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鈥淲OW, I just went out and risked my life. How cool is that?鈥� So said Kristen
Ulmer after flying almost 25 metres through the air, somersaulting, landing and
careering down a sheer slope on skis.

For more than 10 years, Ulmer has considered herself a full-time 鈥渁drenalin
sports鈥� athlete. As well as being voted the craziest skier in North America, she
rock climbs, paraglides and dabbles in ice climbing and mountain biking. 鈥淚鈥檓
addicted to excitement,鈥� she says. 鈥淵ou don鈥檛 know what鈥檚 going to happen, and
when you pull it off you just feel so good.鈥�

Extreme emotion, whether the result of skiing on thin air, conquering
corporate skyscrapers or betting on risky stocks, causes a surge in adrenalin.
For a select group of thrill seekers, like Ulmer, it makes for a killer buzz.
But for most of us mere mortals, that kind of excitement falls into the realm of
nightmare.

While some thrive under pressure, others crumble under the strain. A pounding
heart, throbbing temples, dry mouth and sweaty palms are about as far from
feeling good as it gets. So what is it that makes one person鈥檚 high another鈥檚
hell?

According to David McCobb, a neurobiologist at Cornell University, our
response to pressure has to do with an internal stress alarm in each of us. And
just one tiny part of one protein molecule could determine when that alarm goes
off. It might be the key to why some of us want to jump off buildings with a
bungee cord attached, while others are content to take the stairs.

Any unusual exertion or crisis situation triggers an avalanche of hormones in
the body. The cells in the adrenal glands store adrenalin in little packets and,
when the signals reaching them from the brain scream 鈥渁ction鈥�, the contents of
the packets are released into the blood. This rush of extra adrenalin makes the
heart beat faster and harder. Blood flow increases and the airways in the lungs
dilate, letting more oxygen flood in. You get a rush of energy and alertness
(see Diagram).

But just how stressful do things need to get before the adrenal gland cells
start emptying their contents? Not only does this differ between individuals,
the threshold of one person may change with time. And McCobb believes he has
discovered why.

Adrenalin is released when potassium channels in the membrane of adrenal
cells open. What McCobb has found is that the proteins making up these channels
can come in two forms鈥攐ne with a small, additional part. Channels without
it are called 鈥淶ERO鈥�, and the extra piece is a protein chain that McCobb calls
STREX, for STRess hormone regulated EXon. If the STREX chain is present, the
channel opens more easily, favouring the rapid secretion of adrenalin.

McCobb says that all mammals make both forms of the channels and, judging by
the animals he鈥檚 looked at so far, probably all vertebrates, too. But the
proportion of channels with the STREX chain varies widely between different
animals. So far he has measured the levels in baboons, cows and seven genetic
strains of rats鈥攅ach with a different degree of nervousness.

He found that anxious rat strains鈥攖he ones that huddle timidly in the
corner of a new cage鈥攈ave more STREX than the relaxed strains. Baboons
have slightly lower levels than the rats. And calm cows have the
lowest鈥攐nly about 15 per cent of a cow鈥檚 channels are of the STREX type.
In contrast, about half of a nervous rat鈥檚 channels have the STREX chain.

鈥淭he whole cow system is tuned down in the mellow direction compared to the
rat,鈥� says McCobb. It makes sense that a large grazer isn鈥檛 too nervy. A small
rodent, on the other hand, is easy prey and needs to be more excitable. After
all, a rustle in the leaves might be a cat ready to pounce.

And it鈥檚 not just between species where McCobb has found differences. Looking
at members of the seven rat strains, he has found that STREX levels can vary
quite widely among individuals of the same species. While rats are a far cry
from humans, these findings might tell us a little about our own love or
loathing of adventure. 鈥淚t鈥檚 inevitable that different individuals have
different set points in general stress response,鈥� he says.

鈥淭he level of STREX will affect how quickly [the] adrenal glands dump large
quantities of adrenalin. And that may have some effect on what [people] get
thrills out of and the propensity to do this again,鈥� says McCobb. Perhaps thrill
seekers need bigger thrills to get the same adrenalin-pumping rush as a more
timid individual, he says. The risk-averse, on the other hand, don鈥檛 need to
push their safety limits as far before they鈥檙e flushed with adrenalin, keeping
them on the straight and narrow.

The key to the perfect high seems to be a modest level of stress hormones.
鈥淲e find a moderate stressor pleasurable,鈥� says neurobiologist Robert Sapolsky
of Stanford University. For high STREXers, watching the shower scene from
Psycho or winning a chess tournament might do the trick, whereas for those
with low STREX levels it might take a little more鈥攁 little whitewater
rafting, say.

Too much of a good thing can be bad, however. Even a rat knows that,
according to Sapolsky. Rats will work like mad, pressing a lever over and over,
to get an intravenous infusion of stress hormones, he says. But they will only
work hard enough to generate the sort of levels that they would secrete in
response to a hefty but not overwhelming stressor.

Stress hormones themselves don鈥檛 actually make you feel good though, Sapolsky
points out. 鈥淚nstead, they evolved to make you feel less awful during physical
stress,鈥� he says. 鈥淚f some predator has torn your guts open, it鈥檚 better to
still try to get out of there, instead of just going into shock.鈥�

McCobb acknowledges the possibility that human thrill seekers may also be
driven by an addiction or need for ever-increasing levels of adrenalin. But
rather than adrenalin itself being addictive鈥� it can鈥檛 get into the brain
and doesn鈥檛 directly change your mood鈥攊t seems to be the resulting cascade
of responses that can hook people. In addition to adrenalin, cortisol and other
members of the glucocorticoid family of stress hormones are released. They in
turn trigger the release of the chemical messenger dopamine. 鈥淣ow dopamine is as
trendy as you can get in neurochemistry as it seems to be all about pleasure,鈥�
says Sapolsky.

Dump lots of dopamine in a short space of time and you get that pleasant
buzz. But with chronic stress, stores of dopamine and endorphins鈥攁nother
family of pleasure chemicals鈥攑lummet. Any pain you had will come back and
depression sets in. 鈥淗ighs can be pretty addictive and downs pretty deflating,鈥�
Sapolsky says.

When the stress ends, you go back to baseline. Or perhaps even below
baseline, if rats are anything to go by. Coming up short after a thrill could
trap you in an addictive cycle, as next time it鈥檒l take even more to reach the
same level of bliss. 鈥淛ust remember, somewhere back when, the youthful Evel
Knievel got a charge out of racing a yellow light鈥攐nly to find it didn鈥檛
quite do it for him next time,鈥� says Sapolsky.

George Koob, a neuropharmacologist at the Scripps Research Institute in
California, sees the pleasure of stress as a two-part process, known in
psychology as the opponent process. Part one is the actual stress event, jumping
out of a plane, for instance. 鈥淭hat鈥檚 unbelievably stressful for anyone,鈥� says
Koob. The pleasure comes once you鈥檝e landed safely on the ground and endorphins
gush through your body. Who goes back to jump out of the plane a second time has
a lot to do with how well they tolerate the first part of the process.

McCobb鈥檚 STREX theory fits with the opponent process very well, says Koob.
鈥淧eople with a huge stress response are less likely to get past part one.鈥� The
low STREXers, on the other hand, don鈥檛 have a problem doing it again. 鈥淎nd the
more they push the envelope,鈥� points out Koob, 鈥渢he bigger part two is.鈥�

Frank Farley, a former president of the American Psychological Association
and now a psychologist at Temple University in Philadelphia, has coined a name
for these envelope pushers鈥攖ype T (for 鈥渢hrill鈥�). The most visible members
of the type T group nowadays are the extreme athletes, who are getting braver
and bolder. But you can get your adrenalin going without jumping off a building
or hang-gliding, Farley remarks. 鈥淵ou can get it in sex or gambling.鈥� And those
who push the frontiers of the mind rather than the body are also thrill
seekers.

Researchers have, for example, measured adrenalin surges during chess
competitions. And Farley refers to Albert Einstein as a mental T type. 鈥淓instein
was way beyond the handrails,鈥� says Farley. 鈥淗e was literally creating his own
new vision of the Universe. What sustained his mental life was the thrill of it
all.鈥� Type Ts need to push their abilities鈥攑hysical stamina or
intellect鈥攖o the limits.

What makes a type T may be at least partly due to two 鈥渢hrill-seeking鈥� genes
that have been described, which affect the dopamine receptors in the brain. One
study found that people with certain addictive and impulsive personality
traits had fewer dopamine receptors in their brain to record sensations of
pleasure and satisfaction. That, goes the theory, drives them to overindulge in
substances or activities that stimulate their existing receptors.

But this is clearly not the whole story. 鈥淪ome people are suggesting there
could be anywhere from five to ten genes,鈥� Farley says. And he agrees that
McCobb may be onto something. One of the additional genes just might be
Slo, which encodes the potassium channels where STREX makes its home.

One compelling reason to suspect that STREX is at least part of what makes a
type T personality is that McCobb鈥檚 low-STREX animals seem to share the
remarkable coolness of human thrill seekers. Cows don鈥檛 dart about and low-STREX
rats are far less nervous than their scurrying cage-mates. 鈥淭hrill seekers are
amazingly calm in the face of adversity,鈥� says Farley. 鈥淭hey鈥檙e low in fear and
low in anxiety.鈥� Just before a big ski run or climb, Ulmer says, 鈥淚鈥檓 not
scared. I鈥檓 completely calm.鈥�

Another type T who was amazingly calm in a dangerous situation, says Farley,
is Bill Clinton. 鈥淒uring the height of the impeachment trials and Monica
Lewinsky affair last year, he gave his State of the Union address. He did a
masterful performance. He even ad-libbed. And sitting in front of him were a
substantial number of people who wanted his head on a platter.鈥�

鈥淗uman progress demands risk taking,鈥� Farley says. 鈥淭he basis of creativity,
of risk taking, of exploring, of adventure, of all those things鈥攖here鈥檚 a
very simple motive: thrill.鈥� In the past, the risk takers were the ones who
explored new lands, led the hunt and fought off enemies. 鈥淎ll those things were
important to the survival of the species,鈥� says Marvin Zuckerman, a psychologist
at the University of Delaware.

For other animals too, taking the odd risk can be well worth the effort.
Sapolsky has been studying wild baboons in the Serengeti in East Africa for
nearly 20 years. It鈥檚 a world in which subordinate male baboons have to
challenge a superior to a fight鈥攁nd win鈥攖o climb the ladder of
success and improve their chances of mating. Sapolsky found that males at the
bottom consistently had higher blood levels of glucocorticoid stress
hormones鈥攗p to two-thirds higher鈥攖han those at the top.

Within the senior ranks, there were also differences that might influence who
remains king of the hill. A cluster of these high-ranking males had
glucocorticoid levels half that of the other high rankers, and behaved
differently. When a rival threatened them from close by, for example, they got
agitated and vigilant, but if their rival鈥檚 behaviour was not threatening, they
remained calm. In contrast, other top males became agitated in both types of
interaction. It may be that those baboons with the highest glucocorticoid levels
are also those with the highest STREX levels鈥攎ore twitchy and averse to
risk.

And that may not be solely due to their genes, says McCobb. Prolonged stress
can crank up the intensity of the adrenalin response by increasing the
proportion of channels with the STREX chain. STREX is more likely to be
incorporated into potassium channels if cortisol levels are high (Science, vol 280, p 443).

鈥淭he effect of stress is to enhance the cell鈥檚 excitability, and tendency to
secrete adrenalin, for days or perhaps longer after a stressful event,鈥� McCobb
says. So the more stress you鈥檝e had in the recent past, the more adrenalin
you鈥檙e likely to secrete during the next crisis. In humans, that might increase
the likelihood of a heart attack or stroke.

鈥淲hether our stress responses are controlled by genes or lifestyle,鈥� says
McCobb, 鈥渨e鈥檇 like to have more control. We don鈥檛 want to secrete more adrenalin
than necessary. There鈥檚 a definite adaptive advantage to being able to back
辞蹿蹿.鈥�

Harnessing the powers of STREX could be an answer. 鈥淏y realising that there
are prolonged or lasting consequences of a stress experience and what they
involve in terms of adrenalin responses, we can advise people to avoid stress
not just for immediate reasons,鈥� he says. Indeed, understanding how experience
affects adrenal function could help us control our response to stress on a
day-to-day basis, he adds. And one day, perhaps, yoga, meditation and soothing
music will give way to gene therapy as the preferred form of stress
management.

But life shouldn鈥檛 be too dull. Risk does give you a sense of excitement, of
being alive, McCobb admits. 鈥淚鈥檝e jumped out of an airplane before and I felt it
for days,鈥� he remembers. 鈥淚 just wanted to go up and do it again.鈥�