Sleep deprivation increases rat hypothalamic growth hormone-releasing hormone mRNA

Effects of growth hormone-releasing hormone on sleep and brain interstitial fluid amyloid-β in an APP transgenic mouse model

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by impairment of cognitive function, extracellular amyloid plaques, intracellular neurofibrillary tangles, and synaptic and neuronal loss. There is substantial evidence that the aggregation of amyloid β (Aβ) in the brain plays a key role in the pathogenesis of AD and that Aβ aggregation is a concentration dependent process. Recently, it was found that Aβ levels in the brain interstitial fluid (ISF) are regulated by the sleep-wake cycle in both humans and mice; ISF Aβ is higher during wakefulness and lower during sleep. Intracerebroventricular infusion of orexin increased wakefulness and ISF Aβ levels, and chronic sleep deprivation significantly increased Aβ plaque formation in amyloid precursor protein transgenic (APP) mice. Growth hormone-releasing hormone (GHRH) is a well-documented sleep regulatory substance which promotes non-rapid eye movement sleep. GHRHR(lit/lit) mice that lack functional GHRH receptor have shorter sleep duration and longer wakefulness during light periods. The current study was undertaken to determine whether manipulating sleep by interfering with GHRH signaling affects brain ISF Aβ levels in APPswe/PS1ΔE9 (PS1APP) transgenic mice that overexpress mutant forms of APP and PSEN1 that cause autosomal dominant AD. We found that intraperitoneal injection of GHRH at dark onset increased sleep and decreased ISF Aβ and that delivery of a GHRH antagonist via reverse-microdialysis suppressed sleep and increased ISF Aβ. The diurnal fluctuation of ISF Aβ in PS1APP/GHRHR(lit/lit) mice was significantly smaller than that in PS1APP/GHRHR(lit/+) mice. However despite decreased sleep in GHRHR deficient mice, this was not associated with an increase in Aβ accumulation later in life. One of several possibilities for the finding is the fact that GHRHR deficient mice have GHRH-dependent but sleep-independent factors which protect against Aβ deposition.

Localized suppression of cortical growth hormone-releasing hormone receptors state-specifically attenuates electroencephalographic delta waves

Growth hormone-releasing hormone (GHRH) promotes non-rapid eye movement sleep (NREMS), in part via a well characterized hypothalamic sleep-promoting site. However, GHRH may also act in the cortex to influence sleep. Application of GHRH to the surface of the cortex changes electroencephalographic (EEG) delta power. GHRH and the GHRH receptor (GHRHR) mRNAs are detectable in the rat cortex, and the expression of cortical GHRHR is activity dependent. Here, we microinjected a GHRH antagonist or GHRHR small interfering RNA (siGHRHR) onto the somatosensory cortex surface in rats. The unilateral application of the GHRH antagonist ipsilaterally decreased EEG delta wave power during NREMS, but not wakefulness, during the initial 40 min after injection. Similarly, the injection of siGHRHR reduced cortical expression of GHRHR and suppressed NREMS EEG delta wave power during 20-24 h after injection. Using the fura-2 calcium imaging technique, cultured cortical cells responded to GHRH by increasing intracellular calcium. Approximately 18% of the GHRH-responsive cells were GABAergic as illustrated by glutamic acid decarboxylase-67 (GAD67) immunostaining. Double labeling for GAD67 and GHRHR in vitro and in vivo indicated that only a minority of cortical GHRHR-containing cells were GABAergic. Our data suggest that endogenous cortical GHRH activates local cortical cells to affect EEG delta wave power state-specifically. Results are also consistent with the hypothesis that GHRH contributes to local network state regulation.

Cellular and chemical neuroscience of mammalian sleep

Extraordinary strides have been made toward understanding the complexities and regulatory mechanisms of sleep over the past two decades thanks to the help of rapidly evolving technologies. At its most basic level, mammalian sleep is a restorative process of the brain and body. Beyond its primary restorative purpose, sleep is essential for a number of vital functions. Our primary research interest is to understand the cellular and molecular mechanisms underlying the regulation of sleep and its cognitive functions. Here I will reflect on our own research contributions to 50 years of extraordinary advances in the neurobiology of slow-wave sleep (SWS) and rapid eye movement (REM) sleep regulation. I conclude this review by suggesting some potential future directions to further our understanding of the neurobiology of sleep.

Growth hormone-releasing hormone activates sleep regulatory neurons of the rat preoptic hypothalamus

We examined whether growth hormone-releasing hormone (GHRH) may promote non-rapid eye movement (NREM) sleep via activation of GABAergic neurons in the preoptic area. Male Sprague-Dawley rats were implanted with EEG, EMG electrodes and a unilateral intracerebroventricular cannula. Groups of rats received injections (3 microl icv) with gonadotropin-releasing hormone (GHRH) (0.1 nmol/100 g body wt) or equal volume of physiological saline at the onset of the dark period and were permitted spontaneous sleep for 90 min. Separate groups of rats were sleep deprived by gentle handling for 90 min, beginning at the time of GHRH or saline injection, at the onset of the dark period. Other groups of rats received intracerebroventricular octreotide (somatostatin analog OCT) injections, intracerebroventricular injection of one of two doses of competitive GHRH antagonist, or intracerebroventricular saline injection at light onset and were then permitted 90 min spontaneous sleep-waking. Rats were killed immediately after the 90-min sleep/wake monitoring period. Brain tissue was processed for immunohistochemistry for c-Fos protein and glutamic acid decarboxylase (GAD). Single c-Fos and dual Fos-GAD cell counts were determined in the median preoptic nucleus (MnPN), and in the core and the extended parts of the ventrolateral preoptic nucleus (cVLPO and exVLPO). Intracerebroventricular GHRH elicited a significant increase in NREM sleep amount. Double-labeled Fos+GAD cell counts were significantly elevated after GHRH injection in the MnPN and VLPO in both undisturbed and sleep-deprived groups. OCT and GHRH antagonist significantly decreased NREM sleep amount compared with control rats. OCT injection increased single c-Fos-labeled cell counts in the MnPN, but not in the VLPO. Double-labeled cell counts were significantly reduced after OCT and the high dose of GHRH antagonist injection in all areas examined. These findings identify GABAergic neurons in the MnPN and VLPO as potential targets of the sleep-regulatory actions of GHRH.

Deficiency of growth hormone-releasing hormone signaling is associated with sleep alterations in the dwarf rat

The somatotropic axis, and particularly growth hormone-releasing hormone (GHRH), is implicated in the regulation of sleep-wake activity. To evaluate sleep in chronic somatotropic deficiency, sleep-wake activity was studied in dwarf (dw/dw) rats that are known to have a defective GHRH signaling mechanism in the pituitary and in normal Lewis rats, the parental strain of the dw/dw rats. In addition, expression of GHRH receptor (GHRH-R) mRNA in the hypothalamus/preoptic region and in the pituitary was also determined by means of reverse transcription-PCR, and GHRH content of the hypothalamus was measured. Hypothalamic/preoptic and pituitary GHRH-R mRNA levels were decreased in the dw/dw rats, indicating deficits in the central GHRHergic transmission. Hypothalamic GHRH content in dw/dw rats was also less than that found in Lewis rats. The dw/dw rats had less spontaneous nonrapid eye movement sleep (NREMS) (light and dark period) and rapid eye movement sleep (REMS) (light period) than did the control Lewis rats. After 4 hr of sleep deprivation, rebound increases in NREMS and REMS were normal in the dw/dw rat. As determined by fast Fourier analysis of the electroencephalogram (EEG), the sleep deprivation-induced enhancements in EEG slow-wave activity in the dw/dw rats were only one-half of the response in the Lewis rats. The results are compared with sleep findings previously obtained in GHRH-deficient transgenic mice. The alterations in NREMS are attributed to the defect in GHRH signaling, whereas the decreases in REMS might result from the growth hormone deficiency in the dw/dw rat.

Nociception in cyclooxygenase isozyme-deficient mice

Prostaglandins formed by cyclooxygenase-1 (COX-1) or COX-2 produce hyperalgesia in sensory nerve endings. To assess the relative roles of the two enzymes in pain processing, we compared responses of COX-1- or COX-2-deficient homozygous and heterozygous mice with wild-type controls in the hot plate and stretching tests for analgesia. Preliminary observational studies determined that there were no differences in gross parameters of behavior between the different groups. Surprisingly, on the hot plate (55 degrees C), the COX-1-deficient heterozygous groups showed less nociception, because mean reaction time was longer than that for controls. All other groups showed similar reaction times. In the stretching test, there was less nociception in COX-1-null and COX-1-deficient heterozygotes and also, unexpectedly, in female COX-2-deficient heterozygotes, as shown by a decreased number of writhes. Measurements of mRNA levels by reverse transcription-PCR demonstrated a compensatory increase of COX-1 mRNA in spinal cords of COX-2-null mice but no increase in COX-2 mRNA in spinal cords of COX-1-null animals. Thus, compensation for the absence of COX-1 may not involve increased expression of COX-2, whereas up-regulation of COX-1 in the spinal cord may compensate for the absence of COX-2. The longer reaction times on the hot plate of COX-1-deficient heterozygotes are difficult to explain, because nonsteroid anti-inflammatory drugs have no analgesic action in this test. Reduction in the number of writhes of the COX-1-null and COX-1-deficient heterozygotes may be due to low levels of COX-1 at the site of stimulation with acetic acid. Thus, prostaglandins made by COX-1 mainly are involved in pain transmission in the stretching test in both male and female mice, whereas those made by COX-2 also may play a role in the stretching response in female mice.

Intrapreoptic microinjection of GHRH or its antagonist alters sleep in rats

Previous reports indicate that growth hormone-releasing hormone (GHRH) is involved in sleep regulation. The site of action mediating the nonrapid eye movement sleep (NREMS)-promoting effects of GHRH is not known, but it is independent from the pituitary. GHRH (0.001, 0. 01, and 0.1 nmol/kg) or a competitive antagonist of GHRH (0.003, 0.3, and 14 nmol/kg) was microinjected into the preoptic area, and the sleep-wake activity was recorded for 23 hr after injection in rats. GHRH elicited dose-dependent increases in the duration and in the intensity of NREMS compared with that in control records after intrapreoptic injection of physiological saline. The antagonist decreased the duration and intensity of NREMS and prolonged sleep latency. Consistent alterations in rapid eye movement sleep (REMS) and in brain temperature were not found. The GHRH antagonist also attenuated the enhancements in NREMS elicited by 3 hr of sleep deprivation. Histological verification of the injection sites showed that the majority of the effective injections were in the preoptic area and the diagonal band of Broca. The results indicate that the preoptic area mediates the sleep-promoting activity of GHRH.