The Spark of Life: Electricity in the Human Body (9 page)

Hodgkin and Huxley’s experiments generated much excitement and led to an annual migration of scientists to the marine laboratories at Plymouth and Woods Hole. As squid are migratory and the scientists had teaching duties this inevitably turned into ‘summer camp for scientists’, and – particularly at Woods Hole – led to a hothouse of experiments and ideas. Squid were in short supply and the best squid were keenly fought for so that a pecking order quickly developed. By the mid-1960s the hectic scramble for squid was so considerable it motivated some scientists to find a place to work in the winter, and Montemar, near Valparaiso in Chile, provided the perfect place. There was an added bonus, as the Chilean squid – and their axons – are much larger.

Although many other cell types are used to investigate the mechanism of the nerve impulse today, including mammalian brain cells, the squid axon remains a valuable preparation for scientific study. In Plymouth during the 1940s the few squid caught were so mangled by the trawlers’ nets that they did not survive long once they arrived back at the lab, which meant that experiments had to be conducted immediately. Because the boats did not return until late afternoon, this usually meant working throughout the night. Consequently, Hodgkin and Huxley spent their mornings catching up on sleep and planning experiments. This was also true when I visited Woods Hole in the 1980s, when many scientists crawled into bed around 4 a.m. after a hard night at the bench. In Chile today, many squid are caught by rod and line and consequently suffer less damage. But their huge size means they are less easy to keep in holding tanks, so scientists still face the night shift.

I vividly recall from the time I spent at Woods Hole that the axons which generated the best results were commemorated in a most singular fashion. At the end of the experiment, they were flicked onto the ceiling of the laboratory, which eventually acquired a pattern of dried-out squiggles, somewhat reminiscent of a Jackson Pollock painting. Only the very best axons, however, were ‘sent to Heaven’.

Fire!

Sodium and potassium channels that open in response to changes in the voltage gradient across the cell membrane are the keystone of electrical signalling in our brain, heart and muscle. In resting nerve cells, both kinds of channel are tightly shut. When the nerve is stimulated, first the sodium channels and then, with a short delay, the potassium channels swing into action producing a transient change in membrane potential – the nerve impulse. But what triggers the whole thing off?

Crucially, the sodium and potassium channels that are involved in the action potential are sensitive to voltage and they open if the membrane potential is made more positive (depolarized). This is exactly what happens when a nerve cell is excited by an incoming signal from another nerve cell, or by an externally applied electric shock. The larger the change in membrane potential this produces, the more sodium channels open and the more sodium ions flood into the cell. You may recall that Ohm’s law dictates that a change in current will produce a concomitant change in voltage. In a nerve cell the sodium current drives the voltage more positive, which opens more sodium channels, which makes the membrane more positive, which opens more channels, and so on and so on in a positive feedback cycle. This explains the explosive, all-or-none nature of the action potential.

Two things return the membrane potential to its resting level. The sodium channels do not remain open forever at positive membrane potentials, but eventually close, a process known as inactivation. Secondly, the potassium channels open, so that potassium ions rush out of the cell, restoring the charge imbalance and sending the potential negative once more. It’s just as well that the potassium channels open later than the sodium channels because if they opened at the same time the sodium and potassium currents would cancel each other out and there would be no nerve impulse, and no thoughts or actions.

Terrible Stuff

The importance of sodium and potassium channels in generating the nerve impulse is demonstrated by the fact that a vast array of poisons from spiders, shellfish, sea anemones, frogs, snakes, scorpions and many other exotic creatures interact with these channels and thereby modify the function of nerve and muscle. Many are highly specific and target a single kind of ion channel. Which brings us back to Captain Cook and the puffer fish.

The tetrodotoxin contained in the liver and other tissues of this fish is a potent blocker of the sodium channels found in your nerves and skeletal muscles. It causes numbness and tingling of the lips and mouth within as little as thirty minutes after ingestion. This sensation of ‘pins and needles’ spreads rapidly to the face and neck, moves on to the fingers and toes, and is then followed by gradual paralysis of the skeletal muscles, resulting in loss of balance, incoherent speech, and an inability to move one’s limbs. Ultimately, the respiratory muscles are paralysed, which can be fatal. The heart is not affected, as it has a different type of sodium channel that is not targeted by tetrodotoxin. The toxin is also unable to cross the blood–brain barrier so that, rather horrifyingly, although unable to move and near death, the patient remains conscious. There is no antidote and death usually occurs within two to twenty-four hours. In 1845, the surgeon on board the Dutch brig
Postilion
, sailing off the Cape of Good Hope, observed that two seamen ‘died scarcely seventeen minutes after partaking of the liver of the fish’. However, victims can recover completely if they are given artificial respiratory support until the toxin has washed out of the body – which takes a few days.

Hiroshige’s ‘Amberjack and Fugu’. The puffer fish (fugu) is the smaller fish.

In Japan, the puffer fish is known as fugu, and is considered a great delicacy. Unfortunately, the fish is expensive in more ways than one, as unless it is carefully prepared the flesh can be toxic, and every year several people die from tetrodotoxin poisoning. Most incidents arise from fishermen eating their own catch. Restaurant casualties are far rarer because all fugu chefs must now be specially trained and licensed, which involves passing a rigorous test. Nevertheless, it occasionally happens. One of fugu’s most celebrated victims was the famous Japanese kabuki actor Bando Mitsugoro who died after eating it in 1975: he had demanded four servings of the liver, which is especially dangerous, and the restaurant had felt unable to refuse such a distinguished customer. Perhaps this is why fugu is forbidden to the Emperor of Japan. Properly prepared, the fish is supposed to cause a very mild intoxication and produce a stimulating, tingling sensation in the mouth. On the single occasion when I tried it myself, I found it rather insipid: it was the spice of danger that enlivened the dish.

Not all cases of fugu poisoning are due to the deliberate ingestion of the fish. In 1977, three people died in Italy after eating imported puffer fish mislabelled as anglerfish. Ten years later, two people in Illinois developed symptoms resembling those of tetrodotoxin poisoning after eating soup made from imported frozen ‘monkfish’. Analysis by the FDA confirmed the presence of the toxin and triggered a mass recall of all sixty-four crates of the imported product. Claims lawyers instantly leapt into action. Poisoning from commercially cooked shellfish is also worryingly common in China and Taiwan: between 1997 and 2001 three hundred people were intoxicated and sixteen died.

A wide variety of animals contain tetrodotoxin, from reef fish, crabs and starfish to marine flatworms, salamanders, frogs and toads. Most use it as a biological defence, but some, like the deadly blue-ringed octopus, package it in venom to poison their prey. It was a mystery why so many different kinds of animal should make tetrodotoxin until it was discovered that it is actually made by a bacterium (
Psuedoalteromonas tetraodonia
) that the animal eats or harbours within its intestine. Puffer fish reared in the absence of such bacteria do not contain tetrodotoxin. Whether such fish, in which the element of Russian roulette is removed, will be as highly prized by aficionados as the native fish is an interesting question.

The fictional British agent James Bond (007) appears to have a special attraction for tetrodotoxin, for he has been poisoned with it on no less than two occasions.
From Russia with Love
ends on a moment of high drama when the SMERF agent Rosa Klebb kicks him with a poison-filled spike mounted on the tip of her boot and he is left to die. Bond, of course, is invincible and the next novel (
Dr No
) begins with him recovering from what we learn is a near fatal dose of tetrodotoxin – ‘terrible stuff and very quick’. He survives only because his companion administered artificial respiration until medical help arrived. Bond also has an encounter with a blue-ringed octopus, whose bite is laced with tetrodotoxin, in the film
Octopussy
. As ever, 007 emerges from these incidents shaken but not stirred.

Red Tides and Suicide Potions

When conditions are right, spectacular blooms of the algae
Alexandrium
can occur that turn the sea the colour of blood. The deleterious effects of such red tides have been known for centuries and the biblical account of one of the great plagues of Egypt paints a vivid picture: ‘all the waters that were in the river were turned to blood. And the fish that was in the river died; and the river stank and the Egyptians could not drink of the water of the river.’ Such red tides are composed of millions of minute algae, known as dinoflagellates, which produce a number of virulent neurotoxins, including saxitoxin. Like tetrodotoxin, saxitoxin blocks sodium channels. Filter-feeding molluscs like mussels and clams may ingest the dinoflagellates, thereby concentrating the toxins they produce, and creatures that in turn feed on them may be poisoned. As much as 20,000 micrograms of saxitoxin per 100 grams of tissue (250 times the legally allowed limit) has been recorded in Alaskan mussels: at this level, consumption of a single mussel can kill you. Even more frighteningly, a single green shawl crab from the Great Barrier Reef can contain enough toxin to kill 3,000 people. Dinoflagellates are most abundant in spring and summer, due to the higher sunlight levels and warmer waters, which may be the origin of the old adage ‘Do not eat shellfish unless there is an R in the month’.

In developed countries shellfish poisoning is very rare, due to intensive surveillance programmes and stringent regulations which ensure that, once detected, the affected areas are quarantined and shellfish sales prohibited. In the last decade, seasonal outbreaks of paralytic shellfish poisoning (mainly due to saxitoxins) have led to temporary bans on the sale of shellfish from waters around the world. The Alaskan shellfish industry has been radically affected as the butter clam is toxic for large parts of the year. But while commercial seafood is safe, this is not necessarily the case for shellfish that people collect themselves. Between 1973 and 1992 there were 117 cases of paralytic shellfish poisoning in Alaska, 75 per cent of them between May and July. Fortunately only one person died, but many required hospitalization. The most dramatic outbreak in recent years happened in 1987 in Guatemala, when 187 people were affected by eating clams and 26 of them died.

Tetrodotoxin and saxitoxin are molecular mimics. Each physically plugs the external mouth of the sodium channel pore, is almost equally potent at inhibiting channel function, and produces similar physiological responses. Both are also valuable research tools because they block sodium channels rather specifically, leaving most other channels untouched. Tetrodotoxin is routinely used in scientific studies today to block sodium channels and enable other channels to be studied in isolation. Its action was discovered by Toshio Narahashi in 1962, working round the clock throughout Christmas and New Year with John Moore and William Scott. Narahashi recalls that the reviewer of their manuscript jotted down a request for some of the toxin at the bottom of his report. It was to be the first of many such requests.

By now you might be wondering why butter clams are not affected by the saxitoxin they contain and why puffer fish swim happily around, despite high tetrodotoxin levels. The answer is that the affinity of their own sodium channels for the toxin is dramatically reduced because evolution has changed one or more of the amino acids in the toxin-binding site. A similar mutation is found in the cardiac sodium channel, which helps explain why your heart continues to beat even when your respiratory muscles are totally paralysed.

Saxitoxin was exploited by US agents engaged in covert government operations both as a suicide and an assassination agent. It has the advantage that it is highly poisonous so that only tiny amounts (which can be easily concealed) are needed, and it is faster and more effective than cyanide. Because it is stable, water soluble and about a thousand times more toxic than synthetic nerve gases such as sarin, saxitoxin (known as agent SS or TZ) was also stockpiled by the US government as a chemical weapon. It was extracted from thousands of butter clams laboriously collected by hand in Alaska. In 1969/70, President Nixon halted the US biological weapons programme and ordered existing stocks to be destroyed, in accordance with a United Nations agreement. But five years later, Senator Frank Church, Chair of a Select Committee on Intelligence investigating the CIA, discovered that a middle-level official had failed to do so. About 10 grams of the toxin, enough to kill several thousand people, still remained in downtown Washington in direct violation of the presidential order. It had been packed into two one-gallon cans and stored in a small freezer under a workbench, which must have caused some consternation to its discoverer.