Mechanisms of action and effects on the CNS
The action of hypnotic drugs is dose-dependent. The fundamental mechanism involves the inhibition of synaptic transmission in the nervous system. Based on the degree of selectivity of this inhibition, two types of action are distinguished:
- Non-narcotic type. Characterized by a relatively selective effect: the drugs inhibit only specific brain structures and functions.
- Narcotic type. Leads to general, total, and non-selective depression of the central nervous system.
Classification of hypnotics
The systematization of drugs is based on their chemical structure and type of action on the central nervous system.
1. Drugs with a non-narcotic type of action:
- Benzodiazepine receptor agonists. Include classical benzodiazepine derivatives (nitrazepam, flunitrazepam, triazolam, midazolam) and the so-called Z-drugs of non-benzodiazepine structure (zopiclone, zolpidem, zaleplon).
- $H_1$-histamine receptor blockers. A typical representative is doxylamine.
- Melatonin receptor agonists. This group includes ramelteon.
2. Drugs with a narcotic type of action:
- Barbituric acid derivatives (barbiturates). The main representative is phenobarbital.
- Aliphatic compounds. This group includes chloral hydrate.
Effects on sleep architecture (pharmacodynamics)
It is important to understand that drug-induced sleep differs from natural (physiological) sleep. Primarily, the rapid eye movement (REM) sleep phase is distorted. Under the influence of drugs, its latency period (time to onset) increases, and its overall duration significantly decreases.
Various drug groups disrupt the ratio of REM and non-REM phases to varying degrees:
- Pronounced disruption: barbituric acid derivatives.
- Moderate disruption: benzodiazepines (distort sleep architecture to a lesser extent than barbiturates).
- Minimal effect: zolpidem and zopiclone alter the structure very little, while chloral hydrate has no effect on it at all, providing the most physiological sleep.
Withdrawal syndrome ("Rebound phenomenon")
Upon abrupt discontinuation of hypnotics, a compensatory reaction of the nervous system occurs. Sleep phases change again: the REM sleep latency period is sharply shortened, and its duration increases abnormally.
Clinically, this condition manifests as severe discomfort for the patient. Frequent night awakenings, an abundance of vivid dreams, and severe nightmares occur.
Requirements for modern drugs
To date, a drug that fully meets the criteria of an "ideal" hypnotic does not exist. However, clear requirements for such agents have been developed:
- Pharmacokinetics: ensuring rapid sleep onset and maintaining its optimal duration.
- Physiological profile: absence of effect on sleep architecture (preserving the natural ratio of phases).
- Safety: absence of respiratory center depression and negative impact on the patient's memory.
- Absence of dependence: low risk of tolerance development ( habituation ), as well as the exclusion of physical and psychological drug dependence formation.
Briefly: answers to topical questions
What is the molecular mechanism of action of benzodiazepines?
The molecular mechanism of action of benzodiazepines involves the stimulation of specific allosteric benzodiazepine receptors coupled to the $\gamma$-subunit of the supramolecular $GABA_A$-receptor complex. This leads to allosteric modulation of the GABA receptor, increased sensitivity to the neurotransmitter GABA, and an increased frequency of chloride channel opening. As a result, there is a massive influx of $Cl^-$ ions into the neuron, membrane hyperpolarization, and the development of neuronal inhibition.
What are the specific side effects of Z-drugs?
Specific side effects of zopiclone include a bitter and metallic taste in the mouth. Gastrointestinal disorders, particularly abdominal pain, diarrhea, and nausea, are also possible. Upon awakening, post-somnic manifestations may occur: dizziness and coordination impairment.
Which $H_1$-histamine receptor blockers are used as hypnotics?
As hypnotics, $H_1$-histamine receptor blockers capable of crossing the blood-brain barrier and exerting a depressing effect on the central nervous system are used. Representatives of this group include the specialized hypnotic agent doxylamine, as well as diphenhydramine (dimedrol), which is an antiallergic agent but possesses a pronounced hypnotic effect.
Modern non-benzodiazepine agonists (Z-drugs) are easy to remember: they all start with the letter "Z" (Zopiclone, Zolpidem, Zaleplon), like the sleep symbol "Z-z-z..."
Class and exam questions
What is the difference between narcotic and non-narcotic types of action?
The narcotic type of action (e.g., in barbiturates) causes total, non-selective depression of the CNS. The non-narcotic type acts more subtly, selectively suppressing only individual brain structures.
How do hypnotics alter the REM sleep phase?
Most hypnotics increase the latency period (time to onset) of the REM sleep phase and shorten its total duration, making sleep non-physiological.
What happens upon abrupt drug withdrawal?
A compensatory "rebound phenomenon" develops. The duration of REM sleep sharply increases, leading to nightmares, an abundance of dreams, and frequent awakenings.
What else to study on this topic
- Mechanisms of drug tolerance and dependence development
- Features of respiratory center depression in overdosage
- Effects of hypnotics on cognitive functions and memory
- Specific receptor targets of benzodiazepines and Z-drugs
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