Elsevier

Peptides

Volume 26, Issue 5, May 2005, Pages 767-777
Peptides

Enhanced antinociception by intracerebroventricularly and intrathecally-administered orexin A and B (hypocretin-1 and -2) in mice

https://doi.org/10.1016/j.peptides.2005.01.001Get rights and content

Abstract

Orexins are neuropeptides located exclusively in neurons of the lateral hypothalamic area, which send projections to most monoaminergic nuclei, such as noradrenergic locus coeruleus, dopaminergic ventral tegmental areas, and histaminergic tuberomammillary nuclei. The present work was carried out to examine the role of orexins in nociception in mice. C57BL/6 mice were administered with orexin A and B intracerebroventricularly (i.c.v.), intrathecally (i.t.) and subcutaneously (s.c.) to reveal the sites of action of these peptides and to examine the pain thresholds using four kinds of nociceptive tasks. Orexins showed antinociceptive effects in all four types of assays for thermal (hot-plate, tail-flick, paw-withdrawal), mechanical (tail-pressure), chemical (formalin, capsaicin and abdominal stretch) nociceptions and nociceptin-induced behavioral responses, when administered i.c.v. or i.t., whereas the s.c. administration was ineffective. The antinociceptive effects of orexin A were more remarkable than those of orexin B. The i.c.v. administration of orexin A was as effective as, or more potent than the i.t. administration. The effects of orexin A were completely blocked by adenosine type 1 receptor antagonists, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) and theophylline, but not by naloxone, suggesting a possible involvement of the adenosine-containing neurons and/or the adenosine pathway in these orexin actions. The i.c.v. administration of nociceptin had no significant effects on orexin expression in the brain and spinal cord. The present findings suggest that orexins have an antinociceptive role in at least four different types of pains, probably acting on both the brain and spinal cord.

Introduction

The nociceptive nervous system expresses a variety of signaling molecules with pronociceptive or antinociceptive activity, such as substance P, opioid peptides and orexins. Periaqueductal gray (PAG), the dorsal horn of the spinal cord and dorsal root ganglion (DRG) neurons have been the focus of intense research in order to identify molecular targets of pain neurotransmission and to develop potent analgesic compounds because they are primarily involved in pain processing [5], [30].

Orexins (hypocretins) are neuropeptides that are specifically expressed in neurons of the lateral hypothalamic area [8], [28]. Orexins consist of two structurally related peptides, orexin A and B that are generated from the same precursor by post-translational enzymatic processing. Immunocytochemistry, in situ hybridization and Northern blot analyses demonstrated that cell bodies of orexin neurons were restricted to the lateral hypothalamic regions of the brain [8], [9], [19], [28]. Localization of the fibers from orexin neurons in the hypothalamus to the intermediolateral column and lamina X as well as the innervation of the caudal regions of the sacral cord suggests that orexins may participate in the regulation of the autonomic nervous system [37]. Orexins act on at least two types of G-protein coupled receptors, orexin receptor 1 and receptor 2, and stimulate appetite and arousal [8], [16], [22], [28]. Defect in orexin A or orexin receptor 2 results in narcolepsy, a sleep disorder characterized by excessive daytime sleepiness, cataplexy, sleep paralysis, and hypnagogic hallucinations [4], [20], [22].

In addition to these actions of orexins, recent studies have shown that orexin A and B produce a variety of centrally mediated effects [23], [29], including antinociceptive activity [2], [3], [10]. For example, stimulation of the lateral hypothalamus in the region of orexin neurons produced analgesia, raising the interesting question of whether orexins might possess antinociceptive properties [2]. Furthermore, orexin A is antinociceptive in animal models of acute pain [3].

Distribution of orexin-containing fibers is consistent with the antinociceptive activity of orexins [6], [37]. Van den Pol [37] referred to the putative functions of orexins deduced on the basis of the fact that the laminae in the spinal cord are preferentially innervated by orexin-containing neurons. The innervation by orexin-containing neurons to the marginal zone and lamina I throughout all segments of the cord suggests that orexin may be involved in pain modulation and thermal sensation. Previous research showed that the fine caliber primary afferents terminate in lamina I [14]. Three types of cells in laminae I and II were found by physiological studies: those that respond to thermal and mechanical nociception, respectively. The other remaining cells respond to both noxious stimuli and non-pain related temperature changes. These facts suggest that orexin-containing fibers selectively innervate subpopulations of neurons in the marginal zone, perhaps either sensory neurons such as C fibers bearing pain information, or specific ascending projections innervating the locus coeruleus or contributing to the spinothalamic tract. Orexin-containing fibers may therefore modulate pain sensation.

Initially, hot-plate test and formalin test were used to study the analgesic effects of orexin A [3], [38]. Recently, analgesic effects of orexins have been studied in rats with various types of stimuli to cause pain, such as the incision of the hind paw [5], partial sciatic nerve ligation [40], injection of lambda carrageenan [39], and chronic constriction injury of the sciatic nerve [31]. Although orexins have analgesic effects in most of these studies, Yamamoto et al. [39] reported that orexin A suppressed mechanical information transmission of pain, but not thermal information transmission in carrageenan test. Furthermore, nociceptin was reported to have hyperalgesic actions [27]. The effects of orexins on nociceptin-induced behavioral hyperalgesia, however, have not been studied. It has not been clarified whether nociceptin has effects on expression of endogenous orexins. The aim of this study is therefore to study antinociceptive effects of orexins on four types of pains: thermal (hot-plate, tail-flick, paw-withdrawal), mechanical (tail-pressure), chemical (formalin, capsaicin and abdominal stretch) nociceptions and nociceptin-induced behavioral responses. We used three routes of orexin administration; intracerebroventricularly (i.c.v.), intrathecally (i.t.) and subcutaneously (s.c.), to reveal the sites of action of these peptides. Furthermore, we wished to clarify whether orexin expression in brain is changed by i.c.v. administration of nociceptin.

Section snippets

Drugs

Drugs were obtained from the following sources: orexin A was purchased from American Peptide Company, Inc. (Sunnyvale, CA, USA). Orexin B was synthesized and purified at the Stanford University Peptide Facility and it was given by the Center for Narcolepsy Stanford Sleep Research Center CA, USA. Naloxone hydrochloride, theophylline and 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) were purchased from Research Biochemical International (MA and Natick, USA). Nociceptin, sendide [Tyr6, D-Phe7,

The effects of i.c.v.- and i.t.-administered orexin A and B on pain responses to the thermal and mechanical stimuli

The i.c.v. and i.t. administration of both orexin A and B significantly decreased the behavioral responses to thermal and mechanical pain stimuli. The antinociceptive effects of i.t.- and i.c.v.-administered orexin A and B on the hot-plate, tail-flick, paw-withdrawal and tail-pressure tests are shown in Fig. 1. The responses to behavioral nociceptive tasks were significantly different between orexin A and B in C57BL/6 mice. Subcutaneous orexin A and B did not affect the response to all

Discussion

The present study has shown antinociceptive activities of orexin A and B against four kinds of nociceptive tasks: thermal (hot-plate, tail-flick and paw-withdrawal), mechanical (tail-pressure), chemical nociception (formalin, capsaicin and abdominal stretch) and nociceptin-induced behavioral hyperalgesia. Because pain is not a unitary phenomenon, nociceptive assays of different mechanisms should be performed to evaluate exactly the antinociceptive efficacy. It is interesting to note that orexin

Acknowledgements

This work was supported in part by Grants-in-Aid for scientific research from the Japan Society for the Promotion of Science (JSPS), Goho Life Science Foundation and a 21st Century COE program (Bio-nanotechnology) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. We would like to express our appreciation to Professor S. Shibahara, Professor H. Kondo (Tohoku University School of Medicine) and Dr. A. Haeri (Pasteur Institute of Iran) for their useful suggestions and

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