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Query: UNIPROT:P01178 (oxytocin)
 15,767 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Studies performed in conscious female rats confirmed that iv injection of cholecystokinin octapeptide (CCK; 20 mu/kg) increased the circulating concentration of oxytocin but not that of vasopressin, and confirmed that the stimulation of oxytocin release was markedly facilitated after iv administration of naloxone (1 mg/kg), indicating attenuation of oxytocin release by endogenous opioids. To investigate the site of action of the endogenous opioids, the electrical activity of putative oxytocin neurones in the supraoptic nucleus was recorded in urethane-anaesthetised female rats. Oxytocin neurones responded to CCK injection with an increase in firing rate lasting 5-15 min, but this response was not facilitated by prior injection of naloxone. The results suggest that the opioid influence upon CCK-induced oxytocin release operates at the level of the neurosecretory terminals in the neurohypophysis rather than centrally. Since CCK does not elevate vasopressin release, it appears unlikely that dynorphin, the opioid peptide co-existing with vasopressin, is responsible in these circumstances for the cross-inhibition of oxytocin release. It is suggested that products of proenkephalin A, the met-enkephalin precursor present in the supraoptic nucleus and in the neurohypophysis itself, may be active in the regulation of oxytocin release.
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PMID:Naloxone potentiates the release of oxytocin induced by systemic administration of cholecystokinin without enhancing the electrical activity of supraoptic oxytocin neurones. 157 6

Expression of arginine-vasopressin (AVP), oxytocin (OT), dynorphin and enkephalin genes was studied with the in situ hybridization technique in embryonic rat brain serum-free cultures. Neurones were prepared from hypothalamus and extrahypothalamic structures of 16-day-old rat embryos. After 7 days in culture, AVP gene expression occurred in hypothalamic cultures only, whereas ProOT mRNAs were undetectable. By contrast, prodynorphin and proenkephalin mRNAs could be detected in both hypothalamic and extrahypothalamic cultures, however, with a higher number of cells containing proenkephalin mRNAs. These observations demonstrated that AVP, dynorphin and enkephalin, but not OT genes, can be expressed in cultures prepared from embryonic rat brain as young as 16 days old. This is the first report of an early expression of opioid peptide genes within the central nervous system suggesting that opioids could be involved in the early phases of nervous system development.
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PMID:Expression of vasopressin and opiates but not of oxytocin genes studied by in situ hybridization in embryonic rat brain primary cultures. 198 Jun 42

Carboxypeptidase H is one of several enzymes required for the processing of peptide hormone precursors. In this study, inhibition of carboxypeptidase H by its peptide products was investigated. Carboxypeptidase H activity in bovine adrenal medulla chromaffin granules and rat adrenal medulla homogenate was inhibited by the peptides Met- and Leu-enkephalin, vasopressin, oxytocin, luteinizing hormone-releasing hormone, substance P, and thyrotropin-releasing hormone, with oxytocin and ACTH 1-14 having the least effect, at concentrations of 2-20 mM. Inhibition by amidated peptide products (vasopressin, oxytocin, luteinizing hormone-releasing hormone, substance P, and thyrotropin-releasing hormone) show that the final products of the precursor processing pathway can regulate carboxypeptidase H. These levels of peptides are similar to known intragranular peptide concentrations indicating that product and feedback inhibition of carboxypeptidase H may play a role in the control of neuropeptide synthesis. The proenkephalin-derived peptides Met-enkephalin, Leu-enkephalin, Met-enkephalin-Arg6-Gly7-Leu8, and Met-enkephalin-Arg6-Phe7 competitively inhibited bovine and rat carboxypeptidase H with Ki values of 12.0, 6.5, 7.0, and 5.5 mM, respectively. The significantly greater Ki for Met-enkephalin may reflect the effects of higher intragranular concentration of Met-enkephalin, since one proenkephalin molecule contains four copies of Met-enkephalin and only one copy of each of the other enkephalin peptides. Thus, the products from one multivalent precursor molecule may equivalently inhibit carboxypeptidase H activity. Product inhibition of carboxypeptidase H and perhaps other processing enzymes may serve to limit the maximum peptide concentration within the secretory vesicle.
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PMID:Product inhibition of carboxypeptidase H. 288 69

[he concentrations of immunoreactive (ir-) peptides derived from the opioid peptide precursors proenkephalin A (Met-enkephalin), proenkephalin B [dynorphin (DYN)-(1-17), dynorphin-(1-8), dynorphin B, alpha-neoendorphin (alpha-NEO-E), beta-NEO-E] and proopiomelanocortin [beta-endorphin (beta-END)], and of the neurosecretory hormones vasopressin and oxytocin increased between approximately 10-fold and 50-fold from birth to adulthood in the rate hypothalamus. Gel filtration and HPLC analysis of proenkephalin B-derived opioid peptides revealed that in 3-day-old rats the predominant portion of ir-dynorphin-(1-17) and a substantial part of ir-dynorphin B consisted of a high (6000) mol wt species, a common precursor peptide for DYN-(1-17) and DYN B. In adults rats, however, authentic DYN-(1-17) and DYN B were found to be the major ir-forms. The mol wt patterns of ir-DYN-(1-8), ir-alpha-NEO-E and ir-beta-NEO-E did not differ between 3-day-old and adult rats and reflected predominantly the respective authentic opioid peptides. Taking into consideration the developmental changes in the mol wt pattern of ir-DYN-(1-17), authentic DYN-(1-17) was 5 times lower in concentration than DYN-(1-8) in 3-day-old rats, whereas in adults these opioid peptides occurred in equimolar concentrations. These findings suggest that the posttranslational processing of the precursor proenkephalin B changes in the course of postnatal development. Ir-beta-END in the hypothalamus of newborn and adult rats consisted exclusively of beta-END-sized peptides which were not (unlike those in the intermediate pituitary lobe) alpha-N-acetylated. Thus, in the hypothalamus, the enzymatic processing of the opioid peptide precursor proopiomelanocortin to beta-END seems to be fully active at birth, in contrast to that of proenkephalin B.
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PMID:Evidence for a differential postnatal development of proenkephalin B (= prodynorphin)-derived opioid peptides in the rat hypothalamus. 654 67

The distribution of proenkephalin and [Met]enkephalin immunoreactivities in the bovine hypothalamo-neurohypophyseal system was studied by use of specific antisera. Proenkephalin and [Met]enkephalin immunoreactivities were found in magnocellular neuronal cell bodies in the dorsal part of the supraoptic nuclei and in the peripheral part of the paraventricular nuclei. A densely staining network of nerve terminals was found in the external part of the median eminence and in the posterior hypophysis. This general distribution is identical to that of the neurohypophyseal hormone oxytocin. The precise localization of proenkephalin and [Met]enkephalin immunoreactivities was compared to the distribution of oxytocin and vasopressin in serial 5-micron sections through the magnocellular nuclei. Oxytocin immunoreactivity was nearly always present in cells that were stained with proenkephalin and [Met]enkephalin antisera. The vasopressin-immunoreactive cells were never stained with either the proenkephalin or the [Met]enkephalin antisera.
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PMID:Proenkephalin, [Met]enkephalin, and oxytocin immunoreactivities are colocalized in bovine hypothalamic magnocellular neurons. 657 80

The study of the biochemical and physiological functions of the enkephalinergic cell has greatly extended our understanding of peptidergic cells in general. In the adrenal gland, the major part of the proenkephalin-derived peptides is present as intermediates in the processing of the precursor. These peptides are contained within the adrenergic chromaffin granules, from which they are released in response to stimulation of the cell. The nature of the products released depends on the nature of the stimulus, but it appears that mature granules containing completely processed peptides are preferentially released under physiological conditions. In the brain, the presence and release of the heptapeptide that comprises the carboxyl terminus of adrenal proenkephalin suggest that similar mechanisms are operating centrally. The identity of brain and adrenal proenkephalin is further supported by the purification from brain of a large fragment of the proenkephalin molecule, synenkephalin , and the occurrence in brain of this and the other proenkephalin-derived peptides in a molar ratio close to that found in the sequence of the adrenal precursor. The processing of proenkephalin in brain appears to follow the classical models first proposed for peptide hormones (Steiner et al. 1980), which may thus be generalized to include peptide neurotransmitters/neuroregulators. In addition, the results presented in this paper indicate that enkephalins may be cotransmitters in at least two diverse systems. Enkephalins and catecholamines are colocalized in the adrenergic granules of the adrenal gland. In the brain, enkephalins and oxytocin are colocalized in the magnocellular neurons of the hypothalamo-neurohypophyseal oxytocinergic pathway. In both of these systems, the enkephalins are present in a molar concentration that is less than 1% of the concentration of the principal messenger. Such colocalization , coupled with the numerous active peptides that may arise from proenkephalin, suggests many elegant but complex schemes of neurotransmitter interactions. For example, release of enkephalins in the neurohypophysis may regulate oxytocin release through an action on autoreceptors of the oxytocinergic terminal. In the adrenal the coreleased enkephalins may act by regulating presynaptically the cholinergic output of the splanchnic nerve. However, further studies are needed to define clearly the physiological roles of such cotransmission . From the abundance of proenkephalin-derived peptides in the basal ganglia, it appears that enkephalins may represent the principal transmitter in some central neurons.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:The enkephalinergic neuron: implications of a polyenkephalin precursor. 658 60

At the neurosecretory terminals in the neural lobe, oxytocin secretion is restrained by co-secreted endogenous opioids, which act via kappa-receptors. The co-secreted opioids include products of pro-dynorphin (released by both vasopressin and oxytocin terminals) and proenkephalin (released by oxytocin terminals). In morphine-tolerant rats this opioid mechanism is more effective, but in late pregnancy it is less effective. Opioids also act directly on oxytocin cell bodies, via separate mu- and kappa-receptors, inhibiting excitation by all stimuli tested, and also exert presynaptic and more distal actions on afferent systems. During chronic morphine exposure, tolerance and dependence develop in oxytocin neurones; the former involves reduction in mu-opioid receptor density, while the latter may involve compensatory upregulation of mechanisms regulating Ca2+ influx. In mid-pregnancy, the effectiveness of opioid mechanisms in the neural lobe increases, assisting the accumulation of oxytocin stores in advance of parturition, but by the end of pregnancy the effectiveness of these mechanisms is reduced. At this time, a separate endogenous opioid system, acting via mu-receptors, actively restrains the electrical activity of oxytocin neurones. Release of this endogenous opioid inhibition may contribute to the increase in activity during parturition analogous to that occurring during morphine withdrawal excitation. Central opioid mechanisms retain the ability to control oxytocin neurones during parturition, and can interrupt established parturition by inhibiting oxytocin neurone firing rate in disadvantageous environmental circumstances.
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PMID:Opioid tolerance and dependence in the magnocellular oxytocin system: a physiological mechanism? 764 4

Since stress and opioids affect oxytocin secretion we investigated the involvement of corticotrophin-releasing hormone (CRH) and enkephalin on the regulation of oxytocin production during pregnancy and parturition by measuring CRH, proenkephalin A (PENK) and oxytocin mRNA's in the paraventricular nucleus (PVN) using quantitative in situ hybridisation. On day 21 of pregnancy and during parturition CRH mRNA content and the number of parvocellular neurones expressing CRH were significantly decreased but neither PENK nor oxytocin mRNA was altered. The suppression of the CRH gene in late pregnancy and parturition may lead to the previously reported reduction in stress responses during lactation.
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PMID:Corticotrophin-releasing hormone, proenkephalin A and oxytocin mRNA's in the paraventricular nucleus during pregnancy and parturition in the rat. 854 31

In the female sheep opioids act centrally to influence both oxytocin release and maternal behaviour. We have used in situ hybridization and histochemistry to investigate the changes in mRNA expression of the two opioid precursor genes, pro-opiomelanocortin (POMC) and pre-proenkephalin (PPE), in discrete hypothalamic nuclei as a function of pregnancy, parturition and lactation and following treatment with oestrogen and progesterone. Quantitative in situ hybridization histochemistry demonstrated that POMC mRNA expression in the arcuate nucleus (ARC) decreased at parturition and increased during lactation compared to late pregnant and ovariectomized animals. Oestradiol and progesterone treatments increased POMC mRNA expression compared to ovariectomized controls. Pre-proenkephalin mRNA expression was quantified in three discrete hypothalamic nuclei, the ventromedial nucleus (VMN), the paraventricular nucleus (PVN) and the suprachiasmatic nucleus (SCN). In the VMN, PPE mRNA expression increased during lactation compared to late pregnancy and parturition. Expression levels during late pregnancy and parturition were decreased compared to ovariectomized animals. Oestradiol increased, and progesterone decreased, PPE mRNA levels compared to ovariectomized controls. Combined progesterone followed by oestrogen treatment produced significant increases in PPE mRNA expression. In the PVN, PPE expression increased at parturition compared to late pregnant, lactating and ovariectomized animals. Expression levels in late pregnant animals were decreased compared to lactating or ovariectomized ones. However, sex steroid treatment produced no changes in PPE expression in the PVN. No changes were observed in PPE mRNA expression in the SCN in response to any of the experimental conditions. This data shows that both POMC and PPE mRNA levels are altered in the sheep brain during pregnancy, parturition and lactation and in response to sex steroids, although the direction of the changes is not always the same and in the case of PPE only the VMN and PVN are affected.
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PMID:Changes in pro-opiomelanocortin and pre-proenkephalin mRNA levels in the ovine brain during pregnancy, parturition and lactation and in response to oestrogen and progesterone. 868 Apr 46

Oxytocin is secreted during parturition to stimulate myometrial contractions and birth. Prior to the start of labour, oxytocin neurones undergo changes to prepare for optimal secretion during labour. Thus, during late pregnancy oxytocin secretion is limited by endogenous opioid inhibition. This does not appear to act at the oxytocin nerve terminals in the neural lobe since they in fact become desensitised to opioid inhibition, responding less to either the general opioid antagonist, naloxone, or to the specific kappa-opioid agonist U50,488, and kappa-receptor binding decreases. However, removal of opioid inhibition on oxytocin neurones by naloxone activates oxytocin cell bodies and there is an increase in the number of cells expressing Fos protein in the supraoptic nucleus. This action is mediated via mu- and not kappa-opioid receptors since norBinaltorphimine (kappa-antagonist) is ineffective. Endogenous opioids are likely to act pre-synaptically on inputs to oxytocin neurones, especially those from the brainstem since naloxone potentiates the firing rate response of oxytocin neurones to intravenous cholecystokinin administration which acts via noradrenergic neurones. The endogenous opioid, beta-endorphin may be responsible for inhibition of oxytocin neurones as both the peptide content and its precursor proopiomelanocortin mRNA content increase in the arcuate nucleus during pregnancy, whereas expression of the co-localized opioids, prodynorphin or proenkephalin A, in magnocellular neurones does not alter.
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PMID:Pathways to parturition. 871 93


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