Pituitary portal plasma levels of oxytocin during the estrous cycle, lactation, and hyperprolactinemia

Estrogen-dependent oxytocin expression in the hypothalamus and estrogen-dependent vasopressin in the median eminence

The posterior pituitary (PP) hormones oxytocin (OXT) and arginine vasopressin (AVP) are synthesized within the hypothalamic nucleus and released from the PP into systemic circulation. Hypothalamic AVP projects its axons into the external layer of median eminence (eME) and regulates anterior pituitary hormone secretion during stress responses. Although similar as PP hormones, we demonstrate distinct regulatory roles of estrogen in hypothalamic OXT and AVP dynamics. OXT dynamics in the hypothalamus exhibit sex-dependent variations and that estrogen may influence dynamic OXT level changes, as observed in OXT-mRFP1 transgenic rats. Estrogen was also observed to modulate dynamic changes in AVP levels in the axon terminals of eME in female AVP-eGFP transgenic rats. Although OXT and AVP are produced within the similar hypothalamic region, both exhibit distinct dynamics within the hypothalamus. Estrogen acts on the hypothalamus, and further effects of estrogen replacement therapy can be expected.

Prolactin Is Associated With Insulin Resistance and Beta-Cell Dysfunction in Infertile Women With Polycystic Ovary Syndrome

BACKGROUND

Our study aimed to investigate if serum prolactin (PRL) levels associated with insulin resistance and beta-cell dysfunction in infertile patients with polycystic ovary syndrome (PCOS).

METHODS

This was a retrospective cross-sectional study performed in the reproductive medicine center of the first affiliated hospital of Wenzhou Medical University. From January 2007 to August 2018, a total of 792 PCOS and 700 non-PCOS infertile women were included. All patients' prolactin levels were in the normal range. PCOS was diagnosed according to the Rotterdam Criteria. Anthropometric parameters, blood pressure, serum prolactin levels, sex hormones, fasting lipids, fasting plasma glucose (FPG), fasting insulin (FINS) and hepatic biological parameters were measured in all subjects.

RESULTS

Serum prolactin levels in PCOS women were significantly decreased compared with levels in non-PCOS women after adjusting for age and BMI (P < 0.05). Moreover, we found that prolactin levels were positively associated with high-density lipoprotein cholesterol (HDL-C) and negatively associated with age, BMI, waist circumference (WC), hip circumference (HC), luteinizing hormone/follicle stimulating hormone (LH/FSH), estradiol (E₂), FINS, homeostasis model assessment of insulin resistance (HOMA-IR), homeostasis model assessment of β (HOMA-β), triglyceride (TG) and alanine aminotransferase (ALT) (P < 0.05). After adjusting for age and BMI, multiple linear regression analysis revealed that LH, LH/FSH, E₂, FINS, HOMA-IR, and HOMA-β were negatively associated with serum PRL (P < 0.05).

CONCLUSIONS

Low serum PRL levels within the normal range associates with a higher incidence of insulin resistance and beta-cell dysfunction in infertile women with PCOS.

Sex differences of oxytocin and vasopressin in social behaviors

The neuropeptides oxytocin (OT) and vasopressin (VP) are known to mediate social cognition and behaviors in a sex-dependent manner. This chapter reviews the sex-dependent influence of OT and VP on social behaviors, focusing on (1) partner preference and sexual orientation, (2) memory modulation, (3) emotion regulation, and (4) trust-related behaviors. Most studies suggest that OT promotes familiar (opposite-sex) partner preference, strengthens memory, relieves anxiety, and increases trust. However, VP-regulated social cognition has been studied less than OT. VP facilitates familiar (opposite-sex) partner preference, enhances memory, induces anxiety, and influences happiness/anger perception. Detailed sex differences of these effects are reviewed. There is a male preponderance in the use of animal models and many study results are too complex to draw firm conclusions. Clarifying the complex interplay between the OT/VP system and sex hormones in the regulation of social behaviors is needed.

Hyperprolactinemic African elephant (Loxodonta africana) females exhibit elevated dopamine, oxytocin and serotonin concentrations compared to normal cycling and noncycling, low prolactin elephants†

Many zoo elephants do not cycle normally, and for African elephants, it is often associated with hyperprolactinemia. Dopamine agonists successfully treat hyperprolactinemia-induced ovarian dysfunction in women, but not elephants. The objective of this study was to determine how longitudinal dopamine, serotonin, and oxytocin patterns in African elephants are related to ovarian cycle function. We hypothesized that dopamine concentrations are decreased, while oxytocin and serotonin are increased in non-cycling, hyperprolactinemic African elephants. Weekly urine and serum samples were collected for eight consecutive months from 28 female African elephants. Females were categorized as follows: (1) non-cycling with average prolactin concentrations of 15 ng/ml or greater (HIGH; n = 7); (2) non-cycling with average prolactin concentrations below 15 ng/ml (LOW; n = 13); and (3) cycling with normal progestagen and prolactin patterns (CYCLING; n = 8). Both oxytocin and serotonin were elevated in hyperprolactinemic elephants. Thus, we propose that stimulatory factors may play a role in the observed hyperprolactinemia in this species. Interestingly, rather than being reduced as hypothesized, urinary dopamine was elevated in hyperprolactinemic elephants compared to CYCLING and LOW prolactin groups. Despite its apparent lack of regulatory control over prolactin, this new evidence suggests that dopamine synthesis and secretion are not impaired in these elephants, and perhaps are augmented.

The Association Between Prolactin and Metabolic Parameters in PCOS Women: A Retrospective Analysis

Background: The aim of this retrospective study was to analyze the association between prolactin (PRL) and metabolic parameters in infertile patients with polycystic ovary syndrome (PCOS). Methods: A total of 2,052 patients with PCOS and 9,696 patients with tubal infertility (non-PCOS) undergoing in vitro fertilization and embryo transfer (IVF-ET) at the reproductive medicine center of the first affiliated hospital of Wenzhou Medical University from January 2007 to July 2017 were enrolled in this study. Serum PRL, basic endocrine hormones, fasting plasma lipid, fasting plasma glucose (FPG), liver function, thyroid hormone and other parameters were measured and analyzed. Result: PRL levels were significantly lower in PCOS patients than controls over all age groups (p < 0.05). In the PCOS patients, serum PRL was significantly and positively correlated with FPG, serum TSH and serum FT4, and significantly and negatively correlated with LH, LH/FSH, TC, TG, LDL-C, AST, ALT, γ-GGT, FT3, and FT3/FT4 (p < 0.05 or 0.01). After adjusted for age and body mass index (BMI), serum PRL was positively correlated with FPG, TSH, and FT4, and negatively correlated with LH and LH/FSH. Conclusion: Low serum PRL may be an important cause of metabolic risk in infertile patients with PCOS.

Update on Comparative Biology of Elephants: Factors Affecting Reproduction, Health and Welfare

Asian (Elephas maximus) and African (Loxodonta africana) elephants serve as important keystone, umbrella and flagship species. Despite that, population numbers are declining, due mainly to poaching and habitat destruction. Understanding reproductive mechanisms is vital to effective management, particularly insurance populations in captivity, and to that end, long-term biological databases are key to understanding how intrinsic and extrinsic factors affect reproductive function at individual and population levels. Through decades of hormonal and ultrasonographic monitoring, many unique aspects of zoo elephant reproduction have been identified, including differences in luteal steroidogenic activity, follicular maturation, pituitary gonadotropin secretion, fetal development and reproductive tract anatomy. Reproductive problems also hamper captive propagation efforts, particularly those related to abnormal or lack of ovarian cyclicity. Recent large-scale, multi-institutional studies and use of epidemiological approaches have identified factors important for good welfare and reproduction, which include enrichment, feeding diversity, good elephant-keeper relations, social compatibility, exercise, and not being obese. There are notable differences in reproductive mechanisms between Asian and African elephants, as well as the factors that influence reproduction and welfare, suggesting species-targeted management approaches are needed to maximize fitness. In the first edition, we discussed reproductive function in male and female elephants. Since then, a number of significant advances have been made primarily in female elephants, which will be the focus of this updated review.

Sexual dimorphism of oxytocin and vasopressin in social cognition and behavior

The neuropeptides oxytocin (OT) and vasopressin (VP) are hormones that are known to mediate social behavior and cognition, but their influence may be sex-dependent. This paper aims to provide a comprehensive review of the sex-related influence of OT and VP on social cognition, focusing on partner preference and sexual orientation, trust and relevant behaviors, memory modulation, and emotion regulation. Most studies have suggested that OT facilitates familiar-partner preference in both sexes, with females being more significant, increased trust in others, especially for male, enhanced memory in either sex, and reduced anxious emotion in males. However, VP-regulated social cognition has been less studied. Other relevant studies have indicated that VP facilitated familiar-partner preference, improved memory, induced empathy formation, increased positive-emotion recognition, and induced anxiety without any sex difference. However, there was a male preponderance among studies, and results were often too complex to draw firm conclusions. Clarifying the interplay between OT/VP and sex hormones in the regulation of social cognition is necessary for further applications.

Menstrual cycle-related fluctuations in oxytocin concentrations: A systematic review and meta-analysis

Oxytocin affects physiological and psychological functions that are often expressed sex-specifically, suggesting interactions between oxytocin and sex hormones. As female sex hormone concentrations change during the menstrual cycle, oxytocin might fluctuate, too. This systematic review and meta-analysis investigated endogenous oxytocin concentrations across menstrual cycle phases in healthy women. Data from 13 studies (120 women) showed a significant increase of oxytocin concentrations from the early follicular phase to ovulation (g = 0.39 [0.25; 0.53], p < .001) and a significant decrease from ovulation to the mid-luteal phase (g = -0.50 [-0.81; -0.18], p < .001). There were no significant differences between the early follicular and mid-luteal phase (g = -0.19 [-0.70; -0.32], p = .471). These findings contribute to a deeper understanding of differences in normal and abnormal psychobiological processes in women. They highlight the necessity to consider the menstrual cycle phase in studies on oxytocin in women.

Rapid effects of estrogens on short-term memory: Possible mechanisms

Contribution to Special Issue on Fast effects of steroids. Estrogens affect learning and memory through rapid and delayed mechanisms. Here we review studies on rapid effects on short-term memory. Estradiol rapidly improves social and object recognition memory, spatial memory, and social learning when administered systemically. The dorsal hippocampus mediates estrogen rapid facilitation of object, social and spatial short-term memory. The medial amygdala mediates rapid facilitation of social recognition. The three estrogen receptors, α (ERα), β (ERβ) and the G-protein coupled estrogen receptor (GPER) appear to play different roles depending on the task and brain region. Both ERα and GPER agonists rapidly facilitate short-term social and object recognition and spatial memory when administered systemically or into the dorsal hippocampus and facilitate social recognition in the medial amygdala. Conversely, only GPER can facilitate social learning after systemic treatment and an ERβ agonist only rapidly improved short-term spatial memory when given systemically or into the hippocampus, but also facilitates social recognition in the medial amygdala. Investigations into the mechanisms behind estrogens' rapid effects on short term memory showed an involvement of the extracellular signal-regulated kinase (ERK) and the phosphoinositide 3-kinase (PI3K) kinase pathways. Recent evidence also showed that estrogens interact with the neuropeptide oxytocin in rapidly facilitating social recognition. Estrogens can increase the production and/or release of oxytocin and other neurotransmitters, such as dopamine and acetylcholine. Therefore, it is possible that estrogens' rapid effects on short-term memory may occur through the regulation of various neurotransmitters, although more research is need on these interactions as well as the mechanisms of estrogens' actions on short-term memory.

Sex differences in the brain: Implications for behavioral and biomedical research

Biological differences between males and females are found at multiple levels. However, females have too often been under-represented in behavioral neuroscience research, which has stymied the study of potential sex differences in neurobiology and behavior. This review focuses on the study of sex differences in the neurobiology of social behavior, memory, emotions, and recovery from brain injury, with particular emphasis on the role of estrogens in regulating forebrain function. This work, presented by the authors at the 2016 meeting of the International Behavioral Neuroscience Society, emphasizes varying approaches from several mammalian species in which sex differences have not only been documented, but also become the focus of efforts to understand the mechanistic basis underlying them. This information may provide readers with useful experimental tools to successfully address recently introduced regulations by granting agencies that either require (e.g. the National Institutes of Health in the United States and the Canadian Institutes of Health Research in Canada) or recommend (e.g. Horizon 2020 in Europe) the inclusion of both sexes in biomedical research.

Oxytocin system dysfunction as a common mechanism underlying metabolic syndrome and psychiatric symptoms in schizophrenia and bipolar disorders

There is growing interest in using intranasal oxytocin (OT) to treat social dysfunction in schizophrenia and bipolar disorders (i.e., psychotic disorders). While OT treatment results have been mixed, emerging evidence suggests that OT system dysfunction may also play a role in the etiology of metabolic syndrome (MetS), which appears in one-third of individuals with psychotic disorders and associated with increased mortality. Here we examine the evidence for a potential role of the OT system in the shared risk for MetS and psychotic disorders, and its prospects for ameliorating MetS. Using several studies to demonstrate the overlapping neurobiological profiles of metabolic risk factors and psychiatric symptoms, we show that OT system dysfunction may be one common mechanism underlying MetS and psychotic disorders. Given the critical need to better understand metabolic dysregulation in these disorders, future OT trials assessing behavioural and cognitive outcomes should additionally include metabolic risk factor parameters.

60 YEARS OF NEUROENDOCRINOLOGY: The hypothalamo-prolactin axis

The hypothalamic control of prolactin secretion is different from other anterior pituitary hormones, in that it is predominantly inhibitory, by means of dopamine from the tuberoinfundibular dopamine neurons. In addition, prolactin does not have an endocrine target tissue, and therefore lacks the classical feedback pathway to regulate its secretion. Instead, it is regulated by short loop feedback, whereby prolactin itself acts in the brain to stimulate production of dopamine and thereby inhibit its own secretion. Finally, despite its relatively simple name, prolactin has a broad range of functions in the body, in addition to its defining role in promoting lactation. As such, the hypothalamo-prolactin axis has many characteristics that are quite distinct from other hypothalamo-pituitary systems. This review will provide a brief overview of our current understanding of the neuroendocrine control of prolactin secretion, in particular focusing on the plasticity evident in this system, which keeps prolactin secretion at low levels most of the time, but enables extended periods of hyperprolactinemia when necessary for lactation. Key prolactin functions beyond milk production will be discussed, particularly focusing on the role of prolactin in inducing adaptive responses in multiple different systems to facilitate lactation, and the consequences if prolactin action is impaired. A feature of this pleiotropic activity is that functions that may be adaptive in the lactating state might be maladaptive if prolactin levels are elevated inappropriately. Overall, my goal is to give a flavour of both the history and current state of the field of prolactin neuroendocrinology, and identify some exciting new areas of research development.

Prolactin regulation of kisspeptin neurones in the mouse brain and its role in the lactation-induced suppression of kisspeptin expression

Hyperprolactinaemia is a major cause of infertility in both males and females, although the mechanism by which prolactin inhibits the reproductive axis is not clear. The aim of the present study was to test the hypothesis that elevated prolactin causes suppression of kisspeptin expression in the hypothalamus, resulting in reduced release of gonadotrophin-releasing hormone (GnRH) and consequent infertility. In oestrogen-treated ovariectomised mice, chronic prolactin-treatment prevented the rise in luteinising hormone (LH) seen in vehicle-treated mice. Kiss1 mRNA was significantly suppressed in both the rostral periventricular region of the third ventricle (RP3V) and arcuate nucleus after prolactin treatment. Exogenous prolactin treatment induced phosphorylated signal transducer and activator of transcription 5 (pSTAT5) in kisspeptin neurones, and suppression of endogenous prolactin using bromocriptine reduced levels of pSTAT5 in kisspeptin neurones, suggesting that prolactin acts directly on kisspeptin neurones. By contrast, fewer than 1% of GnRH neurones expressed pSTAT5 in either dioestrous or lactating mice. As reported previously, there was significant suppression of kisspeptin mRNA and protein in the RP3V on day 7 of lactation, although not in the arcuate nucleus. Bromocriptine treatment significantly increased Kiss1 mRNA expression in the RP3V, although not to dioestrous levels. Unilateral thelectomy, aiming to eliminate sensory inputs from nipples on one side of the body, failed to alter the reduction in the number of kisspeptin neurones observed in the RP3V. These data demonstrate that chronic prolactin administration suppressed serum LH, and reduced Kiss1 mRNA levels in both the RP3V and arcuate nucleus, consistent with the hypothesis that prolactin-induced suppression of kisspeptin secretion might mediate the inhibitory effects of prolactin on GnRH secretion. During lactation, however, the suppression of Kiss1 mRNA in the RP3V was only partially reversed by the administration of bromocriptine to block elevated levels of prolactin, suggesting that, although elevated prolactin contributes to lactational anovulation, additional non-neural factors must also contribute to the lactation-induced suppression of kisspeptin neurones.

Estrogenic involvement in social learning, social recognition and pathogen avoidance

Sociality comes with specific cognitive skills that allow the proper processing of information about others (social recognition), as well as of information originating from others (social learning). Because sociality and social interactions can also facilitate the spread of infection among individuals the ability to recognize and avoid pathogen threat is also essential. We review here various studies primarily from the rodent literature supporting estrogenic involvement in the regulation of social recognition, social learning (socially acquired food preferences and mate choice copying) and the recognition and avoidance of infected and potentially infected individuals. We consider both genomic and rapid estrogenic effects involving estrogen receptors α and β, and G-protein coupled estrogen receptor 1, along with their interactions with neuropeptide systems in the processing of social stimuli and the regulation and expression of these various socially relevant behaviors.

Prolactin induces a hyperpolarising current in rat paraventricular oxytocinergic neurones

Prolactin and oxytocin are important reproductive hormones implicated in several common adaptive functions during pregnancy, pseudopregnancy and lactation. Recently, extracellular recordings of supraoptic neurones have shown that prolactin may modulate the electrical activity of oxytocinergic neurones. However, no study has been conducted aiming to establish whether prolactin directly influences this activity in oxytocinergic paraventricular neurones. In the present study, we addressed this question by studying the effects of prolactin on the electrical activity and voltage-current relationship of identified paraventricular neurones in rat brain slices. Whole-cell recordings were obtained and neurones were classified on the basis of their morphological and electrophysiological fingerprint (i.e. magnocellular or parvicellular) and neuropeptide phenotype (i.e. oxytocinergic or non-oxytocinergic). We report that prolactin elicited a hyperpolarising current in 37% of the neurones in this nucleus, of which the majority (67%) were identified as putative magnocellular oxytocin neurones and the reminder (33%) were regarded as oxytocin-negative, parvicellular neuroendocrine neurones. Our results suggest that, in addition to the well-established negative feedback loop between prolactin-secreting lactotrophs and dopaminergic neurones in the arcuate nucleus, an inhibitory feedback loop also exists between lactotrophs and oxytocinergic paraventricular neurones.

Oxytocin and social motivation

Humans are fundamentally social creatures who are ‘motivated’ to be with others. In this review we examine the role of oxytocin (OT) as it relates to social motivation. OT is synthesized in the brain and throughout the body, including in the heart, thymus, gastrointestinal tract, as well as reproductive organs. The distribution of the OT receptor (OTR) system in both the brain and periphery is even more far-reaching and its expression is subject to changes over the course of development. OTR expression is also sensitive to changes in the external environment and the internal somatic world. The OT system functions as an important element within a complex, developmentally sensitive biobehavioral system. Other elements include sensory inputs, the salience, reward, and threat detection pathways, the hypothalamic-pituitary-gonadal axis, and the hypothalamic-pituitary-adrenal stress response axis. Despite an ever expanding scientific literature, key unresolved questions remain concerning the interplay of the central and peripheral components of this complex biobehavioral system that dynamically engages the brain and the body as humans interact with social partners over the course of development.

A tale of two rhythms: the emerging roles of oxytocin in rhythmic prolactin release

Hormone secretion often occurs in a pulsatile manner. In this review, we discuss two rhythms of in vivo prolactin release in female rats and the ongoing research that we and others have performed aiming to understand the mechanisms underlying them. The peptide hormone oxytocin appears to play an important role in both rhythms. One rhythm occurs during the first half of pregnancy, but can also be induced in ovariectomised rats. This is characterised by a circadian pattern with two prolactin surges per day. Two methods for triggering this rhythm are discussed, each utilising a unique physiological pathway that includes oxytocin action, presumably on pituitary lactotrophs. The second rhythm occurs during the oestrous cycle and is characterised by a surge of prolactin on the afternoon of pro-oestrus. We discuss recent findings that oxytocin is more effective at stimulating prolactin release from lactotrophs taken from animals on the afternoon of pro-oestrus than from those of animals on the morning of dioestrus 1, raising the possibility that this hormone plays a physiological role in the regulation of prolactin secretion during the oestrous cycle.

Variations in the response of pituitary lactotrophs to oxytocin during the rat estrous cycle

Although removal of dopamine inhibition is established as a major factor in prolactin (PRL) release, a large body of evidence suggests that hypothalamic oxytocin (OT) may serve as a PRL-releasing hormone in the rat. PRL release is modulated by estradiol (E2), which rises between diestrus and proestrus of the estrous cycle, causing a PRL surge in the afternoon of proestrus. Given that E2 strongly modulates OT actions in both central and peripheral tissues, OT action on lactotrophs might also be modulated by the stage of the estrous cycle. To test this hypothesis, we have monitored PRL release and intracellular calcium levels ([Ca(2+)](i)) induced by OT in pituitary lactotrophs obtained from female rats in either diestrus 1 or proestrus. We found that both secretory and [Ca(2+)](i) responses to OT are significantly increased in lactotrophs obtained on proestrus. Moreover, we show that these differences are due to an increase in both the number of OT-responding lactotrophs and the magnitude of their individual [Ca(2+)](i) responses. Both secretory and [Ca(2+)](i) responses were abolished by a specific OT antagonist. Finally, dose-dependent studies show that the increased PRL-releasing effect of OT on proestrus is significant over a wide range of concentrations, particularly those observed in hypophyseal portal plasma. These results suggest that the rising E2 titers that culminate on proestrus facilitate the stimulatory action of OT on lactotrophs and support the notion that OT is a PRL-releasing hormone with an important role in the production of the proestrous surge of PRL.

Neuroendocrinology of social information processing in rats and mice

We reviewed oxytocin (OT), arginine-vasopressin (AVP) and gonadal hormone involvement in various modes of social information processing in mice and rats. Gonadal hormones regulate OT and AVP mediation of social recognition and social learning. Estrogens foster OT-mediated social recognition and the recognition and avoidance of parasitized conspecifics via estrogen receptor (ER) alpha (ERalpha) and ERbeta. Testosterone and its metabolites, including estrogens, regulate social recognition in males predominantly via the AVP V1a receptor. Both OT and AVP are involved in the social transmission of food preferences and ERalpha has inhibitory, while ERbeta has enhancing, roles. OT also enhances mate copying by females. ERalpha mediates the sexual, and ERbeta the recognition, aspects of the risk-taking enhancing effects of females on males. Thus, androgens and estrogens control social information processing by regulating OT and AVP. This control is finely tuned for different forms of social information processing.

Prolactin: a pleiotropic neuroendocrine hormone

The neuroendocrine control of prolactin secretion is unlike that of any other pituitary hormone. It is predominantly inhibited by the hypothalamus and, in the absence of a regulatory feedback hormone, it acts directly in the brain to suppress its own secretion. In addition to this short-loop feedback action in the brain, prolactin has been reported to influence a wide range of other brain functions. There have been few attempts to rationalise why a single hormone might exert such a range of distinct and seemingly unrelated neuroendocrine functions. In this review, we highlight some of the original studies that first characterised the unusual features of prolactin neuroendocrinology, and then attempt to identify areas of new progress and/or controversy. Finally, we discuss a hypothesis that provides a unifying explanation for the pleiotrophic actions of prolactin in the brain.

Prolactin regulation of gonadotropin-releasing hormone neurons to suppress luteinizing hormone secretion in mice

Hyperprolactinemia causes infertility, but the mechanisms involved are not known. The present study aimed to determine whether and how prolactin may influence LH secretion in the adult female mouse. Using ovariectomized, estrogen-treated (OVX+E) mice, we found that 7 d of intracerebroventricular prolactin potently suppressed serum LH levels (P < 0.05). To examine whether this central action of prolactin may involve the GnRH neurons, the effects of acute and chronic prolactin on cAMP response element-binding protein phosphorylation (pCREB) in GnRH neurons were examined using dual-label immunocytochemistry. In diestrous and OVX+E mice, a single sc injection of ovine prolactin resulted in a significant (P < 0.05) doubling of the number of GnRH neurons expressing pCREB. OVX+E mice treated with five injections of ovine prolactin over 48 h showed a 4-fold increase in the number of GnRH neurons with pCREB. To determine whether GnRH neurons might be regulated directly by prolactin, we examined prolactin receptor (PRL-R) mRNA expression in green fluorescent protein-tagged GnRH neurons by single-cell RT-PCR. As a positive control, PRL-R mRNA was measured in arcuate dopaminergic neurons obtained from green fluorescent protein-tagged tyrosine hydroxylase neurons. Three of 23 GnRH neurons (13%) were identified to express PRL-R transcripts, whereas nine of 11 arcuate dopaminergic neurons (82%) were found to coexpress PRL-R mRNA. These data demonstrate that prolactin suppresses LH levels in the mouse, as it does in other species, and indicate that it acts centrally to regulate intracellular signaling within GnRH neurons. This is likely to occur, at least in part, through the direct regulation of a subpopulation of GnRH neurons.

Suckling-induced change in oxytocin but not in alpha-MSH concentrations of the median eminence, the neural-, intermediate- and anterior lobes of the pituitary gland

Previous reports have implicated that pituitary-derived prolactin (PRL) is secreted from two distinct zones of mammotropes within the anterior lobe (AL). The inner zone (AL-IZ), located adjacent to the neuro-intermediate lobe (NIL), is supposed to be involved in the rapid and massive discharge of PRL from the pituitary gland due to suckling stimulus. Whereas the outer-zone (AL-OZ) gives the basal secretion and it does not play a role in the acute secretory response during nursing. Anatomically, the AL-IZ has an intimate contact with the NIL because the blood passing through the short portal vessels (SPV) bathes it first. Based on this fact it would be hypothesized that locally released and/or produced compounds, like OXY and alpha-MSH, can be delivered to the AL-IZ. In conjunction, OXY and alpha-MSH have already been implicated to play a role in the regulation of PRL release during suckling. Therefore, the purpose of this study was to examine the possible local transportation of these hormones into the median eminence and various regions of the pituitary gland of lactating rats. We have measured the concentrations of OXY and alpha-MSH from tissue samples of nonsuckled (NS) and 10 or 30 min after suckling (S) was initiated using specific RIAs. It has been shown that there are no changes in the concentration of OXY and alpha-MSH in theAL-IZ and AL-OZ due to suckling stimulus. In contrast, our data provide compelling evidence that OXY is transported into the IL, which can be further increased by suckling stimulus. These data suggest that blood transfusing NL passes through the IL before it is drained into the cavernous sinus, which opens the road for OXY into the general circulation. In addition, our data have unequivocally shown a lack of local delivery of either alpha MSH or OXY into the AL that raises serious doubt about their possible role in PRL secretion during suckling stimulus.

The effect of melatonin on suckling-induced oxytocin and prolactin release in the rat

There is growing evidence that melatonin (MEL) inhibits oxytocin (OT) release when used in a low dose, while higher doses stimulate the release of the hormone in the rat. In the present study we investigated the effect of exogenous MEL, administered intracerebroventricularly (ICV), on suckling-induced OT and prolactin (PRL) release in the urethane-anesthetized rat. Lactating rats suckled by 8-12 pups were studied on days 8-12 of postpartum, and lactating pups-deprived rats on the same days of postpartum served as a control. Plasma OT and PRL levels as well as hypothalamic and neurohypophyseal OT contents were measured by RIA. Suckling stimulated the secretion of both OT and PRL. The ICV injection of 1 ng/ml MEL produced a significant inhibition of suckling-induced OT as well as PRL secretion. Melatonin in doses of 100 ng/ml or 10 micrograms/ml did not modify the OT release but significantly inhibited PRL release brought about by suckling; 10 pg/ml of MEL was not effective in this regard. Thus, exogenous MEL seems to inhibit suckling-induced OT as well as PRL secretion when applied at doses regarded to be in the range of the physiological level; when applied in higher doses, it was shown not to influence the release of OT following physiological stimulation such as suckling.