Antipsychotic potential of CCK-based treatments: an assessment using the prepulse inhibition model of psychosis

Reduced Striatal Dopamine Transporter Availability and Heightened Response to Natural and Pharmacological Stimulation in CCK-1R-Deficient Obese Rats

Alterations in dopamine neurotransmission are associated with obesity and food preferences. Otsuka Long-Evans Tokushima Fatty (OLETF) rats that lack functional cholecystokinin receptor type-1 (CCK-1R), due to a natural mutation, exhibit impaired satiation, are hyperphagic, and become obese. In addition, compared to lean control Long-Evans Tokushima (LETO) rats, OLETF rats have pronounced avidity for over-consuming palatable sweet solutions, have greater dopamine release to psychostimulants, reduced dopamine 2 receptor (D2R) binding, and exhibit increased sensitivity to sucrose reward. This supports altered dopamine function in this strain and its general preference for palatable solutions such as sucrose. In this study, we examined the relationship between OLETF's hyperphagic behavior and striatal dopamine signaling by investigating basal and amphetamine stimulated motor activity in prediabetic OLETF rats before and after access to sucrose solution (0.3 M) compared to non-mutant control LETO rats, as well as availability of dopamine transporter (DAT) using autoradiography. In the sucrose tests, one group of OLETF rats received ad libitum access to sucrose while the other group received an amount of sucrose equal to that consumed by the LETO. OLETFs with ad libitum access consumed significantly more sucrose than LETOs. Sucrose exerted a biphasic effect on basal activity in both strains, i.e., reduced activity for 1 week followed by increased activity in weeks 2 and 3. Basal locomotor activity was reduced (-17%) in OLETFs prior to sucrose, compared to LETOs. Withdrawal of sucrose resulted in increased locomotor activity in both strains. The magnitude of this effect was greater in OLETFs and the activity was increased in restricted compared to ad-libitum-access OLETFs. Sucrose access augmented AMPH-responses in both strains with a greater sensitization to AMPH during week 1, an effect that was a function of the amount of sucrose consumed. One week of sucrose withdrawal sensitized AMPH-induced ambulatory activity in both strains. In OLETF with restricted access to sucrose, withdrawal resulted in no further sensitization to AMPH. DAT availability in the nucleus accumbens shell was significantly reduced in OLETF compared with aged-matched LETO. Together, these findings show that OLETF rats have reduced basal DA transmission and a heightened response to natural and pharmacological stimulation.

Roman high and low avoidance rats differ in their response to chronic olanzapine treatment at the level of body weight regulation, glucose homeostasis, and cortico-mesolimbic gene expression

Olanzapine, an antipsychotic agent mainly used for treating schizophrenia, is frequently associated with body weight gain and diabetes mellitus. Nonetheless, studies have shown that not every individual is equally susceptible to olanzapine's weight-gaining effect. Therefore, Roman high and low avoidance rat strains were examined on their responsiveness to olanzapine treatment. The Roman high avoidance rat shares many behavioral and physiological characteristics with human schizophrenia, such as increased central dopaminergic sensitivity, whereas the Roman low avoidance rat has been shown to be prone to diet-induced obesity and insulin resistance. The data revealed that only the Roman high avoidance rats are susceptible to olanzapine-induced weight gain and attenuated glucose tolerance. Here it is suggested that the specific olanzapine-induced weight gain in Roman high avoidance rats could be related to augmented dopaminergic sensitivity at baseline through increased expression of prefrontal cortex dopamine receptor D1 mRNA and nucleus accumbens dopamine receptor D2 mRNA expression. Regression analyses revealed that olanzapine-induced weight gain in the Roman high avoidance rat is above all related to increased prolactin levels, whereas changes in glucose homeostasis is best explained by differences in central dopaminergic receptor expressions between strains and treatment. Our data indicates that individual differences in dopaminergic receptor expression in the cortico-mesolimbic system are related to susceptibility to olanzapine-induced weight gain.

5-HT2C Agonists Modulate Schizophrenia-Like Behaviors in Mice

All FDA-approved antipsychotic drugs (APDs) target primarily dopamine D₂ or serotonin (5-HT_2A) receptors, or both; however, these medications are not universally effective, they may produce undesirable side effects, and provide only partial amelioration of negative and cognitive symptoms. The heterogeneity of pharmacological responses in schizophrenic patients suggests that additional drug targets may be effective in improving aspects of this syndrome. Recent evidence suggests that 5-HT_2C receptors may be a promising target for schizophrenia since their activation reduces mesolimbic nigrostriatal dopamine release (which conveys antipsychotic action), they are expressed almost exclusively in CNS, and have weight-loss-promoting capabilities. A difficulty in developing 5-HT_2C agonists is that most ligands also possess 5-HT_2B and/or 5-HT_2A activities. We have developed selective 5-HT_2C ligands and herein describe their preclinical effectiveness for treating schizophrenia-like behaviors. JJ-3-45, JJ-3-42, and JJ-5-34 reduced amphetamine-stimulated hyperlocomotion, restored amphetamine-disrupted prepulse inhibition, improved social behavior, and novel object recognition memory in NMDA receptor hypofunctioning NR1-knockdown mice, and were essentially devoid of catalepsy. However, they decreased motivation in a breakpoint assay and did not promote reversal learning in MK-801-treated mice. Somewhat similar effects were observed with lorcaserin, a 5-HT_2C agonist with potent 5-HT_2B and 5-HT_2A agonist activities, which is approved for treating obesity. Microdialysis studies revealed that both JJ-3-42 and lorcaserin reduced dopamine efflux in the infralimbic cortex, while only JJ-3-42 decreased it in striatum. Collectively, these results provide additional evidence that 5-HT_2C receptors are suitable drug targets with fewer side effects, greater therapeutic selectivity, and enhanced efficacy for treating schizophrenia and related disorders than current APDs.

Differential effects of the antidepressant mirtazapine on amphetamine- and dizocilpine-induced PPI deficits

Prepulse inhibition (PPI) refers to the decrease in motor startle response to salient sensory stimuli (pulses) when they are closely preceded in time by another more modest sensory stimulus (prepulse). PPI deficits can be induced by stimulation of dopamine receptors (e.g., amphetamine or apomorphine) or blockade of NMDA glutamate receptors (e.g., dizocilpine or PCP). Previously we found that antagonists of α(2)-noradrenergic and H(1)-histaminergic receptors significantly attenuate PPI impairments caused by amphetamine or dizocilpine. In the current study we assessed the effects of the antidepressant mirtazapine, which has combined antagonist effects at α(2)-noradrenergic, H(1)-histaminergic and 5-HT serotonergic receptors, on amphetamine- and dizocilpine-induced PPI deficits. In Experiment 1, rats were tested for PPI of the startle response to a tactile air-puff stimulus after auditory prepulses of three different intensities. Drug treatments consisted of combinations of amphetamine (0 and 1mg/kg) and mirtazapine (0, 0.5, 1, 2, and 5mg/kg), with all rats receiving all drug doses and combinations with different counterbalanced orders. In Experiment 2, a different group of rats was tested with drug treatments consisting of combinations of dizocilpine (0 and 0.05 mg/kg) and mirtazapine (0, 0.5, 1, 2, and 5 mg/kg). In Experiment 1 amphetamine (1 mg/kg) significantly reduced PPI whereas mirtazapine caused the opposite effect, with the highest dose of mirtazapine (5 mg/kg) effectively reversing the amphetamine-induced PPI deficit. In Experiment 2 dizocilpine (0.05 mg/kg) significantly reduced PPI, but mirtazapine did not have a significant effect on the inhibition of the startle response. These results indicate that the potential beneficial effects of combined α-adrenergic, 5-HT, and H(1) receptor blockade in counteracting PPI deficits may be associated to cases of sensorimotor gating disorders mediated by dopamine, but not necessarily to NMDA glutamate-induced PPI impairments.

Peptide therapeutics for CNS indications

Neuropeptides play a crucial role in the normal function of the central nervous system and peptide receptors hold great promise as therapeutic targets for the treatment of several CNS disorders. In general, the development of peptide therapeutics has been limited by the lack of drug-like properties of peptides and this has made it very difficult to transform them into marketable therapeutic molecules. Some of these challenges include poor in vivo stability, poor solubility, incompatibility with oral administration, shelf stability, cost of manufacture. Recent technical advances have overcome many of these limitations and have led to rapid growth in the development of peptides for a wide range of therapeutic indications such as diabetes, cancer and pain. This review examines the therapeutic potential of peptide agonists for the treatment of major CNS disorders such as schizophrenia, anxiety, depression and autism. Both clinical and preclinical data has been accumulated supporting the potential utility of agonists at central neurotensin, cholecystokinin, neuropeptide Y and oxytocin receptors. Some of the successful approaches that have been developed to increase the stability and longevity of peptides in vivo and improve their delivery are also described and potential strategies for overcoming the major challenge that is unique to CNS therapeutics, penetration of the blood-brain barrier, are discussed.

Newer molecules in the treatment of schizophrenia: A clinical update

Schizophrenia is a heterogeneous psychiatric disorder in which multiple neurotransmitter systems have been implicated. Increased and decreased dopamine transmission in the subcortical meso-limbic and meso-cortical systems is closely linked to the “positive” and “negative” symptoms of schizophrenia, respectively. Important roles have also been found for serotonin and acetylcholine, both of which are closely linked to dopamine. An abnormality in glutamate functioning involving N-methyl-D-aspartic acid as well as other receptor subtypes may underlie the dopamine dysfunction observed in schizophrenia. Since the discovery of chlorpromazine in 1952, researchers have been developing new molecules targeting various neurotransmitter systems to maximize their efficacy and tolerability. The advancements in molecular genetics have opened up new horizons to manipulate the post-receptor protein cascade and gene expression. Although the magic-wand still eludes us, the newer molecules hold a lot of promise in this condition.

p-Hydroxyamphetamine causes prepulse inhibition disruptions in mice: contribution of dopamine neurotransmission

It is well known that amphetamine induces disrupted prepulse inhibition (PPI) in humans and rodents. We have previously reported that intracerebroventricular (i.c.v.) administration of p-hydroxyamphetamine (p-OHA) induces multiple behavioral responses, such as increased locomotor activity and head-twitch response in rodents. To reveal the characteristics of p-OHA on sensorimotor function in rodents, herein we tested the effects of p-OHA on PPI in mice. i.c.v. administration of p-OHA dose-dependently induced PPI disruptions for all prepulse intervals tested. This effect of p-OHA on PPI was attenuated by pretreatment with haloperidol or clozapine. p-OHA-induced PPI disruptions were also attenuated by pretreatment with L-741,626 (a selective D(2) receptor antagonist), L-745,870 (a selective D(4) receptor antagonist) or 6-hydroxydopamine (a neurotoxin which targets DA-containing neurons), but not by SCH 23390 (a selective D(1) receptor antagonist), eticlopride (a D(2)/D(3) receptor antagonist) or GBR 12909 (a DA-reuptake inhibitor). These results indicate that selective blockade of either the D(2) or D(4) receptor subtype may prevent disruption of PPI induced by p-OHA via presynaptic DA release.

Differences in the cellular mechanism underlying the effects of amphetamine on prepulse inhibition in apomorphine-susceptible and apomorphine-unsusceptible rats

BACKGROUND

Amphetamine is often used to mimic certain aspects of schizophrenia in laboratory animals, such as a decreased prepulse inhibition.

MATERIALS AND METHODS

Apomorphine-susceptible and apomorphine-unsusceptible rats represent a well-characterized animal model for individual differences in the sensitivity to dopaminergic drugs. Moreover, apomorphine-susceptible rats show a wide variety of schizophrenia-like abnormalities. The differential response to administration of amphetamine (1-4 mg/kg, i.p.) was investigated in these two rat lines using the prepulse inhibition paradigm. Because amphetamine promotes dopamine release, the cellular mechanism underlying the line-specific effects of amphetamine was investigated by administration of alpha-methyl-para-tyrosine (aMpT) and reserpine, substances that are known to deplete the cytosolic dopamine pool and the vesicular dopamine pool, respectively, the former being primarily implicated in mediating the effects of amphetamine.

RESULTS

All doses of amphetamine decreased prepulse inhibition in apomorphine-susceptible rats, whereas only the highest doses (2 and 4 mg/kg, i.p.) of amphetamine decreased prepulse inhibition in apomorphine-unsusceptible rats. Alpha-methyl-para-tyrosine, but not reserpine, blocked the amphetamine-induced disruption in prepulse inhibition in apomorphine-unsusceptible rats, whereas both substances alone had no effect in apomorphine-susceptible rats. However, the combination of alpha-methyl-para-tyrosine and reserpine did block the amphetamine-induced effects in the latter rat line.

DISCUSSION

The present study suggests that apomorphine-susceptible rats are more sensitive to systemic administration of amphetamine than apomorphine-unsusceptible rats. In addition, the data show that the cellular mechanism underlying the effects of amphetamine differs between apomorphine-susceptible and apomorphine-unsusceptible rats. Whereas the effects of amphetamine on prepulse inhibition in apomorphine-unsusceptible rats just require the alpha-methyl-para-tyrosine sensitive dopamine pool, the effects in apomorphine-susceptible rats require both the alpha-methyl-para-tyrosine sensitive and the reserpine sensitive dopamine pool. Because apomorphine-susceptible rats share many features with schizophrenic patients, these data open the perspective that in these patients amphetamine may induce dopamine release from both types of dopamine pool. This might provide an explanation for the increased dopamine release after this psychostimulant drug in patients vs controls.

Cocaine strongly reduces prepulse inhibition in apomorphine-susceptible rats, but not in apomorphine-unsusceptible rats: Regulation by dopamine D2 receptors

Dopaminergic agonists, such as apomorphine and amphetamine, have been shown to drastically reduce prepulse inhibition of the acoustic startle reflex. The effects of the indirect dopamine agonist cocaine on prepulse inhibition have only been described in a few reports and have yielded conflicting results, possibly due to individual differences within and between rat strains. In this study we therefore used apomorphine-susceptible and apomorphine-unsusceptible rats, as an animal model for individual differences, to study the effects of cocaine (20, 30 mg/kg i.p.) on prepulse inhibition. In addition we tested whether the cocaine-induced deficit in prepulse inhibition could be reversed by the D2-antagonist remoxipride (5 mg/kg i.p.), the alpha-1 adrenoceptor antagonist prazosin (2.5 mg/kg i.p.) and the 5-HT2-antagonist ketanserin (2.0 mg/kg i.p.). Cocaine strongly reduced prepulse inhibition in apomorphine-susceptible rats, but had no effect at all on apomorphine-unsusceptible rats. Remoxipride had no effect on prepulse inhibition, but prazosin and ketanserin increased prepulse inhibition. Both remoxipride and prazosin reversed the cocaine-induced deficit in prepulse inhibition, whereas ketanserin did not. We conclude that apomorphine-susceptible rats are extremely sensitive to the effects of cocaine on prepulse inhibition, while apomorphine-unsusceptible rats are not. The effects of cocaine on prepulse inhibition in apomorphine-susceptible rats were mediated by D2-receptors, but not by 5-HT2-receptors or alpha-1 adrenoceptors.

Brief intermittent access to sucrose differentially modulates prepulse inhibition and acoustic startle response in obese CCK-1 receptor deficient rats

Otsuka Long-Evans Tokushima Fatty (OLETF) rats lack the CCK-1 receptor and are hyperphagic and obese. CCK-1 receptors play a role in prepulse inhibition (PPI) by modulating mesolimbic dopamine transmission, a modulator of sensorimotor gating. Therefore, the present study assessed the effects of brief, daily sucrose access on PPI and acoustic startle response (ASR) in OLETF rat and age-matched non-mutant Long-Evans Tokushima Otsuka (LETO) rats. The results revealed that OLETF rats with sucrose access showed an increased ASR [F(1,16) = 6.84; P < 0.01)], relative to sucrose receiving LETO rats. No significant sucrose effect (P = 0.283) on PPI was noted in OLETF rats, whereas sucrose receiving LETO rats had a significantly lower (P < 0.05) PPI percentage than non-sucrose controls. In contrast, sucrose-receiving OLETF rats expressed significantly higher PPI percentage than LETO rats with identical sucrose presentation (P < 0.01). Taken together, these results suggest that sucrose access alters PPI and ASR in general, and the CCK-1 receptors play a modulatory role in facilitating or inhibiting these responses, respectively. A similar effect may be contributory to the hyperphagic behavioral phenotype of obese animal models with altered central dopamine regulation.

Partial reversal of phencyclidine-induced impairment of prepulse inhibition by secretin

BACKGROUND

Secretin is a "gut-brain" peptide whose neural function is as yet poorly understood. Several clinical studies have reported modestly increased social interaction in autistic children following intravenous secretin administration. Very recently secretin also was administered to schizophrenic patients and found to increase social interaction in some individuals.

METHODS

In light of this finding, we assessed the ability of secretin to reverse phencyclidine- (PCP) induced impairment in prepulse inhibition (PPI), a leading animal model of sensorimotor gating deficits in schizophrenia.

RESULTS

Similar to atypical antipsychotics, secretin (1, 3, 10, 30, and 100 microg/kg) partially and dose-dependently reversed the PCP-induced deficit in PPI without significantly affecting baseline startle when administered intraperitoneally (IP) 10 minutes following IP administration of PCP (3 mg/kg).

CONCLUSIONS

This finding may be relevant to observations of antipsychotic efficacy of secretin in schizophrenic patients as well as our previous report that systemically administered secretin is capable of modulating conditioned fear, even at quite low doses.

Cholecystokinin inhibits evoked inhibitory postsynaptic currents in the rat nucleus accumbens indirectly through gamma-aminobutyric acid and gamma-aminobutyric acid type B receptors

We recently reported that cholecystokinin (CCK) excited nucleus accumbens (NAc) cells and depressed excitatory synaptic transmission indirectly through gamma-aminobutyric acid (GABA), acting on presynaptic GABAB receptors (Kombian et al. [2004] J. Physiol. 555:71-84). The present study tested the hypothesis that CCK modulates inhibitory synaptic transmission in the NAc. Using in vitro forebrain slices containing the NAc and whole-cell patch recording, we examined the effects of CCK on evoked inhibitory postsynaptic currents (IPSCs) recorded at a holding potential of -80 mV throughout CCK-8S caused a reversible inward current accompanied by a concentration-dependent decrease in evoked IPSC amplitude. Maximum IPSC depression was approximately 25% at 10 microM, with an estimated EC50 of 0.1 microM. At 1 microM, CCK-8S induced an inward current of 28.3 +/- 4.8 pA (n=6) accompanied by an IPSC depression of -18.8% +/- 1.6% (n=6). This CCK-induced IPSC depression was blocked by pretreatment with proglumide (100 microM; -3.7% +/- 6.9%; n=4) and by LY225910 (100 nM), a selective CCKB receptor antagonist (4.4% +/- 2.6%; n=4). It was not blocked by SCH23390 (10 microM; -23.5% +/- 1.3%; P < 0.05; n=7) or sulpiride (10 microM; -21.8% +/- 5.1%; P <0.05; n=4), dopamine receptor antagonists. By contrast, it was blocked by CGP55845 (1 microM; -0.4% +/- 3.4%; n=5) a potent GABAB receptor antagonist, and by forskolin (50 microM; 9.9% +/- 5.2%; n=4), an adenylyl cyclase activator, and H-89 (1 microM; 6.9% +/- 3.9%; n=4), a protein kinase A (PKA) inhibitor. These results indicate that CCK acts on CCKB receptors to increase extracellular levels of GABA, which then acts on GABAB receptors to decrease IPSC amplitude.

Behavioral and neurotoxicological effects of subchronic manganese exposure in rats

In male Wistar rats, behavioral and electrophysiological investigations, and blood and brain manganese level determinations, were performed; during 10 weeks treatment with low-dose manganese chloride and a 12 weeks post-treatment period. Three groups of 16 animals each received daily doses of 14.84 and 59.36mg/kg b.w. MnCl(2) (control: distilled water) via gavage. During treatment period, Mn accumulation was seen first in the blood, then in the brain samples of the high-dose animals. Short- and long-term spatial memory performance of the treated animals decreased, spontaneous open field activity (OF) was reduced. The number of acoustic startle responses (ASR), and the pre-pulse inhibition (PPI) of these, diminished. In the cortical and hippocampal spontaneous activity, power spectrum was shifted to higher frequencies. The latency of the sensory evoked potentials increased, and their duration, decreased. By the end of the post-treatment period, Mn levels returned to the control in all samples. The impairment of long-term spatial memory remained, as did the number of acoustic startle responses. Pre-pulse inhibition, however, returned to the pre-treatment levels. The changes of the open field activity disappeared but a residual effect could be revealed by administration of d-amphetamine. The electrophysiological effects were partially reversed. By applying a complex set of methods, it was possible to obtain new data for a better-based relationship between the known effects of Mn at neuronal level and the behavioral and electrophysiological outcomes of Mn exposure.

A possible association between the CCK-AR gene and persistent auditory hallucinations in schizophrenia

Recent studies have suggested that DNA variations in the CCK-AR gene might predispose individuals to schizophrenia and particularly to auditory hallucinations (AH). The aim of this study is to assess the association between AH, using a specific scale for AH in schizophrenia (PSYRATS), and the CCK-AR polymorphism at 779 in a Spanish sample. A total of 105 DSM-IV schizophrenic patients with AH and 93 unrelated controls were studied. Twenty-two patients were considered as persistent auditory hallucinators, which showed similar clinical and demographic characteristic than patients with episodic AH, but with the exception of the PSYRATS values. The persistent AH group showed an excess of the A1 allele when was compared with episodic or control groups. Our data support the possible role of the CCK-AR gene in the development of persistent AH in schizophrenic patients.

Reduced effects of amphetamine on prepulse inhibition of startle in gastrin-deficient mice

The present study was aimed at investigating the role of gastrin in startle, startle habituation and prepulse inhibition (PPI). There were no significant differences between gastrin knockout mice and their wildtype controls in any of these baseline parameters. The disruption of PPI by treatment with 5 mg/kg of amphetamine was absent in gastrin knockout mice. However, a higher dose of amphetamine disrupted PPI in both genotypes. Similarly, treatment with the dopamine receptor agonist, apomorphine, the N-methyl-D-aspartate receptor antagonist, MK-801, and the serotonin-1A receptor agonist, 8-hydroxy-di-propylaminotetralin (8-OH-DPAT) modulated PPI similarly in gastrin knockout mice and wildtype controls. These data suggest a role of gastrin in the brain in modulating dopamine release in areas involved in PPI.

Acute amygdalar activation induces an upregulation of multiple monoamine G protein coupled pathways in rat hippocampus

A "partial" rodent model for schizophrenia has been used to characterize the regulation of hippocampal genes in response to amygdalar activation. At 96 h after the administration of picrotoxin into the basolateral nucleus, we have observed an increase in the expression of genes associated with 18 different monoamine (ie adrenergic alpha 1, alpha 2 and beta 2, serotonergic 5HT5b and 5HT6, dopamine D4 and muscarinic m1, m2 and m3) and peptide (CCK A and B, angiotensin 1A, mu and kappa opiate, FSH, TSH, LH, GNRH, and neuropeptide Y) G-protein coupled receptors (GPCRs). These latter receptors are associated with three different G protein signaling pathways (Gq, Gs, and Gi) in which significant changes in gene expression were also noted for adenylate cyclase (AC4), phosphodiesterase (PDE4D), protein kinase A (PKA), and protein kinase C (PKC). Quantitative RT-PCR was used to validate the results and demonstrated that there were predictable increases of three GPCRs selected for this analysis, including the dopamine D4, alpha 1b, and CCK-B receptors. Eight out of the nine monoamine receptors showing these changes have moderate to high affinity for the atypical antipsychotic, clozapine. Taken together, these results suggest that amygdalar activation may play a role in the pathophysiology and treatment of psychosis by regulating the activity of multiple GPCR and metabolic pathways in hippocampal cells.

Cholecystokinin activates CCKB receptors to excite cells and depress EPSCs in the rat rostral nucleus accumbens in vitro

The peptide cholecystokinin (CCK) is abundant in the rat nucleus accumbens (NAc). Although it is colocalized with dopamine (DA) in afferent terminals in this region, neurochemical and behavioural reports are equally divided as to whether CCK enhances or diminishes DA's actions in this nucleus. To better understand the role of this peptide in the physiology of the NAc, we examined the effects of CCK on excitatory synaptic transmission and tested whether these are dependent on DA and/or other neuromodulators. Using whole-cell recording in rat forebrain slices containing the NAc, we show that sulphated CCK octapeptide (CCK-8S), the endogenously active neuropeptide, consistently depolarized cells and depressed evoked excitatory postsynaptic currents (EPSCs) in the rostral NAc. It caused a reversible, dose-dependent decrease in evoked EPSC amplitude that was accompanied by an increase in the decay constant of the EPSC but with no apparent change in paired pulse ratio. It was mimicked by unsulphated CCK-8 (CCK-8US), a CCK(B) receptor-selective agonist, and blocked by LY225910, a CCK(B) receptor-selective antagonist. Both CCK-8S and CCK-8US induced an inward current with a reversal potential around -90 mV that was accompanied by an increase in input resistance and action potential firing. The CCK-8S-induced EPSC depression was slightly reduced in the presence of SCH23390 but not in the presence of sulpiride or 8-cyclopentyltheophylline. By contrast, it was completely blocked by CGP55845, a potent GABA(B) receptor-selective antagonist. These results indicate that CCK excites NAc cells directly while depressing evoked EPSCs indirectly, mainly through the release of GABA.

Effects of the 5-HT(6) receptor antagonist, SB-271046, in animal models for schizophrenia

The 5-HT(6) receptor is targeted by several new antipsychotics such as clozapine, olanzapine, and sertindole. We studied the effect of SB-271046 [5-chloro-N-(4-methoxy-3-piperazin-1-yl-phenyl)-3-methyl-2-benzothiophenesulfonamide], a specific 5-HT(6) receptor antagonist, in three models for the positive symptoms of schizophrenia---D-amphetamine-induced hyperactivity, and D-amphetamine- or phencyclidine (PCP)-disrupted prepulse inhibition (PPI). We also tested this compound in a model for the negative symptoms of schizophrenia, PCP-disrupted social interaction (SIT) in rats. Induction of side effects by this compound was evaluated by testing its potency to reduce spontaneous motility, and to induce catalepsy in rats. The effect of SB-271046 was compared to clozapine in all models tested. This study showed that SB-271046 had no beneficial effect in PCP-disrupted SIT. However, SB-271046 dose-dependently normalised D-amphetamine-disrupted PPI, but did not reverse PCP-disrupted PPI. In addition, SB-271046 did not antagonise D-amphetamine-induced hyperactivity. Thus, this specific 5-HT(6) receptor antagonist was associated with a clear positive outcome in only one model for the positive symptoms of schizophrenia, and had no beneficial effect in the model for negative symptoms. Consequently, it is clear that SB-271046 is not expected to have an antipsychotic efficacy, at least when given as monotherapy.

Decreased haloperidol-induced potentiation of zif268 mRNA expression in the nucleus accumbens shell and the dorsal lateral striatum of rats lacking cholecystokinin-A receptors

Evidence suggests that endogenous cholecystokinin (CCK), a neuropeptide that modulates brain dopamine function, may contribute to the therapeutic and motor effects of antipsychotic drugs via activation of CCK-A receptors in the mesolimbic and nigrostriatal pathways, respectively. To determine if CCK modulates the effects of antipsychotic drugs through CCK-A receptors, we measured the haloperidol-induced zif268 mRNA response in the nucleus accumbens (NA) shell, NA core, and dorsal lateral striatum (DLS) in Otsuka Long Evans Tokushima Fatty (OLETF) rats that lack CCK-A receptors due to a spontaneous mutation. OLETF rats and normal Long Evans rats were treated with subcutaneous (s.c.) injections of saline or haloperidol (2 mg/kg). In situ hybridization was performed and zif268 mRNA expression was quantified. The haloperidol-induced expression of zif268 mRNA was significantly decreased in the DLS (P < 0.01) and the NA shell (P < 0.05), but not in the NA core, in OLETF rats compared to LETO rats. These data suggest that CCK-A receptor mechanisms may contribute to the therapeutic and the extrapyramidal motor effects associated with antipsychotic drug treatment.

Cholecystokinin modulation of apomorphine- or amphetamine-induced stereotypy in rats: opposite effects

Stereotyped behavior can be induced by the dopamine agonist apomorphine or by the releasing agent amphetamine. Cholecystokinin influence on dopamine-mediated behaviors has been extensively studied but a real controversy remains. Our purpose was to further characterize the dopamine-cholecystokinin interaction in apomorphine- and amphetamine-induced stereotyped behavior using sulphated cholecystokinin octapeptide (CCK8) and cholecystokinin tetrapeptide (CCK4) treatments. The results showed that CCK8 decreases apomorphine-induced stereotyped behavior and CCK4 has no effect. CCK4 and CCK8 increased the amphetamine-induced stereotyped behavior; CCK4 was more effective. The results confirm the opposite modulation of apomorphine or amphetamine-induced stereotyped behavior by CCK. These data suggest that this modulation is mediated by both CCK receptors on apomorphine-induced and only by CCK(2) receptors on amphetamine-induced stereotyped behavior.

The inability of CCK to block (or CCK antagonists to substitute for) the stimulus effects of chlordiazepoxide

To further examine the relationship between cholecystokinin (CCK) and GABA, the present study assessed the ability of the CCK-A antagonist devazepide and the CCK-B antagonist L-365,260 to substitute for the stimulus effects of chlordiazepoxide (CDP), as well as the ability of CCK-8s to block these effects, in female Long-Evans rats within the conditioned taste aversion baseline of drug discrimination learning. Both devazepide and L-365,260 failed to substitute for the discriminative stimulus properties of CDP, and CCK-8s failed to block its stimulus effects. The benzodiazepine diazepam did substitute for, and the benzodiazepine antagonist flumazenil did block, the stimulus effects of CDP. This suggests that the lack of substitution for, or antagonism of, CDP by the CCK antagonists and CCK-8s, respectively, was not due to the inability of the present design to assess such effects. Possible bases for the current findings, e.g., necessity of an anxiogenic baseline, drug and receptor specificity, as well as the dose-response nature of the interaction, were discussed. Given that a relationship between CCK and GABA has been reported in other designs, the present results suggest that such a relationship may be preparation specific.

Opposite modulation of apomorphine- or amphetamine-induced stereotypy by antagonists of CCK receptors

Stereotyped behavior is elicited by activation of dopaminergic systems with drugs such as apomorphine and amphetamine. In previous studies, we have reported that the sulfated cholecystokinin octapeptide (CCK-8) decreased apomorphine-induced stereotypy in animals with normal and supersensitive dopamine receptors. The aim of the present study was to evaluate the effects of CCK(1) and CCK(2) receptor antagonists on stereotyped behavior induced by apomorphine or amphetamine. Rats were pretreated with the CCK(1) (SR 27897B; 1-[[2-(4-(2-chlorophenyl) thiazol-2-yl) aminocarbonyl]indolyl]acetic acid; 500 microg/kg; i.p.) or CCK(2) (L-365,260; 3R-(+)-N-(2,3-dihydro-1-methyl-2-oxo-5 phenyl-1H-1, 4-benzodiazepine-3-yl)-N'-(3-methyl phenyl)-urea; 500 microg/kg; i.p. ) receptor antagonists or saline 15 min before apomorphine (0.6 mg/kg; s.c.) or amphetamine (9.0 mg/kg; i.p.) injection. Both CCK(1) and CCK(2) receptor antagonists significantly increased apomorphine-induced stereotypy. In contrast, only the blockade of CCK(2) receptors significantly decreased amphetamine-induced stereotypy. The results suggest a dual opposite mechanism for CCK-dopamine interactions. These data also suggest that both apomorphine- and amphetamine-induced stereotypy should be used whenever effects of drugs acting on dopaminergic systems are being assessed.

Localizing haloperidol effects on sensorimotor gating in a predictive model of antipsychotic potency

The degree to which a startle response to a loud noise is inhibited by a weak prestimulus is an operational measure of sensorimotor gating. Prepulse inhibition (PPI) can be measured across species and is reduced in schizophrenia patients and dopamine (DA)-activated rats. The ability of DA antagonists to restore PPI in apomorphine (APO)-treated rats correlates highly with their clinical antipsychotic potency. We compared the ability of systemic- vs. intracerebrally (i.c.)-administered haloperidol (HAL) to restore PPI in APO-treated rats. Consistent with previous studies, systemic administration of HAL completely restored PPI in rats treated with APO (0.5 mg/kg s.c.), with an ED50 of approximately 0.02 mg/kg. In an otherwise identical paradigm, HAL failed to fully restore PPI after infusion into either the nucleus accumbens (NACcore or NACshell), NACcore + caudate nucleus (CN), ventral subiculum (VS), medial prefrontal cortex (MPFC), or ventral tegmentum (VTA). A subtotal, but statistically significant restoration of PPI was achieved after HAL infusion into all regions, except the NACshell. Statistically significant effects of i.c. HAL tended to be observed at doses that were only approximately 5-10-fold lower than those at which significant effects were observed after systemic administration. The results suggest that systemically administered HAL may restore PPI in APO-treated rats through its action distributed throughout multiple levels of PPI-regulatory circuitry.