Endomorphin-1 improves scopolamine-induced impairment of short-term memory via mu1-opioid receptor in mice

Effects of β-Casomorphin-5 on Passive Avoidance Response in Mice

The effects of intracerebroventricular (i.c.v.) injection of bovine beta-casomorphin-5 (beta-CM-5: Tyr-Pro-Phe-Pro-Gly), a micro-opioid agonist derived from milk beta-casein, on step-down type passive avoidance tasks were investigated in mice. Intracerebroventricular administration of a high dose (10 microg) of beta-CM-5 produced a significant decrease in step-down latency. beta-Funaltrexamine (5 microg, i.c.v.) almost completely reversed the beta-CM-5-induced shortening of step-down latency, although neither naltrindole (5 ng, i.c.v.) nor nor-binaltorphimine (5 microg, i.c.v.) had any significant influence on the effect of beta-CM-5. Meanwhile, a low dose (0.5 microg, i.c.v.) of beta-CM-5 inhibited scopolamine (1 mg/kg)-induced impairment of passive avoidance response. These results indicated that a high dose of beta-CM-5 induces amnesia, whereas a low dose ameliorates scopolamine-induced amnesia.

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.

Circadian-Dependent Learning and Memory Enhancement in Nociceptin Receptor-Deficient Mice with a Novel KUROBOX Apparatus Using Stress-Free Positive Cue Task

Using the novel apparatus KUROBOX, learning and memory behaviors, as well as various parameters of movement activity, were reevaluated in mice deficient for nociceptin/orphanin FQ receptor (NOP-/- mice) or mu-opioid receptor (MOP-/- mice). This method has the advantages that no handling procedures are required throughout the experiments performed over 3 days, positive cue paradigms are used without water or shock stress, and the method does not disturb the nocturnal habit of mice. NOP-/- mice displayed a significant enhancement of learning and memory under stress-free conditions, but there were no changes in the various physical and psychological parameters of movement activity (nest stay ratio, distance moved, speed and angle in the movement) and biological rhythm that were measured. Enhancement of nocturnal learning was observed during the first 12-h dark cycle, and enhancement of memory was observed at the beginning of the second dark cycle in NOP-/- mice. In contrast, MOP-/- mice showed no significant change in learning and memory behaviors or in physical and psychological parameters of movement activity, except for speed, MOP-/- mice showed a significant decrease in speed of movement. Thus, the KUROBOX apparatus provides a useful alternative method to evaluate learning and memory activity under the more physiological conditions. In addition, this apparatus has an advantage that various physical and psychological parameters of movement activity affecting learning and memory behavior are also evaluated at the same time.

Protective effects of endomorphins, endogenous opioid peptides in the brain, on human low density lipoprotein oxidation

Neurodegenerative disorders are associated with oxidative stress. Low density lipoprotein (LDL) exists in the brain and is especially sensitive to oxidative damage. Oxidative modification of LDL has been implicated in the pathogenesis of neurodegenerative diseases. Therefore, protecting LDL from oxidation may be essential in the brain. The antioxidative effects of endomorphin 1 (EM1) and endomorphin 2 (EM2), endogenous opioid peptides in the brain, on LDL oxidation has been investigated in vitro. The peroxidation was initiated by either copper ions or a water-soluble initiator 2,2'-azobis(2-amidinopropane hydrochloride) (AAPH). Oxidation of the LDL lipid moiety was monitored by measuring conjugated dienes, thiobarbituric acid reactive substances, and the relative electrophoretic mobility. Low density lipoprotein oxidative modifications were assessed by evaluating apoB carbonylation and fragmentation. Endomorphins markedly and in a concentration-dependent manner inhibited Cu2+ and AAPH induced the oxidation of LDL, due to the free radical scavenging effects of endomorphins. In all assay systems, EM1 was more potent than EM2 and l-glutathione, a major intracellular water-soluble antioxidant. We propose that endomorphins provide protection against free radical-induced neurodegenerative disorders.

Effects of systemic administration of beta-casomorphin-5 on learning and memory in mice

The effects of systemic administration of bovine beta-casomorphin-5 (Tyr-Pro-Phe-Pro-Gly), a mu-opioid receptor agonist derived from milk beta-casein, on spontaneous alternation behavior in the Y-maze (spatial short-term memory) and step-down-type passive avoidance response (non-spatial long-term memory) were investigated in mice. Intraperitoneal (i.p.) administration of beta-casomorphin-5 (0.1-20 mg/kg) did not have a significant effect on either spontaneous alternation behavior or passive avoidance response. However, a low dose (1 mg/kg, i.p.) of beta-casomorphin-5 improved scopolamine (1 mg/kg, s.c.)-induced impairment of spontaneous alternation behavior and passive avoidance response. Pretreatment with intracerebroventricular injections of beta-funaltrexamine (a mu-opioid receptor antagonist, 0.1 microg/mouse) and naloxonazine (a mu(1)-opioid antagonist, 5 microg/mouse), which did not improve scopolamine-induced impairment, prevented the ameliorating effect of beta-casomorphin-5 on scopolamine-induced impairment of passive avoidance response. These results indicated that systemic administration of a low dose (1 mg/kg, i.p.) of beta-casomorphin-5 improves the disturbance of learning and memory resulting from cholinergic dysfunction through central mediation involving mu(1)-opioid receptors.

The value of spontaneous alternation behavior (SAB) as a test of retention in pharmacological investigations of memory

Because of its reliance on memory, the tendency for rats, mice and other animals to alternate successive choices of T- or Y-maze arms has assumed considerable popularity in pharmacological studies of spatial memory as a quick and simple measure of retention that avoids the need for extensive training and the use of conventional reinforcers. Two forms of this tendency have been utilized, namely two-trial and continuous spontaneous alternation behavior (SAB). However, as the behavior can also reflect drug-related changes in sensory/attentional, motivational and performance processes, SAB should not be unquestionably accepted as a measure of memory alone. While assessments of post-acquisition drug effects on longer term memory may be possible through the appropriate timing of drug administration, this is more problematic if SAB is used as a measure of shorter term memory. Even though SAB can be a useful index of responsiveness to novelty, its value as a measure of retention is less certain. In this latter respect, a possible alternative to SAB testing might be the recently developed form of the related procedure, responsiveness to change.

Endomorphins, endogenous opioid peptides, induce apoptosis in human leukemia HL-60 cells

Opioids play a role in the apoptosis machinery. We studied the induction of apoptosis in endomorphin 1 (EM1) and endomorphin 2 (EM2), 2 newly isolated endogenous µ-opioid receptor agonists. These endomorphins were able to reduce the viability of cultured HL-60 cells. The antiproliferative properties of endomorphins appeared to be attributable to their induction of apoptotic cell death as determined by ultrastructural change, internucleosomal DNA fragmentation, and increased proportion of the subdiploid cell population. To elucidate molecular events in the apoptosis, protein expressions of Bcl-2, Bax, Fas, and FasL were measured by western blotting using specific antibodies in HL-60 cells. The level of Bcl-2 indicated down-regulation, but the Bax, Fas, and FasL expression showed up-regulation as compared with the untreated control cells. These data support the idea that endomorphins induce apoptosis in HL-60 cells through the activation of the Bcl-2–Bax and the Fas–FasL pathway. We suggest that endomorphins may play an important role in the regulation of tumor cell death.Key words: endomorphins, HL-60 cell, apoptosis, Bcl-2, Fas.

Effects of Pre-Germinated Brown Rice on .BETA.-Amyloid Protein-Induced Learning and Memory Deficits in Mice

We evaluated the effects of pre-germinated brown rice (hatsuga genmai, PGR) on learning and memory and compared them with those of polished rice or cornstarch. In mice that were fed pellets of polished rice or PGR for two weeks, the learning ability in the Morris water maze test was significantly enhanced compared with mice that were fed cornstarch pellets. In the Y-maze test, the intake of food pellets for two weeks failed to affect spontaneous alternation behavior. Beta-amyloid(25-35) (Abeta(25-35): 3 nmol/mouse, i.c.v.) protein impaired spontaneous alternation behavior in mice that were fed pellets of cornstarch or polished rice. In contrast, PGR pellets prevented the Abeta(25-35)-induced impairment of spontaneous alternation behavior. These results suggest that polished rice and PGR have facilitating effects on spatial learning. In particular, it is surmised that PGR may prevent Alzheimer's disease associated with Abeta.

Endomorphins, endogenous opioid peptides, provide antioxidant defense in the brain against free radical-induced damage

Oxidative stress has been considered to be a major cause of cellular injuries in a variety of chronic health problems, such as carcinogenesis and neurodegenerative disorders. The brain appears to be more susceptible to oxidative damage than other organs. Therefore, the existence of antioxidants may be essential in brain protective systems. The antioxidative and free radical scavenging effects of endomorphin 1 (EM1) and endomorphin 2 (EM2), endogenous opioid peptides in the brain, have been investigated in vitro. The oxidative damage was initiated by a water-soluble initiator 2,2'-azobis(2-amidinopropane hydrocholoride) (AAPH) and hydrogen peroxide (H2O2). The linoleic acid peroxidation, DNA and protein damage were monitored by formation of hydroperoxides, by plasmid pBR 322 DNA nicking assay and single-cell alkaline electrophoresis, and by SDS-polyacrylamide gel electrophoresis. Endomorphins can inhibit lipid peroxidation, DNA strand breakage, and protein fragmentation induced by free radical. Endomorphins also reacted with galvinoxyl radicals in homogeneous solution, and the pseudo-first-order rate constants were determined spectrophotometrically by following the disappearance of galvinoxyl radicals. In all assay systems, EM1 was more potent than EM2 and GSH, a major intracellular water-soluble antioxidant. We propose that endomorphins are one of the protective systems against free radical-induced damage in the brain.

Endomorphin-1, an endogenous mu-opioid receptor agonist, improves apomorphine-induced impairment of prepulse inhibition in mice

The present study was designed to examine the effects of the endogenous mu-opioid receptor agonist endomorphin-1 on prepulse inhibition (PPI) in mice. Although apomorphine (1mg/kg) produced a marked decrease in PPI, endomorphin-1 (17.5 microg) had no marked effects on PPI or startle amplitude in normal mice. Endomorphin-1 (17.5 microg) inhibited the apomorphine (1mg/kg)-induced decrease in PPI. beta-Funaltrexamine (5 microg), a mu-opioid receptor antagonist, did not significantly antagonize the effects of endomorphin-1 (17.5 microg). Naloxonazine (35 mg/kg), a mu(1)-opioid receptor antagonist, antagonized the effects of endomorphin-1 (17.5 microg) on the apomorphine (1mg/kg)-induced decrease in PPI, whereas naloxonazine (35 mg/kg) itself was without significant effects on the apomorphine (1mg/kg)-induced decrease. These results suggest that endomorphin-1 alleviates the impairment of PPI resulting from the hyperactivity of dopaminergic neurotransmission through the mediation of mu(1)-opioid receptors.

Ethanol improves short-term social memory in rats. Involvement of opioid and muscarinic receptors

Some human and animal studies have demonstrated enhancement of memory processes when ethanol was administered immediately after training and subjects were later tested in the drug-free state. The aim of this study was to evaluate the effect of acute ethanol administration (0.5, 1.0 and 2.0 g/kg) by intraperitoneal (i.p.) and oral route on short-term memory, using the social recognition test in rats. The actions of scopolamine (0.06 and 0.5 mg/kg, i.p.) and naloxone (1.0 mg/kg, i.p.) and their interaction with ethanol in relation to short-term memory were also studied. The doses of ethanol used did not show any sedative effect, which was assessed by measuring locomotor activity. The results indicate that acute low doses of ethanol (0.5 and 1.0 g/kg, i.p.) improve the short-term olfactory memory in rats in a specific and time-dependent manner, and that this action is, at least in part, related to opioid, but not to muscarinic receptors. In addition, these findings confirm that the social recognition test in rats is a useful and reliable model to investigate short-term memory affected by ethanol.

Endogenous opiates and behavior: 2001

This paper is the twenty-fourth installment of the annual review of research concerning the opiate system. It summarizes papers published during 2001 that studied the behavioral effects of the opiate peptides and antagonists. The particular topics covered this year include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology(Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Endomorphins 1 and 2 induce amnesia via selective modulation of dopamine receptors in mice

The involvement of dopamine receptors in the amnesic effects of the endogenous micro-opioid receptor agonists endomorphins 1 and 2 was investigated by observing step-down type passive avoidance learning in mice. Although the dopamine D1 receptor agonist R(+)-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine-7,8-diol hydrochloride (R(+)-SKF38393) (0.05 and 0.1 mg/kg), the dopamine D1 receptor antagonist R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (R(+)-SCH23390) (2.5 and 5 microg/kg) or the dopamine D2 receptor agonist N-n-phenethyl-N-propylethyl-p-(3-hydroxyphenyl)-ethylamine (RU24213) (0.3 and 1 mg/kg) had no significant effects on the endomorphin-1 (10 microg)- or endomorphin-2 (10 microg)-induced decrease in step-down latency of passive avoidance learning, (-)-sulpiride (10 mg/kg), a dopamine D2 receptor antagonist, significantly reversed the decrease in step-down latency evoked by endomorphin-2 (10 microg), but not by endomorphin-1 (10 microg). Taken together, it is likely that stimulation of dopamine D2 receptors results in the endomorphin-2-but not endomorphin-1-induced impairment of passive avoidance learning.