Tatsushi Onaka

Evidence That Oxytocin Exerts Anxiolytic Effects via Oxytocin Receptor Expressed in Serotonergic Neurons in Mice

The oxytocin receptor has been implicated in the regulation of reproductive physiology as well as social and emotional behaviors. The neurochemical mechanisms by which oxytocin receptor modulates social and emotional behavior remains elusive, in part because of a lack of sensitive and selective antibodies for cellular localization. To more precisely characterize oxytocin receptor-expressing neurons within the brain, we generated an oxytocin receptor-reporter mouse in which part of the oxytocin receptor gene was replaced with Venus cDNA (a variant of yellow fluorescent protein). Examination of the Venus expression revealed that, in the raphe nuclei, about one-half of tryptophan hydroxylase-immunoreactive neurons were positive for Venus, suggesting a potential role for oxytocin in the modulation of serotonin release. Oxytocin infusion facilitated serotonin release within the median raphe nucleus and reduced anxiety-related behavior. Infusion of a 5-HT2A/2C receptor antagonist blocked the anxiolytic effect of oxytocin, suggesting that oxytocin receptor activation in serotonergic neurons mediates the anxiolytic effects of oxytocin. This is the first demonstration that oxytocin may regulate serotonin release and exert anxiolytic effects via direct activation of oxytocin receptor expressed in serotonergic neurons of the raphe nuclei. These results also have important implications for psychiatric disorders such as autism and depression in which both the oxytocin and serotonin systems have been implicated.

Nesfatin-1-Regulated Oxytocinergic Signaling in the Paraventricular Nucleus Causes Anorexia through a Leptin-Independent Melanocortin Pathway

The hypothalamic paraventricular nucleus (PVN) functions as a center to integrate various neuronal activities for regulating feeding behavior. Nesfatin-1, a recently discovered anorectic molecule, is localized in the PVN. However, the anorectic neural pathway of nesfatin-1 remains unknown. Here we show that central injection of nesfatin-1 activates the PVN and brain stem nucleus tractus solitarius (NTS). In the PVN, nesfatin-1 targets both magnocellular and parvocellular oxytocin neurons and nesfatin-1 neurons themselves and stimulates oxytocin release. Immunoelectron micrographs reveal nesfatin-1 specifically in the secretory vesicles of PVN neurons, and immunoneutralization against endogenous nesfatin-1 suppresses oxytocin release in the PVN, suggesting paracrine/autocrine actions of nesfatin-1. Nesfatin-1-induced anorexia is abolished by an oxytocin receptor antagonist. Moreover, oxytocin terminals are closely associated with and oxytocin activates pro-opiomelanocortin neurons in the NTS. Oxytocin induces melanocortin-dependent anorexia in leptin-resistant Zucker-fatty rats. The present results reveal the nesfatin-1-operative oxytocinergic signaling in the PVN that triggers leptin-independent melanocortin-mediated anorexia.

Oxytocin-gaze positive loop and the coevolution of human-dog bonds

Gaze into my eyes Humans bond emotionally as we gaze into each others eyes—a process mediated by the hormone oxytocin. Nagasawa et al. show that such gaze-mediated bonding also exists between us and our closest animal companions, dogs (see the Perspective by MacLean and Hare). They found that mutual gazing increased oxytocin levels, and sniffing oxytocin increased gazing in dogs, an effect that transferred to their owners. Wolves, who rarely engage in eye contact with their human handlers, seem resistant to this effect. Science, this issue p. 333; see also p. 280 The human-dog bond is facilitated by the interaction of oxytocin feedback loops that emerged over the course of domestication. [Also see Perspective by MacLean and Hare] Human-like modes of communication, including mutual gaze, in dogs may have been acquired during domestication with humans. We show that gazing behavior from dogs, but not wolves, increased urinary oxytocin concentrations in owners, which consequently facilitated owners’ affiliation and increased oxytocin concentration in dogs. Further, nasally administered oxytocin increased gazing behavior in dogs, which in turn increased urinary oxytocin concentrations in owners. These findings support the existence of an interspecies oxytocin-mediated positive loop facilitated and modulated by gazing, which may have supported the coevolution of human-dog bonding by engaging common modes of communicating social attachment.

Oxytocin receptor-deficient mice developed late-onset obesity.

The oxytocin receptor has been suggested to be involved in energy metabolism, such as food intake and energy consumption. Here, we demonstrate that oxytocin receptor-deficient (Oxtr−/−) male mice exhibited late-onset obesity with increases in abdominal fat pads and fasting plasma triglycerides. Daily food intake and spontaneous motor activity of Oxtr−/− mice were not significantly different as compared with wild-type mice. In contrast, brown adipose tissue in Oxtr−/− mice contained large lipid droplets and cold-induced thermogenesis was impaired. This study demonstrates that oxytocin receptor plays essential roles in the regulation of energy homeostasis.

An intrinsic vasopressin system in the olfactory bulb is involved in social recognition

Many peptides, when released as chemical messengers within the brain, have powerful influences on complex behaviours. Most strikingly, vasopressin and oxytocin, once thought of as circulating hormones whose actions were confined to peripheral organs, are now known to be released in the brain, where they have fundamentally important roles in social behaviours. In humans, disruptions of these peptide systems have been linked to several neurobehavioural disorders, including Prader–Willi syndrome, affective disorders and obsessive–compulsive disorder, and polymorphisms of V1a vasopressin receptor have been linked to autism. Here we report that the rat olfactory bulb contains a large population of interneurons which express vasopressin, that blocking the actions of vasopressin in the olfactory bulb impairs the social recognition abilities of rats and that vasopressin agonists and antagonists can modulate the processing of information by olfactory bulb neurons. The findings indicate that social information is processed in part by a vasopressin system intrinsic to the olfactory system.

Long-Term Administration of Intranasal Oxytocin Is a Safe and Promising Therapy for Early Adolescent Boys with Autism Spectrum Disorders

OBJECTIVE Oxytocin (OT) has been a candidate for the treatment of autism spectrum disorders (ASD), and the impact of intranasally delivered OT on ASD has been investigated. However, most previous studies were conducted by single-dose administration to adults; and, therefore, the long-term effect of nasal OT on ASD patients and its effect on children remain to be clarified. METHODS We conducted a singled-armed, open-label study in which OT was administered intranasally over the long term to eight male youth with ASD (10-14 years of age; intelligence quotient [IQ] 20-101). The OT administration was performed in a stepwise increased dosage manner every 2 months (8, 16, 24 IU/dose). A placebo period (1-2 weeks) was inserted before each step. The outcome measures were autism diagnostic observation schedule--generic (ADOS-G), child behavior checklist (CBCL), and the aberrant behavior checklist (ABC). In addition, side effects were monitored by measuring blood pressure and examining urine and blood samples. RESULTS Six of the eight participants showed improved scores on the communication and social interaction domains of the ADOS-G. However, regarding the T-scores of the CBCL and the scores of the ABC, we could not find any statistically significant improvement, although several subcategories showed a mild tendency for improvement. Caregivers of five of the eight participants reported certain positive effects of the OT therapy, especially on the quality of reciprocal communication. All participants showed excellent compliance and no side effects. CONCLUSIONS Although our results on the efficacy of long-term nasal OT therapy still remain controversial, to the best of our knowledge, this is the first report documenting the safety of long-term nasal OT therapy for children with ASD. Even though our data are too preliminary to draw any definite conclusions about efficacy, they do suggest this therapy to be safe, promising, and worthy of a large-scale, double-blind placebo-controlled study.

Neural pathways controlling central and peripheral oxytocin release during stress.

Oxytocin is released from the pituitary gland in response to a variety of stressful stimuli, including noxious stimuli, conditioned fear and exposure to novel environments. These responses are believed to be mediated, at least in part, by noradrenergic projections from the medulla oblongata, and some of these noradrenergic neurones also contain prolactin‐releasing peptide (PrRP). Central administration of either PrRP or noradrenaline stimulates oxytocin secretion into the circulation. Stressful stimuli activate PrRP‐containing noradrenergic neurones in the medulla oblongata, and it is thus possible that PrRP/noradrenergic projections to the hypothalamus mediate oxytocin responses to stressful stimuli. Here, the roles of brainstem PrRP/noradrenergic projections to the hypothalamus in oxytocin responses to different kinds of stressful stimuli are reviewed, with a particular emphasis on conditioned fear. Roles of dendritic oxytocin release during stress and metabolic factors affecting stress pathways are also discussed.

Role of noradrenergic projections to the bed nucleus of the stria terminalis in neuroendocrine and behavioral responses to fear-related stimuli in rats

The bed nucleus of the stria terminalis (BNST) receives dense noradrenergic projections from the brainstem and has been claimed to play a role in expression of a variety of stress responses. Fear-related stimuli suppress vasopressin and facilitate oxytocin release from the neurohypophysis and induce behavioral suppression. Here we investigated in male rats whether conditioned fear stimuli increase noradrenergic activity in the BNST and whether depletion of epinephrine content in the BNST prevents neuroendocrine and behavioral responses to fear stimuli. Environmental stimuli previously paired with electric footshocks increased the ratio of 3-methoxy-4-hydroxyphenylglycol to norepinephrine contents in the BNST, suggesting that the stimuli activated noradrenergic projections to the BNST. 5-Amino-2, 4-dihydroxy-alpha-methylphenylethylamine, a neurotoxin relatively selective for noradrenergic fibers, when injected into the BNST 7 days before measurement, decreased the content of norepinephrine by 95% and that of dopamine or serotonin by about 50%. In the rats that received the neurotoxin, the suppressive vasopressin but not the augmentative oxytocin response to intermittent footshocks was abolished. In the experiments with conditioned fear stimuli, the neurotoxin given before training partially but significantly impaired the suppressive vasopressin and behavioral responses to testing stimuli. The neurotoxin given after training, however, did not prevent the vasopressin, oxytocin or behavioral responses. The results suggest that noradrenergic fibers in the BNST mediate the suppressive vasopressin but not the augmentative oxytocin response to nonassociatively applied fear stimuli and that they modulate, in a facilitative fashion, acquisition but not retention or recall of the emotional memory associated with the vasopressin and behavioral responses to conditioned fear stimuli.

Roles of Oxytocin Neurones in the Control of Stress, Energy Metabolism, and Social Behaviour

Oxytocin neurones are activated by stressful stimuli, food intake and social attachment. Activation of oxytocin neurones in response to stressful stimuli or food intake is mediated, at least in part, by noradrenaline/prolactin‐releasing peptide (PrRP) neurones in the nucleus tractus solitarius, whereas oxytocin neurones are activated after social stimuli via medial amygdala neurones. Activation of oxytocin neurones induces the release of oxytocin not only from their axon terminals, but also from their dendrites. Oxytocin acts locally where released or diffuses and acts on remote oxytocin receptors widely distributed within the brain, resulting in anxiolytic, anorexic and pro‐social actions. The action sites of oxytocin appear to be multiple. Oxytocin shows anxiolytic actions, at least in part, via serotoninergic neurones in the median raphe nucleus, has anorexic actions via pro‐opiomelanocortin neurones in the nucleus tractus solitarius and facilitates social recognition via the medial amygdala. Stress, obesity and social isolation are major risk factors for mortality in humans. Thus, the oxytocin–oxytocin receptor system is a therapeutic target for the promotion of human health.

Long-term infusion of brain-derived neurotrophic factor reduces food intake and body weight via a corticotrophin-releasing hormone pathway in the paraventricular nucleus of the hypothalamus.

Brain‐derived neurotrophic factor (BDNF) has been implicated in learning, depression and energy metabolism. However, the neuronal mechanisms underlying the effects of BDNF on energy metabolism remain unclear. The present study aimed to elucidate the neuronal pathways by which BDNF controls feeding behaviour and energy balance. Using an osmotic mini‐pump, BDNF or control artificial cerebrospinal fluid was infused i.c.v. at the lateral ventricle or into the paraventricular nucleus of the hypothalamus (PVN) for 12 days. Intracerebroventricular BDNF up‐regulated mRNA expression of corticotrophin‐releasing hormone (CRH) and urocortin in the PVN. TrkB, the receptor for BDNF, was expressed in the PVN neurones, including those containing CRH. Both i.c.v. and intra‐PVN‐administered BDNF decreased food intake and body weight. These effects of BDNF on food intake and body weight were counteracted by the co‐administration of α‐helical‐CRH, an antagonist for the CRH and urocortin receptors CRH‐R1/R2, and partly attenuated by a selective antagonist for CRH‐R2 but not CRH‐R1. Intracerebroventricular BDNF also decreased the subcutaneous and visceral fat mass, adipocyte size and serum triglyceride levels, which were all attenuated by α‐helical‐CRH. Furthermore, BDNF decreased the respiratory quotient and raised rectal temperature, which were counteracted by α‐helical‐CRH. These results indicate that the CRH‐urocortin‐CRH‐R2 pathway in the PVN and connected areas mediates the long‐term effects of BDNF to depress feeding and promote lipolysis.