Hypnotics benzodiazepine-receptor agonists

A medication that causes induction of sleep. The majority of currently available hypnotics (for example benzodiazepine receptor agonists) act via potentiating the brain s inhibitory GABAergic systems, in turn reducing the activity of arousal (i.e. wake promoting) neurotransmitter systems. 

Zopiclone is widely used as a sedative-hypnotic. It is metabolized to an inactive N-desmethylated derivative and an active N-oxide compound, both of which contain chiral centres. S-Zopiclone has a 50-fold higher affinity for the benzodiazepine receptor site than the R-enantiomer. This could be therapeutically important, particularly if the formation and the urinary excretion of the active metabolite benefits the S-isomer, which appears to be the case. As the half-life of the R-enantiomer is longer than that of the S-form, it would seem advantageous to use the R-isomer in order to avoid the possibility of daytime sedation and hangover effects which commonly occur with long-acting benzodiazepine receptor agonists. 

Alprazolam, 269, a popular sedative/hypnotic agent and benzodiazepine receptor agonist, has been labelled257 with 11C in metabolically stable position in reaction of the amidrazone, 7-chloro-5-phenyl-3//-l,4-benzodiazepine-2-yl hydrazone, 270, with 1-[ Cjacetyl chloride followed by pyrolysis of the resulting 1-acetylhydrazone 271. Compound 269 has been obtained in 43-65% yield during 40-55 min at specific activity 0.93-2.18 Ci /miol-1, and used for metabolism and tissue distribution studies257 (equation 107). 

Zolpidem (an imidazopyridine) and zopiclone (a cyclopyrrolone) are hypnotics that, despite their different chemical structure, possess agonist activity at the benzodiazepine receptor (p. 226). 

Zolpidem is a non-benzodiazepine hypnotic of the imidazopyridine class. It is a GABA receptor agonist selective for omega-1 receptor subunit. 

It belongs to azapirones which is chemically and pharmacologically distinct class. It acts as a partial agonist at serotonin and dopamine receptors and having no hypnotic and sedative action. It does not interact with benzodiazepine receptor or modify GABA-ergic transmission. 

Several drugs with novel chemical structures have been introduced more recently for use in sleep disorders. Zolpidem, an imidazopyridine, zaleplon, a pyrazolopyrimidine, and eszopiclone, a cyclopyrrolone (Figure 22-4), although structurally unrelated to benzodiazepines, share a similar mechanism of action, as described below. Eszopiclone is the (S) enantiomer of zopiclone, a hypnotic drug that has been available outside the United States since 1989. Ramelteon, a melatonin receptor agonist, is a new hypnotic drug (see Ramelteon). Buspirone is a slow-onset anxiolytic agent whose actions are quite different from those of conventional sedative-hypnotics (see Buspirone). 

Zaleplon has a pharmacological profile similar to benzodiazepines. Zaleplon is a full agonist for the benzodiazepine oq receptor located on the GABAa receptor ionophore complex in the brain, with lower affinity for the a2 and a3 subtypes. It selectively enhances the action of GABA similar to but more selectively than benzodiazepines. Zaleplon, although not benzodiazepine-like in chemical structure, induces sedative-hypnotic, anticonvulsant, and anticonflict effects via its binding to the central nervous system (CNS)-type benzodiazepine receptors [33-36]. 

Ramelteon is a melatonin receptor agonist selective for the MTj and MT2 receptors. The dose is 8 mg at bedtime. It is well tolerated, but side effects include headache, dizziness, and somnolence. It is not a controlled substance. Zolpidem, chemically unrelated to benzodiazepines or barbiturates, acts selectively at the y-aminobutyric acid (GABA )-receptor and has minimal anxiolytic and no muscle relaxant or anticonvulsant effects. It is comparable in effectiveness to benzodiazepine hypnotics, and it has htde effect on sleep stages. Its duration is approximately 6 to 8 hours, and it is metabolized to inactive metabolites. Common side effects are drowsiness, amnesia, dizziness, headache, and GI complaints. Rebound effects when discontinued and tolerance with prolonged use are minimal, but theoretical concerns about abuse exist. It appears to have minimal effects on next-day psychomotor performance. The usual dose is 10 mg (5 mg in the elderly or those with liver impairment), which can be increased up to 20 mg nightly. Cases of psychotic reactions and sleep-eating have been reported. 

Other drugs The anxiolytic ding buspirone interacts with the S-HT, subclass of brain serotonin receptors as a partial agonist, but the precise mechanism of its anxiolytic effect is unknown. The hypnotics zolpidem and zaleplon are not benzodiazepines but appear to exert their CNS effects via interaction with certain benzodiazepine receptors, classified as BZ, or omega, subtypes their CNS depressant effects are antagonized by flumazenil. 

Using molecular biological techniques, point mutations of the a subunits have revealed that the sedative effects of the benzodiazepines likely result from an Interaction with the ai subunit, whereas the anxiolytic effects result from an interaction at the 02 subunit (27,28), as shown in Figure 15.10 and Table 15.3. Nonbenzodiazepine receptor agonists, such as the sedative-hypnotics indiplon, zaleplon, zolpicone, and zolpidem (see Chapter 19), are ai subunit-preferring ligands, as shown in Table 15.3 (29). 

If only a low level of receptor activation is required in order to obtain a full response, one would predict that receptor desensitization would be a smaller problem with THIP, muscimol, and other GABAa agonists used as hypnotics. In an animal study, Lancel and Langebartels showed that 5-d treatment with THIP produced sustained effects on sleep parameters, whereas a benzodiazepine in a similar paradigm induced tolerance-like effects after only 5 d of treatment (52). Accepting that only... 

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