Psychopharmacological profile of cholecystokinin using the self-stimulation and the drug discrimination paradigms

The effects of cholecystokinin on stimulation-induced feeding and self-stimulation

Cholecystokinin (CCK) is a peptide hormone which controls a number of important functions during the process of digestion. It is present in the gut and the central nervous system, although its exact role in the latter is not yet clear. Our interest was in the effects of intraperitoneal and intracerebral injections of CCK on brain stimulation reward and stimulation-induced feeding. Period thresholds for rewarding stimulation were unaffected by either route of peptide administration, whereas stimulation-induced feeding thresholds were weakly increased by centrally injected CCK. In addition, we evaluated stimulation-induced feeding using a more resolved measure and found it to provide clearer results. By actually measuring the amount of food eaten during a stimulation-induced feeding session, and not only the occurrence of feeding, CCK was shown to systematically decrease the total intake as a function of dose in an inverse manner. The efficiency of food utilization was calculated for each animal during the different phases of the experiment in order to monitor the effects of CCK on the animals' overall health. Centrally administered CCK appeared to increase the animals' efficiency and, furthermore, this level was sustained for the entire post-injection phase, about 2 weeks, suggesting a relatively enduring increase in metabolic rate. While the functional role of central CCK and other gastric peptides requires clarification, analyses which exploit the stimulation-induced feeding paradigm need to make use of more clearly defined microstructural variables.

Effects of microinjections of cholecystokinin and neurotensin into lateral hypothalamus and ventral mesencephalon on intracranial self-stimulation

Changes in intracranial self-stimulation (ICSS) evoked from ventral tegmental area-substantia nigra (VTA-SN) and lateral hypothalamus-medial forebrain bundle (LH-MFB) before and after microinjections of sulfated cholecystokinin octapeptide (CCK-8S) and unsulfated cholecystokinin (CCK-8US), neurotensin tridecapeptide ([D-Tyr11]NT(1-13) or [DTrp11]NT(1-13)) into either VTA-SN or LH-MFB were assessed. The current intensity was fixed at a level to obtain 60-70% of the maximal asymptotic rate. CCK-8S (0.10 microg/0.5 microl and 0.25 microg/0.5 microl) into VTA-SN resulted in dose-dependent decreases in VTA-SN ICSS of 38-42% and 78-92%, respectively, without affecting the ICSS of LH-MFB. Similar doses of CCK-8S injected into LH-MFB changed neither LH-MFB ICSS nor VTA-SN ICSS. CCK-8Us injected into VTA-SN or LH-MFB had no effect on ICSS in either site. Intra-VTA-SN injections of the neurotensin-1 (NT1) receptor agonist [DTyr11]NT(1-13) and the NT1 receptor antagonist [D-Trp11]NT(1-13) at doses of 5 microg/0.5 microl and 10 microg/0.5 microl decreased VTA-SN ICSS. NT1 receptor agonist and antagonist injections did not alter LH-MFB ICSS in any significant manner. Similar injections of these peptides into LH-MFB did not change the responding rates for LH-MFB ICSS or VTA-SN ICSS. Increasing the current intensity reversed the inhibitory effect of CCK-8S and [D-Trp11]NT(1-13) on VTA-SN ICSS and restored basal preinjection rates of responding. These results suggest that CCK(A) and NT1 receptor mechanisms in the ventral tegmentum in association with dopamine neurotransmission may be important in mediating the rewarding effects of VTA-SN ICSS but not LH-MFB ICSS.

Cholecystokinin as a stimulus in drug discrimination learning

Animals were trained to discriminate a relatively low dose of the octapeptide cholecystokinin (CCK) from distilled water within the conditioned taste aversion baseline of drug discrimination learning. Specifically, rats were injected with CCK (5.6 micrograms/kg) prior to the presentation of saccharin-LiCl pairings and with the CCK vehicle prior to the presentation of saccharin alone. After 10 conditioning trials (40 days), subjects acquired the discrimination, avoiding saccharin consumption following administration of CCK and consuming the same saccharin solution following the drug vehicle. Once the discrimination was acquired, a generalization function was determined for doses above and below that of the training stimulus. At doses below the training dose of CCK (i.e., 0, 3.2, and 4.2 micrograms/kg), subjects drank at control levels, whereas at the training dose and above (10 micrograms/kg) subjects significantly reduced consumption. That a relatively low dose of CCK can be used as a discriminative stimulus within a drug discrimination design may be important in that the procedure can now be used in the assessment of the pharmacological characteristics of CCK at a dose similar to that used in other behavioral assessments of the compound.

Balance of glutamate and dopamine in the nucleus accumbens modulates self-stimulation behavior after injection of cholecystokinin and neurotensin in the rat brain

Subpopulations of dopamine (DA) neurons in the ventral mesencephalon have been reported to contain cholecystokinin (CCK) and neurotensin (NT), giving rise to DA, DA/NT, NT/CCK and DA/CCK/NT projections. More precisely, colocalized DA/CCK neurons project mainly to the caudal part of the medial nucleus accumbens, whereas its rostral portion receives CCK and DA nerve terminal networks that are structurally independent. We investigated the respective effects of both CCK and NT on the intracranial self-stimulation behavior (ICSS) from the posterolateral hypothalamus after their direct administration into the lateral ventricle (ICV), into both portions of the nucleus accumbens, into the ventral tegmental area (VTA), and into the subiculum of the hippocampal formation (SUB). The ICV injection of 150 pmol CCK8 induced a decrease in the rate of ICSS. By contrast, the direct administration of 150 pmol CCK8 into the mediocaudal part of the nucleus accumbens induced an enhanced rate of ICSS while a similar injection into its rostral portion gave rise to a slight transient decrease of ICSS. When injected into the SUB, both CCK8 and glutamate produced decreased rates of ICSS at femtomolar doses one thousand-fold under the picomolar concentrations used for ICV injections. Neurotensin induced similar behavioral profiles to that observed after the ICV injection of CCK8 or into both portions of the nucleus accumbens. Neurotensin and CCK8 displayed opposite effects on ICSS when administered into the SUB or into the VTA, suggesting they may regulate ICSS most probably through different synaptic mechanisms and through different anatomical pathways.

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.

Facilitation and inhibition of feeding by a single dose of amphetamine: relationship to baseline intake and accumbens cholecystokinin

Amphetamine (AMP) administered in high doses suppresses feeding. However, in low doses AMP has been shown to both suppress and facilitate feeding. Further, there is some indication of individual differences in the feeding response to low doses of AMP. Evidence indicates that AMP's effects on feeding are dopamine-mediated and that the nucleus accumbens (Acb) may be an important site of action. Of interest here is the fact that CCK terminals exist within the Acb and CCK modulates DA activity. Experiment 1 investigated the effects of intra-Acb CCK administration as a function of individual differences in the feeding response to a low dose of systemic AMP. Results indicate that response to AMP was baseline dependent. AMP stimulated feeding in low baseline feeders and suppressed feeding in high baseline feeders. Intra-Acb CCK blocked the AMP-induced increase in feeding but not the AMP-induced anorexia. In experiment 2, the effects of intra-Acb CCK administration on baseline feeding were assessed. Intra-Acb CCK suppressed baseline feeding, but only when there was a high level of intake. It is speculated that Acb-DAergic activity may play a role in the observed feeding effects of both AMP and CCK.

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.

The effects of cholecystokinin (CCK-8) on dopamine-containing nerve terminals in the caudate nucleus and nucleus accumbens of the anesthetized rat: an in vivo electrochemical study

The action of cholecystokinin (CCK) on presynaptic function of dopaminergic nerve terminals has been the subject of much debate in the literature. In efforts to resolve some of the reported ambiguities, high speed in vivo electrochemical recordings were carried out in the caudate nucleus and nucleus accumbens of the urethane anesthetized rat, to determine effects of locally applied sulfated (CCK-8S) and unsulfated (CCK-8US) CCK octapeptide. Locally-applied CCK-8S and CCK-8US caused no increase in the baseline electrochemical signals recorded from either brain region. However, locally applied CCK-8S potentiated the potassium-evoked overflow of dopamine (DA) into the extracellular space in both the caudate and nucleus accumbens. In contrast, pressure ejection of CCK-8US produced no significant effects on the potassium-evoked overflow of DA in either structure. These data support a facilitatory effect of CCK-8S on potassium-evoked overflow from DA-containing nerve terminals in the urethane anesthetized rat that is likely mediated through a peripheral type CCK receptor.

Adrenocorticotropic hormone fragment ACTH/MSH(4-10 can act as a discriminative stimulus in rats

Five of eight male albino rats learned to discriminate the effects of ACTH/MSH(4-10) (100 micrograms/kg, SC) from those of saline vehicle in a food-reinforced T-maze task. Generalization tests showed that the ACTH/MSH(4-10) cue was dose related with an ED50 of approximately 68.0 micrograms/kg. These findings indicate that ACTH/MSH(4-10) has interoceptive stimulus properties discriminable to some animals, which might provide a useful model for studying the pharmacology of this neuroactive peptide. Moreover, since ACTH/MSH(4-10) induces effects similar to those of ACTH/MSH in the central nervous system but not hormonally, these data suggest that endogenous ACTH or MSH may also have interoceptive stimulus effects unrelated to peripheral endocrine activity.

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.

Pharmacological and mechanistic studies of cholecystokinin-facilitated [3H]dopamine efflux from rat nucleus accumbens

Cholecystokinin (CCK) is co-localized with dopamine (DA) in mesolimbic neurons of the CNS and appears to selectively regulate the output of this system. In an attempt to characterize the nature of CCK interactions with mesolimbic DA-containing nerve terminals, we have investigated CCK regulation of [3H]DA overflow from rat nucleus accumbens slices. CCK-8 produced a saturable and potent (EC50 = 3 nM) facilitation of KCl (35 mM)-evoked [3H]DA efflux from nucleus accumbens, but failed to significantly alter [3H]DA efflux from striatum: The stimulatory action of CCK-8 was unaffected by the muscarinic antagonist atropine, the opiate antagonist naloxone, or the selective ion channel blockers tetrodotoxin and nifedipine. Pharmacological studies revealed that non-sulfated CCK-8 and CCK-4 (up to low micromolar concentrations) did not facilitate [3H]DA efflux from accumbens slices. Furthermore, the effect of CCK-8 was selectively and potently (IC50 = 300 nM) inhibited by the Type A-selective CCK antagonist CR-1409. Taken together, these results indicate that CCK regulates DA efflux from mesolimbic nerve terminals via a direct presynaptic action on receptors which display a pharmacological profile that is similar to Type A CCK receptors in gastrointestinal tissues.

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.

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.

Clarification of the behavioral functions of peripheral and central cholecystokinin: two separate peptide pools

Cholecystokinin is localized in two distinct systems, the gastrointestinal and the central nervous systems. At both locations, cholecystokinin (CCK) is synthesized, stored, and released, and high affinity binding sites for CCK have been identified. The blood-brain barrier is thought to completely block passage of CCK from the gut to the brain compartment, although CCK can pass from the cerebrospinal fluid out into the blood. A variety of behavioral actions of CCK have been described. Evidence that the actions of CCK on feeding and behavioral sedation are mediated through the peripheral site is reviewed. Recent studies demonstrating behavioral effects of CCK administered in nanogram doses directly into brain nuclei are reviewed. Caveats against administration of microgram doses of CCK into the cerebral ventricles are raised, emphasizing the interpretational difficulties inherent in this approach.

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.