Peterfi Z, McGinty D, Sarai E, Szymusiak R. Growth hormone-releasing hormone activates sleep regulatory neurons of the rat preoptic hypothalamus. Am J Physiol Regul Integr Comp Physiol 298: R147-R156, 2010. First published November 4, 2009 doi:10.1152/ajpregu.00494.2009.-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 l icv) with gonadotropinreleasing 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 FosϩGAD 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. preoptic area; growth hormone-releasing hormone; somatostatin; octreotide; median preoptic nucleus; ventrolateral preoptic nucleus GROWTH HORMONE-RELEASING HORMONE (GHRH) is the main stimulatory neuropeptide that regulates growth hormone (GH) release by the pituitary. Exogenous GHRH stimulates sleep, especially, non-rapid eye movement (NREM) sleep (reviewed in Ref. 39). Intracerebroventricular (icv) administration of GHRH increases NREM sleep and enhances EEG slow-wave activity (SWA) in the neocortical EEG during NREM sleep in rats and rabbits (11,37,38). Intravenous administration of GHRH promotes sleep and EEG SWA in humans (25,33,54).In humans, the major daily pulse in GH release occurs early in the nocturnal sleep period, coinciding with the o...