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Antihistamines have a clear effect on histamine-mediated allergic diseases. However, antihistamines still have many adverse reactions. One day passes and one sleeps and sleeps. Therefore, it is very necessary to correctly and reasonably grasp the use of antihistamines.

Histamine source and metabolism

The source of histamine

Precursor: Histidine (rich in histidine in fish and shrimp seafood) is stored in the form of granules in mast cells, basophils, and platelets; histamine can also be synthesized in gastric parietal cells and nerve endings.

Histamine metabolism

The main metabolic site is the skin, the main route: histamine (histamine methyltransferase) – methyl histamine (monoamine oxidase) – methyl imidazole acid; second route: histamine (diamino oxidase ) – imidazole propionic acid – oxazole imidazole (extracted from the body)

 

Histamine receptor classification and function

H1 receptor

(1) (bronchial, gastrointestinal, uterine, etc.) smooth muscle contraction: snarl, abdominal pain;

(2) Skin vasodilation: erythema, wheal, edema;

(3) Pain-producing substances: acting on pain receptors (a kind of free nerve endings): pruritus;

(4) Atria, atrioventricular node: Increased contraction, slowed conduction.

 

H2 receptor

(1) Vasodilation: Decreased blood pressure and shock (heamin a large number of blood);

(2) Gastric parietal cells: Increased gastric acid secretion;

(3) Ventricular and sinoatrial node: The heart rate is increased and the myocardial contractility is increased.

 

H3 receptor

(1) negative feedback regulates the synthesis and release of histamine in the central and peripheral neuronal cells;

(2) reduce histamine release from mast cells;

(3) decrease the release of tachykinin from the airway without myelin C;

(4) inhibit airway cholinergic and non-cholinergic nerve fiber excitation.

 

H4 receptor

Unlike the HI1, H2, and H3 receptors, which are present in peripheral blood lymphocytes and HL-60 cell lines, their function is unknown.

Unfinished class

Unclassified histamine receptor (H5) is present in cells.

 

The central role of histamine

Histamine receptors in the brain mainly include H1, H2, and autoreceptor H3 that feedback inhibits the synthesis and release of histamine. Brain histamine plays an important regulatory role in many central activities such as: neuroendocrine regulation, drinking water regulation, body temperature regulation, learning and memory, awakening-sleep, exercise and aggressive behavior.

1. Histamine can improve cognitive function: the central inhibitory effect of H-R blockers. The reason may be that histamine enhances excitatory synaptic transmission in the hippocampus (which is related to memory), thereby improving the decline in cognitive function caused by Alzheimer’s disease. Our experiments also found that histamine, histidine and the cholinesterase inhibitor tacrine can improve memory impairment in rats caused by nucleus basalis damage, so histamine may also enhance the activity of cholinergic nerves in the basal ganglia. Increased choline content in the hippocampus improves cognitive performance.

2. Intracerebral histamine has antiepileptic effects: long-term administration of histamine H1 receptor antagonists pyratin and ketotifen are prone to cause epileptic seizure frequency in children with febrile convulsions and epilepsy; the center of chlorpheniramine (chlorpheniramine) Excitement. The antiepileptic effect of histamine in the brain is mainly related to histamine H1 receptor and histamine H3 receptor, but not to histamine H2 receptor.

3. Histamine can delay the death of late neurons: this effect may be mediated by H1, H2 receptors, and its underlying mechanism remains unclear, pending further study.

 

Rational use of histamine

medication should be based on the condition

When life threatens during the emergency phase of an emergency, when histamine has been released in large amounts, physiological antagonists such as epinephrine should be used first.

When an allergic reaction is imminent, that is, administration of an antagonist before histamine release is more effective than histamine release or after release.

General allergy: The first generation of traditional antihistamines such as chlorpheniramine, cyproheptadine, and promethazine can be used. Severe allergies: Efficient, rapid-acting second-generation antihistamines such as astemizole, terfenadine (mildanti), cetirizine, kaempitam, and avastin (xin minole) can be used . Chronic allergy: High-efficiency, long-acting antihistamines can be used, such as astemizole (astemizole), tranilast, ketotifen, and dactylamine.

use alone / combination therapy

1. Single use: Mainly used for the treatment of urticaria, especially acute urticaria, often an antihistamine can effectively control the condition. The principle is to use the simplest drugs to control the disease. It can be used alone and used alone. Artificial keratitis uses ketotifen more, cold urticaria uses cyproheptadine, and stress urticaria has more choices of hydroxyzine, but individual differences should be noted.

2. Joint application: There are several joint forms

(1) The combination of more than two antihistamines, the most commonly used is the combination of the first generation of antihistamines and the second generation of antihistamines, such as the combination of chlorpheniramine and loratadine, which is usually taken in the morning. In the second generation of antihistamines, patients can continue to work and study during the day and take the first generation of antihistamines at night so that patients can have a good nighttime sleep;

(2) Combinations of anti-H1 receptor drugs and anti-H2 receptor drugs, which can be used for individual chronic urticaria, such as cetirizine plus ranitidine, are often better than anti-H1 receptor drugs alone ;

(3) The combination of antihistamines and other anti-allergic drugs, this combination of methods used most, such as antihistamines plus vitamin C, antihistamines plus calcium gluconate, antihistamines plus glucocorticoids, etc. In urticaria, eczema, neurodermatitis, pruritus, drug eruption and other skin allergic diseases.

 

Precautions

1. Disable: Coma, has taken a large number of central nervous system inhibitors, patients with epilepsy or hepatic insufficiency should be used with caution or disabled;

2. Be careful about career taboos. Drive, work at heights or other workers who require high concentration;

3. Do not use with alcohol or other sedatives; do not metabolize with liver drug enzymes with imidazole antifungals, macrolide antibiotics, cimetidine;

4. Promethazine, cyproheptadine, and diphenhydramine have anticholinergic effects and should be avoided with doxepin; caution should be given to patients with glaucoma and prostatic hypertrophy;

5. Can produce drug resistance, long-term use is best to be different kinds of antihistamines to be compatible; the use of two or more antihistamines can enhance the treatment effect, the selected drugs should belong to different categories: during the day Use drugs that have no sedative effect and have a sedative and hypnotic effect after dinner or before going to bed. Those who take a long time to take drugs should gradually reduce the dose after they are effective, or take a period of time after the symptoms are fully controlled, which can reduce the recurrence of the disease.

6. If the conventional dosage is invalid or the effect is not obvious, as long as there is no serious adverse reaction, a larger dose can also be used (astemizole, terfenadine should not be used excessively). But children and the elderly should pay attention to the amount. It is also possible to use one of the other antihistamines instead of the same drug.

7. Piperazines can be teratogenic;

8. Avoid use within 1 to 2 days before skin allergy test, otherwise it will affect the results;

9. Generally not for external use (external use sensitization). People who are allergic to certain antihistamines are likely to also have allergies to other drugs in this category.

10. The use of antihistamines and corticosteroids can reduce the latter’s therapeutic effect.

11. When using antihistamines, do not use drugs that cause non-immunogenic release of histamine, such as quinine and vitamin B1. At the same time, do not eat beverages and foods that can cause histamine release, such as ethanol, aquatic shellfish, and foods containing proteolytic enzymes.

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