Emetics

Physiology of the Vomiting Reflex as the Basis of Therapy

Vomiting is not merely a symptom, but a vital protective reflex act. Its physiological purpose is the emergency removal of various toxic or irritating substances from the stomach that could cause irreparable harm to the body. This process represents a complex combination of motor and secretory reactions.

The central structure coordinating this act is the vomiting center, anatomically located in the medulla oblongata. However, it does not function alone. A crucial role is played by the initiation, or chemoreceptor trigger zone (chemoreceptor trigger zone), which is localized on the floor of the fourth ventricle of the brain.

Stimulation of the vomiting center can occur via three primary pathways:

1. Signals from the vestibular apparatus. This exact mechanism triggers motion sickness, frequently termed kinetosis in medicine.
2. Peripheral stimulation. In this case, nerve impulses originate from sensory receptors located directly in the gastric mucosa and pharynx.
3. Activation of trigger zone chemoreceptors. This zone is unique because it directly responds to the presence of chemical substances (toxins, poisons, drugs) in the systemic circulation or cerebrospinal fluid.

Receptor Map of the Vomiting Reflex

To understand how pharmacological agents work, one must know the exact localization of the receptors that serve as drug targets. The receptor map is structured as follows:

• Vomiting center: Its activity is regulated via $M_1$ muscarinic receptors.
• Trigger zone: Concentrated in this area are $D_2$ dopamine and $5 ext{-}HT_3$ serotonin receptors. These receptors detect chemical changes in the blood and cerebrospinal fluid.
• Vestibular nuclei: Here, nerve impulse transmission is mediated by $M_1$ muscarinic and $H_1$ histamine receptors.
• Visceral afferent system: Neural pathways extending from the pharynx and stomach to the brain transmit signals predominantly via $5 ext{-}HT_3$ receptors.

Pharmacological Profile of Emetics

The primary and most well-known representative of the emetic group is apomorphine. Chemically, it is a semisynthetic alkaloid derived from morphine.

The mechanism of action of apomorphine involves the direct and selective stimulation of $D_2$ dopamine receptors, which, as noted previously, are located in the chemoreceptor trigger zone on the floor of the fourth ventricle.

Pharmacokinetic features determine the speed of the drug's clinical effect:

• Administration is performed strictly subcutaneously.
• The drug exhibits very rapid penetration into the central nervous system.
• The latent period is minimal: a pronounced vomiting reflex occurs just 5–10 minutes after injection.
• Elimination from the body occurs in two stages: first, the substance undergoes hepatic metabolism, and subsequently, its metabolites are excreted by the kidneys.

Indications and Absolute Contraindications

In modern medical practice, the use of emetics is strictly limited to specific indications. Apomorphine is utilized in two main scenarios:

1. Acute poisonings. The drug is administered to rapidly eliminate swallowed poisons from the stomach before they can be absorbed into the systemic circulation.
2. Treatment of chronic alcoholism. In addiction medicine, apomorphine is used to establish a strong conditioned aversive reflex to the taste and smell of alcohol.

Due to its powerful systemic impact and the intensity of the emetic act itself, apomorphine has a significant list of contraindications. It is strictly prohibited in:

• Severe cardiac diseases.
• Active forms of tuberculosis and any pulmonary hemorrhages.
• Peptic ulcer disease of the stomach and duodenum.
• Organic lesions of the central nervous system.