Why isn't regular dopamine used?
Endogenous dopamine performs a vital physiological function in endocrine regulation. At the level of the anterior pituitary, it acts as a natural brake: it suppresses the incretion of prolactin (lactotropic hormone) and reduces the production of growth hormone (somatotropic hormone, GH).
It would seem logical to use dopamine itself in the presence of an excess of these hormones. However, the native hormone has strict pharmacological limitations and is not used to correct endocrine disorders. There are two reasons for this: it has an extremely short duration of action and is completely incapable of crossing the blood-brain barrier (BBB). To solve this problem, pharmacology uses dopamine receptor agonists—drugs that mimic the action of the natural mediator but lack its pharmacokinetic drawbacks.
Bromocriptine: the classical approach
Bromocriptinum is a classical representative of the group, being a semisynthetic derivative of ergot alkaloids. Its pharmacodynamics is based on the direct stimulation of postsynaptic dopamine receptors in the central nervous system.
The drug effectively reduces the pathological secretion of prolactin and GH, and also possesses pronounced antiparkinsonian activity.
Indications for use:
- Acromegaly (prescribed in situations where surgical treatment is impossible for any reason).
- Hyperprolactinemic states accompanied by galactorrhea and prolactin-dependent amenorrhea.
- The need for medical cessation of physiological lactation.
Strict contraindications for administration include severe forms of arterial hypertension, occlusive vascular disease, and Huntington's chorea.
Side effects of bromocriptine
Since bromocriptine acts non-selectively, its use is often accompanied by side effects. Their pathogenesis is directly related to the localization of stimulated receptors in various body structures:
- Trigger zone of the vomiting center: activation of this area naturally causes nausea and vomiting.
- Cerebral cortex: excessive stimulation can provoke psychotic reactions, including delusions and hallucinations.
- Extrapyramidal system: interference with its function leads to the development of motor disorders—dyskinesias.
- Peripheral circulation: the drug causes orthostatic reactions and peripheral arterial spasm. Clinically, this spasm often manifests as painful calf muscle cramps.
Other, less specific adverse reactions include dry mouth and constipation.
Cabergoline: a modern alternative
Cabergolinum (best known under the trade name Dostinex) is a modern drug that is superior to bromocriptine in many ways.
Its key difference lies in high selectivity—cabergoline is a selective $D_2$ receptor agonist. The second important advantage is a significantly longer duration of action. After taking the drug, the hypoprolactinemic effect persists for 7–28 days, making therapy much more convenient for the patient.
It is important to consider the dose-dependent nature of cabergoline's effects:
- When used in therapeutic doses, the drug works with maximum precision, inhibiting the secretion exclusively of lactotroph cells of the pituitary.
- When prescribed in high doses, selectivity decreases, and the patient develops systemic effects of central $D_2$ receptor stimulation.
Indications for the use of cabergoline are completely analogous to those for bromocriptine.
Briefly: answers to topical questions
What is the mechanism of the antiparkinsonian action of bromocriptine?
The antiparkinsonian effect of bromocriptine is due to the direct stimulation of dopamine receptors in the CNS. The drug causes direct excitation of dopamine receptors in the neostriatum without prior metabolic conversion.
What other drugs belong to the group of dopamine receptor agonists?
The group of dopamine receptor agonists includes:
- Cabergoline — a selective D2 receptor agonist.
- Pramipexole — an agonist stimulating D2 and D3 receptors.
- Pergolide — a D1 and D2 receptor agonist, an ergot alkaloid derivative.
To remember the side effects of non-selective dopaminergic drugs, recall the acronym PERC: Periphery (vascular spasm, orthostasis), Extrapyramidal system (dyskinesias), Retinum / Vomiting center (nausea), Cortex (psychosis).
Class and exam questions
Why doesn't native dopamine treat hyperprolactinemia?
It does not cross the blood-brain barrier and is degraded in the body too quickly to provide a sustained therapeutic effect.
What is the main advantage of cabergoline over bromocriptine?
Cabergoline acts selectively on $D_2$ receptors and works much longer—its effect lasts up to 4 weeks, whereas bromocriptine requires frequent administration.
Why does taking bromocriptine cause calf cramps?
The drug stimulates dopamine receptors in the peripheral circulation, causing arterial spasm, which clinically manifests as calf muscle cramps.
How does the dose of cabergoline affect its selectivity?
In standard therapeutic doses, it affects only the lactotroph cells of the pituitary. When the dose is increased, selectivity is lost, and the effects of general central $D_2$ receptor stimulation occur.
What else to study on this topic
- Mechanisms regulating the secretion of trophic hormones of the anterior pituitary gland.
- Pharmacotherapy of acromegaly: the place of dopaminergic drugs in clinical practice.
- Antiparkinsonian activity of ergot alkaloid derivatives.
- Pathogenesis of dyskinesias during extrapyramidal system stimulation.
- Occlusive vascular diseases as a contraindication to the administration of ergot alkaloids.
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