Endogenous opiates: 1988

The nociceptin/orphanin FQ-like opioid peptide in nervous periesophageal ganglia of land snail Helix aspersa

The neuropeptide nociceptin/orphanin FQ (N/OFQ) and its receptor are members of the endogenous opioid peptide family. In mammals N/OFQ modulates a variety of biological functions such as nociception, food intake, endocrine, control of neurotransmitter release, among others. In the molluscs Cepea nemoralis and Helix aspersa the administration of N/OFQ produces a thermopronociceptive effect. However, little is known about its existence and anatomic distribution in invertebrates. The aim of this study was to provide a detailed anatomical distribution of N/OFQ like peptide immunoreactivity (N/OFQ-IL), to quantify the tissue content of this peptide, as well as to demostrate molecular evidence of N/OFQ mRNA in the nervous tissue of periesophageal ganglia of the land snail H. aspersa. Immunohistochemical, immunocytochemical, radioimmunoanalysis (RIA) and reverse transcription-polymerase chain reaction (RT-PCR) techniques were used. With regard to RT-PCR, the primers to detect expression of mRNA transcripts from H. aspersa were derived from the rat N/OFQ opioid peptide. We show a wide distribution of N/OFQ-IL in neurons and fibers in all perioesophageal ganglia, fibers of the neuropile, nerves, periganglionar connective tissue, aortic wall and neurohemal sinuses. The total amount of N/OFQ-IL in the perioesophageal ganglia (7.75 ± 1.75 pmol/g of tissue) quantified by RIA was similar to that found in mouse hypothalamus (10.1 ± 1.6 pmol/g of tissue). In this study, we present molecular evidence of N/OFQ mRNA expression. Some N/OFQ-IL neurons have been identified as neuroendocrine or involved in olfaction, hydro-electrolyte regulation, feeding, and thermonociception. Therefore, we suggest that N/OFQ may participate in these snail functions.

Food cravings, endogenous opioid peptides, and food intake: a review

Extensive research indicates a strong relationship between endogenous opioid peptides (EOPs) and food intake. In the present paper, we propose that food cravings act as an intervening variable in this opioid-ingestion link. Specifically, we argue that altered EOP activity may elicit food cravings which in turn may influence food consumption. Correlational support for this opioidergic theory of food cravings is provided by examining various clinical conditions (e.g. pregnancy, menstruation, bulimia, stress, depression) which are associated with altered EOP levels, intensified food cravings, and increased food intake.

Endometrial corticotropin-releasing hormone. Its potential autocrine and paracrine actions

Corticotropin-releasing hormone (CRH) is expressed at several peripheral tissues including normal epithelial cells of human and rodent uterus. However, its biological role is unknown in both species. To clarify this role we studied the regulation of CRH promoter in endometrial cells. We performed homologous transfection experiments in Ishikawa cells, a human endometrial cell line, using a 0.9-kb fragment of the 5'-flanking region of human CRH gene coupled to luciferase. We found that the activity of the 5'-flanking region of the CRH gene is stimulated by cAMP and EGF and inhibited in a receptor-mediated, dose-dependent fashion by estradiol and dexamethasone. The antiglucocorticoid RU 486 acted as a glucocorticoid agonist suppressing CRH gene activation, whereas progesterone was devoid of any activity. Prostaglandin E2 and interleukins-1 and -6 stimulated CRH activation, and the prostanoid inhibitor indomethacin suppressed it, most probably by inhibiting endogenous prostaglandins. These findings suggest that endometrial CRH gene expression may be under the negative control of estrogens and glucocorticoids and under the positive control of PGE2, IL-1, and IL-6. Considering the involvement of CRH in proinflammatory phenomena, we postulate that endometrial CRH, in association with uterine prostanoids and cytokines, may participate in intrauterine inflammatory processes of early pregnancy, such as decidualization and blastocyst implantation.

kappa opioid receptors in human microglia downregulate human immunodeficiency virus 1 expression

Microglial cells, the resident macrophages of the brain, play an important role in the neuropathogenesis of human immunodeficiency virus type 1 (HIV-1), and recent studies suggest that opioid peptides regulate the function of macrophages from somatic tissues. We report herein the presence of kappa opioid receptors (KORs) in human fetal microglia and inhibition of HIV-1 expression in acutely infected microglial cell cultures treated with KOR ligands. Using reverse transcriptase-polymerase chain reaction and sequencing analyses, we found that mRNA for the KOR was constitutively expressed in microglia and determined that the nucleotide sequence of the open reading frame was identical to that of the human brain KOR gene. The expression of KOR in microglial cells was confirmed by membrane binding of [3H]U69,593, a kappa-selective ligand, and by indirect immunofluorescence. Treatment of microglial cell cultures with U50,488 or U69,593 resulted in a dose-dependent inhibition of expression of the monocytotropic HIV-1 SF162 strain. This antiviral effect of the kappa ligands was blocked by the specific KOR antagonist, nor-binaltrophimine. These findings suggest that kappa opioid agonists have immunomodulatory activity in the brain, and that these compounds could have potential in the treatment of HIV-1-associated encephalopathy.

Absence of opioid stress-induced analgesia in mice lacking beta-endorphin by site-directed mutagenesis

A physiological role for beta-endorphin in endogenous pain inhibition was investigated by targeted mutagenesis of the proopiomelanocortin gene in mouse embryonic stem cells. The tyrosine codon at position 179 of the proopiomelanocortin gene was converted to a premature translational stop codon. The resulting transgenic mice display no overt developmental or behavioral alterations and have a normally functioning hypothalamic-pituitary-adrenal axis. Homozygous transgenic mice with a selective deficiency of beta-endorphin exhibit normal analgesia in response to morphine, indicating the presence of functional mu-opiate receptors. However, these mice lack the opioid (naloxone reversible) analgesia induced by mild swim stress. Mutant mice also display significantly greater nonopioid analgesia in response to cold water swim stress compared with controls and display paradoxical naloxone-induced analgesia. These changes may reflect compensatory upregulation of alternative pain inhibitory mechanisms.

Fetal ethanol exposure: hypothalamic-pituitary-adrenal and beta-endorphin responses to repeated stress

Previous studies provide evidence that fetal ethanol exposure induces hypothalamic-pituitary-adrenal (HPA) and pituitary beta-endorphin (beta-EP) hyperresponsiveness to acute stressors. The present study demonstrates significant effects of in utero ethanol exposure on the parallel response patterns of the HPA axis and the pituitary beta-EP system to repeated exposures to a stressor, restraint stress, and indicates sex differences in response. Together, data from the two experiments indicate that, after repeated restraint exposures, fetal ethanol-exposed (E) males and females both show significantly increased plasma levels of adrenocorticotropin (ACTH), and E males also show significantly increased plasma levels of beta-endorphin-like immunoreactivity (beta-EPLIR), compared with their respective pair-fed and control counterparts. Marginal increases in the corticosterone response of E males and the beta-EPLIR response of E females, compared with their controls, were also observed. In addition, delayed or deficient habituation to restraint stress was observed in the beta-EPLIR response of E males and the ACTH response of E females. These data demonstrate that fetal E-exposed males and females both exhibit hormonal hyperresponsiveness and/or deficits in recovery after repeated exposures to restraint stress, but that the patterns of response may differ depending on the number and duration of restraint exposures, the time course measured, and whether the endpoint measured is corticosterone, ACTH, or beta-EPLIR. In addition, the finding that E and pair-fed animals both differed from their respective controls in certain developmental and hormonal measures suggests that prenatal nutritional factors may play a role in mediating some of the changes that are observed.

A modulating effect of beta-endorphin on pigs' coping strategies during transport

Pigs were selected at random from three genetic lines (homozygote halothane positive and negative, heterozygotes), and were transported twice. Before and after each transport, blood samples were taken from the jugular vein to measure plasma cortisol and beta-endorphin content. During each transport behaviour and heart rate of the pigs were measured. Descriptive statistics of behavioural, endocrinological and heart rate data provided no evidence for a predetermined idiosyncratic response pattern (i.e., active vs. passive) with respect to transport stress, nor an interaction from the halothane gene, nor an habituation effect. However, from the endocrinological point of view a potentially of passive and active coping strategy (i.e., high vs. low basal plasma cortisol content) was observed before transport. But classification of pigs according to an active and passive coping strategy was unrepeatable during successive transports, when based on heart rate and plasma cortisol content measurements during successive transports. This was explained by a potential modulating effect of beta-endorphin on the relationship between cortisol and heart rate.

mu- and delta-opioid receptor-mediated contractile effects on rat aortic vascular smooth muscle

The actions of opioid receptor agonists and antagonists were studied in isolated rat aortic strips. Morphine (10(-7)-10(-6) M) had no contractile effect on resting strips but when added during the relaxation of the contractions induced by 10(-9) M noradrenaline, it induced a contractile response which was blocked by naloxone. The selective mu-opioid receptor agonist, [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAMGO, 10(-7)-10(-6) M), induced an increase in basal tension which remained after removal of endothelium or in Ca(2+)-free solution, but was inhibited by beta-flunaltrexamine. beta-Flunaltrexamine also inhibited the contractile response induced by DAMGO added during the relaxation of the contractions induced by noradrenaline. The delta-opioid receptor agonist, [D-Pen2,D-Pen5]enkephalin, had no effect on resting tension but potentiated the contractions induced by noradrenaline; these effects were abolished by naltrindol. The selective kappa-opioid receptor agonist, bremazocine, had no effect on resting tension and did not modify the amplitude of the contractions induced by noradrenaline. These results suggest that, at low concentrations, agonists of mu- and delta-opioid receptors may act as modulators of noradrenaline-induced responses, whereas at higher concentrations, mu-opioid receptor stimulation may have a direct contractile effect in isolated rat aorta.

The Tyr-MIF-1 family of peptides

A review of research on the Tyr-MIF-1 family of peptides is presented with emphasis on Tyr-MIF-1 and its structure, passage through the blood-brain barrier, and both opiate antagonist and agonist properties. Family members MIF-1, Tyr-W-MIF-1 and Tyr-K-MIF-1 are also included.

Enhanced immunogenicity of leucine enkephalin following coupling to anti-immunoglobulin and anti-CD3 antibodies

Leucine enkephalin (Leu-enk) was coupled to both T and B cell antibodies in order to investigate the possibility of enhanced immunogenicity via targeted immunization. The two antibodies used were Hm x Mo CD3 and Gt x Mo Ig, respectively. The data indicate that while both antibody carriers enhanced the immunogenicity of Leu-enk, the use of the Hm x Mo CD3 antibody resulted in a greater number of mice with positive Leu-enk specific serum titers. 12 Leu-enk cell lines were produced and one, LE4H8, was chosen for characterization.

Plasma levels of beta-endorphin and ACTH during labor and immediate puerperium

We measured concentrations of beta-endorphin (beta-EP) and adrenocorticotropic hormone (ACTH) in maternal peripheral plasma during dilation, the expulsive period and immediate puerperium, and in the umbilical vein. Plasma levels of both peptides increased markedly and were directly correlated during labor, and decreased 24 h after birth; they were no longer correlated during immediate puerperium. In the umbilical vein, beta-EP and ACTH were also directly correlated. Moreover, the concentration of each of the two peptides in maternal plasma during the expulsive period was correlated with the corresponding concentration in umbilical vein. We discuss the importance of placental corticotropin-releasing factor in the regulation of maternal and fetal stress.

Ejaculation induced changes in escape latency in the hot plate test: pharmacological analysis of anxiolytic versus analgesic effect

The possible changes in nociception at various stages of male sexual behaviour were explored in the hot plate test. Although other authors have reported an antinociceptive effect of mating, we failed to find this effect on the hot plate test after several sexual behaviour events. To further explore the possible antinociceptive action of copulation we administered a suboptimal analgesic dose of morphine (0.3 mg/kg i.p.). No change in nociception were observed in animals treated with a subthreshold dose of morphine and tested in the nociception test after ejaculation were observed. Since previous reports have refered that ejaculation produces hypoalgesia when measured in the hot plate test, we attempted to replicate these findings. We found that one ejaculation produces an increase in the latency to escape in male rats previously habituated to the hot plate test. These results suggest a hypoalgesic effect. However, such changes could also be interpreted as alterations in the animals' emotionality. Thus, a group of habituated animals was tested on the switched off plate. An important increase in the escape latency behaviour after ejaculation was observed, while no differences between control animals, without sexual behaviour display, tested on switched on and switched off plate were observed. The participation of the benzodiazepine and opiod systems in the ejaculation effect on the switched off plate was explored. A similar increase in escape latency to that induced by ejaculation was caused by diazepam (1.0 and 2.0 mg/kg). The increase in escape latency induced by ejaculation was prevented by the benzodiazepine antagonist, flumazenil (20.0 mg/kg).(ABSTRACT TRUNCATED AT 250 WORDS)

CNS effects of peptides: a cross-listing of peptides and their central actions published in the journal Peptides, 1986-1993

The centrally mediated effects of peptides as published in the journal Peptides from 1986 to 1993 are tabulated in two ways. In one table, the peptides are listed alphabetically. In another table, the effects are arranged alphabetically. Most of the effects observed after administration of peptides are grouped, wherever possible, into categories such as cardiovascular and gastrointestinal. The species used in most cases has been rats; where other animals were used, the species is noted. The route of administration of peptides and source of information also are included in the tables, with a complete listing provided at the end. Many peptides have been shown to exert a large number of centrally mediated effects.

Cloning and functional comparison of kappa and delta opioid receptors from mouse brain

While trying to identify new members of the somatostatin receptor family of G protein-coupled receptors, we isolated cDNAs from a mouse brain library encoding two related receptor-like proteins, designated msl-1 and msl-2, of 380 and 372 amino acids, respectively. There was 61% identity and 71% similarity between the sequences of msl-1 and msl-2. Among members of the G protein-coupled receptor superfamily, the sequences of both msl-1 and msl-1 were most closely related to those of the somatostatin receptors (SSTRs), having approximately 35% identity with the sequence of SSTR1. Transient expression in COS-1 cells showed that msl-1 and msl-2 did not bind somatostatin. Rather they bound opioids selectively and with high affinity and had the pharmacological properties of kappa and delta opioid receptors, respectively. Indeed, the sequence of msl-2 was identical to that of a delta opioid receptor recently cloned by other workers. Functional characterization of kappa/msl-1 and delta/msl-2 opioid receptors showed that they were coupled to G proteins and mediated opioid receptor class-specific agonist inhibition of forskolin-stimulated cAMP formation. RNA blotting studies and in situ hybridization histochemistry showed that kappa opioid receptor mRNA was expressed at high levels in brain in the neocortex, hippocampus, amygdala, medial habenula, hypothalamus (arcuate and paraventricular nuclei), locus ceruleus, and parabrachial nucleus, suggesting that this receptor may play a role in arousal and regulation of autonomic and neuroendocrine functions.

Quantitative autoradiographic demonstration of changes in binding to delta opioid, but not mu or kappa receptors, in chick forebrain 30 minutes after passive avoidance training

Day-old domestic chicks (Gallus domesticus) were trained on a one-trial passive avoidance task in which the aversive stimulus was a bitter tasting substance, methylanthranilate. Thirty minutes later, localization of binding of highly specific ligands (([D-Ala2, Gly-ol]-enkephalin ([3H]DAGO) for mu (mu) receptor sites, [D-Pen2,D-Pen5]-enkephalin ([3H]-DPDPE) for delta (delta) sites, and [3H]-U- 69593 for kappa (kappa 1) sites) to opioid receptors in various regions of the forebrain of methyl-anthranilate trained (M-) and control (water trained (W-)) chicks was determined using quantitative receptor autoradiography. Significant differences in binding to delta ([3H]-DPDPE), but not mu or kappa receptors, were found in several regions of the forebrain, of trained compared to control chicks. There were decreases in binding in the hyperstriatum dorsale of the left hemisphere (14%) and a decrease in binding in the lateral hyperstriatum ventrale of the right hemisphere (14%). However, significant increases were observed in delta binding in the paleostriatum augmentatum of the right hemisphere (16%) and the lobus parolfactorius of both hemispheres (left, 20%; right, 21%). In a control experiment designed to determine whether the taste of methylanthranilate contributed to the increase in 3H-DPDPE binding, there was no significant difference in the level of binding between blindfolded birds in which methylanthranilate was placed in the beak, and blindfolded birds in which water was placed on the bead and inserted into the beak. These findings demonstrate that changes occur in an opioid receptor sub-type in specific regions of forebrain of the chick following passive avoidance training which may be related to events concerned with the process of memory formation.

Modifications of striatal D2 dopaminergic postsynaptic sensitivity during development of morphine tolerance-dependence in mice

Alterations in the activity of striatal dopaminergic neurons have been implicated in the development of morphine tolerance-dependence in rodents. To further explore this possibility, we examined the activity of these neurons in mice exposed to morphine during 4 days (addiction group) and subsequently treated with naloxone (withdrawal group). The efficiency of opiate treatment was assessed behaviorally. Striatal dopaminergic activity was evaluated by measuring: a) the ratio between the amounts of L-3,4-dihydroxyphenylacetic acid (DOPAC), the main intraneuronal metabolite of dopamine (DA), and the neurotransmitter itself, as an index of presynaptic activity; and b) the number and affinity of D1 and D2 dopaminergic receptors, as well as the amount of their coupled second messenger, cyclic adenosine monophosphate (cAMP), as postsynaptic parameters. Spontaneous motor activity was decreased in chronically morphine-exposed mice. In these animals, the number of striatal D2 receptors also decreased, with no changes in their affinity, whereas the number and affinity of D1 receptors remained unchanged. This hyposensitivity of D2 receptors was paralleled by an increase in the amount of cAMP with a good statistical correlation between both parameters. Treatment with naloxone of morphine-exposed mice resulted in the typical jumping behavior indicative of opiate withdrawal. The differences in D2 receptors between placebo- and morphine-exposed mice disappeared after naloxone-induced opiate withdrawal, although this effect was due more to the inhibitory effect of naloxone on the density of these receptors in placebo-exposed mice rather than to a stimulatory effect in morphine-addicted mice. The morphine-induced increase in cAMP content also disappeared after naloxone treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Low doses of morphine reduce voluntary alcohol consumption in rhesus monkeys

Experimental opioid modulation has been found to influence the consumption of alcohol in animals. Whereas it has generally been agreed upon that opiate antagonists reduce alcohol consumption, the results with opiate agonists are less consistent. The present study reports on the effect of low doses of morphine in 8 adult male rhesus monkeys that had a free choice in drinking water, a 16% and a 32% ethanol/water solution, (a) during continuous ad libitum access (Experiment I), and (b) after 2 days of alcohol abstinence (Experiment II). In both experiments each monkey received a single morphine injection (i.m.) in 5 different doses (0.03, 0.06, 0.17, 0.50, 1.50 mg.kg-1); each morphine injection (i.m.) was placebo-controlled in a cross-over design. Consumption was measured from 16.00 h in the afternoon (30 min after injection) to 08.30 h the next morning. In Experiment I after 0.50 and 1.50 mg.kg-1 of morphine ethanol intake and water consumption were both reduced during the first hours after injection; only ethanol intake remained reduced during the subsequent night. Effects lasted not longer than 24 h. In Experiment II, morphine administered 30 min before reintroduction of ethanol solutions reduced ethanol intake at doses of 0.17, 0.50 and 1.50 mg.kg-1; water consumption was unaffected. The reduction lasted for the subsequent night after the 2 highest doses. Records obtained of various spontaneous behavioural activities made it unlikely that the used dose range had induced some aspecific sedation; monkeys remained alert and active. The results are contradictory with studies in which low doses of morphine stimulated alcohol drinking in rats. The present results seem to support the hypothesis that at least in monkeys morphine can compensate for some effects of alcohol.

Opioid receptors and their pharmacological profiles

Opioid receptors can be divided into three major classes, which are called mu, delta and kappa-receptors. The molecular basis of the receptors is discussed and a hypothesis of the binding of bivalent ligands to the receptor is presented. Furthermore the mechanism of action, the distribution and the probable function of these classes is overviewed. Increasing evidence is accumulating that the classical binding model cannot explain completely the interaction of opioids with their receptors. In addition to the mu-receptors, high affinity mu 1 binding sites have been demonstrated. Similarly, the delta receptors may be divided in delta 1 and delta 2. The significance of these subclasses is not yet fully understood. The high affinity mu 1-binding sites, however, represent probably an activated receptor complex, e.g. the complex between the receptor and the guanine-nucleotide-binding protein.

Peripheral opioid secretory pattern in anorexia nervosa

The peripheral secretion of endogenous opioids was studied in 10 women with restrictive anorexia nervosa and 10 age- and sex-matched healthy controls. The circadian rhythm of beta-endorphin (beta-EP) and beta-lipotropin (beta-LPH), and their responses to the administration of corticotropin releasing hormone (CRH, 1 micrograms/kg body weight, i.v.), clonidine (150 microgram, i.v.), domperidone (10 mg, i.v.), and 5-hydroxytryptophan (5-HTP, 200 mg, p.o.) were examined in patients and controls. The results revealed increased nocturnal secretion of beta-EP and diurnal-nocturnal secretion of beta-LPH with loss of circadian rhythmicity of both peptides, normal response to CRH stimulation, blunted response to clonidine and domperidine, and normal beta-EP and blunted beta-LPH response to 5-HTP stimulation. The data suggest a complex alteration of peripheral opioids and of central aminergic mechanisms that regulate proopiomelanocortin-derived peptide secretion and eating behavior.

Effects of cold-restraint and swim stress on convulsions induced by pentylenetetrazol and electroshock: influence of naloxone pretreatment

The influence of two stressogenic conditions, restraint at 4 degrees C for 30 min (cold-restraint stress; CRS) or swimming at 20 degrees C for 3 min (swim stress; SS), on nociception and on convulsions triggered by different agents was assessed in mice. In saline-pretreated mice CRS and SS caused analgesia (hot-plate test, 56 degrees C), delayed the onset of convulsions induced by pentylenetetrazol (PTZ, 100 mg/kg, IP) and aggravated convulsions elicited by maximal transcorneal electroshock (150 mA pulses at 60 Hz for 0.2 s). Pretreatment with naloxone (10 mg/kg, SC, 30 min prior to testing), which did not affect the responsiveness of nonstressed mice to the hot plate or to the convulsant treatments, attenuated the development of analgesia following CRS, but not SS, and further prolonged the latency to onset of PTZ-induced convulsions in both stressed groups. Thus the extent to which CRS and SS can each delay the onset of PTZ-triggered convulsion appears to be limited by activation of a proconvulsant opioid system. In contrast, naloxone pretreatment did not modify the effects of CRS or SS on the severity of electroshock-induced seizures. In conclusion, CRS and SS can each, simultaneously, exert anticonvulsant and proconvulsant influences on responsiveness to PTZ and electroshock, respectively. Also, both forms of stress can activate an opioid system modulating the onset of PTZ-induced seizures, which is distinct from that controlling nociception. These findings, together with those of other stress, convulsions and opioid systems, which depends on the characteristics of the stressogenic condition, species, convulsant agent and parameter considered.

Sleep deprivation decreases mu and delta opioid receptor binding in the rat limbic system

Sleep deprivation induced by the platform technique is considered to be a heavy stressful situation in rats. At the end of the sleep deprivation period (72 h) the rat displayed particular behavior characterized by wakefulness, a high degree of motor and exploratory activity, increased alertness and reactivity to environmental stimuli. Our previous results indicated that this behavior was antagonized by the administration of the opioid receptor antagonist naloxone and increased by opioid agonists. In this paper we show that concomitantly with this behavior, a decreased Bmax of mu and delta opioid receptors is present in the limbic system of these animals. These data suggest an active role of limbic mu and delta receptors in the generation of arousal and insomnia related to sleep deprivation induced stress.