Role of genotype in the adaptation of the brain dopamine system to stress

Unpredictable chronic mild stress differentially impairs social and contextual discrimination learning in two inbred mouse strains

Alterations in the social and cognitive domain are considered important indicators for increased disability in many stress-related disorders. Similar impairments have been observed in rodents chronically exposed to stress, mimicking potential endophenotypes of stress-related psychopathologies such as major depression disorder (MDD), anxiety, conduct disorder, and posttraumatic stress disorder (PTSD). Data from numerous studies suggest that deficient plasticity mechanisms in hippocampus (HC) and prefrontal cortex (PFC) might underlie these social and cognitive deficits. Specifically, stress-induced deficiencies in neural plasticity have been associated with a hypodopaminergic state and reduced neural plasticity persistence. Here we assessed the effects of unpredictable chronic mild stress (UCMS) on exploratory, social and cognitive behavior of females of two inbred mouse strains (C57BL/6J and DBA/2J) that differ in their dopaminergic profile. Exposure to chronic stress resulted in impaired circadian rhythmicity, sociability and social cognition in both inbred strains, but differentially affected activity patterns and contextual discrimination performance. These stress-induced behavioral impairments were accompanied by reduced expression levels of brain derived neurotrophic factor (BDNF) in the prefrontal cortex. The strain-specific cognitive impairment was coexistent with enhanced plasma corticosterone levels and reduced expression of genes related to dopamine signaling in hippocampus. These results underline the importance of assessing different strains with multiple test batteries to elucidate the neural and genetic basis of social and cognitive impairments related to chronic stress.

Reduced Ventral Tegmental Area-Hippocampal Connectivity in Children and Adolescents Exposed to Early Threat

BACKGROUND

Preclinical data suggest that early life stress has detrimental effects on the brain's dopaminergic system, particularly the mesocorticolimbic pathway. Altered dopamine function is thought to contribute to the development of stress-related pathologies; yet, little is known about the impact of early stress on dopamine systems during childhood and adolescence, when stress-related disorders frequently emerge. Here, we evaluate the impact of early threat exposure (violence, abuse) on functional connectivity of putative dopaminergic midbrain regions, the ventral tegmental area (VTA) and substantia nigra (SN), giving rise to mesocorticolimbic and nigrostriatal pathways, respectively.

METHODS

Resting-state functional magnetic resonance imaging scans were completed in 43 trauma-exposed and 43 matched comparison youth (ages 7-17). Functional connectivity of the VTA and SN were compared between groups.

RESULTS

The trauma group demonstrated lower functional connectivity between the VTA and hippocampus. No group differences in SN connectivity were observed. Across all participants, there were age-related decreases in connectivity of both VTA and SN with the hippocampus, suggesting that age-related attenuations in VTA-hippocampal circuitry may be exacerbated in trauma-exposed youth. Higher levels of anxiety symptomology were associated with reduced SN-nucleus accumbens connectivity.

CONCLUSIONS

Prior research suggests that VTA-hippocampal circuitry is critical for the gating of new information into long-term memory. Lower connectivity in this circuitry suggests a novel mechanism that may serve to adaptively prevent the overwriting of a previously stored trauma memory, but at the same time contribute to the broad range of cognitive and emotional difficulties linked to early stress exposure.

Altered consolidation of extinction-like inhibitory learning in genotype-specific dysfunctional coping fostered by chronic stress in mice

Genetic and stress-related factors interact to foster mental disorders, possibly through dysfunctional learning. In a previous study we reported that a temporary experience of reduced food availability increases forced swim (FS)-induced helplessness tested 14days after a first experience in mice of the standard inbred C57BL/6(B6) strain but reduces it in mice of the genetically unrelated DBA/2J (D2) strain. Because persistence of FS-induced helplessness influences adaptive coping with stress challenge and involve learning processes the present study tested whether the behavioral effects of restricted feeding involved altered consolidation of FS-related learning. First, we demonstrated that restricted feeding does not influence behavior expressed on the first FS experience, supporting a specific effect on persistence rather then development of helplessness. Second, we found that FS-induced c-fos expression in the infralimbic cortex (IL) was selectively enhanced in food-restricted (FR) B6 mice and reduced in FR D2 mice, supporting opposite alterations of consolidation processes involving this brain area. Third, we demonstrated that immediate post-FS inactivation of IL prevents 24h retention of acquired helplessness by continuously free-fed mice of both strains, indicating the requirement of a functioning IL for consolidation of FS-related learning in either mouse strain. Finally, in line with the known role of IL in consolidation of extinction memories, we found that restricted feeding selectively facilitated 24h retention of an acquired extinction in B6 mice whereas impairing it in D2 mice. These findings support the conclusion that an experience of reduced food availability strain-specifically affects persistence of newly acquired passive coping strategies by altering consolidation of extinction-like inhibitory learning.

Effect of chronic social defeat stress on behaviors and dopamine receptor in adult mice

Victims of bullying often undergo depression, low self-esteem, high anxiety and post-traumatic stress disorder symptoms. The social defeat model has become widely accepted for studying experimental animal behavior changes associated with bullying; however, differences in the effects in susceptible and unsusceptible individuals have not been well studied. The present study investigated the effects of social defeat stress on behavior and the expression of dopamine receptors D1 and D2 in the brains of adult mice. Adult mice were divided into susceptible and unsusceptible groups after 10days of social defeat stress. Behavioral tests were conducted, and protein levels in the brains were assessed by Western blotting. The results indicate that all mice undergo decreased locomotion and increased anxiety behavior. However, decreased social interaction and impaired memory performance were only observed in susceptible mice. A significantly decreased expression of D1 was observed in the prefrontal cortex and amygdala of susceptible mice only. No significant differences in D2 expression were shown between control and defeated mice in any area studied. These data indicate that depression-like behavior and cognition impairment caused by social defeat stress in susceptible mice may be related to changes in the dopamine receptor D1.

Temperament and arousal systems: A new synthesis of differential psychology and functional neurochemistry

This paper critically reviews the unidimensional construct of General Arousal as utilised by models of temperament in differential psychology for example, to underlie 'Extraversion'. Evidence suggests that specialization within monoamine neurotransmitter systems contrasts with the attribution of a "general arousal" of the Ascending Reticular Activating System. Experimental findings show specialized roles of noradrenaline, dopamine, and serotonin systems in hypothetically mediating three complementary forms of arousal that are similar to three functional blocks described in classical models of behaviour within kinesiology, clinical neuropsychology, psychophysiology and temperament research. In spite of functional diversity of monoamine receptors, we suggest that their functionality can be classified using three universal aspects of actions related to expansion, to selection-integration and to maintenance of chosen behavioural alternatives. Monoamine systems also differentially regulate analytic vs. routine aspects of activities at cortical and striatal neural levels. A convergence between main temperament models in terms of traits related to described functional aspects of behavioural arousal also supports the idea of differentiation between these aspects analysed here in a functional perspective.

Modulation of sensorimotor gating in prepulse inhibition by conditional brain glycine transporter 1 deletion in mice

Inhibition of glycine transporter 1 (GlyT1) augments N-methyl-D-aspartate receptor (NMDAR)-mediated transmission and represents a potential antipsychotic drug target according to the NMDAR hypofunction hypothesis of schizophrenia. Preclinical evaluation of GlyT1 inhibiting drugs using the prepulse inhibition (PPI) test, however, has yielded mixed outcomes. Here, we tested for the first time the impact of two conditional knockouts of GlyT1 on PPI expression. Complete deletion of GlyT1 in the cerebral cortices confers resistance to PPI disruption induced by the NMDAR blocker MK-801 (0.2mg/kg, i.p.) without affecting PPI expression in unchallenged conditions. In contrast, restricting GlyT1 deletion to neurons in forebrain including the striatum significantly attenuated PPI, and the animals remained sensitive to the PPI-disruptive effect of MK-801 at the same dose. These results demonstrate in mice that depending on the regional and/or cell-type specificity, deletion of the GlyT1 gene could yield divergent effects on PPI.

A comparison of the discriminative stimulus properties of the atypical antipsychotic drug clozapine in DBA/2 and C57BL/6 inbred mice

Inbred mouse strain comparisons are an important aspect of pharmacogenetic research, especially in strains known to differ in regard to specific neurotransmitter systems. DBA/2 mice differ from C57BL/6 mice in terms of both functional and anatomical characteristics of dopamine systems. Given the importance of D2 antagonism in the action of antipsychotic drugs and in theories regarding schizophrenia (i.e. the dopamine hypothesis), this study compared the discriminative stimulus properties of the atypical antipsychotic drug clozapine (CLZ) in C57BL/6 and DBA/2 inbred mice. DBA/2 and C57BL/6 mice were trained to discriminate 2.5 mg/kg of CLZ from vehicle in a two-lever drug discrimination procedure and tested with a variety of antipsychotic drugs and selective ligands. Both strains of mice readily acquired the CLZ discrimination. The atypical antipsychotic drugs olanzapine and risperidone fully substituted for CLZ in both DBA/2 and C57BL/6 mice, but ziprasidone fully substituted only in the C57BL/6 mice. The typical antipsychotic drug haloperidol produced partial substitution for CLZ in the DBA/2 mice, and the dopamine agonist amphetamine required a higher dose to reduce response rates significantly in DBA/2 mice as compared with C57BL/6 mice. Antagonism of serotonergic (5-HT2A/2B/2C) receptors with ritanserin and alpha1-adrenergic receptors with prazosin engendered CLZ-appropriate responding only in the C57BL/6 mice. Thus, while serotonergic and alpha-adrenergic antagonism were shown to be important for CLZ's discriminative cue in C57BL/6 mice, none of the selective ligands produced CLZ-appropriate responding in DBA/2 mice. Differences in dopamine-mediated functions between the two strains of mice may explain some of the findings in this study.

Comparative immunohistochemical study of the dopaminergic systems in two inbred mouse strains (C57BL/6J and DBA/2J)

This study investigated possible neurochemical differences in the brain of two inbred mouse strains, C57BL/6J (C57) and DBA/2J (DBA) that in behavioral, memorization and learning tasks under normal and experimental conditions perform differently or often in an opposite manner. The immunohistochemical study, designed to investigate the dopaminergic system, identified many differences within the midbrain A10 area and less marked differences in areas A9 and A8. The number of dopamine transporter (DAT), vesicular monoamine transporter of type 2 (VMT) and tyrosine hydroxylase (TH) immunoreactive cell bodies was significantly higher in the midbrain of DBA mice than in C57 mice (on average +21.5%, P<0.001 in A10: +9.4% in A9, P<0.05: and +5.9% in A8, P<0.1). The distribution patterns of nerve fibres immunoreactive for same antisera also differed significantly in the two strains, especially at prelimbic, infralimbic and anterior cingulate cortical levels. In C57 mice these fibres were scanty whereas in DBA mice they were well represented. In the nucleus accumbens, also the territorial distribution of DAT immunoreactive nerve fibres differed in the two strains. In the midbrain, the galanin immunoreactive axons were more densely distributed in DBA than in C57 mice whereas neurotensin immunoreactive axons were more densely distributed in C57 than in DBA. These distinct immunohistochemical patterns could help to explain why performance differs in the two mouse strains.

Exposure to chronic mild stress alters thresholds for lateral hypothalamic stimulation reward and subsequent responsiveness to amphetamine

Chronic mild stress in rodents has been proposed to model some of the environmental factors that contribute to the induction of depressive disorders in humans. This model is based on the hypothesis that chronic mild stress induces a change in brain reward function that resembles the symptomatology of major depression, namely, a decrease in responsiveness to rewarding stimuli. The purpose of the first experiment was to investigate whether chronic mild stress affects brain reward function as measured by alterations in lateral hypothalamic self-stimulation behavior in rats. Exposure to chronic mild stress induces a reduction in body weight which might affect brain reward function on its own. Therefore, the potential contribution of a reduction in body weight to the chronic mild stress-induced alterations in brain reward function was examined in a separate group of food-restricted rats. Thresholds for lateral hypothalamic self-stimulation were slightly but significantly lowered in animals exposed to chronic mild stress, indicating an enhancement of stimulation reward efficacy. Food restriction had no effect on brain reward function. The second experiment examined the interaction between prior exposure to chronic mild stress or food restriction and responsiveness to a pharmacological challenge, amphetamine, that enhances brain reward function. Acute administration of amphetamine produced a greater enhancement of lateral hypothalamic self-stimulation reward in animals exposed to chronic stress relative to non-stressed and food-restricted animals. Taken together, the present findings indicate that chronic mild stress sensitizes the neural substrates that mediate both lateral hypothalamic stimulation and psychostimulant drug reward. These findings support the hypothesis that prior exposure to stress affects the vulnerability for drug-taking behavior by increasing the positive reinforcing properties of drug of abuse.

Immunoreactive neurons in the brain of two mouse strains after incubation with an antiserum recognizing Asp-Val-Val-Gly.NH2 (DVVG), the C-terminal fragment of (D-Ala2)-deltorphin I

D-Ala(2)-deltorphin I (DADTI) is a heptapeptide amide first extracted from frog skin that displays a high selectivity and affinity for delta opioid receptors. Previous studies using a polyclonal antiserum specific for its C-terminal tetrapeptide-amide (DVVG) have already described in rat and mouse brain the presence of immunoreactive neurons, most of them belonging to the mesencephalic dopaminergic neurons. C57BL/6J (C57) and DBA/2J (DBA) are two inbred strains of mice well known for showing marked genotype-dependent differences for phenotypes related to differential brain dopamine functioning. Brain specimens of both inbred mouse strains were frozen, cut and immunostained using the same antiserum. Some sections were also double immunostained with monoclonal anti-tyrosine hydroxylase (TH). DVVG-immunoreactive neurons were observed among both dopaminergic and non-dopaminergic neurons. DVVG- and TH-immunoreactive neurons were observed among the dopaminergic A8, A9 and A10 mesencephalic nuclei. They were on average 21.9% more numerous in DBA than in C57 mice. DVVG-immunoreactive nerve fibres could be seen in limbic, striatal, cortical and thalamic areas. The distribution patterns of DVVG-IR and TH-IR nerve fibres differed most conspicuously within the infralimbic, prelimbic and cingulate cortices, forming a dense network in DBA but rare in C57 mice. Non-dopaminergic DVVG-immunoreactive neurons did not differ significantly in the two strains. Our finding that the number and distribution pattern of this dopaminergic neuronal subpopulation differed in the two mouse strains could provide morphological support for the known behavioural differences between the DBA and C57 strains under normal and experimental conditions.

Handling, genetic and housing effects on the mouse stress system, dopamine function, and behavior

This research was designed to examine how early stimulation (i.e., handling), subsequent housing conditions and genetic factors interact to produce adult differences in stress regulation. High-aggressive (NC900) and low-aggressive (NC100) mice were handled for 3 weeks potspartum and were subsequently isolated or grouped until observed as adults in an open field or a dyadic test. In NC100, handling abolished the temporal variations seen in open-field activity among the nonhandled subjects and reduced corticosterone (CORT) activation. In NC900, these two measures were unaffected by handling. Only among handled NC100 did subsequent group rearing further reduce CORT activation. By contrast, handling caused an up-regulation of D1 dopamine receptors in both lines, and, in NC100, this effect was increased by group rearing. In a dyadic encounter with another male mouse, subjects of both lines showed handling effects. NC100 froze less rapidly and NC900 attacked more rapidly. This multifactorial design showed that the systemic effects of handling are modulated by genetic background, and that measures of these effects are affected by experience beyond infancy. Our findings also showed that the effects of handling vary when assessed across different physiological systems and across social and nonsocial testing conditions.

A dopamine D(1/5) receptor antagonist, SCH23390, prevents stress-induced sudden death in cardiomyopathic hamsters

Stress is known to have an impact on the development of life-threatening cardiovascular dysfunction. We have previously demonstrated that repeated exposure to cold-immobilization stress had lethal effects on cardiomyopathic Syrian hamsters (BIO 14.6), and that stress-induced sudden death was prevented by daily treatment with propranolol, suggesting an important role of sympathetic nerves in the etiology of stress-induced cardiac sudden death. In an attempt to clarify further the mechanisms of the sudden death, in the present study we investigated the effects of D(1/5) receptor blockade by SCH23390 on the sudden death of cardiomyopathic hamsters. In accordance with our previous results, repeated exposure for 5 days to cold-immobilization stress induced a lethal effect in the cardiomyopathic hamsters but not in control healthy hamsters. SCH23390 (0.1-10 mg/kg, IP), administered just before the exposure for 5 consecutive days, dose-dependently and significantly prevented the lethal effects of the stress. Furthermore, it was demonstrated that the drug significantly reduced the increase in the weights of the adrenal and kidneys observed in the stressed-cardiomyopathic hamsters. On the other hand, specific D(2) antagonist haloperidol (0. 1-10 mg/kg) failed to prevent the stress-induced sudden death and minimally affected the increase in organ weights. Collectively, these results suggest that D(1/5) receptors had an important role in the etiology of stress-induced cardiac sudden death of the cardiomyopathic hamsters, and provide the first experimental evidence of the potential therapeutic values of D(1/5) antagonists against cardiac sudden death associated with stress.

Indication of a genetic basis of stereotypies in laboratory-bred bank voles (Clethrionomys glareolus)

The development of stereotypies was studied in two successive laboratory-bred generations of bank voles representing F1 (n=248) and F2 (n=270) of an originally wild caught stock. It was shown that the propensity to develop stereotypies under barren housing conditions strongly relates to the same propensity of the parents. Stereotypies were approximately seven times more frequent in the offspring of stereotyping parents than in the offspring of permanent non-stereotypers. This held true even when only one of the parents was stereotyper. The paternal and maternal contributions to stereotypies in the offspring appeared to be equal. Males showing stereotypies but prevented from any physical contact with the offspring were as potent as stereotyping females in producing stereotyping offspring. Moreover, the specific type of stereotypy appearing in the offspring after isolation was very much related to the type of stereotypy developed in the mothers. We found no support for the possible importance of social facilitation from littermates, in that the development of stereotypies was independent of the length of time the voles were kept socially with littermates before isolation. We suggest that the possible genetic basis of individual differences in the propensity to develop stereotypies in captivity may result from differences in genetic predispositions and their interactions with discrete frustrating stimuli early in life and/or to genetically different predispositions to cope with frustrating experiences later in life.

Chronic psychosocial stress reduces the density of dopamine transporters

The effect of different types of physical stress on brain dopaminergic function has been well established in rodents; however, the role of the dopaminergic system in more naturalistic stress situations is poorly understood. Therefore, the aim of the current study was to investigate the effect of chronic psychosocial stress on the dopamine transporter, which is an important component in the regulation of dopaminergic neurotransmission. For this purpose, we used the well-characterized paradigm of subordination stress in male tree shrews (Tupaia belangeri). In the present study, the animals were subjected to psychosocial stress for 28 days. Animals were daily videotaped and locomotor activity was quantified. In subordinate animals, urinary cortisol and noradrenaline, as well as adrenal weight, were increased, whereas body weight, locomotor activity and testicular function were decreased. Brain dopamine transporter binding sites were quantified by in vitro autoradiography using [3H] WIN 35,428 as ligand. Chronic stress reduced the number of binding sites (Bmax) in the caudate nucleus and the putamen without affecting the affinity (Kd). Stress did not influence the binding parameters in the nucleus accumbens, the substantia nigra or the ventral tegmental area. Furthermore, we found a positive correlation between locomotor activity and the Bmax values for [3H] WIN 35,428 binding in the caudate nucleus, the putamen and the nucleus accumbens. The present study shows that a naturalistic stressor, such as chronic psychosocial conflict, decreases dopamine transporter binding sites in motor-related brain areas, suggesting that the reduction in locomotor activity in subordinate tree shrews is related to the downregulation of dopamine transporter binding sites.

The role of stress in the pathophysiology of the dopaminergic system

In this review, we will examine the most recent preclinical evidence in support of the fact that both acute and chronic stress may have a detrimental impact on the normal function of the dopaminergic system. In recent decades, the term stress has changed its meaning from that of a 'non-specific body response' to a 'monitoring system of internal and external cues'; that is a modality of reaction of the mammalian central nervous system (CNS) which is critical to the adaptation of the organism to its environment. Compelling results have demonstrated that the dopaminergic system is important not only for hedonic impact or reward learning but also, in a broader sense, for reactivity to perturbation in environmental conditions, for selective information processing, and for general emotional responses, which are essential functions in the ability (or failure) to cope with the external world. In this, stress directly influences several basic behaviors which are mediated by the dopaminergic system such as locomotor activity, sexual activity, appetite, and cross sensitization with drugs of abuse. Studies using rat lines which are genetically different in dopamine (DA) physiology, have shown that even small alterations in the birth procedure or early life stress events may contribute to the pathophysiology of psychiatric disorders-in particular those involving central DA dysfunction-and may cause depression or psychotic derangement in the offspring. Finally, the fact that the dopaminergic system after stress responds, preferentially, in the medial prefrontal cortex (MFC), is thought to serve, in humans, as a protection against positive psychotic symptoms, since the increased DA activity in the MFC suppresses limbic DA transmission. However, excessive MFC dopaminergic activity has a negative impact on the cognitive functions of primates, making them unable to select and process significant environmental stimuli. Thus it appears that a critical range of DA turnover is necessary for optimal cognitive functioning after stress, in the response of the CNS to ever-changing environmental demands. Molecular Psychiatry (2000) 5, 14-21.

Stress promotes major changes in dopamine receptor densities within the mesoaccumbens and nigrostriatal systems

This study investigated the effects of stress on brain dopamine receptor densities in two inbred strains of mice. Analysis of [3H]SCH23390 binding by quantitative autoradiography revealed that repeated restraint stress significantly increases D1-like receptor density in the nucleus accumbens of mice of the DBA/2 strain whist reducing it in the caudate-putamen of C57BL/6 mice. No significant changes in D2-like receptor quantified by [3H](-)-sulpiride binding were observed in caudate, substantia nigra and accumbens of stressed C57BL/6 mice. Instead, in DBA/2 mice, stress significantly increased D2-like receptor density in the nucleus accumbens whilst reducing it in the substantia nigra. Finally, stress significantly increased D2-like receptor density within the ventral tegmental area of C57BL/6 mice whilst significantly reducing it in mice of the DBA/2 strain. These results indicate that stress promotes major changes in mesoaccumbens and nigrostriatal dopamine receptor densities. The direction of these changes depends on receptor subtype, brain area and strain. Moreover, the opposite changes of D2-like receptor densities promoted by stress in the ventral tegmental area of the two inbred strains of mice suggest that mesoaccumbens dopamine autoreceptors density might be controlled by a major genotype x stress interaction.

Extracellular serotonin in the striatum increased after immobilization stress only in the nighttime

The release of serotonin (5-HT) in the striatum and the motor activity of rats given the immobilization stress were measured using in vivo microdialysis techniques and locomotion counts. Serotonin release in the striatum did not change in the daytime and nighttime, but motor activity in the nighttime was significantly higher than in the daytime. Serotonin release was not significantly increased during immobilization stress in the daytime or nighttime. In the nighttime, however, after the end of stress, 5-HT release was significantly increased from 0.002 ng/dialysate to 0.47 ng/dialysate. The motor activities were also significantly increased after the stress in the nighttime. These results suggest that the effects of immobilization stress on serotonin release in the striatum were different in the daytime and nighttime. In the nighttime, serotonin release in the striatum and the motor activities increased not during but after immobilization stress.

Psychopharmacology of dopamine: the contribution of comparative studies in inbred strains of mice

Comparative studies of behavioral responses to centrally acting drugs in inbred strains of mice which show differences in brain neurotransmitter activity represent a major strategy in the investigation of the neurochemical bases underlying behavioural expression. Moreover, these studies represent a preliminary stage in behavioral genetic research since they allow quantitative scales to be established and suggest correlations to be tested in recombinant inbred strains. The present review evaluates results obtained in mice of the C57BL/6 (C57) and DBA/2 (DBA) inbred strains which have been used for studies of the behavioral pharmacology of dopamine (DA) and investigated for the functional and anatomical characteristics of their brain DA systems. Differences between C57 and DBA strain involve susceptibility and sensitivity as well as qualitative differences in the type or direction of the behavioral effects of DA agonists. Moreover, data on strain-dependent differences for DA metabolism, release and receptor densities and distribution provide important indications about the relationship between behavioral and central effects of DA agonists and, more generally, about the involvement of brain DA in behavior. Comparative studies in C57 and DBA mice have also revealed differences in susceptibility to context-dependent, context-independent and stress-induced behavioral sensitization to psychostimulants. Consequently, they support the view that the term "behavioral sensitization" may define different phenomena in which different, independent genotype-related factors play a major role. Finally, studies on the behavioral and central effects of stressful experiences in C57 and DBA mice together with psychopharmacogenetic analyses, indicate that different symptomatological profiles may derive from genotype-dependent adaptation of brain DA receptors to environmental pressure.

Parallel strain-dependent susceptibility to environmentally-induced stereotypies and stress-induced behavioral sensitization in mice

Food restriction (9 days) promoted stereotyped behavior in drug-free mice of the DBA/2 (DBA), but not in those of the C57BL/6 (C57), inbred strain. Indeed, behavior presented by food-restricted mice of the DBA strain within the home cage was characterised by a very high response rate within a single response: cage cover climbing. Moreover, enhanced climbing in food-restricted mice of the DBA strain was also observed in a test designed to detect stereotypic effects of drugs in mice. Stereotypic behavior in DBA mice did not depend on nutritional status because: 1. No stereotypies were observed in DBA mice food-deprived for 15 h; 2. no strain-dependent differences in weight loss were observed; and 3. enhanced cage cover climbing was still evident in DBA mice following 24 h of free feeding. Finally, food-restricted DBA mice showed long-lasting sensitization to the locomotor effects of systemic amphetamine, indicating stress-induced behavioral sensitization in this strain of mice. By contrast, no sign of behavioral sensitization was observed in food-restricted mice of the C57 strain. These results indicate that restricted feeding promotes stereotyped behavior, as well as behavioral sensitization to amphetamine, in mice. Moreover, the observed parallelism between strain-dependent susceptibility to environmentally-induced stereotypies and behavioral sensitization supports the hypothesis that these phenomena share common neuro-biological bases.

CRH-R1 mRNA expression in two strains of inbred mice and its regulation after repeated restraint stress

Using in-situ hybridization histochemistry we investigated the distribution of CRH receptor 1 (CRH-R1) mRNA in the cortex of C57BL/6 (C57) and DBA/2 (DBA) strains and its regulation after repeated restraint stress. We show that DBA mice have a higher concentration of CRH-R1 mRNA than C57 mice. Repeated restraint stress produced an increase in CRH-R1 mRNA expression of C57 mice, whereas it was uneffective in DBA mice.

A comparison of the behavioral effects of minaprine, amphetamine and stress

Different types of clinically effective antidepressants prevent the behavioral effects of experimental stress, and some of these treatments affect mesolimbic dopamine (DA) functioning. Animal studies have demonstrated that repeated psychostimulant administration and repeated or chronic stressful experiences also affect mesolimbic DA functioning. These results could suggest homologies among stress, psychostimulants and antidepressants. The present experiments show that either repeated stress (120 min restraint daily for 10 consecutive days) or subchronic treatment with the antidepressant minaprine (5 mg/kg daily for 10 consecutive days) significantly reduced the inhibitory effect of 120 min of restraint on climbing, a behavioral response dependent on mesolimbic DA functioning. However, the antidepressant did not induce the altered sensitivity of presynaptic DA receptors promoted by repeated stress. Chronic stressful experience (13 days of food restriction) and repeated amphetamine (2.5 mg/kg daily for 10 consecutive days) were as effective as subchronic minaprine in reducing immobility in the Porsolt's swimming test. However, whilst both stress and amphetamine enhanced struggling, minaprine promoted swimming. Finally, chronically stressed mice and mice pretreated with amphetamine showed enhanced sensitivity to amphetamine-induced locomotion, whilst this effect was absent in animals pretreated with the antidepressant. These results indicate that although chronic and repeated stress as well as amphetamine have some antidepressant-like behavioral effects, their mode of action could be different from that of clinically active substances.

Central catecholamine alterations induced by stressor exposure: analyses in recombinant inbred strains of mice

Stressors increase plasma corticosterone concentrations and the turnover of norepinephrine (NE) and dopamine (DA) in some brain regions. However, appreciable interindividual and interstrain differences exist in this respect. The present investigation assessed the NE and DA changes induced by uncontrollable footshock in the BALB/cByJ and C57BL/6ByJ mice and seven recombinant inbred strains. Stressor exposure indeed provoked strain dependent alterations of plasma corticosterone, hypothalamic and mesocortical NE, as well as mesocortcolimbic DA. The profile of stressor effects in the recombinant strains with respect to the NE alterations, the mesocortical DA changes, as well as plasma corticosterone was commensurate with a polygenic mode of inheritance. The data were related to behavioral changes induced by stressors in these strains of mice, as well as to the contribution of the brain amines to stressor-induced behavioral changes.

Escape deficits induced by uncontrollable foot-shock in recombinant inbred strains of mice

Although uncontrollable stressors reliably induce numerous behavioral disturbances, considerable interindividual variability exists in this respect. Inasmuch as genetic factors may be fundamental in determining vulnerability to stressor effects, the present investigation assessed alterations in escape performance following exposure to uncontrollable foot-shock in the BALB/cByJ and C57BL/6ByJ mice and seven recombinant inbred strains. Exposure to uncontrollable foot-shock disrupted shuttle escape performance in a strain-specific manner; however, any differences due to gender were not particularly remarkable. The profile of stressor effects in the recombinant strains (i.e., performance deficits greater, lesser or intermediate to the progenitor strains) suggest that the stressor effects on escape performance may be subserved by two or more genetic determinants. The findings are related to central mechanisms that may potentially account for strain differences.

Effects of postnatal stress on dopamine mesolimbic system responses to aversive experiences in adult life

The effects of postnatal stress on mesolimbic dopamine (DA) functioning in 90-day-old mice were investigated. Postnatal stress consisted of 15 min daily exposure to clean bedding (CB) in the absence of the mother for the first two weeks of life. Controls were daily exposed to home cage bedding (HCB) in the absence of the mother. A single brief (5-10 min) exposure to restraint produced a clear-cut increase in DA metabolites (3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 3-methoxytyramine (3-MT)) in the nucleus accumbens septi (NAS) of adult HCB but not CB mice. Moreover, when tested in an elevated plus maze, CB mice showed more exploration and reduced fearfulness in comparison with HCB mice. Taken together, these results indicate reduced emotional reactivity in adult mice repeatedly stressed during postnatal development. Moreover, HCB mice but not CB mice showed altered behavioral responsiveness to apomorphine following repeated restraint stress (10 daily 120 min) in adult life, although no difference in the behavioral response to either a low or a high dose of apomorphine was observed in adult unstressed mice of the CB and HCB groups. These results indicate that the effects of early experiences on brain DA functioning may not be evident in basal conditions and be revealed only under environmental pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Repeated stressful experiences differently affect the time-dependent responses of the mesolimbic dopamine system to the stressor

The increase in mesolimbic dopamine (DA) release observed during the first 40 min of 120 min restraint in naive rats is not evident in repeatedly stressed animals (daily 60 min restraint, for 5 days). However, repeatedly stressed rats show a significant decrease in DA release from 80 min of restraint onwards which is not observable in naive rats. These results indicate that repeated stressful experiences do not produce habituation but alter the response of mesolimbic DA system to the stressor. Moreover, they point to a possible neuronal mechanism underlying stress-induced depression.

Nonhuman behavioral models in the genetics of disturbed behavior

The development of the association method in which genetic markers match quantitative traits had led to quantitative trait loci (QTL) interval mapping. The association method has been extensively used in animal behavior genetics. Animal research allows more suitable linkage studies and detailed assessment of cellular and subcellular components of the central nervous system that may play a crucial role in the development susceptibility to behavioral disorders. Moreover, experimental designs in the laboratory setting allow genotype x environment interactions to be controlled, thus possibly providing more information on the role of nongenetic factors in gene expression. Experimental results are discussed which indicate that animal studies will provide a sort of test for hypotheses arising in clinical settings, allowing gene-product and product-behavior pathways to be examined at molecular levels when the gene accounts for a very small amount of genetic variance. In such a perspective, new molecular biology approaches and behavior genetics in nonhuman species could provide useful tools in the assessment of the genetic as well as nongenetic factors that lead to psychopathology.

Chronic stress induces strain-dependent sensitization to the behavioral effects of amphetamine in the mouse

Following 10 days of daily restraint stress, sensitization developed to the stimulatory effect of amphetamine on locomotion in DBA/2 but not in C57BL/6 mice tested 24 h after the last stressful experience regardless of their being naive or habituated to the test cages. Saline-injected C57BL/6 mice, however, showed an increase of locomotion 24 h after chronic stress treatment. Chronically stressed mice of the two strains did not exhibit any alteration of dopamine and metabolites (3-4-dihydroxphenylacetic acid, homovanillic acid, and 3-methoxytyramine) levels in the frontal cortex, caudatus putamen, or nucleus accumbens septi, thus ruling out that stress-induced alteration of basal dopamine metabolism affected the behavioral response to amphetamine challenging in DBA/2 mice. Ten daily amphetamine injections (5 mg/kg) did not significantly modify the behavioral response to amphetamine in either strain of mice tested 24 h after the end of the chronic treatment and did not increase locomotion in saline-injected C57BL/6 mice. Finally, chronically stressed hybrids B6D2F1 did not show sensitization to the locomotor effects of amphetamine, suggesting a dominant mode of inheritance in the response to chronic stress of the C57BL/6 strain.

Repeated stressful experiences differently affect limbic dopamine release during and following stress

The effects of repeated restraint stress exposures (daily 60 min, for 6 days) on extracellular dopamine in the nucleus accumbens, during and after the stress experience, have been investigated in rats by in vivo microdialysis. On the first day, restraint increased dopamine release during the first 40 min followed by a return to basal levels (50-60 min later). As soon as restraint ceased and the rats were set free, there was another increase in dopamine release lasting 40 min. On the second and third day, restraint produced only a slight increase in dopamine release, while no significant changes were evident from the fourth to the sixth day. By contrast, from the second to the sixth day the increase in dopamine release observed once rats were freed, was unchanged in comparison to the first day. The present results show that the activation of the mesolimbic dopaminergic system induced by aversive stimuli adapts to repeated experiences differently from that produced by pleasurable events, suggesting that aversive and rewarding experiences involve different neural systems.

Acute stress induces time-dependent responses in dopamine mesolimbic system

Exposure to either restraint or footshock (3-60 min) induced similar biphasic alterations of 3-methoxytyramine (3-MT) concentrations (initial increase followed by decrease below control levels) in the nucleus accumbens septi (NAS) of mice, as revealed by tissue analysis. The only difference between the two stressors was the earlier onset of the decrease phase in the restrained mice. In both stressful conditions acid metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) increased throughout stress, while no significant changes in dopamine (DA) concentrations occurred. These data suggest biphasic alteration of DA release during prolonged stress exposure. The analysis of release in restrained conscious rats by in vivo microdialysis (10-240 min) showed a similar biphasic DA evolution (initial increase followed by decrease below baseline levels) in the NAS. The only difference from the previous experiment was the delayed onset of the decrease phase. Similar changes in DOPAC and HVA were also evident. Moreover, freed rats showed an immediate increase of DA release over baseline levels, also indicating that depletion of the neurotransmitter cannot account for the reduction of released DA. Taken together, these results support the hypothesis that biphasic alteration of DA transmission in the mesolimbic system is a general response to stress and suggest that the initial increase of DA release represents an arousal response while the subsequent decrease in DA release may be related to coping failure.

Repeated stressful experiences differently affect brain dopamine receptor subtypes

The binding of tritiated spiperone (D2 antagonist) and tritiated SCH 23390 (D1 antagonist), in vivo, was investigated in the caudatus putamen (CP) and nucleus accumbens septi (NAS) of mice submitted to ten daily restraint stress sessions. Mice sacrificed 24 hr after the last stressful experience presented a 64% decrease of D2 receptor density (Bmax) but no changes in D1 receptor density in the NAS. In the CP a much smaller (11%) reduction of D2 receptor density was accompanied by a 10% increase of D1 receptors. These results show that the two types of dopamine (DA) receptors adapt in different or even opposite ways to environmental pressure, leading to imbalance between them.