A genetic analysis of stereotypy in the mouse: dopaminergic plasticity following chronic stress

When chocolate seeking becomes compulsion: gene-environment interplay

BACKGROUND

Eating disorders appear to be caused by a complex interaction between environmental and genetic factors, and compulsive eating in response to adverse circumstances characterizes many eating disorders.

MATERIALS AND METHODS

We compared compulsion-like eating in the form of conditioned suppression of palatable food-seeking in adverse situations in stressed C57BL/6J and DBA/2J mice, two well-characterized inbred strains, to determine the influence of gene-environment interplay on this behavioral phenotype. Moreover, we tested the hypothesis that low accumbal D2 receptor (R) availability is a genetic risk factor of food compulsion-like behavior and that environmental conditions that induce compulsive eating alter D2R expression in the striatum. To this end, we measured D1R and D2R expression in the striatum and D1R, D2R and α1R levels in the medial prefrontal cortex, respectively, by western blot.

RESULTS

Exposure to environmental conditions induces compulsion-like eating behavior, depending on genetic background. This behavioral pattern is linked to decreased availability of accumbal D2R. Moreover, exposure to certain environmental conditions upregulates D2R and downregulates α1R in the striatum and medial prefrontal cortex, respectively, of compulsive animals. These findings confirm the function of gene-environment interplay in the manifestation of compulsive eating and support the hypothesis that low accumbal D2R availability is a "constitutive" genetic risk factor for compulsion-like eating behavior. Finally, D2R upregulation and α1R downregulation in the striatum and medial prefrontal cortex, respectively, are potential neuroadaptive responses that parallel the shift from motivated to compulsive eating.

Parameters of haematology, clinical chemistry and lipid metabolism in the common marmoset and alterations under stress conditions

Common marmosets are suitable non-human primate models for many human diseases. Standard values for blood parameters are required to evaluate physiological and pathological situations. Two studies were conducted: study I to determine standard values and study II to examine these under changed housing conditions. In study I, all parameters for clinical chemistry were similar in range for both genders with these specifics: male marmosets had significantly higher total and LDL cholesterol levels than females, whereas the mean corpuscular volume and the mean corpuscular haemoglobin were significantly lower than in females. In study II, glucose, lymphocytes and salivary cortisol were significantly lower, and faecal cortisol was increased during the change of housing conditions. In conclusion, standard values for haematology and clinical chemistry for the common marmoset were determined. Further on, parameters that are influenced by relocation stress and its importance for experimental results are described.

Strain-specific proportion of the two isoforms of the dopamine D2 receptor in the mouse striatum: associated neural and behavioral phenotypes

Genetic variability in the proportion of the two alternative dopamine D2 receptor (D2R) mRNA splice variants, D2R-long (D2L) and D2R-short (D2S), influence corticostriatal functioning and could be implicated in liability to psychopathology. This study compared mesostriatal D2L/D2S ratios and associated neural and behavioral phenotypes in mice of the DBA/2J and C57BL/6J-inbred strains, which differ for schizophrenia- and addiction-like phenotypes. Results showed that DBA/2J mice lack the striatal predominance of D2L that has been reported in the rat and in C57BL/6J mice and confirmed in the latter strain by this study. Only C57BL/6J mice showed enhanced striatal c-Fos expression under D1R and D2/3R co-stimulation, indicating synergistic interaction between the subtypes of DA receptors. Instead, DBA/2J mice were characterized by opposing effects of D2/3R and D1R stimulation on striatal c-Fos expression, in line with a more pronounced influence of D2S isoform, and did not express stereotyped climbing under D1R and D2/3R co-stimulation, as reported for D2L-/- mice. Finally, strain-specific modulation of c-Fos expression by D1R and D2/3R co-stimulation was selectively observed in striatal compartments receiving inputs from the prefrontal cortex and involved in the control of motivated behaviors. These results show differences in tissue-specific D2R splicing in mice with intact genotypes and support a role for this phenotype in individual variability of corticostriatal functioning and in liability to psychopathology.

Extraversion. Interaction between D2 dopamine receptor polymorphisms and parental alcoholism

Both molecular genetic factors (the D2 dopamine receptor (DRD2) and the D4 dopamine receptor (DRD4) polymorphisms) and environmental influences of living in an alcoholic or nonalcoholic home on the personality traits of Extraversion and Neuroticism were assessed in drug-naive, young adolescent boys. There were no significant main effects of genetic or environmental factors on either Neuroticism or Extraversion as measured by the Junior Eysenck Personality Inventory (JEPI). However, a significant interaction between DRD2 (but not DRD4) alleles and environmental variables was observed on Extraversion. Specifically, children with the minor alleles of the DRD2 gene showed a significantly greater Extraversion score when living in an alcoholic than in a nonalcoholic home. In contrast, children with the major alleles of the DRD2 gene showed a trend in the opposite direction. Although the results are preliminary and pending replication, they nevertheless provide the first report of a specific gene-environment interaction involving a human personality trait.

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.

The role of endogenous sensitization in the pathophysiology of schizophrenia: implications from recent brain imaging studies

Long-term sensitization is a process whereby exposure to a given stimulus such as a drug or a stressor results in an enhanced response at subsequent exposures. Sensitization of mesolimbic dopamine systems has been postulated by several authors to underlie the development of dopaminergic abnormalities associated with schizophrenia. In this review, core features of stimulant-induced sensitization of dopamine systems in rodents are briefly reviewed, as well as the behavioral and clinical evidence suggesting the relevance of this process to drug-induced psychosis and schizophrenia. Results of recent brain imaging studies relevant to the question of sensitization in schizophrenia are then discussed. These studies indicate that schizophrenia is associated with increased amphetamine-induced dopamine release. This exaggerated response was detected in patients experiencing an episode of clinical deterioration but not in clinically stable patients. Since increased stimulant-induced dopamine release is a hallmark of sensitization, these results support the view that schizophrenia is associated with a process of endogenous sensitization. Based on the preclinical evidence that dopamine projection to the prefrontal cortex acts as a buffer that oppose the development of sensitization in subcortical dopamine projections, we propose that, in schizophrenia, neurodevelopmental abnormalities of prefrontal dopaminergic systems might result in a state of enhanced vulnerability to sensitization during late adolescence and early adulthood. It is also proposed that D(2) receptor blockade, if sustained, might allow for an extinction of this sensitization process, with possible re-emergence upon treatment discontinuation. A better understanding of the neurocircuitry associated with endogenous sensitization and its consequence in schizophrenia might be important for the development of better treatment and relapse prevention strategies.

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.

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.

Effects of the NMDA-antagonist, MK-801, on stress-induced alterations of dopamine dependent behavior

The effects of pretreatment with the non-competitive NMDA antagonist (+)MK-801 on the behavioral alterations induced by repeated restraint stress were investigated. Repeatedly stressed (restraint stress 2 h a day x 10 days) mice showed enhanced sensitivity to the inhibitory effects of a low dose of direct dopamine agonist, apomorphine (0.25 mg/kg), on climbing behavior. On the other hand, no changes were observed for the stimulatory effect of the high dose of apomorphine (3 mg/kg) on this behavioral response. Mice pretreated with MK-801 (0.15 mg/kg) before the stressful experience did not show altered response to the low dose of apomorphine (0.25 mg/kg). Finally, ten daily injections with 0.15 mg/kg MK-801 did not affect the behavioral response to the low dose of apomorphine, but enhanced the stimulatory effect of the high dose of the dopaminergic agonist on climbing behavior. Therefore, it is possible that the protective action of MK-801 against stress-induced behavioral alteration is due to changes in sensitivity of postsynaptic receptors.

The distribution of catecholamines and beta-endorphin in the brains of three behaviorally distinct breeds of dogs and their F1 hybrids

This study examines neurochemical and behavioral differences among three types of domestic dogs and F1 hybrids derived from them. Purebred dogs included Border Collies, representing herding dogs, Shar Plaininetz, representing livestock protecting dogs, and Siberian Huskies, representing Northern dogs. Composite behavioral scores were derived from frequency measures of various components of predatory behavior observed when the dogs were tested with mice. Catecholamine levels, including norepinephrine (NE), dopamine (DA), and epineprine (EPI), were determined in various brain regions by high-performance liquid chromatography (HPLC) with electrochemical detection. beta-endorphin levels were determined in the same regions by RIA. Collies showed the highest levels of non-consummatory behaviors and Huskies the highest levels of consummatory behaviors. Shars were found to have lower levels of NE and DA than Collies and Huskies in several brain regions, including those comprising the nigrostriatal DA system. Positive correlations between neurochemical and behavioral characteristics could be made between Shars and Collies. Comparisons of F1 hybrids with their respective parental breeds revealed no clear pattern of inheritance for these characteristics but suggested that multiple factors, both independent and epistatic, are involved. Based on previous studies on nigrostriatal DA and behavior, the levels of DA in this system may be causally related to the levels of predatory behavior expressed by Collies and Shars.

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.

Strain-dependent effects of post-training dopamine receptor agonists and antagonists on memory storage in mice

Post-training administration of the selective D1 or D2 agonists SKF 38393 and LY 171555 dose dependently impairs retention of an inhibitory avoidance response in DBA/2 mice. In agreement, the selective D1 or D2 antagonists SCH 23390 and (-)-sulpiride improve retention. These effects are opposite to those observed in the C57BL/6 strain, as previously reported. Moreover, B6D2F1 hybrids present a response to SKF 38393, LY 171555, SCH 23390, and (-)-sulpiride that parallels that of the C57BL/6 strain, thus suggesting that the neural mechanisms underlying the effects of DA agonists or antagonists on memory processes may be inherited through a dominant mode of inheritance.

Effects of cocaine and footshock stress on extracellular dopamine levels in the ventral striatum

Behavioral and neurochemical cross-sensitization between cocaine and stress was examined. The effects of stress and cocaine on extracellular levels of dopamine and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), were measured by in vivo microdialysis in the rostral ventral striatum, including the nucleus accumbens. Pretreatment with a daily 20 min footshock stress (0.45 mA/200 ms/s) for 5 days enhanced the cocaine-induced increase in extracellular dopamine levels in shock compared to sham shock-pretreated rats. The motor stimulant response to acute cocaine was also augmented in shock-pretreated rats. There was a slight but significant decrease in the levels of DOPAC and HVA in both groups following cocaine but no differences between shock and sham shock animals. In contrast, in the converse experiment, pretreatment with daily cocaine (15 mg/kg, i.p.) for 5 days did not significantly alter the stress-induced levels of extracellular dopamine compared to controls. The levels of DOPAC and HVA were not different between cocaine- and saline-pretreated groups although there was a trend towards enhanced metabolite levels in cocaine-pretreated animals. These data in part support a role for enhanced dopamine neutrotransmission in mediating behavioral cross-sensitization between psychostimulants and stress.

Genotype-dependent effects of chronic stress on apomorphine-induced alterations of striatal and mesolimbic dopamine metabolism

After 10 daily consecutive restraint experiences, DBA/2 (DBA) mice showed an increase of climbing behavior after injection of 0.25 mg/kg of the dopamine (DA) agonist apomorphine (APO), while no changes were observed following vehicle or 1 mg/kg of APO. By contrast, chronically stressed C57BL/6 (C57) mice showed a clear-cut decrease of climbing behavior at the dose of 0.25 mg/kg of APO and a similar, although less pronounced, effect of stress on the behavior of mice injected either with vehicle or with 1 mg/kg APO. The DA agonist at these same doses decreased 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 3-methoxytyramine (3-MT) concentrations in the caudatus putamen (CP) and nucleus accumbens septi (NAS) of both strains. Higher DOPAC, HVA and 3-MT concentrations were evident in stressed DBA mice receiving 0.25 mg/kg but not 1 mg/kg of APO, in both CP and NAS. Concerning C57 mice, lower concentrations of the 3 metabolites were present at both doses of APO in the NAS of stressed mice in comparison with non-stressed animals, while no significant stress-related effects were evident in the CP. Non-significant differences between control and stressed mice of both strains were evident as regards DA concentrations in CP and NAS. These results suggest that repeated stressful experiences lead to a hyposensitivity of DA presynaptic receptors in DBA mice while they produce a sensitization of mesolimbic DA presynaptic receptors possibly accompanied by down-regulation of postsynaptic DA receptors in the C57 strain.

Effects of acute and chronic stress and of genotype on oxotremorine-induced locomotor depression of mice

The locomotor behavior of unstressed and stressed mice of two inbred strains, DBA/2 and C57/BL6, was investigated. Animals were tested in a toggle-floor box apparatus, 30 min after saline or oxotremorine treatment (ip). A dose of oxotremorine that did not depress the activity of naïve mice (0.01 mg/kg) was chosen. Stressed mice were injected 24 h after either a single 2-h stress session (acute stress) or the last of 14 daily stress sessions of tube restraining (chronic stress). Acute stress did not modify the depressant effect of oxotremorine on locomotor behavior in either strain. On the contrary, chronic stress induced a clear sensitization of DBA but not C57 mice to the depressant effect of oxotremorine. These findings show that chronic stress may result in modifications of the cholinergic function, and its behavioral correlates, and that these changes are modulated by the genetic makeup.

Role of genotype and dopamine receptors in behaviour of inbred mice in a forced swimming test

The role of genotype in the effects of selective D1 and D2 dopamine agonists and antagonists on behavioural despair (Porsolt's test) was studied. Mice of nine inbred strains showed significant interstrain differences in duration of immobility. The influence of dopaminergic drugs was assessed in six strains characterized by different levels of swimming activity. SKF 38393 (10 mg/kg), an agonist at D1 dopamine receptors, increased swimming activity, while the D1 antagonist SCH 23390 (0.2 and 0.5 mg/kg) reduced it, the effects being genotype dependent. The involvement of D2 dopamine receptors in the regulation of mouse behaviour in the forced swimming test was not so evident; the D2 agonist bromocriptine (10 mg/kg) produced no significant effect. The D2 agonist quinpirole (2.5 mg/kg) increased immobility in the majority of the mouse strains studied, while in CBA mice it resulted in a marked reduction of immobility. The D2 antagonist sulpiride (20 mg/kg) decreased immobility and increased active swimming only in two strains. The present results suggest a different role for D1 and D2 dopamine receptors in the regulation of swimming in the mouse.

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.

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

Behavioral and biochemical analysis of the effects of stress on brain dopamine (DA) functioning in two inbred strains of mice reveals opposite patterns of adaptation to chronic stress. Chronically stressed mice of the C57BL/6 (C57) strain are characterized by hypersensitive mesolimbic DA autoreceptors and by a dramatic increase of D1/D2 DA receptor ratio (possibly postsynaptic) in the nucleus accumbens septi (NAS) as revealed by in vivo binding of 3H-spiperone and 3H-SCH23390. Chronically stressed DBA/2 (DBA) mice present, on the contrary, hyposensitive DA autoreceptors and no changes in the D1/D2 DA receptors ratio in this brain area. The analysis of the behavioral responses of chronically stressed mice of the C57 strain to the mixed D1/D2 receptor agonist apomorphine, to the selective D2 agonist LY171555 and to the selective D1 agonist SKF 38393 suggest a close relationship between the behavioral alterations produced by chronic stress and the alterations of sensitivity of D2 pre- and postsynaptic receptors in the mesolimbic system. Furthermore, chronically stressed C57 mice present a marked decrease of spontaneous-climbing behavior which is not observed in the mice of the DBA strain and is dependent on the alteration of the biphasic evolution of this behavior during exposure to the test situation which, for these mice, represents a novel environment. Acute exposure to aversive environmental conditions induces a biphasic alteration of DA transmission (initial increase of DA release followed by a decrease under control levels) in the NAS.(ABSTRACT TRUNCATED AT 250 WORDS)

Stress-induced decrease of 3-methoxytyramine in the nucleus accumbens of the mouse is prevented by naltrexone pretreatment

Pretreatment with naltrexone (2.5 and 5 mg/kg) prevented the decrease of 3-methoxytyramine (3-MT)/dopamine (DA) ratio induced by 2 h immobilization stress in the nucleus accumbens (NAS) of the mouse while it did not affect the stress-induced decrease of 3-MT/DA ratio in caudatus putamen (CP). Naltrexone also produced a slight antagonism of homovanillic acid (HVA)/DA ratio increase produced by stress in the frontal cortex (FC). These results point to an involvement of endogenous opioids in the effects of stress on DA metabolism in the mesolimbic system of the mouse.

Pharmacological evidence for a role of D2 dopamine receptors in the defensive behavior of the mouse

In this study the role of the DA system in the expression of defensive behavior of the mouse was investigated. C57BL/6 mice subjected to three daily defeat experiences (24 h apart) exhibited an increase of defensive behaviors (upright and sideways postures and escape) as well as a decrease of activity and a decrease of social investigation compared with undefeated mice (controls) when confronted with nonaggressive Swiss mice 24 h after the last aggressive confrontation. The selective D2 DA receptor antagonist (-)-sulpiride administered before confrontation with nonaggressive opponents (fourth day) dramatically decreased defensive behaviors and produced an increase of social investigation. The selective D1 DA receptor antagonist SCH 23390 did not affect either defence or social investigation. In further experiments the behavioral effects of the selective D1 agonist SKF 38393 and of the selective D2 agonist LY171555 on naive C57BL/6 mice interacting with nonaggressive opponents of the same strain were assessed. SKF 38393 in doses up to 30 mg/kg did not produce any significant behavioral changes while LY171555 produced a clear-cut dose-dependent increase of defensive behavior as well as a decrease of social investigation and activity and an increase of immobility. The behavioral profile produced by the D2 agonist did not differ from that produced by defeat experiences. These results indicate that D2 receptors play a major role in the expression of defensive behavior in the mouse. The hypothesis that alteration in D2 receptor functioning may produce hyperdefensiveness possibly due to altered perceptive processes is discussed.

Opioids and behavior: genetic aspects

Three animal models, based on genetic differences in endogenous opioid peptides and opioid receptors, are described. Obese mice and rats, whose pituitary opioid content is elevated, may be used to investigate eating disorders. Recombinant inbred strains of mice, which differ in brain opioid receptors and analgesic responsiveness, can be used for study of opioid- and nonopioid-mediated mechanisms of pain inhibition. Individual reactivity to opioids can be examined in C57BL/6 and DBA/2 inbred strains of mice. A model that combines a variety of opioid effects is offered and suggests the existence of a genetically determined dissociation of opioid effects on locomotor activity and pain inhibition. In addition, stimulatory locomotor responses in the C57BL/6 reaction type are linked to a high risk of drug addiction and facilitatory effects on adaptive processes, while high analgesic potency in the DBA/2 reaction type is accompanied by a low proneness to drug abuse and amnesic properties of opioids.

A classical genetic analysis of two apomorphine-induced behaviors in the mouse

Apomorphine (3 mg/kg) produced in C57BL/6 (C57) mice a clear-cut increase in locomotor activity and climbing behavior in comparison with saline, while in DBA/2 (DBA) mice it produced a clear-cut decrease in locomotion and a small reduction in climbing behavior. Genetic analysis involving F1 and F2 hybrids and the backcross populations (F1 X C57; F1 X DBA) indicated that apomorphine-induced locomotion and climbing are inherited through different modes of inheritance. With regard to climbing behavior the mean analysis of apomorphine parameters showed that the additive-dominance model fitted adequately, while this single model did not fit the locomotor activity data for which the best fitting model involved epistatic parameter. Moreover, a zero correlation between the two behaviors in the F2 generation resulted, indicating that no relationship exists between these apomorphine-induced behaviors under our experimental conditions. These results suggest that the horizontal locomotion and climbing are distinct behaviors controlled, at least in part, by different genetic factors related to different dopaminergic mechanisms.

Effects of immobilization stress on dopamine and its metabolites in different brain areas of the mouse: role of genotype and stress duration

Immobilization stress induced, in mice of both C57BL/6 (C57) and DBA/2 (DBA) strains, an increase in dihydroxyphenylacetic acid (DOPAC)/dopamine (DA) and homovanillic acid (HVA)/DA ratios and a reduction of 3-methoxytyramine (3-MT)/DA ratio in the caudatus putamen (CP) and nucleus accumbens septi (NAS). These effects were already evident after 30 min stress in the NAS, while in the CP 120 min were needed in order to show the effects of stress. Immobilization did not produce any effects on dopaminergic metabolism in the frontal cortex (FC) of the C57 strain either after 30 or after 120 min stress while in mice of the DBA strain a time-dependent effect of stress on the HVA/DA ratio was evident. When B6D2F1 hybrids were considered, the effects produced by 120 min immobilization in the CP and the NAS paralleled those observed in parental strains, while in the FC 120 min stress induced the same increase of HVA observed in DBA mice, thus suggesting that the pattern of response in the FC that characterizes the DBA strain may be inherited through a dominant pattern of inheritance.

Different effects of acute and chronic stress on two dopamine-mediated behaviors in the mouse

After two hours of immobilization stress, C57BL/6 mice presented a significant reduction of spontaneous locomotion and a slight reduction of spontaneous climbing. The effect of stress on locomotor activity disappeared after ten daily sessions of immobilization while this chronic treatment increased the effect of stress on spontaneous climbing. Twenty-four hr after the last stressful experience the mice showed an increase of spontaneous locomotion and a decrease of spontaneous climbing in comparison with unstressed mice. Following a single exposure to immobilization stress, an increase of DOPAC/DA and HVA/DA ratios was found in the striatum and in the nucleus accumbens. These effects were still evident following repeated exposure to this stressor but disappeared 24 hr after the last of ten daily stressful experiences. Finally, chronically stressed mice, tested 24 hr after the last stressful experience, showed an increased sensitivity to the inhibitory effects of low doses of apomorphine on climbing behavior and a decreased sensitivity to the inhibitory effects of the same doses of the dopamine agonist on locomotion. These results are discussed in terms of altered sensitivity of different populations of dopamine receptors following chronic stress.

Chronic exposure to a novel odor increases pups' vocalizations, maternal care, and alters dopaminergic functioning in developing mice

This study was designed to assess the stress effect of manipulation of the olfactory environment in developing mice. In a first experiment it was found that mouse pups could be stressed (as measured by an increase in ultrasonic calls) by removing the litter from the dam for 15 min/day for the first 14 days of life and exposing them to a novel odor (clean bedding). This stress procedure also produced a long-term modification in maternal behavior. The stress response (ultrasounds) and the modification of maternal behavior were prevented by providing the litter with home cage bedding during maternal separation. In a second experiment it was demonstrated that early stress influenced apomorphine-induced wall climbing behavior in 15-day-old mice, suggesting stress-induced alterations in the dopaminergic system. Pups exposed to clean bedding during infancy exhibited more wall climbing behavior than pups never separated from the mother. Moreover, preventing the early stress response during mother-offspring separation, by providing pups with home cage bedding, eliminated the increase in apomorphine-induced wall climbing. Taken together these results suggest that olfactory cues are decisive in characterizing stressful situations inducing both immediate and long-lasting effects in mouse pups.