Neurotransmitters and peptides: whispered secrets and public announcements

The central oxytocinergic system of the prairie vole

Oxytocin (OXT) is a peptide hormone and a neuropeptide that regulates various peripheral physiological processes and modulates behavioral responses in the central nervous system. While the humoral release occurs from the axons arriving at the median eminence, the neuropeptide is also released from oxytocinergic cell axons in various brain structures that contain its receptor, and from their dendrites in hypothalamic nuclei and potentially into the cerebrospinal fluid (CSF). Understanding oxytocin's complex functions requires the knowledge on patterns of oxytocinergic projections in relationship to its receptor (OXTR). This study provides the first comprehensive examination of the oxytocinergic system in the prairie vole (Microtus ochrogaster), an animal exhibiting social behaviors that mirror human social behaviors linked to oxytocinergic functioning. Using light and electron microscopy, we characterized the neuroanatomy of the oxytocinergic system in this species. OXT+ cell bodies were found primarily in the hypothalamus, and axons were densest in subcortical regions. Examination of the OXT+ fibers and their relationship to oxytocin receptor transcripts (Oxtr) revealed that except for some subcortical structures, the presence of axons was not correlated with the amount of Oxtr across the brain. Of particular interest, the cerebral cortex that had high expression of Oxtr transcripts contained little to no fibers. Electron microscopy is used to quantify dense cored vesicles (DCV) in OXT+ axons and to identify potential axonal release sites. The ependymal cells that line the ventricles were frequently permissive of DCV-containing OXT+ dendrites reaching the third ventricle. Our results highlight a mechanism in which oxytocin is released directly into the ventricles and circulates throughout the ventricular system, may serve as the primary source for oxytocin that binds to OXTR in the cerebral cortex.

The interplay of oxytocin and sex hormones

The neuropeptide oxytocin has historically been associated with reproduction and maternal behavior. However, more recent research has uncovered that oxytocin has a much wider range of roles in physiology and behavior. Despite the excitement surrounding potential therapeutical applications of intranasally administered oxytocin, the results of these intervention studies have been inconsistent. Various reasons for these mixed results have been proposed, which tend to focus on methodological issues, such as study design. While methodological issues are certainly important, emerging evidence suggests that the interaction between oxytocin and sex hormones may also account for these varied findings. To better understand the purpose and function of the interaction of oxytocin with sex hormones, with a focus on estrogens, progesterone, and testosterone, we conducted a comprehensive thematic review via four perspectives: evolutionary, developmental, mechanistic, and survival. Altogether, this synergistic approach highlights the critical function of sex hormone activity for accomplishing the diverse roles of oxytocin via the modulation of oxytocin release and oxytocin receptor activity, which is also likely to contribute to the heterogeneity of outcomes after oxytocin administration.

Neuropeptides affecting social behavior in mammals: Oxytocin

Oxytocin (OXT), a neuropeptide consisting of only nine amino acids, is synthesized in the paraventricular and supraoptic nuclei of the hypothalamus. Although OXT is best known for its role in lactation and parturition, recent research has shown that it also has a significant impact on social behaviors in mammals. However, a comprehensive review of this topic is still lacking. In this paper, we systematically reviewed the effects of OXT on social behavior in mammals. These effects of OXT from the perspective of five key behavioral dimensions were summarized: parental behavior, anxiety, aggression, attachment, and empathy. To date, researchers have agreed that OXT plays a positive regulatory role in a wide range of social behaviors, but there have been controversially reported results. In this review, we have provided a detailed panorama of the role of OXT in social behavior and, for the first time, delved into the underlying regulatory mechanisms, which may help better understand the multifaceted role of OXT. Levels of OXT in previous human studies were also summarized to provide insights for diagnosis of mental disorders.

Systematic review: pain, cognition, and cardioprotection-unpacking oxytocin's contributions in a sport context

Introduction

This systematic review investigates the interplay between oxytocin and exercise; in terms of analgesic, anti-inflammatory, pro-regenerative, and cardioprotective effects. Furthermore, by analyzing measurement methods, we aim to improve measurement validity and reliability.

Methods

Utilizing PRISMA, GRADE, and MECIR protocols, we examined five databases with a modified SPIDER search. Including studies on healthy participants, published within the last 20 years, based on keywords "oxytocin," "exercise" and "measurement," 690 studies were retrieved initially (455 unique records). After excluding studies of clinically identifiable diseases, and unpublished and reproduction-focused studies, 175 studies qualified for the narrative cross-thematic and structural analysis.

Results

The analysis resulted in five categories showing the reciprocal impact of oxytocin and exercise: Exercise (50), Physiology (63), Environment (27), Social Context (65), and Stress (49). Exercise-induced oxytocin could promote tissue regeneration, with 32 studies showing its analgesic and anti-inflammatory effects, while 14 studies discussed memory and cognition. Furthermore, empathy-associated OXTR rs53576 polymorphism might influence team sports performance. Since dietary habits and substance abuse can impact oxytocin secretion too, combining self-report tests and repeated salivary measurements may help achieve precision.

Discussion

Oxytocin's effect on fear extinction and social cognition might generate strategies for mental training, and technical, and tactical development in sports. Exercise-induced oxytocin can affect the amount of stress experienced by athletes, and their response to it. However, oxytocin levels could depend on the type of sport in means of contact level, exercise intensity, and duration. The influence of oxytocin on athletes' performance and recovery could have been exploited due to its short half-life. Examining oxytocin's complex interactions with exercise paves the way for future research and application in sports science, psychology, and medical disciplines.

Systematic Review Registration

https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=512184, identifier CRD42024512184.

The Yin and Yang of the oxytocin and stress systems: opposites, yet interdependent and intertwined determinants of lifelong health trajectories

During parturition and the immediate post-partum period there are two opposite, yet interdependent and intertwined systems that are highly active and play a role in determining lifelong health and behaviour in both the mother and her infant: the stress and the anti-stress (oxytocin) system. Before attempting to understand how the environment around birth determines long-term health trajectories, it is essential to understand how these two systems operate and how they interact. Here, we discuss together the hormonal and neuronal arms of both the hypothalamic-pituitary-adrenal (HPA) axis and the oxytocinergic systems and how they interact. Although the HPA axis and glucocorticoid stress axis are well studied, the role of oxytocin as an extremely powerful anti-stress hormone deserves more attention. It is clear that these anti-stress effects depend on oxytocinergic nerves emanating from the supraoptic nucleus (SON) and paraventricular nucleus (PVN), and project to multiple sites at which the stress system is regulated. These, include projections to corticotropin releasing hormone (CRH) neurons within the PVN, to the anterior pituitary, to areas involved in sympathetic and parasympathetic nervous control, to NA neurons in the locus coeruleus (LC), and to CRH neurons in the amygdala. In the context of the interaction between the HPA axis and the oxytocin system birth is a particularly interesting period as, for both the mother and the infant, both systems are very strongly activated within the same narrow time window. Data suggest that the HPA axis and the oxytocin system appear to interact in this early-life period, with effects lasting many years. If mother-child skin-to-skin contact occurs almost immediately postpartum, the effects of the anti-stress (oxytocin) system become more prominent, moderating lifelong health trajectories. There is clear evidence that HPA axis activity during this time is dependent on the balance between the HPA axis and the oxytocin system, the latter being reinforced by specific somatosensory inputs, and this has long-term consequences for stress reactivity.

Single-neuron projectomes of mouse paraventricular hypothalamic nucleus oxytocin neurons reveal mutually exclusive projection patterns

Oxytocin (OXT) plays important roles in autonomic control and behavioral modulation. However, it is unknown how the projection patterns of OXT neurons align with underlying physiological functions. Here, we present the reconstructed single-neuron, whole-brain projectomes of 264 OXT neurons of the mouse paraventricular hypothalamic nucleus (PVH) at submicron resolution. These neurons hierarchically clustered into two groups, with distinct morphological and transcriptional characteristics and mutually exclusive projection patterns. Cluster 1 (177 neurons) axons terminated exclusively in the median eminence (ME) and have few collaterals terminating within hypothalamic regions. By contrast, cluster 2 (87 neurons) sent wide-spread axons to multiple brain regions, but excluding ME. Dendritic arbors of OXT neurons also extended outside of the PVH, suggesting capability to sense signals and modulate target regions. These single-neuron resolution observations reveal distinct OXT subpopulations, provide comprehensive analysis of their morphology, and lay the structural foundation for better understanding the functional heterogeneity of OXT neurons.

Social buffering in rats reduces fear by oxytocin triggering sustained changes in central amygdala neuronal activity

The presence of a companion can reduce fear, but the neural mechanisms underlying this social buffering of fear are incompletely known. We studied social buffering of fear in male and female, and its encoding in the amygdala of male, auditory fear-conditioned rats. Pharmacological, opto,- and/or chemogenetic interventions showed that oxytocin signaling from hypothalamus-to-central amygdala projections underlied fear reduction acutely with a companion and social buffering retention 24 h later without a companion. Single-unit recordings with optetrodes in the central amygdala revealed fear-encoding neurons (showing increased conditioned stimulus-responses after fear conditioning) inhibited by social buffering and blue light-stimulated oxytocinergic hypothalamic projections. Other central amygdala neurons showed baseline activity enhanced by blue light and companion exposure, with increased conditioned stimulus responses that persisted without the companion. Social buffering of fear thus switches the conditioned stimulus from encoding "fear" to "safety" by oxytocin-mediated recruitment of a distinct group of central amygdala "buffer neurons".

The neurohypophyseal hormone oxytocin and eating behaviors: a narrative review

BACKGROUND

The neuropeptide oxytocin (OT) is crucial in several conditions, such as lactation, parturition, mother-infant interaction, and psychosocial function. Moreover, OT may be involved in the regulation of eating behaviors.

METHODS

This review briefly summarizes data concerning the role of OT in eating behaviors. Appropriate keywords and medical subject headings were identified and searched for in PubMed/MEDLINE. References of original articles and reviews were screened, examined, and selected.

RESULTS

Hypothalamic OT-secreting neurons project to different cerebral areas controlling eating behaviors, such as the amygdala, area postrema, nucleus of the solitary tract, and dorsal motor nucleus of the vagus nerve. Intracerebral/ventricular OT administration decreases food intake and body weight in wild and genetically obese rats. OT may alter food intake and the quality of meals, especially carbohydrates and sweets, in humans.

DISCUSSION

OT may play a role in the pathophysiology of eating disorders with potential therapeutic perspectives. In obese patients and those with certain eating disorders, such as bulimia nervosa or binge/compulsive eating, OT may reduce appetite and caloric consumption. Conversely, OT administered to patients with anorexia nervosa may paradoxically stimulate appetite, possibly by lowering anxiety which usually complicates the management of these patients. Nevertheless, OT administration (e.g., intranasal route) is not always associated with clinical benefit, probably because intranasally administered OT fails to achieve therapeutic intracerebral levels of the hormone.

CONCLUSION

OT administration could play a therapeutic role in managing eating disorders and disordered eating. However, specific studies are needed to clarify this issue with regard to dose-finding and route and administration time.

Cancer and the Vasopressin Gene: Radioimmunoassay Values and Commentary on Copeptin as a Plasma Marker

Background

Vasopressin gene expression has been demonstrated to be a common feature of all small-cell lung cancer (SCLC) and breast cancer. Provasopressin (ProVP) is a component of the cancer cell membrane and a likely target for treatment. However, a measurable fraction of this cancer provasopressin is also normally processed and products are released into the circulation. Vasopressin (VP) and vasopressin-associated human neurophysin (VP-HNP), two of three products of processing, were earlier shown to be reliable plasma markers for determining the presence of SCLC and monitoring response to treatment.

Material and Methods

In this study, copeptin, the third product of provasopressin processing, was preliminarily evaluated as a plasma marker for SCLC or breast cancer using radioimmunoassay (RIA). Antibodies directed against the 18 residue C-terminal peptide fragment of copeptin were used to avoid interference from the large-carbohydrate component of this endogenous glycopeptide.

Results

The levels of copeptin in 8 male and 6 female patients with SCLC before treatment ranged from 16 to 319 pmol/L, and these levels were elevated (>2.5 times) in 10 of 14 cases (70%) when compared with healthy volunteers (normal mean, 18 ± 6 pmol/L). Volunteer values for males were smaller than for females (15± 4 pmol/L and 20± 9 pmol/L), but numbers were small. Patients with breast cancer had plasma levels ranging from 12 to 68 pmol/L, with only three of the six elevated.

Conclusion

While cancer patients displayed a wide range of plasma copeptin levels over 70% with SCLC and 50% with breast cancer had clearly elevated levels. This finding indicates that for such patients, plasma copeptin, like plasma VP and VP-HNP, could be used to detect disease. The control values found for healthy volunteers using our RIA were in a range predictable from established normal plasma levels of both VP and VP-HNP.

Changes in neuropeptide large dense core vesicle trafficking dynamics contribute to adaptive responses to a systemic homeostatic challenge

Neuropeptides are packed into large dense core vesicles (LDCVs) that are transported from the soma out into their processes. Limited information exists regarding mechanisms regulating LDCV trafficking, particularly during challenges to bodily homeostasis. Addressing this gap, we used 2-photon imaging in an ex vivo preparation to study LDCVs trafficking dynamics in vasopressin (VP) neurons, which traffic and release neuropeptide from their dendrites and axons. We report a dynamic bidirectional trafficking of VP-LDCVs with important differences in speed and directionality between axons and dendrites. Acute, short-lasting stimuli known to alter VP firing activity and axonal/dendritic release caused modest changes in VP-LDCVs trafficking dynamics. Conversely, chronic/sustained systemic osmotic challenges upregulated VP-LDCVs trafficking dynamic, with a larger effect in dendrites. These results support differential regulation of dendritic and axonal LDCV trafficking, and that changes in trafficking dynamics constitute a novel mechanism by which peptidergic neurons can efficiently adapt to conditions of increased hormonal demand.

Connectomics of the Octopus vulgaris vertical lobe provides insight into conserved and novel principles of a memory acquisition network

Here, we present the first analysis of the connectome of a small volume of the Octopus vulgaris vertical lobe (VL), a brain structure mediating the acquisition of long-term memory in this behaviorally advanced mollusk. Serial section electron microscopy revealed new types of interneurons, cellular components of extensive modulatory systems, and multiple synaptic motifs. The sensory input to the VL is conveyed via~1.8 × 106 axons that sparsely innervate two parallel and interconnected feedforward networks formed by the two types of amacrine interneurons (AM), simple AMs (SAMs) and complex AMs (CAMs). SAMs make up 89.3% of the~25 × 106VL cells, each receiving a synaptic input from only a single input neuron on its non-bifurcating primary neurite, suggesting that each input neuron is represented in only~12 ± 3.4SAMs. This synaptic site is likely a 'memory site' as it is endowed with LTP. The CAMs, a newly described AM type, comprise 1.6% of the VL cells. Their bifurcating neurites integrate multiple inputs from the input axons and SAMs. While the SAM network appears to feedforward sparse 'memorizable' sensory representations to the VL output layer, the CAMs appear to monitor global activity and feedforward a balancing inhibition for 'sharpening' the stimulus-specific VL output. While sharing morphological and wiring features with circuits supporting associative learning in other animals, the VL has evolved a unique circuit that enables associative learning based on feedforward information flow.

Measuring oxytocin release in response to gavage: Computational modelling and assay validation

In the present experiments, we tested the conclusion from previous electrophysiological experiments that gavage of sweet food and systemically applied insulin both stimulate oxytocin secretion. To do so, we measured oxytocin secretion from urethane-anaesthetised male rats, and demonstrated a significant increase in secretion in response to gavage of sweetened condensed milk but not isocaloric cream, and a significant increase in response to intravenous injection of insulin. We compared the measurements made in response to sweetened condensed milk with the predictions from a computational model, which we used to predict plasma concentrations of oxytocin from the published electrophysiological responses of oxytocin cells. The prediction from the computational model was very closely aligned to the levels of oxytocin measured in rats in response to gavage.

Translational and post-translational dynamics in a model peptidergic system

The cell bodies of hypothalamic magnocellular neurones are densely packed in the hypothalamic supraoptic nucleus whereas their axons project to the anatomically discrete posterior pituitary gland. We have taken advantage of this unique anatomical structure to establish proteome and phosphoproteome dynamics in neuronal cell bodies and axonal terminals in response to physiological stimulation. We have found that proteome and phosphoproteome responses to neuronal stimulation are very different between somatic and axonal neuronal compartments, indicating the need of each cell domain to differentially adapt. In particular, changes in the phosphoproteome in the cell body are involved in the reorganisation of the cytoskeleton and in axonal terminals the regulation of synaptic and secretory processes. We have identified that prohormone precursors including vasopressin and oxytocin are phosphorylated in axonal terminals and are hyperphosphorylated following stimulation. By multi-omic integration of transcriptome and proteomic data we identify changes to proteins present in afferent inputs to this nucleus.

Oxytocin increased intragastric pressure in the forestomach of rats via the dorsal vagal complex

We previously reported that appetite-enhancing peptides facilitated phasic contractions of the distal stomach and relaxed the forestomach via the dorsal vagal complex (DVC). The present study investigated the effects of anorectic substances on gastric reservoir function. The effects of oxytocin on the motility of the forestomach were examined in rats anesthetized with urethane-chloralose. Gastric motor responses were measured using an intragastric balloon. The fourth ventricular administration of oxytocin (0.1 - 1.0 nmol) increased intragastric pressure (IGP) in the forestomach in a dose-dependent manner. Conversely, the administration of oxytocin (0.3 nmol) suppressed phasic contractions of the distal stomach. These responses were opposite to those of appetite-enhancing peptides in previous studies. The oxytocin response in the forestomach was not observed after bilateral cervical vagotomy. The effects of oxytocin on forestomach motility were examined in animals that underwent ablation of the area postrema (AP) to clarify its involvement. Although the magnitude of the response to the fourth ventricular administration of oxytocin decreased, a significant response was still observed. A microinjection of oxytocin (3 pmol) into the AP, the left medial nucleus of the nucleus tractus solitarius (mNTS), the left commissural part of the NTS, or the left dorsal motor nucleus of the vagus was performed. The oxytocin injection into the AP and/or mNTS induced a rapid and large increase in IGP in the forestomach. Prior injection of L-368,899, an oxytocin receptor antagonist, into both the AP and mNTS attenuated the oxytocin response of the forestomach induced by fourth ventricular administration of oxytocin. These results indicate that oxytocin acts on the AP and/or mNTS to increase IGP in the forestomach via vagal preganglionic neurons.

[Emergent role of astrocytes in oxytocin-mediated modulatory control of neuronal circuits and brain functions]

The neuropeptide oxytocin has been in the focus of scientists for decades due to its profound and pleiotropic effects on physiology, activity of neuronal circuits and behaviors. Until recently, it was believed that oxytocinergic action exclusively occurs through direct activation of neuronal oxytocin receptors. However, several studies demonstrated the existence and functional relevance of astroglial oxytocin receptors in various brain regions in the mouse and rat brain. Astrocytic signaling and activity are critical for many important physiological processes including metabolism, neurotransmitter clearance from the synaptic cleft and integrated brain functions. While it can be speculated that oxytocinergic action on astrocytes predominantly facilitates neuromodulation via the release of gliotransmitters, the precise role of astrocytic oxytocin receptors remains elusive. In this review, we discuss the latest studies on the interaction between the oxytocinergic system and astrocytes, and give details of underlying intracellular cascades.

Identification of oxytocin expression in human and murine microglia

BACKGROUND

Oxytocin is a neuropeptide synthesized in the hypothalamus. In addition to its role in parturition and lactation, oxytocin mediates social behavior and pair bonding. The possibility of using oxytocin to modify behavior in neurodevelopmental disorders, such as autism spectrum disorder, is of clinical interest. Microglia are tissue-resident macrophages with roles in neurogenesis, synapse pruning, and immunological mediation of brain homeostasis. Recently, oxytocin was found to attenuate microglial secretion of proinflammatory cytokines, but the source of this oxytocin was not established. This prompted us to investigate whether microglia themselves were the source.

METHODS

We examined oxytocin expression in human and murine brain tissue in both sexes using immunohistochemistry. Oxytocin mRNA expression and secretion were examined in isolated murine microglia from wild type and oxytocin-knockout mice. Also, secretion of oxytocin and cytokines was measured in cultured microglia (MG6) stimulated with lipopolysaccharide (LPS).

RESULTS

We identified oxytocin expression in microglia of human brain tissue, cultured microglia (MG6), and primary murine microglia. Furthermore, LPS stimulation increased oxytocin mRNA expression in primary murine microglia and MG6 cells, and oxytocin secretion as well. A positive correlation between oxytocin and IL-1β, IL-10 secretion emerged, respectively.

CONCLUSION

This may be the first demonstration of oxytocin expression in microglia. Functionally, oxytocin might regulate inflammatory cytokine release from microglia in a paracrine/autocrine manner.

Arginine Vasopressin, Synaptic Plasticity, and Brain Networks

The arginine vasopressin (AVP), a neurohypophysial hormone, is synthesized within specific sites of the central nervous system and axonally transported to multiple areas, acting as a neurotransmitter/ neuromodulator. In this context, AVP acts primarily through vasopressin receptors A and B and is involved in regulating complex social and cognition behaviors and basic autonomic function. Many earlier studies have shown that AVP as a neuromodulator affects synaptic plasticity. This review updates our current understanding of the underlying molecular mechanisms by which AVP affects synaptic plasticity. Moreover, we discuss AVP modulatory effects on event-related potentials and blood oxygen level-dependent responses in specific brain structures, and AVP effects on the network level oscillatory activity. We aimed at providing an overview of the AVP effects on the brain from the synaptic to the network level.

Electrical signaling in cochlear efferents is driven by an intrinsic neuronal oscillator

Efferent neurons are believed to play essential roles in maintaining auditory function. The lateral olivocochlear (LOC) neurons-which project from the brainstem to the inner ear, where they release multiple transmitters including peptides, catecholamines, and acetylcholine-are the most numerous yet least understood elements of efferent control of the cochlea. Using in vitro calcium imaging and patch-clamp recordings, we found that LOC neurons in juvenile and young adult mice exhibited extremely slow waves of activity (∼0.1 Hz). These seconds-long bursts of Na⁺ spikes were driven by an intrinsic oscillator dependent on L-type Ca²⁺ channels and were not observed in prehearing mice, suggesting an age-dependent mechanism underlying the intrinsic oscillator. Using optogenetic approaches, we identified both ascending (T-stellate cells of the cochlear nucleus) and descending (auditory cortex) sources of synaptic excitation, as well as the synaptic receptors used for such excitation. Additionally, we identified potent inhibition originating in the glycinergic medial nucleus of trapezoid body (MNTB). Conductance-clamp experiments revealed an unusual mechanism of electrical signaling in LOC neurons, in which synaptic excitation and inhibition served to switch on and off the intrinsically generated spike burst mechanism, allowing for prolonged periods of activity or silence controlled by brief synaptic events. Protracted bursts of action potentials may be essential for effective exocytosis of the diverse transmitters released by LOC fibers in the cochlea.

Oxytocin-a social peptide? Deconstructing the evidence

In this paper, we analyse the claim that oxytocin is a 'social neuropeptide'. This claim originated from evidence that oxytocin was instrumental in the initiation of maternal behaviour and it was extended to become the claim that oxytocin has a key role in promoting social interactions between individuals. We begin by considering the structure of the scientific literature on this topic, identifying closely interconnected clusters of papers on particular themes. We then analyse this claim by considering evidence of four types as generated by these clusters: (i) mechanistic studies in animal models, designed to understand the pathways involved in the behavioural effects of centrally administered oxytocin; (ii) evidence from observational studies indicating an association between oxytocin signalling pathways and social behaviour; (iii) evidence from intervention studies, mainly involving intranasal oxytocin administration; and (iv) evidence from translational studies of patients with disorders of social behaviour. We then critically analyse the most highly cited papers in each segment of the evidence; we conclude that, if these represent the best evidence, then the evidence for the claim is weak. This article is part of the theme issue 'Interplays between oxytocin and other neuromodulators in shaping complex social behaviours'.

Emerging role of astrocytes in oxytocin-mediated control of neural circuits and brain functions

The neuropeptide oxytocin has been in the focus of scientists for decades due to its profound and pleiotropic effects on physiology, activity of neuronal circuits and behaviors, among which sociality. Until recently, it was believed that oxytocinergic action exclusively occurs through direct activation of neuronal oxytocin receptors. However, several studies demonstrated the existence and functional relevance of astroglial oxytocin receptors in various brain regions in the mouse and rat brain. Astrocytic signaling and activity is critical for many important physiological processes including metabolism, neurotransmitter clearance from the synaptic cleft and integrated brain functions. While it can be speculated that oxytocinergic action on astrocytes predominantly facilitates neuromodulation via the release of specific gliotransmitters, the precise role of astrocytic oxytocin receptors remains elusive. In this review, we discuss the latest studies on the interaction between the oxytocinergic system and astrocytes, including detailed information about intracellular cascades, and speculate about future research directions on astrocytic oxytocin signaling.

The physiological control of eating: signals, neurons, and networks

During the past 30 yr, investigating the physiology of eating behaviors has generated a truly vast literature. This is fueled in part by a dramatic increase in obesity and its comorbidities that has coincided with an ever increasing sophistication of genetically based manipulations. These techniques have produced results with a remarkable degree of cell specificity, particularly at the cell signaling level, and have played a lead role in advancing the field. However, putting these findings into a brain-wide context that connects physiological signals and neurons to behavior and somatic physiology requires a thorough consideration of neuronal connections: a field that has also seen an extraordinary technological revolution. Our goal is to present a comprehensive and balanced assessment of how physiological signals associated with energy homeostasis interact at many brain levels to control eating behaviors. A major theme is that these signals engage sets of interacting neural networks throughout the brain that are defined by specific neural connections. We begin by discussing some fundamental concepts, including ones that still engender vigorous debate, that provide the necessary frameworks for understanding how the brain controls meal initiation and termination. These include key word definitions, ATP availability as the pivotal regulated variable in energy homeostasis, neuropeptide signaling, homeostatic and hedonic eating, and meal structure. Within this context, we discuss network models of how key regions in the endbrain (or telencephalon), hypothalamus, hindbrain, medulla, vagus nerve, and spinal cord work together with the gastrointestinal tract to enable the complex motor events that permit animals to eat in diverse situations.

Optogenetic stimulation of Kiss1ARC terminals in the AVPV induces surge-like luteinizing hormone secretion via glutamate release in mice

Kisspeptin neurons are mainly located in the arcuate (Kiss1^ARC, vis-à-vis the GnRH pulse generator) and anteroventral periventricular nucleus (Kiss1^AVPV, vis-à-vis the GnRH surge generator). Kiss1^ARC send fibre projections that connect with Kiss1^AVPV somata. However, studies focused on the role of Kiss1^ARC neurons in the LH surge are limited, and the role of Kiss1^ARC projections to AVPV (Kiss1^ARC→AVPV) in the preovulatory LH surge is still unknown. To investigate its function, this study used optogenetics to selectively stimulate Kiss1^ARC→AVPV and measured changes in circulating LH levels. Kiss1^ARC in Kiss-Cre-tdTomato mice were virally infected to express channelrhodopsin-2 proteins, and optical stimulation was applied selectively via a fibre optic cannula in the AVPV. Sustained 20 Hz optical stimulation of Kiss1^ARC→AVPV from 15:30 to 16:30 h on proestrus effectively induced an immediate increase in LH reaching peak surge-like levels of around 8 ng/ml within 10 min, followed by a gradual decline to baseline over about 40 min. Stimulation at 10 Hz resulted in a non-significant increase in LH levels and 5 Hz stimulation had no effect in proestrous animals. The 20 Hz stimulation induced significantly higher circulating LH levels on proestrus compared with diestrus or estrus, which suggested that the effect of terminal stimulation is modulated by the sex steroid milieu. Additionally, intra-AVPV infusion of glutamate antagonists, AP5+CNQX, completely blocked the increase on LH levels induced by Kiss1^ARC→AVPV terminal photostimulation in proestrous animals. These results demonstrate for the first time that optical stimulation of Kiss1^ARC→AVPV induces an LH surge-like secretion via glutamatergic mechanisms. In conclusion, Kiss1^ARC may participate in LH surge generation by glutamate release from terminal projections in the AVPV.

Intranasal oxytocin reduces attentional bias to food stimuli

Attentional biases to food-related stimuli have been demonstrated in response to hunger as well as during restrained eating. Such biases are often associated with obesity, but healthy-weight individuals who do not self-report hunger have also demonstrated attentional biases to stimuli signalling food using laboratory-based cognitive tasks. Levels of the anorectic neuropeptide oxytocin are elevated by food intake and, when administered intranasally, oxytocin inhibits food intake in the laboratory. To investigate whether oxytocin can affect appetite via an action on attentional processes, 40 adults (29 women; mean age 24.0 years old) self-administered 24 IU of oxytocin or placebo intranasally. Forty minutes after administration, participants ate a small snack to maintain alertness and ameliorate deprivation-induced hunger before starting a computerized dot-probe attentional bias task that presented 180 trials of paired visual stimuli comprising neutral, food, social and/or romantic images (500 ms presentation time). Reaction times to probe stimuli that appeared after the offset of the visual images indicated a significant attentional bias to food pictures after placebo; this effect was significantly attenuated by oxytocin, p < .001. The effect of oxytocin on attentional bias to the food pictures was not altered by the type of stimulus paired with the food image, and was independent of BMI, age, sex, self-rated eating behaviour, and self-reported parental bonding; however, the effect was modulated by self-reported food cravings and trait stress. The findings support and extend previous work which has suggested that oxytocin can counteract attentional biases to food-related stimuli in a sample with anorexia by demonstrating the same effect for the first time in a cohort who do not have an eating disorder.

Gonadotrophin-releasing hormone and kisspeptin: It takes two to tango

Kisspeptin (Kp), a family of peptides comprising products of the Kiss1 gene, was discovered 20 years ago; it is recognised as the major factor controlling the activity of the gonadotrophin-releasing hormone (GnRH) neurones and thus the activation of the reproductive axis in mammals. It has been widely documented that the effects of Kp on reproduction through its action on GnRH neurones are mediated by the GPR54 receptor. Kp controls the activation of the reproductive axis at puberty, maintains reproductive axis activity in adults and is involved in triggering ovulation in some species. Although there is ample evidence coming from both conditional knockout models and conditional-induced Kp neurone death implicating the Kp/GPR54 pathway in the control of reproduction, the mechanism(s) underlying this process may be more complex than a sole direct control of GnRH neuronal activity by Kp. In this review, we provide an overview of the recent advances made in elucidating the interplay between Kp- and GnRH- neuronal networks with respect to regulating the reproductive axis. We highlight the existence of a possible mutual regulation between GnRH and Kp neurones, as well as the implication of Kp-dependent volume transmission in this process. We also discuss the capacity of heterodimerisation between GPR54 and GnRH receptor (GnRH-R) and its consequences on signalling. Finally, we illustrate the role of mathematical modelling that accounts for the synergy between GnRH-R and GPR54 in explaining the role of these two receptors when defining GnRH neuronal activity and GnRH pulsatile release.

Oxytocin as an Anti-obesity Treatment

Obesity is a growing health concern, as it increases risk for heart disease, hypertension, type 2 diabetes, cancer, COVID-19 related hospitalizations and mortality. However, current weight loss therapies are often associated with psychiatric or cardiovascular side effects or poor tolerability that limit their long-term use. The hypothalamic neuropeptide, oxytocin (OT), mediates a wide range of physiologic actions, which include reproductive behavior, formation of prosocial behaviors and control of body weight. We and others have shown that OT circumvents leptin resistance and elicits weight loss in diet-induced obese rodents and non-human primates by reducing both food intake and increasing energy expenditure (EE). Chronic intranasal OT also elicits promising effects on weight loss in obese humans. This review evaluates the potential use of OT as a therapeutic strategy to treat obesity in rodents, non-human primates, and humans, and identifies potential mechanisms that mediate this effect.

Stress Adaptation and the Brainstem with Focus on Corticotropin-Releasing Hormone

Stress adaptation is of utmost importance for the maintenance of homeostasis and, therefore, of life itself. The prevalence of stress-related disorders is increasing, emphasizing the importance of exploratory research on stress adaptation. Two major regulatory pathways exist: the hypothalamic–pituitary–adrenocortical axis and the sympathetic adrenomedullary axis. They act in unison, ensured by the enormous bidirectional connection between their centers, the paraventricular nucleus of the hypothalamus (PVN), and the brainstem monoaminergic cell groups, respectively. PVN and especially their corticotropin-releasing hormone (CRH) producing neurons are considered to be the centrum of stress regulation. However, the brainstem seems to be equally important. Therefore, we aimed to summarize the present knowledge on the role of classical neurotransmitters of the brainstem (GABA, glutamate as well as serotonin, noradrenaline, adrenaline, and dopamine) in stress adaptation. Neuropeptides, including CRH, might be co-localized in the brainstem nuclei. Here we focused on CRH as its role in stress regulation is well-known and widely accepted and other CRH neurons scattered along the brain may also complement the function of the PVN. Although CRH-positive cells are present on some parts of the brainstem, sometimes even in comparable amounts as in the PVN, not much is known about their contribution to stress adaptation. Based on the role of the Barrington’s nucleus in micturition and the inferior olivary complex in the regulation of fine motoric—as the main CRH-containing brainstem areas—we might assume that these areas regulate stress-induced urination and locomotion, respectively. Further studies are necessary for the field.

Central and peripheral release of oxytocin: Relevance of neuroendocrine and neurotransmitter actions for physiology and behavior

The hypothalamic neuropeptide oxytocin (OT) is critically involved in the modulation of socio-emotional behavior, sexual competence, and pain perception and anticipation. While intracellular signaling of OT and its receptor (OTR), as well as the functional connectivity of hypothalamic and extra-hypothalamic OT projections, have been recently explored, it remains elusive how one single molecule has pleotropic effects from cell proliferation all the way to modulation of complex cognitive processes. Moreover, there are astonishing species-dependent differences in the way OT regulates various sensory modalities such as touch, olfaction, and vision, which can be explained by differences in OTR expression in brain regions processing sensory information. Recent research highlights a small subpopulation of OT-synthesizing cells, namely, parvocellular cells, which merely constitute 1% of the total number of OT cells but act as "master cells' that regulate the activity of the entire OT system. In this chapter, we summarize the latest advances in the field of OT research with a particular focus on differences between rodents, monkeys and humans and highlight the main differences between OT and its "sister" peptide arginine-vasopressin, which often exerts opposite effects on physiology and behavior.

Brain oxytocin: how puzzle stones from animal studies translate into psychiatry

The neuropeptide oxytocin has attracted great attention of the general public, basic neuroscience researchers, psychologists, and psychiatrists due to its profound pro-social, anxiolytic, and "anti-stress" behavioral and physiological effects, and its potential application for treatment of mental diseases associated with altered socio-emotional competence. During the last decade, substantial progress has been achieved in understanding the complex neurobiology of the oxytocin system, including oxytocinergic pathways, local release patterns, and oxytocin receptor distribution in the brain, as well as intraneuronal oxytocin receptor signaling. However, the picture of oxytocin actions remains far from being complete, and the central question remains: "How does a single neuropeptide exert such pleotropic actions?" Although this phenomenon, typical for many of about 100 identified neuropeptides, may emerge from the anatomical divergence of oxytocin neurons, their multiple central projections, distinct oxytocin-sensitive cell types in different brain regions, and multiple intraneuronal signaling pathways determining the specific cellular response, further basic studies are required. In conjunction, numerous reports on positive effects of intranasal application of oxytocin on human brain networks controlling socio-emotional behavior in health and disease require harmonic tandems of basic researchers and clinicians. During the COVID-19 crisis in 2020, oxytocin research seems central as question of social isolation-induced inactivation of the oxytocin system, and buffering effects of either activation of the endogenous system or intranasal application of synthetic oxytocin need to be thoroughly investigated.

New light on cortical neuropeptides and synaptic network plasticity

Neuropeptides, members of a large and evolutionarily ancient family of proteinaceous cell-cell signaling molecules, are widely recognized as extremely potent regulators of brain function and behavior. At the cellular level, neuropeptides are known to act mainly via modulation of ion channel and synapse function, but functional impacts emerging at the level of complex cortical synaptic networks have resisted mechanistic analysis. New findings from single-cell RNA-seq transcriptomics now illuminate intricate patterns of cortical neuropeptide signaling gene expression and new tools now offer powerful molecular access to cortical neuropeptide signaling. Here we highlight some of these new findings and tools, focusing especially on prospects for experimental and theoretical exploration of peptidergic and synaptic networks interactions underlying cortical function and plasticity.

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.

An Allostatic Theory of Oxytocin

Oxytocin has garnered considerable interest for its role in social behavior, as well as for the potential of intranasal administration to treat social difficulties. However, current theoretical models for the role of oxytocin in social behavior pay little consideration to its evolutionary and developmental history. This article aims to broaden our understanding of the role of oxytocin in social behavior by adopting an ethological approach through the lens of Nikolaas Tinbergen's 'four questions' - how does oxytocin work; how does the role of oxytocin change during development; how does oxytocin enhance survival; and how did the oxytocin system evolve? We argue that oxytocin is most accurately described as an allostatic hormone that modulates both social and non-social behavior by maintaining stability through changing environments.

Can oxytocin inhibit stress-induced hyperalgesia?

Stress-induced hyperalgesia is a problematic condition that lacks an effective therapeutic measure, and hence impairs health-related quality of life. The regulation of stress by oxytocin (OT) has overlapping effects on pain. OT can alleviate pain directly mainly at the spinal level and the peripheral tissues. Additionally, OT plays an analgesic role by dealing with stress and fear learning. When OT relieves stress by targeting the prefrontal brain regions and the hypothalamic-pituitary-adrenal axis, the body's sensitivity to pain is attenuated. Meanwhile, OT facilitates fear learning and may, in turn, enhance the anticipatory actions to painful stimulation. The unique therapeutic value of OT in patients suffering from stress and stress-related hyperalgesia conditions is worth considering. We reviewed recent advances in animal and human studies involving the effects of OT on stress and pain, and discussed the possible targets of OT within the descending and ascending pathways in the central nervous system. This review provides an overview of the evidence on the role of OT in alleviating stress-induced hyperalgesia.

Intradermal Injection of Oxytocin Aggravates Chloroquine-Induced Itch Responses via Activating the Vasopressin-1a Receptor/Nitric Oxide Pathway in Mice

Oxytocin (OT), a hormone synthesized within the paraventricular nucleus and supraoptic nucleus of the hypothalamus, when given intracerebroventricularly, induces strong scratching behaviors. However, it is not clear whether intradermal injection (ID) of OT elicits itch sensation. Herein, we found that OT (0.02 mg/ml) did not elicit an itch-scratching response in mice but aggravated chloroquine (CQ, 3 mmol/L)-elicited scratching behavior. Similar to OT, arginine vasopressin (AVP, 0.02 mg/ml), which is structurally related to OT, also enhanced CQ-induced scratching behavior but did not directly induce scratching behavior in mice. Mechanistically, OT-mediated enhancement of CQ-induced scratching behavior was significantly suppressed by conivaptan (0.05 mg/ml), a vasopressin-1a receptor (V1AR) antagonist and 1,400 W (3 mg/kg), inhibitor of inducible nitric oxide synthase (iNOS), but not OT receptor (OTR) antagonist L-368,899 (0.05 mg/ml). Notably, conivaptan also directly decreased CQ-induced scratching. In conclusion, OT plays a role in CQ-induced scratching behavior via V1AR binding events. V1AR antagonists could be used as possible treatments for CQ-induced itch.

P2X2 Receptor Expression and Function Is Upregulated in the Rat Supraoptic Nucleus Stimulated Through Refeeding After Fasting

Magnocellular neurons in the supraoptic nucleus (SON), which synthesize and release arginine vasopressin (AVP) and oxytocin (OT), express several subtypes of ATP-stimulated purinergic P2X receptors (P2XR) that modulate neuronal activity as well as neurotransmitter and hormone release. However, the physiological impact of this modulation is not well understood. Here, we tested a hypothesis that P2XRs play a role in the sustained release of hormones from SON neurons stimulated through fasting/refeeding. We studied the effect of 2 h of refeeding after 48 h of fasting on P2XR and P2YR mRNA expression and ATP-induced presynaptic and postsynaptic responses in the SON of 30-day-old rats. Quantitative real-time PCR revealed that the expression of P2X2R and AVP mRNA was upregulated, whereas P2X4R, P2X7R, P2Y2R, and OT mRNA levels were not significantly changed and P2Y1R mRNA expression was decreased. Whole-cell patch clamp recordings performed on isolated rat brain slices showed that the amplitude of the ATP-stimulated somatic current and the ATP-induced increases in the frequency of spontaneous GABAergic inhibitory postsynaptic currents were significantly higher in SON neurons from fasted/refed rats than in SON neurons from normally fed rats. No evidence was found for changes in the presynaptic effect of ATP in SON neurons not expressing somatic P2XRs. These results suggest that the increased activity of SON neurons synthesizing AVP is associated with enhanced expression of P2X2Rs on neuronal cell bodies and their GABAergic presynaptic nerve terminals.

Robo2 regulates synaptic oxytocin content by affecting actin dynamics

The regulation of neuropeptide level at the site of release is essential for proper neurophysiological functions. We focused on a prominent neuropeptide, oxytocin (OXT) in the zebrafish as an in vivo model to visualize and quantify OXT content at the resolution of a single synapse. We found that OXT-loaded synapses were enriched with polymerized actin. Perturbation of actin filaments by either cytochalasin-D or conditional Cofilin expression resulted in decreased synaptic OXT levels. Genetic loss of robo2 or slit3 displayed decreased synaptic OXT content and robo2 mutants displayed reduced mobility of the actin probe Lifeact-EGFP in OXT synapses. Using a novel transgenic reporter allowing real-time monitoring of OXT-loaded vesicles, we show that robo2 mutants display slower rate of vesicles accumulation. OXT-specific expression of dominant-negative Cdc42, which is a key regulator of actin dynamics and a downstream effector of Robo2, led to a dose-dependent increase in OXT content in WT, and a dampened effect in robo2 mutants. Our results link Slit3-Robo2-Cdc42, which controls local actin dynamics, with the maintenance of synaptic neuropeptide levels.

The osmoresponsiveness of oxytocin and vasopressin neurones: Mechanisms, allostasis and evolution

In the rat supraoptic nucleus, every oxytocin cell projects to the posterior pituitary, and is involved both in reflex milk ejection during lactation and in regulating uterine contractions during parturition. All are also osmosensitive, regulating natriuresis. All are also regulated by signals that control appetite, including the neural and hormonal signals that arise from the gut after food intake and from the sites of energy storage. All are also involved in sexual behaviour, anxiety-related behaviours and social behaviours. The challenge is to understand how a single population of neurones can coherently regulate such a diverse set of functions and adapt to changing physiological states. Their multiple functions arise from complex intrinsic properties that confer sensitivity to a wide range of internal and environmental signals. Many of these properties have a distant evolutionary origin in multifunctional, multisensory neurones of Urbilateria, the hypothesised common ancestor of vertebrates, insects and worms. Their properties allow different patterns of oxytocin release into the circulation from their axon terminals in the posterior pituitary into other brain areas from axonal projections, as well as independent release from their dendrites.

Sex-dependent regulation of social reward by oxytocin: an inverted U hypothesis

The rewarding properties of social interactions are essential for the expression of social behavior and the development of adaptive social relationships. Here, we review sex differences in social reward, and more specifically, how oxytocin (OT) acts in the mesolimbic dopamine system (MDS) to mediate the rewarding properties of social interactions in a sex-dependent manner. Evidence from rodents and humans suggests that same-sex social interactions may be more rewarding in females than in males. We propose that there is an inverted U relationship between OT dose, social reward, and neural activity within structures of the MDS in both males and females, and that this dose-response relationship is initiated at lower doses in females than males. As a result, depending on the dose of OT administered, OT could reduce social reward in females, while enhancing it in males. Sex differences in the neural mechanisms regulating social reward may contribute to sex differences in the incidence of a large number of psychiatric and neurodevelopmental disorders. This review addresses the potential significance of a sex-dependent inverted U dose-response function for OT's effects on social reward and in the development of gender-specific therapies for these disorders.

The endocrinology of the brain

The brain hosts a vast and diverse repertoire of neuropeptides, a class of signalling molecules often described as neurotransmitters. Here I argue that this description entails a catalogue of misperceptions, misperceptions that feed into a narrative in which information processing in the brain can be understood only through mapping neuronal connectivity and by studying the transmission of electrically conducted signals through chemical synapses. I argue that neuropeptide signalling in the brain involves primarily autocrine, paracrine and neurohormonal mechanisms that do not depend on synaptic connectivity and that it is not solely dependent on electrical activity but on mechanisms analogous to secretion from classical endocrine cells. As in classical endocrine systems, to understand the role of neuropeptides in the brain, we must understand not only how their release is regulated, but also how their synthesis is regulated and how the sensitivity of their targets is regulated. We must also understand the full diversity of effects of neuropeptides on those targets, including their effects on gene expression.

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.

Differential Roles of the Two Raphe Nuclei in Amiable Social Behavior and Aggression - An Optogenetic Study

Serotonergic mechanisms hosted by raphe nuclei have important roles in affiliative and agonistic behaviors but the separate roles of the two nuclei are poorly understood. Here we studied the roles of the dorsal (DR) and median raphe region (MRR) in aggression by optogenetically stimulating the two nuclei. Mice received three 3 min-long stimulations, which were separated by non-stimulation periods of 3 min. The stimulation of the MRR decreased aggression in a phasic-like manner. Effects were rapidly expressed during stimulations, and vanished similarly fast when stimulations were halted. No carryover effects were observed in the subsequent three trials performed at 2-day intervals. No effects on social behaviors were observed. By contrast, DR stimulation rapidly and tonically promoted social behaviors: effects were present during both the stimulation and non-stimulation periods of intermittent stimulations. Aggressive behaviors were marginally diminished by acute DR stimulations, but repeated stimulations administered over 8 days considerably decreased aggression even in the absence of concurrent stimulations, indicating the emergence of carryover effects. No such effects were observed in the case of social behaviors. We also investigated stimulation-induced neurotransmitter release in the prefrontal cortex, a major site of aggression control. MRR stimulation rapidly but transiently increased serotonin release, and induced a lasting increase in glutamate levels. DR stimulation had no effect on glutamate, but elicited a lasting increase of serotonin release. Prefrontal serotonin levels remained elevated for at least 2 h subsequent to DR stimulations. The stimulation of both nuclei increased GABA release rapidly and transiently. Thus, differential behavioral effects of the two raphe nuclei were associated with differences in their neurotransmission profiles. These findings reveal a surprisingly strong behavioral task division between the two raphe nuclei, which was associated with a nucleus-specific neurotransmitter release in the prefrontal cortex.

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.

Role of oxytocin in parental behaviour

Both animal and human studies have provided conclusive evidence that oxytocin (OXT) acts in the brain (eg, medial preoptic area, ventral tegmental area, nucleus accumbens) to promote parental behaviour under different reproductive and physiological conditions. OXT appears to accelerate and strengthen the neural process that makes newborns attractive or rewarding. Furthermore, OXT reduces stress/anxiety and might improve mood and well being, resulting in indirect benefits for parents. However, OXT also plays a role in the development of species reproductive and social strategies, making some species or individuals more prone to display caring activities in nonreproductive contexts. There are important differences in the development of the OXT system and its regulation by gonadal hormones that can make individuals or species very different. Those intra- and interspecific differences in the OXT system have been associated with differences in parental behaviour. For example, differences in OXT levels in body fluids and genetic variants for the OXT and OXT receptor genes have been associated with variability in parental mood and behaviour in humans. Thus, OXT has received much attention as a potential therapeutic agent for affective, emotional and behavioural problems. Despite many preliminary studies indicating promising findings, several unknown aspects of the OXT system remain to be addressed before we can achieve a complete understanding of its function in the brain. The enormous interest that this area of study has attracted in the last decade will likely continually contribute to advancing our understanding of the role of OXT in parental behaviour and other behavioural and physiological functions.

The role of vasopressin in olfactory and visual processing

Neural vasopressin is a potent modulator of behaviour in vertebrates. It acts at both sensory processing regions and within larger regulatory networks to mediate changes in social recognition, affiliation, aggression, communication and other social behaviours. There are multiple populations of vasopressin neurons within the brain, including groups in olfactory and visual processing regions. Some of these vasopressin neurons, such as those in the main and accessory olfactory bulbs, anterior olfactory nucleus, piriform cortex and retina, were recently identified using an enhanced green fluorescent protein-vasopressin (eGFP-VP) transgenic rat. Based on the interconnectivity of vasopressin-producing and sensitive brain areas and in consideration of autocrine, paracrine and neurohormone-like actions associated with somato-dendritic release, we discuss how these different neuronal populations may interact to impact behaviour.