Pharmacological characteristics and anatomical distribution of [3H]oxytocin-binding sites in the Wistar rat brain studied by autoradiography

A circuit from the locus coeruleus to the anterior cingulate cortex modulates offspring interactions in mice

Social sensitivity to other individuals in distress is crucial for survival. The anterior cingulate cortex (ACC) is a structure involved in making behavioral choices and is influenced by observed pain or distress. Nevertheless, our understanding of the neural circuitry underlying this sensitivity is incomplete. Here, we reveal unexpected sex-dependent activation of ACC when parental mice respond to distressed pups by returning them to the nest ("pup retrieval"). We observe sex differences in the interactions between excitatory and inhibitory ACC neurons during parental care, and inactivation of ACC excitatory neurons increased pup neglect. Locus coeruleus (LC) releases noradrenaline in ACC during pup retrieval, and inactivation of the LC-ACC pathway disrupts parental care. We conclude that ACC maintains sex-dependent sensitivity to pup distress under LC modulation. We propose that ACC's involvement in parenting presents an opportunity to identify neural circuits that support sensitivity to the emotional distress of others.

Erectile Dysfunction: Treatments, Advances and New Therapeutic Strategies

Erectile dysfunction (ED) is the inability to get and maintain an adequate penile erection for satisfactory sexual intercourse. Due to its negative impacts on men's life quality and increase during aging (40% of men between 40 and 70 years), ED has always attracted researchers of different disciplines, from urology, andrology and neuropharmacology to regenerative medicine, and vascular and prosthesis implant surgery. Locally and/or centrally acting drugs are used to treat ED, e.g., phosphodiesterase 5 inhibitors (first in the list) given orally, and phentolamine, prostaglandin E1 and papaverine injected intracavernously. Preclinical data also show that dopamine D4 receptor agonists, oxytocin and α-MSH analogues may have a role in ED treatment. However, since pro-erectile drugs are given on demand and are not always efficacious, new strategies are being tested for long lasting cures of ED. These include regenerative therapies, e.g., stem cells, plasma-enriched platelets and extracorporeal shock wave treatments to cure damaged erectile tissues. Although fascinating, these therapies are laborious, expensive and not easily reproducible. This leaves old vacuum erection devices and penile prostheses as the only way to get an artificial erection and sexual intercourse with intractable ED, with penile prosthesis used only by accurately selected patients.

Neuromodulatory functions exerted by oxytocin on different populations of hippocampal neurons in rodents

Oxytocin (OT) is a neuropeptide widely known for its peripheral hormonal effects (i.e., parturition and lactation) and central neuromodulatory functions, related especially to social behavior and social, spatial, and episodic memory. The hippocampus is a key structure for these functions, it is innervated by oxytocinergic fibers, and contains OT receptors (OTRs). The hippocampal OTR distribution is not homogeneous among its subregions and types of neuronal cells, reflecting the specificity of oxytocin's modulatory action. In this review, we describe the most recent discoveries in OT/OTR signaling in the hippocampus, focusing primarily on the electrophysiological oxytocinergic modulation of the OTR-expressing hippocampal neurons. We then look at the effect this modulation has on the balance of excitation/inhibition and synaptic plasticity in each hippocampal subregion. Additionally, we review OTR downstream signaling, which underlies the OT effects observed in different types of hippocampal neuron. Overall, this review comprehensively summarizes the advancements in unraveling the neuromodulatory functions exerted by OT on specific hippocampal networks.

Probing the Spatiotemporal Dynamics of Oxytocin in the Brain Tissue Using a Simple Peptide Alkyne-Tagging Approach

The dynamics of oxytocin and its site of action in the brain are poorly understood due to the lack of appropriate tools, despite the interest in the central action of oxytocin signaling. Here, we develop and apply an oxytocin analogue probe by conjugating it with an alkyne via a widely applicable simple coupling reaction. Alkyne-tagged oxytocin behaves similarly to endogenous oxytocin while allowing specific and highly sensitive detection of extracellularly applied oxytocin. Using this probe, we find the existence of high-affinity specific binding sites of oxytocin in the hippocampus. Furthermore, characterization of oxytocin dynamics reveals the cellular basis of its volume transmission in the brain tissue. Finally, we show the wide applicability of this technique for other centrally acting peptides. Thus, the alkyne tagging strategy provides a unique opportunity to characterize the spatiotemporal dynamics of oxytocin and other small-sized peptides in the brain tissue.

Using Receptor Autoradiography to Visualize and Quantify Oxytocin and Vasopressin 1a Receptors in the Human and Nonhuman Primate Brain

Despite its development almost 40 years ago, receptor autoradiography remains a regular and reliable practice for the localization of oxytocin and vasopressin receptors in brain tissue sections. It is used across many laboratories, institutions, and animal species to characterize and quantify the distribution and density of these receptors at baseline and/or in response to experimental manipulations or lived experience. This powerful tool and the neuroanatomical receptor maps that it generates have allowed researchers to more accurately investigate and understand the neural substrates upon which oxytocin and vasopressin act to affect behavior. Researchers have used these maps to design site-specific pharmacological manipulations and electrophysiological recordings in animal studies to directly probe the underlying neural mechanisms in this system. This methods chapter describes the specific procedures by which a pharmacologically optimized, competitive binding modification to receptor autoradiography can be used to reliably localize oxytocin and vasopressin receptors in the human brain and in the brains of nonhuman primates. The ability to reliably perform receptor autoradiography for these targets in human brain tissue can finally inform our interpretation of past intranasal oxytocin neuroimaging studies and allows us to move past the reliance on transcriptomic studies using brain tissue homogenates so that we can directly investigate the involvement of oxytocin and vasopressin receptors in human behavior, physiology, and neuropsychiatric disease.

Oxytocin, Erectile Function and Sexual Behavior: Last Discoveries and Possible Advances

A continuously increasing amount of research shows that oxytocin is involved in numerous central functions. Among the functions in which oxytocin is thought to be involved are those that play a role in social and sexual behaviors, and the involvement of central oxytocin in erectile function and sexual behavior was indeed one of the first to be discovered in laboratory animals in the 1980s. The first part of this review summarizes the results of studies done in laboratory animals that support a facilitatory role of oxytocin in male and female sexual behavior and reveal mechanisms through which this ancient neuropeptide participates in concert with other neurotransmitters and neuropeptides in this complex function, which is fundamental for the species reproduction. The second part summarizes the results of studies done mainly with intranasal oxytocin in men and women with the aim to translate the results found in laboratory animals to humans. Unexpectedly, the results of these studies do not appear to confirm the facilitatory role of oxytocin found in male and female sexual behavior in animals, both in men and women. Possible explanations for the failure of oxytocin to improve sexual behavior in men and women and strategies to attempt to overcome this impasse are considered.

Activation of Oxytocin Receptors Excites Subicular Neurons by Multiple Signaling and Ionic Mechanisms

Oxytocin (OXT) is a nonapeptide that serves as a neuromodulator in the brain and a hormone participating in parturition and lactation in the periphery. The subiculum is the major output region of the hippocampus and an integral component in the networks that process sensory and motor cues to form a cognitive map encoding spatial, contextual, and emotional information. Whilst the subiculum expresses the highest OXT-binding sites and is the first brain region to be activated by peripheral application of OXT, the precise actions of OXT in the subiculum have not been determined. Our results demonstrate that application of the selective OXT receptor (OXTR) agonist, [Thr4,Gly7]-oxytocin (TGOT), excited subicular neurons via activation of TRPV1 channels, and depression of K+ channels. The OXTR-mediated excitation of subicular neurons required the functions of phospholipase Cβ, protein kinase C, and degradation of phosphatidylinositol 4,5-bisphosphate (PIP2). OXTR-elicited excitation of subicular neurons enhanced long-term potentiation via activation of TRPV1 channels. Our results provide a cellular and molecular mechanism to explain the physiological functions of OXT in the brain.

Central oxytocin signaling inhibits food reward-motivated behaviors and VTA dopamine responses to food-predictive cues in male rats

Oxytocin potently reduces food intake and is a potential target system for obesity treatment. A better understanding of the behavioral and neurobiological mechanisms mediating oxytocin's anorexigenic effects may guide more effective obesity pharmacotherapy development. The present study examined the effects of central (lateral intracerebroventricular [ICV]) administration of oxytocin in rats on motivated responding for palatable food. Various conditioning procedures were employed to measure distinct appetitive behavioral domains, including food seeking in the absence of consumption (conditioned place preference expression), impulsive responding for food (differential reinforcement of low rates of responding), effort-based appetitive decision making (high-effort palatable vs. low-effort bland food), and sucrose reward value encoding following a motivational shift (incentive learning). Results reveal that ICV oxytocin potently reduces food-seeking behavior, impulsivity, and effort-based palatable food choice, yet does not influence encoding of sucrose reward value in the incentive learning task. To investigate a potential neurobiological mechanism mediating these behavioral outcomes, we utilized in vivo fiber photometry in ventral tegmental area (VTA) dopamine neurons to examine oxytocin's effect on phasic dopamine neuron responses to sucrose-predictive Pavlovian cues. Results reveal that ICV oxytocin significantly reduced food cue-evoked dopamine neuron activity. Collectively, these data reveal that central oxytocin signaling inhibits various obesity-relevant conditioned appetitive behaviors, potentially via reductions in food cue-driven phasic dopamine neural responses in the VTA.

Oxytocin increases inhibitory synaptic transmission and blocks development of long-term potentiation in the lateral amygdala

Oxytocin (OT) is a neuroactive peptide that influences the processing of fearful stimuli in the amygdala. In the central nucleus of the amygdala, the activation of OT receptors alters neural activity and ultimately suppresses the behavioral response to a fear conditioned stimulus. Receptors for OT are also found in the lateral amygdala (LA), and infusion of OT into the basolateral amygdala complex affects the formation and consolidation of fear memories. Yet, how OT receptor activation alters neurons and neural networks in the LA is unknown. In this study we used whole cell electrophysiological recordings to determine how OT-receptor activation changes synaptic transmission and synaptic plasticity in the LA of Sprague-Dawley rats. Our results demonstrate that OT-receptor activation results in a 200% increase in spontaneous inhibitory transmission in the LA that leads to the activation of presynaptic GABAB receptors. The activation of these receptors inhibits excitatory transmission in the LA, blocking long-term potentiation of cortical inputs onto LA neurons. Hence, this study provides the first demonstration that OT influences synaptic transmission and plasticity in the LA, revealing a mechanism that could explain how OT regulates the formation and consolidation of conditioned fear memories in the amygdala.

NEW & NOTEWORTHY This study investigates modulation of synaptic transmission by oxytocin (OT) in the lateral amygdala (LA). We demonstrate that OT induces transient increases in spontaneous GABAergic transmission by activating interneurons in the basolateral amygdala. The resultant increase in GABA release in the LA activates presynaptic GABAB receptors on both inhibitory and excitatory inputs onto LA neurons, reducing release probability at these synapses. We subsequently demonstrate that OT modulates synaptic plasticity at cortical inputs to the LA.

Variation in the density of oxytocin receptors in the brain as mechanism of adaptation to specific social and reproductive strategies

Most species have predominant forms of social and reproductive behavior driven by many years of selection pressures and evolution. For example, rodent species can live in small or large groups, behave more tolerant or aggressively toward conspecifics (including newborns), and form or not bonds with other members of the group (including sexual partners). Any of those behavioral adaptations could result in good fitness for the species, but could also require compromises such as sharing resources, greater parental investment, increased risk of predation, etc. We propose that the oxytocin (OXT) system, among others neuroendocrine peptides, is at the basis of a neural mechanism that adapts and predisposes species to a particular social and reproductive form of living. In this review we will show evidence that the variability in the density of receptors for OXT (OXTR) in the nucleus accumbens (NAc) and the lateral septum (LS) predisposes species to adopt at least 4 different social and reproductive strategies in rodents. Large or medium size groups with lower conspecific spacing (preferred separation distance maintained by adult conspecifics), and high levels of promiscuity are characterized by low levels of OXTR in the NAc and LS (e.g. Ratus norvegicus, Ctenomys sociabilis, Scotinomys teguina, Cavia porcellus); small size groups with higher conspecific spacing and low levels of promiscuity are characterized by high OXTR in the NAc and the LS (e.g. Peromyscus californicus); large or medium groups with lower conspecific spacing and low levels of promiscuity characterized by high levels of OXTR in the NAc but low levels in the LS (e.g. Microtus ochrogaster, Heterocephalus glaber, Microtus kikuchii); and small or medium size groups with higher conspecific spacing and high levels of promiscuity characterized by low levels of OXTR in the NAc and high OXTR in the LS (e.g. Mus musculus, Ctenomys haigi, Peromyscus maniculatus, Microtus pennsylvanicus, Microtus montanus). Careful analysis of the distribution of OXTR, and other peptides receptors, in the brain can contribute to understand its function but also to predict reproductive and social strategies of species.

Comparing vasopressin and oxytocin fiber and receptor density patterns in the social behavior neural network: Implications for cross-system signaling

Vasopressin (AVP) and oxytocin (OXT) regulate social behavior by binding to their canonical receptors, the vasopressin V1a receptor (V1aR) and oxytocin receptor (OTR), respectively. Recent studies suggest that these neuropeptides may also signal via each other's receptors. The extent to which such cross-system signaling occurs likely depends on anatomical overlap between AVP/OXT fibers and V1aR/OTR expression. By comparing AVP/OXT fiber densities with V1aR/OTR binding densities throughout the rat social behavior neural network (SBNN), we propose the potential for cross-system signaling in four regions: the medial amygdala (MeA), bed nucleus of the stria terminalis (BNSTp), medial preoptic area, and periaqueductal grey. We also discuss possible implications of corresponding sex (higher in males versus females) and age (higher in adults versus juveniles) differences in AVP fiber and OTR binding densities in the MeA and BNSTp. Overall, this review reveals the need to unravel the consequences of potential cross-system signaling between AVP and OXT systems in the SBNN for the regulation of social behavior.

High-yielding 18F radiosynthesis of a novel oxytocin receptor tracer, a probe for nose-to-brain oxytocin uptake in vivo

A novel Al18F labelled peptide tracer for PET imaging of oxytocin receptor has been accessed through a high radiochemical yield approach. This tracer showed comparable affinity and higher selectivity and stability compared to oxytocin, and was used to demonstrate direct nose-to-brain uptake following intranasal administration, a common yet controversial delivery route for oxytocin-based therapeutics.

The Oxytocin Receptor: From Intracellular Signaling to Behavior

The many facets of the oxytocin (OXT) system of the brain and periphery elicited nearly 25,000 publications since 1930 (see FIGURE 1 , as listed in PubMed), which revealed central roles for OXT and its receptor (OXTR) in reproduction, and social and emotional behaviors in animal and human studies focusing on mental and physical health and disease. In this review, we discuss the mechanisms of OXT expression and release, expression and binding of the OXTR in brain and periphery, OXTR-coupled signaling cascades, and their involvement in behavioral outcomes to assemble a comprehensive picture of the central and peripheral OXT system. Traditionally known for its role in milk let-down and uterine contraction during labor, OXT also has implications in physiological, and also behavioral, aspects of reproduction, such as sexual and maternal behaviors and pair bonding, but also anxiety, trust, sociability, food intake, or even drug abuse. The many facets of OXT are, on a molecular basis, brought about by a single receptor. The OXTR, a 7-transmembrane G protein-coupled receptor capable of binding to either Gαior Gαqproteins, activates a set of signaling cascades, such as the MAPK, PKC, PLC, or CaMK pathways, which converge on transcription factors like CREB or MEF-2. The cellular response to OXT includes regulation of neurite outgrowth, cellular viability, and increased survival. OXTergic projections in the brain represent anxiety and stress-regulating circuits connecting the paraventricular nucleus of the hypothalamus, amygdala, bed nucleus of the stria terminalis, or the medial prefrontal cortex. Which OXT-induced patterns finally alter the behavior of an animal or a human being is still poorly understood, and studying those OXTR-coupled signaling cascades is one initial step toward a better understanding of the molecular background of those behavioral effects.

Hippocampal GABAergic Inhibitory Interneurons

In the hippocampus GABAergic local circuit inhibitory interneurons represent only ~10-15% of the total neuronal population; however, their remarkable anatomical and physiological diversity allows them to regulate virtually all aspects of cellular and circuit function. Here we provide an overview of the current state of the field of interneuron research, focusing largely on the hippocampus. We discuss recent advances related to the various cell types, including their development and maturation, expression of subtype-specific voltage- and ligand-gated channels, and their roles in network oscillations. We also discuss recent technological advances and approaches that have permitted high-resolution, subtype-specific examination of their roles in numerous neural circuit disorders and the emerging therapeutic strategies to ameliorate such pathophysiological conditions. The ultimate goal of this review is not only to provide a touchstone for the current state of the field, but to help pave the way for future research by highlighting where gaps in our knowledge exist and how a complete appreciation of their roles will aid in future therapeutic strategies.

Radiosynthesis and evaluation of 1-substituted 3-(2,3-dihydro-1H-inden-2-yl)-6-(1-ethylpropyl)-(3R,6R)-2,5-piperazinedione derivatives as PET tracers for imaging the central oxytocinergic system

Oxytocin is known to be implicated in a variety of functions, such as learning, stress, anxiety, feeding, and pain perception. Oxytocin is also important for social memory and attachment, human bonding, sexual and maternal behaviour, and aggression. Human disorders characterized by aberrant social interactions, such as autism and schizophrenia, may also involve abnormal oxytocin levels. GSK712043, GSK711320, and GSK664004, three antagonists exhibiting subnanomolar affinity for the human oxytocin receptor (hOTR) and high selectivity over vasopressin receptors were successfully labelled with carbon-11 with suitable yields (0.5-1GBq @EOS), high molar activity (275-700 GBq/μmol), and radiochemical purities. The in vivo regional uptake of these radiotracers was determined in porcine brain. [¹¹ C]GSK711320 baseline scan showed no significant brain uptake, and limited initial uptake was observed following administration of [¹¹ C]GSK712043 or [¹¹ C]GSK664004. The [¹¹ C]GSK712043 and [¹¹ C]GSK664004 kinetics were slow and peaked at around 2%ID/L at 90 minutes post-injection. For both tracers, the distribution of activity was homogeneous throughout the brain. All the tracers showed high uptake in the pituitary gland, especially [¹¹ C]GSK711320; however, its uptake could not be blocked by pretreatment with the known OTR antagonist, L368,899. In vivo evaluation of these candidates demonstrated that they are not suitable as central OTR PET imaging agents.

High-Sugar, but Not High-Fat, Food Activates Supraoptic Nucleus Neurons in the Male Rat

Oxytocin is a potent anorexigen and is believed to have a role in satiety signaling. We developed rat models to study the activity of oxytocin neurons in response to voluntary consumption or oral gavage of foods using c-Fos immunohistochemistry and in vivo electrophysiology. Using c-Fos expression as an indirect marker of neural activation, we showed that the percentage of magnocellular oxytocin neurons expressing c-Fos increased with voluntary consumption of sweetened condensed milk (SCM). To model the effect of food in the stomach, we gavaged anesthetized rats with SCM. The percentage of supraoptic nucleus and paraventricular nucleus magnocellular oxytocin-immunoreactive neurons expressing c-Fos increased with SCM gavage but not with gastric distention. To further examine the activity of the supraoptic nucleus, we made in vivo electrophysiological recordings from SON neurons, where anesthetized rats were gavaged with SCM or single cream. Pharmacologically identified oxytocin neurons responded to SCM gavage with a linear, proportional, and sustained increase in firing rate, but cream gavage resulted in a transient reduction in firing rate. Blood glucose increased after SCM gavage but not cream gavage. Plasma osmolarity and plasma sodium were unchanged throughout. We show that in response to high-sugar, but not high-fat, food in the stomach, there is an increase in the activity of oxytocin neurons. This does not appear to be a consequence of stomach distention or changes in osmotic pressure. Our data suggest that the presence of specific foods with different macronutrient profiles in the stomach differentially regulates the activity of oxytocin neurons.

Oxytocin modulation of neural circuits for social behavior

Oxytocin is a hypothalamic neuropeptide that has gained attention for the effects on social behavior. Recent findings shed new light on the mechanisms of oxytocin in synaptic plasticity and adaptively modifying neural circuits for social interactions such as conspecific recognition, pair bonding, and maternal care. Here, we review several of these newer studies on oxytocin in the context of previous findings, with an emphasis on social behavior and circuit plasticity in various brain regions shown to be enriched for oxytocin receptors. We provide a framework that highlights current circuit-level mechanisms underlying the widespread action of oxytocin. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 169-189, 2017.

Awake canine fMRI predicts dogs' preference for praise vs food

Dogs are hypersocial with humans, and their integration into human social ecology makes dogs a unique model for studying cross-species social bonding. However, the proximal neural mechanisms driving dog–human social interaction are unknown. We used functional magnetic resonance imaging in 15 awake dogs to probe the neural basis for their preferences for social interaction and food reward. In a first experiment, we used the ventral caudate as a measure of intrinsic reward value and compared activation to conditioned stimuli that predicted food, praise or nothing. Relative to the control stimulus, the caudate was significantly more active to the reward-predicting stimuli and showed roughly equal or greater activation to praise vs food in 13 of 15 dogs. To confirm that these differences were driven by the intrinsic value of social praise, we performed a second imaging experiment in which the praise was withheld on a subset of trials. The difference in caudate activation to the receipt of praise, relative to its withholding, was strongly correlated with the differential activation to the conditioned stimuli in the first experiment. In a third experiment, we performed an out-of-scanner choice task in which the dog repeatedly selected food or owner in a Y-maze. The relative caudate activation to food- and praise-predicting stimuli in Experiment 1 was a strong predictor of each dog’s sequence of choices in the Y-maze. Analogous to similar neuroimaging studies of individual differences in human social reward, our findings demonstrate a neural mechanism for preference in domestic dogs that is stable within, but variable between, individuals. Moreover, the individual differences in the caudate responses indicate the potentially higher value of social than food reward for some dogs and may help to explain the apparent efficacy of social interaction in dog training.

Oxytocin in hypothalamic supraoptic nucleus is transferred to the caudate nucleus to influence pain modulation

Oxytocin (OXT), which is synthesized and secreted in the hypothalamic supraoptic nucleus (SON), is the most important bioactive substance in SON regulating pain process. Our previous study has pointed that OXT in the caudate nucleus (CdN) plays a role in pain modulation. The communication was designed to investigate the source of OXT in the rat CdN during pain process using the methods of push-pull perfusion and radioimmunoassay. The results showed that (1) pain stimulation increased the OXT concentration in the CdN perfusion liquid; (2) SON cauterization inhibited the increase of OXT concentration in CdN perfusion liquid induced by the pain stimulation, which role in both sides of SON cauterization was stronger than that in one side of SON cauterization; and (3) SON microinjection of l-glutamate sodium, which excited the SON neurons, increased OXT concentration in the CdN perfusion liquid. The data suggested that OXT in the CdN was influenced by SON during pain process, i.e., OXT in the SON might be transferred to the CdN to influence pain modulation.

Oxytocin Enhances Social Recognition by Modulating Cortical Control of Early Olfactory Processing

Oxytocin promotes social interactions and recognition of conspecifics that rely on olfaction in most species. The circuit mechanisms through which oxytocin modifies olfactory processing are incompletely understood. Here, we observed that optogenetically induced oxytocin release enhanced olfactory exploration and same-sex recognition of adult rats. Consistent with oxytocin's function in the anterior olfactory cortex, particularly in social cue processing, region-selective receptor deletion impaired social recognition but left odor discrimination and recognition intact outside a social context. Oxytocin transiently increased the drive of the anterior olfactory cortex projecting to olfactory bulb interneurons. Cortical top-down recruitment of interneurons dynamically enhanced the inhibitory input to olfactory bulb projection neurons and increased the signal-to-noise of their output. In summary, oxytocin generates states for optimized information extraction in an early cortical top-down network that is required for social interactions with potential implications for sensory processing deficits in autism spectrum disorders.

Comparative Perspectives on Oxytocin and Vasopressin Receptor Research in Rodents and Primates: Translational Implications

In the last several decades, sophisticated experimental techniques have been used to determine the neurobiology of the oxytocin and vasopressin systems in rodents. Using a suite of methodologies, including electrophysiology, site-specific selective pharmacology, receptor autoradiography, in vivo microdialysis, and genetic and optogenetic manipulations, we have gained unprecedented knowledge about how these neuropeptides engage neural circuits to regulate behaviour, particularly social behaviour. Based on this foundation of information from rodent studies, we have started generating new hypotheses and frameworks about how the oxytocin and vasopressin systems could be acting in humans to influence social cognition. However, despite the recent inundation of publications using intranasal oxytocin in humans, we still know very little about the neurophysiology of the oxytocin system in primates more broadly. Furthermore, the design and analysis of these human studies have remained largely uninformed of the potential neurobiological mechanisms underlying their findings. Although the methods available for studying the oxytocin and vasopressin systems in humans are incredibly limited as a result of practical and ethical considerations, there is great potential to fill the gaps in our knowledge by developing better nonhuman primate models of social functioning. Behavioural pharmacology and receptor autoradiography have been used to study the oxytocin and vasopressin systems in nonhuman primates, and there is now great potential to broaden our understanding of the neurobiology of these systems. In this review, we discuss comparative findings in receptor distributions in rodents and primates, with perspectives on the functionality of conserved regions of expression in these distinct mammalian clades. We also identify specific ways that established technologies can be used to answer basic research questions in primates. Finally, we highlight areas of future research in nonhuman primates that are experimentally poised to yield critical insights into the anatomy, physiology and behavioural effects of the oxytocin system, given its remarkable translational potential.

Chronic Methamphetamine Self-Administration Dysregulates Oxytocin Plasma Levels and Oxytocin Receptor Fibre Density in the Nucleus Accumbens Core and Subthalamic Nucleus of the Rat

The neuropeptide oxytocin attenuates reward and abuse for the psychostimulant methamphetamine (METH). Recent findings have implicated the nucleus accumbens (NAc) core and subthalamic nucleus (STh) in oxytocin modulation of acute METH reward and relapse to METH-seeking behaviour. Surprisingly, the oxytocin receptor (OTR) is only modestly involved in both regions in oxytocin attenuation of METH-primed reinstatement. Coupled with the limited investigation of the role of the OTR in psychostimulant-induced behaviours, we primarily investigated whether there are cellular changes to the OTR in the NAc core and STh, as well as changes to oxytocin plasma levels, after chronic METH i.v. self-administration (IVSA) and after extinction of drug-taking. An additional aim was to examine whether changes to central corticotrophin-releasing factor (CRF) and plasma corticosterone levels were also apparent because of the interaction of oxytocin with stress-regulatory mechanisms. Male Sprague-Dawley rats were trained to lever press for i.v. METH (0.1 mg/kg/infusion) under a fixed-ratio 1 schedule or received yoked saline infusions during 2-h sessions for 20 days. An additional cohort of rats underwent behavioural extinction for 15 days after METH IVSA. Subsequent to the last day of IVSA or extinction, blood plasma was collected for enzyme immunoassay, and immunofluorescence was conducted on NAc core and STh coronal sections. Rats that self-administered METH had higher oxytocin plasma levels, and decreased OTR-immunoreactive (-IR) fibres in the NAc core than yoked controls. In animals that self-administered METH and underwent extinction, oxytocin plasma levels remained elevated, OTR-IR fibre density increased in the STh, and a trend towards normalisation of OTR-IR fibre density was evident in the NAc core. CRF-IR fibre density in both brain regions and corticosterone plasma levels did not change across treatment groups. These findings demonstrate that oxytocin systems, both centrally within the NAc core and STh, as well as peripherally through plasma measures, are dysregulated after METH abuse.

Oxytocin in the nucleus accumbens core reduces reinstatement of methamphetamine-seeking behaviour in rats

The psychostimulant methamphetamine (METH) is an addictive illicit drug. Systemic administration of the neuropeptide oxytocin modulates METH-related reward and METH-seeking behaviour. Recent findings demonstrated a reduction in METH-induced reward by oxytocin administration into the nucleus accumbens (NAc) core. It is not known, however, if oxytocin acts in this region to reduce relapse to METH-seeking behaviour. Using the drug reinstatement paradigm in rats experienced at METH self-administration, we aimed to determine whether oxytocin pre-treatment within the NAc core would reduce relapse to METH use and if this could be reversed by the co-administration of the oxytocin receptor (OTR) antagonist desGly-NH2,d(CH2)5[D-Tyr2,Thr4]OVT. Male Sprague-Dawley rats underwent surgery to implant an intravenous jugular vein catheter and bilateral microinjection cannulae in the NAc core. Rats were then trained to self-administer intravenous METH (0.1 mg/kg/infusion) by lever press during 2-hour fixed ratio 1 scheduled sessions for 20 days. Following extinction of lever press activity, the effect of microinjecting saline, oxytocin (0.5 pmol, 1.5 pmol, 4.5 pmol) or co-administration of oxytocin (1.5 pmol) and desGly-NH2,d(CH2)5[D-Tyr2,Thr4]OVT (1 nmol, 3 nmol) in the NAc core (500 nl/side) was examined on METH-primed (1 mg/kg, i.p.) reinstatement of drug-seeking behaviour. Our results showed oxytocin directly administered into the NAc core decreased METH-primed reinstatement in a dose-dependent manner. Co-administration of the selective OTR antagonist did not specifically reverse the inhibitory effects of oxytocin on METH priming, suggesting mediation by receptors other than the OTR. These findings highlight an important modulatory effect of oxytocin in the NAc core on relapse to METH seeking.

Oxytocin Protects Hippocampal Memory and Plasticity from Uncontrollable Stress

The hippocampus is vulnerable to uncontrollable stress and is enriched with oxytocin receptors, but their interactive influences on hippocampal functioning are unknown. This study aimed to determine the effects of intranasal oxytocin administration on stress-induced alterations in synaptic plasticity and spatial memory in male rats. While vehicle-administered stressed rats showed impairment in long-term potentiation, enhancement in long-term depression, and weakened spatial memory, these changes were not observed in oxytocin-administered stressed rats. To reveal the potential signaling mechanism mediating these effects, levels of phosphorylated extracellular signal-regulated kinases (pERK) in the hippocampus was examined. Western blotting showed that oxytocin treatment blocked stress-induced alterations of pERK. Additionally, the oxytocin receptor antagonist L-368,899 inhibited the oxytocin’s protective effects on hippocampal memory to stress. Thus, intranasal administration of oxytocin reduced stress effects on hippocampal synaptic plasticity and memory in rats via acting on oxytocin receptors and regulating ERK activity. This study suggests that exogenous oxytocin may be a therapeutically effective means to counter the detrimental neurocognitive effects of stress.

Distinct BOLD Activation Profiles Following Central and Peripheral Oxytocin Administration in Awake Rats

A growing body of literature has suggested that intranasal oxytocin (OT) or other systemic routes of administration can alter prosocial behavior, presumably by directly activating OT sensitive neural circuits in the brain. Yet there is no clear evidence that OT given peripherally can cross the blood-brain barrier at levels sufficient to engage the OT receptor. To address this issue we examined changes in blood oxygen level-dependent (BOLD) signal intensity in response to peripheral OT injections (0.1, 0.5, or 2.5 mg/kg) during functional magnetic resonance imaging (fMRI) in awake rats imaged at 7.0 T. These data were compared to OT (1 μg/5 μl) given directly to the brain via the lateral cerebroventricle. Using a 3D annotated MRI atlas of the rat brain segmented into 171 brain areas and computational analysis, we reconstructed the distributed integrated neural circuits identified with BOLD fMRI following central and peripheral OT. Both routes of administration caused significant changes in BOLD signal within the first 10 min of administration. As expected, central OT activated a majority of brain areas known to express a high density of OT receptors, e.g., lateral septum, subiculum, shell of the accumbens, bed nucleus of the stria terminalis. This profile of activation was not matched by peripheral OT. The change in BOLD signal to peripheral OT did not show any discernible dose-response. Interestingly, peripheral OT affected all subdivisions of the olfactory bulb, in addition to the cerebellum and several brainstem areas relevant to the autonomic nervous system, including the solitary tract nucleus. The results from this imaging study do not support a direct central action of peripheral OT on the brain. Instead, the patterns of brain activity suggest that peripheral OT may interact at the level of the olfactory bulb and through sensory afferents from the autonomic nervous system to influence brain activity.

Ontogenesis of oxytocin pathways in the mammalian brain: late maturation and psychosocial disorders

Oxytocin (OT), the main neuropeptide of sociality, is expressed in neurons exclusively localized in the hypothalamus. During the last decade, a plethora of neuroendocrine, metabolic, autonomic and behavioral effects of OT has been reported. In the urgency to find treatments to syndromes as invalidating as autism, many clinical trials have been launched in which OT is administered to patients, including adolescents and children. However, the impact of OT on the developing brain and in particular on the embryonic and early postnatal maturation of OT neurons, has been only poorly investigated. In the present review we summarize available (although limited) literature on general features of ontogenetic transformation of the OT system, including determination, migration and differentiation of OT neurons. Next, we discuss trajectories of OT receptors (OTR) in the perinatal period. Furthermore, we provide evidence that early alterations, from birth, in the central OT system lead to severe neurodevelopmental diseases such as feeding deficit in infancy and severe defects in social behavior in adulthood, as described in Prader-Willi syndrome (PWS). Our review intends to propose a hypothesis about developmental dynamics of central OT pathways, which are essential for survival right after birth and for the acquisition of social skills later on. A better understanding of the embryonic and early postnatal maturation of the OT system may lead to better OT-based treatments in PWS or autism.

Inhibition of ejaculation by the non-peptide oxytocin receptor antagonist GSK557296: a multi-level site of action

BACKGROUND AND PURPOSE

Oxytocin (OT) plays a major role in the control of male sexual responses. Notably, blockade of OT receptors has been reported to inhibit ejaculation in animals. The study aimed to investigate the action of a highly selective, non-peptide OT antagonist GSK557296 in a model of pharmacologically induced ejaculation in anaesthetized rats. The site of action was assessed by investigating different delivery routes for this compound.

EXPERIMENTAL APPROACH

Urethane-anaesthetized Wistar rats were implanted with a cerebral ventricle cannula for i.c.v. injections or with a subdural catheter for intrathecal (i.t.) GSK557296 injections. Occurrence of ejaculation was assessed following i.v. 7-hydroxy-2-(di-N-propylamino)tetralin (7-OH-DPAT), a dopamine D3 receptor agonist. In addition, seminal vesicle pressures (SVP) and bulbospongiosus muscle (BS) EMG were recorded as physiological markers of emission and expulsion phases of ejaculation respectively.

KEY RESULTS

Highest i.v. GSK557296 dose reduced occurrence of ejaculation and increases in SVP but had no effect on BS-EMG. I.c.v. GSK557296 dose dependently inhibited ejaculation, increases in SVP and BS contractions. At spinal thoracic level, GSK557296 dose dependently inhibited ejaculation and increases in SVP but BS-EMG was impaired only with the highest dose. When delivered at lumbar level, GSK557296 dose dependently inhibited ejaculation, increases in SVP and BS contractions.

CONCLUSIONS AND IMPLICATIONS

In the 7-OH-DPAT-induced ejaculation model, GSK557296 acts peripherally and centrally to inhibit ejaculation with different modalities. Blockade of brain OT receptors seems to be the most effective mechanism of action. Targeting central OT receptors with highly selective antagonist seems a promising approach for the treatment of premature ejaculation.

Overexpression of oxytocin receptors in the hypothalamic PVN increases baroreceptor reflex sensitivity and buffers BP variability in conscious rats

BACKGROUND AND PURPOSE

The paraventricular nucleus (PVN) of the hypothalamus is an important integrative site for neuroendocrine control of the circulation. We investigated the role of oxytocin receptors (OT receptors) in PVN in cardiovascular homeostasis.

EXPERIMENTAL APPROACH

Experiments were performed in conscious male Wistar rats equipped with a radiotelemetric device. The PVN was unilaterally co-transfected with an adenoviral vector (Ad), engineered to overexpress OT receptors, and an enhanced green fluorescent protein (eGFP) tag. Control groups: PVN was transfected with an Ad expressing eGFP alone or untransfected, sham rats (Wt). Recordings were obtained without and with selective blockade of OT receptors (OTX), during both baseline and stressful conditions. Baroreceptor reflex sensitivity (BRS) and cardiovascular short-term variability were evaluated using the sequence method and spectral methodology respectively.

KEY RESULTS

Under baseline conditions, rats overexpressing OT receptors (OTR) exhibited enhanced BRS and reduced BP variability compared to control groups. Exposure to stress increased BP, BP variability and HR in all rats. In control groups, but not in OTR rats, BRS decreased during stress. Pretreatment of OTR rats with OTX reduced BRS and enhanced BP and HR variability under baseline and stressful conditions. Pretreatment of Wt rats with OTX, reduced BRS and increased BP variability under baseline and stressful conditions, but only increased HR variability during stress.

CONCLUSIONS AND IMPLICATIONS

OT receptors in PVN are involved in tonic neural control of BRS and cardiovascular short-term variability. The failure of this mechanism could critically contribute to the loss of autonomic control in cardiovascular disease.

Decoding ventromedial hypothalamic neural activity during male mouse aggression

The ventromedial hypothalamus, ventrolateral area (VMHvl) was identified recently as a critical locus for inter-male aggression. Optogenetic stimulation of VMHvl in male mice evokes attack toward conspecifics and inactivation of the region inhibits natural aggression, yet very little is known about its underlying neural activity. To understand its role in promoting aggression, we recorded and analyzed neural activity in the VMHvl in response to a wide range of social and nonsocial stimuli. Although response profiles of VMHvl neurons are complex and heterogeneous, we identified a subpopulation of neurons that respond maximally during investigation and attack of male conspecific mice and during investigation of a source of male mouse urine. These "male responsive" neurons in the VMHvl are tuned to both the inter-male distance and the animal's velocity during attack. Additionally, VMHvl activity predicts several parameters of future aggressive action, including the latency and duration of the next attack. Linear regression analysis further demonstrates that aggression-specific parameters, such as distance, movement velocity, and attack latency, can model ongoing VMHvl activity fluctuation during inter-male encounters. These results represent the first effort to understand the hypothalamic neural activity during social behaviors using quantitative tools and suggest an important role for the VMHvl in encoding movement, sensory, and motivation-related signals.

Oxytocin and vasopressin modulation of the neural correlates of motivation and emotion: results from functional MRI studies in awake rats

Oxytocin and vasopressin modulate a range of species typical behavioral functions that include social recognition, maternal-infant attachment, and modulation of memory, offensive aggression, defensive fear reactions, and reward seeking. We have employed novel functional magnetic resonance mapping techniques in awake rats to explore the roles of these neuropeptides in the maternal and non-maternal brain. Results from the functional neuroimaging studies that are summarized here have directly and indirectly confirmed and supported previous findings. Oxytocin is released within the lactating rat brain during suckling stimulation and activates specific subcortical networks in the maternal brain. Both vasopressin and oxytocin modulate brain regions involved unconditioned fear, processing of social stimuli and the expression of agonistic behaviors. Across studies there are relatively consistent brain networks associated with internal motivational drives and emotional states that are modulated by oxytocin and vasopressin. This article is part of a Special Issue entitled Oxytocin and Social Behav.

Neuropeptides and central control of sexual behaviour from the past to the present: a review

Of the numerous neuropeptides identified in the central nervous system, only a few are involved in the control of sexual behaviour. Among these, the most studied are oxytocin, adrenocorticotropin, α-melanocyte stimulating hormone and opioid peptides. While opioid peptides inhibit sexual performance, the others facilitate sexual behaviour in most of the species studied so far (rats, mice, monkeys and humans). However, evidence for a sexual role of gonadotropin-releasing hormone, corticotropin releasing factor, neuropeptide Y, galanin and galanin-like peptide, cholecystokinin, substance P, vasoactive intestinal peptide, vasopressin, angiotensin II, hypocretins/orexins and VGF-derived peptides are also available. Corticotropin releasing factor, neuropeptide Y, cholecystokinin, vasopressin and angiotensin II inhibit, while substance P, vasoactive intestinal peptide, hypocretins/orexins and some VGF-derived peptide facilitate sexual behaviour. Neuropeptides influence sexual behaviour by acting mainly in the hypothalamic nuclei (i.e., lateral hypothalamus, paraventricular nucleus, ventromedial nucleus, arcuate nucleus), in the medial preoptic area and in the spinal cord. However, it is often unclear whether neuropeptides influence the anticipatory phase (sexual arousal and/or motivation) or the consummatory phase (performance) of sexual behaviour, except in a few cases (e.g., opioid peptides and oxytocin). Unfortunately, scarce information has been added in the last 15 years on the neural mechanisms by which neuropeptides influence sexual behaviour, most studied neuropeptides apart. This may be due to a decreased interest of researchers on neuropeptides and sexual behaviour or on sexual behaviour in general. Such a decrease may be related to the discovery of orally effective, locally acting type V phosphodiesterase inhibitors for the therapy of erectile dysfunction.

Oxytocin, motivation and the role of dopamine

The hypothalamic neuropeptide oxytocin has drawn the attention of scientists for more than a century. The understanding of the function of oxytocin has expanded dramatically over the years from a simple peptide adept at inducing uterine contractions and milk ejection to a complex neuromodulator with a capacity to shape human social behavior. Decades of research have outlined oxytocin's ability to enhance intricate social activities ranging from pair bonding, sexual activity, affiliative preferences, and parental behaviors. The precise neural mechanisms underlying oxytocin's influence on such behaviors have just begun to be understood. Research suggests that oxytocin interacts closely with the neural pathways responsible for processing motivationally relevant stimuli. In particular, oxytocin appears to impact dopaminergic activity within the mesocorticolimbic dopamine system, which is crucial not only for reward and motivated behavior but also for the expression of affiliative behaviors. Though most of the work performed in this area has been done using animal models, several neuroimaging studies suggest similar relationships may be observed in humans. In order to introduce this topic further, this paper will review the recent evidence that oxytocin may exert some of its social-behavioral effects through its impact on motivational networks.

Intranasal administration of oxytocin: behavioral and clinical effects, a review

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The mechanisms behind the effects of IN-applied substances need more attention.

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The mechanisms involved in the brain-distribution of IN-OT are completely unexplored.

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The possibly cascading effects of IN-OT on the intrinsic OT-system require serious investigation.

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IN-OT induces clear and specific changes in neural activation.

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IN-OT is a promising approach to treat certain clinical symptoms.

The intranasal (IN-) administration of substances is attracting attention from scientists as well as pharmaceutical companies. The effects are surprisingly fast and specific. The present review explores our current knowledge about the routes of access to the cranial cavity. ‘Direct-access-pathways’ from the nasal cavity have been described but many additional experiments are needed to answer a variety of open questions regarding anatomy and physiology.

Among the IN-applied substances oxytocin (OT) has an extensive history. Originally applied in women for its physiological effects related to lactation and parturition, over the last decade most studies focused on their behavioral ‘prosocial’ effects: from social relations and ‘trust’ to treatment of ‘autism’.

Only very recently in a microdialysis study in rats and mice, the ‘direct-nose-brain-pathways’ of IN-OT have been investigated directly, implying that we are strongly dependent on results obtained from other IN-applied substances. Especially the possibility that IN-OT activates the ‘intrinsic’ OT-system in the hypothalamus as well needs further clarification.

We conclude that IN-OT administration may be a promising approach to influence human communication but that the existing lack of information about the neural and physiological mechanisms involved is a serious problem for the proper understanding and interpretation of the observed effects.

Effects of noradrenergic alpha-2 receptor antagonism or noradrenergic lesions in the ventral bed nucleus of the stria terminalis and medial preoptic area on maternal care in female rats

RATIONALE

Maternal behavior in laboratory rats requires a network of brain structures including the ventral bed nucleus of the stria terminalis (BSTv) and medial preoptic area (mPOA). Neurotransmitter systems in the BSTv and mPOA influencing maternal behaviors are not well understood, although norepinephrine is an excellent candidate because the BSTv contains the densest noradrenergic fiber plexus in the forebrain and norepinephrine in the mPOA is known to influence other female reproductive functions.

OBJECTIVES

We hypothesized that downregulated noradrenergic activity in the BSTv and mPOA is necessary for mothering.

METHODS

Postpartum mother-litter interactions were observed after BSTv infusion of yohimbine (an α2 autoreceptor antagonist that increases norepinephrine release), and after BSTv or mPOA infusion of the more selective α2 autoreceptor antagonist idazoxan. Lastly, noradrenergic input to the BSTv/mPOA was selectively lesioned in nulliparous rats with anti-DBH-saporin to determine if this would facilitate mothering.

RESULTS

BSTv yohimbine almost abolished retrieval of pups but did not significantly affect dams' ability to initiate contact, lick, or nurse them. BSTv idazoxan disrupted retrieval somewhat less than yohimbine, but significantly reduced nursing. mPOA idazoxan impaired retrieval more severely than that found after BSTv infusion. Anti-DBH-saporin almost eliminated noradrenergic terminals in the BSTv and reduced them by over 60% in the mPOA, but did not promote maternal responding. It also did not affect females' anxiety-related behavior.

CONCLUSIONS

Downregulated noradrenergic activity in the BSTv and mPOA is necessary for postpartum maternal behavior in rats, but eliminating this system alone is insufficient to promote maternal behaviors in nulliparous females.

Oxytocin-induced yawning: sites of action in the brain and interaction with mesolimbic/mesocortical and incertohypothalamic dopaminergic neurons in male rats

Oxytocin (80 ng) induces yawning when injected into the caudal part of the ventral tegmental area, the hippocampal ventral subiculum and the posteromedial nucleus of the amygdala of male rats. The behavioural response occurred concomitantly with an increase in the concentration of extracellular dopamine and its main metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the dialysate obtained from the shell of the nucleus accumbens and of the prelimbic medial prefrontal cortex by means of intracerebral microdialysis. Both oxytocin responses were significantly reduced by d(CH₂)₅Tyr(Me)²-Orn⁸-vasotocin, a selective oxytocin receptor antagonist, injected in the above brain areas 15 min before oxytocin. Similar results were obtained by activating central oxytocinergic neurons originating in the paraventricular nucleus of the hypothalamus and projecting to the ventral tegmental area, the hippocampus and the amygdala, with the dopamine agonist apomorphine given at a dose that induces yawning when injected into the paraventricular nucleus. Since oxytocin is considered a key regulator of emotional and social reward that enhances amygdala-dependent, socially reinforced learning and emotional empathy, mesolimbic and mesocortical dopamine neurons play a key role in motivation and reward, and yawning in mammals is considered a primitive, unconscious form of empathy, the present results support the hypothesis that oxytocinergic neurons originating in the paraventricular nucleus of the hypothalamus and projecting to the above brain areas and mesolimbic and mesocortical dopaminergic neurons participate in the complex neural circuits that play a role in the above mentioned functions.

Oxytocin regulates changes of extracellular glutamate and GABA levels induced by methamphetamine in the mouse brain

Oxytocin (OT), a neurohypophyseal neuropeptide, affects adaptive processes of the central nervous system. In the present study, we investigated the effects of OT on extracellular levels of glutamate (Glu) and γ-aminobutyric acid (GABA) induced by methamphetamine (MAP) in the medial prefrontal cortex (mPFC) and dorsal hippocampus (DHC) of freely moving mice, using in vivo microdialysis coupled to high-performance liquid chromatography and fluorescence detection. The results showed that OT had no effect on basal Glu levels, but attenuated MAP-induced Glu increase in the mPFC and decrease in the DHC. OT increased the basal levels of extracellular GABA in mPFC and DHC of mice, and inhibited the MAP-induced GABA decrease in DHC. Western blot results indicated that OT significantly inhibited the increased glutamatergic receptor (NR1 subunit) levels in the PFC after acute MAP administration, whereas OT further enhanced the elevated levels of glutamatergic transporter (GLT1) induced by MAP in the hippocampus of mice. Atosiban, a selective inhibitor of OT receptor, antagonized the effects of OT. The results provided the first neurochemical evidence that OT, which exerted its action via its receptor, decreased Glu release induced by MAP, and attenuated the changes in glutamatergic neurotransmission partially via regulation of NR1 and GLT1 expression. OT-induced extracellular GABA increase also suggests that OT acts potentially as an inhibitory neuromodulator in mPFC and DHC of mice.

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.

Oxytocin in the rat caudate nucleus influences pain modulation

Our previous studies have demonstrated that oxytocin (OXT) in the central nervous system plays a role in pain modulation. Many studies have found that caudate nucleus (CdN) enriches OXT and OXT receptors by the methods of historadioautograph and gene expression. The communication was designed to investigate OXT effect in the rat CdN on pain modulation. The results showed that (1) intra-CdN microinjection of OXT receptor antagonist, desGly-NH(2), d(CH(2))(5)[D-Tyr(2), Thr-sup-4]OVT decreased the pain threshold, whereas the local administration of OXT increased the pain threshold in a dose-dependent manner; (2) OXT receptor antagonist can attenuate the analgesic role induced intra-CdN administration of OXT; and (3) pain stimulation could increase OXT concentration in the CdN perfusion liquid. The data suggested that OXT in the CdN was involved in this pain process via OXT receptors.

Mechanisms of penile erection and basis for pharmacological treatment of erectile dysfunction

Erection is basically a spinal reflex that can be initiated by recruitment of penile afferents, both autonomic and somatic, and supraspinal influences from visual, olfactory, and imaginary stimuli. Several central transmitters are involved in the erectile control. Dopamine, acetylcholine, nitric oxide (NO), and peptides, such as oxytocin and adrenocorticotropin/α-melanocyte-stimulating hormone, have a facilitatory role, whereas serotonin may be either facilitatory or inhibitory, and enkephalins are inhibitory. The balance between contractant and relaxant factors controls the degree of contraction of the smooth muscle of the corpora cavernosa (CC) and determines the functional state of the penis. Noradrenaline contracts both CC and penile vessels via stimulation of α₁-adrenoceptors. Neurogenic NO is considered the most important factor for relaxation of penile vessels and CC. The role of other mediators, released from nerves or endothelium, has not been definitely established. Erectile dysfunction (ED), defined as the "inability to achieve or maintain an erection adequate for sexual satisfaction," may have multiple causes and can be classified as psychogenic, vasculogenic or organic, neurologic, and endocrinologic. Many patients with ED respond well to the pharmacological treatments that are currently available, but there are still groups of patients in whom the response is unsatisfactory. The drugs used are able to substitute, partially or completely, the malfunctioning endogenous mechanisms that control penile erection. Most drugs have a direct action on penile tissue facilitating penile smooth muscle relaxation, including oral phosphodiesterase inhibitors and intracavernosal injections of prostaglandin E₁. Irrespective of the underlying cause, these drugs are effective in the majority of cases. Drugs with a central site of action have so far not been very successful. There is a need for therapeutic alternatives. This requires identification of new therapeutic targets and design of new approaches. Research in the field is expanding, and several promising new targets for future drugs have been identified.

Ovarian hormone-induced reorganization of oxytocin-labeled dendrites and synapses lateral to the hypothalamic ventromedial nucleus in female rats

Central oxytocin (OT) modulates many social behaviors, including female rat sexual receptivity, quantified as the copulatory stance known as lordosis. The expression of the lordosis response is modulated by OT action in the ventromedial nucleus of the hypothalamus (VMH), as demonstrated by behavioral pharmacology experiments. However, the subcellular localization of OT in this brain region has been unclear. We tested the hypothesis that ovarian hormones reorganize OT-labeled pre- or postsynaptic elements in the fiber complex lateral to the VMH by using immunoelectron microscopy. OT immunolabeling occurred in axonal boutons identified by the presence of small, clear synaptic vesicles and double labeling with the presynaptic markers synaptophysin and vesicular glutamate transporter 2. OT immunoreactivity also was observed in dendritic profiles, verified with double labeling for the dendrite-specific marker microtubule-associated protein 2. Ovarian hormones did not alter the density of axonal boutons; however, estradiol treatment reduced the density of dendritic profiles by 34%. This effect was reversed when progesterone was given subsequent to estradiol. The effect of estradiol treatment was specific to dendrites that lacked OT immunostaining; the density of OT-labeled dendritic profiles remained constant during estradiol treatment. With the estradiol-induced exit of non-OT-labeled dendritic profiles, the remaining OT-labeled dendritic profiles experienced an increase in their number of synaptic contacts. Thus, hormone treatments that mimic the 4-day rat estrous cycle provoke a chemically coded reorganization of dendrite innervation in the fiber plexus lateral to the VMH that may underlie the hormone-specific effect of OT on reproductive behavior.

Short- and long-term consequences of different early environmental conditions on central immunoreactive oxytocin and arginine vasopressin levels in male rats

Numerous studies have provided evidence for an important role for the neuropeptides oxytocin (OT) and arginine vasopressin (AVP) in establishment of social behaviour early in life, such as mother-pup interactions. However, there are few reports examining the consequences of early-life experiences on OT and AVP in male offspring. We have used the maternal separation (MS) model to study the effect of different early environmental conditions in rats. The purpose was to study OT and AVP in rats subjected to prolonged daily MS (360 min, MS360), short daily MS (15 min, MS15) and conventional animal facility rearing (AFR) during postnatal days 1-21. In addition, the influence of the presence or absence of littermates during MS, i.e. litter-wise (l) or individual (i) MS, was assessed. The immunoreactive (ir) peptide levels were measured in the hypothalamus, amygdala and pituitary gland of 3 and 10 weeks old male rats. Assessment in 3-week-old rats revealed that MS15 was associated with low ir OT levels in the hypothalamus and amygdala and high levels in the pituitary gland compared with the MS360 and AFR condition. In the amygdala, differences between groups were also detected in adulthood. MS studies commonly use either MS15 or AFR as a control for prolonged MS. The present results show differences in MS360 rats as compared to MS15 but not AFR rats. Consequently, comparisons between prolonged MS with either short periods of MS or AFR will generate divergent results, hence, making the outcome of MS difficult to compare between studies. Moreover, the different early environments had no effect on ir AVP levels. In conclusion, OT in the amygdala was most sensitive to MS. Besides both short- and long-term consequences, distinct effects were seen after litter and individual separation, respectively. We propose that environmentally induced alterations in OT transmission due to disrupted mother-pup interactions early in life may cause altered susceptibility to challenges later in life.

Dopamine and oxytocin interactions underlying behaviors: potential contributions to behavioral disorders

Dopamine is an important neuromodulator that exerts widespread effects on the central nervous system (CNS) function. Disruption in dopaminergic neurotransmission can have profound effects on mood and behavior and as such is known to be implicated in various neuropsychiatric behavioral disorders including autism and depression. The subsequent effects on other neurocircuitries due to dysregulated dopamine function have yet to be fully explored. Due to the marked social deficits observed in psychiatric patients, the neuropeptide, oxytocin is emerging as one particular neural substrate that may be influenced by the altered dopamine levels subserving neuropathologic-related behavioral diseases. Oxytocin has a substantial role in social attachment, affiliation and sexual behavior. More recently, it has emerged that disturbances in peripheral and central oxytocin levels have been detected in some patients with dopamine-dependent disorders. Thus, oxytocin is proposed to be a key neural substrate that interacts with central dopamine systems. In addition to psychosocial improvement, oxytocin has recently been implicated in mediating mesolimbic dopamine pathways during drug addiction and withdrawal. This bi-directional role of dopamine has also been implicated during some components of sexual behavior. This review will discuss evidence for the existence dopamine/oxytocin positive interaction in social behavioral paradigms and associated disorders such as sexual dysfunction, autism, addiction, anorexia/bulimia, and depression. Preliminary findings suggest that whilst further rigorous testing has to be conducted to establish a dopamine/oxytocin link in human disorders, animal models seem to indicate the existence of broad and integrated brain circuits where dopamine and oxytocin interactions at least in part mediate socio-affiliative behaviors. A profound disruption to these pathways is likely to underpin associated behavioral disorders. Central oxytocin pathways may serve as a potential therapeutic target to improve mood and socio-affiliative behaviors in patients with profound social deficits and/or drug addiction.

Oxytocin-messages via the cerebrospinal fluid: behavioral effects; a review

The cerebrospinal fluid (CSF) usually is considered as a protective 'nutrient and waste control' system for the brain. Recent findings suggest, however, that the composition of CSF is actively controlled and may play an influential role in the changes in brain activity, underlying different behavioral states. In the present review, we present an overview of available data concerning the release of oxytocin into the CSF, the location of the oxytocin-receptive brain areas and the behavioral effects of intracerebroventricular oxytocin. About 80% of the oxytocin-receptive areas are located close to the ventricular or subarachnoid CSF, including the hypothalamic 'Behavior Control Column' (L.W.Swanson, 2003). As a conclusion we suggest that 'CSF-oxytocin' contributes considerably to the non-synaptic communication processes involved in hypothalamic-, brainstem- and olfactory brain areas and behavioral states and that the flowing CSF is used as a 'broadcasting system' to send coordinated messages to a wide variety of nearby and distant brain areas.