alpha-Melanocyte-stimulating hormone and oxytocin: a peptide signalling cascade in the hypothalamus

Oxytocin treatment rescues irritability-like behavior in Cc2d1a conditional knockout mice

Irritability, a state of excessive reactivity to negative emotional stimuli, is common in individuals with autism spectrum disorder (ASD). Although it has a significant negative impact of patients' disease severity and quality of life, the neural mechanisms underlying irritability in ASD remain largely unclear. We have previously demonstrated that male mice lacking the Coiled-coil and C2 domain containing 1a (Cc2d1a) in forebrain excitatory neurons recapitulate numerous ASD-like behavioral phenotypes, including impaired social behaviors and pronounced repetitive behaviors. Here, using the bottle-brush test (BBT) to trigger and evaluate aggressive and defensive responses, we show that Cc2d1a deletion increases irritability-like behavior in male but not female mice, which is correlated with reduced number of oxytocin (OXT)-expressing neurons in the paraventricular nucleus (PVN) of the hypothalamus. Intranasal OXT administration or chemogenetic activation of OXT neurons in the PVN rescues irritability-like behavior in Cc2d1a conditional knockout (cKO) mice. Administration of a selective melanocortin receptor 4 agonist, RO27-3225, which potentiates endogenous OXT release, also alleviates irritability-like behavior in Cc2d1a cKO mice, an effect blocked by a specific OXT receptor antagonist, L-368,899. We additionally identify a projection connecting the posterior ventral segment of the medial amygdala (MeApv) and ventromedial nucleus of the ventromedial hypothalamus (VMHvl) for governing irritability-like behavior during the BBT. Chemogenetic suppression of the MeApv-VMHvl pathway alleviates irritability-like behavior in Cc2d1a cKO mice. Together, our study uncovers dysregulation of OXT system in irritability-like behavior in Cc2d1a cKO mice during the BBT and provide translatable insights into the development of OXT-based therapeutics for clinical interventions.

Melanocortin agonism in a social context selectively activates nucleus accumbens in an oxytocin-dependent manner

Social deficits are debilitating features of many psychiatric disorders, including autism. While time-intensive behavioral therapy is moderately effective, there are no pharmacological interventions for social deficits in autism. Many studies have attempted to treat social deficits using the neuropeptide oxytocin for its powerful neuromodulatory abilities and influence on social behaviors and cognition. However, clinical trials utilizing supplementation paradigms in which exogenous oxytocin is chronically administered independent of context have failed. An alternative treatment paradigm suggests pharmacologically activating the endogenous oxytocin system during behavioral therapy to enhance the efficacy of therapy by facilitating social learning. To this end, melanocortin receptor agonists like Melanotan II (MTII), which induces central oxytocin release and accelerates formation of partner preference, a form of social learning, in prairie voles, are promising pharmacological tools. To model pharmacological activation of the endogenous oxytocin system during behavioral therapy, we administered MTII prior to social interactions between male and female voles. We assessed its effect on oxytocin-dependent activity in brain regions subserving social learning using Fos expression as a proxy for neuronal activation. In non-social contexts, MTII only activated hypothalamic paraventricular nucleus, a primary site of oxytocin synthesis. However, during social interactions, MTII selectively increased oxytocin-dependent activation of nucleus accumbens, a site critical for social learning. These results suggest a mechanism for the MTII-induced acceleration of partner preference formation observed in previous studies. Moreover, they are consistent with the hypothesis that pharmacologically activating the endogenous oxytocin system with a melanocortin agonist during behavioral therapy has potential to facilitate social learning.

Oestrogen-dependent hypothalamic oxytocin expression with changes in feeding and body weight in female rats

Oxytocin (OXT) is produced in the hypothalamic nuclei and secreted into systemic circulation from the posterior pituitary gland. In the central nervous system, OXT regulates behaviours including maternal and feeding behaviours. Our aim is to evaluate whether oestrogen regulates hypothalamic OXT dynamics. Herein, we provide the first evidence that OXT dynamics in the hypothalamus vary with sex and that oestrogen may modulate dynamic changes in OXT levels, using OXT-mRFP1 transgenic rats. The fluorescence intensity of OXT-mRFP1 and expression of the OXT and mRFP1 genes in the hypothalamic nuclei is highest during the oestrus stage in female rats and decreased significantly in ovariectomised rats. Oestrogen replacement caused significant increases in fluorescence intensity and gene expression in a dose-related manner. This is also demonstrated in the rats' feeding behaviour and hypothalamic Fos neurons using cholecystokinin-8 and immunohistochemistry. Hypothalamic OXT expression is oestrogen-dependent and can be enhanced centrally by the administration of oestrogen.

Elevated Neuropeptides in Dry Eye Disease and Their Clinical Correlations

PURPOSE

The goal of this study was to assess neuropeptide levels in patients with dry eye disease (DED) and investigate their correlations with clinical characteristics.

METHODS

This study included 38 eyes of 38 patients diagnosed with DED (DED group) and 38 eyes of 38 healthy volunteers without DED (control group). Ocular surface evaluation was performed. The severity of dry eye symptoms and signs in the DED group was graded. Neuropeptides [substance P (SP), alpha-melanocyte-stimulating hormone (α-MSH), β-endorphin, neurotensin, and oxytocin] and inflammatory cytokines levels were measured in basal tears. The link between neuropeptides and clinical parameters was investigated using Spearman rank correlation.

RESULTS

Overall, 76.3% of patients in the DED group showed dry eye symptoms and signs that were inconsistent in severity. Compared with the control group, the DED group showed higher levels of SP, α-MSH, and oxytocin in tears (P = 0.012, P = 0.030, and P = 0.006, respectively), but similar levels of β-endorphin and neurotensin (P = 0.269 and P = 0.052). The levels of SP, α-MSH, and oxytocin were elevated in DED patients with higher grading of symptoms than clinical signs (all P < 0.05). SP, α-MSH, and oxytocin levels in tears were positively correlated with Ocular Surface Disease Index scores, frequency of sensitivity to light, and frequency of blurred vision (all P < 0.05).

CONCLUSIONS

The increased tear levels of SP, α-MSH, and oxytocin may be linked to ocular discomfort in DED. Neuropeptides may play a key role in the development of DED, especially in DED patients with more severe symptoms than clinical signs.

Dendritic osmosensors modulate activity-induced calcium influx in oxytocinergic magnocellular neurons of the mouse PVN

Hypothalamic oxytocinergic magnocellular neurons have a fascinating ability to release peptide from both their axon terminals and from their dendrites. Existing data indicates that the relationship between somatic activity and dendritic release is not constant, but the mechanisms through which this relationship can be modulated are not completely understood. Here, we use a combination of electrical and optical recording techniques to quantify activity-induced calcium influx in proximal vs. distal dendrites of oxytocinergic magnocellular neurons located in the paraventricular nucleus of the hypothalamus (OT-MCNs). Results reveal that the dendrites of OT-MCNs are weak conductors of somatic voltage changes; however, activity-induced dendritic calcium influx can be robustly regulated by both osmosensitive and non-osmosensitive ion channels located along the dendritic membrane. Overall, this study reveals that dendritic conductivity is a dynamic and endogenously regulated feature of OT-MCNs that is likely to have substantial functional impact on central oxytocin release.

Detection of neuropeptides in vivo and open questions for current and upcoming fluorescent sensors for neuropeptides

During a stress response, various neuropeptides are secreted in a spatiotemporally coordinated way in the brain. For a precise understanding of peptide functions in a stress response, it is important to investigate when and where they are released, how they diffuse, and how they are broken down in the brain. In the past two decades, genetically encoded fluorescent calcium indicators have greatly advanced our knowledge of the functions of specific neuronal activity in regulation of behavioral changes and physiological responses during stress. In addition, various kinds of structural information on G-protein-coupled receptors (GPCRs) for neuropeptides have been revealed. Recently, genetically encoded fluorescent sensors have been developed for detection of neurotransmitters by making use of conformational changes induced by ligand binding. In this review, we summarize the recent and upcoming advances of techniques for detection of neuropeptides and then present several open questions that will be solved by application of recent or upcoming technical advances in detection of neuropeptides in vivo.

The promiscuity of the oxytocin-vasopressin systems and their involvement in autism spectrum disorder

Oxytocin and vasopressin systems have been studied separately in autism spectrum disorder (ASD). Here, we provide evidence from an evolutionary and neuroscience perspective about the shared mechanisms and the common roles in regulating social behaviors. We first discuss findings on the evolutionary history of oxytocin and vasopressin ligands and receptors that highlight their common origin and clarify the evolutionary background of the crosstalk between them. Second, we conducted a comprehensive review of the increasing evidence for the role of both neuropeptides in regulating social behaviors. Third, we reviewed the growing evidence on the associations between the oxytocin/vasopressin systems and ASD, which includes oxytocin and vasopressin dysfunction in animal models of autism and in human patients, and the impact of treatments targeting the oxytocin or the vasopressin systems in children and in adults. Here, we highlight the potential of targeting the oxytocin/vasopressin systems to improve social deficits observed in ASD and the need for further investigations on how to transfer these research innovations into clinical applications.

Somato-dendritic vasopressin and oxytocin secretion in endocrine and autonomic regulation

Somato-dendritic secretion was first demonstrated over 30 years ago. However, although its existence has become widely accepted, the function of somato-dendritic secretion is still not completely understood. Hypothalamic magnocellular neurosecretory cells were among the first neuronal phenotypes in which somato-dendritic secretion was demonstrated and are among the neurones for which the functions of somato-dendritic secretion are best characterised. These neurones secrete the neuropeptides, vasopressin and oxytocin, in an orthograde manner from their axons in the posterior pituitary gland into the blood circulation to regulate body fluid balance and reproductive physiology. Retrograde somato-dendritic secretion of vasopressin and oxytocin modulates the activity of the neurones from which they are secreted, as well as the activity of neighbouring populations of neurones, to provide intra- and inter-population signals that coordinate the endocrine and autonomic responses for the control of peripheral physiology. Somato-dendritic vasopressin and oxytocin have also been proposed to act as hormone-like signals in the brain. There is some evidence that somato-dendritic secretion from magnocellular neurosecretory cells modulates the activity of neurones beyond their local environment where there are no vasopressin- or oxytocin-containing axons but, to date, there is no conclusive evidence for, or against, hormone-like signalling throughout the brain, although it is difficult to imagine that the levels of vasopressin found throughout the brain could be underpinned by release from relatively sparse axon terminal fields. The generation of data to resolve this issue remains a priority for the field.

Leu8 and Pro8 oxytocin agonism differs across human, macaque, and marmoset vasopressin 1a receptors

Oxytocin (OXT) is an important neuromodulator of social behaviors via activation of both oxytocin receptors (OXTR) and vasopressin (AVP) 1a receptors (AVPR1a). Marmosets are neotropical primates with a modified OXT ligand (Pro⁸-OXT), and this ligand shows significant coevolution with traits including social monogamy and litter size. Pro⁸-OXT produces more potent and efficacious responses at primate OXTR and stronger behavioral effects than the consensus mammalian OXT ligand (Leu⁸-OXT). Here, we tested whether OXT/AVP ligands show differential levels of crosstalk at primate AVPR1a. We measured binding affinities and Ca²⁺ signaling responses of AVP, Pro⁸-OXT and Leu⁸-OXT at human, macaque, and marmoset AVPR1a. We found that AVP binds with higher affinity than OXT across AVPR1a, and marmoset AVPR1a show a 10-fold lower OXT binding affinity compared to human and macaque AVPR1a. Both Leu⁸-OXT and Pro⁸-OXT produce a less efficacious response than AVP at human AVPR1a and higher efficacious response than AVP at marmoset AVPR1a. These data suggest that OXT might partially antagonize endogenous human AVPR1a signaling and enhance marmoset AVPR1a signaling. These findings aid in further understanding inconsistencies observed following systemic intranasal administration of OXT and provide important insights into taxon-specific differences in nonapeptide ligand/receptor coevolution and behavior.

Role of oxytocin in the control of stress and food intake

Oxytocin neurones in the hypothalamus are activated by stressful stimuli and food intake. The oxytocin receptor is located in various brain regions, including the sensory information-processing cerebral cortex; the cognitive information-processing prefrontal cortex; reward-related regions such as the ventral tegmental areas, nucleus accumbens and raphe nucleus; stress-related areas such as the amygdala, hippocampus, ventrolateral part of the ventromedial hypothalamus and ventrolateral periaqueductal gray; homeostasis-controlling hypothalamus; and the dorsal motor complex controlling intestinal functions. Oxytocin affects behavioural and neuroendocrine stress responses and terminates food intake by acting on the metabolic or nutritional homeostasis system, modulating emotional processing, reducing reward values of food intake, and facilitating sensory and cognitive processing via multiple brain regions. Oxytocin also plays a role in interactive actions between stress and food intake and contributes to adaptive active coping behaviours.

Cross-talk among oxytocin and arginine-vasopressin receptors: Relevance for basic and clinical studies of the brain and periphery

Oxytocin (OT) and arginine-vasopressin (AVP) act in the brain to regulate social cognition/social behavior and in the periphery to influence a variety of physiological processes. Although the chemical structures of OT and AVP as well as their receptors are quite similar, OT and AVP can have distinct or even opposing actions. Here, we review the increasing body of evidence that exogenously administered and endogenously released OT and AVP can activate each other's canonical receptors (i.e., cross-talk) and examine the possibility that receptor cross-talk following the synaptic and non-synaptic release of OT and AVP contributes to their distinct roles in the brain and periphery. Understanding the consequences of cross-talk between OT and AVP receptors will be important in identifying how these peptides control social cognition and behavior and for the development of drugs to treat a variety of psychiatric disorders.

Ventromedial hypothalamic nucleus in regulation of stress-induced gastric mucosal injury in rats

Previous studies showed that restraint water-immersion stress (RWIS) increases the expression of Fos protein in the ventromedial hypothalamic nucleus (VMH), indicating the VMH involving in the stress-induced gastric mucosal injury (SGMI). The present study was designed to investigate its possible neuro-regulatory mechanisms in rats receiving either VMH lesions or sham surgery. The model for SGMI was developed by restraint and water (21 ± 1 °C) immersion for 2 h. Gastric mucosal injury index, gastric motility, gastric acid secretion and Fos expression in the hypothalamus and brainstem were examined on the 15th postoperative day in RWIS rats. Gastric mucosal injury in VMH-lesioned rats was obviously aggravated compared to the control. Gastric acidity under RWIS was obviously higher in VMH-lesioned rats than that in sham rats. Meantime, the VMH-lesioned rats exhibited marked increases in the amplitude of gastric motility in the VMH lesions group after RWIS. In VMH-lesioned rats, Fos expression significantly increased in the dorsal motor nucleus of the vagus (DMV), the nucleus of the solitary tract (NTS), the area postrema (AP), the paraventricular nucleus (PVN) and the supraoptic nucleus (SON) in response to RWIS. These results indicate that VMH lesions can aggravate the stress-induced gastric mucosal injury through the VMH-dorsal vagal complex (DVC)-vagal nerve pathway.

Rebalancing the Addicted Brain: Oxytocin Interference with the Neural Substrates of Addiction

Drugs that act on the brain oxytocin (OXT) system may provide a much-needed treatment breakthrough for substance-use disorders. Targeting the brain OXT system has the potential to treat addiction to all major classes of addictive substance and to intervene across all stages of the addiction cycle. Emerging evidence suggests that OXT is able to interfere with such a wide range of addictive behaviours for such a wide range of addictive substances by rebalancing core neural systems that become dysregulated over the course of addiction. By improving our understanding of these interactions between OXT and the neural substrates of addiction, we will not only improve our understanding of addiction, but also our ability to effectively treat these devastating disorders.

Targets for Drug Therapy for Autism Spectrum Disorder: Challenges and Future Directions

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by persistent deficits in social communication and interaction and restricted, repetitive patterns of behavior, interests, and activities. Various factors are involved in the etiopathogenesis of ASD, including genetic factors, environmental toxins and stressors, impaired immune responses, mitochondrial dysfunction, and neuroinflammation. The heterogeneity in the phenotype among ASD patients and the complex etiology of the condition have long impeded the advancement of the development of pharmacological therapies. In the recent years, the integration of findings from mouse models to human genetics resulted in considerable progress toward the understanding of ASD pathophysiology. Currently, strategies to treat core symptoms of ASD are directed to correct synaptic dysfunctions, abnormalities in central oxytocin, vasopressin, and serotonin neurotransmission, and neuroinflammation. Here, we present a survey of the studies that have suggested molecular targets for drug development for ASD and the state-of-the-art of medicinal chemistry efforts in related areas.

Functional consequences of neuropeptide and small-molecule co-transmission

Colocalization of small-molecule and neuropeptide transmitters is common throughout the nervous system of all animals. The resulting co-transmission, which provides conjoint ionotropic ('classical') and metabotropic ('modulatory') actions, includes neuropeptide- specific aspects that are qualitatively different from those that result from metabotropic actions of small-molecule transmitter release. Here, we focus on the flexibility afforded to microcircuits by such co-transmission, using examples from various nervous systems. Insights from such studies indicate that co-transmission mediated even by a single neuron can configure microcircuit activity via an array of contributing mechanisms, operating on multiple timescales, to enhance both behavioural flexibility and robustness.

Rol Modulador de la Oxitocina en la Interacción Social y el Estrés Social

La Oxitocina es un neuropéptido conocido por facilitar funciones del sistema nervioso periférico, relacionadas específicamente con el sistema reproductivo. Sin embargo, en las últimas décadas se ha reconocido la función moduladora de la Oxitocina en el comportamiento social, a través de su liberación en el sistema nervioso central. Así mismo, estudios han mencionado que la Oxitocina es un potencial ansiolítico cuando un individuo ha sido sometido a estrés social. Por lo tanto, el objetivo de esta revisión es presentar una caracterización de la Oxitocina y su relación con distintas formas de interacción social y el estrés social; a través de los resultados presentados en distintos estudios, tanto en modelos animales como en humanos. Además, se intenta mostrar la importancia de continuar con el estudio de la Oxitocina, dados los posibles vacíos teóricos y experimentales existentes, teniendo en cuenta las potenciales cualidades ansiolíticas de esta hormona.

Oxytocin - The Sweet Hormone?

Mammalian neurons that produce oxytocin and vasopressin apparently evolved from an ancient cell type with both sensory and neurosecretory properties that probably linked reproductive functions to energy status and feeding behavior. Oxytocin in modern mammals is an autocrine/paracrine regulator of cell function, a systemic hormone, a neuromodulator released from axon terminals within the brain, and a 'neurohormone' that acts at receptors distant from its site of release. In the periphery oxytocin is involved in electrolyte homeostasis, gastric motility, glucose homeostasis, adipogenesis, and osteogenesis, and within the brain it is involved in food reward, food choice, and satiety. Oxytocin preferentially suppresses intake of sweet-tasting carbohydrates while improving glucose tolerance and supporting bone remodeling, making it an enticing translational target.

Attenuated hypothalamic responses to α-melanocyte stimulating hormone during pregnancy in the rat

KEY POINTS

Increased appetite and weight gain occurs during pregnancy, associated with development of leptin resistance, and satiety responses to the anorectic peptide α-melanocyte stimulating hormone (α-MSH) are suppressed. This study investigated hypothalamic responses to α-MSH during pregnancy, using c-fos expression in specific hypothalamic nuclei as a marker of neuronal signalling, and in vivo electrophysiology in supraoptic nucleus (SON) oxytocin neurons, as a representative α-MSH-responsive neuronal population that shows a well-characterised α-MSH-induced inhibition of firing. While icv injection of α-MSH significantly increased the number of c-fos-positive cells in the paraventricular, supraoptic, arcuate and ventromedial hypothalamic nuclei in non-pregnant rats, this response was suppressed in pregnant rats. Similarly, SON oxytocin neurons in pregnant rats did not demonstrate characteristic α-MSH-induced inhibition of firing that was observed in non-pregnant animals. Given the known functions of α-MSH in the hypothalamus, the attenuated responses are likely to facilitate adaptive changes in appetite regulation and oxytocin secretion during pregnancy.

ABSTRACT

During pregnancy, a state of positive energy balance develops to support the growing fetus and to deposit fat in preparation for the subsequent metabolic demands of lactation. As part of this maternal adaptation, the satiety response to the anorectic peptide α-melanocyte stimulating hormone (α-MSH) is suppressed. To investigate whether pregnancy is associated with changes in the response of hypothalamic α-MSH target neurons, non-pregnant and pregnant rats were treated with α-MSH or vehicle and c-fos expression in hypothalamic nuclei was then examined. Furthermore, the firing rate of supraoptic nucleus (SON) oxytocin neurons, a known α-MSH responsive neuronal population, was examined in non-pregnant and pregnant rats following α-MSH treatment. Intracerebroventricular injection of α-MSH significantly increased the number of c-fos-positive cells in the paraventricular, arcuate and ventromedial hypothalamic nuclei in non-pregnant rats, but no significant increase was observed in any of these regions in pregnant rats. In the SON, α-MSH did induce expression of c-fos during pregnancy, but this was significantly reduced compared to that observed in the non-pregnant group. Furthermore, during pregnancy, SON oxytocin neurons did not demonstrate the characteristic α-MSH-induced inhibition of firing rate that was observed in non-pregnant animals. Melanocortin receptor mRNA levels during pregnancy were similar to non-pregnant animals, suggesting that receptor down-regulation is unlikely to be a mechanism underlying the attenuated responses to α-MSH during pregnancy. Given the known functions of α-MSH in the hypothalamus, the attenuated responses will facilitate adaptive changes in appetite regulation and oxytocin secretion during pregnancy.

Exogenous and evoked oxytocin restores social behavior in the Cntnap2 mouse model of autism

Mouse models of neuropsychiatric diseases provide a platform for mechanistic understanding and development of new therapies. We previously demonstrated that knockout of the mouse homolog of CNTNAP2 (contactin-associated protein-like 2), in which mutations cause cortical dysplasia and focal epilepsy (CDFE) syndrome, displays many features that parallel those of the human disorder. Because CDFE has high penetrance for autism spectrum disorder (ASD), we performed an in vivo screen for drugs that ameliorate abnormal social behavior in Cntnap2 mutant mice and found that acute administration of the neuropeptide oxytocin improved social deficits. We found a decrease in the number of oxytocin immunoreactive neurons in the paraventricular nucleus (PVN) of the hypothalamus in mutant mice and an overall decrease in brain oxytocin levels. Administration of a selective melanocortin receptor 4 agonist, which causes endogenous oxytocin release, also acutely rescued the social deficits, an effect blocked by an oxytocin antagonist. We confirmed that oxytocin neurons mediated the behavioral improvement by activating endogenous oxytocin neurons in the paraventricular hypothalamus with Designer Receptors Exclusively Activated by Designer Drugs (DREADD). Last, we showed that chronic early postnatal treatment with oxytocin led to more lasting behavioral recovery and restored oxytocin immunoreactivity in the PVN. These data demonstrate dysregulation of the oxytocin system in Cntnap2 knockout mice and suggest that there may be critical developmental windows for optimal treatment to rectify this deficit.

Species, sex and individual differences in the vasotocin/vasopressin system: relationship to neurochemical signaling in the social behavior neural network

Arginine-vasotocin (AVT)/arginine vasopressin (AVP) are members of the AVP/oxytocin (OT) superfamily of peptides that are involved in the regulation of social behavior, social cognition and emotion. Comparative studies have revealed that AVT/AVP and their receptors are found throughout the "social behavior neural network (SBNN)" and display the properties expected from a signaling system that controls social behavior (i.e., species, sex and individual differences and modulation by gonadal hormones and social factors). Neurochemical signaling within the SBNN likely involves a complex combination of synaptic mechanisms that co-release multiple chemical signals (e.g., classical neurotransmitters and AVT/AVP as well as other peptides) and non-synaptic mechanisms (i.e., volume transmission). Crosstalk between AVP/OT peptides and receptors within the SBNN is likely. A better understanding of the functional properties of neurochemical signaling in the SBNN will allow for a more refined examination of the relationships between this peptide system and species, sex and individual differences in sociality.

Neonatal melanocortin receptor agonist treatment reduces play fighting and promotes adult attachment in prairie voles in a sex-dependent manner

The melanocortin receptor (MCR) system has been studied extensively for its role in feeding and sexual behavior, but effects on social behavior have received little attention. α-MSH interacts with neural systems involved in sociality, including oxytocin, dopamine, and opioid systems. Acute melanotan-II (MTII), an MC3/4R agonist, potentiates brain oxytocin (OT) release and facilitates OT-dependent partner preference formation in socially monogamous prairie voles. Here we examined the long-term impact of early-life MCR stimulation on hypothalamic neuronal activity and social development in prairie voles. Male and female voles were given daily subcutaneous injections of 10 mg/kg MTII or saline between postnatal days (PND) 1-7. Neonatally-treated males displayed a reduction in initiated play fighting bouts as juveniles compared to control males. Neonatal exposure to MTII facilitated partner preference formation in adult females, but not males, after a brief cohabitation with an opposite-sex partner. Acute MTII injection elicited a significant burst of the immediate early gene EGR-1 immunoreactivity in hypothalamic OT, vasopressin, and corticotrophin releasing factor neurons, when tested in PND 6-7 animals. Daily neonatal treatment with 1 mg/kg of a more selective, brain penetrant MC4R agonist, PF44687, promoted adult partner preferences in both females and males compared with vehicle controls. Thus, developmental exposure to MCR agonists lead to a persistent change in social behavior, suggestive of structural or functional changes in the neural circuits involved in the formation of social relationships.

Annexin A1 complex mediates oxytocin vesicle transport

Oxytocin is a major neuropeptide that modulates the brain functions involved in social behaviour and interaction. Despite of the importance of oxytocin for the neural control of social behaviour, little is known about the molecular mechanism(s) by which oxytocin secretion in the brain is regulated. Pro-oxytocin is synthesised in the cell bodies of hypothalamic neurones in the supraoptic and paraventricular nuclei and processed to a 9-amino-acid mature form during post-Golgi transport to the secretion sites at the axon terminals and somatodendritic regions. Oxytocin secreted from the somatodendritic regions diffuses throughout the hypothalamus and its neighbouring brain regions. Some oxytocin-positive axons innervate and secrete oxytocin to the brain regions distal to the hypothalamus. Brain oxytocin binds to its receptors in the brain regions involved in social behaviour. Oxytocin is also secreted from the axon terminal at the posterior pituitary gland into the blood circulation. We have discovered a new molecular complex consisting of annexin A1 (ANXA1), A-kinase anchor protein 150 (AKAP150) and microtubule motor that controls the distribution of oxytocin vesicles between the axon and the cell body in a protein kinase A (PKA)- and protein kinase C (PKC)-sensitive manner. ANXA1 showed significant co-localisation with oxytocin vesicles. Activation of PKA enhanced the association of kinesin-2 with ANXA1, thus increasing the axon-localisation of oxytocin vesicles. Conversely, activation of PKC decreased the binding of kinesin-2 to ANXA1, thus attenuating the axon-localisation of oxytocin vesicles. The result of the present study suggest that ANXA1 complex coordinates the actions of PKA and PKC to control the distribution of oxytocin vesicles between the axon and the cell body.

Anxiolytic drug discovery: what are the novel approaches and how can we improve them?

INTRODUCTION

Contemporary biological psychiatry uses experimental (animal) models to increase our understanding of affective disorder pathogenesis. Despite the well-recognized spectrum nature of affective disorders, modern anxiolytic drug discovery mainly targets specific pathways and molecular determinants within a single phenotypic domain. However, greater understanding of the integrative mechanisms and pathogenesis is essential in order to develop new effective therapies.

AREAS COVERED

In this review, the authors emphasize the importance of a 'domain interplay-oriented' approach to experimental affective research. They also highlight the need to expand the scope of anxiolytic drug targets to better understand the pathogenesis of anxiety-spectrum disorders.

EXPERT OPINION

There is the potential to markedly improve the utility of animal models for affective disorders. First, the authors suggest that one such way would be by analyzing the systems of several domains and their interplay to better understand disease pathogenesis. Further, it could also be improved by expanding the range of model species and by extending the spectrum of anxiolytic drug targets; this would help to focus on emerging and unconventional systems to better develop new therapies.

The role of oxytocin in psychiatric disorders: a review of biological and therapeutic research findings

LEARNING OBJECTIVES

After participating in this educational activity, the physician should be better able to 1. Identify the biological role of oxytocin in forming attachments. 2. Evaluate the relationship between various neuropsychiatric disorders and oxytocin. 3. Identify clinical implications of using oxytocin to treat various neuropsychiatric disorders. Oxytocin is a peptide hormone integral in parturition, milk letdown, and maternal behaviors that has been demonstrated in animal studies to be important in the formation of pair bonds and in social behaviors. This hormone is increasingly recognized as an important regulator of human social behaviors, including social decision making, evaluating and responding to social stimuli, mediating social interactions, and forming social memories. In addition, oxytocin is intricately involved in a broad array of neuropsychiatric functions and may be a common factor important in multiple psychiatric disorders such as autism, schizophrenia, and mood and anxiety disorders. This review article examines the extant literature on the evidence for oxytocin dysfunction in a variety of psychiatric disorders and highlights the need for further research to understand the complex role of the oxytocin system in psychiatric disease and thus pave the way for developing new therapeutic modalities. Articles were selected that involved human participants with various psychiatric disorders and that either compared oxytocin biology to healthy controls or examined the effects of exogenous oxytocin administration.

Systemic leptin increases the electrical activity of supraoptic nucleus oxytocin neurones in virgin and late pregnant rats

In the rat hypothalamus, fasting attenuates the expression of oxytocin and this can be reversed by exogenous leptin administration. In the present study, we investigated the effects of systemically administered leptin on the electrical activity of magnocellular neurones in the supraoptic nucleus of urethane-anaesthetised rats. In virgin female rats, systemic leptin significantly excited identified oxytocin neurones with no detected effects on the patterning of activity, as reflected by hazard function analyses. The lowest dose that was consistently effective was 100 μg/i.v., and this dose had no significant effect on vasopressin neurones. In virgin rats fasted overnight, the spontaneous firing rate of oxytocin neurones was significantly lower than in unfasted rats, although leptin had a similar excitatory effect as in unfasted rats. In late pregnant rats (days 19-21 of pregnancy), spontaneous firing rates of oxytocin neurones were higher than in virgins, and the initial response to leptin was similar to that in virgin rats, although the increase in activity was more persistent. In fasted pregnant rats, the mean spontaneous firing rate of oxytocin neurones was again lower than in unfasted rats, although leptin had no significant effect even at the higher dose of 1 mg/rat. Thus, fasting reduced the spontaneous firing rates of oxytocin neurones in nonpregnant rats, and this effect could be reversed by the excitatory effects of leptin. Pregnant rats showed some evidence of leptin resistance but only after an overnight fast.

Helping oxytocin deliver: considerations in the development of oxytocin-based therapeutics for brain disorders

Concerns regarding a drought in psychopharmacology have risen from many quarters. From one perspective, the wellspring of bedrock medications for anxiety disorders, depression, and schizophrenia was serendipitously discovered over 30 year ago, the swell of pharmaceutical investment in drug discovery has receded, and the pipeline's flow of medications with unique mechanisms of action (i.e., glutamatergic agents, CRF antagonists) has slowed to a trickle. Might oxytocin (OT)-based therapeutics be an oasis? Though a large basic science literature and a slowly increasing number of studies in human diseases support this hope, the bulk of extant OT studies in humans are single-dose studies on normals, and do not directly relate to improvements in human brain-based diseases. Instead, these studies have left us with a field pregnant with therapeutic possibilities, but barren of definitive treatments. In this clinically oriented review, we discuss the extant OT literature with an eye toward helping OT deliver on its promise as a therapeutic agent. To this end, we identify 10 key questions that we believe future OT research should address. From this overview, several conclusions are clear: (1) the OT system represents an extremely promising target for novel CNS drug development; (2) there is a pressing need for rigorous, randomized controlled clinical trials targeting actual patients; and (3) in order to inform the design and execution of these vital trials, we need further translational studies addressing the questions posed in this review. Looking forward, we extend a cautious hope that the next decade of OT research will birth OT-targeted treatments that can truly deliver on this system's therapeutic potential.

The contribution of brain reward circuits to the obesity epidemic

One of the defining characteristics of the research of Ann E. Kelley was her recognition that the neuroscience underlying basic learning and motivation processes also shed significant light upon mechanisms underlying drug addiction and maladaptive eating patterns. In this review, we examine the parallels that exist in the neural pathways that process both food and drug reward, as determined by recent studies in animal models and human neuroimaging experiments. We discuss contemporary research that suggests that hyperphagia leading to obesity is associated with substantial neurochemical changes in the brain. These findings verify the relevance of reward pathways for promoting consumption of palatable, calorically dense foods, and lead to the important question of whether changes in reward circuitry in response to intake of such foods serve a causal role in the development and maintenance of some cases of obesity. Finally, we discuss the potential value for future studies at the intersection of the obesity epidemic and the neuroscience of motivation, as well as the potential concerns that arise from viewing excessive food intake as an "addiction". We suggest that it might be more useful to focus on overeating that results in frank obesity, and multiple health, interpersonal, and occupational negative consequences as a form of food "abuse".

Oxytocin secretion is associated with severity of disordered eating psychopathology and insular cortex hypoactivation in anorexia nervosa

CONTEXT

Animal data suggest that oxytocin is a satiety hormone. We have demonstrated that anorexia nervosa (anorexia), a disorder characterized by food restriction, low weight, and hypoleptinemia, is associated with decreased nocturnal oxytocin secretion. We have also reported functional magnetic resonance imaging (fMRI) hypoactivation in anorexia in brain regions involved in food motivation. The relationships between oxytocin, food-motivation neurocircuitry, and disordered eating psychopathology have not been investigated in humans.

OBJECTIVE

The objective of the study was to determine whether the oxytocin response to feeding in anorexia differs from healthy women and to establish the relationship between oxytocin secretion and disordered eating psychopathology and food-motivation neurocircuitry.

DESIGN

This was a cross-sectional study.

SETTING

The study was conducted at a clinical research center.

PARTICIPANTS

Participants included 35 women: 13 anorexia (AN), nine weight-recovered anorexia (ANWR), and 13 healthy controls (HC).

MEASURES

Peripheral oxytocin and leptin levels were measured fasting and 30, 60, and 120 min after a standardized mixed meal. The Eating Disorder Examination-Questionnaire was used to assess disordered eating psychopathology. fMRI was performed during visual processing of food and nonfood stimuli to measure brain activation before and after the meal.

RESULTS

Mean oxytocin levels were higher in AN than HC at 60 and 120 min and lower in ANWR than HC at 0, 30, and 120 min and AN at all time points. Mean oxytocin area under the curve (AUC) was highest in AN, intermediate in HC, and lowest in ANWR. Mean leptin levels at all time points and AUC were lower in AN than HC and ANWR. Oxytocin AUC was associated with leptin AUC in ANWR and HC but not in AN. Oxytocin AUC was associated with the severity of disordered eating psychopathology in AN and ANWR, independent of leptin secretion, and was associated with between-group variance in fMRI activation in food motivation brain regions, including the hypothalamus, amygdala, hippocampus, orbitofrontal cortex, and insula.

CONCLUSIONS

Oxytocin may be involved in the pathophysiology of anorexia.

Nesfatin-1, a unique regulatory neuropeptide of the brain

Nesfatin-1, a newly discovered NUCB2-derived satiety neuropeptide is expressed in several neurons of forebrain, hindbrain, brainstem and spinal cord. This novel anorexigenic substance seems to play an important role in hypothalamic pathways regulating food intake and energy homeostasis. Nesfatin-1 immunoreactive cells are detectable in arcuate (ARC), paraventricular (PVN) and supraoptic nuclei (SON), where the peptide is colocalized with POMC/CART, NPY, oxytocin and vasopressin. The nesfatin-1 molecule interacts with a G-protein coupled receptor and its cytophysiological effect depends on inhibitory hyperpolarization of NPY/AgRP neurons in ARC and melanocortin signaling in PVN. Administration of nesfatin-1 significantly inhibits consumatory behavior and decreases weight gain in experimental animals. These recent findings suggest the evidence for nesfatin-1 involvement in other important brain functions such as reproduction, sleep, cognition and anxiety- or stress-related responses. The neuroprotective and antiapoptotic properties of nesfatin-1 were also reported. From the clinical viewpoint it should be noteworthy, that the serum concentration of nesfatin-1 may be a sensitive marker of epileptic seizures. However, the details of nesfatin-1 physiology ought to be clarified, and it may be considered suitable in the future, as a potential drug in the pharmacotherapy of obesity, especially in patients treated with antipsychotics and antidepressants. On the other hand, some putative nesfatin-1 antagonists may improve eating disorders.

The role of oxytocin in mating and pregnancy

The hormone oxytocin (OT) is released both centrally and peripherally during and after mating. Although research in humans suggests a central role in sexuality, the most reliable findings to date involve peripheral activation. This review will discuss these results and will particularly focus on understanding the most recent findings from fMRI data and the effects of exogenous peripheral OT administration. We will then consider hypotheses of the roles played by central and systemic OT release as well as their control and modulation in the female, summarizing recent findings from animal research. Finally, we will discuss the contribution of OT to the initiation of pregnancy in rodents. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

The oxytocin system in drug discovery for autism: animal models and novel therapeutic strategies

Animal models and behavioral paradigms are critical for elucidating the neural mechanism involved in complex behaviors, including social cognition. Both genotype and phenotype based models have implicated the neuropeptide oxytocin (OT) in the regulation of social behavior. Based on the findings in animal models, alteration of the OT system has been hypothesized to play a role in the social deficits associated with autism and other neuropsychiatric disorders. While the evidence linking the peptide to the etiology of the disorder is not yet conclusive, evidence from multiple animal models suggest modulation of the OT system may be a viable strategy for the pharmacological treatment of social deficits. In this review, we will discuss how animal models have been utilized to understand the role of OT in social cognition and how those findings can be applied to the conceptualization and treatment of the social impairments in ASD. Animal models with genetic alterations of the OT system, like the OT, OT receptor and CD38 knock-out mice, and those with phenotypic variation in social behavior, like BTBR inbred mice and prairie voles, coupled with behavioral paradigms with face and construct validity may prove to have predictive validity for identifying the most efficacious methods of stimulating the OT system to enhance social cognition in humans. The widespread use of strong animal models of social cognition has the potential yield pharmacological, interventions for the treatment social impairments psychiatric disorders. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

Reward systems and food intake: role of opioids

Humans eat for many reasons, including the rewarding qualities of foods. A host of neurotransmitters have been shown to influence eating behavior and some of these appear to be involved in reward-induced eating. Endogenous opioid peptides and their receptors were first reported more than 30 years ago, and studies suggesting a role of opioids in the regulation of food intake date back nearly as far. Opioid agonists and antagonists have corresponding stimulatory and inhibitory effects on feeding. In addition to studies aimed at identifying the relevant receptor subtypes and sites of action within the brain, there has been a continuing interest in the role of opioids on diet/taste preferences, food reward, and the overlap of food reward with others types of reward. Data exist that suggest a role for opioids in the control of appetite for specific macronutrients, but there is also evidence for their role in the stimulation of intake based on already-existing diet or taste preferences and in controlling intake motivated by hedonics rather than by energy needs. Finally, various types of studies indicate an overlap between mechanisms mediating drug reward and palatable food reward. Preference or consumption of sweet substances often parallels the self-administration of several drugs of abuse, and under certain conditions, the termination of intermittent access to sweet substances produces symptoms that resemble those observed during opiate withdrawal. The overconsumption of readily available and highly palatable foods likely contributes to the growing rates of obesity worldwide. An understanding of the role of opioids in mediating food reward and promoting the overconsumption of palatable foods may provide insights into new approaches for preventing obesity.

Effects of cholecystokinin in the supraoptic nucleus and paraventricular nucleus are negatively modulated by leptin in 24-h fasted lean male rats

Cholecystokinin (CCK) and leptin are two important satiety factors that are considered to act in synergy to reduce meal size. Peripheral injection of CCK activates neurones in several hypothalamic nuclei, including the supraoptic (SON) and paraventricular (PVN) nuclei and neurones in the brainstem of fed rats. We investigated whether peripheral leptin would modulate the effects of CCK on neuronal activity in the hypothalamus and brainstem of fasted rats by investigating Fos expression in the PVN, SON, arcuate nucleus, ventromedial hypothalamus (VMH), dorsomedial hypothalamus (DMH), area postrema (AP) and the nucleus tractus solitarii (NTS). Male rats, fasted for 24 h, received either one i.p. injection of vehicle, leptin or CCK-8 alone, or received one injection of vehicle or leptin before an i.p. injection of CCK-8. We found that CCK increased Fos expression in the PVN and SON as well as in the NTS and AP, but had no effect on Fos expression in the arcuate nucleus, VMH or DMH compared to vehicle. Leptin injected alone significantly increased Fos expression in the arcuate nucleus but had no effect on Fos expression in the VMH, DMH, SON, PVN, AP or NTS compared to vehicle. Fos expression was significantly increased in the AP in rats injected with both leptin and CCK compared to rats injected with vehicle and CCK. Unexpectedly, there was significantly less Fos expression in the PVN and SON of fasted rats injected with leptin and CCK than in rats injected with vehicle and CCK, suggesting that leptin attenuated CCK-induced Fos expression in the SON and PVN. However, Fos expression in the NTS was similar in fasted rats injected with vehicle and CCK or with leptin and CCK. Taken together, these results suggest that leptin dampens the effects of CCK on Fos expression in the SON and PVN, independently from NTS pathways, and this may reflect a direct action on magnocellular neurones.

CD38/cyclic ADP-ribose system: a new player for oxytocin secretion and regulation of social behaviour

Oxytocin is important for regulating a number of physiological processes. Disruption of the secretion, metabolism or action of oxytocin results in an impairment of reproductive function, social and sexual behaviours, and stress responses. This review discusses current views on the regulation and autoregulation of oxytocin release in the hypothalamic-neurohypophysial system, with special focus on the activity of the CD38/cADP-ribose system as a new component in this regulation. Data from our laboratories indicate that an impairment of this system results in alterations of oxytocin secretion and abnormal social behaviour, thus suggesting new clues that help in our understanding of the pathogenesis of neurodevelopmental disorders.

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 and the neural mechanisms regulating social cognition and affiliative behavior

Oxytocin is produced in the hypothalamus and released into the circulation through the neurohypophyseal system. Peripherally released oxytocin facilitates parturition and milk ejection during nursing. Centrally released oxytocin coordinates the onset of maternal nurturing behavior at parturition and plays a role in mother-infant bonding. More recent studies have revealed a more general role for oxytocin in modulating affiliative behavior in both sexes. Oxytocin regulates alloparental care and pair bonding in female monogamous prairie voles. Social recognition in male and female mice is also modulated by oxytocin. In humans, oxytocin increases gaze to the eye region of human faces and enhances interpersonal trust and the ability to infer the emotions of others from facial cues. While the neurohypopheseal oxytocin system has been well characterized, less is known regarding the nature of oxytocin release within the brain. Here we review the role of oxytocin in the regulation of prosocial interactions, and discuss the neuroanatomy of the central oxytocin system.

Neurotransmitter regulation of c-fos and vasopressin gene expression in the rat supraoptic nucleus

Acute increases in plasma osmotic pressure produced by intraperitoneal injection of hypertonic NaCl are sensed by osmoreceptors in the brain, which excite the magnocellular neurons (MCNs) in the supraoptic nucleus (SON) and the paraventricular nucleus (PVN) in the hypothalamus inducing the secretion of vasopressin (VP) into the general circulation. Such systemic osmotic stimulation also causes rapid and transient increases in the gene expression of c-fos and VP in the MCNs. In this study we evaluated potential signals that might be responsible for initiating these gene expression changes during acute hyperosmotic stimulation. We use an in vivo paradigm in which we stereotaxically deliver putative agonists and antagonists over the SON unilaterally, and use the contralateral SON in the same rat, exposed only to vehicle solutions, as the control SON. Quantitative real time-PCR was used to compare the levels of c-fos mRNA, and VP mRNA and VP heteronuclear (hn)RNA in the SON. We found that the ionotropic glutamate agonists (NMDA plus AMPA) caused an approximately 6-fold increase of c-fos gene expression in the SON, and some, but not all, G-coupled protein receptor agonists (e.g., phenylephrine, senktide, a NK-3-receptor agonist, and alpha-MSH) increased the c-fos gene expression in the SON from between 1.5 to 2-fold of the control SONs. However, none of these agonists were effective in increasing VP hnRNA as is seen with acute salt-loading. This indicates that the stimulus-transcription coupling mechanisms that underlie the c-fos and VP transcription increases during acute osmotic stimulation differ significantly from one another.

Oxytocin deficiency mediates hyperphagic obesity of Sim1 haploinsufficient mice

Single-minded 1 (Sim1) encodes a transcription factor essential for formation of the hypothalamic paraventricular nucleus (PVN). Sim1 haploinsufficiency is associated with hyperphagic obesity and increased linear growth in humans and mice, similar to the phenotype of melanocortin 4 receptor (Mc4r) mutations. PVN neurons in Sim1(+/-) mice are hyporesponsive to the melanocortin agonist melanotan II. PVN neuropeptides oxytocin (Oxt), TRH and CRH inhibit feeding when administered centrally. Consequently, we hypothesized that altered PVN neuropeptide expression mediates the hyperphagia of Sim1(+/-) mice. To test this hypothesis, we measured hypothalamic expression of PVN neuropeptides in Sim1(+/-) and wild-type mice. Oxt mRNA and peptide were decreased by 80% in Sim1(+/-) mice, whereas TRH, CRH, arginine vasopressin (Avp), and somatostatin mRNAs were decreased by 20-40%. Sim1(+/-) mice also showed abnormal regulation of Oxt but not CRH mRNA in response to feeding state. A selective Mc4r agonist activated PVN Oxt neurons in wild-type mice, supporting involvement of these neurons in melanocortin feeding circuits. To test whether Oxt itself regulates feeding, we measured the effects of central administration of an Oxt receptor antagonist or repeated doses of Oxt on food intake of Sim1(+/-) and wild-type mice. Sim1(+/-) mice were hypersensitive to the orexigenic effect of the Oxt receptor antagonist. Oxt decreased the food intake and weight gain of Sim1(+/-) mice at a dose that did not affect wild-type mice. Our results support the importance of Oxt neurons in feeding regulation and suggest that reduced Oxt neuropeptide is one mechanism mediating the hyperphagic obesity of Sim1(+/-) mice.

Nesfatin-1 neurons in paraventricular and supraoptic nuclei of the rat hypothalamus coexpress oxytocin and vasopressin and are activated by refeeding

Nesfatin-1, a newly discovered satiety molecule, is located in the hypothalamic nuclei, including the paraventricular nucleus (PVN) and supraoptic nucleus (SON). In this study, fine localization and regulation of nesfatin-1 neurons in the PVN and SON were investigated by immunohistochemistry of neuropeptides and c-Fos. In the PVN, 24% of nesfatin-1 neurons overlapped with oxytocin, 18% with vasopressin, 13% with CRH, and 12% with TRH neurons. In the SON, 35% of nesfatin-1 neurons overlapped with oxytocin and 28% with vasopressin. After a 48-h fast, refeeding for 2 h dramatically increased the number of nesfatin-1 neurons expressing c-Fos immunoreactivity by approximately 10 times in the PVN and 30 times in the SON, compared with the fasting controls. In the SON, refeeding also significantly increased the number of nesfatin-1-immunoreactive neurons and NUCB2 mRNA expression, compared with fasting. These results indicate that nesfatin-1 neurons in the PVN and SON highly overlap with oxytocin and vasopressin neurons and that they are activated markedly by refeeding. Feeding-activated nesfatin-1 neurons in the PVN and SON could play a role in the postprandial regulation of feeding behavior and energy homeostasis.

Endogenous modulators of synaptic transmission: cannabinoid regulation in the supraoptic nucleus

The magnocellular neurons of the hypothalamic supraoptic nucleus (SON) are a major source of both systemic and central release of the neurohypophyseal peptides, oxytocin (OXT) and arginine-vasopressin (AVP). Both OXT and AVP are released from the somatodendritic compartment of magnocellular neurons and act within the SON to modulate the electrophysiological function of these cells. Cannabinoids (CBs) affect hormonal output and the SON may represent a neural substrate through which CBs exert specific physiological and behavioural effects. Dynamic modulation of synaptic inputs is a fundamental mechanism through which neuronal output is controlled. Dendritically released OXT acts on autoreceptors to generate endocannabinoids (eCBs) which modify both excitatory and inhibitory inputs to OXT neurons through actions on presynaptic CB receptors. As such, OXT and eCBs cooperate to shape the electrophysiological properties of magnocellular OXT neurons, regulating the physiological function of this nucleus. Further study of eCB signalling in the SON, including its interaction with AVP neurons, promises to extend our understanding of the synaptic regulation of SON physiological function.

Central release of oxytocin and the ventromedial hypothalamus

Recent studies on the regulation of social behaviours by neuropeptides indicate that it is the distribution of peptide receptor expression in particular brain areas that determines the specificity of peptide actions; and that, accordingly, peptides can evoke specific behaviours when administered centrally without temporal or spatial selectivity of administration. The release of neuropeptides at synaptic sites appears irrelevant, and in the brain, some peptides are released mainly from dendrites rather than from nerve endings. Dendritic peptide release can be long lasting, semi-independent of electrical activity, and allows the diffusion of peptides to distant targets. The peptide oxytocin regulates many behaviours; in particular, it inhibits food intake. Centrally, oxytocin is released in large amounts by the dendrites of hypothalamic magnocellular neurons. This mini-review considers the possible involvement of dendritically released oxytocin in the regulation of food intake by its actions on the ventromedial hypothalamus.

FMRFamide-like neuropeptides and mechanosensory touch receptor neurons regulate male sexual turning behavior in Caenorhabditis elegans

Caenorhabditis elegans male mating provides a powerful model to study the relationship between the nervous system, genes, and innate sexual behaviors. Male mating is the most complex behavior exhibited by the nematode C. elegans and involves the steps of response, backing, turning, vulva location, spicule insertion, and sperm transfer. Because neuropeptides are important neural regulators of many complex animal behaviors, we explored the function of the FMRFamide-like neuropeptide (flp) gene family in regulating male copulation. We found that peptidergic signaling mediated by FMRF-amide like neuropeptides (FLPs) FLP-8, FLP-10, FLP-12, and FLP-20 is required for the sensory transduction involved in male turning behavior. flp-8, flp-10, flp-12, and flp-20 mutant males significantly increase repetition of substep(s) of turning behavior compared with wild-type males. Genes controlling neuropeptide processing and secretion in general, including egl-3, egl-21, ida-1, and unc-31, are also required for inhibiting repetitive turning behavior. Neuropeptidergic signaling adjusts the repetitiveness of turning independently of serotonergic modulation of the timing of turning. Surprisingly, the mechanosensitive touch receptor neurons are found to be part of the neural circuitry regulating male turning behavior, indicating the existence of functional dimorphisms in the nervous system with regard to sex-specific behaviors.

Neuroendocrine mechanisms of change in food intake during pregnancy: a potential role for brain oxytocin

During pregnancy body weight, and particularly adiposity, increase, due to hyperphagia rather than decreased energy metabolism. These physiological adaptations provide the growing fetus(es) with nutrition and prepare the mother for the metabolically-demanding lactation period following birth. Mechanisms underlying the hyperphagia are still poorly understood. Although the peripheral signals that drive appetite and satiety centers of the brain are increased in pregnancy, the brain may become insensitive to their effects. For example, leptin secretion increases but hypothalamic resistance to leptin actions develops. However, several adaptations in hypothalamic neuroendocrine systems may converge to increase ingestive behavior. Oxytocin is one of the anorectic hypothalamic neuropeptides. Oxytocin neurons, both centrally-projecting parvocellular oxytocin neurons and central dendritic release of oxytocin from magnocellular neurons, may play a key role in regulating energy intake. During feeding in non-pregnant rats, magnocellular oxytocin neurons, especially those in the supraoptic nucleus, become strongly activated indicating their imminent role in meal termination. However, in mid-pregnancy the excitability of these neurons is reduced, central dendritic oxytocin release is inhibited and patterns of oxytocin receptor binding in the brain alter. Our recent data suggest that lack of central oxytocin action may partly contribute to maternal hyperphagia. However, although opioid inhibition is a major factor in oxytocin neuron restraint during pregnancy and opioids enhance food intake, an increase in opioid orexigenic actions were not observed. While changes in several central input pathways to oxytocin neurons are likely to be involved, the high level of progesterone secretion during pregnancy is probably the ultimate trigger for the adaptations.