Tyr-MIF-1 acts as an opiate antagonist in the tail-flick test

Crosstalk between Opioid and Anti-Opioid Systems: An Overview and Its Possible Therapeutic Significance

Opioid peptides and receptors are broadly expressed throughout peripheral and central nervous systems and have been the subject of intense long-term investigations. Such studies indicate that some endogenous neuropeptides, called anti-opioids, participate in a homeostatic system that tends to reduce the effects of endogenous and exogenous opioids. Anti-opioid properties have been attributed to various peptides, including melanocyte inhibiting factor (MIF)-related peptides, cholecystokinin (CCK), nociceptin/orphanin FQ (N/OFQ), and neuropeptide FF (NPFF). These peptides counteract some of the acute effects of opioids, and therefore, they are involved in the development of opioid tolerance and addiction. In this work, the anti-opioid profile of endogenous peptides was described, mainly taking into account their inhibitory influence on opioid-induced effects. However, the anti-opioid peptides demonstrated complex properties and could show opioid-like as well as anti-opioid effects. The aim of this review is to detail the phenomenon of crosstalk taking place between opioid and anti-opioid systems at the in vivo pharmacological level and to propose a cellular and molecular basis for these interactions. A better knowledge of these mechanisms has potential therapeutic interest for the control of opioid functions, notably for alleviating pain and/or for the treatment of opioid abuse.

From blood to brain through BBB and astrocytic signaling

In this Festschrift, I discuss the career and guiding principles to which Abba J. Kastin has adhered during the last 20 years we worked together. I briefly describe the history of our joint laboratory group, the context of studies of peptide permeation across the blood-brain barrier (BBB), and newer developments in the BBB Group as Abba steps down after serving 35 years as the founding Editor-in-Chief for Peptides. Abba's BBB studies on peptides have contributed to concepts in the neuroendocrinology of feeding and developed information on molecular trafficking across BBB cells. The astroglial leptin signaling studies and the interactions of sleep and BBB are two major directions, whereas the long-term MIF-1 project demarcates a tortuous road on translational research.

Isolation of a heptapeptide Val-Val-Tyr-Pro-Trp-Thr-Gln (valorphin) with some opiate activity

Bovine hypothalamic tissue was extracted and purified by solid phase extraction and several reversed-phase HPLC steps. The amino acid sequence of the purified peptide was determined by Edman degradation to be Val-Val-Tyr-Pro-Trp-Thr-Gln. This was confirmed by comparison of its chromatographic behavior with that of the synthetic peptide, and mass spectrometric analysis resulted in a mass identical to the calculated mass for this peptide. This heptapeptide shows homology with residues 32-38 of the beta-chain of bovine hemoglobin. The peptide inhibited the electrically induced contractions of the guinea pig ileum muscle preparation; this inhibition was reversible by naloxone. It also inhibited the binding of 125I-DAMGO (selective for mu receptors) to rat brain with an IC50 of 10 microM and the binding of 3H-DPDPE (selective for sigma receptors) with an IC50 of 185 microM. With two valines at the N-terminus and some opiate activity, valorphin seems a suitable name for this newly isolated peptide.

Conformational studies of N-Tyr-MIF-1 in aqueous solution by 1H nuclear magnetic resonance spectroscopy

N-Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) is an endogenous brain peptide with multiple effects on animal behavior. However, there have been no studies on the conformation of this tetrapeptide. In this report, we studied the conformation of N-Tyr-MIF-1 in aqueous solution by conventional one-dimensional and two-dimensional (COSY and NOESY) 1H nuclear magnetic resonance spectroscopy at 300 MHz. A complete set of assignments for the resolved resonances and approximate assignments for the overlapping resonances were made. The results demonstrate that N-Tyr-MIF-1 is in slow exchange between two conformers, most likely determined by the cis and trans states of the proline residue. The minor conformation represents 30 +/- 3% of the population over the temperature range from 3 degrees to 73 degrees. In the major conformation, the tyrosine aromatic ring appears to be close enough to interact directly with the proline pyrrolidine ring, as indicated by a strong temperature dependence of the proline C beta H, C delta H and C delta H' chemical shifts. In contrast, this interaction of the tyrosine and proline rings is not present in the minor conformation.

From MIF-1 to endomorphin: the Tyr-MIF-1 family of peptides

The Tyr-MIF-1 family of small peptides has served a prototypic role in the introduction of several novel concepts into the peptide field of research. MIF-1 (Pro-Leu-Gly-NH(2)) was the first hypothalamic peptide shown to act "up" on the brain, not just "down" on the pituitary. In several situations, including clinical depression, MIF-1 exhibits an inverted U-shaped dose-response relationship in which increasing doses can result in decreasing effects. This tripeptide also can antagonize opiate actions, and the first report of such activity also correctly predicted the discovery of other endogenous antiopiate peptides. The tetrapeptide Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH(2)) not only shows antiopiate activity, but also considerable selectivity for the mu-opiate binding site. Tyr-W-MIF-1 (Tyr-Pro-Trp-Gly-NH(2)) is an even more selective ligand for the mu receptor, leading to the discovery of two more Tyr-Pro tetrapeptides that have the highest specificity and affinity for this site. These are the endomorphins: endomorphin-1 is Tyr-Pro-Trp-Phe-NH(2) and endomorphin-2 is Tyr-Pro-Phe-Phe-NH(2). Tyr-MIF-1 proved, contrary to the then prevailing dogma, that peptides can be saturably transported across the blood-brain barrier by a quantifiable transport system. Unexpectedly, the Tyr-MIF-1 transporter is shared with Met-enkephalin. In the era in which it was doubtful whether a peripheral peptide could exert CNS effects, the Tyr-MIF-1 family of peptides also explicitly showed that they can exert more than one central action that persists longer than their half-lives in blood. These peptides clearly illustrate that the name of a peptide restricts neither its actions nor its conceptual implications.

Why study transport of peptides and proteins at the neurovascular interface

The blood-brain barrier (BBB) is an immense neurovascular interface. In neurodegenerative, ischemic, and traumatic disorders of the central nervous system (CNS), the BBB may hinder the delivery of many therapeutic peptides and proteins to the brain and spinal cord. Fortunately, the mistaken dogma that peptides and proteins do not cross the BBB has been corrected during the past two decades by the accumulating evidence that peptides and proteins in the periphery exert potent effects in the CNS. Not only can peptides and proteins serve as carriers for selective therapeutic agents, but they themselves may directly cross the BBB after delivery into the bloodstream. Their passage may be mediated by simple diffusion or specific transport, both of which can be affected by interactions in the blood compartment (outside the BBB) and within the endothelial cells (at the BBB level). Although the majority of current delivery strategies focuses on modification of the molecule to be delivered, understanding the mechanisms of transport will eventually facilitate regulation of the BBB directly. We review the different aspects of interactions and discuss recent advances in the cell biology of peptide/protein transport across the BBB. Better understanding of the nature and regulation of the transport systems at the BBB will provide a new direction to enhance the interactions of peripheral peptides and proteins with the CNS.

Relation of beta-casomorphin to apnea in sudden infant death syndrome

Sudden infant death syndrome (SIDS) is the most common cause of death in infants and its pathogenesis is complex and multifactorial. The aim of this review is to summarize recent novel findings regarding the possible association of beta-casomorphin (beta-CM) to apnea in SIDS, which has not been widely appreciated by pediatricians and scientists. beta-CM is an exogenous bioactive peptide derived from casein, a major protein in milk and milk products, which has opioid activity. Mechanistically, circulation of this peptide into the infant's immature central nervous system might inhibit the respiratory center in the brainstem leading to apnea and death. This paper will review the possible relationship between beta-CM and SIDS in the context of passage of beta-CM through the gastrointestinal tract and the blood-brain barrier (BBB), permeability of the BBB to peptides in infants, and characterization of the casomorphin system in the brain.

The role of spinal cholecystokinin in chronic pain states

It is well established that cholecystokinin (CCK) reduces the antinociceptive effect of opioids. The level of CCK and CCK receptors, as well as CKK release, exhibits considerable plasticity after nerve injury and inflammation, conditions known to be associated with chronic pain. Such altered CCK release coupled in some situation with changes in CCK receptor levels may underlie the clinical phenomenon of varying opioid sensitivity in different clinical pain conditions. In particular, neuropathic pain after injury to the peripheral and central nervous system does not respond well to opioids, which is likely to be caused by increased activity in the endogenous CCK system. CCK receptor antagonists may thus be useful as analgesics in combination with opioids to treat neuropathic pain.

Neuropeptides in neuropathic and inflammatory pain with special emphasis on cholecystokinin and galanin

Neuropeptides present in primary afferents and the dorsal horn of the spinal cord have an important role in the mediation of nociceptive input under normal conditions. Under pathological conditions, such as chronic inflammation or following peripheral nerve injury, the production of peptides and peptide receptors is dramatically altered, leading to a number of functional consequences. In this review, the role of two neuropeptides that undergo such altered expression under pathological conditions, cholecystokinin (CKK) and galanin, is reviewed.

Cholecystokinin/opioid interactions

Cholecystokinin (CCK) acts as an anti-opioid peptide. The mechanisms of CCK-opioid interaction under normal and pathological conditions were examined with various techniques. Nerve injury induces upregulation of CCK mRNA and CCK2 receptors in sensory neurons. The involvement of CCK in spinal nociception in normal and axotomized rats was examined. The CCK2 receptor antagonist CI-988 did not reduce spinal hyperexcitability following repetitive C-fiber stimulation in normal or axotomized rats, suggesting that CCK is probably not released from injured primary afferents. With in vivo microdialysis intravenous (i.v.) or intrathecal (i.t.) morphine increased the extracellular level of CCK in the dorsal horn in a naloxone reversible manner. Morphine also released CCK after axotomy, but not during carrageenan-induced inflammation. In contrast, K(+)-stimulation failed to increase extracellular levels of CCK in axotomized rats, but did so in inflamed rats. Double-coloured immunofluorescence technique revealed partial co-localization between CCK-like immunoreactivity (LI) and mu-opioid receptor (MOR)-LI in superficial dorsal horn neurons. The presence of MOR in CCK containing neurons suggests a possible direct influence of opioids on CCK release in the spinal cord. Axotomy, but not inflammation, induced a moderate decrease in CCK- and MOR-LI in the dorsal horn. I.v. morphine further temporarily reduced CCK- and MOR-LIs in axotomized, but not in normal or inflamed, rats. While the effect of morphine on CCK-LI can be interpreted as the result of increased CCK release, the effect on MOR-LI may be related to changes in the microenvironment of the dorsal horn induced by nerve injury.

Effects of Tyr-MIF-1 on stress-induced analgesia and the blockade of development of morphine tolerance by stress in mice

The role of Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) in biological responses to stress exposure was examined in mice. Intraperitoneal or intracerebroventricular administration of Tyr-MIF-1 attenuated not only footshock (FS)- and forced swimming (SW)-stress-induced analgesia (SIA) but also socio-psychological (PSY)-SIA that, when using the communication box, is produced without any direct physical nociceptions. Tyr-MIF-1 also disrupted the suppressive effect of concurrent exposure to FS- and PSY-stress on the development of morphine antinociceptive tolerance. In elevated-plus-maze tests, mice treated with Tyr-MIF-1 tended to spend more time in the open arms compared with the control group, suggesting the anxiolytic properties of the peptide. Thus, the finding that Tyr-MIF-1 modulates these stress responses suggests that the peptide regulates an endogenous biological alert system responding to stress exposure, perhaps, counteracting the excessive response of the system. Furthermore, Tyr-MIF-1, in the case of PSY-stress, through the attenuation of emotional factors such as fear and anxiety, may suppress PSY-SIA and inhibition by PSY-stress of the development of morphine tolerance.

Opioid binding profile of morphiceptin, Tyr-MIF-1 and dynorphin-related peptides in rat brain membranes

Opioid properties of several morphiceptin- (Tyr-Pro-Phe-Pro-NH2), Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) and dynorphin-derivatives were characterized in rat brain in vitro receptor binding assay and in electrically stimulated longitudinal muscle strip preparation of guinea pig ileum. In the case of morphiceptin-related peptides, an excellent correlation was found between the [3H]-naloxone binding displacement data and the agonist potencies determined in the bioassay. The "turning point' was the C-terminal amidation in the tri- and tetrapeptide pairs in both series. Tyr-MIF-1 derivatives showed weak affinity in the opioid receptor binding assay and none of them had any remarkable effect in the bioassay either as agonist or antagonist. The dynorphin A(1-10)-peptides modified at positions 5 and 8 retained their affinity with Pro5-, Pro8-, and Ala8-substituents, whereas some loss of affinity was observed in the case of Gly8-Dyn A(1-10).

Opioid and anti-opioid peptides

The numerous endogenous opioid peptides (beta-endorphin, enkephalins, dynorphins ... ) and the exogenous opioids (such as morphine) exert their effects through the activation of receptors belonging to four main types, mu, delta, kappa and epsilon. Opioidergic neurones and opioid receptors are largely distributed centrally and peripherally. It is thus not surprising that opioids have numerous pharmacological effects and that endogenous opioids are thought to be involved in the physiological control of various functions, among which nociception is particularly emphasized. Some opioid targets may be components of homeostatic systems tending to reduce the effects of opioids. "Anti-opioid" properties have been attributed to various peptides, especially cholecystokinin (CCK), neuropeptide FF (NPFF) and melanocyte inhibiting factor (MIF)-related peptides. In addition, a particular place should be attributed, paradoxically, to opioid peptides themselves among the anti-opioid peptides. These peptides can oppose some of the acute effects of opioids, and a hyperactivation of anti-opioid peptidergic neurones due to the chronic administration of opioids may be involved in the development of opioid tolerance and/or dependence. In fact, CCK, NPFF and the MIF family of peptides have complex properties and can act as opioid-like as well as anti-opioid peptides. Thus, "opioid modulating peptides" would be a better term to designate these peptides, which probably participate, together with the opioid systems, in multiple feed-back loops for the maintenance of homeostasis. "Opioid modulating peptides" have generally been shown to act through the activation of their own receptors. For example, CCK appears to exert its anti-opioid actions mainly through the activation of CCK-B receptors, whereas its opioid-like effects seem to result from the stimulation of CCK-A receptors. However, the partial agonistic properties at opioid receptors of some MIF-related peptides very likely contribute to their ability to modulate the effects of opioids. CCK- and NPFF-related drugs have potential therapeutic interest as adjuncts to opioids for alleviating pain and/or for the treatment of opioid abuse.

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.

Improvement in major depression after low subcutaneous doses of MIF-1

In this double-blind pilot study, 20 significantly depressed patients who all met the DSM-III R criteria for major depression were given a single subcutaneous injection of either 10 mg MIF-1 (Pro-Leu-Gly-NH2) or placebo on each of 5 consecutive days. Treatments were reversed for a second week of 5 consecutive daily injections. At the end of the first week, the group receiving MIF-1 was significantly improved on all rating scales as compared with the group receiving placebo. Eight out of 9 patients receiving MIF-1 showed marked improvement (score < or = 7 on the Hamilton Scale) as compared with only 2 of 11 patients receiving saline (P < 0.01). Administration of MIF-1 during the second week to the patients who had received placebo during the first week resulted in substantial improvement so that by the end of the second week the two groups were indistinguishable.

Opioid and nonopioid interactions in two forms of stress-induced analgesia

Stressful environmental events activate endogenous mechanisms of pain inhibition. Under some circumstances the analgesia is blocked by naloxone/naltrexone ("opioid"), while under others it is not ("nonopioid"). The existence of these two categories of analgesia leads to the question of how they are related. In a collateral inhibition model proposed by Kirshgessner, Bodnar, and Pasternak (1982), opiate and nonopiate mechanisms were viewed as acting in a mutually inhibitory fashion. In the present experiments, rats were exposed to either of two environmental stressors that produce a nonopioid stress-induced analgesia (SIA) following injections of the opiate antagonist naltrexone or agonist morphine. In the presence of naltrexone, SIA produced by either cold water swim (CWS) or social defeat was enhanced. These same SIAs were found to attenuate the analgesic effect of morphine, demonstrating that an activation of opioid systems can inhibit nonopioid analgesias. These results support an inhibitory interaction of opioid and nonopioid mechanisms in some forms of stress-induced analgesia.

A review of the role of anti-opioid peptides in morphine tolerance and dependence

Studies on the mechanisms of tolerance and dependence have mostly focused on changes at the receptor level. These experiments, conducted with model systems ranging from clonal cell lines to whole animals, have identified a number of important adaptive mechanisms which occur at the receptor level. However, none of these adaptive mechanisms can completely account for the phenomena which serve to define the state of morphine tolerance and dependence, especially the observation that as an animal becomes more tolerant to morphine, less naloxone is required to trigger withdrawal. The data reviewed in this paper provide strong support for the hypothesis that the brain synthesizes and secretes neuropeptides which act as part of a homeostatic system to attenuate the effects of morphine and endogenous opioid peptides. According to this model, administration of morphine releases anti-opioid peptides (AOP), which then attenuate the effects of morphine. As more morphine is given, more AOP are released, thereby producing tolerance to the effects of morphine. Cessation of morphine administration, or administration of naloxone, produces a relative excess of anti-opioid, which is in part responsible for the withdrawal syndrome. Since endogenous and exogenous antagonists might together produce synergistic effects, less naloxone might be required to trigger withdrawal in the presence of higher levels of AOPs. Although the study of AOP is in its infancy, a deeper understanding of the central nervous system (CNS) anti-opioid systems may lead to new treatments for chronic pain, substance abuse, and psychiatric disorders.

Tyr-MIF-1 and hemorphin can act as opiate agonists as well as antagonists in the guinea pig ileum

The brain peptide Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) was tested for its effects on electrically stimulated contractions in the guinea pig ileum assay. Tyr-MIF-1 acted as an opiate agonist in reducing these contractions. Its IC50 was about 9 microM, and its effects were reversed by naloxone and CTOP. The ability of Tyr-MIF-1 also to antagonize the inhibitory effects of opiates on electrically stimulated contractions was more evident in the ileum removed from a guinea pig tolerant to morphine or after partial inactivation of opiate receptors with beta-CNA. Similar results were observed with hemorphin. The endogenous peptide Tyr-MIF-1 and the blood-derived peptide hemorphin, therefore, can act as agonists as well as antagonists in the guinea pig ileum. The effects as antagonists are best observed in preparations of ileum with reduced receptor reserve (tolerant or beta-CNA treated) and are consistent with the idea that properties of endogenous peptides as opiate antagonists are enhanced in the tolerant state.

Isolation of a novel tetrapeptide with opiate and antiopiate activity from human brain cortex: Tyr-Pro-Trp-Gly-NH2 (Tyr-W-MIF-1)

A novel tetrapeptide, Tyr-Pro-Trp-Gly-NH2 (Tyr-W-MIF-1), was purified from extracts of frontal cortex of human brain tissue by several consecutive reversed-phase high performance liquid chromatographic steps followed by a radioimmunoassay originally developed for Tyr-Pro-Leu-Gly-NH2 (Tyr-MIF-1). Sequencing, mass spectrometric analysis, and comparison of its chromatographic behavior with that of the synthetic peptide confirmed the structure. Like Tyr-MIF-1, which was previously isolated from human brain tissue, Tyr-W-MIF-1 can inhibit the binding of 3H-DAMGO (selective for mu opiate receptors) to rat brain and can act as an opiate agonist as well as antagonist. Tyr-W-MIF-1 was a more potent opiate agonist than Tyr-MIF-1, the free acid of Tyr-W-MIF-1, and the structurally related hemoglobin-derived opiate peptide hemorphin-4 (Tyr-Pro-Trp-Thr) in the guinea pig ileum. Each of these peptides acted as opiate antagonists on the ileum from morphine-tolerant guinea pigs; the free acid of Tyr-W-MIF-1 was the most potent antagonist in inhibiting the activity of DAMGO. The results demonstrate the presence in human brain of a new member of the Tyr-MIF-1 family of biologically active peptides.

Oral administration of Tyr-MIF-1 stimulates gastric emptying and gastrointestinal motility in rodents

The effects of orally administered Tyr-MIF-1, an agonist of an endogenous antiopiate system, were examined on gastric emptying in mice and gastrointestinal myoelectric activity in rats. Tyr-MIF-1 (5 mg/kg in mice, 20 mg/kg in rats) accelerated gastric emptying of a methylcellulose test meal, increased the frequency of antral spike bursts, and disrupted intestinal migrating myoelectric complexes. These effects were reproduced by a subcutaneous administration of Tyr-MIF-1 at the same dosage. They were blocked by naloxone (1 mg/kg) but not by the kappa receptor subtype antagonist MR 2266 (1 mg/kg). The GABAA antagonist bicuculline (0.5 mg/kg), but not the GABAB antagonist 2-hydroxysaclofen (4 mg/kg), also antagonized the effects of Tyr-MIF-1. These data demonstrate that oral Tyr-MIF-1 stimulates gastric emptying and gastrointestinal motility through a systemic or central action that involves opioid and GABA systems.

Chronic treatment with MIF-1 prevents the painful stimuli threshold elevation induced by neonatal handling in mice

Chronic postnatal stressful handling results in a hyposensitivity to thermal nociceptive stimuli. This phenomenon is strongly affected by manipulations of the opioid system. In the present experiment, we report that chronic treatment with MIF-1 during the neonatal period prevents the behavioral alterations induced by handling while it is completely ineffective if injected acutely before antinociceptive testing by the tail flick test at 45 days of life.

Dopamine receptor antagonists block the effect of Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) on the opiate form of footshock-induced analgesia

Previous studies have shown that some of CNS actions of an endogenous peptide Tyr-MIF-1, are mediated by dopamine (DA) receptors. To study the effect of DA receptor blockade on the antiopiate properties of Tyr-MIF-1, the opiate form of footshock-induced analgesia was elicited in the rat. The nociceptive responses were determined using the hot-plate test (52.5 degrees C). Intraperitoneal pre-treatment with haloperidol (500 micrograms/kg), SCH 23390 (150 micrograms/kg), or spiroperidol (150 micrograms/kg) potentiated the antinociceptive effect of the footshock and blocked the antagonistic action of Tyr-MIF-1 (200 micrograms/kg and 2.0 mg/kg). A dose of haloperidol too small to potentiate the antinociceptive effect of the footshock (100 micrograms/kg) was still able to block the action of Tyr-MIF-1 (200 micrograms/kg). The results suggest that activation of DA receptors mediates the antagonizing effect of Tyr-MIF-1 on the opiate form of footshock-induced analgesia in the rat.

Effects of Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) on GH, LH, prolactin, FSH, and TSH secretion in rats with and without morphine

Plasma concentrations of GH, LH, prolactin, FSH, and TSH were evaluated in adult rats after administration of Tyr-MIF-1. Male rats were killed 0, 15, 30, and 60 min after Tyr-MIF-1 (0.02, 0.2 and 2.0 mg/kg) and ovariectomized females 15 min after injection of the peptide (0.2 and 2.0 mg/kg). The effect of Tyr-MIF-1 on pituitary hormonal secretion in morphine-treated ovariectomized rats also was studied. After 15 min, Tyr-MIF-1 (0.2 mg/kg) increased plasma concentrations of LH in males (p less than 0.05) and, at 2.0 mg/kg, in ovariectomized rats (p less than 0.05). Tyr-MIF-1 (0.2 mg/kg) decreased plasma concentrations of GH as compared with diluent at 15 min in males (p less than 0.05) but was ineffective in ovariectomized females not receiving morphine. Plasma concentrations of prolactin, FSH, and TSH remained unchanged both in males and in ovariectomized females by any of the administered doses of the peptide at any of the times tested. When administered to ovariectomized rats injected earlier with morphine sulfate, Tyr-MIF-1 (0.2 and 2.0 mg/kg) reduced (p less than 0.05) the effect of morphine (5 mg/kg) on GH secretion and tended (p = 0.061) to partially inhibit the effect of morphine (10 mg/kg) on prolactin secretion at a dose of 2.0 mg/kg. The decrease in plasma concentrations of TSH after morphine at a dose of 10 mg/kg (p less than 0.001) remained unaffected by any of administered doses of Tyr-MIF-1. The results suggest that Tyr-MIF-1 may affect the regulation of the secretion of some anterior pituitary hormones.

Upregulation of the opioid receptor complex by the chronic administration of morphine: a biochemical marker related to the development of tolerance and dependence

Studies conducted after the development of the rapid filtration assay for opiate receptors, and before the recognition of multiple opioid receptors, failed to detect changes in opioid receptors induced by chronic morphine. Recent experiments conducted in our laboratories were designed to examine the hypothesis that only one of several opioid receptor types might be altered by chronic morphine. Using binding surface analysis and irreversible ligands to increase the "resolving power" of the ligand binding assay, the results indicated that chronic morphine increased both the Bmax and Kd of the opioid receptor complex, labeled with either [3H][D-Ala2,D-Leu5]enkephalin, [3H][D-Ala2-MePhe4,Gly-ol5]enkephalin or [3H]6-desoxy-6 beta-fluoronaltreone. In the present study rats were pretreated with drugs known to attenuate the development of tolerance and dependence [the irreversible mu-receptor antagonist, beta-funaltrexamine (beta-FNA), and the inhibitor of tryptophan hydroxylase, para-chlorophenylalanine], prior to subcutaneous implantation of morphine pellets. The results demonstrated that 1) unlike chronic naltrexone, beta-FNA failed to upregulate opioid receptors and 2) both beta-funaltrexamine and PCPA pretreatment attenuated the chronic morphine-induced increase in the Bmax, but not the Kd, of the opioid receptor complex. These results provide evidence that naltrex-one-induced upregulation of the opioid receptor complex might occur indirectly as a consequence of interactions at beta-funaltrexamine-insensitive opioid receptors and that morphine-induced upregulation (increased Bmax) of the opioid receptor complex is a relevant in vitro marker related to the development of tolerance and dependence. These data collectively support the hypothesis that endogenous antiopiate peptides play an important role in the development of tolerance and dependence to morphine.

Evidence for presence of Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) in human brain cortex

Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) was previously isolated from bovine hypothalamus. We have now purified it from the parietal cortex of human brain tissue by gel filtration chromatography and four subsequent high performance liquid chromatographic steps. During isolation, the peptide content was followed by radioimmunoassay and compared with the elution of synthetic Tyr-MIF-1 in identical chromatographic systems. This extends evidence for the presence of Tyr-MIF-1 from bovine to human brain tissue and from hypothalamus to cortex.

Uptake of peptides containing Tyr-Pro by human and mouse erythrocytes

Red blood cells (RBCs) harvested from mice were used to investigate the possible existence of an uptake system for peptides in these cells. The radioactively iodinated tetrapeptide Tyr-MIF-1 (Tyr-Pro-Leu-Gly-amide) was incubated with RBCs for varying lengths of time with or without inhibitors. The RBCs showed saturable uptake that could be inhibited by Tyr-Pro containing peptides. Uptake was also found in human RBCs, but was more robust in the mouse. Uptake by mouse RBCs was temperature dependent and magnesium sensitive but did not require sodium, potassium, or glucose. With the exception of some enkephalin- and dynorphin-related peptides that partially inhibited uptake, most substances tested were without effect. The results of HPLC showed internalization of the N-Tyr-Pro containing peptides, with accumulation of degradation products over time. The degradation products, however, did not inhibit transport, suggesting that peptides were transported intact into the RBCs with degradation occurring after internalization. This suggestion was strengthened by the finding that only the cytosol of the RBC, not its membranes, rapidly degraded Tyr-MIF-1 to free iodine and iodotyrosine. Nevertheless, the cytosol contained a large amount of immunoreactive material that eluted at the position of intact Tyr-MIF-1 on HPLC. These findings show that RBCs can take up, store, and degrade Tyr-Pro containing peptides.

MIF-1 can accelerate neuromotor, EEG and behavioral development in mice

Newborn mice were injected SC daily with 1 mg/kg of MIF-1 or saline during the first 19 days of life. The progress of each pup was monitored for physical (body weight, eye and ear opening), neurobehavioral (reflexes) and neurophysiological (EEG) development until the weaning stage. In early adulthood (40 days of age) mice were tested on a maze learning task. Results indicate that MIF-1 can accelerate neurologic (days 3-9), somatic (days 10-14) and electroencephalographic (days 16-19) parameters, and that the effects of treatment last into the early adult stage with increased learning abilities in an appetitive task.

Hemorphins, cytochrophins, and human beta-casomorphins bind to antiopiate (TYR-MIE-1) as well as opiate binding sites in rat brain

Novel peptides with opiate activity, derived from endogenous sources (human and bovine casomorphins from milk, hemorphins from hemoglobin, and cytochrophins from mitochondrial cytochrome b), were tested for their ability to inhibit binding of the brain peptide Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) to its high affinity sites in rat brain. The order of potency in inhibiting binding of 125I-Tyr-MIF-1 was: hemorphin and bovine casomorphins greater than Tyr-MIF-1 greater than cytochrophins greater than human casomorphins. Naloxone and DAMGO were ineffective at inhibiting Tyr-MIF-1 binding. The results provide evidence that, in addition to their ability to bind to mu opiate receptors, these novel endogenous peptides with opiate activity and a peptide (Tyr-MIF-1) with antiopiate properties also bind to a non-opiate site labeled by Tyr-MIF-1. These sites could be involved in a balance between opiate and antiopiate peptides.

In vitro studies of Tyr-MIF-1 with human lymphocytes

Our previous report showed that the brain peptide Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) blocks the inhibitory effect of morphine sulfate on E-rosette formation by human peripheral blood lymphocytes (PBL). In this study, additional in vitro effects of Tyr-MIF-1 on human PBL were studied. The percentages of positive cells for CD 2, a sheep erythrocyte receptor, CD 4 and CD 8 were unchanged after incubation of PBL with morphine or morphine plus Tyr-MIF-1. Tyr-MIF-1 was not mitogenic by itself. The addition of Tyr-MIF-1 did not increase the proliferative response of PBL to Con A, although morphine did. Tyr-MIF-1 did not activate PBL to produce IL 2 nor did it affect the production of IL 2 by Con A-stimulated PBL. The results suggest that Tyr-MIF-1 does not directly modulate CD 2, CD 4 and CD 8 expression, does not alter the proliferative response of PBL, and does not affect the production of IL 2.

Passage of Tyr-MIF-1 from blood to brain

Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) has been shown to be transported from the brain to blood by a saturable system shared with Met-enkephalin and a few other substances. It is not known whether a similar system exists in the opposite direction. Accordingly, the entry rate of 125I-Tyr-MIF-1 from blood to brain was measured by a method involving perfusion of the test substances into the common carotid artery. The rate of entry was obtained from the slope of the line determined by brain to blood ratios at multiple points of time. Penetration of 125I-Tyr-MIF-1 across the blood-brain barrier was found to be 4.444 x 10(-3) ml/g/min, an entry rate significantly higher than that of the vascular marker 125I-albumin. Competition with Tyr-MIF-1 or nonradioactively labeled 127I-Tyr-MIF-1 showed no difference in rate of entry, indicating that the penetration of 125I-tyr-MIF-1 was not saturable. Met-enkephalin and Leu-enkephalin also failed to affect entry of 125I-Tyr-MIF-1. The results indicate that Tyr-MIF-1 can enter the brain from the blood to a greater extent than does albumin, but that this penetration does not involve a saturable system.

Differential effects of Tyr-MIF-1 and naloxone in two animal models involving benzodiazepine

It has been shown previously that the endogenous brain peptide Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) can act as an antiopiate and can also increase binding and function at the GABAA/benzodiazepine receptor complex. We now describe the effects of this tetrapeptide in two models in which the antiopiate naloxone has been reported to decrease the activity of benzodiazepines. Unlike naloxone, Tyr-MIF-1 and MIF-1 neither prevented chlordiazepoxide-induced locomotor hyperactivity in mice on a tilting floor nor suppressed chlordiazepoxide-induced eating in rats. Thus, in these two systems, Try-MIF-1 did not act as an antiopiate or alter the effects of a benzodiazepine, indicating a selectivity in the actions of Tyr-MIF-1.

Aluminum-induced neurotoxicity: alterations in membrane function at the blood-brain barrier

Aluminum is established as a neurotoxin, although the basis for its toxicity is unknown. It recently has been shown to alter the function of the blood-brain barrier (BBB), which regulates exchanges between the central nervous system (CNS) and peripheral circulation. The BBB owes its unique properties to the integrity of the cell membranes that comprise it. Aluminum affects some of the membrane-like functions of the BBB. It increases the rate of transmembrane diffusion and selectively changes saturable transport systems without disrupting the integrity of the membranes or altering CNS hemodynamics. Such alterations in the access to the brain of nutrients, hormones, toxins, and drugs could be the basis of CNS dysfunction. Aluminum is capable of altering membrane function at the BBB; many of its effects on the CNS as well as peripheral tissues can be explained by its actions as a membrane toxin.

Chronic, but not acute, administration of morphine alters antiopiate (Tyr-MIF-1) binding sites in rat brain

Opiate addiction could involve a change in the binding of endogenous antiopiates. A candidate for such a role is Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2), a brain peptide that can antagonize exogenous and endogenous opiates and bind to opiate receptors. Its primary action, however, may be through its own binding site in brain, which we now report is altered by chronic administration of morphine. Rats given morphine pellets had reduced binding of both iodinated and tritiated Tyr-MIF-1 on day 5, when substantial tolerance is evident. In contrast, mu and delta opiate receptors were increased. Acute injection of an analgesic dose of morphine did not reduce Tyr-MIF-1 binding, indicating that chronic administration is required for the change. These findings open new approaches to the study of addiction by focusing on antiopiate activity.

Twenty-one hormones fail to inhibit the brain to blood transport system for Tyr-MIF-1 and the enkephalins in mice

Tyr-MIF-1 (Tyr-Pro-Leu-Gly-amide) and methionine enkephalin are transported intact across the blood-brain barrier by a saturable, stereospecific system. This system has been found to be modulated by a few non-peptide substances and by certain conditions such as ageing and some stresses. We investigated the possibility that hormones structurally unrelated to Tyr-MIF-1 and the enkephalins might also be capable of modulating this transport. Twenty-one hormones were tested including steroids, proteins, glycoproteins, peptides, and thyroid hormones, in doses ranging from 0.01 pmol to 1 nmol/mouse by injecting each hormone directly into the lateral ventricle simultaneously with [125I]Tyr-MIF-1. No clear effect on transport could be established for any of the substances at the doses tested. None of these substances seemed able to act as competitive inhibitors, to share their respective transport systems with Tyr-MIF-1, or to modulate immediately the saturable transport system.

Analgesia and the blood-brain barrier transport system for Tyr-MIF-1/enkephalins: evidence for a dissociation

The blood-brain barrier is capable of transporting peptides with anti-opiate (Tyr-MIF-1) and opiate (enkephalins) activity out of the central nervous system. The relationship of this transport system to the various actions of opiates remains unexplored. This study examined the relationship between the rate of transport and opiate-induced analgesia. Both restraint, a stress that provokes an opiate-mediated analgesia, and the administration of morphine (12 mg/kg, i.p.) each induced an inhibition in the rate of transport. Such inhibition exhibited specificity, since the saturable, brain to blood transport of iodide remained unaltered. However, it was possible to dissociate analgesia and inhibition of transport. The onset and peak of analgesia, as measured by tail-flick latency induced by morphine, preceded the onset and peak of the inhibition of transport. Naltrexone, which blocks opiate-mediated analgesia, also induced inhibition of transport without any significant effect on tail-flick latency. (-) Naloxone but not (+) naloxone also weakly inhibited transport. Deprivation of food and water, associated with analgesia possibly mediated by the opiate, beta-endorphin, which is not transported out of the brain by this system, did not alter transport. These results suggest that while inhibition of transport and analgesia may occur together, these events probably represent two separate aspects of the action of opiates, that may even be mediated by separate receptor sites or peptides in the opiate family.

Pharmacological activities of the MIF-1 analogues Pro-Leu-Gly, Tyr-Pro-Leu-Gly and pareptide

The pharmacological activities of the related free acid analogues of MIF-1, Pro-Leu-Gly (PLG) and Tyr-Pro-Leu-Gly (YPLG), were investigated because of the possibility that they may be formed during the digestion of milk and wheat proteins in vivo. The amino acid sequences -Tyr-Pro-Leu-Gly- and -Pro-Leu-Gly- are present in proteins from these foods. Chronic administration of either PLG (0.25 mg/kg, SC, BID) or the control substance, pareptide (0.25 mg/kg, SC, BID), antagonized the development of tolerance to the cataleptic effects of haloperidol in mice. The effect of YPLG (0.25 mg/kg, SC, BID) on the development of this tolerance was borderline and not statistically significant. Nanomolar concentrations of PLG, YPLG, and pareptide each increased the in vitro binding of 3H-apomorphine to rat striatal receptors. In this in vitro system, bell-shaped dose response curves were observed for each peptide. The effects of these peptides on tolerance development and apomorphine binding are similar to those previously reported for MIF-1 and demonstrate that amidation at the carboxyl terminus is not required for biological activity.

Brain peptide reverses effect of morphine on human lymphocytes

E-rosette formation by human lymphocytes incubated with sheep red blood cells (sRBC) is inhibited by morphine. We studied the ability of the opiate antagonists naloxone and Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) to block this action. Active E-rosette formation by lymphocytes incubated with morphine was reduced from the control of 35.7 +/- 1.7% to 23.7 +/- 1.5% (p less than 0.001). Similarly, total E-rosette formation was reduced by morphine from the control of 65.8 +/- 1.3% to 53.2 +/- 2.9% (p less than 0.001). These effects were blocked by co-incubation of the lymphocytes with either Tyr-MIF-1 or naloxone (p less than 0.05). Tyr-MIF-1 was active (p less than 0.05) at concentrations as dilute as 10(-13) M. These results indicate that the neuropeptide Tyr-MIF-1 exerts an antiopiate effect at the human T-lymphocyte.

Saturable transport of peptides across the blood-brain barrier

Peptides can be transported across the blood-brain barrier by saturable transport systems. One system, characterized with radioactively labeled Tyr-MIF-1 (Tyr-Pro-Leu-Gly-amide), is specific for some of the small peptides with an N-terminal tyrosine, including Tyr-MIF-1, the enkephalins, beta-casomorphin, and dynorphin (1-8). Another separate system transports vasopressin-like peptides. The choroid plexus has at least one system distinguishable from those above that is capable of uptake and possibly transport of opiate-like peptides. The possibility of saturable transport of other peptides has been investigated to a varying degree. Specificity, stereo-specificity, saturability, allosteric regulation, modulation by physiologic and pharmacologic manipulations, and noncompetitive inhibition have been demonstrated to occur in peptide transport systems and suggest a role for them in physiology and disease.

Tyr-MIF-1 and Met-enkephalin share a saturable blood-brain barrier transport system

Previous studies have shown that methionine enkephalin and Tyr-MIF-1 are transported from the brain to the blood by a saturable, stereospecific, carrier-mediated process. It was not established by these studies whether Tyr-MIF-1 and methionine enkephalin were transported by the same system or by separate, but overlapping systems. This issue was investigated in anesthetized mice receiving injections containing both 131I-methionine enkephalin and 125I-Tyr-MIF-1 into the lateral ventricle of the brain. Mice were decapitated and the brain to blood transport rate was derived from the residual counts in the brain. It was found that in individual mice, the transport rate for Tyr-MIF-1 correlated highly with the transport rate for methionine enkephalin but not with the transport of iodide. This shows that the transport of Tyr-MIF-1 is closely coupled to the transport of methionine enkephalin but dissociable from the brain to blood transport of iodide. Furthermore, the inability of varying doses of Tyr-MIF-1 or of methionine enkephalin to preferentially self-inhibit is radiolabeled form in comparison with the other peptide shows that, functionally, only a single system exists. Aluminum, a noncompetitive inhibitor of Tyr-MIF-1 transport, was also without preferential inhibition. Thus, under the conditions of these studies, only a single system could be functionally demonstrated for the transport of both Tyr-MIF-1 and methionine enkephalin.

Long-term hyperalgesia induced by neonatal beta-endorphin and morphiceptin is blocked by neonatal Tyr-MIF-1

Male rats were injected s.c. once daily during the first week of life with beta-endorphin (BE), morphiceptin, the antiopiate Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2), or one of the two opiate peptides in combination Tyr-MIF-1. Pups treated with neonatal BE removed their tails from a series of increasingly hot water baths significantly faster than controls on day 9, confirming our earlier studies. In addition, we found that Tyr-MIF-1 blocked this effect of BE. At 4.5 months, latency to lick a hindpaw in the hot-plate test was significantly faster in groups given BE alone, morphiceptin alone, or the control vehicle than in any of the 3 groups given Tyr-MIF-1. At 6 months the two groups given opiate peptides alone showed faster tail-flick latencies than the controls and the groups given Tyr-MIF-1. These results indicated that the long-term nociceptive changes induced by the opiate peptides were opposite to those induced by Tyr-MIF-1. Mean tail-flick latencies of the groups on day 9 correlated well with hot-plate and tail-flick scores in adulthood, indicating that the effects of the peptides were persistent. The neonatal peptide treatments did not differentially affect the analgesia induced by the stress of footshock or warm-water swim. Rats given either of the opiate peptides alone tended to fall off a rotorod faster than those in the other groups. These results support the role of Tyr-MIF-1 as an antiopiate and further illustrate the long-term effects of neonatally administered peptides.

Tyr-MIF-1 attenuates antinociceptive responses induced by three models of stress-analgesia

Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2), a biologically active brain peptide, has previously been shown to antagonize the analgesia induced by morphine. In this report experiments are described in which mice were tested on the hot-plate in three models of antinociception - shock, novel environment, and warm-water swim - after the administration of various doses of Tyr-MIF-1 without any exogenous opiates. The peptide reduced the antinociception produced by all three methods of inducing endogenous antinociception. These results add further support for the existence of peptides like Tyr-MIF-1 that act as opiate antagonists.

MIF-1 and Tyr-MIF-1 antagonize morphine and opioid but not non-opioid stress-induced analgesia in the snail, Cepaea nemoralis

The effects of prolyl-leucyl-glycinamide (MIF-1, PLG), tyrosine-prolyl-leucyl-glycinamide (Tyr-MIF-1, YPLG) and naloxone on morphine and warm and cold stress-induced increases in the latency of the thermal (40 degrees C hot plate) avoidance behaviors of the terrestrial snail, Cepaea nemoralis, were examined. All three substances blocked the morphine- and warm stress-induced opioid analgesia, while having no effects on non-opioid cold stress-induced analgesia. Tyr-MIF-1 had a significantly greater inhibitory effect than MIF-1. These results indicate that MIF-1 and Tyr-MIF-1 antagonize the antinociceptive effects of exogenous opiates and opioid-mediated analgesia in snails in a manner analogous to that described for mammals. This raises the possibility of an evolutionary conservation of functional opioid antagonists.

Existence of antiopiate systems as illustrated by MIF-1/Tyr-MIF-1

Evidence is presented that the small peptides MIF-1/Tyr-MIF-1 are part of an endogenous antiopiate system that may function to balance the opiate system. We review the biological activity, behavioral activity, and functional effects of this proposed opiate antagonist system. In addition, we suggest, based on antinociceptive mechanisms, that the individual components of the antiopiate system might function differently from naloxone.

Interactions of Tyr-MIF-1 at opiate receptor sites

Binding of Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) to mu and delta opiate receptors was compared with other putative opiate antagonist peptides by displacement of iodinated ligands selective for mu (DAGO, FK33824, and morphiceptin) and delta (DPDPE) receptors. Tyr-MIF-1 and ACTH (1-24 and 1-39) inhibited binding of 125I-DAGO with IC50's of about 1 microM. FMRF-NH2 was about an order of magnitude weaker while CCK-8 and MIF-1 failed to inhibit 50% of binding at concentrations up to 100 microM. Morphiceptin, Tyr-MIF-1, and ACTH were less potent but more efficacious than DAGO, FK33824, morphine, or naloxone in inhibiting the binding of 125I-morphiceptin. Tyr-MIF-1 appeared to have a more selective action at opiate receptors than ACTH; in contrast to their effects at 125I-DAGO-labeled sites, morphiceptin and Tyr-MIF-1 inhibited less than 50% of 125I-DPDPE binding at concentrations up to 10 and 50 microM, while ACTH 1-39 and 1-24 inhibited more than 80% of the binding at 2.5 and 5 microM, respectively. The results indicate that at relatively high concentrations Tyr-MIF-1, like ACTH, can affect binding to the opiate receptor, but unlike ACTH, binding of Tyr-MIF-1 appears relatively selective for the mu site.

Binding of Tyr-MIF-1 to isolated brain capillaries

Tyr-MIF-1 (Tyr-Pro-Leu-Gly-amide) and the enkephalins have previously been shown with in vivo studies to be transported out of the brain by a saturable, carrier-mediated system. The possibility that this transport system occurs at the level of the endothelial cells was tested by measuring the ability of isolated brain capillaries to bind radioiodinated Tyr-MIF-1. The binding of labeled Tyr-MIF-1 was displaced by unlabeled Tyr-MIF-1 (50% inhibition at 1.5 X 10(-6) M) and by nonradioactive (I127) iodinated Tyr-MIF-1, but not by tyrosine, MIF-1 (Pro-Leu-Gly-amide), Tyr-Pro, or D-Tyr-MIF-1. This pattern of inhibition is similar to that found for the in vivo transport of Tyr-MIF-1 out of the brain. Tyr-MIF-1 did not appear to be bound by albumin or degraded by cells aged 3-7 days at 4 degrees C. These results suggest that the transport system previously described in vivo for Tyr-MIF-1 and the enkephalins may be associated with the brain endothelial cells.