Tuesday, May 7, 2013
Orexin and Insomnia
If Valium makes you groggy, and Ambien makes you sleepwalk…
A compound that blocks a brain receptor you probably have never heard of may hold the key to the next generation of sleeping pills—and there is always a next generation of sleeping pills.
A new class of hypnotic compounds that serve as antagonists for the neurotransmitter orexin may combat insomnia without the “confusional arousals” that have come to plague some users of zolpidem, otherwise known as Ambien. Sleepwalking, sleep driving, and sleep sex are common among the reports. Orexin is involved in central nervous system arousal. So-called DORAs, or dual orexin receptor antagonists, discovered in 1998, are being seen as potential therapies for insomnia, without the daytime drowsiness and rebound insomnia typical of existing treatments. The sleep disorder narcolepsy, which is in many ways the exact opposite of insomnia, is caused by “an autoimmune attack against neurons that express orexin,” according to Emmanuel Mignot of Stanford’s Center for Sleep Sciences, in an article for Science magazine.
If these drugs are already being thought of as potential insomnia therapies, their addiction potential will have to be much lower than that of earlier generations of sleep medications. But that just might be the case, since DORA-style drugs don’t appear to promote sleep by inhibiting brain activity through neurotransmitter systems for GABA, as most existing treatments do. A study last month in Science Translational Medicine by J.M. Uslaner and coworkers asserted that DORA-type drugs caused less cognition and memory impairment in rats than Valium or Ambien, and are effective at lower doses. With drugs that modulate the orexin system, the hope is that there would be less rebound insomnia, less memory loss, less addiction, and less weird wandering around like a zombie in the middle of the night.
The search for safer sleep drugs was recently given a shot in the arm by a disturbing report from the U.S. Substance Abuse & Mental Health Services Administration (SAMHSA). Emergency room visits caused by Ambien more than doubled from 2005 to 2010, and patients 45 years and up accounted for 74% of adverse zolpidem reactions. Overall, male ER visits went up by 144%, whereas female ER visits went up almost twice as much. Overall, women made up two-thirds of all Ambien-related emergency visits—a bald fact that led the Food & Drug Administration (FDA) in January 2013 to cut the recommended dose for females in half. Lower doses were also recommended for men as well.
But a closer look at the report shows the typical confusion of polydrug use: 50% of emergency department visits for Ambien involved its use in combination with other drugs. And in 37% of cases, Ambien was used specifically in combination with other central nervous system depressants. “Although short-term medications can help patients,” SAMSHA Administrator Pamela Hyde said in a prepared statement, “it is exceedingly important that they be carefully used and monitored.”
A new class of medications based on orexin-active drugs would follow three earlier generations of sleeping pills. And each new generation of sleeping pills seems to bring its own history of unintended consequences.
In the beginning, there was meprobamate, the postwar tranquilizer known as Miltown. In additional, powerful barbiturates like phenobarbital were marketed as miracle drugs for the anxious insomniac. By the 1950s, it had become clear that these drugs were seriously addictive, and dangerous in overdose. Emmanuel Mignot noted that in high doses, barbiturates “lead to pulmonary arrest and death, outcomes that gained further notoriety with the deaths of celebrities Marilyn Monroe and Jimi Hendrix.” Barbiturates do their work by activating chloride channel receptors for the inhibitory neurotransmitter GABA. In addition, strong sedatives were increasingly being used on psychotic patients in the 1950s, and found their way into the treatment of insomnia.
The continued promise of a safe and effective hypnotic for insomnia drove research that led to the development of the first benzodiazepines, and eventually to Valium. The benzodiazepines like Valium were safer in overdose, came in a bewildering variety of flavors, and found widespread use for sleep induction—but also as anticonvulsants, anti-anxiety agents, and muscle relaxers. The benzos bound to GABA receptor sites just like the barbiturates, but the effects were less extensive. Still, it was not long before Valium, another “perfect” drug, showed it’s adverse side, in the form of sedation, memory problems, and addiction.
Then, in 2007 came the 3rd generation, in the form of the now wildly popular “Z-drugs”—zolpidem, (Ambien) zopiclone (used overseas), and zaleplon (Sonata). And again, the side effect profiles looked better in testing, the effective dose was lower, and the binding site—GABA again—looked like the right place to bring on slower brain activity and more inclination toward sleep without the “knockout effect” of earlier barbiturates and benzos. These drugs are now the default treatment for insomnia. But over time, predictably, problems revealed themselves: “Occasional problems with dependence, tolerance, and ‘confusional arousals’ are still reported with Z-drugs…. And viewed with some suspicion by doctors and patients,” writes Mignot.
Put simply, any sleep treatment that relies on the broad-brush inhibition of GABA will likely produce a range of unwelcome side effects. There are only about 70,000 orexin-producing neurons in the hypothalamus, researchers have found. But this small band of neurons has projections to histamine systems, as well as “the adrenergic locus coeruleus, and various cholinergic and aminergic cell groups,” as Mignot sums up the research. “Blocking orexin may thus be closer to treating the underlying issue of excess alertness in insomnia compared to promoting sleep by inhibiting brain activity.”
That’s the idea, at least. And it may represent a change in thinking. Researchers are no longer looking for a better knockout drug by bludgeoning the brain into inactivity. Instead, they are looking for ways to combat hyper-alertness as a key component of insomnia.
Uslaner J.M., Tye S.J., Eddins D.M., Wang X., Fox S.V., Savitz A.T., Binns J., Cannon C.E., Garson S.L. & Yao L. & (2013). Orexin Receptor Antagonists Differ from Standard Sleep Drugs by Promoting Sleep at Doses That Do Not Disrupt Cognition, Science Translational Medicine, 5 (179) 179ra44-179ra44. DOI: 10.1126/scitranslmed.3005213
Photo Credit: F Delventhal under license from Creative Commons.