Inhibitory influences of MIF-1 (PLG) and Tyr-MIF-1 (YPLG) on aggression and defeat-induced analgesia in mice

Microtubule stabilising peptides: new paradigm towards management of neuronal disorders

Neuronal cells made of soma, axon, and dendrites are highly compartmentalized and possess a specialized transport system that can convey long-distance electrical signals for the cross-talk. The transport system is made up of microtubule (MT) polymers and MT-binding proteins. MTs play vital and diverse roles in various cellular processes. Therefore, defects and dysregulation of MTs and their binding proteins lead to many neurological disorders as exemplified by Parkinson's disease, Alzheimer's disease, amyotrophic lateral sclerosis, Huntington's disease, and many others. MT-stabilising agents (MSAs) altering the MT-associated protein connections have shown great potential for several neurodegenerative disorders. Peptides are an important class of molecules with high specificity, biocompatibility and are devoid of side effects. In the past, peptides have been explored in various neuronal disorders as therapeutics. Davunetide, a MT-stabilising octapeptide, has entered into phase II clinical trials for schizophrenia. Numerous examples of peptides emerging as MSAs reflect the emergence of a new paradigm for peptides which can be explored further as drug candidates for neuronal disorders. Although small molecule-based MSAs have been reviewed in the past, there is no systematic review in recent years focusing on peptides as MSAs apart from davunetide in 2013. Therefore, a systematic updated review on MT stabilising peptides may shed light on many hidden aspects and enable researchers to develop new therapies for diseases related to the CNS. In this review we have summarised the recent examples of peptides as MSAs.

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.

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 peptides on animal and human behavior: a review of studies published in the first twenty years of the journal Peptides

This review catalogs effects of peptides on various aspects of animal and human behavior as published in the journal Peptides in its first twenty years. Topics covered include: activity levels, addiction behavior, ingestive behaviors, learning and memory-based behaviors, nociceptive behaviors, social and sexual behavior, and stereotyped and other behaviors. There are separate tables for these behaviors and a short introduction for each section.

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.

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.

Differential metabolism of Tyr-MIF-1 and MIF-1 in rat and human plasma

The metabolism of the endogenous brain peptides Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) and MIF-1 (Pro-Leu-Gly-NH2) was determined by HPLC after incubation of the tritiated peptides in human and rat plasma. Degradation of Tyr-MIF-1 was rapid in the plasma from both species, in contrast to the slightly delayed degradation of MIF-1 in rat plasma and the extremely prolonged persistence of MIF-1 in human plasma. In rat plasma, more than half of the intact Tyr-MIF-1 and MIF-1 was degraded within 5 min, in contrast to the 5 days required for 50% degradation of MIF-1 in human plasma at 37 degrees. To slow the rapid rate of metabolism, studies were then performed at 0 degree. Incubation of Tyr-MIF-1 in human plasma at 0 degree for 2 hr resulted in HPLC identification of more Tyr-Pro than Tyr at all times. At 0 degree in rat plasma, however, more Tyr than Tyr-Pro was formed after the first 5 min of incubation of the Tyr-MIF-1 that was labeled on the Tyr. This raised the possibility that the tetrapeptide Tyr-MIF-1 might be serving as a precursor of the tripeptide MIF-1. Incubation of Tyr-MIF-1 tritiated at the Pro under the same conditions with and without Tyr-MIF-1 tritiated at the Tyr showed that Tyr-Pro, not MIF-1, was the predominant degradation product of Tyr-MIF-1. In addition to the metabolism of Tyr-MIF-1 being slower at lower temperatures, it was also slowed by some enzyme inhibitors. After 10 min of incubation at 37 degrees, EDTA appeared to be more effective than bestatin, p-chloromercuribenzoic acid (PCMB), pepstatin, or aprotinin, but after 30 min, bestatin was more effective. Intravenous injection of the tritiated peptides into rats showed short half-time disappearances; again, MIF-1 persisted in blood longer than Tyr-MIF-1. Thus, the results show the rapid metabolism of Tyr-MIF-1 in human and rat plasma, the slightly slower metabolism of MIF-1 in rat plasma, the predominant formation of Tyr-Pro rather than MIF-1 from Tyr-MIF-1, and the markedly delayed metabolism of MIF-1 in human plasma.

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.

Sex differences in naloxone- and Tyr-MIF-1-induced hypoalgesia

Reproductive adult male and female deer mice that received daily (7 days) injections of either the prototypic exogenous opiate antagonist, naloxone (1.0 mg/kg), or the endogenous putative antiopioid tetrapeptide, Tyr-MIF-1 (Tyr-Pro-Leu-Gly amide; 1.0 and 10 mg/kg), followed by determinations of thermal nociceptive sensitivity (hot-plate response) developed hypoalgesia. There were significant sex differences in this opioid blockade-induced or associated analgesia, with male mice displaying significantly greater hypoalgesia than females. Mice that received daily injections of either naloxone or Tyr-MIF-1 for 7 days without any accompanying determinations of nociceptive sensitivity (days 2-6 of treatment) failed to show any hypoalgesia on day 7 when they received the antagonist followed by a measurement of nociception. These results indicate that there are sex differences in both naloxone- and Tyr-MIF-1-induced hypoalgesia, and suggest that this pattern may be associated with sexually dimorphic opioid modulation of aversive conditioning.

Sex differences in the effects of Tyr-MIF-1 on morphine- and stress-induced analgesia

There is evidence suggesting that the endogenous tetrapeptide, Tyr-MIF-1 (Tyr-Prol-Leu-Gly-amide), has antagonistic or modulatory effects on opioid-mediated analgesia. There is also substantial evidence for sex differences in opioid effects, whereby male rodents display greater levels of opioid-mediated analgesia than females. In the present study, determinations were made of the effects of Tyr-MIF-1 on morphine- and restraint stress-induced opioid analgesia in adult male and female deer mice, Peromyscus maniculatus. Intraperitoneal treatment with Tyr-MIF-1 (0.10-10 mg/kg) reduced morphine- and stress-induced analgesia in both male and female mice, with Tyr-MIF-1 having markedly greater antagonistic effects in male than female mice. These results indicate that there are sex differences in the modulatory (antiopiate) effects of Tyr-MIF-1 on opioid-mediated analgesia.

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.

Modifications of social conflict-induced analgesic and activity responses in male mice receiving chronic opioid agonist and antagonist treatments

This study examined the effects of chronic (7 day) administrations of opioid agonists, via osmotic minipumps (20 micrograms/microliters/h, or 2 mg/kg/h for each agent) on: 1) nociception and activity, and 2) the analgesic and locomotor responses of subordinate male mice experiencing social conflict (aggression without defeat) and defeat in a "resident-intruder" paradigm. Chronic infusion of the mu opioid antagonist, naltrexone, resulted in a hypoanalgesic response and a decrease in basal locomotor activity on days 3-7 postimplantation which returned to the basal levels of saline-implanted control mice after termination of the infusions on day 9. Naltrexone reduced defeat-induced analgesia on the second day after implantation, but had no consistent effects on analgesia on test days 6 and 9 or on the aggression-induced (nondefeat) analgesia and increases in activity. The delta opioid antagonist ICI-154, 129, while having no significant effects on basal nociception or locomotor activity, augmented nondefeat-induced analgesia (day 2) and reduced the defeat-induced increases in activity (days 2 and 6). The mu agonist, levorphanol, resulted in a significant analgesia on the first two days after infusion, followed by the development of tolerance to the analgesic effects over days 3-7. On day 9, a hypoanalgesic response indicative of withdrawal was evident. Levorphanol also induced a marked decrease in locomotor activity over days 3-7 postimplantation, with no evidence of the development of tolerance or withdrawal following termination of infusion.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.

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.

Analgesic effects of the putative FSH-suppressing gonadal steroid, 3 alpha-hydroxy-4-pregnen-20-one: possible modes of action

The effects of intracerebroventricular (i.c.v.) administrations of the putative follicle stimulating hormone (FSH) suppressing gonadal steroid, 3 alpha-hydroxy-4-pregnen-20-one (3A4P) on the nociceptive responses of male mice were examined. This allylic steroid elicited significant, dose-dependent (0.001-1.0 micrograms) analgesic responses for 90-150 min after injection. These analgesic effects of 3A4P were stereospecific, the stereoisomer, 3 beta-hydroxy-4-pregnen-20-one (3B4P) failing to affect the nociceptive responses. The analgesic effects of 3A4P were blocked by peripheral administrations of the GABA antagonists, bicuculline and picrotoxin, and reduced by the benzodiazepine antagonist, Ro 15-1788. The exogenous opiate antagonist, naloxone, and the putative endogenous opioid antagonist, Tyr-MIF-1 (Pro-Leu-Gly-amide), also reduced 3A4P-induced analgesia, while i.c.v. administration of 3A4P (0.001 and 0.01 micrograms) itself attenuated the analgesic effects arising from peripheral administrations of opiate receptor agonist, morphine. In addition, the calcium channel antagonists, nifedipine and verapamil, enhanced 3A4P-induced analgesia but had no evident effects on the actions of 3B4P. These results suggest that the central analgesic effects of the FSH-suppressing steroid, 3A4P, arise via benzodiazepine--GABA--opiate mechanisms and calcium channels. These findings also suggest possible central modes of action whereby 3A4P may elicit selective suppression of FSH.

Endogenous opiates: 1986

This is the ninth installment of our annual review of research involving the endogenous opiate peptides. It is restricted to the non-analgesic and behavioral studies of the opiate peptides published in 1986. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic processes; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; activity; sex, pregnancy, and development; and some other behaviors.

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.