COUNT slowly backwards from 10. Before you reach seven, you’ll be out like a light.
Without anaesthesia, surgery would be, and once was, excruciating. Yet, as anyone who has been put under will attest, general anaesthesia is a pretty drastic medical intervention itself: a sudden and total shutdown of consciousness. It’s not hard to see it as a little foretaste of death. General anaesthesia was first used for surgery in the 1840s. The shocking thing is that we still don’t really know how it works.
We do know that anaesthetic agents suppress signalling between neurons in the brain. We think we know which molecules the agents hit. But just how they do their silencing job is a mystery.
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Fortunately, this lack of knowledge doesn’t stop anaesthetists wielding the drugs effectively. However, a clearer picture of what happens could not only help to avoid the rare but very real dangers of anaesthesia, but also help us develop more precisely targeted drugs and give us a better idea of consciousness itself, and what it means to toggle it on and off.
Recently, we have uncovered a few more clues as to how it all works, through research in biochemistry and more surprisingly in biophysics. But just how close are we to solving the mystery of how anaesthesia turns out the lights?
Many drugs work more or less in a “lock-and-key” fashion. They block biochemical processes because they precisely match the shape of specific molecules’ binding sites. And this seems to be how some classes of anaesthetics, including barbiturates, interrupt communication between our neurons.
But there is a befuddling diversity to the substances that can knock us out: from large-molecule steroids to untethered individual atoms (see diagram, right). Consider xenon, a gas that exists as lone atoms that don’t undergo ordinary chemical interactions with anything else. These bland, unresponsive balls are about as far away as you can imagine from the exquisitely shaped molecules of most drugs. Yet today xenon is a fairly common anaesthetic. How can such a simple substance have such a remarkable effect?
Xenon isn’t very compatible with water, which prefers charged particles such as salt ions. Because it doesn’t have concentrations of positive or negative charge, xenon sits more easily in a nonpolar environment, like the fatty insides of the membranes around our cells.
This, it turns out, is a common feature of many anaesthetics, from nitrous oxide to chloroform, as German pharmacologist Hans Meyer and British physiologist Charles Overton discovered more than a century ago. The pair independently found that the potency of many anaesthetics corresponds with how readily they dissolve in olive oil: the more soluble the substance, the less of it you need to induce unconsciousness. This relationship is even stronger when it comes to their solubility in real membranes like those of cells, made from fatty acids called lipids. Meyer and Overton both figured that anaesthetic agents must accumulate within the membranes and make them swell or distort, altering their ability to transmit signals.
Electrical signals pass along nerve cells through the movement of positively or negatively charged ions. These flow in and out of the cell via ion channels – proteins in the cell membrane that are arranged into the shape of a tunnel. When the difference in voltage between the inside and outside of the cell reaches a critical threshold, this triggers a signal and neurotransmitters are released at the synapse, or junction, with neighbouring cells. These chemicals flow across the gap and latch onto ion channels at the next cell, where, depending on the type of neurotransmitter, they either boost or dampen onward signalling.
The Meyer-Overton model suggested that anaesthetic molecules might be absorbed into cell membranes at the synapses and block cell-to-cell signalling, possibly by causing the sheets of fatty molecules to swell and shut the ion channels.
“There is a befuddling diversity to the substances that can knock us out”
It was a neat idea, but too simple. In 1997, chemist Robert Cantor of Dartmouth College in New Hampshire suggested a . He argued that rather than indiscriminately swelling the membrane, anaesthetics jostle the molecules around ion channels, affecting how they are packed together and changing the curvature of the membrane itself. But details of how this might work remained sketchy, and the resulting changes would be so small that it was hard to see how they could make much difference.
But now there are hints that such small effects can indeed have big consequences. Physicist Ben Machta at Princeton University and biophysicist Sarah Veatch at the University of Michigan in Ann Arbor think that anaesthetics may affect the “critical temperature” of the cell membrane, making the system sensitive to slight changes. This is a basic concept in physics: at a critical point or critical temperature, a system can undergo an abrupt change of its state. A magnet can lose its magnetism, or a mixture of two liquids can separate, for example.
In 2012, Machta and Veatch, working with physicist James Sethna at Cornell University in Ithaca, New York, argued that close to a critical temperature, the molecules that make up the cell membrane are constantly rearranging themselves. In the membrane there are “rafts” of regularly packed molecules, mostly cholesterol and saturated fats, that drift within a more disorderly matrix of unsaturated fats.
The researchers assumed that some ion channels only open, or open most easily, when surrounded by particular molecules – a cluster of cholesterol, for instance. Close to the critical point, these molecules are more active, so the rafts are constantly appearing and dissolving all through the membrane. In this case, there is a good chance the ion channel will acquire the surroundings it needs to open.
Tipping point
If, however, anaesthetic molecules join the membrane and alter the temperature needed to reach this critical state, the ion channels won’t get exposed to the same variety of environments and may stay shut. “Cells, or drugs, could fine-tune the activity of channels by changing the membrane,” says Veatch. “Maybe this is what happens in general anaesthesia.”
In a series of experiments, Machta and Veatch showed that alcohols with anaesthetic properties, such as ethanol, do indeed , meaning the membrane would have to be colder than usual for the rafts to acquire such dynamic variation in shape and form. What’s more, they also found that two lipid-loving drugs that ought to act as anaesthetics according to the Meyer-Overton rule but don’t, fail to alter this critical temperature.
In contrast, some compounds, such as hexadecanol, raise the critical temperature. So would they suppress anaesthesia? Last year, that’s just what Machta and Veatch found. In tadpoles at least, hexadecanol can .
Machta says they are hoping to test their ideas further by finding ways of looking directly at the environment surrounding ion channels at the molecular scale.
A more exotic possibility is proposed by biophysicist Luca Turin of the Alexander Fleming Biomedical Sciences Research Centre in Vari, Greece. Turin is no stranger to big, if controversial, ideas. He has previously made the case that our sense of smell works by quantum physics. Instead of picking up scent from the shape of particular molecules, he argued that we pick up on their vibrations, which influence electrons jumping across gaps in our olfactory receptors.
Now he thinks some general anaesthetics do something similar. Xenon “has no chemistry and no shape, but it has physics”, Turin says. He suggests xenon might insert itself directly into proteins and influence signalling. It could do this by providing new, energetically favourable pathways along which individual electrons would use the magic of quantum physics to jump from one part of the molecule to another. If an anaesthetic did use such electron currents, this should show up as changes in a property called spin, which is detectable for lone hopping electrons but not when they are paired up in chemical bonds. And in fruit flies knocked out by drugs including xenon, nitrous oxide and chloroform, Turin and his team have indeed .
It’s hard to know what to make of such broad-brush measurements. But Turin’s challenge to conventional wisdom doesn’t stop there. He points to some experimental evidence that , but instead bind to membrane proteins in mitochondria, the compartments within cells that produce energy. “What is the connection between mitochondria and neuronal function?” Turin says. “Nobody knows, but there the treasure is buried.”
The mainstream view, though, sticks with synaptic ion channels – and largely with the notion that anaesthetic molecules hit them directly, not via some influence on their membrane environment. Some of the most potent, intravenously delivered anaesthetics do seem to bind directly to such ion channels. And the activity of these drugs is highly sensitive to shape – mirror-image forms of the same molecules have different potencies – which points to a conventional lock-and-key mechanism.
Ethereal ideas
Stuart Forman, an anaesthesiologist at Massachusetts General Hospital in Boston, says there is some evidence that even inhaled, small-molecule anaesthetic agents bind directly to ion channels. Besides, says neuroscientist Peter Århem of the Karolinska Institute in Stockholm, Sweden, the fact that anaesthetic molecules prefer fatty environments doesn’t necessarily mean that they head for the membranes as some of the other theories suggest. They might instead stick to water-repellent cavities on the proteins in the ion channels. “But I would happily welcome better explanations,” he says.
So, however intriguing these new physics-based ideas may be, they have their work cut out if they are to persuade many experts. Forman has learned to be wary of theories based on biophysics rather than biochemistry. “I call them zombie theories,” he says. “Experiments can’t breathe any life into them, but neither can they be definitively killed off.”
Zombie theories or not, solving the mystery of anaesthesia is more than an academic matter. Sometimes there are serious complications (see “Putting you under: When anaesthesia goes wrong“). Death from anaesthesia is rare, but it does happen; less serious side effects are fairly common. Knowing what is really going on should let us design better drugs. Here Forman thinks that the more potent lock-and-key molecules are the most promising, because they are more amenable to design – you can’t, after all, do much to tune the effects of an agent like xenon. What’s more, these more powerful drugs can be used in lower doses, and can hit their targets more selectively.
The case isn’t yet cracked, but the latest theories could provide opportunities to limit potential harms. At the very least, they show how inventive we are forced to be in trying to understand what really goes on when the lights go out.
This article appeared in print under the headline “When the lights go out”
