Table of Contents

Autacoids

1. Introduction to Autacoids

The word "Autacoid" comes from the Greek words Auto (meaning "self") and Coids (meaning "healing/remedy"). They are frequently referred to as Local Hormones.

Conceptual Check

Autacoids vs. Classic Hormones

Unlike classical hormones (like insulin or thyroid hormone) which are produced by a specific gland, secreted into the blood, and travel long distances to reach a target, Autacoids are produced locally by many different tissues, act locally near their site of synthesis, and have a very brief lifespan.

Analogy: Think of them as the body's "neighborhood watch" system. If a house is broken into (tissue trauma), you don't wait for the national army (classical hormones) to arrive; the local neighborhood watch (autacoids) acts immediately at the exact site of injury to raise the alarm (inflammation/pain) and start repairs.

Why are Autacoids Important? (Functions)

Physiological: Regulate normal baseline organ functions (e.g., gastric acid secretion, local blood flow).
Pathophysiological (Reaction to Injuries): They are the primary drivers of inflammation, pain, allergy, and the body's response to tissue trauma.
Transmission and Modulation: They act as mediators that fine-tune pain signals and nerve responses.

Everyday Clinical Example: When you take an NSAID like Ibuprofen for a sprained ankle, you are specifically blocking the production of a lipid autacoid called a Prostaglandin. By shutting down this local autacoid, you stop the localized pain and swelling!

Classification of Autacoids

Autacoids are categorized by their chemical structure:

Chemical Class	Examples & Origin
A. Amine Derivatives	Histamine (derived from the amino acid Histidine) Serotonin (derived from the amino acid Tryptophan)
B. Lipid Derivatives	Eicosanoids: Prostaglandins, Thromboxane, Leukotrienes. Others: Interleukins, Platelet Activating Factor (PAF).
C. Peptide Derivatives	Kinins: Bradykinin. Renin-Angiotensin system. Neuropeptides.

2. Neuropeptides

Neuropeptides are small, protein-like molecules (short chains of amino acids) used by neurons to communicate with each other. They act in an autocrine (acting on the cell that released it) or paracrine (acting on immediate neighboring cells) manner.

Exam Trap: Neuropeptides vs. Classical Neurotransmitters

Classical neurotransmitters (like dopamine, serotonin, glutamate) are fired into the synapse and then quickly sucked back up by reuptake pumps to be recycled and used again.

NEUROPEPTIDES ARE NOT RECYCLED. Once they are secreted, they are broken down by specific enzymes (peptidases) and destroyed. The neuron must synthesize entirely new ones from the cell body (which takes time) and transport them down the axon. Do not forget this distinction!

General Functions of Neuropeptides

They are heavily responsible for higher-order brain functions and systemic regulation, including:

Analgesia (pain regulation)
Food intake (appetite stimulation/suppression)
Learning & Memory
Metabolism & Reproduction
Social Behaviors

Key Examples include: Neuropeptide Y (NPY), Cholecystokinin (CCK), Tachykinins (Substance P, Neurokinin), Arginine Vasopressin (AVP), and Corticotropin-Releasing Factor (CRF).

Neuropeptide Y (NPY)

NPY is a 36-amino acid peptide that acts as a potent neurotransmitter in both the Brain and the Autonomic Nervous System (ANS).

Location	Source	Physiological Actions
Brain (Central NPY)	Produced mainly by the Hypothalamus.	↑ Food intake (Potent appetizer/orexigenic) ↑ Storage of energy as fat ↓ Anxiety and stress ↓ Voluntary alcohol intake ↓ Blood pressure and pain perception Regulates circadian rhythm and controls epileptic seizures.
ANS (Peripheral NPY)	Produced mainly by sympathetic neurons.	Strong Vasoconstrictor Promotes the growth of fat tissue.

NPY Receptors & Mechanisms

NPY acts on G-Protein Coupled Receptors (GPCRs). Mammals have 5 types (Y1-Y5), but humans only express 4 functional types.

Y1 (NPY1R) & Y5 (NPY5R): These are the Feeding Stimulators (Appetizers). Activation leads to massive hunger.
Y2 (NPY2R) & Y4 (NPY4R): These act as Appetite Inhibitors (Anorectic).
Mechanism of Action: NPY receptors are Gi-coupled (Inhibitory G-protein). When NPY binds, the Gi subunit is released, which inhibits the enzyme adenylate cyclase. This stops the conversion of ATP into the 2nd messenger cAMP.

Clinical Scenario

Anti-Obesity Drugs and NPY

Because Y1 and Y5 receptors powerfully drive hunger and fat storage, pharmaceutical companies are actively researching Y1/Y5 Antagonists as therapeutic targets for obesity. Blocking these receptors could shut off the brain's unnatural drive to overeat. Conversely, chronic stress increases NPY release in the periphery, which promotes the growth of visceral fat (explaining why chronic stress often leads to weight gain!).

3. Tachykinins (TAC) & Substance P

Tachykinins form the largest family of neuropeptides. They get their name because they induce a rapid ("tachy") contraction of gut tissues.

Chemical Characteristic: All tachykinins share a common "C-terminal" sequence: "Phe-X-Gly-Leu-Met-NH2" (Where 'X' is either an aromatic or aliphatic amino acid, and COOH-terminus is the end of the protein chain).
Synthesis Pathway: Preprotachykinin → Protachykinin → Tachykinin.

Tachykinin Genes and Products

TAC-1 Gene produces: Neurokinin A, Neurokinin K, Neuropeptide γ, and Substance P (SP).
TAC-3 Gene produces: Neurokinin B.

Tachykinin Receptors (GPCRs)

Tachykinin receptors are Gq-coupled. Activation leads to the activation of Phospholipase C (PLC), which chops PIP2 into IP3 and DAG. This ultimately causes a massive release of intracellular Calcium. There are three main receptors, each with a preferred agonist:

NK1R: Prefers Substance P.
NK2R: Prefers Neurokinin A.
NK3R: Prefers Neurokinin B.

Substance P (SP)

Substance P is an Undecapeptide (a chain of 11 amino acids). It is a highly potent mediator of pain signaling and inflammation.

Receptor: Primarily binds to NK1R. The binding occurs via specific amino acid residues on the extracellular loops and transmembrane regions of the NK1 receptor.
Physiological Roles:
- Promotes wound healing in humans (especially non-healing ulcers).
- Acts as a potent vasodilator. This vasodilation is entirely dependent on the release of Nitric Oxide (NO) from the endothelium.
- Transmits intense, burning pain signals to the brain (Neurogenic Inflammation).

Clinical Application

Substance P Antagonists (SPA)

By blocking or depleting Substance P, we can block pain and severe nausea.

Capsaicin: The active ingredient in chili peppers! Clinically used as a topical analgesic cream for arthritis and diabetic neuropathy.
Mechanism: It initially causes a burning sensation (triggering SP release), but it eventually forces the nerve to release ALL of its Substance P. Because neuropeptides take a long time to synthesize (they aren't recycled), the nerve is left empty of Substance P, rendering it completely unable to transmit pain signals for weeks!

Oncology Magic

The "-pitant" Drugs

Aprepitant: Used heavily in oncology as an antiemetic drug to treat severe, delayed nausea and vomiting caused by cancer chemotherapy.
Fosaprepitant: An IV prodrug form of Aprepitant used for adult chemo patients.
Casopitant: Has dual antidepressant and antiemetic activities.
Vestipitant: Under trial for treating tinnitus (ringing in ears) and insomnia.
Maropitant: FDA-approved veterinary antiemetic for dog/cat motion sickness.

Exam Hint: If a drug ends in "-pitant", it is an NK1 Receptor Antagonist used to stop Puking (Emesis)!

Neurokinin A (Substance K)

Binds primarily to NK2R (Gq coupled → Inositol phosphate + Calcium 2nd messengers).

Oncology Role: High circulating levels of Neurokinin A serve as an independent indicator of poor prognosis in certain cancers, specifically carcinoid tumors.
Asthma Role: Neurokinin A is a powerful bronchoconstrictor. Therefore, selective NK2 receptor antagonists (like MEN 11420) are being studied to suppress bronchial constriction in asthmatics. They may also possess anti-inflammatory effects.

Note: Standard asthma drugs like fluticasone (corticosteroid) and montelukast (leukotriene antagonist) also happen to indirectly reduce NKA-induced bronchoconstriction.

4. Kinins & Bradykinin

Kinins are potent peptide autacoids involved in the inflammatory response. The most famous and clinically relevant is Bradykinin.

Synthesis and Metabolism

Synthesis: Bradykinin is not stored; it is created on-demand. An enzyme called Kallikrein acts as molecular scissors, cutting (proteolytic cleavage) a circulating protein called Kininogen to form active Bradykinin.
Metabolism (Breakdown): Because it is so potent, Bradykinin must be destroyed quickly. It is broken down by three "kininase" enzymes:
1. Angiotensin-Converting Enzyme (ACE) - This is the most clinically important one!
2. Aminopeptidase P (APP)
3. Carboxypeptidase N (CPN)

Receptors and Actions

Kinins activate B1, B2, and B3 receptors, which are linked to Phospholipase C / A2 (PLC/A2). The B2 receptor mediates the majority of Bradykinin's classic effects:

Cardiovascular:
- Potent Vasodilation: It forces the endothelium to release Prostacyclin (PGI2), Nitric Oxide (NO), and Endothelium-Derived Hyperpolarizing Factor (EDHF). This leads to a massive drop in blood pressure.
- Cardiac Stimulation: The sudden drop in BP triggers a compensatory reflex tachycardia (fast heart rate) and increased cardiac output.
- Coronary Vasodilation: Acts as a cardiac anti-ischemic agent (protects the heart from lack of oxygen).
Smooth Muscle: Causes contraction of NON-vascular smooth muscle, leading to bronchoconstriction (lungs) and gut cramps.
Inflammation & Pain: Radically increases vascular permeability (causing fluids to leak out into tissues = edema/swelling) and directly stimulates and sensitizes pain nerve endings (nociceptors).
Kidneys: Causes natriuresis (excretion of sodium in urine), further dropping BP.

Crucial Board Exam Concept: ACE Inhibitors and Bradykinin

Scenario: A 55-year-old patient with hypertension is prescribed Lisinopril (an ACE Inhibitor). Weeks later, they return complaining of a relentless, dry, hacking cough. In a worst-case scenario, they return with massive, life-threatening swelling of their lips, tongue, and throat. What happened?

The Science: The enzyme ACE has two jobs in the body. Job 1 is to create Angiotensin II (which raises BP). Job 2 is to destroy Bradykinin.

When you give a patient an ACE Inhibitor, you block the destruction of Bradykinin. Bradykinin levels skyrocket. This is actually good for blood pressure (because Bradykinin is a vasodilator), but it also causes fluid leakage and bronchoconstriction in the lungs, triggering a dry cough (affecting up to 20% of patients). In severe, rare cases, this excessive Bradykinin causes massive facial and airway swelling known as Angioedema, which is a medical emergency requiring immediate airway management.

Pharmacological Manipulation of Kinins

We can manipulate this system by either stopping Bradykinin from being made, or blocking its receptors.

1. Kallikrein Inhibitors (Stop the synthesis of Bradykinin)

Aprotinin: Used to treat acute pancreatitis, carcinoid syndrome (which dumps excessive peptides), and hyperfibrinolysis.
Ecallantide: A human plasma kallikrein inhibitor given via subcutaneous injection to treat severe inflammation (like hereditary angioedema).

2. Bradykinin Antagonists (Block the B2 Receptor)

Deltibant: A novel antagonist used for Severe Systemic Inflammatory Response Syndrome (SIRS) and Sepsis.
Icatibant: A synthetic decapeptide that acts as a potent, competitive antagonist of the B2 receptor. Used primarily for Hereditary Angioedema (a genetic condition causing severe, unprovoked swelling underneath the skin because the body overproduces bradykinin).
Pharmacokinetics of Antagonists: Usually given SubQ (30mg). Half-life is 1-2 hours. Rapid onset within an hour. Local injection site reactions are common but transient. Drug Interaction: ACE inhibitors block B2 receptor desensitization, potentiating bradykinin effects far beyond just blocking its hydrolysis!

Natural Note: Bromelain, an extract from pineapple stems/leaves, suppresses trauma-induced swelling by preventing the release of bradykinin into the bloodstream.

5. Ergot Alkaloids

Ergot alkaloids are a fascinating and dangerous class of compounds produced by Claviceps purpurea, a fungus that infects grains, particularly rye.

Historical & Toxicological Context

St. Anthony's Fire (Ergotism)

Accidental ingestion of grain contaminated with this fungus leads to a horrific disease known as Ergotism. In the Middle Ages, this was called "St. Anthony's Fire" because victims felt a burning pain in their limbs and sought help from St. Anthony's monks. Symptoms include:

Dementia and florid hallucinations (Ergot compounds mimic serotonin/LSD).
Prolonged, severe vasospasm which completely cuts off blood supply to the limbs, eventually resulting in dry gangrene and requiring amputation.
Uterine smooth muscle stimulation resulting in violent cramps and spontaneous abortion.

Epidemiology: Epidemics mandate continuous grain surveillance (e.g., the Karamoja incidence in Uganda). Poisoning of grazing animals is also common.

Chemistry and Major Families

All ergot alkaloids share a tetracyclic ergoline nucleus. The fungus naturally synthesizes acetylcholine, histamine, and tyramine alongside the unique alkaloids. There are two major families:

Amine Alkaloids: Lysergic acid diethylamide (LSD), Ergonovine, Methysergide, 6-methylergoline, Lysergic acid.
Peptide Alkaloids: Ergotamine, α-ergocryptine, Bromocriptine.

Pharmacokinetics: They are variably absorbed from the GI tract. Oral absorption of ergotamine is significantly improved by co-administering Caffeine (caffeine also acts as a cranial vasoconstrictor, helping with migraines). They are extensively metabolized in the liver.

Pharmacodynamics & Receptor Action

Ergots are considered "dirty drugs" because they lack specificity. They act as agonists, partial agonists, and antagonists across three major receptor families:

Alpha-adrenoceptors: Causes massive vasoconstriction.
Serotonin (5-HT) Receptors: Especially 5-HT1A, 5-HT1D, and 5-HT2.
Dopamine (D2) Receptors: In the CNS, primarily acting as agonists.

Ergot Alkaloid	α-Adrenoceptor	Dopamine Receptor	Serotonin (5-HT2)	Uterine Stimulation
Bromocriptine	-	+++ (Strong Agonist)	-	0
Ergonovine	+	+	- (Partial Agonist)	+++ (Very Strong)
Ergotamine	-- (Partial Agonist)	0	+ (Partial Agonist)	+++
LSD	0	+++	-- (Peripheral Antagonist) ++ (CNS Agonist)	+

6. Clinical Uses of Ergot Alkaloids

1. Central Nervous System & Hyperprolactinemia

LSD: A powerful hallucinogen. Acts as a potent peripheral 5-HT2 antagonist, but behavioral effects are mediated by agonist effects at pre/postjunctional 5-HT2 receptors in the CNS.
Bromocriptine & Cabergoline: These are highly selective Dopamine (D2) Agonists. Dopamine naturally suppresses the pituitary gland from releasing Prolactin. Therefore, these drugs are given to treat Hyperprolactinemia (excess prolactin usually caused by pituitary secreting tumors or antipsychotic drugs).
Clinical note: High prolactin causes amenorrhea (loss of periods) and infertility in women, and galactorrhea (milky discharge) in both sexes. Bromocriptine (2.5mg 2-3x daily) suppresses the secretion and can even shrink pituitary tumors.

Neurology

2. Migraine Treatment

Migraines involve massive, painful vasodilation of cranial blood vessels. Ergotamine potently constricts human blood vessels (partial agonist at alpha-receptors and 5-HT2 receptors). Its antimigraine action is also linked to action on prejunctional neuronal 5-HT receptors.

Ergotamine: Highly specific for migraine pain, but only effective if given early in the attack. It becomes progressively less effective if delayed. Often combined with caffeine to enhance GI absorption.
The Danger: Because ergotamine dissociates very slowly from the alpha-receptor, the vasoconstriction is long-lasting and cumulative. Max dose limits: No more than 6mg per attack, and NO MORE than 10mg per week, or the patient risks gangrene.
Dihydroergotamine: Given IV (0.5-1mg) or intranasally for intractable, severe migraines lasting >72 hours.

Obstetrics

3. Postpartum Hemorrhage

The uterus possesses alpha-1 and serotonin receptors. During pregnancy, the dominance of alpha-1 receptors increases dramatically, making the uterus at term extremely sensitive to ergot alkaloids.

Ergot derivatives induce a powerful, prolonged spasm of the uterine muscle (unlike natural, rhythmic labor contractions).
ABSOLUTE CONTRAINDICATION: Never give ergots before delivery, as the prolonged tetanic contraction will suffocate the fetus or rupture the uterus.
Use: Used strictly for the control of late uterine bleeding (Postpartum hemorrhage) after the placenta has been delivered. Note: Oxytocin is the 1st line drug, but if it fails, Ergonovine maleate (0.2 mg IM) is the Drug of Choice among ergots because it works within 1-5 minutes and is less toxic than ergotamine.

Toxicity & Contraindications of Ergots

GI Disturbances: Diarrhea, nausea, vomiting (due to activation of medullary vomiting center and GI serotonin receptors).
Prolonged Vasospasm: Overdose of ergotamine leads to ischemia, bowel infarction (requires surgical resection), and gangrene (requires amputation). Treatment: Reversible with massive peripheral vasodilators like Nitroprusside or Nitroglycerin.
Contraindications: Pregnant patients (causes abortion/fetal distress). Patients with obstructive vascular disease (Peripheral Artery Disease, Coronary Artery Disease) and collagen diseases. Crucial Note: Never combine Ergotamine with Triptans (modern migraine drugs) within 24 hours, as both cause massive vasoconstriction and will trigger a heart attack or stroke!

7. Bonus Section: Self-Study Autacoids Guide

Your lecture noted to read up on these. Here is a simplified summary to ensure your knowledge is 100% complete for the exam, complete with clinical context:

Renin-Angiotensin System

Renin (from kidney) converts Angiotensinogen (from liver) to Angiotensin I. ACE (from lungs) converts AT-I to Angiotensin II (AT-II). AT-II is a massive vasoconstrictor and triggers Aldosterone release (retains sodium/water), sharply raising Blood Pressure.

Clinical Context: We block this system with ACE Inhibitors (Lisinopril) or ARBs (Losartan) to treat hypertension and heart failure.

Nitric Oxide (NO)

A gas that acts as a localized autacoid. Synthesized by eNOS in blood vessels. It diffuses into smooth muscle, increases cGMP, and causes profound vasodilation.

Clinical Context: Sildenafil (Viagra) works by preventing the breakdown of cGMP, vastly prolonging the vasodilatory effects of Nitric Oxide to maintain an erection.

Oxytocin & Vasopressin

Peptides from the posterior pituitary. Oxytocin causes rhythmic uterine contractions and milk let-down. Vasopressin (ADH) retains water in the kidney and constricts blood vessels at high doses.

Clinical Context: Synthetic Oxytocin (Pitocin) is used to safely induce labor. Vasopressin is given during cardiac arrest to clamp blood vessels and force blood to the brain.

Endothelins

The exact opposite of NO. They are the most potent naturally occurring vasoconstrictors in the human body.

Clinical Context: Endothelin receptor antagonists (like Bosentan) are used specifically to treat Pulmonary Arterial Hypertension by stopping this massive vessel clamping in the lungs.

Cholecystokinin (CCK)

Found in the gut and brain. In the gut, it stimulates gallbladder contraction and pancreatic secretion (digestion). In the brain, it acts as a satiety signal (tells you to stop eating) and is heavily implicated in anxiety, panic disorders, and social behavior modulation.