This group includes narcotic analgesics (from Greek algos — pain and an — without), which have a pronounced ability to weaken or eliminate the feeling of pain.
Substances with different chemical structures exhibit analgesic activity, and it is realized by various mechanisms. Modern analgesics are divided into two main groups: narcotic and non-narcotic. Narcotic analgesics, having, as a rule, a strong analgesic effect, cause side effects, the main of which is the development of addiction (drug addiction). Non—narcotic analgesics act less strongly than narcotic ones, but they do not cause drug dependence (see Non-narcotic analgesics, including nonsteroidal and other anti-inflammatory drugs).
Opioids are characterized by strong analgesic activity, which makes them possible to use as highly effective painkillers in various fields of medicine, especially in injuries, surgical procedures, wounds, etc. and in diseases accompanied by severe pain (malignant neoplasms, myocardial infarction, etc.). Having a special effect on the central nervous system, opioids cause euphoria, a change in the emotional coloring of pain and reactions to it. Their most significant drawback is the danger of developing mental and physical dependence.
This group of analgesics includes natural alkaloids (morphine, codeine) and synthetic compounds (trimeperidine, fentanyl, tramadol, nalbuphine, etc.). Most synthetic drugs are obtained by modifying the morphine molecule while preserving the elements of its structure or simplifying it. Substances that are its antagonists (naloxone, naltrexone) have also been obtained by chemical modification of the morphine molecule.
According to the severity of the analgesic effect and side effects, drugs differ from each other, which is due to the peculiarities of their chemical structure and physico-chemical properties and, accordingly, to the interaction with receptors involved in the implementation of their pharmacological effects.
The discovery of specific opiate receptors and their endogenous peptide ligands, enkephalins and endorphins, played an important role in understanding the neurochemical mechanisms of opioid action in the 1970s. Opiate receptors are concentrated mainly in the central nervous system, but are also found in peripheral organs and tissues. In the brain, opiate receptors are found mainly in structures directly related to the transmission and encoding of pain signals. Depending on the sensitivity to different ligands, subpopulations are distinguished among opiate receptors: 1-(mu), 2-(kappa), 3-(delta), 4-(sigma), 5-(epsilon), having different functional significance.
According to the nature of the interaction with opiate receptors, all opioidergic drugs are divided into:
– agonists (activate all types of receptors) — morphine, trimeperidine, tramadol, fentanyl, etc.;
– partial agonists (activate mainly mu receptors) – buprenorphine;
– agonists-antagonists (activate kappa and sigma and block mu and delta opiate receptors) — pentazocine, nalorphine (blocks mainly mu opiate receptors and is not used as an analgesic);
– antagonists (block all types of opiate receptors) — naloxone, naltrexone.
In the mechanism of action of opioids, a depressing effect on the thalamic centers of pain sensitivity, which conduct pain impulses to the cerebral cortex, plays a role.
A number of opioids are used in medical practice. In addition to morphine, its prolonged dosage forms have been created. A significant number of synthetic highly active analgesics of this group (trimeperidine, fentanyl, buprenorphine, butorphanol, etc.) with high analgesic activity with varying degrees of “drug addiction potential” (the ability to cause addiction) have also been obtained.
In case of poisoning or overdose with narcotic analgesics, antagonists that block all types of opioid receptors (naloxone and naltrexone) are used.
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