Cholecystokinin2 receptor-deficient mice display altered function of brain dopaminergic system

Novel Molecular Targets of Antidepressants

Antidepressants target a variety of proteins in the central nervous system (CNS), the most important belonging to the family of G-protein coupled receptors and the family of neurotransmitter transporters. The increasing number of crystallographic structures of these proteins have significantly contributed to the knowledge of their mechanism of action, as well as to the design of new drugs. Several computational approaches such as molecular docking, molecular dynamics, and virtual screening are useful for elucidating the mechanism of drug action and are important for drug design. This review is a survey of molecular targets for antidepressants in the CNS and computer based strategies to discover novel compounds with antidepressant activity.

The role of cholecystokinin in the induction of aggressive behavior: a focus on the available experimental data (review)

Cholecystokinin (CCK) is a neuropeptide that is (among others) reportedly involved in the pathophysiology of psychiatric disorders. The excitatory role of CCK in negative affective emotions as well as in aversive reactions, antisocial behaviors and memories, has been indicated by numerous electrophysiological, neurochemical and behavioral methodologies on both animal models for anxiety and human studies. The current review article summarizes the existing experimental evidence with regards to the role of CCK in the induction of aggressive behavior, and: (a) synopsizes the anatomical circuits through which it could potentially mediate all types of aggressive behavior, as well as (b) highlights the potential use of these experimental evidence in the current research quest for the clinical treatment of mood and anxiety disorders.

Effects of exogenous cholecystokinin octapeptide on acquisition of naloxone precipitated withdrawal induced conditioned place aversion in rats

Cholecystokinin octapeptide (CCK-8), a gut-brain peptide, regulates a variety of physiological behavioral processes. Previously, we reported that exogenous CCK-8 attenuated morphine-induced conditioned place preference, but the possible effects of CCK-8 on aversively motivated drug seeking remained unclear. To investigate the effects of endogenous and exogenous CCK on negative components of morphine withdrawal, we evaluated the effects of CCK receptor antagonists and CCK-8 on the naloxone-precipitated withdrawal-induced conditioned place aversion (CPA). The results showed that CCK2 receptor antagonist (LY-288,513, 10 µg, i.c.v.), but not CCK1 receptor antagonist (L-364,718, 10 µg, i.c.v.), inhibited the acquisition of CPA when given prior to naloxone (0.3 mg/kg) administration in morphine-dependent rats. Similarly, CCK-8 (0.1–1 µg, i.c.v.) significantly attenuated naloxone-precipitated withdrawal-induced CPA, and this inhibitory function was blocked by co-injection with L-364,718. Microinjection of L-364,718, LY-288,513 or CCK-8 to saline pretreated rats produced neither a conditioned preference nor aversion, and the induction of CPA by CCK-8 itself after morphine pretreatments was not significant. Our study identifies a different role of CCK1 and CCK2 receptors in negative affective components of morphine abstinence and an inhibitory effect of exogenous CCK-8 on naloxone-precipitated withdrawal-induced CPA via CCK1 receptor.

Neuropeptidergic mediators of spontaneous physical activity and non-exercise activity thermogenesis

Lean individuals have high levels of spontaneous physical activity (SPA) and the energy expenditure derived from that activity, termed non-exercise activity thermogenesis or NEAT, appears to protect them from obesity. Conversely, obesity in different human populations is characterized by low levels of SPA and NEAT. Like in humans, elevated SPA in rats appears to protect against obesity: obesity-resistant rats have significantly greater SPA and NEAT than obesity-prone rats. We review the literature on brain mechanisms important in mediating SPA and NEAT. The focus is on neuropeptides, including cholecystokinin, corticotropin-releasing hormone (also known as corticotropin-releasing factor), neuromedin U, neuropeptide Y, leptin, agouti-related protein, orexin-A (also known as hypocretin-1), and ghrelin. We also review information regarding interactions between these neuropeptides and dopamine, a neurotransmitter important in mediating motor function. Finally, we present evidence that elevated signaling of pathways mediating SPA and NEAT may protect against weight gain and obesity.

Behavioral genetic contributions to the study of addiction-related amphetamine effects

Amphetamines, including methamphetamine, pose a significant cost to society due to significant numbers of amphetamine-abusing individuals who suffer major health-related consequences. In addition, methamphetamine use is associated with heightened rates of violent and property-related crimes. The current paper reviews the existing literature addressing genetic differences in mice that impact behavioral responses thought to be relevant to the abuse of amphetamine and amphetamine-like drugs. Summarized are studies that used inbred strains, selected lines, single-gene knockouts and transgenics, and quantitative trait locus (QTL) mapping populations. Acute sensitivity, neuroadaptive responses, rewarding and conditioned effects are among those reviewed. Some gene mapping work has been accomplished, and although no amphetamine-related complex trait genes have been definitively identified, translational work leading from results in the mouse to studies performed in humans is beginning to emerge. The majority of genetic investigations have utilized single-gene knockout mice and have concentrated on dopamine- and glutamate-related genes. Genes that code for cell support and signaling molecules are also well-represented. There is a large behavioral genetic literature on responsiveness to amphetamines, but a considerably smaller literature focused on genes that influence the development and acceleration of amphetamine use, withdrawal, relapse, and behavioral toxicity. Also missing are genetic investigations into the effects of amphetamines on social behaviors. This information might help to identify at-risk individuals and in the future to develop treatments that take advantage of individualized genetic information.

Differences in behavioural effects of amphetamine and dopamine-related gene expression in wild-type and homozygous CCK2 receptor deficient mice

Neuropeptide cholecystokinin (CCK) interacts with dopamine in the regulation of motor activity and motivations. Therefore, in CCK(2) receptor deficient mice the behavioural effects of repeated amphetamine administration and changes in dopamine-related gene expression were studied. Four-day amphetamine (1 mg/kg) treatment induced a significantly stronger motor sensitization in homozygous mice compared to their wild-type littermates. However, in the conditioned place preference test the action of amphetamine was more pronounced in wild-type animals. As opposed to wild-type mice, amphetamine (1-3 mg/kg) did not cause a significant conditioned place preference in homozygous mice. The expression of Tyhy gene was elevated in the mesolimbic structures and Drd2 gene was down-regulated in the mesencephalon of saline-treated homozygous mice in comparison with respective wild-type group. Four-day treatment with amphetamine induced a significant increase in the expression of Tyhy in the mesencephalon, striatum and mesolimbic structures of wild-type mice, whereas in homozygous mice a similar change was evident only in the mesencephalon. Also, the expression of Drd1 gene in the striatum and Drd2 gene in the mesolimbic structures of wild-type mice were up-regulated under the influence of amphetamine. In conclusion, the present study established differences in the behavioural effects of amphetamine in wild-type and homozygous mice. The increased tone of dopaminergic projections from the mesencephalon to mesolimbic structures is probably related to increased amphetamine-induced motor sensitization in homozygous mice. The lack of development of up-regulation of Drd1 and Drd2 genes after repeated treatment with amphetamine probably explains the reduced place conditioning in CCK(2) receptor deficient mice.

Is the CCK2 receptor essential for normal regulation of body weight and adiposity?

Cholecystokinin (CCK) is a gastrointestinal satiety signal released from the duodenum to terminate feeding, via CCK1 receptors. CCK2 receptors are considered to be involved in anxiety. CCK2 receptor knockout mice have increased body weight and food intake. Little is known regarding the effects of CCK2 receptor deficiency on adipose distribution and hypothalamic feeding regulators such as neuropeptide Y (NPY), a powerful stimulator of feeding. Adult (10 week) CCK2 receptor knockout and wild-type mice were anaesthetized and killed by decapitation. Brain sections, organs and fat tissue were dissected. Plasma leptin, insulin and brain NPY content were measured by radioimmunoassay. Female CCK2 receptor knockout mice weighed more than control mice (22.0 +/- 0.2 vs. 19.9 +/- 0.4 g, P < 0.05), with this difference being less marked in male mice (26.4 +/- 0.4 vs. 25.6 +/- 0.6 g). Fat masses in all locations sampled were significantly smaller in CCK2 receptor knockout mice of both genders (P < 0.05), resulting in lower plasma leptin and insulin levels. NPY concentrations were significantly increased in arcuate nucleus and anterior hypothalamus in both male and female CCK2 receptor knockout mice, and total hypothalamic NPY content was increased by 7 and 9% in males and females, respectively (P < 0.05). CCK2 receptor deletion was associated with increased body weight and hypothalamic NPY content, but reduced fat masses and plasma leptin and insulin. Increased NPY might contribute to increased food intake in CCK2 receptor knockout mice. Further work needs to focus on the metabolic changes.

Very low levels of cholecystokinin octapeptide activate Na‐pump in the cerebral cortex of CCK2receptor‐deficient mice

This study provides the first evidence that CCK-8 (0.01 pM to 0.1 mM) stimulates Na,K-ATPase in the cortical membranes of wild-type and CCK(2) receptor-deficient mice. In each genotype, the maximal stimulation was about 40%. Homozygous mice revealed substantially lower EC50 (4 pM) than heterozygous (37 pM) or wild-type animals (682 pM). In homozygous CCK2 receptor-deficient mice, the expression of CCK1 receptor gene was 5-fold higher than in wild-type animals. CCK1 receptor antagonist devazepide counteracted effect of CCK-8 in all three genotypes, whereas CCK2 receptor antagonist L-365, 260 showed significant antagonism in wild-type and heterozygous mice. The cooperativity of Na,K-ATPase for Na+, but not for K+, was lost in homozygous mice. Altogether, very low concentrations of CCK-8 via CCK1 and CCK2 receptors stimulate Na,K-ATPase in the cerebral cortex. CCK2 receptor-deficiency leads to the altered functionality of Na,K-ATPase that might be compensated by CCK1 receptor mediated influence of CCK (and its agonists) on the enzyme.

Targeted invalidation of CCK2 receptor gene induces anxiolytic-like action in light-dark exploration, but not in fear conditioning test

RATIONALE

Evidence suggests that gamma-aminobutyric acid (GABA) and cholecystokinin (CCK) have opposite roles in the regulation of anxiety.

OBJECTIVES

The aim of our work was to study the behaviour of CCK(2) receptor deficient mice in light-dark exploration and fear conditioning tests. Moreover, the action of diazepam and methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), having the opposite effect on GABA(A) receptors, was evaluated on the exploratory behaviour in these mice. Expression levels of GABA(A) receptor subunit genes were also measured.

METHODS

Light-dark exploration and fear conditioning tests were used to determine changes in anxiety of mice. The action of diazepam (0.5-2 mg/kg i.p.) and DMCM (0.25-1 mg/kg i.p.) was studied in the light-dark box. The effect of DMCM was also evaluated in the motor activity test to demonstrate that its anti-exploratory action was not related to motor suppression. Expression levels of GABA(A) receptor subunit genes were determined by means of real-time polymerase chain reaction (qRT-PCR).

RESULTS

Female mice lacking CCK(2) receptors displayed increased exploratory activity in the light-dark box compared to their wild-type (+/+) littermates. Locomotor activity in the motility boxes and the intensity of freezing did not differ in wild-type (+/+) and homozygous (-/-) mice. Treatment with diazepam (0.5 mg/kg) increased the number of transitions in wild-type (+/+) animals, whereas in homozygous (-/-) mice diazepam (0.5-2 mg/kg) reduced exploratory activity. Administration of DMCM (0.25-1 mg/kg) induced an anxiogenic-like effect in homozygous (-/-) mice, but did not change their locomotor activity. Gene expression analysis established a 1.6-fold increase in the expression of the alpha2 subunit of GABA(A) receptors in the frontal cortex of homozygous (-/-) mice.

CONCLUSION

Genetic invalidation of CCK(2) receptors induced an anxiolytic-like action in exploratory, but not in conditioned models of anxiety. The observed reduction in anxiety in homozygous (-/-) mice is probably related to an increased function of GABAergic system in the brain.

Targeted mutation of CCK(2) receptor gene antagonises behavioural changes induced by social isolation in female, but not in male mice

Neuropeptide cholecystokinin (CCK) regulates the adaptation of rodents in the novel environment. In the present study we analysed the behavioural changes induced by the individual housing in mice, lacking CCK(2) receptors. The wild-type (+/+) and homozygous (-/-) CCK(2) receptor deficient mice of both gender were used throughout the study. The weight gain during the 21-day isolation period and changes in the locomotor activity following the social separation were measured. The elevated plus-maze and resident/intruder tests were also performed to test alterations in the emotional behaviour. Social isolation induced locomotor hyperactivity, reduced weight gain and increased aggressiveness in the wild-type (+/+) and homozygous (-/-) male mice. In the wild-type (+/+) female mice the significant reduction of exploratory activity in the plus-maze was evident. By contrast, in female mice, lacking CCK(2) receptors, the exploration of the plus-maze was not significantly affected by the individual housing. This finding demonstrates that the social isolation does not cause anxiety-like state in the CCK(2) receptor deficient mice. Moreover, the targeted invalidation of CCK(2) receptors increased in male mice the affinity of dopamine D(2) receptors in the sub-cortical structures, whereas in female mice the increased affinity of 5-hydroxytryptamine(2) (5-HT(2)) receptors in the frontal cortex was established. The increased affinity of 5-HT(2) receptors is probably the compensatory change to the lack of CCK(2) receptors in female mice and probably reflects the reduced sensitivity of these animals to the anxiogenic manipulations. In conclusion, targeted mutation of CCK(2) receptors selectively antagonised the behavioural changes induced by the individual housing in females, but not in male mice.

Effect of CCK1 and CCK2 receptor blockade on amphetamine-stimulated exploratory behavior and sensitization to amphetamine

Interactions between dopaminergic neurotransmission and cholecystokinin (CCK) in the CNS may be important in the pathogenesis of psychotic disorders and substance abuse. In this study, the effect of coadministration of the selective CCK receptor antagonists devazepide and L-365,260 (for selectively blocking CCK1 and CCK2 receptors, respectively), on the effect of amphetamine on the rat exploratory behavior, and on sensitization of locomotor response to amphetamine, were studied. Amphetamine (0.5 mg/kg) increased exploratory activity in the exploration box for 5 consecutive testing days, while devazepide (10 microg/kg) blocked and L-365,260 (10 microg/kg) enhanced amphetamine-induced stimulation of activity. Devazepide coadministration prevented the development of sensitization to amphetamine, while coadministration of L-365,260 with amphetamine potentiated the locomotor effect of a challenge dose of amphetamine. These results suggest that endogenous CCK, released during exploratory activity, shapes behavioral responses to amphetamine by acting on both receptor subtypes, and modulates the development of sensitization to amphetamine.

Distinct changes in the behavioural effects of morphine and naloxone in CCK2 receptor-deficient mice

The effects of morphine, mu-opioid receptor agonist, and naloxone, a non-selective opioid receptor antagonist, in the locomotor activity and place conditioning tests were studied in the CCK(2) receptor-deficient male mice. The exposure of mice to the motility boxes for 3 consecutive days induced a significant inhibition of locomotor activity in the wild-type (+/+) mice compared to homozygous (-/-) animals. The administration of naloxone (10 mg/kg i.p.) to animals, adapted to the motility boxes, induced a significant reduction of locomotor activity in the homozygous (-/-), but not in the wild-type (+/+) mice. Treatment of habituated mice with morphine (10 mg/kg i.p.) caused a stronger increase of locomotor activity in the wild-type (+/+) mice compared to the homozygous (-/-) littermates. In the place preference test the pairing of the preferred side with naloxone (1 and 10 mg/kg i.p.) induced a dose-dependent place aversion in the wild-type (+/+) mice. The treatment with naloxone was less effective in the homozygous (-/-) mice, because the high dose of naloxone (10 mg/kg) tended to shift the preference. The pairing of morphine (3 mg/kg i.p.) injections with the non-preferred side induced a significant place preference both in the wild-type (+/+) and homozygous (-/-) mice. The increased density of opioid receptors was established in the striatum of homozygous (-/-) mice, but not in the other forebrain structures. In conclusion, the targeted invalidation of CCK(2) receptors induces a dissociation of behavioural effects of morphine and naloxone. Morphine-induced place preference remained unchanged, whereas hyper-locomotion was less pronounced in the mutant mice compared to the wild-type (+/+) littermates. By contrast, naloxone-induced place aversion was weaker, but naloxone caused a stronger inhibition of locomotor activity in the homozygous (-/-) mice than in the wild-type (+/+) animals. These behavioural alterations can be explained in the light of data that the targeted mutation of CCK(2) receptors induces distinct changes in the properties of opioid receptors in various brain structures.

Targeted mutation of CCK2 receptor gene modifies the behavioural effects of diazepam in female mice

RATIONALE

Evidence suggests that GABA and CCK have opposite roles in the regulation of anxiety.

OBJECTIVE

The aim of the present work was to study diazepam-induced anxiolytic-like action and impairment of motor co-ordination, and the parameters of benzodiazepine receptors in mice lacking CCK2 receptors.

METHODS

The action of diazepam (0.5-3 mg/kg i.p.) was studied in the elevated plus-maze model of anxiety and rotarod test using mice lacking CCK2 receptors. The parameters of benzodiazepine receptors were analysed using [3H]-flunitrazepam binding.

RESULTS

In the plus-maze test, the exploratory activity of the homozygous (-/-) mice was significantly higher compared to their wild-type (+/+) littermates. However, the wild-type (+/+) mice displayed higher sensitivity to the anxiolytic-like action of diazepam. Even the lowest dose of diazepam (0.5 mg/kg) induced a significant increase of open arm entries in the wild-type (+/+) mice. A similar effect in the homozygous (-/-) mice was established after the administration of diazepam 1 mg/kg. The highest dose of diazepam (3 mg/kg) caused a prominent anxiolytic-like effect in the wild-type (+/+) mice, whereas in the homozygous (-/-) animals suppression of locomotor activity was evident. The performance of the homozygous (-/-) mice in the rotarod test did not differ from that of the wild-type (+/+) littermates. However, a difference between the wild-type (+/+) and homozygous (-/-) animals became evident after treatment with diazepam. Diazepam (0.5 and 3 mg/kg) induced significantly stronger impairment of motor co-ordination in the homozygous (-/-) mice compared to their wild-type (+/+) littermates. The density of benzodiazepine binding sites was increased in the cerebellum, but not in the cerebral cortex and hippocampus, of the homozygous (-/-) mice.

CONCLUSIONS

Female mice lacking CCK2 receptors are less anxious than their wild-type (+/+) littermates. The reduced anxiety in homozygous (-/-) mice probably explains why the administration of a higher dose of diazepam is necessary to induce an anxiolytic-like action in these animals. The highest dose of diazepam (3 mg/kg) induced significantly stronger suppression of locomotor activity and impairment of motor co-ordination in the homozygous (-/-) mice compared to the wild-type (+/+) littermates. The increase in the action of diazepam is probably related to the elevated density of benzodiazepine receptors in the cerebellum of homozygous (-/-) mice. The present study seems to be in favour of increased tone of the GABAergic system in mice without CCK2 receptors.

What we know and what we need to know about the role of endogenous CCK in psychostimulant sensitization

The unique distribution of CCK and its receptors and its co-localization with dopamine makes it ideally situated to pay a role in dopamine-mediated reward and psychostimulant sensitization. A number of studies support the hypothesis that CCK acting through the CCK 1 and CCK 2 receptors is an endogenous modulator of dopamine neurotransmission. Behavioral studies with CCK antagonists and CCK 1 receptor mutant rats support a role for endogenous CCK in behavioral sensitization to psychostimulants. CCK microdialysis studies in the nucleus accumbens (NAC) have demonstrated that extracellular CCK is increased in the NAC by psychostimulants, providing neurochemical evidence that CCK could be involved in the behavioral response to psychostimulants. A model for how CCK may be acting in multiple brain regions to foster sensitization is presented and the gaps in our knowledge about the role of CCK in psychostimulant sensitization are described.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Cholecystokinin, cholecystokinin-A receptor and cholecystokinin-B receptor gene polymorphisms in Parkinson's disease

Cholecystokinin modulates the release of dopamine and dopamine-related behaviours in the mesolimbic pathway, where cholecystokinin and dopamine coexist in dopaminergic neurones. Because cholecystokinin and its receptors (A and B) have a functional interaction with dopaminergic neurotransmission, alterations in them may constitute a predisposition for Parkinson's disease. We performed a case-control study to investigate the association between the cholecystokinin system and Parkinson's disease using genetic markers for three genes: cholecystokinin and its two receptors (A and B). One hundred and sixty patients with Parkinson's disease and 160 controls, matched for age, gender, ethnic origin and area of residence, were recruited. Cholecystokinin -45C>T, cholecystokinin-A receptor 779T>C and cholecystokinin-B receptor 1550G>A gene polymorphisms were studied using polymerase chain reaction-restriction fragment length polymorphism analyses. These three gene polymorphisms showed no correlation with risk of Parkinson's disease; however, the cholecystokinin CT/TT genotype was associated with a 4.429-fold increased risk for visual hallucinations in Parkinson's disease. Cholecystokinin-A receptor and B receptor polymorphisms, considered alone, showed no correlation with hallucinations in Parkinson's disease; however, a combined effect was found in patients with hallucinations harboring both the cholecystokinin CT/TT and cholecystokinin-A receptor TC/CC genotypes. Parkinson's disease patients harboring this genotype have a 5.922-fold increased risk for developing visual hallucinations. These results suggest that, in Chinese, visual hallucinations in Parkinson's disease are associated with cholecystokinin -45C>T polymorphism, and this association was still observed in the presence of the cholecystokinin-A receptor TC/CC genotype, indicating a possible interaction of these two genes in the visual hallucinogenesis in Parkinson's disease.

Influence of task parameters on rotarod performance and sensitivity to ethanol in mice

Motor performance in mice can be assessed with multiple apparatus and protocols. Use of the rotarod (a.k.a. rotorod, rota-rod, roto-rod, or accelerod) is very common, and it is often used with the apparent assumption by the experimenters that it is a straightforward and simple assay of coordination. The rotarod is sensitive to drugs that affect motor coordination, including ethanol. However, there are few systematic data assessing the range of "normal" performance in mice. There are also few data exploring optimal task parameters (e.g. the influence of different speeds of rotation). In these experiments, we show that both accelerating and fixed-speed rotarod (FSRR) performance vary under different test protocols and conditions, and that moderate to high doses of ethanol disrupt performance. Under certain conditions, low doses of ethanol were found to enhance performance on the accelerating rotarod (ARR). Therefore, it is not possible to characterize individual differences fully using a single set of test parameters. For example, because of the biphasic effect of ethanol on performance, at least two doses of the drug are necessary to explore individual sensitivity differences. We offer recommendations of parameters we believe to be generally suitable for exploring the performance of new genotypes using the rotarod. We suggest that other putative tests of "ataxia" are similarly complex, and that characterizing the contribution of genetic differences will require similar attention to the details of task apparatus and protocols. These data also underscore the need to employ multiple behavioral assays in order to model a complex domain such as "ataxia" or "coordination."

Assessment of genetic susceptibility to ethanol intoxication in mice

Increased use of gene manipulation in mice (e.g., targeted or random mutagenesis) has been accompanied by increased reliance on a very few rapid and simple behavioral assays, each of which aspires to model a human behavioral domain. Yet, each assay comprises multiple traits, influenced by multiple genetic factors. Motor incoordination (ataxia), a common characteristic of many neurological disorders, may reflect disordered balance, muscle strength, proprioception, and/or patterned gait. Impaired motor performance can confound interpretation of behavioral assays of learning and memory, exploration, motivation, and sensory competence. The rotarod is one of the most commonly used tests to measure coordination in mice. We show here that exactly how the rotarod test is performed can markedly alter the apparent patterns of genetic influence both in undrugged performance and sensitivity to ethanol intoxication. However, when tested with well chosen parameters, the accelerating rotarod test showed very high inter- and intralaboratory reliability. Depending on test conditions, ethanol can either disrupt or enhance performance in some strains. Genetic contribution to performance on the accelerating versus the fixed-speed rotarod assay can be completely dissociated under some test conditions, and multiple test parameters are needed to assess the range of genetic influence adequately.

Altered pain sensitivity and morphine-induced anti-nociception in mice lacking CCK2 receptors

RATIONALE

Cholecystokinin (CCK) interacts with the endopioid system in the regulation of various physiological functions, including the control of pain sensitivity, motor activity and emotional behaviour.

OBJECTIVE

The aim of the present work was to study the pain sensitivity, morphine-induced antinociception and density of opioid receptors in mice lacking CCK(2) receptors.

METHODS

Plantar analgesia and hotplate tests were used to evaluate pain sensitivity and morphine-induced antinociception. The parameters of opioid receptors were analysed by using [(3)H]-diprenorphine binding.

RESULTS

In the plantar analgesia test the latency of hind paw withdrawal was significantly increased in CCK(2) receptor deficient mice compared to wild-type (+/+) littermates. The treatment with saline reversed the reduced pain sensitivity in heterozygous (+/-) and homozygous (-/-) mice. The administration of morphine (1 mg/kg) induced a significantly stronger antinociceptive effect in homozygous (-/-) mice compared with wild-type (+/+) animals. In the hotplate test, only homozygous (-/-) mutant mice displayed the delayed latency of hind paw licking/shaking in comparison with wild-type (+/+) mice. The injection of saline and isolation of mice for 30 min reversed the delayed response in homozygous (-/-) mice. However, in this test, the anti-nociceptive action of morphine (5-10 mg/kg) in mutant mice did not differ from that in wild-type (+/+) littermates. By contrast, the jump latency was decreased in both homozygous (-/-) and heterozygous (+/-) mice in the hotplate test. The increased density of opioid receptors was established in the striatum of homozygous (-/-) mice.

CONCLUSION

It is apparent that the targeted mutagenesis of the CCK(2) receptor gene has different effects on the sensitivity of opioid receptors in various brain structures. This is a probable reason for the altered pain sensitivity and morphine-induced antinociception in mutant mice compared to wild-type (+/+) littermates.

Experimental genetic approaches to addiction

Drugs of abuse are able to elicit compulsive drug-seeking behaviors upon repeated administration, which ultimately leads to the phenomenon of addiction. Evidence indicates that the susceptibility to develop addiction is influenced by sources of reinforcement, variable neuroadaptive mechanisms, and neurochemical changes that together lead to altered homeostasis of the brain reward system. Addiction is hypothesized to be a cycle of progressive dysregulation of the brain reward system that results in the compulsive use and loss of control over drug taking and the initiation of behaviors associated with drug seeking. The view that addiction represents a pathological state of reward provides an approach to identifying the factors that contribute to vulnerability, addiction, and relapse in genetic animal models.

Phenotypes of mice with invalidation of cholecystokinin (CCK(1) or CCK(2)) receptors

Cholecystokinin (CCK) is a peptide originally discovered in the gastrointestinal tract, but also found in high density in the mammalian brain. This peptide has been shown to be involved in numerous physiological functions such as feeding behavior, central respiratory control and cardiovascular tonus, vigilance states, memory processes, nociception, emotional and motivational responses. CCK interacts with nanomolar affinites with two different receptors designated CCK(1) and CCK(2). Primarily, the functional role of these binding sites in the brain and the periphery has been investigated thanks to the development of potent and selective CCK receptor antagonists and agonists. However, several studies have yielded conflicting data. Knockout mice provide unique opportunities to analyse diverse aspects of gene function in vivo. This review highlights recent progress in our understanding of the role of CCK(1) and CCK(2) receptors obtained by using mice with genetic invalidation of CCK(1) or CCK(2) receptors or natural CCK receptors mutants. The limits of this approach is discussed and some results were compared to those obtained by pharmacological blockade of CCK receptors by selective antagonists.