Sedative and hypnotic drugs are central nervous system depressants. They depress behavior, moderate excitement, induce calmness, and may produce drowsiness or even loss of consciousness. The sedative-hypnotics are used clinically as antianxiety agents, muscle relaxants, antiepileptics, and as preanesthetic medications. Drugs in this category include barbiturates, benzodiazepines, and anesthetics.

• History of Sedative-hypnotics
• Behavioral Effects in Humans
• Mechanism of Action
• Thought Questions and Quick Quiz
• Additional Sources of Information

In 1903, barbital, a derivative of barbituric acid, was introduced. Its sleep-inducing and anxiolytic effects made it very popular in clinical medicine. It soon became popular as a treatment for anxiety and as the first "sleeping pill". In 1912, phenobarbital was introduced. Phenobarbital, in addition to sedative-hypnotic properties, has anticonvulsant properties and has become one of the most important pharmacological treatments for epilepsy. The success of barbital and phenobarbital spawned the synthesis of over 2,500 barbiturates. Of these many barbiturate analogues, only about 20 are still on the market. The effects of these various barbiturates are generally similar, differing primarily in potency and duration of action.

The partial separation of sedative-hypnotic-anesthetic from anticonvulsant properties, found in phenobarbital, led researchers to search for agents with more selective effects on the functions of the CNS. In the late 1930's, relatively nonsedative anticonvulsants were developed (e.g., phenytoin and trimethadione). In 1957 the first benzodiazepine, chlordiazepoxide (Librium) was synthesized. Benzodiazepines have demonstrated the ability to relieve symptoms of anxiety with relatively little interference with cognitive function or wakefulness. Benzodiazepines and barbiturates share very similar properties but the benzodiazepines have demonstrated to have a much safer pharmacological profile. Benzodiazepines have therefore replaced barbiturates for most uses, particularly for treatment of anxiety and sleep disturbances.

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Sedative-hypnotic drugs depress behavior, moderate excitement, and induce calmness. These drugs depress the central nervous system, however, they usually produce therapeutic benefits at far lower doses than those causing substantial generalized depression of behavior.

Barbiturates have a wider and more powerful effect on the central nervous system than the other sedatives. The barbiturates can produce varying degrees of depression of the CNS, ranging from mild sedation to general anesthesia. In low doses barbiturates have a calming effect, and some of the barbiturates (e.g., phenobarbital) have demonstrated selective anticonvulsant properties. In moderate doses they produce a drunken euphoric state, similar to alcohol. Sedation and sleep result from increased doses, and even higher doses produce surgical anesthesia. Because of their ability to produce sedation and decrease sleep latency, barbiturates were popular in the treatment of insomnia prior to the advent of benzodiazepines. However, because of the high incidence of tolerance and physical dependence following chronic use and the relatively high danger of overdose, these drugs are rarely used today for the treatment of anxiety or sleep disturbances.

Benzodiazepines share the sedative-hypnotic properties, but produce fewer side effects than barbiturates. Like barbiturates, benzodiazepines have also been reported to produce anticonvulsant effects. In addition, these drugs are used clinically as muscle relaxants, antiepilieptic agents, and to produce sedation before operative procedures. The antianxiety effects of benzodiazepines are more selective than those of other sedative-hypnotics -- they relieve anxiety at lower doses and thus produce minimal sedation and motor impairment. The benzodiazepines are currently the most important class of drugs for treatment of anxiety and sleep disorders.

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Barbiturates and benzodiazepines act similarly to produce depression of central nervous system function and behavior. Both classes of drugs enhance the ability of the inhibitory neurotransmitter, gamma aminobutyric acid (GABA), to activate a type of receptors known as GABA-A receptors. These drugs increase the effectiveness of GABA by altering the receptor so that GABA can bind more easily, an effect known as allosteric regulation. Activation of the GABA-A receptor opens an ion channel, allowing negatively charged chloride ions to enter the cell, producing an inhibition of neuronal activity.

In addition to barbiturates and benzodiazepines, ethanol appears to produce depression of central nervous system function, in part by enhancing the ability of GABA to bind to the GABA-A receptor. This may explain why these three classes of drugs potentiate each others' effects and why tolerance to one results in cross-tolerance to another.

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Anxiolytic and Sedative-hypnotics	Generic Name	Trade Name
Benzodiazepines	Alprazolam	Xanax
	Chlordiazepoxide	Librium
	Clonazepam	Klonopin
	Clorazepate	Tranxene
	Diazepam	Valium
	Estazolam	ProSom
	Flurazepam	Dalmane
	Lorazepam	Ativan
	Midazolam	Versed
	Oxazepam	Serax
	Quazepam	Doral
	Temazepam	Restoril
	Triazolam	Halcion
	Halazepam	Paxipam
	Prazepam	Centrax
Non-Benzodiazepine anxiolytics and hypnotics	Buspirone	BuSpar
	Chloral hydrate	Noctec
	Zolpidem tartrate	Ambien

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Thought Questions

• Drugs, such as benzodiazepines and alcohol, that are generally depressant on behavior often produce excitatory effects, particularly at low doses. How would you explain these apparently paradoxical effects?

• Barbiturates and benzodiazepines are effective for a variety of clinical problems, ranging from anxiety and sleep disturbances to seizures. Given their pharmacological effects, why do you suppose this is the case?

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