Histamine and Antihistamines, Pharmacology, Animation

Summary

Antihistamines are medications that counteract the action of histamine, a compound involved in allergic reactions, immune response, gastric acid secretion, sleep-wake cycles, and cognitive functions. Histamine binds to H-receptors in various tissues, with H1 and H4 receptors being key players in allergies. H1-antihistamines are primarily used for allergy treatment, while H2-antihistamines treat gastric acid disorders. First-generation antihistamines cause sedation due to their ability to cross the blood-brain barrier, whereas second-generation antihistamines are less sedating and more selective for H1-receptors, offering a targeted treatment for allergies.

Takeaways

• 😀 Antihistamines are medications that counteract the action of histamine, a key player in allergic reactions and several physiological processes.
• 😀 Histamine is synthesized from histidine and is found in tissues like the skin, lungs, and gastrointestinal tract, where it's stored in mast cells.
• 😀 Histamine functions as a neurotransmitter in the brain and is involved in processes like immune response, gastric acid secretion, and the sleep-wake cycle.
• 😀 Histamine exerts its action by binding to histamine receptors (H-receptors), which are G-protein-coupled, with four types (H1 to H4) having distinct functions.
• 😀 H1 and H4 receptors are involved in allergic inflammation, with H1-antihistamines currently used to treat allergies.
• 😀 H2-receptors stimulate gastric acid secretion, and H2-antihistamines are used to treat conditions like gastric reflux and peptic ulcers.
• 😀 The term 'antihistamine' typically refers to H1-antihistamines used for allergy treatment.
• 😀 Allergic reactions occur when mast cells, sensitized to an allergen, release histamine, leading to symptoms such as watery eyes, runny nose, and difficulty breathing.
• 😀 Histamine causes vasodilation, increased blood vessel permeability, sensory nerve stimulation, and smooth muscle contraction, leading to various allergy symptoms.
• 😀 First-generation H1-antihistamines can cross the blood-brain barrier and cause drowsiness, cognitive impairment, and increased appetite, while some are used as sedatives.
• 😀 Second-generation antihistamines are less likely to cross the blood-brain barrier, causing fewer sedative effects, and are highly selective for the H1-receptor.

Q & A

What are antihistamines and what do they do?

Antihistamines are medications that counteract the action of histamine, a compound involved in allergic reactions, immune responses, gastric acid secretion, and other physiological processes.

What is histamine and what role does it play in the body?

Histamine is a compound synthesized from the amino acid histidine. It is involved in various physiological processes, including immune response, sleep-wake cycle, gastric acid secretion, cognitive function, and food intake.

Where is histamine primarily stored in the body?

Histamine is mostly stored as granules inside mast cells, with the highest concentration found in the skin, lungs, and gastrointestinal tract.

How does histamine function as a neurotransmitter?

In the brain, histamine functions as a neurotransmitter and is found in histaminergic neurons in the hypothalamus, whose axons project throughout the brain.

What are histamine receptors and how do they work?

Histamine receptors (H-receptors) are G-protein-coupled receptors that histamine binds to in order to exert its effects. There are four types of H-receptors, each with distinct tissue expression patterns and functions.

What is the primary function of H2-receptors?

The primary function of H2-receptors is to stimulate gastric acid secretion, and H2-antihistamines are used to treat conditions like gastric reflux and peptic ulcers.

Why are only H1-antihistamines used for allergy treatment?

H1-antihistamines are used for allergy treatment because H1-receptors are primarily involved in allergic inflammation, while other histamine receptors (such as H2 and H4) play different roles in the body.

How do allergies typically occur in the body?

Allergies occur when a person is repeatedly exposed to an allergen, leading to sensitization of mast cells. These cells release histamine and other chemicals, triggering allergic symptoms.

What effects does histamine have during an allergic reaction?

Histamine causes dilation and increased permeability of blood vessels, stimulation of sensory nerves, smooth muscle contraction, and is responsible for symptoms like watery eyes, runny nose, sneezing, itching, swelling, hives, and difficulty breathing.

How do first-generation H1-antihistamines work, and what side effects do they have?

First-generation H1-antihistamines bind to H1-receptors and stabilize them in an inactive state. These medications can cross the blood-brain barrier, leading to side effects like drowsiness, cognitive impairment, and increased appetite.

What are the differences between first-generation and second-generation antihistamines?

Second-generation antihistamines are more selective for H1-receptors and are less likely to cross the blood-brain barrier, making them less sedating. They also do not have the anti-cholinergic effects seen with first-generation antihistamines.