Mobilization of calcium from intracellular stores as one of the mechanisms underlying the antiopioid effect of cholecystokinin octapeptide

A1-adenosine acute withdrawal response and cholecystokinin-8 induced contractures are regulated by Ca(2+)- and ATP-activated K(+) channels

In isolated guinea-pig ileum (GPI), the A1-adenosine acute withdrawal response is under the control of several neuronal signalling systems, including the μ/κ-opioid and the cannabinoid CB1 systems. It is now well established that after the stimulation of the A1-adenosine system, the indirect activation of both μ/κ-opioid and CB1 systems is prevented by the peptide cholecystokinin-8 (CCk-8). In the present study, we have investigated the involvement of the Ca(2+)/ATP-activated K(+) channels in the regulation of both acute A1-withdrawal and CCk-8-induced contractures in the GPI preparation. Interestingly, we found that: (a) the A1-withdrawal contracture is inhibited by voltage dependent Ca(2+)-activated K(+) channels, Kv, while it is enhanced by the voltage independent Ca(2+)-activated K(+) channels, SKCa; (b) in the presence of CCk-8, the inhibitory effect of the A1 agonist, CPA, on the peptide induced contracture is significantly enhanced by the voltage independent Ca(2+)-activated K(+) channel, SKCa; and (c) the A1-withdrawal contracture precipitated in the presence of CCk-8 is controlled by the ATP-sensitive potassium channels, KATP. Our data suggest, for the first time, that both Ca(2+)- and ATP-activated K(+) channels are involved in the regulation of both A1-withdrawal precipitated and CCk-8 induced contractures.

The NOP receptor involvement in both withdrawal- and CCk-8-induced contracture responses of guinea pig isolated ileum after acute activation of κ-opioid receptor

In isolated guinea-pig ileum (GPI), the κ-opioid acute withdrawal response is under the control of several neuronal signaling systems, including the μ-opioid, the A(1)-adenosine and the CB(1) receptors, which are involved in the inhibitory control of the κ-withdrawal response. After κ-opioid system stimulation, indirect activation of μ-opioid, A(1)-adenosine and CB(1) systems is prevented by the peptide cholecystokinin-8 (CCk-8). In the present study, we have investigated whether the NOP system is also involved in the regulation of the acute κ-withdrawal response. Interestingly, we found that in GPI preparation, the NOP system is not indirectly activated by the κ-opioid receptor stimulation, but instead this system is able by itself to directly regulate the acute κ-withdrawal response. Specifically, our results clearly highlight first the existence of an endogenous tone of the NOP system in GPI, and second that it behaves as a functional anti-opioid system. We also found that, the NOP receptor system is involved in the regulation of the CCk-8-induced contracture intensity, only when in the presence of the κ-opioid receptor stimulation. This effect seems to be regulated by an activation threshold mechanism. In conclusion, the NOP system could act as neuromodulatory system, whose action is strictly related to the modulation of both excitatory and inhibitory neurotransmitters released in GPI enteric nervous system.

Biphasic regulation of the acute μ-withdrawal and CCk-8 contracture responses by the ORL-1 system in guinea pig ileum

The cloning of the opioid-receptor-like receptor (ORL-1) and the identification of the orphaninFQ/nociceptin (OFQ/N) as its endogenous agonist has revealed a new G-protein-coupled receptor signalling system. The structural and functional homology of ORL-1 to the opioid receptor systems has posed a number of challenges in the understanding the often competing physiological responses elicited by these G-protein-coupled receptors. We had previously shown that in guinea pig ileum (GPI), the acute μ-withdrawal response is under the inhibitory control of several systems. Specifically, we found that the exposure to a μ-opioid receptor agonist activates indirectly the κ-opioid, the A(1)-adenosine and the cannabinoid CB(1) systems, that in turn inhibit the withdrawal response. The indirect activation of these systems is prevented by the peptide cholecystokinin-8 (CCk-8). In the present study, we have investigated whether the ORL-1 system is also involved in the regulation of the acute μ-withdrawal response. Interestingly, we found that in GPI preparation, the ORL-1 system is not indirectly activated by the μ-opioid receptor stimulation, but instead the system is able by itself to directly regulate the acute μ-withdrawal response. Moreover, we have demonstrated that the ORL-1 system behaves both as anti-opioid or opioid-like system based on the level of activation. The same behaviour has also been observed in presence of CCk-8. Furthermore, in GPI, the existence of an endogenous tone of the ORL-1 system has been demonstrated. We concluded that the ORL-1 system acts as a neuromodulatory system, whose action is strictly related to the modulation of excitatory neurotrasmitters released in GPI enteric nervous system.

Inhibitory Control of the Acute Mu-Withdrawal Response by Indirectly Activated Adenosine A1 and Kappa-Opioid Systems in the Guinea-Pig Ileum; Reversal by Cholecystokinin

In the isolated guinea-pig ileum (GPI), the acute mu-opioid withdrawal response is inhibited by the kappa-opioid system, indirectly activated by the opioid agonist; yet, other inhibitory mechanisms are probably operating. On the other hand, cholecystokinin (CCK-8) strongly enhances the withdrawal response. In this study, we have shown that the adenosine A1 antagonist 8-cyclopenthyl-1,3-dimethylxantine (CPT) increased the withdrawal response in dermorphin/naloxone (NLX) tests but lacked any effect if the withdrawal tests were carried out in presence of CCK-8. In tissue preparations coming from a same animal both CPT and the kappa-opioid antagonist, nor-binaltorphimine (BNI), increased the intensity of the withdrawal responses; the effects of the two antagonists were additive. The intensity of withdrawal contractile responses in presence of CCK-8 was similar to those obtained in presence of the two antagonists. Tissue preparations tested with dermorphin/CCK-8/NLX and then washed out yielded contractile responses when subsequently challenged with CPT, BNI or BNI+CPT, with a percentage markedly higher than the percentage of the response to NLX challenge. BNI+CPT also increased the intensity of the response to NLX challenge. These data suggest that acute exposure of GPI to dermorphin induces the activation of both the adenosine A1 and kappa-opioid systems, which in turns inhibit the mu-withdrawal response. CCK-8 antagonises the inhibitory effect of the indirectly activated systems.

Cholecystokinin fails to block the spinal inhibitory effects of nociceptin in sham operated and neuropathic rats

Cholecystokinin (CCK) has a number of roles in the central nervous system and can reduce the analgesic effect of activation of mu (micro), delta (delta) and kappa (kappa) opioid receptors. CCK has been proposed to be a major reason for reduced effects of morphine after nerve injury. This study examines if CCK modulates the effect of the Opioid Receptor Like-1 (ORL1) agonist, nociceptin on dorsal horn neurone activity in vivo in the spinal nerve ligation model of neuropathic pain compared with sham-operated and naive rats. In naive and neuropathic rats nociceptin alone inhibited the C-fibre evoked response, post-discharge, wind-up and input, while in sham operated rats nociceptin did not cause any inhibition but by contrast caused a facilitation of post-discharge and wind-up. CCK alone had no significant effect, although did cause slight facilitation in the three groups. In the presence of CCK the inhibitory effect of nocieceptin was blocked in naive animals, but in contrast the inhibitory effect of nociceptin was enhanced by CCK in sham and neuropathic rats. These results emphasize the differences between ORL1 and other opioid receptors. This loss of the inhibitory effect of CCK on nociceptin after nerve injury may be of clinical interest in the treatment of neuropathic pain.

The role of spinal cholecystokinin in chronic pain states

It is well established that cholecystokinin (CCK) reduces the antinociceptive effect of opioids. The level of CCK and CCK receptors, as well as CKK release, exhibits considerable plasticity after nerve injury and inflammation, conditions known to be associated with chronic pain. Such altered CCK release coupled in some situation with changes in CCK receptor levels may underlie the clinical phenomenon of varying opioid sensitivity in different clinical pain conditions. In particular, neuropathic pain after injury to the peripheral and central nervous system does not respond well to opioids, which is likely to be caused by increased activity in the endogenous CCK system. CCK receptor antagonists may thus be useful as analgesics in combination with opioids to treat neuropathic pain.

Neuropeptides in neuropathic and inflammatory pain with special emphasis on cholecystokinin and galanin

Neuropeptides present in primary afferents and the dorsal horn of the spinal cord have an important role in the mediation of nociceptive input under normal conditions. Under pathological conditions, such as chronic inflammation or following peripheral nerve injury, the production of peptides and peptide receptors is dramatically altered, leading to a number of functional consequences. In this review, the role of two neuropeptides that undergo such altered expression under pathological conditions, cholecystokinin (CKK) and galanin, is reviewed.

Non-opioid actions of lamotrigine within the rat dorsal horn after inflammation and neuropathic nerve damage

Some opioid-resistant pain conditions can be alleviated by voltage-dependent Na(+) channel blockers such as lamotrigine. The mu-opioid-receptor agonist morphine can modulate cation entry into cells to affect overall cellular excitability, an effect which can in turn be endogenously antagonised by the neuropeptide cholecystokinin (CCK). However, lamotrigine may also modulate cellular excitability by non-specifically blocking voltage-dependent ion channels. We have looked for interactions of lamotrigine with the opioid/CCK pathway within the spinal dorsal horn, to rule out the possibility that lamotrigine may attenuate nociceptive responses via actions on this pathway. Both lamotrigine and the mu-opioid agonist DAMGO inhibited mustard oil-evoked cell firing by approximately 50% compared with control levels. Co-application of CCK8S reversed DAMGO-, but not lamotrigine-induced inhibition of cell firing and this reversal was prevented with the selective CCK(B) receptor antagonist PD 135158. Although lamotrigine inhibited both brush- and cold-evoked cell firing in neuropathic animals, lamotrigine inhibition of mustard oil-evoked cell firing in the same animals was not significantly greater than that observed in controls. These results suggest that the antinociceptive properties of lamotrigine within the spinal dorsal horn are unlikely to be mediated via interactions with the opioid/CCK pathway.

The role of spinal cholecystokinin B receptors in thermal allodynia and hyperalgesia in diabetic mice

We examined the tail-flick response to various heat intensities in diabetic and non-diabetic mice. Heat intensities were set to one of six values by adjusting the source of voltage for a 50-W projection bulb to 20, 25, 35, 50, 65 and 80 V. Tail-flick latencies at source voltages of 35 and 50 V in diabetic mice were significantly shorter than those in non-diabetic mice. However, tail-flick latencies at 25, 65 and 80 V in diabetic mice were not significantly altered. Although tail-flick latencies in non-diabetic mice were not affected by i.t. pre-treatment with CI-988, a selective cholecystokinin B (CCK(B)) receptor antagonist, those at 35 and 50 V in diabetic mice were significantly increased. In non-diabetic mice, i.t. pre-treatment with cholecystokinin octapeptide (CCK-8), at a dose of 0.3 ng, decreased tail-flick latencies at 35 and 50 V. Furthermore, the attenuation of tail-flick latencies induced by i.t. pre-treatment with CCK-8 in non-diabetic mice was reversed by i.t. pre-treatment with CI-988. Protein kinase C (PKC) activator phorbol-12, 13-dibutyrate (PDBu)-induced reduction in the tail-flick latencies at heat intensities of 35 and 50 V in non-diabetic mice was dose-dependently and significantly reversed by i.t. pre-treatment with CI-988. On the other hand, the CCK-8-induced thermal hyperalgesia and allodynia at heat intensities of 35 and 50 V in non-diabetic mice were inhibited when PKC activity was inhibited by i.t. pre-treatment with calphostin C. These results indicate that the thermal allodynia and hyperalgesia in diabetic mice may be due, at least in part, to the activation of CCK(B) receptors followed by the activation of PKC in the spinal cord.

Synergistic effect of cholecystokinin octapeptide and angiotensin II in reversal of morphine induced analgesia in rats

The aim of this paper is to study the synergistic anti-analgesic effect of angiotensin II (Ang II) plus cholecystokinin octapeptide (CCK-8). Our previous studies have shown that both CCK-8 and Ang II are potent anti-opioid substances. Intracerebroventricular (i.c.v.) injection of CCK-8 or Ang II dose-dependently antagonizes morphine-induced analgesia (MIA). In the present study, we observed the combined effect of CCK-8 and Ang II in antagonizing MIA. CCK-8 and Ang II were injected intracerebroventricularly to rats in various proportions and doses. The results were analyzed with isobolographic analysis. Combined injection of CCK-8 and Ang II in a ratio of 1 ng: 2.5 microg or 1 ng: 5 microg produced significantly greater effect in antagonizing MIA. The ED(50) of the two ratios are only 18.5% and 27.5%, respectively, of the theoretical dose of simple addition. We conclude that CCK-8 and Ang II used in such dose ratios may antagonize MIA synergistically.

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.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

CI 988, an antagonist of the cholecystokinin-B receptor, potentiates endogenous opioid-mediated antinociception at spinal level

The effects of RB 101 {N-[(R, S)-2-benzyl-3[(S)(2-amino-4-methylthio)butyl dithio]-1-oxo-propyl]-L-phenylalanine benzyl ester}, a complete inhibitor of enkephalin-degrading enzymes and CI 988, a selective antagonist of the cholecystokinin (CCK)-B receptors, on the flexor reflex in decerebrate, spinalized, unanaesthetized rats were assessed. Intravenous RB 101 induced a dose-dependent depression of the flexor reflex with a threshold dose of 20 mg/kg and an ED50 of 25.3 mg/kg. Subcutaneous CI 988 at 1 mg/kg, which by itself did not influence the flexor reflex, strongly enhanced the reflex depressive effect of RB 101. The dose-response curve for RB 101 was shifted to the left and the duration of reflex depression was significantly prolonged. The results confirmed and extended previous behavioural data indicating that blockade of CCK-B receptors potentiated antinociception elicited by endogenous opioids protected from enzymatic degradation. Furthermore, the spinal cord is an important site of interaction between the endogenous opioid and CCK systems.

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.

Spinal opioid systems in inflammation

Until recently, basic science studies, both behavioural and electrophysiological, have concentrated on the antinociceptive actions of opioids primarily gauged against acute nociceptive responses. However, of more relevance to clinical situations are the actions of opioids in more persistent/prolonged pain states. This review sets out to examine the central actions of opioids against nociception of inflammatory origins. The first section deals with the response of the endogenous opioid system to the development of an inflammatory state and the second examines the ability of exogenous opioids to modulate inflammatory nociception. There are complex changes in the roles of endogenous opioids, in particular dynorphin, at the spinal level after inflammation although the physiological consequences remain unclear. With regard to exogenous opioids, the effectiveness of spinal morphine is rapidly enhanced after inflammation, likely to be due to changes in the interaction between the peptide cholecystokinin and the mu opioid receptor. The ability of inflammatory processes to alter both endogenous opioids and morphine analgesia at the spinal level illustrates the considerable degree of plasticity observed in opioid function.

Differential modulation of alpha 2-adrenergic and opioid spinal antinociception by cholecystokinin and cholecystokinin antagonists in the rat dorsal horn: an electrophysiological study

Cholecystokinin (CCK) has been shown to reduce the spinal antinociceptive effects of opioid agonists such as morphine. The present study examined the effect of CCK and CCKB antagonists on the spinal antinociception mediated by the selective alpha 2-adrenergic agonist dexmedetomidine. Extracellular recordings of noxious-evoked C fibre responses of dorsal horn convergent neurones were made in the halothane-anaesthetized rat. Alone, intrathecal dexmedetomidine (5 micrograms) profoundly inhibited C fibre-evoked responses (92 +/- 7%). In the presence of 1 microgram intrathecal CCK the antinociceptive effect of dexmedetomidine was reduced to 27 +/- 11%. Inhibitions of C fibre-evoked responses mediated by submaximal doses (0.5 and 2.5 micrograms) dexmedetomidine were not altered by CCKB antagonists L365,260 (0.2 mg/kg subcutaneous) or PD135158 (10 micrograms intrathecal). Both CCKB antagonists did increase the inhibition of C fibre-evoked responses by the mu opioid agonists DAGOL and morphine. The results suggest CCK is able to inhibit spinal antinociception mediated via the activation of alpha 2-adrenergic receptors in addition to its well-documented interaction with spinal opioid analgesia. However the antagonist studies indicate an endogenous CCK control of spinal mu opioid mediated antinociception which does not extend to alpha 2-adrenergic antinociception.

Brain cholecystokinin and beta-endorphin systems may antagonistically interact to regulate tissue DNA synthesis in rat pups

Previously, we have shown that intracisternal (i.c.) administration of beta-endorphin suppresses brain and liver DNA synthesis in rat pups. This finding is consistent with the view that endogenous CNS beta-endorphin plays an important role in controlling postnatal growth. Recent evidence suggests that brain CCK8, the sulfated carboxyterminal octapeptide fragment of cholecystokinin, may function physiologically as an endogenous opioid antagonist. We now report that CCK8 injected i.c. together with beta-endorphin effectively prevented beta-endorphin from inhibiting brain and liver DNA synthesis in 10-day-old rats. CCK8 blocked the liver DNA effect of beta-endorphin via actions within the brain, as subcutaneous administration of CCK8 was ineffective. In contrast to CCK8, i.c. administration of CCK8U (the unsulfated form of CCK8) together with beta-endorphin did not prevent beta-endorphin from inhibiting liver DNA synthesis, and only slightly reversed the brain DNA effect. The results obtained support a role for endogenous brain CCK8 in the modulation of tissue DNA responses to CNS beta-endorphin and possibly to other endogenous opioids. If so, interference with brain CCK function could disrupt tissue growth. Thus, normal mammalian development may require a close functional interaction between the cholecystokinin and beta-endorphin systems in the brain.

CNS effects of peptides: a cross-listing of peptides and their central actions published in the journal Peptides, 1986-1993

The centrally mediated effects of peptides as published in the journal Peptides from 1986 to 1993 are tabulated in two ways. In one table, the peptides are listed alphabetically. In another table, the effects are arranged alphabetically. Most of the effects observed after administration of peptides are grouped, wherever possible, into categories such as cardiovascular and gastrointestinal. The species used in most cases has been rats; where other animals were used, the species is noted. The route of administration of peptides and source of information also are included in the tables, with a complete listing provided at the end. Many peptides have been shown to exert a large number of centrally mediated effects.