With which type of G proteins are H1 histamine receptors coupled?

H1 histamine receptors are coupled to G proteins; the specific G protein subtype is not detailed in the referenced sources. Their stimulation leads to the activation of phospholipase C and an increase in intracellular calcium ion concentration. The receptors are localized predominantly on the postsynaptic membrane; activation of this cascade causes smooth muscle contraction and increased vascular permeability.

How does mebhydrolin differ from cetirizine?

Mebhydrolin belongs to the first generation, crosses the BBB, acts for a short duration, and causes a sedative effect. Cetirizine is a second-generation drug; it is more selective and lacks pronounced sedative effects.

Which intracellular mechanism does mebhydrolin interrupt?

It blocks $H_1$-receptors coupled to G proteins, preventing the activation of phospholipase C and the subsequent increase in intracellular $Ca^{2+}$ ion concentration.

Does the drug block all types of histamine receptors?

No, antihistamines possess specificity. Mebhydrolin predominantly blocks $H_1$-receptors responsible for allergic reactions without affecting other types.

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Mebhydrolin

Diazolinum

For medical students2 min readUpdated 2026-10-10

Mebhydrolin (Diazolinum) is a classic antiallergic agent belonging to first-generation $H_1$-histamine receptor blockers. The drug suppresses inflammation and affects immune processes by eliminating the tissue effects of histamine.

Drug ClassH1-histamine receptor blockers

GenerationFirst generation (sedative)

FormulationsCoated tablets (dragees) of 0.05 and 0.1 g

Route of AdministrationOral (0.05–0.2 g)

Classification and Place in Therapy

Mebhydrolin (commonly known as Diazolin) is a representative of agents that affect immune processes and suppress inflammation. In pharmacological classification, it belongs to first-generation antihistamines ($H_1$-receptor blockers).

This same subgroup includes diphenhydramine, clemastine, and quifenadine. It is essential to clearly distinguish them from second-generation drugs (e.g., cetirizine), which exhibit higher selectivity for $H_1$-receptors and do not cause pronounced sedative effects.

Pharmacodynamics and Cellular Mechanism

The action of mebhydrolin is based on interfering with histamine receptors, which are structurally coupled to G proteins.

Normally, the pathogenesis of an allergic reaction at the cellular level proceeds as follows: free histamine stimulates an $H_1$-receptor $\rightarrow$ the enzyme phospholipase C is activated $\rightarrow$ intracellular calcium ion ($Ca^{2+}$) concentration sharply increases.

Mebhydrolin possesses specificity: it selectively blocks $H_1$-receptors specifically (rather than all types of histamine receptors simultaneously, and it does not affect $H_2$-, $M_1$-, or $\alpha_1$-receptors). By occupying the receptor, the drug prevents histamine from initiating the aforementioned cascade involving phospholipase C and calcium.

Pharmacological Effects

By blocking the intracellular release of $Ca^{2+}$, mebhydrolin prevents the development of the tissue effects of histamine. Primarily, it:

Prevents smooth muscle contraction (which is characteristic of allergic spasms).
Hinders increased vascular permeability (which underlies the development of allergic edema).

Thus, the drug effectively suppresses inflammation. However, it is characterized by a short duration of action, which is a typical feature of first-generation antihistamines.

Side Effects and Special Characteristics

The main pharmacokinetic feature of mebhydrolin is its ability to cross the blood-brain barrier (BBB).

Penetration into the central nervous system causes a sedative effect. This is a key side effect of all first-generation drugs that must be considered when prescribing (especially to patients whose activities require high concentration).

Formulation and Clinical Use

According to prescription references, Diazolinum is available in a solid oral dosage form — as coated tablets (dragees).

Dosages: 0.05 g and 0.1 g.
Route of Administration: Strictly oral.
Therapeutic Range: 0.05 to 0.2 g per single dose.

Mnemonic

First generation = Crosses the BBB = Sedation. Mebhydrolin has a short duration of action: the patient sleeps, the allergy sleeps.

Frequently asked questions

With which type of G proteins are H1 histamine receptors coupled?: H1 histamine receptors are coupled to G proteins; the specific G protein subtype is not detailed in the referenced sources. Their stimulation leads to the activation of phospholipase C and an increase in intracellular calcium ion concentration. The receptors are localized predominantly on the postsynaptic membrane; activation of this cascade causes smooth muscle contraction and increased vascular permeability.
How does mebhydrolin differ from cetirizine?: Mebhydrolin belongs to the first generation, crosses the BBB, acts for a short duration, and causes a sedative effect. Cetirizine is a second-generation drug; it is more selective and lacks pronounced sedative effects.
Which intracellular mechanism does mebhydrolin interrupt?: It blocks $H_1$-receptors coupled to G proteins, preventing the activation of phospholipase C and the subsequent increase in intracellular $Ca^{2+}$ ion concentration.
Does the drug block all types of histamine receptors?: No, antihistamines possess specificity. Mebhydrolin predominantly blocks $H_1$-receptors responsible for allergic reactions without affecting other types.

Go deeper

Comparative profile of first- and second-generation antihistamines.
The role of phospholipase C and calcium ions in smooth muscle contraction.
Mechanisms of increased vascular permeability in allergic inflammation.
Mechanisms of drug penetration across the blood-brain barrier (BBB).
Prescription writing rules for coated tablets.