Understanding Autocoids: The “Local Hormones” of the Body
In the world of pharmacology, autocoids occupy a unique space. Derived from the Greek words autos (self) and akos (healing), these are chemical signals produced by cells that act locally at the site where they are released. Unlike systemic hormones that travel through the blood, autocoids are like “neighborhood messengers.”
In this guide, we’ll dive into the three heavyweights of the autocoid world: Histamine, Serotonin, and Prostaglandins.
1. Histamine: The Immune Responder
Histamine is an amine found in nearly all animal tissues, primarily stored in mast cells. It is famous for its role in allergic reactions, but its job is much broader.
Physiological Roles:
- Cardiovascular: It dilates small arterioles (causing a drop in BP) and increases capillary permeability, leading to Oedema (swelling).
- Gastric Acid: It is a potent stimulator of stomach acid secretion via H₂ receptors.
- Nervous System: It stimulates sensory nerves, causing the classic “itch” or pain associated with insect bites.
- Smooth Muscle: It causes the bronchial muscles to constrict, which is why it’s a major factor in Asthma.
The “Triple Response” of Lewis:
When histamine is injected into the skin, it produces three distinct signs:
- Red Spot: Localized vasodilation.
- Flare: A brighter red flush caused by axon reflexes.
- Wheal: Swelling caused by fluid leaking from capillaries.
2. Antihistamines: H₁ vs. H₂ Blockers
We use drugs called antihistamines to block these effects. They are generally categorized by the receptor they target:
| Class | Examples | Primary Use |
|---|---|---|
| 1st Gen H₁ Blockers | Diphenhydramine, Promethazine | Allergies, Motion Sickness, Sleep aid (cause drowsiness). |
| 2nd Gen H₁ Blockers | Cetirizine, Loratadine, Fexofenadine | “Non-drowsy” allergy relief. |
| H₂ Blockers | Ranitidine, Famotidine, Cimetidine | Treating Peptic Ulcers and Gastric Acidity. |
3. Serotonin (5-HT): The Mood and Vessel Regulator
5-Hydroxytryptamine (5-HT), commonly known as Serotonin, is a neurotransmitter derived from the amino acid Tryptophan.
Key Functions:
- CVS: Complex actions that can cause both vasoconstriction and vasodilation.
- Digestive: Increases intestinal motility; stimulation of 5-HT₃ receptors in the brain leads to nausea and vomiting.
- CNS: Regulates sleep, mood, appetite, and body temperature.
5-HT Drugs in Clinical Practice:
- Triptans (Sumatriptan): Selective 5-HT agonists used to stop acute Migraine attacks.
- Ondansetron: A 5-HT₃ antagonist used to prevent vomiting in cancer patients.
- SSRIs (Fluoxetine): Antidepressants that increase serotonin activity in the brain.
4. Prostaglandins: The Lipid Signals
Prostaglandins are unique because they are derived from fatty acids (Arachidonic Acid). They act as high-power local signals for inflammation and repair.
Biological Impact:
- Reproduction: They facilitate labor by dilating the cervix and contracting the uterus.
- Stomach: They protect the gastric lining by decreasing acid and increasing mucus.
- Kidneys: They maintain blood flow and regulate the release of renin.
- Pain: Prostaglandins sensitize nerve endings to pain, which is why Aspirin and NSAIDs work by blocking their synthesis.
Dr. Saint Paul is a pharmacy educator, Pharm.D graduate, and academic content creator from Jawaharlal Nehru Technological University Kakinada (JNTUK), where he completed his Doctor of Pharmacy (Pharm.D) degree between 2015 and 2021.
He has more than 7 years of experience creating pharmacy educational content, writing study materials, and reviewing academic articles for pharmacy students. He has also contributed guest articles to pharmacy education platforms, including PharmD Guru.
At D.PharmGuru, his work focuses on simplifying complex Diploma in Pharmacy (D.Pharmacy) subjects into easy-to-understand notes, practical explanations, and exam-oriented educational resources for students across India.
His areas of focus include Human Anatomy and Physiology, Pharmaceutics, Pharmacology, Pharmaceutical Chemistry, Hospital and Clinical Pharmacy, and other core D.Pharmacy subjects.
