Define autocoids and classify them. Describe clinical use of antihistaminic drugs.

Defining Autocoids and Their Classification

Autocoids are short-acting, locally released mediators that influence cellular activity in the immediate vicinity of their release. They are synthesized and released rapidly, and their effects are typically transient. Their discovery significantly broadened the understanding of physiological and pathological processes beyond the traditional endocrine system.

Classification of Autocoids

Autocoids can be classified based on their chemical structure and function. The major classes include:

• Lipid Mediators: Derived from fatty acids.
• Amino Acid Derivatives: Derived from amino acids.
• Peptides: Short chains of amino acids.

Autocoid Class	Specific Examples	Primary Function
Lipid Mediators	Prostaglandins, Leukotrienes, Thromboxanes	Inflammation, Pain, Bronchoconstriction, Platelet Aggregation
Amino Acid Derivatives	Histamine, Serotonin	Vasodilation, Bronchoconstriction, Itching, Nausea
Peptides	Bradykinin, Angiotensin II	Vasodilation, Blood Pressure Regulation

Clinical Use of Antihistaminic Drugs

Antihistaminic drugs are pharmacological agents designed to block the action of histamine, a key autocoid involved in allergic reactions. They are primarily used to alleviate symptoms associated with allergies, such as itching, sneezing, runny nose, and hives. They act by competitively binding to histamine receptors (H1, H2, H3, and H4), preventing histamine from exerting its effects.

Mechanism of Action

Most commonly used antihistaminics are H1-receptor antagonists. They bind to H1 receptors on various cells, including mast cells, basophils, and smooth muscle cells, preventing histamine from triggering downstream signaling cascades. Newer antihistaminics exhibit greater selectivity for H1 receptors and have fewer side effects compared to older generations.

Types of Antihistaminic Drugs

• First-Generation Antihistaminics: (e.g., Diphenhydramine, Chlorpheniramine). These are lipophilic and readily cross the blood-brain barrier, causing sedation and anticholinergic effects.
• Second-Generation Antihistaminics: (e.g., Loratadine, Cetirizine, Fexofenadine). These are less lipophilic, have poor blood-brain barrier penetration, and are less likely to cause sedation.

Clinical Applications

• Allergic Rhinitis: Relief of sneezing, runny nose, and itchy eyes.
• Urticaria (Hives): Reduction of itching and swelling.
• Motion Sickness: Some first-generation antihistaminics (e.g., Dimenhydrinate) are effective in preventing motion sickness.
• Anxiety: Historically, some antihistaminics were used for anxiety, though this is now less common due to side effects.
• Insomnia: Diphenhydramine is commonly used as a sleep aid, although it is not a recommended long-term solution.
• Veterinary Medicine: Antihistaminics like diphenhydramine are used in animals (especially dogs and cats) to manage allergic reactions, pruritus (itching), and anxiety.

Side Effects and Precautions

While generally safe, antihistaminics can cause side effects. First-generation antihistaminics are associated with sedation, dry mouth, blurred vision, and urinary retention due to their anticholinergic properties. Second-generation antihistaminics have a better safety profile but can still cause drowsiness in some individuals. Caution is advised in patients with glaucoma, urinary retention, or cardiovascular disease.

Case Study: Managing Seasonal Allergic Rhinitis in a Dairy Farm Worker A dairy farm worker experiencing severe seasonal allergic rhinitis (hay fever) was prescribed Loratadine, a second-generation antihistamine. The drug effectively reduced his sneezing and nasal congestion, allowing him to perform his duties without significant impairment. The lack of sedation was a crucial factor, ensuring his safety while operating machinery on the farm.