Compensatory sleep response to 12 h wakefulness in young and old rats

Hypocretin-2-saporin lesions of the lateral hypothalamus produce narcoleptic-like sleep behavior in the rat

Hypocretins (Hcrts) are recently discovered peptides linked to the human sleep disorder narcolepsy. Humans with narcolepsy have decreased numbers of Hcrt neurons and Hcrt-null mice also have narcoleptic symptoms. Hcrt neurons are located only in the lateral hypothalamus (LH) but neither electrolytic nor pharmacological lesions of this or any other brain region have produced narcoleptic-like sleep, suggesting that specific neurons need to be destroyed. Hcrt neurons express the Hcrt receptor, and to facilitate lesioning these neurons, the endogenous ligand hypocretin-2/orexin B (Hcrt2) was conjugated to the ribosome-inactivating protein saporin (SAP). In vitro binding studies indicated specificity of the Hcrt2-SAP because it preferentially bound to Chinese hamster ovary cells containing the Hcrt/orexin receptor 2 (HcrtR2/OX(2)R) or the Hcrt/orexin receptor 1 (HcrtR1/OX(1)R) but not to Kirsten murine sarcoma virus transformed rat kidney epithelial (KNRK) cells stably transfected with the substance P (neurokinin-1) receptor. Administration of the toxin to the LH, in which the receptor is known to be present, eliminated some neurons (Hcrt, melanin-concentrating hormone, and adenosine deaminase-containing neurons) but not others (a-melanocyte-stimulating hormone), indicating specificity of the toxin in vivo. When the toxin was administered to the LH, rats had increased slow-wave sleep, rapid-eye movement (REM) sleep, and sleep-onset REM sleep periods. These behavioral changes were negatively correlated with the loss of Hcrt-containing neurons but not with the loss of adenosine deaminase-immunoreactive neurons. These findings indicate that damage to the LH that also causes a substantial loss of Hcrt neurons is likely to produce the multiple sleep disturbances that occur in narcolepsy.

Effects of a 25-h sleep deprivation on daytime sleep in the middle-aged

Our understanding of the mechanisms by which sleep deteriorates with age almost exclusively stems from comparisons of young and elderly subjects. The present study investigated the different effects of a 25-h sleep deprivation on the recovery sleep initiated in the morning (when circadian sleep propensity decreases) of young (20-39 y) and middle-aged subjects (40-60 y). Middle-aged subjects showed a steeper increase in the duration of wakefulness during daytime recovery sleep than the young subjects. Slow-wave sleep (SWS) and EEG slow-wave activity (SWA: spectral power between 0.5-4.5 Hz) were potentiated in both groups following sleep deprivation. However, the rebound of SWS and SWA was significantly less pronounced in the middle-aged than in the young. This reduction in homeostatic recuperative drive in middle-aged subjects might account for the decrease in their ability to maintain sleep when they have to recuperate at an abnormal circadian phase. These results helps to understand the increase in complaints related to shift work and jet lag in the middle years of life.

Effects of prolonged wakefulness on c-fos and AP1 activity in young and old rats

Recent studies have demonstrated that the immediate-early gene c-fos is induced in neuronal populations responsible for specific sleep-wake states. The induction of this gene may be functionally relevant to sleep homeostasis since without the gene mice (c-fos null) take longer to fall asleep and have a selective reduction in slow-wave sleep. This suggests that a build-up of c-fos during wakefulness increases the drive to sleep and lack of c-fos is associated with reduced sleep. Sleep also has an effect on c-Fos serving to eliminate the protein rapidly. Waxing and waning of transcription factors such as c-Fos may influence slow, oscillating events such as sleep and wakefulness. To further examine what role c-Fos may play in regulating sleep, the present study examined the effects of prolonged wakefulness on c-Fos and AP-1 activity in young (3.5 months old) and old (21.5 months old) Sprague--Dawley rats. Previously we found that old rats slept less even after prolonged wakefulness, and other investigators have found that aging is also associated with a decline in c-Fos. In the present study, we reasoned that prolonged wakefulness would also fail to increase c-Fos in old versus young rats. The baseline levels of c-Fos and AP-1 activity were not different between young and old rats. However, in response to 6 or 12 h of prolonged wakefulness, old rats demonstrated significantly less c-Fos and AP-1 activity compared to young rats. These findings suggest that in old rats the mechanism responsible for c-Fos induction in response to wakefulness is deficient. Such a decline at the molecular level could contribute to the decline in sleep that typically occurs with age.