Occurrence of two cholecystokinin binding sites in guinea-pig brain cortex

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.

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.

Pharmacological comparison of the alternatively spliced short and long CCK2 receptors

(1) The alternatively spliced, short and long cholecystokinin receptors (CCK2S and CCK2L) were expressed in NIH3T3 cells, and compared using radioligand-binding assays with identical buffer and incubation conditions. (2) As judged by a saturation analysis, the selective CCK2-receptor antagonist radioligand [3H]-JB93182 did not discriminate between the CCK2S or CCK2L receptors. (3) A global analysis of competition studies, using a range of structurally diverse, CCK-receptor selective ligands, provided further evidence that these receptor subtypes were pharmacologically indistinguishable. However, when analysed individually a number of small, yet significant differences were observed with some of the compounds. (4) These data are consistent with previous study that suggested a possible pharmacological difference between these isoforms, at least in terms of the CCK2-receptor antagonist, L-365,260. However, it would appear that the pharmacological profile of these compounds is not consistent with their affinity at the putative G1/G2 receptors previously described by Harper et al.

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.

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.

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.

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.

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.

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.

Role of fluidity of membranes on the guanyl nucleotide-dependent binding of cholecystokinin-8S to rat brain cortical membranes

The binding of [3H]cholecystokinin octapeptide (sulphated) ([3H]CCK-8S), an agonist of the cholecystokinin receptors, to rat cortical membranes was fast, specific and saturable, with pH optimum at 6.5-7.0. The divalent cations Mg2+ and Ca2+ clearly enhanced [3H]CCK-8S binding, whereas the monovalent cations Na+ and K+ were inhibitors. Inactivation of the ligand binding ability of these membranes was dependent on the incubation temperature and corresponding tau1/2 values were 11 days at 4 degrees , 12 hr at 21 degrees , 154 min at 30 degrees and 51 min at 37 degrees , which revealed the apparent activation energy of this process to be 130+/-4 kJ/mol. Scatchard analysis of the saturation curves of [3H]CCK-8S binding was best described by a one site binding model with a Kd = 0.63+/-0.18 nM and a maximum binding of 32+/-2 fmol/mg protein. The stable GTP analogue guanosin-5'-O-(3-thiotriphosphate) (GTPgammaS) decreased the affinity of [3H]CCK-8S binding only up to 2-fold without significant influence on maximal binding. Modulation of membrane properties by different detergents revealed that only in the case of digitonin (0.03-0.04%) did the GTP-dependence of [3H]CCK-8S binding considerably increase without significant influence on the ligand binding properties in the absence of GTPgammaS. Other detergents studied (sodium cholate, sodium deoxycholate, 3-(3-cholamidopropyl)dimethylammonio-1-propanesulfonate (CHAPS), sucrose monolaurate, series Triton X and Tween) either had little influence on GTP-gammaS-dependence of [3H]CCK-8S binding or inactivated the receptor. Parallel studies of fluorescent polarization of diphenylhexatriene (DPH) in rat cortical membranes indicated that digitonin was the only detergent which at low concentrations caused a rapid increase in membrane fluidity and thereafter stabilized it at a certain level. Other detergents studied had only moderate influence on membrane fluidity (CHAPS, cholate, deoxycholate) or caused fast and continuous increase of membrane fluidity (Triton X-100, Tween 80). These data together point to the essential influence of the fluidity of membranes on the regulation of the interactions between G proteins and CCK receptors in rat cortical membranes. Under standard experimental conditions (temperature lower than 30 degrees), the CCK receptor-G protein complex is active for quantitative characterization of the receptors, but the membranes are too rigid for natural communication and regulation.

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 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.

Pharmacological characterization of CCKB receptors in human brain: no evidence for receptor heterogeneity

In this paper, cholecystokinin (CCK) B-type binding sites were characterized with receptor binding studies in different human brain regions (various parts of cerebral cortex, basal ganglia, hippocampus, thalamus, cerebellar cortex) collected from 22 human postmortem brains. With the exception of the thalamus, where no specific CCK binding sites were found, a pharmacological characterization demonstrated a single class of high affinity CCK sites in all brain areas investigated. Receptor densities ranged from 0.5 fmol/mg protein (hippocampus) to 8.4 fmol/mg protein (nucleus caudatus). These CCK binding sites displayed a typical CCKA binding profile as shown in competition studies by using different CCK-related compounds and non peptide CCK antagonists discriminating between CCKA and CCKB sites. The rank order of agonist or antagonist potency in inhibiting specific sulphated [propionyl-3H]cholecystokinin octapeptide binding was similar and highly correlated for the brain regions investigated as demonstrated by a computer-assisted analysis. Therefore it is concluded that CCKB binding sites in human cerebral cortex, basal ganglia, cerebellar cortex share identical ligand binding characteristics.

Novel CCK-B receptor agonists: diketopiperazine analogues derived for CCK4 bioactive conformation

Recently, we proposed a CCK-B agonist bioactive conformation characterized by an 'S' shape of the peptidic backbone which was derived from structure-activity relationships and conformational analysis of CCK4 (Trp-Met-Asp-Phe-NH2) analogues. Using this template, we report here the synthesis of cyclic CCK4 analogues which contain, in place of the Trp-Met dipeptide, a diketopiperazine moiety resulting from a cyclization between Nle and N-substituted (D)Trp residues and coupled with a small linker to Asp-Phe-NH2. Some of these compounds displayed good affinities and selectivities for the CCK-B receptor. The results are discussed in terms of size, hydrophobicity and spatial orientation of the side-chains on the diketopiperazine ring. The most potent ligand exhibited potent and full CCK-B receptor agonist properties in promoting the hydrolysis of inositol phosphates (EC50 = 8 nM) in CHO cells, stably transfected with the rat brain CCK-B receptor. This compound was also shown to be a potent selective CCK-B/gastrin receptor agonist since, it increased gastric acid secretion measured in anesthetized rats on i.v. administration. These compounds provide a rigid template for the design of non-peptide CCK-B agonists, by modification of the remaining peptide moiety.

Effects of guanyl nucleotides on CCKB receptor binding in brain tissue and continuous cell lines: a comparative study

The effects of non-hydrolyzable guanyl nucleotide analogue GTP-gamma S on CCKB receptor binding in human and guinea-pig cortex, Jurkat T-cells, rat pituitary GH3 cells, rat glioma C6 cells and human small cell lung cancer NCI-H69 cells were investigated by using [3H]CCK-8S saturation and competition binding studies. GTP-gamma S caused inhibition of specific [3H]CCK-8S binding in a concentration dependent manner with a plateau at 10-25 microM. 25 microM GTP-gamma S resulted in a small but significant increase in Kd and IC50 values with amount very similar in all CCKB receptor models tested. However, the maximal number of specific [3H]CCK-8S binding sites (Bmax) was unaffected. Results suggest that CCKB receptors are G-protein coupled in a similar way to human and guinea-pig cortex, Jurkat cells, GH3 cells, C6 cells and NCI-H69 cells.

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

The effects of the selective CCK-B agonists, BC 264 and BC 197, and the nonselective CCK agonist BDNL were investigated in the elevated plus-maze in rats. BDNL and BC 197 induced anxiogeniclike effects, in contrast to BC 264, which had no effect. The behavioral responses induced by BDNL were not significantly blocked by L-365,260, but were suppressed by CI-988, another selective CCK-B antagonist, and by high doses of L-364,718, a selective CCK-A antagonist. BC 197-induced effects were also blocked by CI-988. Competition experiments performed with [3H]pBC 264 using brain membranes of guinea pig, mouse, and rat were significantly better fitted when analyzed by a two site model than by a one site model with BC 197 but not with BC 264. Moreover, BC 264 produced anxiogeniclike effects when administered with increasing doses of L-365,260 and opposing effects with increasing doses of CI-988. Together these results give pharmacological and behavioral evidence for the existence of CCK-B receptor subtypes.

Species specificity of pharmacological characteristics of CCK-B receptors

Novel CCK-B receptor antagonists, tetronothiodin and L-156,586, showed different affinities for CCK-B receptors in brain membranes from human, rat, guinea pig and mouse. [125I]CCK-8 bound to these membranes with a similar affinity. However, tetronothiodin was most potent in rat (IC50 = 3.6 nM), followed by guinea pig (96 nM), human (210 nM) and mouse (280 nM). L-156,586 bound with highest affinity to membranes from guinea pig (11 nM), and with lowest affinity to membranes from mouse (220 nM). These results suggest the existence of species specificity of CCK-B receptors, and that these two compounds are useful tools for discrimination between these receptors.

Examination of the effects of cholecystokinin 26-33 and neuropeptide Y on responses of visual cortical neurons of the cat

Extracellular recordings were made from 160 neurons in area 17 (n = 120) and area 18 (n = 40) of the visual cortex of anesthetized cats. Cells were classified according to their receptive field properties and their intracortical positions were evaluated histologically. Cholecystokinin 26-33, antagonists, (cholecystokinin 27-32, cholecystokinin 27-33 and proglumide), amino acids, neuropeptide Y and solvent vehicle (control), were administered to cells by microiontophoresis (cholecystokinin and neuropeptide Y) or by pressure (neuropeptide Y). The results of the tests with cholecystokinin 26-33 fell into four categories: enhancement (31%), suppression (24%), mixed, i.e. either biphasic responses or dose-related alterations in the direction of effect (20%), and no effect (25%). Enhancements of the visually elicited response were more prevalent in simple (43%) and unimodal/movement-sensitive (34%) cells than in complex (7%) cells. The converse was true for suppressions: 19% of simple cells, 24% of unimodal/movement-sensitive cells, and 31% of complex cells were suppressed. Thirty per cent of the unaffected cells were complex or unimodal/movement-sensitive; only 14% were simple. Cells in layers II-IV were more likely to have firing enhanced than suppressed by cholecystokinin 26-33. The converse was true for cells in layers V and VI, where 50% of responses were suppressed and only 22% were enhanced. Unaffected cells were found predominantly in layer III of areas 17, and the lower part of layer III and layer IV of area 18. Cholecystokinin 26-33 sometimes exerted delayed, response-suppressant effects; it also occasionally elevated responsiveness preferentially within the upper ranges (10-20 degrees/s) of velocity tuning curves. Cholecystokinin 26-33 altered the response-suppressant action of GABA in 11 of 19 visually sensitive cells. The peptide potentiated the visual responsiveness in half of the cells where cholecystokinin 26-33 diminished the GABA-induced suppressions (n = 8). The presumed antagonists either exerted no effect on firing or on cholecystokinin 26-33-induced effects, or had cholecystokinin 26-33-like actions themselves. There was a reversible partial antagonism of the effects of cholecystokinin 26-33 on only two of 11 cells tested. Neuropeptide Y injected by pressure or administered iontophoretically had variable and inconsistent effects on the visually evoked responses of 29 additional neurons from those described above. These effects were indistinguishable from those of the vehicle whether spontaneous activity, magnitude of the visually elicited response, spatial integrity of the RF substructure, orientation or velocity tuning was assessed.(ABSTRACT TRUNCATED AT 400 WORDS)

Age-related differences of cholecystokinin receptor binding in the rat brain

1. Cholecystokinin and benzodiazepine receptor binding was evaluated in 2-, 9- and 18-month old rats in the brain regions where cholecystokinin octapeptide and gamma-aminobutyric acid are known to coexist in common nerve cells (frontal cortex, hippocampus). 2. There was a regionally selective alteration of hippocampal 3H-CCK-8 binding in the oldest age group, if compared to both young and adult animals. Non-linear regression analysis of binding data revealed significantly lower apparent number of binding sites (Bmax), and twofold (but not statistically significantly) higher binding affinity for the radiolabelled ligand. No differences between any age groups in 3H-flunitrazepam binding to benzodiazepine receptors were found. 3. The results suggest that changes in cholecystokinin receptor characteristics might contribute to the behavioural impairments in aged rats.

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.

Replacement of Tyr-SO3H by a p-carboxymethyl-phenylalanine in a CCK8-derivative preserves its high affinity for CCK-B receptor

The sulfated tyrosine present in the sequence of CCK8 Asp26-Tyr(SO3H)-Met-Gly-Trp-Met-Asp-PheNH2, seems to play a critical role in the recognition of CCK-A binding sites. In this work, we have investigated whether the presence of an anionic charge on the tyrosine moiety is strictly necessary and whether the sulfate moiety interacts with a divalent cation in the receptor subsite. For this purpose, the novel amino acids (L,D)Phe(p-CH2CO2H) and (L,D) Phe(p-CH2CONHOH), as well as their L-resolved forms were introduced into the sequence of Ac[X27, Nle28, Nle31]-CCK27-33 by solid phase method. The biological activities of these new derivatives were compared to two almost equiactive analogues of CCK8, Ac[Phe(p-CH2SO3H)27, Nle28, Nle31]-CCK27-33 and Boc[Nle28, Nle31]-CCK27-33 (BDNL) and to the nonsulfated analogue of the latter peptide (BDNL NS). All these new CCK-related analogues behave as agonists in stimulating pancreatic amylase release and display high affinity for brain binding sites (KI approximately 3-11 nM) but the only peptides which retain affinity for CCK-A receptors (KI approximately 20 nM) are those containing a p-carboxymethyl phenylalanine. Thus, introduction of this amino acid under an esterified form on the side chain, into specific and potent CCK-B agonists could allow compounds endowed with good bioavailabilities to be obtained.

In vivo binding affinities of cholecystokinin agonists and antagonists determined using the selective CCKB agonist [3H]pBC 264

The respective role of central vs. peripheral CCK-B receptors in the recently reported anxiolytic effects of CCK-B antagonists remains to be firmly established. We therefore investigated the in vivo binding properties of cerebral CCK receptors after i.c.v. injection into mice of [3H]pBC 264 ([3H]propionyl-Tyr(SO3H)-gNle-mGly- Trp-(NMe)Nle-Asp-Phe-NH2), a highly potent, peptidase-resistant and selective CCK-B agonist. The specific binding of [3H]pBC 264 was reversible and saturable. The dose producing 50% receptor occupancy was 25 pmol and the Bmax was 0.9 pmol/brain 15 min after injection. I.c.v. administered CCK8 (ID50 8500 pmol) was 200-fold less potent than pBC 264 (ID50 43 pmol) in inhibiting specific [3H]pBC 264 binding; CCK8NS, CCK5 and CCK4 being slightly less potent than CCK8. Aminopeptidases play a major role in degrading CCK8 since the protected analog pCCK8 or CCK8 in the presence of an aminopeptidase inhibitor exhibited higher affinities than CCK8. I.v. administration of pBC 264 (20 mg/kg) inhibited [3H]pBC 264 specific binding by about 72%, confirming its ability to enter the brain. In contrast, CCK4 was unable to modify [3H]pBC 264 binding. As expected, the CCK-A antagonist (L364,718) did not inhibit [3H]pBC 264 binding, while at the highest dose used (40 mg/kg i.p.) the CCK-B antagonist (L365,260) inhibited binding by 20%. Several hypotheses are discussed to account for the very low i.v. doses of CCK4 and L365,260 needed to produce anxiogenic and anxiolytic responses, respectively.

Somatostatin 28 interacts with CCK receptor in brain and pancreas

The ability of somatostatin analogs to interact with the binding of cholecystokinin has been studied in pancreatic and brain cortical membranes. Only the 28 amino-acid forms of somatostatin (S28), [Nle8]S28 and [Des Lys14,DTrp22]S28 were found to inhibit the binding of cholecystokinin to rat pancreatic plasma membranes and to increase the amylase release from pancreatic acini. This effect was independent of somatostatin receptor and resulted from an interaction between S28 and CCK receptor. This interaction was not observed with [Leu8, DTrp22, Tyr25]S28, indicating that this analog does not possess the biological activity of the native peptide and that the iodinated peptide could not label specific S28 receptors. S28 interacted also with CCK receptors in cortical brain membranes. Our results support the concept that S28, but not S14, may function as a regulatory molecule at CCK receptors and emphasize that S28 and S14 may be distinct neuromodulators.

Distribution of cholecystokinin-like-immunoreactive neurons in the guinea pig forebrain

The distribution of cholecystokinin (CCK)-immunoreactive nerve fibers and cell bodies was studied in the forebrain of control and colchicine-treated guinea pigs by using an antiserum directed against the carboxyterminus of CCK octapeptide (CCK-8) in the indirect immunoperoxidase technique. Virtually all forebrain areas examined contained immunoreactive nerve fibers. A dense innervation was visualized in; neocortical layers II-III, piriform cortex, the medial amygdala, the medial preoptic area, a circumventricular organ-like structure located at the top of the third ventricle in the preoptic area, the subfornical organ, the posterior bed nucleus of the stria terminalis, the posterior globus pallidus (containing labeled woolly fiber-like profiles), the ventromedial hypothalamus, the median eminence, and the premammillary nucleus. A moderately dense innervation was visualized elsewhere excepted in the septum and thalamus where labeled axons were comparatively few. Immunoreactive perikarya were abundant in: neocortex (especially layers II-III), piriform cortex, amygdala, the median preoptic nucleus, the bed nucleus of the stria terminalis, the hypothalamic paraventricular (parvicellular part), arcuate, and dorsomedial (pars compacta) nuclei, the dorsal and perifornical hypothalamic areas, and throughout the thalamus. Areas also containing a moderate number of labeled cell bodies were the medial preoptic area, the globus pallidus, the caudate-putamen, and the periventromedial area in the hypothalamus. Immunostained perikarya were absent or only occasionally observed in the septum, the suprachiasmatic nucleus, the magnocellular hypothalamoneurohypophyseal nuclei, and the ventral mesencephalon. In the adenohypophysis, corticomelanotrophs were labeled in both males and females, and thyrotrophs were labeled in females only. This distribution pattern of CCK-8 immunoreactivity is compared to those previously recorded in other mammals. This shows that very few features are peculiar to the the guinea pig. It is discussed whether some interspecific differences in immunostaining are real rather than methodological.

Electrophysiological studies with new CCK analogs: correlation with binding affinity on B-type receptors

The electrophysiological effects of Boc-D-Asp-Tyr(SO3H)-Nle-D-Lys-Trp-Nle-Asp-Phe-NH2 (compound I) and Boc-gamma-D-Glu-Tyr(SO3H)-Nle-D-Lys-Trp-Nle-Asp-Phe-NH2 (compound II), two cyclic cholecystokinin analogs with high selectivity for CCK-B receptors, as well as the effects of the linear enzyme-resistant analog Boc-[Nle28,Nle31]-CCK7 (BDNL), were compared with those of CCK8 using extracellular recordings in rat hippocampal slices in vitro. Bath applications of the three synthetic compounds resulted in concentration-dependent and reversible increases in single-unit activity. Comparison of equieffective concentrations yielded the following potency rank order: BDNL greater than CCK8 greater than compound II greater than compound I. There was a close correlation (r = .96, slope = 0.98) between the excitatory activities of the analogs and their potencies in displacing radiolabelled CCK8 from CCK-B receptors on rat brain membranes.

Similar potencies of CCK-8 and its analogue BOC(Nle28;Nle31)CCK27-33 on the self-stimulation behaviour both are antagonized by a newly synthesized cyclic CCK analogue

Neurons with co-localized cholecystokinin (CCK) and dopamine (DA) are present predominantly in the ventral tegmental area (VTA) and project mainly to the caudal part of the medial nucleus accumbens. The activity of this dopaminergic system can be evaluated by means of the intracranial self-stimulation behaviour (ICSS) on male Wistar rats having chronic electrodes implanted into the medial forebrain bundle in the postero-lateral area of the hypothalamus. The direct injection of the CCK analogue BOC(Nle28;Nle31)CCK27-33 (BDNL-CCK7) into a lateral ventricle decreased the electrical self-stimulation of the medial forebrain bundle. Nevertheless, this decrease in self-stimulation was steeper (immediately after the injection vs a delay of +/- 5-10 min.) than the CCK8-induced ICSS depletion. The intracerebroventricular (ICV) injection of 150 pmol and 1000 pmol BC-197 (BOC-D.Asp-Tyr(SO3H)-Nle-D.Lys-Trp-Nle-Asp-Phe-NH2) was ineffective to modify the self-stimulation behaviour when administered alone while a 150 pmol BC-197 dosage was able to antagonize the decreasing effect of 150 pmol CCK-8 on ICSS. Nevertheless, a dosage 6 times as important, i.e. 1000 pmol BC-197, was needed to antagonize the depression induced by 150 pmol BDNL-CCK7 on ICSS behaviour. These results support the equipotence of BDNL-CCK7 to CCK-8 in decreasing the self-stimulation behaviour after their direct administration into the lateral ventricle. They further give evidence of the relevance of BC-197 in antagonizing the respective effects of both compounds on the ICSS.

Characterization of [3H] CCK4 binding sites in mouse and rat brain

We have investigated the possible occurrence of distinct CCK8 and CCK4 binding sites in the brain by comparing the binding characteristics of [3H] CCK4 to those of the CCK8 analogue, [3H] Boc (Nle28,31]CCK27-33 (BDNL-CCK7). [3H] CCK4 and [3H] BNDL-CCK7 were shown to interact with mouse brain membranes with very similar maximal binding capacities 31.7 +/- 2.1 fmol/mg prot (KD = 3.78 +/- 0.47 nM) and 38.9 +/- 2.2 fmol/mg prot (KD = 0.26 +/- 0.02 nM) respectively. The apparent affinities of five CCK analogues for the sites labelled by both probes were almost identical. Autoradiographic studies revealed that the distribution of [3H] CCK4 binding sites in rat forebrain was the same as that of [3H] BDNL-CCK7, with high densities of receptors in the cortex, nucleus accumbens, olfactory bulb and the medial striatum, moderate densities in the amygdala, the hippocampus, several nuclei of the thalamus and hypothalamus. However in the interpenduncular nucleus where there was moderate binding of [3H]BDNL-CCK7, no [3H]CCK4 labelling was observed. These studies demonstrated the occurrence of one class of high affinity binding sites for [3H] CCK4 in mouse and rat brain, with characteristics similar to those already reported with CCK33, CCK8 and pentagastrin probes. Nevertheless the presence of a small amount of very high affinity binding sites for [3H]CCK4 cannot be excluded.

Discrimination between CCK receptors of guinea-pig and rat brain by cyclic CCK8 analogues

The properties of high affinity CCK8 binding sites of guinea-pig and rat brain cortex were compared using [3H]pCCK8. Large differences were observed, with the KD value being significantly higher in the rat (KD = 1.25 nM) than in guinea-pig brain (KD = 0.18 nM). Both sites exhibited different specificities for various CCK8 analogues, the selectivity factors KI rat/KI guinea-pig varied from 0.9 for CCK4 to 64 for cyclic CCK8-related compounds. Significant differences in the inhibition of [3H]pCCK8 binding by monovalent and nucleotides cations were also observed. These results could be explained by a difference in receptor environment or by a species difference in the proportion of CCK8 receptor-subtypes.

Enzyme-resistant CCK analogs with high affinities for central receptors

Based on the results of the in vitro metabolism of CCK8 by various peptidases, we have synthesized three CCK analogs: Boc-Tyr(SO3H)-Nle-Gly-Trp-(N- Me)Nle-Asp-Phe-NH2 (compound I), Boc-Tyr(SO3H)-gNle-mGly-Trp-Nle-Asp-Phe-Nh2 (compound II), Boc-Tyr(SO3H)-gNle-mGly-Trp-(N-Me)Nle-Asp-Phe-NH2 (compound III). In in vitro enzymatic degradation studies, these compounds showed a high stability toward either enkephalinase or the enzymes present in crude rat brain membranes preparations. Moreover, in binding studies on guinea pig tissues, these CCK-related peptides were characterized by high apparent affinities for brain CCK receptors and by a broader range of affinities for pancreatic CCK receptors. This broad range of affinities was reflected by their pharmacological potencies in the guinea pig pancreatic amylase release and ileum contraction assays. These enzyme-resistant CCK analogs provide therefore valuable tools to investigate the pharmacology of CCK.

Regulation of central cholecystokinin recognition sites by guanyl nucleotides

Guanyl nucleotides affected the binding of radiolabeled cholecystokinin (CCK) octapeptide to rodent cortical binding sites. Micromolar quantities of a stable GTP analogue, guanylyl-imidodiphosphate (GppNp), resulted in a plateau where binding was decreased by 30%. In the presence of 25 microM GppNp, binding analysis revealed a decrease in affinity (increase in KD), without an apparent effect on the maximal number of binding sites. Ki values for CCK-related peptides shifted up to 1.6-fold. The rate of peptide association decreased by threefold, and the rapid component of peptide dissociation increased. The collective data suggest that a class of central CCK binding sites is linked to nucleotide regulatory proteins. The evidence is discussed with regard to multiple receptor populations and to possible interconversions between receptor types.

Characterization of cholecystokinin receptors in toad retina

The binding characteristics, structure, and pharmacologic properties of a cholecystokinin binding protein in toad retinal membranes have been studied. In competition binding studies using 125I-CCK-8, toad retinal membranes exhibited a high affinity binding site having a Ki50 of 1.5 nM using CCK-8 as competitive ligand. The relative potencies of CCK-related peptides in inhibiting radioligand binding were caerulein greater than gastrin II approximately equal to CCK-8 approximately equal to CCK-33 greater than CCK-8-DS approximately equal to gastrin I. L-364,718, a potent inhibitor of peripheral CCK receptors, was ineffective at competition binding at concentrations up to 1 microM; dibutyryl cyclic GMP was modestly effective at competing (KD approximately 10 mM). Covalent binding of 125I-CCK-33 to toad retinal membranes using chemical cross-linkers or UV irradiation resulted in the labeling of a major Mr 62,000 protein and the intermittent labeling of minor components of Mr 105,000 and Mr 40,000 as determined by SDS-PAGE and autoradiography. The binding of 125I-CCK-33 to retinal membranes and the concomitant labeling of the Mr 62,000 component was specifically inhibited by CCK-8 (KD approximately 1.5 nM). Reduction of membranes with DTT abolished specific binding of 125I-CCK. SDS-PAGE analysis of affinity cross-linked membranes under non-reducing conditions revealed that the Mr 62,000 protein migrated with an apparently lower molecular weight. These results suggest that the Mr 62,000 CCK binding protein in the toad retina contains an intramolecular disulfide bond(s). The Mr 62,000 protein was retained on a wheat germ agglutinin-agarose column and eluted with N-acetyl D-glucosamine, suggesting the glycoprotein nature of this protein. Digestion of the Mr 62,000 protein with neuraminidase together with O-glycanase resulted in a discrete product of Mr approximately 60,000. These results indicate that the Mr 62,000 protein is a glycoprotein with O-linked oligosaccharide chains. Taken together, these data indicate that the CCK receptor in toad retina has a distinct structure compared to that described in rat pancreas or brain. It will be important to establish whether this difference is reflected in differences in signal transduction mechanisms.

Cyclic cholecystokinin analogues with high selectivity for central receptors

Taking as a model the N-terminal folding of the cholecystokinin tyrosine-sulfated octapeptide [CCK-8; Asp-Tyr(SO3H)-Met-Gly-Trp-Met-Asp-Phe-NH2] deduced from conformational studies, two cyclic cholecystokinin (CCK) analogues were synthesized by conventional peptide synthesis: Boc-D-Asp-Tyr(SO3H)-Ahx-D-Lys-Trp-Ahx-Asp-Phe-NH2 [compound I (Ahx, 2-aminohexanoic acid)] and Boc-gamma-D-Glu-Tyr(SO3H)-Ahx-D-Lys-Trp-Ahx-Asp-Phe-NH2 (compound II). The binding characteristics of these peptides were investigated on brain cortex membranes and pancreatic acini of guinea pig. Compounds I and II were competitive inhibitors of [3H]Boc[Ahx28,31]CCK-(27-33) binding to central CCK receptors and showed a high degree of selectivity for these binding sites (compound I: Ki for pancreas/Ki for brain, 179; compound II: Ki for pancreas/Ki for brain, 1979). This high selectivity was associated with a high affinity for central CCK receptors (compound I: Ki, 5.1 nM; compound II: Ki, 0.49 nM). Similar affinities and selectivities were found when 125I Bolton-Hunter-labeled CCK-8 was used as a ligand. Moreover, these compounds were only weakly active in the stimulation of amylase release from guinea pig pancreatic acini (EC50 greater than 10,000 nM) and were unable to induce contractions in the guinea pig ileum (to 10(-6) M). The two cyclic CCK analogues, therefore, appear to be synthetic ligands exhibiting both high affinity and high selectivity for central CCK binding sites. These compounds could help clarify the respective role of central and peripheral receptors for various CCK-8-induced pharmacological effects.

Characteristics of 125I-Bolton-Hunter labelled cholecystokinin binding in human brain

The high affinity binding of 125I-Bolton-Hunter labelled CCK-8 (125I-BHCCK) was studied in human brain using ligand binding and autoradiographic techniques. High levels of 125I-BHCCK binding were observed in cortex, striatum and cerebellum. In cerebellar membranes 125I-BHCCK binding was inhibited by CCK analogues with an order of potency, caerulein greater than CCK-8 greater than CCK-33 greater than gastrin 1-17 = CCK-8 non-sulphated. CCK-4 was inactive. It is suggested that in human brain 125I-BHCCK labels a population of CCK receptors similar to those observed in guinea pig brain. The properties and distribution of these receptors differ from those of rat brain.

Species differences in the localization of 'peripheral' type cholecystokinin receptors in rodent brain

A comparison of cholecystokinin (CCK) receptor binding to sections of rat, mouse and guinea pig brain has been performed using 125I-Bolton Hunter CCK and the selective peripheral CCK receptor antagonist L-365,031. In both rat and mouse, 125I-Bolton Hunter CCK binding in the region of the interpeduncular nucleus (IPN) was inhibited by L-365,031 indicating that these receptors resemble CCK receptors found in peripheral tissues. In the mouse especially, dense regions of peripheral CCK receptors were detected either side of the IPN. By contrast, in the guinea pig IPN no evidence of L-365,031-sensitive binding could be found. The present reports shows that in different species, regional variations in brain CCK receptor binding occur not only in the case of classical 'brain' receptors, but also for the more discretely localised 'peripheral' type CCK receptors.