Autacoids — Histamine, Serotonin and Prostaglandin Drugs

The exam value of autacoids lies in connecting a mediator to its receptor or synthetic pathway, then to a clinical use and a characteristic adverse effect. Histamine, serotonin and prostaglandins appear in several organ systems, so revising them only as a list of drug names makes mechanism questions unnecessarily difficult. Build a chain: mediator → target → effect → indication → limitation.

Local action does not mean a mediator has only one function or that a medicine produces only local adverse effects. Histamine also participates in central wakefulness, serotonin is a neurotransmitter as well as a gastrointestinal mediator, and prostanoid effects vary by tissue and receptor. The same broad family therefore includes agents used in very different clinical settings.

Histamine receptor effects explain why H1 and H2 drugs are not interchangeable. H1-mediated effects include allergic symptoms, pruritus, vasodilation and bronchoconstriction, with a central role in wakefulness. H2 blockade is used to reduce gastric acid secretion. These are receptor-defined clinical effects, not merely separate brand categories of the same allergy medicine.

Histamine receptors belong to the G protein-coupled receptor family. For routine revision, prioritise the link between H1 and allergic symptoms, H2 and acid suppression, and the central effects that distinguish antihistamine generations. A receptor-name question may also ask about H3 or H4, but do not transfer every peripheral H1 effect to these other subtypes.

The distinction also helps with adverse effects. Blocking central H1 signalling can produce sedation, while acid suppression is the intended effect of H2 blockers. When a question pairs an allergy complaint with a requirement to remain alert, CNS entry becomes the decision point rather than the severity of itching alone.

First-generation H1 antihistamines are relatively lipophilic and cross the blood-brain barrier readily. Sedation, impaired psychomotor performance and anticholinergic adverse effects are the major revision anchors. Diphenhydramine, chlorpheniramine and promethazine are familiar examples. Their additional pharmacological effects explain why some are used for motion sickness or as antiemetics.

Second-generation H1 antihistamines have less central penetration and are preferred for many patients with allergic rhinitis or urticaria. Examples include cetirizine, levocetirizine, loratadine, desloratadine and fexofenadine. “Less sedating” is a safer description than claiming every drug in this group can never cause drowsiness; patient response and the selected agent still matter.

Anticholinergic burden becomes particularly relevant in older adults and patients susceptible to urinary retention, confusion or angle-closure glaucoma. Alcohol and other sedating medicines can worsen psychomotor impairment. A question about a driver, machine operator or student who needs alertness therefore favours a less sedating option, while a question about motion sickness may deliberately favour a first-generation agent.

Antihistamines can improve itching and other allergic symptoms, but they do not replace epinephrine in anaphylaxis. A patient with airway compromise, shock or a systemic anaphylactic reaction requires emergency treatment directed at those problems. Choosing an H1 blocker because the patient also has urticaria misses the severity of the syndrome.

Famotidine and cimetidine act at H2 receptors to reduce acid secretion. They do not provide the same clinical role as an H1 drug for allergic rhinitis. Cimetidine is associated with endocrine adverse effects, including gynaecomastia, and clinically relevant interactions. In a vignette combining acid suppression with a new endocrine complaint, identify the particular H2 agent rather than blaming histamine blockade as a whole.

Antiemetic questions can cross over into this topic. First-generation H1 agents can be useful for vestibular symptoms and motion sickness, whereas serotonin antagonists are associated with other emetic pathways. The origin of the nausea matters: the best answer for motion-triggered symptoms is not automatically the best answer for chemotherapy-related vomiting.

Serotonin, or 5-hydroxytryptamine, is synthesised in the raphe nuclei and in enterochromaffin cells of the intestinal mucosa. It participates in mood, gastrointestinal homeostasis and multiple other physiological functions. It can be stored in platelet granules and contributes to platelet aggregation. A serotonin question is therefore not necessarily a psychiatry question.

Serotonin synthesis begins with tryptophan. Hydroxylation by tryptophan hydroxylase is the rate-limiting step, followed by decarboxylation. After release, serotonin can be taken up through the serotonin transporter and then stored or metabolised by monoamine oxidase. Reuptake inhibition and receptor activation are different mechanisms, even though both can increase a serotonergic effect.

Most serotonin receptors are G protein-coupled. 5-HT3 is the ligand-gated ion-channel exception. This is an important contrast with histamine receptors and a useful mechanism cue for ondansetron. Avoid treating all receptors for an amine transmitter as structurally identical; receptor families determine the type of signalling as well as the clinical drug association.

Triptans affect trigeminovascular pathways, inhibit release of vasoactive peptides and promote vasoconstriction. This mechanism explains both their migraine role and their limitations in patients with coronary artery disease or coronary vasospasm. Do not convert “drug for migraine” into “appropriate for every patient with a headache”; diagnosis and vascular contraindications are part of the answer.

Buspirone is a partial agonist at serotonin receptors and does not act through benzodiazepine GABA receptors. Its anxiolytic benefit develops with ongoing treatment, so it is not simply a substitute for an immediately acting sedative in an acute emergency. The combination of a serotonin receptor target and a delayed clinical effect is a useful discriminator.

Ondansetron blocks 5-HT3 receptors in emetic pathways. Its adverse effects include headache and constipation, and QT prolongation is a clinically important concern. It is not an H1 antihistamine and does not owe its antiemetic action to sedation. A mechanism question may deliberately place all these antiemetic classes in the same list of options.

Drugs affecting serotonin require attention to other serotonergic medicines and monoamine oxidase inhibitors. The key revision habit is to consider the entire medication list rather than evaluating each agent in isolation. A recognised receptor association establishes the mechanism; it does not remove the need to check interactions and contraindications.

Prostaglandins are made from arachidonic acid released from membrane lipids by phospholipase A2. Cyclooxygenase enzymes produce an intermediate from which tissue-specific enzymes form different prostanoids. These include prostaglandins, prostacyclin and thromboxane. Effects depend on the product, receptor and tissue, so “prostaglandin” does not describe a single uniform physiological action.

The pathway provides a useful distinction between reducing synthesis and giving an analogue. NSAIDs reduce cyclooxygenase-dependent prostanoid production. Prostaglandin analogues instead activate selected receptors and are used when a particular prostaglandin-like effect is useful. A gastric-protection question about misoprostol is consequently asking for a different pharmacological strategy from an analgesia question about an NSAID.

Protective prostaglandin effects in the stomach and kidney explain important NSAID adverse effects. Loss of gastric protection can contribute to ulceration, and reduced renal vasodilator support can be problematic in susceptible patients. Prostacyclin and thromboxane also differ in their vascular and platelet effects. Avoid memorising inflammation as the only function of the entire pathway.

Misoprostol reduces acid secretion and increases gastric mucus and bicarbonate protection. It also stimulates uterine contraction, which explains both its obstetric uses and why pregnancy is a contraindication when it is being prescribed for gastric-ulcer prevention. The indication matters: an intended uterine effect in a supervised obstetric setting is a serious unwanted effect in a different setting.

Carboprost is associated with gastrointestinal adverse effects and bronchospasm. Its use requires screening for pulmonary and other organ disease. Alprostadil maintains a patent ductus arteriosus when ductal flow is needed in a neonate with suitable congenital heart disease. It should not be confused with a drug used to close the ductus.

Latanoprost and related glaucoma analogues increase uveoscleral outflow. This distinguishes them from medicines that primarily reduce aqueous production. Local adverse effects can include changes in pigmentation and eyelash growth. Epoprostenol belongs to the prostacyclin group and is used in pulmonary arterial hypertension; do not transfer the uterotonic role of carboprost to every prostaglandin analogue.

• Allergic symptoms with a need to remain alert: consider the CNS penetration of the chosen H1 antihistamine.
• Anaphylaxis with hypotension or airway involvement: select epinephrine rather than antihistamine-only management.
• Migraine mechanism: triptan agonism at 5-HT1B/1D; vomiting mechanism: ondansetron antagonism at 5-HT3.
• Anxiety without a benzodiazepine receptor mechanism: buspirone partial agonism at 5-HT1A.
• Ductal patency: alprostadil; cervical ripening: dinoprostone; postpartum haemorrhage: carboprost.
• Glaucoma through increased uveoscleral outflow: latanoprost; pulmonary arterial hypertension: epoprostenol.

For each answer, say the mechanism before the drug name. Then add the adverse effect or contraindication that could change the choice. This ties the most reusable facts together and prevents a familiar indication from overriding a clinically important clue in the stem.