cDNA libraries, to mouse cells to compare the genetic
activity of cells from a region of the amygdala called the lateral
nucleus, with cells from another region of the brain that is not known
to be involved in learned fear. The comparison revealed two candidate
genes for fear-related learning that are highly expressed in the
amygdala.
The researchers decided to focus further study on one of the genes,
Grp, which encodes a short protein called gastrin-releasing
peptide (GRP), because they found that this protein has an unusual
distribution in the brain and is known to serve as a neurotransmitter.
Shumyatsky’s analysis revealed that the Grp gene was highly
enriched in the lateral nucleus, and in other regions of the brain that
feed auditory inputs into the amygdala.
"Gleb’s finding that this gene was active not only in the
lateral nucleus but also in a number of regions that projected into the
lateral nucleus was interesting because it suggested that a whole
circuit was involved," said Kandel. Shumyatsky next showed that GRP is
expressed by excitatory principal neurons and that its receptor, GRPR,
is expressed by inhibitory interneurons. The researchers then undertook
collaborative studies with co-author Vadim Bolshakov at Harvard Medical
School to characterize cells in the amygdala that expressed receptors
for GRP. Those studies in mouse brain slices revealed that GRP acts in
the amygdala by exciting a population of inhibitory interneurons in the
lateral nucleus that provide feedback and inhibit the principal
neurons.
The researchers next explored whether eliminating GRP's activity
could affect the ability to learn fear by studying a strain of knockout
mice that lacked the receptor for GRP in the brain.
In behavioral experiments, they first trained both the knockout mice
and normal mice to associate an initially neutral tone with a
subsequent unpleasant electric shock. As a result of the training, the
mouse learns that the neutral tone now predicts danger. After the
training, the researchers compared the degree to which the two strains
of mice showed fear when exposed to the same tone alone — by measuring
the duration of a characteristic freezing response that the animals
exhibit when fearful.
"When we compared the mouse strains, we saw a powerful enhancement
of learned fear in the knockout mice," said Kandel. Also, he said, the
knockout mice showed an enhancement in the learning-related cellular
process known as long-term potentiation.
"It is interesting that we saw no other disturbances in these mice," he
said. "They showed no increased pain sensitivity; nor did they exhibit
increased instinctive fear in other behavioral studies. So, their
defect seemed to be quite specific for the learned aspect of fear," he
said. Tests of instinctive fear included comparing how both normal and
knockout mice behaved in mazes that exposed them to anxiety-provoking
environments such as open or lighted areas.
"These findings reveal a biological basis for what had only been
previously inferred from psychological studies — that instinctive
fear, chronic anxiety, is different from acquired fear," said
Kandel.
In additional behavioral studies, the researchers found that the
normal and knockout mice did not differ in spatial learning abilities
involving the hippocampus, but not the amygdala, thus genetically
demonstrating that these two anatomical structures are different in
their function.
According to Kandel, further understanding of the fear-learning
pathway could have important implications for treating anxiety
disorders. "Since GRP acts to dampen fear, it might be possible in
principle to develop drugs that activate the peptide, representing a
completely new approach to treating anxiety," he said. However, he
emphasized, the discovery of the action of the Grp gene is only
the beginning of a long research effort to reveal the other genes in
the fear-learning pathway.
More broadly, said Kandel, the fear-learning pathway might provide
an invaluable animal model for a range of mental illnesses. "Although
one would ultimately like to develop mouse models for various mental
illnesses such as schizophrenia and depression, this is very hard to do
because we know very little about the biological foundations of most
forms of mental illness," he said. "However, we do know something about
the neuroanatomical substrates of anxiety states, including both
chronic fear and acute fear. We know they are centered in the
amygdala.
"And while I don’t want to overstate the case, in studies of
fear learning we could well have an excellent beginning for animal
models of a severe mental illness. We already knew quite a lot about
the neural pathways in the brain that are involved in fear learning.
And now, we have a way to understand the genetic and biochemical
mechanisms underlying those pathways."
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