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

Cholecystokinin Modulates Corticostriatal Transmission and Plasticity in Rodents

Recent findings have shifted the view of cholecystokinin (CCK) from being a cellular neuronal marker to being recognized as a crucial neuropeptide pivotal in synaptic plasticity and memory processes. Despite its now appreciated importance in various brain regions and abundance in the basal ganglia, its role in the striatum, which is vital for motor control, remains unclear. This study sought to fill this gap by performing a comprehensive investigation of the role of CCK in modulating striatal medium spiny neuron (MSN) membrane properties, as well as the secondary somatosensory cortex S2 to MSN synaptic transmission and plasticity in rodents. Using in vivo optopatch-clamp recording in mice on identified MSNs, we showed that the application of CCK receptor Type 2 (CCK2R) antagonists decreases corticostriatal transmission in both direct and indirect pathway MSNs. Moving to an ex vivo rat preparation to maximize experimental access, we showed that CCK2R inhibition impacts MSN membrane properties by reducing spike threshold and rheobase, suggesting an excitability increase. Moreover, CCK modulates corticostriatal transmission mainly via CCK2R, and CCK2R blockage shifted spike-timing-dependent plasticity from long-term potentiation to long-term depression. Our study advances the understanding of CCK's importance in modulating corticostriatal transmission. By showing how CCK2R blockade influences synaptic function and plasticity, we provide new insights into the mechanisms underlying striatal functions, opening new paths for exploring its potential relevance to neurological disorders involving basal ganglia-related behaviors.

Gender-Specific Association between Alcohol Consumption and Stress Perception, Depressed Mood, and Suicidal Ideation: The 2010-2015 KNHANES

OBJECTIVE

Alcohol drinking can cause various psychiatric and medical diseases. Although women generally consume less alcohol than men, they may be at a greater risk for alcohol-related psychological distress. The aim of the current study is to evaluate whether the association between alcohol consumption and psychological distress, including stress, depressed mood, and suicidal ideation and the risks of psychological distress differ based on gender.

METHODS

The cross-sectional study included 31,657 participants (17,915 women and 13,742 men) from the 2010-2013 and 2015 Korea National Health and Nutrition Examination Survey. Alcohol drinking and Alcohol Use Disorders Identification Test (AUDIT) levels were assessed for evaluating the amount of alcohol intake and alcohol-related problems. Self-perception of stress, depressed mood, and suicidal ideation were assessed for evaluating psychological distress. Odds ratio and 95% confidence intervals for psychological distress were calculated using multiple logistic regression models.

RESULTS

The risks of psychological distress were not significantly associated with drinking level in both sexes. However, the risks of psychological distress were associated with an increase in AUDIT levels, and there were gender differences in the psychological consequences of alcohol-related problems. The association was more drastic in women, and women showed a significant association even though the severity of drinking problem was low with the exception of stress perception.

CONCLUSION

The risks of psychological distress were associated with the severity of alcohol-related problems and women were more likely to be susceptible. Therefore, it is recommended that women even at low-risk for problematic drinking should be screened for psychological distress.

Differential effects of cholecystokinin (CCK-8) microinjection into the ventrolateral and dorsolateral periaqueductal gray on anxiety models in Wistar rats

Cholecystokinin (CCK) is one of the main neurohormone peptide systems in the brain, and a major anxiogenic mediator. The periaqueductal gray (PAG) is a key midbrain structure for defensive behaviors, which could include anxiety, fear, or even panic. The CCK system has wide distribution in the PAG, where the dorsolateral region (DL) participates in active defensive behavior and the ventrolateral region (VL) in passive defensive behavior. The aim of this study was to assess the effect of CCK-8 microinjection into DL-PAG or VL-PAG on anxiety-like behavior through two tests: elevated plus maze (EPM) and defensive burying behavior (DBB). CCK-8 (0.5 and 1.0 μg/0.5 μL) presently microinjected into the DL-PAG produced an anxiogenic-like effect on the EPM evidenced by decreasing the time spent/number of entries in open arms compared to vehicle group. Additionally, the latency to burying decreased and burying time increased on the DBB test. Contrarily, CCK-8 microinjected into the VL-PAG resulted in greater open-arm time and more open-arm entries compared to the vehicle-microinjected group. The results on the DBB test confirmed an anxiolytic-like response of CCK-8 into the VL-PAG. In conclusion, CCK-8 microinjected into DL-PAG produced anxiety-like behavior on EPM, and for first time reported on DBB. Contrarily, CCK-8 microinjected into the VL-PAG reduced anxiety-like behavior also for first time reported using both behavioral models EPM and DBB.

Control of Food Intake by Gastrointestinal Peptides: Mechanisms of Action and Possible Modulation in the Treatment of Obesity

This review focuses on the control of appetite by food intake-regulatory peptides secreted from the gastrointestinal tract, namely cholecystokinin, glucagon-like peptide 1, peptide YY, ghrelin, and the recently discovered nesfatin-1 via the gut-brain axis. Additionally, we describe the impact of external factors such as intake of different nutrients or stress on the secretion of gastrointestinal peptides. Finally, we highlight possible conservative—physical activity and pharmacotherapy—treatment strategies for obesity as well as surgical techniques such as deep brain stimulation and bariatric surgery also altering these peptidergic pathways.

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.

Neuronal network of panic disorder: the role of the neuropeptide cholecystokinin

Panic disorder (PD) is characterized by panic attacks, anticipatory anxiety and avoidance behavior. Its pathogenesis is complex and includes both neurobiological and psychological factors. With regard to neurobiological underpinnings, anxiety in humans seems to be mediated through a neuronal network, which involves several distinct brain regions, neuronal circuits and projections as well as neurotransmitters. A large body of evidence suggests that the neuropeptide cholecystokinin (CCK) might be an important modulator of this neuronal network. Key regions of the fear network, such as amygdala, hypothalamus, peraqueductal grey, or cortical regions seem to be connected by CCKergic pathways. CCK interacts with several anxiety-relevant neurotransmitters such as the serotonergic, GABA-ergic and noradrenergic system as well as with endocannabinoids, NPY and NPS. In humans, administration of CCK-4 reliably provokes panic attacks, which can be blocked by antipanic medication. Also, there is some support for a role of the CCK system in the genetic pathomechanism of PD with particularly strong evidence for the CCK gene itself and the CCK-2R (CCKBR) gene. Thus, it is hypothesized that genetic variants in the CCK system might contribute to the biological basis for the postulated CCK dysfunction in the fear network underlying PD. Taken together, a large body of evidence suggests a possible role for the neuropeptide CCK in PD with regard to neuroanatomical circuits, neurotransmitters and genetic factors. This review article proposes an extended hypothetical model for human PD, which integrates preclinical and clinical findings on CCK in addition to existing theories of the pathogenesis of PD.

Co-administration of subeffective anxiolytic doses of diazepam and hydroxyzine in elevated zero-maze in mice

OBJECTIVE

Benzodiazepines are from the most common drugs which are used for treatment of anxiety disorders. There are other drugs with antianxiety properties including antihistamines such as hydroxyzine, too. Body of evidence show that co-administration of two drugs which act through different mechanisms, makes the dose of each drug to be reduced, while preserving the desired effect with less adverse drug reactions. The aim of this study was to see whether co-administration of subeffective antianxiety doses of diazepam and hydroxyzine has any antianxiety effect in elevated zero-maze (EZM) in mice.

METHODS

To find the highest subeffective dose of each drug, different doses of hydroxyzine from 1.5 to 24 mg/kg and diazepam in doses of 0.25, 0.5 and 1 mg/kg were injected to male mice. Thirty minutes later, the animals were placed on EZM and various parameters of anxiety were recorded by a camera to assess later. After determination of subeffective antianxiety dose of the drugs, co-administration of hydroxyzine and diazepam was done and the anxiety parameters were measured.

RESULTS

In co-administration of 0.25 mg/kg of diazepam and 12 mg/kg hydroxyzine, as subeffective antianxiety doses of either drug, there were not any significant differences in main anxiety parameters, i.e., time spent in open areas and open area entries compared to control group. Hence, no anxiolytic effect was seen.

CONCLUSION

It seems that subeffective doses of diazepam and hydroxyzine may not have any facilitating or synergistic effect on each other in antianxiety responses in mice.

Sex difference in alcoholism: who is at a greater risk for development of alcoholic complication?

AIMS

Alcohol abuse and alcoholism are among the major medical problems afflicting both men and women. While men display a higher prevalence for alcoholism, it is women who suffer a much greater risk for alcoholism-associated bodily damage. Although women generally consume less alcohol compared to men, females usually suffer more severe brain and other organ damage following binge or chronic alcohol abuse.

MAIN METHODS AND KEY FINDINGS

Although many biological (i.e., genetic risk and neurological abnormalities) and psychosocial (i.e., impact of positive drinking expectancies, personality characteristics and deviance proneness) factors appear to impact men and women equally. These factors especially social and environmental, physiological, genetic and neurobiological ones have been demonstrated to contribute to the sex difference in response to alcohol intake, as well as the development of alcoholic complications. A number of neurotransmitters and growth factors may be partially involved in these differences between men and women. The mesolimbic dopamine system is implicated in the development of addictive behaviors. Differences in dopamine receptor density are found between sexes where gonadal steroid hormones may play a role. Inhibitory GABAergic and stimulatory glutamatergic systems also act as neuromodulators in the brain and differences in their specific receptors have been identified between men and women (particularly GABA(A) receptors and NMDA receptors).

SIGNIFICANCE

Given the variety of factors contributing to the sex difference in response to alcohol intake, alcoholism treatment should take sex dimorphism into consideration. Furthermore, future research needs to be directed towards a better understanding of the mechanism of action of alcohol in both men and women.

Wiring and volume transmission in rat amygdala. Implications for fear and anxiety

The amygdala plays a key role in anxiety. Information from the environment reaches the amygdaloid basolateral nucleus and after its processing is relayed to the amygdaloid central nucleus where a proper anxiogenic response is implemented. Experimental evidence indicates that in this information transfer a GABAergic interface controls the trafficking of impulses between the two nuclei. Recent work indicates that interneuronal communication can take place by classical synaptic transmission (wiring transmission) and by volume transmission in which the neurotransmitter diffuses and flows through the extracellular space from its site of release and binds to extrasynaptic receptors at various distances from the source. Based on evidence from our laboratory the concept is introduced that neurotransmitters in the amygdala can modulate anxiety involving changes in fear learning and memories by effects on receptor mosaics in the fear circuits through wiring and volume transmission modes of communication.

Neuroendocrine pathways in benzodiazepine dependence: new targets for research and therapy

Benzodiazepines are known to modulate the activity of the hypothalamo-pituitary-adrenocortical (HPA) axis by antagonizing the effects of corticotropin-releasing factor (CRH). Besides regulating the HPA axis CRH evolves properties of a neurotransmitter in the limbic system that is closely involved in the delivery of the emotional consequences of the stress response. At a superordinated level Neuropeptide Y (NPY) and Cholecystokinin (CCK) affect the release of CRH and modulate thereby the intensity of the physiological stress response. Benzodiazepine treatment interferes not only with the release of CRH but also with the release of NPY and CCK. Alterations in the intracortical ratio of NPY, CCK and CRH are correlated with behavioural changes like increased respectively decreased anxiety and subsequent alterations in the activity of the HPA axis. Recent research offers the possibility that the alterations of plasma levels of these neuropeptides are not only a secondary phenomenon due to drug intake, but that low levels of those neuropeptides that modulate anxiety and fear can possibly explain addiction to substances that counterbalance these deficits. Depending on the available results possible implications of NPY and CCK on benzodiazepine addiction and withdrawal symptoms are reviewed, thereby providing topics for further research.

Role of the amygdaloid cholecystokinin (CCK)/gastrin-2 receptors and terminal networks in the modulation of anxiety in the rat. Effects of CCK-4 and CCK-8S on anxiety-like behaviour and [3H]GABA release

The amygdala plays a key role in fear and anxiety. The intercalated islands are clusters of glutamate-responsive GABAergic neurons rich in cholecystokinin (CCK)-2 receptors which control the trafficking of nerve impulses from the cerebral cortex to the central nucleus of amygdala. In this study, the nature of the CCK-glutamate-GABA interactions within the rat rostral amygdala, and their relevance for anxiety, were studied. CCK/gastrin-like immunoreactive nerve terminals were found to be mainly restricted to the paracapsular intercalated islands and the rostrolateral part of the main intercalated island. Behaviourally, the bilateral microinjection of CCK-4 (0.043-4.3 pmol/side) or CCK-8S (4.3 pmol/side) into the rostrolateral amygdala reduced the open-arm exploration in the elevated plus-maze without affecting locomotion. In contrast, neither CCK-4 nor CCK-8S (0.043-4.3 pmol/side) had any effects in the shock-probe burying test as compared with their saline-treated controls. Biochemically, CCK-4 (0.3 and 1.5 microm), unlike CCK-8S, enhanced significantly the K(+)-stimulated release of [(3)H]GABA from amygdala slices. These effects were fully prevented by prior superfusion of the slices with either the selective CCK-2 receptor antagonist CR2945 (3 microm), or 6,7-dinitroquinoxaline-2,3(1H,4H)-dione (DNQX), 10 microm, a glutamatergic (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate receptor antagonist. It is suggested that CCK modulates glutamate-GABA mechanisms by acting on CCK-2 receptors via volume transmission occurring at the level of the basolateral amygdaloid nucleus and/or by synaptic or perisynaptic volume transmission in the region of the rostrolateral main and paracapsular intercalated islands, resulting in subsequent disinhibition of the central amygdaloid nucleus and anxiety or panic-like behaviour.

Rats displaying distinct exploratory activity also have different expression patterns of gamma-aminobutyric acid- and cholecystokinin-related genes in brain regions

The aim of the present study was to evaluate the expression of genes in relation to the exploratory activity of rats. Limbic system-associated membrane protein (LsAMP) gene, gamma-aminobutyric acid-(GABA)- and cholecystokinin-(CCK)-related genes were analyzed in the frontal cortex, amygdala and periaqueductal gray (PAG) after exposure of rats to exploratory challenge. Two groups of animals displaying low and high exploratory activity in the elevated plus-maze were selected for gene expression studies from the population of 43 male Wistar rats. Eight rats were taken randomly from the same cages as animals exposed to the plus-maze, but they were not subjected to the exploratory test. This home-cage control group was also used for gene expression analysis in order to explore a possible impact of the plus-maze exposure. Rats with low and high exploratory activity displayed clearly distinct profiles in gene expression. Most prominent alterations were established in the amygdala where almost all GABA-related and CCK receptor genes were two- to five-fold up-regulated in low exploratory activity rats compared to high exploratory activity and home-cage control group. The expression of several GABA-related genes was also increased in the PAG of animals displaying low exploratory activity compared to the other groups. Moreover, we found reduced expression of GABA- and CCK-related genes in all three brain regions in animals with high exploratory activity compared to the home-cage control group. In addition to these findings, we established a significantly increased expression of the LsAMP gene in the amygdala and PAG of low exploratory activity animals compared with the other groups. In conclusion, low and high exploratory activity rats differed not only by their exploratory activity but also displayed opposite gene expression patterns. Low exploratory activity of rats correlated with the up-regulation of LsAMP and GABA-related genes in the amygdala and PAG and with the up-regulation of CCK receptors in the amygdala. High exploratory activity of rats was related to a significant down-regulation of CCK receptor genes in the amygdala and PAG. These rats also displayed the reduced expression of GABA-related genes in the frontal cortex and PAG.

Gender specific effects of ethanol in mice, lacking CCK2 receptors

Neuropeptide cholecystokinin (CCK) has been reported to suppress ethanol intake, but there is contradictory evidence about the role of CCK(2) receptors. In the present study anxiolytic, hypolocomotor and sedative effects of acute ethanol administration, but also voluntary ethanol consumption were studied in male and female mice, lacking CCK(2) receptors (-/-). Ethanol (1.0 and 2.0 g/kg) induced a significant reduction of anxiety-related behaviours in the elevated plus-maze, but this effect was statistically significant only in female homozygous mice (-/-). In male mice, lacking CCK(2) receptors (-/-), but not in their wild-type littermates (+/+), the suppression of vertical locomotor activity was caused by ethanol at a dose 0.5 g/kg. The highest dose of ethanol (2.0 g/kg) produced statistically significant reduction of horizontal locomotor activity only in female wild-type (+/+) mice, but this effect was related to increased basal activity when compared to female mutant (-/-) mice. Duration of the loss of righting reflex was not significantly affected by genotype or gender, but blood ethanol levels at regain of righting reflex were significantly lower in female homozygous mice (-/-) compared to their wild-type (+/+) littermates, indicating increased sensitivity to the sedative effect of ethanol. Ethanol intake, but not preference, at concentration 10% was significantly increased in female mice, lacking CCK(2) receptors (-/-). The present study revealed an altered response to the acute effects of ethanol in CCK(2) receptor deficient mice (-/-). These changes are gender-specific and could be attributed to the altered activity of dopaminergic system in male mice and increased activity of GABA-ergic system in female mice as established in our previous studies.

Cat odour exposure decreases exploratory activity and alters neuropeptide gene expression in CCK2 receptor deficient mice, but not in their wild-type littermates

An attempt was made to establish whether the anxiogenic effect of cat odour differs in female wild-type and CCK(2) receptor deficient mice, having different exploratory activity in the elevated plus-maze. The exposure of wild-type and homozygous CCK(2) receptor deficient mice to cat odour did not reveal substantial differences between the two genotypes. The number of contacts with the cat odour impregnated cloth was reduced and the frequency of stretch-attend postures was increased similarly in wild-type and homozygous mice. However, the following exposure of mice to the elevated plus-maze established differences as homozygous mice displayed increased exploratory activity in the plus-maze. The cat odour exposure significantly reduced exploratory activity only in homozygous mice. Together with the increased exploratory activity we established in homozygous mice significantly increased expression of the Oprm1 gene in the frontal cortex and mesencephalon. The exposure of mice to cat odour caused only minor changes in the gene expression of wild-type mice, whereas in homozygous animals a significantly increased expression of the Mc3r gene in the frontal cortex and temporal lobe, and the Pomc1 gene in the temporal lobe, mesencephalon and mesolimbic area was established. In conclusion, CCK(2) receptor deficient mice displayed reduced anxiety compared to their wild-type littermates in the plus-maze test. This behavioural effect seems to be related, at least partly, to an increased tone of opioid system in the brain. Moreover, homozygous mice respond to the exposure of cat odour with an increased anxiety. This effect seems to be related to the increased function of the melanocortin system in the brain structures of genetically modified mice.

Cholecystokinin and endogenous opioid peptides: interactive influence on pain, cognition, and emotion

It is well documented that stressful life experiences contribute to the etiology of human mood disorders. Cholecystokinin (CCK) is a neuropeptide found in high concentrations throughout the central nervous system, where it is involved in numerous physiological functions. A role for CCK in the induction and persistence of anxiety and major depression appears to be conspicuous. While increased CCK has been associated with motivational loss, anxiety and panic attacks, an increase in mesocorticolimbic opioid availability has been associated with coping and mood elevation. The close neuroanatomical distribution of CCK with opioid peptides in the limbic system suggests that there may be an opioid-CCK link in the modulation and expression of anxiety or stressor-related behaviors. In effect, while CCK induces relatively protracted behavioral disturbances in both animal and human subjects following stressor applications, opioid receptor activation may change the course of psychopathology. The antagonistic interaction of CCK and opioid peptides is evident in psychological disturbances as well as stress-induced analgesia. There appears to be an intricate balance between the memory-enhancing and anxiety-provoking effects of CCK on one hand, and the amnesic and anxiolytic effects of opioid peptides on the other hand. Potential anxiogenic and mnemonic influences of site-specific mesocorticolimbic CCK and opioid peptide availability, the relative contributions of specific CCK and opioid receptors, as well as the time course underlying neuronal substrates of long-term behavioral disturbances as a result of stressor manipulations, are discussed.

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.

Cholecystokinin-2 (CCK2) receptor-mediated anxiety-like behaviors in rats

Cholecystokinin (CCK) is a neurotransmitter in the brain closely related to anxiety. Of the two CCK receptor subtypes, CCK(2) receptors are most implicated in the control of anxiety-related behavior. CCK(2) receptor activation causes anxiogenic effects while the blockade of this receptor has anxiolytic effects. This review focuses on the molecular mechanisms of CCK(2) receptors underlying anxiety-related behaviors of PVG hooded and Spraque-Dawley (SD) rats in two anxiety models (elevated plus-maze [EPM] and cat exposure test). PVG hooded rats showed prolonged freezing behavior in the cat exposure test while SD rats showed very low levels of freezing. A CCK(2) receptor antagonist (LY225910) attenuated freezing behavior in PVG hooded rats while a CCK(2) receptor agonist (CCK-4) increased freezing behavior in SD rats. In contrast, the two strains behaved similarly on the EPM. CCK-4 caused a pronounced anxiogenic effect in PVG hooded rats but only a slight effect in SD rats. CCK(2) antagonists also showed more pronounced anxiolytic effects in PVG hooded rats than in SD rats. CCK(2) receptor expression was greater in PVG hooded than in SD rats in the cortex and hippocampus. Genetic studies also demonstrated four differences in the DNA sequence of the CCK(2) receptor gene between the two rat strains.

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.