Modulation of the mesolimbic dopaminergic system by beta-endorphin-(1-27) as assessed by microdialysis

Peripheral Antinociception Induced by Aripiprazole Is Mediated by the Opioid System

Background

Aripiprazole is an antipsychotic drug used to treat schizophrenia and related disorders. Our previous study showed that this compound also induces antinociceptive effects. The present study aimed to assess the participation of the opioid system in this effect.

Methods

Male Swiss mice were submitted to paw pressure test and hyperalgesia was induced by intraplantar injection of prostaglandin E₂ (PGE₂, 2 μg). Aripiprazole was injected 10 min before the measurement. Naloxone, clocinnamox, naltrindole, nor-binaltorphimine, and bestatin were given 30 min before aripiprazole. Nociceptive thresholds were measured in the 3rd hour after PGE₂ injection.

Results

Aripiprazole (100 μg/paw) injected locally into the right hind paw induced an antinociceptive effect that was blocked by naloxone (50 μg/paw), a nonselective opioid receptor antagonist. The role of μ-, δ-, and κ-opioid receptors was investigated using the selective antagonists, clocinnamox (40 μg/paw), naltrindole (15, 30, and 60 μg/paw), and nor-binaltorphimine (200 μg/paw), respectively. The data indicated that only the δ-opioid receptor antagonist inhibited the peripheral antinociception induced by aripiprazole. Bestatin (400 μg), an aminopeptidase-N inhibitor, significantly enhanced low-dose (25 μg/paw) aripiprazole-induced peripheral antinociception.

Conclusion

The results suggest the participation of the opioid system via δ-opioid receptor in the peripheral antinociceptive effect induced by aripiprazole.

The effect of Gly-Gln [ß-endorphin30-31] on morphine-evoked serotonin and GABA efflux in the nucleus accumbens of conscious rats

Glycyl-L-glutamine (Gly-Gln; β-endorphin30-31) is an endogenous dipeptide synthesized through the post-translational processing of β-endorphin1-31. Central Gly-Gln administration inhibits the rewarding properties of morphine and attenuates morphine tolerance, dependence and withdrawal although it does not interfere with morphine analgesia. In an earlier study, we found that Gly-Gln inhibits morphine-induced dopamine efflux in the nucleus accumbens (NAc), consistent with its ability to inhibit morphine reward. To further investigate the mechanism responsible for its central effects we tested whether i.c.v. Gly-Gln administration influences the rise in extracellular serotonin and GABA concentrations evoked by morphine in the NAc. Conscious rats were treated with Gly-Gln (100nmol/5μl) or saline i.c.v. followed, 2min later, by morphine (2.5mg/kg) or saline i.p. and extracellular serotonin and GABA concentrations were analyzed by microdialysis and HPLC. Morphine administration increased extracellular serotonin and GABA concentrations significantly within 20min, as shown previously. Unexpectedly, Gly-Gln also increased extracellular serotonin concentrations significantly in control animals. Combined treatment with Gly-Gln+morphine also elevated extracellular serotonin concentrations although the magnitude of the response did not differ significantly from the effect of Gly-Gln or morphine, given alone suggesting that Gly-Gln suppressed morphine induced serotonin efflux. Gly-Gln abolished the morphine-induced rise in extracellular GABA concentrations but had no effect on extracellular GABA when given alone to otherwise untreated animals. These data show that Gly-Gln stimulates NAc serotonin efflux and, together with earlier studies, support the hypothesis that Gly-Gln inhibits the rewarding effects of morphine by modulating morphine induced dopamine, GABA and serotonin efflux in the NAc.

Effects of N-terminal fragments of beta-endorphin on feeding in chicks

It is known that N-terminal fragments of beta-endorphin have biological activities, such as an antagonism effect of beta-endorphin (1-31) on the secretion of hormones or thermoregulation in mammals. We studied the effects of the N-terminal fragments on feeding behavior in male broiler chicks. Intracerebroventricular administration of beta-endorphin (1-27) (0.4nmol) stimulated feeding behavior compared with saline control during the 60-min experimental period. beta-Endorphin (1-17) (2.0nmol) also increased food intake at 30min postinjection. Co-injection of either beta-endorphin (1-27) or (1-17) was effective in reducing full-length beta-endorphin-induced feeding in chicks. These data suggest that the N-terminal fragments of beta-endorphin act as a partial agonist, and may regulate the activity of the central opioidergic system in chicks.

Glycyl-glutamine reduces ethanol intake at three reward sites in P rats

beta-endorphin, implicated in modulation of ethyl alcohol reward, has neuron terminals in several reward sites. Alcohol consumption was reduced after ventricular or site-specific injections into the nucleus accumbens of an opioid-derived dipeptide, glycyl-glutamine. The current study examined the effects of this dipeptide after site-specific injections into additional reward sites. Alcohol-preferring (P) rats, stereotaxically implanted with bilateral guide cannulae into the nucleus accumbens, ventral tegmental area, and the central nucleus of the amygdala were given 30% alcohol and water in a 24h voluntary two-bottle choice paradigm. Upon achieving stable baseline intakes, glycyl-glutamine (GQ) doses were injected bilaterally, and the alcohol and water intakes and body weight recorded for the response and recovery. The data show reduced alcohol intake by 32-49.5% after 100-pmol glycyl-glutamine into reward sites (nucleus accumbens, ventral tegmental area, and central nucleus of the amygdala), but not after injections into control sites dorsal to reward sites. The order of sensitivity to the 1-fmol dose was amygdala > or = ventral tegmental area > accumbens. GQ was effective in reducing ethanol intake at reported beta-endorphin terminal regions in each of the three reward sites tested. The effective doses were similar to reported endogenous GQ levels, consistent with the notion that it may function as part of an endogenous counter regulatory mechanism and represent a "stop drinking" signal in the high drinking, P rats at these three reward sites.

Glycyl-glutamine in nucleus accumbens reduces ethanol intake in alcohol preferring (P) rats

Opioid peptides and glycyl-glutamine (Gly-Gln) have been implicated in the control of ethanol consumption. A recognized beta-endorphin cleavage product, Gly-Gln, inhibits voluntary alcohol consumption when microinjected into the nucleus accumbens (AcbSh) of P rats. To evaluate the site-specific efficacy of Gly-Gln on ethanol consumption following AcbSh application, ethanol preferring (P) rats were allowed to establish individual baseline ethanol/water consumption utilizing a voluntary self-administration paradigm. Subsequent to baseline ethanol consumption being established, bilateral guide cannulae were stereotaxically implanted +1 mm dorsal to the AcbSh for subsequent Gly-Gln (100 nmol/microl) or saline vehicle (1 microl) injections. Alcohol intake, body weight, and water intake were measured at 24 h post-injection intervals. Unilateral Gly-Gln injections reduced ethanol consumption 35.6% (P < 0.05) from pre-established baseline consumption (6.24 +/- 0.64 g/kg to 4.06 +/- 0.28 g/kg). Bilateral Gly-Gln injections further reduced consumption to 51.9% (6.4 +/- 1.0 g/kg to 3.08 +/- 0.65 g/kg at 24 h (P < 0.01) below established baseline values within 24 h without significant changes in body weight or water consumption. Also, the amino acid constituents of the dipeptide had no influence on ethanol consumption behavior; however, Gly-Gln efficacy was shown to be comparable to central beta-endorphin-(1-27) or intraperitoneal (i.p.) naltrexone-induced suppression of ethanol intake. These data indicate that the AcbSh exhibits a site-specific sensitivity to the suppressive actions of Gly-Gln or beta-endorphin-(1-27) injections that modulate voluntary ethanol consumption in P rats. These findings support the broader concept that select forebrain opioid-responsive neural sites may influence the development or expression of alcohol abuse syndromes in animal models or humans.

Glycyl-Glutamine, an Endogenous β-Endorphin-Derived Peptide, Inhibits Morphine-Induced Conditioned Place Preference, Tolerance, Dependence, and Withdrawal

Glycyl-glutamine (Gly-Gln; beta-endorphin(30-31)) is an endogenous dipeptide synthesized from beta-endorphin(1-31). Previous investigations have shown that Gly-Gln inhibits the cardiovascular and respiratory depression caused by morphine and beta-endorphin(1-31), but it does not interfere with opioid analgesia. In this study, we tested whether Gly-Gln administration would influence morphine-induced conditioned place preference, tolerance, dependence, or withdrawal. For place preference experiments, rats were conditioned with morphine sulfate (2.5 mg/kg i.p.) or saline on alternate days for 6 days and tested on day 7. Glycyl-glutamine (1-100 nmol i.c.v.) pretreatment inhibited acquisition of a conditioned place preference to morphine significantly. Glycyl-glutamine (100 nmol i.c.v.) also blocked expression of a pre-established morphine place preference, but it did not interfere with acquisition of a conditioned place preference to palatable food, and it did not produce place preference or aversion when given alone to morphine-naive animals. To induce antinociceptive tolerance, rats were treated with morphine (10 mg/kg i.p.) twice daily for 7 days, and morphine antinociception was evaluated with the tail-flick test. Glycyl-glutamine (100 nmol i.c.v.) pretreatment delayed the onset of morphine tolerance significantly and partially reversed pre-established tolerance. Morphine dependence and withdrawal were assessed by measuring naloxone-precipitated withdrawal symptoms. Glycyl-glutamine inhibited the development of morphine dependence when given to rats twice daily immediately before they received morphine (10 mg/kg i.p.) and suppressed withdrawal symptoms of rats with subcutaneously implanted morphine pellets when administered 5 min before withdrawal was induced with naloxone. Glycyl-glutamine thus attenuates morphine-induced conditioned place preference, tolerance, dependence, and withdrawal without compromising morphine analgesia.

Glycyl-glutamine inhibits the respiratory depression, but not the antinociception, produced by morphine

Glycyl-glutamine (Gly-Gln; beta-endorphin(30-31)) is an endogenous dipeptide that is synthesized through the posttranslational processing of beta-endorphin in brain stem regions that control respiration and autonomic function. This study tested the hypothesis that Gly-Gln administration to conscious rats will prevent the respiratory depression caused by morphine without affecting morphine antinociception. Rats were administered Gly-Gln (1-100 nmol) or saline (10 microl) intracerebroventricularly followed, 5 min later, by morphine (40 nmol icv). Arterial blood gases and pH were measured immediately before Gly-Gln and 30 min after morphine injection. Gly-Gln pretreatment inhibited morphine-induced hypercapnia, hypoxia, and acidosis significantly. The response was dose dependent and significant at Gly-Gln doses as low as 1 nmol. In contrast, Gly-Gln (1-300 nmol) had no effect on morphine-evoked antinociception in the paw withdrawal test. When given alone to otherwise untreated animals, Gly-Gln did not affect nociceptive latencies or blood gas values. These data indicate that Gly-Gln inhibits morphine-induced respiratory depression without compromising morphine antinociception.

Glycyl-L-glutamine injected centrally suppresses alcohol drinking in P rats

Recent reports show that central beta-endorphin (1-31) injection augments the volitional intake of alcohol. Correspondingly, alcohol drinking stimulates beta-endorphin (1-31) release from the hypothalamus of the rat. Glycyl-l-glutamine (Gly-Gln) is produced in beta-endorphin-containing neurons and is co-released with beta-endorphin(1-31) and other processing products. Because Gly-Gln is apparently an endogenous antagonist of beta-endorphin(1-31) in several systems, the present study was designed to investigate the hypothesis that Gly-Gln injected i.c.v. would alter voluntary alcohol drinking in the genetic, high-alcohol-preferring P rat. After a guide tube was implanted stereotaxically above the lateral cerebral ventricle, the rats were offered 3-30% alcohol over 10 days, and then given their maximally preferred concentration of alcohol in the presence of water for the remainder of the experiment. Gly-Gln or artificial cerebrospinal fluid (CSF) vehicle then was injected i.c.v. in a dose of 10 or 100 nmol for 3 consecutive days, which was followed by a 7-day postinjection interval. Gly-Gln suppressed significantly the intakes of alcohol in terms of both g/kg and proportion to total fluid. During the postinjection days, alcohol drinking continued to be suppressed, whereas neither the daily intakes of food or water nor the body weights of the rats were changed. The present results are consistent with the concept of a functional antagonism by Gly-Gln of the role of beta-endorphin(1-31) in mediating certain central functions. These results demonstrate that alcohol consumption is suppressed by the direct intracerebral application of this unique peptide.

Dopamine and opioid regulation of the memory retrieval recovery in mice

The reactivation effects of the delta-opioid receptor blockade and D2 dopamine receptor activation on the detention-induced memory deficit in mice were investigated, in order to study possible interactions between opioid and dopamine systems in memory retrieval. Animals were trained in a one-trial passive-avoidance task. Pretesting treatment with ICI 174,864 (1, 3 or 5 mg/kg, i.p.) or quinpirole (0.5, 1 or 2 mg/kg, i.p.) facilitated retrieval of memory trace in saline-pretreated mice. Pretraining injection of the dopamine autoreceptor agonist, (+)-3PPP (2 mg/kg), having no effect alone in learning, prevented the ability of ICI 174,864 to produce the memory-enhancing effect. It is suggested that the normal functioning of the dopamine system was critical for the facilitation of retrieval by delta-antagonist. Quinpirole-induced reactivátion of memory retrieval was enhanced by pretreatment with Leu-enkephalin (0.2 mg/kg), inducing increased retention. We discuss these results in the context of an important interactions between D2 dopamine and delta-opioid receptors.

The apomorphine test in heroin addicts

Chronic administration of opiates to laboratory animals induces supersensitivity of the dopamine receptors in the cerebral areas innervated by the mesotelencephalic dopamine pathways. In humans, the in vivo study of the sensitivity of the dopamine neurotransmitter system in Parkinson's patients can be done by means of the apomorphine test, which consists of measuring the number of yawns induced by the subcutaneous administration of low doses of apomorphine (0.005 mg/kg). If chronic opiate use in humans, as in experimental animals, results in supersensitivity of the dopamine systems, the apomorphine test could differentiate between heroin addicts and healthy volunteers, with the former showing greater number of yawns. In order to test this hypothesis we carried out the apomorphine test in two groups of subjects: a group of male heroin addicts attending our Addiction Treatment Centre for detoxification and the other group consisting of healthy volunteer male university students. Results showed that subcutaneous apomorphine administration induced a greater number of yawns (p < 0.05) in the group of heroin addicts as compared with the group of healthy volunteers, suggesting that heroin addicts present an enhanced sensitivity of the dopamine nuerotransmitter system.