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Analgesics

Analgesics, or analgesics (from the Greek. Algos - pain and an - without), are called medicines that have a specific ability to weaken or eliminate the feeling of pain.
Analgesic (analgesic) effect can have not only the actual analgesics, but also other substances belonging to different pharmacological groups. Thus, drugs used for anesthesia (general anesthesia) may have an analgesic effect, and some of them at appropriate concentrations and doses (for example, trichlorethylene, nitrous oxide) are used specifically for analgesia. Local anesthetics are essentially analgesic in essence. For pain associated with smooth muscle spasms, antispasmodic and anticholinergic agents can have an analgesic effect.
By analgesic (analgesic) in the proper sense of the word means means, the dominant effect of which is analgesia, resulting from resorptive action and not accompanied in therapeutic doses by deactivation of consciousness and marked impairment of motor functions.
According to the chemical nature, nature and mechanisms of pharmacological activity, modern analgesics are divided into two main groups. A. Narcotic analgesics, including morphine and close to it alkaloids (opiates) and synthetic compounds with opiate-like properties (opioids). B. Non-narcotic analgesics, including synthetic derivatives of salicylic acid, pyrazolone, aniline and other compounds.
For narcotic analgesics characterized by the following main features.
1. Strong analgesic activity, providing the possibility of their use as highly effective painkillers in various fields of medicine, especially for injuries (surgical interventions, injuries, etc.) and for diseases accompanied by severe pain syndrome (malignant neoplasms, myocardial infarction, etc.) .
2. A special influence on the human central nervous system, expressed in the development of euphoria and the appearance of the syndromes of mental and physical dependence (drug addiction) with repeated use, which limits the possibility of long-term use of these drugs.
3. The development of the disease state (withdrawal syndrome) in individuals with developed syndrome of physical dependence while depriving them of the analgesic drug.
4. Removal of acute toxic effects caused by them (respiratory depression, cardiac abnormalities, etc.), as well as the analgesic effect by specific antagonists (see Naloxone).
With repeated use of narcotic analgesics, addiction (tolerance) usually develops, i.e., a weakening of the action, when ever higher doses of the drug are required to obtain an analgesic effect.
The effect of narcotic analgesics is not limited to the analgesic effect. In one degree or another, they have a hypnotic effect, inhibit breathing and cough reflex, increase the tone of the intestine and bladder, can cause nausea, vomiting, constipation and other side effects.
In connection with a pronounced narcotic potential (i.e., the ability to cause addiction with the addiction syndrome) and significant changes in the activity of the central nervous system and other body systems associated with it, all narcotic analgesics are to be stored, prescribed and dispensed from pharmacies according to special rules.
According to the severity of the analgesic action and side effects, different drugs of this group differ from each other, which is associated with the characteristics of their chemical structure and physicochemical properties and, accordingly, with the interaction with the receptors involved in the implementation of their pharmacological effects.
In recent years, a number of previously used narcotic analgesics with pronounced narcogenic potential (tecodine, hydrocodone phosphate, fenadone, some finished dosage forms containing opium or codeine) have been excluded from the nomenclature of drugs.
At the same time, a number of new synthetic highly effective narcotic analgesics have been created.
According to the sources of production and chemical structure, modern narcotic analgesics are divided into 3 groups: a) natural alkaloids - morphine and codeine, contained in a hypnotic poppy; b) semi-synthetic compounds obtained by chemical modification of the morphine molecule (ethyl morphine and others); c) compounds resulting from the complete chemical synthesis (promedol, fentanyl, pentazocine, nalbuphine, butorphanol, tramadol, etc.). Most of the synthetic drugs obtained by the principle of modification of the morphine molecule with the preservation of the elements of its structure or its simplification. In this way, a set of drugs was obtained, differing in the strength and duration of the analgesic effect, in the speed and degree of addiction, addiction and other side effects, in their ability to inhibit the intestinal motor activity and exert an antidiabetic effect (see Loperamide), as well as for the antitussive effect, etc. Some drugs have a spasmolytic effect on smooth muscles (see Promedol).
By chemical modification of the morphine molecule, compounds that are its pharmacological antagonists have also been obtained (see Naloxone).
The mechanism of action of narcotic analgesics is not well understood.
Neurophysiological studies indicate the inhibition of narcotic analgesics by thalamic pain sensitivity centers and blocking the transmission of pain impulses to the cerebral cortex. This effect is, in all likelihood, the leading in the physiological mechanism of action of analgesics of this group.
In the neurochemical aspect, it is important to study the effect of these substances on the neurotransmitter processes of the brain and peripheral organs. There is evidence of the inhibitory effect of morphine on the hydrolysis of acetylcholine and its release from nerve endings. It is established that morphine activates serotonin biosynthesis. At the same time, reserpine, which decreases serotonin content in the brain, weakens the analgesic effect of morphine and its analogues.
In general, there is insufficient data on the role of this neurotransmitter in the mechanism of action of narcotic analgesics. Data on the role of adrenergic processes and the GABAergic system are also contradictory. Shifts in the GABA system under the influence of various opiates and similar substances are ambiguous.
Of particular importance for understanding the mechanism of action of opiates are recent data on the presence of specific <opiate> receptors in the brain and other organs. Endogenous ligands, i.e. The neuropeptides enkephalins and endorphins that bind to these specific receptors, which are formed in the body by physiologically active compounds, are neuropeptides. Enkephalins are pentapeptides containing five amino acid residues. The so-called met-enkephalin (methionine-enkephalin) has the structure: H-tyrosyl-glycyl-glycyl-phenylalanyl-methionyl-OH. Endorphins have a slightly more complex structure. So,
-endorphin (one of the most active endorphins) consists of 31 amino acids. Enkephalins and endorphins, binding to opiate receptors, have an analgesic effect, and their effect is blocked by specific opiate antagonists. The binding of morphine to these receptors is ensured by the fact that a certain part of its molecule has a structural and conformational similarity with a part of the molecules (the tyrosine residue) of enkephalins and endorphins. Thus, exogenous analgesic morphine (as well as other opiates and opioids close to it in structure) interacts with the same <binding sites> (receptors), which are intended for binding endogenous analgesic compounds - enkephalins and endorphins.
It is possible that the action of exogenous analgesics is also associated with the stabilization of endogenous neuropeptides (enkephalins, etc.) by inactivating enkephalin-destroying enzymes, enkephalinases.
It has now been established that endogenous ligands for opiate receptors are not only enkephalins and endorphins, but also other peptides (dinorphins, etc.). It is also shown that opiate receptors exist in the form of different subpopulations: (mi), (kappa), (delta), (sigma), with different functional significance. The receptors are believed to mediate supraspinal analgesia, euphoria, respiratory depression and physical dependence; - receptors mediate spinal analgesia, miosis, sedation, etc. Different endogenous peptides and narcotic analgesics can bind mainly to one or another subgroup of receptors, which can determine the features of their pharmacological action.
Different narcotic analgesics also differ in the nature of binding to opiate receptors. Some of them (morphine, promedol, fentanyl, etc.) are <pure> (full) agonists; by binding to receptors, they have a physiological (pharmacological) effect characteristic of endogenous ligands. Others are <pure> antagonists (see Naloxone). By binding to receptors, they block the action of endogenous ligands and exogenous opiates. The third group includes drugs of a mixed type of action (antagonist agonists), which bind differently to different subgroups of opiate receptors and have in this connection an agonistic effect on one type of action, and antagonistic effect on others (nalorphin, pentazocine, nalbuphin, etc.) .
The effect of narcotic analgesics on peripheral organs (intestines, etc.) is also associated with the interaction with opiate receptors located in them.
The pronounced local anesthetic effect is not observed in the majority of opiates. However, in recent years it has been found that they have a strong general analgesic effect when epidural and subarachnondal administration. This effect is associated with direct effects on the neuronal systems of the spinal cord, which are involved in the formation of a painful flow of impulses. This method of administration of opiates has recently become increasingly used for the relief of severe acute and chronic pain.
The following main features are characteristic of non-narcotic analgesics.
1. Analgesic activity, manifested in certain types of pain: mainly with neuralgia, muscle, joint pain, headache and toothache. With severe pain associated with injuries, abdominal surgery, etc., they are practically ineffective.
2. Antipyretic effect, manifested in febrile conditions, and anti-inflammatory effect, expressed in varying degrees in different compounds of this group.
3. The absence of a depressant effect on the respiratory and cough centers.
4. The absence in their application of euphoria and phenomena of mental and physical dependence.
The main representatives of non-narcotic analgesics are:
a) salicylic acid derivatives (salicylates) - sodium salicylate, acetylsalicylic acid, salicylamide, etc .;
b) pyrazolone derivatives - antipyrine, amidopyrine, analgin;
c) para-aminophenol (or aniline) derivatives - phenacetin, paracetamol.
In the mechanism of action of non-narcotic analgesics, the influence on the thalamic centers plays a role, which leads to inhibition of the conduction of pain impulses to the cortex. According to the central action, these analgesics differ, however, from narcotic analgesics by a number of features (for example, they do not affect the ability of the central nervous system to sum up subliminal pulses).
In the mechanism of action of salicylates, an important role is played by the inhibition of the biosynthesis of prostaglandins (see acetylsalicylic acid), as well as a stimulating effect on the pituitary-adrenal axis <p>>, which promotes the release of corticosteroids. Essential in the action of non-narcotic analgesics is their influence on the kinin system (antagonism with the action of bradykinin and others).

Subgroup Analgesics include drugs: