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

The influence of dopamine autoreceptors on temperament and addiction risk

As a major regulator of dopamine (DA), DA autoreceptors (DAARs) exert substantial influence over DA-mediated behaviors. This paper reviews the physiological and behavioral impact of DAARs. Individual differences in DAAR functioning influences temperamental traits such as novelty responsivity and impulsivity, both of which are associated with vulnerability to addictive behavior in animal models and a broad array of externalizing behaviors in humans. DAARs additionally impact the response to psychostimulants and other drugs of abuse. Human PET studies of D2-like receptors in the midbrain provide evidence for parallels to the animal literature. These data lead to the proposal that weak DAAR regulation is a risk factor for addiction and externalizing problems. The review highlights the potential to build translational models of the functional role of DAARs in behavior. It also draws attention to key limitations in the current literature that would need to be addressed to further advance a weak DAAR regulation model of addiction and externalizing risk.

Neurosteroids and the mesocorticolimbic system

The mesocorticolimbic system coordinates executive functions, such as working memory and behavioral flexibility. This circuit includes dopaminergic projections from the ventral tegmental area to the nucleus accumbens and medial prefrontal cortex. In this review, we summarize evidence that cells in multiple nodes of the mesocorticolimbic system produce neurosteroids (steroids synthesized in the nervous system) and express steroid receptors. Here, we focus on neuroandrogens (androgens synthesized in the nervous system), neuroestrogens (estrogens synthesized in the nervous system), and androgen and estrogen receptors. We also summarize how (neuro)androgens and (neuro)estrogens affect dopamine signaling in the mesocorticolimbic system and regulate executive functions. Taken together, the data suggest that steroids produced in the gonads and locally in the brain modulate higher-order cognition and executive functions.

Interactions Between Experience, Genotype and Sex in the Development of Individual Coping Strategies

Coping strategies, the first line of defense against adversities, develop through experience. There is consistent evidence that both genotype and sex contribute to the development of dysfunctional coping, leading to maladaptive outcomes of adverse experiences or to adaptive coping that fosters rapid recovery even from severe stress. However, how these factors interact to influence the development of individual coping strategies is just starting to be investigated. In the following review, we will consider evidence that experience, sex, and genotype influence the brain circuits and neurobiological processes involved in coping with adversities and discuss recent results pointing to the specific effects of the interaction between early experiences, genotype, and stress in the development of functional and dysfunctional coping styles.

Involvement of the dopamine system in the effect of chronic social isolation during adolescence on social behaviors in male C57 mice

In the early stage of life, experiencing social isolation can generate long-lasting deleterious effects on behaviors and brain development. However, the effects of chronic social isolation during adolescence on social behaviors and its underlying neurobiological mechanisms remain unclear. The present study found that four weeks of social isolation during adolescence impaired social recognition ability in the three-chamber test and five-trial social recognition test, and increased aggressive-like behaviors, but reduced environmental exploration, as showed in the social interaction test. Chronic social isolation decreased levels of dopamine D2 receptor in the shell of the nucleus accumbens (NAcc) and medial prefrontal cortex. It also reduced TH in the NAcc. Using in vivo fiber photometry, it was also found that isolated mice displayed a reduction in NAcc shell activity upon exploring unfamiliar social stimuli. An injection of a 100 ng dose of the D2R agonist quinpirole into the shell of the NAcc reversed behavioral abnormalities induced by chronic social isolation. These data suggest that the dopamine system is involved in alterations in social behaviors induced by chronic social isolation. This finding sheds light on the mechanism underlying abnormalities in social behavior induced by adolescent chronic social isolation and provides a promising target to treat mental diseases relevant to social isolation.

The neurobiology of environmentally induced stereotypic behaviours in captive animals: assessing the basal ganglia pathways and cortico-striatal-thalamo-cortical circuitry hypotheses

The neurobiology of environmentally induced stereotypic behaviours (SBs) (e.g., pacing in zoo carnivores, crib-biting in horses, tail chasing in dogs) is hypothesized to involve altered functioning within the basal ganglia (‘Basal Ganglia (BG) Pathways Hypotheses’) and/or between the basal ganglia and cortex (‘Cortico-Striatal-Thalamo-Cortical (CSTC) Circuits Hypotheses’). We review four decades of relevant studies, critically assessing support for both hypotheses. Currently no BG Pathways or CSTC Circuits hypothesis is fully supported. While some results are partially consistent with some hypotheses (decreased subthalamic nucleus activity in deer mice and C58 mice); others (nucleus accumbens activity in mink and C57 mice) seem to reflect individual differences in SB, but not environmental effects. Yet others can be tentatively rejected: neither elevated striatal dopamine nor the cortico-striatal connection of the sensorimotor circuit seem to be involved for most species studied to date. Further research is now important for understanding the impact of captivity on animals’ functioning.

Genetics of Equine Behavioral Traits

Behavior is a valuable quantitative trait in the horse because of its impact on performance, work, recreation, and prerequisite close interactions with humans. This article reviews what is known about the genetics of behavior in horses with an emphasis on the genetic basis for temperament traits, neuroendocrine function, and stereotypic behavior. The importance of using modern molecular genetic techniques to the study of equine behavior and recommendations for future research are also discussed. Ultimately, these studies enhance the understanding of the biology of behavior in the horse, improve handler and rider safety, and benefit horse welfare.

Cholinergic Stress Signals Accompany MicroRNA-Associated Stereotypic Behavior and Glutamatergic Neuromodulation in the Prefrontal Cortex

Stereotypic behavior (SB) is common in emotional stress-involved psychiatric disorders and is often attributed to glutamatergic impairments, but the underlying molecular mechanisms are unknown. Given the neuro-modulatory role of acetylcholine, we sought behavioral-transcriptomic links in SB using TgR transgenic mice with impaired cholinergic transmission due to over-expression of the stress-inducible soluble 'readthrough' acetylcholinesterase-R splice variant AChE-R. TgR mice showed impaired organization of behavior, performance errors in a serial maze test, escape-like locomotion, intensified reaction to pilocarpine and reduced rearing in unfamiliar situations. Small-RNA sequencing revealed 36 differentially expressed (DE) microRNAs in TgR mice hippocampi, 8 of which target more than 5 cholinergic transcripts. Moreover, compared to FVB/N mice, TgR prefrontal cortices displayed individually variable changes in over 400 DE mRNA transcripts, primarily acetylcholine and glutamate-related. Furthermore, TgR brains presented c-fos over-expression in motor behavior-regulating brain regions and immune-labeled AChE-R excess in the basal ganglia, limbic brain nuclei and the brain stem, indicating a link with the observed behavioral phenotypes. Our findings demonstrate association of stress-induced SB to previously unknown microRNA-mediated perturbations of cholinergic/glutamatergic networks and underscore new therapeutic strategies for correcting stereotypic behaviors.

Animal models of liability to post-traumatic stress disorder: going beyond fear memory

In this review, we advocate a dimensional approach on the basis of candidate endophenotypes to the development of animal models of post-traumatic stress disorder (PTSD) capable of including genetic liability factors, variations in symptoms profile and underlying neurobiological mechanisms, and specific comorbidities. Results from the clinical literature pointed to two candidate endophenotypes of PTSD: low sensory gating and high waiting impulsivity. Findings of comparative studies in mice of two inbred strains characterized by different expressions of the two candidate endophenotypes showed different strain-specific neural and behavioral effects of stress experiences. Thus, mice of the standard C57BL/6J strain show stress-induced helplessness, stress-learned helplessness, and stress-extinction-resistant conditioned freezing. Instead, mice of the genetically unrelated DBA/2J strain, expressing both candidate endophenotypes, show stress-induced extinction-resistant avoidance and neural and behavioral phenotypes promoted by prolonged exposure to addictive drugs. These strain differences are in line with evidence of associations between genetic variants and specific stress-promoted pathological profiles in PTSD, support a role of genotype in determining different PTSD comorbidities, and offer the means to investigate specific pathogenic processes.

Mutual interactions between cognition and welfare: The horse as an animal model

Research in cognitive psychology has repeatedly shown how much cognition and emotions are mutually related to one another. Psychological disorders are associated with cognitive (attention, memory and judgment) biases and chronic pain may affect attention, learning or memory. Laboratory studies have provided useful insights about the processes involved but observations about spontaneous animal models, living in different stress/welfare conditions may help understand further how cognition and welfare are interrelated in the « real world ». Domestic horses constitute such a model as they live in a variety of conditions that impact differently their welfare state. In the present review, we try and provide an overview of the scientific literature on cognition and welfare of domestic horses and their interrelationship. We address how emotions and welfare may affect cognitive processes in horses and impact the way they perceive their environment (including work). We propose new methods for assessing the relationship between welfare and cognition and open up the discussion on the evolution of the brain and the part domestication may have played.

Comparison and Functional Genetic Analysis of Striatal Protein Expression Among Diverse Inbred Mouse Strains

C57BL/6J (B6) and DBA/2J (D2) inbred mouse strains are highly variable genetically and differ in a large number of behavioral traits related to striatal function, including depression, anxiety, stress response, and response to drugs of abuse. The genetic basis of these phenotypic differences are, however, unknown. Here, we present a comparison of the striatal proteome between B6 and D2 and relate differences at the protein level to strain differences at the mRNA level. We also leverage a recombinant inbred BXD population derived from B6 and D2 strains to investigate the role of genetic variation on the regulation of mRNA and protein levels. Finally, we test the hypothesis that differential protein expression contributes to differential behavioral responses between the B6 and D2 strain. We detected the expression of over 2,500 proteins in membrane-enriched protein fractions from B6 and D2 striatum. Of these, 160 proteins demonstrated significant differential expression between B6 and D2 strains at a 10% false discovery level, including COMT, GABRA2, and cannabinoid receptor 1 (CNR1)—key proteins involved in synaptic transmission and behavioral response. Similar to previous reports, there was little overlap between protein and transcript levels (25%). However, the overlap was greater (51%) for proteins demonstrating genetic regulation of cognate gene expression. We also found that striatal proteins with significantly higher or lower relative expression in B6 and D2 were enriched for dopaminergic and glutamatergic synapses and processes involved in synaptic plasticity [e.g., long-term potentiation (LTP) and long-term depression (LTD)]. Finally, we validated higher expression of CNR1 in B6 striatum and demonstrated greater sensitivity of this strain to the locomotor inhibiting effects of the CNR1 agonist, Δ9-tetrahydrocannabinol (THC). Our study is the first comparison of differences in striatal proteins between the B6 and D2 strains and suggests that alterations in the striatal proteome may underlie strain differences in related behaviors, such as drug response. Furthermore, we propose that genetic variants that impact transcript levels are more likely to also exhibit differential expression at the protein level.

Chronic Social Isolation Stress during Peri-Adolescence Alters Presynaptic Dopamine Terminal Dynamics via Augmentation in Accumbal Dopamine Availability

Chronic peri-adolescent stress in humans increases risk to develop a substance use disorder during adulthood. Rats reared in social isolation during peri-adolescence (aSI; 1 rat/cage) period show greater ethanol and cocaine intake compared to group housed (aGH; 4 rats/cage) rats. In addition, aSI rats have a heightened dopamine response in the nucleus accumbens (NAc) to rewarding and aversive stimuli. Furthermore, single pulse electrical stimulation in slices containing NAc core elicits greater dopamine release in aSI rats. Here, we further investigated dopamine release kinetics and machinery following aSI. Dopamine release, across a wide range of stimulation intensities and frequencies, was significantly greater in aSI rats. Interestingly, subthreshold intensity stimulations also resulted in measurable dopamine release in accumbal slices from aSI but not aGH rats. Extracellular [Ca²⁺] manipulations revealed augmented calcium sensitivity of dopamine release in aSI rats. The readily releasable pools of dopamine, examined by bath application of Ro-04-1284/000, a vesicular monoamine transporter 2 (VMAT2) inhibitor, were depleted faster in aGH rats. Western blot analysis of release machinery proteins (VMAT2, Synaptogyrin-3, Syntaxin-1, and Munc13-3) showed no difference between the two groups. Tyrosine hydroxylase (TH) protein expression levels, however, were elevated in aSI rats. The greater dopamine release could potentially be explained by higher levels of TH, the rate-limiting step for dopamine synthesis. This augmented responsivity of the dopamine system and heightened dopamine availability post-aSI may lead to an increased risk of addiction vulnerability.

Stereotypic horses (Equus caballus) are not cognitively impaired

Stereotypies in animals are thought to arise from an interaction between genetic predisposition and sub-optimal housing conditions. In domestic horses, a well-studied stereotypy is crib-biting, an abnormal behaviour that appears to help individuals to cope with stressful situations. One prominent hypothesis states that animals affected by stereotypies are cognitively less flexible compared to healthy controls, due to sensitization of a specific brain area, the basal ganglia. The aim of this study was to test this hypothesis in crib-biting and healthy controls, using a cognitive task, reversal learning, which has been used as a diagnostic for basal ganglia dysfunction. The procedure consisted of exposing subjects to four learning tasks; first and second acquisition, and their reversals. For each task, we measured the number of trials to reach criterion and heart rate and heart-rate variability. Importantly, we did not try to prevent crib-biters from executing their stereotypic behaviour. We found that the first reversal learning task required the largest number of trials, confirming its challenging nature. Interestingly, the second reversal learning task required significantly fewer trials to reach criterion, suggesting generalisation learning. However, we did not find any performance differences across groups; both stereotypic and control animals required a similar numbers of trials and did not differ in their physiological responses. Our results thus challenge the widely held belief that crib-biting horses, and stereotypic animals more generally, are cognitively impaired. We conclude that cognitive underperformance may occur in stereotypic horses if they are prevented from crib-biting to cope with experienced stress.

Regulation of somatostatin receptor 2 in the context of antidepressant treatment response in chronic mild stress in rat

RATIONALE

The role of somatostatin and its receptors for the stress-related neuropsychiatric disorders has been widely raised. Recently, we have also demonstrated the involvement of somatostatin receptor type 2-sst2R and dopamine receptor type 2-D2R in stress.

OBJECTIVE

In this context, we decided to find if these receptors are involved in response to antidepressant treatment in animal model of depression-chronic mild stress (CMS).

METHODS

Here, we report data obtained following 7-week CMS procedure. The specific binding of [125I]Tyr3-Octreotide to sst2R and [3H]Domperidone to D2R was measured in the rat brain, using autoradiography. Additionally, the level of dopamine and metabolites was measured in the rat brain.

RESULTS

In the final baseline test after 7 weeks of stress, the reduced consumption of sucrose solution was observed (controls vs the stressed animals (6.25 0.16 vs. 10.39 0.41; p < 0.05). Imipramine was administered for the next 5 weeks, and it reversed anhedonia in majority of animals (imipramine-reactive); however, in some animals, it did not (imipramine-non-reactive). Two-way repeated measures ANOVA revealed significant effects of stress and treatment and time interaction [F(16, 168) = 3.72; p < 0.0001], n = 10 per groups. We observed decreased binding of [125I]Tyr3-Octreotide in most of rat brain regions in imipramine non-reactive groups of animals. The decrease of D2R after stress in striatum and nucleus accumbens and no effect of imipramine were observed. In the striatum and prefrontal cortex, the significant role of stress and imipramine in dopamine levels was observed.

CONCLUSIONS

The results obtained in binding assays, together with dopamine level, indicate the involvement of sst2R receptors for reaction to antidepressant treatment. Besides, the stress context itself changes the effect of antidepressant drug.

Stress-Induced Reduction of Dorsal Striatal D2 Dopamine Receptors Prevents Retention of a Newly Acquired Adaptive Coping Strategy

The inability to learn an adaptive coping strategy in a novel stressful condition leads to dysfunctional stress coping, a marker of mental disturbances. This study tested the involvement of dorsal striatal dopamine receptors in the dysfunctional coping with the Forced Swim test fostered by a previous experience of reduced food availability. Adult male mice were submitted to a temporary (12 days) reduction of food availability [food-restricted (FR)] or continuously free-fed (FF). Different groups of FF and FR mice were used to evaluate: (1) dorsal striatal mRNA levels of the two isoforms of the dopamine D2 receptor (D2S, D2L). (2) Forced Swim-induced c-fos expression in the dorsal striatum; (3) acquisition and 24 h retention of passive coping with Forced Swim. Additional groups of FF mice were tested for 24 h retention of passive coping acquired during a first experience with Forced Swim immediately followed by intra-striatal infusion of vehicle or two doses of the dopamine D2/D3 receptors antagonist sulpiride or the D1/D5 receptors antagonist SCH23390. Previous restricted feeding selectively reduced mRNA levels of both D2 isoforms and abolished Forced Swim-induced c-fos expression in the left Dorsolateral Striatum and selectively prevented 24 h retention of the coping strategy acquired in a first experience of Forced Swim. Finally, temporary blockade of left Dorsolateral Striatum D2/D3 receptors immediately following the first Forced Swim experience selectively reproduced the behavioral effect of restricted feeding in FF mice. In conclusion, the present results demonstrate that mice previously exposed to a temporary reduction of food availability show low striatal D2 receptors, a known marker of addiction-associated aberrant neuroplasticity, as well as liability to relapse into maladaptive stress coping strategies. Moreover, they offer strong support to a causal relationship between reduction of D2 receptors in the left Dorsolateral Striatum and impaired consolidation of newly acquired adaptive coping.

Emerging Role for Nucleus Accumbens Medium Spiny Neuron Subtypes in Depression

The ventral striatum (nucleus accumbens) and its role in mood, reward, and motivation has been the focus of significant research. Despite this interest, little work has addressed cell type-specific distinctions in medium spiny neurons (MSNs), the main projection neurons in the nucleus accumbens and dorsal striatum, and their function in relation to stress and depression. Previous work has shown opposing roles for D₁ and D₂ receptor MSN subtypes in depression-like outcomes to stress, particularly in regard to repeated neuronal stimulation and excitatory transmission. Yet the mechanisms of action are still unknown. We discuss potential mechanisms by which MSN subtype function promotes dichotomous behavioral outcomes caused by differences in cellular plasticity, subcellular signaling pathways, and genetic expression. This review aims to address our current understanding about the role of nucleus accumbens MSN subtypes in stress-related depression behavior and speculates on how currently understood mechanisms contribute to factors that control the activity of MSNs.

Neural modulators of temperament: A multivariate approach to personality trait identification in the horse

A relationship between dopamine and temperament has previously been described in human cases of dopaminergic dysfunction. Adjustment in temperament prior to disease manifestation can enable the early identification of individuals at risk of such conditions, and scope exists to extend this application of temperament alterations to cases of dopaminergic dysfunction in horses. A multivariate and mixed-methods approach utilising a questionnaire along with two inferred measurements of dopamine activity (Spontaneous Blink Rate [SBR] and behavioral initiation rate [BIR]) were recorded from direct observation of animals (n=99) to identify the potential relationship between dopamine and temperament in horses. Principal components analysis (PCA) of 36 temperament variables revealed nine principal components, including 'Anxiety' and 'Docility', which accounted for 72.4% of the total variance. Component scores were calculated and correlated with SBR and BIR utilising Spearman rank correlation coefficient analysis. The component 'Anxiety' was found to have a significant positive relationship with SBR, whereas 'Docility' was observed to have a significant negative relationship with SBR. These results indicate a relationship between dopamine and temperament within the horse that is certainly worthy of further study. Potential mechanisms involving neural dopaminergic and GABAergic systems are presented, in addition to how such alterations could be utilised to probe for equine dopamine dysfunction pending future research.

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.

Maternal separation and early stress cause long-lasting effects on dopaminergic and endocannabinergic systems and alters dendritic morphology in the nucleus accumbens and frontal cortex in rats

A considerable amount experimental studies have shown that maternal separation (MS) is associated with adult offspring abnormal behavior and cognition disorder. Accordingly, this experimental procedure has been proposed as a predictor for alcohol and drug dependence based on the neurodevelopmental soon after birth. Endocannabinoid system (eCBs) has been implicated in reward processes, including drug abuse and dependence. MS and associated stress causes changes in the eCBs that seem to facilitate alcohol consumption. In this study, we seek to evaluate potential morphological changes in neurons of the frontal cortex (FCx) and nucleus accumbens (NAcc), in the expression of receptors and enzymes of the endocannabinoid and dopamine systems and in second messengers, such as Akt, in adult rats subjected to MS and early stress (MS + ES; 2 × 180 min daily) vs. nonseparated rats (NMS). Results showed that MS + ES induces higher D2R expression and lower D3R, FAAH, and MAGL expression compared with NMS rats. Alterations in total dendritic length were also detected and were characterized by increases in the NAcc while there were decreases in the FCx. We believe MS + ES-induced changes in the dopaminergic and endocannabinergic systems and in the neuronal microstructure might be contributing to alcohol seeking behavior and, potential vulnerability to other drugs in rats. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 819-831, 2016.

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.

Different contributions of dopamine D1 and D2 receptor activity to alcohol potentiation of brain stimulation reward in C57BL/6J and DBA/2J mice

C57BL/6J (C57) and DBA/2J (DBA) mice respond differently to drugs that affect dopamine systems, including alcohol. The current study compared effects of D1 and D2 receptor agonists and antagonists, and the interaction between D1/D2 antagonists and alcohol, on intracranial self-stimulation in male C57 and DBA mice to determine the role of dopamine receptors in the effects of alcohol on brain stimulation reward (BSR). In the initial strain comparison, dose effects on BSR thresholds and maximum operant response rates were determined for the D1 receptor agonist SKF-82958 (±-6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine; 0.1-0.56 mg/kg) and antagonist SCH 23390 (+-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepinehydrochloride; 0.003-0.056 mg/kg), and the D2 receptor agonist quinpirole (0.1-3.0 mg/kg) and antagonist raclopride (0.01-0.56 mg/kg). For the alcohol interaction, SCH 23390 (0.003 mg/kg) or raclopride (0.03 mg/kg) was given before alcohol (0.6-2.4 g/kg p.o.). D1 antagonism dose-dependently elevated and SKF-82958 dose-dependently lowered BSR threshold in both strains; DBA mice were more sensitive to SKF-82958 effects. D2 antagonism dose-dependently elevated BSR threshold only in C57 mice. Low doses of quinpirole elevated BSR threshold equally in both strains, whereas higher doses of quinpirole lowered BSR threshold only in C57 mice. SCH 23390, but not raclopride, prevented lowering of BSR threshold by alcohol in DBA mice. Conversely, raclopride, but not SCH 23390, prevented alcohol potentiation of BSR in C57 mice. These results extend C57 and DBA strain differences to D1/D2 sensitivity of BSR, and suggest differential involvement of D1 and D2 receptors in the acute rewarding effects of alcohol in these two mouse strains.

The disrupted basal ganglia and behavioural control: an integrative cross-domain perspective of spontaneous stereotypy

Spontaneous stereotypic behaviour (SB) is common in many captive animal species, as well as in humans with some severe psychiatric disorders, and is often cited as being related to general basal ganglia dysfunction. Despite this assertion, there is little in the literature examining SB specifically in terms of the basal ganglia mechanics. In this review, we attempt to fill this gap by offering an integrative, cross-domain perspective of SB by linking what we currently understand about the SB phenotype with the ever-growing literature on the anatomy and functionality of the basal ganglia. After outlining current models of SB from different theoretical perspectives, we offer a broad but detailed overview of normally functioning basal ganglia mechanics, and attempt to link this with current neurophysiological evidence related to spontaneous SB. Based on this we present an empirically derived theoretical framework, which proposes that SB is the result of a dysfunctional action selection system that may reflect dysregulation of excitatory (direct) and inhibitory (indirect and hyperdirect) pathways as well as alterations in mechanisms of behavioural switching. This approach also suggests behaviours that specifically become stereotypic may reflect inbuilt low selection threshold behavioural sequences associated with early development and the species-specific ethogram or, low threshold behavioural sequences that are the result of stress-induced dopamine exposure at the time of performance.

Initial d2 dopamine receptor sensitivity predicts cocaine sensitivity and reward in rats

The activation of dopamine receptors within the mesolimbic dopamine system is known to be involved in the initiation and maintenance of cocaine use. Expression of the D₂ dopamine receptor subtype has been implicated as both a predisposing factor and consequence of chronic cocaine use. It is unclear whether there is a predictive relationship between D₂ dopamine receptor function and cocaine sensitivity that would enable cocaine abuse. Therefore, we exploited individual differences in behavioral responses to D₂ dopamine receptor stimulation to test its relationship with cocaine-mediated behaviors. Outbred, male Sprague-Dawley rats were initially characterized by their locomotor responsiveness to the D₂ dopamine receptor agonist, quinpirole, in a within-session ascending dose-response regimen (0, 0.1, 0.3 & 1.0 mg/kg, sc). Rats were classified as high or low quinpirole responders (HD₂ and LD₂, respectively) by a median split of their quinpirole-induced locomotor activity. Rats were subsequently tested for differences in the psychostimulant effects of cocaine by measuring changes in cocaine-induced locomotor activity (5 and 15 mg/kg, ip). Rats were also tested for differences in the development of conditioned place preference to a low dose of cocaine (7.5 mg/kg, ip) that does not reliably produce a cocaine conditioned place preference. Finally, rats were tested for acquisition of cocaine self-administration and maintenance responding on fixed ratio 1 and 5 schedules of reinforcement, respectively. Results demonstrate that HD₂ rats have enhanced sensitivity to the locomotor stimulating properties of cocaine, display greater cocaine conditioned place preference, and self-administer more cocaine compared to LD₂ animals. These findings suggest that individual differences in D₂ dopamine receptor sensitivity may be predictive of cocaine sensitivity and reward.

Partial extinction of a conditioned context enhances preference for elements previously associated with cocaine but not with chocolate

Drug-associated stimuli are crucial to reinstatement of drug-seeking after periods of abstinence, representing a central problem in treatment of addiction. The present study investigated the influence of partial extinction of the conditioned context on the expression of conditioned place preference (CPP). Mice of the inbred DBA/2J strain were conditioned with cocaine or chocolate in a context identified by multiple elements (A+B) and subsequently CPP expression was evaluated in a context containing only one element (A or B) or both (A+B). Cocaine- and chocolate-conditioned mice showed CPP in presence of the original compound stimulus. However, cocaine-conditioned mice did not show CPP when tested in A or B context, while chocolate-conditioned mice did show CPP to single element context. After conditioning mice were exposed to extinction training of the context A or B and then tested for CPP 1 and 9 days after the end of the extinction (days 9 and 18). Cocaine-conditioned mice showed CPP 9 days after extinction while chocolate-conditioned mice were relatively insensitive to the extinction procedure on day 1 after extinction, but they did not show CPP for the partial or the original compound 9 days after extinction. Cocaine-conditioned mice not submitted to the extinction training (simple passage of time) or submitted to a Sham-extinction procedure (saline injections and confinement in a new environment) did not show CPP on day 9 or 18. Cocaine-conditioned mice exposed to extinction training showed increased c-Fos expression in several brain areas in comparison to mice exposed to Sham-extinction. The extinction procedure did not specifically reduce behavioral sensitization. The results suggest that extinction training involving only elements of a drug-associated context can result in increased associative strength of those elements.

Mesolimbic dopamine D₂ receptor plasticity contributes to stress resilience in rats subjected to chronic mild stress

RATIONALE

Few studies have investigated neurobiological and biochemical differences between stress-resilient and stress-vulnerable experimental animals.

OBJECTIVES

We investigated alterations in mesolimbic dopamine D2 receptor density and mRNA expression level in stressed rats at two time points, i.e. after 2 and 5 weeks of chronic mild stress (CMS).

METHODS

We used the chronic mild stress paradigm because it is a well-established animal model of depression. Two groups of stressed rats were distinguished during CMS experiments: (1) stress reactive (70 %), which displayed a decrease in the drinking of a palatable sucrose solution during the stress regimen, and (2) stress resilient (30 %), which exhibited an unaltered drinking profile when compared with the unchallenged control group. [(3)H]Domperidone was used as a ligand to label dopamine D2 receptors, and a mixture of three specific oligonucleotides was used to evaluate dopamine D2 receptor mRNA changes in various regions of the rat brain.

RESULTS

CMS strongly affected the mesolimbic dopamine circuit in stress-resilient group after 2 weeks and stress-reactive group of rats after 5 weeks which exhibited a decrease in the level of dopamine D2 receptor protein without alterations in D2 mRNA expression. Stress-resilient animals, but not stress-reactive animals, effectively adapted to the extended stress and coped with it. The increase in D2 mRNA expression returned the dopamine D2 receptor density to control levels in stress-resilient rats after 5 weeks of CMS, but not in stress-reactive animals.

CONCLUSIONS

These results clearly demonstrate that, despite earlier blunting, the activation of dopamine receptor biosynthesis in the dopamine mesoaccumbens system in stress-resilient rats is involved in active coping with stressful experiences, and it exhibits a delay in time.

Activating effects of chronic variable stress in rats with different exploratory activity: association with dopamine d(1) receptor function in nucleus accumbens

BACKGROUND/AIMS

Rats display persistent behavioural phenotypes of low (LE) versus high (HE) exploratory activity in the exploration box paradigm. LE rats that prefer passive coping strategies show differential dopaminergic activity in the striatum. The main hypothesis of this study was that chronic variable stress (CVS) would have a higher impact on LE rats.

METHODS

Animals were submitted to a CVS regimen lasting 32 days that was followed by a behavioural test battery. The functional states of their dopamine D(1) and D(2) receptors were measured in the striatum and nucleus accumbens (NAcc). Cerebral oxidative metabolism was assessed via cytochrome c oxidase histochemistry in 65 brain regions.

RESULTS

CVS decreased weight gain, to a higher extent in LE rats, and lowered the sucrose preference after the first week, but habituation to the anhedonic effect had developed by the end of the experiment. CVS did not change the behavioural phenotypes initially assigned. No effect of stress on D(2) receptor function was found. Chronically stressed animals exhibited higher levels of social interaction and D(1) receptor-mediated cAMP accumulation in the NAcc, but not in the striatum. CVS was associated with higher oxidative metabolism levels in the anteroventral thalamus, median raphe nuclei and central periaqueductal grey matter. These changes after stress did not depend upon the exploratory phenotype.

CONCLUSION

This study revealed changes in brain biochemistry after habituation to CVS that might be implicated in successful adaptation to chronic stress.

Mouse strain- and age-dependent effects of binge methamphetamine on dopaminergic signaling

We have shown that a single "binge" dose of methamphetamine (Meth) in mice has long-lasting effects on open-field behavior dependent on mouse strain and age. Here we further investigated the impact of genotype and age on tyrosine hydroxylase (TH) loss and dopamine (DA) metabolism due to a high binge dose of Meth (4 × 5 mg/kg × 2 h × 2 days). Administration of high dose Meth or saline (Sal) to adolescent (PND 40) and adult (PND 80) C57BL/6 (B6), DBA/2 (DBA), and 129S6SvEv/Tac (129) mice was followed by a 1mg/kg Meth or Sal (control) challenge 40 days later. Striatal and prefrontal cortex tissues were collected 1h following the challenge. Meth-pretreated adolescent B6 and DBA mice exhibited losses in striatal DA concentrations; DBA adolescents also showed losses in striatal 3,4-dihydroxyphenylacetic acid (DOPAC) and increased DA turnover. Pre-exposed B6 and 129 adults demonstrated significant decreases in striatal DA, DOPAC, and increased DA turnover; DBA adults showed significant losses in striatal DA and increased DA turnover. 129 and DBA adults exhibited increases and decreases, respectively, in prefrontal cortex DA. Adult pretreated B6 mice produced significant losses in striatal TH. The results again show age and genotype dependent differences in Meth-induced DA alterations.

The mesoaccumbens dopamine in coping with stress

Mesoaccumbens dopamine (DA) is involved in the stress response. Although neural mechanisms involved in stress are of paramount importance for both clinical and preclinical research, the results of studies on the stress response by mesoaccumbens DA have received little attention. Therefore, we aimed to review these results and propose a role for mesoaccumbens DA in coping with stress. The data reviewed support the view that fluctuations of tonic levels characterize the mesoaccumbens DA stress response. Stress-induced increase of tonic DA levels in nucleus accumbens (NAc) supports expression of responses aimed at removing and avoiding the stressor through activation of DA D2 receptors, whereas inhibition of DA is associated with cessation of active defensive responses. In novel unescapable/uncontrollable stressful conditions tonic levels of DA in NAc show an initial increase followed by a decrease below pre-stress levels that lasts as long as the stressful situation. This biphasic response fits with the dynamics of the primary and secondary appraisal of a stressor that cannot be removed, escaped or controlled by the organism. In fact, NAc DA fluctuations are controlled by the medial pre-frontal cortex, which is involved in stress appraisal. We propose that enhanced mesoaccumbens DA supports expression of active coping strategies against an event appraised as a stressor and that inhibition of DA is required for passive coping with stressful situations appraised as unescapable/uncontrollable.

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.

Comparison of toluene-induced locomotor activity in four mouse strains

The mechanisms by which abused inhalants exert their neurobehavioral effects are only partially understood. In research with other drugs of abuse, specific inbred mouse strains have been useful in exploring genetic loci important to variation in behavioral reactions to these drugs. In the present investigation, mice from three inbred strains (Balb/cByj, C57BL/6J and DBA/2J) and one outbred strain (Swiss Webster) were studied for their acute and chronic sensitivity to toluene-induced changes in locomotor activity. Mice were exposed to toluene (0, 100, 2000, 8000, and 10,000 ppm) for 30 min in static exposure chambers equipped with activity monitors. In the acute condition, concentrations of toluene <8000 ppm increased ambulatory distance while the concentrations of > or =8000 ppm induced temporally biphasic effects with initial increases in activity followed by hypoactivity. Between-group differences in absolute locomotor activity levels were evident. The inbred Balb/cByj and DBA/2J strains as well as the outbred Swiss Webster strain of mice showed greater increases in activity after an acute challenge exposure to 2000 ppm than the inbred C57BL/6J strain. The same animals were then exposed 30 min/day to 8000 ppm toluene for 14 consecutive days. Re-determination of responses to 2000-ppm challenge exposures revealed that sensitization developed in locomotor activity and that the DBA/2J strain showed the greatest increase in sensitivity. These baseline differences in acute sensitivity and the differential shifts in sensitivity after repeated exposures among the inbred mouse strains suggest a genetic basis for the behavioral effects to toluene. The results support the notion that like for other drugs of abuse, using various strains of mice may be useful for investigating mechanisms that underlie risk for inhalant abuse.

Voluntary alcohol consumption alters stress-induced changes in dopamine-2 receptor binding in Wistar-Kyoto rat brain

The Wistar-Kyoto (WKY) rat has been proposed as an animal model of depressive behavior and exhibits hyper-responsiveness to stressful stimulation when compared to other rat strains. We have demonstrated that WKY rats consume 200% more alcohol under naïve conditions as compared to their outbred counterparts, Wistar (WIS) rats. The present study was designed to understand the influence of stress and alcohol consumption on central dopamine type-2 (D2) receptor sites in these two behaviorally distinct rat strains. The first part of this study examined the effects of chronic stress on alcohol consumption, while the second part examined the binding of [(125)I]-Iodosulpiride to D2 receptors in control, stressed or stress and alcohol co-treated WKY compared to WIS rats. Exposure to chronic stress led to an increase in the amount of alcohol consumed by both rat strains, with WKY rats consuming significantly more alcohol than WIS rats with or without stress exposure. Quantitative autoradiography experiments showed that chronic stress increased D2 receptor binding in the caudate putamen (CPu), nucleus accumbens (NAc), substantia nigra (SN) and ventral tegmental area (VTA) of WKY rats, and reduced receptor binding in the CPu and SN of WIS rats. Compared to the stressed animals, WKY rats co-treated with stress and alcohol demonstrated a reduction in D2 receptor sites in the cell body regions (SN and VTA), while WIS rats showed no changes in receptor binding. The observed changes in D2 receptor sites may indicate altered DA neurotransmission following stress and alcohol exposure. Since stressed WKY rats consumed more alcohol, it is possible that consumption of alcohol reverses the stress-induced D2 receptor alterations in the cell body regions, suggestive of a self medicating phenotype.

Differential response of central dopaminergic system in acute and chronic unpredictable stress models in rats

We aimed to evaluate the response of dopaminergic system in acute stress (AS) and chronic unpredictable stress (CUS) by measuring dopamine (DA) levels, its receptor densities in the frontal cortex, striatum, hippocampus, amygdala and orbito-frontal cortex regions of rat brain, and investigated the corresponding behavioral locomotor changes. Involvement of D(1) receptor was also examined during AS and CUS using A 68930, a D(1) selective agonist. Rats were exposed to AS (single immobilization for 150 min) and CUS (two different stressors for 7 days). AS significantly decreased the DA levels in the striatum and hippocampus, and A 68930 pretreatment significantly reverted these changes. However, in the frontal cortex significantly increased DA levels were remain unchanged following A 68930. CUS led to a decrease of DA levels in the frontal cortex, striatum and hippocampus, which were normalized by A 68930. Saturation radioligand binding assays revealed a significant decrease in the number of D(1)-like receptors in the frontal cortex during CUS, which were further decreased by A 68930 pretreatment. However, in the striatum and hippocampus, A 68930 pretreatment reduced the CUS induced increase in the number of D(1)-like receptors. No significant changes were observed in the amygdala and orbito-frontal cortex during AS and CUS, while D(2)-like receptors were unchanged in all the brain regions studied. Locomotor activity was significantly decreased in both the stress models, A 68930 pretreatment significantly increased stereotypic counts and horizontal activity. Thus, present investigation provide insights into the differential regional response of dopaminergic system during AS and CUS. Further, neurochemical and behavioral effects of D(1) agonist pretreatment suggest specific modulatory role of D(1) receptor under such stressful episodes.

Differential place and response learning in horses displaying an oral stereotypy

Significant similarities exist between the neural and behavioural features of environmentally and drug-induced stereotypy. For example, exposure to dopamine agonists, such as amphetamine, induces stereotypy and causes alterations in midbrain neurophysiology similar to those observed following chronic stress. An additional behavioural feature of these neural changes in the drug-induced phenotype is an enhanced rate of switching from response-outcome (R-O) to stimulus-response (S-R) learning. The aim of the current experiment was to examine R-O and S-R learning in horses displaying environmentally induced oral stereotypies. This was achieved by employing variations of the place/response paradigm. In Experiment 1, we found that crib-biting horses displayed 'response' learning after 20 learning trials, whereas non-crib-biting controls tended to display 'place' learning throughout the experiment. In Experiment 2, we used a modified version of the procedure, in which the subjects were introduced to the maze from different start points and forced always to turn the same way. We found that the crib-biters acquired the task at a faster rate suggesting again that this group was displaying 'response' learning. Finally, in Experiment 3, we carried out an arena test to ensure that crib-biters were capable of 'place' learning. These results are the first to show that horses displaying an oral stereotypy, a behavioural phenotype previously associated with stress-induced perturbations of the basal ganglia, preferentially use 'response' learning. The findings are discussed in relation to the search for an aetiological model of stereotypy.

Genetic variance contributes to dopamine receptor antagonist-induced inhibition of sucrose intake in inbred and outbred mouse strains

Preference and intake of sucrose varies across inbred and outbred strains of mice. Pharmacological analyses revealed that the greatest sensitivity to naltrexone-induced inhibition of sucrose (10%) intake was observed in C57BL10/J and C57BL/6J strains, whereas 129P3/J, SWR/J and SJL/J strains displayed far less sensitivity to naltrexone-induced inhibition of sucrose intake. Given that dopamine D1 (SCH23390) and D2 (raclopride) receptor antagonism potently reduce sucrose intake in outbred rat and mouse strains, the present study examined the possibility of genetic variance in the dose-dependent (50-1600 nmol/kg) and time-dependent (5-120 min) effects of these antagonists upon sucrose (10%) intake in the eight inbred (BALB/cJ, C3H/HeJ, C57BL/6J, C57BL/10J, DBA/2J, SJL/J, SWR/J and 129P3/J) and one outbred (CD-1) mouse strains previously tested with naltrexone. SCH23390 significantly reduced sucrose intake across all five doses in 129P3/J and SJL/J mice, across four doses in C57BL/6J and BALB/cJ mice, across three doses in DBA/2J, SWR/J, C3H/HeJ and C57BL/10J mice, but only at the two highest doses in CD-1 mice. SCH23390 was 2-3-fold more potent in inhibiting sucrose intake in 129P3/J and SJL/J mice relative to CD-1 mice. In contrast, only the highest equimolar 1600 nmol/kg dose of raclopride significantly reduced sucrose intake in the BALB/cJ, C3H/HeJ, C57BL/6J, C57BL/10J, DBA/2J, SJL/J and 129P3/J, but not the SWR/J and CD-1 strains. The present and previous data demonstrate specific and differential patterns of genetic variability in inhibition of sucrose intake by dopamine and opioid antagonists, suggesting that distinct neurochemical mechanisms control sucrose intake across different mouse strains.

Genetic liability increases propensity to prime-induced reinstatement of conditioned place preference in mice exposed to low cocaine

RATIONALE

Relapse to drug use after periods of forced or self-imposed abstinence is a central problem in the treatment of addiction; therefore, identification of factors modulating the risk to relapse is a relevant goal of preclinical research.

OBJECTIVES

These experiments evaluated the influence of the amount of drug experienced, the duration of drug withdrawal, and individual liability on the propensity to cocaine-induced reinstatement of conditioned place preference (CPP).

MATERIALS AND METHODS

Mice from the inbred strains C57BL/6J and DBA/2J were trained for CPP with a high (20 mg/kg) or low (5 mg/kg) effective dose of cocaine. After CPP testing, all groups underwent extinction. Twenty-four hours after the extinction test, mice were challenged with saline, a cocaine dose unable to induce CPP (2.5 mg/kg) or an intermediate effective dose (10 mg/kg), and tested for CPP reinstatement. Additional groups of mice trained with the low cocaine dose were left undisturbed for 8 days after extinction test (long withdrawal), retested for extinction, and evaluated for prime-induced reinstatement (0, 2.5, 10 mg/kg of cocaine).

RESULTS

Mice trained with the high cocaine dose, but not with the low one, showed prime-induced reinstatement 24 h after the extinction test; DBA/2J mice trained with the low dose showed reinstatement after long withdrawal.

CONCLUSIONS

These results indicate that reinstatement of CPP by cocaine prime depends on the amount of drug experienced and on an interaction between individual liability and duration of drug abstinence and suggest that the risk to relapse into drug seeking is not prevented by moderated drug consumption.

Repeated social defeat stress-induced sensitization to the locomotor activating effects of d-amphetamine: role of individual differences

RATIONALE

In this study, we sought to examine individual differences in stress-induced behavioral sensitization to d-amphetamine after repeated social defeat stress. In an effort to understand what mechanisms underlie stress-induced sensitization to d-amphetamine, we examined striatal gene expression of the dopamine receptor D(2). Additionally, we investigated if repeated social defeat was associated with changes in dendritic spine density in the hippocampus, prefrontal cortex, and nucleus accumbens of rats that exhibit stress-induced sensitization.

METHODS

Male rats were classified into high responders (HR) and low responders (LR) based on their locomotor response to a novel environment. Then, rats were either handled as a control or defeated on four occasions by aggressive rats. Two weeks after the last defeat, animals were challenged with one of three doses of d-amphetamine and their locomotor activity was recorded.

RESULTS

Non-defeated HR rats exhibited higher locomotor activity in response to d-amphetamine when compared to LR non-defeated rats. Fourteen days from the last repeated social defeat, LR rats and HR rats were behaviorally identical in response to acute injections of amphetamine. Furthermore, HR non-defeated rats had less D(2) mRNA expression in the nucleus accumbens core and dorsal striatum than do LR non-defeated rats. However, after repeated social defeat, HR and LR rats had identical D(2) mRNA expression in both the core and dorsal striatum. Finally, there were no changes in dendritic spine density in any of the brain areas examined in LR rats.

CONCLUSION

Repeated social defeat abolishes individual differences in behavioral responses to d-amphetamine which may be due to a down-regulation of striatal dopamine D(2) receptors in LR rats.

Impaired instrumental choice in crib-biting horses (Equus caballus)

Horses displaying an oral stereotypy were tested on an instrumental choice paradigm to examine differences in learning from non-stereotypic counterparts. Stereotypic horses are known to have dysfunction of the dorsomedial striatum, and lesion studies have shown that this region may mediate response-outcome learning. The paradigm was specifically applied in order to examine learning that requires maintenance of response-outcome judgements. The non-stereotypic horses learned, over three sessions, to choose a more immediate reinforcer, whereas the stereotypic horses failed to do so. This suggests an initial behavioural correlate for dorsomedial striatum dysregulation in the stereotypy phenotype.