Heterogeneity of CCK-B receptors involved in animal models of anxiety

Neuropeptides at the crossroad of fear and hunger: a special focus on neuropeptide Y

Survival in a natural environment forces an individual into constantly adapting purposive behavior. Specified interoceptive neurons monitor metabolic and physiological balance and activate dedicated brain circuits to satisfy essential needs, such as hunger, thirst, thermoregulation, fear, or anxiety. Neuropeptides are multifaceted, central components within such life‐sustaining programs. For instance, nutritional depletion results in a drop in glucose levels, release of hormones, and activation of hypothalamic and brainstem neurons. These neurons, in turn, release several neuropeptides that increase food‐seeking behavior and promote food intake. Similarly, internal and external threats activate neuronal pathways of avoidance and defensive behavior. Interestingly, specific nuclei of the hypothalamus and extended amygdala are activated by both hunger and fear. Here, we introduce the relevant neuropeptides and describe their function in feeding and emotional‐affective behaviors. We further highlight specific pathways and microcircuits, where neuropeptides may interact to identify prevailing homeostatic needs and direct respective compensatory behaviors. A specific focus will be on neuropeptide Y, since it is known for its pivotal role in metabolic and emotional pathways. We hypothesize that the orexigenic and anorexigenic properties of specific neuropeptides are related to their ability to inhibit fear and anxiety.

Non-sulfated cholecystokinin-8 reduces meal size and prolongs the intermeal interval in male Sprague Dawley rats

The current study measured seven feeding responses by non-sulfated cholecystokinin-8 (NS CCK-8) in freely fed adult male Sprague Dawley rats. The peptide (0, 0.5, 1, 3, 5 and 10 nmol/kg) was given intraperitoneally (ip) prior to the onset of the dark cycle, and first meal size (MS), second meal size, intermeal interval (IMI) length, satiety ratio (SR = IMI/MS), latency to first meal, duration of first meal, number of meals and 24-hour food intake were measured. We found that NS CCK-8 (0.5 and 1.0 nmol/kg) reduced MS, prolonged IMI length and increased SR during the dark cycle. Furthermore, the specific CCK-B receptor antagonist L365, 260 (1 mg/kg, ip) attenuated these responses. These results support a possible role for NS CCK-8 in regulating food intake.

Neuropeptide Y attenuates anxiety- and depression-like effects of cholecystokinin-4 in mice

We investigated the involvement of neuropeptide Y (NPY) in the modulation of cholecystokinin-4 (CCK-4)-evoked anxiety and depression. Adult male mice were injected with vehicle, CCK-4, NPY, NPY Y1 receptor agonist [Leu(31), Pro(34)]-NPY or antagonist BIBP3226, via intracerebroventricular route, and subjected to social interaction or forced swim test (FST) for the evaluation of anxiety- and depression-like phenotypes, respectively. To assess the interactions between the two systems, if any, NPYergic agents were administered prior to CCK-4 and the animals were subjected to these behavioral tests. Treatment with CCK-4 or BIBP3226 dose-dependently reduced social interaction time, while NPY or [Leu(31), Pro(34)]-NPY produced opposite effect. CCK-4 treatment increased immobility time in FST. This effect was reversed by NPY and [Leu(31), Pro(34)]-NPY, although BIBP3226 per se did not alter the immobility time. In a combination study, the anxiogenic or depressive effects of CCK-4 were attenuated by NPY or [Leu(31), Pro(34)]-NPY and potentiated by BIBP3226. The brains of CCK-4 treated rats were processed for NPY immunohistochemistry. Following CCK-4 treatment, the nucleus accumbens shell (AcbSh), ventral part of lateral division of the bed nucleus of stria terminalis (BSTLV), hypothalamic paraventricular nucleus and locus coeruleus showed a reduction in NPY-immunoreactive fibers. Population of NPY-immunopositive cells was also decreased in the AcbSh, BSTLV, prefrontal cortex and hypothalamic arcuate nucleus (ARC). However, NPY-immunoreaction in the fibers of the ARC and cells of the central nucleus of amygdala was unchanged. We conclude that, inhibition of NPY signaling in the brain by CCK-4 might be causal to anxiety- and depression-like behaviors.

Neuropeptide regulation of fear and anxiety: Implications of cholecystokinin, endogenous opioids, and neuropeptide Y

The neural circuitry of fear likely underlies anxiety and fear-related disorders such as specific and social phobia, panic disorder, and posttraumatic stress disorder. The primary pharmacological treatments currently utilized for these disorders include benzodiazepines, which act on the GABAergic receptor system, and antidepressants, which modulate the monamine systems. However, recent work on the regulation of fear neural circuitry suggests that specific neuropeptide modulation of this system is of critical importance. Recent reviews have examined the roles of the hypothalamic-pituitary-adrenal axis neuropeptides as well as the roles of neurotrophic factors in regulating fear. The present review, instead, will focus on three neuropeptide systems which have received less attention in recent years but which are clearly involved in regulating fear and its extinction. The endogenous opioid system, particularly activating the μ opioid receptors, has been demonstrated to regulate fear expression and extinction, possibly through functioning as an error signal within the ventrolateral periaqueductal gray to mark unreinforced conditioned stimuli. The cholecystokinin (CCK) system initially led to much excitement through its potential role in panic disorder. More recent work in the CCK neuropeptide pathway suggests that it may act in concordance with the endogenous cannabinoid system in the modulation of fear inhibition and extinction. Finally, older as well as very recent data suggests that neuropeptide Y (NPY) may play a very interesting role in counteracting stress effects, enhancing extinction, and enhancing resilience in fear and stress preclinical models. Future work in understanding the mechanisms of neuropeptide functioning, particularly within well-known behavioral circuits, are likely to provide fascinating new clues into the understanding of fear behavior as well as suggesting novel therapeutics for treating disorders of anxiety and fear dysregulation.

Neuronal network of panic disorder: the role of the neuropeptide cholecystokinin

Panic disorder (PD) is characterized by panic attacks, anticipatory anxiety and avoidance behavior. Its pathogenesis is complex and includes both neurobiological and psychological factors. With regard to neurobiological underpinnings, anxiety in humans seems to be mediated through a neuronal network, which involves several distinct brain regions, neuronal circuits and projections as well as neurotransmitters. A large body of evidence suggests that the neuropeptide cholecystokinin (CCK) might be an important modulator of this neuronal network. Key regions of the fear network, such as amygdala, hypothalamus, peraqueductal grey, or cortical regions seem to be connected by CCKergic pathways. CCK interacts with several anxiety-relevant neurotransmitters such as the serotonergic, GABA-ergic and noradrenergic system as well as with endocannabinoids, NPY and NPS. In humans, administration of CCK-4 reliably provokes panic attacks, which can be blocked by antipanic medication. Also, there is some support for a role of the CCK system in the genetic pathomechanism of PD with particularly strong evidence for the CCK gene itself and the CCK-2R (CCKBR) gene. Thus, it is hypothesized that genetic variants in the CCK system might contribute to the biological basis for the postulated CCK dysfunction in the fear network underlying PD. Taken together, a large body of evidence suggests a possible role for the neuropeptide CCK in PD with regard to neuroanatomical circuits, neurotransmitters and genetic factors. This review article proposes an extended hypothetical model for human PD, which integrates preclinical and clinical findings on CCK in addition to existing theories of the pathogenesis of PD.

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.

Cell-type-specific CCK2 receptor signaling underlies the cholecystokinin-mediated selective excitation of hippocampal parvalbumin-positive fast-spiking basket cells

Parvalbumin-positive (PV+) fast-spiking basket cells are thought to play key roles in network functions related to precise time keeping during behaviorally relevant hippocampal synchronous oscillations. Although they express relatively few receptors for neuromodulators, the highly abundant and functionally important neuropeptide cholecystokinin (CCK) is able to selectively depolarize PV+ basket cells, making these cells sensitive biosensors for CCK. However, the molecular mechanisms underlying the CCK-induced selective and powerful excitation of PV+ basket cells are not understood. We used single and paired patch-clamp recordings in acute rat hippocampal slices, in combination with post hoc identification of the recorded interneurons, to demonstrate that CCK acts via G-protein-coupled CCK2 receptors to engage sharply divergent intracellular pathways to exert its cell-type-selective effects. In contrast to CCK2 receptors on pyramidal cells that signal through the canonical G(q)-PLC pathway to trigger endocannabinoid-mediated signaling events, CCK2 receptors on neighboring PV+ basket cells couple to an unusual, pertussis-toxin-sensitive pathway. The latter pathway involves ryanodine receptors on intracellular calcium stores that ultimately activate a nonselective cationic conductance to depolarize PV+ basket cells. CCK has highly cell-type-selective effects even within the PV+ cell population, as the PV+ dendrite-targeting bistratified cells do not respond to CCK. Together, these results demonstrate that an abundant ligand such as CCK can signal through the same receptor in different neurons to use cell-type-selective signaling pathways to provide divergence and specificity to its effects.

Anxiolytic-like effects of the neurokinin 1 receptor antagonist GR-205171 in the elevated plus maze and contextual fear-potentiated startle model of anxiety in gerbils

Gerbils show a neurokinin (NK)1 receptor pharmacological profile, which is similar to that observed in humans, and thus have become a commonly used species to test efficacy of NK1 receptor antagonists. The aim of this study was to determine whether systemic administration of the NK1 receptor antagonist GR-205171 produced anxiolytic-like effects in the elevated plus maze and in a novel contextual conditioned fear test using fear-potentiated startle (FPS). On the elevated plus maze, treatment with GR-205171 at 0, 0.3, 1.0, and 5.0 mg/kg doses, 30 min before testing produced anxiolytic-like effects in an increasing dose-response manner as measured by the percentage of open arm time and percentage of open arm entries. For contextual fear conditioning, gerbils were given 10 unsignaled footshocks (0.6 mA) at a 2-min variable interstimulus interval in a distinctive training context. Twenty-four hours after training, gerbils received treatment of GR-205171 at 0, 0.3, 1.0, and 5.0 mg/kg doses, 30 min before testing in which startle was elicited in the same context in which they were trained. Contextual FPS was defined as an increase in startle over pretraining baseline values. All drug dose levels (0.3, 1.0, and 5.0 mg/kg) significantly attenuated contextual FPS when compared with the vehicle control group. A control group, which received testing in a different context, showed little FPS. These findings support other evidence for anxiolytic activity of NK1 receptor antagonists and provide a novel conditioned fear test that may be an appropriate procedure to test other NK1 antagonists for preclinical anxiolytic activity in gerbils.

Role of the amygdaloid cholecystokinin (CCK)/gastrin-2 receptors and terminal networks in the modulation of anxiety in the rat. Effects of CCK-4 and CCK-8S on anxiety-like behaviour and [3H]GABA release

The amygdala plays a key role in fear and anxiety. The intercalated islands are clusters of glutamate-responsive GABAergic neurons rich in cholecystokinin (CCK)-2 receptors which control the trafficking of nerve impulses from the cerebral cortex to the central nucleus of amygdala. In this study, the nature of the CCK-glutamate-GABA interactions within the rat rostral amygdala, and their relevance for anxiety, were studied. CCK/gastrin-like immunoreactive nerve terminals were found to be mainly restricted to the paracapsular intercalated islands and the rostrolateral part of the main intercalated island. Behaviourally, the bilateral microinjection of CCK-4 (0.043-4.3 pmol/side) or CCK-8S (4.3 pmol/side) into the rostrolateral amygdala reduced the open-arm exploration in the elevated plus-maze without affecting locomotion. In contrast, neither CCK-4 nor CCK-8S (0.043-4.3 pmol/side) had any effects in the shock-probe burying test as compared with their saline-treated controls. Biochemically, CCK-4 (0.3 and 1.5 microm), unlike CCK-8S, enhanced significantly the K(+)-stimulated release of [(3)H]GABA from amygdala slices. These effects were fully prevented by prior superfusion of the slices with either the selective CCK-2 receptor antagonist CR2945 (3 microm), or 6,7-dinitroquinoxaline-2,3(1H,4H)-dione (DNQX), 10 microm, a glutamatergic (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate receptor antagonist. It is suggested that CCK modulates glutamate-GABA mechanisms by acting on CCK-2 receptors via volume transmission occurring at the level of the basolateral amygdaloid nucleus and/or by synaptic or perisynaptic volume transmission in the region of the rostrolateral main and paracapsular intercalated islands, resulting in subsequent disinhibition of the central amygdaloid nucleus and anxiety or panic-like behaviour.

Lesions of the nucleus accumbens shell can reduce activity in the elevated plus-maze

Across different behavioural tasks, nucleus accumbens (n.acc) lesions have generated conflicting effects on locomotor activity and in particular, the relative roles of the n.acc shell and core subfields in this have been controversial. To date there is only one study examining effects of lesions to the medial n.acc on elevated plus-maze (EPM) behaviour; these lesions were shown to increase both locomotor and exploratory activity. Given the well-documented distinction between shell and core, the present study sought to extend previous research by testing lesions selective to each n.acc subfield in the EPM. Results showed no statistical differences between core lesioned and sham-operated animals on any measure. In contrast, shell lesions consistently reduced locomotion and exploratory activity. This direction of effects is opposite to that previously observed after medial n.acc. lesions. In conclusion, locomotion and exploratory activity were clearly reduced by shell but not core lesions, consistent with other evidence for the functional heterogeneity of n.acc shell and core.

Cholecystokinin and endogenous opioid peptides: interactive influence on pain, cognition, and emotion

It is well documented that stressful life experiences contribute to the etiology of human mood disorders. Cholecystokinin (CCK) is a neuropeptide found in high concentrations throughout the central nervous system, where it is involved in numerous physiological functions. A role for CCK in the induction and persistence of anxiety and major depression appears to be conspicuous. While increased CCK has been associated with motivational loss, anxiety and panic attacks, an increase in mesocorticolimbic opioid availability has been associated with coping and mood elevation. The close neuroanatomical distribution of CCK with opioid peptides in the limbic system suggests that there may be an opioid-CCK link in the modulation and expression of anxiety or stressor-related behaviors. In effect, while CCK induces relatively protracted behavioral disturbances in both animal and human subjects following stressor applications, opioid receptor activation may change the course of psychopathology. The antagonistic interaction of CCK and opioid peptides is evident in psychological disturbances as well as stress-induced analgesia. There appears to be an intricate balance between the memory-enhancing and anxiety-provoking effects of CCK on one hand, and the amnesic and anxiolytic effects of opioid peptides on the other hand. Potential anxiogenic and mnemonic influences of site-specific mesocorticolimbic CCK and opioid peptide availability, the relative contributions of specific CCK and opioid receptors, as well as the time course underlying neuronal substrates of long-term behavioral disturbances as a result of stressor manipulations, are discussed.

Cholecystokinin-2 (CCK2) receptor-mediated anxiety-like behaviors in rats

Cholecystokinin (CCK) is a neurotransmitter in the brain closely related to anxiety. Of the two CCK receptor subtypes, CCK(2) receptors are most implicated in the control of anxiety-related behavior. CCK(2) receptor activation causes anxiogenic effects while the blockade of this receptor has anxiolytic effects. This review focuses on the molecular mechanisms of CCK(2) receptors underlying anxiety-related behaviors of PVG hooded and Spraque-Dawley (SD) rats in two anxiety models (elevated plus-maze [EPM] and cat exposure test). PVG hooded rats showed prolonged freezing behavior in the cat exposure test while SD rats showed very low levels of freezing. A CCK(2) receptor antagonist (LY225910) attenuated freezing behavior in PVG hooded rats while a CCK(2) receptor agonist (CCK-4) increased freezing behavior in SD rats. In contrast, the two strains behaved similarly on the EPM. CCK-4 caused a pronounced anxiogenic effect in PVG hooded rats but only a slight effect in SD rats. CCK(2) antagonists also showed more pronounced anxiolytic effects in PVG hooded rats than in SD rats. CCK(2) receptor expression was greater in PVG hooded than in SD rats in the cortex and hippocampus. Genetic studies also demonstrated four differences in the DNA sequence of the CCK(2) receptor gene between the two rat strains.

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.

Synthesis and biological characterisation of [3H]BBL454, a new CCK2 selective radiolabelled agonist displaying original pharmacological properties

[(3)H]BBL454, a new CCK(2) selective tritiated agonist was prepared via the reductive tritiation of a 5-aminopentyn-1-yl moiety introduced on the N-terminal end of a pentapeptide derivative of cholecystokinin. The binding properties of this labelled compound were determined on CHO cells transfected with the rat CCK(2) receptor. [(3)H]BBL454 is able to discriminate two affinity states of the CCK(2) receptor a supplementary indication of its validity for further exploring the heterogeneity of this receptor.

Atypical behavioural responses to CCK-B receptor ligands in Fawn-Hooded rats

At present there is an increasing literature demonstrating heterogeneity of the CCK-B receptor. Recent reports by our laboratory have demonstrated that the Fawn-Hooded rat demonstrates atypical neurochemical responses to CCK4, in vitro. Since the ability of CCK-B receptor ligands to modulate affective state is dependent on the putative receptor subtype activated, the aim of the present study was to examine the behavioural effects of the CCK-B receptor agonist, t-boc-CCK4, and the CCK-B receptor antagonist, Ci-988 in Fawn-Hooded and Wistar Kyoto rats. Interestingly, both t-boc-CCK4 and Ci-988 produced an anxiolytic profile in FH rats as determined by an increased time spent on the open arms of an elevated plus maze, while both drugs were devoid of any behavioural effect in WKY rats, lending further support to the theory that the FH rat strain has an atypical relative proportion of these putative subtypes apparently resulting in a predominantly CCK-B2 receptor effect.

Cholecystokinin antagonists: pharmacological and therapeutic potential

Cholecystokinin (CCK) is a regulatory peptide hormone, predominantly found in the gastrointestinal tract, and a neurotransmitter present throughout the nervous system. In the gastrointestinal system CCK regulates motility, pancreatic enzyme secretion, gastric emptying, and gastric acid secretion. In the nervous system CCK is involved in anxiogenesis, satiety, nociception, and memory and learning processes. Moreover, CCK interacts with other neurotransmitters in some areas of the CNS. The biological effects of CCK are mediated by two specific G protein coupled receptor subtypes, termed CCK(1) and CCK(2). Over the past fifteen years the search of CCK receptor ligands has evolved from the initial CCK structure derived peptides towards peptidomimetic or non-peptide agonists and antagonists with improved pharmacokinetic profile. This research has provided a broad assortment of potent and selective CCK(1) and CCK(2) antagonists of diverse chemical structure. These antagonists have been discovered through optimization programs of lead compounds which were designed based on the structures of the C-terminal tetrapeptide, CCK-4, or the non-peptide natural compound, asperlicin, or derived from random screening programs. This review covers the main pharmacological and therapeutic aspects of these CCK(1) and CCK(2) antagonist. CCK(1) antagonists might have therapeutic potential for the treatment of pancreatic disorders and as prokinetics for the treatment of gastroesophageal reflux disease, bowel disorders, and gastroparesis. On the other hand, CCK(2) antagonists might have application for the treatment of gastric acid secretion and anxiety disorders.

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.

Further evidence that the CCK2 receptor is coupled to two transduction pathways using site-directed mutagenesis

A heterogeneity of CCK2 receptors has been reported which could correspond to different states of coupling to G proteins and/or association with different second messenger systems. To investigate these hypotheses, the wild-type CCK2 receptor and three mutants F347A, D100N and K333M/K334T/R335L, expected to modify the coupling of the G protein with the third intracellular loop of the receptor, were transfected into Cos-7 cells and their binding and signalling properties were evaluated using the natural ligand CCK8. Activation of wild-type as well as F347A, D100N or K333M/K334T/R335L CCK2 receptors by this ligand led to a similar arachidonic acid release which was blocked by pertussis toxin and the phospholipase A2 inhibitor, mepacrine. Nevertheless, in contrast to the wild-type CCK2 receptor, addition of CCK8 to cells transfected with the F347A or K333M/K334T/R335L mutants did not result in the production of inositol phosphates while the maximum increase in this second messenger formation was reduced by 30% with the D100N mutant. Taken together, these results suggest that the CCK2 receptor is coupled to two G proteins and that Phe347 and the cluster of basic residues K333/K334/R335 probably play a key role in Gq protein stimulation leading to inositol phosphate production but not in activation of the G protein coupled to phospholipase A2. These data bring additional support at the molecular level to the existence of different affinity states of CCK2 receptors suggested from the results of binding assays and behavioural studies.

The inability of CCK to block (or CCK antagonists to substitute for) the stimulus effects of chlordiazepoxide

To further examine the relationship between cholecystokinin (CCK) and GABA, the present study assessed the ability of the CCK-A antagonist devazepide and the CCK-B antagonist L-365,260 to substitute for the stimulus effects of chlordiazepoxide (CDP), as well as the ability of CCK-8s to block these effects, in female Long-Evans rats within the conditioned taste aversion baseline of drug discrimination learning. Both devazepide and L-365,260 failed to substitute for the discriminative stimulus properties of CDP, and CCK-8s failed to block its stimulus effects. The benzodiazepine diazepam did substitute for, and the benzodiazepine antagonist flumazenil did block, the stimulus effects of CDP. This suggests that the lack of substitution for, or antagonism of, CDP by the CCK antagonists and CCK-8s, respectively, was not due to the inability of the present design to assess such effects. Possible bases for the current findings, e.g., necessity of an anxiogenic baseline, drug and receptor specificity, as well as the dose-response nature of the interaction, were discussed. Given that a relationship between CCK and GABA has been reported in other designs, the present results suggest that such a relationship may be preparation specific.

Insights into peptide and protein function: a convergent approach

From viruses to multicellular organisms, life is inseparable from the genetic instructions aimed at regulating its maintenance, division, multiplication, differentiation and death (apoptosis). Over the past 15 years, structural studies have begun to resolve the complex reactions involved in these fundamental processes in biology. The three-dimensional representations of the complexes formed with peptides and/or proteins have allowed interpretation of the biochemical data and formulation of novel hypotheses about the control and execution of these processes. Moreover, they have opened the way to rational approaches for designing compounds able to interfere with these crucial events in normal or pathological conditions. Various results obtained in our laboratory in these fields are briefly summarized in this review.

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.

The effect of a cholecystokinin agonist on masseter muscle activity in the cat

The CCK(B) agonist, pentagastrin, has been shown to induce anxiety in human subjects. Similarly, in the cat model, pentagastrin facilitates the expression of hypothalamically activated emotional behavior. Because hypothalamically mediated emotional behavior is also accompanied by increased EMG activity in the jaw muscles, these experiments were designed to examine the combined effects of administration of pentagastrin with activation of hypothalamically mediated emotional behavior upon jaw muscle EMG activity. Electrodes were carefully lowered through previously placed guide tubes overlying the hypothalamus until a behavioral site was identified. Following the establishment of a stable threshold current for eliciting an emotional behavioral response, the skin overlying the ipsilateral masseter muscle was shaved and cleaned with alcohol, and surface electrodes were attached. The EMG was recorded, amplified, digitized, and stored in a microcomputer for analysis. Mean power frequencies (MPF) and latencies for behavior were calculated for baseline prior to infusion of all drugs. Following this, the effects of intravenous administration of pentagastrin and the CCK(B) antagonist LY288513 on the MPF were determined. The infusion of the CCK(B) agonist, pentagastrin (0.77, 1.92, and 3.84 microg/kg), decreased MPF in a time-related manner. The effects of pentagastrin 1.92 microg/kg were blocked by the CCK(B) antagonist, LY288513 (6.54 microg/kg). In addition, the infusion of LY288513 alone increased MPF. These results are surprising in that pentagastrin's anxiogenic properties would appear to make it likely to facilitate motor activity, not suppress it.

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.

5-(Tryptophyl)amino-1,3-dioxoperhydropyrido[1,2-c]pyrimidine-based potent and selective CCK(1) receptor antagonists: structural modifications at the tryptophan domain

Analogues of the previously reported potent and highly selective CCK(1) receptor antagonist (4aS, 5R)-2-benzyl-5-(N-Boc-tryptophyl)amino-1,3-dioxoperhydropyrido-[1, 2-c]pyrimidine (2a) were prepared to explore the structural requirements at the Boc-tryptophan domain for CCK(1) receptor affinity. Structural modifications of 2a involved the Trp side chain, its conformational freedom, the Boc group, and the carboxamide bond. Results of the CCK binding and in vitro functional activity evaluation showed three highly strict structural requirements: the type and orientation of the Trp side chain, the H-bonding acceptor carbonyl group of the carboxamide bond, and the presence of the Trp amino protection Boc. Replacement of this acid-labile group with 3, 3-dimethylbutyryl or tert-butylaminocarbonyl conferred acid stability to analogues 14a and 15a, which retained a high potency and selectivity in binding to CCK(1) receptors, as well as an in vivo antagonist activity against the acute pancreatitis induced by caerulein in rats. Oral administration of compounds 14a and 15a also produced a lasting antagonism to the hypomotility induced by CCK-8 in mice, suggesting a good bioavailability and metabolic stability.

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.

Peripheral stimulation of CCK-B receptors by BC264 induces a hyperexploration, dependent on the delta opioid system in the nucleus accumbens of rat

This study analyses the influence of the CCKergic system on the enkephalinergic system in the exploratory behavior of rats, using both behavioral and biochemical approaches. The results show that the increase of the spontaneous alternation behavior induced by the selective CCKB agonist, BC264 (3 microg/kg) was not suppressed by the opioid antagonists, naloxone (100 microg/kg), or naltrindole (300 microg/kg). In contrast, BC264 injected at the same dose induced a hyperlocomotor activity measured in the open-field test, which was antagonized by the selective delta opioid antagonist, naltrindole. BC264 (3 microg/kg) significantly increased the extracellular levels of Met-LI in the anterior part of the nucleus accumbens. Furthermore, local injection of naltrindole (0.25 microg/0.5 microl) in the anterior nucleus accumbens completely suppressed the hyperlocomotion induced by BC264. The behavioral effects induced by BC264 cannot be explained by its interaction with gastrinic receptors mediating gastric acid release, since BC264 produced a long-lasting increase of gastric acid output from conscious gastric fistula rats only at doses 100 times higher than those inducing behavioral modifications. The hyperlocomotion obtained after stimulation by BC264 of probably peripheral CCKB receptors, indicates that this receptor type could participate in the transmission of information between the peripheral system and some regions of the CNS involved in motivations and emotions.

The cholecystokininB receptor is coupled to two effector pathways through pertussis toxin-sensitive and -insensitive G proteins

Previous binding studies have suggested the existence of two affinity states for type B cholecystokinin receptors (CCK(B)R), which could correspond to different coupling states of the receptor to G proteins. To test this hypothesis, we have further investigated signal transduction pathways coupled to rat CCK(B)R stably transfected in Chinese hamster ovary cells. We show that CCK(B)R are coupled to two distinct transduction pathways involving two different G proteins, a pertussis toxin-insensitive/phospholipase C pathway leading to the production of inositol phosphate and arachidonic acid, and a pertussis toxin-sensitive/phospholipase A2 pathway leading to the release of arachidonic acid. We further demonstrate that the relative degree of activation of each effector pathway by different specific CCK(B)R agonists is the same, and that a specific CCK(B)R antagonist, RB213, can differentially antagonize the two signal transduction pathways elicited by these agonists. Taken all together, these data could be explained by the recently proposed theory assuming that the receptor can exist in a three-state model in which two active conformations corresponding to the complex formed by the receptor with two different G proteins coexist. According to this model, agonists or antagonists could recognize preferentially either conformation of the activated receptor, leading to variable behavior in a system containing a single receptor type.

Is there a future for neuropeptide receptor ligands in the treatment of anxiety disorders?

This review provides an overview of preclinical and clinical evidence of a role for the neuroactive peptides cholecystokinin (CCK), corticotropin-releasing factor (CRF), neuropeptide Y (NPY), tachykinins (i.e., substance P, neurokinin [NK] A and B), and natriuretic peptides in anxiety and/or stress-related disorders. Results obtained with CCK receptor antagonists in animal studies have been highly variable, and clinical trials with several of these compounds in anxiety disorders have been unsuccessful so far. However, future investigations using CCK receptor antagonists with better pharmacokinetic characteristics and animal models other than those validated with the classical anxiolytics benzodiazepines may permit a more precise evaluation of the potential of these compounds as anti-anxiety agents. Results obtained with peptide CRF receptor antagonists in animal models of anxiety convincingly demonstrated that the blockade of central CRF receptors may yield anxiolytic-like activity. However, the discovery of nonpeptide and more lipophilic CRF receptor antagonists is essential for the development of these agents as anxiolytics. Similarly, there is clear preclinical evidence that the central infusion of NPY and NPY fragments selective for the Y1 receptor display anxiolytic-like effects in a variety of tests. However, synthetic nonpeptide NPY receptor agonists are still lacking, thereby hampering the development of NPY anxiolytics. Unlike selective NK1 receptor antagonists, which have variable effects in anxiety models, peripheral administration of selective NK2 receptor antagonists and central infusion of natriuretic peptides produce clear anxiolytic-like activity. Taken as a whole, these findings suggest that compounds targeting specific neuropeptide receptors may become an alternative to benzodiazepines for the treatment of anxiety disorders.

Utility of ethological analysis to overcome locomotor confounds in elevated maze models of anxiety

The elevated plus-maze is a commonly used model to identify putative anxiolytic and anxiogenic drugs. However, the validity of elevated plus-maze and other recently developed variants such as the elevated zero-maze has recently been questioned on the grounds that both the reference anxiolytic drug chlordiazepoxide and the psychostimulant d-amphetamine increase open arm exploration and stimulate locomotor activity. These findings suggest that measures of "anxiety" in the elevated maze cannot be adequately dissociated from simple changes in locomotor activity, which may confound the interpretation of results obtained using these models. A variety of approaches to assess drug effects on locomotor activity in the elevated maze have been suggested, including the use of total and closed arm entries, as well as supplementary tests such as exploration of the holeboard apparatus. However, all these approaches utilise the measurement of exploration in a novel environment, and as such, could potentially be influenced by either changes in anxiety or locomotor activity. Recently, it has been shown that ethological measures of "risk assessment", such as stretched-attend postures and head-dipping, are sensitive indicators of drug-effects in the elevated maze. The present study assessed the utility of ethological analysis in dissociating locomotor activity from "anxiety" by comparing the effects of d-amphetamine to those of chlordiazepoxide in the rat elevated zero-maze. The results showed that both chlordiazepoxide and d-amphetamine increase the amount of time spent in the open arms and reduce "risk assessment" without increasing line crossing or rearing. These results confirm that under certain test conditions, psychostimulants are capable of producing "false-positives" in elevated maze models, and that both traditional methods and the ethological measures used in this study fail to unequivocally dissociate drug effects on anxiety from effects on locomotor activity. Further studies using other species and different classes of psychostimulants are warranted to determine the generality of these findings.

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.

Opposite effects of CCK(B) agonists in grooming behaviour in rats: further evidence for two CCK(B) subsites

1. The hypothesis of the existence of two CCK(B) receptor subsites, CCK(B1) and CCK(B2) corresponding probably to different coupling states of CCK(B) receptors, was studied by measuring grooming behaviour in rats. 2. The B1 receptor agonist, BC197 (300 microg kg(-1), i.p.) produced a 45-50% decrease in grooming activity, which was prevented by both the CCK(B) receptor antagonists CI-988 (20 microg kg(-1) i.p.) and L-365,260 (200 microg kg(-1), i.p.). 3. In contrast, 3, 10 and 30 microg kg(-1), i.p., of the potent B2 receptor agonist, BC264, enhanced grooming (150-190%). This effect was prevented by previous injection of 75 microg kg(-1) of L-365,260 while higher doses (200 microg kg(-1), i.p.) produced only a partial antagonism. Moreover, CI-988 (20 microg kg(-1), i.p.), showed an opposite effect in potentiating the responses induced by BC264. However, 200 microg kg(-1) of CI-988 tended to suppress the increase of grooming induced by BC264. 4. The effects of BC264 were prevented by the D1 receptor (SCH 23390) and D2 receptor (sulpiride) antagonists, while those of BC197 were only antagonized by sulpiride, emphasizing the existence of a link between peptidergic (CCK) and dopaminergic systems. 5. This study brings additional evidence for the existence of the two CCK(B) receptor subsites and suggests that particular attention should be focused on the selectivity of CCK(B) receptor agonists, notably to explain the fact that some compounds such as Boc-CCK4 induce anxiogenic-like effects while others, including BC264, are devoid of these effects.

Novel constrained CCK-B dipeptoid antagonists derived from pipecolic acid

A new series of 4-substituted pipecolic acid derivatives was prepared and incorporated into dipeptoids. The resulting products behave as moderately potent CCK-B antagonists but their constrained structure and its comparison with structurally related compounds yield valuable information about the conformational requirements for optimal recognition of the CCK-B receptor by antagonists.

CCK(B) antagonists protect against some aspects of the ethanol withdrawal syndrome

Effects of the CCK(B) antagonists, CAM1028 and C1988, and the CCK(A) antagonist, CAM1481, were studied on the ethanol withdrawal syndrome. When handling-induced behavior was measured hourly for 12 h from withdrawal of ethanol, a small, but significant, protective effect was seen with 3 mg/kg CAM1028, but not with 0.3, 1, or 10 mg/kg. C1988 (0.3 1,3, or 10 mg/kg), or CAM1481 (0.1 or 1 mg/kg), had no effects. At 16 h from ethanol withdrawal, these ratings were significantly decreased by 3 mg/kg CAM1028 or C1988, but not by lower doses. At 16 h, CAM1481 had very small, but significant, protective effects. At 3 mg/kg, CAM1028, increased the latencies to audiogenic seizures, but had only small effects on convulsion incidence. CAM1481 did not alter the audiogenic convulsions. The decrease in convulsion thresholds to NMDLA, at 16 h from ethanol withdrawal, was completely prevented by CAM1028 or C1988, at 1 and at 3 mg/kg, but not by lower doses; CAM1481 had no significant effects. The results suggest change in CCK(B) receptors may be involved in the later stages of the ethanol withdrawal syndrome.

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.

Modulation of memory, reinforcement and anxiety parameters by intra-amygdala injection of cholecystokinin-fragments Boc-CCK-4 and CCK-8s

This series of experiments examined the effects of the cholecystokinin (CCK) fragments Boc-CCK-4 and CCK-8s on memory, reinforcement and anxiety following unilateral injection into the central nucleus of the amygdala (CeA). In experiment 1, rats with chronically implanted cannulae were injected with CCK-8s or Boc-CCK-4 and were tested on a one-trial uphill avoidance task. Post-trial injection of 20 ng Boc-CCK-4 or 1 ng CCK-8s was found to improve the retention performance, whereas lower and higher doses had no effect. The hypermnestic effects of Boc-CCK-4 and CCK-8s were no longer evident when injection was performed 5 h, rather than immediately, after the learning trial. In experiment 2, the elevated plus-maze was used to gauge anxiogenous properties of intra-amygdala injections of Boc-CCK-4 and CCK-8s in memory-enhancing doses. The treatment with 20 ng Boc-CCK-4 and 1 ng CCK-8s did not influence the number of entries into and time spent on the open and enclosed arms of the maze as well as other anxiety-related behaviors. In experiment 3, possible reinforcing effects of the CCK-fragments were examined. After intra-amygdala injection of Boc-CCK-4 or CCK-8s in memory-enhancing doses the rats were placed into one of four restricted quadrants of a circular open field (closed corral) for a single conditioning trial. Subsequent tests for conditioned corral preference revealed no evidence for reinforcing or aversive effects of the CCK-fragments. In sum, these findings indicate that Boc-CCK-4 and CCK-8s facilitate memory processing upon injection into the CeA without exerting reinforcing or anxiogenous effects.

Synthesis and biological properties of new constrained CCK-B antagonists: discrimination of two affinity states of the CCK-B receptor on transfected CHO cells

To improve our knowledge of the bioactive conformation of CCK-B antagonists, we have developed a new series of constrained dipeptoids whose synthesis and biochemical properties are reported here. These compounds, of general structure N alpha-[(2-adamantyloxy)carbonyl]-alpha-methyltryptophanyl-(4 -X)-proline, were designed by introducing a cyclization in the structure of the previously described CCK-B/peptoid antagonist RB 210, N-[N-[(2-adamantyloxy)carbonyl]-DL-alpha-methyltryptophanyl] -N-(2-phenylethyl)glycine (Blommaert et al. J. Med. Chem. 1993, 36, 2868-2877), by means of a five-membered ring. Structure-affinity relationship studies showed that an R configuration of Trp-C alpha and a cis configuration of the pyrrolidine substituents were favorable for receptor recognition. The most potent compounds of this new series had similar affinities for the CCK-B receptor as RB 210 and proved to be far more efficient in inhibiting inositol phosphate production in CHO cells stably transfected with rat brain CCK-B receptor, with IC50 values approaching those of the commonly used antagonists L-365,260 and PD-134,308. Moreover, binding studies performed using transfected CHO cells showed that two affinity states of the CCK-B receptor can be discriminated by some of these compounds which also have different biological profiles and are therefore highly interesting tools for the biochemical and pharmacological characterization of CCK-B receptor heterogeneity.

The effects of CCK-4 on dopamine D1 agonist-induced grooming are blocked by a CCK(A) receptor antagonist: evidence for a novel CCK receptor subtype?

The neuropeptide cholecystokinin (CCK) has been shown to interact with dopamine in various ways, including attenuation of dopamine D1 receptor-mediated vacuous chewing and grooming. While we have demonstrated a clear role for the CCK(A) receptor in the attenuation of dopamine D1 agonist-induced vacuous chewing, studies of grooming yielded anomalous results. We examined the effects of selective CCK receptor antagonists on the attenuation of SKF 38393-induced grooming by the CCKB agonist CCK-4. Administration of SKF 38393 (5 mg/kg s.c.) to male Sprague-Dawley rats resulted in a significant increase in grooming which was reduced to control levels by CCK-4 (20 mg/kg i.p.). Pretreatment with either the CCKA receptor antagonist devazepide or the CCK(B) receptor antagonist L-365,260 significantly attenuated this effect over a range of doses (20, 100, 500 microg/kg i.p.). The suppression of dopamine D1 agonist-induced grooming by CCK-4 does not appear to reflect a non-specific effect of anxiogenesis, as it was unaffected by the anxiolytic diazepam. The CCK receptor antagonists alone were without behavioural effect. Taken together with previous studies in models of anxiety and analgesia, our findings lend further support to the hypothesis that CCK-4 may act at a novel receptor subtype.

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.

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.

Structure-based design of new constrained cyclic agonists of the cholecystokinin CCK-B receptor

New constrained cyclic pseudopeptide cholecystokinin-B (CCK-B) agonists have been designed on the basis of conformational characteristics of the potent and selective CCK-B agonist Boc-Trp-(NMe)Nle-Asp-Phe-NH2 (Ki = 0.8 nM, selectivity ratio CCK-A/CCK-B > 6000) (Goudreau et al. Biopolymers, 1994, 34, 155-169). These compounds are among the first successful examples of macrocyclic constrained CCK4 analogs endowed with agonist properties and as such may be of value for the development of nonpeptide CCK-B agonists. The affinities and selectivities of these compounds for CCK-B and CCK-A receptors have been determined in vitro by measuring the displacement of [3H]pCCK8 binding to guinea pig cortex and pancreas membranes, respectively. The most potent compound, 8b, N-(cycloamido)-alpha-Me(R)Trp-[(2S)-2-amino-9- ((cycloamido)carbonyl)nonanoyl]-Asp-Phe-NH2, has a Ki value of 15 +/- 1 nM for guinea pig cortex membranes with a good CCK-B selectivity ratio (CCK-A/CCK-B = 147). Furthermore, 8b behaved as a potent and full agonist in a functional assay which measures the stimulation of inositol phosphate accumulation in CHO cells transfected with the rat CCK-B receptor (EC50 = 7 nM). The in vivo affinity of 8b for mouse brain CCK-B receptors was determined following intracerebroventricular injection (ID50 approximately 29 nmol/kg). 8b was also shown to cross the blood-brain barrier (0.16%), after intravenous administration in mice. 8b also increased gastric acid secretion measured in anesthetized rats after intravenous injection. Therefore, 8b appears to be an interesting pharmacological tool and is currently under investigation as a lead for further development of nonpeptide CCK-B agonists.

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.

Short-term consumption of a diet rich in fat decreases anxiety response in adult male rats

Short- and long-term changes in the composition of dietary macronutrients [protein (P), carbohydrate (C), and fat (F)] alter neurochemistry and behavior in animals. We examined whether short-term intake of a diet rich in P, C, or F affected their anxiety response (AR). AR of Sprague-Dawley rats was measured in an elevated plus maze. Rats were placed in the black compartment facing the wall opposite the aperture, and the time (max. 360 s) it took to enter the white compartment with all four paws was noted. Rats were fed Purina chow and tap water unless otherwise indicated. On repeated testing (three times on the same day) AR increased and, consequently, most rats spent the entire 360 s in the dark. Whereas most rats exhibited low anxiety response in trial 1, which increased during successive trials (low-high group), some exhibited high initial anxiety that remained unchanged (high-high group). To determine whether macronutrients may alter AR, groups of low-high and high-high rats were tested three times on the same day and then put on a P, C, or F diet for 7 days. On day 8, they were again tested for AR in a single trial and the results compared with those of the third trial of the previous test (preC: 302 +/- 39, post-C: 294 +/- 42, p > 0.05; pre-P: 305 +/- 35, post-P: 297 +/- 43, p > 0.05; pre-F: 321 +/- 17, post-F: 241 +/- 24sec, p = 0.009; n = 30; mean +/- SEM). The results show that a diet rich in F, but not P or C, decreases AR in rats.

Plus-maze retest profile in mice: importance of initial stages of trail 1 and response to post-trail cholinergic receptor blockade

Recent research has shown that a single undrugged prior experience of the elevated plus-maze produces significant behavioural changes upon 24-h retest in rats and mice. Typically, when reexposed to the maze, animals display an increased avoidance of the open arms and a corresponding preference for the enclosed sections of the apparatus. Using ethological analyses, the present series of experiments sought to further characterize this phenomenon in mice and to determine whether or not it involves cholinergic receptor mechanisms. Results confirmed that behaviour during Trial 2 is markedly different to that seen on initial exposure, and that such changes are independent of the duration of Trial 1 (2 vs. 5 min). Retest behavioural changes included reduced entry latencies, reduced open arm entries, less time on the open arms and centre platform, lower levels of exploratory head-dipping, and increased entries into and time spent in the closed arms. The importance to the retest phenomenon of the first few minutes of initial exposure was further suggested by min-by-min analyses of the behaviour of animals naive to the maze. Results showed that behaviour during the first min is characterized by high levels of risk assessment from the centre platform and relatively low, but equal, levels of open- and closed-arm exploration. From min 2 onwards, however, behaviour showed a marked change with increasing open arm/centre platform avoidance, increasing closed-arm preference, and decreasing levels of risk assessment and exploratory head-dipping. Thus, it would appear that this within-session aversive learning transfers between sessions to account for behavioural profiles on retest. Irrespective of the duration of Trial 1 (2 or 5 min), posttrial administration of the muscarinic antagonist, scopolamine (0.1-1.0 mg/kg), failed to significantly alter the behavioural changes seen between trials. Data are discussed in relation to the apparent sensitization of fear produced by plus-maze exposure, its possible relation to phobia acquisition, and the need for further research on underlying mechanisms.