The rhythmic secretion of mating-induced prolactin secretion is controlled by prolactin acting centrally

Impact of Chronic Prenatal Stress on Maternal Neuroendocrine Function and Embryo and Placenta Development During Early-to-Mid-Pregnancy in Mice

Psychological stress, both leading up to and during pregnancy, is associated with increased risk for negative pregnancy outcomes. Although the neuroendocrine circuits that link the stress response to reduced sexual motivation and mating are well-described, the specific pathways by which stress negatively impacts gestational outcomes remain unclear. Using a mouse model of chronic psychological stress during pregnancy, we investigated 1) how chronic exposure to stress during gestation impacts maternal reproductive neuroendocrine circuitry, and 2) whether stress alters developmental outcomes for the fetus or placenta by mid-pregnancy. Focusing on the stress-responsive neuropeptide RFRP-3, we identified novel contacts between RFRP-3-immunoreactive (RFRP-3-ir) cells and tuberoinfundibular dopaminergic neurons in the arcuate nucleus, thus providing a potential pathway linking the neuroendocrine stress response directly to pituitary prolactin production and release. However, neither of these cell populations nor circulating levels of pituitary hormones were affected by chronic stress. Conversely, circulating levels of steroid hormones relevant to gestational outcomes (progesterone and corticosterone) were altered in chronically-stressed dams across gestation, and those dams were qualitatively more likely to experience delays in fetal development. Together, these findings suggest that, up until at least mid-pregnancy, mothers appear to be relatively resilient to the effects of elevated glucocorticoids on reproductive neuroendocrine system function. We conclude that understanding how chronic psychological stress impacts reproductive outcomes will require understanding individual susceptibility and identifying reliable neuroendocrine changes resulting from gestational stress.

Dopamine: A Modulator of Circadian Rhythms in the Central Nervous System

Circadian rhythms are daily rhythms that regulate many biological processes – from gene transcription to behavior – and a disruption of these rhythms can lead to a myriad of health risks. Circadian rhythms are entrained by light, and their 24-h oscillation is maintained by a core molecular feedback loop composed of canonical circadian (“clock”) genes and proteins. Different modulators help to maintain the proper rhythmicity of these genes and proteins, and one emerging modulator is dopamine. Dopamine has been shown to have circadian-like activities in the retina, olfactory bulb, striatum, midbrain, and hypothalamus, where it regulates, and is regulated by, clock genes in some of these areas. Thus, it is likely that dopamine is essential to mechanisms that maintain proper rhythmicity of these five brain areas. This review discusses studies that showcase different dopaminergic mechanisms that may be involved with the regulation of these brain areas’ circadian rhythms. Mechanisms include how dopamine and dopamine receptor activity directly and indirectly influence clock genes and proteins, how dopamine’s interactions with gap junctions influence daily neuronal excitability, and how dopamine’s release and effects are gated by low- and high-pass filters. Because the dopamine neurons described in this review also release the inhibitory neurotransmitter GABA which influences clock protein expression in the retina, we discuss articles that explore how GABA may contribute to the actions of dopamine neurons on circadian rhythms. Finally, to understand how the loss of function of dopamine neurons could influence circadian rhythms, we review studies linking the neurodegenerative disease Parkinson’s Disease to disruptions of circadian rhythms in these five brain areas. The purpose of this review is to summarize growing evidence that dopamine is involved in regulating circadian rhythms, either directly or indirectly, in the brain areas discussed here. An appreciation of the growing evidence of dopamine’s influence on circadian rhythms may lead to new treatments including pharmacological agents directed at alleviating the various symptoms of circadian rhythm disruption.

The role of orgasm in the development and shaping of partner preferences

BACKGROUND

The effect of orgasm on the development and shaping of partner preferences may involve a catalysis of the neurochemical mechanisms of bonding. Therefore, understanding such process is relevant for neuroscience and psychology.

METHODS

A systematic review was carried out using the terms Orgasm, Sexual Reward, Partner Preference, Pair Bonding, Brain, Learning, Sex, Copulation.

RESULTS

In humans, concentrations of arousing neurotransmitters and potential bonding neurotransmitters increase during orgasm in the cerebrospinal fluid and the bloodstream. Similarly, studies in animals indicate that those neurotransmitters (noradrenaline, oxytocin, prolactin) and others (e.g. dopamine, opioids, serotonin) modulate the appetitive and consummatory phases of sexual behavior and reward. This suggests a link between the experience of orgasm/sexual reward and the neurochemical mechanisms of pair bonding. Orgasm/reward functions as an unconditioned stimulus (UCS). Some areas in the nervous system function as UCS-detection centers, which become activated during orgasm. Partner-related cues function as conditioned stimuli (CS) and are processed in CS-detector centers.

CONCLUSIONS

Throughout the article, we discuss how UCS- and CS-detection centers must interact to facilitate memory consolidation and produce recognition and motivation during future social encounters.

Direct stimulatory effects of oxytocin in female rat gonadotrophs and somatotrophs in vitro: comparison with lactotrophs

The peptide oxytocin (OT) is secreted by hypothalamic neurons and exerts numerous actions related to reproduction. OT stimulation of prolactin secretion in female rats is important during the estrous cycle, pregnancy, and lactation. Here we report that OT also stimulates transients of intracellular Ca(2+) concentration in somatotrophs and gonadotrophs as well as the release of GH and LH in a dose-dependent manner with EC50 values that closely correspond to the ligand affinity of the OT receptor (OTR). Remarkably, the hormone-releasing effect of OT in these two cell types is 2 orders of magnitude more sensitive than that in lactotrophs. The specific OTR agonist [Thr(4),Gly(7)]-oxytocin acutely stimulated the release of LH, GH, and prolactin from female rat pituitary cells in primary culture and increased intracellular Ca(2+) concentration in gonadotrophs, somatotrophs, and lactotrophs. In these three cell types, the effects on hormone release and intracellular Ca(2+) of both OT and [Thr(4),Gly(7)]oxytocin were abolished by the specific OT receptor antagonist desGly-NH2-d(CH2)5[D-Tyr(2),Thr(4)]OVT but not by the highly selective vasopressin V1a receptor antagonist, d(CH2)5[Tyr(Me)(2),Dab(5)]AVP. Furthermore, 10 nM arginine vasopressin stimulated LH and GH release comparably with a dose of OT that was at least 10 times lower. Finally, the presence of the OTR-like immunoreactivity could be observed in all three cell types. Taken together, these results show that OT directly stimulates gonadotrophs, somatotrophs, and lactotrophs through OT receptors and suggest that OT signaling may serve to coordinate the release of different pituitary hormones during specific physiological conditions.

Oxytocin: an emerging regulator of prolactin secretion in the female rat

In the female rat, a complex interplay of both stimulatory and inhibitory hypothalamic factors controls the secretion of prolactin. Prolactin regulates a large number of physiological processes from immunity to stress. Here, we have chosen to focus on the control of prolactin secretion in the female rat in response to suckling, mating and ovarian steroids. In all three of these states, dopamine, released from neurones in the mediobasal hypothalamus, is a potent inhibitory signal regulating prolactin secretion. Early research has determined that the relief of dopaminergic tone is not sufficent to account for the full surge of prolactin secretion observed in response to the suckling stimulus, launching a search for possible prolactin-releasing factors. This research has subsequently broadened to include searching for prolactin-releasing factors controlling prolactin secretion after mating or ovarian steroids. A great deal of literature has suggested that this prolactin-releasing factor may include oxytocin. Oxytocin receptors are present on lactotrophs. These oxytocin receptors respond to exogenous oxytocin and antagonism of endogenous oxytocin inhibits lactotroph activity. In addition, the pattern of oxytocin neuronal activity and oxytocin release correlate with the release of prolactin. Here, we suggest not only that oxytocin is stimulating prolactin secretion, but also that prolactin secretion is controlled by a complex network of positive (oxytocin) and negative (dopamine) feedback loops. We discuss the available literature and attempt to describe the circuitry we believe may be responsible for controlling prolactin secretion.

Central prolactin modulates insulin sensitivity and insulin secretion in diabetic rats

BACKGROUND

Prolactin secretion is self-regulating as it acts upon hypothalamic dopaminergic systems which inhibit prolactin release from the anterior pituitary. Circulating prolactin improves glucose homeostasis by increasing insulin action and secretion, but central prolactin effects on glucose homeostasis have not been examined. Here, we determined that chronic central infusion of prolactin modulates insulin resistance and β-cell function and mass in 90% of pancreatectomized diabetic male rats.

METHODS

Diabetic rats were divided into three groups according to the dose of intracerebroventricular infusion of prolactin during 4 weeks: (1) low-dose prolactin (Low-PRL; 0.1 µg/h), (2) high-dose prolactin (High-PRL; 1 µg/h) and (3) vehicle only (cerebrospinal fluid). Nondiabetic rats were centrally infused with the vehicle.

RESULTS

Chronic intracerebroventricular infusion of Low-PRL lowered body weight and epididymal fat pads by increasing hypothalamic dopamine levels that reduced serum prolactin levels and potentiated leptin signaling. However, High-PRL slightly exacerbated energy dysregulation, decreased hypothalamic dopamine levels, and elevated serum prolactin levels. Both dosages promoted β-cell mass but in a different manner: Low-PRL decreased β-cell apoptosis, whereas High-PRL increased its proliferation. However, only Low-PRL enhanced first-phase insulin secretion and improved insulin sensitivity at a hyperglycemic state in comparison to the control. Low-PRL also increased glucose infusion rates and decreased hepatic glucose output in hyperinsulinemic states, signifying an improvement in hepatic insulin sensitivity. However, High-PRL exacerbated hepatic insulin resistance compared with the control diabetic rats.

CONCLUSIONS

In contrast to the exacerbation of insulin resistance caused by High-PRL, Low-PRL may improve energy and glucose metabolism by increasing hypothalamic dopamine levels in diabetic rats.

Mother-offspring dialogue in early pregnancy: impact of adverse environment on pregnancy maintenance and neurobiology

The mother-offspring dialogue begins even before implantation and is essential to signal pregnancy, establish robust contact, and maintain embryo growth and development. Any circumstance that disrupts the dialogue risks pregnancy problems. A new look at how stress impacts on pregnancy involves its adverse effects on the key pregnancy hormones of progesterone and prolactin. These effects have far-reaching consequences on pregnancy maintenance, maternal anxiety and embryo programming. This review focuses on early pregnancy and how stress might compromise the multi-layer, two-way communication between mother and embryo.

Systemic oxytocin induces a prolactin secretory rhythm via the pelvic nerve in ovariectomized rats

We have shown previously that an intravenous injection of oxytocin (OT) in ovariectomized (OVX) rats initiates a circadian rhythm of prolactin (PRL) secretion similar to that observed after cervical stimulation (CS). In this study, we investigated the pathway through which OT triggers the PRL rhythm. We first tested whether an intracerebroventricular injection of OT could trigger the PRL secretory rhythm. As it did not, we injected OT intravenously while an OT receptor antagonist was infused intravenously. This antagonist completely abolished the PRL surges, suggesting that a peripheral target of OT is necessary for triggering the PRL rhythm. We hypothesized that OT may induce PRL release, which would be transported into the brain and trigger the rhythm. In agreement with this, OT injection increased circulating PRL by 5 min. To test whether this acute increase in PRL release would induce the PRL rhythm, we compared the effect of intravenously administered thyrotropin-releasing hormone (TRH) and OT. Although TRH injection also increased PRL to a comparable level after 5 min, only OT-injected animals expressed the PRL secretory rhythm. Motivated by prior findings that bilateral resection of the pelvic nerve blocks CS-induced pseudopregnancy and OT-induced facilitation of lordosis, we then hypothesized that the OT signal may be transmitted through the pelvic nerve. In fact, OT injection failed to induce a PRL secretory rhythm in pelvic-neurectomized animals, suggesting that the integrity of the pelvic nerve is necessary for the systemic OT induction of the PRL secretory rhythm in OVX rats.

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.

Baby on board: do responses to stress in the maternal brain mediate adverse pregnancy outcome?

Stress and adverse environmental surroundings result in suboptimal conditions in a pregnant mother such that she may experience poor pregnancy outcome including complete pregnancy failure and preterm labor. Furthermore her developing baby is at risk of adverse programming, which confers susceptibility to long term ill health. While some mechanisms at the feto-maternal interface underlying these conditions are understood, the underlying cause for their adverse adaptation is often not clear. Progesterone plays a key role at many levels, including control of neuroendocrine responses to stress, procuring the required immune balance and controlling placental and decidual function, and lack of progesterone can explain many of the unwanted consequences of stress. How stress that is perceived by the mother inhibits progesterone secretion and action is beginning to be investigated. This overview of maternal neuroendocrine responses to stress throughout pregnancy analyses how they interact to compromise progesterone secretion and precipitate undesirable effects in mother and offspring.

Short term hypothyroidism affects ovarian function in the cycling rat

BACKGROUND

Rats made hypothyroid with propilthyouracil start showing abnormal cycling on the second cycle after the start of the treatment, with a high proportion of spontaneous pseudopregnancies and reduced fertility.

METHODS

To investigate some of the mechanisms involved in these reproductive abnormalities, hypothyroidism was induced in virgin rats by propilthyouracil (0.1 g/L in the drinking water) and we determined circulating hormones by radioimmunoassay and whole ovary expression of ovarian hormone receptors, growth factors and steroidogenic enzymes using semi-quantitative RT-PCR.The study was performed on days 6 to 9 of treatment, corresponding to diestrus I (at 20.00-22.00 h), diestrus II (at 20.00-22.00 h), proestrus and estrus (both at 8.00-10.00 h and 20.00-22.00 h) of the second estrous cycle after beginning propilthyouracil treatment. Another group of rats was mated on day 8 and the treatment continued through the entire pregnancy to evaluate reproductive performance.

RESULTS

Hypothyroidism increased circulating prolactin and estradiol on estrus 5 to 7-fold and 1.2 to 1.4-fold respectively. Growth hormone and insulin-like growth factor 1 diminished 60 and 20% respectively on proestrus morning. Hypothyroidism doubled the ovarian mRNA contents of estrogen receptor-beta on proestrus and estrus evenings, cyp19A1 aromatase mRNA on estrus evening and of growth hormone receptor on proestrus evening. Hypothyroidism did not influence ovulation rate or the number of corpora lutea at term, but a diminished number of implantation sites and pups per litter were observed (Hypothyroid: 11.7 +/- 0.8 vs.

CONTROL

13.9 +/- 0.7).

CONCLUSIONS

Short term hypothyroidism alters normal hormone profile in the cycling rat increasing the expression of estrogen receptor-beta and cyp19A1 aromatase on estrus, which in turn may stimulate estradiol and prolactin secretion, favouring corpus luteum survival and the subsequent instauration of pseudopregnancy.