The wake-promoting effects of hypocretin-1 are attenuated in old rats

doi:10.1016/j.neurobiolaging.2009.07.017

Neurobiology of Aging

Volume 32, Issue 8, August 2011, Pages 1514-1527

https://doi.org/10.1016/j.neurobiolaging.2009.07.017 Get rights and content

Abstract

Disruption of sleep is a frequent complaint among elderly humans and is also evident in aged laboratory rodents. The neurobiological bases of age-related sleep/wake disruption are unknown. Given the critical role of the hypocretins in sleep/wake regulation, we sought to determine whether the wake-promoting effect of hypocretin changes with age in Wistar rats, a strain in which age-related changes in both sleep and hypocretin signaling have been reported. Intracerebroventricular infusions of hypocretin-1 (10 and 30 μg) significantly increased wake time relative to vehicle in both young (3 mos) and old (25 mos) Wistar rats. However, the magnitude and duration of the wake-promoting effects were attenuated with age. An increase of parameters associated with homeostatic sleep recovery after sleep deprivation, including non-rapid eye movement (NR) sleep time, NR delta power, the ratio of NR to rapid eye movement (REM) sleep, and NR consolidation, occurred subsequent to Hcrt-induced waking in young but not old rats. ICV infusions of hypocretin-2 (10 and 30 μg) produced fewer effects in both young and old rats. These data demonstrate that activation of a major sleep/wake regulatory pathway is attenuated in old rats.

Introduction

Disruption of sleep is a common complaint among elderly humans (Ballinger, 1976, Bliwise et al., 1992, Karacan et al., 1976). While sleep disruption may be a secondary indication to other medical conditions in a portion of the aging population, even healthy aging is associated with reduced consolidation of sleep (Espiritu, 2008). The broad array of ill-effects that are associated with sleep disruption militate for a more thorough understanding of the mechanisms that underlie changes in sleep with aging.

The hypothalamic neuropeptides hypocretin-1 (Hcrt-1) and Hcrt-2 (also known as orexin-A and orexin-B), derived from a common prepro-peptide precursor (de Lecea et al., 1998, Sakurai et al., 1998), are key regulators of sleep and wakefulness (Sakurai, 2007). Hcrt-synthesizing cells project broadly to the forebrain and to mid- and hindbrain sites implicated in the regulation of wake and sleep. The Hcrt peptides act through two receptors, Hcrt receptor 1 (HcrtR1) and HcrtR2, to increase the excitability of postsynaptic targets. The importance of Hcrt signaling in maintaining sleep and wake consolidation is demonstrated by the effects of deficiencies in either Hcrt-synthesis or Hcrt receptor signaling. Hcrt-deficient human narcoleptics (Thannickal et al., 2000), Hcrt cell- (Hara et al., 2001) and gene (Chemelli et al., 1999) knockout mice, and HcrtR2-deficient dogs (Lin et al., 1999) exhibit a profound loss of sleep and wake state continuity (for review see Sakurai, 2007). Reductions in Hcrt peptide levels and/or Hcrt cell loss has also been documented in a number of other neurological disorders characterized by sleep disruption, including Parkinson's disease (Drouot et al., 2003, Fronczek et al., 2007, Thannickal et al., 2007), Alzheimer's disease (Friedman et al., 2007) and Huntington's disease (Petersen et al., 2005, Aziz et al., 2008). Accordingly, the emergence of sub-clinical deficits in Hcrt signaling might contribute to the disruption of sleep and wake continuity that is seen with aging. We have previously found that mRNA for the HcrtR2 is significantly reduced in the pons, medulla, thalamus and hippocampus of aged mice (Terao et al., 2002).

Intracerebroventricular (ICV) administration of Hcrt produces dose-dependent increases in waking and decreases in non-rapid eye movement sleep (NR) and rapid eye movement sleep (REM) (Huang et al., 2001, Akanmu and Honda, 2005, Bourgin et al., 2000, Espana et al., 2001, Fujiki et al., 2003, Piper et al., 2000, Vogel et al., 2002). The wake-promoting effect of ICV Hcrt may thus be used as an indicator of the function of the Hcrt signaling system. Since we (Terao et al., 2002) and others (Porkka-Heiskanen et al., 2004) have observed reductions in Hcrt receptor expression in the aged rodent brain, we hypothesized that aged animals would have a blunted response to the wake-promoting effects of Hcrt peptides. In the current study, we investigated the effects of Hcrt-1 and Hcrt-2 infusion on the sleep-wake patterns of old and young Wistar rats, a strain known to exhibit disruption of sleep with aging (Clement et al., 2003, Schiffelholz and Lancel, 2001). Our data confirm the wake-promoting effects of ICV Hcrt in young rats and indicate that this effect of Hcrt-1 infusion on behavioral arousal is significantly blunted in aged Wistar rats.

Section snippets

Animals and surgical procedures

Thirteen male aged (25 mos, a gift from Roche Palo Alto LLC) and 7 young (3 mos, Charles River, Wilmington, MA) Wistar rats were maintained on a 12/12 LD cycle in a temperature-controlled recording room (23 ± 1 °C) with food and water available ad libitum. All experimental procedures complied with institutional animal care and use committee regulations at SRI International and National Institutes of Health guidelines for the care and use of experimental animals.

Animals were implanted for continuous

Latency to sleep onset

Latency to sleep onset subsequent to Hcrt-1 infusion was significantly affected by treatment in both young and old rats (F_2,9 = 56.44, P < 0.0001 for young and F_2,27 = 6.45, P = 0.0051 for old; Fig. 1A). While latency to sleep onset was significantly increased by 10 and 30 μg Hcrt-1 infusions relative to vehicle infusion for both young and old rats, the effect was considerably blunted in the old rats compared to the young rats. Latency to sleep onset was significantly shorter for old rats for both the

Discussion

Sleep in the elderly is generally recognized as being of poorer quality than in younger adults (Ballinger, 1976, Bliwise et al., 1992, Karacan et al., 1976). Many of the aspects that characterize poor sleep in elderly humans, such as sleep fragmentation, reduced amplitude of the diurnal sleep/wake rhythm, and decrease in delta frequencies in the EEG (for review see Espiritu, 2008), have been found in studies of aged rats (Li and Satinoff, 1995, Mendelson and Bergmann, 1999a, Mendelson and

Conflict of interest

There are no actual or potential conflict of interests for any of the authors.

Acknowledgements

The authors would like to thank Judith Flores for her outstanding technical assistance during the data collection process for this study. We would also like to thank Daniel Valladao for his assistance with data processing and statistics checking. This work supported by NIH R01 AG02584.

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The wake-promoting effects of hypocretin-1 are attenuated in old rats

Abstract

Introduction

Section snippets

Animals and surgical procedures

Latency to sleep onset

Discussion

Conflict of interest

Acknowledgements

Brain Res.

J. Psychosom. Res.

Soc. Sci. Med.

Cell

Neuroscience

Clin. Geriatr. Med.

Neuron

Soc. Sci. Med.

Neurosci. Lett.

Cell

Neurobiol. Aging

Neuroscience

Neurobiol. Aging

Cell

Physiol. Behav.

Neurosci. Lett.

Neuron

Brain Res.

J. Neurosci. Methods

Brain Res. Bull.

Neurosci. Lett.

Behav. Biol.

Brain Res.

Differences in the appetite-stimulating effect of orexin, neuropeptide Y and ghrelin among young, adult and old rats

Neuroendocrinology