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 Kandels 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. Shumyatskys 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.
"Glebs 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 dont 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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