Sedatives and Hypnotics

Sedative-Hypnotics: Pharmacological Intervention in Sleep Disorders

Background on Sleep Disorders

  • Definition of Insomnia:

    • Inability to fall asleep and/or remain asleep for the desired time.

    • Impairs normal functioning during social or occupational tasks.

  • Causes of Insomnia:

    • Medically-related:

      • Conditions such as asthma, diabetes, neurological disorders, urological issues.

    • Psychiatric:

      • Mood disorders, anxiety disorders.

    • Environmental:

      • Factors such as noise, stress, light, etc.

  • Epidemiology:

    • Approximately 30-50% of adults experience symptoms of insomnia.

    • Insomnia is about 40% more prevalent in women than men.

    • Around 10% have impaired daytime functioning associated with insomnia.

Sedation Continuum

  • Sedative-Hypnotic Agents:

    • Central Nervous System (CNS) depressants that progress in a dose-dependent manner through clinical effects including:

      • Calming/Reduce Excitement (Sedation): Initial calming effect.

      • Drowsiness/Sleep (Hypnosis): Increased propensity to sleep.

      • Unconsciousness: Loss of consciousness.

      • Coma: Prolonged state of unresponsiveness.

      • Death: Potential fatality due to cardiorespiratory depression.

    • Different classes of drugs have unique propensities to reach these endpoints; some drugs achieve them alone while others depend on combinations.

Neurobiology and Chemistry

  • Normal Sleep:

    • Occurs in cyclical stages characterized by distinct brain wave activities, observable via electroencephalogram (EEG).

  • REM Deprivation Effects:

    • Associated clinical effects include:

      • Anxiety, depression, increased irritability/aggression, changes in eating habits, deficits in memory formation.

Neurotransmitters and Brain Regions

  • Key Neurotransmitters and Their Effects:

    • Acetylcholine (Ach):

      • Location: Pons

      • Effect: Promotes arousal and wakefulness.

    • Histamine:

      • Location: Posterior hypothalamus

      • Effect: Promotes arousal and wakefulness.

    • Serotonin (5-HT):

      • Location: Raphe nuclei

      • Effect: Promotes arousal and wakefulness.

    • Norepinephrine (NE):

      • Location: Locus coeruleus

      • Effect: Promotes arousal and wakefulness.

    • Orexin (Hypocretin):

      • Location: Lateral hypothalamus

      • Effect: Promotes arousal and wakefulness during NREM sleep.

Wake-Promoting vs. Sleep-Promoting Systems

  • Wake-Active Neurotransmitters:

    • Acetylcholine, Dopamine

  • Sleep-Active Neurotransmitters:

    • GABA (γ-aminobutyric acid), Histamine, Norepinephrine, Serotonin

    • Sleep-Wake Switch:

      • Hypocretin acts as a stabilizing neuromodulator influencing the switch between sleep and wake states.

GABA Receptors

  • Role of GABA:

    • Primary inhibitory neurotransmitter in the brain influencing sedative effects.

  • Types of GABA Receptors:

    • GABAB Receptor:

      • Mechanism: Gai-coupled GPCR leading to the opening of potassium channels, hyperpolarizing the cell membrane.

      • Agonists include baclofen (muscle relaxant) and gamma-hydroxybutyrate (GHB).

    • GABAA Receptor:

      • Mechanism: Ligand-gated chloride channel composed of a heteropentameric structure that promotes inward chloride flux (Cl-i), reducing the likelihood of action potential firing.

GABAA Receptor Characteristics

  • Binding Sites:

    • Assembled GABAA receptors have multiple binding sites for various drug classes, all distinct from the two GABA binding sites.

    • Binding can either activate the channel directly (agonists) or serve as allosteric regulators of GABA binding.

Overview of Drug Classes

  • Objectives of Sedative-Hypnotic Therapy:

    • Induce sedation while minimizing toxicity, abuse, misuse, and dependency risks.

  • Classes of Sedative-Hypnotic Agents:

    • Benzodiazepines

    • Barbiturates

    • Z-drugs

    • Miscellaneous Others:

      • Antihistamines

      • Trazodone (antidepressant)

      • Orexin Antagonist (Suvorexant®)

      • Melatonin Agonist (Ramelteon®)

      • Alcohol

Central Nervous System Effects

  • Sedation Continuum:

    • Coma (Barbiturates)

    • Medullary depression

    • Anesthesia

    • Hypnosis (Benzodiazepines & Z-drugs)

    • Possible selective effects: Disinhibition, anxiolysis, anticonvulsant and muscle-relaxing activities

    • Increasing dosage leads to increased sedative-hypnotic effects.

Benzodiazepines (BZDs)

  • General Characteristics:

    • Exhibit anxiolytic, muscle relaxant, and anticonvulsant activities.

  • Indications:

    • Short-term treatment of insomnia (2-4 week course to reduce dependence risk, classified as Schedule IV).

    • Drawbacks include tolerance, rebound insomnia, and disruption of sleep cycles (decrease of stages 3-4 and REM sleep).

  • Mechanism of Action:

    • Allosteric activation of GABAA receptors leading to enhanced potency of endogenous GABA without enhancing its efficacy independently.

    • BZD binding increases frequency of channel openings, improving safety profile significantly compared to other agents, with rare life-threatening outcomes.

Choice of Benzodiazepine

  • Pharmacokinetics (t1/2):

    • Short t1/2: 1-5 hours (e.g. triazolam): Ideal for difficulty falling asleep; minimal next-day sedation, risk of rebound insomnia and amnesia.

    • Intermediate t1/2: 8-20 hours (e.g. temazepam): Suitable for sleep maintenance; risk of rebound insomnia and next-day sedation.

    • Long t1/2: 40-250 hours (e.g. flurazepam): Beneficial in cases of comorbid anxiety; lower risk of rebound insomnia but increased daytime sedation; caution advised in elderly or patients with liver dysfunction due to the risk of drug accumulation.

Adverse Events of Benzodiazepines

  • Physiological Effects:

    • Respiratory System: Typical sedative doses do not depress function in normal patients, but caution is advised in those with hepatic impairment.

    • Higher Doses: CNS depression occurs primarily when combined with other depressants (e.g. opioids, ethanol).

    • Extrinsic Risks: Can exacerbate sleep apnea, especially in patients with COPD.

    • Cardiovascular Effects: Minimal risks with negligible effects under normal use; however, overdose can induce lowered blood pressure (BP) and heart rate (HR).

  • Common Side Effects:

    • Lightheadedness, headache, impaired cognition/motor control, amnesia, vertigo, nausea/vomiting.

    • Chronic use relates to tolerance, dependence, and withdrawal symptoms.

Benzodiazepine Antagonist: Flumazenil

  • Flumazenil (Romazicon®):

    • Only available GABAA receptor antagonist in the U.S.

    • Mechanism of Action: Competitive antagonist at the benzodiazepine binding site with no intrinsic activity; effectively reverses effects specifically of benzodiazepines and similar agents (e.g. Z-drugs).

    • Limitation: Does not reverse effects of GABA receptor regulators at non-benzodiazepine sites (e.g. barbiturates, alcohol).

Novel Benzodiazepine-receptor Agonists: Z-drugs

  • Characteristics:

    • Bind to GABAA receptor at benzodiazepine sites, eliciting similar sedative effects; non-benzodiazepine chemicals with benzodiazepine-like activity.

    • Similar mechanism to benzodiazepines; enhance GABA's sedative efficacy yet with lesser anticonvulsant and muscle relaxant activities.

    • Termed “Z-drugs” due to nomenclature beginning with the letter “Z”.

      • Examples: Ambien® (zolpidem), Sonata® (zaleplon), Lunesta® (eszopiclone).

Indications and Pharmacokinetics of Z-drugs

  • Recommended Use:

    • Indicated for short-term treatment of insomnia; rapid kinetics help reduce time to sleep onset (~15 minutes; t1/2 = 1-2 hours).

    • Classified as Schedule IV controlled substances with associated risks of dependency, tolerance, and withdrawal.

  • Adverse Effects:

    • Similar adverse effects to benzodiazepines; short half-life minimizes next-day effects.

    • Advantageous in reducing impacts on sleep dynamics and less rebound insomnia risk.

Barbiturates

  • Historical Context:

    • Initially used as sedatives in the early 1900s but largely replaced by benzodiazepines.

    • Remaining use includes general anesthesia, treatment for epilepsy, and management of cluster headaches or migraines.

    • Chemistry:

      • Derived from the heterocyclic compound barbituric acid, with pharmacophore defined by various aromatic and/or aliphatic hydrocarbon chains enhancing lipophilicity, thus increasing CNS penetration and accumulation.

Mechanism of Action of Barbiturates

  • GABA A Receptor Activity:

    • Allosterically activate GABAA receptors, increasing GABA efficacy via prolonged channel opening events induced by GABA binding, enhancing chloride flux (Cl-), and therefore increasing overall efficacy.

    • At higher doses, can function independently of endogenous GABA.

    • Due to these characteristics, barbiturates present a much narrower therapeutic index, linking them to more severe overdose risks than benzodiazepines.