Narcotics

• ANATOMY OR SYSTEM AFFECTED: Brain, nervous system, psychic-emotional system
• SPECIALTIES AND RELATED FIELDS: Pharmacology
• DEFINITION: The use of drugs from the opiate family, which mimic the action of the body’s own painkilling substances, to treat pain, anxiety, coughing, diarrhea, and insomnia.

The Effects of Narcotics

Narcotics are drugs commonly used to treat pain (analgesics), suppress coughing, control diarrhea, and aid in anesthesia. These drugs are some of the oldest and most used agents. Most drugs are able to alter the effects of body functions by mimicking naturally occurring chemicals (as with agonists) or by blocking the effects of these chemicals (as with antagonists). Today, narcotics are more commonly referred to as "opioids." Examples of opioids include the illicit drug heroin and pharmaceutical drugs such as codeine, fentanyl, OxyContin, morphine, and methadone.

Researchers have examined the many effects of the substances derived from the opium poppy, including morphine, codeine, and heroin. They have identified opioid-like chemicals called endorphins, dynorphins, and enkephalins that act as neurotransmitters; that is, some opioid compounds are normally found in the human as substances that allow nerve cells to communicate with one another. The synthetic opioid morphine mimics the actions of endorphins, dynorphins, and enkephalins by taking the place of these neurotransmitters. Scientists and physicians now know that morphine and related compounds produce their major effects by acting on the central nervous system, which includes the and spinal cord. To understand how affect the body’s response to pain, one must first understand the of pain.

Any time tissues are damaged, they release chemical substances into the space outside the damaged cell, known as the extracellular space. Sensory neurons that have the ability to detect these chemicals are known as pain neurons. Once the chemicals bind to receptors on a pain neuron, the is stimulated to send an electrical message to the spinal cord. Two actions occur once this message arrives. The first is an immediate initiation of a reflex which attempts to remove the tissue from the source of injury. For example, when one accidentally places an arm on a hot stove, a neural reflex causes the muscles of the limb to retract the arm from the burner. This is accomplished when the pain neuron releases a chemical message (neurotransmitter) in the spinal cord to stimulate neurons that control the muscles of the affected limb. This is known as substance P. The second action of the pain neuron is to inform the brain of the tissue damage, so that appropriate behavioral modification can take place. For example, one may become more cautious around the kitchen after burning one’s arm on the stove. Notification of the brain is also accomplished by activating a second neuron using the neurotransmitter substance P, which will carry this electrical message toward the brain. Morphine and related opioids are very effective analgesics that seem to alter this pathway, thus dampening the of pain messages.

Morphine-like drugs act at several sites in the nervous system. One of the most clinically important places is within the spinal cord at the region where the pain neurons release substance P. Opioids are known to reduce the amount of substance P that is released and thereby to decrease the stimulatory message in the neural pathway to the brain. If the pain impulses traveling to the brain are reduced, so is one’s perception of pain. The second area of the nervous system known to be involved in regulating the perception of pain is a diffuse area of neurons located between the brain and spinal cord referred to as the brain stem. When researchers stimulate a region of the brain stem, the pain impulses traveling to the brain are reduced. Opioid peptides have been identified in this area which, as in the spinal cord, are probably responsible for reducing the pain message.

Because opioids must act to mimic endogenous opioids, one may wonder why there is a need for narcotic drugs if the body already produces the opioid-like endorphins and enkephalins. The reason is that every individual has a different degree of pain tolerance. How much pain one can endure also changes with certain circumstances. For example, one hardly notices the pain of a cut when participating in an exciting outdoor game. If the same wound occurs while one’s attention is focused on it, however, the cut becomes noticeably painful. Perhaps the best explanation for the differing interpretation of pain during these activities and among different people is the endogenous opioid system. It is likely that the acupuncture pins used to block pain messages cause neurons to release increasing amounts of endorphins, enkephalins, and dynorphins. In the same way, with the administration of narcotics, one artificially increases the amount of opioids in the body in order to block pain impulses.

Opioids act on the to affect several systems other than the one associated with pain. They suppress coughing in a way that is similar to their effect on neural signals to decrease pain messages to the brain. Narcotics seem to inhibit release of the neurotransmitters responsible for the cough reflex. Unfortunately, opioids can activate another area in the brain stem to produce nausea and vomiting. This unwanted effect is related to the dose and type of drug used. Therefore, physicians can usually diminish the response with appropriate treatment selections. Perhaps the most dangerous problem with opioid usage is the effect that opioids have on the brain stem’s regulation of respiration. When the brain stem senses that the level of carbon dioxide is too high, breathing is increased to rid the body of this excess waste gas. Narcotics decrease the responsiveness of the brain stem to carbon dioxide. Therefore, breathing rates tend to be inappropriately low, causing a buildup of carbon dioxide.

Constriction of the pupils of the eyes is a very common effect of opioids on the visual system. In fact, this constriction serves as an important diagnostic clue in examining a patient who has taken an overdose of a narcotic.

Opioids have a constipating effect, indirectly through the and directly through their influence on the intestines. Opioids cause a decrease in peristalsis, the series of muscular contractions of the intestinal wall that would normally move food toward the anus.

Most opioid analgesics have no direct effect on the heart and blood vessels. Thus, they do not alter or rhythm or blood pressure to any significant degree. The only noticeable effect of narcotics on the system is a flushing and warming of the skin because of a slight increase in blood flow to the skin. Occasionally, this is accompanied by sweating. Kidney function tends to be depressed by opioids, which may be attributable to a decrease in the amount of blood that is filtered through the kidneys. There is also a decrease in the ability to urinate, as these drugs increase contraction of the muscle that prevents urine from leaving the bladder.

Uses and Complications

Medical personnel use this knowledge of how narcotics alter the body to the patient’s advantage. Narcotics are used in the relief of pain and anxiety, as sedatives and anesthetics, to reduce coughing, and as a way to control diarrhea.

Opioid analgesics are among the most effective and valuable medications for the treatment of serious pain. Morphine-like drugs dampen the pain response but do not affect to a great extent other senses such as vision and hearing. They are often used to treat pain in the period, in which they effectively reduce or eliminate the short-term pain from tissue that is caused by surgery. When pain is reduced, patients tend to eat, sleep, and recover much more rapidly. Physicians often prescribe narcotics such as meperidine (Demerol) or codeine on an as-needed basis. In this way, the patient, who knows firsthand the effectiveness of the drug, can control the frequency of administration. In fact, patients are usually advised to administer a small dose before the pain becomes too intense, thus decreasing the pain message before it reaches a high level and requires a relatively high dose to make the patient comfortable again.

A painful sensation consists of the neural response to the tissue damage and the patient’s reaction to the stimulus. The analgesic properties of narcotics are related to their ability to diminish both pain perception and the reaction of the patient to pain. These drugs effectively raise the threshold for pain, perhaps because of the euphoria experienced by patients given opioids. For example, a patient in pain who is given morphine experiences a pleasant floating sensation with a great in distress and anxiety. It is interesting to note, however, that some subjects do not experience euphoria when given morphine. In fact, they tend to have an unpleasant response known as dysphoria, which often includes restlessness and a feeling of general discomfort.

Physicians and other health care workers must achieve a delicate balance between alleviating pain from known causes and masking pain as a warning signal from unexpected sources. For example, a patient having abdominal surgery would likely require relatively high doses of narcotic analgesics to reduce the postoperative pain. Yet the administration of an analgesic could mask the pain from an unexpected abdominal infection. Therefore, if used excessively, narcotics may prevent the early recognition of complications.

In addition to their analgesic effects, opioids tend to have a sedative effect and are often used as a preanesthetic drug or as an anesthetic. Potent opioids are used in relatively large doses to achieve general anesthesia, particularly in patients undergoing heart surgery. These narcotics are also commonly used during other surgeries in which it is important that heart function be affected only minimally. Examples of narcotic agents used in include fentanyl (Sublimaze), sufentanil, alfentanil, and propafol.

Suppression of the cough reflex is a clinically useful effect of narcotics. The therapeutic doses of opioids needed to reduce coughing are much lower than the doses to achieve analgesia. The opioid derivatives most commonly used to suppress the cough reflex are codeine, dextromethorphan, and noscapine. How these agents work to reduce coughing is not known, but they are thought to act on the brain stem.

Diarrhea from almost any cause can be controlled with opioids. Diphenoxylate (Lomotil) and loperamide (Imodium), narcotics commonly used to treat diarrhea, do not possess analgesic properties. These drugs appear to act on the nerves within the intestinal tract to decrease muscular activity.

Like all drugs, narcotics have both beneficial and undesired effects. The toxic effects of an opioid depend on the dosage, the agent used, the clinical condition in which it is used, and an individual patient’s response to the drug. Some of the more common unwanted effects include restlessness and hyperactivity instead of sedation, respiratory depression, nausea and vomiting, increased pressure within the brain, low blood pressure, constipation, urinary retention, and around the nose. Most of these conditions are of short duration and resolve themselves after the drug has been discontinued.

Patients, or more often narcotic drug abusers, may become tolerant and dependent upon these agents. These individuals, such as heroin abusers, have a strong craving for the drugs. These agents are abused for their euphoric effect at relatively high doses. The human body is very efficient in tolerating the effects of opioids. Their effects lessen somewhat with each succeeding dose, so that a higher dose must be taken to achieve the same effect. Physiological to the long-term use of opioids (two to three weeks) causes the development of tolerance for these drugs.

Exogenous opioids take the place of endogenous ones. Therefore, the nervous system and other physiological systems attempt to bring the levels of these neurotransmitters back to normal. First, the liver speeds up its of the drugs to eliminate them from the system more rapidly. Second, the regions of the nervous system that respond to opioids become desensitized by reducing the number of neural receptors that are available. Finally, after a few weeks of high levels of opioids, changes in other areas of the brain attempt to compensate for the rising opioid levels. Individuals who abruptly stop taking the drugs enter a period of withdrawal in which the symptoms are similar to a bad case of influenza. Morphine and heroin withdrawal symptoms usually start within twelve hours of the last dose. Peak symptoms of narcotic withdrawal occur after one to two days. Most symptoms gradually subside and are usually gone after one week. It should be emphasized that, under a physician’s direction, the abuse potential of narcotics is very low.

There are certain clinical conditions in which opioid drugs should not be used or should be used with extreme caution. Because of the potential for respiratory with opioid treatment, these drugs should not be administered to patients with head injuries or impaired lung function. Most opioid drugs can cross the placenta and therefore should be avoided during pregnancy; with long-term use, the infant can be born addicted to narcotics.

Fortunately, some drugs can reverse the effects of narcotics. Three opioid antagonists are nalmefene, naloxone (Narcan), and naltrexone (Trexan). When these agents are given in the absence of an opioid agonist, they have no noticeable effect. When administered to a morphine-treated patient, however, they completely reverse the opioid effects almost immediately. These narcotic antagonists are particularly useful in treating patients who have taken an overdose of opioids. Such patients often arrive in the hospital emergency room not breathing and in a coma. These antagonists will normalize respiration, restore consciousness, and counteract other opioid effects. Interestingly, individuals who have become tolerant to and dependent upon opioids will immediately experience withdrawal symptoms when given naloxone or naltrexone.

Perspective and Prospects

Narcotic drugs were originally found in the opium poppy five thousand years ago. Opium is obtained from the milky of the unripe seed capsules of the poppy plant. The juice is dried in the air and forms a brown, sticky substance. With continued drying, the mass can be pulverized into powder. It is this powder that contains opioids. Morphine, codeine, and papaverine are the natural opioids that are used clinically. Most other narcotics are chemically derived.

The opium poppy, Papaver somniferum, was named after the Roman god of sleep, Somnis. Ancient Egyptian medical texts listed opium as a cure for illness and as a poison. Although opium was used extensively, the abuse potential was low because the poppy has a very bitter taste. Smoking opium became popular in eighteenth century China as a treatment for severe and was also used as a socially acceptable drug mainly for its euphoric effects.

The opium poppy contains more than twenty distinct agents with a variety of potencies and unwanted effects. In 1806, a pharmacist refined opium into one active substance, morphine, which was found to be ten times as potent. Morphine was named after Morpheus, the Greek god of dreams, because the drug has powerful sedative effects. The discovery of other medically active agents quickly followed. Codeine and papaverine were identified next and found to be slightly less potent than morphine. At this time, clinicians used these purified products rather than the crude opium juice.

Shortly after purified narcotics became available, so did the widespread use of hypodermic needles. This allowed physicians to administer narcotics directly into the bloodstream. The injected opioids would travel via the blood to the brain in as short a time as twenty seconds. In the United States, morphine found widespread use as an analgesic for wounded soldiers during the Civil War. It was one of the most powerful painkillers available to physicians, but its unrestricted availability created great potential for with long-term use.

Opioids became so popular that hundreds of medications became available to the public. These tonics promised to cure everything from “tired blood” to common aches and pains. Their widespread, unregulated use produced a large number of addicts. At the beginning of the twentieth century, the US government attempted to reduce the number of addicts by making it illegal to buy any opioid-containing without a prescription. Medical scientists tried to synthesize compounds with morphinelike characteristics but without the addictive effects.

Physicians now have available to them a wide range of narcotics with different pharmacological properties. For example, there are drugs without addictive, euphoric, or sedative properties that can treat coughing or diarrhea. Some of these are available without a prescription for the treatment of occasional coughing and diarrhea. Narcotic analgesics, however, are given only under the direction of a physician. For example, morphine is still used as a potent pain reliever; when it is used appropriately, there is little chance of addiction. It is likely that other clinical uses will be found for narcotic drugs as researchers learn more about the human body’s own endogenous narcotics, the endorphins.

• Key terms:
• agonist a drug that acts in a fashion similar to that of a hormone or neurotransmitter normally found in the body
• analgesia the absence of pain; analgesics are compounds that stop the neurotransmission of pain messages
• antagonist a drug that acts to block the effects of a hormone or neurotransmitter normally found in the body
• brain stem the region between the brain and spinal cord that controls such functions as respiration and heart rate
• central nervous system the brain and spinal cord
• dependence a craving for a drug
• endogenous something naturally found in the body, such as neurotransmitters
• exogenous something originating outside the body and administered orally or by injection
• neuron a nerve cell that can conduct electrical impulses from one region of the body to another; it is capable of releasing neurotransmitters
• neurotransmitter a chemical substance released by one nerve cell to stimulate or inhibit the function of an adjacent nerve cell; a chemical message released from a neuron
• opioids drugs derived from opium; also known as narcotics or opiates
• tolerance the ability to endure ever-increasing amounts of a drug

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