Dopamine
Norepinephrine
Serotonin and Kynurenine
Androgens
Cortisol and DHEA
The GTS gene isn’t the only gene involved with Tourette syndrome. Tourette’s is actually caused by many genes (no one knows how many yet) that are
slightly abnormal. The GTS gene seems to act as a catalyst for all the other genes, and makes their effects more pronounced. Each of these minor genes codes for a trait too, and the abnormalities in the genes cause an abnormality in its product.
Every thought that you have and action that you perform, begins in the brain
. Billions of brain cells, called neurons, connect to each other to form
pathways. When a thought is had, an electrical pulse starts at one neuron
and passes from one to another down a long predetermined pathway. (To be
honest, no one has
worked out how this actually translates into
conscious thought...but we’ll leave that to psychologists and philosophers
to fight out.)
However, neurons aren’t actually connected directly to each other. Between
the end of the dendrites (the little “tentacles” coming out of the cell body) and the body of the next cell there is a gap of about 10-20 nm. (1 nm is
one billionth of a metre). This gap is called a synapse. The electrical
impulse cannot cross the synapse, so chemical transmitters are used –
neurotransmitters. When the electrical impulse arrives at the end of the
dendrite, a chemical neurotransmitter is released which crosses the gap and
sparks off another electrical impulse on the other side. This is how
messages get around the brain.
It had been thought for ages that many of the tics in Tourette’s are
caused by
too much of the neurotransmitter Dopamine.
The theory says:
The brain tends to randomly fire off weak impulses of electricity just to
check that everything is connected and healthy. These pulses are so weak
that the thought or action they code for does not actually occur. In
Tourette’s, however, when some of these random pulses reach the synapses,
a larger than normal amount of dopamine is released. And the more dopamine,
the stronger the signal released when the chemical
reaches the other side. And this stronger pulse would be amplified again
at the next synapse, and so on. Very soon, the action would actually be
performed.
However, this theory was not quite right. Post-mortem studies of adults who
were diagnosed with Tourette syndrome, showed a large increase in dopamine
receptor sites.
Dr Comings (1991) then discovered that defects in the genes D2A1 and DAT2
increases the
number of Dopamine Type Two receptors (D2) while another gene (unidentified)
affects the D3 receptors. The increase was about 44% more on average across
the whole brain. This would have the same effect
as excess dopamine on multiplying the signals passed through the brain, but
would do it in a different way.
When the neurotransmitters are released into the synapse, a lot is wasted.
At the far end of the synapse there are uptake carrier sites. These sites
grab onto the neurotransmitter and, once the chemical is bound to the site,
it releases the impulse. But not all of the neurotransmitter reaches the
uptake sites. It’s like shooting arrows at a target while blindfolded –
only some of them are going to hit it. If you increase the number of
targets though, you’re more likely to hit one. It’s the same with
dopamine uptake sites, if you have more sites; more dopamine is going
to be absorbed. And more dopamine means a stronger impulse at the
other end.
Many medications for Tourette’s are dopamine blockers. Once in the body
they get into the synapses and bind with the dopamine uptake sites. The
dopamine blockers don’t trigger an impulse, they just sit there and stop
dopamine from using the uptake sites. This effectively reduces the number
of uptake sites in the synapse.
The reason you get sluggish when on dopamine blockers is that ALL movements
require the passage of dopamine, not just tics. The blockers do not
discriminate between the purpose behind movements .
Dopamine is also converted in the body to norepinephrine.
Brain norepinephrine plays a role in general arousal level. Norepinephrine
is at lowest levels while asleep, and is released in large quantities to
wake you up in the morning.
Hormonal norepinephrine is released from the adrenal glands (located just
above the kidneys) during stress. It is part of the “fight or flight”
response to danger and increases heart rate and blood pressure, relaxes
smooth muscle to allow more efficient breathing and increases the conversion
of food and sugar into energy. This
may cause part of the excess energy Touretters have, as well as things like
difficulty sleeping and increased appetite.
Hormonal norepinephrine is also important for learning and the formation of
memory.
Because norepinephrine and dopamine are part of the same chemical pathway,
the body has to make a choice between which of the two chemicals it needs.
The body is very clever and has its own ideas about how much of each chemical it
needs, focusing primarily on the balance in the most vital area, the
brain. When a Touretter is in an unmedicated state the body detects that
it needs to produce a lot less dopamine because the messages are getting through
very easily. So the dopamine is converted to norepinephrine in an attempt
to lower dopamine levels to the body's idea of normal (actually lower
levels than in non-Touretters). So unmedicated Touretters have higher than
average norepinephrine levels.
When a Touretter is on a dopamine blocker
medication, however, the body feels that messages that should be getting through
are not.
This is a weird thought to grasp but for Touretters, tics are
the desirable normal state for the body, and it will fight to keep them.
So
the body will stop producing as much norepinephrine in order to increase dopamine
levels and help the messages get through more easily.
The problem with this is not only are you getting sedation from the lack of
dopamine getting through (because producing more dopamine won't get it past the
blockers, just like having more people on one side of a wall won't get them
through it) you get sedated by the sudden drop in brain norepinephrine.
Some
people feel their memory gets weaker if they are on large doses of anti-psychotics
and this would be why. Lower norepinephrine levels mean that the body has
a bit more trouble forming new memories.
Serotonin, often known as 5HT (but actually not), is another type of
neurotransmitter. It affects many of our positive and negative experiences
of life, so it is often known as the “happy drug”. Very generally,
high levels of
serotonin bring on good moods - low levels bring on low moods. For this reason antidepressant drugs work to delay the breakdown of
serotonin and thus lift our mood.
Serotonin is produced in the body from Tryptophan, a chemical found naturally in
high carbohydrate foods. As can be seen in the diagram to the left, about
10% of the body's tryptophan intake is converted to serotonin via the
stage of 5HT (5-hydroxytryptophan). The other 90% is converted to
kynurenine.
Serotonin acts on three different receptors numbered 5HT1, 5HT2 and 5HT3.
When serotonin acts on a 5HT1 receptor it mainly causes disruption of neurochemicals. This is the receptor pathway that ensures you are not flooded with serotonin, the 5HT1 receptors stop production of any more serotonin, both in the body and in the brain. It also causes the levels of noradrenaline released into the body to drop.
When a 5HT2 receptor comes into contact with serotonin it causes a stress management reaction. The human body can be stressed in many ways; injury, hard work, fear. The reaction of 5HT2 is to cause inflammation of any injured area (e.g. bruising), create faster breathing, send more energy to muscles, contract the smooth muscle in the body (i.e. the stomach to slow down digestion) and basically speed up the metabolism.
5HT3 receptors are the ones that make this the "happy" drug. Serotonin is converted in the brain through a neurochemical pathway into a feeling of well being. (Don't ask me how this works...no-one knows yet.)
What this means in Tourette syndrome is quite convoluted. It has been
shown by many researchers that the levels of serotonin in Touretters is quite
low. This is immediately made obvious by the fact that anti-depressants
can help control tics.
Dr Comings(1990,1991) went further to find that there is a problem in the
Tryptophan to 5HT to serotonin pathway in Touretters. Instead of turning
10% of our dietary tryptophan into serotonin we only use 0 - 4%. This means
our serotonin production is very low and our kynurenine levels are high.
|
Oh crud! I always guessed kynurenine was somewhat toxic...after
all, it does have the word "urine" in it and that's one of the body's most
toxic wastes. But recent research seems to show that where you find
high levels of kynurenine, you find brain damage. Most of the
research has been done on HIV patients (HIV causes the higher levels of
kynurenine), but there have also been studies that show kynurenine
inhibitors (things that stop more of it being made) can reduce the risk of
Alzheimer's disease, inflammatory brain conditions, and dementia. |
So what does this cause physically?
If a low amount of serotonin hits the 5HT1 receptors then norepinephrine is
released in larger quantities, making the whole norepinephrine/dopamine problem
worse than it already is. This is probably the main reason why SSRI's work
on tics, because they would reduce the amount of norepinephrine in the body.
If a low amount of serotonin hits the 5HT2 receptors the body is less able to cope with stress. You are more likely to react badly to temperature changes, low food levels, and injury.
If a low amount of serotonin hits the 5HT3 receptors, you get depressed. This tends to also make you eat more in an attempt to raise serotonin levels.
All this means that it is important to have decent serotonin levels in the
body and the brain. This is easy for the body, you can just take a serotonin
tablet. But the brain is protected by the BBB (blood brain barrier) which
screens out most chemicals from the blood, including serotonin. The way we
deal with this is to use SSRI's. Selective Serotonin Re-uptake Inhibitors stop
the serotonin in the brain from being broken down, so there ends up being more
around.
However, SSRI's won't work forever.
As has been said before, the body has a preset idea of what is a
"normal" amount of neurochemical to have. With serotonin in
Touretters, that level is quite low. But if you put more serotonin into
the system via SSRI's, then the brain will make even less natural
serotonin. The artificially higher levels will also begin to "burn
out" the serotonin receptors, so more is needed to get the same result.
This is the reason why SSRI levels have to be upped occasionally as the drug
just stops working. It also explains the huge withdrawal after stopping
SSRI's, the body is not producing serotonin, but you need high levels to get any
reaction...in Touretters this translates to depression and lots of
tics.
To understand a bit how this "burn out" works:
"Imagine a pool. It starts off full of salt water. By the side of the pool sits
a life guard, his job is to make sure that (a) the pool is full and (b) that
there isn’t too much salt in the pool. However this particular pool has a slow
leak from the bottom. To counter this ten men are hired to walk to a nearby sea,
fill a bucket with sea water, and walk back to pour the water into the pool.
In a normal pool, the men fill their buckets from the Mediterranean sea, which
has exactly the right level of salt for the pool. The pool stays full, and the
salt level stays correct.
In a Tourette pool, however, the men fill their buckets from the dead sea. The
dead sea has 4 times the salt of the Mediterranean, so as time goes on, the salt
levels in the pool rise. The life guard notices that the pool is getting too
salty, so as the next man comes to tip in his bucketful, the life guard talks to
him, and he puts down his bucket and heads to the bar. When the second guy comes
and tips in his bucket, the lifeguard also talks to him, and one by one the men
put down their buckets and go to the pool bar.
Because the pool is leaking, the salt levels in it begin to drop, but so does
the water level. It actually starts to drop quite suddenly after the last bucket
is put in, and soon the water level has gone from full to too shallow to swim.
The life guard runs over to the bar, grabs the men and they run back to the
pool, pick up their buckets, and begin running to the sea to fill them up. The
water level in the pool begins to rise at a moderate speed, but soon the men are
drunk from the bar and are tired from all the running. So the life guard hires
another 5 men to help bring in the water. So now there are 15 men lugging water
from the sea to the pool. The pool level rises a bit, until the lifeguard
notices that the pool is getting to salty…"
So although SSRI's are very effective, they should be considered carefully before any are taken.
Dr Comings (1990, 1991) believed that the serotonin gene at 4q31 was the GTS gene, but that has been disproved.
The best way to raise serotonin levels is exercise. Not only are levels raised during the activity, they stay high for a couple of days afterwards.
It has long been known that androgens (such as testosterone, the male sex hormone) affect Tourette syndrome. This is why Tourette syndrome affects three times more males than females. Also, Shapiro et al. (1988) and Scahill (1990) reported that anabolic steroids made Tourette syndrome symptoms worse.
The reason these androgens aggravate Tourette syndrome is probably related to serotonin. It is known that steroids lower
serotonin levels, and it is probable that high levels of other androgens will as well.
Cortisol is known as the “stress hormone”. It maintains blood pressure and cardiovascular function, reduces the immune system’s inflammatory response, balances the effect of insulin in breaking down sugar for energy, and regulates the metabolism of proteins, carbohydrates and fats. When the body is stressed, the level of cortisol in the body increases. (e.g. athletes, pregnant women, depressed people, alcoholics, malnourished people and those with panic disorders have higher than normal levels of cortisol.)
It is believed by many that cortisol has a negative effect on Tourette syndrome; that is, it makes tics worse. So it may be cortisol that makes tics worse under stress.
Discoveries (Biol. Psychiatry 1996 – as cited by J. M. Howard 1997) have shown that Touretters have significantly higher levels of corticotropin-releasing factor, the precursor to cortisol, than normal.
Dehydroepiandrosterone (DHEA) is an “anti-cortisol” hormone according to Howard. DHEA has been known to have calming effects and is packaged by some companies as a “wonder anti-aging drug!” It is believed that low levels of DHEA will add to the negative effect of high cortisol levels.
The strange behaviour of antipsychotics and neuroleptics when used to treat Tourette syndrome can be partially explained by DHEA/cortisol reactions. It is believed that these drugs cause the release of prolactin, the precursor to DHEA. This increased DHEA counters the high cortisol levels. The drugs, unfortunately, overwhelm the natural production of DHEA and “burn out” the adrenal glands. This then means the levels of cortisol rise again, usually to a higher level than where they started.
This means the brain is not getting enough DHEA. Howard suggests that tics are actually small seizures designed to stimulate DHEA production. How this works:
"...the increased melatonin found in untreated epileptics builds
up and is released so that nerves are shut down. Individuals susceptible to
epilepsy must have entire sections of the brain shut down so much that they
"rebound" and call up a large response of DHEA. It is this rebound response that
is the large area of stimulated nerves that cause the seizures. Once the brain
has stimulated sufficient DHEA, then the seizure stops."
So TSers may find that (directly affected by DHEA levels) this rebound in small
sections of the brain (instead of large sections as in epilepsy) causes not an
overall muscle contraction seisure; they cause a small nerve impulse to be sent;
which gets multiplied by the Dopamine receptor abnormalities described above. So
the brain's attempt to create DHEA, in order to relieve stress, may cause tics
that way.
This (in my mind) is actually the most plausible explanation for how tics start.
Stress → need for more DHEA → mini seisure → small electrical impulse +
excess dopamine receptors
= TICS
DHEA levels are known to start low in childhood and increase during adolescence to peak at about age 25. It is also common knowledge that Tourette syndrome often decreases in severity or disappears following the same pattern. DHEA then may be the reason for this decrease.
Also, in an article on epilepsy, Howard states:
"I suggest migraines result from low MLT and increased DHEA.
Women produce more DHEA from birth than men. This extra DHEA should have most
effect on migraines prior to the onset of interfering sex hormones, before
puberty. It has been found that "when the onset [of migraines] is below the age
of puberty there is a striking predominance of women over men in a ratio of
3:1," (Headache 1994; 34: S8). It is part of my theory that the
hormone, testosterone, causes DHEA to be used for "testosterone target tissues."
This use of DHEA by these tissues should reduce the availability of DHEA, i.e.,
increased testosterone should decrease migraines. "
This is of interest to us as the ratio of Women over Men in Tourette
syndrome is the exact opposite; 1:3. If testosterone does cause DHEA to be
used for "testosterone target tissues" (parts of the body that make men men) and
reduces overall DHEA then not only will women have 3 times more DHEA to
aggravate migraines, they will have 3 times more DHEA to stop tics!