Histamine and Its Receptors Quiz

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Questions and Answers

What is the primary characteristic of histamine in its bound form within tissues?

It acts as a neurotransmitter in neural tissues.

It is biologically active and stimulates receptors.

It is biologically inactive and stored in granules. (correct)

It can easily migrate to different tissues.

Which physiological function is NOT associated with histamine?

Regulating gastric acid secretion.

Serving as an energy source for cells. (correct)

Acting as a neurotransmitter in the brain.

Mediating allergic responses.

Where are mast cells primarily found, leading to histamine release in response to tissue injury?

In blood plasma exclusively.

In all body tissues uniformly.

Concentrated at potential sites of injury. (correct)

Only in neural tissue.

What defines histamine as a biological amine?

It contains an imidazole ring. Signup and view all the answers

Which type of cells contain the majority of tissue histamine?

Mast cells and basophils. Signup and view all the answers

What is the primary role of H2 receptors in the body?

Mediate gastric acid secretion Signup and view all the answers

Which of the following receptors is mainly found on leukocytes and plays a role in inflammation?

H4 receptors Signup and view all the answers

What mechanism do H3 receptors utilize to reduce transmitter release?

Decrease in calcium influx Signup and view all the answers

What physiological function is NOT associated with histamine?

Inhibition of gastric acid secretion Signup and view all the answers

What is one way in which histamine is released from mast cells during injury?

Direct release associated with mast cell degranulation Signup and view all the answers

In the brain, which function is NOT attributed to the action of endogenous histamine?

Production of gastrointestinal hormones Signup and view all the answers

Which of the following drugs can lead to the release of histamine without mast cell injury?

Morphine Signup and view all the answers

What is the result of H4 receptor activation in the immune system?

Modulation of leukocyte production Signup and view all the answers

What is one of the most common undesirable effects of certain medications mentioned?

Sedation Signup and view all the answers

What is the main purpose of H2-receptor antagonists?

To decrease gastric acid secretion Signup and view all the answers

Which selective ligands may be beneficial for sleep disorders?

H3 ligands Signup and view all the answers

Which medication class is most frequently used in the US due to their low toxicity?

H2-receptor antagonists Signup and view all the answers

Which mechanism do histamine antagonists NOT utilize?

H3 receptor agonists Signup and view all the answers

Which receptor type is responsible for increasing intracellular calcium levels through phosphoinositol hydrolysis?

H1 receptor Signup and view all the answers

What is the common G protein mechanism coupled with H2 receptors?

Gs Signup and view all the answers

What is the primary pharmacological action of first-generation antihistamines like promethazine?

H1-receptor antagonism Signup and view all the answers

Which agonist acts on H4 receptors but is an antagonist at H3 receptors?

Clobenpropit Signup and view all the answers

Where are H3 receptors predominantly located?

Presynaptic neurons Signup and view all the answers

Which of the following drugs is specifically mentioned as having anti-serotonin activity?

Ciproheptadine Signup and view all the answers

Which of the following statements is true regarding the structural relationship of H1 and H2 receptors?

They are more closely related to muscarinic and 5-HT1 receptors respectively. Signup and view all the answers

Which drug has a usual adult dose of 60 mg?

Fexofenadine Signup and view all the answers

What is a common side effect associated with promethazine due to its anticholinergic action?

Marked sedation Signup and view all the answers

Which cells are H4 receptors mainly associated with?

Eosinophils and CD4 T cells Signup and view all the answers

What effect do H1 receptors have on phosphoinositol hydrolysis?

Increase Signup and view all the answers

What type of pharmacodynamic effect do H1-receptor antagonists inhibit?

Histamine-induced contraction of smooth muscle Signup and view all the answers

Which of the following is a characteristic feature of H3 receptors?

They are predominantly presynaptic. Signup and view all the answers

Which second-generation antihistamine is noted for having a longer action?

Loratadine Signup and view all the answers

What major risk factor is associated with second-generation antihistamines when combined with certain drugs?

Altered metabolization Signup and view all the answers

Which of the following is a use of H1-antagonist drugs during pregnancy?

Management of morning sickness Signup and view all the answers

Study Notes

Histamine

Histamine is a biological amine with a hydrophilic structure.
It's composed of an imidazole ring and an amino acid connected by two methylene groups.
Found in non-neural tissue, including plant and animal tissues, some venoms, and stinging secretions.
Most tissue histamine is contained within granules (vesicles) inside mast cells and basophils.
Stimuli trigger the release of histamine from mast cells, enabling it to act on surrounding tissues.

Histamine Receptors

Four subtypes: H1, H2, H3, and H4.
All are coupled with G proteins (GPCR).
Different receptor subtypes are coupled to different G protein subtypes, resulting in varying intracellular signaling pathways.
H1 receptors are found in endothelium, smooth muscle cells, and nerve endings, increasing phosphoinositol hydrolysis and intracellular calcium levels (Ca²⁺).
H2 receptors are present in gastric mucosa, cardiac muscle cells, some immune cells, and increase intracellular cyclic adenosine monophosphate (cAMP) through Gs signaling.
H3 receptors are mainly presynaptic, reducing neurotransmitter release by decreasing calcium influx through N-type calcium channels.
H4 receptors are primarily found on leukocytes, modulating the production of these cells and potentially mediating histamine's effects on cytokine production.

Histamine's Physiological Functions

Mediator of immediate allergic and inflammatory reactions.
Significant role in gastric acid secretion.
Functions as a neurotransmitter and neuromodulator.
Plays a role in chemotaxis of white blood cells (WBC).
No clinical application as a therapeutic agent.

Release of Histamine

Release occurs through chemical and mechanical methods.
Certain drugs, like morphine and tubocurarine, displace histamine from its complex with heparin and proteins.
Loss of granules from mast cells also releases histamine.
Chemical and mechanical injury to mast cells leads to degranulation and histamine release.
Non-mast cell histamine is also found in various tissues, including the brain.
Endogenous neurotransmitter histamine plays a role in brain functions, including neuro-endocrine control, cardiovascular regulation, thermal and body weight regulation, and arousal.

Antihistamines

Two main generations: First and second generation.
First generation:
- Examples: promethazine, ciproheptadine.
- Often have anticholinergic effects and can cause sedation.
Second generation:
- Examples: fexofenadine, loratidine, cetirizine.
- Less likely to cause sedation.
- Minimal anticholinergic effects.

Pharmacology of Antihistamines

Act through a reversible competitive antagonism at the H1 receptor.
Clinically used to prevent and treat allergic reactions, such as allergic rhinitis and urticaria.
H1 antagonists are also effective in preventing motion sickness when combined with ephedrine or amphetamine.

Adverse Effects of Antihistamines

Common: sedation and antimuscarinic actions.
Less common but serious: excitation and convulsions in children, postural hypotension, and allergic responses.

H2-receptor Antagonists

Significant in treating peptic ulcer disease and related GI complaints.
Decrease gastric acid secretion.
Have low toxicity and are widely used in the US.
Available over-the-counter (OTC).

H3 and H4 Ligands

No selective H3 or H4 ligands currently available for clinical use.
Selective H3 ligands have potential for treating sleep disorders, obesity, cognitive and psychiatric disorders.
H4 blockers may prove useful in treating chronic inflammatory conditions, such as asthma.

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Description

Test your knowledge on histamine, its structure, and functions. This quiz covers the four subtypes of histamine receptors and their roles in various physiological processes. Evaluate your understanding of how histamine interacts with different tissues and the implications for health and disease.