The behavioral effects of CCK8 injected into the medial nucleus accumbens are dependent on the motivational state of the rat

The unappreciated roles of the cholecystokinin receptor CCK(1) in brain functioning

The CCK(1) receptor is a G-protein-coupled receptor activated by the sulfated forms of cholecystokinin (CCK), a gastrin-like peptide released in the gastrointestinal tract and mammal brain. A substantial body of research supports the hypothesis that CCK(1)r stimulates gallbladder contraction and pancreatic secretion in the gut, as well as satiety in brain. However, this receptor may also fulfill relevant roles in behavior, thanks to its widespread distribution in the brain. The strategic location of CCK(1)r in mesolimbic structures and specific hypothalamic and brainstem nuclei lead to complex interactions with neurotransmitters like dopamine, serotonin, and glutamate, as well as hypothalamic hormones and neuropeptides. The activity of CCK(1)r maintains adequate levels of dopamine and regulates the activity of serotonin neurons of raphe nuclei, which makes CCK(1)r an interesting therapeutic target for the development of adjuvant treatments for schizophrenia, drug addiction, and mood disorders. Unexplored functions of CCK(1)r, like the transmission of interoceptive sensitivity in addition to the regulation of hypothalamic hormones and neurotransmitters affecting emotional states, well-being, and attachment behaviors, may open exciting roads of research. The absence of specific ligands for the CCK(1) receptor has complicated the study of its distribution in brain so that research about its impact on behavior has been published sporadically over the last 30 years. The present review reunites all this body of evidence in a comprehensive way to summarize our knowledge about the actual role of CCK in the neurobiology of mental illness.

Central neural and endocrine mechanisms of non-exercise activity thermogenesis and their potential impact on obesity

The rise in obesity is associated with a decline in the amount of physical activity in which people engage. The energy expended through everyday non-exercise activity, called non-exercise activity thermogenesis (NEAT), has a considerable potential impact on energy balance and weight gain. Comparatively little attention has been paid to the central mechanisms of energy expenditure and how decreases in NEAT might contribute to obesity. In this review, we first examine the sensory and endocrine mechanisms through which energy availability and energy balance are detected that may influence NEAT. Second, we describe the neural pathways that integrate these signals. Lastly, we consider the effector mechanisms that modulate NEAT through the alteration of activity levels as well as through changes in the energy efficiency of movement. Systems that regulate NEAT according to energy balance may be linked to neural circuits that modulate sleep, addiction and the stress response. The neural and endocrine systems that control NEAT are potential targets for the treatment of obesity.

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.

The open field as a paradigm to measure the effects of drugs on anxiety-like behaviors: a review

The open field is a very popular animal model of anxiety-like behavior. An overview of the literature on the action elicited by effective or putative anxiolytics in animal subjected to this procedure indicates that classical treatments such as benzodiazepine receptor full agonists or 5-HT(1A) receptor full or partial agonists elicit an anxiolytic-like effect in this procedure in most cases (approximately 2/3). However, compounds (triazolobenzodiazepines such as adinazolam and alprazolam, selective serotonin reuptake inhibitors) that have a different spectrum of therapeutic efficacy in anxiety disorders such as panic attacks, generalized anxiety disorder or obsessive-compulsive disorder were poorly effective as anxiolytics in the open field test, suggesting that this paradigm may not model features of anxiety disorders. The procedure is also relevant for the study of compounds endowed with anxiogenic effects, as such effects were detected after treatments with benzodiazepine receptor inverse agonists or with corticotropin releasing factor (CRF) receptor agonists.

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.

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.

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.

Behavioural evidence for cholecystokinin-dopamine D1 receptor interactions in the rat

The effects of cholecystokinin (CCK) on behavioural responses to the dopamine D1 receptor agonist (+/- )SKF 38393 ((+/- )-2,3,4,5-tetrahydro-7,8- dihydroxy-1-phenyl-1H-3-benzazepine HCl) were studied in the rat. SKF 38393 (5 mg/kg s.c.) induced stereotyped grooming and vacuous chewing movements. Both responses were inhibited by CCK-8S (10-50 micrograms/kg i.p.), but the preferential CCKB receptor agonist CCK-4 (20-100 micrograms/kg i.p.) attenuated SKF 38393-induced grooming only. Suppression of SKF 38393-induced grooming and vacuous chewing movements by CCK-8S was blocked by the selective CCKA receptor antagonist MK-329 (also known as devazepide or L-364,718) (0.1, 0.3 mg/kg i.p.) but unaffected by the CCKB receptor antagonist L-365,260 (0.1, 0.3 mg/kg i.p.). We conclude that CCK can modify dopamine-mediated behavioural responses, possibly reflecting an action post-synaptic to dopamine terminals. The effect on dopamine D1 receptor agonist-induced vacuous chewing movements is probably mediated by CCKA receptors, while the effect on grooming may reflect an interaction with the CCKA receptor and/or a novel CCKB receptor subtype.

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

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

Antidepressant-like effects of CCKB antagonists in mice: antagonism by naltrindole

1. The effects of selective CCKB agonists, BC 264 and BC 197 were investigated in the conditioned suppression of motility test in mice, an animal model used to select antidepressant drugs. The results showed that both CCKB agonists at doses of 3 and 30 micrograms kg-1, accentuated the suppression of motility in shocked mice and did not modify the behaviour of non-shocked mice. The effects of BC 264 were suppressed by L-365,260. 2. L-365,260 alone, at doses of 0.2 and 2 mg kg-1 decreased motor inhibition in shocked mice and had no effect in non-shocked mice. 3. The effects of L-365,260 observed in shocked mice were suppressed by naltrindole, a selective antagonist for delta-opioid receptors, suggesting the occurrence of physiological adverse interactions between CCK and opioid systems. 4. Together, these results suggest that CCKB antagonists could block centrally located CCKB receptors to produce antidepressant-like effects which could indirectly involve delta-opioid receptor stimulation.

Biological actions of cholecystokinin

Cholecystokinin (CCK) has emerged as an important mammalian neuropeptide, localized in peripheral organs and in the central nervous system. This review presents an overview of the molecular aspects of CCK peptides and CCK receptors, the anatomical distribution of CCK, the neurophysiological actions of CCK, release of CCK and effects of CCK on release of other neurotransmitters, and the actions of CCK on digestion, feeding, cardiovascular function, respiratory function, neurotoxicity and seizures, cancer cell proliferation, analgesia, sleep, sexual and reproductive behaviors, memory, anxiety, and dopamine-mediated exploratory and rewarded behaviors. Human clinical studies of CCK in feeding disorders and panic disorders are described. New findings are presented on potent, nonpeptide CCK antagonists, selective for the two CCK receptor subtypes, which demonstrate that endogenous CCK has biologically important effects on physiology and behavior.

Involvement of D2 dopaminergic receptors in the emotional and motivational responses induced by injection of CCK-8 in the posterior part of the rat nucleus accumbens

When CCK-8 was injected in the rat posterior nucleus accumbens, where it is in part co-localized with dopamine, a decrease in exploration of the four hole box and the elevated plus maze was observed. In this study, a selective destruction of the dopaminergic mesoaccumbens pathway induced by local injection of 6-hydroxydopamine (6-OHDA) in the nucleus accumbens was found to suppress the CCK-8-evoked behavioral effects. Moreover, an ex vivo measurement of the dopaminergic metabolism has been performed after injection of CCK-8 in the posterior nucleus accumbens by electrochemical detection of dopamine and its metabolites extracted from punches of brain tissue. The results showed that CCK-8 decreased the turnover of dopamine in the posterior part but not in the anterior part of the nucleus accumbens or in the ventral tegmental area. Furthermore, sulpiride, a selective antagonist for D2 dopamine receptors, but not SCH 23390, a selective antagonist for D1 dopamine receptors, prevented CCK-8-induced behavioral responses. Taken together, these results suggest that CCK-8 could be involved in behavioral adaptation to situations producing change in emotional and/or motivational states through modulation of presynaptic D2 receptor functioning.

Alterations in brain cholecystokinin receptors in suicide victims

Cholecystokinin (CCK) and benzodiazepine receptor binding characteristics were analyzed in the brain tissue samples from 19 suicide victims and 23 control cases. In the frontal cortex, significantly higher apparent number of CCK receptors and affinity constants were found in the series of suicide victims. These differences between suicides and controls were present in similar proportions when the suicide cases with depressive syndrome or violent or non-violent means of self-killing were compared to matched controls. However, when the samples were split into subgroups consisting of persons either below or over the age of 60 years, significant differences in the CCK receptor characteristics in the frontal cortex were observed only between younger suicides and controls. Furthermore, the younger suicide victims had a higher density of CCK receptors in the cingulate cortex, whereas in older suicides the value was lower as compared to age-matched controls. No difference in benzodiazepine receptor binding was found between control and suicide groups. The results of this investigation suggest that CCK-ergic neurotransmission is linked to self-destructive behaviour, probably through its impact on anxiety and adaptational deficits.

The CCK-A and CCK-B receptor antagonists, devazepide and L-365,260, enhance morphine antinociception only in non-acclimated rats exposed to a novel environment

Devazepide, a potent CCK-A receptor antagonist, and L-365,260, a selective CCK-B receptor antagonist, have been introduced as pharmacologic tools for differentiating the physiologic roles of CCK-A and CCK-B receptor subtypes. In the present study, we tested the effects of devazepide and L-365,260, on morphine antinociception in rats using the thermal sensorimotor tail flick test. Both devazepide and L-365,260 significantly enhanced the antinociceptive action of morphine, but only in rats that had not been acclimated to the laboratory environment or habituated to investigator handling. When tested with fully acclimated animals, devazepide and L-365,260 had no effect whatsoever; they neither enhanced nor attenuated morphine-induced antinociception. These observations indicate that the effects of devazepide and L-365,260, CCK antagonists, on morphine antinociception appear to be dependent on the animal's response to a new environment or to the stress induced by an unaccustomed experimental paradigm.

Cholecystokinin-induced anxiety: how is it reflected in studies on exploratory behaviour?

Central cholecystokinin (CCK)-ergic neurotransmission has been implicated in the genesis of negative emotions. Most animal studies on the neurochemical background of CCK-induced anxiety have, up to date, exploited exploratory activity paradigms. The interaction of CCK with GABAergic inhibitory neurotransmission, mediated probably through CCK-B receptors, could be the neurochemical substrate for anxious type of exploratory behaviour. However, the CCK-A and CCK-B receptor-mediated interactions of this neuropeptide with mesencephalic dopaminergic regulation of motivation for locomotor activity have the potential to interfere with the behavioural outcome from routine exploratory activity tests. Systemic treatment with CCK receptor antagonists is likely to influence both GABA- and dopamine-linked CCK-ergic neurotransmission, and therefore their effects in exploratory activity tests should be interpreted with caution.

Effects of dopamine agonists on excitatory inputs to nucleus accumbens neurons from the amygdala: modulatory actions of cholecystokinin

The interaction of the cholecystokinin octapeptide (CCK-8) with dopamine (DA) and dopamine agonists on neurons in the nucleus accumbens was investigated using single unit recording and iontophoretic techniques in urethane-anaesthetized rats. Neurons in the nucleus accumbens were activated by single pulse stimulation of amygdala. Using seven-barrel microelectrodes, the effects of iontophoretic application of CCK-8, DA, dopamine D1 and/or D2 receptor agonists (SKF 38393 and LY 171555 respectively) were compared. The iontophoretic application of DA, LY 171555 and LY 171555 + SKF 38393 attenuated by 50-60% the excitatory responses of accumbens neurons to electrical stimulation of basolateral amygdala whereas SKF 38393 attenuated the response by less than 30%. The iontophoretic application of CCK reduced these attenuating effects of DA, LY 171555 and SKF 38393 + LY 171555. With CCK there was a rather small reduction of the attenuating effect of SKF 38393. These observations provide additional electrophysiological evidence of the interaction of CCK and dopamine and suggest that the interaction is associated mainly with dopamine D2 mechanisms.

Specific binding of D-Tyr25 (Nle28,31)-CCK(25-33) to cortical membranes from rat brain

The 125I-labeled sulfonated cholecystokinin (CCK) analogue, D-Tyr25(Nle28,31)-CCK(25-33), bound saturably and reversibly to membranes of rat cerebral cortex. The dissociation constant Kd was estimated to be 0.14 nM by Scatchard analysis and 0.40 nM from analyses of dissociation/association rates. Hill coefficients of 0.99, Scatchard plots and drug competition studies were consistent with a single population of binding sites. Dependence on divalent cations and inhibition by guanylyl-imidodiphosphate suggested binding to a receptor coupled to a G protein. Competition studies with CCK-analogues and non-peptide specific antagonists indicated binding to receptor sites of the B-subtype (CCKB).

Investigation of behavioral and electrophysiological responses induced by selective stimulation of CCKB receptors by using a new highly potent CCK analog, BC 264

The new CCKB analog, Boc-Tyr (SO3H)-gNle-mGly-Trp-(NMe)-Nle-Asp-PheNH2 (BC 264) exhibited a high affinity (KI = 0.39 +/- 0.15 nM) and selectivity for central (B) versus peripheral (A) receptors (KI CCKA/KI CCKB = 910) in the rat. In agreement with these binding studies, BC 264 was at least 50 times more potent than CCK8 in stimulating the firing of rat CA hippocampal neurones. Furthermore stereotaxic injection of BC 264 or CCK8 in the VTA of rats resulted in potentiation of the dopamine-induced hypolocomotion. These two types of CCK8 responses have been previously shown in involve CCKB receptors. In contrast, after administration into the postero-median nucleus accumbens, the hypoexploration, the increase of emotionality of rats, or the potentiation of dopamine-induced hyperlocomotion were obtained after injection of CCK8 but not of BC 264, supporting the involvement of peripheral CCKA receptors in these CCK8 responses. Owing to its resistance to peptidases, BC 264 appears to be of great interest in the investigation of the still uncertain functional roles of CCK in the central nervous system.

CCK8 effects on motivational and emotional states of rats involve CCKA receptors of the postero-median part of the nucleus accumbens

Administration of 3 fmol of cholecystokinin octapeptide (CCK8) into the postero-median nucleus accumbens (NAS) induced an hypoexploration measured using the four-hole box and an increase in the emotional states of rats observed in the elevated plus maze. These effects seem likely to involve CCKA receptors since they were reversed by the selective CCKA antagonist L364,718 (100 micrograms/kg, 200 micrograms/kg IP) and not observed after injection of 0.1 to 1000 fmol unsulfated cholecystokinin octapeptide (CCK8NS) in the same region. On the other hand, CCK8 or CCK8NS injected into the anterior NAS did not significantly modify these behaviors. These results support the neuroanatomical heterogeneity in the distribution of CCK and its binding sites in the NAS, but raise the question of the presence of CCKA receptors not detected in binding studies and of the behavioral effects mediated by CCKB receptor stimulation in this structure.