Altered extracellular dopamine concentration in the brains of cholecystokinin-A receptor deficient rats

Cholecystokinin neurotransmission in the central nervous system: Insights into its role in health and disease

Cholecystokinin (CCK) plays a key role in various brain functions, including both health and disease states. Despite the extensive research conducted on CCK, there remain several important questions regarding its specific role in the brain. As a result, the existing body of literature on the subject is complex and sometimes conflicting. The primary objective of this review article is to provide a comprehensive overview of recent advancements in understanding the central nervous system role of CCK, with a specific emphasis on elucidating CCK's mechanisms for neuroplasticity, exploring its interactions with other neurotransmitters, and discussing its significant involvement in neurological disorders. Studies demonstrate that CCK mediates both inhibitory long-term potentiation (iLTP) and excitatory long-term potentiation (eLTP) in the brain. Activation of the GPR173 receptor could facilitate iLTP, while the Cholecystokinin B receptor (CCKBR) facilitates eLTP. CCK receptors' expression on different neurons regulates activity, neurotransmitter release, and plasticity, emphasizing CCK's role in modulating brain function. Furthermore, CCK plays a pivotal role in modulating emotional states, Alzheimer's disease, addiction, schizophrenia, and epileptic conditions. Targeting CCK cell types and circuits holds promise as a therapeutic strategy for alleviating these brain disorders.

Cholecystokinin (CCK): a neuromodulator with therapeutic potential in Alzheimer's and Parkinson's disease

Cholecystokinin (CCK) is a neuropeptide modulating digestion, glucose levels, neurotransmitters and memory. Recent studies suggest that CCK exhibits neuroprotective effects in Alzheimer's disease (AD) and Parkinson's disease (PD). Thus, we review the physiological function and therapeutic potential of CCK. The neuropeptide facilitates hippocampal glutamate release and gates GABAergic basket cell activity, which improves declarative memory acquisition, but inhibits consolidation. Cortical CCK alters recognition memory and enhances audio-visual processing. By stimulating CCK-1 receptors (CCK-1Rs), sulphated CCK-8 elicits dopamine release in the substantia nigra and striatum. In the mesolimbic pathway, CCK release is triggered by dopamine and terminates reward responses via CCK-2Rs. Importantly, activation of hippocampal and nigral CCK-2Rs is neuroprotective by evoking AMPK activation, expression of mitochondrial fusion modulators and autophagy. Other benefits include vagus nerve/CCK-1R-mediated expression of brain-derived neurotrophic factor, intestinal protection and suppression of inflammation. We also discuss caveats and the therapeutic combination of CCK with other peptide hormones.

Intestinal microbiome dysbiosis in alcohol-dependent patients and its effect on rat behaviors

IMPORTANCE

Intestinal microbiome dysbiosis is associated with psychiatric disease through the "microbiota-gut-brain" axis. Here, we revealed that there was obvious intestinal microbiome (including bacterial and fungal) dysbiosis in alcohol-dependent patients. Alcohol consumption seriously disturbs the gut equilibrium between bacteria and fungi, reduces the interactions among bacterial-fungal trans-kingdom, and increases intestinal permeability. Gut microbiota should be considered as a whole to study the development of alcohol dependence. The gut microbiome of alcohol-dependent patients increased the anxiety- and depression-like behavior in rats. The gut microbiota dysbiosis may promote the development of alcohol dependence by regulating the endogenous cholecystokinin (CCK) and related receptors. Hence, regulating the balance of gut microbiota and the endogenous CCK may be a potential strategy for reducing the risk of relapse in alcohol addiction patients.

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.

Neural circuit mechanisms of the cholecystokinin (CCK) neuropeptide system in addiction

Given historical focus on the roles for cholecystokinin (CCK) as a peripheral hormone controlling gastrointestinal processes and a brainstem peptide regulating food intake, the study of CCK as a limbic neuromodulator coordinating reward-seeking and emotional behavior remains underappreciated. Furthermore, localization of CCK to specialized interneurons throughout the hippocampus and cortex relegated CCK to being examined primarily as a static cell type marker rather than a dynamic functional neuromodulator. Yet, over three decades of literature have been generated by efforts to delineate the central mechanisms of addiction-related behaviors mediated by the CCK system across the striatum, amygdala, hypothalamus, and midbrain. Here, we cover fundamental findings that implicate CCK neuron activity and CCK receptor signaling in modulating drug intake and drug-seeking (focusing on psychostimulants, opioids, and alcohol). In doing so, we highlight the few studies that indicate sex differences in CCK expression and corresponding drug effects, emphasizing the importance of examining hormonal influences and sex as a biological variable in translating basic science discoveries to effective treatments for substance use disorders in human patients. Finally, we point toward understudied subcortical sources of endogenous CCK and describe how continued neurotechnology advancements can be leveraged to modernize understanding of the neural circuit mechanisms underlying CCK release and signaling in addiction-relevant behaviors.

Molecular Imaging of Central Dopamine in Obesity: A Qualitative Review across Substrates and Radiotracers

Dopamine is a neurotransmitter that plays a crucial role in adaptive behavior. A wealth of studies suggests obesity-related alterations in the central dopamine system. The most direct evidence for such differences in humans comes from molecular neuroimaging studies using positron emission tomography (PET) and single-photon emission computed tomography (SPECT). The aim of the current review is to give a comprehensive overview of molecular neuroimaging studies that investigated the relation between BMI or weight status and any dopamine target in the striatal and midbrain regions of the human brain. A structured literature search was performed and a summary of the extracted findings are presented for each of the four available domains: (1) D2/D3 receptors, (2) dopamine release, (3) dopamine synthesis, and (4) dopamine transporters. Recent proposals of a nonlinear relationship between severity of obesity and dopamine imbalances are described while integrating findings within and across domains, after which limitations of the review are discussed. We conclude that despite many observed associations between obesity and substrates of the dopamine system in humans, it is unlikely that obesity can be traced back to a single dopaminergic cause or consequence. For effective personalized prevention and treatment of obesity, it will be crucial to identify possible dopamine (and non-dopamine) profiles and their functional characteristics.

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

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

Heterogeneities in Axonal Structure and Transporter Distribution Lower Dopamine Reuptake Efficiency

Efficient clearance of dopamine (DA) from the synapse is key to regulating dopaminergic signaling. This role is fulfilled by DA transporters (DATs). Recent advances in the structural characterization of DAT from Drosophila (dDAT) and in high-resolution imaging of DA neurons and the distribution of DATs in living cells now permit us to gain a mechanistic understanding of DA reuptake events in silico. Using electron microscopy images and immunofluorescence of transgenic knock-in mouse brains that express hemagglutinin-tagged DAT in DA neurons, we reconstructed a realistic environment for MCell simulations of DA reuptake, wherein the identity, population and kinetics of homology-modeled human DAT (hDAT) substates were derived from molecular simulations. The complex morphology of axon terminals near active zones was observed to give rise to large variations in DA reuptake efficiency, and thereby in extracellular DA density. Comparison of the effect of different firing patterns showed that phasic firing would increase the probability of reaching local DA levels sufficiently high to activate low-affinity DA receptors, mainly owing to high DA levels transiently attained during the burst phase. The experimentally observed nonuniform surface distribution of DATs emerged as a major modulator of DA signaling: reuptake was slower, and the peaks/width of transient DA levels were sharper/wider under nonuniform distribution of DATs, compared with uniform. Overall, the study highlights the importance of accurate descriptions of extrasynaptic morphology, DAT distribution, and conformational kinetics for quantitative evaluation of dopaminergic transmission and for providing deeper understanding of the mechanisms that regulate DA transmission.

The unappreciated roles of the cholecystokinin receptor CCK(1) in brain functioning

The CCK(1) receptor is a G-protein-coupled receptor activated by the sulfated forms of cholecystokinin (CCK), a gastrin-like peptide released in the gastrointestinal tract and mammal brain. A substantial body of research supports the hypothesis that CCK(1)r stimulates gallbladder contraction and pancreatic secretion in the gut, as well as satiety in brain. However, this receptor may also fulfill relevant roles in behavior, thanks to its widespread distribution in the brain. The strategic location of CCK(1)r in mesolimbic structures and specific hypothalamic and brainstem nuclei lead to complex interactions with neurotransmitters like dopamine, serotonin, and glutamate, as well as hypothalamic hormones and neuropeptides. The activity of CCK(1)r maintains adequate levels of dopamine and regulates the activity of serotonin neurons of raphe nuclei, which makes CCK(1)r an interesting therapeutic target for the development of adjuvant treatments for schizophrenia, drug addiction, and mood disorders. Unexplored functions of CCK(1)r, like the transmission of interoceptive sensitivity in addition to the regulation of hypothalamic hormones and neurotransmitters affecting emotional states, well-being, and attachment behaviors, may open exciting roads of research. The absence of specific ligands for the CCK(1) receptor has complicated the study of its distribution in brain so that research about its impact on behavior has been published sporadically over the last 30 years. The present review reunites all this body of evidence in a comprehensive way to summarize our knowledge about the actual role of CCK in the neurobiology of mental illness.

Neurotransmitters: The Critical Modulators Regulating Gut-Brain Axis

Neurotransmitters, including catecholamines and serotonin, play a crucial role in maintaining homeostasis in the human body. Studies on these neurotransmitters mainly revolved around their role in the "fight or flight" response, transmitting signals across a chemical synapse and modulating blood flow throughout the body. However, recent research has demonstrated that neurotransmitters can play a significant role in the gastrointestinal (GI) physiology. Norepinephrine (NE), epinephrine (E), dopamine (DA), and serotonin have recently been a topic of interest because of their roles in the gut physiology and their potential roles in GI and central nervous system pathophysiology. These neurotransmitters are able to regulate and control not only blood flow, but also affect gut motility, nutrient absorption, GI innate immune system, and the microbiome. Furthermore, in pathological states, such as inflammatory bowel disease (IBD) and Parkinson's disease, the levels of these neurotransmitters are dysregulated, therefore causing a variety of GI symptoms. Research in this field has shown that exogenous manipulation of catecholamine serum concentrations can help in decreasing symptomology and/or disease progression. In this review article, we discuss the current state-of-the-art research and literature regarding the role of neurotransmitters in regulation of normal GI physiology, their impact on several disease processes, and novel work focused on the use of exogenous hormones and/or psychotropic medications to improve disease symptomology. J. Cell. Physiol. 232: 2359-2372, 2017. © 2016 Wiley Periodicals, Inc.

Trait and state binge eating predispose towards cocaine craving

Binge eating (BE) and drug seeking share similar behavioral features, including loss of control over consumption and compulsive seeking of the craved substance. Previous studies in animal models have demonstrated a complex interaction between 'state' BE, produced by intermittent access to a palatable diet, and 'trait' BE, a phenotypical proneness towards overeating. In the present study, we examined the relationship between state and trait BE and cocaine seeking. We used Otsuka Long Evans Tokushima Fatty rats, a genetic model for obesity that demonstrates BE-like behavior, and Long Evans Tokushima Otsuka controls. They received a schedule of limited access to a palatable diet (3 days/week or 5 days/week access to Ensure for a month). Next, they underwent cocaine self-administration training (1 mg/kg, 1 hour/day for 10 days) followed by extinction sessions (7 days). We found that the degree of BE-like behavior and the state and trait BE combination predicted cocaine craving patterns. Lower levels of dopamine D2 receptors in the prefrontal cortex were correlated with increased drug craving. Moreover, restricted access to an attractive diet was found to be a risk factor for heightened cocaine craving, particularly in trait binge eaters, as rats on the 3 days/week access schedule persistently failed to cease cocaine seeking throughout extinction. Hence, we postulate a joint role of state and trait BE as risk factors for heightened cocaine craving.

Cholecystokinin A receptor (CCKAR) gene variation is associated with language lateralization

Schizophrenia is a psychiatric disorder associated with atypical handedness and language lateralization. However, the molecular mechanisms underlying these functional changes are still poorly understood. Therefore, the present study was aimed at investigating whether variation in schizophrenia-related genes modulates individual lateralization patterns. To this end, we genotyped 16 single nucleotide polymorphisms that have previously been linked to schizophrenia on a meta-analysis level in a sample of 444 genetically unrelated healthy participants and examined the association of these polymorphisms with handedness, footedness and language lateralization. We found a significant association of the cholecystokinin-A receptor (CCKAR) gene variation rs1800857 and language lateralization assessed using the dichotic listening task. Individuals carrying the schizophrenia risk allele C of this polymorphism showed a marked reduction of the typical left-hemispheric dominance for language processing. Since the cholecystokinin A receptor is involved in dopamine release in the central nervous system, these findings suggest that genetic variation in this receptor may modulate language lateralization due to its impact on dopaminergic pathways.

The gut-brain dopamine axis: a regulatory system for caloric intake

Post-ingestive factors are known to strongly modulate feeding behavior by providing feedback signals to the central nervous system on the current physiological state of the organism. Of particular interest is the identification of the physiological pathways that permit the brain to sense post-ingestive signals. We will review recent evidence supporting the concept that direct stimulation of the gastrointestinal tract with nutrients induces release of the catecholamine neurotransmitter dopamine. In addition, changes in dopamine efflux produced by direct stimulation of the gastrointestinal tract were found to reflect the caloric load of the infusates, suggesting that dopamine signaling may function as a central caloric sensor that mediates adjustments in intake according to the caloric density of a meal. Consistent with the above, blockade of dopamine signaling disrupts flavor-nutrient associations and impairs the regulatory capacity to maintain constant caloric intake during intra-gastric feeding. Future research must determine the exact pathways linking gut nutrient administration to dopamine efflux. Current evidence points to parallel contributions by pre- and post-absorptive pathways, indicating that dopamine systems constitute a site of convergence through which distinct physiological signals can exert control over ingestive behaviors.

Feeding and reward: ontogenetic changes in an animal model of obesity

Given that food is a natural reinforcement, deficits in the reward system can lead to disordered eating behavior, inducing or worsening an already existing pre-obese phenotype. In order to evaluate developmental, food-reward-related measures we used the OLETF rat, an animal model of early-onset overeating-induced obesity, and a natural CCK-1 receptor knockout. Dopamine-like-receptor type 1 (D1R) and D2R levels were examined in a reward-related brain area (Nac shell) and sucrose preference was assessed at selected time points from weaning to adulthood (postnatal day [PND]90). In addition, a group of OLETF was pair fed (PF) to the amount of food consumed by same-age LETO controls (from weaning to PND 90) to examine the contribution of overweight to the alteration in DR expression. In addition, we examined food "craving"-like behavior by analyzing microstructural patterns of licking a palatable liquid diet. OLETF rats expressed significantly lower D2R levels than LETO controls only on PND 90. In PF OLETF, weight and D2R levels were normalized. In addition, OLETF presented exaggerated preference for the high sucrose concentration. After 30-day abstinence, OLETF rats presented significant increased initial rate of licking, suggesting food "craving". Thus, adult OLETF rats demonstrated altered D2R signaling similar to drug-induced sensitization, suggesting a link with their avidity for sucrose and their abnormal craving response. However, the current findings of a late deficit appearance and the novel PF results suggest that deficits in the motivation/regulatory systems of the OLETF rat are a developing process (at least from weaning and on) depending on the overeating and obese phenotype of the rats and not only on the CCK mutation.

Regulation of fat intake in the absence of flavour signalling

Animals, including humans, can achieve precise regulation of caloric intake by adjusting consumption in response to covert changes in energy density. It remains unknown, however, whether the presence of flavour cues are required for the ability to maintain constant caloric intake. Also unknown are the brain circuits that may function as the central calorie monitors that control adaptive adjustments in energy intake. Here we show that mice trained to lick a dry spout in order to receive intra-gastric infusions of a fat emulsion maintained constant hourly caloric intake by adjusting the number of dry licks in response to changes in caloric density. Animals also increased dry licking according to hunger levels, and developed conditioned preferences for dry sippers associated with high calorie infusions. Importantly, striatal dopamine levels were closely associated with the amount of calories ingested, rather than with the number of dry licks produced. Dopamine levels in dorsal and ventral striatum also reflected caloric density in mice passively receiving intra-gastric infusions of fat emulsions. Consistent with the above, systemic administration of the dopamine receptor blocker haloperidol markedly increased the production of dry licks needed to obtain high-calorie infusions, as if the caloric density of the infusions had been diluted. Conversely, haloperidol markedly decreased the production of dry licks needed to obtain low-calorie infusions. Taken together, our results support the proposition that brain dopamine circuits function as one central sensor of calorie ingestion, since (1) extracellular striatal dopamine levels fluctuate in proportion to the caloric density of nutrients infused in the gut; and (2) inhibiting dopamine receptor signalling disrupts the animals' ability to maintain constant caloric intake across experimental sessions.

Attenuation of obesity by early-life food restriction in genetically hyperphagic male OLETF rats: peripheral mechanisms

The alarming increase in childhood, adolescent and adult obesity has exposed the need for understanding early factors affecting obesity and for treatments that may help prevent or moderate its development. In the present study, we used the OLETF rat model of early-onset hyperphagia induced obesity, which become obese as a result of the absence of CCK(1) receptors, to examine the influence of partial food restriction on peripheral adiposity-related parameters during and after chronic and early short-term food restriction. Pair feeding (to the amount of food eaten by control, LETO rats) took place from weaning until postnatal day (PND) 45 (early) or from weaning until PND90 (chronic). We examined fat pad weight (brown, retroperitoneal, inguinal and epididymal); inguinal adipocyte size and number; and plasma leptin, oxytocin and creatinine levels. We also examined body weight, feeding efficiency and spontaneous intake after release from food-restriction. The results showed that chronic food restriction produced significant reductions in adiposity parameters, hormones and body weight, while early food restriction successfully reduced long-term body weight, intake and adiposity, without affecting plasma measurements. Early (and chronic) dieting produced promising long-term effects that may imply the reorganization of both peripheral and central mechanisms that determine energy balance and further support the theory suggesting that early interventions may effectively moderate obesity, even in the presence of a genetic tendency.

Altered orosensory sensitivity to oils in CCK-1 receptor deficient rats

CCK-1 receptor deficient Otsuka Long Evans Tokushima Fatty (OLETF) rats are hyperphagic, which leads to subsequent obesity and diabetes. Additionally, they have increased sham intake and enhanced preference for sucrose solutions relative to control, Long Evans Tokushima Otsuka (LETO) rats. To determine the effects of oil on ingestion, we first measured real feeding of various concentrations of oil emulsions (12.5, 25, 50, 75, and 100%) in rats that were fed ad libitum. Secondly, to isolate the orosensory compontent of oils from post-ingestive consequences, as well as determine the contribution of energy status, we measured sham feeding in OLETF and LETO rats using one-bottle acceptance tests while non-deprived and overnight food deprived. Finally, to assess the orosensory effects of nutritive and non-nutritive oils, we used two-bottle preference tests in sham fed OLETF and LETO rats. We found that real feeding resulted in increased intake of high oil concentrations for OLETF rats relative to LETO rats. Similarly, OLETF rats consumed significantly more of higher concentration corn oils than LETO while non-deprived sham feeding. Conversely, OLETF rats overconsumed low concentration corn oil compared to LETO during overnight deprived sham-feeding tests. In two-bottle sham-feeding preference tests, both non-deprived OLETF and LETO rats preferred corn to mineral oil. Collectively, these results show that increased oil intake in OLETF rats is driven by both peripheral deficits to satiation and altered orosensory sensitivity.

Parabrachial coding of sapid sucrose: relevance to reward and obesity

Cumulative evidence in rats suggests that the pontine parabrachial nuclei (PBN) are necessary for assigning hedonic value to taste stimuli. In a series of studies, our laboratory has investigated the parabrachial coding of sapid sucrose in normal and obese rats. First, using chronic microdialysis, we demonstrated that sucrose intake increases dopamine release in the nucleus accumbens, an effect that is dependent on oral stimulation and on concentration. The dopamine response was independent of the thalamocortical gustatory system but was blunted substantially by lesions of the PBN. Similar lesions of the PBN but not the thalamic taste relay diminished cFos activation in the nucleus accumbens caused by sucrose ingestion. Recent single-neuron recording studies have demonstrated that processing of sucrose-evoked activity in the PBN is altered in Otsuka Long Evans Tokushima Fatty (OLETF) rats, which develop obesity due to chronic overeating and express increased avidity to sweet. Compared with lean controls, taste neurons in OLETF rats had reduced overall sensitivity to sucrose and altered concentration responses, with decreased responses to lower concentrations and augmented responses to higher concentrations. The decreased sensitivity to sucrose was specific to NaCl-best neurons that also responded to sucrose, but the concentration effects were carried by the sucrose-specific neurons. Collectively, these findings support the hypothesis that the PBN enables taste stimuli to engage the reward system and, in doing so, influences food intake and body weight regulation. Obesity, in turn, may further alter the gustatory code via forebrain connections to the taste relays or hormonal changes consequent to weight gain.

Association between CCK-AR gene and schizophrenia with auditory hallucinations

OBJECTIVE

Previous studies on a possible association between CCK-AR polymorphisms and schizophrenia have been controversial. The aim of the present study was to assess a potential association between schizophrenic patients with auditory hallucinations and polymorphisms of the CCK-AR gene.

METHODS

A set of single nucleotide polymorphisms mainly located in the regulatory region of the CCK-AR gene was analysed in a sample of 163 Diagnostic and statistical manual of mental disorders-IV-diagnosed schizophrenic patients and 162 healthy controls.

RESULTS

Significant differences in the genotype (P=0.011) and allele (P=0.0009) frequencies of the +121C/G SNP (located in the 5' regulatory region) were found between patients and controls. The excess of the C allele in the patient group remained significant after Bonferroni correction (P=0.03). However, functional in vitro assays, did not reveal significant differences on gene expression between +121G and +121C alleles of this SNP. Further investigations revealed two risk haplotypes: +121C/+978A/+984T (P=0.01) and +121C/+978T/+984C (P=0.0091) as well as a protective haplotype: +121G/+978T/+984T (P=0.0001).

CONCLUSION

Our data support a possible role of the CCK-AR gene in the vulnerability to schizophrenia in patients with auditory hallucinations, and suggest remarkable allele heterogeneity.

Obese OLETF rats exhibit increased operant performance for palatable sucrose solutions and differential sensitivity to D2 receptor antagonism

CCK-1-receptor-deficient Otsuka Long-Evans Tokushima fatty (OLETF) rats are hyperphagic and exhibit a greater preference for sucrose compared with lean controls [Long-Evans Tokushima Otsuka (LETO)]. To directly assess motivation to work for sucrose reward in this model of obesity and type 2 diabetes, we examined the operant performance of OLETF rats at nondiabetic and prediabetic stages (14 and 24 wk of age, respectively) on fixed-ratio (FR) and progressive-ratio (PR) schedules of reinforcement. To evaluate the involvement of dopamine systems, the effects of the D1 receptor antagonist SCH23390 (100 and 200 nmol/kg ip) and the D2 receptor antagonist raclopride (200 and 400 nmol/kg ip), were also tested on PR responding for sucrose. Compared with age-matched LETO rats, 14-wk-old OLETF rats emitted more licks on the "active" empty spout operant on the FR-10 schedule of reinforcement to obtain 0.01 M and 0.3 M sucrose and completed higher ratio requirements on the PR schedule to gain access to 0.3 M and 1.0 M sucrose. At 24 wk, this effect was limited to 1.0 M sucrose. Both antagonists were potent in reducing operant responding to 0.3 M sucrose in both strains at both ages, and there was no strain effect to SCH23390 at either age. OLETF rats, on the other hand, showed an increased sensitivity to the higher dose of raclopride, resulting in reduced responding to sucrose reinforcement at 24 wk. Taken together, these findings provide the first direct evidence for an increased motivation for sucrose reward in the OLETF rats and suggest altered D2 receptor regulation with the progression of obesity and prediabetes.

Altered dopamine D2 receptor function and binding in obese OLETF rat

A decrease in D2-like receptor (D2R) binding in the striatum has been reported in obese individuals and drug addicts. Although natural and drug rewards share neural substrates, it is not clear whether such effects also contribute to overeating on palatable meals as an antecedent of dietary obesity. Therefore, we investigated receptor density and the effect of the D2R agonist quinpirole (0.05, 0.5 mg/kg, S.C.) on locomotor activity and sucrose intake in a rat model of diet-induced obesity, the CCK-1 receptor-deficient Otsuka Long Evans Tokushima Fatty (OLETF) rat. Compared to age-matched lean controls (LETO), OLETF rats expressed significantly lower [125I]-iodosulpride binding in the accumbens shell (-16%, p<0.02). Whereas the high dose of quinpirole increased motor activity in both strains equally, the low dose reduced activity more in OLETF. Both doses significantly reduced sucrose intake in OLETF but not LETO rats. These findings demonstrate an altered D2R signaling in obese OLETF rats similar to drug-induced sensitization and suggest a link between this effect and avidity for sucrose in this model.

Altered basal and stimulated accumbens dopamine release in obese OLETF rats as a function of age and diabetic status

The Otsuka Long-Evans Tokushima Fatty (OLETF) rat lacking the CCK-1 receptor is hyperphagic, prefers palatable and high-calorie meals, and gradually develops obesity and type 2 diabetes. To determine dopamine levels in this strain, we used in vivo quantitative (no net flux) microdialysis at three different ages representing nondiabetic (8 wk), prediabetic (18 wk), and diabetic (56 wk) stages in OLETF and age-matched lean Long-Evans Tokushima Otsuka (LETO) controls. Results showed significantly elevated basal dopamine levels in the caudomedial nucleus accumbens of OLETF rats compared with LETO at younger ages (8 wk: 20.10 +/- 5.61 nM vs. 15.85 +/- 5.63 nM; 18 wk: 7.37 +/- 3.71 nM vs. 4.75 +/- 1.25 nM, means +/- SD). In contrast, at 56 wk of age, a profound decline in extracellular dopamine concentrations was seen in both strains with a tendency for a greater effect in OLETF rats (1.78 +/- 0.40 nM vs. 2.39 +/- 0.42 nM). Further, extracellular fraction, an index for reuptake, was higher in 56-wk-old OLETF compared with LETO (0.648 +/- 0.049 vs. 0.526 +/- 0.057). Potassium-stimulated dopamine efflux revealed an increased capacity of vesicular pool in OLETF rats compared with LETO across all age groups with an accentuated strain difference at 56 wk. These findings demonstrate altered striatal dopamine functions (i.e., increased stimulated release and uptake) in obese OLETF rat. This could be due to the lack of functional CCK-1 receptors, or metabolic and hormonal factors associated with the development of obesity and insulin resistance, or both.

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.

Independent ingestion and microstructure of feeding patterns in infant rats lacking CCK-1 receptors

Otsuka Long-Evans Tokushima fatty (OLETF) rats are a strain of Long-Evans Tokushima Otsuka (LETO) rats that do not express CCK-1 receptors, developing in adulthood, hyperphagia, obesity, and non-insulin-dependent diabetes mellitus (NIDDM). We examined weight gain and meal patterns during a 30-min independent ingestion test on postnatal days 2-4 and again on days 9-11 in OLETF and LETO rat pups. OLETF pups were significantly heavier compared with their LETO controls at both ages, and they consumed significantly more of the sweet milk diet. The difference in intake can be attributed to a significant increase in meal size and duration. Number of clusters and bursts of licking within a meal were greater in OLETF rat pups, with no difference between strains in burst and cluster size. Interlick interval (ILI) was not significantly different between OLETF and LETO pups. This measure decreased on days 9-11 compared with days 2-4 in both strains. Latency to start feeding was significantly shorter on days 2-4 in OLETF vs. LETO pups, but this difference disappeared at the second test at the older age. Two- to four-day-old OLETF pups consumed a larger volume of milk during the first minute of feeding, and their initial lick rate and decay of lick rate were significantly larger compared with their LETO controls. Lack of CCK-1 receptors, or other OLETF-related abnormalities, therefore, resulted in a satiation deficit, leading to increased meal size, hyperphagia, and increased weight gain as early as 2-4 postnatal days.

Altered taste sensitivity in obese, prediabetic OLETF rats lacking CCK-1 receptors

Otsuka Long-Evans Tokushima fatty (OLETF) rats lack the CCK-1 receptor, are hyperphagic, progressively become obese, and develop type-2 diabetes. We recently demonstrated an increased preference for both real and sham feeding of sucrose in this strain, suggesting altered orosensory sensitivity. To investigate taste functions, we used an automated gustometer with 10-s access to different concentrations of various sapid stimuli. Tests were repeated at 10 and 18 wk of age to assess the early and advanced stages of prediabetes, respectively. Compared with age-matched, nonmutant controls, the OLETF rats showed higher avidity for sucrose at both ages. This difference increased as a function of age and tastant concentration. An exaggerated response also occurred for saccharin, alanine, and fructose, but not for Polycose. Similarly, OLETF rats consumed monosodium-glutamate more at the lower concentrations compared with controls, an effect that age also accentuated. In contrast, there was no statistical strain or age differences in responses to NaCl, MgCl2, citric acid, quinine-HCl, and the trigeminal stimulus capsaicin. These findings demonstrate that compared with controls, OLETF rats differ in their gustatory functions with an overall augmented sensitivity for sweet that progresses during prediabetes. This effect explains their overconsumption of sweet solutions and may contribute to the overall hyperphagia and obesity in this strain.

Involvement of endogenous CCK and CCK1 receptors in colonic motor function

Cholecystokinin (CCK) is a brain-gut peptide; it functions both as a neuropeptide and as a gut hormone. Although the pancreas and the gallbladder were long thought to be the principal peripheral targets of CCK, CCK receptors are found throughout the gut. It is likely that CCK has a physiological role not only in the stimulation of pancreatic and biliary secretions but also in the regulation of gastrointestinal motility. The motor effects of CCK include postprandial inhibition of gastric emptying and inhibition of colonic transit. It is now evident that at least two different receptors, CCK(1) and CCK(2) (formerly CCK-A and CCK-B, respectively), mediate the actions of CCK. Both localization and functional studies suggest that the motor effects of CCK are mediated by CCK(1) receptors in humans. Since CCK is involved in sensory and motor responses to distension in the intestinal tract, it may contribute to the symptoms of constipation, bloating and abdominal pain that are often characteristic of functional gastrointestinal disorders in general and irritable bowel syndrome (IBS), in particular. CCK(1) receptor antagonists are therefore currently under development for the treatment of constipation-predominant IBS. Clinical studies suggest that CCK(1) receptor antagonists are effective facilitators of gastric emptying and inhibitors of gallbladder contraction and can accelerate colonic transit time in healthy volunteers and patients with IBS. These drugs are therefore potentially of great value in the treatment of motility disorders such as constipation and constipation-predominant IBS.

Involvement of endogenous CCK and CCK1 receptors in colonic motor function

Cholecystokinin (CCK) is a brain-gut peptide; it functions both as a neuropeptide and as a gut hormone. Although the pancreas and the gallbladder were long thought to be the principal peripheral targets of CCK, CCK receptors are found throughout the gut. It is likely that CCK has a physiological role not only in the stimulation of pancreatic and biliary secretions but also in the regulation of gastrointestinal motility. The motor effects of CCK include postprandial inhibition of gastric emptying and inhibition of colonic transit. It is now evident that at least two different receptors, CCK(1) and CCK(2) (formerly CCK-A and CCK-B, respectively), mediate the actions of CCK. Both localization and functional studies suggest that the motor effects of CCK are mediated by CCK(1) receptors in humans. Since CCK is involved in sensory and motor responses to distension in the intestinal tract, it may contribute to the symptoms of constipation, bloating and abdominal pain that are often characteristic of functional gastrointestinal disorders in general and irritable bowel syndrome (IBS), in particular. CCK(1) receptor antagonists are therefore currently under development for the treatment of constipation-predominant IBS. Clinical studies suggest that CCK(1) receptor antagonists are effective facilitators of gastric emptying and inhibitors of gallbladder contraction and can accelerate colonic transit time in healthy volunteers and patients with IBS. These drugs are therefore potentially of great value in the treatment of motility disorders such as constipation and constipation-predominant IBS.

Increased oral and decreased intestinal sensitivity to sucrose in obese, prediabetic CCK-A receptor-deficient OLETF rats

CCK-A receptor-deficient Otsuka Long-Evans Tokushima fatty (OLETF) rats are hyperphagic and develop obesity and Type 2 diabetes. In this strain, taste preference functions have not been investigated. Therefore, a series of short-access, two-bottle tests were performed in age-matched prediabetic OLETF and nonmutant Long-Evans Tokushima Otsuka (LETO) rats to investigate preference for sucrose (0.03, 0.1, 0.3, or 1.0 M) presented with a choice of water. To discern orosensory from postgastric factors that may contribute to this preference, in a separate experiment, rats were allowed to sham feed sucrose in the absence or presence of duodenal sucrose infusion (0.3, 0.6, or 1.0 M). In the two-bottle real-feeding tests, OLETF rats exhibited a greater preference for 0.3 M sucrose (91.2 +/- 1.7 and 78.5 +/- 3.4% for OLETF and LETO, respectively; P < 0.01) and 1.0 M sucrose (65.3 +/- 1.2 and 57.5 +/- 2.7% for OLETF and LETO, respectively; P < 0.05) than LETO rats. OLETF rats also sham fed less of the lowest (0.03 M; 33.8 +/- 4.8 and 58.3 +/- 7.3 ml for OLETF and LETO, respectively; P < 0.05) and more of the highest (1.0 M; 109.9 +/- 6.5 and 81.0 +/- 3.9 ml for OLETF and LETO, respectively; P < 0.01) concentration of sucrose relative to LETO rats. Finally, intraduodenal sucrose infusions (0.6 and 1.0 M) produced a smaller reduction of 0.3 M sham sucrose intake [14.1 +/- 8.1 vs. 52.5 +/- 3.3 ml and 49.4 +/- 8.0 vs. 82.4 +/- 3.2 ml for 0.6 M (P < 0.01) and 1.0 M (P < 0.05) infusions in OLETF and LETO, respectively]. These findings demonstrate that OLETF rats display an increased preference for sucrose, an effect that is at least partially influenced by the orosensory stimulating effect of sucrose. This enhanced responsiveness to oral stimulation, coupled with the deficit in responding to the postingestive feedback of intestinal sucrose, may contribute additively to the development of hyperphagia and weight gain in OLETF rats.