Endomorphins 1 and 2, endogenous mu-opioid receptor agonists, impair passive avoidance learning in mice

Therapeutic Potential of Orally Administered Rubiscolin-6

Rubiscolins are naturally occurring opioid peptides derived from the enzymatic digestion of the ribulose bisphosphate carboxylase/oxygenase protein in spinach leaves. They are classified into two subtypes based on amino acid sequence, namely rubiscolin-5 and rubiscolin-6. In vitro studies have determined rubiscolins as G protein-biased delta-opioid receptor agonists, and in vivo studies have demonstrated that they exert several beneficial effects via the central nervous system. The most unique and attractive advantage of rubiscolin-6 over other oligopeptides is its oral availability. Therefore, it can be considered a promising candidate for the development of a novel and safe drug. In this review, we show the therapeutic potential of rubiscolin-6, mainly focusing on its effects when orally administered based on available evidence. Additionally, we present a hypothesis for the pharmacokinetics of rubiscolin-6, focusing on its absorption in the intestinal tract and ability to cross the blood-brain barrier.

Non-nociceptive roles of opioids in the CNS: opioids' effects on neurogenesis, learning, memory and affect

Mortality due to opioid use has grown to the point where, for the first time in history, opioid-related deaths exceed those caused by car accidents in many states in the United States. Changes in the prescribing of opioids for pain and the illicit use of fentanyl (and derivatives) have contributed to the current epidemic. Less known is the impact of opioids on hippocampal neurogenesis, the functional manipulation of which may improve the deleterious effects of opioid use. We provide new insights into how the dysregulation of neurogenesis by opioids can modify learning and affect, mood and emotions, processes that have been well accepted to motivate addictive behaviours.

Medial prefrontal cortex oxytocin-opioid receptors interaction in spatial memory processing in rats

Medial prefrontal cortex (mPFC), a forebrain structure, is involved in many brain functions such as learning and memory. In the present study, the effect of intra-mPFC microinjection of oxytocin, atosiban, morphine and naloxone was investigated on memory processing. Two guide cannulas were implanted into the right and left sides of the mPFC in ketamine and xylazine-anesthetized rats. To assess spatial memory function MWM test was performed by four training sessions of four trials. On day 5, a probe test was conducted after drugs microinjection. Significant differences were observed in learning activities during training days before microinjection of drugs. Intra-mPFC microinjections of oxytocin (5 and 10 ng/site) significantly increased memory related activities. This effect of oxytocin was inhibited by prior microinjection of atosiban (20 ng/site). On the other hand, morphine microinjection at doses of 5 and 10 μg/site into the mPFC significantly decreased memory related activities that were prevented by prior administration of naloxone (5 μg/site) and oxytocin (5 and 10 ng/site). In addition, intra-mPFC combined microinjections of low doses of oxytocin (2.5 ng/site) and naloxone (1 μg/site) improved memory function. By increasing the doses of oxytocin (5 ng/site) and naloxone (5 μg/site), a more documented improving effect was observed. These results showed that memory performance was impaired by activation of mPFC opioid receptors in rats. In addition, oxytocin in the mPFC improved memory function and prevented memory impairment-induced by morphine. Moreover, an interaction between oxytocin and opioid systems was also appeared in the present study.

Naltrexone Facilitates Learning and Delays Extinction by Increasing AMPA Receptor Phosphorylation and Membrane Insertion

BACKGROUND

The opioid antagonists naloxone/naltrexone are involved in improving learning and memory, but their cellular and molecular mechanisms remain unknown. We investigated the effect of naloxone/naltrexone on hippocampal α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) trafficking, a molecular substrate of learning and memory, as a probable mechanism for the antagonists activity.

METHODS

To measure naloxone/naltrexone-regulated AMPAR trafficking, pHluorin-GluA1 imaging and biochemical analyses were performed on primary hippocampal neurons. To establish the in vivo role of GluA1-Serine 845 (S845) phosphorylation on the behavioral effect induced by inhibition of the endogenous μ-opioid receptor (MOR) by naltrexone, MOR knockout, and GluA1-S845A mutant (in which Ser(845) was mutated to Ala) mice were tested in a water maze after chronic naltrexone administration. Behavioral responses and GluA1 levels in the hippocampal postsynaptic density in wild-type and GluA1-S845A mutant mice were compared using western blot analysis.

RESULTS

In vitro prolonged naloxone/naltrexone exposure significantly increased synaptic and extrasynaptic GluA1 membrane expression as well as GluA1-S845 phosphorylation. In the MOR knockout and GluA1-S845A mutant mice, naltrexone did not improve learning, which suggests that naltrexone acts via inhibition of endogenous MOR action and alteration of GluA1 phosphorylation. Naltrexone-treated wild-type mice had significantly increased phosphorylated GluA1-S845 and GluA1 levels in their hippocampal postsynaptic density on the third day of acquisition, which is the time when naltrexone significantly improved learning.

CONCLUSIONS

The beneficial effect of naltrexone on spatial learning and memory under normal conditions appears to be the result of increasing GluA1-S845 phosphorylation-dependent AMPAR trafficking. These results can be further explored in a mouse model of memory loss.

Effect of morphine on the persistence of long-term memory in rats

RATIONALE

Current evidence suggests that pharmacological manipulation around 12 h after training alters the persistence of long-term memory. However, no study has addressed whether opioids modulate the persistence of fear. The current study examined whether morphine alters the persistence of the memory of contextual fear conditioning.

METHODS

Male adult Wistar rats were injected with saline (NaCl 0.9 %, intraperitoneally (i.p.)) or morphine (3 and/or 10 mg/kg, i.p.) 6, 9, 12, or 24 h post-training and tested 2 or 7 days after training, when freezing responses were assessed. The involvement of state dependence and opioid receptors in the effect of morphine was investigated by respectively injecting naloxone (1 mg/kg, i.p.) 30 min before morphine, and morphine (10 mg/kg, i.p.) 30 min before testing.

RESULTS

Morphine (10 mg/kg, i.p., 12 h post-training) did not alter freezing to context in animals tested 2 days after training but impaired freezing to context when testing was carried out 7 or 14 days after training. Morphine (10 mg/kg, i.p.) administration 6, 9, or 24 h post-training did not alter freezing measured 2 or 7 days after training. Pre-test morphine improved recall but did not alter the deleterious effect of 12 h post-training morphine. The deleterious effect of morphine was prevented by naloxone, indicating that opioid receptors are involved in this effect.

CONCLUSIONS

Our findings indicate an inhibitory role for opioid receptors in memory persistence. This is relevant from both the experimental and clinical point of views, since it may have implications for the prevention of post-traumatic stress disorder (PTSD).

Neuroprotective effects of sulforaphane on cholinergic neurons in mice with Alzheimer's disease-like lesions

Alzheimer’s disease (AD) is a common neurodegenerative disease in elderly individuals, and effective therapies are unavailable. This study was designed to investigate the neuroprotective effects of sulforaphane (an activator of NF-E2-related factor 2) on mice with AD-like lesions induced by combined administration of aluminum and d-galactose. Step-down-type passive avoidance tests showed sulforaphane ameliorated cognitive impairment in AD-like mice. Immunohistochemistry results indicated sulforaphane attenuated cholinergic neuron loss in the medial septal and hippocampal CA1 regions in AD-like mice. However, spectrophotometry revealed no significant difference in acetylcholine level or the activity of choline acetyltransferase or acetylcholinesterase in the cerebral cortex among groups of control and AD-like mice with and without sulforaphane treatment. Sulforaphane significantly increased the numbers of 5-bromo-2'-deoxyuridine-positive neurons in the subventricular and subgranular zones in AD-like mice which were significantly augmented compared with controls. Atomic absorption spectrometry revealed significantly lower aluminum levels in the brains of sulforaphane-treated AD-like mice than in those that did not receive sulforaphane treatment. In conclusion, sulforaphane ameliorates neurobehavioral deficits by reducing cholinergic neuron loss in the brains of AD-like mice, and the mechanism may be associated with neurogenesis and aluminum load reduction. These findings suggest that phytochemical sulforaphane has potential application in AD therapeutics.

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.

Nicotine restores morphine-induced memory deficit through the D1 and D2 dopamine receptor mechanisms in the nucleus accumbens

Involvement of the dopamine D1 and D2 receptors in the nucleus accumbens (NAc) with interaction between morphine and nicotine on inhibitory avoidance (IA) memory was investigated. A step-through type of inhibitory avoidance tasks was used to assess memory in male Wistar rats. The results showed that subcutaneous (s.c.) administration of morphine (7.5 mg/kg) after training decreased retrieval of IA memory in the animals when tested 24 h later. Pre-test administration of the same dose of morphine significantly reversed the deficiency in retrieval. The results also showed that pre-test administration of nicotine (0.2 and 0.4 mg/kg, s.c.) by itself mimicked the effect of pre-test morphine, and lower doses of nicotine (0.1 and 0.2 mg/kg) also improved the effect of a low dose of morphine (2.5 mg/kg) on retrieval of IA memory. Pre-test intra-NAc administration of the dopamine D1 receptor antagonist, SCH 23390 (0.001 and 0.01 µg/rat), and the dopamine D2 receptor antagonist, sulpiride (0.5 and 1 µg/rat) caused no significant effects on IA memory by themselves, but both prevented reinstatement of the retrieval of IA memory by the effective dose of nicotine (0.4 mg/kg). It can be concluded that the dopaminergic mechanism(s) in the NAc is a crosslink for the effect of morphine and nicotine on reinstatement of retrieval of IA memory impaired by post-training administration of morphine.

Endomorphin 1 and endomorphin 2 suppress in vitro antibody formation at ultra-low concentrations: anti-peptide antibodies but not opioid antagonists block the activity

Endomorphin 1 (EM-1) and endomorphin 2 (EM-2) were tested for their capacity to alter immune function. Addition of either of these peptides to murine spleen cells in vitro inhibited antibody formation to sheep red blood cells in a bi-phasic dose dependent manner. Maximal inhibition was achieved at doses in the range of 10(-13) to 10(-15)M. Neither naloxone (general opioid receptor antagonist) nor CTAP (selective mu opioid receptor antagonist) blocked the immunosuppressive effect. To show that there was specificity to the immunosuppressive activity of the peptides, affinity-purified rabbit antibodies were raised against each of the synthetic EM peptides haptenized to KLH and tested for capacity to inhibit immunosuppression. Antibody responses were monitored by a standard solid phase antibody capture ELISA, and antibodies were purified by immunochromatography using the synthetic peptides coupled to a Sepharose 6B resin. Verification of the specificity of affinity-purified antisera was performed by immunodot-blot and solid-phase RIA assays. The antisera specific for both EM-1 and EM-2 neutralized the immunosuppressive effects of their respective peptides in a dose-related manner. Control normal rabbit IgG had no blocking activity on either EM-1 or EM-2. These studies show that the endomorphins are immunomodulatory at ultra-low concentrations, but the data do not support a mechanism involving the mu-opioid receptor.

N-Methyl-d-aspartate receptors in the ventral tegmental area are involved in retrieval of inhibitory avoidance memory by nicotine

The interaction of opiate, cholinergic, glutamatergic and (possibly) dopaminergic inputs in the ventral tegmental area (VTA) influencing a learned behavior is certainly a topic of great interest. In the present study, the effect of intra-VTA administration of N-methyl-d-aspartate (NMDA) receptor agents on nicotine's effect in morphine state-dependent learning was investigated. An inhibitory avoidance (IA) task was used for memory assessment in male Wistar rats. Subcutaneous (s.c.) administration of morphine (5 and 7.5mg/kg) immediately after training decreased IA response on the test day, which was reinstated by pre-test administration of the same doses of the opioid; this is known as state-dependency. Moreover, pre-test administration of nicotine (0.2, 0.4 and 0.6 mg/kg, s.c.) also reversed the decrease in IA response because of post-training morphine (5mg/kg). Here, we also show that when infused into the VTA before testing, NMDA (0.01 and 0.1 microg/rat) reverse the post-training morphine effect on memory. In addition, the sub-effective doses of NMDA (0.0001 and 0.001 microg/rat) in combination with a low dose of nicotine (0.1mg/kg) which had no effects by themselves, synergistically improved retrieval of IA memory on the test day. In contrast, pre-test administration of a competitive NMDA receptor antagonist D-AP5 (0.5, 1 and 2 microg/rat) which had no effect alone prevented the nicotine reversal of morphine effect on memory. Our data indicate that NMDA receptors in the VTA are involved in the reversing effect of nicotine on morphine induced state-dependency.

Rubiscolin-6, a delta opioid peptide derived from spinach Rubisco, has anxiolytic effect via activating sigma1 and dopamine D1 receptors

Rubiscolin-6 (Tyr-Pro-Leu-Asp-Leu-Phe) is a delta opioid peptide derived from the large subunit of spinach d-ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). We previously reported that rubiscolin-6 had an analgesic effect and stimulated memory consolidation. Here we show that intraperitoneally (i.p.) or orally administered rubiscolin-6 has an anxiolytic effect at a dose of 10 mg/kg or 100 mg/kg, respectively, in the elevated plus-maze test in mice. The anxiolytic effects of rubscolin-6 after i.p. (10 mg/kg) and oral (100 mg/kg) administration were blocked by a delta opioid receptor antagonist, naltrindole (1 mg/kg, s.c.), suggesting that the anxiolytic activity of rubiscolin-6 is mediated by delta opioid receptor. The anxiolytic effect of rubiscolin-6 (10 mg/kg, i.p.) was also blocked by a dopamine D(1) antagonist, SCH23390 (30 microg/kg, i.p.), but not by a dopamine D(2) antagonist, raclopride (15 microg/kg, i.p.). The anxiolytic effect of rubiscolin-6 (10 mg/kg, i.p.) was blocked by sigma(1) receptor antagonist, BMY14802 (0.5 mg/kg, i.p.) or BD1047 (10 mg/kg, i.p.). Taken together, the anxiolytic effect of rubiscolin-6 is mediated by sigma(1) and dopamine D(1) receptors downstream of delta opioid receptor.

The endomorphin system and its evolving neurophysiological role

Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) are two endogenous opioid peptides with high affinity and remarkable selectivity for the mu-opioid receptor. The neuroanatomical distribution of endomorphins reflects their potential endogenous role in many major physiological processes, which include perception of pain, responses related to stress, and complex functions such as reward, arousal, and vigilance, as well as autonomic, cognitive, neuroendocrine, and limbic homeostasis. In this review we discuss the biological effects of endomorphin-1 and endomorphin-2 in relation to their distribution in the central and peripheral nervous systems. We describe the relationship between these two mu-opioid receptor-selective peptides and endogenous neurohormones and neurotransmitters. We also evaluate the role of endomorphins from the physiological point of view and report selectively on the most important findings in their pharmacology.

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.

Nicotine improves morphine-induced impairment of memory: Possible involvement ofN-methyl-D-aspartate receptors in the nucleus accumbens

The possible involvement of N-methyl-D-aspartate (NMDA) receptors in the nucleus accumbens (NAc) in nicotine's effect on impairment of memory by morphine was investigated. A passive avoidance task was used for memory assessment in male Wistar rats. Subcutaneous (s.c.) administration of morphine (5 and 10 mg/kg) after training impaired memory performance in the animals when tested 24 h later. Pretest administration of the same doses of morphine reversed impairment of memory because of post-training administration of the opioid. Moreover, administration of nicotine (0.2 and 0.4 mg/kg, s.c.) before the test prevented impairment of memory by morphine (5 mg/kg) given after training. Impairment of memory performance in the animals because of post-training administration of morphine (5 mg/kg) was also prevented by pretest administration of a noncompetitive NMDA receptor antagonist, MK-801 (0.75 and 1 microg/rat). Interestingly, an ineffective dose of MK-801 (0.5 microg/rat) in combination with low doses (0.075 and 0.1 mg/kg) of nicotine, which had no effects alone, synergistically improved memory performance impaired by morphine given after training. On the other hand, pretest administration of NMDA (0.1 and 0.5 microg/rat), which had no effect alone, in combination with an effective dose (0.4 mg/kg, s.c.) of nicotine prevented the improving effect of nicotine on memory impaired by pretreatment morphine. The results suggest a possible role for NMDA receptors of the NAc in the improving effect of nicotine on the morphine-induced amnesia.

Interactions of buprenorphine and dipotassium clorazepate on anxiety and memory functions in the mouse

Buprenorphine, a partial mu-receptor agonist widely substituted for heroin in the treatment of addiction, is often misused in combination with benzodiazepines. Improved hedonic properties may result, but only at the cost of increased buprenorphine toxicity. In order to elucidate the appeal of the benzodiazepine-buprenorphine combination, the present study looked at its neuropsycho-pharmacological effects on various emotional and cognitive parameters in the mouse. On the basis of previous dose-response studies, the regimen used was buprenorphine 0.3mg/kg, s.c. plus dipotassium clorazepate 1, 4 and 16 mg/kg, i.p. Anxiety-like behaviour was assessed using the black and white test box, and memory processes were examined via the spontaneous alternation paradigm in the Y-maze, and passive avoidance tests. Spontaneous locomotor activity was also evaluated. High doses of clorazepate impaired buprenorphine-induced hyperactivity and anxiogenic-like effects. They also increased buprenorphine-induced spontaneous alternation impairment, but did not modify its impact on long-term memory processes. These results suggest that the positive reinforcement experienced with the buprenorphine-benzodiazepine combination may be attributable, at least in part, to an increase in buprenorphine's sedative effect associated with a decrease in anxiogenicity.

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.

Habituation to test procedure modulates the involvement of dopamine D2- but not D1-receptors in ethanol-induced locomotor stimulation in mice

RATIONALE

Novelty associated with behavioral testing has been shown to enhance psychostimulant- and morphine-induced locomotor stimulation. Evidence has demonstrated that novelty increases dopamine (DA) activity, and habituation to a novel environment reduces such activation. However, it is not clear whether novelty modulates ethanol-induced behavioral stimulation and whether DA plays a role in this effect.

OBJECTIVES

The present work sought to demonstrate a role of habituation to test procedure as a factor that could modulate the involvement of DA in ethanol-induced locomotor stimulation.

METHODS

Non-habituated (NH) and habituated (H) Swiss mice pretreated with DA D1- (SCH23390; 0-0.045 mg/kg) or D2-receptor (sulpiride; 0-50 mg/kg) antagonists were tested for ethanol (0-2.5 g/kg)-induced locomotor stimulation. Experiments with amphetamine (0-4 mg/kg), morphine (0-5 mg/kg) and caffeine (0-15 mg/kg)were designed to compare their results to those obtained with ethanol. The effect of the non-selective opioid receptor antagonist naltrexone (0-1.5 mg/kg) was also tested on ethanol-induced locomotor stimulation.

RESULTS

NH and H animals did not differ in their locomotor response to ethanol or caffeine; however, amphetamine- and morphine-induced stimulation was greater in NH than in H mice. SCH23390 only reduced ethanol-induced stimulation at doses that also reduced spontaneous activity in both NH and H mice. Sulpiride decreased ethanol-stimulated behavior only in the NH condition. Habituation did not modify the effect of sulpiride on amphetamine-, morphine- or caffeine-induced activation. Naltrexone (0-1.5 mg/kg) reduced ethanol-induced stimulation regardless of habituation.

CONCLUSIONS

The present data suggest that the participation of DA D2-receptors in ethanol-induced behavioral stimulation requires the presence of novelty. Results also support the involvement of neurotransmitter systems other than DA (i.e., endogenous opioid system) as important substrates mediating ethanol-induced locomotor activation.

Effects of naloxone on the long-term potentiation of EPSPs from the pathway of Schaffer collateral to CA1 region of hippocampus in aged rats with declined memory

Morris water maze (MWM) was employed to distinguish the aged rats with declined memory to investigate the effect of naloxone on the synaptic plasticity of hippocampus in declined memory aged rats. After administration with naloxone for 7 days, LTP of excitatory post-synaptic potentials (EPSPs) from Schaffer collateral to CA1 region was recorded. The results showed that the maintenance of LTP of EPSPs from Schaffer collateral to CA1 subfield in isolate hippocampal brain slice was prolonged by naloxone with improved Morris water maze performance and reduced threshold of EPSPs. It is suggested that naloxone can improve learning and memory through enhancement of the synaptic plasticity of hippocampus in aged rats with declined memory.

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.

Effect of rubiscolin, a delta opioid peptide derived from Rubisco, on memory consolidation

Rubiscolin-6 (YPLDLF) is a delta selective opioid peptide isolated from the enzymatic digests of ribulose bisphosphate carboxylase/oxygenase (Rubisco) from spinach leaves. In a step-through type passive avoidance test in ddY mice, rubiscolin-6 enhanced memory consolidation at doses of 3nmol/mouse after intracerebroventricular administration, and at 100mg/kg after oral administration. These doses are smaller than the optimal doses for an analgesic effect. The memory enhancing effect of rubiscolin-6 was blocked by pretreatment with the delta antagonist naltrindole, suggesting the involvement of the delta opioid receptor.

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.

Involvement of mu(1)-opioid receptors and cholinergic neurotransmission in the endomorphins-induced impairment of passive avoidance learning in mice

The effects of naloxonazine, a mu(1)-opioid receptor antagonist, and physostigmine, a cholinesterase inhibitor, on the endomorphins-induced impairment of passive avoidance learning were investigated in mice. Endomorphin-1 (10 microg) and endomorphin-2 (10 microg) significantly impaired passive avoidance learning, while naloxonazine (35 mg/kg, s.c.), a mu(1)-opioid receptor antagonist, which alone failed to influence passive avoidance learning significantly inhibited the endomorphin-1 (10 microg)- but not endomorphin-2 (10 microg)-induced disturbance of such learning. A rather nonselective higher dose (50 mg/kg, s.c.) of naloxonazine almost completely antagonized the endomorphin-1 (10 microg)- and endomorphin-2 (10 microg)-induced impairment of passive avoidance learning. In contrast, physostigmine (0.025 and 0.05 mg/kg, i.p.) significantly reversed the endomorphin-1 (10 microg)- and endomorphin-2 (10 microg)-induced disturbance of passive avoidance learning, whereas physostigmine (0.025 and 0.05 mg/kg, i.p.) alone did not influence such learning. These results suggest that endomorphin-1 but not endomorphin-2 impairs learning and memory resulting from cholinergic dysfunction, and from activation of mu(1)-opioid receptors.

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

The effects of intracerebroventricular injection of endomorphin-1 and 2, endogenous mu-opioid receptor agonists, on the scopolamine-induced impairment of spontaneous alternation performance associated with short-term memory were investigated in mice. Endomorphin-1 (0.03 microg) inhibited scopolamine (1 mg/kg)-induced impairment of spontaneous alternation performance without affecting total arm entries, while endomorphin-2 (0.01-10 microg) failed to significantly influence the scopolamine (1 mg/kg)-induced impairment. Endomorphin-1 (0.03 microg) itself had no marked effects on spontaneous alternation performance in intact mice. Although beta-funaltrexamine (5 microg), a mu-opioid receptor antagonist, did not significantly affect the inhibitory effects of endomorphin-1 (0.03 microg) on the scopolamine (1 mg/kg)-induced impairment, naloxonazine (35 mg/kg), a mu1-opioid receptor antagonist, significantly reversed the inhibitory effects of endomorphin-1 (0.03 microg) on the impairment. Naloxonazine (35 mg/kg) unlike beta-funaltrexamine (5 microg) did not significantly influence the scopolamine (1 mg/kg)-induced impairment of spontaneous alternation performance. These results suggest that endomorphin-1 improves the disturbance of short-term memory resulting from cholinergic dysfunction through the mediation of mu1-opioid receptors.