Unlocking Pain's Secrets

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sensory information can be processed by the nervous system at any given moment. When too much information is sent through, certain cells hi the spinal column interrupt the signal, as if closing a gate. Thus, it is reasoned, pain can be prevented from getting through the gate when there is competition from other sensations, like toe rubbing. The theory served as the basis for a now widely used analgesic therapy known as transcutaneous electrical nerve stimulation, or TENS. Electrodes are attached to the skin above a painful area, and a mild current is generated to compete with the pain signals. The stimulation of acupuncture needles is also believed to work, in part, by shutting the pain gate.

The gate-control theory, which has given rise to much new research, has been useful in explaining some of the puzzling psychological aspects of pain. Take for instance the familiar wartime phenomenon of a soldier so immersed in the heat of battle that he does not feel the mine going off beneath his foot. Or the football player so absorbed in making a play that he does not realize he has dislocated a shoulder until the game is over. Neurosurgeon Charles Poletti, of Massachusetts General Hospital, has his own favorite example. As a youth, fresh from the innocence of an all-boys boarding school, Poletti discovered the pleasures of necking with a Wellesley girl one wintry night. Only when he got up off the icy wall where they had been sitting did he feel an excruciating pain in his rump. "I was almost frostbitten," he says. "We must have been sitting there for 45 minutes, and I didn't feel a thing. Something hi my system was suppressing that pain."

About ten years ago, scientists in Scotland discovered what that something might be. Building on the work of American and Swedish researchers, University of Aberdeen Pharmacologists John Hughes and Hans Kosterlitz isolated powerful pain-blocking chemicals that occur naturally in the brain and spinal cord. The substances, called endorphins, switch off the pain alarm, rather like a key fitting into a lock. The locks are receptors on the surface of nerve cells. Opiate drugs like morphine and heroin also fit these locks, activating the body's pain-relief system. "All these years people had been using an extract of the poppy for pain relief," muses Fields. "Finally we have a plausible explanation for how it works."

The endorphin system is just one mechanism that can shut the gate to pain. Since the first endorphin was discovered in 1975, several types of natural opiates have been identified, along with other nonopiates produced in the body that can alter the pain message. Some of these chemicals, called neurotransmitters, not only are associated with pain but are also involved in emotional responses like depression. Doctors have learned that drugs developed to treat depression can also be used, in small doses, as analgesics. Ronald Dubner, of the National Institutes of Health, who spends most of his time trying to unravel the chemistry of pain and analgesia, has come to appreciate the fact that "pain is a complex experience that involves emotions, previous experiences with pain, and what the pain means to us at any given tune." In short, the borderline between the physiology and psychology of pain is a blurry one.

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