Autofeedback effects of progressively rising oxytocin concentrations on supraoptic oxytocin neuronal activity in slices from lactating rats

Potential of Endogenous Oxytocin in Endocrine Treatment and Prevention of COVID-19

Coronavirus disease 2019 or COVID-19 caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has become a significant threat to the health of human beings. While wearing mask, maintaining social distance and performing self-quarantine can reduce virus spreading passively, vaccination actively enhances immune defense against COVID-19. However, mutations of SARS-CoV-2 and presence of asymptomatic carriers frustrate the effort of completely conquering COVID-19. A strategy that can reduce the susceptibility and thus prevent COVID-19 while blocking viral invasion and pathogenesis independent of viral antigen stability is highly desirable. In the pathogenesis of COVID-19, endocrine disorders have been implicated. Correspondingly, many hormones have been identified to possess therapeutic potential of treating COVID-19, such as estrogen, melatonin, corticosteroids, thyroid hormone and oxytocin. Among them, oxytocin has the potential of both treatment and prevention of COVID-19. This is based on oxytocin promotion of immune-metabolic homeostasis, suppression of inflammation and pre-existing comorbidities, acceleration of damage repair, and reduction of individuals' susceptibility to pathogen infection. Oxytocin may specifically inactivate SARS-COV-2 spike protein and block viral entry into cells via angiotensin-converting enzyme 2 by suppressing serine protease and increasing interferon levels and number of T-lymphocytes. In addition, oxytocin can promote parasympathetic outflow and the secretion of body fluids that could dilute and even inactivate SARS-CoV-2 on the surface of cornea, oral cavity and gastrointestinal tract. What we need to do now is clinical trials. Such trials should fully balance the advantages and disadvantages of oxytocin application, consider the time- and dose-dependency of oxytocin effects, optimize the dosage form and administration approach, combine oxytocin with inhibitors of SARS-CoV-2 replication, apply specific passive immunization, and timely utilize efficient vaccines. Meanwhile, blocking COVID-19 transmission chain and developing other efficient anti-SARS-CoV-2 drugs are also important. In addition, relative to the complex issues with drug applications over a long term, oxytocin can be mobilized through many physiological stimuli, and thus used as a general prevention measure. In this review, we explore the potential of oxytocin for treatment and prevention of COVID-19 and perhaps other similar pathogens.

α-Melanocyte-stimulating hormone inhibition of oxytocin neurons switches to excitation in late pregnancy and lactation

Oxytocin is secreted into the periphery by magnocellular neurons of the hypothalamic supraoptic and paraventricular nuclei (SON and PVN) to trigger uterine contraction during birth and milk ejection during suckling. Peripheral oxytocin secretion is triggered by action potential firing, which is regulated by afferent input activity and by feedback from oxytocin secreted into the extracellular space from magnocellular neuron somata and dendrites. A prominent input to oxytocin neurons arises from proopiomelanocortin neurons of the hypothalamic arcuate nucleus that secrete an alpha-melanocyte-stimulating hormone (α-MSH), which inhibits oxytocin neuron firing in non-pregnant rats by increasing somato-dendritic oxytocin secretion. However, α-MSH inhibition of oxytocin neuron firing is attenuated in mid-pregnancy and somato-dendritic oxytocin becomes auto-excitatory in late-pregnancy and lactation. Therefore, we hypothesized that attenuated α-MSH inhibition of oxytocin neuron firing marks the beginning of a transition from inhibition to excitation to facilitate peripheral oxytocin secretion for parturition and lactation. Intra-SON microdialysis administration of α-MSH inhibited oxytocin neuron firing rate by 33 ± 9% in non-pregnant rats but increased oxytocin neuron firing rate by 37 ± 12% in late-pregnant rats and by 28 ± 10% in lactating rats. α-MSH-induced somato-dendritic oxytocin secretion measured ex vivo with oxytocin receptor-expressing "sniffer" cells, was of similar amplitude in PVN slices from non-pregnant and lactating rats but longer-lasting in slices from lactating rats. Hence, α-MSH inhibition of oxytocin neuron activity switches to excitation over pregnancy while somato-dendritic oxytocin secretion is maintained, which might enhance oxytocin neuron excitability to facilitate the increased peripheral secretion that is required for normal parturition and milk ejection.

Oxytocin in Women's Health and Disease

Oxytocin (OT) is a nonapeptide mainly produced in the supraoptic and paraventricular nuclei. OT in the brain and blood has extensive functions in both mental and physical activities. These functions are mediated by OT receptors (OTRs) that are distributed in a broad spectrum of tissues with dramatic sexual dimorphism. In both sexes, OT generally facilitates social cognition and behaviors, facilitates parental behavior and sexual activity and inhibits feeding and pain perception. However, there are significant differences in OT levels and distribution of OTRs in men from women. Thus, many OT functions in men are different from women, particularly in the reproduction. In men, the reproductive functions are relatively simple. In women, the reproductive functions involve menstrual cycle, pregnancy, parturition, lactation, and menopause. These functions make OT regulation of women's health and disease a unique topic of physiological and pathological studies. In menstruation, pre-ovulatory increase in OT secretion in the hypothalamus and the ovary can promote the secretion of gonadotropin-releasing hormone and facilitate ovulation. During pregnancy, increased OT synthesis and preterm release endow OT system the ability to promote maternal behavior and lactation. In parturition, cervix expansion-elicited pulse OT secretion and uterine OT release accelerate the expelling of fetus and reduce postpartum hemorrhage. During lactation, intermittent pulsatile OT secretion is necessary for the milk-ejection reflex and maternal behavior. Disorders in OT secretion can account for maternal depression and hypogalactia. In menopause, the reduction of OT secretion accounts for many menopausal symptoms and diseases. These issues are reviewed in this work.

Neural Functions of Hypothalamic Oxytocin and its Regulation

Oxytocin (OT), a nonapeptide, has a variety of functions. Despite extensive studies on OT over past decades, our understanding of its neural functions and their regulation remains incomplete. OT is mainly produced in OT neurons in the supraoptic nucleus (SON), paraventricular nucleus (PVN) and accessory nuclei between the SON and PVN. OT exerts neuromodulatory effects in the brain and spinal cord. While magnocellular OT neurons in the SON and PVN mainly innervate the pituitary and forebrain regions, and parvocellular OT neurons in the PVN innervate brainstem and spinal cord, the two sets of OT neurons have close interactions histologically and functionally. OT expression occurs at early life to promote mental and physical development, while its subsequent decrease in expression in later life stage accompanies aging and diseases. Adaptive changes in this OT system, however, take place under different conditions and upon the maturation of OT release machinery. OT can modulate social recognition and behaviors, learning and memory, emotion, reward, and other higher brain functions. OT also regulates eating and drinking, sleep and wakefulness, nociception and analgesia, sexual behavior, parturition, lactation and other instinctive behaviors. OT regulates the autonomic nervous system, and somatic and specialized senses. Notably, OT can have different modulatory effects on the same function under different conditions. Such divergence may derive from different neural connections, OT receptor gene dimorphism and methylation, and complex interactions with other hormones. In this review, brain functions of OT and their underlying neural mechanisms as well as the perspectives of their clinical usage are presented.

Involvement of Supraoptic Astrocytes in Basilar Artery Occlusion-Evoked Differential Activation of Vasopressin Neurons and Vasopressin Secretion in Rats

Vasopressin (VP) is a key factor in the development of brain injury in ischemic stroke. However, the regulation of VP secretion in basilar artery occlusion (BAO) remains unclear. To clarify the regulation of VP secretion in BAO and the underlying mechanisms, we performed this study in a rat model of BAO with (BC) or without common carotid artery occlusion (CCAO). The results showed that BAO and BC time-dependently increased neurological scores and that BC also increased water contents in the medulla at 2 h and in the pontine at 8 h. Moreover, plasma VP level increased significantly at BAO-8 h, CCAO and BC-2 h but not at BC-8 h; however, VP expressions increased in the supraoptic nucleus (SON) at BC-8 h. The neurological scores were highly correlated with pontine water contents and plasma VP levels. The number of phosphorylated extracellular signal-regulated protein kinase1/2-positive VP neurons increased significantly in the SON at BC-8 h. Similarly, the number of c-Fos-positive VP neurons increased significantly in the SON at BAO-8 h and BC-8 h. In addition, the length of glial fibrillary acidic protein (GFAP) filaments increased significantly in BC compared to BAO only. Aquaporin 4 (AQP4) puncta around VP neurons increased significantly at BC-8 h relative to BC-2 h, which had negative correlation with plasma VP levels. These findings indicate that BAO facilitates VP secretion and increases VP neuronal activity in the SON. The peripheral VP release is possibly under a negative feedback regulation of central VP neuronal activity through increasing GFAP and AQP4 expression in astrocytic processes.

Oxytocin Modulation of Maternal Behavior and Its Association With Immunological Activity in Rats With Cesarean Delivery

Oxytocin (OT), a neuropeptide produced in the supraoptic (SON) and paraventricular (PVN) nuclei, is not only essential for lactation and maternal behavior but also for normal immunological activity. However, mechanisms underlying OT regulation of maternal behavior and its association with immunity around parturition, particularly under mental and physical stress, remain unclear. Here, we observed effects of OT on maternal behavior in association with immunological activity in rats after cesarean delivery (CD), a model of reproductive stress. CD significantly reduced maternal interests to the pups throughout postpartum day 1-8. On postpartum day 5, CD decreased plasma OT levels and thymic index but increased vasopressin, interleukin (IL)-1β, IL-6 and IL-10 levels. CD had no significant effect on plasma adrenocorticotropic hormone and corticosterone levels. In the hypothalamus, CD decreased corticotropin-releasing hormone contents in the PVN but increased OT contents in the PVN and SON and OT release from hypothalamic implants. CD also increased c-Fos expression, particularly in the cytoplasm of OT neurons. Lastly, CD depolarized resting membrane potential and increased spike width while increasing the variability of the firing rate of OT neurons in brain slices. Thus, CD can increase hypothalamic OT contents and release but reduce pituitary release of OT into the blood, which is associated with depressive-like maternal behavior, increased inflammatory cytokine release and decreased relative weight of the thymus.

Intranasal Oxytocin Restores Maternal Behavior and Oxytocin Neuronal Activity in the Supraoptic Nucleus in Rat Dams with Cesarean Delivery

Oxytocin (OT) is a key factor for maternal behavior. However, neurochemical regulation of OT neurons, the major source of OT, remains incompletely understood. Here we report the effect of intranasally-applied OT (IAO) on OT neuronal activity in the supraoptic nucleus (SON) and on maternal behavior in a rat model of cesarean delivery (CD) at day 4-5 (stage I) and day 8-9 (stage II) following delivery. We found that at stage I, CD dams exhibited significantly longer latency of pup retrieval, lower number of anogenital licks and smaller acinar area of the mammary glands. In the SON, the number of OT neurons expressing phosphorylated extracellular signal-regulated protein kinase 1/2 (pERK 1/2) decreased significantly. IAO reversed the depressive-like maternal behavior and involution-like change in the mammary glands, and restored the number of pERK1/2-positive OT neurons in CD dams. At stage II, CD did not significantly influence the latency of retrieval and pERK1/2 expression in the SON. However, CD still reduced the number of anogenital licks during suckling, which was reversed by IAO. Notably, IAO but not hypodermic OT application in CD dams significantly increased litter's body weight gains. In brain slices, CD but not CD plus IAO significantly depolarized membrane potential and increased spike duration in OT neurons. In vasopressin neurons, CD, but not CD plus IAO, significantly depolarized membrane potential and increased the firing rate. Thus, decreased OT neuronal activity and increased vasopressin neuronal activity impair maternal behavior in CD dams, which can be prevented by IAO .

Involvement of Protein Kinase A in Oxytocin Neuronal Activity in Rat Dams with Pup Deprivation

Oxytocin (OT) neuronal activity is the key factor for breastfeeding and it can be disrupted by mother-baby separation. To explore cellular mechanisms underlying OT neuronal activity, we studied the role of protein kinase A (PKA) in OT neuronal activity in the supraoptic nucleus (SON) using a rodent model of pup deprivation (PD) Intermittent (IPD) or continuous (CPD) PD significantly reduced suckling duration and number of milk ejections in lactating rats, particularly those with CPD. In Western blots of the SON, PD increased expressions of OT receptor (OTR) and its immediate downstream effectors, Gαq and Gβ subunits, particularly IPD, but reduced the expression of catalytic subunit of PKA (cPKA). In brain slices, inhibition of PKA blocked prostaglandin E2-evoked increase in firing activity including burst firing in OT neurons. In IPD dams, filamentous actin formed ring-like structures in the cytoplasmic region of OT neurons, which was reduced in CPD. Moreover, molecular association between actin and cPKA also reduced in PD dams. Incubation of brain slices with OT reduced the expression of cPKA, which was blocked by pretreatment with atosiban, an antagonist of OTR. These results indicate that PD disrupts OT neuronal activity through dissociating the Gq proteins and PKA in OTR-associated signaling cascade, which couples with reduced interactions between filamentous actin and PKA in OT neurons in the SON. This study highlights that PKA can be a novel target treating abnormal OT neuronal activity and its associated diseases.

Cardiovascular protective properties of oxytocin against COVID-19

SARS-CoV-2 infection or COVID-19 has become a worldwide pandemic; however, effective treatment for COVID-19 remains to be established. Along with acute respiratory distress syndrome (ARDS), new and old cardiovascular injuries are important causes of significant morbidity and mortality in COVID-19. Exploring new approaches managing cardiovascular complications is essential in controlling the disease progression and preventing long-term complications. Oxytocin (OXT), an immune-regulating neuropeptide, has recently emerged as a strong candidate for treatment and prevention of COVID-19 pandemic. OXT carries special functions in immunologic defense, homeostasis and surveillance. It suppresses neutrophil infiltration and inflammatory cytokine release, activates T-lymphocytes, and antagonizes negative effects of angiotensin II and other key pathological events of COVID-19. Additionally, OXT can promote γ-interferon expression to inhibit cathepsin L and increases superoxide dismutase expression to reduce heparin and heparan sulphate fragmentation. Through these mechanisms, OXT can block viral invasion, suppress cytokine storm, reverse lymphocytopenia, and prevent progression to ARDS and multiple organ failures. Importantly, besides prevention of metabolic disorders associated with atherosclerosis and diabetes mellitus, OXT can protect the heart and vasculature through suppressing hypertension and brain-heart syndrome, and promoting regeneration of injured cardiomyocytes. Unlike other therapeutic agents, exogenous OXT can be used safely without the side-effects seen in remdesivir and corticosteroid. Importantly, OXT can be mobilized endogenously to prevent pathogenesis of COVID-19. This article summarizes our current understandings of cardiovascular pathogenesis caused by COVID-19, explores the protective potentials of OXT against COVID-19-associated cardiovascular diseases, and discusses challenges in applying OXT in treatment and prevention of COVID-19.

Chemical compounds

Angiotensin-converting enzyme 2 (ACE2); atrial natriuretic peptide (ANP); cathepsin L; heparan sulphate proteoglycans (HSPGs); interferon; interleukin; oxytocin; superoxide dismutase; transmembrane serine protease isoform 2 (TMPRSS2).

Key Roles of Cyclooxygenase 2-Protein Kinase A-Hyperpolarization-activated Cyclic Nucleotide-gated Channel 3 Pathway in the Regulation of Oxytocin Neuronal Activity in Lactating Rats with Intermittent Pup-Deprivation

Suckling-evoked pulsatile release of oxytocin (OT) from the posterior pituitary plays a key role in breastfeeding, which relies on burst-like discharges of OT neurons. To explore cellular mechanisms regulating OT neuronal activity, using lactating rats with pup-deprivation (PD) during postpartum day 1-5, we observed the involvement of prostaglandin, cyclic AMP/protein kinase A (PKA) and hyperpolarization-activated cyclic nucleotide-gated channel 3 (HCN3) signaling pathway in OT neuronal activity. PD gradually reduced lactation efficiency. Intermittent PD (IPD) was largely reversed by intranasally-applied OT (IAO) but not by hypodermically-applied OT. IPD caused involution-like histological changes in the mammary glands, increased hypothalamic OT release but did not influence plasma OT concentrations. In the supraoptic nucleus, IPD increased OT receptor (OTR) expressions in OT neurons as well as Gαq subunit, Gβ subunit and cyclooxygenase 2 (Cox-2). These effects except that on Gβ subunit were reversed by IAO. Notably, IPD increased the expression of catalytic subunit of PKA in the SON, specifically in vasopressin neurons but not in OT neurons. In addition, IPD increased the expression of HCN3. IAO partially reversed these changes in the SON. Lastly, blocking HCN3 blocked excitation and burst firing in OT neurons-evoked by prostaglandin E₂, a key mediator of OT-evoked burst firing; blocking Cox-2 or PKA reduced the molecular association between OTR and HCN3. Thus, there is a prostaglandin-cAMP/PKA-HCN3 pathway in the regulation of OT neuronal activity. PD disrupts lactation performance through uncoupling OTR and PKA-HCN3 signaling. The reversal effect of IAO highlights its therapeutic potential in PD-evoked hypogalactia.

Oxytocin Inhibition of Metastatic Colorectal Cancer by Suppressing the Expression of Fibroblast Activation Protein-α

Oxytocin (OXT) and its receptor (OXTR) are present in the gastrointestinal system and are involved in gastrointestinal tumorigenesis. However, the effect of OXTR signaling on the development of colorectal cancer (CRC) and its underlying mechanisms remain unexplored. To address these issues, we first examined the expressions of OXT, OXTR, and several cancer-associated proteins using colon “tissue chips” from a spectrum of malignant progression of the colon, which included normal colon tissue, chronic colitis, colorectal adenoma, and colorectal adenocarcinoma (CAC). The results showed that the expressions of OXT and OXTR decreased gradually with the malignant progression of the disease. Stimulation of CAC tissues with OXT increased OXTR expression while down-regulated FAPα and CCL-2 protein expressions in a concentration- and time-dependent manner. Moreover, cell invasion experiment showed that OXT treatment reduced the invasion ability of colon cancer cells and blocking OXTR with atosiban blocked OXT-reduced invasion ability of human colon cancer cell lines Ls174T and SW480. The results indicate that OXT has the potential to inhibit CRC development via down-regulating the immunosuppressive proteins FAPα and CCL-2. When the OXTR signaling is weakened, colon tissues may transform to CRC. These findings also highlight the possibility of applying OXT to inhibit CRC development directly.

Involvement of Hyperpolarization-Activated Cyclic Nucleotide-Gated Channel 3 in Oxytocin Neuronal Activity in Lactating Rats With Pup Deprivation

Oxytocin, a hypothalamic neuropeptide essential for breastfeeding, is mainly produced in oxytocin neurons in the supraoptic nucleus (SON) and paraventricular nucleus. However, mechanisms underlying oxytocin secretion, specifically the involvement of hyperpolarization-activated cyclic nucleotide-gated channel 3 (HCN3) in oxytocin neuronal activity, remain unclear. Using a rat model of intermittent and continuous pup deprivation (PD) at the middle stage of lactation, we analyzed the contribution of HCN3 in oxytocin receptor (OTR)-associated signaling cascade to oxytocin neuronal activity in the SON. PD caused maternal depression, anxiety, milk shortage, involution of the mammary glands, and delays in uterine recovery, particularly in continuous PD. PD increased hypothalamic but not plasma oxytocin levels in enzyme-linked immunosorbent assay. In the SON, PD increased c-Fos expression but reduced expressions of cyclooxygenase-2 and HCN3 in Western blots and/or immunohistochemistry. Moreover, PD significantly increased the molecular association of OTR with HCN3 in coimmunoprecipitation. In brain slices, inhibition of HCN3 activity with DK-AH269 blocked prostaglandin E₂-evoked increase in the firing activity and burst discharge in oxytocin neurons in patch-clamp recordings. In addition, oxytocin-evoked increase in the molecular association between OTR and HCN3 in brain slices of the SON was blocked by pretreatment with indomethacin, an inhibitor of cyclooxygenase-2. These results indicate that normal activity of oxytocin neurons is under the regulation of an oxytocin receptor-cyclooxygenase-2-HCN3 pathway and that PD disrupts maternal behavior through increasing intranuclear oxytocin secretion in the SON but likely reducing bolus oxytocin release into the blood through inhibition of HCN3 activity.

Role of Connexin 36 in Autoregulation of Oxytocin Neuronal Activity in Rat Supraoptic Nucleus

In the supraoptic nucleus (SON), the incidence of dye coupling among oxytocin (OT) neurons increases significantly in nursing mothers. However, the type(s) of connexin (Cx) involved is(are) unknown. In this study, we specifically investigated whether Cx36 plays a functional role in the coupling between OT neurons in the SON of lactating rats. In this brain region, Cx36 was mainly coimmunostained with vasopressin neurons in virgin female rats, whereas in lactating rats, Cx36 was primarily colocalized with OT neurons. In brain slices from lactating rats, application of quinine (0.1 mM), a selective blocker of Cx36, significantly reduced dye coupling among OT neurons as well as the discharge/firing frequency of spikes/action potentials and their amplitude, and transiently depolarized the membrane potential of OT neurons in whole-cell patch-clamp recordings. However, quinine significantly reduced the amplitude, but not frequency, of inhibitory postsynaptic currents in OT neurons; the duration of excitatory postsynaptic currents was reduced but not their frequency and amplitude. Furthermore, the excitatory effect of OT (1 pM) on OT neurons was significantly weakened and delayed by quinine, and burst firing was absent in the presence of this inhibitor. Lastly, Western blotting analysis revealed that the presence of combined, but not alone, quinine and OT significantly reduced the amount of Cx36 in the SON. Thus, Cx36-mediated junctional communication plays a crucial role in autoregulatory control of OT neuronal activity, likely by acting at the postsynaptic sites. The level of Cx36 is modulated by its own activity and the presence of OT.

Astroglial Modulation of Hydromineral Balance and Cerebral Edema

Maintenance of hydromineral balance (HB) is an essential condition for life activity at cellular, tissue, organ and system levels. This activity has been considered as a function of the osmotic regulatory system that focuses on hypothalamic vasopressin (VP) neurons, which can reflexively release VP into the brain and blood to meet the demand of HB. Recently, astrocytes have emerged as an essential component of the osmotic regulatory system in addition to functioning as a regulator of the HB at cellular and tissue levels. Astrocytes express all the components of osmoreceptors, including aquaporins, molecules of the extracellular matrix, integrins and transient receptor potential channels, with an operational dynamic range allowing them to detect and respond to osmotic changes, perhaps more efficiently than neurons. The resultant responses, i.e., astroglial morphological and functional plasticity in the supraoptic and paraventricular nuclei, can be conveyed, physically and chemically, to adjacent VP neurons, thereby influencing HB at the system level. In addition, astrocytes, particularly those in the circumventricular organs, are involved not only in VP-mediated osmotic regulation, but also in regulation of other osmolality-modulating hormones, including natriuretic peptides and angiotensin. Thus, astrocytes play a role in local/brain and systemic HB. The adaptive astrocytic reactions to osmotic challenges are associated with signaling events related to the expression of glial fibrillary acidic protein and aquaporin 4 to promote cell survival and repair. However, prolonged osmotic stress can initiate inflammatory and apoptotic signaling processes, leading to glial dysfunction and a variety of brain diseases. Among many diseases of brain injury and hydromineral disorders, cytotoxic and osmotic cerebral edemas are the most common pathological manifestation. Hyponatremia is the most common cause of osmotic cerebral edema. Overly fast correction of hyponatremia could lead to central pontine myelinolysis. Ischemic stroke exemplifies cytotoxic cerebral edema. In this review, we summarize and analyze the osmosensory functions of astrocytes and their implications in cerebral edema.

The Action Radius of Oxytocin Release in the Mammalian CNS: From Single Vesicles to Behavior

The hypothalamic neuropeptide oxytocin (OT) has attracted the attention both of the scientific community and a general audience because of its prosocial effects in mammals, and OT is now seen as a facilitator of mammalian species propagation. Furthermore, OT is a candidate for the treatment of social deficits in several neuropsychiatric and neurodevelopmental conditions. Despite such possibilities and a long history of studies on OT behavioral effects, the mechanisms of OT actions in the brain remain poorly understood. In the present review, based on anatomical, biochemical, electrophysiological, and behavioral studies, we propose a novel model of local OT actions in the central nervous system (CNS) via focused axonal release, which initiates intracellular signaling cascades in specific OT-sensitive neuronal populations and coordinated brain region-specific behaviors.

Oxytocin-secreting system: A major part of the neuroendocrine center regulating immunologic activity

Interactions between the nervous system and immune system have been studied extensively. However, the mechanisms underlying the neural regulation of immune activity, particularly the neuroendocrine regulation of immunologic functions, remain elusive. In this review, we provide a comprehensive examination of current evidence on interactions between the immune system and hypothalamic oxytocin-secreting system. We highlight the fact that oxytocin may have significant effects in the body, beyond its classical functions in lactation and parturition. Similar to the hypothalamo-pituitary-adrenal axis, the oxytocin-secreting system closely interacts with classical immune system, integrating both neurochemical and immunologic signals in the central nervous system and in turn affects immunologic defense, homeostasis, and surveillance. Lastly, this review explores therapeutic potentials of oxytocin in treating immunologic disorders.

Neuroendocrine regulation of lactation and milk production

Prolactin (PRL) released from lactotrophs of the anterior pituitary gland in response to the suckling by the offspring is the major hormonal signal responsible for stimulation of milk synthesis in the mammary glands. PRL secretion is under chronic inhibition exerted by dopamine (DA), which is released from neurons of the arcuate nucleus of the hypothalamus into the hypophyseal portal vasculature. Suckling by the young activates ascending systems that decrease the release of DA from this system, resulting in enhanced responsiveness to one or more PRL-releasing hormones, such as thyrotropin-releasing hormone. The neuropeptide oxytocin (OT), synthesized in magnocellular neurons of the hypothalamic supraoptic, paraventricular, and several accessory nuclei, is responsible for contracting the myoepithelial cells of the mammary gland to produce milk ejection. Electrophysiological recordings demonstrate that shortly before each milk ejection, the entire neurosecretory OT population fires a synchronized burst of action potentials (the milk ejection burst), resulting in release of OT from nerve terminals in the neurohypophysis. Both of these neuroendocrine systems undergo alterations in late gestation that prepare them for the secretory demands of lactation, and that reduce their responsiveness to stimuli other than suckling, especially physical stressors. The demands of milk synthesis and release produce a condition of negative energy balance in the suckled mother, and, in laboratory rodents, are accompanied by a dramatic hyperphagia. The reduction in secretion of the adipocyte hormone, leptin, a hallmark of negative energy balance, may be an important endocrine signal to hypothalamic systems that integrate lactation-associated food intake with neuroendocrine systems.

In vitro differentiation between oxytocin- and vasopressin-secreting magnocellular neurons requires more than one experimental criterion

The phenotypic differentiation between oxytocin (OT)- and vasopressin (VP)-secreting magnocellular neurosecretory cells (MNCs) from the supraoptic nucleus is relevant to understanding how several physiological and pharmacological challenges affect their electrical activity. Although the firing patterns of OT and VP neurons, both in vivo and in vitro, may appear different from each other, much is assumed about their characteristics. These assumptions make it practically impossible to obtain a confident phenotypic differentiation based exclusively on the firing patterns. The presence of a sustained outward rectifying potassium current (SOR) and/or an inward rectifying hyperpolarization-activated current (IR), which are presumably present in OT neurons and absent in VP neurons, has been used to distinguish between the two types of MNCs in the past. In this study, we aimed to analyze the accuracy of the phenotypic discrimination of MNCs based on the presence of rectifying currents using comparisons with the molecular phenotype of the cells, as determined by single-cell RT-qPCR and immunohistochemistry. Our results demonstrated that the phenotypes classified according to the electrophysiological protocol in brain slices do not match their molecular counterparts because vasopressinergic and intermediate neurons also exhibit both outward and inward rectifying currents. In addition, we also show that MNCs can change the relative proportion of each cell phenotype when the system is challenged by chronic hypertonicity (70% water restriction for 7 days). We conclude that for in vitro preparations, the combination of mRNA detection and immunohistochemistry seems to be preferable when trying to characterize a single MNC phenotype.

Performance, properties and plasticity of identified oxytocin and vasopressin neurones in vitro

The neurohypophysial hormones oxytocin (OT) and vasopressin (VP) originate from hypothalamic neurosecretory cells in the paraventricular and supraoptic (SON) nuclei. The firing rate and pattern of action potentials arising from these neurones determine the timing and quantity of peripheral hormone release. We have used immunochemical identification of biocytin-filled SON neurones in hypothalamic slices in vitro to uncover differences between OT and VP neurones in membrane and synaptic properties, firing patterns, and plasticity during pregnancy and lactation. In this review, we summarise some recent findings from this approach: (i) VP neuronal excitability is influenced by slow (sDAP) and fast (fDAP) depolarising afterpotentials that underlie phasic bursting activity. The fDAP may relate to a transient receptor potential (TRP) channel, type melastatin (TRPM4 and/or TRPM5), both of which are immunochemically localised more to VP neurones, and especially, to their dendrites. Both TRPM4 and TRPM5 mRNAs are found in the SON, but single cell reverse transcriptase-polymerisation suggests that TRPM4 might be the more prominent channel. Phasic bursting in VP neurones is little influenced by spontaneous synaptic activity in slices, being shaped largely by intrinsic currents. (ii) The firing pattern of OT neurones ranges from irregular to continuous, with the coefficient of variation determined by randomly distributed, spontaneous GABAergic, inhibitory synaptic currents (sIPSCs). These sIPSCs are four- to five-fold more frequent in OT versus VP neurones, and much more frequent than spontaneous excitatory synaptic currents. (iii) Both cell types express Ca(2+)-dependent afterhyperpolarisations (AHPs), including an apamin-sensitive, medium duration AHP and a slower, apamin-insensitive AHP (sAHP). In OT neurones, both AHPs are enhanced during pregnancy and lactation. During pregnancy, the plasticity of the sAHP is blocked by antagonism of central OT receptors. AHP enhancement is mimicked by exposing slices from day 19 pregnant rats to OT and oestradiol, suggesting that central OT and sex steroids programme this plasticity during pregnancy by direct hypothalamic actions. In conclusion, the differences in VP and OT neuronal function are underlain by differences in both membrane and synaptic properties, and differentially modulated by reproductive state.

The adaptive brain: Glenn Hatton and the supraoptic nucleus

In December 2009, Glenn Hatton died, and neuroendocrinology lost a pioneer who had done much to forge our present understanding of the hypothalamus and whose productivity had not faded with the passing years. Glenn, an expert in both functional morphology and electrophysiology, was driven by a will to understand the significance of his observations in the context of the living, behaving organism. He also had the wit to generate bold and challenging hypotheses, the wherewithal to expose them to critical and elegant experimental testing, and a way with words that gave his papers and lectures clarity and eloquence. The hypothalamo-neurohypophysial system offered a host of opportunities for understanding how physiological functions are fulfilled by the electrical activity of neurones, how neuronal behaviour changes with changing physiological states, and how morphological changes contribute to the physiological response. In the vision that Glenn developed over 35 years, the neuroendocrine brain is as dynamic in structure as it is adaptable in function. Its adaptability is reflected not only by mere synaptic plasticity, but also by changes in neuronal morphology and in the morphology of the glial cells. Astrocytes, in Glenn's view, were intimate partners of the neurones, partners with an essential role in adaptation to changing physiological demands.

Chronic vs. acute interactions between supraoptic oxytocin neurons and astrocytes during lactation: role of glial fibrillary acidic protein plasticity

In this article, we review studies of astrocytic-neuronal interactions and their effects on the activity of oxytocin (OXT) neurons within the magnocellular hypothalamo-neurohypophysial system. Previous work over several decades has shown that withdrawal of astrocyte processes increases OXT neuron excitability in the hypothalamic supraoptic nucleus (SON) during lactation. However, chronically disabling astrocyte withdrawal does not significantly affect the functioning of OXT neurons during suckling. Nevertheless, acute changes in a cytoskeletal element of astrocytes, glial fibrillary acidic protein (GFAP), occur in concert with changes in OXT neuronal activity during suckling. Here, we compare these changes in GFAP and related proteins with chronic changes that persist throughout lactation. During lactation, a decrease in GFAP levels accompanies retraction of astrocyte processes surrounding OXT neurons in the SON, resulting from high extracellular levels of OXT. During the initial stage of suckling, acute increases in OXT levels further strengthen this GFAP reduction and facilitate the retraction of astrocyte processes. This change, in turn, facilitates burst discharges of OXT neurons and leads to a transient increase in excitatory neurochemicals. This transient neurochemical surge acts to reverse GFAP expression and results in postburst inhibition of OXT neurons. The acute changes in astrocyte GFAP levels seen during suckling likely recur periodically, accompanied by rhythmic changes in glutamate metabolism, water transport, gliotransmitter release, and spatial relationships between astrocytes and OXT neurons. In the neurohypophysis, astrocyte retraction and reversal with accompanying GFAP plasticity also likely occur during lactation and suckling, which facilitates OXT release coordinated with its action in the SON. These studies of the dynamic interactions that occur between astrocytes and OXT neurons mediated by GFAP extend our understanding of astrocyte functions within the central nervous system.

Emergent synchronous bursting of oxytocin neuronal network

When young suckle, they are rewarded intermittently with a let-down of milk that results from reflex secretion of the hormone oxytocin; without oxytocin, newly born young will die unless they are fostered. Oxytocin is made by magnocellular hypothalamic neurons, and is secreted from their nerve endings in the pituitary in response to action potentials (spikes) that are generated in the cell bodies and which are propagated down their axons to the nerve endings. Normally, oxytocin cells discharge asynchronously at 1–3 spikes/s, but during suckling, every 5 min or so, each discharges a brief, intense burst of spikes that release a pulse of oxytocin into the circulation. This reflex was the first, and is perhaps the best, example of a physiological role for peptide-mediated communication within the brain: it is coordinated by the release of oxytocin from the dendrites of oxytocin cells; it can be facilitated by injection of tiny amounts of oxytocin into the hypothalamus, and it can be blocked by injection of tiny amounts of oxytocin antagonist. Here we show how synchronized bursting can arise in a neuronal network model that incorporates basic observations of the physiology of oxytocin cells. In our model, bursting is an emergent behaviour of a complex system, involving both positive and negative feedbacks, between many sparsely connected cells. The oxytocin cells are regulated by independent afferent inputs, but they interact by local release of oxytocin and endocannabinoids. Oxytocin released from the dendrites of these cells has a positive-feedback effect, while endocannabinoids have an inhibitory effect by suppressing the afferent input to the cells.

When young suckle, they are rewarded intermittently with a let-down of milk that results from reflex secretion of the hormone oxytocin. Oxytocin is a neuropeptide made by specialised neurons in the hypothalamus, and is secreted from nerve endings in the pituitary gland. During suckling, every 5 min or so, each of these neurons discharges a brief, intense burst of action potentials; these are propagated down the axons, and release a pulse of oxytocin into the circulation. Here, we have built a computational model to understand how these bursts arise and how they are synchronized. In our model, bursting is an emergent behaviour of a complex system, involving both positive and negative feedbacks, between many, sparsely connected cells. The oxytocin cells are regulated by independent afferent inputs, but they interact by local release of oxytocin and endocannabinoids. Oxytocin released from the dendrites of these cells has a positive-feedback effect, while endocannabinoids have an inhibitory effect by suppressing the afferent input to the cells. Many neurons make peptides that act as messengers within the brain, and many of these are also released from dendrites, so this model may reflect a common pattern-generating mechanism in the brain.

Epigenetic programming of phenotypic variations in reproductive strategies in the rat through maternal care

Studies across multiple organisms reveal considerable phenotypic variation in reproductive tactics. In some species, this variation is associated with maternal effects in which variation in maternal investment results in stable individual differences in reproductive function. Recent studies with the rat suggest that maternal effects can alter the function of neuroendocrine systems associated with female sexual behaviour as well as maternal behaviour. These maternal effects appear to be mediated by epigenetic modifications at the promoter for oestrogen receptor alpha (ERalpha) and subsequent effects on gene expression. The tissue-specific nature of such effects may underlie the co-ordinated variation in multiple forms of reproductive function, resulting in distinct reproductive strategies.

Dominant role of betagamma subunits of G-proteins in oxytocin-evoked burst firing

Pulsatile neuropeptide secretion is associated with burst firing patterns; however, intracellular signaling cascades leading to bursts remain unclear. We explored mechanisms underlying burst firing in oxytocin (OT) neurons in the supraoptic nucleus in brain slices from lactating rats. Application of 10 pm OT for 30 min or progressively rising OT concentrations from 1 to 100 pm induced burst firing in OT neurons in patch-clamp recordings. Burst generation was blocked by OT antagonist and ionotropic glutamate receptor blockers or tetanus toxin. Blocking G-protein activation with suramin or intracellular GDP-beta-S, but not intracellularly administered antibody against the OT-receptor (OTR) C terminus, blocked bursts. Moreover, pretreatment of slices with pertussis toxin, an inhibitor of G(i/o)-proteins, did not block OT-evoked bursts, suggesting that G(i)/G(o) activation is unnecessary for burst generation. Thus, we further examined G alpha(q/11)-associated signaling pathways in OT-evoked bursts. Inhibition of phospholipase C or RhoA/Rho kinase did not block bursts. Activation of G betagamma subunits using myristoylated G betagamma-binding peptide (mSIRK) caused bursts, whereas intracellularly loaded antibody against G beta subunit blocked OT-evoked bursts. Blocking Src family kinase, but not phosphatidylinositol 3-kinase, occluded OT-evoked bursts. Similar to the effects of OT on EPSCs, mSIRK inhibited tonic EPSCs and elicited EPSC clustering. Finally, suckling caused dissociation of OTRs and G beta subunits from G alpha(q/11) subunits shown by coimmunoprecipitation and immunocytochemistry, supporting crucial roles for OTRs and G betagamma subunits in the milk-ejection reflex. We conclude that G betagamma subunits play a dominant role in burst firing evoked by applied OT or by suckling.