Accelerated expression of cholecystokinin gene in the brain of rats rendered tolerant to morphine

The Involvement of Descending Pain Inhibitory System in Electroacupuncture-Induced Analgesia

Chronic pain is a major health problem, which can impair quality of life and reduce productivity. Electroacupuncture (EA), a modality of medicine based on the theories of Traditional Chinese Medicine (TCM), presents great therapeutic effects on chronic pain. Its clinical application has gained increasing popularity, and in parallel, more research has been performed on the mechanisms of EA-induced analgesia. The past decades have seen enormous advances both in neuronal circuitry of needle-insertion and in its molecular mechanism. EA may block pain by activating the descending pain inhibitory system, which originates in the brainstem and terminates at the spinal cord. This review article synthesizes corresponding studies to elucidate how EA alleviate pain via the mediation of this descending system. Much emphasis has been put on the implication of descending serotonergic and noradrenergic pathways in the process of pain modulation. Also, other important transmitters and supraspinal regions related to analgesic effects of EA have been demonstrated. Finally, it should be noticed that there exist some shortcomings involved in the animal experimental designed for EA, which account for conflicting results obtained by different studies.

Over-expression of CCK1 Receptor Reverse Morphine Dependence

Studies demonstrated that cholecystokinin (CCK) system involved in morphine dependence and withdrawal. Our previous study showed that endogenous CCK system were up-regulated after chronic morphine exposure. Additionally, CCK1 receptor significantly blocked the inhibitory effect of exogenous CCK-8 on morphine dependence, but CCK2 receptor appears to be necessary for low concentrations of endogenous CCK to potentiate morphine dependence. Therefore, CCK1R and CCK2R function differently in chronic morphine dependence, but the mechanism is still unclear. In this study, HEK-293 cells co-transfected with µ-opioid receptors (HEK293-hMOR) and CCK1R or CCK2R were established. Cells were treated with 10 µM morphine for 6, 12, 16, 24 h and 100 µM naloxone precipitation for 15 min. cAMP overshoot was appeared at 12 h and was increased time dependently after morphine exposure in HEK293-hMOR cells. The cAMP overshoot did not appear in CCK1R-overexpressing HEK293-hMOR cells, while still appeared in CCK2R-overexpressing HEK293-hMOR cells. Over-expression of CCK1R reversed CREB and ERK1/2 activation in HEK293-hMOR cells exposed to morphine. Our study identifies over-expression of CCK1R significantly blocked morphine dependence, which was related with phosphorylation of CREB, and ERK1/2 signaling activation. While over-expression of CCK2R promoted morphine dependence, which was related with phosphorylation of CREB but not ERK1/2 signaling activation.

The relationship between opioids and immune signalling in the spinal cord

Opioids are considered the gold standard for the treatment of moderate to severe pain. However, heterogeneity in analgesic efficacy, poor potency and side effects are associated with opioid use, resulting in dose limitations and suboptimal pain management. Traditionally thought to exhibit their analgesic actions via the activation of the neuronal G-protein-coupled opioid receptors, it is now widely accepted that neuronal activity of opioids cannot fully explain the initiation and maintenance of opioid tolerance, hyperalgesia and allodynia. In this review we will highlight the evidence supporting the role of non-neuronal mechanisms in opioid signalling, paying particular attention to the relationship of opioids and immune signalling.

Exogenous and endogenous opioid-induced pain hypersensitivity in different rat strains

BACKGROUND

Opioid-induced hyperalgesia (OIH) is a recognized complication of opioid use that may facilitate the development of exaggerated postoperative pain.

OBJECTIVE

To examine the role of genetic factors on OIH by comparing four rat strains. Because the authors previously reported that the endogenous opioids released during non-nociceptive environmental stress induce latent pain sensitization, genetic and environmental factor interactions were also evaluated.

METHODS

First, the propensity of Sprague Dawley, Wistar, Lewis and Fischer rats to develop OIH following single or repeated fentanyl exposures was compared by measuring the nociceptive threshold using the paw pressure vocalization test. Second, Sprague Dawley and Fischer rats were exposed to a series of three non-nociceptive environmental stress sessions to evaluate the ability of endogenous opioids to enhance hyperalgesia associated with a carrageenan-induced hind-paw inflammation test performed two weeks later.

RESULTS

Sprague Dawley, Wistar and Lewis rats exhibited OIH, although differences were observed. OIH was not observed in Fischer rats. Inflammatory hyperalgesia enhancement induced through previous stress in Sprague Dawley rats was not observed in Fischer rats.

CONCLUSIONS

The pain level not only reflects nociceptive inputs but also depends on both the history and genetic factors of the individual. Genetic and environmental models may provide new insights into the mechanisms that underlie individual differences observed in postoperative pain.

Complications of long-term opioid therapy for management of chronic pain: the paradox of opioid-induced hyperalgesia

While opioids remain a valid and effective analgesic strategy for patients suffering from a wide variety of painful conditions, they are not a panacea. Increasingly, physicians must balance patient expectations of adequate pain control with known limitations of opioid pharmaceuticals including adverse effects, tolerance, addiction, withdrawal, and drug diversion. Further complicating the issue over the last decade is a growing body of evidence suggesting chronic opioid use may unexpectedly worsen the perception of pain in some individuals. This syndrome, termed opioid-induced hyperalgesia (OIH), fundamentally changes our understanding of opioid pharmacodynamics and may influence our approach to management of chronic pain. This manuscript describes the concept OIH and provides an overview of basic science and clinical research to date attempting to characterize this syndrome, as well as ascertain its clinical relevance. The potential existence of OIH in humans is framed within the context of our current understanding of opioids and our prescribing patterns so that physicians may begin to incorporate these ideas into their philosophy of pain management as further information develops. Animal studies reliably validate OIH in controlled models. Rigorous research protocols in humans are lacking, and we cannot yet confidently conclude that OIH manifests in clinically significant ways. However, clinicians should consider the possibility of OIH when evaluating outcomes of patients on chronic opioid therapy.

Cholecystokinin receptors mediate tolerance to the analgesic effect of TENS in arthritic rats

Transcutaneous electrical nerve stimulation (TENS) is a treatment for pain that involves placement of electrical stimulation through the skin for pain relief. Previous work from our laboratory shows that repeated application of TENS produces analgesic tolerance by the fourth day and a concomitant cross-tolerance at spinal opioid receptors. Prior pharmacological studies show that blockade of cholecystokinin (CCK) receptors systemically and spinally prevents the development of analgesic tolerance to repeated doses of opioid agonists. We therefore hypothesized that systemic and intrathecal blockade of CCK receptors would prevent the development of analgesic tolerance to TENS, and cross-tolerance at spinal opioid receptors. In animals with knee joint inflammation (3% kaolin/carrageenan), high (100Hz) or low frequency (4Hz) TENS was applied daily and the mechanical withdrawal thresholds of the muscle and paw were examined. We tested thresholds before and after inflammation, and before and after TENS. Animals treated systemically, prior to TENS, with the CCK antagonist, proglumide, did not develop tolerance to repeated application of TENS on the fourth day. Spinal blockade of CCK-A or CCK-B receptors blocked the development of tolerance to high and low frequency TENS, respectively. In the same animals we show that spinal blockade of CCK-A receptors prevents cross-tolerance at spinal delta-opioid receptors that normally occurs with high frequency TENS; and blockade of CCK-B receptors prevents cross-tolerance at spinal mu-opioid receptors that normally occurs with low frequency TENS. Thus, we conclude that blockade of CCK receptors prevents the development of analgesic tolerance to repeated application of TENS in a frequency-dependent manner.

Naloxone, not proglumide or MK-801, alters effects of morphine preexposure on morphine-induced taste aversions

Both cholecystokinin (CCK) antagonists and N-methyl-D-aspartate (NMDA) antagonists block or reduce the development of morphine tolerance in several analgesic assays. The present experiments were performed to assess the ability of the CCK antagonist proglumide and the NMDA antagonist MK-801 to affect tolerance to the aversive properties of morphine as indexed by conditioned taste aversion (CTA) learning. Specifically, male Sprague-Dawley rats were exposed to either vehicle or morphine (5 mg/kg) in combination with either proglumide (5 mg/kg; Experiment 1), MK-801 (0.1 mg/kg; Experiment 2) or naloxone (1, 3.2 mg/kg; Experiment 3). Saccharin was then presented and was followed by an injection of either vehicle or morphine (10 mg/kg). Animals preexposed to and conditioned with morphine acquired an attenuated morphine-induced aversion to saccharin. While neither proglumide nor MK-801 had an effect on this attenuation, naloxone blocked the effects of morphine preexposure, suggesting that neither CCK nor NMDA may be involved in the aversive effects of morphine (or their modulation by drug exposure). That the attenuating effects of morphine preexposure on a morphine-induced CTA can be blocked suggests that the weakening of the aversive effects of morphine with chronic use can be prevented, an effect that may have implications for overall drug acceptability.

Is paradoxical pain induced by sustained opioid exposure an underlying mechanism of opioid antinociceptive tolerance?

Opiates are the primary treatment for pain management in cancer patients reporting moderate to severe pain, and are being increasingly used for non-cancer chronic pain. However, prolonged administration of opiates is associated with significant problems including the development of antinociceptive tolerance, wherein higher doses of the drug are required over time to elicit the same amount of analgesia. High doses of opiates result in serious side effects such as constipation, nausea, vomiting, dizziness, somnolence, and impairment of mental alertness. In addition, sustained exposure to morphine has been shown to result in paradoxical pain in regions unaffected by the initial pain complaint, and which may also result in dose escalation, i.e. 'analgesic tolerance'. A concept that has been gaining considerable experimental validation is that prolonged use of opioids elicits paradoxical, abnormal pain. This enhanced pain state requires additional opioids to maintain a constant level of antinociception, and consequently may be interpreted as antinociceptive tolerance. Many substances have been shown to block or reverse antinociceptive tolerance. A non-inclusive list of examples of substances reported to block or reverse opioid antinociceptive tolerance include: substance P receptor (NK-1) antagonists, calcitonin gene-related peptide (CGRP) receptor antagonists, nitric oxide (NO) synthase inhibitors, calcium channel blockers, cyclooxygenase (COX) inhibitors, protein kinase C inhibitors, competitive and non-competitive antagonists of the NMDA (N-methyl-D-aspartate) receptor, AMPA (alpha-amino-3-hydroxy-5-methyl-4 isoxazolepropionic acid) antagonists, anti-dynorphin antiserum, and cholecystokinin (CCK) receptor antagonists. Without exception, these substances are also antagonists of pain-enhancing agents. Prolonged opiate administration indeed induces upregulation of substance P (SP) and calcitonin gene-related peptide (CGRP) within sensory fibers in vivo, and this is accompanied by an enhanced release of excitatory neurotransmitters and neuropeptides from primary afferent fibers upon stimulation. The enhanced evoked release of neuropeptides is correlated with the onset of abnormal pain states and opioid antinociceptive tolerance. Importantly, the descending pain modulatory pathway from the brainstem rostral ventromedial medulla (RVM) via the dorsolateral funiculus (DLF) is critical for maintaining the changes observed in the spinal cord, abnormal pain states and antinociceptive tolerance, because animals with lesion of the DLF did not show enhanced evoked neuropeptide release, or develop abnormal pain or antinociceptive tolerance upon sustained exposure to opiates. Microinjection of either lidocaine or a CCK antagonist into the RVM blocked both thermal and touch hypersensitivity as well as antinociceptive tolerance. Thus, prolonged opioid exposure enhances a descending pain facilitatory pathway from the RVM that is mediated at least in part by CCK activity and is essential for the maintenance of antinociceptive tolerance.

Cholecystokinin and endogenous opioid peptides: interactive influence on pain, cognition, and emotion

It is well documented that stressful life experiences contribute to the etiology of human mood disorders. Cholecystokinin (CCK) is a neuropeptide found in high concentrations throughout the central nervous system, where it is involved in numerous physiological functions. A role for CCK in the induction and persistence of anxiety and major depression appears to be conspicuous. While increased CCK has been associated with motivational loss, anxiety and panic attacks, an increase in mesocorticolimbic opioid availability has been associated with coping and mood elevation. The close neuroanatomical distribution of CCK with opioid peptides in the limbic system suggests that there may be an opioid-CCK link in the modulation and expression of anxiety or stressor-related behaviors. In effect, while CCK induces relatively protracted behavioral disturbances in both animal and human subjects following stressor applications, opioid receptor activation may change the course of psychopathology. The antagonistic interaction of CCK and opioid peptides is evident in psychological disturbances as well as stress-induced analgesia. There appears to be an intricate balance between the memory-enhancing and anxiety-provoking effects of CCK on one hand, and the amnesic and anxiolytic effects of opioid peptides on the other hand. Potential anxiogenic and mnemonic influences of site-specific mesocorticolimbic CCK and opioid peptide availability, the relative contributions of specific CCK and opioid receptors, as well as the time course underlying neuronal substrates of long-term behavioral disturbances as a result of stressor manipulations, are discussed.

Cholecystokinin in the rostral ventromedial medulla mediates opioid-induced hyperalgesia and antinociceptive tolerance

Opioid-induced hyperalgesia is characterized by hypersensitivity to innocuous or noxious stimuli during sustained opiate administration. Microinjection of lidocaine into the rostral ventromedial medulla (RVM), or dorsolateral funiculus (DLF) lesion, abolishes opioid-induced hyperalgesia, suggesting the importance of descending pain facilitation mechanisms. Here, we investigate the possibility that cholecystokinin (CCK), a pronociceptive peptide, may drive such descending facilitation from the RVM during continuous opioid administration. In opioid-naive rats, CCK in the RVM produced acute tactile and thermal hypersensitivity that was antagonized by the CCK2 receptor antagonist L365,260 or by DLF lesion. CCK in the RVM also acutely displaced the spinal morphine antinociceptive dose-response curve to the right. Continuous systemic morphine elicited sustained tactile and thermal hypersensitivity within 3 d. Such hypersensitivity was reversed in a time-dependent manner by L365,260 in the RVM, and blockade of CCK2 receptors in the RVM also blocked the rightward displacement of the spinal morphine antinociceptive dose-response curve. Microdialysis studies in rats receiving continuous morphine showed an approximately fivefold increase in the basal levels of CCK in the RVM when compared with controls. These data suggest that activation of CCK2 receptors in the RVM promotes mechanical and thermal hypersensitivity and antinociceptive tolerance to morphine. Enhanced, endogenous CCK activity in the RVM during sustained morphine exposure may diminish spinal morphine antinociceptive potency by activating descending pain facilitatory mechanisms to exacerbate spinal nociceptive sensitivity. Prevention of opioid-dose escalation in chronic pain states by CCK receptor antagonism represents a potentially important strategy to limit unintended enhanced clinical pain and analgesic tolerance

Involvement of cholecystokinin in the opioid tolerance induced by dipyrone (metamizol) microinjections into the periaqueductal gray matter of rats

The analgesic effect of non-steroidal anti-inflammatory drugs (NSAIDs) is partly due to an action upon the periaqueductal gray matter (PAG), which triggers the descending pain control system and thus inhibits nociceptive transmission. This action of NSAIDs engages endogenous opioids at the PAG, the nucleus raphe magnus and the spinal cord. Repeated administration of NSAIDs such as dipyrone (metamizol) and acetylsalicylate thus induces tolerance to these compounds and cross-tolerance to morphine. Since cholecystokinin plays a key role in opioid tolerance, the present study in rats investigated whether PAG cholecystokinin is also responsible for tolerance to PAG-microinjected dipyrone. Microinjection of cholecystokinin (1 ng/0.5 microl) into PAG blocked the antinociceptive effect of a subsequent microinjection of dipyrone (150 microg/0.5 microl) into the same site, as evaluated by the tail flick and hot plate tests. Microinjection of proglumide (0.4 microg/0.5 microl), a non-selective cholecystokinin antagonist, into PAG prevented the development of tolerance to subsequent microinjections of dipyrone, as well as cross-tolerance to microinjection of morphine (5 microg/0.5 microl) into the same site. In rats tolerant to PAG dipyrone, a PAG microinjection of proglumide restored the antinociceptive effect of a subsequent microinjection of dipyrone or morphine. These results suggest that PAG-microinjected dipyrone triggers and/or potentiates local opioidergic circuits leading to descending inhibition of nociception, on the one hand, and to a local antiopioid action by cholecystokinin, on the other. Reiteration of these events would then result in an enhancement of cholecystokinin's antiopioid action and thus tolerance to opioids and dipyrone in the PAG.

Antinociceptive and nociceptive actions of opioids

Although the opioids are the principal treatment options for moderate to severe pain, their use is also associated with the development of tolerance, defined as the progressive need for higher doses to achieve a constant analgesic effect. The mechanisms which underlie this phenomenon remain unclear. Recent studies revealed that cholecystokinin (CCK) is upregulated in the rostral ventromedial medulla (RVM) during persistent opioid exposure. CCK is both antiopioid and pronociceptive, and activates descending pain facilitation mechanisms from the RVM enhancing nociceptive transmission at the spinal cord and promoting hyperalgesia. The neuroplastic changes elicited by opioid exposure reflect adaptive changes to promote increased pain transmission and consequent diminished antinociception (i.e., tolerance).

Induction of pain facilitation by sustained opioid exposure: relationship to opioid antinociceptive tolerance

Opioid analgesics are frequently used for the long-term management of chronic pain states, including cancer pain. The prolonged use of opioids is associated with a requirement for increasing doses to manage pain at a consistent level, reflecting the phenomenon of analgesic tolerance. It is now becoming clearer that patients receiving long-term opioid therapy can develop unexpected abnormal pain. Such paradoxical opioid-induced pain, as well as tolerance to the antinociceptive actions of opioids, has been reliably measured in animals during the period of continuous opioid delivery. Several recent studies have demonstrated that such pain may be secondary to neuroplastic changes that result, in part, from an activation of descending pain facilitation mechanisms arising from the rostral ventromedial medulla (RVM). One mechanism which may mediate such pain facilitation is through the increased activity of CCK in the RVM. Secondary consequences from descending facilitation may be produced. For example, opioid-induced upregulation of spinal dynorphin levels seem to depend on intact descending pathways from the RVM reflecting spinal neuroplasticity secondary to changes at supraspinal levels. Increased expression of spinal dynorphin reflects a trophic action of sustained opioid exposure which promotes an increased pain state. Spinal dynorphin may promote pain, in part, by enhancing the evoked release of excitatory transmitters from primary afferents. In this regard, opioids also produce trophic actions by increasing CGRP expression in the dorsal root ganglia. Increased pain elicited by opioids is a critical factor in the behavioral manifestation of opioid tolerance as manipulations which block abnormal pain also block antinociceptive tolerance. Manipulations that have blocked enhanced pain and antinociceptive tolerance include reversible and permanent ablation of descending facilitation from the RVM. Thus, opioids elicit systems-level adaptations resulting in pain due to descending facilitation, upregulation of spinal dynorphin and enhanced release of excitatory transmitters from primary afferents. Adaptive changes produced by sustained opioid exposure including trophic effects to enhance pain transmitters suggest the need for careful evaluation of the consequences of long-term opioid administration to patients.

Involvement of local cholecystokinin in the tolerance induced by morphine microinjections into the periaqueductal gray of rats

The ventrolateral periaqueductal gray (PAG) is a key structure for the development of opioid tolerance. An increased activity of 'anti-opioids' like cholecystokinin (CCK) has been proposed as a possible mechanism for opioid tolerance. The present study evaluates the role of PAG-located CCK in the opioid tolerance induced by repeated microinjections of morphine (MOR) into PAG. Male rats were implanted with chronic guide cannulae aimed at the PAG. Microinjection of MOR (0.5 microg in 0.5 microl) into PAG caused antinociception as quantified with the tail flick and the hot plate tests. When MOR microinjection was repeated twice daily, the antinociceptive effect disappeared within 2 days (tolerance). However, if each MOR microinjection was preceded (within 15 min) by a microinjection of the non-selective CCK receptor antagonist proglumide (PRO), (0.4 microg in 0.5 microl) into the same PAG site, the microinjections of MOR always produced antinociception and did not induce tolerance. If PRO microinjections were suspended, subsequent MOR microinjections induced tolerance. In MOR-tolerant rats, a single PRO microinjection into the same PAG site was enough to restore the antinociceptive effect of MOR. On the other hand, if CCK (1 ng in 0.5 microl) was microinjected into PAG, then MOR microinjection administered 15 min later into the same PAG site did not elicit antinociception. These results show that CCK has anti-opioid activity in PAG and that tolerance to MOR in PAG can be prevented or reversed if CCK receptors are blocked with PRO. Finally, opioid tolerance induced by repeated systemic MOR injections (5mg/kg intraperitoneal ) was reversed by a single microinjection of PRO into PAG. This emphasizes the central importance of PAG in the MOR/CCK interactions that lead to opioid tolerance.

The role of cholecystokinin in conditional compensatory responding and morphine tolerance in rats

As elaborated in the conditioning analysis of tolerance, cues present at the time of drug administration become associated with the drug effect. A particularly salient cue that may become associated with the drug effect is the pharmacological drug-onset cue inherent to drug administration. Drug-associated cues contribute to tolerance by eliciting a conditional compensatory response that attenuates the drug effect. For example, the early drug effect, having been paired with the subsequent larger drug effect, may elicit the release of antiopioid peptides that counter opioid effects. The role of a putative antiopioid peptide, cholecystokinin-8 (CCK), in the associative mechanisms of opiate tolerance was evaluated. The results of these experiments suggest that a CCK2 receptor antagonist attenuates both the expression of opiate tolerance and the conditional compensatory response hypothesized to mediate such tolerance.

Cholecystokinin/opioid interactions

Cholecystokinin (CCK) acts as an anti-opioid peptide. The mechanisms of CCK-opioid interaction under normal and pathological conditions were examined with various techniques. Nerve injury induces upregulation of CCK mRNA and CCK2 receptors in sensory neurons. The involvement of CCK in spinal nociception in normal and axotomized rats was examined. The CCK2 receptor antagonist CI-988 did not reduce spinal hyperexcitability following repetitive C-fiber stimulation in normal or axotomized rats, suggesting that CCK is probably not released from injured primary afferents. With in vivo microdialysis intravenous (i.v.) or intrathecal (i.t.) morphine increased the extracellular level of CCK in the dorsal horn in a naloxone reversible manner. Morphine also released CCK after axotomy, but not during carrageenan-induced inflammation. In contrast, K(+)-stimulation failed to increase extracellular levels of CCK in axotomized rats, but did so in inflamed rats. Double-coloured immunofluorescence technique revealed partial co-localization between CCK-like immunoreactivity (LI) and mu-opioid receptor (MOR)-LI in superficial dorsal horn neurons. The presence of MOR in CCK containing neurons suggests a possible direct influence of opioids on CCK release in the spinal cord. Axotomy, but not inflammation, induced a moderate decrease in CCK- and MOR-LI in the dorsal horn. I.v. morphine further temporarily reduced CCK- and MOR-LIs in axotomized, but not in normal or inflamed, rats. While the effect of morphine on CCK-LI can be interpreted as the result of increased CCK release, the effect on MOR-LI may be related to changes in the microenvironment of the dorsal horn induced by nerve injury.

CCK-B receptor: chemistry, molecular biology, biochemistry and pharmacology

Cholecystokinin (CCK) is a peptide originally discovered in the gastrointestinal tract but also found in high density in the mammalian brain. The C-terminal sulphated octapeptide fragment of cholecystokinin (CCK8) constitutes one of the major neuropeptides in the brain; CCK8 has been shown to be involved in numerous physiological functions such as feeding behavior, central respiratory control and cardiovascular tonus, vigilance states, memory processes, nociception, emotional and motivational responses. CCK8 interacts with nanomolar affinities with two different receptors designated CCK-A and CCK-B. The functional role of CCK and its binding sites in the brain and periphery has been investigated thanks to the development of potent and selective CCK receptor antagonists and agonists. In this review, the strategies followed to design these probes, and their use to study the anatomy of CCK pathways, the neurochemical and pharmacological properties of this peptide and the clinical perspectives offered by manipulation of the CCK system will be reported. The physiological and pathological implication of CCK-B receptor will be confirmed in CCK-B receptor deficient mice obtained by gene targeting (Nagata el al., 1996. Proc. Natl. Acad. Sci. USA 93, 11825-11830). Moreover, CCK receptor gene structure, deletion and mutagenesis experiments, and signal transduction mechanisms will be discussed.

Extracellular cholecystokinin levels in the rat spinal cord following chronic morphine exposure: an in vivo microdialysis study

Conflicting results concerning the issue of whether or not chronic morphine exposure induces an increase in CCK biosynthesis have been found in many CNS sites, including the spinal cord, where CCK activity may contribute to the facilitation of the development of opiate tolerance. The present study was undertaken in order to monitor the extracellular level of CCK under spontaneous and stimulus-evoked release in the spinal cord dorsal horn of drug naive and morphine tolerant rats. Tolerance was induced by implantation of two morphine pellets (2x75 mg) which induced a stable morphine plasma concentration after 48 h post-implantation. The tail-flick test and naloxone precipitated withdrawal were used as indexes of tolerance and dependence to morphine. The effect of morphine-pellet implantation on basal and K+-induced release of CCK-like immunoreactivity (CCK-LI) in the rat dorsal horn were monitored with in vivo microdialysis 96 h after implantation of morphine or placebo pellets, when rats showed tolerance and dependence. Basal CCK levels were below the detection limit of the assay (0.6 pM) in both tolerant and normal animals. K+ (100 mM) in the perfusion medium induced a more than 3-fold increase of the extracellular level of CCK-LI in control animals, and a more than 4-fold increase on CCK-LI in morphine-pellet implanted animals. However, this difference was not significant. In addition, naloxone (2 mg/kg; i.v.), did not induce any change in the extracellular level of CCK in either group. The present study suggests that the modulatory interaction between CCK and opioids in the development of tolerance in the spinal cord may occur without necessarily increasing the extracellular level of CCK. Another possible explanation of the finding is that the microdialysis technique is not sensitive enough to detect differences in unstimulated CCK levels in normal and tolerant animals.

Reversal of tolerance to the antitransit effects of morphine during acute intestinal inflammation in mice

1. The aim of investigation was to establish and compare the reversibility of tolerance to the antitransit effects of morphine by three different procedures: (a) acute inflammation of the gut, (b) lorglumide a cholecystokininA (CCKA) receptor antagonist, or (c) MK-801, an N-methyl-D-aspartate (NMDA) receptor ion channel blocker. The type of interaction between morphine and lorglumide or MK-801 on the inhibition of gastrointestinal transit (GIT) in naive animals was also evaluated. 2. Male Swiss CD-1 mice were implanted with 75 mg of morphine base or placebo pellets. Gastrointestinal transit was assessed with a charcoal meal and results expressed as % inhibition of GIT. Inflammation was induced by the intragastric (p.o.) administration of croton oil (CO), while controls received castor oil (CA) or saline (SS). Morphine was administered by subcutaneous (s.c.) or intracerebroventricular (i.c.v.) injection, to naive and tolerant animals treated with CO, CA or SS. Dose-response curves for s.c. morphine were also performed in naive and tolerant mice receiving 5.2 or 7.4 nmol (s.c.) lorglumide or MK-801, respectively. 3. The ED50 values for inhibition of GIT by s.c. morphine were: 45.9 +/- 2.7 and 250.1 +/- 3.1 nmol in naive and tolerant animals, respectively, demonstrating a five fold decrease in the potency of morphine. In naive animals, inflammation (CO) decreased the ED50 of morphine three times (14.4 +/- 2.2 nmol). However, no tolerance to s.c. morphine (ED50 16.4 +/- 2.6 nmol) was manifested during intestinal inflammation. After i.c.v. administration, a similar degree of tolerance to morphine was observed (4.8 fold decrease in potency). Intestinal inflammation had no effect on the ED50 values of i.c.v. morphine in naive and tolerant animals, showing that reversal of tolerance is related to local mechanism/s. Mean values for intestinal pH were 6.9 +/- 0.04 and 6.2 +/- 0.04 in SS and CO treated mice, respectively. In addition, morphine was 74 times more potent by the i.c.v. than by the s.c. route (naive-SS). 4. Morphine and lorglumide interacted synergistically in naive animals; in addition, the administration of lorglumide reversed tolerance to s.c. morphine. No interaction (additivity) was observed in naive animals when morphine and MK-801 were administered in combination. However, the drug completely reversed tolerance to the antitransit effects of morphine. 5. The present investigation shows that acute inflammation of the gut reverses tolerance to the antitransit effects of s.c. morphine by a peripheral mechanism. Qualitatively similar results were obtained after the administration of lorglumide or MK-801. Our results suggest that a local decrease in pH could play an important role during inflammation, while antagonism of endogenous compensatory systems would explain the reversal of tolerance induced by lorglumide or MK-801.

Association of enkephalin catabolism inhibitors and CCK-B antagonists: a potential use in the management of pain and opioid addiction

The overlapping distribution of opioid and cholecystokinin (CCK) peptides and their receptors (mu and delta opioid receptors; CCK-A and CCK-B receptors) in the central nervous system have led to a large number of studies aimed at clarifying the functional relationships between these two neuropeptides. Most of the pharmacological studies devoted to the role of CCK and enkephalins have been focused on the control of pain. Recently the existence of regulatory mechanisms between both systems have been proposed, and the physiological antagonism between CCK and endogenous opioid systems has been definitely demonstrated by coadministration of CCK-B selective antagonists with RB 101, a systemically active inhibitor, which fully protects enkephalins from their degradation. Several studies have also been done to investigate the functional relationships between both systems in development of opioid side-effects and in behavioral responses. This article will review the experimental pharmacology of association of enkephalin-degrading enzyme inhibitors and CCK-B antagonists to demonstrate the interest of these molecules in the management of both pain and opioid addiction.

Similar decrease in spontaneous morphine abstinence by methadone and RB 101, an inhibitor of enkephalin catabolism

1. The dual inhibitor of enkephalin degrading enzymes, RB 101, is able to block endogenous enkephalin metabolism completely, leading to potent antinociceptive responses potentiated by blockade of CCKB receptors. In this study we have investigated the effects induced by RB 101 given alone, or with the CCKB antagonist, PD-134,308, on a model of spontaneous morphine withdrawal and substitutive maintenance in rats. 2. Animals were chronically treated with morphine for 7 days followed, 36 h after the interruption of drug administration, by a maintenance treatment for 5 days with methadone (2 mg kg-1, i.p.), clonidine (0.025 mg kg-1, i.p.), RB 101 (40 mg kg-1, i.p.), PD-134,308 (3 mg kg-1, i.p.) or a combination of RB 101 plus PD-134,308. Several behavioural observations were made during this period in order to evaluate the acute effects as well as the consequence of chronic maintenance induced on spontaneous withdrawal by the different treatments. 3. Methadone was the most effective compound in decreasing the spontaneous withdrawal syndrome after acute administration. Both, methadone and RB 101 had similar effectiveness in reducing opiate abstinence during the period of substitutive treatment. PD-134,308 did not show any effect when administered alone and did not modify the effect of RB 101. 4. Naloxone (1 mg kg-1, s.c.) failed to precipitate any sign of withdrawal when injected at the end of the chronic maintenance treatment suggesting that, under the present conditions, methadone and RB 101 did not induce significant physical opiate-dependence. 5. The mildness of the side effects induced by chronic RB 101, suggests that systemically active inhibitors of enkephalin catabolism could represent a promising treatment in the maintenance of opiate addicts.

Changes in cholecystokinin mRNA expression after amygdala kindled seizures: an in situ hybridization study

Cholecystokinin (CCK) can be a potent anticonvulsant neuropeptide in certain seizure models. Therefore, we examined whether seizures produced by electrical kindling of the amygdala or electroconvulsive seizures (ECS) would affect the expression of CCK mRNA in rat brain. Following a single kindled seizure, CCK mRNA expression was decreased about 20-58% in the amygdala. In contrast, after multiple consecutive kindled seizures, CCK mRNA expression was increased in the amygdala, cerebral cortex, CA1 pyramidal cell layer of the hippocampus and dentate hilus. A single ECS produced no effect on CCK mRNA expression, but multiple ECS increased expression in the interneurons of the hippocampus 24 h after the last seizure. Since seizures produced by ECS can be anticonvulsant to further ECS or kindled seizures, the CCK increases in the hippocampus may represent a compensatory anticonvulsant adaptation observed in both models. Overall, the kindling-induced alterations in CCK expression appear to be more complex involving multiple brain regions and distinct temporal properties.

Opioid and anti-opioid peptides

The numerous endogenous opioid peptides (beta-endorphin, enkephalins, dynorphins ... ) and the exogenous opioids (such as morphine) exert their effects through the activation of receptors belonging to four main types, mu, delta, kappa and epsilon. Opioidergic neurones and opioid receptors are largely distributed centrally and peripherally. It is thus not surprising that opioids have numerous pharmacological effects and that endogenous opioids are thought to be involved in the physiological control of various functions, among which nociception is particularly emphasized. Some opioid targets may be components of homeostatic systems tending to reduce the effects of opioids. "Anti-opioid" properties have been attributed to various peptides, especially cholecystokinin (CCK), neuropeptide FF (NPFF) and melanocyte inhibiting factor (MIF)-related peptides. In addition, a particular place should be attributed, paradoxically, to opioid peptides themselves among the anti-opioid peptides. These peptides can oppose some of the acute effects of opioids, and a hyperactivation of anti-opioid peptidergic neurones due to the chronic administration of opioids may be involved in the development of opioid tolerance and/or dependence. In fact, CCK, NPFF and the MIF family of peptides have complex properties and can act as opioid-like as well as anti-opioid peptides. Thus, "opioid modulating peptides" would be a better term to designate these peptides, which probably participate, together with the opioid systems, in multiple feed-back loops for the maintenance of homeostasis. "Opioid modulating peptides" have generally been shown to act through the activation of their own receptors. For example, CCK appears to exert its anti-opioid actions mainly through the activation of CCK-B receptors, whereas its opioid-like effects seem to result from the stimulation of CCK-A receptors. However, the partial agonistic properties at opioid receptors of some MIF-related peptides very likely contribute to their ability to modulate the effects of opioids. CCK- and NPFF-related drugs have potential therapeutic interest as adjuncts to opioids for alleviating pain and/or for the treatment of opioid abuse.

Immediate early gene expression during morphine withdrawal

The expression of immediate early genes (IEG)s c-fos, c-jun and zif/268 was studied during naloxone-precipitated morphine withdrawal in various organs of the rat. Dependence was induced over a period of 6 days by a graded regimen of 6-hourly injections. Northern analysis revealed peak expression of all IEGs occurred in the forebrain plus cerebellum at 20 min and at 60 min in the brain stem following morphine withdrawal. Increased levels of c-fos and c-jun mRNA were observed in the spinal cord at 40 min of morphine withdrawal. An increase in c-fos and c-jun but not zif/268 mRNAs was seen in the jejunum between 20 and 60 min. Elevated levels of the IEG protein products in the cerebral cortex, hippocampus, thalamus, cerebellum, brain stem and spinal cord were observed at 60 min following morphine withdrawal. These data emphasize the temporal and spatial variation in IEG expression in different tissues during opiate withdrawal.

Opposite role of CCKA and CCKB receptors in the modulation of endogenous enkephalin antidepressant-like effects

Systemic administration of RB 101, a complete inhibitor of the enkephalin degrading enzymes, has been reported to induce naltrindole-reversed antidepressant-like effects in the conditioned suppression of motility (CSM) test in mice. The selective CCKB antagonist L-365,260 also elicits the same naltrindole-blocked responses on CSM. The aim of this study was therefore to investigate the possible modulation of RB 101 induced behavioral responses by activation or blockade of CCK receptors. Thus, the effects induced by RB 101 administered alone or associated with an ineffective dose of a selective CCKB agonist (BC 264), a CCKB antagonist (L-365,260) or a CCKA antagonist (L-364,718), were evaluated on the CSM in mice. RB 101 alone decreased the stress-induced loss of motility, as previously reported. The antidepressant-like effect of RB 101 was potentiated by L-365,260, and suppressed by BC 264 and to a lesser extent by L-364,718. The facilitatory effect induced by L-365,260 on RB 101 responses was blocked by the delta selective antagonist naltrindole. All these effects occurred only in shocked animals. The present results suggest that the activation of CCKA and CCKB receptors by endogenous CCK, could play an opposite role in the control of behavioral responses induced by endogenous enkephalins. Delta opioid receptors seem to be selectively involved in this interaction.

Cholecystokinin octapeptide (CCK-8) antagonizes morphine analgesia in nucleus accumbens of the rat via the CCK-B receptor

The analgesic effect of systemic morphine (4 mg/kg, s.c.) was antagonized in a dose-dependent manner by cholecystokinin octapeptide (CCK-8) (0.1-0.5 ng) administered bilaterally to the nucleus accumbens of the rat. This effect of CCK-8 could be reversed by devazepide, a CCK-A receptor antagonist, at 50 ng and 200 ng and by L-365,260, a CCK-B receptor antagonist, at 5 ng administered bilaterally to the nucleus accumbens. A marked potentiation of morphine analgesia was achieved by intra-nucleus accumbens injection of 200 ng devazepide or 5 ng L-365,260. Since the effect of L-365,260 in antagonizing the anti-opioid effect of CCK-8 in the nucleus accumbens is 40 times more potent than devazepide, it is suggested that the anti-opioid effect of CCK-8 is mediated by CCK-B receptors. In conclusion, nucleus accumbens is a strategic site where CCK-8 exerts an anti-opioid activity, most probably via the CCK-B receptors.

Mu and delta opioid receptors mediate opposite modulations by morphine of the spinal release of cholecystokinin-like material

The possible modulations by morphine and various opioids of the spinal release of cholecystokinin-like material (CCKLM) evoked by 30 mM K+ was studied in vitro, using slices of the dorsal part of the rat lumbar enlargement superfused with an artificial cerebrospinal fluid. Addition of the mu agonist, DAGO (0.1-10 microM), to the perfusing fluid produced a concentration-dependent decrease in the peptide release, which could be prevented by the preferential mu antagonist, naloxone. Complex modulations were induced by the delta agonist, DTLET, as this drug inhibited CCKLM release when added at 10 nM-3 microM to the perfusing fluid, but enhanced it at 10 microM. Both effects were preventable by the delta antagonists naltrindole and ICI 154129, suggesting that delta receptors, possibly of different subtypes, mediated the inhibition and stimulation by DTLET. Morphine also exerted a biphasic effect, as the alkaloid decreased CCKLM release at 0.01-0.1 microM and enhanced it at 10 microM. Morphine-induced inhibition was preventable by naloxone, whereas its stimulatory effect could be blocked by naltrindole and ICI 154129. Although inactive on its own on CCKLM release, the selective kappa 1 agonist U 50488H (1 microM) prevented the inhibitory effects of both DAGO (10 microM) and morphine (0.1 microM), suggesting the existence of interactions between kappa 1 and mu receptors within the dorsal zone of the rat spinal cord. These data indicate that low concentrations of morphine exert an inhibitory influence on spinal CCKergic neurons that depends on the stimulation of mu opioid receptors. The excitatory influence of 10 microM morphine likely results from the simultaneous stimulation of mu, delta and kappa receptors, as the inhibitory effect of mu receptor stimulation can be masked by that of kappa 1 receptors, allowing only the expression of a delta-dependent excitatory effect similar to that induced by 10 microM DTLET.

Regulation of calbindin-D28K gene expression in response to acute and chronic morphine administration

The effect of acute and chronic morphine administration on calbindin-D28K (calbindin) gene expression has been studied. One group of adult male rats received a single injection of morphine (10 mg/kg, s.c.) or saline and were sacrificed 1 or 4 h later. Another group was injected with escalating doses of morphine sulfate twice daily for 15 days to induce tolerance and physical dependence. Rats were sacrificed 1 h after the last injection. In a third group, the effect of naloxone-precipitated withdrawal on gene expression in morphine-addicted rats was also analyzed 1 h after naloxone (1 mg/kg, i.p.). The cerebellum and remaining brain (minus the cerebellum) were removed, and total RNA was extracted and used for analysis. Calbindin mRNA levels in cerebellum were decreased to 30%-40% control at 1 and 4 h after a single morphine injection. Co-administration of the opiate antagonist, naloxone, reversed the effect of morphine. Tolerance developed to the acute effects in that levels were not altered significantly 1 h after morphine injection in chronically-treated rats. Unlike the cerebellum, calbindin mRNA in the remainder of the brain (minus the cerebellum) was unchanged 1 and 4 h following morphine administration to drug-naive rats, but was increased more than 2-fold compared to controls 1 h after morphine injection in chronically treated animals. Naloxone-precipitated withdrawal caused a small (20%) but significant decrease in calbindin mRNA in the cerebellum, with no change in the brain (minus the cerebellum).(ABSTRACT TRUNCATED AT 250 WORDS)