Increase in hypothalamic cholecystokinin following acute and chronic morphine

The difference in mRNA expressions of hypothalamic CCK and CCK-A and -B receptors between responder and non-responder rats to high frequency electroacupuncture analgesia

The present study was performed to determine whether the expression levels of the hypothalamic cholecystokinin (CCK) and its receptors are associated with the responsiveness to high frequency electroacupuncture (EA) analgesia in rats. EA stimulation (100 Hz, 0.5 ms pulse width, 0.2-0.3 mA) was delivered to the Zusanli (ST36) acupoint of male Sprague-Dawley rats for 20 min without anesthetics or holder restraint. The analgesic effect of EA was quantified using a tail flick latency test, and subsequently animals were allocated to responder or non-responder groups. The hypothalamus of rats in each group was dissected and RNA was purified. The mRNA expressions of CCK, and CCK-A and -B receptor were determined by real-time RT-PCR. CCK mRNA levels were not significantly different in the two groups, whereas both CCK-A and -B receptors were significantly more expressed in non-responders. These results suggest that the level of CCK receptor mRNA expression in the hypothalamus, rather than CCK mRNA, has an important relationship with the individual variations to high frequency EA analgesia in rats.

Cholecystokinin modulation of locomotor behavior in rats is sensitized by chronic amphetamine and chronic restraint stress exposure

DA release in the nucleus accumbens (NAcc) is a critical substrate mediating locomotor behavior. Cholecystokinin (CCK) is co-localized with dopamine (DA) in up to 90% of mesolimbic DA neurons. We have previously shown that while CCKA receptor antagonists generally do not affect locomotor behaviors, systemic administration of a CCKA receptor antagonist attenuates amphetamine (AMPH)-induced locomotion in animals previously treated chronically with AMPH, suggesting that chronic stimulant pretreatment may sensitize CCK systems. The present studies examined this issue by testing the effects of CCKA antagonists on AMPH- and novel environment-induced locomotor activity following two manipulations which are known to alter mesolimbic system function: Chronic AMPH administration and chronic restraint stress (RS). Additionally, CCK immunoreactivity in the mesolimbic system following these manipulations was examined using immunohistochemistry. Results indicated that intra-NAcc microinjections of the selective CCKA receptor antagonist PD-140548 attenuated AMPH-induced and novel environment-induced locomotion only in animals which had previously been exposed to chronic AMPH or chronic RS pretreatment. However, chronic AMPH and chronic RS did not produce detectable changes in the number of CCK-immunostained neurons in the ventral tegmental area (VTA) or substantia nigra (SN), or in CCK levels in any of the subregions of the NAcc. Together, these results suggest that the role of endogenous CCK in the modulation of locomotor behaviors is sensitized following chronic psychostimulant or chronic RS exposure. However, this sensitization does not appear to be accompanied by changes in the overall basal levels of CCK or in the number of CCK-positive cells within the mesoaccumbens system.

The brain decade in debate: VIII. Peptide hormones and behavior: cholecystokinin and prolactin

This article is a transcription of an electronic symposium held on November 28, 2000 in which active researchers were invited by the Brazilian Society of Neuroscience and Behavior (SBNeC) to discuss the advances of the last decade in the peptide field with particular focus on central actions of prolactin and cholecystokinin. The comments in this symposium reflect the diversity of prolactin and cholecystokinin research and demonstrate how the field has matured. Since both peptides play a role in reproductive behaviors, particularly mother-infant interactions, this was the starting point of the discussion. Recent findings on the role of the receptor subtypes as well as interaction with other peptides in this context were also discussed. Another issue discussed was the possible role of these peptides in dopamine-mediated rewarding systems. Both prolactin and cholecystokinin are involved in mechanisms controlling food intake and somatic pain thresholds. The role of peripheral inputs through vagal afferents modulating behavior was stressed. The advent of knockout animals as potential generators of new knowledge in this field was also addressed. Finally, interactions with other neuropeptides and investigation of the role of these peptides in other fields such as immunology were mentioned. Knowledge about the central functions of prolactin and cholecystokinin has shown important advances. The role of these peptides in neurological and psychiatric syndromes such as anorexia, drug abuse and physiological disturbances that lead to a compromised maternal behavior seems relevant.

Morphine tolerance and dependence in nociceptin/orphanin FQ transgenic knock-out mice

It has been hypothesized that morphine tolerance and dependence in mice following chronic exposure may reflect increased compensatory activity of antiopioid systems. The endogenous peptide nociceptin/orphanin FQ has been shown to have anti-opioid effects, for example antagonizing morphine analgesia. Moreover, chronic morphine administration increases synthesis of the peptide, and morphine tolerance and dependence can be attenuated or reversed by antagonists and agonists of the nociceptin/orphanin FQ receptor, respectively. The present study seeks to confirm a role for nociceptin/orphanin FQ in opioid tolerance and dependence by comparing morphine ED(50) values and naloxone-precipitated withdrawal jumping in mice homozygous (knock-out) and heterozygous for a null mutation of the Npnc1 gene encoding the nociceptin/orphanin FQ propeptide, and their wild type littermates, following chronic morphine exposure. Relative to morphine-naive control mice, significant rightward shifts in the morphine dose-response curve, resulting in increased morphine ED(50) values (approximately two to three-fold), was observed for all genotypes following three days of repeated systemic morphine injections. However, no differences between genotypes in the magnitude of tolerance were observed. In contrast, knock-out mice displayed significantly increased naloxone-precipitated withdrawal jumping relative to heterozygous and wild-type mice following implantation with a morphine pellet (25mg) for 72h. Use of nociception/orphaninFQ transgenic knock-out mice thus demonstrate the differential involvement of nociceptin/orphanin FQ in morphine tolerance and dependence.

FMRFamide-like immunoreactivity in the olfactory system responds to morphine treatment in the teleost Clarias batrachus: involvement of opiate receptors

In view of the close relationship between the FMRF-related peptides and the central opiate-sensitive system, we investigated the effects of morphine, alone and in combination with naloxone, on the FMRFamide-like immunoreactivity in the olfactory system of the teleost, Clarias batrachus. In the olfactory system of normal and untreated fish, FMRFamide-like immunoreactivity was confined to the ganglion cells and fibers of the terminal nerve; the cells in the olfactory epithelium per se or the olfactory nerve were not immunoreactive. Intensely immunoreactive cells appeared in the olfactory epithelium following 2 h of intracranial morphine administration. FMRFamide-like immunoreactivity also appeared in the olfactory nerve fibers as they ran caudally and arborized in the glomerular layer of the bulb. However, immunoreactivity in the ganglion cells of the terminal nerve and the ensuing fibers was abolished, suggesting the transport/release of the immunoreactive material. Pretreatment with naloxone, a potent opiate receptor antagonist, reversed the effects of morphine, suggesting the involvement of opiate receptors in the regulation of the ganglion cells of the terminal nerve. The results provide initial immunocytochemical evidence in favor of a relationship between the opiates and FMRFamide-containing systems within the framework of the olfactory system.

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.

Opiate receptors modulate estrogen-induced cholecystokinin and tachykinin but not enkephalin messenger RNA levels in the limbic system and hypothalamus

Cholecystokinin, substance P and methionine enkephalin all regulate the display of reproductive behaviour. Their expression is exquisitely regulated by estrogen in the limbic-hypothalamic circuit, a circuit that regulates the display of estrogen-sensitive female reproductive behavior. Relatively little is known, however, about the interaction of endogenous opioid peptides with cholecystokinin and substance P in the limbic-hypothalamic circuit. Opiates antagonize the release of cholecystokinin and substance P in the hypothalamus and periaqueductal gray and stimulate cholecystokinin messenger RNA levels in the amygdala. To determine the effect of endogenous opioid input on estrogen-induced cholecystokinin, enkephalin and substance P expression, in situ hybridization histochemistry was used to examine estrogen-induced messenger RNA levels of these neuropeptides in specific nuclei of the limbic system and hypothalamus in the presence of opiate receptor antagonists. Estrogen treatment of ovariectomized rats significantly elevated cholecystokinin messenger RNA levels in the central portion of the medial preoptic nucleus, the encapsulated portion of the bed nucleus of the stria terminalis and the posterodorsal medial amygdala, as well as increased preproenkephalin and preprotachykinin messenger RNA levels in the ventromedial hypothalamic nucleus and the posterodorsal medial amygdala. The universal opiate receptor antagonist naltrexone and the delta-opiate receptor antagonist naltrindole each potentiated the estrogen-induced increase and elevated cholecystokinin messenger RNA levels an additional 1.9- to 2.8-fold depending on the nucleus examined, but had no effect on the estrogen-induced expression of either preproenkephalin or preprotachykinin messenger RNA. beta-Funaltrexamine, a mu-opiate receptor antagonist, had no effect on the medial preoptic or medial amygdaloid cholecystokinin messenger RNA levels or on the estrogen-induced expression of preproenkephalin messenger RNA but did cause a decrease in estrogen-induced cholecystokinin messenger RNA levels in the bed nucleus of the stria terminalis and a decrease in the preprotachykinin messenger RNA levels in the ventromedial hypothalamic nucleus. These results indicate that endogenous opioids, acting on the delta-opiate receptor within nuclei of the limbic-hypothalamic circuit, restrain the estrogen-induced increase of cholecystokinin messenger RNA expression. Activation of the mu-opiate receptor, however, may facilitate cholecystokinin messenger RNA expression in the bed nucleus of the stria terminalis and preprotachykinin messenger RNA expression in the ventromedial hypothalamic nucleus. Thus, endogenous opioid peptides may act in a site- and receptor-specific manner to modulate estrogen-induced neuropeptide levels in the limbic system and hypothalamus.

Weak tolerance to the antinociceptive effect induced by the association of a peptidase inhibitor and a CCKB receptor antagonist

We have recently shown that CCKB receptor antagonists such as PD-134,308, 4-([2-[[3-(1H-indol-3-yl)-2-methyl-1-oxo-2-[[tricyclo[3.3.1.1]dec - 2-yloxy)carbonyl]amino]propyl]amino]-1-phenylethyl]amino)-4-oxo[R- (R*,R*)]-butanoate-N-methyl-D-glucamine, are able to strongly potentiate antinociception induced by endogenous enkephalins, protected from degrading enzymes by the mixed inhibitor RB 101, N-[(R,S)-2-benzyl-3[(S)-(2-amino-4- methylthio)butyldithio]-1-oxopropyl)-L-phenylalanine benzyl ester, at both spinal and supraspinal levels. In this study, the duration of this facilitatory response and the possible development of tolerance to this synergistic effect were investigated in the rat tail-flick test after acute and chronic treatment with PD-134,308 and RB 101. PD-134,308 facilitated and prolonged the antinociceptive responses induced by RB 101 (20 mg/kg, i.v.). The duration of the effect induced by PD-134,308 was also investigated by injecting this compound at different times before RB 101 administration. In the case of the tail-flick test, the improvement of RB 101 antinociceptive response was still significant 6 h after PD-134,308 (3 mg/kg, i.p.), whereas in the hot-plate test, this enhancement was only effective for 3 h after CCKB receptor antagonist administration. In the case of a repeated administration of RB 101, the potentiation induced by PD-134,308 on the antinociceptive effect produced by the first injection of RB 101 (20 mg/kg, i.v.), was found almost identical after a second administration of RB 101 performed 190 min later. Chronic administration of RB 101 (20 mg/kg, i.v.) plus PD-134,308 (3 mg/kg, i.p.) administered for 5 days both once or twice per day, did not induce the development of tolerance to antinociception at the peak effect time. However, a decrease in the duration of the antinociceptive response was observed. These results indicate that the potent and long-lasting antinociceptive response induced by the coadministration of the peptidase inhibitor and the CCKB receptor antagonist could have interesting perspectives in the clinical treatment of pain.

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.

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.

Increases of CCK mRNA and peptide in different brain areas following acute and chronic administration of morphine

The present study examined whether either acute or chronic administration of morphine resulted in changes in the content of CCK mRNA and CCK immunoactive peptide in selective areas of the rat brain and spinal cord. Two hours after a single injection of morphine (10 mg/kg, s.c.), CCK mRNA significantly increased in the hypothalamus (0.8-fold) and spinal cord (2-fold) relative to the CCK mRNA content in saline-injected controls. No significant differences in CCK mRNA were observed in the frontal cortex, hippocampus, midbrain or brainstem. There were no significant alterations in CCK immunoreactivity in any brain regions and spinal cord after the acute treatment with morphine. Upon repeated morphine administration, the content of CCK mRNA in both the hypothalamus and the spinal cord was further elevated by at least 3-fold. A significant increase of CCK mRNA content in brain stem (2.8-fold) was also observed following chronic morphine administration. In contrast to the acute exposure to morphine, chronic administration resulted in significant increases in CCK immunoactive peptide in hypothalamus (2.6-fold), spinal cord (2.1-fold) and brainstem (1.6-fold), but not in the other brain areas. These results demonstrate that morphine, especially following repeated administrations, stimulates endogenous CCK biosynthesis in selective brain regions.

CI988, a selective antagonist of cholecystokininB receptors, prevents morphine tolerance in the rat

1. The effect of chronic treatment with CI988, a recently developed selective antagonist of cholecystokinin type-B receptors (CCKB receptors) on the tolerance to morphine analgesia was studied in rats with the hot plate test. 2. Morphine tolerance was induced with the use of two paradigms. Morphine was injected i.p. either in a schedule of increasing doses (1-32 mg kg-1) twice daily for 6 days or at a fixed dose (3 mg kg-1) daily for 29 days. 3. In both series of experiments, tolerance to the analgesic effect of morphine was prevented by simultaneous treatment with i.p. CI988. Chronic treatment with only CI988 daily for up to 29 days did not reduce the analgesic effect of a weekly injection of morphine. 4. CI988 did not diminish the physical dependence to morphine, as examined with naloxone precipitated withdrawal. 5. The present results provide evidence that chronic treatment with a selective CCKB receptor antagonist could prevent tolerance to the analgesic effect of morphine without affecting morphine-induced physical dependence. Application of CCK antagonists may be clinically important in treating chronic pain patients by preventing morphine tolerance and by eliminating the need to increase morphine doses to unacceptable levels.

Differential cholecystokinin gene expression in brain and gut of the fasted rat

Cholecystokinin (CCK) gene expression has been compared in the brain and duodenum of control and 5 days fasted rats. To study transcription, CCK mRNA was quantified using a solution hybridization assay. Large and small molecular weight CCK peptides were separated using a sequential extraction process and subsequently quantified by radioimmunoassay. In the duodenum, a fall in weight was paralleled by a decrease in CCK mRNA and in the large forms of CCK peptides. Small molecular species of CCK peptides did not change. There was no change in weight, CCK transcriptional or translational products in the brain as a whole. These data indicate location-specific differential regulation of the products of CCK gene expression in the fasted rat.

Caerulein-induced antinociception: interaction with morphine and opioid antagonists in the rat

The relationship between CCK- and opioid-activated systems in antinociception is not clear. The effects of morphine, naloxone and naltrexone on the antinociceptive effect of systemically administered caerulein was determined using the paw pressure test in the rat. Caerulein treatment significantly increased paw pressure threshold with an ED50 of 30 micrograms/kg (22.2 nmol/kg) and was considerably more potent than morphine in this respect (ED50 of 882 nmol/kg). The opioid antagonists naloxone and naltrexone were found to potentiate the antinociceptive effect of caerulein (30 micrograms/kg) whilst abolishing the effect of morphine at doses of 3 micrograms/kg and above. Co-administration of caerulein (30 micrograms/kg) with low doses of morphine, normally ineffective in the paw pressure test, abolished caerulein-induced antinociception. However the effects of antinociceptive doses of morphine were depressed by caerulein (30 micrograms/kg), showing a mutual antagonism between caerulein and opioid activated effects. In contrast to these observations, morphine pre-injection (3h before testing) was found to significantly potentiate caerulein-induced antinociception revealing a differential interaction between opioid and CCK systems at different time points. The results indicate that CCK and opioids produce antinociception by separate, yet overlapping mechanisms.

Cholecystokinin interferes with the thermoregulatory effect of exogenous and endogenous opioids

Intraperitoneal (ip) injection of cholecystokinin octapeptide sulfate ester (CCK; 5, 10 and 50 micrograms/kg) reduced the hypothermic response to 8 mg/kg and 32 mg/kg systemic morphine in restrained and freely moving rats, respectively. The hyperthermia elicited by a subcutaneous (sc) injection of 8 mg/kg morphine to freely moving rats was not diminished by CCK pretreatment. CCK (5 micrograms/kp ip) completely prevented the restraint stress-induced hyperthermia. Naloxone (1 mg/gk sc) was effective in antagonizing both the hyperthermic and the hypothermic effects of morphine and the stress-induced emotional hyperthermia. These results support the hypothesis that CCK may contribute to regulation of the endogenous opioid system.

Morphine action on cholecystokinin octapeptide release from rat periaqueductal grey slices: sensitisation by naloxone

Cholecystokinin (CCK) has potent antinociceptive properties when given either peripherally or centrally. An interaction between opiate and CCK-induced antinociception is indicated as CCK-induced analgesia is potentiated by naloxone. Since CCK cells in Periaqueductal grey (PAG) are known to be sensitive to both noxious stimuli and i.v. morphine, the possibility that the PAG was the site of such an interaction was investigated by an in vitro study of the effects of morphine and naloxone on CCK release in PAG. The K+-evoked release of CCK from tissue slices of PAG was unaffected by a wide range of concentrations of morphine. However, after pretreatment with naloxone (10(-9) M), morphine (10(-7)-10(-6) M) caused a significant, dose dependent attenuation of CCK release (70% inhibition at 10(-6) M). These results suggest that the release of CCK in PAG is modulated by opioid systems.