Differential inhibitory/stimulatory modulation of spinal CCK release by mu and delta opioid agonists, and selective blockade of mu-dependent inhibition by kappa receptor stimulation

Why mu-opioid agonists have less analgesic efficacy in neuropathic pain?

Injury to peripheral nerves often leads to abnormal pain states (hyperalgesia, allodynia and spontaneous pain), which can remain long after the injury heals. Although opioid agonists remain the gold standard for the treatment of moderate to severe pain, they show reduced efficacy against neuropathic pain. In addition to analgesia, opioid use is also associated with hyperalgesia and analgesia tolerance, whose underlying mechanisms share some commonalities with nerve injury-induced hypersensitivity. Here, we reviewed up-to-day research exploring the contribution of mu-opioid receptor (MOR) on the pathophysiology of neuropathic pain and on analgesic opioid actions under these conditions. We focused on the specific contributions of MOR populations at peripheral, spinal and supraspinal level. Moreover, evidences of neuroplastic changes that may underlie the low efficacy of MOR agonists under neuropathic pain conditions are reviewed and discussed. Sensitization processes leading to pain hypersensitivity, molecular changes in signalling pathways triggered by MOR and glial activation are some of these mechanisms elicited by both nerve injury and opioid exposure. Nerve injury-induced pain hypersensitivity might be masking the initial analgesic effects of opioid agonists, and alternatively, sustained opioid treatment to individuals already suffering from neuropathic pain could aggravate their pathophysiological state. Finally, some combined therapies that can increase opioid analgesic effectiveness in neuropathic pain treatment are highlighted. SIGNIFICANCE: This review provides evidence of the low benefit of opioid monotherapy in neuropathic pain and analyses the reasons of this reduced effectiveness. Opioid agonists along with drugs targeted to block the sensitization processes induced by MOR stimulation might result in a better management of neuropathic pain.

The impact of opioid-induced hyperalgesia for postoperative pain

Clinical evidence suggests that--besides their well known analgesic activity - opioids can increase rather than decrease sensitivity to noxious stimuli. Based on the observation that opioids can activate pain inhibitory and pain facilitatory systems, this pain hypersensitivity has been attributed to a relative predominance of pronociceptive mechanisms. Acute receptor desensitization via uncoupling of the receptor from G-proteins, upregulation of the cAMP pathway, activation of the N-methyl-D-aspartate (NMDA)-receptor system, as well as descending facilitation, have been proposed as potential mechanisms underlying opioid-induced hyperalgesia. Numerous reports exist demonstrating that opioid-induced hyperalgesia is observed both in animal and human experimental models. Brief exposures to micro-receptor agonists induce long-lasting hyperalgesic effects for days in rodents, and also in humans large-doses of intraoperative micro-receptor agonists were found to increase postoperative pain and morphine consumption. Furthermore, the prolonged use of opioids in patients is often associated with a requirement for increasing doses and the development of abnormal pain. Successful strategies that may decrease or prevent opioid-induced hyperalgesia include the concomitant administration of drugs like NMDA-antagonists, alpha2-agonists, or non-steroidal anti-inflammatory drugs (NSAIDs), opioid rotation or combinations of opioids with different receptor/selectivity.

Stimulation of δ-Opioid Receptors Reduces the In Vivo Binding of the Cholecystokinin (CCK)-B-Selective Agonist [3H]pBC 264: Evidence for a Physiological Regulation of CCKergic Systems by Endogenous Enkephalins

Cholecystokinin (CCK) and enkephalins appear to be colocalized in several brain structures, and a physiological interaction between these peptides has been suggested by a large number of pharmacological studies. In this work we have shown, by in vivo binding experiments, that the endogenous enkephalins, protected from degrading enzymes by mixed inhibitors such as kelatorphan and N-[(R,S)-2-benzyl-3-[(S)-2-amino-4-methylthiobutyldithio]-1-oxo pro pyl]- L-phenylalanine benzyl ester (RB 101), a systemically active prodrug, modulate CCK release in mouse brain, leading to an overall increase in the extracellular levels of CCK. This was quantified by measuring the effects of both inhibitors on the in vivo binding of [3H]propionyl-Tyr(SO3H)-gNle-mGly-Trp-(N-Me)Nle-Asp-Phe-NH2 ([3H]pBC 264), a selective and highly potent CCK-B agonist. Thus, intracerebroventricular injection of kelatorphan produced a dose-dependent inhibition of the in vivo binding of [3H]pBC 264 with a maximal effect (40%) at 50 nmol. A similar response was observed after intravenous injection of RB 101 (40 mg/kg). The specific binding of [3H]pBC 264 was also inhibited (25%) by intravenous injection of the selective delta-opioid agonist H-Tyr-D-Cys(StBu)-Gly-Phe-Leu-Thr(OtBu)-OH (BUBUC; 2 mg/kg) but not by the mu-agonist H-Tyr-D-Ala-Gly-(N-Me)Phe-Gly-ol (5 mg/kg), suggesting a preferential involvement of delta-opioid receptors in the modulation of CCK release. This was confirmed by using the selective delta-opioid antagonist naltrindole, which prevented the inhibitory effects of BUBUC and of enkephalin-degrading enzyme inhibitors on [3H]pBC 264 binding.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Modulation of visceral hyperalgesia by morphine and cholecystokinin from the rat rostroventral medial medulla

Using a model of visceral nociception, we examined whether cholecystokinin (CCK) acts as an anti-opioid peptide in the rat rostral ventromedial medulla (RVM). Because such interaction may be affected by inflammation, rats with and without inflamed colons were studied. The visceromotor response to noxious colorectal distension (CRD), quantified electromyographically, was recorded before and after intra-RVM administration of CCK, CCK receptor antagonists, and morphine. Either 50% ethanol/saline (vehicle) or 2,4,6-trinitrobenzenesulfonic acid (TNBS), which inflames the colon, was instilled into the colon 5 days before experiments. Intra-RVM morphine dose-dependently attenuated responses to CRD in intracolonic vehicle-treated rats. In TNBS-treated rats with inflamed colons, responses to CRD were significantly increased and 0.3, 3.0 and 6.0 microg doses of intra-RVM morphine reduced responses to control (i.e. were anti-hyperalgesic); the greatest dose tested (30 microg) further reduced responses to 40% control. In intracolonic vehicle-treated rats, intra-RVM pre-treatment with a selective CCK(B) (but not CCK(A)) receptor antagonist dose-dependently and significantly enhanced the effect of a low dose of morphine. Intra-RVM CCK-8 peptide enhanced responses to CRD in intracolonic vehicle-treated, but not TNBS-treated rats. Intra-RVM naloxone was without effect in intracolonic vehicle-or TNBS-treated rats, suggesting an absence of tonic opioid activity in RVM. These results document a CCK-opioid interaction in RVM, suggesting that colon inflammation leads to tonic activity at CCK(B) receptors in RVM.

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.

Estrogen and endogenous opioids regulate CCK in reproductive circuits

This review focuses on the interaction of estrogen with the cholecystokinin (CCK) and endogenous opioid peptide systems in the medial preoptic nucleus, and how these interactions result in alterations of a stereotypic female reproductive behavior--lordosis. The medial preoptic nucleus is an integral part of a circuit controlling lordosis that extends from the limbic system through the hypothalamus. Estrogen alters the integration of sensory information in the circuit that results in the display of sexually receptive behavior. Estrogen determines the activity of CCK and endogenous opioid peptide systems through regulation of expression, release and interaction with specific receptors. Studies of each system individually have indicated that they are pivotal to the expression of lordosis. Recent studies demonstrate an estrogen-dependent interaction between endogenous opioid and CCK systems that control reproductive behavior.

Morphine-induced in vivo release of spinal cholecystokinin is mediated by delta-opioid receptors--effect of peripheral axotomy

Morphine and other opioid agonists induce spinal in vivo release of cholecystokinin (CCK), a neuropeptide with anti-opioid properties. However, so far the opioid receptor subtype responsible for this effect has not been determined. In the present in vivo microdialysis study, the morphine-induced release of cholecystokinin-like immunoreactivity (CCK-LI) in the dorsal horn was completely blocked by the delta-opioid antagonist naltrindole (10 microM in the perfusion fluid). Neither the mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr amide (CTOP; 10 microM in the perfusion fluid), nor the kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI); 10 microM in the perfusion fluid) had any significant effect in this respect. In addition, systemic administration of the delta-opioid receptor agonist BW373U86 (1 mg/kg, s.c.) and spinal administration of the delta(2)-opioid receptor agonist, Tyr-D-Ala-Phe-Glu-Val-Val-Gly amide ([D-Ala(2)] deltorphin II) (1 microM in the perfusion fluid) induced a significant increase of the CCK-LI level. The effect of BW373U86 on spinal CCK-LI release was completely blocked by spinal administration of naltrindole. The mu-opioid receptor agonist [D-ala(2)-N-Me-Phe(4)-Gly(5)-ol]-enkephalin (DAMGO) (1 microM in the perfusion fluid or 1 mg/kg, s.c.) failed to alter the CCK-LI level. Peripheral nerve lesions have previously been shown to down-regulate mu- and delta-opioid receptors in the dorsal horn, to increase the gene-expression of CCK and CCK-receptor mRNA in dorsal root ganglion neurons and to alter the potassium-induced spinal CCK-LI release. After complete sciatic nerve transection, administration of the two selective delta-opioid receptor agonists induced a significant release of CCK-LI, which was comparable to controls. In contrast, neither systemic nor spinal administration of morphine and DAMGO altered the spinal CCK-LI release in axotomized animals. The present data indicate that the delta-opioid receptor mediates morphine-induced CCK-LI release in the spinal cord.

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.

Delta(2)-opioid receptor mediation of morphine-induced CCK release in the frontal cortex of the freely moving rat

Numerous pharmacological data have been accumulated in support of the existence of physiological interactions between cholecystokinin (CCK) and opioids in the central nervous system. With the aim of further characterizing these interactions, an in vivo microdialysis approach was used to directly assess the possible influence of opioids on the extracellular levels of CCK-like material (CCKLM) in the frontal cortex of the awake, freely moving rat. Systemic administration of a high dose of morphine (10 mg/kg i.p.) produced a marked increase (up to +200%) of cortical CCKLM outflow, and this effect could be completely prevented by systemic (1.5 mg/kg i.p.) as well as intracortical (10 microM) administration of the opioid receptor antagonist naloxone. The opioid receptors activated by morphine appeared to be of the delta type because the intracortical infusion of naltrindole (10 microM) also prevented the effect of morphine, whereas CTOP (10 microM), a selective mu-opioid receptor antagonist, and nor-binaltorphimine (10 microM), a selective kappa-opioid receptor antagonist, were inactive. In addition, naltriben (10 microM), which acts selectively at the delta(2) subtype, also abolished the stimulatory effect of morphine on cortical CCKLM outflow, whereas 7-benzylidenenaltrexone (10 microM), a selective delta(1)-opioid receptor antagonist (10 microM), did not alter the morphine effect. Conversely, the direct stimulation of cortical delta(2)-opioid receptors by local infusion of [D-Ala(2)] deltorphin II mimicked the stimulatory effect of systemic morphine on CCKLM outflow. These data indicate that delta(2)-opioid receptors play a key role in opioid-CCK interactions in the rat frontal cortex.

Pharmacological characterization of morphine-induced in vivo release of cholecystokinin in rat dorsal horn: effects of ion channel blockers

Previous studies indicate that an increased release of cholecystokinin (CCK) in response to morphine administration may counteract opioid-induced analgesia at the spinal level. In the present study we used in vivo microdialysis to demonstrate that systemic administration of antinociceptive doses of morphine (1-5 mg/kg, s.c.) induces a dose-dependent and naloxone-reversible release of CCK-like immunoreactivity (CCK-LI) in the dorsal horn of the spinal cord. A similar response could also be observed following perfusion of the dialysis probe for 60 min with 100 microM but not with 1 microM morphine. The CCK-LI release induced by morphine (5 mg/kg, s.c.) was found to be calcium-dependent and tetrodotoxin-sensitive (1 microM in the perfusion medium). Topical application of either the L-type calcium channel blocker verapamil (50 microg) or the N-type calcium channel blocker omega-conotoxin GVIA (0.4 microg) onto the dorsal spinal cord completely prevented the CCK-LI release induced by morphine (5 mg/kg, s.c.). Our data indicate that activation of L- and N-type calcium channels is of importance for morphine-induced CCK release, even though the precise site of action of morphine in the dorsal horn remains unclear. The present findings also suggest a mechanism for the potentiation of opioid analgesia by L- and N-type calcium channel blocking agents.

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.

Estrogen modulation of opioid and cholecystokinin systems in the limbic-hypothalamic circuit

The display of lordosis behavior has been correlated with the estrogen-induced expression of cholecystokinin (CCK) and enkephalin within the limbic-hypothalamic circuit. These neuropeptides have opposing effects on lordosis; for example, in the medial preoptic nucleus, CCK facilitates and opiates inhibit lordosis. Antisense oligodeoxynucleotide blockade of receptor expression indicated that CCK modulates lordosis in the medial preoptic nucleus through the CCK(A)-receptor. Sequence-specific antibodies directed against delta- and mu-opiate receptor proteins labeled fibers in the medial preoptic nucleus. Estrogen treatment of ovariectomized rats or etorphine (a nonselective opiate agonist) treatment altered the appearance of the immunoreactivity from a diffuse pattern to one of distinctly stained mu-opiate receptor immunoreactive cells and varicose fibers in the medial preoptic nucleus. Such a pattern of staining reflects an internalization of mu-opiate receptors following agonist stimulation. This type of internalization has been used as an indication of synaptic activity. The distribution of receptor internalization surrounds the distribution of CCK cells in the medial preoptic nucleus, suggesting that endogenous opioid peptides may modulate estrogen-induced CCK mRNA expression. Interestingly, nonselective and delta-opiate receptor selective antagonists potentiated the estrogen-induced CCK mRNA expression in the medial preoptic nucleus. Together, these results suggest that endogenous opioid peptides may modulate the estrogenic upregulation of CCK mRNA expression and demonstrate an important level of regulation of gene expression in which synaptic activity modifies hormonal input.

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.

Effects of mu- and delta-opioid-receptor antagonists on the stimulus properties of cholecystokinin

Melton and Riley recently reported that the relatively selective mu-opioid-antagonist naloxone potentiated the stimulus properties of the gut peptide cholecystokinin (CCK). To assess whether such opioid potentiation is limited to activity at the mu-receptor subtype, in the present experiment the effects of the highly selective delta-antagonist naltrindole on CCK's stimulus properties were examined. Because in the initial report of naloxone's potentiation of CCK a relatively high, nonphysiologic dose of CCK (i.e., 13 micrograms/kg) was used as the training drug, in the current analysis subjects were trained to discriminate 5.6 micrograms/kg CCK from its vehicle and the assessments and comparisons of the effects of naloxone and naltrindole were based on this dose. Specifically, rats were administered 5.6 micrograms/kg CCK before saccharin-LiCl pairings and the CCK vehicle before saccharin alone. With such training, they rapidly acquired the drug discrimination, avoiding saccharin consumption when it was preceded by CCK and consuming the same saccharin solution when it was preceded by its vehicle. In subsequent generalization tests, doses of CCK that were ineffective in suppressing saccharin consumption (i.e., did not substitute for the training dose of CCK) did result in the suppression of saccharin consumption when combined with doses of the mu antagonist naloxone that alone had no effect on saccharin intake. On the other hand, the highly selective delta-opioid-receptor antagonist naltrindole was ineffective in potentiating the effects of CCK. Specifically, when naltrindole was combined with ineffective doses of CCK, subjects drank at control levels. The ability of naloxone to potentiate CCK's stimulus effects is consistent with a range of other demonstrations of the role of the mu-opioid-receptor subtype in CCK-opioid interactions, although the specific basis for the interaction remains unknown. Given recent findings on the effects of delta agonists and antagonists on CCK-induced activity, the failure of naltrindole to potentiate CCK's stimulus effects may be due to the absence of delta activity within this preparation, rather than the absence of delta mediation of CCK-opioid interactions in general.

Antidepressant-like effects of CCK(B) receptor antagonists: involvement of the opioid system

RB 101 (N-[(R,S)-2-benzyl-3-[(S)-2-amino-4-methylthiobutyldithio]-1-oxopr opyl]-L -phenylalaninebenzyl ester), a systemically active inhibitor of enkep halin catabolism, has been shown to elicit antidepressant-like effects in mice, both in the forced-swimming and in the conditioned suppression of the mobility tests. The same type of response has been also observed following administration of the cholecystokinin CCK(B) receptor antagonist L-365,260 ((3R)-(+)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepin -3-yl)-3 -methylphenylurea). In terestingly, the delta-opioid receptor antagonist naltrindole (17-cyclopropylmethyl-6,7-dehydro-4,5alpha-epoxy-3,14-dihydroxy-6, 7,2'-3'-indolomorphinan) blocks the effect of both RB 101 and L-365,260 in the conditioned suppression of the motility test. In this work we have investigated the involvement of the opioid system in the antidepressant response to the CCK(B) receptor antagonist L-365,260 in the forced-swimming test in mice. The effect of L-365,260 was decreased by the delta-opioid receptor antagonist naltrindole. Furthermore, the CCK(B) receptor agonist, BC 264 (Boc-Tyr(OSO3H)-gNle-mGly-Trp-(NMe)Nle-Asp-Phe-NH2), blocked the antidepressant-like effect of RB 101 while CCK-8 (H-Asp-Tyr(OSO3H)-Met-Gly-Trp-Met-Asp-Phe-NH2) enhanced the effect of this drug, probably through stimulation of central CCK(A) receptors, since the CCK(A) receptor antagonist devazepide ((3S)-(-)-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepin++ +-3-yl)-1H-indole-2 -carboxamide) abolished the CCK-8-induced potentiation of the RB 101 effect. In addition, RB 101 enhanced the effect of L-365,260. Such an effect was blocked by the delta-opioid receptor antagonist naltrindole. These data further support the involvement of opioid receptors in the antidepressant-type effect induced by CCK(B) receptor blockers and support the hypothesis of a regulatory role of CCK in the activity of the endogenous opioid system. As in other experimental paradigms, CCK(A) and CCK(B) receptor stimulation appears to have opposite effects in modulating opioidergic activity.

Opposite role of delta 1- and delta 2-opioid receptors activated by endogenous or exogenous opioid agonists on the endogenous cholecystokinin system: further evidence for delta-opioid receptor heterogeneity

Using the mouse caudate-putamen, where delta-opioid receptor subtypes have been shown to regulate adenylyl cyclase activity, we show in this study that endogenous enkephalins inhibit enzyme activity through activation of delta 1- and delta 2-opioid receptors. Thus, naltriben or 7-benzylidenenaltrexone as well as the delta-selective antagonist naltrindole (mixed delta 1 and delta 2 antagonist) antagonized inhibition of adenylyl cyclase activity induced by methionine- or leucine-enkephalin, while the micro-antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) was without effect. Furthermore, we have previously shown that activation of delta-opioid receptors increases cholecystokinin release in the central nervous system, resulting in a potentiation of micro-opioid antinociceptive responses, and the respective role of delta 1- and delta 2-opioid receptors in this facilitatory effect has now been evaluated. Activation of delta 2-opioid receptors, either by endogenous enkephalins protected from catabolism by the complete enkephalin-degrading enzyme inhibitor N-((R,S)-2-benzyl-3((S)(2-amino-4-methyl-thio) butyldithio)-1-oxopropyl)-L-phenyl-alanine benzyl ester (RB 101), or by the delta 2-selective agonist Tyr-D-Ser(O-tert-butyl)-Gly-Phe-Leu-Thr(O-tert-butyl) (BUBU), potentiated micro-opioid antinociceptive responses in the hot-plate test in mice. This effect was antagonized by a selective cholecystokinin-A antagonist. Activation of delta 1-opioid receptors by endogenous opioid peptides decreased the micro-opioid responses. These results suggest that stimulation of delta 2-opioid receptors potentiates micro-opioid analgesia in the hot-plate test in mice through an increase in endogenous cholecystokinin release, while activation of delta 1-opioid receptors could decrease it. Thus, the pre-existing physiological balance between opioid and cholecystokinin systems seems to be modulated in opposite directions depending on whether delta 1- or delta 2-opioid receptors are selectively activated. This is the first demonstration that endogenous enkephalins, methionine- and leucine-enkephalin, are the natural ligands of delta-opioid receptor subtypes, and that delta 2-opioid receptor activation may facilitate the endogenous cholecystokinin-related modulation of micro-opioid analgesia, while the delta 1-opioid receptors may have an inhibitory role. These results could have important applications for the characterization of opioid delta 1 and delta 2 as subtypes or subsites and in pain alleviation.

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.

Simultaneous activation of spinal antiopioid system (neuropeptide FF) and pain facilitatory circuitry by stimulation of opioid receptors in rats

Neuropeptide FF (NPFF) is a mammalian FMRFamide-like octapeptide with antiopioid properties that inhibits morphine-induced analgesia but also produces hyperalgesia. In the present study, a series of three experiments was carried out to investigate the interactions between opioid receptor stimulation and antiopioid systems. First, by using in vitro superfusion system with rat spinal cord slices, we showed that morphine stimulated NPFF release in a dose-dependent manner. The stimulating effect which was observed with morphine concentrations as low as 100 fM reached a maximum at 0.1 nM, then decreased and was ineffective at 10 microM. The morphine-induced release of NPFF was abolished by naloxone (1 microM) but unaltered by tetrodotoxin. Second, by an in vivo approach, we showed that a single heroin administration (2.5 mg/kg, s.c.) elicited in 30 min a drastic drop (38%) in spinal NPFF content. In a third experiment, we evaluated the capacity of naloxone in revealing an antiopioid component associated with opioid receptor stimulation. The administration of naloxone (1 mg/kg, s.c..) 25 min following that of heroin (2.5 mg/kg, s.c.) not only abolished the heroin-induced increase of tail-flick latency, but also lowered it under the basal value by 30%. These results indicate that opioid receptor stimulation activates both pain inhibitory and pain facilitatory systems in which NPFF may play a significant role and that opiate-induced analgesia is always partly masked.

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.

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.

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.

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.

Opioidergic control of the spinal release of neuropeptides. Possible significance for the analgesic effects of opioids

Several neuropeptides play a key role in the transfer (substance P, calcitonin gene-related peptide, etc) and control (enkephalins, cholecystokinin, etc) of nociceptive messages from primary afferent fibres to spino-thalamic neurones in the dorsal horn of the spinal cord. This first relay in nociceptive pathways has been shown to be a major target for opioids such as analgesic drugs, and the effects of exogenous (mainly morphine) and endogenous opioids on the release of neuropeptides within the dorsal horn are reviewed here for a better understanding of the cellular mechanisms responsible for their antinociceptive action. Complex modulations of the in vitro (from tissue slices) and in vivo (in halothane-anaesthetized rats whose intrathecal space was perfused with an artificial cerebrospinal fluid) release of substance P and calcitonin gene-related peptide by opioids have been reported, depending on the opioid receptor (mu, delta, kappa, and their subtypes) stimulated by these compounds. In particular, the inhibition by delta agonists of substance P release from primary afferent fibres, and that by the concomitant stimulation of mu and kappa receptors of the release of calcitonin gene-related peptide are very probably involved in the analgesic action of specific opioids and morphine at the level of the spinal cord. Furthermore, the negative modulation (through presynaptic opioid autoreceptors) by delta and mu agonists of the spinal release of met-enkephalin, and the complex inhibitory/excitatory influence of delta, mu and kappa receptor ligands on the release of cholecystokinin within the dorsal horn very likely also contribute to the antinociceptive action of these drugs and morphine. The reviewed data strongly support the existence of functional interactions between mu and kappa receptors within the spinal cord, and their key role in the analgesic action of non specific opiates (acting on mu, delta and kappa receptors) such as morphine.

Morphine reduces the release of met-enkephalin-like material from the rat spinal cord in vivo by acting at delta opioid receptors

The modulation by morphine of the spinal release of met-enkephalin-like material (MELM) was investigated in anaesthetized rats whose intrathecal space was perfused with an artificial CSF (ACSF). Morphine (10 microM in the ACSF), as well as a mu- (DAGO, 10 microM) or delta opioid receptor agonist (DTLET, 10 microM), significantly decreased the outflow of MELM. The effects of morphine and DTLET were prevented by the delta antagonist, naltrindole (10 microM), but not by naloxone (10 microM). Conversely, naloxone, but not naltrindole, prevented the inhibitory effect of DAGO. Although neither the kappa 1 agonist, U 50488H (10 microM), nor the kappa 1 antagonist, norbinaltorphimine (10 microM), exerted on their own any significant effect, norbinaltorphimine enhanced the inhibitory action of morphine. In contrast to the inhibition induced by morphine (with or without naloxone) which was preventable by 10 microM naltrindole, the inhibition of MELM release by morphine plus norbinaltorphimine was only partly reduced by naltrindole. Thus, concomitant stimulation of mu, delta and kappa 1 receptors might account for the apparent delta opioid receptor-dependent inhibition of MELM release by morphine. Indeed, its potential inhibitory effect through the stimulation of mu receptors (normally prevented by the concomitant stimulation of kappa 1 receptors) becomes efficient only when kappa 1 receptors are blocked.

An assessment of the interaction between cholecystokinin and the opiates within a drug discrimination procedure

Recently, cholecystokinin (CCK) has been reported to antagonize a variety of opiate-induced effects, including nociception, body shaking, thermoregulation, and locomotion. Consistent with these results, a number of CCK antagonists potentiate the opiates in a range of behavioral and physiological assessments. The present study further examined the interaction between CCK and the opiates within the conditioned taste aversion baseline of drug discrimination learning, a design that utilizes the stimulus properties of the drug to control consummatory behavior. Specifically, animals injected with CCK prior to saccharin-LiCl pairings and the CCK vehicle prior to saccharin alone rapidly acquired the CCK-vehicle discrimination, avoiding saccharin consumption following the administration of CCK and consuming the same saccharin solution following the vehicle. Although the stimulus properties of CCK did not generalize to either naloxone or diprenorphine, morphine blocked and naloxone potentiated CCK's stimulus effects. These data are thus consistent with a physiological (rather than a pharmacological) interaction between CCK and the opiates.

Modulation of opioid antinociception by CCK at the supraspinal level: evidence of regulatory mechanisms between CCK and enkephalin systems in the control of pain

1. Much evidence in the literature supports the idea that cholecystokinin (CCK) interacts with opioids in pain mechanisms. In this work, we have investigated the supraspinal interactions between enkephalins and CCK, using the hot plate test in mice. 2. Intracerebroventricular (i.c.v.) administration of BDNL (a mixed CCKA/CCKB agonist) induced dose-dependent antinociceptive responses on both paw lick and jump responses. In contrast, using the same test, the i.c.v. injection of BC 264 (a selective CCKB agonist) induced a hyperalgesic effect, which was restricted to paw licking and occurred only at a high dose of 2.5 nmol. 3. In addition, i.c.v. administration of BDNL potentiated the antinociceptive effects of the mixed inhibitor of enkephalin degrading enzymes, RB 101 and of the mu-agonist, DAMGO, while BC 264 reduced these effects. 4. Furthermore, at a dose where it interacts selectively with delta-opioid receptors, the opioid agonist BUBU reversed the hyperalgesic responses of BC 264 (2.5 nmol) but was unable to modify the effects induced by BDNL. 5. Taken together, these results suggest the existence of regulatory mechanisms between CCK and enkephalin systems in the control of pain. These regulatory loops could enhance the antinociceptive effects of morphine allowing the opiate doses used to be reduced and thus, possibly, the side-effects to be minimized.

Opioid control of the release of calcitonin gene-related peptide-like material from the rat spinal cord in vivo

The possible control by opioids of the spinal release of calcitonin gene-related peptide-like material (CGRPLM) was investigated in halothane-anaesthetized rats whose intrathecal space was perfused with an artificial cerebrospinal fluid. Morphine (20 mg/kg i.v.; or at 10-100 microM added to the perfusing fluid), the mu selective agonist DAGO (10 microM) and the kappa selective agonist U 50488 H (10 microM) did not affect the spontaneous outflow of the CGRPLM. In contrast, the selective delta agonist DTLET (10 microM) significantly increased CGRPLM release. The latter effect could be prevented by the selective delta antagonist naltrindole (10 microM) as expected from the involvement of this class of opioid receptors. However, the addition of naltrindole alone to the perfusing fluid did not modify CGRPLM outflow, indicating that endogenous opioids do not exert a tonic control of CGRP-containing fibers through the stimulation of delta receptors. In contrast, intrathecal perfusion with naloxone (10 microM) or nor-binaltorphimine (10 microM), a selective antagonist of kappa receptors, produced a marked increase in spinal CGRPLM release, suggesting that endogenous opioids acting at mu and kappa receptors, respectively, exert a tonic inhibitory control of CGRP-containing fibers. Indeed, a significant decrease in the spinal release of CGRPLM release could be evoked by the combined addition of U 50488 H (10 microM) plus DAGO (10 microM) to the perfusing medium, indicating that the simultaneous stimulation of both kappa and mu receptors is required for this negative control to occur. This could notably be achieved with morphine (10 microM) in the presence of naltrindole (10 microM) which also produced a significant reduction in the spinal release of CGRPLM. In conclusion, morphine per se did not change CGRPLM release because this drug triggers opposite positive (through the stimulation of delta receptors) and negative (through the concomitant stimulation of both kappa and mu receptors) control mechanisms within the rat spinal cord.

Increased release of immunoreactive cholecystokinin octapeptide by morphine and potentiation of mu-opioid analgesia by CCKB receptor antagonist L-365,260 in rat spinal cord

This is the first report showing, in an in vivo study, that systemic morphine produced a marked (89%, P < 0.01) increase of the cholecystokinin octapeptide (CCK-8) immunoreactivity in the perfusate of the rat spinal cord, an effect completely reversed by naloxone. Since CCK-8 has been shown to possess potent anti-opioid activity at a spinal level, a blockade of the spinal cholecystokinin effect would be expected to potentiate opiate analgesia. With tail flick latency as a nociceptive index, it was found that intrathecal (i.t.) injection of a novel CCKB antagonist L-365,260 produced a marked potentiation of the analgesic effect induced by the mu-opioid agonists morphine (4 mg/kg s.c.) or ohmefentanyl (32 ng i.t.). Similar effects were obtained with the CCKA antagonist devazepide at a dose 40-50 times higher than that of L-365,260. Both devazepide and L-365,260 showed a bell-shaped dose-response curve. The results confirm the notion that an increased release of CCK-8 may constitute a self-limiting process for opioid effects at the spinal level, and that it is the CCKB receptor which mediates the anti-opioid effect of CCK-8 in the rat spinal cord.

Cholecystokinin as a factor in the enhanced potency of spinal morphine following carrageenin inflammation

1. Cholecystokinin (CCK) has been shown to diminish opioid analgesia. Here we investigate whether changes in the physiological levels of spinal CCK are responsible for the enhanced potency of spinal morphine in animals following carrageenin inflammation, as compared with normal animals. 2. Single dorsal horn nociceptive neurones were recorded in intact halothane-anaesthetized rats in the presence and absence of carrageenin-induced inflammation and comparisons were made between the two groups of animals. Inflammation was induced by the injection of 100 microliters of 2% lambda-carrageenin into the hind paw. 3. The inhibitory effect of intrathecal morphine on the C-fibre-evoked responses of the neurones was enhanced in the carrageenin-treated animals such that the effects of 0.25 microgram and 10 micrograms of morphine in normal animals were comparable to those of 0.01 microgram and 2.5 micrograms in the carrageenin animals. The effect of 0.2 mg kg-1 of the CCKB antagonist, L-365,260, on the antinociceptive potency of intrathecal morphine was examined in both groups of animals. In normal animals, L-365,260 produced a significant enhancement in the effect of morphine indicating a tonic CCK modulation in these animals, but it had no effect on the inhibitions produced by either dose of morphine in the carrageenin animals. 4. The inhibition of the C-fibre-evoked response produced by intrathecal morphine in the presence of 1 microgram of CCK was examined in both groups of animals. CCK attenuated the effects of morphine only in animals with carrageenin inflammation, having no effect on the action of morphine in normal animals. 5. The effects of both CCK and L-365,260 were therefore dependent on the inflammatory state of the animal, with each drug being active in opposite situations.6. We propose that in normal animals, morphine may produce a maximal stimulation of the release of CCK such that exogenous CCK is unable to reduce further the analgesic effects under these conditions.However, the differential effects of the agonist and antagonist in the normal and inflamed rats points to a role of CCK in the enhanced opiate actions. This enhancement of the potency of spinal morphine in inflammation is best explained by a reduction in spinal CCK release by morphine in this state.

Morphine activates omega-conotoxin-sensitive Ca2+ channels to release adenosine from spinal cord synaptosomes

Morphine-induced release of adenosine from the spinal cord is believed to contribute to spinal antinociception. Although this release is Ca2+ dependent, little is known of the nature of this dependence. In this study, the effects of the dihydropyridine L-type Ca2+ channel agonist Bay K 8644 and the antagonist nifedipine, the N-type Ca2+ channel antagonist omega-conotoxin, and ruthenium red, a blocker of Ca2+ influx induced by capsaicin, on release of adenosine evoked by morphine were determined. The effect of partial depolarization with a minimally effective concentration of K+ on morphine-evoked release of adenosine also was examined. Morphine 10(-5)-10(-4) M produced a dose-dependent enhancement of adenosine release from dorsal spinal cord synaptosomes. Following the addition of 6 mM K+ (total K+ concentration of 10.7 mM), 10(-6) M morphine also enhanced release, and an additional component of action at 10(-8) M was revealed. Release was Ca(2+)-dependent as it was not observed in the absence of Ca2+ and presence of EGTA. Bay K 8644 (10 nM) and nifedipine (100 nM) had no effect on the release of adenosine evoked by morphine, but omega-conotoxin (100 nM) markedly reduced such release in both the absence and the presence of the additional 6 mM K+. Morphine-evoked adenosine release was not altered in the presence of a partially effective dose of capsaicin, nor by ruthenium red.(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous opiates: 1991

This paper is the fourteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1991 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.

Kappa-/mu-receptor interactions in the opioid control of the in vivo release of substance P-like material from the rat spinal cord

The possible involvement of mu and kappa receptors in the opioid control of the spinal release of substance P-like material was assessed in vivo, in halothane-anaesthetized rats whose intrathecal space was continuously perfused with an artificial cerebrospinal fluid supplemented with various opioid receptor agonists and antagonists. Whereas the intrathecal perfusion with the mu agonist DAGO (10 microM) significantly enhanced (approximately + 50%) the spontaneous release of substance P-like material, that with the kappa agonist U 50488 H (10 microM) produced no change in the peptide outflow. The respective antagonists naloxone (10 microM) for the mu receptors and nor-binaltorphimine (10 microM) for the kappa receptors did not affect the spontaneous release of substance P-like material, indicating that endogenous opioids acting at mu and kappa receptors do not exert a tonic control on substance P-containing neurons in the spinal cord of halothane-anaesthetized rats. However, as expected from the involvement of mu receptors, the stimulatory effect of DAGO on the peptide outflow could be prevented by naloxone but not norbinaltorphimine. Furthermore, instead of an increase with DAGO alone, a significant decrease in the spinal release of substance P-like material was observed upon the intrathecal perfusion with DAGO plus U 50488 H. Additional experiments with the respective mu and kappa antagonists naloxone and nor-binaltorphimine demonstrated that this effect actually resulted from the simultaneous stimulation of mu and kappa receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

GABA, acting at both GABAA and GABAB receptors, inhibits the release of cholecystokinin-like material from the rat spinal cord in vitro

Superfusion of slices of the dorsal zone of the lumbar enlargement of the rat spinal cord with an artificial cerebrospinal fluid allowed the collection of cholecystokinin-like material (CCKLM) whose Ca(2+)-dependent release could be evoked by tissue depolarization with 30 mM K+. Studies on the possible influence of GABA and related agonists on this process showed that the amino acid, the GABAA agonist, muscimol, and the GABAB agonist, baclofen, inhibited the K(+)-evoked release of CCKLM from the rat spinal cord in a concentration-dependent manner. Maximal inhibition did not exceed -40% with either agonist. Furthermore, the effects of GABAA and GABAB receptor stimulation were not additive. Whereas the effects of muscimol (10 microM) and baclofen (1 microM) could be completely antagonized by bicuculline (1 microM) and phaclofen (10 microM), respectively, complete blockade of the inhibition by GABA (1 microM) could only be achieved in the presence of both antagonists. These data indicate that both GABAA and GABAB receptors are involved in the negative influence of GABA onto CCK-containing neurones within the dorsal horn of the rat spinal cord. Apparently, these receptors are not located on CCK-containing neurones themselves, since the inhibitory effect of GABA on the K(+)-evoked release of CCKLM could be completely prevented by tetrodotoxin (1 microM). As CCK acts centrally as an endogenous opioid antagonist, such a GABA-inhibitory control of spinal CCK-containing neurones might participate in the analgesic action of the amino acid via the intrathecal route.

Cholecystokinin (CCK)-like material and CCK mRNA levels in the rat brain and spinal cord after acute or repeated morphine treatment

The effects of a single or repeated administrations of morphine on the tissue levels of cholecystokinin-like material (CCKLM) and pre pro cholecystokinin mRNA (CCK mRNA) were examined in various brain and spinal cord regions (cerebral cortex, cerebellum, hippocampus, septum, substantia nigra, lumbar enlargement) in adult rats using a specific radioimmunoassay and 'Northern blot' analysis, respectively. Although a clear parallelism existed between the regional distribution of CCKLM (septum greater than cerebral cortex greater than or equal to hippocampus much greater than lumbar enlargement, dorsal zone greater than substantia nigra greater than lumbar enlargement, ventral zone much much greater than cerebellum) and that of CCK mRNA, some mismatch was found notably in the septum where CCK mRNA levels were less than in other regions except the cerebellum. Neither CCKLM nor CCK mRNA levels were altered one hour after an acute administration of morphine (5 mg/kg i.p.). Similarly, morphine addiction after a four-day treatment with this drug was not associated with any change in the tissue levels of CCKLM and CCK mRNA. These data indicate that the previously reported modulatory action of opioids on central CCKergic systems could occur without affecting the preproCCK gene transcription and the tissue peptide concentrations.

Kappa-opioid receptor stimulation abolishes mu- but not delta-mediated inhibitory control of spinal Met-enkephalin release

The possible opioid control through delta, mu and kappa receptors of the spinal release of Met-enkephalin-like material (MELM) was investigated in halothane-anaesthetized rats. The intrathecal perfusion of the delta agonist DTLET (10 microM) or the mu agonist DAGO (10 microM) resulted in a marked inhibition of MELM release, which could be prevented by the selective antagonists naltrindole and naloxone, respectively. Although the kappa agonist U 50488 H (10 microM) was inactive per se, it completely suppressed the inhibitory effect of DAGO, without affecting that of DTLET. As the selective kappa antagonist norbinaltorphimine blocked the action of U 50488 H, it can be concluded that kappa receptors modulate the mu- (but not the delta-) mediated feed back control of spinal enkephalinergic neurones.