Ergot Alkaloid Pharmacology

Chemistry, receptor pharmacology, and toxicology

The ergot alkaloids are unusual in pharmacology because their clinical problems and their clinical applications are generated by the same receptor activity. The vasoconstriction that makes ergotamine dangerous in overdose is the same vasoconstriction that terminates a migraine attack. Understanding how this dual character arises requires the receptor map — which is where the infographic begins.

The lysergic acid core is what links all the pharmacological diversity in this class. Every ergot alkaloid is built on it, and that shared scaffold explains why these drugs hit serotonin, dopamine, and alpha-adrenergic receptors simultaneously rather than selectively. The partial agonist/antagonist character at each receptor type — shown in the receptor activity table — is a consequence of that structural constraint, not a feature that was designed in.

The receptor table should be read column by column before reading it row by row. The columns are receptor types; the rows are agents. Each cell shows whether the agent agonizes or antagonizes that receptor. Ergotamine's clinical profile falls out of the combination: serotonin 5-HT1B/1D agonism aborts migraine; alpha-adrenergic agonism constricts peripheral vessels; both together produce the vasoconstriction that threads the line between therapeutic and toxic.

Ergotism — the poisoning syndrome — has two historical and mechanistic faces. Convulsive ergotism from central serotonin and dopamine excess produces seizures and hallucinations. Gangrenous ergotism from sustained peripheral vasoconstriction produces ischemic limb necrosis. Modern toxicity almost always presents as the gangrenous form, and it requires vasodilator treatment rather than supportive care alone.

The contraindication and interaction card closes the infographic because these are the limits that govern how ergotamine is used in practice. The pregnancy contraindication follows from the uterotonic activity that makes other ergot alkaloids useful in obstetrics. The cytochrome P450 3A4 interaction with azole antifungals and macrolides multiplies the vasoconstriction risk to a dangerous degree.

Receptor Activity Profile

Multi-Receptor Class Ergot Alkaloid Receptor Activity

Serotonin 5-HT1B/1D: Agonism — cranial vasoconstriction, migraine termination (ergotamine, dihydroergotamine)
Serotonin 5-HT2A: Partial agonism/antagonism — vasoconstriction, uterine contraction
Dopamine D2: Agonism — prolactin inhibition, anti-Parkinson effect (bromocriptine, cabergoline)
Alpha-1/2 adrenergic: Partial agonism + antagonism — peripheral vasoconstriction; dihydroergotamine has more alpha-antagonism than ergotamine
Structural basis: All share a lysergic acid core; partial agonist/antagonist activity at each receptor type reflects shared scaffold

Clinical Applications

Migraine Ergotamine and Dihydroergotamine

Mechanism: 5-HT1B/1D agonism → cranial vasoconstriction and trigeminovascular inhibition
Largely replaced by triptans (more selective, better tolerated)
Still used for prolonged migraine and status migrainosus
Dihydroergotamine: intravenous or intranasal; faster than ergotamine
Risk: medication overuse headache with frequent use

Dopaminergic Ergots Bromocriptine and Cabergoline

Dopamine D2 agonists — covered fully in PARK chapter
Bromocriptine: Parkinson disease, hyperprolactinemia, acromegaly
Cabergoline: preferred for prolactinoma (longer half-life, better tolerated)
Cardiac valvulopathy risk with long-term use (5-HT2B agonism)

Ergotism — Toxicology

Gangrenous Form (Modern) Peripheral Vasospasm

Sustained alpha-adrenergic and serotonin 2A-mediated vasoconstriction
Ischemia of extremities → burning sensation → gangrene
Historical name: St. Anthony's Fire
Treatment: stop ergot, vasodilators (nitroprusside, nifedipine)

Convulsive Form (Historical) Central Serotonin / Dopamine Excess

Seizures, hallucinations, psychosis
Associated with lysergic acid diethylamide-like serotonergic activity
Primary concern in epidemic rye grain contamination
Rare in modern therapeutic use

Contraindications and Critical Interactions

Absolute contraindications: Pregnancy (uterotonic → fetal ischemia and miscarriage); peripheral vascular disease; coronary artery disease; uncontrolled hypertension; hepatic or renal impairment.

Critical drug interaction: Cytochrome P450 3A4 inhibitors (azole antifungals, macrolide antibiotics, ritonavir) markedly increase ergot levels → severe ergotism at therapeutic doses. Combination is contraindicated.

Serotonin syndrome risk: Avoid concurrent use with triptans, selective serotonin reuptake inhibitors, serotonin-norepinephrine reuptake inhibitors.

Author / Organization	Title	Source
Katzung BG, ed.	Basic and Clinical Pharmacology. 15th ed.	McGraw-Hill; 2021
Brunton LL, Knollmann BC, eds.	Goodman & Gilman's The Pharmacological Basis of Therapeutics. 14th ed.	McGraw-Hill; 2023
Schiff PL	Ergot and its alkaloids	Am J Pharm Educ. 2006;70(5):98
Boissier JR	General pharmacology of ergot alkaloids	Pharmacology. 1978;16(Suppl 1):12–26
Missale C, Nash SR, Robinson SW, Jaber M, Caron MG	Dopamine receptors: from structure to function	Physiol Rev. 1998;78(1):189–225
Silberstein SD, McCrory DC	Ergotamine and dihydroergotamine: history, pharmacology, and efficacy	Headache. 2003;43(2):144–166
Tfelt-Hansen P, Saxena PR, Dahlof C, et al.	Ergotamine in the acute treatment of migraine: a review and European consensus	Brain. 2000;123(1):9–18
de Groot AN, van Dongen PW, Vree TB, Hekster YA, van Roosmalen J	Ergot alkaloids: current status and review of clinical pharmacology and therapeutic use compared with other oxytocics in obstetrics and gynaecology	Drugs. 1998;56(4):523–535
Matossian MK	Poisons of the Past: Molds, Epidemics, and History	Yale University Press; 1989
Dresser GK, Spence JD, Bailey DG	Pharmacokinetic-pharmacodynamic consequences and clinical relevance of cytochrome P450 3A4 inhibition	Clin Pharmacokinet. 2000;38(1):41–57
Diener HC, Limmroth V	Medication-overuse headache: a worldwide problem	Lancet Neurol. 2004;3(8):475–483
Aellig WH, Nuesch E	Comparative pharmacokinetic investigations with tritium-labelled ergot alkaloids after oral and intravenous administration in man	Int J Clin Pharmacol Biopharm. 1977;15(3):106–112

Author / Organization

Title

Source

Katzung BG, ed.

Basic and Clinical Pharmacology. 15th ed.

McGraw-Hill; 2021

Brunton LL, Knollmann BC, eds.

Goodman & Gilman's The Pharmacological Basis of Therapeutics. 14th ed.

McGraw-Hill; 2023

Schiff PL

Ergot and its alkaloids

Am J Pharm Educ. 2006;70(5):98

Boissier JR

General pharmacology of ergot alkaloids

Pharmacology. 1978;16(Suppl 1):12–26

Missale C, Nash SR, Robinson SW, Jaber M, Caron MG

Dopamine receptors: from structure to function

Physiol Rev. 1998;78(1):189–225

Silberstein SD, McCrory DC

Ergotamine and dihydroergotamine: history, pharmacology, and efficacy

Headache. 2003;43(2):144–166

Tfelt-Hansen P, Saxena PR, Dahlof C, et al.

Ergotamine in the acute treatment of migraine: a review and European consensus

Brain. 2000;123(1):9–18

de Groot AN, van Dongen PW, Vree TB, Hekster YA, van Roosmalen J

Ergot alkaloids: current status and review of clinical pharmacology and therapeutic use compared with other oxytocics in obstetrics and gynaecology

Drugs. 1998;56(4):523–535

Matossian MK

Poisons of the Past: Molds, Epidemics, and History

Yale University Press; 1989

Dresser GK, Spence JD, Bailey DG

Pharmacokinetic-pharmacodynamic consequences and clinical relevance of cytochrome P450 3A4 inhibition

Clin Pharmacokinet. 2000;38(1):41–57

Diener HC, Limmroth V

Medication-overuse headache: a worldwide problem

Lancet Neurol. 2004;3(8):475–483

Aellig WH, Nuesch E

Comparative pharmacokinetic investigations with tritium-labelled ergot alkaloids after oral and intravenous administration in man

Int J Clin Pharmacol Biopharm. 1977;15(3):106–112