Lack of effect of CCKB receptor antagonists in ethological and conditioned animal screens for anxiolytic drugs

Differential effects of cholecystokinin (CCK-8) microinjection into the ventrolateral and dorsolateral periaqueductal gray on anxiety models in Wistar rats

Cholecystokinin (CCK) is one of the main neurohormone peptide systems in the brain, and a major anxiogenic mediator. The periaqueductal gray (PAG) is a key midbrain structure for defensive behaviors, which could include anxiety, fear, or even panic. The CCK system has wide distribution in the PAG, where the dorsolateral region (DL) participates in active defensive behavior and the ventrolateral region (VL) in passive defensive behavior. The aim of this study was to assess the effect of CCK-8 microinjection into DL-PAG or VL-PAG on anxiety-like behavior through two tests: elevated plus maze (EPM) and defensive burying behavior (DBB). CCK-8 (0.5 and 1.0 μg/0.5 μL) presently microinjected into the DL-PAG produced an anxiogenic-like effect on the EPM evidenced by decreasing the time spent/number of entries in open arms compared to vehicle group. Additionally, the latency to burying decreased and burying time increased on the DBB test. Contrarily, CCK-8 microinjected into the VL-PAG resulted in greater open-arm time and more open-arm entries compared to the vehicle-microinjected group. The results on the DBB test confirmed an anxiolytic-like response of CCK-8 into the VL-PAG. In conclusion, CCK-8 microinjected into DL-PAG produced anxiety-like behavior on EPM, and for first time reported on DBB. Contrarily, CCK-8 microinjected into the VL-PAG reduced anxiety-like behavior also for first time reported using both behavioral models EPM and DBB.

Differential effects of MPEP and diazepam in tests of conditioned emotional response and Pavlovian-to-instrumental transfer suggests 'anxiolytic' effects are mediated by different mechanisms

BACKGROUND

The selective mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) is reported to be anxiolytic in several animal models of anxiety, including the conditioned emotional response (CER) paradigm. Suppression of responding during conditioned stimulus (CS) presentation in CER may reflect behavioural competition between lever pressing and adopting a shock-avoidance posture, or it may alternatively reflect altered value of the food reward following its association with a footshock, thus reducing its ability to motivate responding. If this is the case, then drugs that reduce the CER may interfere with the mechanism by which CSs are able to motivate responding, rather than by reducing anxiety. The standard test of the ability of Pavlovian cues to motivate responding is the Pavlovian-to-instrumental transfer (PIT) paradigm and it has recently been suggested that CER may be 'negative PIT'.

MATERIALS AND METHODS

We compared the effect of MPEP (0, 3, 10 and 30 mg/kg) and diazepam (0, 1, 3 and 10 mg/kg) in CER and PIT.

RESULTS

Both MPEP and diazepam significantly reduced conditioned suppression in the CER paradigm. MPEP, but not diazepam, significantly reduced PIT.

CONCLUSION

The findings support the hypothesis that MPEP may reduce expression of anxiety in the CER paradigm by interfering with the way in which emotionally salient cues are able to affect behaviour, but do not support such an analysis of the effect of diazepam. Diazepam and MPEP may therefore achieve their effects in CER by influencing different psychological processes.

Functional interactions between endocannabinoid and CCK neurotransmitter systems may be critical for extinction learning

The endocannabinoid system and the cannabinoid type 1 receptor (CB1R) are required for the extinction of conditioned fear. CB1 antagonists have been shown to prevent extinction when delivered both systemically and within the amygdala. Anatomical studies suggest that CB1Rs in the basolateral amygdala (BLA) are expressed on GABAergic interneurons expressing the anxiogenic peptide cholecystokinin (CCK). Pre-synaptic CB1Rs inhibit neurotransmitter release, suggesting that CB1R activation during extinction may decrease CCK peptide release as well as GABA release. Thus, we examined whether extinction involves the CB1R modulation of CCK2 receptor activation. We found that intracerebroventricular administration of the CCK2 agonist pentagastrin dose-dependently impaired extinction of conditioned fear. Systemic administration of a CB1 antagonist, rimonabant (SR141716), also potently inhibited extinction learning. This effect was ameliorated with systemic administration of a CCK2 antagonist, CR2945. Furthermore, the extinction blockade by systemic SR141716 was reversed with intra-BLA, but not intrastriatal, infusion of CR2945. Lastly, as extinction usually leads to an increase in Akt phosphorylation, a biochemical effect antagonized by systemic CB1 antagonist treatment, we examined whether CR2945 co-administration would increase extinction-induced p-Akt levels. We observed that extinction-trained animals showed increased Akt phosphorylation following extinction, CB1 antagonist-treated animals showed p-Akt levels similar to those of non-extinction trained animals, and co-administration of CR2945 with SR141716 led to levels of p-Akt similar to those of vehicle-treated, extinction-trained controls. Together, these data suggest that interactions between the endocannabinoid and CCKergic transmitter systems may underlie the process of extinction of conditioned fear.

CCK and NPY as anti-anxiety treatment targets: promises, pitfalls, and strategies

Short CCK peptides elicit panic attacks in humans and anxiogenic-like effects in some animal models, but CCK receptor antagonists have not been found clinically effective. Yet CCK overactivity appears to be involved in submissive behaviour, and CCKB receptor expression and binding are increased in suicide victims and animal models of anxiety. Preliminary data suggest that involvement of CCK and its receptor subtypes in anxiety can be better described when focusing on distinct endophenotypes, and considering environmental contingencies and confounds originating from interactions with dopamin-, opioid- and glutamatergic neurotransmission. In contrast, NPY is an anti-anxiety peptide with robust effects in various animal models when administrated into several brain regions. Studies with non-peptide antagonists selective for receptor subtypes have revealed the role of endogenous NPY in active coping. At least Y1, Y2 and Y5 receptors in various brain regions are involved, with the strongest evidence for contribution of Y1.

Social defeat increases alcohol preference of C57BL/10 strain mice; effect prevented by a CCKB antagonist

RATIONALE

In humans, social stress over long and short term can increase alcohol consumption, but the mechanisms involved are not understood.

OBJECTIVES

This study was conducted to examine the effects of social defeat, using the resident/intruder paradigm, on the alcohol preference of "low alcohol drinking" individuals in a colony of C57BL/10 strain mice and the effects of two anxiolytic drugs.

METHODS

Alcohol preference, in a two-bottle choice (8% v/v alcohol or water), was measured, in separate experiments, after either a single experience of social defeat by a resident male mouse, five consecutive daily defeat experiences or one experience per week for 4 weeks. Comparison was made with effects of repeated social defeat on the preference for dilute sucrose. In addition, the actions of the CCKB receptor antagonist, CAM1028, and of diazepam were examined on the effects of repeated defeat experiences.

RESULTS

Five consecutive daily defeat experiences had a slow onset effect in increasing alcohol preference and consumption, compared with five daily exposures to a novel environment. A single defeat, or one defeat per week, did not significantly alter alcohol preference or intake. There were no effects of five daily defeat experiences on sucrose preference or consumption. The effect of repeated defeats on alcohol preference was significantly decreased by administration of the CCKB receptor antagonist, CAM1028, prior to each experience, but not by corresponding administration of diazepam.

CONCLUSION

The results show that social stress increases alcohol intake in low alcohol preference C57BL/10 mice and suggest that CCK transmission may be involved in this effect.

Targeted invalidation of CCK2 receptor gene induces anxiolytic-like action in light-dark exploration, but not in fear conditioning test

RATIONALE

Evidence suggests that gamma-aminobutyric acid (GABA) and cholecystokinin (CCK) have opposite roles in the regulation of anxiety.

OBJECTIVES

The aim of our work was to study the behaviour of CCK(2) receptor deficient mice in light-dark exploration and fear conditioning tests. Moreover, the action of diazepam and methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), having the opposite effect on GABA(A) receptors, was evaluated on the exploratory behaviour in these mice. Expression levels of GABA(A) receptor subunit genes were also measured.

METHODS

Light-dark exploration and fear conditioning tests were used to determine changes in anxiety of mice. The action of diazepam (0.5-2 mg/kg i.p.) and DMCM (0.25-1 mg/kg i.p.) was studied in the light-dark box. The effect of DMCM was also evaluated in the motor activity test to demonstrate that its anti-exploratory action was not related to motor suppression. Expression levels of GABA(A) receptor subunit genes were determined by means of real-time polymerase chain reaction (qRT-PCR).

RESULTS

Female mice lacking CCK(2) receptors displayed increased exploratory activity in the light-dark box compared to their wild-type (+/+) littermates. Locomotor activity in the motility boxes and the intensity of freezing did not differ in wild-type (+/+) and homozygous (-/-) mice. Treatment with diazepam (0.5 mg/kg) increased the number of transitions in wild-type (+/+) animals, whereas in homozygous (-/-) mice diazepam (0.5-2 mg/kg) reduced exploratory activity. Administration of DMCM (0.25-1 mg/kg) induced an anxiogenic-like effect in homozygous (-/-) mice, but did not change their locomotor activity. Gene expression analysis established a 1.6-fold increase in the expression of the alpha2 subunit of GABA(A) receptors in the frontal cortex of homozygous (-/-) mice.

CONCLUSION

Genetic invalidation of CCK(2) receptors induced an anxiolytic-like action in exploratory, but not in conditioned models of anxiety. The observed reduction in anxiety in homozygous (-/-) mice is probably related to an increased function of GABAergic system in the brain.

Cholecystokinin and panic disorder

Evidence for implication of cholecystokinin (CCK) in the neurobiology of panic disorder is reviewed through animal and human pharmacological studies. The results of these investigations raise two issues: (i) selectivity of action of CCK-2 agonists in anxiety disorders; and (ii) aberrations of the CCK system in anxiety disorders, both of which are discussed.

Targeted mutation of CCK2 receptor gene modifies the behavioural effects of diazepam in female mice

RATIONALE

Evidence suggests that GABA and CCK have opposite roles in the regulation of anxiety.

OBJECTIVE

The aim of the present work was to study diazepam-induced anxiolytic-like action and impairment of motor co-ordination, and the parameters of benzodiazepine receptors in mice lacking CCK2 receptors.

METHODS

The action of diazepam (0.5-3 mg/kg i.p.) was studied in the elevated plus-maze model of anxiety and rotarod test using mice lacking CCK2 receptors. The parameters of benzodiazepine receptors were analysed using [3H]-flunitrazepam binding.

RESULTS

In the plus-maze test, the exploratory activity of the homozygous (-/-) mice was significantly higher compared to their wild-type (+/+) littermates. However, the wild-type (+/+) mice displayed higher sensitivity to the anxiolytic-like action of diazepam. Even the lowest dose of diazepam (0.5 mg/kg) induced a significant increase of open arm entries in the wild-type (+/+) mice. A similar effect in the homozygous (-/-) mice was established after the administration of diazepam 1 mg/kg. The highest dose of diazepam (3 mg/kg) caused a prominent anxiolytic-like effect in the wild-type (+/+) mice, whereas in the homozygous (-/-) animals suppression of locomotor activity was evident. The performance of the homozygous (-/-) mice in the rotarod test did not differ from that of the wild-type (+/+) littermates. However, a difference between the wild-type (+/+) and homozygous (-/-) animals became evident after treatment with diazepam. Diazepam (0.5 and 3 mg/kg) induced significantly stronger impairment of motor co-ordination in the homozygous (-/-) mice compared to their wild-type (+/+) littermates. The density of benzodiazepine binding sites was increased in the cerebellum, but not in the cerebral cortex and hippocampus, of the homozygous (-/-) mice.

CONCLUSIONS

Female mice lacking CCK2 receptors are less anxious than their wild-type (+/+) littermates. The reduced anxiety in homozygous (-/-) mice probably explains why the administration of a higher dose of diazepam is necessary to induce an anxiolytic-like action in these animals. The highest dose of diazepam (3 mg/kg) induced significantly stronger suppression of locomotor activity and impairment of motor co-ordination in the homozygous (-/-) mice compared to the wild-type (+/+) littermates. The increase in the action of diazepam is probably related to the elevated density of benzodiazepine receptors in the cerebellum of homozygous (-/-) mice. The present study seems to be in favour of increased tone of the GABAergic system in mice without CCK2 receptors.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Anxiety-related behaviors in cholecystokinin-A, B, and AB receptor gene knockout mice in the plus-maze

Cholecystokinin (CCK)-A receptor (AR) and B receptor (BR) share highly amino acid sequence homology and overlap in their tissue distribution. We examined the anxiety-related behavior of CCK-AR, CCK-BR, and CCK-ARBR gene knockout (-/-) mice in the elevated plus-maze. CCK-AR(-/-) mice showed a significantly higher frequency of open-arm entries than wild-type and CCK-BR(-/-) mice, whereas the percentage open-arm entry values in CCK-AR(-/-) mice did not differ from those in wild-type mice. Thus, this increased frequency in open-arm entries for CCK-AR(-/-) mice was interpreted to be due to an increase in locomotor activity, rather than to a reduction in anxiety. By contrast, CCK-BR(-/-) mice showed significantly lower percentage open-arm entry values and spent significantly less time in the open- arms than wild-type and CCK-AR(-/-) mice. We therefore conclude that a lack of CCK-BR increases the anxiety-related behavior of the mouse in the elevated plus- maze.

Validation of a behavioral recording automated system in the elevated plus-maze test

The elevated plus-maze test has been widely used for screening of anxiolytic drugs and for exploring neurobiological bases of anxiety. In this study, we validated a new automated system that enables to record exploratory behavior in the elevated plus-maze test. This system, called cyberplus, consisted of ten pairs of photoelectric cells strategically located in several parts of the apparatus, and seemed to be sensitive to the position of the animal's forepaws, so it would yield scores in anxiety measurements and locomotor activity similar to those obtained by following the traditional procedure, that is, by analyzing videotapes by experienced observers. In order to assess this hypothesis, we exposed rats to the elevated plus-maze test and compared the scores obtained by cyberplus with the values recorded by two independent observers, conducting a correlational study with both kinds of recording procedures. The results obtained suggest the utility of cyberplus as a behavioral recording automated system in the elevated plus-maze test, making data collection and data analysis easier in exploring pharmacological and neurobiological bases of anxiety.

Placebo-controlled trial of the CCK-B antagonist, CI-988, in panic disorder

BACKGROUND

Based on the induction of panic-like symptoms by infusion of cholecystokinin (CCK) peptide in normals and panic disorder patients, it has been proposed that CCK may play a role in the disease mechanisms underlying anxiety disorders. Selective antagonists of CCK-B receptors can block the challenge-induced symptoms in a dose-dependent manner, leading to the hypothesis that these compounds may have anxiolytic effects.

METHODS

A randomized, double-blind study was carried out to compare the effects of placebo with CI-988, a selective antagonist of the CCK-B receptors. Following a one-week placebo lead-in, patients with Panic Disorder with or without Agoraphobia received either placebo or CI-988 100 mg TID for six weeks. Panic attacks were recorded by a daily diary method.

RESULTS

A total sample of 88 patients was planned but and interim analysis was carried out when about half the patients had been enrolled (n = 41). All patients improved during treatment and no difference in the weekly rate of panic attacks was seen between the treatment groups. The study was terminated at this point due to the remote likelihood of showing a treatment difference.

CONCLUSIONS

CI-988 was not superior to placebo in reducing panic attacks. Several explanations are possible, including the poor pharmacokinetic characteristics of CI-988 which may make it unsuitable to test the CCK hypothesis of anxiety.

Concurrent chronic administration of a CCK(B) antagonist can decrease tolerance to the ataxic effects of ethanol

The effects of chronic administration of a CCK(B) antagonist, CAM1028, were examined on the development of tolerance to ethanol and the appearance of withdrawal signs. Ethanol was given chronically by liquid diet, and none of the dose schedules of CAM1028 affected the amount of ethanol taken in during the chronic treatment. Brain ethanol concentrations were not altered by the administration of CAM1028.The ataxic effects of ethanol were tested on a rotorod, 3 hours after cessation of the ethanol administration, and the last injections of CAM1028 were given 8 hours before withdrawal from the ethanol treatment. When administered at 0.03, 0.1 or 0.3 mg/kg, CAM1028 decreased the extent of tolerance development. Higher doses, 1 and 3 mg/kg, did not alter the tolerance development. Chronic administration of CAM1028 had a small effect in protecting against the effects of ethanol withdrawal that, in contrast to the effects on the tolerance, was seen only at the highest dose, 10 mg/kg, of CAM1028.

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.

CCK receptor antagonists

1. The peptide hormone and neurotransmitter, cholecystokinin, is widely distributed throughout the gastrointestinal tract and central nervous system and mediates a diverse number of biological functions. 2. Two receptor subtypes, CCK-A and CCK-B, have been identified by both pharmacological characterization and molecular cloning. The CCK-A receptor is the predominant peripheral CCK receptor subtype and the CCK-B receptor is the predominant central CCK receptor. In addition, there are discrete populations of CCK-A receptors in the brain and CCK-B receptors are present in gastric mucosa. 3. Subtype selective antagonists have been developed which discriminate between the two receptor subtypes. One of the major chemical classes has exploited a benzodiazepine template present in asperlicin which was initially discovered in a natural product screen for CCK receptor antagonists. 4. The structurally related benzodiazepines L-365,260, L-740,093, and YM022 are selective antagonists of the CCK-B receptor subtype. Their in vitro pharmacological profiles have been characterized using the human CCK-B receptor expressed in CHO cells. 5. L-365,260 behaves in a manner consistent with that of a competitive antagonist and both L-740,093 and YM022 behave as insurmountable CCK-B receptor antagonists in vitro.

Involvement of GABA(A)-benzodiazepine receptor in the anxiolytic effect induced by hexanic fraction of Rubus brasiliensis

To investigate the ability of hexanic ethanolic fraction of Rubus brasiliensis Martius (Roseceae), to induce anxiolytic effect and also the possible involvement of the GABA(A)-benzodiazepine receptor complex, male Wistar rats and Swiss mice behaviour were tested in the elevated plus maze (EPM). All the doses of the extract, 50, 100 and 150 mg/kg, administered per gavage (vo), 30 min before the behavioural evaluation, induced an anxiolytic effect expressed by: increased number of entries in and time spent in the open arms and percentage of open arm entries; and decreased number of entries and time spent in the closed arms. The treatment of mice with flumazenil (Ro 15-1788), 0.5, 1.0 and 1.5 mg/kg, i.p., 15-min before the administration of hexanic fraction, 100 mg/kg, vo, blocked the hexanic fraction-induced anxiolytic effect. The LD50 for the hexanic fraction was 1512 mg/kg. In conclusion, it was shown that the hexanic fraction of R. brasiliensis induced an anxiolytic effect in rats and mice. This effect can be attributed to a liposoluble principle with low toxicity which may be acting as an agonist on GABA(A)-benzodiazepine receptor complex.

Cholecystokinin and psychiatric disorders : role in aetiology and potential of receptor antagonists in therapy

Cholecystokinin (CCK) is one of the most abundant neuropeptides in the brain. It is found in the highest levels in cortical and limbic structures and also in the basal ganglia. Two subtypes of CCK receptors have been described in the brain and gastrointestinal tissues. CCK(A) (alimentary subtype) receptors are mainly located in the gastrointestinal tract, regulating secretion of enzymes from the pancreas and emptying of the gallbladder. However, CCK(A) receptors are also found in several brain regions, with the highest densities in structures poorly protected by the haematoencephalic barrier (the area postrema, nucleus tractus solitarius and hypothalamus). The distribution of CCK(B) (brain subtype) receptors overlaps with the localisation of CCK and its mRNA in different brain areas, with the highest densities in the cerebral cortex, basal ganglia, nucleus accumbens and forebrain limbic structures.Both subtype of CCK receptor belong to the guanine nucleotide-binding protein-(G protein)-linked receptor superfamily containing 7 transmembrane domains. Signal transduction at CCK receptors is mediated via G(q) protein-related activation of phospholipase C and the formation of inositol 1,4,5-triphosphate (IP(3)) and 1,2-diacylglycerol (DAG). Recent cloning of CCK(A) and CCK(B) receptors has shown that mRNA for both receptors is distributed in the same tissues as established in radioligand binding and receptor autoradiography studies, with few exceptions.The existence of multiple CCK receptors has fuelled the development of selective CCK(A) and CCK(B) receptor antagonists. These antagonists belong to distinct chemical groups, including dibutyryl derivatives of cyclic nucleotides, amino acid derivatives, partial sequences and derivatives of the -COOH terminal sequence heptapeptides of CCK, benzodiazepine derivatives, 'peptoids' based on fragments of the CCK molecule, and pyrazolidinones. At the present time, the compounds of choice for blockade of the CCK(A) receptor are lorglumide, devazepide and lintitript (SR27897). L-365,260, CI-988, L-740,093 and LY288513 are the drugs most widely used to block CCK(B) receptors.Studies with CCK antagonists (and agonists) in animals and humans suggest a role for CCK in the regulation of anxiety and panic. The administration of CCK agonists [ceruletide (caerulein), CCK-4, pentagastrin] has an anxiogenic action in various animal models and in different animal species. However, the anxiogenic action of CCK agonists is restricted to nonconditioned (ethological) models of anxiety, with very limited activity in the 'classical' conditioned models. Pharmacological studies have revealed that CCK(B) receptors are the key targets in the anxiogenic action of CCK agonists. Nevertheless, CCK(B) antagonists displayed very little activity, if any at all, in these models, but strongly antagonised the effects of CCK(B) agonists. The anxiogenic/panicogenic action of CCK(B) agonists (CCK-4, pentagastrin) is even more pronounced in human studies, but the effectiveness of CCK(B) antagonists as anxiolytics remains unclear. Clinical trials performed to date have provided inconclusive data about the anxiolytic potential of CCK(B) receptor antagonists, probably because of limiting pharmacokinetic factors.The results of some animal experiments suggest a role for CCK in depression. The administration of CCK(B) antagonists causes antidepressant-like action in mouse models of depression. However, human studies replicating this result have yet to be carried out.A prominent biochemical alteration in schizophrenia is a reduction of CCK levels in the cerebral cortex. This change may be related to the loss of cortical neurons, due to the schizophrenic process itself. In animal studies (mainly in mice), administration of CCK agonists and antagonists has been shown to be effective in several models, reflecting a possible antipsychotic activity of these drugs. However, the data obtained in human studies suggest that CCK agonists and antagonists do not improve the symptoms of schizophrenia. Taking into account the reduced levels of CCK and its receptors found in schizophrenia, treatments increasing, but not blocking, brain CCK activity may be more appropriate.

The effects of cholecystokinin A and B receptor antagonists on exploratory behaviour in the elevated zero-maze in rat

The aim of the present study was to investigate the effects of cholecystokinin (CCK) CCK(A) and CCKB receptor antagonists SR 27897 B, devazepide, L 365260 and PD 135158 (CAM 1028) on exploratory behaviour in the elevated zero-maze in the rat. For further validation of the elevated zero-maze, the effects of a reference anxiolytic diazepam (0.25, 0.5, 1.0, 2.0 mg/kg), a non-benzodiazepine (BDZ) anxiolytic buspirone (0.04, 0.2, 1.0, 5.0 mg/kg), BDZ receptor inverse agonists FG 7142 (5, 10, 15, 20 mg/kg) and DMCM (0.1, 0.5, 1.0, 1.5 mg/kg), and a BDZ receptor antagonist flumazenil (10 mg/kg) were studied. Diazepam decreased the number of stretched-attend postures in all doses used and increased the percentage of time spent exploring in open parts at doses of 0.5 and 1.0 mg/kg. The effects of diazepam were blocked by flumazenil. FG 7142 and DMCM had effects only in subconvulsive doses (20 mg/kg and 1.5 mg/kg). Flumazenil and buspirone failed to show any effect. The CCK(A) receptor antagonists were also without any effect. The CCK(B) receptor antagonists L 365260 (1.0 and 5.0 mg/kg) and PD 135158 (100 microg/kg) had a significant anxiolytic-like effect. The CCK(B) receptor antagonists increased the number of open part entries, the number of head dips, the percentage of time spent exploring in the open part and decreased the number of stretched-attend postures. These data support the hypothesis of the involvement of the CCK(B) receptor subtype in the neurobiological mechanisms of anxiety.

Cholecystokinin in anxiety

Cholecystokinin (CCK) plays an important role in both the alimentary tract and the central nervous system (CNS). At present it seems to be the most abundant neuropeptide in the CNS. This paper reviews the CCK neuronal system and its interactions with gamma-aminobutyric acid (GABA) and serotonin (5-hydroxytryptamine; 5-HT). In addition, its putative role in anxiety will be discussed on the basis of animal data and studies in healthy volunteers and panic disorder patients. According to these investigations, the CCK4 challenge test fulfills most criteria for an ideal panicogenic agent and evidence has been found that CCKB receptor antagonists might possess anxiolytic properties in man.

Behavioral effects of CCKB receptor ligands in a validated simulation of panic anxiety in rats

Animals or human subjects receiving brain stimulation in the dorsal periaqueductal gray matter (dPAG) show sudden fear-suggestive behavioral reactions and physical signs of autonomic activation which are reminiscent of the symptom profile characterizing a panic attack. An experimental situation in rats measuring dPAG stimulation self-interruption thresholds has been validated as realistically simulating several aspects of panic anxiety with objective signs of symptomatic and predictive validity using established antipanic and panicogenic agents; it was utilized here to evaluate the effects of various cholecystokinin B receptor ligands. A dose-dependent increase in self-interruption thresholds (antipanic-like effect) was recorded following injection of L-365,260 (3.2, 10 and 32 mg/kg i.p.), a CCKB receptor antagonist with good brain penetration, whereas no significant changes in thresholds were recorded following CI-988 (3.2, 10 and 32 mg/kg i.p.), a dipeptoid CCKB receptor antagonist with poor brain penetration. Latencies for self-interruption were not modified, suggesting that motor functions remained intact. No significant changes in self-interruption thresholds were recorded following peripheral administration of the CCKB receptor agonists CCK4 (0.03 to 0.32 mg/kg i.v.; 0.01 to 3.2 mg/kg i.p.) or the metabolically stabilized analog Boc-CCK4 (0.1 to 10 mg/kg i.p.). Systemic administration of the panicogenic compounds caffeine and yohimbine enhance acute anxiety in this model. These data indicate that, in the dPAG simulation of panic anxiety, central CCKB receptor blockade by L-365,260 induces antiaversive effects analogous to those observed following benzodiazepine receptor activation by clonazepam or alprazolam. Potency and efficacy of L-365,260 were lower than those of clonazepam or alprazolam, suggesting modest, but nonetheless authentic, antiaversive properties for this CCKB receptor antagonist. Lack of effects observed following peripheral administration of the agonists CCK4, and Boc-CCK4 or of the dipeptoid antagonist CI-988 is likely to reflect restricted brain penetration of those compounds in rats; it furthermore excludes a contribution of peripheral gastrin and CCKA receptors to the antipanic-like properties of selective CCKB receptor antagonists such as L-365,260.

Inhibition of pentagastrin-induced pressor response in conscious rats by the CCK-B receptor antagonist CI-988 and chlordiazepoxide

Elevation of blood pressure is a characteristic feature of spontaneous and pharmacologically induced panic attacks in man. We examined whether this symptom could be reproduced in conscious rats and, if so, whether the cardiovascular changes induced by anxiogenic agents could be employed as a functional in vivo screen for CCK-B receptor antagonists which might be predictive of anxiolytic activity. Bolus i.v. administration of pentagastrin (0.1-100 micrograms/kg) or the beta-carboline FG-7142 (0.001-3 mg/kg) caused transient ( < or = 5 min) dose-dependent increases in arterial blood pressure of up to 41 mmHg. The benzodiazepine receptor agonist chlordiazepoxide (10 mg/kg i.v, 15 min previously) attenuated the pressor response induced by either pentagastrin (1 microgram/kg i.v.) or FG-7142 (0.3 mg/kg i.v.). In contrast, the CCK-B receptor antagonist CI-988 (3 mg/kg i.v., 15 min previously) inhibited the pressor response induced by pentagastrin, but not FG-7142. Antagonism of the pressor response elicited by pentagastrin in animals provides a simple method to establish the active dose range for CCK-B receptor antagonists in vivo. Since cardiovascular parameters can be readily monitored in man, this approach may assist in guiding clinical dose ranging studies to establish therapeutically beneficial effect of these compounds in panic disorder.