Release Of Met-enkephalin Research Articles

Kappa 1/mu interactions in the control of the in vivo release of neuropeptides from the rat spinal cord

The possible modulation by κ 1 opioid receptor stimulation of the effects of μ receptor stimulation on the spinal release of met-enkephalin- (ME), substance P- (SP) μ calcitonin gene-related peptide- (CGRP) like materials (LM) was investigated in vivo in the rat. The stimulation of κ 1 receptors by 10 μM U 50488 H exerted no effect per se on the release processes. The stimulation of μ opioid receptors by 10 UM DAGO significantly reduced (-40%) the spinal outflow of MELM, enhanced (+44%) that of SPLM and did not affect CGRPLM outflow. In the presence of both DAGO and U 50488 H, the spinal release of MELM

Antisense oligodeoxynucleotide to a δ-opioid receptor given intrathecally blocks i.c.v. administered β-endorphin-induced antinociception in the mouse

The antinociception induced by supraspinally administered β-endorphin is mediated by the release of Met-enkephalin and subsequent stimulation of δ-opioid receptors from the spinal cord in mice. Repeated intrathecal administration of an antisense oligodeoxynucleotide against δ-opioid receptors selectively attenuated i.c.v. administered β-endorphin-induced antinociception without any effect on the antinociception induced by μ-opioid receptor agonists, morphine and DAMGO, or κ-opioid receptor agonists, U50,488H. A random sequence oligodeoxynucleotide was inactive against β-endorphin-induced antinociception. The study confirms previous findings that the antinociception induced by β-endorphin is mediated by the stimulation of the δ-opioid receptors in the spinal cord.

Optimization of adrenal medullary allograft conditions for pain alleviation.

Previous findings in our laboratory have demonstrated that transplants of adrenal medullary tissue into the spinal subarachnoid space can alleviate pain, most likely via sustained local release of pain-reducing neuroactive substances from the transplanted chromaffin cells. The success of this work in animal models has led to preliminary clinical trials with promising results. However, before large-scale clinical studies are undertaken, numerous issues should be resolved, many of which can be readily addressed initially in the laboratory. One of these is the amount of donor adrenal medullary tissue necessary to produce long-term antinociception. Although tissue from two adrenal glands has generally been used, it is unknown whether less would be equally effective, or more could increase analgesic potency. To address this, various amounts of adrenal medullary tissue, ranging from one to ten donor glands, were used. Results showed lowered antinociceptive benefits when only one adrenal medulla was used, but only small and short-lived increases when donor material was increased substantially. In addition, assays of atecholamine and Met-enkephalin release in the host spinal CSF revealed only slight further increases following the transplantation of more than 2-4 glands. These results indicate that a small amount of adrenal medullary tissue is necessary and sufficient to produce sustained antinociception, and suggest that higher amounts may result in tolerance development or feedback inhibition. Another important issue is the ability to retain antinociceptive potency if donor tissue is maintained in culture prior to transplantation, since the coordination of donor harvesting and recipient availability is often difficult. To address this, donor adrenal medullary tissue was maintained in explant culture for various periods following dissection. Results indicated that adrenal medullary tissue can be maintained in culture for up to 30 days prior to transplantation without decrement in antinociceptive potency, and that a period of 7-21 days results in improved graft viability. Results of this study indicate that some of the critical issues for successful neural transplantation outcomes can be initially addressed in pre-clinical studies.

Release of β-endorphin and methionine-enkephalin into cerebrospinal fluid during deep brain stimulation for chronic pain

The authors systematically studied the release of the endogenous opioid peptides beta-endorphin and methionine (met)-enkephalin into the cerebrospinal fluid (CSF) during deep brain stimulation in patients suffering from otherwise intractable chronic pain. Nine patients were included in the study; six had stimulation electrodes placed in both the periventricular gray matter (PVG) and the thalamic nucleus ventralis posterolateralis (VLP) and three in the PVG only. Immunoreactivity of beta-endorphin and met-enkephalin (beta-EPir and MEir, respectively) was measured by radioimmunoassays in ventricular and lumbar CSF samples obtained before, during, and after stimulation. Prestimulation concentrations of beta-EPir and MEir were lower in ventricular than in lumbar CSF (6.6 +/- 0.5 vs. 13.7 +/- 1.0 pmol/liter, p = 0.0001, for beta-EPir; 33.6 +/- 5.1 vs. 48.3 +/- 3.2 pmol/liter, p < 0.05, for MEir). Ventricular CSF concentrations of both beta-EPir and MEir increased significantly during PVG stimulation, whereas VPL stimulation was without effect. No changes were seen in lumbar CSF levels of the peptides during stimulation in either site. A significant inverse relationship was found between the "during:before stimulation" ratios of visual analog scale ratings and beta-EPir levels during PVG stimulation. The beta-EPir and MEir concentration during:before stimulation ratios were positively correlated, whereas no correlation was present in prestimulation samples from ventricular or lumbar CSF. High-performance liquid chromatography of ventricular CSF pools obtained during PVG stimulation revealed that major portions of beta-EPir and MEir eluted as synthetic beta-endorphin and met-enkephalin, respectively, thus documenting the release of beta-endorphin and met-enkephalin into ventricular CSF during PVG stimulation. The finding of a direct relationship between beta-EPir release and pain alleviation may suggest a role for beta-endorphin in the analgesic mechanism of PVG stimulation.

Opioid peptide release evoked by noxious stimulation of the hind instep of rats

Previous studies showed that opioids and opioid peptides modulate vascular reactions such as plasma extra-vasation and vasodilatation through inhibition of substance P release from peripheral nerve endings of primary afferent fibers, and suggested the existence of endogenous opioid peptide activity related to regulation of inflammatory responses. Here we examined the effect of heat stimulation and antidromic stimulation of primary afferent fibers on the release of immunoreactive opioid peptides into the perfusate of the subcutaneous space in the rat instep. Antidromic stimulation of sectioned sciatic and saphenous nerves did not have any significant effect on the release of Met-enkephalin (Met-EK), while immersion of the hind paw in hot water (47°C) for 30 min caused an increase in Met-EK release into the perfusate. High-pressure liquid chromatography of the perfusate revealed that noxious heat stimulation induced increase in release of Leu-enkephalin (Leu-EK) as well as Met-EK, although the maximal concentration of Leu-EK was less than 16% of that of Met-EK. No β-endorphin was detected in the perfusate before, during or after heating. We conclude that noxious heat stimulation mainly leads to increase in Met-EK, and that this peptide originates mainly, from peripheral cells containing opioid peptides such as immune cells and/or Merkel cells, not from primary afferent fibers.

Pharmacologic characterization of opioid peptide release from chromaffin cell transplants using a brain slice superfusion method.

A simple chamber and an inexpensive superfusion system for studying mammalian brain slices containing neural transplants is described. With this method, rat brain slices containing bovine chromaffin cell transplants can be maintained for several hours, allowing for the determination of neurochemical characteristics and pharmacologic responsiveness of the grafted cells. Using this technique, basal and nicotine-stimulated release of metenkephalin from rat periaqueductal gray slices containing bovine chromaffin cell transplants were measured. Results showed that met-enkephalin release can be increased by nicotinic stimulation in slices containing chromaffin cell, but not control implants, for at least 8 weeks postimplantation. Furthermore, this response was dose-related. These results are in good agreement with previous behavioral studies and provide corroborative evidence for the mechanism of pain reduction by the release of opioid peptides from chromaffin cell transplants in the periaqueductal gray. This study demonstrates that neurochemical and pharmacologic analyses of neural transplants using a superfused brain slice method can be a complementary approach in determining the underlying mechanisms of neural transplants in the central nervous system.

Influence of labor and neonatal hypoxia on sympathoadrenal activation and methionine enkephalin release in calves

Summary Labor and delivery stimulate increased release of catecholamines and endogenous opioid peptides in neonates. Catecholamines promote adaptation to the extrauterine environment after birth. Enkephalins are stored together with catecholamines in the adrenal medulla and have an inhibitory effect on catecholamine release. We investigated the influence of labor and neonatal hypoxia on epinephrine, norepinephrine, and met-enkephalin release in calves. Blood samples were taken from the umbilical artery before rupture of the umbilical cord and from the jugular vein repeatedly after birth. Highest plasma norepinephrine concentration was found in calves delivered at the end of gestation (term calves) before umbilical cord rupture. In calves delivered before the physiologic end of gestation (preterm calves), norepinephrine values increased after cord rupture, but remained lower than values in term calves. Epinephrine release followed a similar pattern, but norepinephrine was clearly predominant. In term calves, met-enkephalin values were significantly higher than values in preterm calves. In calves of both groups, met-enkephalin release increased after cord rupture. During birth, the increase in catecholamine release seems to take place earlier than that of enkephalins. Norepinephrine-dominated stimulation during expulsion of the calf might be followed by increasing enkephalinergic inhibition after cord rupture and onset of respiration. Reduced release of catecholamines and enkephalins in preterm calves may be connected with delayed adaptation to the extrauterine environment.

Transplants of immunologically isolated xenogeneic chromaffin cells provide a long-term source of pain-reducing neuroactive substances

Adrenal medullary chromaffin cells are a potential source of neuroactive substances for transplantation into the CNS to alleviate neurochemical deficits. In particular, work in our laboratory has suggested that adrenal medullary transplants in the spinal subarachnoid space can alleviate pain by providing sustained local delivery of catecholamines and opioid peptides. One of the major limitations for clinical application of neural transplantation is the availability of donor material in sufficient quantities. This limitation may be overcome by the use of xenogeneic donors if long-term graft rejection can be prevented. The purpose of this study was to assess whether xenogeneic chromaffin cells immunologically isolated by semipermeable membranes could survive and continue to reduce pain when transplanted into the CNS. Isolated bovine chromaffin cells were encapsulated by semipermeable polymer membranes and implanted into the rat spinal subarachnoid space. Pain sensitivity was assessed at several intervals up to 3 months following implantation. Results indicated that encapsulated bovine chromaffin cell implants, but not empty control capsules, could repeatedly reduce pain sensitivity with nicotine stimulation for the duration of the study. This response was dose related, indicating that pharmacologic integrity of the transplanted chromaffin cells is retained. The analgesia induced by encapsulated chromaffin cell implants could be attenuated by the opiate antagonist naloxone and the alpha-adrenergic antagonist phentolamine, suggesting the involvement of both opioid peptides and catecholamines in mediating this response. In addition, in vitro neurochemical studies of recultured capsules revealed sustained release of Met-enkephalin and catecholamines from encapsulated cells 3 months following implantation into the spinal subarachnoid space.(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous dynorphin modulates calcium-mediated antinociception in mice

We previously reported that calcium administered IT produces antinociception by stimulating spinal Met-enkephalin release. However, at times the antinociceptive effects of calcium in the tail-flick test are greatly diminished. The results of this study indicates that during these periods calcium also stimulates endogenous dynorphin release. Dynorphin has been reported to block opiate-induced antinociception. Calcium-injected mice (150–600 nmol, IT) pretreated with vehicle IP displayed a poor degree of antinociception. Alternatively, pretreating mice with pentobarbital (45 mg/kg, IP) restored the antinociceptive effects of calcium. Low doses of naloxone and norbinaltorphimine (BNI) did not produce antinociception but restored the antinociceptive effects of calcium. Dynorphin (1–17) (Dyn 1–17), and Dyn (1–13), but not Dyn (1–8), blocked the antinociceptive effects of calcium restored with pentobarbital. These results indicate that calcium-mediated antinociception was sensitive to injected dynorphins. In additional experiments, antiserum to Dyn (1–13) was found to restore the antinociceptive effects of calcium, presumably by binding dynorphin released by calcium.

L-arginine exerts a dual role in nociceptive processing in the brain: involvement of the kyotorphin-Met-enkephalin pathway and NO-cyclic GMP pathway.

1. Intracerebroventricular (i.c.v.) administration of L-arginine (L-Arg), at 10-100 micrograms per mouse, produced antinociception in mice, as assessed by the tail flick test; this antinociception was reversed by pretreatment (s.c.) with naltrindole (NTI), a delta-selective opioid antagonist, and by co-administered L-leucyl-L-arginine (Leu-Arg), a kyotorphin (endogenous Met-enkephalin releaser) receptor antagonist. 2. L-NG-nitroarginine methyl ester (L-NAME), a NO synthase inhibitor, but not D-NG-nitroarginine methyl ester, given i.c.v. at 3-10 micrograms per mouse, exhibited antinociceptive activity that was resistant to naloxone (s.c.), NTI (s.c.) and Leu-Arg (i.c.v.). 3. The L-NAME (i.c.v.)-induced antinociception was not reversed by L-Arg (i.c.v.), which was antinociceptive by itself, but was abolished by combined injection of L-Arg plus Leu-Arg (i.c.v.) or by L-Arg (i.c.v.) after NTI (s.c.). 4. Methylene blue (MB), a soluble guanylate cyclase inhibitor, at 0.1-1 microgram per mouse, produced antinociception by i.c.v. administration. The antinociception induced by MB (i.c.v.) or L-NAME (i.c.v.) was reversed by co-administered dibutyryl cyclic GMP. 5. These findings suggest that L-Arg plays a dual role in nociceptive processing in the brain, being antinociceptive via the kyotorphin-Met-enkephalin pathway and nociceptive via the NO-cyclic GMP pathway.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) effects on enkephalinergic neurons in various regions of mouse brain

In order to elucidate the effects of MPTP on enkephalinergic neurons, dopamine (DA), norepinephrine (NE), proenkephalin (PE) mRNA and met-enkephalin (ME) were measured in striatum, olfactory tubercle, and prefrontal cortex of C57/B16 mice 1 day–2 weeks following treatment with 96 mg/kg MPTP-HCl (24 mg/kg i.p., twice/day for 2 days). DA and its metabolites were depleted 70% in striatum and 40% in olfactory tubercle within 1 day. In cortex, DA was unchanged, whereas homovanillic acid and NE were depleted 50 and 40% respectively by 3 days. ME increased in all three brain regions at different times whereas PE mRNA showed a different pattern in each region, with an increase in olfactory tubercle, a decrease in cortex, and in striatum, a decrease at 1 day followed by an increase at 3 days. Thus enkephalinergic neurons in each region respond differently to MPTP treatment. In striatum and olfactory tubercle, DA is depeleted sufficiently to release its tonic inhibition on the enkephalinergic neurons, thereby leading to increased enkephalin synthesis. In cortex, the change in NE metabolism appears to cause a decrease of ME release and thereby a depression of PE synthesis. The possible relationship between these results and the changes observed in Parkinson's disease are discussed.

Spinal δ2 but not δ1 opioid receptors are involved in intracereboventricular β-endorphin-induced antinociception in the mouse

The antinociception induced by β-endorphin given intracerebroventricularly (i.c.v.) has been previously demonstrated to be mediated by the release of Met-enkephalin and subsequent stimulation of δ receptors in the spinal cord for antinociception. The present study was designed to determine what type of opioid receptor, δ1 or δ2, in the spinal cord is involved in i.c.v. β-endorphin-induced antinociception. Antinociception was assessed by the tail-flick test in male ICR mice. NTB (0.2–20 nmol) and NTI0 (0.22–2.2 nmol),selective δ2 receptor antagonists, given intrathecally (i.t.) dose-dependently attenuated i.c.v. β-endorphin-induced inhibition of the tail-flick response. On the other hand, BNTX (0.02–2.2 nmol), a selective δ1 receptor antagonist, given i.t., did not block i.c.v. β-endorphin-induced antinociception. The tail-flick inhibition induced by DAMGO, a μ receptor agonist, or U50,488H, a к receptor agonist, was not blocked by i.t. BNTX, NTB or NTI. It is concluded that δ2 but not δ1 receptors in the spinal cord are involved in i.c.v. β-endorphin-induced antinociception.

Met-enkephalin secretion from mixed cultures of hypothalamic neurons and astrocytes.

Release of the endogenous opioid pentapeptide, met-enkephalin, from primary cultures of dissociated fetal rat hypothalamic cells was studied using an assay system which could both measure and differentiate between free met-enkephalin and the larger enkephalin-containing peptides (ECPs), which are the processing intermediates of the enkephalin precursor. The cultures were maintained in fully defined, serum-free medium and contained both neurons and astrocytes. Free met-enkephalin was secreted from the cultures in significant quantities in response to nonspecific depolarisation with 56 mM potassium, by a mechanism dependent upon extracellular calcium. Under basal conditions, barely detectable amounts of free peptide were released, whereas ECPs were secreted in significant quantities which were not reduced by the removal of extracellular calcium. As the period of culture increased, so did the quantitative importance of this constitutive ECP secretion, relative to the stimulated release of free peptide. Treatment of the cultures with the cytotoxic agent, cytosine arabinoside, attenuated this temporal increase of ECP secretion, whilst leaving the stimulated release of free met-enkephalin relatively unaffected. This suggested that the met-enkephalin secretion seen within the cultures reflected the presence of at least two distinct enkephalinergic cell types and that the change in the nature of the secreted enkephalin was at least in part, due to the proliferation of one of these cell populations. These results are consistent with secretion of met-enkephalin from both neurons and astrocytes within these cultures. We propose that the neurons secreted essentially fully processed peptide in a regulated manner, whilst the mitotic glial cells constitutively secreted non- or partially processed precursor peptides.(ABSTRACT TRUNCATED AT 250 WORDS)

The mixed antinociceptive agonist-antagonist activity of β-endorphin(1–27) in mice

β-Endorphin(1–27) (i.c.v.) has been reported to inhibit the antinociceptive activity of i.c.v. administered β-endorphin in mice. In this study the antagonist activity of β-endorphin(1–27) has been confirmed and the antagonism appears to be mediated at δ 1 opioid receptors. At higher doses than that used for antagonism, i.c.v. administered β-endorphin(1–27) was a full antinociceptive agonist. The antinociceptive activity of β-endorphin is attributed to the release of met-enkephalin in the spinal cord and is antagonized by the selective δ 2 opioid receptor antagonist, naltriben (NTB) but not by the selective δ 1 opioid receptor antagonist, 7-benzylidenenaltrexone (BNTX). In contrast, the antinociceptive activity of i.c.v. administered β-endorphin(1–27) was not affected by either NTB or BNTX administered i.c.v. or i.t. Also, the antinociceptive activity of β-endorphin(1–27) was unaffected by the selective μ opioid receptor antagonist, β-funaltrexamine (β-FNA) or the selective κ opioid receptor antagonist, norbinaltorphimine (norBNI). Thus, β-endorphin(1–27) appears to mediate antinociception supraspinally through the interaction of a unique receptor, i.e. a receptor that is different from μ, κ, δ 1 or δ 2 opioid receptors. Alternatively, a non-opioid mechanism may be considered.

Characterization of the release of Met-enkephalin from isolated nerve terminals: release kinetics and cation-dependence

The release of the neuropeptide Met-enkephalin (Met-ENK) from isolated nerve terminals (synaptosomes) of the rat forebrain was characterized with respect to the subcellular distribution, the release upon addition of various stimulatory agents, the release kinetics, the cation-dependence of release and the relationship between Met-ENK release and elevations of the intraterminal free Ca 2+-concentration ([Ca] i). A highly specific radioimmunoassay was developed for determination of Met-ENK (H-Tyr-Gly-Gly-Phe-Met-OH). Truncated and elongated forms of Met-ENK Leu-enkephalin, β-endorphin and dynorphin displayed negligible cross-reactivity. Met-ENK-like immunoreactivity (Met-ENK-LI) is enriched in the purified synaptosomal fraction of rat forebrain homogenates and is released in a strictly Ca 2+-dependent manner upon chemical depolarization or stimulation with the Ca 2+ ionophore, ionomycin. A correlation exists between the release of Met-ENK-LI and the elevations of [Ca] i. Barium ions are able to replace Ca 2+ in triggering Met-ENK-LI release. The release of Met-ENK-LI is initiated within 20 s after depolarization and is terminated after 3–5 min, although depolarization and [Ca] i elevation are maintained. At this time, 90% of the initial Met-ENK-LI is still present inside the synaptosomes. Repolarization and renewed stimulation again evokes Ca 2+-dependent release of this retained Met-ENK-LI. It is concluded that Met-ENK release from isolated nerrve terminals is exocytosis is terminated by a regulatory mechanism in synaptosomes after 3–5 min of depolarization, a process which can be reversed by repolarization. The characteristics of Met-ENK release are compared to those of other neuropeptides, of catecholamines and of amino acid transmitters, in similar preparations.

Tetanus toxin and botulinum toxins type A and B inhibit glutamate, gamma-aminobutyric acid, aspartate, and met-enkephalin release from synaptosomes. Clues to the locus of action.

Tetanus toxin (100 nM) when preincubated with guinea pig cerebrocortical synaptosomes for 45 min reduces the final extent of the KCl-evoked, Ca(2+)-dependent, glutamate transmitter release to 30% of non-intoxicated controls. Similarly, 100 nM Botulinum neurotoxins, types A and B, preincubated for 90 min inhibit release to 45-60% of non-intoxicated controls. The toxins preferentially attenuate a slow phase of KCl-evoked glutamate release which may be associated with synaptic vesicle mobilization. Tetanus toxin additionally inhibits the release of aspartate, gamma-aminobutyric acid and met-enkephalin from the same preparation. Since amino acids and neuropeptides are released by distinct mechanisms, this indicates that the toxin affects a step common to both exocytotic pathways. When Ba2+ (which does not interact with calmodulin) is substituted for Ca2+, the control KCl-evoked release of each transmitter is unaffected and tetanus toxin is still inhibitory. Taken together these results implicate a calmodulin-independent locus (or loci) of action common to small- and large-dense-core vesicles and associated with vesicle transport.

Feedback inhibition of met-enkephalin release from the rat spinal cord in vivo.

The possible existence of a feedback control by endogenous opioids of the spinal release of met-enkephalin-like material was assessed in vivo, in halothane-anesthetized rats whose intrathecal space was continuously perfused with an artificial cerebrospinal fluid supplemented with various opioid-related drugs. Both the intrathecal perfusion of the mu agonist D-Ala2-D-MePhe4-Gly-ol5-enkephalin (DAGO) (10 microM) and the delta agonist Tyr-D-Thr-Gly-Phe-Leu-Thr (DTLET) (10 microM) produced a significant inhibition of the spinal outflow of met-enkephalin-like material. The effect of DAGO, but not that of DTLET, could be prevented by naloxone (10 microM), and, conversely, the effect of DLTET, but not that of DAGO, was no longer observed in the presence of naltrindole (10 microM). Therefore naloxone and naltrindole acted as potent and selective mu and delta antagonists, respectively, when perfused at 10 microM in the intrathecal space of halothane-anesthetized rats. As expected from the lack of a tonic opioid control of spinal enkephalinergic neurones, neither naloxone nor naltrindole alone affected the spontaneous outflow of met-enkephalin-like material. However, naltrindole, but not naloxone, markedly increased the spinal overflow of met-enkephalin-like material due to intrathecal administration of either porcine calcitonin (10 microM) or the peptidase inhibitors thiorphan (10 microM) plus bestatin (20 microM). These data suggest that delta, but not mu, receptors are involved in a phasic opioid inhibitory control of the release of met-enkephalin-like material in the rat spinal cord.(ABSTRACT TRUNCATED AT 250 WORDS)

Intrathecal cholecystokinin octapeptide attenuates the antinociception and release of immunoreactive Met-enkephalin induced by intraventricular β-endorphin in the rat

The effects of cholecystokinin octapeptide (CCK8s) given intrathecally (i.t.) on antinociception and the release of immunoreactive Met-enkephalin in the spinal perfusate induced by intraventricular (i.vt.) injection of β-endorphin were studied in anesthetized rats. β-Endorphin (5 μg) given i.vt. inhibited the tail-flick response. The inhibition of the tail-flick response induced by β-endorphin was blocked dose-dependently by CCK8s (0.1–7 μg) given i.t. The antagonistic effect of CCK8s on β-endorphin-induced inhibition was blocked dose dependently by co-intrathecal injection of proglumide (3 and 10 μg), a CCK8s receptor antagonist. β-Endorphin (5 μg) given i.vt. elicited a release of immunoreactive Met-enkephalin in the spinal perfusate. Repeated injections of the same dose of β-endorphin released about the same amount of the immunoreactive Met-enkephalin in the spinal perfusate. CCK8s at concentrations from 1 × 10 −9 to 1 × 10 −6 M added into the spinal perfusate decreased the release of Met-enkephalin induced by β-endorphin given i.vt. in a dose-dependent manner. The results suggest that CCK8s may attenuate β-endorphin-induced inhibition of the tail-flick response by inhibiting the release of Met-enkephalin from the spinal cord.

β-Endorphin and met-enkephalin in cattle during late gestation and parturition — release from uteroplacental tissues

Plasma concentrations of β-endorphin and met-enkephalin during term and pre-term caesarean sections, and the release of these peptides by bovine uteroplacental tissues, were investigated. Blood samples were taken from the jugular vein and from a branch of the uterine vein. A release of metenkephalin into the maternal circulation by the uterus or placenta could be demonstrated.

Barium-evoked glutamate and met-enkephalin release from synaptosomes: Insights into the locus of action of tetanus toxin

Barium-evoked glutamate and met-enkephalin release from synaptosomes: Insights into the locus of action of tetanus toxin