Involvement of beta-endorphin and mu-opioid receptors in mediating the aversive effect of lithium in the rat

Delta-opioid receptor antagonist naltrindole reduces oxycodone addiction and constipation in mice

Oxycodone, a widely prescribed and very potent oral opioid analgesic agent, is highly addictive and has many side effects, including troublesome constipation. Our studies in mice indicated that pretreatment of naltrindole did not significantly affect the analgesic efficacy of oxycodone but attenuated the tolerance and withdrawal induced by chronic oxycodone administration. Naltrindole also attenuated the oxycodone-induced rewarding and re-instatement behaviors, as shown by the conditioned place preference test. Further, oxycodone-induced decrease in intestinal transit (i.e., constipation) was reduced by naltrindole. However, naltrindole did not block the respiratory depression produced by oxycodone. Taken together, these data suggest that naltrindole can attenuate some major side effects while retaining the analgesic efficacy of oxycodone in mice. Naltrindole and oxycodone may have the potential to be a potent analgesic combination with much lower levels of oxycodone's side effects of addictive liability and constipation.

Effects of acute morphine withdrawal on ultrasonic vocalizations in adult rats: unchanged 50-kHz call rate and altered subtype profile

RATIONALE

Adult rat 22- and 50-kHz ultrasonic vocalizations (USVs) are commonly considered as indices of negative and positive affect, respectively. More specifically, we have proposed that positive affective states are revealed by a predominance of trill over flat 50-kHz call subtypes. However, the 50-kHz call subtypes emitted during aversive drug states remain largely uninvestigated.

OBJECTIVES

To determine whether acute morphine withdrawal affects 50-kHz call rates or alters the relative prevalence of trill and flat calls.

METHODS

In experiment 1, adult male rats were given saline or morphine (6 mg/kg SC), then acutely challenged 4 h later with saline or naloxone (1 mg/kg SC), and recorded 10-30 min post-injection. In experiments 2 and 3, rats received saline or morphine (6 mg/kg), followed 4 h later by acute saline or naloxone (0.1 mg/kg) challenge; USVs were subsequently recorded during 30-min place conditioning sessions.

RESULTS

Naloxone (0.1 mg/kg) produced a strong conditioned place aversion only after acute morphine pretreatment, consistent with antagonist-precipitated morphine withdrawal. The morphine-naloxone combination decreased the relative prevalence of trills and promoted flat calls. Naloxone given alone (0.1 and 1 mg/kg) inhibited trill calls but did not significantly alter the prevalence of flat calls, whereas morphine given alone (4 h pre-session) was largely without effect. Fifty-kHz call rates were inhibited by naloxone given alone, but otherwise unaffected. Twenty-two-kHz calls were sparse.

CONCLUSIONS

The 50-kHz call subtype shift seen during antagonist-precipitated morphine withdrawal was opposite in direction to that previously associated with rewards, and hence may reveal negative affect.

Lithium reverses mechanical allodynia through a mu opioid-dependent mechanism

Background

Lithium is widely used to treat bipolar disorders and displays mood stabilizing properties. In addition, lithium relieves painful cluster headaches and has a strong analgesic effect in neuropathic pain rat models.

Objectives

To investigate the analgesic effect of lithium on the cuff model of neuropathic pain.

Methods

We used behavioral and pharmacological approaches to study the analgesic effect of a single injection of lithium in wild-type and mu opioid receptor (MOR) null cuffed neuropathic mice. Mass spectrometry and enzyme-linked immunosorbent assay allowed to measure the levels of endogenous MOR agonist beta-endorphin as well as monoamines in brain and plasma samples 4 h after lithium administration.

Results

A single injection of lithium chloride (100 mg/kg, ip) alleviated mechanical allodynia for 24 h, and this effect was absent in MOR null neuropathic mice. Biochemical analyses highlight a significant increase in beta-endorphin levels by 30% in the brain of lithium-treated mice compared to controls. No variation of beta-endorphin was detected in the blood.

Conclusions

Together, our results provide evidence that lithium induces a long-lasting analgesia in neuropathic mice presumably through elevated brain levels of beta-endorphin and the activation of MORs.

Behavioral and molecular effects of dopamine D1 receptor stimulation during naloxone-precipitated morphine withdrawal

Morphine dependence is characterized by somatic and motivational signs of withdrawal that likely contribute to the maintenance of addictive behavior. The nucleus accumbens (NAc) receives extensive dopaminergic input and is an important substrate for mediating these aversive states. In the NAc, the function of the transcription factor cAMP response element binding protein (CREB) and AMPA glutamate receptor subunit, type 1 (GluR1) can be regulated by dopamine (DA) D1 receptor-mediated phosphorylation (P-CREB, P-GluR1). However, the roles of D1 receptors, CREB, and GluR1 in morphine dependence are not well understood. Here, we show that somatic signs of naloxone-precipitated withdrawal were associated with increased P-CREB, but not P-GluR1, in the NAc of morphine-dependent rats. The D1 receptor agonist chloro-APB hydrobromide (SKF 82958) was rewarding in morphine-dependent rats and blocked naloxone-induced place aversions and somatic signs of withdrawal. Surprisingly, SKF 82958 increased P-GluR1, but not P-CREB, in the NAc, and naloxone reduced SKF 82958-mediated P-GluR1 induction specifically in morphine-dependent rats. Together, these results confirm that aversive treatments can increase CREB function in the NAc. Furthermore, they suggest a dependence-associated shift in the molecular mechanisms that regulate the consequences of D1 receptor stimulation, favoring activation of GluR1 rather than CREB. These data raise the possibility that the rewarding effects of SKF 82958 in morphine-dependent rats involve increased P-GluR1 in the NAc, although the involvement of other brain regions cannot be ruled out. Regardless, these findings suggest for the first time that D1 agonists might be useful for the treatment of withdrawal symptoms that contribute to the maintenance of opiate addiction in humans.

Endogenous enkephalins, not endorphins, modulate basal hedonic state in mice

The aversive response to naloxone administration observed in human and animal studies suggests the presence of an endogenous opioid tone regulating hedonic state but the class(es) of opioid peptides mediating such opioid hedonic tone is uncertain. We sought to address this question using mice deficient in either beta-endorphin or pro-enkephalin in a naloxone-conditioned place aversion paradigm. Mice received saline in the morning in one chamber and either saline or naloxone (0.1, 1 or 10 mg/kg, s.c.) in the afternoon in another chamber, each day for 3 days. On the test day they were given free access to the testing chambers in the afternoon and the time spent in each chamber was recorded. Whereas wild-type and beta-endorphin-deficient mice exhibited a robust conditioned place aversion to naloxone, pro-enkephalin knockout mice failed to show aversion to naloxone at any dose tested. In contrast, these mice showed a normal conditioned aversion to the kappa opioid receptor agonist, U50,488 (5 mg/kg), and to LiCl (100 mg/kg) indicating that these mice are capable of associative learning. In a separate experiment, pro-enkephalin knockout mice, similar to wild-type and beta-endorphin-deficient mice, demonstrated a significant conditioned place preference to morphine (2.5, 5 and 10 mg/kg s.c.). These data suggest that enkephalins, but not endorphins, may mediate an endogenous opioid component of basal affective state and also indicate that release of neither endogenous enkephalins nor endorphins is critical for the acquisition or expression of the association between contextual cues and the rewarding effect of exogenously administered opiates.

Molecular targets of lithium action

Lithium is an effective drug for both the treatment and prophylaxis of bipolar disorder. However, the precise mechanism of lithium action is not yet well understood. Extensive research aiming to elucidate the molecular mechanisms underlying the therapeutic effects of lithium has revealed several possible targets. The behavioral and physiological manifestations of the illness are complex and are mediated by a network of interconnected neurotransmitter pathways. Thus, lithium's ability to modulate the release of serotonin at presynaptic sites and modulate receptor-mediated supersensitivity in the brain remains a relevant line of investigation. However, it is at the molecular level that some of the most exciting advances in the understanding of the long-term therapeutic action of lithium will continue in the coming years. The lithium cation possesses the selective ability, at clinically relevant concentrations, to alter the PI second-messenger system, potentially altering the activity and dynamic regulation of receptors that are coupled to this intracellular response. Subtypes of muscarinic receptors in the limbic system may represent particularly sensitive targets in this regard. Likewise, preclinical data have shown that lithium regulates arachidonic acid and the protein kinase C signaling cascades. It also indirectly regulates a number of factors involved in cell survival pathways, including cAMP response element binding protein, brain-derived neurotrophic factor, bcl-2 and mitogen-activated protein kinases, and may thus bring about delayed long-term beneficial effects via under-appreciated neurotrophic effects. Identification of the molecular targets for lithium in the brain could lead to the elucidation of the pathophysiology of bipolar disorder and the discovery of a new generation of mood stabilizers, which in turn may lead to improvements in the long-term outcome of this devastating illness (1).

Acute and conditioned sickness reduces morphine analgesia

Animals made ill by intraperitoneal injection with toxins, such as lithium chloride (LiCl) or lipopolysaccharides (LPS), or presented with cues associated with LiCl become hyperalgesic [Pain 56 (1994) 227]. The descending pronociceptive neurocircuitry and spinal pharmacology that underlie these effects bear the same features as those that mediate analgesic tolerance to morphine [Neurosci. Biobehav. Rev. 23 (1999) 1059]. Thus, we examined whether LiCl, LPS or cues paired with LiCl could reduce morphine analgesia. Morphine analgesia in the tail flick test was reduced 24 h but not 7 days following injection with LiCl, and 24 h following injection with LPS. In addition, morphine analgesia was reduced in the hot plate test 40 min and 24 h following LiCl. Furthermore, these effects occurred in the absence of detectable hyperalgesia indicating that illness-induced tolerance was not the result of an increase in pain sensitivity offsetting analgesia. Finally, rats tested in a context associated with LiCl demonstrated less morphine analgesia than rats tested in a context not associated with LiCl or rats naive to LiCl suggesting that illness activates descending mechanisms that antagonize analgesia rather than simply desensitizing opioid receptors. Thus, in addition to provoking hyperalgesia, illness-inducing agents also activate endogenous antianalgesic mechanisms.

Naloxone fails to produce conditioned place aversion in mu-opioid receptor knock-out mice

There is growing evidence that tonic activity of the opioid system may be important in the modulation of affective state. Naloxone produces a conditioned place aversion in rodents, an effect that is centrally mediated. Previous pharmacological data using antagonists with preferential actions at mu-, delta-, and kappa-opioid receptors indicate the importance of the mu-opioid receptor in mediating this effect. We sought to test the mu-opioid receptor selectivity of naloxone aversion using mu-opioid receptor knock-out mice. mu-Opioid receptor knock-out and wild-type mice were tested for naloxone (10 mg/kg, s.c.) aversion using a place conditioning paradigm. As a positive control for associative learning, knock-out mice were tested for conditioned place aversion to a kappa agonist, U50,488H (2 mg/kg, s.c.). Naloxone produced a significant place aversion in wild-type mice, but failed to have any effect in mu-opioid receptor knock-out mice. On the other hand, both knock-out and wild-type mice treated with U50,488H spent significantly less time in the drug-paired chamber compared to their respective vehicle controls. We conclude that the mu-opioid receptor is crucial for the acquisition of naloxone-induced conditioned place aversion. Furthermore, in a separate experiment using C57BL/6 mice, the delta-selective antagonist naltrindole (10 or 30 mg/kg, s.c.) failed to produce conditioned place aversion.Taken together, these data further support the notion that naloxone produces aversion by antagonizing tonic opioid activity at the mu-opioid receptor.

A peripheral, intracerebral, or intrathecal administration of an opioid receptor antagonist blocks illness-induced hyperalgesia in the rat

We used the tail-flick response of rats to study the role of opioid receptors in illness-induced hyperalgesia. An intraperitoneal injection of lithium chloride (LiCl) produced hyperalgesia that was blocked in a dose-dependent manner by subcutaneous injection of the opioid antagonist naloxone. Neither hyperalgesia nor its blockade by naloxone were due to variations in tail-skin temperature induced by LiCl. Hyperalgesia was also blocked when opioid receptor antagonism was restricted to (a) the periphery, by intraperitoneal administration of the quaternary opioid receptor antagonist naloxone methiodide; (b) the brain, by intracerebroventricular microinjection of naloxone; or (c) the spinal cord, by intrathecal microinjection of naloxone. These results document a pain facilitatory role of opioid receptors in both the peripheral and central nervous systems and are discussed with reference to their analgesic and motivational functions.

Chronic cadmium exposure attenuates conditioned place preference produced by cocaine and other drugs

Adult male rats were exposed ad lib for 40 days to 100 ppm dietary cadmium chloride (group cadmium) or an identical diet with no added cadmium (group control). Conditioned place preference (CPP) was conducted in a two-chamber apparatus in which all drugs were paired with the least-preferred side as determined by a pretest. In Experiment 1, animals received 0, 2.5, or 5 mg/kg cocaine HCl (IP) for 4 days and vehicle only for 4 days. Control animals showed a place preference for the drug side at 2.5 and 5 mg/kg, while the cadmium-exposed animals showed a preference at 5 mg/kg only. In Experiment 2, animals received 0, 5, or 10 mg/kg of the D1/D2 dopamine receptor agonist apomorphine HCl (SC) for 4 days and vehicle only for 4 days. Control animals showed a place preference at 5 and 10 mg/kg, while metal-exposed animals showed a preference at 10 mg/kg only. To determine the possible effects of alterations of learning mechanisms by cadmium, a conditioned place aversion (CPA) procedure was employed for Experiment 3. Animals received 0, 10, or 40 mg/kg lithium chloride (IP) for 4 days or vehicle only for 4 days. Control animals showed a significant place aversion at 40 mg/kg, while cadmium-exposed animals did not. These findings are discussed within a framework of possible metal-induced disturbance of neurochemical function and/or associative processing.

Measuring reward with the conditioned place preference paradigm: a comprehensive review of drug effects, recent progress and new issues

This review gives an overview of recent findings and developments in research on brain mechanisms of reward and reinforcement from studies using the place preference conditioning paradigm, with emphasis on those studies that have been published within the last decade. Methodological issues of the paradigm (such as design of the conditioning apparatus, biased vs unbiased conditioning, state dependency effects) are discussed. Results from studies using systemic and local (intracranial) drug administration, natural reinforcers, and non-drug treatments and from studies examining the effects of lesions are presented. Papers reporting on conditioned place aversion (CPA) experiments are also included. A special emphasis is put on the issue of tolerance and sensitization to the rewarding properties of drugs. Transmitter systems that have been investigated with respect to their involvement in brain reward mechanisms include dopamine, opioids, acetylcholine, GABA, serotonin, glutamate, substance P, and cholecystokinin, the motivational significance of which has been examined either directly, by using respective agonist or antagonist drugs, or indirectly, by studying the effects of these drugs on the reward induced by other drugs. For a number of these transmitters, detailed studies have been conducted to delineate the receptor subtype(s) responsible for the mediation of the observed drug effects, particularly in the case of dopamine, the opioids, serotonin and glutamate. Brain sites that have been implicated in the mediation of drug-induced place conditioning include the 'traditional' brain reward sites, ventral tegmental area and nucleus accumbens, but the medial prefrontal cortex, ventral pallidum, amygdala and the pedunculopontine tegmental nucleus have also been shown to play important roles in the mediation of place conditioning induced by drugs or natural reinforcers. Thus, although the paradigm has also been criticized because of some inherent methodological problems, it is clear that during the past decade place preference conditioning has become a valuable and firmly established and very widely used tool in behavioural pharmacology and addiction research.

The effects of naloxone on expression and acquisition of ethanol place conditioning in rats

Naloxone has been shown to facilitate extinction of ethanol-induced conditioned place preference (CPP) in mice. The present-study extended these findings by examining naloxone's effect on the expression (Experiment 1) and acquisition (Experiment 2) of place conditioning with ethanol in rats. In Experiment 1, after place conditioning with ethanol (1.8 g/kg, I.P.), groups N0, N1.5, and N10 received 0, 1.5, or 10 mg/kg naloxone before testing. As expected, ethanol produced a robust conditioned place aversion (CPA). However, naloxone had no effect on expression of CPA. In contrast to studies with mice, the endogenous opioid system does not appear to be involved in the conditioned motivational effects of ethanol in rats. In Experiment 2, groups SE1 and SE2, NS(1.5), NE(1.5), and NE(10), received ethanol alone (1.2 g/kg), naloxone alone (1.5 mg/kg), naloxone 1.5 mg/kg plus ethanol, and naloxone 10 mg/kg plus ethanol during acquisition, respectively. All naloxone-treated groups exhibited CPA. Moreover, group NE(1.5) showed a stronger CPA than group NS(1.5). The CPA produced by coadministration of naloxone and ethanol was attributed to naloxone's effects on the neural processes underlying ethanol's unconditioned aversive effects, or to other nonspecific effects on ethanol's motivational properties.

Blockade of lithium chloride-induced conditioned place aversion as a test for antiemetic agents: comparison of metoclopramide with combined extracts of Zingiber officinale and Ginkgo biloba

The present study tests the hypothesis that the blockade of lithium chloride-induced conditioned place aversion might be a suitable model to assess antiemetic properties of drugs, especially in species that do not vomit, like rats. The effects of the known antiemetic compound metoclopramide were compared with those of zingicomb, a combination preparation of extracts of Ginkgo biloba and Zingiber officinale, also presumed to have antiemetic properties. Place conditioning was performed using a conventional three-compartment test procedure. On three successive conditioning trials, rats received an intraperitoneal (i.p.) injection of lithium chloride (125 mg/kg) and were placed into the compartment that they had preferred over three baseline trials. During the test, rats treated with lithium chloride (LiCl) spent less time in the treatment compartment, indicative of a conditioned place aversion (CPA). In the first experiment, metoclopramide (MCP) was administered intragastrically (IG) in doses of 2 or 10 mg/kg 60 min prior to LiCl injection. The pretreatment with 50 and 100 mg/kg zingicomb attenuated the LiCl-produced CPA, whereas a dosage of 10 mg/kg had no effect. These findings suggest that LiCl-induced CPA is a viable procedure with which to assess the antiemetic properties of metoclopramide. Furthermore, the data confirm the hypothesis that the phytopharmacon zingicomb might have antiemetic properties that are comparable to those of metoclopramide.

The delta-opioid receptor antagonist naltrindole prevents sensitization to the conditioned rewarding effects of cocaine

A conditioned place preference paradigm was used to determine whether: (i) prior exposure to cocaine results in an enhancement of its rewarding effects, and (ii) the delta-opioid receptor antagonist naltrindole can prevent the development of this response. Rats received daily injections of saline or cocaine (10.0 mg/kg i.p.) for 5 days in the colony room. Additional animals received naltrindole (0.03-0.3 mg/kg s.c.), lithium chloride (20 mg/kg s.c.) or vehicle prior to i.p. injections. Conditioning sessions (2 drug; 2 vehicle) commenced 3 days later. Cocaine (1.0-10.0 mg/kg) was ineffective as a conditioning stimulus in saline pre-exposed rats. In cocaine pre-exposed animals, however, doses of 5.0 and 10.0 mg/kg cocaine resulted in significant drug-induced place preferences. Significant cocaine-induced place preferences were also observed in animals which had received lithium chloride with the cocaine treatment regimen. In animals which had received naltrindole together with the chronic cocaine treatment regimen, cocaine failed to produce a conditioned response. These data demonstrate that the repeated administration of cocaine results in an enhancement of its rewarding effects (e.g. sensitization) and that this phenomenon is prevented by a delta-opioid receptor antagonist. Furthermore, the finding that naltrindole does not modify the acute rewarding effects of cocaine suggests a specific role of delta-opioid receptors in the sensitization process.

Trends in place preference conditioning with a cross-indexed bibliography; 1957-1991

The purpose of this work is to present a perspective of the conditioned place preference (CPP) test by offering an overview of the empirical research from 1957-1991. The intent is not to extensively analyze the controversies inherent to any behavioral technique but rather to present a survey of research using a descriptive statistics approach to explore topical issues. The objectives of this work are three-fold: (a) to provide an exhaustive bibliography of the CPP literature including articles, journal abstracts, book chapters and critical reviews; (b) to provide a cross-index of identified key words/drugs tested; and (c) to give an overview of selected procedural issues underlying CPP testing.

Stimulation of hypothalamic opioid peptide release by lithium is mediated by opioid autoreceptors: evidence from a combined in vitro, ex vivo study

The effect of both chronic and acute lithium treatment on hypothalamic opioid peptides was investigated. Acute treatment with lithium was found to stimulate the release of beta-endorphin, dynorphin and Met-enkephalin from perfused rat hypothalamic slices. Application of tetrodotoxin was found to have no effect upon the stimulation indicating it to be mediated at the nerve terminal level. The release of hypothalamic opioid peptides is known to be under the chronic control of a system of inhibitory autoreceptors. Blockade of these autoreceptors with, for example, the opioid receptor antagonist naloxone causes a release of all three opioid peptides. Simultaneous addition of naloxone and lithium was found to have no additive effect on the release of any opioid, suggesting lithium acts via an inhibition of the inhibitory autoreceptor. Preincubation with pertussis toxin prevented the lithium stimulation of dynorphin and Met-enkephalin, but not beta-endorphin, release, indicating lithium interacts with a G-protein to affect the autoreceptor controlling the release of dynorphin and Met-enkephalin. Chronic treatment with lithium in vivo (10 days) had no effect on the basal release or hypothalamic content of any of the opioids, but prevented the naloxone-stimulated release of dynorphin and Met-enkephalin in vitro. Long-term treatment with lithium would thus appear to inactivate the autoreceptor(s) controlling their release. These data demonstrate a lithium-stimulated release of hypothalamic beta-endorphin, Met-enkephalin and dynorphin, apparently mediated via an inhibition of the autoreceptors controlling their release. Chronic treatment with lithium permanently inactivated the autoreceptor(s) controlling the release of dynorphin and Met-enkephalin but not beta-endorphin. Lithium would appear to mediate its effects upon Met-enkephalin and dynorphin release via an interaction with a pertussis toxin-sensitive G-protein. The mechanisms underlying its release of beta-endorphin are at present uncertain.

Endogenous opiates: 1988

This paper is the eleventh installment in our annual review of the research during the past year involving the endogenous opiate system. It is concerned with nonanalgesic and behavioral studies of the opiate peptides that were published during 1988. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical activity; locomotor activity; sex, pregnancy, and development; immunology and cancer; and other behavior.

Infusion of gamma 2-MSH produce a conditioned taste aversion in morphine-dependent rats

A two flavour, unbiased, taste preference conditioning procedure was used to test for possible motivating effects of gamma 2-MSH. Three training trials failed to produce any significant effect with doses ranging from 2.4 to 40 micrograms/ICV infusion in drug-naive, non-operated or placebo-implanted rats. However, in rats made dependent by SC implantation of a morphine pellet 4 days earlier 15 micrograms gamma 2-MSH/infusion produced a taste aversion that was comparable to that produced by infusion of a low dose of the competitive opioid receptor antagonist naloxone (0.32 micrograms). The findings confirm with a conditioning procedure and with opiate-dependent animals the naloxone-like effects of gamma 2-MSH. They also suggest that this endogenously-located peptide may acquire an aversive property as a result of chronic morphine treatment.