Effects of CCK8 and of the CCKB-selective agonist BC264 on extracellular dopamine content in the anterior and posterior nucleus accumbens: a microdialysis study in freely moving rats

Fischer 344 and Lewis Rat Strains as a Model of Genetic Vulnerability to Drug Addiction

Today it is well acknowledged that both nature and nurture play important roles in the genesis of psychopathologies, including drug addiction. Increasing evidence suggests that genetic factors contribute for at least 40–60% of the variation in liability to drug dependence. Human genetic studies suggest that multiple genes of small effect, rather than single genes, contribute to the genesis of behavioral psychopathologies. Therefore, the use of inbred rat strains might provide a valuable tool to identify differences, linked to genotype, important in liability to addiction and related disorders. In this regard, Lewis and Fischer 344 inbred rats have been proposed as a model of genetic vulnerability to drug addiction, given their innate differences in sensitivity to the reinforcing and rewarding effects of drugs of abuse, as well their different responsiveness to stressful stimuli. This review will provide evidence in support of this model for the study of the genetic influence on addiction vulnerability, with particular emphasis on differences in mesolimbic dopamine (DA) transmission, rewarding and emotional function. It will be highlighted that Lewis and Fischer 344 rats differ not only in several indices of DA transmission and adaptive changes following repeated drug exposure, but also in hypothalamic-pituitary-adrenal (HPA) axis responsiveness, influencing not only the ability of the individual to cope with stressful events, but also interfering with rewarding and motivational processes, given the influence of corticosteroids on dopamine neuron functionality. Further differences between the two strains, as impulsivity or anxiousness, might contribute to their different proneness to addiction, and likely these features might be linked to their different DA neurotransmission plasticity. Although differences in other neurotransmitter systems might deserve further investigation, results from the reviewed studies might open new vistas in understanding aberrant deviations in reward and motivational functions.

Relationship between vulnerability to reinforcing effects of morphine and activity of the endogenous cholecystokinin system in Lewis and Fischer rats

A great number of studies have shown the presence of physiological interactions between brain neurotransmitter systems in behavioural responses. This is the case for opioid, cholecystokinin (CCK) and dopamine systems. However, so far the role that the CCK system may play in vulnerability to consumption of drugs of abuse is not clear. This was investigated in this study using Lewis rats that are more sensitive to the reinforcing properties of drugs of abuse than Fischer rats. The extraneuronal CCK(8) levels and brain CCK(2) receptors were found higher in Fischer than in Lewis rats in the nucleus accumbens, one of the most important structures involved in drug consumption. Moreover, pharmacological modulation of the CCK system by administration of a selective CCK(2) agonist blocked, in the conditioned place preference, the reinforcing effects of morphine in Lewis rats, whereas a selective CCK(2) antagonist revealed reinforcing effects of the alkaloid in Fischer rats. These results obtained following systemic administrations of the CCK ligands were confirmed following microinjection into the nucleus accumbens. Thus, a low level of CCK efflux in the nucleus accumbens could be one of the many factors involved in drug reinforcing effects, whereas a high level of CCK efflux could attenuate it.

Cholecystokinin is necessary for the expression of morphine conditioned place preference

There is evidence that the neuropeptide cholecystokinin (CCK) is important for the rewarding effects of drugs of abuse. However, less is known regarding the role of CCK in drug seeking and craving. The present study investigated whether the CCK(B) antagonist L-365, 260 could block morphine-induced drug seeking using the conditioned place preference paradigm and whether the dopaminergic reward pathway contributes to the effect of L-365, 260 on expression of morphine place preference. We found that systemic administration of the CCK(B) antagonist L-365, 260 attenuates the expression of morphine-induced drug seeking as assessed using conditioned place preference (CPP) and shows that this effect is mediated by CCK(B) receptors in the anterior nucleus accumbens (NAcc). Additionally, we demonstrate that this effect is dependent on D(2) receptor activation in the anterior nucleus accumbens (NAcc). These results indicate that endogenous CCK modulates the incentive-salience of morphine-associated cues and suggest that CCK antagonists may be useful in the treatment of drug craving.

[Effect of a cholecystokinin tetrapeptide analogue on opioid reception under acute and chronic morphine administration]

Effects of a modified CCK-4, a tetrapeptide fragment of cholecystokinin, on opioid reception and cAMP level were studied. The modified CCK-4 changed the ligand binding of the opioid receptors of mu- and sigma-types in vitro. In vivo, it prevented changes in opioid reception caused by a single morphine injection or by morphine withdrawal after its long-term introduction. The CCK-4 analogue did not exert any effect in the state of intoxication after a long-term introduction of morphine or even promoted the morphine effect. The introduction of the CCK-4 analogue alone or together with morphine changed the forskoline-stimulated level of cAMP. These changes depended on the brain structure and the duration of the introduction of morphine and the CCK-4 analogue. The English version of the paper: Russian Journal of Bioorganic Chemistry, 2006, vol. 32, no. 3; see also http://www.maik.ru.

New CCK2 agonists confirming the heterogeneity of CCK2 receptors: characterisation of BBL454

Pharmacological studies were undertaken with a new series of cholecystokinin(2) CCK(2) agonists in order to assign to them a CCK(2A) or CCK(2B) pharmacological profile. The open-field test was chosen as the discrimination test of CCK(2B) agonists. The most interesting agonist, BBL454 (0.03-300 microg/kg) induced hyperactivity which was blocked by a CCK(2) antagonist, the D1 antagonist SCH23390, the delta-opioid antagonist naltrindole, but not a CCK(1) antagonist. All compounds active in the open-field test are characterised by a common structural feature, -COCH(2)CO-Trp-NMeNle-Asp-Phe-NH(2), whereas inactive compounds do not possess such a motive. Therefore, this feature can be considered crucial for CCK(2B) activity. BBL454 (0.03-3 microg/kg) improved memory in a two-trial memory test while it was very weakly active on the peripheral CCK(2) receptor, and did not evoke anxiogenic effects in the plus-maze test. The synthesis of BBL454 is simple, its minimal active dose is 30 ng/kg and no "bell-shaped" responses were observed. These results suggest that BBL454 could be considered to be the new CCK(2B) reference agonist.

Pain management by a new series of dual inhibitors of enkephalin degrading enzymes: long lasting antinociceptive properties and potentiation by CCK2 antagonist or methadone

The discovery that the endogenous morphine-like peptides named enkephalins are inactivated by two metallopeptidases, neutral endopeptidase and aminopeptidase N, which can be blocked by dual inhibitors, represents a promising way to develop 'physiological' analgesics devoid of the side effects of morphine. A new series of dual aminophosphinic inhibitors of the two enkephalin-catabolizing enzymes has been recently designed. In this study, one of these inhibitors, RB3007, was tested in various assays commonly used to select analgesics (mouse hot-plate test, rat tail-flick test, writhing and formalin tests in mice, and paw pressure test in rats), and the extracellular levels of the endogenous enkephalins in the ventrolateral periaqueductal grey have been measured by microdialysis after systemic administration of RB3007. In the mouse hot-plate test, the dual inhibitor induced long-lasting (2 h) antinociceptive effects with a maximum of 35% analgesia 60 min after i.v. or i.p. administration. These antinociceptive responses were antagonized by prior injection of naloxone (0.1 mg/kg, s.c.). Similar long lasting effects were observed in the other animal models used. Very interestingly, injection of RB3007 (50 mg/kg, i.p.) significantly increased (82%) the extracellular levels of Met-enkephalin with a peak 60 min after i.p. injection. This increase parallels the antinociceptive responses observed. In addition, strong facilitatory effects of subanalgesic doses of the CCK(2) receptor antagonist, PD-134,308 or the synthetic opioid agonist, methadone on RB3007-induced antinociceptive responses were observed. These findings may constitute promising data for future development of a new class of analgesics that could be of major interest in a number of severe and persistent pain syndromes.

What we know and what we need to know about the role of endogenous CCK in psychostimulant sensitization

The unique distribution of CCK and its receptors and its co-localization with dopamine makes it ideally situated to pay a role in dopamine-mediated reward and psychostimulant sensitization. A number of studies support the hypothesis that CCK acting through the CCK 1 and CCK 2 receptors is an endogenous modulator of dopamine neurotransmission. Behavioral studies with CCK antagonists and CCK 1 receptor mutant rats support a role for endogenous CCK in behavioral sensitization to psychostimulants. CCK microdialysis studies in the nucleus accumbens (NAC) have demonstrated that extracellular CCK is increased in the NAC by psychostimulants, providing neurochemical evidence that CCK could be involved in the behavioral response to psychostimulants. A model for how CCK may be acting in multiple brain regions to foster sensitization is presented and the gaps in our knowledge about the role of CCK in psychostimulant sensitization are described.

The effect of isolation rearing on volitional ethanol consumption and central CCK/dopamine systems in Fawn-Hooded rats

Numerous studies have demonstrated that socially isolating rats (from weaning) produces a sustained anxious phenotype and an enhanced response to psychostimulant drugs such as amphetamine and cocaine. In addition, isolation rearing has been shown to induce significant changes in the mesolimbic dopamine system. These data indicate that isolation rearing not only induces an anxiogenic phenotype but also induces neurochemical changes in reward nuclei of the brain, which is correlated with an enhanced response to psychostimulants. For these reasons, the effect of isolation rearing on volitional ethanol consumption was examined in Fawn-Hooded (FH) rats and correlated with neurochemical changes in central dopamine and cholecystokinin systems. Social isolation from weaning produced an anxiogenic phenotype as measured by a decreased time spent on the open arms of an elevated plus-maze. Interestingly, isolation-rearing induced a greater proportion of FH rats to acquire preference for ethanol while having no effect on the amount of ethanol consumed by alcohol-preferring rats. In addition, isolation rearing induced a number of changes in central CCK/dopamine systems.

The neurochemical effects of anxiolytic drugs are dependent on rearing conditions in Fawn-Hooded rats

There is a vast literature examining the neurochemical effects of anxiolytics throughout the rat brain; however, although the behavioural actions of anxiolytic drugs are routinely assessed in animal models of anxiety, the majority of neurochemical studies have been performed in rats with relatively 'normal' behavioural phenotypes. Since there is significant evidence that an anxious phenotype is associated with numerous neurochemical alterations, it is feasible that the central effects of anxiolytics may vary depending on the underlying behavioural state (and corresponding neuropathology) of the experimental animal. For this reason, the aim of the present study was to examine the effect of chronic anxiolytic drug administration on the central CCK and dopamine systems in anxious (isolated from weaning) and nonanxious (group-housed) Fawn-Hooded (FH) rats. It is important to note that these studies were performed in rats with continued access to ethanol, which may affect the responses to anxiolytic treatment. Chronic anxiolytic treatment with the selective CCK-B (CCK(2)) receptor antagonist, Ci-988 (0.3 mg/kg/day ip) or diazepam (2 mg/kg/day ip), induced numerous effects throughout the central nervous system (CNS), with Ci-988 inducing significant changes in the density of dopamine D(2) receptors, and diazepam producing marked changes in both dopamine D(2) and CCK-B receptor binding density as well as preproCCK mRNA expression. Interestingly, the neurochemical effects of these anxiolytic drugs varied significantly depending on the rearing conditions of the rats, demonstrating the importance of using adequate animal models when correlating the behavioural and central effects of drugs acting throughout the CNS.

The entorhinal cortex-nucleus accumbens pathway and latent inhibition: a behavioral and neurochemical study in rats

Latent inhibition (LI) refers to the decrease in conditioned response produced by the repeated nonrein-forced preexposure to the to-be-conditioned stimulus. Experiment I investigated the effects of electrolytic lesions of the entorhinal cortex on LI in a conditioned emotional response procedure. Entorhinal cortex lesions attenuated LI. Experiments 2 and 3 investigated whether this attenuation of LI could result from a modification in nucleus accumbens (NAcc) dopamine (DA) release. Rats with entorhinal cortex lesions displayed normal spontaneous and amphetamine-induced locomotor activity, as well as normal basal and amphetamine-induced release of DA within the NAcc (assessed by microdialysis). Taken together, these results show that entorhinal cortex lesions disrupt LI in a way that is unlikely to be due to an alteration of DA release within the NAcc.

Cholecystokinin modulation of apomorphine- or amphetamine-induced stereotypy in rats: opposite effects

Stereotyped behavior can be induced by the dopamine agonist apomorphine or by the releasing agent amphetamine. Cholecystokinin influence on dopamine-mediated behaviors has been extensively studied but a real controversy remains. Our purpose was to further characterize the dopamine-cholecystokinin interaction in apomorphine- and amphetamine-induced stereotyped behavior using sulphated cholecystokinin octapeptide (CCK8) and cholecystokinin tetrapeptide (CCK4) treatments. The results showed that CCK8 decreases apomorphine-induced stereotyped behavior and CCK4 has no effect. CCK4 and CCK8 increased the amphetamine-induced stereotyped behavior; CCK4 was more effective. The results confirm the opposite modulation of apomorphine or amphetamine-induced stereotyped behavior by CCK. These data suggest that this modulation is mediated by both CCK receptors on apomorphine-induced and only by CCK(2) receptors on amphetamine-induced stereotyped behavior.

Replacement of glycine with dicarbonyl and related moieties in analogues of the C-terminal pentapeptide of cholecystokinin: CCK(2) agonists displaying a novel binding mode

Recent advances in the field of cholecystokinin have indicated the possible occurrence of multiple affinity states of the CCK(2) receptor. Besides, numerous pharmacological experiments performed "in vitro" and "in vivo" support the eventuality of different pharmacological profiles associated to CCK(2) ligands. Indeed, some agonists are essentially anxiogenic and uneffective in memory tests, whereas others are not anxiogenic and appear as able to reinforce memory. The reference compound for the latter profile is the CCK-8 analogue BC 264 (Boc-Tyr(SO(3)H)-gNle-mGly-Trp-(NMe)Nle-Asp-Phe-NH(2)). However, although tetrapeptide ligands based on CCK-4 (Trp-Met-Asp-Phe-NH(2)) are known to possess sufficient structural features for CCK(2) recognition, none shares the properties of BC 264. Hence we have developed new short peptidic or pseudo-peptidic derivatives containing the C-terminal tetrapeptide of BC 264. Our results indicate that some compounds characterized by the presence of two carbonyl groups at the N-terminus, as in 2b (HO(2)C-CH(2)-CONH-Trp-(NMe)Nle-Asp-Phe-NH(2)), are likely to show a BC 264-like profile, bind to the CCK(2) receptor in a specific way, and display remarkable affinities (2b: 0.28 nM on guinea-pig cortex membrane preparations). This original binding mode is discussed and further enlightened by NMR and molecular modeling studies.

CCK/dopamine interactions in Fawn-Hooded and Wistar-Kyoto rat brain

The aim of this study was to compare the actions of CCK neuropeptides within the nucleus accumbens (N.Acc) of alcohol preferring (Fawn-Hooded, FH) and alcohol nonpreferring (Wistar-Kyoto, WKY) rats. CCK-8S (30-300 nM) facilitated the K(+) stimulated release of [(3)H]dopamine (DA) from N.Acc prisms in both rat strains, whereas CCK-4 (30 nM-1 microM) caused a significant decrease of evoked [(3)H]DA in the FH rat only. A scattered distribution of CCK-A and -B receptor immunopositive varicose fibers were visualized throughout the N.Acc of both rat strains along with a topographic distribution of CCK receptor positive cells throughout the ventral mesencephalon.

Opposite modulation of apomorphine- or amphetamine-induced stereotypy by antagonists of CCK receptors

Stereotyped behavior is elicited by activation of dopaminergic systems with drugs such as apomorphine and amphetamine. In previous studies, we have reported that the sulfated cholecystokinin octapeptide (CCK-8) decreased apomorphine-induced stereotypy in animals with normal and supersensitive dopamine receptors. The aim of the present study was to evaluate the effects of CCK(1) and CCK(2) receptor antagonists on stereotyped behavior induced by apomorphine or amphetamine. Rats were pretreated with the CCK(1) (SR 27897B; 1-[[2-(4-(2-chlorophenyl) thiazol-2-yl) aminocarbonyl]indolyl]acetic acid; 500 microg/kg; i.p.) or CCK(2) (L-365,260; 3R-(+)-N-(2,3-dihydro-1-methyl-2-oxo-5 phenyl-1H-1, 4-benzodiazepine-3-yl)-N'-(3-methyl phenyl)-urea; 500 microg/kg; i.p. ) receptor antagonists or saline 15 min before apomorphine (0.6 mg/kg; s.c.) or amphetamine (9.0 mg/kg; i.p.) injection. Both CCK(1) and CCK(2) receptor antagonists significantly increased apomorphine-induced stereotypy. In contrast, only the blockade of CCK(2) receptors significantly decreased amphetamine-induced stereotypy. The results suggest a dual opposite mechanism for CCK-dopamine interactions. These data also suggest that both apomorphine- and amphetamine-induced stereotypy should be used whenever effects of drugs acting on dopaminergic systems are being assessed.

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

Numerous pharmacological data indirectly support the idea that interactions between cholecystokinin (CCK) and opioids participate in the development of tolerance to morphine. Biochemical investigations were performed with the aim of directly assessing the status of such interactions in morphine treated rats. Tolerance to the alkaloid after s.c. implantation of morphine pellets for three days was not associated with any change in the levels of both CCK like-material (CCKLM) and proCCK mRNA in the frontal cortex. However, microdialysis in the freely moving rat showed that this morphine treatment produced a significant increase (+40%) of the cortical spontaneous CCKLM outflow, which could be completely prevented by intracortical infusion of naloxone (10 microM). The opioid receptors responsible for morphine-induced cortical CCKLM overflow appeared to be of the delta type because intracortical infusion of selective delta-opioid receptor antagonists such as naltriben (10 microM) and 7-benzylidenenaltrexone (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 K-opioid receptor antagonist, were inactive. These data indicate that morphine tolerance is associated with delta-opioid receptor mediated activation of cortical CCKergic systems in rats.

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.

Rat hippocampal neurons are critically involved in physiological improvement of memory processes induced by cholecystokinin-B receptor stimulation

The involvement in memory processes of the neuropeptide cholecystokinin (CCK) through its interaction with the CCK-B receptors was studied. The two-trial recognition memory task was used. Control animals showed recognition memory after a 2 hr time interval but not after a 6 hr time interval between the two trials. The improving effect of a selective CCK-B agonist, BC 264, intraperitoneally administered (0.3 microgram/kg) in the retrieval phase of the task (6 hr time interval), was also observed after its injection (1 pmol/0.5 microliter) in the dorsal subiculum/CA1 of the hippocampus but not in the caudate/putamen nucleus or in the prefrontal cortex of rats. The CCK-B antagonist L-365,260 injected (10 ng/0.5 microliter) into this region of the hippocampus abolished the improving effect of BC 264 injected intraperitoneally. Furthermore, L-365,260 injected in the hippocampus suppressed the recognition of the novel arm normally found in the controls (2 hr time interval) when it was injected before the acquisition or the retrieval phase of the task. In addition, an increase of the extracellular levels of CCK-like immunoreactivity in the hippocampus of rats during the acquisition and retention phase of the task was observed. Finally, CCK-B receptor-deficient mice have an impairment of performance in the memory task (2 hr time interval). Together, these results support the physiological involvement of the CCKergic system through its interaction with CCK-B receptors in the hippocampus to improve performance of rodents in the spatial recognition memory test.

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.

Convergence and segregation of ventral striatal inputs and outputs

The ventral striatum, which prominently includes the nucleus accumbens (Acb), is a heterogeneous area. Within the Acb of rats, a peripherally located shell and a centrally situated core can be recognized that have different connectional, neurochemical, and functional identities. Although the Acb core resembles in many respects the dorsally adjacent caudate-putamen complex in its striatal character, the Acb shell has, in addition to striatal features, a more diverse array of neurochemical characteristics, and afferent and efferent connections. Inputs and outputs of the Acb, in particular of the shell, are inhomogeneously distributed, resulting in a mosaical arrangement of concentrations of afferent fibers and terminals and clusters of output neurons. To determine the precise relationships between the distributional patterns of various afferents (e.g., from the prefrontal cortex, the basal amygdaloid complex, the hippocampal formation, and the midline/intralaminar thalamic nuclei) and efferents to the ventral pallidum and mesencephalon, neuroanatomical anterograde and retrograde tracing experiments were carried out. The results of the double anterograde, double retrograde, and anterograde/retrograde tracing experiments indicate that various parts of the shell (dorsomedial, ventromedial, ventral, and lateral) and the core (medial and lateral) have different input-output characteristics. Furthermore, within these Acb regions, various populations of neurons can be identified, arranged in a cluster-like fashion, onto which specific sets of afferents converge and that project to particular output stations, distinct from the input-output relationships of neighboring, cluster-like neuronal populations. These results support the idea that the nucleus accumbens may consist of a collection of neuronal ensembles with different input-output relationships and, presumably, different functional characteristics.

CCK in anxiety and cognitive processes

Extensive studies were carried out on the involvement of the CCKergic system in anxiety-, panic- and stress-related behaviour. The stimulation of CCK-A or CCK-B receptors is implicated in the physical and psychological responses of CCK to stress. Furthermore, several selective CCK-B agonists produce anxiogenic-like effects, while CCK-B antagonists induce anxiolytic-like responses in several models of anxiety. However, BC264 a highly selective CCK-B agonist, does not produce anxiogenic-like effects but increases attention and/or memory. These effects are dependent on the dopaminergic systems. Together with biochemical data, this led to the hypothesis of the existence of two CCK-B binding sites, CCK-B1 and CCK-B2, which could correspond to different activation states of a single molecular entity. Investigations into CCK-B1 and CCK-B2 systems might be of critical interest, since only one site, CCK-B1, appears to be responsible for the effects of anxiety. Furthermore, the improvement of attention and/or memory processes by CCK, through CCK-B2 receptors, could offer a new perspective in the treatment of attention and/or memory disorders.

Electrical stimulation of the prefrontal cortex increases cholecystokinin, glutamate, and dopamine release in the nucleus accumbens: an in vivo microdialysis study in freely moving rats

In vivo microdialysis, radioimmunoassay, and HPLC with electrochemical or fluorometric detection were used to investigate the release of cholecystokinin (CCK), glutamate (Glu), and dopamine (DA) in nucleus accumbens septi (NAS) as a function of ipsilateral electrical stimulation of medial prefrontal cortex (mPFC). CCK was progressively elevated by mPFC stimulation at 50-200 Hz. Stimulation-induced CCK release was intensity-dependent at 250-700 microA. NAS Glu and DA levels were each elevated by stimulation at 25-400 Hz; the dopamine metabolites DOPAC and homovanillic acid were increased by stimulation at 100-400 Hz. When rats were trained to lever press for mPFC stimulation, the stimulation induced similar elevations of each of the three transmitters to those seen with experimenter-administered stimulation. Perfusion of 1 mM kynurenic acid (Kyn) into either the ventral tegmental area (VTA) or NAS blocked lever pressing for mPFC stimulation. VTA, but not NAS, perfusion of Kyn significantly attenuated the increases in NAS DA levels induced by mPFC stimulation. Kyn did not affect NAS CCK or Glu levels when perfused into either the VTA or NAS. The present results are consistent with histochemical evidence and provide the first in vivo evidence for the existence of a releasable pool of CCK in the NAS originating from the mPFC. Although dopamine is the transmitter most closely linked to reward function, it was CCK that showed frequency-dependent differences in release corresponding most closely to rewarding efficacy of the stimulation. Although not essential for the reward signal itself, coreleased CCK may modulate the impact of the glutamatergic action in this behavior.

Cholecystokinin modulates the aversive component of morphine withdrawal syndrome in rats

The conditioned place aversion paradigm was used to investigate the role of cholecystokinin in the aversive/dysphoric component of morphine abstinence. Several cholecystokinin ligands were chronically administered during the development of morphine dependence: the CCKA antagonist devazepide, the CCKB antagonists PD-134,308 and L-365,260, and the CCKB agonist BC 264. The CCK-B antagonists L-365,260 and PD-134,308 decreased and completely blocked (respectively) the place aversion induced by naloxone in morphine dependent animals whereas BC 264 and devazepide were inactive in this model. No effect was observed in non-dependent animals after chronic administration of these CCK-ligands. These results show a distinct role for CCK receptors in the regulation of the motivational component of morphine abstinence, probably related to their differential effects in the regulation of limbic dopaminergic neurons.

Relation of exploratory behavior of rats in elevated plus-maze to brain receptor binding properties and serum growth hormone levels

Forty-five male Wistar rats were selected according to their behavior in the elevated plus-maze. They were separated as follows: animals with low exploratory activity ('anxious'), an 'intermediate' group and animals having high exploratory activity ('non-anxious'). Various receptor binding studies and hormonal assays were also performed in these selected rats. The affinity of 5-hydroxytryptamine 5-HT2A receptors in the frontal cortex was lower in the 'anxious' rats compared to home-cage controls and 'non-anxious' animals. Moreover, the number of cholecystokinin (CCK) receptors in the hippocampus was significantly elevated in the 'anxious' group compared to home-cage control animals. The blood levels of growth hormone (GH) were significantly lower in the 'non-anxious' rats compared to 'anxious' counterparts. In conclusion, it seems likely that the decreased exploratory activity of rats is related to the increased 5-hydroxytryptamine (5-HT) and CCK mediated neurotransmission in the brain. The different serum levels of GH in the selected rats probably reflect alterations in the activity of 5-HT and CCK.

The CCK-B agonist, BC264, increases dopamine in the nucleus accumbens and facilitates motivation and attention after intraperitoneal injection in rats

Although it is known that panic attacks are triggered by the cholecystokinin fragment CCK4, the specific involvement of peripheral or central cholecystokinin CCK receptors in various adaptive processes such as emotion, memory and anxiety has yet to be demonstrated. With this aim, we have investigated the biochemical and pharmacological effects resulting from the administration of BC264, a highly potent and selective CCK-B agonist able to cross the blood-brain barrier. Very low doses of BC264 (microg/kg i.p.), increased the exploration of animals submitted to an unknown territory but were devoid of anxiogenic properties in the elevated plus maze. BC264 increased locomotion and rearings of rats newly placed in an open field and improved their spontaneous alternation in a Y-maze. The use of vagotomized animals showed that the increased alternation induced by BC264 did not require an intact vagus nerve, unlike the locomotor activation. These behavioural effects, prevented by the prior i.p. administration of the CCK-B antagonist L-365,260 but not by the CCK-A antagonist L-364,718, were shown to depend on dopaminergic systems, since they were blocked by D1 (SCH23390, 25 microg/kg i.p.) or D2 (sulpiride, 50 or 100 mg/kg i.p.) antagonists. In addition, bilateral perfusion in freely moving rats of BC264 at pharmacologically active doses, using a newly designed microdialysis system, was found to increase the extracellular levels of DA, DOPAC and HVA in the anterior part of the nucleus accumbens. These results show that activation of CCK-B receptors by BC264 does not produce anxiogenic-like effects but appears to improve motivation and attention, whereas other CCK-B agonists such as BocCCK4 induce anxiogenic responses. Several explanations, including the existence of different sub-sites of the CCK-B receptor, could account for these differential effects.

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.

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.

Novel CCK-B receptor agonists: diketopiperazine analogues derived for CCK4 bioactive conformation

Recently, we proposed a CCK-B agonist bioactive conformation characterized by an 'S' shape of the peptidic backbone which was derived from structure-activity relationships and conformational analysis of CCK4 (Trp-Met-Asp-Phe-NH2) analogues. Using this template, we report here the synthesis of cyclic CCK4 analogues which contain, in place of the Trp-Met dipeptide, a diketopiperazine moiety resulting from a cyclization between Nle and N-substituted (D)Trp residues and coupled with a small linker to Asp-Phe-NH2. Some of these compounds displayed good affinities and selectivities for the CCK-B receptor. The results are discussed in terms of size, hydrophobicity and spatial orientation of the side-chains on the diketopiperazine ring. The most potent ligand exhibited potent and full CCK-B receptor agonist properties in promoting the hydrolysis of inositol phosphates (EC50 = 8 nM) in CHO cells, stably transfected with the rat brain CCK-B receptor. This compound was also shown to be a potent selective CCK-B/gastrin receptor agonist since, it increased gastric acid secretion measured in anesthetized rats on i.v. administration. These compounds provide a rigid template for the design of non-peptide CCK-B agonists, by modification of the remaining peptide moiety.

Evidence for a differential cholecystokinin-B and -A receptor regulation of GABA release in the rat nucleus accumbens mediated via dopaminergic and cholinergic mechanisms

In the present study we characterized the cholecystokinin receptor regulation of (i) the dopamine D2 agonist binding sites in striatal sections including the nucleus accumbens and (ii) GABA and dopamine release in the central part of the rat nucleus accumbens, by combining the in vitro filter wipe-off and the in vivo microdialysis techniques. In the binding study we demonstrate that sulphated cholecystokinin octapeptide (1 nM) increased (219 +/- 30%) the KD value of the D2 agonist [3H]N-propylnorapomorphine binding sites in sections from the striatum including the accumbens. This effect was counteracted by the cholecystokinin-B antagonist PD134308 (50 nM). In a parallel study using microdialysis in the central nucleus accumbens, we found that local perfusion with sulphated cholecystokinin octapeptide (1 microM) induced an increase in GABA (135 +/- 7%) and dopamine (146 +/- 8%) release which was unaffected by the cholecystokinin-A antagonist L-364,718 (10 nM). In contrast, when the cholecystokinin-B antagonist PD134308 (10 nM) was co-perfused with the peptide it prevented the increase in dopamine and decreased GABA release (-24 +/- 2%). This reduction was counteracted by the addition to the perfusate medium of the cholecystokinin-A antagonist or the cholinergic muscarinic M2 receptor antagonist AF-DX 116 (0.1 microM). Taken together, these data demonstrate that the facilitation by sulphated cholecystokinin octapeptide of GABA and dopamine release in the central accumbens probably reflects an inhibitory effect of the peptide on both pre- and postsynaptic D2 receptors, mediated via cholecystokinin-B receptor activation. In addition, for the first time we provide evidence for a differential cholecystokinin-A and -B receptor-mediated regulation of GABA transmission in the central accumbens, where the cholecystokinin-B receptor exerts a dominant excitatory influence while the cholecystokinin-A receptor mediates an inhibition of GABA release via a local muscarinic M2 receptor.

PD-135158, a CCKB receptor antagonist, microinjected into the nucleus accumbens and the expression of conditioned rewarded behavior

Cholecystokinin (CCK) has been localized in the nucleus accumbens (NAC) where it may interact with dopamine neurotransmission. NAC dopamine is involved in the control over behavior produced by conditioned rewards. The present experiment examines whether the blockade of CCKB receptors in the NAC with microinjection of PD-135158 (10 micrograms in 0.5 microliter) potentiates bar-pressing for stimuli previously associated with food reward. Intra-NAC microinjections of amphetamine (10 micrograms in 0.5 microliter) increased the number of bar presses for conditioned reward presentation. Furthermore, similar administration of PD-135158 produced no significant effect on responding when administered alone but potentiated the level of amphetamine responding. These findings suggest that endogenous CCKB mechanisms in the NAC may normally inhibit dopamine function in reward-related behaviors.

The CCKB antagonist PD-134,308 facilitates rewarding effects of endogenous enkephalins but does not induce place preference in rats

The interaction between cholecystokinin and endogenous opioid systems on rewarding responses was examined. Motivational effects induced by peripheral administration of a complete inhibitor of enkephalin catabolism, RB 101 or the CCKB antagonist PD-134,308, and by both compounds in combination were evaluated in the conditioned place preference test in rats. RB 101 (5, 10, 20, 40 and 80 mg/kg, IP, and 20 mg/kg, IV) given alone produced a bell-shaped dose-effect function. A significant increase of the preference for the drug-associated compartment was only observed at doses of 10 and 20 mg/kg (IP). The effect observed with morphine was stronger, and all the doses used of this compound (1.25, 2.5 and 5 mg/kg, SC) were found to be active. These results suggest that the inhibitor of enkephalin catabolism has weak rewarding properties. Pretreatment with the CCKB antagonist PD-134,308 (0.1, 0.3, 1 and 3 mg/kg, IP) alone failed to produce a reliable aversion or preference on the paradigm studied. When PD-134,308 (0.3 mg/kg, IP) was coadministered with a subthreshold dose of morphine (0.6 mg/kg, SC) or RB 101 (5 mg/kg, IP), a conditioned place preference was observed, indicating that the CCKB antagonist facilitated the motivational responses induced by endogenous enkephalins as compared to morphine. This suggests that endogenous cholecystokinin, acting through CCKB receptors, modulates the rewarding effects of endogenous enkephalins.

Cholecystokinin-induced release of dopamine in the nucleus accumbens of the spontaneously hypertensive rat

Changes in dopamine neurotransmission in the nucleus accumbens of the spontaneously hypertensive rat (SHR) may be involved in the pathogenesis of hypertension. This investigation tested the hypothesis that the sulfated octapeptide cholecystokinin (CCK8S) induced release of dopamine is greater in the SHR than in its normotensive control, the Wistar-Kyoto rat (WKY). Dopamine and its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were sampled using microdialysis in the caudal half of the nucleus accumbens of 10-week-old anesthetized SHRs and WKYs. Samples were collected in the following order: 3 baseline, 3 CCK8S (10 mumol/l), and 3 postdrug samples. The samples were then analyzed using high pressure liquid chromatography with electrochemical detection. CCK8S increased dopamine and DOPAC levels in both the SHR and WKY with a larger increase in basal dopamine in the SHR (greater than 200%). Perfusion of the nucleus accumbens with 1 mumol/l of CCK8S or the nonsulfated form of CCK8 (CCK8US, 10 mumol/l) produced no significant increase in the release of dopamine in the SHR. These results indicate that CCK8S-induced release of dopamine in the nucleus accumbens is greater in the SHR. Changes in CCK8S neurotransmission/receptor function may be responsible for the alterations in dopaminergic function of the SHR and the pathogenesis of hypertension.

Effects of administration of cholecystokinin into the VTA on DA overflow in nucleus accumbens and amygdala of freely moving rats

The carboxyterminal octapeptide of cholecystokinin (CCK-8) coexists with dopamine (DA) in mesolimbic neurons of the ventral tegmental area (VTA). In the present study, in vivo microdialysis in freely moving rats was used to assess the relative effects of sulfated CCK-8 (CCK-8S), unsulfated CCK-8 (CCK-8US) and CCK tetrapeptide (CCK-4), focally injected into the VTA, on DA overflow in two mesolimbic DA/CCK-8S terminal regions, the nucleus accumbens and the amygdala. Consistent with electrophysiological findings, microinjection of CCK-8S, but not CCK-8US or CCK-4, elicited increases in DA overflow in both terminal regions. In the absence of anatomical evidence of CCK-containing fibers in the VTA region, it seems reasonable to conclude that the modulation of terminal DA overflow by CCK-8S through actions at the somatodendritic region represents a form of autoregulation of these cells. Whereas CCK-8US and CCK-4 are preferential CCK-B receptor agonists, CCK-8S binds non-selectively to CCK-A and CCK-B receptors. Thus, these results implicate CCK-A receptors in the stimulatory effects of CCK-8S on VTA DA neurons.

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.

Systemic administration of CCK-8S, but not CCK-4, enhances dopamine turnover in the posterior nucleus accumbens: a microdialysis study in freely moving rats

The present study was carried out to examine the effects of peripheral administration of sulfatedcholecystokinin octapeptide (CCK-8S) on dopamine (DA) turnover in the posterior nucleus accumbens (PNAc) and the caudate-putamen (CP) in awake rats. Microdialysis was used to quantify the extracellular concentrations of DA and its two metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). Intraperitoneal injections of CCK-8S (0.3 mg/kg b.wt.) caused a significant increase in DOPAC and HVA concentrations in the PNAc, but did not affect the DA level. Such increases in the metabolite contents were not found in the CP. Similar injections of vehicle (1% NaHCO3 solution, 1 ml/kg b.wt.) did not have an effect in either brain region. In an attempt to determine the type of receptor involved in the CCK-8S-induced changes, CCK tetrapeptide (CCK-4, 0.3 mg/kg b.wt.) known to have high affinity for CCKB subtype or vehicle (10% DMSO-saline, 1 ml/kg b.wt.) was administered intraperitoneally. Neither CCK-4 nor vehicle caused significant changes in any of extracellular DA, DOPAC and HVA contents in the PNAc. These results suggest that peripherally administered CCK-8S has stimulatory effects on the dopaminergic system in the PNAc, and raise the possibility that the effect appears to be mediated via CCKA receptors.

Different modifications of the dopamine metabolism in the core and shell parts of the nucleus accumbens following CCK-A receptor stimulation in the shell region

After the injection of CCK8 into the posterior N. Acc. of rats DA, DOPAC HVA contents were determined from punches of the anterior and posterior N. Acc. and VTA. CCK8 (20 pmol/side) modified these levels only in the posterior N. Acc. and these responses were inhibited by the CCK-A antagonist devazepide. Five min after treatment, DA, DOPAC and HVA were increased in the N. Acc.shell and 10 min later they were decreased in the N. Acc.core. These data suggest that in these regions CCK8 could both abolish the influence of DA from the core on the transmission of motor information and favor that of DA from the shell on emotional-like responses.