Insomnia is a highly prevalent condition, and due to ongoing demand from patients suffering with this condition, new pharmacological treatments are actively being sought. As our neurophysiological understanding of insomnia grows, so too do the available treatment options. A significant advance in the treatment of insomnia came with the development of the nonbenzodiazepine hypnotic medications, zolpidem, zaleplon and eszopiclone. These medications have shorter durations of action than many traditional benzodiazepines and may be associated with less risk of tolerance and abuse. They have also been demonstrated to be helpful in cases in which insomnia is comorbid with depression or anxiety, leading to beneficial effects not only with sleep but also with mood and anxiety symptoms. Melatonin, a naturally-occurring substance intimately involved in the regulation of the circadian rhythm, has also been explored as a hypnotic. Little data to date support its use; however, there is evidence that ramelteon, a melatonin agonist, may be helpful for sleep initiation difficulties. Tri-cyclic antidepressants have long been used for insomnia, but use has been limited by unwanted anticholinergic side effects. Low-dose doxepin, at doses of 3 and 6mg, has been demonstrated to have the unique property among its class of being free of anticholinergic effects at those doses. In addition, it seems to have particular efficacy for sleep maintenance insomnia, exhibiting the most robust effects in the latter third of the night. The hypocretin/orexin system has been identified as a possible target. Almorexant, a hypocretin/orexin antagonist displayed evidence of efficacy during Phase III clinical trials but these trials were recently discontinued due to its side effect profile. Another hypocretin/orexin antagonist with a different mechanism of action, MK-4035, is presently in clinical trials. Serotonin antagonists and inverse agonists have been investigated; however, a recent trial with APD125 was discontinued due to lack of efficacy. Sodium oxybate has been used off-label for insomnia by some providers, although data supporting its use are limited. While the sleep-promoting effects of GABAA (nonbenzodiazepines and benzodiazepines) enhancement is well-established, newer research examining other mechanisms of action suggest that agents which modulate the histaminergic, serotonergic, melontonergic, and hypocretin/orexin and perhaps GABA-B systems show promise.
Keywords: Insomnia, pharmacology, hypnotic, benzodiazepines, histaminergic, serotonergic, melontonergic, orexin, anticholinergic, zaleplon, zolpidem, eszopiclone, polysomnographic, amygdala, myorelaxant, fluoxetine
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