Neuropharmacological profile of cholecystokinin-like peptides

The potential role of cholecystokinin in schizophrenia: review and update

The role of the neuropeptide cholecystokinin in schizophrenia has been widely explored because of its modulating action on midbrain dopamine neurons. The recent discovery of more specific receptor subtype cholecystokinin antagonists should be considered as potential treatment for schizophrenia with fewer side effects. This paper reviews cholecystokinin/dopamine interactions in animal and human studies. Clinical trials with cholecystokinin agonists and antagonists in schizophrenia are updated.

Nitric oxide mediates caerulein-induced suppression of locomotor activity

Caerulein, a non-selective agonist of cholecystokinin (CCK) receptors, is shown to suppress locomotor activity in rodents via stimulation of CCK(A) receptors. In the present study we examined the possible involvement of nitric oxide (NO) in caerulein-induced hypolocomotion in rats. Caerulein (10 microg/kg) markedly decreased the horizontal and vertical components of locomotor activity in rats measured in dark motility boxes. Pretreatment with a nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME), at 5 mg/kg i.p., abolished the inhibiting action of caerulein on the horizontal activity, but did not affect the reduced frequency of rearing. The other doses of L-NAME (1, 10 and 20 mg/kg) were ineffective against caerulein. As L-NAME at this dose range does not stimulate locomotor activity, it is likely that NO is involved in the motor suppressant effect of systemically administered caerulein.

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

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

In search of black bile

Historically Western medicine has been divided into two main schools that were based on the ancient Greek tradition. These are the Hygeian school, based on the views of Hippocrates (born 460 BC), and the Asclepian school which is named after the Greek god of medicine but probably based on the physician Asclepius who was said to have performed miracles!

In brief, the Hygeian school of medicine views health as a natural state of the body. The body is believed to be endowed with inherent healing powers which, if one lives in harmony with these powers, maintains health and helps to restore it should it become impaired. Disease is seen as a manifestation of a weakness of the inherent healing powers of the body and the function of the physician is to help the patients to live within the natural law (vis medicatrix naturae) and to remove impediments to those mechanisms that maintain and restore health.

The second school that has profoundly influenced the development of modern medicine is the Asclepian school which arose in about 1200 BC around the teaching of Asclepius. This school focuses on diseases, their causes and cures. Each disease is considered to be the effect of, or response to, a specific cause that primarily affects a specific organ system. For every disease it is postulated that there is a specific drug or procedure which can alleviate the symptoms or cure the disease. Thus, the successful physician is the one who can make the correct diagnosis and prescribe the correct therapy.

Analgesic effect of ceruletide in men is limited to specific pain qualities

Cholecystokinin (CCK) and related peptides are supposed to be potent analgesic neuropeptides. Studies in rodents suggest a dose-dependent biphasic effect. The present study aimed to examine the pain modulating effect of different doses (0.5 microgram and 5 micrograms) of ceruletide (CRL), infused i.v. for 30 min. Pain thresholds were obtained for ischemic, mechanical, and thermal pain. In addition, pain tolerance was measured for mechanical pain. According to a placebo-controlled double-blind within-subject design 25 healthy men attended three experimental sessions each. Pain perception was measured as a baseline and twice after the infusion. The effect of both doses of CRL to enhance the pain threshold for thermal stimuli is in line with former studies. However, perception of heat stimuli above or below the threshold was not substantially affected by CRL treatment. Algesic properties of CRL are also indicated, because the tolerance for mechanical pain decreased after administration of the high dose of CRL. Perception of ischemic pain was not obviously influenced by any of the treatments. The role of CRL in human pain modulation seems to vary, depending on the type of experimental pain.

Effects of ceruletide and haloperidol on auditory evoked potentials in the cat brain

The influence of cholecystokinin-like peptide, ceruletide, on EEG and auditory evoked potentials (AEPs) was studied in nine cats. The cats were bearing electrodes implanted in the auditory cortex, hippocampus, reticular formation and cerebellum. Reference drugs used were haloperidol and neostigmine. The hippocampus showed the strongest effect of ceruletide, whereas the cerebellum was virtually unresponsive. The amplitude of AEPs was increased by peptide, an effect lasting up to 21 days which, according to amplitude frequency analysis (AFC) was due to an augmented theta response. The latter possibly indicates increased signal transfer to, or through, the brain structure in question, particularly in the hippocampal neurons. The effects of haloperidol and neostigmine did not reflect those of ceruletide and lasted only a few hours.

Improved event-related potential signs of selective attention after the administration of the cholecystokinin analog ceruletide in healthy persons

Cholecystokinin (CCK) is co-localized with dopamine (DA) in neurons of the mesolimbic-frontocortical dopamine (DA) system, considered essential for the pathology of psychotic behavior and associated attention deficits. The present experiments in 13 healthy men aimed at examining the effects of the CCK analog ceruletide on attention as reflected by event-related brain potentials (ERPs). Subjects were tested according to a double-blind cross-over design on three occasions, following intravenous infusion of placebo, 0.5 microgram ceruletide, and 2.5 micrograms ceruletide. ERPs were recorded during the subject's performance on an auditory selective attention task including the concurrent presentation of frequent standard tones and infrequent deviant tones which the subject had to listen to, or to ignore. The processing negativity (PN) over frontocentral cortical areas, reflecting selective attention, was higher after ceruletide than placebo, this increase being most pronounced after the 2.5 micrograms dose (placebo -1.29 +/- 0.38 microV versus ceruletide -3.02 +/- 0.65 microV, p < .05). ERP signs of a general increase in cortical arousal after ceruletide did not reach significance. Likewise, mismatch negativity, an indicator of preattentive processing of stimulus deviance, was not significantly affected by the peptide. The results indicate that ceruletide affects human brain function primarily by improving selective attention.

Receptor-receptor interactions and their relevance for receptor diversity. Focus on neuropeptide/dopamine interactions

Receptor diversity in combination with receptor-receptor subtype specific interactions, which can be antagonistic or synergistic in character, markedly increase plasticity in WT and VT in the nervous system. In this way switching among transmission lines for the various DA receptor subtypes becomes possible. Some of these aspects are supported by our work on selective modulation of D2 receptors by CCK and NT. Selective regulation of D2 receptors via CCK-8 receptor subtypes and NT receptors may underlie CCK/DA interactions and NT/DA interactions in the basal ganglia. These studies underline the importance of receptor-receptor interactions exerted at the membrane level between neuropeptide receptors and D2 receptors, which are determined at least in part by the ongoing activity at D1 receptors. In the case of both CCK/D2 and NT/D2 receptor interactions, it has been possible, by means of intrastriatal and intraaccumbens microdialysis, to obtain a functional correlate to the receptor interactions found in the membrane preparations from the striatum. Schizophrenia may be in part related to reduced release of CCK and/or NT peptides or to alterations in their receptor interactions with the D2 receptor. This view may lead to new therapeutic approaches.

Cholecystokinin octapeptide in vitro and ex vivo strongly modulates striatal dopamine D2 receptors in rat forebrain sections

Receptor autoradiographic experiments together with the filter wipe-off technique were performed to investigate the effects of cholecystokinin octapeptide (CCK-8) on dopamine D2 receptors. In vitro studies showed that 1 nM CCK-8 significantly increased the KD value of binding sites for the D2 agonist [3H]N-propylnorapomorphine (NPA) in the rostral and caudal parts of the nucleus accumbens by 48 and 148% respectively. In contrast, 1 nM CCK-8 significantly decreased the IC50 value of dopamine for binding sites for the D2 antagonist [125I]iodosulpride in the rostral and caudal parts of the caudate-putamen by 46 and 56% respectively, and in the rostral and caudal parts of the nucleus accumbens (areas of CCK-dopamine coexistence) by 57 and 75% respectively. Ex vivo studies demonstrated that 30 min after an intraventricular injection of 1 nmol/rat CCK-8 the KD value of [3H]NPA binding sites in the caudal part of the forebrain and the IC50 value of dopamine for [125I]iodosulpride binding sites in the caudal part of the nucleus accumbens were significantly increased by 160% and decreased by 77% respectively. These results indicate for the first time that in sections CCK-8 in vitro and ex vivo can strongly regulate D2 receptor affinity in the striatum. The present studies also provide evidence for stronger modulation of D2 receptors by CCK-8 in the area of CCK/dopamine coexistence in the nucleus accumbens than in other basal ganglion areas, supporting the existence of CCK/D2 receptor interactions in cotransmission. The stronger interactions found in sections than in membrane preparations may indicate the requirement of intracellular mechanisms and/or a more intact membrane structure for optimal receptor-receptor interactions.

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.

3'-Phosphoadenosine 5'-phosphosulfate biosynthesis and the sulfation of cholecystokinin by the tyrosylprotein-sulfotransferase in rat brain tissue

This article resumes the work we have accomplished in the past few years. Cholecystokinin sulfation is an important post-translational modification necessary for the biological activity of this peptide hormone. The tyrosyl protein sulfotransferase (TPST) activity from rat cerebral cortex was characterized. TPST activity is most probably responsible for the endogenous sulfation of CCK. TPST reaction kinetic properties were studied using radiolabeled 3'-phosphoadenosine 5'-phosphosulfate (PAPS) and the non-sulfated peptide acceptor terbutyloxycarbonyl-cholecystokinin octapeptide (BocCCK-8(ns)) as substrates, and brain microsomes as the enzyme source. The BocCCK-8 sulfating reaction data is consistent with the idea that TPST forward reaction follows an ordered Bi Bi mechanism. PAPS biosynthesis and availability was studied in slices from rat cerebral cortex incubated in the presence of [35S]sulfate. There is a rapid and dynamic turnover of the steady-state level of PAPS in brain cells which is decreased by depolarizing agents such as potassium, veratridine and glutamate. Furthermore, the presence of a membrane-bound PAPS biosynthesis inhibitor was observed. These results are discussed in view of the biological importance that the cell sulfating pathways might play in nerve cell activity.

Receptor-receptor interactions as an integrative mechanism in nerve cells

Several lines of evidence indicate that interactions among transmission lines can take place at the level of the cell membrane via interactions among macromolecules, integral or associated to the cell membrane, involved in signal recognition and transduction. The present view will focus on this last subject, i.e., on the interactions between receptors for chemical signals at the level of the neuronal membrane (receptor-receptor interaction). By receptor-receptor interaction we mean that a neurotransmitter or modulator, by binding to its receptor, modifies the characteristics of the receptor for another transmitter or modulator. Four types of interactions among transmission lines may be considered, but mainly intramembrane receptor-receptor interactions have been dealt with in this article, exemplified by the heteroregulation of D2 receptors via neuropeptide receptors and A2 receptors. The role of receptor-receptor interactions in the integration of signals is discussed, especially in terms of filtration of incoming signals, of integration of coincident signals, and of neuronal plasticity.

Subcellular and developmental studies of the tyrosyl protein sulfotransferase in rat brain

1. Tyrosyl protein sulfotransferase (TPS) activity in the newborn and mature rat brain was studied using the cholecystokinin derivative terbutyloxycarbonyl-Asp-Tyr-Met-Gly-Trp-Met-Asp-PheNH2, BocCCK-8(ns), as the peptide substrate. 2. TPS activity was enriched 4 times in the microsomal and synaptic vesicular enriched fractions of rat cerebral cortex. 3. CCK-8 content, in the subcellular fractions and the peptide sulfation activity distribution was in accord with the hypothesis that tyrosyl protein sulfotransferase plays a key role in the maturation process of bioactive CCK. 4. TPS activity measured in membranes from newborn brain was 2.5 times higher than the activity observed in the mature brain membranes with a Vmax = 0.83 +/- 0.05 and 0.31 +/- 0.02 respectively. The apparent KM for the sulfate donor, 3'-phosphoadenosine 5'-phosphosulfate (PAPS), was similar, 94 +/- 4 nM and 90 +/- 6 nM and the KM for the peptide substrate, BocCCK-8(ns), was 234 +/- 16 microM and 160 +/- 12 microM in the newborn and adult brain membranes respectively. 5. TPS activity reached normal mature values within 20 days of age. 6. These data support the idea that tyrosyl protein sulfation is an important process in the secretion mechanism and in the CCK maturation.

Cholecystokinin receptors and memory: a radial maze study

CCK receptor agonists and antagonists have repeatedly been demonstrated to improve and impair, respectively, learning and memory functions. However, all studies to date have exploited avoidance paradigms. In the present study, the effect of some CCK receptor agonists and antagonists on the ability to learn an appetitively motivated task and to influence spatial working memory was investigated. In the first experiment, drugs were given immediately after each training session in the radial maze and the animals were tested, drug-free, during a 2-week period. After the initial treatments with caerulein, an unselective CCK receptor agonist (100 ng/kg SC), the animals were slightly less successful to obtain food pellets during the sessions on the first 2 days; whereas proglumide, an unselective CCK receptor antagonist (1 mg/kg SC) was without any effect. However, on the following days, all the three groups of rats (saline, caerulein, and proglumide) performed in a similar way. In the second experiment, drugs were given before each test session to well-trained animals. Scopolamine (0.15 and 0.3 mg/kg IP), the reference amnestic drug, produced dose-dependent impairment of working memory in the radial maze test. Proglumide (1 and 10 mg/kg SC) and devazepide, (a selective CCK-A receptor antagonist; 0.01 and 1 mg/kg SC), as well as caerulein (0.01, 0.1 and 1 microgram/kg SC) and CCK-4 (a selective CCK-B receptor agonist; 25 and 50 micrograms/kg SC) had no reliable effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of diurnal phase and pimozide on cholecystokinin-elicited hypoactivity in the hamster

Locomotor activity in golden Syrian hamsters was measured following IP injections of cholecystokinin (CCK; 25 micrograms/kg) and pimozide (0.5 mg/kg), the dopamine receptor antagonist. In addition, animals were tested during either the dark or light phase of the diurnal cycle in either dark or light running wheel environments. Results indicated that CCK-elicited hypoactivity was blocked by pimozide and that the effect of CCK was evident only among animals tested during the light phase of the daily cycle. Ambient lighting conditions in the test environment did not modify the drug effects. Independently of any drug effect, locomotor activity was affected by diurnal phase and ambient lighting in the test environment. Animals were more active when tested during the dark phase than during the light phase and locomotor activity was higher under dark than under light ambient conditions. It is concluded that diurnal phase modulates CCK's effect on hamster locomotion and that CCK's effect on locomotion is mediated, in part, by dopaminergic mechanisms.

Coactivation of dopamine D1 and D2 receptors increases the affinity of cholecystokinin-8 receptors in membranes from post-mortem human caudate-putamen

The effects of dopamine in vitro were investigated on the binding sites for cholecystokinin-8 (sulphated, CCK-8) and neurotensin in membrane preparations of the caudate-putamen and nucleus accumbens of post-mortem human brains. Dopamine reduced the IC50 value of competition curves with CCK-8 for [125I]CCK-8 binding in membranes from the caudate-putamen, but not the nucleus accumbens, with a maximal decrease of -25 +/- 9% at 300 nM of dopamine. This decrease could be antagonized by 100 nM of SCH 23390 or 100 nM of raclopride. Kinetic analysis of [125I]CCK-8 binding showed a decrease in the first order dissociation rate constant and in the kinetic Kd (-22 +/- 6% and -24 +/- 6%, respectively) at 300 nM of dopamine, without any significant effect on the apparent or actual association rate constant. Competition curves with neurotensin versus [125I]neurotensin were not affected by dopamine (10-1000 nM) in membranes from the caudate-putamen or the nucleus accumbens. These results suggest that dopamine, by synergistic stimulation of both D1 and D2 receptors, selectively increases the affinity of CCK-8 receptors in the human caudate-putamen, by a selective inhibition of ligand dissociation. This increase may reflect a positive feed-back mechanism, further enhancing the modulatory effects of CCK-8 on dopamine neurotransmission.

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.

Changes in motor activity and forebrain [propionyl-3H]propionylated-CCK-8 binding in mice after repeated administration of drugs affecting cholecystokinin receptors

The effects of acute or repeated treatment of male albino BKW mice with caerulein, a cholecystokinin octapeptide (CCK-8) agonist, and with devazepide (MK-329) and L-365,260, antagonists at CCKA ('peripheral') and CCKB ('central') receptors respectively, on motor activity and [propionyl-3H]propionylated-CCK-8 ([3H]pCCK-8) binding were studied. Acute treatment with a large dose of caerulein (100 micrograms/kg s.c.) suppressed motor activity (line crossings and rearings) whereas devazepide (2 mg/kg i.p.) had the opposite action. L-365,260 (2 mg/kg i.p.) increased only the number of rearings. Tolerance developed to the locomotor effects of caerulein and devazepide when these same doses were administered once daily (caerulein) or twice daily (devazepide) for 10 days. Twice daily administration of L-365,260 (2 mg/kg) for 10 days did not significantly alter the locomotor activity of mice. The sedative effect of caerulein (20 micrograms/kg s.c.) was markedly reduced in mice receiving repeated injections of either a larger amount of caerulein (100 micrograms/kg) or devazepide but not after L-365,260. The stimulant effect of (+)-amphetamine (2 mg/kg s.c.) on motor activity was increased by subchronic administration of either devazepide or caerulein, but not by L-365,260. All three compounds (caerulein, devazepide and L-365,260) increased the number of [( 3H]pCCK-8 binding sites in mouse forebrain but the increase was only significant after L-365,260. The effects of long-term treatment with caerulein are probably related to the stimulation of CCKA receptors, whereas the paradoxically similar action of devazepide may be linked to the blockade of both subtypes of the CCK-8 receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

The behavioural properties of CI-988, a selective cholecystokininB receptor antagonist

1. The behavioural effects of a selective cholecystokininB (CCKB) receptor antagonist CI-988 were investigated in rodents. 2. In three rodent tests of anxiety (rat elevated X-maze, rat social interaction test and mouse light/dark box) CI-988 over the dose range 0.001-10.0 mg kg-1, (i.p.) produced an anxiolytic-like action. The magnitude of this effect was similar to that of chlordiazepoxide (CDP). In contrast, the selective CCKA receptor antagonist, devazepide, was inactive. CI-988 also showed anxiolytic-like action in the rat conflict test but the magnitude of this effect was about 2.5 fold less than that of CDP. 3. Central but not peripheral administration of the selective CCKB receptor agonist, pentagastrin, like FG 7142, produced an anxiogenic-like action. 4. The pentagastrin-induced anxiety was dose-dependently antagonized by CI-988, whereas devazepide was inactive. However, ten times higher doses of CI-988 were required to block a similar action of FG 7142. 5. In contrast to CDP, CI-988 up to 3000 fold higher doses than those inducing anxiolysis was inactive in tests measuring sedation and ataxia. It also failed to antagonize pentylenetetrazol-induced tonic seizures. Furthermore, CI-988 did not interact with alcohol or barbiturates. Thus, CI-988 appears to be an anxioselective compound. 6. The anxiolytic-like action of CDP in the rat elevated X-maze was dose-dependently antagonized by flumazenil. In contrast, the benzodiazepine receptor antagonist failed to block a similar effect of CI-988. 7. Thus, CI-988 shows anxiolytic-like activity in several animal models of anxiety. The anxiolytic-like effect of CI-988 involves a novel mechanism of action, that is likely to be mediated by selective antagonism of the brain CCKB receptor. It is suggested that CI-988 should have a better side-effect profile in man than the benzodiazepines.

Effects of cholecystokinin on morphine-elicited hyperactivity in hamsters

The effects of the octapeptide cholecystokinin (CCK) on hamster locomotor activity were investigated in three experiments. In Experiment 1, the effect of CCK (25, 50, 75 micrograms/kg) on morphine (2.5 mg/kg)-elicited hyperactivity was studied. Results indicated that CCK antagonized morphine-elicited hyperactivity and that CCK alone elicited hypoactivity. There were no effects of dose of CCK. In Experiment 2, the effects of intraperitoneal (IP) and subcutaneous (SC) routes of administration of CCK (25 micrograms/kg) on locomotor activity were studied. Compared to saline controls, CCK induced hypoactivity that was of greater magnitude and of longer duration when administered IP than SC. Experiment 3 was designed to replicate the route of administration effect observed in Experiment 2 and to determine whether sensitization to CCK-induced hypoactivity develops over the course of a few injections. Results indicated that CCK-induced hypoactivity was greater after IP than SC administration but that sensitization was not detectable. It is concluded that CCK antagonizes morphine-elicited hyperactivity in the hamster by acting, in part, independently of morphine to produce opposite behavioral effects.

CCK-8 modulates D2 receptor agonist-induced hypermotility in the nucleus accumbens

The influence of CCK-8 on locomotor effects associated with independent D2 receptor stimulation was studied. To selectively stimulate mesolimbic D2 receptors LY 171555 was injected into the nucleus accumbens of awake rats. Locomotor activity was measured in the open-field test. LY 171555 induced a biphasic effect: low doses stimulated, whereas higher doses inhibited locomotor activity. CCK-8 injected into the posteromedial part of the nucleus accumbens suppressed hyperlocomotion induced by LY 171555. The CCK-8 effect was prevented by the CCK-antagonist L 364,718. Our results indicate that CCK-8 modulates D2 receptor-mediated effects in the mesolimbic system.

Differential involvement of CCK-A and CCK-B receptors in the regulation of locomotor activity in the mouse

The influence of the CCK-A antagonist devazepide and the CCK-B/gastrin antagonist L-365,260 on the locomotor activity of mice was studied. Devazepide and L-365,260 had opposite effects on spontaneous locomotor activity, and on caerulein- and apomorphine-induced hypomotility in the mouse. Devazepide in high doses (0.1-1 mg/kg IP) reduced spontaneous motor activity, whereas L-365,260 at a high dose (1 mg/kg IP) increased the activity of mice. Devazepide (0.1-10 micrograms/kg) moderately antagonized the sedative effect of apomorphine (0.1 mg/kg SC) and caerulein (25 micrograms/kg SC), whereas L-365,260 (1-10 micrograms/kg) significantly potentiated the actions of dopamine and CCK agonists. Concomitant administration of caerulein (15 micrograms/kg SC) and apomorphine (0.1 mg/kg SC) caused an almost complete loss of locomotor activity in the mouse. Devazepide and L-365,260 (0.1-10 micrograms/kg) were completely ineffective against caerulein-induced potentiation of apomorphine hypomotility. Devazepide in high doses (0.1-1 mg/kg), reducing the spontaneous motor activity of mice, counteracted the motor excitation induced by d-amphetamine (5 mg/kg IP). The CCK agonist caerulein (100 micrograms/kg SC) had a similar antiamphetamine effect. Devazepide (1-100 micrograms/kg) and L-365,260 (1 micrograms/kg) reversed completely the antiamphetamine effect of caerulein. The results of present study reflect apparently distinct role of CCK-A and CCK-B receptors in the regulation of motor activity. The opposite effect of devazepide and L-365,260 on caerulein- and apomorphine-induced hypolocomotion is probably related to the antagonistic role of CCK-A and CCK-B receptor subtypes in the regulation of mesencephalic dopaminergic neurons. The antiamphetamine effect of caerulein is possibly linked to the stimulation of CCK-A receptors in the mouse brain, whereas the blockade of both subtypes of the CCK-8 receptor is involved in the antiamphetamine effect of devazepide.

Similar behavioral and biochemical effects of long-term haloperidol and caerulein treatment in albino mice

Behavioral and biochemical experiments on male albino mice have revealed similar effects after the cessation of repeated (15 days) haloperidol (0.5 mg/kg daily IP) and caerulein (0.1 mg/kg daily SC) treatment. Tolerance developed to the action of muscimol (a GABA-A agonist, 1 mg/kg IP), caerulein (a CCK-8 agonist, 15 micrograms/kg SC) and flumazenil (a benzodiazepine antagonist, 10 mg/kg IP). Muscimol and caerulein were not able to suppress the motor activity of mice after 15 days treatment with haloperidol and caerulein. Flumazenil, which increased motor activity in saline-treated animals, also failed to affect activity after extended haloperidol or caerulein treatment. In contrast, the motor excitation induced by amphetamine (an indirect dopamine agonist, 3 mg/kg IP) was increased after haloperidol or caerulein administration. In radioligand binding studies the density of dopamine-2-receptors in striatum, opioid receptors in mesolimbic structures, and benzodiazepine and GABA-A receptors in brainstem was significantly elevated after long-term haloperidol or caerulein treatment. Simultaneously, the number of CCK-8, benzodiazepine and GABA-A receptors in cerebral cortex was decreased. It is probable that CCK-8-ergic mechanisms are involved closely in the action of repeated haloperidol treatment. CCK-8 seems to modulate the action of haloperidol through altering the sensitivity of dopamine, opioid, GABA-A and benzodiazepine receptors.

Anxiogenic-like action of caerulein, a CCK-8 receptor agonist, in the mouse: influence of acute and subchronic diazepam treatment

Effects of caerulein, a cholecystokinin octapeptide (CCK-8) receptor agonist, on exploratory activity of mice were investigated. Exploratory and locomotor activity of animals were measured using elevated plus-maze and open field tests. The systemic administration of caerulein at non-sedative doses (100 ng/kg-1 micrograms/kg i.p.) resulted in a significant decrease in the exploratory activity of mice. This effect was completely blocked by proglumide, a CCK-8 receptor. Acute treatment with low doses (0.1-0.75 mg/kg i.p.) of diazepam did not attenuate the anxiogenic-like effect of caerulein, but at more high doses of diazepam the coadministration depressed locomotor activity in mice. After subchronic diazepam treatment (2.5 mg/kg once a day, 10 days, i.p.) tolerance was developed toward the sedative effect of diazepam, and 72 h after withdrawal of the drug the animals showed increased anxiety in the plus-maze test. 30 min after the last injection procedure the anxiogenic-like effect of caerulein (500 ng/kg i.p.) on exploration was absent in both diazepam or vehicle groups. However, 72 h after the last pretreatment injection caerulein (500 ng/kg i.p.) reduced significantly the exploratory activity in control group, whereas it was inactive after diazepam withdrawal. The results obtained in this study support the hypothesis that endogenous CCK-8 an CCK-8 receptors are involved in the neurochemistry of anxiety and the anxiolytic action of benzodiazepine tranquillizers.

Cholecystokinin and satiation with alcohol

Release of the brain-gut peptide cholecystokinin (CCK) is stimulated by intragastric instillation of ethanol, and peripheral administration of CCK inhibits ethanol consumption. To assess the temporal specificity of the inhibitory effect of CCK on alcohol intake, water-deprived rats were given 5% ethanol at 20, 10 or 0 min after intraperitoneal injections of CCK octapeptide. Delaying access to ethanol for 20 min prevented a significant effect of CCK on intake. CCK's temporally constrained inhibitory action on alcohol consumption is consistent with an ethanol satiation effect. To test the motivational specificity of CCK's effect on fluid intake, rats were allowed a 2-bottle choice of 2% ethanol and water after CCK injections. Ethanol solution intake was suppressed by CCK, and total water intake was unaffected. The putative alcohol satiation action of CCK is appropriately specific to ethanol solution in free-choice tests. Hungry, but not fluid-deprived rats that were either ethanol experienced or naive received a 2-bottle choice of 4% ethanol or water after CCK or saline injections. CCK again specifically inhibited ethanol intake, but this effect required prior ethanol experience. Doses of CCK and naloxone, an opioid receptor blocker, combined to inhibit ethanol intake in an infra-dose-additive manner in water-deprived rats. CCK may act endogenously, in part on opioid receptor-mediated processes, as a preabsorptive satiety signal of ethanol. The full expression of this action appears to depend on prior conditioning of nutritive expectancy of the postingestive effects of alcohol.

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

The effects of CCK8 injected into the rat median nucleus accumbens on exploration and locomotion were measured in the four hole box and open-field tests. CCK8 (1 fmol to 100 pmol) induced hypoexploration in the four hole box (duration, frequency), with the pattern of the responses remaining unchanged compared to those of the control group. These effects were reversed by injection of the CCK antagonist proglumide (20 micrograms) into the nucleus accumbens. In the open-field test, CCK8 (100 pmol) only decreased locomotion and rearing when the rats were not habituated to the environment. These results show that the behavior of rats exposed to a novel external stimuli is highly sensitive to CCK8.

A behavioural pharmacological study on intracerebroventricularly administered CCK-8 related peptides in mice

The sulfated form of cholecystokinin octapeptide (CCK-8) and ceruletide (CER), but not their non-sulfated forms of CCK-4, significantly decreased the rates of locomotor activity and rearing during the first 10 min test session 10 min after intracerebroventricular (ICV) administration at doses more than 25 and 3.125 mg, respectively. CER-S antagonized methylphenidate-induced hypermotility after ICV administration at a dose of 800 ng. Plasma levels of CER-like immunoreactivity (CER-LI) measured at 120 min after subcutaneous injection, when the locomotor suppressive activity induced by 100 and 200 micrograms was no longer observed, were similar to or much higher than that 30 min after ICV administration at a dose of 800ng, suggesting that the effects of ICV CER-S are not mediated by a peripheral redistribution. These findings indicate that (1) the structural requirement for the locomotor suppressive activity is sulfated tyrosine residue; (2) the behavioural effects of ICV-administered CCK-8-S and CER-S are due to their central actions and mediated by the/inhibition of the central dopamine (DA) function; and (3) CCK-8-S in the brain is functionally associated with the central DA system.

A subpopulation of dopaminergic neurons in rat ventral mesencephalon contains both neurotensin and cholecystokinin

The coexistence of the neuropeptides neurotensin and cholecystokinin and the catecholamine-synthesizing enzyme tyrosine hydroxylase within neurons of the ventral mesencephalon was analyzed using an immunofluorescence triple-labeling technique. Virtually all of the neurotensin-positive cell bodies in the ventral tegmental area, medial substantia nigra pars compacta, retrorubral field, and rostral and caudal linear raphe nuclei were found to contain both cholecystokinin and tyrosine hydroxylase immunoreactivities. The degree of colocalization was lower and more variable in other regions including the ventral and central periaqueductal grey matter and dorsal raphe nucleus. It appeared that immunoreactivities for these 3 neuroactive substances were not contained within the same axonal-like fibers and terminals in the ventral midbrain. These results demonstrate that a subpopulation of dopaminergic neurons, which presumably comprise part of the ascending mesotelencephalic system, contains the two peptides neurotensin and cholecystokinin. Thus, the data suggest a morphological basis for some of the reported functional interactions of these 3 putative neurotransmitters/neuromodulators within this system.

Dose-additive inhibition of intake of ethanol by cholecystokinin and bombesin

Cholecystokinin (CCK) and bombesin (BBS) are neuropeptides of the brain and gut which have been shown to inhibit intake of ethanol. CCK octapeptide and BBS tetradecapeptide were injected intraperitoneally in both single doses and combinations of doses to determine interactions of the two peptides in the control of consumption of ethanol. Water-deprived rats were given access to 5% w/v ethanol for 30 min, followed by a 30-min access to water, daily. One minute before presentation of ethanol, rats were injected with either saline or one of ten peptide solutions (three of CCK alone, three of BBS alone, and four combinations of both). Results from the injections of single peptides were used to determine predicted inhibitions of the peptide combinations, assuming perfect additivity of doses. None of the actual values of inhibition of intake of ethanol by peptide combinations differed significantly from its predicted additive value. Endogenous CCK-like and BBS-like peptides may suppress intake of ethanol by an additive mechanism of inhibition.

Effect of ceruletide on discriminated avoidance behavior in rats

To clarify the central action of peripherally administered ceruletide, we examined the effects of ceruletide and four different classes of neuroleptics, i.e. haloperidol, chlorpromazine, oxypertine and sulpiride, on the discriminated avoidance response (DAR) in rats. Ceruletide and sulpiride did not suppress the DAR over a broad range of doses, whereas other three neuroleptics caused a dose-related decrease in both avoidance and response rates. In addition, the combined administration of ceruletide (100 micrograms/kg s.c.) and neuroleptics at critical doses that suppress the DAR caused a significant reduction in the avoidance rate without affecting the response rate, compared with neuroleptics alone. These findings suggest that ceruletide influences the central dopaminergic system, potentiating the central effects of neuroleptics and producing the favorable therapeutic effects observed in the clinical trials.

CCK-8 elicits grooming: cross tolerance to ACTH1-24

Rats received lateral intracerebroventricular (icv) injection of sulfated or desulfated cholecystokinin octapeptide (CCK-8-S or CCK-8-D, respectively) and subsequent grooming behaviors were observed using a behavioral sampling technique. CCK-8-S (2.0 to 4.0 micrograms in 10 microliter) elicited a significant increase in grooming behaviors relative to the controls, but the relatively inactive CCK-8-D did not. Excessive grooming induced by CCK-8-S produced less grooming than did the adrenocorticotropic hormone fragment, ACTH1-24 (1.0 microgram in 10 microliter). An icv injection of CCK-8-S 3 h before the icv injection of ACTH1-24 inhibited the usual excessive grooming produced by the ACTH. Conversely, prior injection of ACTH1-24 abolished the increased grooming induced by CCK-8-S. Thus, the sulfated cholecystokinin octapeptide and ACTH1-24 exhibited short-term cross tolerance for excessive grooming. These results suggest that the colocalization of corticotropin releasing hormone and CCK-8 in neurons may have functional significance in connection with stress-related neuronal systems.

Bilateral subdiaphragmatic vagotomy does not prevent the behavioral effects of systematically administered ceruletide in mice

To clarify the vagal mediation of behavioral changes following systematically administered CCK-like peptides, we examined the effects of subcutaneously injected ceruletide on several behavioral parameters. Ceruletide at a dose of 100 micrograms/kg reduced the rates of spontaneous locomotor activity and rearing, and also inhibited methylphenidate- and methamphetamine-induced hyperactivity in both sham-operated and vagotomized mice to same extent, whereas bilateral subdiaphragmatic vagotomy attenuated these behavioral parameters. These results indicate that the ascending sensory pathway mediating a peripheral CCK-elicited signal may not be responsible for producing the behavioral effects of systematically administered CCK-like peptides.

A neuroendocrinological study of responders and non-responders to ceruletide treatment in chronic neuroleptic-resistant schizophrenia

To define the characteristics of chronic schizophrenic patients who had responded poorly to treatment with ceruletide, several neuroendocrinological tests were carried out in 5 responders and 5 non-responders. Both thyroid-stimulating hormone and prolactin responses to thyrotropin-releasing hormone stimulation were within normal ranges in all subjects. The luteinizing hormone response to luteinizing hormone-releasing hormone, was slightly weaker in non-responders. There were no differences in the insulin tolerance test between responders and non-responders. Unusually delayed or prolonged growth hormone response to arginine infusion was observed in non-responders. These findings suggest dysfunction of the hypothalamo-pituitary system, especially of the arcuate nucleus and its functionally related areas, in non-responders.

Antistereotype effects of ceruletide and some neuroleptics differentiated by interactions with clonazepam, muscimol, scopolamine and clonidine

Compulsive gnawing was produced in mice by administration of either methylphenidate or (after sensitizing pretreatment with the neuroleptic, tetflutixol) apomorphine. Drugs which antagonise stereotypy, such as ceruletide (CER, a sulphated decapeptide related to cholecystokinin octapeptide), haloperidol, zuclopenthixol and fluphenazine were applied in equipotent doses (reducing stereotypy by 80%). Clonazepam, muscimol, clonidine and scopolamine (but not methylscopolamine) antagonized to a different extent the antistereotype effect of ceruletide and the neuroleptics. The ED50s for clonazepam and other drugs, were determined; clonazepam had the greatest potency. Regarding the antagonism of the antistereotype effect, ceruletide was similar to but by no means congruent with haloperidol. The antagonism of the antistereotype effect was specific because other effects of ceruletide and cholecystokinin octapeptide (inhibition of exploratory rearing activity, ptosis, antinociception, hypothermia) were not antagonized by clonazepam and only weakly modified by scopolamine. Methylscopolamine was ineffective throughout, indicating a central site for the mechanism of the actions studied of scopolamine. In conclusion, the antistereotype effect of ceruletide is different from that of conventional neuroleptic drugs and functionally independent of other behavioural effects of the cholecystokinin-like peptides.

Cholecystokinin antagonizes morphine induced hypoactivity and hyperactivity in hamsters

Three experimental replications were used to test the effects of three doses (25, 50 or 75 micrograms/kg) of cholecystokinin octapeptide (CCK-8) on morphine induced changes in activity. For each dose of CCK-8, running wheel activity of golden Syrian hamsters was monitored for three hours following a series of two injections. The first injection consisted of either saline or CCK-8, the second of either saline or morphine sulfate (15 mg/kg). Thus, in each replication four groups were created: Group SAL/SAL (n = 8) received two saline injections, Group CCK/SAL (n = 8) an injection of CCK-8 followed by an injection of saline, Group SAL/MS (n = 8) an injection of saline followed by an injection of morphine and Group CCK/MS (n = 8) an injection of CCK-8 followed by an injection of morphine. Results indicated that a 25 micrograms/kg dose of CCK-8 blocked the hypoactivity elicited by morphine 40-60 min after opiate injection, whereas a 75 micrograms/kg dose of CCK-8 blocked the hyperactivity elicited by morphine 80-100 min after opiate injection. These findings are consistent with previous reports that CCK-8 antagonizes the effects of opiate agonists on a variety of behaviors and is supportive of the hypothesis that endogenous CCK-8 may antagonize endogenous opioid peptides in the control of behavior.

The cholecystokinin-like peptide, ceruletide, reduces isolation-induced aggression in mice

Ceruletide, a non-mammalian decapeptide related to cholecystokinin octapeptide, reduced the aggressiveness of isolated NMRI mice. In this respect, its ED50 (mumol/kg, s.c.) was 0.024 (95% confidence range, 0.0133-0.0447) and that of haloperidol 3.38 (1.488-7.669). An approximately equipotent dose of diazepam was 17.5 mumol/kg.

Ceruletide inhibits water intake in deprived mice: comparison with morphine and the enkephalin analogue, FK 33-824

Subcutaneous injections of ceruletide (caerulein diethylammonium hydrate, CER) reduced dose-dependently the water intake in male NMRI mice deprived of water for 18 h. The ED50 for this effect was 5.5 (3.70-7.94) nmol/kg, which is 3.7 times more than the corresponding food intake inhibiting dose. Also inhibitory but much less potent than CER were (in decreasing order) FK 33-824, morphine and naloxone. Naloxone was an antagonist to both FK 33-824 and morphine but not to CER, thereby separating CER from the opioids. When water intake reducing doses of CER (15 nmol/kg) and FK 33-824 (850 nmol/kg) were combined, the two peptides were not additive but antagonized each other. Together, the present and previous results suggest that pharmacological inhibition of food and water intake have different characteristics.

Clonidine sensitizes mice for apomorphine-induced stereotypic gnawing: antagonism by neuroleptics and cholecystokinin-like peptides

In mice sensitized for apomorphine by either scopolamine or teflutixol, clonidine antagonized the antistereotypic effect of ceruletide and haloperidol. The same effect of clonidine occurred in normal mice with methylphenidate-induced gnawing. In naive mice, clonidine alone had a sensitizing effect for the action of apomorphine leading to wire-gnawing. Yohimbine and rauwolscine (but not corynanthine) antagonized this effect of clonidine. The gnawing-inducing effect of methylphenidate was also enhanced by clonidine but not to the same extent as that of apomorphine. The stereotypic effect of apomorphine (in mice sensitized by either scopolamine or clonidine) was antagonized by yohimbine and rauwolscine but not by corynanthine. Apomorphine-induced wire gnawing was used as test of the antistereotypic effect of haloperidol, trifluoperazine, teflutixol, CCK-8, ceruletide and 8 related peptides. Ceruletide and 2 of its analogues were more potent than the neuroleptics; CCK-8 was 7 times less active than ceruletide. In conclusion, clonidine sensitized mice for the stereotypic effect (wire-gnawing) of apomorphine and methylphenidate. The clonidine-apomorphine effect permits the estimation of antistereotypic effects.

Caerulein and its analogues: neuropharmacological properties

The decapeptide from the frog Hyla caerulea, caerulein (caerulein diethylammonium hydrate, ceruletide, CER) is chemically closely related to the C-terminal octapeptide of cholecystokinin (CCK-8). Like CCK-8, CER and some of its analogues produce many behavioural effects in mammals: inhibition of intake of food and water; antinociception; sedation; catalepsy; ptosis, antistereotypic, anticonvulsive and tremorolytic effects; inhibition of self-stimulation. Effects of CER in man comprise sedation, satiety, changes in mood, analgesia and antipsychotic effects. A modulation of central dopaminergic functions appears to be one possible mechanism of CER and its analogues. A common denominator for all effects of CER is, at present, not evident.