Cholecystokinin antagonizes morphine induced hypoactivity and hyperactivity in hamsters

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.

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.

Cholecystokinin potentiates morphine anticonvulsant action through both CCK-A and CCK-B receptors

Recent studies have suggested that cholecystokinin may have a role in modulating the effects of the endogenous opioid system in physiological functions such as thermoregulation and pain control. However, the possible interaction of cholecystokinin and morphine in epileptogenesis is unknown. We studied the effect of subcutaneous morphine and intracerebroventricularly administered cholecystokinin octapeptide sulphate ester and receptor antagonists CCK-A (MK 329) and CCK-B (L 365,260) on seizures provoked by maximal electroshock in male Sprague-Dawley rats. Seizures were induced through electrode-gel-coated ear clip electrodes by a high voltage, high internal resistance constant current generator, 30 minutes after morphine administration and 10 minutes after cholecystokinin-8-SE, CCK-A and CCK-B infusion. Morphine decreased the length of the tonic component of the seizure and cholecystokinin potentiated this decrease. Cholecystokinin antagonists blocked the effects of both cholecystokinin and morphine. The results suggest that cholecystokinin acts as an endogenous agonist with opioids in the regulation of seizure susceptibility through both CCK-A and B receptors and may be responsible for part of the anticonvulsant action of morphine.

Effects induced by BC 264, a selective agonist of CCK-B receptors, on morphine-dependent rats

The aim of this study was to investigate the possible interaction between neuronal cholecystokinin (CCK) and opiate dependence. Rats were made dependent to morphine and the ability of cholecystokinin-octapeptide (CCK-8) and Tyr(SO3H)-gNle-mGly-Trp-(NMe)Nle-Asp-Phe-NH2 (BC 264), a selective agonist of CCK-B receptors, to induce signs of morphine withdrawal after ICV injection was tested. Behavioral responses were compared to those occurring during the naloxone-precipitated morphine withdrawal syndrome. In contrast to naloxone, CCK-8 (0.1, 1, and 10 micrograms, ICV) did not precipitate any sign of withdrawal. BC 264 (0.1, 1, and 10 micrograms, ICV) induced a strong hyperlocomotion and wet dog shakes in morphine-dependent rats, the latter effect also observed in nondependent animals. In rats receiving acute morphine, BC 264 induced an opposite effect (i.e., blockade of morphine-induced hyperactivity). Taken together, these results suggest that CCK plays only a minor role in the expression of morphine physical dependence.

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.

Effects of diurnal phase and pimozide on cholecystokinin-elicited hypoactivity in the hamster

Locomotor activity in golden Syrian hamsters was measured following IP injections of cholecystokinin (CCK; 25 micrograms/kg) and pimozide (0.5 mg/kg), the dopamine receptor antagonist. In addition, animals were tested during either the dark or light phase of the diurnal cycle in either dark or light running wheel environments. Results indicated that CCK-elicited hypoactivity was blocked by pimozide and that the effect of CCK was evident only among animals tested during the light phase of the daily cycle. Ambient lighting conditions in the test environment did not modify the drug effects. Independently of any drug effect, locomotor activity was affected by diurnal phase and ambient lighting in the test environment. Animals were more active when tested during the dark phase than during the light phase and locomotor activity was higher under dark than under light ambient conditions. It is concluded that diurnal phase modulates CCK's effect on hamster locomotion and that CCK's effect on locomotion is mediated, in part, by dopaminergic mechanisms.

Effects of cholecystokinin on morphine-elicited hyperactivity in hamsters

The effects of the octapeptide cholecystokinin (CCK) on hamster locomotor activity were investigated in three experiments. In Experiment 1, the effect of CCK (25, 50, 75 micrograms/kg) on morphine (2.5 mg/kg)-elicited hyperactivity was studied. Results indicated that CCK antagonized morphine-elicited hyperactivity and that CCK alone elicited hypoactivity. There were no effects of dose of CCK. In Experiment 2, the effects of intraperitoneal (IP) and subcutaneous (SC) routes of administration of CCK (25 micrograms/kg) on locomotor activity were studied. Compared to saline controls, CCK induced hypoactivity that was of greater magnitude and of longer duration when administered IP than SC. Experiment 3 was designed to replicate the route of administration effect observed in Experiment 2 and to determine whether sensitization to CCK-induced hypoactivity develops over the course of a few injections. Results indicated that CCK-induced hypoactivity was greater after IP than SC administration but that sensitization was not detectable. It is concluded that CCK antagonizes morphine-elicited hyperactivity in the hamster by acting, in part, independently of morphine to produce opposite behavioral effects.

Failure of cholecystokinin to precipitate withdrawal in morphine-treated rats

In a test of the possible antagonistic interaction between cholecystokinin (CCK) and morphine, morphine-dependent rats were injected with one of three doses of CCK or with naloxone immediately following the consumption of a novel saccharin solution. Whereas opiate-dependent rats injected with the opiate antagonist naloxone acquired an aversion to the saccharin solution (and displayed a dramatic weight loss), CCK was without effect. These data were discussed in relation to the possible pharmacological antagonism between CCK and the opiates.

Cholecystokinin and satiation with alcohol

Release of the brain-gut peptide cholecystokinin (CCK) is stimulated by intragastric instillation of ethanol, and peripheral administration of CCK inhibits ethanol consumption. To assess the temporal specificity of the inhibitory effect of CCK on alcohol intake, water-deprived rats were given 5% ethanol at 20, 10 or 0 min after intraperitoneal injections of CCK octapeptide. Delaying access to ethanol for 20 min prevented a significant effect of CCK on intake. CCK's temporally constrained inhibitory action on alcohol consumption is consistent with an ethanol satiation effect. To test the motivational specificity of CCK's effect on fluid intake, rats were allowed a 2-bottle choice of 2% ethanol and water after CCK injections. Ethanol solution intake was suppressed by CCK, and total water intake was unaffected. The putative alcohol satiation action of CCK is appropriately specific to ethanol solution in free-choice tests. Hungry, but not fluid-deprived rats that were either ethanol experienced or naive received a 2-bottle choice of 4% ethanol or water after CCK or saline injections. CCK again specifically inhibited ethanol intake, but this effect required prior ethanol experience. Doses of CCK and naloxone, an opioid receptor blocker, combined to inhibit ethanol intake in an infra-dose-additive manner in water-deprived rats. CCK may act endogenously, in part on opioid receptor-mediated processes, as a preabsorptive satiety signal of ethanol. The full expression of this action appears to depend on prior conditioning of nutritive expectancy of the postingestive effects of alcohol.

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.

Dose-additive inhibition of intake of ethanol by cholecystokinin and bombesin

Cholecystokinin (CCK) and bombesin (BBS) are neuropeptides of the brain and gut which have been shown to inhibit intake of ethanol. CCK octapeptide and BBS tetradecapeptide were injected intraperitoneally in both single doses and combinations of doses to determine interactions of the two peptides in the control of consumption of ethanol. Water-deprived rats were given access to 5% w/v ethanol for 30 min, followed by a 30-min access to water, daily. One minute before presentation of ethanol, rats were injected with either saline or one of ten peptide solutions (three of CCK alone, three of BBS alone, and four combinations of both). Results from the injections of single peptides were used to determine predicted inhibitions of the peptide combinations, assuming perfect additivity of doses. None of the actual values of inhibition of intake of ethanol by peptide combinations differed significantly from its predicted additive value. Endogenous CCK-like and BBS-like peptides may suppress intake of ethanol by an additive mechanism of inhibition.