Autacoids and Histamine Functions

Autacoids

• Autacoids are locally acting hormones produced by tissues, not endocrine glands
• Prostaglandins are unsaturated fatty acids that act on tissues where they are synthesized
• Misoprostol is a prostaglandin E1 analogue used in abortions and peptic ulcers
• Histamine is a chemical messenger involved in allergic and inflammatory reactions, gastric acid secretion, and neurotransmission
• High histamine concentrations are found in tissues with many mast cells, such as skin, lungs, and gut

Histamine Receptors

• Histamine exerts its effects by binding to H1, H2, H3, and H4 receptors
• H1 receptors mediate smooth muscle contraction, increased vascular permeability and vasodilation
• H2 receptors are involved in gastric acid secretion
• H3 and H4 receptors are found in a limited number of cells and their roles in drug action are not fully understood

Histamine - Actions and Reactions

• Histamine release in response to various stimuli leads to a range of effects
• Symptoms of histamine injection resemble those of anaphylaxis, including smooth muscle contraction, increased secretions, and vasodilation
• The triple response of Lewis describes the characteristic reaction to intradermal histamine injection: reddening, flare, and wheal

H1 Receptor Antagonists

• H1 receptor antagonists block the receptor-mediated response to histamine but do not affect its formation or release
• First-generation H1 antagonists can cause central nervous system stimulation or depression
• Some antihistamines have diverse effects, such as appetite stimulant, anti-antiemetic, and sedative properties

Antihistamine Uses

• Antihistamines are effective in treating allergic rhinitis and urticaria
• Antihistamines are not the primary treatment for asthma because histamine is only one of many mediators
• Ophthalmic antihistamines are used for allergic conjunctivitis
• Antihistamines such as diphenhydramine and dimenhydrinate are used to treat motion sickness and nausea
• Some first-generation antihistamines are used for insomnia due to their sedative effects
• Promethazine has local anesthetic properties

Pharmacokinetics

• Antihistamines are well absorbed after oral administration with peak serum levels reached 2-3 hours after dosing
• First-generation antihistamines have a high bioavailability and are distributed in all tissues, including the central nervous system
• Antihistamines are metabolized by the hepatic cytochrome P450 system
• The duration of action is typically 24 hours, allowing for once-daily dosing
• Tolerance to antihistamines has not been observed

Adverse Effects

• First-generation antihistamines lack specificity and interact with various receptors (histamine, muscarinic cholinergic, adrenergic, and serotonin)
• Sedation is the most common adverse effect of first-generation antihistamines, and can be exacerbated by alcohol and other CNS depressants
• Second-generation antihistamines have less sedation and central nervous system effects due to poor CNS penetration
• Anticholinergic effects of antihistamines can lead to dry mouth and blurred vision
• Antihistamines can cross the placenta, caution is advised during pregnancy
• In acute poisoning, antihistamines can cause a syndrome resembling atropine poisoning
• Second-generation antihistamines are recommended for elderly patients, especially those with cognitive impairment due to the sedative effects of first-generation drugs
• First-generation antihistamines are not recommended for children due to their sedative effects

Explore the roles of autacoids, particularly histamine, and their receptors in the body. This quiz covers the actions of histamine, its implications in allergies and gastric secretion, and the functions of various histamine receptors. Test your knowledge on these locally acting hormones and their significance.