Microinjection of melanin concentrating hormone into the lateral preoptic area promotes non-REM sleep in the rat
Highlights
► Rats were prepared for sleep recordings and intracerebral microinjections. ► Unilateral administration of MCH into the VLPO did not affect sleep. ► Bilateral microinjections of MCH into the VLPO promoted non-REM sleep. ► Bilateral administration of MCH into the VLPO did not affect REM sleep.
Introduction
Since von Economo's classical report, a number of studies have confirmed the proposal that the preoptic area of the hypothalamus (POA) is involved in the generation of non-REM (NREM) sleep [27], [34]. The use of several experimental approaches, including restricted lesions, electrical and chemical stimulation procedures, and the study of neuronal activity by unit recordings or c-fos expression have allowed the identification of POA subregions, such as the median preoptic nucleus (MnPN) and the ventrolateral preoptic region (VLPO), as the critical structures responsible for NREM sleep occurrence [2], [6], [7], [8], [18], [29]. In the rat, the VLPO has been identified as a cluster of neurons that show Fos immunoreactivity during sleep. Among other brain areas, these neurons project to the waking-related histaminergic tuberomamillary nucleus (TMN) [25], [26].
Melanin concentrating hormone (MCH), a 19 aminoacid cyclic peptide initially described in teleost fish [11], is a neuromodulator synthesized in neurons localized in the lateral hypothalamus and incerto-hypothalamic area of mammals [3], [32]. The MCHergic system projects widely throughout the central nervous system (CNS) and sends relatively dense projections to the VLPO [3]. MCH acts through G-proteins via two types of receptors, MCHR1 and MCHR2. The former is the only functional subtype in rodents [28].
Classically, MCH has been involved in the central regulation of feeding and energy homeostasis [23]. However, we and other groups have demonstrated the importance of this neuropeptide in the regulation of sleep (for review, see [30]). Recently, we characterized the role of MCH in the modulation of sleep and wakefulness (W) in different brain areas, such as the dorsal raphe nucleus (DRN) and the lateral basal forebrain of the rat, as well as the nucleus pontis oralis (NPO) of the cat. Microinjection of MCH into these nuclei resulted in an increase in REM sleep [14], [31]. To date, no attempts have been made to determine whether MCH intervenes in the VLPO-induced generation of NREM sleep. Thus, the aim of the present study was to define the effects of MCH microinjection into the VLPO on sleep variables in the rat.
Section snippets
Materials and methods
Fourteen adult male Wistar rats (240–260 g) were employed in this study. All rats were used in strict accordance with the “Guide to the care and use of laboratory animals” (8th edition, National Academy Press, Washington, DC, 2011) and the Institutional Animal Care Committee approved the experimental procedures. In addition, adequate measures were taken to minimize the pain, discomfort and stress of the animals, and all efforts were made to use the minimal number of animals necessary to produce
Results
In eleven animals the tip of the microinjection cannulae was placed in the region of the VLPO; in most of the animals also included an adjacent dorsal region of the lateral preoptic area. Fig. 1A shows a photomicrograph illustrating the bilateral sites of microinjection at the level of the VLPO in one representative animal. Fig. 1B summarizes the sites of the successful bilateral microinjections. Three animals were excluded because the cannulae were misplaced.
The time spent in wakefulness and
Discussion
In the present report, we examined the role of MCH on the regulation of sleep, after it was microinjected into the VLPO. In these experiments, MCH (50 and 100 ng) were microinjected into the VLPO of animals prepared for chronic sleep recordings; the microinjections also included adjacent dorsal regions of the lateral preoptic area. We found that bilateral microinjections of the neuropeptide produced an increase in the time spent in NREM sleep, whereas REM sleep time as well as the number and
Conclusions
The present study demonstrates that MCH can induce NREM sleep by modulation of the VLPO neuronal activity.
Acknowledgments
This study was supported by the “Proyecto de cooperación bilateral Uruguay- Brazil, Dicyt-CNPq” and by the “Programa de Desarrollo de Ciencias Básicas” (PEDECIBA), Uruguay.
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