The Dual Hypocretin Receptor Antagonist Almorexant is Permissive for Activation of Wake-Promoting Systems

Neuropsychopharmacology. 2016 Mar;41(4):1144-55. doi: 10.1038/npp.2015.256. Epub 2015 Aug 20.

Abstract

The dual hypocretin receptor (HcrtR) antagonist almorexant (ALM) may promote sleep through selective disfacilitation of wake-promoting systems, whereas benzodiazepine receptor agonists (BzRAs) such as zolpidem (ZOL) induce sleep through general inhibition of neural activity. Previous studies have indicated that HcrtR antagonists cause less-functional impairment than BzRAs. To gain insight into the mechanisms underlying these differential profiles, we compared the effects of ALM and ZOL on functional activation of wake-promoting systems at doses equipotent for sleep induction. Sprague-Dawley rats, implanted for EEG/EMG recording, were orally administered vehicle (VEH), 100 mg/kg ALM, or 100 mg/kg ZOL during their active phase and either left undisturbed or kept awake for 90 min after which their brains were collected. ZOL-treated rats required more stimulation to maintain wakefulness than VEH- or ALM-treated rats. We measured Fos co-expression with markers for wake-promoting cell groups in the lateral hypothalamus (Hcrt), tuberomammillary nuclei (histamine; HA), basal forebrain (acetylcholine; ACh), dorsal raphe (serotonin; 5HT), and singly labeled Fos(+) cells in the locus coeruleus (LC). Following SD, Fos co-expression in Hcrt, HA, and ACh neurons (but not in 5HT neurons) was consistently elevated in VEH- and ALM-treated rats, whereas Fos expression in these neuronal groups was unaffected by SD in ZOL-treated rats. Surprisingly, Fos expression in the LC was elevated in ZOL- but not in VEH- or ALM-treated SD animals. These results indicate that Hcrt signaling is unnecessary for the activation of Hcrt, HA, or ACh wake-active neurons, which may underlie the milder cognitive impairment produced by HcrtR antagonists compared to ZOL.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Acetamides / administration & dosage*
  • Animals
  • Brain / drug effects
  • Brain / metabolism
  • Brain / physiology*
  • Cholinergic Neurons / drug effects
  • Cholinergic Neurons / metabolism
  • Electroencephalography
  • Electromyography
  • Histamine / metabolism
  • Isoquinolines / administration & dosage*
  • Male
  • Neurons / drug effects
  • Neurons / metabolism
  • Neurons / physiology*
  • Orexin Receptor Antagonists / administration & dosage*
  • Orexins / metabolism
  • Proto-Oncogene Proteins c-fos / metabolism
  • Pyridines / administration & dosage*
  • Rats
  • Rats, Sprague-Dawley
  • Serotonergic Neurons / drug effects
  • Serotonergic Neurons / metabolism
  • Sleep Stages / drug effects*
  • Wakefulness / drug effects*
  • Zolpidem

Substances

  • Acetamides
  • Hcrt protein, rat
  • Isoquinolines
  • Orexin Receptor Antagonists
  • Orexins
  • Proto-Oncogene Proteins c-fos
  • Pyridines
  • Zolpidem
  • Histamine
  • almorexant