Three years ago, Israeli archaeologists stumbled upon a 1600-year-old tragedy: the remains of a narrow-hipped teenage girl with the skeleton of a full-term fetus still cradled in her abdomen. With her were grey ashes that contained traces of tetra-hydrocannabinol, the active ingredient of marijuana. Could it be that the midwife had administered the plant in a last-ditch effort to bring on labour or to ease her pain?
Today, in nearby Jerusalem, another chemical is in the news -- this one extracted not from ancient ashes but from fresh, pulverised pig brain. It is anadamide, a newly christened chemical that might do naturally in our heads what marijuana does when we choose to smoke it. Anandamide's discovery, along with that of the molecule it binds to in the brain, has marijuana researchers buzzing with the best high they have had in years. The findings provide new hope for therapies that draw on the weed's long list of anecdotal medical uses: as a painkiller, appetite stimulant or nausea suppressant, to name a few. They also throw open windows onto the mysterious workings of our brains.
More recently came other exciting finds: in 1988, Allyn Howlett of St Louis University Medical School discovered a specific protein receptor for THC in mouse nerve cells -- a protein that only THC and its relatives dock onto. Two years later, Tom Bonner's group at the National Institute of Mental Health pinpointed the DNA that encodes the same receptor in rats. It is now known that humans have the receptor, too.
Finding a cannabinoid receptor implies that THC -- unlike alcohol -- has a quite precise modus operandi that taps into a specific brain function. Presumably the drug binds to nerves that have the receptor, and the nerves respond in turn by altering their behaviour. The classic effects of marijuana smoking are the consequences: changes in mood, memory, appetite, movement and perception, including pain. Researchers think THC affects so many mental processes because receptors are found in many brain regions, especially in those that perform tasks known to be disturbed during THC intoxication: in the banana-shaped hippocampus, crucial for proper memory; in the crumpled cerebral cortex, home of higher thinking; and in the primitive basal ganglion, controller of movement.
Once a specially tailored receptor was found, the next step was simple -- in theory, anyway. "The receptor had to be there for a purpose -- presumably it didn't evolve so that people could smoke cannabis and get high," says Roger Pertwee, a pharmacologist at Aberdeen University. Instead, there had to be a natural chemical inside of us that fitted onto the receptor and sent some biochemical signal cascading through the nerve cell to do who knows what. But plucking that one chemical out of a brain stuffed with millions of others was never going to be easy.
Several laboratories set to work on the problem and, fittingly, Mechoulam's was the first to come up with an answer, in the form of a greasy, hairpin-shaped chemical. The researchers dubbed it anandamide, from "ananda", the Sanskrit word for bliss. "The guy discovers the active ingredient of marijuana back in the 1960s, and now, almost 30 years later to the day, he discovers anandamide," says Paul Consroe, a neuropharmacologist at the University of Arizona. "Isn't that great?"
Mechoulam's strategy was to chase after chemicals that, like THC, are soluble in fat. By teasing these substances away from those that are water soluble, his group extracted a substance from pig brain that did indeed bind to the cannabinoid receptor. But did it act like THC? To find out they sent their specimen to Pertwee who had devised a sensitive test for cannabinoids that involved monitoring a substance's ability to stop muscle-twitching in mouse tissue, when dropped on certain nerves. "When it arrived, there was so little of it in the phial I couldn't even see it," Pertwee recalls. "We didn't know what it was - just that it was a greasy substance." But the tests went well: anandamide depressed the twitch just like THC, and last December the researchers published their results in "Science".
The mouse result gave Mechoulam and his group the encouragement they needed to extract more anandamide from pig brains and then analyse and synthesis the chemical in the lab. They also wanted more evidence that anandamide docked specifically onto the cannabinoid receptor and acted like THC, which has a very different molecular structure. And so, with Zvi Vogel and colleagues at the Weizmann Institute near Tel Aviv, they came up with a plan. They would add the DNA encoding the cannabinoid receptor to hamster or monkey cells growing in dishes. The cells equipped with this DNA would then produce masses of receptor, which would sit in the cell membrane ready and available for any chemical "key" that should happen along. Vogel's researchers would add anandamide to the cells and watch what happened.
The results, published in July's issue of the "Journal of Neurochemistry," were clear: anandamide acted as a key, and a precise one at that, sticking only to the cells containing the receptor, and not to others. What's more, when anandamide stuck to the cells, it triggered biochemical changes similar to those associated with THC and related chemicals. Not only did anandamide fit the same lock as THC, but it appeared to open similar doors in the brain.
More tests followed in a number of laboratories, and those researchers found that in every way that has been tested so far, anandamide acts very much like THC. But why would we want such a mind-altering substance in our brains?
Studies on another class of drugs provide a useful parallel. Opiates such as morphine and heroin act upon the body's nervous system to cause euphoria and block pain. In 1973, natural opioids, which behave in the same way as opiates, but have a different structure, were pulled out of the body. It appears that when the body is under serious assault, nerve cells spit out these opioids, which promptly bind to other nerve cells to stop pain signals dead in their tracks. At the same time, they fasten onto sites in the brain to induce a feeling of wellbeing.
Anandamide, like the natural opioids, will probably have its own specific set of jobs to perform in the brain and body. The effects of THC give a rough guide to what these might be: involvement in mood, memory and pain are obvious examples.
But what would the brain be like without anandamide? Researchers intend to find out. Bonner is gearing up to produce a genetically engineered mouse that has no cannabinoid receptors: no receptors, no anandamide function. Others want to tinker with anandamide to make a version that binds to the receptor but doesn't trigger any change in the nerve's behaviour. Added to a mouse, it would stop the body's real, internal anandamide from doing its job. Researchers are also excited by anandamide's possible role in mental and neurological disease. There are also other questions to be asked. If anandamide, like THC, hampers memory, could a drug with the opposite effects -- a "memory pill" -- be made? "It's all speculation for now," says Steven Childers, a pharmacologist at Bowman Gray School of Medicine, North Carolina, "but we like to think about these things."
It will take more time before anandamide is firmly established as the bona fide partner to the cannabinoid receptor. Meanwhile, Mechoulam's lab has two other anandamide-like chemicals waiting in the wings. And in the US, Howlett and Childers both have chemicals of an entirely different kind that bind to the receptor: they are water soluble, not fat soluble. The importance of each remains to be seen.
Whatever anandamide turns out to be, it provides pharmacologists with a fresh plan of attack in their hunt for drugs that act like the cannabinoids. Such drugs could be valuable to help keep at bay the nausea of cancer chemotherapy; to stimulate appetite in AIDS patients; to dampen tremors in neurological disorders; to reduce eye pressure in patients with glaucoma; and to dull pain in those for whom other painkillers do not work.
Cannabinoids can do at least some of these things, with one small drawback [sic.]: they also make the recipient high. The holy grail of cannabinoid therapeutics has been to separate what causes the high from the source of the desired effects, by chemical tinkering with THC or its relations -- shortening a side group on one part of the molecule, lengthening a carbon chain in another -- in the hope that the "undesirable" effects will be lost in the reshuffle. Despite the drug's dubious reputation, several US pharmaceuticals spent several years trying to make this work, but without success. Nor did they reach another equally sought after goal: an antagonist that will block the effects of THC and similar substances when taken.
Until marijuana researchers succeed in doing something along these lines, it is unlikely that drugs companies will pay much attention. "There is a real stigma with working with drugs of abuse," says Billy Martin, a pharmacologist at the Medical College of Virginia. "If drugs companies had three choices of classes of drugs to work on and one was a drug of abuse, they're just not going to work on the drug of abuse." This view is shared by Larry Melvin, who worked on the Pfizer pharmaceuticals company's now defunct cannabinoid therapeutics programme. "What will ultimately legitimise the field in a big way is if researchers can come up with a really good therapeutic ability. Then you'll see the companies turn around."
But Gabriel Nahas, an anaesthetist from Columbia University in New York, who has spoken out against marijuana use for many years, maintains that THC's effects on the brain are too general and too toxic for this route ever to work. The discovery of anandamide and its receptor have not changed his mind. "The brain is a computer," he says. "To put THC in the brain is akin to putting a bug in the computer. I'm sticking to my guns about its harmful effects -- not only to man but to society."
Only time will reveal the value of anandamide and its receptor to drug therapy. But the importance of these discoveries to brain research is not in doubt. "We're no longer just dealing with the pharmacology of a recreational drug," says Pertwee. "We're dealing with the physiology of a newly discovered system in the brain. And that's an enormously bigger field."
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