Mu and delta opiate receptors in rat brain are affected by GTP but not by MIF-1

Mass spectrometric quantification of MIF-1 in mouse brain by multiple reaction monitoring

MIF-1 (Pro-Leu-Gly-NH(2)) has potent therapeutic effects in depression and Parkinson's disease, but its CNS sites of production are not yet clear. In this study, the concentration of MIF-1 in different brain regions was measured by the multiple reaction monitoring technique on a 4000 QTRAP mass spectrometer. The limit of quantification was 300 fg of MIF-1, and limit of detection was 60 fg. The low molecular weight fractions of tissue homogenates from different regions of mouse brain were analyzed. The concentration of MIF-1 ranged from 22+/-3 fg/microg protein in cerebral cortex to 930+/-60 fg/microg protein in the hypothalamus. Moderate concentrations were also detected in all other regions tested, including the striatum, thalamus, and hippocampus. By incubation of stable isotope-labeled oxytocin with tissue preparations, it was also confirmed that oxytocin at least partially contributed to the production of MIF-1 in the hypothalamus by action of peptidases. Regional differences were also found. The results are the first to show the ultrasensitive quantification of MIF-1 in different brain regions, and support the neuromodulatory actions of MIF-1 in the striatum.

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.

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.

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.

Isolation of a novel peptide with a unique binding profile from human brain cortex: Tyr-K-MIF-1 (Tyr-Pro-Lys-Gly-NH2)

Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2), Tyr-W-MIF-1 (Tyr-Pro-Trp-Gly-NH2), and MIF-1 (Pro-Leu-Gly-NH2) are biologically active peptides previously isolated from brain tissue. We now have used size exclusion chromatography and several consecutive rp-HPLC steps monitored by RIA to isolate a structurally related peptide from human brain cortex with the sequence Tyr-Pro-Lys-Gly-NH2 (Tyr-K-MIF-1). Determination of the sequence, electrospray mass spectrometry, and comparison of its chromatographic behavior with synthetic Tyr-K-MIF-1 confirmed the structure. Unlike Tyr-MIF-1 and Tyr-W-MIF-1, Tyr-K-MIF-1 does not bind to the mu opiate site; unlike MIF-1, Tyr-K-MIF-1 can bind to the Tyr-MIF-1 site. Of these peptides, only Tyr-K-MIF-1 binds to its own site in brain tissue prepared in Tris buffer. Thus, a new member of the Tyr-MIF-1 family of peptides, with a unique profile of binding, has been isolated from human brain cortex.

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.

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.

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.

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.

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

The effects of prolyl-leucyl-glycinamide (MIF-1, PLG), tyrosine-prolyl-leucyl-glycinamide (Tyr-MIF-1, YPLG) and the exogenous opiate antagonist, naloxone, on aggressive interactions and defeat-induced analgesia were examined in male mice. All three substances reduced the number of bites required to obtain defeat in subordinate mice during aggressive encounters, as well as blocking subsequent defeat-induced analgesia. Tyr-MIF-1 had significantly greater inhibitory effects than MIF. These results suggest that both MIF and Tyr-MIF-1 may function as endogenous opioid antagonists and have inhibitory influences on aggression, with the antagonistic effects of Tyr-MIF-1 being more potent than those of MIF-1.

Inhibitory influences of FMRFamide and PLG on stress-induced opioid analgesia and activity

The effects of i.c.v. administration of the peptide FMRFamide (Phe-Met-Arg-Phe-NH2), as well as i.p. injections of PLG (Pro-Leu-Gly-NH2) and the opiate antagonist, naloxone, on immobilization-induced analgesia and locomotor activity were examined in CF-1 and C57BL strains of mice. Both naloxone (1.0 mg/kg) and FMRFamide (0.10-1.0 microgram) blocked the experimentally induced analgesia and activity, whereas PLG (0.10-10 mg/kg) suppressed only analgesia. These results indicate that FMRFamide (or FMRFamide-like neuropeptides) and PLG may function as differential antagonists of the behavioral and physiological consequences of endogenous opioid activation.

Immunoreactive plasma concentrations of an endogenous antiopiate are higher in spontaneously hypertensive rats than in Wistar-Kyoto rats

Tyrosine-MIF-1 (Tyr-Pro-Leu-Gly-NH2) is present in rat brain in varying concentrations throughout the day and can act as an opiate antagonist. Since altered sensitivity to pain is known to occur in hypertension, plasma and brain concentrations of Tyr-MIF-1--like immunoreactivity were measured in spontaneously hypertensive rats (SHR) and compared every 4 hours for 24 hours with the concentrations in control Wistar-Kyoto rats (WKY). The Tyr-MIF-1--like immunoreactivity in plasma was significantly higher in SHR than in the WKY at each interval; the mean difference was 62% (p less than 0.001). High-performance liquid chromatography demonstrated that peak immunoreactivity eluted in the same position as the synthetic tetrapeptide. Brain concentrations of the peptide were not reliably different between SHR and WKY. The diurnal rhythm was particularly evident in SHR: the highest concentrations of peptide in both brain and plasma occurred at 2000 hours. These results suggest the presence of another difference between SHR and WKY.

Tyr-MIF-1, identified in brain tissue, and its analogs are active in two models of antinociception

The antiopiate activities of Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) and some of its representative analogs were tested in two animal models of antinociception. Doses of the tetrapeptides as low as 0.001 mg/kg injected peripherally could block the analgesic effects of morphine in both the tail-flick test of mild thermal pain induced by heat and the scratching test of mild chemical pain induced by hypertonic saline. These tetrapeptides showed cross-reactivity in the radio-immunoassay (RIA) used to identify the presence of Tyr-MIF-1 in brain extracts and in the brain membrane binding assay. Only Tyr-MIF-1, however, eluted at the position of the immunoreactive peak after gel filtration chromatography and high performance liquid chromatography (HPLC). The results support the concept that peptides with anti-opiate activity can exist in the brain.

Interactions between the antiopiate Tyr-MIF-1 and the mu opiate morphiceptin at their respective binding sites in brain

The interactions between Tyr-MIF-1, a brain peptide with antiopiate activity, and the beta-casomorphins, a family of peptides derived from milk protein with opiate activity, were investigated by in vitro binding assays. Specific binding of 125I-Tyr-MIF-1 to rat brain membranes was displaced with high potency by beta-casomorphin, morphiceptin, and the morphiceptin analog PL017 but not by the analgesically inactive analog D-Pro2-morphiceptin or by several other ligands for classical delta, kappa, or sigma opiate receptors. In addition, Tyr-MIF-1 displaced 125I-morphiceptin from its binding sites in brain with affinities similar to those of unlabeled morphiceptin and PL017. These results, which include the first demonstration of a binding site in brain for labeled morphiceptin, indicate that brain antiopiate Tyr-MIF-1 and the beta-casomorphin derived peptides with opiate activity may share a common binding site or cross-react at each other's site. This suggests a possible mechanism of action for endogenous antiopiate-opiate interactions.

Prolyl-leucyl-glycinamide reduces aggression and blocks defeat-induced opioid analgesia in mice

The effects of Prolyl-leucyl-glycinamide (PLG, MIF-1) and the exogenous opiate antagonist naloxone, on aggressive interactions and defeat-induced analgesia were examined in male mice. Both substances reduced the number of bites required to obtain defeat in subordinate mice during aggressive encounters as well as blocking the subsequent defeat-induced analgesia. These results suggest that MIF-1 may function as an endogenous opioid antagonist and have an inhibitory influence on aggression.

Antagonism of morphine analgesia by prolyl-leucyl-glycinamide (MIF-1) in humans

Prolyl-leucyl-glycinamide (MIF-1) has been observed to inhibit the analgesic effect of morphine in a series of animal studies. In the present study, the naloxone-like properties of MIF-1 were assessed in human subjects. Eight men received a capsule containing 60 mg of MIF-1 or placebo followed one hour later by a 10 mg intramuscular injection of morphine in a double-blind, crossover design at two visits 4 weeks apart. Experimental pain was induced by the cold pressor test administered 45, 75, 120 and 180 min after the morphine. Each subject recorded severity of pain on a 100 mm line scale every 5 sec during the 120 sec his foot was immersed in the cold water tank and during the 60 seconds immediately following its removal. On a third visit, baseline values were measured in the absence of morphine, MIF-1 or placebo. Analysis of variance revealed that MIF-1 resulted in significantly higher scores (less analgesia) compared with placebo when measured at 45 and 75 min after morphine during the immersion phase and during all four times the subjects were evaluated during the removal phase. The results indicate that MIF-1 can act in humans as an opiate antagonist.

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

The naturally occurring brain peptide Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) was tested for its ability to block and reverse the actions of morphine in the tail-flick test. Injected peripherally either 10 minutes before or after morphine, Tyr-MIF-1, like MIF-1, was found to significantly reduce the antinociceptive actions of morphine on thermal pain. The results indicate that Tyr-MIF-1 may act, in part, as an endogenous opiate antagonist.

Tyr-MIF-1 and MIF-1 are active in the water wheel test for antidepressant drugs

MIF-1 [Pro-Leu-Gly-NH2] and Tyr-MIF-1 [Tyr-Pro-Leu-Gly-NH2] were tested in a system in which antidepressant drugs are known to result in increased wheel turning as mice attempt to escape from a small tank of water. One hr after injection, both peptides were found to cause a significant increase of the number of rotations of the wheel at doses as low as 0.01 mg/kg IP, the dose-response pattern for MIF-1 resembling an inverted-U. DSIP and morphine, by contrast, decreased the number of rotations. Under the conditions tested, neither MIF-1 nor Tyr-MIF-1 reversed the effect of morphine. The results demonstrate that MIF-1 and Tyr-MIF-1 are active in another test for antidepressants.

Dose-dependent effects of prolyl-leucyl-glycinamide on morphine-induced analgesia, tolerance and dependence

Prolyl-leucyl-glycinamide (PLG) at a low dose (10 ng/mouse) administered intracerebroventricularly (i.c.v.) did not affect morphine analgesia, but produced a greater increase in the ED50 of morphine-pretreated (100 mg/kg of morphine sulfate) mice as compared to control mice. PLG at doses of 10 and 100 micrograms/mouse antagonized morphine analgesia. Development of morphine tolerance was unaffected by 10 micrograms/mouse but antagonized by 100 micrograms/mouse of PLG. Development of morphine dependence was assessed by changes in body weight and temperature during naloxone-induced withdrawal. PLG (10 ng/mouse) potentiated, 10 micrograms/mouse had no effect and 100 micrograms/mouse antagonized development of morphine dependence. PLG at doses of 10 and 100 micrograms/mouse precipitated withdrawal in morphine-dependent mice. When mice were pretreated with 1.0 mg/kg naloxone i.p. 15 min before PLG, all doses of PLG had no effect on morphine analgesia, but potentiated the development of morphine tolerance and dependence. None of the doses of PLG altered whole brain levels of morphine. PLG did not alter the affinity of opioid receptors for etorphine or the maximal number of binding sites but PLG did exhibit a very weak affinity for opioid receptors. These results indicate that PLG potentiated development of morphine tolerance and dependence through a mechanism not involving opioid receptors. However, at very high doses it was a weak opioid receptor antagonist.

MIF-1 does not act like naloxone in antagonizing the cardiovascular activity of leucine-enkephalin in the conscious dog

MIF-1 (Pro-Leu-Gly-NH2), a hypothalamic tripeptide, has been demonstrated to stimulate naloxone in antagonizing the effects of opioid peptides in a number of experimental systems including enkephalin-induced analgesia in the tail-flick assay, beta-endorphin induced hypothermia and hypomotility, deprivation-induced drinking, and analgesia in goldfish. MIF-1, however, has no effect upon the activity of enkephalins in the mouse vas deferens or enkephalin binding in the rat striatum. We have studied the interactions of MIF-1 with Leu5-enkephalin (Leu5-ENK) in the conscious, chronically instrumented dog. Although naloxone inhibits both the elevations of heart rate and blood pressure produced by IV Leu5-ENK in the conscious state and the depressions in these variables produced by Leu5-ENK after pentobarbital anesthesia, MIF-1 has no effect upon the Leu5-ENK response in either state. However, both naloxone and MIF-1 seem to raise mean arterial pressure in the conscious dog. These results indicate that MIF-1 does not act like naloxone in antagonizing the peripheral effects of Leu5-ENK and lend further support to the existence of mechanistic differences among opiate-mediated behavior, analgesia, and cardiovascular activity.

Failure of MIF-1 or naloxone to reverse ischemic-induced neurologic deficits in gerbils

MIF-1 and naloxone exert similar actions in several situations. Since naloxone, at a dose of 1 mg/kg IP, has been reported to reverse the neurologic deficits of gerbils whose right common carotid artery had been occluded, MIF-1 was tested under the same conditions and the effects compared with naloxone. Doses of 0.1, 1.0, and 10.0 mg/kg IP of MIF-1 and naloxone did not significantly alter the signs of either moderate or severe neurologic deficits. Thus, the results of this study with gerbils do not add evidence for the use of these opiate antagonists in strokes.

Characterization of type 2 opiate receptors

Recent evidence suggests that the Type 1 opiate receptor (in rat striatal patches) is a mobile receptor which is able to adopt a mu, delta, or kappa opiate receptor ligand selectivity pattern under appropriate conditions. In this paper, we have investigated such a possibility for Type 2 opiate receptors which are visualized diffusely over rat striatum. Ligand selectivity analysis suggested that the Type 2 opiate binding site is equivalent to a delta opiate receptor. The auto-radiographic distribution of Type 2 opiate binding sites is diffuse over most areas of rat brain. Thus, Type 2 opiate binding sites are different from Type 1 opiate receptors which are very discretely distributed in rat brain. Our results suggest that Type 2 opiate receptors, unlike Type 1 opiate receptors, are receptors locked in a delta-like ligand selectivity conformation.

Similar antagonism of morphine analgesia by MIF-1 and naloxone in Carassius auratus

Prolyl-leucyl-glycinamide (MIF-1), the C-terminal tripeptide of oxytocin, and naloxone were administered intracranially (IC) to goldfish (Carassius auratus) in doses of 0.001, 0.01, 0.1, 1.0 and 10.0 mg/kg and compared to a diluent control group for their ability to reduce the effects of morphine (30 mg/kg IC) in an assay measuring analgesia to electric shock. Threshold levels of pain were determined by the voltage necessary to produce an agitated swimming response (ASR). Both MIF-1 and naloxone were found to significantly reduce the analgesic effects of morphine when compared to the diluent control group. Similar dose-response curves in an apparent sine-wave pattern were noted with both MIF-1 and naloxone when comparisons were made both at 20 minutes after administration of morphine and over the entire 150 minutes of the experiment. The results support the evidence that MIF-1 can act as an opiate antagonist.