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

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.

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.

Effects of neonatal treatment with Tyr-MIF-1 and naloxone on the long-term body weight gain induced by repeated postnatal stressful stimuli

Stressful stimuli repeatedly applied during the first postnatal weeks can induce body weight gain in the mouse during adulthood. This effect can be prevented by injecting naloxone concomitantly with stress. The peptides belonging to the Tyr-MIF-1 family have a great modulating activity on numerous stress-induced phenomena. The aim of the present work was to compare the effect of repeated neonatal injections of Tyr-MIF-1 or naloxone on the long-term body weight gain induced by a stressing procedure applied daily during the first three weeks of life. The results indicate that although naloxone blocked the development of the stress-induced effects, Tyr-MIF-1 potentiated them.

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 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.

Effects of neonatal treatment with Tyr-MIF-1, morphiceptin, and morphine on development, tail flick, and blood-brain barrier transport

Morphine and endogenous peptides can alter developmental processes, inducing changes that can endure into adulthood. Morphiceptin binds to mu opiate receptors and to non-opiate sites labeled by Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2), an endogenous brain peptide known to modulate opiate effects. Morphine, morphiceptin, Tyr-MIF-1, morphine + Tyr-MIF-1, and morphiceptin+Tyr-MIF-1 (50 micrograms, s.c.) were given to rats during their first week of life. Animals given morphine alone or in combination with Tyr-MIF-1 had significantly lower body weights for the first 3 weeks of life and delayed eye opening on day 16. Rats given morphine had hypersensitive tail flick responses on day 9 while those given morphine + Tyr-MIF-1 were hypersensitive on days 3, 8, and 9. Locomotor, passive avoidance, and rotorod behaviors were not altered by the neonatal treatments. Transport of [125I]Tyr-MIF-1 out of the brain was tested on day 23 and found to be increased by neonatal morphine, an effect that was significantly potentiated by neonatal Tyr-MIF-1. The results indicate that neonatal administration of peptides and opiates can affect later peptide transport across the blood-brain barrier as well as selected developmental characteristics.

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.

Cimetidine does not change the effect of Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) on the opiate form of footshock-induced analgesia

It has been demonstrated that cimetidine blocks the effect of naloxone on footshock-induced analgesia. To study the effect of cimetidine on the antiopiate properties of an endogenous peptide Tyr-MIF-1, the opiate form of intermittent footshock-induced analgesia was elicited in the rat. The nociceptive responses were determined using the hot-plate test (52.5 degrees C). Intraperitoneal pretreatment with cimetidine (100 mg/kg) or chlorpheniramine maleate (20 mg/kg) did not affect the footshock-induced analgesia, and did not change the antagonizing effect of Tyr-MIF-1 (0.2 mg/kg) on this model of antinociception. It is concluded that cimetidine and chlorpheniramine maleate do not change the antagonizing effect of Tyr-MIF-1 on the opiate form of intermittent footshock-induced analgesia.

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.

MIF-1 and Tyr-MIF-1 do not alter GABA binding on the GABAA receptor

The endogenous brain peptides MIF-1 and Tyr-MIF-1 have been reported to alter binding at several sites on the GABAA receptor. To determine whether these peptides affect binding at the GABA site, we assessed effects of MIF-1 and Tyr-MIF-1 on [3H]SR 95531 binding in mouse cortical synaptosomal membranes. Neither peptide altered [3H]SR 95531 binding across a wide range of concentrations (10(-12)-10(-6) M). Scatchard analysis indicated no change in either apparent affinity or receptor density at both the high-affinity and low-affinity GABA sites with MIF-1 or Tyr-MIF-1, 10(-9) M. Thus, MIF-1 and Tyr-MIF-1 do not affect binding at the GABA site on the GABAA receptor.

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.

A decade of changing perceptions about neuropeptides

The last decade has seen rapid growth in research with neuropeptides. During this time, we have been actively developing several concepts including the highly controversial one that peptides can cross the blood-brain barrier in intact form. One of the endogenous brain peptides used as a prototype for that concept, Tyr-MIF-1, also was used for the concept of the existence of endogenous antiopiate neuropeptides. As has been true for most novel developments in science, these concepts, as well as some older ones, were met with a great deal of skepticism when first suggested. Eventually, however, amnesia concerning the difficulties initially encountered with the introduction of new concepts occurs, with their subsequent "rediscovery" made easier.

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.

MIF-1 and Tyr-MIF-1 augment muscimol-stimulated chloride uptake in cerebral cortex

The peptides MIF-1 (Pro-Leu-Gly-NH2) and Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) recently have been found to augment the effects of gamma-aminobutyric acid (GABA) on benzodiazepine receptor binding and chloride channel binding (Tyr-MIF-1) at the GABAA receptor complex. To determine whether these peptides affect the function of this complex in chloride transport, we evaluated chloride uptake stimulated by the GABA analog muscimol in synaptoneurosome preparations. In mice treated with either MIF-1 or Tyr-MIF-1 (1 mg/kg IP), maximal chloride uptake in cortex was increased compared with controls. The two peptides had similar effects in cortical preparations, but in cerebellum neither peptide altered chloride uptake. No differences from controls were observed in cortical synaptoneurosomes treated in vitro with either MIF-1 or Tyr-MIF-1. These results suggest that the brain peptides MIF-1 and Tyr-MIF-1 alter function at the GABAA receptor complex, perhaps by binding at a specific peptide receptor.

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.

Differential inhibitory effects of MIF-1, Tyr-MIF-1, naloxone and beta-funaltrexamine on body rotation-induced analgesia in the meadow vole, Microtus pennsylvanicus

The effects of body rotation in a horizontal plane and various opiate antagonists on the nociceptive responses of a day-active microtine rodent, the meadow vole, Microtus pennsylvanicus, were examined. Intermittent rotation (70 rpm, schedule of 30 sec on, 30 sec off) for 30 min induced significant analgesic responses in the voles for 15 min after rotation. These increases in thermal response latency were blocked by intraperitoneal pretreatment with either naloxone or the irreversible mu opiate receptor antagonist beta-funaltrexamine (beta-FNA; 10 mg/kg; 24 hr pretreatment). This antagonistic effect of beta-FNA indicates mu opioid involvement in the mediation of rotation-induced analgesia. The antiopiate peptides MIF-1 (Pro-Leu-Gly-NH2) and Tyr-MIF-1 also significantly reduced, though did not completely block, body rotation-induced opiate analgesia. This suggests that Tyr-MIF-1 and MIF-1 have significant antagonistic effects on mu opioid systems that are involved in the mediation of stress (rotation)-induced analgesia.

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.

Naloxone, but not Tyr-MIF-1, reduces volitional ethanol drinking in rats: correlation with degree of spontaneous preference

The possible relationship between the actions of ethanol and opiates led us to examine the effect of opiate antagonists on ethanol intake in rats with a free choice of water. Naloxone (NAL) significantly reduced intake of ethanol. This effect was much greater in "high-preferring" (ethanol/total fluid intake greater than 60%) than in "low-preferring" (ethanol/total fluid intake less than 30%) rats. Furthermore, a correlation was found between the degree of spontaneous preference (ethanol/total fluid intake ratio) and the reduction of ethanol drinking by NAL. Sensitivity to NAL increased with increased preference for ethanol. Neither Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) nor MIF-1 (Pro-Leu-Gly-NH2) caused a significant modification of ethanol intake. This study shows that NAL can reduce volitional ethanol intake in rats and provides further evidence that Tyr-MIF-1 does not always act like NAL.

Social conflict-induced changes in nociception and beta-endorphin-like immunoreactivity in pituitary and discrete brain areas of C57BL/6 and DBA/2 mice

The present study characterizes the time course of social conflict analgesia and its reversibility by opioid antagonist drugs in the C57BL/6 and DBA/2 inbred strains of mice and examines the relationship between alterations in brain and pituitary levels of beta-endorphin-like immunoreactivity (beta-ELIR) and the antinociception elicited by social stress. Data revealed statistically significant strain differences in regard to beta-ELIR in control animals. The pituitary content of beta-ELIR was higher in DBA/2, while the values in the periaqueductal grey (PAG) and in the amygdala were higher in C57BL/6 mice. No interstrain differences were found in the hypothalamus. Exposure to 50 attack bites resulted in a 6-fold higher analgesia in DBA/2 mice and in a strain-independent fall of beta-ELIR in pituitary (approximately 27%) and PAG (23%). PAG but not pituitary beta-ELIR levels in C57BL/6 mice correlated positively with the increase in tail-flick latency after attack. Mere confrontation with a non-aggressive opponent failed to induce analgesia and was associated in C57BL/6 mice with a significant reduction in the beta-ELIR content of both the pituitary and the PAG. The data are discussed in terms of genotype-dependent sensitivity of the beta-endorphin system to stress and its relation to analgesia.

Differential activity of the endogenous antiopiate Tyr-MIF-1 after various intensities of stress

Three forms of stress-induced analgesia (electric shock, forced water-swim and novelty) were used to examine the nature of the endogenous antiopiate system. It was hypothesized that a role of the antiopiate system may be to regulate the extent of antinociception within varying environments. The antiopiate properties of Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2), which were manifest by reduction of opiate analgesia in mice on a hot-plate, were best expressed within a defined range of intensities. In each of the 3 analgesic situations, pre-administration of Tyr-MIF-1 (0.1 mg/kg) resulted in an antinociceptive effect after low to moderate stress but not after more intense stress. These observations indicate that the antiopiate system can function differentially under various environmental conditions, thus ensuring that the organism's responses to its perception of the immediate environment are appropriate and specific.

Tyr-MIF-1 augments benzodiazepine receptor binding in vivo

Behavioral and limited neurochemical evidence indicates possible links between the endogenous opiate and gamma-aminobutyric acid (GABA)-benzodiazepine receptor systems. A previous study using in vitro techniques indicated that MIF-1 (Pro-Leu-Gly-NH2) and Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2), peptides with anti-opiate activity, enhanced GABA-stimulated benzodiazepine binding. To assess the activity of these peptides under in vivo conditions, we evaluated their effects on benzodiazepine receptor binding as determined by specific uptake of the benzodiazepine antagonist [3H]-Ro15-1788. Tyr-MIF-1, at a dose of 1 mg/kg IP, significantly augmented benzodiazepine binding in cortex and hippocampus but not in cerebellum, hypothalamus, or pons-medulla. Increases in binding were due in large part to increased apparent affinity at the receptor. At none of the doses of MIF-1 (0.1, 1 and 10 mg/kg) or at the highest (10 mg/kg) and lowest (0.1 mg/kg) doses of Tyr-MIF-1 was there any significant alteration in benzodiazepine binding in any region evaluated. These results indicate that peptide-benzodiazepine receptor interactions may also occur in vivo.

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.

MIF-1 and Tyr-MIF-1 augment GABA-stimulated benzodiazepine receptor binding

Behavioral evidence in laboratory animals and human beings indicates possible links between the endogenous opiate and gamma-aminobutyric acid (GABA)-benzodiazepine receptor systems, especially with regard to antagonistic properties. To assess possible interactions between endogenous opiate antagonists and benzodiazepine receptor binding, we evaluated the effects of the peptides MIF-1 and Tyr-MIF-1 on benzodiazepine receptor binding in mouse brain membranes. Neither peptide affected receptor binding in cortex over a broad dose range, but both peptides significantly augmented GABA-stimulated benzodiazepine receptor binding at GABA concentrations of 10(-8) and 10(-7) M. Rosenthal-Scatchard analysis indicated that the increase in binding was largely due to increased apparent affinity. Both peptides augmented GABA-enhanced binding at low doses (MIF-1 10(-11) M, Tyr-MIF-1 10(-13) M) with decreased effects at higher doses. In cerebellum and brainstem, MIF-1 tended to enhance GABA-stimulated binding but Tyr-MIF-1 was inactive. These results indicate benzodiazepine-opiate and benzodiazepine-peptide interactions.

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.