Mari Hondo

Orexin receptor-1 in the locus coeruleus plays an important role in cue-dependent fear memory consolidation.

The noradrenergic (NA) projections arising from the locus ceruleus (LC) to the amygdala and bed nucleus of the stria terminalis have been implicated in the formation of emotional memory. Since NA neurons in the LC (LC-NA neurons) abundantly express orexin receptor-1 (OX1R) and receive prominent innervation by orexin-producing neurons, we hypothesized that an OX1R-mediated pathway is involved in the physiological fear learning process via regulation of LC-NA neurons. To evaluate this hypothesis, we examined the phenotype of Ox1r−/− mice in the classic cued and contextual fear-conditioning test. We found that Ox1r−/− mice showed impaired freezing responses in both cued and contextual fear-conditioning paradigms. In contrast, Ox2r−/− mice showed normal freezing behavior in the cued fear-conditioning test, while they exhibited shorter freezing time in the contextual fear-conditioning test. Double immunolabeling of Fos and tyrosine hydroxylase showed that double-positive LC-NA neurons after test sessions of both cued and contextual stimuli were significantly fewer in Ox1r−/− mice. AAV-mediated expression of OX1R in LC-NA neurons in Ox1r−/− mice restored the freezing behavior to the auditory cue to a comparable level to that in wild-type mice in the test session. Decreased freezing time during the contextual fear test was not affected by restoring OX1R expression in LC-NA neurons. These observations support the hypothesis that the orexin system modulates the formation and expression of fear memory via OX1R in multiple pathways. Especially, OX1R in LC-NA neurons plays an important role in cue-dependent fear memory formation and/or retrieval.

Neurotensin Co-Expressed in Orexin-Producing Neurons in the Lateral Hypothalamus Plays an Important Role in Regulation of Sleep/Wakefulness States

Both orexin and neurotensin are expressed in the lateral hypothalamic area (LHA) and have been implicated in the regulation of feeding, motor activity and the reward system. A double label immunofluorescence and in situ hybridization studies showed that neurotensin colocalizes with orexin in neurons of the LHA. Pharmacological studies suggested that neurotensin excites orexin-producing neurons (orexin neurons) through activation of neurotensin receptor-2 (NTSR-2) and non-selective cation channels. In situ hybridization study showed that most orexin neurons express neurotensin receptor-2 mRNA but not neurotensin receptor-1 (Ntsr-1) mRNA. Immunohistochemical studies showed that neurotensin-immunoreactive fibers make appositions to orexin neurons. A neurotensin receptor antagonist decreased Fos expression in orexin neurons and wakefulness time in wild type mice when administered intraperitoneally. However, the antagonist did not evoke any effect on these parameters in orexin neuron-ablated mice. These observations suggest the importance of neurotensin in maintaining activity of orexin neurons. The evidence presented here expands our understanding of the regulatory mechanism of orexin neurons.

Histamine-1 receptor is not required as a downstream effector of orexin-2 receptor in maintenance of basal sleep/wake states

Aim:  The effect of orexin on wakefulness has been suggested to be largely mediated by activation of histaminergic neurones in the tuberomammillary nucleus (TMN) via orexin receptor‐2 (OX2R). However, orexin receptors in other regions of the brain might also play important roles in maintenance of wakefulness. To dissect the role of the histaminergic system as a downstream mediator of the orexin system in the regulation of sleep/wake states without compensation by the orexin receptor‐1 (OX1R) mediated pathways, we analysed the phenotype of Histamine‐1 receptor (H1R) and OX1R double‐deficient (H1R−/−;OX1R−/−) mice. These mice lack OX1R‐mediated pathways in addition to deficiency of H1R, which is thought to be the most important system in downstream of OX2R.

The NPB/NPW Neuropeptide System and Its Role in Regulating Energy Homeostasis, Pain, and Emotion

Neuropeptide B (NPB) and neuropeptide W (NPW) are neuropeptides that were recently identified as endogenous ligands for the previously orphan G-protein coupled receptors, GPR7 (NPBWR1) and GPR8 (NPBWR2). This neuropeptide system is thought to have a role in regulating feeding behavior, energy homeostasis, neuroendocrine function, and modulating inflammatory pain. Strong and discrete expression of their receptors in the extended amygdala suggests a potential role in regulating stress responses, emotion, anxiety and fear; however, there have been no functional studies to date to support this possibility. Future studies of NPB/NPW using both pharmacological and phenotypic analysis of genetically engineered mice will lead to further elucidation of the physiological role of this novel neuropeptide system.

Orexin Neurons Receive Glycinergic Innervations

Glycine, a nonessential amino-acid that acts as an inhibitory neurotransmitter in the central nervous system, is currently used as a dietary supplement to improve the quality of sleep, but its mechanism of action is poorly understood. We confirmed the effects of glycine on sleep/wakefulness behavior in mice when administered peripherally. Glycine administration increased non-rapid eye movement (NREM) sleep time and decreased the amount and mean episode duration of wakefulness when administered in the dark period. Since peripheral administration of glycine induced fragmentation of sleep/wakefulness states, which is a characteristic of orexin deficiency, we examined the effects of glycine on orexin neurons. The number of Fos-positive orexin neurons markedly decreased after intraperitoneal administration of glycine to mice. To examine whether glycine acts directly on orexin neurons, we examined the effects of glycine on orexin neurons by patch-clamp electrophysiology. Glycine directly induced hyperpolarization and cessation of firing of orexin neurons. These responses were inhibited by a specific glycine receptor antagonist, strychnine. Triple-labeling immunofluorescent analysis showed close apposition of glycine transporter 2 (GlyT2)-immunoreactive glycinergic fibers onto orexin-immunoreactive neurons. Immunoelectron microscopic analysis revealed that GlyT2-immunoreactive terminals made symmetrical synaptic contacts with somata and dendrites of orexin neurons. Double-labeling immunoelectron microscopy demonstrated that glycine receptor alpha subunits were localized in the postsynaptic membrane of symmetrical inhibitory synapses on orexin neurons. Considering the importance of glycinergic regulation during REM sleep, our observations suggest that glycine injection might affect the activity of orexin neurons, and that glycinergic inhibition of orexin neurons might play a role in physiological sleep regulation.

Critical Role of Neuropeptides B/W Receptor 1 Signaling in Social Behavior and Fear Memory

Neuropeptide B/W receptor 1 (NPBWR1) is a G-protein coupled receptor, which was initially reported as an orphan receptor, and whose ligands were identified by this and other groups in 2002 and 2003. To examine the physiological roles of NPBWR1, we examined phenotype of Npbwr1 −/− mice. When presented with an intruder mouse, Npbwr1 −/− mice showed impulsive contact with the strange mice, produced more intense approaches toward them, and had longer contact and chasing time along with greater and sustained elevation of heart rate and blood pressure compared to wild type mice. Npbwr1 −/− mice also showed increased autonomic and neuroendocrine responses to physical stress, suggesting that impairment of NPBWR1 leads to stress vulnerability. We also observed that these mice show abnormality in the contextual fear conditioning test. These data suggest that NPBWR1 plays a critical role in limbic system function and stress responses. Histological and electrophysiological studies showed that NPBWR1 acts as an inhibitory regulator on a subpopulation of GABAergic neurons in the lateral division of the CeA and terminates stress responses. These findings suggest important roles of NPBWR1 in regulating amygdala function during physical and social stress.

Comprehensive Behavioral Analysis of Male Ox1r−/− Mice Showed Implication of Orexin Receptor-1 in Mood, Anxiety, and Social Behavior

Neuropeptides orexin A and orexin B, which are exclusively produced by neurons in the lateral hypothalamic area, play an important role in the regulation of a wide range of behaviors and homeostatic processes, including regulation of sleep/wakefulness states and energy homeostasis. The orexin system has close anatomical and functional relationships with systems that regulate the autonomic nervous system, emotion, mood, the reward system, and sleep/wakefulness states. Recent pharmacological studies using selective antagonists have suggested that orexin receptor-1 (OX1R) is involved in physiological processes that regulate emotion, the reward system, and autonomic nervous system. Here, we examined Ox1r−/− mice with a comprehensive behavioral test battery to screen additional OX1R functions. Ox1r−/− mice showed increased anxiety-like behavior, altered depression-like behavior, slightly decreased spontaneous locomotor activity, reduced social interaction, increased startle response, and decreased prepulse inhibition. These results suggest that OX1R plays roles in social behavior and sensory motor gating in addition to roles in mood and anxiety.

Activation of Bombesin Receptor Subtype-3 Influences Activity of Orexin Neurons by Both Direct and Indirect Pathways

The neuropeptides orexin A and orexin B (also known as hypocretin 1 and hypocretin 2), produced in lateral hypothalamic neurons, are critical regulators of feeding behavior, the reward system, and sleep/wake states. Orexin-producing neurons (orexin neurons) are regulated by various factors involved in regulation of energy homeostasis and sleep/wakefulness states. Bombesin receptor subtype 3 (BRS3) is an orphan receptor that might be implicated in energy homeostasis and is highly expressed in the hypothalamus. However, the neural pathway by which BRS3 regulates energy homeostasis is largely unknown. We examined whether BRS3 is involved in the regulation of orexin neurons. Using a calcium imaging method, we found that a selective BRS3 agonist [Ac-Phe-Trp-Ala-His-(τBzl)-Nip-Gly-Arg-NH2] increased the intracellular calcium concentration of orexin neurons. However, intracellular recordings from slice preparations revealed that the BRS3 agonist hyperpolarized orexin neurons. The BRS3 agonist depolarized orexin neuron in the presence of tetrodotoxin. Moreover, in the presence of GABA receptor blockers, picrotoxin and CGP55845, the BRS3 agonist induced depolarization and increased firing frequency. Additionally, double-label in situ hybridization study revealed that Brs3 mRNA was expressed in almost all orexin neurons and many cells around these neurons. These findings suggest that the BRS3 agonist indirectly inhibited orexin neurons through GABAergic input and directly activated orexin neurons. Inhibition of activity of orexin neurons through BRS3 might be an important pathway for regulation of feeding and sleep/wake states. This pathway might serve as a novel target for the treatment of obesity.

Critical role of neuropeptides B/W receptor 1 signaling in social behavior and fear memory

Neuropeptide B/W receptor 1 (NPBWR1) is a G-protein coupled receptor, which was initially reported as an orphan receptor, and whose ligands were identified by this and other groups in 2002 and 2003. To examine the physiological roles of NPBWR1, we examined phenotype of Npbwr1 mice. When presented with an intruder mouse, Npbwr1 mice showed impulsive contact with the strange mice, produced more intense approaches toward them, and had longer contact and chasing time along with greater and sustained elevation of heart rate and blood pressure compared to wild type mice. Npbwr1 mice also showed increased autonomic and neuroendocrine responses to physical stress, suggesting that impairment of NPBWR1 leads to stress vulnerability. We also observed that these mice show abnormality in the contextual fear conditioning test. These data suggest that NPBWR1 plays a critical role in limbic system function and stress responses. Histological and electrophysiological studies showed that NPBWR1 acts as an inhibitory regulator on a subpopulation of GABAergic neurons in the lateral division of the CeA and terminates stress responses. These findings suggest important roles of NPBWR1 in regulating amygdala function during physical and social stress. Citation: Nagata-Kuroiwa R, Furutani N, Hara J, Hondo M, Ishii M, et al. (2011) Critical Role of Neuropeptides B/W Receptor 1 Signaling in Social Behavior and Fear Memory. PLoS ONE 6(2): e16972. doi:10.1371/journal.pone.0016972 Editor: Xiaoxi Zhuang, University of Chicago, United States of America Received August 12, 2010; Accepted January 10, 2011; Published February 24, 2011 Copyright: 2011 Nagata-Kuroiwa et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported by Grants-in-Aid for Scientific Research on Priority Areas ‘‘System study on higher-order brain functions’’, the 21st Century COE Program for University of Tsukuba and Kanazawa University from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, ERATO from the Japan Science and Technology Agency, Takeda Science Foundation and Uehara Memorial Foundation. M.Y. is an investigator at the Howard Hughes Medical Institute. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: tsakurai@med.kanazawa-u.ac.jp

Glycinergic regulation of orexin neurons

Neurons in the preoptic area (POA), especially the ventral lateral preoptic area and the median preoptic nucleus, fire rapidly during sleep and cease firing during wakefulness. These neurons carry GABA, and thought to play an important role in initiation and maintenance of sleep by sending inhibitory projections to the arousal systems that reside in the brain stem. Recently, several evidence have suggested that orexinergic neurons in the hypothalamus, which play a critical role in maintaining arousal, are also influenced by these neurons. To elucidate the roles of these neurons in regulation of orexin neurons, we optogenetically stimulate these sleep-active neurons. We used Gad1-Cre knock-in mice, in which Cre recombinase is exclusively expressed in GABAergic neurons. We used an adeno-associated viral vector to deliver channelrhodopsin-2-YFP to Cre-expressing neurons in the POA. The axonal projection of the GABAegic neurons of the POA was visualized with double-label immunohistochemistry used anti orexin antiserum combined with an anti-GFP antiserum. Rich immunoreactivities of GFP-ir projections were observed in arousal region including the LHA. Optogenetic stimulation of POA GABAergic neurons resulted in increase of NREM sleep accompanied by inhibition of orexin neurons. These observations suggest that the POA GABAergic neurons are important in inhibition of arousal regions including hypothalamic orexin neurons.