Cholecystokinin receptor subtypes and neuromodulation

Cholecystokinin-Mediated Neuromodulation of Anxiety and Schizophrenia: A "Dimmer-Switch" Hypothesis

Cholecystokinin (CCK), the most abundant brain neuropeptide, is involved in relevant behavioral functions like memory, cognition, and reward through its interactions with the opioid and dopaminergic systems in the limbic system. CCK excites neurons by binding two receptors, CCK1 and CCK2, expressed at low and high levels in the brain, respectively. Historically, CCK2 receptors have been related to the induction of panic attacks in humans. Disturbances in brain CCK expression also underlie the physiopathology of schizophrenia, which is attributed to the modulation by CCK1 receptors of the dopamine flux in the basal striatum. Despite this evidence, neither CCK2 receptor antagonists ameliorate human anxiety nor CCK agonists have consistently shown neuroleptic effects in clinical trials. A neglected aspect of the function of brain CCK is its neuromodulatory role in mental disorders. Interestingly, CCK is expressed in pivotal inhibitory interneurons that sculpt cortical dynamics and the flux of nerve impulses across corticolimbic areas and the excitatory projections to mesolimbic pathways. At the basal striatum, CCK modulates the excitability of glutamate, the release of inhibitory GABA, and the discharge of dopamine. Here we focus on how CCK may reduce rather than trigger anxiety by regulating its cognitive component. Adequate levels of CCK release in the basal striatum may control the interplay between cognition and reward circuitry, which is critical in schizophrenia. Hence, it is proposed that disturbances in the excitatory/ inhibitory interplay modulated by CCK may contribute to the imbalanced interaction between corticolimbic and mesolimbic neural activity found in anxiety and schizophrenia.

Cholecystokinin modulation of mesolimbic dopamine function: regulation of motivated behaviour

This article reviews evidence and presents a hypothesis regarding the effects of stress on motivated behaviour, and in particular the observation that stress can have both motivationally inhibitory and motivationally facilitatory effects. This issue will be addressed with regard to psychostimulant self-administration, and the role that the neurobiological mechanisms underlying motivated behaviour are thought to be involved in the evolution of addictions. Evidence from animal studies shows that stress and stress-related hormones such as corticosterone can facilitate mesolimbic dopamine function and the behavioural effects of psychostimulants, particularly at lower levels of stress. Conversely, higher levels of stress can inhibit motivated behaviour, and evidence is presented that this may occur in part through the effects of the neuropeptide cholecystokinin (CCK), acting through CCK-B receptors in the nucleus accumbens. Individual differences in endogenous CCK and dopamine systems are hypothesized to be important modulators of individual differences in motivated behaviour.

Distribution of cholecystokinin immunoreactivity in the BALB/c mouse forebrain: an immunocytochemical study

The present study describes cholecystokinin (CCK) immunoreactivity (CCK-IR) distribution in the brains of control and colchicine-treated mice. In the brains of control mice, the CCK-IR strongly revealed numerous axons and terminals. Perikarya exhibiting a faint to moderate immunoreactivity were also observed in areas such as cortices, hippocampus, amygdala, septum, and thalamus. The colchicine treatment did not seem to notably affect the brain CCK-IR innervation, but resulted in profound changes of the perikaryal staining. Indeed, the regions, which contained numerous moderately stained perikarya in the control animals, exhibited after colchicine treatment immunoreactive perikarya intensely stained but only in moderate number. This feature obviously appeared in the cortex in which, in addition to strongly stained perikarya, colchicine induced the appearance of numerous CCK-IR hillocks. In the lateral amygdala and thalamus of colchicine-treated animals, the somatic immunoreactivity was considerably decreased. The regions, such as paraventricular hypothalamic nucleus and bed nucleus of the stria terminalis, which in the control animals did not exhibit any stained perikaryon, showed a high number of strongly stained cell bodies after colchicine treatment. This study, mapping the mouse forebrain CCK-IR, demonstrated a wide distribution of this peptide. Moreover, CCK-IR is spontaneously visible in neurons of untreated mouse in some brain areas previously shown in the rat to exhibit CCK mRNA, but no clear perikaryal CCK-IR even after colchicine treatment.

Preparation of dipeptoid mimetics for the tetrapeptide cholecystokinin, CCK(30-33)

The diastereoselective synthesis of 2,3-methanophenylalanine methyl esters (5) has been achieved in 58% yield. The preparation of the dehydropeptides (1, R = Me; 2, R = H) and the cyclopropylpeptides (3, R = Me; 4, R = H) possessing good binding affinities for the CCK-A and CCK-B receptors is described. Conformational studies of the dipeptide esters 1 and 3 indicated the presence of a beta-turn within the peptide backbone, although there was no preference in type. The Phe and Trp moieties, however, did prefer to be situated on the same side of the peptide turn which is favourable for receptor binding.

Whole body autoradiography of CCK-8 in rats

Rats were given i.v., intranasal or intraperitoneal doses of CCK-8 (sulfated) labelled with 125I-labeled Bolton and Hunter reagent. Radioactivity was found mainly in the liver, kidney, and the intestinal contents. No radioactivity was detected in the brain. In animals dosed i.v., specific localization occurred in the tissue of the pyloric region of the stomach, and in the pancreas. Label persisted within the pyloric region of the stomach for longer than 30 min, in spite of the reported half-life of CCK-8 in plasma of approximately 1 min. Intranasal and intraperitoneal doses had limited bioavailability. The binding to the sites in the pyloric region of the stomach, which required systemic delivery, may have identified receptors associated with appetite control.

Dopamine D1 receptors are involved in the modulation of D2 receptors induced by cholecystokinin receptor subtypes in rat neostriatal membranes

The action of cholecystokinin octapeptide (CCK-8) on rat neostriatal dopamine (DA) D2 receptors was evaluated in membrane binding experiments. 0.1 nM of CCK-8 increased the Kd value of the D2 agonist [3H]N-propylnorapomorphine (NPA) binding sites by 42%. The CCKB antagonist PD134308 blocked this action. Kinetic analysis demonstrated that this effect of CCK-8 was related to a reduction by 45% of the association rate constant of [3H]NPA. In contrast, 1 nM of CCK-8 decreased the KH and the KL values of DA for the D2 antagonist [3H]raclopride binding sites by 56% and 50%, respectively. Both the CCKA antagonist L364718 and the CCKB antagonist PD134308 blocked this effect. The D1 antagonist SCH23390 counteracted the CCK-8 induced decrease in the KH and the KL values of DA, and allowed 1 nM of CCK-8 to produce a significant increase in the IC50 value of NPA for the [3H]raclopride binding sites. These results indicate that CCK-8 can reduce the affinity of the neostriatal D2 agonist binding sites, but increase the affinity of D2 receptors for DA. D1 receptors may exert a switching role in the modulation of the neostriatal D2 receptors by the CCK receptors.

Pentagastrin induced panic attacks: enhanced sensitivity in panic disorder patients

The effects of pentagastrin, a synthetic analogue of the cholecystokinin tetrapeptide (CCK4), were studied in 15 patients with panic disorder and 15 healthy controls. Three different intravenous dosages of pentagastrin (0.1, 0.3 and 0.6 microgram/kg) and saline were investigated. Subjects were randomly allocated to two of the four treatment groups and tested on two separate occasions, 1 week apart, using an unbalanced double-blind incomplete block design. The mean panic rate with pentagastrin was 55% (12/22) for patients and 5% (1/22) for controls. None of the subjects panicked with saline. The frequency of panic attacks between the three pentagastrin doses in patients was not different. One control subject had a panic-like attack at the highest dose of pentagastrin. These findings concur with previous studies on the panicogenic effect of CCK4 and pentagastrin and suggest a greater sensitivity for CCK receptor agonists in patients suffering from panic disorder than in healthy controls.

CCK antagonists: pharmacology and therapeutic interest

CCK was first identified and characterized in the digestive tract where it is known to be a factor involved in the control of gut motility. Later, CCK and CCK receptors were identified in regions of the central nervous system that are associated with the control of emotion, motivation and sensory processing. The recent discovery and development of CCK-receptor antagonists having selective affinity for either CCKA or CCKB receptors has led to a better understanding of the functional role of CCK and its binding sites in the brain and periphery. Some of these compounds are being examined in man for their therapeutic usefulness in mental as well as in digestive disorders. This review will highlight the results from both basic and clinical investigations that have examined the effects of selective CCK receptor ligands. The focus will be on the central nervous system pharmacology of CCK antagonists and the involvement of CCK in gastrointestinal and colonic motility.

Effects of anti-CCK-8 antiserum on acquisition and retrieval by mice in an appetitive task

Endogenous cholecystokinin (CCK) was blocked in the posterior cingulate cortex of BALB/c mice using a local injection of anti-CCK-8 antiserum, and memory effects were tested using Go-No Go visual discrimination conditioning. Injection of 0.4 microliter of anti-CCK-8 antiserum diluted to 1:10, 10-15 min before each session, produced substantial learning impairment on the discrimination task. But when injections were stopped, animals began to learn the task normally, showing that the anti-CCK antiserum effect was reversible. When the antiserum was administered at the same dose before a single test session 14 days after the end of the initial training, the retrieval process was also disturbed. These results show that cingulate cholecystokinin is essential for memory processes and suggest that cholecystokinin octapeptide may be a neuromodulator of the thalamo-cingulate pathway of Papez's circuit, which is involved in memory processes.

Effects of bombesin, of a new bombesin agonist (BIM187) and a new antagonist (BIM189) on food intake in rats, in relation to cholecystokinin

To study the mechanism by which bombesin induces satiety, we studied the effect of two new peptides, BIM187, a bombesin agonist, and BIM189, a bombesin antagonist, on food intake in rats fed 6 h a day. BIM187 at 4 micrograms/kg, significantly reduced food intake at 30 min, but did not change the total 6-h food intake. BIM189 (10 mg/kg), had no effect on food intake when administered alone, even at high doses (20 mg/kg). BIM189 selectively reduced bombesin-induced satiety but had no effect on satiety induced by BIM187. To examine the extent to which the satiety effect of bombesin or related peptides depends on the release of cholecystokinin (CCK), we studied the ability of CCK antagonists, BIM18216 and L364718, to reduce satiety induced by bombesin and BIM187. Neither BIM18216 nor L364718 alone had an effect on the 30-min food intake. They were not able to reverse the effect of bombesin on food intake. In our model, bombesin seems to act on satiety by a mechanism independent of CCK.