Neurotransmission systems in the brain are one focus
of the basic science research of Nakanishi and colleagues.
This lecture reviews the types of receptors in the
brain, one aspect of the development of the glutamate
receptors, and some clinical studies.
Neurotransmission occurs via neurotransmitters;
from the terminus of the nerve the neurotransmitter
is released and interacts with the receptors to cause
various functions.
The brain has more than 50 neurotransmitters or substances
modulating transmission, and these may be excitatory,
inhibitory, or modifying transmitters. Additionally,
there are various receptors for the neurotransmitters.
To understand the function of the brain, they elucidated
the receptors. About 15 years ago, they were able
to clone different receptors using a method they developed
that combined genetic engineering and electrophysiology
to isolate receptors.
Glutamate receptors were cloned by this group to
clarify their fundamental function. At least two types
of glutamate receptors exist: ionotropic and metabotropic.
The ionotropic glutamate receptor has an ion channel
that opens when the glutamate response is seen to
allow the ion to flow in and quick excitation of the
nerve to occur.
The ionotropic receptor has an AMPA, kinate, and
NMDA receptor. The NMDA type is important for memory
and learning, and the AMPA receptor for very rapid
excitation of the nerve. All of the receptors have
G proteins, so depending on the condition of the glutamate,
a metabolic-type receptor, metabotropic glutamate
receptor (mGluR), is present. Their work to clone
the mGluR led to the understanding there are different
types of receptors. They have cloned the NMDA receptor
and the metabotropic receptor.
The glutamate receptors comprise more than 20 genes,
and as expected G protein coupled-type ion channels
are present. Presently they are investigating the
role of these receptors in various sites of the brain
and how they act on specific neurons. There are 8
types of G protein-coupled receptors, which are divided
into 3 groupings based on functionality.
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The Basal
Ganglia Neurocircuit |
To understand one aspect of the brain, they are researching
the basal ganglia neurocircuit. The basal ganglia,
comprised by the striatum and nucleus accumbens, is
a key site for human disease. For example, Parkinson's
disease is due to a reduced level of dopamine.
Basically, in this circuit the glutamate transmission
from the cerebral cortex is received by the basal
ganglia. A control mechanism feeds the information
back to the cerebral cortex to control motor movement.
Under normal conditions, there is always a positive
and negative control of this feedback to control motor
movement, a direct and indirect pathway. In this positive
and negative pathway, dopamine positively controls
movement; a dopamine deficiency, as seen in Parkinsons
disease, results in a decline in motor movement.
Acetylcholine, produced by about 1% of the cells
in the basal ganglia, is a physiologic antagonistic
transmitter against dopamine. To study the role and
function of acetylcholine, which is not well understood,
they used cell targeting to create a transgenic mouse
expressing human interleukin-2 receptor (IL-2R). They
injected into the transgenic mouse an anti-IL-2R antibody
fused to pseudomonas toxin, an immunotoxin. The antibody
reacted to the human IL-2R, was taken into the complex,
and the pseudomonas toxin killed the cell. In this
model, the human IL-2R responds, but the mouse IL-2R
does not, and thus the neurons expressing human IL-2R
are eliminated specifically.
Then, they generated transgenic mice expressing
IL-2R/GFP under the control of the mGluR-2 promoter.
By using GFP, they showed that this model specifically
expresses human IL-2R in the ACH-producing cells (choline
acetyl transferase; ChAT). Then, using immunotoxin-mediated
cell targeting, they selectively eliminated the ChAT
cells.
After the selective elimination of the ChAT cells,
the ACh levels are selectively reduced; the dopamine
level is not affected. Further, the elimination of
the ChAT cells results in changes in movement. Specifically,
contralateral rotation towards the site where the
cells were eliminated occurs in about 3 days, a finding
that was somewhat expected. ACh acts antagonistically
against dopamine, and the removal of dopamine results
in ipsilateral turning in the mice. Interestingly,
after the acute phase, movement returns to normal.
Brain function is somehow compensated and results
in recovery.
Why did contralateral abnormal rotation occur in
the acute phase and recovery in the chronic phase?
In a subsequent experiment, they showed that a decrease
in ACh results in dopamine becoming relatively more
dominant; the positive effect of dopamine became stronger
and the negative effect weaker. Thus, in this setting,
the stimulus to the cerebral cortex becomes stronger.
The information from the basal ganglia acted stronger
on the eliminated site.
In the chronic phase, downregulation occurs at the
site of the relatively stronger action of dopamine.
As a result, dopamine becomes stronger because there
is no ACh. But, there is a decrease in the number
of receptors and thus the left and the right sides
maintain a certain balance; compensation occurs.
To treat Parkinson's disease dopamine is administered.
In some cases there is simultaneous administration
of an ACh antagonist. But, administering an ACh antagonist
in the chronic phase is tantamount to reducing ACh.
Therefore, there is some amount of compensation, perhaps
resulting in some amount of imbalance remaining overall.
Their work supports the fact that a change in one
site results in a reaction in another site.
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Drug Addiction
and Brain Function |
In the mesolimbic dopaminergic pathway of ACh, dopamine
and ACh are antagonistic. The nucleus accumbens, found
in the anterior portion of the basal ganglia, is closely
and critically related to drug addiction. Globally
30 million people are addicted to substances.
Cocaine and morphine addiction is closely related
to brain function. The role of ACh in persons with
drug addiction can be studied because the nucleus
accumbens is present in both the mesolimbic dopaminergic
pathway and the basal ganglia neurotransmitting pathway.
Cocaine blocks the dopamine transporter, so the
dopamine level at the nucleus accumbens is potentiated
on a longer-term basis. But the mechanism for this
has not been elucidated. Morphine also elevates the
dopamine level, because of the effect on the nucleus
accumbens, and results in chronic poisoning or intoxication
from morphine. Because dopamine and ACh are antagonists,
they presume that an ACh antagonist will suppress
the excessive dopamine.
This group developed an experimental animal lacking
ACh-producing cells in the nucleus accumbens, because
cocaine works on and binds strongly to the nucleus
accumbens, to study the action of cocaine and morphine
in this model. Other work had shown that the ACh antagonist
and agonist work promiscuously. Thus the role of ACh
is not clearly understood. In their work, they specifically
eliminated ACh-producing cells only at the nucleus
accumbens.
Cocaine was strongly potentiated in this mouse model.
To study this further, they used the conditioned place
preference test. In this test, a saline-injected animal
is placed in one chamber and a drug-injected animal
in another chamber. Each chamber is given a different
appearance and colors. The mice are conditioned to
prefer one room over to the other, because when cocaine
or morphine is given, the animal has a sense of pleasure
and comfort.
Is it possible to suppress the effects of cocaine
and morphine if the animal can preserve ACh production,
despite lacking ACh-producing cells? Various pharmacologic
drugs have been developed to potentiate ACh production.
For example, to suppress the progression of Alzheimer's
disease, an acetylcholinesterase inhibitor was developed,
which suppresses degradation of ACh in the brain.
Two drugs that are being clinically are donepezil
and galanthamine.
In the animal model, pre-treatment with donepezil
completely suppresses the action of cocaine. Further
work showed that the presence of ACh at the nucleus
accumbens is very important to suppress the action
of morphine and cocaine.
Typically, the continued use of cocaine results
in continued hyperactivity. But, even after the action
of cocaine is established, pre-treatment with donepezil
suppresses this action in their animal model. Although
the safety of donepezil in humans has already been
established, whether it can be used for drug addiction
requires further research.
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Paraneoplastic
Cerebellar Ataxia Due to Autoantibody Against a Glutamate
Receptor |
In work in paraneoplastic cerebellar ataxia, this
group found that with some neoplasms there is some
brain disorder. Although this is known to be some
sort of autoimmune disease, it is not understood.
The autoantibody production may work on the cerebellum
and cause motor disorder, specifically gait disorder
in some patients. The presence of a specific anti-MgluR1
antibody has been shown in some patients, causing
cerebellar ataxia and memory and motor disorders.
This group showed that in knockout mice, immunohistochemical
staining strongly suggested that a specific antibody
against mGluR-1. Further study showed that although
mGluR1 and mGluR5 are similar, immunoreactivity was
only observed for mGluR1, but not mGluR5. Inositol
phosphate 3 production was reduced also by this antibody.
This antibody suppresses the action of the mGluR1
and its receptors.
To produce cerebellar ataxia in the mouse model,
antibody serum was injected into mouse cerebellum.
This injection produced gait staggering in the mice
and convulsion-like symptoms. A rota-rod test, using
a rotating bar, was used to quantify the motor disorder.
Normally the mouse can stay on the bar while it rotates.
Injection of serum from a patient with Hodgkins
disease and cerebellar ataxia into the mice cerebella
suppressed mGluR1 function and motor ataxia resulted.
The mice fell off the rotating rod. With time, the
function returned to normal.
So, although the exact mechanism is not understood,
after the onset of Hodgkin's disease, an mGluR-1 antibody
was produced in the two patients studied. Its high
level of expression in the cerebellum caused ataxia.
When the serum is given to mice, similar symptoms
can be reproduced. Therefore, paraneoplastic cerebellar
ataxia is partly caused by selective autoantibody
production against mGluR1. The metabotropic receptors
are important for the function and regulation of the
cerebellum.
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