Delta-opioid receptor-mediated increase in cortical extracellular levels of cholecystokinin-like material by subchronic morphine in rats

Expression and Distribution of Neuropeptide-Expressing Cells Throughout the Rodent Paraventricular Nucleus of the Thalamus

The paraventricular nucleus of the thalamus (PVT) has been shown to make significant contributions to affective and motivated behavior, but a comprehensive description of the neurochemicals expressed in the cells of this brain region has never been presented. While the PVT is believed to be composed of projection neurons that primarily use as their neurotransmitter the excitatory amino acid, glutamate, several neuropeptides have also been described in this brain region. In this review article, we combine published literature with our observations from the Allen Brain Atlas to describe in detail the expression and distribution of neuropeptides in cells throughout the mouse and rat PVT, with a special focus on neuropeptides known to be involved in behavior. Several themes emerge from this investigation. First, while the majority of neuropeptides are expressed across the antero-posterior axis of the PVT, they generally exist in a gradient, in which expression is most dense but not exclusive in either the anterior or posterior PVT, although other neuropeptides display somewhat more equal expression in the anterior and posterior PVT but have reduced expression in the middle PVT. Second, we find overall that neuropeptides involved in arousal are more highly expressed in the anterior PVT, those involved in depression-like behavior are more highly expressed in the posterior PVT, and those involved in reward are more highly expressed in the medial PVT, while those involved in the intake of food and drugs of abuse are distributed throughout the PVT. Third, the pattern and content of neuropeptide expression in mice and rats appear not to be identical, and many neuropeptides found in the mouse PVT have not yet been demonstrated in the rat. Thus, while significantly more work is required to uncover the expression patterns and specific roles of individual neuropeptides in the PVT, the evidence thus far supports the existence of a diverse yet highly organized system of neuropeptides in this nucleus. Determined in part by their location within the PVT and their network of projections, the function of the neuropeptides in this system likely involves intricate coordination to influence both affective and motivated behavior.

Involvement of the N/OFQ-NOP system in rat morphine antinociceptive tolerance: Are astrocytes the crossroad?

The development of tolerance to the antinociceptive effect is a main problem associated with the repeated administration of opioids. The progressively higher doses required to relieve pain reduce safety and exacerbate the side effects of classical opioid receptor agonists like morphine. Nociceptin/orphanin FQ (N/OFQ) and its NOP receptor constitute the fourth endogenous opioid system that is involved in the control of broad spectrum of biological functions, including pain transmission. Aim of this work was to evaluate the relevance of the N/OFQ-NOP system in morphine antinociceptive action and in the development of morphine tolerance in the rat. Continuous spinal intrathecal infusion of morphine (1-3 nmol/h) evoked analgesic effects for 5 days in wild type animals. The same doses infused in NOP(-/-) rats showed a lower analgesic efficacy, while the onset of tolerance was delayed to day 9. N/OFQ (1-3 nmol/h), continuously infused in NOP(+/+) animals, showed an analgesic profile similar to morphine. Immunohistochemical analysis of the dorsal horn of the spinal cord of morphine tolerant NOP(+/+) rats showed an increased number of Iba1- and GFAP-positive cells (microglia and astrocytes, respectively). Interestingly, microglia but not astrocyte activation was observed in NOP(-/-) morphine tolerant rat. A selective activation of astrocytes was observed in the dorsal horn of wild type N/OFQ tolerant rats. The antinociceptive effect of morphine partially depends by the N/OFQ-NOP system that participates in the development of morphine tolerance. In particular, NOP receptors are involved in morphine-induced astrocyte activation, and N/OFQ per se increases astrocyte density.

Lack of Analgesic Synergy of the Cholecystokinin Receptor Antagonist Proglumide and Spinal Cord Stimulation for the Treatment of Neuropathic Pain in Rats

OBJECTIVE

Neuropathic pain is difficult to manage and treat. Spinal cord stimulation (SCS) has become an established procedure for treating chronic neuropathic pain that is refractory to pharmacological therapy. In order to achieve better analgesia, a number of studies have evaluated the effectiveness of combining drug therapy with SCS. Cholecystokinin antagonists, such as proglumide, enhance the analgesic efficacy of endogenous opioids in animal models of pain. We previously reported that both systemic and spinal administration of proglumide enhances analgesia produced by both low- and high-frequency transcutaneous electrical nerve stimulation (TENS). Since SCS produces analgesia through endogenous opioids, we hypothesized that the analgesic effect of SCS would be enhanced through co-administration with proglumide in animals with neuropathic pain.

MATERIALS AND METHODS

Male Sprague-Dawley rats (n = 40) with spared nerve injury were given proglumide (20 mg/kg, i.p.) or saline prior to treatment with SCS (sham, 4 Hz, and 60 Hz). Mechanical withdrawal thresholds of the paw were measured before and after induction of nerve injury, and after SCS. Physical activity levels were measured after SCS.

RESULTS

Both proglumide and SCS when given independently significantly increased withdrawal thresholds two weeks after nerve injury. However, there was no additional effect of combining proglumide and SCS on mechanical withdrawal thresholds or activity levels in animals with nerve injury.

DISCUSSION AND CONCLUSIONS

Proglumide may be a candidate for achieving analgesia for patients with refractory neuropathic pain conditions, but does not enhance analgesia produced by SCS.

Effects of exogenous cholecystokinin octapeptide on acquisition of naloxone precipitated withdrawal induced conditioned place aversion in rats

Cholecystokinin octapeptide (CCK-8), a gut-brain peptide, regulates a variety of physiological behavioral processes. Previously, we reported that exogenous CCK-8 attenuated morphine-induced conditioned place preference, but the possible effects of CCK-8 on aversively motivated drug seeking remained unclear. To investigate the effects of endogenous and exogenous CCK on negative components of morphine withdrawal, we evaluated the effects of CCK receptor antagonists and CCK-8 on the naloxone-precipitated withdrawal-induced conditioned place aversion (CPA). The results showed that CCK2 receptor antagonist (LY-288,513, 10 µg, i.c.v.), but not CCK1 receptor antagonist (L-364,718, 10 µg, i.c.v.), inhibited the acquisition of CPA when given prior to naloxone (0.3 mg/kg) administration in morphine-dependent rats. Similarly, CCK-8 (0.1–1 µg, i.c.v.) significantly attenuated naloxone-precipitated withdrawal-induced CPA, and this inhibitory function was blocked by co-injection with L-364,718. Microinjection of L-364,718, LY-288,513 or CCK-8 to saline pretreated rats produced neither a conditioned preference nor aversion, and the induction of CPA by CCK-8 itself after morphine pretreatments was not significant. Our study identifies a different role of CCK1 and CCK2 receptors in negative affective components of morphine abstinence and an inhibitory effect of exogenous CCK-8 on naloxone-precipitated withdrawal-induced CPA via CCK1 receptor.

Cholecystokinin receptor-1 mediates the inhibitory effects of exogenous cholecystokinin octapeptide on cellular morphine dependence

Background

Cholecystokinin octapeptide (CCK-8), the most potent endogenous anti-opioid peptide, has been shown to regulate the processes of morphine dependence. In our previous study, we found that exogenous CCK-8 attenuated naloxone induced withdrawal symptoms. To investigate the precise effect of exogenous CCK-8 and the role of cholecystokinin (CCK) 1 and/or 2 receptors in morphine dependence, a SH-SY5Y cell model was employed, in which the μ-opioid receptor, CCK1/2 receptors, and endogenous CCK are co-expressed.

Results

Forty-eight hours after treating SH-SY5Y cells with morphine (10 μM), naloxone (10 μM) induced a cAMP overshoot, indicating that cellular morphine dependence had been induced. The CCK receptor and endogenous CCK were up-regulated after chronic morphine exposure. The CCK2 receptor antagonist (LY-288,513) at 1–10 μM inhibited the naloxone-precipitated cAMP overshoot, but the CCK1 receptor antagonist (L-364,718) did not. Interestingly, CCK-8 (0.1-1 μM), a strong CCK receptor agonist, dose-dependently inhibited the naloxone-precipitated cAMP overshoot in SH-SY5Y cells when co-pretreated with morphine. The L-364,718 significantly blocked the inhibitory effect of exogenous CCK-8 on the cAMP overshoot at 1–10 μM, while the LY-288,513 did not. Therefore, the CCK2 receptor appears to be necessary for low concentrations of endogenous CCK to potentiate morphine dependence in SH-SY5Y cells. An additional inhibitory effect of CCK-8 at higher concentrations appears to involve the CCK1 receptor.

Conclusions

This study reveals the difference between exogenous CCK-8 and endogenous CCK effects on the development of morphine dependence, and provides the first evidence for the participation of the CCK1 receptor in the inhibitory effects of exogenous CCK-8 on morphine dependence.

The effects of exogenous CCK-8 on the acquisition and expression of morphine-induced CPP

Cholecystokinin octapeptide (CCK-8) is the most potent endogenous anti-opioid peptide and regulates a variety of physiological processes. In our previous study, we found that exogenous CCK-8 attenuated naloxone-induced withdrawal symptoms, but the possible regulative effects of CCK-8 on the rewarding effects of morphine were not examined. In the present study, we aimed to determine the exact effects of exogenous CCK-8 at various doses on the rewarding action of morphine by utilizing the unbiased conditioned place preference (CPP) paradigm. We therefore examined the effects of CCK-8 on the acquisition, expression and extinction of morphine-induced CPP and on locomotor activity. The results showed that CCK-8 (0.01-1μg, i.c.v.), administered alone, induced neither CPP nor place aversion, but blocked the acquisition of CPP when administered with 10mg/kg morphine. The highest dose of CCK-8 (1μg) administered before CPP testing increased CPP and, along with lower doses (0.1μg), reduced its extinction. In addition, the highest dose (1μg) of CCK-8 suppressed locomotor activity. Our study provides the first behavioral evidence for the inhibitory effects of exogenous CCK-8 on rewarding activity and reveals significant effects of exogenous CCK-8 on various stages of place preference and the development of opioid dependence.

Novel peptide ligands with dual acting pharmacophores designed for the pathophysiology of neuropathic pain

The conventional design of high affinity drugs targeted to a single molecule has not resulted in clinically useful therapies for pain relief. Recent reviews have suggested that newly designed analgesic drugs should incorporate multiple targets. The distributions of cholecystokinin (CCK) and CCK receptors in the central nervous system (CNS) overlap significantly with endogenous opioid systems and can be dually targeted. CCK has been shown to act as an endogenous "anti-analgesic" peptide and neuropathic pain conditions promote endogenous CCK release in CNS regions of pain modulation. Administration of CCK into nuclei of the rostral ventromedial medulla induces pronociceptive behaviors in rats. RSA 504 and RSA 601 are novel bifunctional compounds developed to target neuropathic pain by simultaneously acting as agonists at two distinct opioid receptors and antagonizing CCK receptors in the CNS. RSA 504 and RSA 601 demonstrate agonist activity in vitro and antihypersensitivity to mechanical and thermal stimuli in vivo using the spinal nerve ligation model of neuropathic pain. Intrathecal administration of RSA 504 and RSA 601 did not demonstrate antinociceptive tolerance over 7 days of administration and did not display motor impairment or sedation using a rotarod. These are the first behavioral studies that demonstrate how multi-targeted molecule design can address the pathology of neuropathic pain. These compounds with δ and μ opioid agonist activity and CCK antagonist activity within one molecule offer a novel approach with efficacy for neuropathic pain while lacking the side effects typically caused by conventional opioid therapies.

Arthritis-induced increase in cholecystokinin release in the rat anterior cingulate cortex is reversed by diclofenac

Given a hypothesised role for CCK in the anterior cingulate cortex (ACC) for the sensation of pain, the aim of the present study was to investigate whether the increased CCK release could be affected by two different analgesic drugs, morphine and the non-selective cyclooxygenase inhibitor diclofenac. Since opioids stimulate CCK release in other CNS regions we have also studied the effect of morphine by itself on the CCK-LI release in the ACC of non-arthritic rats. Three to seven hours after intraarticular carrageenan injection, at the time when the animals are known to show pain-related behaviour, in vivo microdialysis in awake rats revealed increased CCK-LI release in the ACC. The CCK-LI release was significantly attenuated by diclofenac (25 mg/kg i.m.), but not by morphine (10 mg/kg s.c.). Neither diclofenac (25 mg/kg i.m.) nor morphine (5 or 10 mg/kg s.c.) affected the CCK-LI release in the ACC in non-arthritic rats. The results obtained with diclofenac indicate that prostaglandins contribute to the increased CCK-LI release in the ACC during monoarthritis. However, the lack of effect of morphine suggests that the CCK release in the ACC is not directly related to the sensation of pain. Further on, the failure of morphine to affect the extracellular level of CCK-LI in the ACC in control animals as well as in animals with carrageenan-induced monoarthritis is in contrast to previous studies on the frontal cortex or the dorsal horn of the spinal cord, in which similar doses of morphine stimulate CCK release. Thus, compared to these regions, CCK release may be differently regulated in the ACC.

Increased cholecystokinin release in the rat anterior cingulate cortex during carrageenan-induced arthritis

Several human and animal studies indicate that the anterior cingulate cortex (ACC) plays an important role in the affective component of pain. The neuropeptide cholecystokinin (CCK) is especially abundant in the ACC. CCK has been suggested to be involved in the mediation of anxiety and in the modulation of opioid effects in the spinal cord and medulla oblongata. However, its possible role in pain transmission or modulation in the brain is far less clear. In this study, a model of subchronic inflammatory pain in rats, carrageenan-induced monoarthritis, was used to study the effect of pain on the release of CCK-like immunoreactivity (CCK-LI) in the ACC. Pain-related behaviour quantified by weight bearing and stance scoring, as well as inflammation measured by ankle oedema, was increased for at least 24 h after carrageenan injection with a maximum at 5 h. Using microdialysis in freely moving rats, extracellular concentrations of CCK-LI was measured in the ACC during a time period when the animals showed significant pain behaviour. In animals with carrageenan-induced arthritis, both basal and potassium-evoked release of CCK-LI were significantly increased compared to controls. HPLC analysis of dialysates from the ACC during potassium stimulation showed that the main part of the immunoreactive material was sulphated CCK-8. Because CCK has been implicated in anxiety, we suggest that an altered CCK-ergic activity in the ACC may be of importance for the affective component of pain, but an involvement in the modulation of nociception is also possible.

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

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.

Effects of morphine administration and its withdrawal on rat brain aminopeptidase activities

The endogenous opioid neuropeptide system seems to be involved in the neural processes which underlie drug addiction. Several studies have reported that the administration of morphine induces changes in the levels and/or activity of endogenous opioid peptides (enkephalin, dynorphin) and their precursors in specific brain regions of the adult CNS. The aim of this work was to study the effects of chronic morphine exposure and its withdrawal on certain aminopeptidases capable of degrading opioid peptides in brain areas including the amygdala, hypothalamus, hippocampus, striatum and brain cortices. In animals treated with morphine, aminopeptidase N presented higher enzyme activity levels in the striatum, the hypothalamus and the amygdala compared to control animals, although statistically significant differences were observed only in the case of the striatum. In addition, the activity of soluble puromycin-sensitive aminopeptidase (PSA) was found to be higher in the frontal cortex of these rats. In contrast, rats experiencing withdrawal symptoms presented decreased levels of aminopeptidase activity in certain brain areas. Thus, the activity of aminopeptidase N in the hippocampus and soluble puromycin-sensitive aminopeptidase in the frontal cortex were found to be lower in rats experiencing naloxone precipitated withdrawal symptoms, compared to the corresponding controls. Finally, the activity of the three studied aminopeptidases in vitro was unaltered by incubation with morphine, suggesting that the observed effects are not due to a direct action of this opioid upon the aminopeptidases. The results of the present report indicate that aminopeptidases may play an important role in the processes of tolerance and withdrawal associated with morphine administration.

Chronic morphine treatment modulates the extracellular levels of endogenous enkephalins in rat brain structures involved in opiate dependence: a microdialysis study

The endogenous opioid system is often assumed to play a role in vulnerability to drug abuse. However, controversial results have been reported regarding the levels of enkephalins or preproenkephalin in neurons of rodent brains after opiate administration. The present study was performed to determine the extracellular levels of enkephalins and its physiological antagonist cholecystokinin (CCK), using in vivo microdialysis in freely moving rats after morphine-induced physical dependence or positive place conditioning. A large increase (340%) of Met-enkephalin was observed in the periaqueductal gray matter, a structure involved in morphine withdrawal syndrome, in morphine-dependent rats. No change in CCK immunoreactivity occurred in these conditions. Moreover, using the conditioning place preference paradigm, we observed for the first time opposite changes of enkephalin outflow in the nucleus accumbens (NAc). Thus, an increase in enkephalin levels was observed in rats placed in the drug-associated compartment and a decrease in the saline-paired side. These changes in opioid peptides in the NAc may reflect an "emotional state" of the animals in relation to the expectation of drug reward (reinforcing effects of morphine). Moreover, the lack of regulation in CCK outflow suggests that CCK-opioid interactions in morphine dependence involve probably post-receptor events.

Orphanin FQ/nociceptin attenuates the development of morphine tolerance in rats

1. Recent evidence from studies in mice lacking the opioid receptor-like (ORL-1) receptor and from experiments using antibodies raised against orphanin FQ/nociceptin (OFQ/N) suggest that this peptide may be involved in morphine tolerance. In the present study we sought to investigate if administration of exogenous OFQ/N would modulate the development of tolerance to the antinociceptive effect of morphine. 2. Rats were treated for 3 days with either saline or morphine (10 mg kg(-1), s.c.) followed, 15 and 75 min later, by two intracerebroventricular injections of either artificial cerebrospinal fluid (aCSF) or OFQ/N. The dose of OFQ/N was doubled each day (7.5, 15, 30 nmol). On day 4, rats were tested on a hot plate apparatus before and 30, 60 and 90 min after morphine administration. 3. Repeated OFQ/N treatment did not affect basal nociceptive responses or morphine-induced antinociception. However, the same treatment significantly attenuated the development of morphine tolerance. 4. Since learning and memory could contribute to the development of morphine tolerance, in subsequent studies, we examined the effect of OFQ/N administered in the CA3 region of the hippocampus, where OFQ/N has been shown to block LTP and impair spatial memory. A greater attenuation of morphine tolerance with no alteration of baseline hot plate latency or morphine-induced antinociception was observed when OFQ/N was administered in this area of the rat brain. 5. Taken together, our results demonstrate that OFQ/N may act in the hippocampus to attenuate morphine tolerance.