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SURGEONS have been putting patients under with general anaesthesia for 150
years, but ask yours how it knocks you out, and she or he will likely reply, 鈥淚
don鈥檛 know, but count from 10 backwards and I鈥檒l see you later in the recovery
room.鈥� Your surgeon won鈥檛 know how general anaesthesia works, because nobody
does.

Which is quite surprising when you think that a few lungfuls of halothane are
all it all takes to induce profound unconsciousness, immobility and an inability
to feel pain or anything else for that matter. Inhaling anaesthetic will also
make muscles relax, blood pressure and heart rate drop, and slow down breathing.
And as a clincher, you won鈥檛 remember anything at all about the experience when
you wake up.

In fact, if you ask most experts to give you one solid fact about these
wonder drugs, probably the best they鈥檒l come up with is an ancient rule of
thumb: the better the anaesthetic, the more soluble it is in olive oil.

That might sound more like salad dressing than science but according to
Richard Lerner, a biochemist and the director of the Scripps Research Institute
in La Jolla, California, that rule, plus some new evidence he鈥檚 gleaned from a
chemical found in the spinal fluid of cats, could solve the mystery of how
anaesthetics work.

One thing is certain, whoever solves it is guaranteed a fair bit of kudos.
After all, understanding consciousness is one of science鈥檚 most coveted goals,
and finding out how anaesthetics stun you into unconsciousness would probably be
a good place to start. Solving the mystery could also lead to better
anaesthetics with fewer side effects.

The unconsciousness induced by an anaesthetic is arguably the ultimate in
altered brain states, but just like the others鈥攕leep, drug-induced
hallucinations and so on鈥攖he effects depend on altering the electrical
signals that pass between nerves. Firing patterns change according to the flow
of charged particles across the nerve cell membranes鈥攕ome will rush in
while others rush out. So most experts think anaesthetics might work their magic
by binding to specific protein receptors that open and shut these ion channels
(see 鈥淕oing for the gap鈥�).

Lerner, however, suspects that an older, more controversial hypothesis is
closer to the truth. He argues that at least some anaesthetics鈥斺�滻t could
be, horror among horrors, that different anaesthetics work in very different
ways,鈥� he says鈥攕lip into the cell membrane and increase the 鈥渞unniness鈥� of
the annuli, rings of extra-stiff membrane encircling the ion channels. Loosen
these annuli, and the channels will change shape, altering the rate at which
ions flow through them. If anaesthetics really do behave in this nonspecific
way, that could explain why such a wide range of volatile chemicals, from
chloroform to paint stripper, can do the job, albeit with some having far more
unpleasant side effects than others.

Lerner鈥檚 ideas would probably have been rejected out of hand a few years ago,
not least because no neuroactive drug is known to affect cell membranes in this
fashion. However, cell biologists are just beginning to realise how little they
know about the cell membrane and how it reacts with the outside world. It turns
out that the membrane is not simply a homogenous shell, but a patchwork of
different types of fats, clustered together with one type of protein here,
another there. 鈥淲e still don鈥檛 fully understand the structure of the lipid
membrane,鈥� says Roderic Eckenhoff, an anaesthesia researcher at the University
of Pennsylvania in Philadelphia. That means, he says, that people are having to
be far more open-minded about how anaesthetics might alter the permeability of
the nerve cell membranes.

Lerner bases his support for the membrane runniness theory on what he鈥檚
learnt about oleamide, the chemical that he and Scripps colleagues Benjamin
Cravatt and Dale Boger discovered a few years ago in the spinal fluid of
sleep-deprived cats. Injected into the bloodstream, oleamide puts cats and other
animals to sleep, and causes a slight drop in their body temperature. Although
its effect on humans has not yet been tested, oleamide is also present in human
spinal fluid.

Most other drugs that act on the nervous system, such as painkillers, affect
only one type of ion channel. But oleamide鈥攁nd general
anaesthetics鈥攊nfluence a whole range of different membrane channels. For
instance, the team found that oleamide makes gap junctions, which pass
electrical signals between nerve cells, close more frequently. It also alters
the activity of some chloride channels used by the neural messenger serotonin.
In fact, it enhances serotonin鈥檚 sleep-inducing effect. Similarly, an
anaesthetic such as ether shuts the sodium channels that the neurotransmitter
acetylcholine uses to excite nerve cells, and at the same time opens chloride
channels that the neurotransmitter GABA uses to inhibit other nerve cells.

It seems unlikely that oleamide or general anaesthetics could affect so many
different channels if they had to bind to their own specific receptors to do so.
Usually, a particular receptor is associated with one particular channel. What鈥檚
more, Lerner and Bernie Gilula, a chemical biologist at Scripps, discovered that
chemicals with a structure similar to oleamide, but with a double bond shifted a
few carbon atoms to the right or left, can still open chloride channels in nerve
cell cultures. This small change has a significant effect on the shape of a
molecule, which once again suggests that oleamide can鈥檛 be binding to a
receptor, because to do so it would need to fit precisely like a key in a
lock.

Oleamide鈥檚 double bond might also be important for a different
reason鈥攂ecause it helps make lipids runny. The double bond would mean that
oleamide took up more room than your average membrane lipid, so the molecules
would be spread out rather than clumped together. Indeed, the food industry adds
double bonds to convert solid fats into oils. 鈥淵ou couldn鈥檛 design a compound
more disruptive to a membrane than oleamide,鈥� says Lerner. Few anaesthetics can
make that claim, but, he says, most have a bulky molecular structure that should
also make membranes more runny.

The Scripps team has also found an enzyme that breaks down oleamide in the
parts of the brain that deal with sleep and regulating body temperature. The
enzyme, oleamide hydrolase, is not free-floating but linked to the cell
membrane: further evidence that oleamide affects the membrane itself. And since
it has its own synthesis and dismantling machinery, oleamide must be important,
says Lerner. 鈥淢aking oleamide is a pretty fancy thing for cells to do
chemically,鈥� he says. 鈥淣ature isn鈥檛 going to make it for nothing.鈥�

At the moment, however, it鈥檚 anybody鈥檚 guess what oleamide is doing in the
body. Its most likely job is to help regulate sleep. Since the beginning of the
century, researchers have suspected that animals nod off when sleep-inducing
substances reach a certain level in the brain. While that view is probably an
oversimplification, several chemicals have been isolated from animals鈥� spinal
fluid, brain and urine that will induce sleep in another animal or alter the
amounts of rapid eye movement (REM) sleep and non-REM sleep. When oleamide is
injected into rats, it induces sleep that look remarkably natural.

It鈥檚 true that being asleep and under an anaesthetic are very different. But
oleamide is one of only a handful of substances in the body that come anywhere
near to mimicking the effects of a general anaesthetic. So the Lerner team
decided to scrutinise the connection more closely, and what they found made them
wonder if oleamide was indeed the body鈥檚 own version of an anaesthetic.

For a start, oleamide is chemically similar to a natural pain killer called
anandamide, and anaesthetics do of course block pain. What鈥檚 more, anaesthetics
and oleamide both affect a range of different ion channels. Both are soluble in
olive oil. And, with their fatty structures, both feel at home in greasy cell
membranes.

Cells don鈥檛 look oily on the surface. And if you shrank yourself down and
stood on a cell membrane, you would see only a water-loving veneer. But plunge
your fist through that facade, and you鈥檇 be up to your elbows in grease. It was
in 1976 that Tony Lee, a membrane biochemist at Southampton University, first
suggested that anaesthetics might alter the runniness of the annulus. At the time,
Lee had little evidence to support his hypothesis, but now, with their oleamide
work, Lerner, Gilula and their colleagues believe they are close to showing that
he was probably right.

Take the gap junctions in nerve cells, says Gilula. They are surrounded by a
particularly tough annulus of tightly packed lipids, enriched with rod-like
cholesterol molecules. The result is a liquid crystal. If a molecule of oleamide
or an anaesthetic slipped into that annulus, it would totally disrupt the
crystal, Gilula says. Without that support, the gap junction would not be able
to maintain the electrical connection between the cells.

In cultured cells, the channels formed by receptors that bind to a chemical
called acetylcholine usually close in response to anaesthetics, but they don鈥檛
if they are inserted into artificial membranes that lack cholesterol. With no
liquid crystal annulus to disrupt, anaesthetics may have no way to affect the
channel, says Gilula. He has started on experiments that are designed to show
once and for all whether oleamide closes gap junctions by making their annuli
more runny.

Lerner would be the first to admit that they have a long way to go before
they can say for sure whether anaesthetics knock you out because they alter the
runniness of bits of the cell membrane. Still, he likes to point out that
finding the endorphins, the brain鈥檚 own pleasure enhancers, has been the key to
solving how heroin and morphine create their highs. Perhaps oleamide will do the
same thing for anaesthetics. Meanwhile, he says, 鈥渙ne of the most important
tools in medicine remains a mystery鈥�.

A CELL membrane is basically a couple of sheets of fat, held together by the
watery soups of the cell鈥檚 cytoplasm and the extracellular fluid. The fatty ends
of the lipid molecules that make up the membrane are so repelled by the water
that the two layers press against each other, generating pressures of up to 400
atmospheres. According to one researcher, it鈥檚 this high pressure environment
and not the runniness of different parts of the membrane that holds the answer
to how general anaesthetics work.

We know that almost any volatile, fat-soluble chemical, be it halothane or
nail polish remover, can knock you unconscious, but how? According to a
mathematical model of the membrane created by Robert Cantor of Dartmouth College
in Hanover, New Hampshire, whenever an anaesthetic molecule muscles its way into
the tight press of lipids, all the molecules reshuffle to distribute the added
pressure throughout the membrane, much like commuters when one more person
crowds into a train.

That suggests, says Cantor, that anaesthetics might work by altering the
pressure in the membrane so that ion channels are either squeezed shut or given
lots of space to open. That would alter the flow of ions from one side of the
membrane to the other and influence neural activity. Because the pressure within
the cell membranes is so immense, just a slight redistribution of pressure
triggered by relatively few molecules of anaesthetic could have a profound
effect on nerve cell activity.

The best general anaesthetics dissolve well in olive oil and slip easily into
fatty cell membranes, suggesting to some researchers that membrane lipids are
key to how the drugs work. But a more popular theory has been that anaesthetics
directly target proteins in the membrane.

The first good evidence for that hypothesis came in the 1980s from
experiments with luciferase, a protein that makes firefly tails glow green and
which is never found in cell membranes. When researchers added anaesthetics such
as halothane to a solution of luciferase, it cut the emissions of green light by
half. Since the luciferase solution was completely free of any bits of cell
membrane, the anaesthetics must have acted directly on the protein.

Still, it seems unlikely that anaesthetics act like most other neuroactive
drugs and fit snugly into specially designed binding sites on receptor proteins,
so throwing the switch that opens or closes the ion channels.

Instead, researchers such as Neil Harrison of the University of Chicago
suspect that anaesthetics work by gluing up the cavities that are part and
parcel of a large protein such as a nerve cell receptor. Once the anaesthetic
molecule is in place, it stops the receptor from changing shape and opening or
closing the channel in response to other chemicals
(see This Week, 30 May 1998, p 12).

EVER since a team led by Richard Lerner at Scripps Research Institute in La
Jolla, California, discovered oleamide, a sleep-inducing chemical, in the spinal
fluid of cats, they鈥檝e been trying to work out what it does. The obvious first
choice is that it regulates sleep, but Lerner thinks it could be doing something
else as well.

Oleamide鈥檚 chemical structure includes a double bond that makes it perfect
for increasing the runniness of lipids. What鈥檚 more, the enzyme that dismantles
oleamide is concentrated in cell membranes, and it can be turned on and off at a
moment鈥檚 notice. So Lerner suggests that oleamide plays some quite general role
in modulating membrane runniness. After all, the membrane must be exactly the
right consistency for ions and signalling molecules such as hormones to be able
to travel across it, and for the cell to stretch, grow and divide.

Next to nothing is known about how mammals keep their membranes at the right
consistency. But some bacteria use enzymes to rapidly add double bonds to the
lipids in the membranes to stop them solidifying in the cold. Carp use a similar
rapid response mechanism to keep their membranes fluid when they swim into icy
water.

True, mammals are warm-blooded and their body temperature varies very little.
Nonetheless, says Lerner, they probably also need some way of stopping membranes
congealing in their extremities when cold weather strikes. Oleamide could also
help compensate for fluctuations in lipid composition during normal membrane
breakdown and repair, he says.