Effects of the kappa-opioid agonist U50,488 on parturition in rats

Ultra-fast genetically encoded sensor for precise real-time monitoring of physiological and pathophysiological peroxide dynamics

Hydrogen Peroxide (H2O2) is a central oxidant in redox biology due to its pleiotropic role in physiology and pathology. However, real-time monitoring of H2O2 in living cells and tissues remains a challenge. We address this gap with the development of an optogenetic hydRogen perOxide Sensor (oROS), leveraging the bacterial peroxide binding domain OxyR. Previously engineered OxyR-based fluorescent peroxide sensors lack the necessary sensitivity and response speed for effective real-time monitoring. By structurally redesigning the fusion of Escherichia coli (E. coli) ecOxyR with a circularly permutated green fluorescent protein (cpGFP), we created a novel, green-fluorescent peroxide sensor oROS-G. oROS-G exhibits high sensitivity and fast on-and-off kinetics, ideal for monitoring intracellular H2O2 dynamics. We successfully tracked real-time transient and steady-state H2O2 levels in diverse biological systems, including human stem cell-derived neurons and cardiomyocytes, primary neurons and astrocytes, and mouse brain ex vivo and in vivo. These applications demonstrate oROS's capabilities to monitor H2O2 as a secondary response to pharmacologically induced oxidative stress and when adapting to varying metabolic stress. We showcased the increased oxidative stress in astrocytes via Aβ-putriscine-MAOB axis, highlighting the sensor's relevance in validating neurodegenerative disease models. Lastly, we demonstrated acute opioid-induced generation of H2O2 signal in vivo which highlights redox-based mechanisms of GPCR regulation. oROS is a versatile tool, offering a window into the dynamic landscape of H2O2 signaling. This advancement paves the way for a deeper understanding of redox physiology, with significant implications for understanding diseases associated with oxidative stress, such as cancer, neurodegenerative, and cardiovascular diseases.

µ-Opioid receptor antagonism facilitates the anxiolytic-like effect of oxytocin in mice

Mood and anxiety disorders are leading causes of disability worldwide and are major contributors to the global burden of diseases. Neuropeptides, such as oxytocin and opioid peptides, are important for emotion regulation. Previous studies have demonstrated that oxytocin reduced depression- and anxiety-like behavior in male and female mice, and opioid receptor activation reduced depression-like behavior. However, it remains unclear whether the endogenous opioid system interacts with the oxytocin system to facilitate emotion regulation in male and female mice. We hypothesized that opioid receptor blockade would inhibit the anxiolytic- and antidepressant-like effects of oxytocin. In this study, we systemically administered naloxone, a preferential μ-opioid receptor antagonist, and then intracerebroventricularly administered oxytocin. We then tested mice on the elevated zero maze and the tail suspension tests, respective tests of anxiety- and depression-like behavior. Contrary to our initial hypothesis, naloxone potentiated the anxiolytic-like, but not the antidepressant-like, effect of oxytocin. Using a selective μ-opioid receptor antagonist, D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2, and a selective κ-opioid receptor antagonist, norbinaltorphimine, we demonstrate that μ-opioid receptor blockade potentiated the anxiolytic-like effect of oxytocin, whereas κ-opioid receptor blockade inhibited the oxytocin-induced anxiolytic-like effects. The present results suggest that endogenous opioids can regulate the oxytocin system to modulate anxiety-like behavior. Potential clinical implications of these findings are discussed.

Release of endogenous dynorphin opioids in the prefrontal cortex disrupts cognition

Following repeated opioid use, some dependent individuals experience persistent cognitive deficits that contribute to relapse of drug-taking behaviors, and one component of this response may be mediated by the endogenous dynorphin/kappa opioid system in neocortex. In C57BL/6 male mice, we find that acute morphine withdrawal evokes dynorphin release in the medial prefrontal cortex (PFC) and disrupts cognitive function by activation of local kappa opioid receptors (KORs). Immunohistochemical analyses using a phospho-KOR antibody confirmed that both withdrawal-induced and optically evoked dynorphin release activated KOR in PFC. Using a genetically encoded sensor based on inert KOR (kLight1.2a), we revealed the in vivo dynamics of endogenous dynorphin release in the PFC. Local activation of KOR in PFC produced multi-phasic disruptions of memory processing in an operant-delayed alternation behavioral task, which manifest as reductions in response number and accuracy during early and late phases of an operant session. Local pretreatment in PFC with the selective KOR antagonist norbinaltorphimine (norBNI) blocked the disruptive effect of systemic KOR activation during both early and late phases of the session. The early, but not late phase disruption was blocked by viral excision of PFC KORs, suggesting an anatomically dissociable contribution of pre- and postsynaptic KORs. Naloxone-precipitated withdrawal in morphine-dependent mice or optical stimulation of pdyn^Cre neurons using Channelrhodopsin-2 disrupted delayed alternation performance, and the dynorphin-induced effect was blocked by local norBNI. Our findings describe a mechanism for control of cortical function during opioid dependence and suggest that KOR antagonism could promote abstinence.

Why is running a marathon like giving birth? The possible role of oxytocin in the underestimation of the memory of pain induced by labor and intense exercise

Pain can be overestimated, underestimated or reported accurately at recall. The way pain is remembered seems to depend on certain factors, including the type of pain or, in other words, its cause, the context, and the meaning it has for the person suffering from it. For instance, episodes of chronic pain, as well as pain related to surgery, are often overestimated at recall. Interestingly, research shows that pain induced by parturition or marathon running is often underestimated at recall despite the fact that both are not only physically grueling but also emotionally intense experiences. However, both processes can likewise be considered positive events, as opposed to most that involve pain. On the neurophysiological level, one of the similarities between giving birth and running a marathon is the particular involvement of the oxytocin system. Oxytocin is involved both in parturition and intense exercise, for various reasons. During labor, oxytocin mediates uterine contractions, while in the case of extensive running it might be involved in the maintenance of fluid balance. It also has well-documented analgesic properties and plays an important role in memory formation and recall. It has been suggested that oxytocin modulates the output of the central nucleus of the amygdala (CeA) during the fear recall. Moreover, it has been demonstrated that oxytocin can impair fear learning and influence the memory of both positive and negative emotionally salient stimuli. We propose that the reason for pain to be remembered in a more favorable light is the central action of oxytocin in the central nucleus of the amygdala, first and foremost during the encoding phase.

Giving a good start to a new life via maternal brain allostatic adaptations in pregnancy

Successful pregnancy requires adjustments to multiple maternal homeostatic mechanisms, governed by the maternal brain to support and enable survival of the growing fetus and placenta. Such adjustments fit the concept of allostasis (stability through change) and have a cost: allostatic load. Allostasis is driven by ovarian, anterior pituitary, placental and feto-placental hormones acting on the maternal brain to promote adaptations that support the pregnancy and protect the fetus. Many women carry an existing allostatic load into pregnancy, from socio-economic circumstances, poor mental health and in 'developed' countries, also from obesity. These pregnancies have poorer outcomes indicating negative interactions (failing allostasis) between pre-pregnancy and pregnancy allostatic loads. Use of animal models, such as adult prenatally stressed female offspring with abnormal neuroendocrine, metabolic and behavioural phenotypes, to probe gene expression changes, and epigenetic mechanisms in the maternal brain in adverse pregnancies are discussed, with the prospect of ameliorating poor pregnancy outcomes.

Allopregnanolone in the brain: protecting pregnancy and birth outcomes

A successful pregnancy requires multiple adaptations in the mother's brain that serve to optimise foetal growth and development, protect the foetus from adverse prenatal programming and prevent premature delivery of the young. Pregnancy hormones induce, organise and maintain many of these adaptations. Steroid hormones play a critical role and of particular importance is the progesterone metabolite and neurosteroid, allopregnanolone. Allopregnanolone is produced in increasing amounts during pregnancy both in the periphery and in the maternal and foetal brain. This review critically examines a role for allopregnanolone in both the maternal and foetal brain during pregnancy and development in protecting pregnancy and birth outcomes, with particular emphasis on its role in relation to stress exposure at this time. Late pregnancy is associated with suppressed stress responses. Thus, we begin by considering what is known about the central mechanisms in the maternal brain, induced by allopregnanolone, that protect the foetus(es) from exposure to harmful levels of maternal glucocorticoids as a result of stress during pregnancy. Next we discuss the central mechanisms that prevent premature secretion of oxytocin and consider a role for allopregnanolone in minimising the risk of preterm birth. Allopregnanolone also plays a key role in the foetal brain, where it promotes development and is neuroprotective. Hence we review the evidence about disruption to neurosteroid production in pregnancy, through prenatal stress or other insults, and the immediate and long-term adverse consequences for the offspring. Finally we address whether progesterone or allopregnanolone treatment can rescue some of these deficits in the offspring.

The effect of continuous morphine administration on maternal plasma oxytocin concentration and uterine contractions after open fetal surgery

OBJECTIVE

A major complication of open fetal surgery is prematurity. We propose that fetal and maternal stress/pain after surgery may affect the concentration of circulating oxytocin and the frequency of uterine contractions, thus increasing the risk of preterm delivery. The objective of this study was to test whether continuous morphine sulfate administration after open fetal surgery has an effect on maternal plasma oxytocin concentration and the frequency of uterine contractions.

METHODS

An established time-pregnant primate model for open fetal surgery was used. From the time of surgery until the end of the three-day study period, three animals received prophylactic antibiotics, a bolus of indomethacin, and a bolus of morphine sulfate (group I). Three other animals received the same prophylactic antibiotics and an i.v. bolus of indomethacin, as well as a continuous i.v. infusion of morphine sulfate throughout the entire study period (group II). Maternal blood samples were collected to determine oxytocin plasma concentrations. Oxytocin was measured by radioimmunoassay. Uterine activity was continuously recorded through an amniotic fluid catheter and quantified as number of contractions (10 mmHg increase from base line in intrauterine pressure) per hour (UCs/h).

RESULTS

The mean maternal plasma oxytocin concentration was higher (p < 0.01) and the number of uterine contractions more frequent (p < 0.05) in the group of animals with intermittent doses of morphine than in the group that received morphine continuously.

CONCLUSIONS

These data suggest that maternal plasma oxytocin concentration and uterine activity after open fetal surgery may be related to inadequate maternal/fetal analgesia/sedation.

The magnocellular oxytocin system, the fount of maternity: adaptations in pregnancy

Oxytocin secretion from the posterior pituitary gland is increased during parturition, stimulated by the uterine contractions that forcefully expel the fetuses. Since oxytocin stimulates further contractions of the uterus, which is exquisitely sensitive to oxytocin at the end of pregnancy, a positive feedback loop is activated. The neural pathway that drives oxytocin neurons via a brainstem relay has been partially characterised, and involves A2 noradrenergic cells in the brainstem. Until close to term the responsiveness of oxytocin neurons is restrained by neuroactive steroid metabolites of progesterone that potentiate GABA inhibitory mechanisms. As parturition approaches, and this inhibition fades as progesterone secretion collapses, a central opioid inhibitory mechanism is activated that restrains the excitation of oxytocin cells by brainstem inputs. This opioid restraint is the predominant damper of oxytocin cells before and during parturition, limiting stimulation by extraneous stimuli, and perhaps facilitating optimal spacing of births and economical use of the store of oxytocin accumulated during pregnancy. During parturition, oxytocin cells increase their basal activity, and hence oxytocin secretion increases. In addition, the oxytocin cells discharge a burst of action potentials as each fetus passes through the birth canal. Each burst causes the secretion of a pulse of oxytocin, which sharply increases uterine tone; these bursts depend upon auto-stimulation by oxytocin released from the dendrites of the magnocellular neurons in the supraoptic and paraventricular nuclei. With the exception of the opioid mechanism that emerges to restrain oxytocin cell responsiveness, the behavior of oxytocin cells and their inputs in pregnancy and parturition is explicable from the effects of hormones of pregnancy (relaxin, estrogen, progesterone) on pre-existing mechanisms, leading through relative quiescence at term inter alia to net increase in oxytocin storage, and reduced auto-inhibition by nitric oxide generation. Cyto-architectonic changes in parturition, involving evident retraction of glial processes between oxytocin cells so they get closer together, are probably a response to oxytocin neuron activation rather than being essential for their patterns of firing in parturition.

Beta-endorphin cells in the arcuate nucleus: projections to the supraoptic nucleus and changes in expression during pregnancy and parturition

Supraoptic nucleus oxytocin neurone activity and secretion are inhibited in late pregnancy and parturition by endogenous opioids. Here, we investigated alterations in the projections and gene expression of beta-endorphin/pro-opiomelanocortin neurones in the arcuate nucleus in the pregnant rat. All regions of the arcuate nucleus were found to contain cells immunoreactive for beta-endorphin fluorescent microbeads retrogradely transported from the supraoptic nucleus, and double-labelled neurones (beta-endorphin plus microbeads), showing that beta-endorphin neurones throughout the arcuate nucleus project to the supraoptic nucleus. There was an increase in the number of beta-endorphin-immunoreactive cells in the arcuate nucleus and an increase in the density of beta-endorphin fibres within the supraoptic nucleus and peri-supraoptic region in late pregnancy and parturition, suggesting enhanced expression of beta-endorphin and increased beta-endorphin innervation of the supraoptic nucleus. Pro-opiomelanocortin mRNA expression in the arcuate nucleus increased in late compared to early pregnancy: the number of positive neurones significantly increased in the caudal region. Fos expression (an indicator of neuronal activation) in the arcuate nucleus was colocalized in beta-endorphin neurones in both proestrus and parturient rats, but the number of positive cells did not increase during parturition, suggesting lack of activation of beta-endorphin neurones at birth. Thus, beta-endorphin cells in the arcuate nucleus project to the supraoptic nucleus and increased innervation during pregnancy may explain the enhanced endogenous opioid inhibition of oxytocin neurones.

Endogenous opioid regulation of oxytocin and ACTH secretion during pregnancy and parturition

Progress of parturition in the rat is optimal when there is increased oxytocin secretion, thus ensuring quick birth and otherwise risking adverse neonatal health. To ensure that the mechanisms for this are available, oxytocin neurons adapt in pregnancy and this includes development of a tonic inhibition by endogenous opioids. Endogenous opioid inhibition of oxytocin secretion increases in pregnancy, initially acting on the nerve terminals in the posterior pituitary and later on oxytocin cell bodies and their inputs. This inhibition enhances stores of oxytocin and enables restraint of oxytocin neuron responsiveness to selected excitatory inputs. The hypothalamic neurons which mediate stress also adapt in late pregnancy so that hypothalamo-pituitary-adrenal axis and oxytocin secretory responses to stressor exposure are attenuated. This is also partly due to endogenous opioid inhibition. Thus, in pregnancy oxytocin and hypothalamo-pituitary-adrenal axis secretion in response to stimulation is restrained, protecting the unborn fetus(es) from premature delivery and glucocorticoid exposure and preparing the oxytocin neurons for their important secretory role during parturition. In parturition itself, endogenous opioids continue to inhibit these neurons. Stress exposure during parturition delays births, probably due to endogenous opioid inhibition of pulsatile oxytocin secretion. On the other hand, basal ACTH and corticosterone secretion are reduced in parturition through inhibition by endogenous opioids. So, opioids continue to regulate the activity of oxytocin and hypothalamo-pituitary-adrenal mechanisms in labor; inhibition of oxytocin neurons at this time may control the spacing of pup births.

The effects of spiradoline (U-62066E), a kappa-opioid receptor agonist, on neuroendocrine function in man

1. Opioid drugs act on specific receptors which are principally classified into mu, delta and kappa subtypes. Spiradoline (U-62066E) is a kappa-selective agent which has been shown to possess potent anti-nociceptive effects but does not show cross tolerance with morphine. 2. We have assessed the neuroendocrine effects of spiradoline in healthy volunteers with two doses (1.6 and 4.0 micrograms kg-1, i.m.) of the compound. Six male non-smokers aged 19-27 years were studied by use of a randomized, double-blind three-limb placebo-controlled cross-over design. Blood was taken from an in-dwelling venous cannula basally and at 15 min intervals for 2 h for determination of serum cortisol, prolactin, growth hormone (GH) and catecholamines. 3. Psychological function was assessed by the Stanford Sleepiness Scale (SSS) and the Addiction Research Centre Inventory (ARCI) administered before the medication and at 35 min, 1 h 25 min and 2 h afterwards. Cardiovascular variables were recorded at 10 min intervals. Results were analysed by analysis of variance. 4. Spiradoline showed a significant (P < 0.05) dose-dependent increase in free water clearance, as predicted for a kappa-opioid agonist. It also caused a dose-dependent stimulation of prolactin, (increment over baseline for higher dose 214%), GH (433%) and cortisol (215%) release (P < 0.05). There were no significant drug-related changes in plasma catecholamines, blood pressure, pulse or psychological variables. 5. We have therefore confirmed that kappa-opioids increase free-water clearance and may participate in the stimulation of prolactin and GH release. In contrast to mu and delta-opioid agonists, this novel kappa-agonist stimulates cortisol release in man.

Cardiovascular and respiratory actions of U50,488H in the unanaesthetized ovine foetus

In an effort to evaluate the feasibility of kappa-opioid receptor agonists for use in pregnancy, we have investigated the actions of U50,488H (trans-3,4-dichloro-N-methyl-N-[2-(1- pyrrolidinyl)cyclohexyl]benzeneacetamide) on cardiovascular and respiratory control in the unanaesthetized ovine foetus. Intravenous administration of U50,488H (1.0 mg/kg) to the foetus resulted in an immediate increase in foetal blood pressure (P < 0.0001) and heart rate (P < 0.0001) which lasted 15 min, followed by a prolonged loss of heart rate variability for up to 3 h. There was also a significant suppression of foetal breathing movements for 2-3 h (P < 0.008). Pretreatment with naloxone (12 mg/h) completely blocked the hypertensive and tachycardiac response to U50,488H, but was unable to prevent the loss of variation in heart rate or respiratory depression. These data suggest that U50,488H can exert direct cardiovascular and respiratory actions in the ovine foetus via both opioid and non-opioid mechanisms. The naloxone-insensitive suppression of foetal breathing would severely limit the use of U50,488H as an obstetrical analgesic.

Opioids and coupling of the anterior peri-third ventricular input to oxytocin neurones in anaesthetized pregnant rats

In the pregnant rat the osmotic drive to oxytocin neurones is reduced and oxytocin secretion itself is inhibited by endogenous opioids. Coupling of the anterior peri-third ventricular input pathway, involved in osmoregulation, to magnocellular oxytocin neurones was studied in urethane-anaesthetized virgin and 21 day pregnant rats using electrical stimulation of the region anterior and ventral to the third cerebral ventricle (AV3V region) to drive the oxytocin neurones, and giving naloxone to prevent the action of any endogenous opioids on the system. Trains of stimuli (0.5 mA, 1 ms pulses, 10 s on 10 s off, at either 10 Hz or 25 Hz for 10 or 2 min respectively) were given at 20 or 30 min intervals via an electrode stereotaxically-implanted in the AV3V region, and femoral arterial blood plasma samples collected immediately before and after each stimulation were radioimmunoassayed for oxytocin concentration. The first (control) AV3V stimulation increased plasma oxytocin concentration reproducibly and similarly in virgin and 21-day pregnant rats. Naloxone administered 10 min before the second stimulus increased basal plasma oxytocin concentration in virgin and pregnant rats and increased the oxytocin secretory response to 25 Hz AV3V stimulation in virgin but not pregnant rats, and the response was significantly greater in virgin rats. Naloxone reveals oxytocin secretion unrestrained by endogenous opioids, therefore it appears that there is an opioid-independent reduction in the excitatory coupling of the AV3V input to oxytocin neurones which may explain the reduced osmoresponsiveness of oxytocin neurones at the end of pregnancy.(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous opiates: 1993

This paper is the sixteenth installment of our annual review of research concerning the opiate system. It is restricted to papers published during 1993 that concern the behavioral effects of the endogenous opiate peptides, and does not include papers dealing only with their analgesic properties. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; development; immunological responses; and other behaviors.