Chronic stress sensitizes frontal cortex dopamine release in response to a subsequent novel stressor: reversal by naloxone

Can social adversity alter the epigenome, trigger oral disease, and affect future generations?

The nature versus nurture debate has intrigued scientific circles for decades. Although extensive research has established a clear relationship between genetics and disease development, recent evidence has highlighted the insufficiency of attributing adverse health outcomes to genetic factors alone. In fact, it has been suggested that environmental influences, such as socioeconomic position (SEP), may play a much larger role in the development of disease than previously thought, with extensive research suggesting that low SEP is associated with adverse health conditions. In relation to oral health, a higher prevalence of caries (tooth decay) exists among those of low SEP. Although little is known about the biological mechanisms underlying this relationship, epigenetic modifications resulting from environmental influences have been suggested to play an important role. This review explores the intersection of health inequalities and epigenetics, the role of early-life social adversity and its long-term epigenetic impacts, and how those living within the lower hierarchies of the socioeconomic pyramid are indeed at higher risk of developing diseases, particularly in relation to oral health. A deeper understanding of these mechanisms could lead to the development of targeted interventions for individuals of low SEP to improve oral health or identify those who are at higher risk of developing oral disease.

Brain Monoamine Dysfunction in Response to Predator Scent Stress Accompanies Stress-Susceptibility in Female Rats

Post-traumatic stress disorder (PTSD) is prevalent in women; however, preclinical research on PTSD has predominantly been conducted in male animals. Using a predator scent stress (PSS) rodent model of PTSD, we sought to determine if stress-susceptible female rats show altered monoamine concentrations in brain regions associated with PTSD: the medial prefrontal cortex (mPFC), nucleus accumbens (NAc), and dorsal (dHIPP) and ventral (vHIPP) hippocampus. Female Sprague-Dawley rats were exposed to a single, 10-min PSS exposure and tested for persistent anhedonia, fear, and anxiety-like behavior over four weeks. Rats were phenotyped as stress-Susceptible based on sucrose consumption in the sucrose preference task and time spent in the open arms of the elevated plus maze. Brain tissue was collected, and norepinephrine, dopamine, serotonin, and their metabolites were quantified using high-performance liquid chromatography. Stress-susceptibility in female rats was associated with increased dopamine and serotonin turnover in the mPFC. Susceptibility was also associated with elevated dopamine turnover in the NAc and increased norepinephrine in the vHIPP. Our findings suggest that stress-susceptibility after a single stress exposure is associated with long-term effects on monoamine function in female rats. These data suggest interventions that decrease monoamine turnover, such as MAOIs, may be effective in the treatment of PTSD in women.

Offensive Behavior, Striatal Glutamate Metabolites, and Limbic-Hypothalamic-Pituitary-Adrenal Responses to Stress in Chronic Anxiety

Variations in anxiety-related behavior are associated with individual allostatic set-points in chronically stressed rats. Actively offensive rats with the externalizing indicators of sniffling and climbing the stimulus and material tearing during 10 days of predator scent stress had reduced plasma corticosterone, increased striatal glutamate metabolites, and increased adrenal 11-dehydrocorticosterone content compared to passively defensive rats with the internalizing indicators of freezing and grooming, as well as to controls without any behavioral changes. These findings suggest that rats that display active offensive activity in response to stress develop anxiety associated with decreased allostatic set-points and increased resistance to stress.

Investigating the Role of Prolactin as a Potential Biomarker of Stress in Castrated Male Domestic Dogs

Prolactin has been recently regarded as a potential biomarker of both acute and chronic stress in several species. Since only few studies until now have focussed on domestic dogs, this study was aimed at evaluating whether prolactin, cortisol and stress behaviour correlated with each other in sheltered dogs. Both cortisol and prolactin analysis were performed in serum samples through a hormone-specific ELISA kit. For each dog, a stress score was calculated by summing the number of occurrences of stress-related behaviours. The presence/absence of fear during the time spent in the collection room was also scored for each individual. Results revealed a weak negative correlation between cortisol and prolactin levels. Neither of the hormones was correlated with the stress score, nor did their values seem to be influenced by showing fear in the collection room. The weak negative correlation found between cortisol and prolactin values agrees with results obtained in other studies, indicating that prolactin response might be an alternative to cortisol response. This, together with the high serum prolactin levels compared to those reported by other authors for healthy domestic dogs, may indicate that prolactin might be a good biomarker of chronic stress, and although further studies are needed to better understand the potential role of prolactin in the evaluation of canine welfare.

Bidirectional Associations Between Stress and Reward Processing in Children and Adolescents: A Longitudinal Neuroimaging Study

BACKGROUND

Aberrations in both neural reward processing and stress reactivity are associated with increased risk for mental illness; yet, how these two factors relate to each other remains unclear. Several studies suggest that stress exposure impacts reward function, thus increasing risk for psychopathology. However, the alternative hypothesis, in which reward dysfunction impacts stress reactivity, has been rarely examined. The current study aimed to test both hypotheses using a longitudinal design.

METHODS

Participants were 38 children (23 girls; 61%) from a prospective cohort study. A standard stress-exposure measure was collected at 7 years of age. Children performed a well-validated imaging reward paradigm at age 10, and a standardized acute psychological stress laboratory protocol was administered both at age 10 and at age 13. Structural equation modeling was used to examine bidirectional associations between stress and neural response to reward anticipation.

RESULTS

Higher exposure to stressful life events at age 7 predicted lower neural response to reward anticipation in regions of the basal ganglia at age 10, which included ventral caudate, nucleus accumbens, putamen, and globus pallidus. Lower response to reward anticipation in medial prefrontal and anterior cingulate cortex predicted higher stress reactivity at age 13.

CONCLUSIONS

Our findings provide support for bidirectional associations between stress and reward processing, in that stress may impact reward anticipation, but also in that reduced reward anticipation may increase susceptibility to stress.

Cued for risk: Evidence for an incentive sensitization framework to explain the interplay between stress and anxiety, substance abuse, and reward uncertainty in disordered gambling behavior

Gambling disorder is an impairing condition confounded by psychiatric co-morbidity, particularly with substance use and anxiety disorders. Yet, our knowledge of the mechanisms that cause these disorders to coalesce remains limited. The Incentive Sensitization Theory suggests that sensitization of neural "wanting" pathways, which attribute incentive salience to rewards and their cues, is responsible for the excessive desire for drugs and cue-triggered craving. The resulting hyper-reactivity of the "wanting' system is believed to heavily influence compulsive drug use and relapse. Notably, evidence for sensitization of the mesolimbic dopamine pathway has been seen across gambling and substance use, as well as anxiety and stress-related pathology, with stress playing a major role in relapse. Together, this evidence highlights a phenomenon known as cross-sensitization, whereby sensitization to stress, drugs, or gambling behaviors enhance the sensitivity and dopaminergic response to any of those stimuli. Here, we review the literature on how cue attraction and reward uncertainty may underlie gambling pathology, and examine how this framework may advance our understanding of co-mordidity with substance-use disorders (e.g., alcohol, nicotine) and anxiety disorders. We argue that reward uncertainty, as seen in slot machines and games of chance, increases dopaminergic activity in the mesolimbic pathway and enhances the incentive value of reward cues. We propose that incentive sensitization by reward uncertainty may interact with and predispose individuals to drug abuse and stress, creating a mechanism through which co-mordidity of these disorders may emerge.

Understanding the role of dopamine in conditioned and unconditioned fear

Pharmacological and molecular imaging studies in anxiety disorders have primarily focused on the serotonin system. In the meantime, dopamine has been known as the neurotransmitter of reward for 60 years, particularly for its action in the nervous terminals of the mesocorticolimbic system. Interest in the mediation by dopamine of the well-known brain aversion system has grown recently, particularly given recent evidence obtained on the role of D2 dopamine receptors in unconditioned fear. However, it has been established that excitation of the mesocorticolimbic pathway, originating from dopaminergic (DA) neurons from the ventral tegmental area (VTA), is relevant for the development of anxiety. Among the forebrain regions innervated by this pathway, the amygdala is an essential component of the neural circuitry of conditioned fear. Current findings indicate that the dopamine D2 receptor-signaling pathway connecting the VTA to the basolateral amygdala modulates fear and anxiety, whereas neural circuits in the midbrain tectum underlie the expression of innate fear. The A13 nucleus of the zona incerta is proposed as the origin of these DA neurons projecting to caudal structures of the brain aversion system. In this article we review data obtained in studies showing that DA receptor-mediated mechanisms on ascending or descending DA pathways play opposing roles in fear/anxiety processes. Dopamine appears to mediate conditioned fear by acting at rostral levels of the brain and regulate unconditioned fear at the midbrain 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.

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.

Ventral tegmental area dopamine revisited: effects of acute and repeated stress

Aversive events rapidly and potently excite certain dopamine neurons in the ventral tegmental area (VTA), promoting phasic increases in the medial prefrontal cortex and nucleus accumbens. This is in apparent contradiction to a wealth of literature demonstrating that most VTA dopamine neurons are strongly activated by reward and reward-predictive cues while inhibited by aversive stimuli. How can these divergent processes both be mediated by VTA dopamine neurons? The answer may lie within the functional and anatomical heterogeneity of the VTA. We focus on VTA heterogeneity in anatomy, neurochemistry, electrophysiology, and afferent/efferent connectivity. Second, recent evidence for a critical role of VTA dopamine neurons in response to both acute and repeated stress will be discussed. Understanding which dopamine neurons are activated by stress, the neural mechanisms driving the activation, and where these neurons project will provide valuable insight into how stress can promote psychiatric disorders associated with the dopamine system, such as addiction and depression.

Perceived life stress exposure modulates reward-related medial prefrontal cortex responses to acute stress in depression

INTRODUCTION

Major depressive disorder (MDD) is often precipitated by life stress and growing evidence suggests that stress-induced alterations in reward processing may contribute to such risk. However, no human imaging studies have examined how recent life stress exposure modulates the neural systems that underlie reward processing in depressed and healthy individuals.

METHODS

In this proof-of-concept study, 12 MDD and 10 psychiatrically healthy individuals were interviewed using the Life Events and Difficulties Schedule (LEDS) to assess their perceived levels of recent acute and chronic life stress exposure. Additionally, each participant performed a monetary incentive delay task under baseline (no-stress) and stress (social-evaluative) conditions during functional MRI.

RESULTS

Across groups, medial prefrontal cortex (mPFC) activation to reward feedback was greater during acute stress versus no-stress conditions in individuals with greater perceived stressor severity. Under acute stress, depressed individuals showed a positive correlation between perceived stressor severity levels and reward-related mPFC activation (r=0.79, p=0.004), whereas no effect was found in healthy controls. Moreover, for depressed (but not healthy) individuals, the correlations between the stress (r=0.79) and no-stress (r=-0.48) conditions were significantly different. Finally, relative to controls, depressed participants showed significantly reduced mPFC gray matter, but functional findings remained robust while accounting for structural differences.

LIMITATION

Small sample size, which warrants replication.

CONCLUSION

Depressed individuals experiencing greater recent life stress recruited the mPFC more under stress when processing rewards. Our results represent an initial step toward elucidating mechanisms underlying stress sensitization and recurrence in depression.

Neurochemical and behavioral effects of chronic unpredictable stress

Chronic stress can influence behaviors associated with medial prefrontal cortex (mPFC) function, such as cognition and emotion regulation. Dopamine in the mPFC is responsive to stress and modulates its behavioral effects. The current study tested whether exposure to 10 days of chronic unpredictable stress (CUS) altered the effects of acute elevation stress on dopamine release in the mPFC and on spatial recognition memory. Male rats previously exposed to CUS or nonstressed controls were tested behaviorally, underwent microdialysis to assess mPFC dopamine levels or underwent blood sampling for corticosterone analysis. Dopamine in the mPFC significantly increased in both groups during acute elevation stress compared with baseline levels, but the level was attenuated in CUS rats compared with controls. Control rats exposed to elevation stress immediately before the T-maze test showed impaired performance, whereas CUS rats did not. No group differences were observed in general motor activity or plasma corticosterone levels following elevation stress. The present results indicate that prior exposure to this CUS procedure reduced dopamine release in the mPFC during acute elevation stress and prevented the impairment of performance on a spatial recognition test following an acute stressor. These findings may contribute to an understanding of the complex behavioral consequences of stress.

Depression, stress, and anhedonia: toward a synthesis and integrated model

Depression is a significant public health problem, but its etiology and pathophysiology remain poorly understood. Such incomplete understanding likely arises from the fact that depression encompasses a heterogeneous set of disorders. To overcome these limitations, renewed interest in intermediate phenotypes (endophenotypes) has resurfaced, and anhedonia has emerged as one of the most promising endophenotypes of depression. Here, a heuristic model is presented postulating that anhedonia arises from dysfunctional interactions between stress and brain reward systems. To this end, we review and integrate three bodies of independent literature investigating the role of (a) anhedonia, (b) dopamine, and (c) stress in depression. In a fourth section, we summarize animal data indicating that stress negatively affects mesocorticolimbic dopaminergic pathways critically implicated in incentive motivation and reinforcement learning. In the last section, we provide a synthesis of these four literatures, present initial evidence consistent with our model, and discuss directions for future research.

Repeated exposure to conditioned fear stress increases anxiety and delays sleep recovery following exposure to an acute traumatic stressor

Repeated stressor exposure can sensitize physiological responses to novel stressors and facilitate the development of stress-related psychiatric disorders including anxiety. Disruptions in diurnal rhythms of sleep-wake behavior accompany stress-related psychiatric disorders and could contribute to their development. Complex stressors that include fear-eliciting stimuli can be a component of repeated stress experienced by human beings, but whether exposure to repeated fear can prime the development of anxiety and sleep disturbances is unknown. In the current study, adult male F344 rats were exposed to either control conditions or repeated contextual fear conditioning for 22 days followed by exposure to no, mild (10), or severe (100) acute uncontrollable tail shock stress. Exposure to acute stress produced anxiety-like behavior as measured by a reduction in juvenile social exploration and exaggerated shock-elicited freezing in a novel context. Prior exposure to repeated fear enhanced anxiety-like behavior as measured by shock-elicited freezing, but did not alter social exploratory behavior. The potentiation of anxiety produced by prior repeated fear was temporary; exaggerated fear was present 1 day but not 4 days following acute stress. Interestingly, exposure to acute stress reduced rapid eye movement (REM) and non-REM (NREM) sleep during the hours immediately following acute stress. This initial reduction in sleep was followed by robust REM rebound and diurnal rhythm flattening of sleep/wake behavior. Prior repeated fear extended the acute stress-induced REM and NREM sleep loss, impaired REM rebound, and prolonged the flattening of the diurnal rhythm of NREM sleep following acute stressor exposure. These data suggest that impaired recovery of sleep/wake behavior following acute stress could contribute to the mechanisms by which a history of prior repeated stress increases vulnerability to subsequent novel stressors and stress-related disorders.

Central, but not basolateral, amygdala involvement in the anxiolytic-like effects of midazolam in rats in the elevated plus maze

The role of the amygdala in the mediation of fear and anxiety has been extensively investigated. However, how the amygdala functions during the organization of the anxiety-like behaviors generated in the elevated plus maze (EPM) is still under investigation. The basolateral (BLA) and the central (CeA) nuclei are the main input and output stations of the amygdala. In the present study, we ethopharmacologically analyzed the behavior of rats subjected to the EPM and the tissue content of the monoamines dopamine (DA) and serotonin (5-HT) and their metabolites in the nucleus accumbens (NAc), dorsal hippocampus (DH), and dorsal striatum (DS) of animals injected with saline or midazolam (20 and 30 nmol/0.2 µL) into the BLA or CeA. Injections of midazolam into the CeA, but not BLA, caused clear anxiolytic-like effects in the EPM. These treatments did not cause significant changes in 5-HT or DA contents in the NAc, DH, or DS of animals tested in the EPM. The data suggest that the anxiolytic-like effects of midazolam in the EPM also appear to rely on GABA-benzodiazepine mechanisms in the CeA, but not BLA, and do not appear to depend on 5-HT and DA mechanisms prevalent in limbic structures.

Blunted accumbal dopamine response to cocaine following chronic social stress in female rats: exploring a link between depression and drug abuse

RATIONALE

Women have twice the risk as men to develop depression. Approximately, 24% of major depression disorder cases have comorbid disorders with substance abuse. Several central systems, including dopaminergic and serotonergic pathways, are thought to be involved in such comorbidity.

OBJECTIVES

The present study established a chronic social stress model in female rats, which produces some cardinal features of depressive-like symptoms. Further, we examined the effects of acute cocaine on dopamine (DA) and serotonin (5-HT) in the nucleus accumbens (NAc) using this model.

METHODS

Female Long-Evans rats confronted a nursing dam in its home cage for 30 min twice daily for 21 days. The non-stressed control group was handled daily throughout the experiment. During the 21 days of stress, behaviors during confrontations, weight, preference for saccharin, and estrous cycles were measured. Ten days after the last confrontation, the experimental rat was challenged with 10 mg/kg of cocaine, and levels of DA and 5-HT in the NAc were measured using in vivo microdialysis.

RESULTS

During the course of daily confrontations for 21 days, the experimental females significantly increased the duration of immobility, reduced weight gain and the preference for saccharin, and disrupted estrous cycles during the stress. Chronic social stress significantly attenuated cocaine-induced DA levels, and to some extent, attenuated a percent change of 5-HT compared to the non-stressed control group.

CONCLUSIONS

Chronic social defeat stress for 21 days induced physiological and behavioral depression-relevant deficits and blunted response of dopaminergic and to some extent, serotonergic neurons to cocaine challenge in females.

Effect of a synthetic appeasing pheromone on behavioral, neuroendocrine, immune, and acute-phase perioperative stress responses in dogs

OBJECTIVE

To study the effects of a synthetic, dog-appeasing pheromone (sDAP) on the behavioral, neuroendocrine, immune, and acute-phase perioperative stress responses in dogs undergoing elective orchiectomy or ovariohysterectomy.

DESIGN

Randomized, controlled clinical trial.

ANIMALS

46 dogs housed in animal shelters and undergoing elective orchiectomy or ovariohysterectomy.

PROCEDURES

Intensive care unit cages were sprayed with sDAP solution or sham treated with the carrier used in the solution 20 minutes prior to use. Dogs (n = 24 and 22 in the sDAP and sham treatment exposure groups, respectively) were placed in treated cages for 30 minutes before and after surgery. Indicators of stress (ie, alterations in behavioral, neuroendocrine, immune, and acute-phase responses) were evaluated perioperatively. Behavioral response variables, salivary cortisol concentration, WBC count, and serum concentrations of glucose, prolactin, haptoglobin, and C-reactive protein were analyzed.

RESULTS

Behavioral response variables and serum prolactin concentration were influenced by sDAP exposure. Dogs exposed to sDAP were more likely to have alertness and visual exploration behaviors after surgery than were dogs exposed to sham treatment. Decreases in serum prolactin concentrations in response to perioperative stress were significantly smaller in dogs exposed to sDAP, compared with findings in dogs exposed to the sham treatment. Variables examined to evaluate the hypothalamic-pituitary-adrenal axis, immune system, and acute-phase responses were unaffected by treatment.

CONCLUSIONS AND CLINICAL RELEVANCE

sDAP appeared to affect behavioral and neuroendocrine perioperative stress responses by modification of lactotropic axis activity. Use of sDAP in a clinical setting may improve the recovery and welfare of dogs undergoing surgery.

The locus coeruleus: A key nucleus where stress and opioids intersect to mediate vulnerability to opiate abuse

The interaction between the stress axis and endogenous opioid systems has gained substantial clinical attention as it is increasingly recognized that stress predisposes to opiate abuse. For example, stress has been implicated as a risk factor in vulnerability to the initiation and maintenance of opiate abuse and is thought to play an important role in relapse in subjects with a history of abuse. Numerous reports indicating that stress alters individual sensitivity to opiates suggest that prior stress can influence the pharmacodynamics of opiates that are used in clinical settings. Conversely, the effects of opiates on different components of the stress axis can impact on individual responsivity to stressors and potentially predispose individuals to stress-related psychiatric disorders. One site at which opiates and stress substrates may interact to have global effects on behavior is within the locus coeruleus (LC), the major brain norepinephrine (NE)-containing nucleus. This review summarizes our current knowledge regarding the anatomical and neurochemical afferent regulation of the LC. It then presents physiological studies demonstrating opposing interactions between opioids and stress-related neuropeptides in the LC and summarizes results showing that chronic morphine exposure sensitizes the LC-NE system to corticotropin releasing factor and stress. Finally, new evidence for novel presynaptic actions of kappa-opioids on LC afferents is provided that adds another dimension to our model of how this central NE system is co-regulated by opioids and stress-related peptides.

Perioperative stress response in dogs undergoing elective surgery: variations in behavioural, neuroendocrine, immune and acute phase responses

The aim of this trial was to describe the behavioural, neuroendocrine, immune and acute phase stress responses in dogs undergoing elective surgery in normal, clinical practice conditions. Sixteen dogs were submitted to elective orchiectomy or ovariohysterectomy using a standardised surgical protocol. Each animal was confined to the Intensive Care Unit during pre- and post-surgery and perioperative behavioural, neuroendocrine, immune and acute phase responses were studied. Behavioural categories, cortisol, prolactin, white blood cell, C-reactive protein and haptoglobin variation were evaluated. Values at different times were compared with basal values shown by the dog in its usual environment. Communicative and explorative behaviours showed high occurrence pre-surgery and were inhibited post-surgery. Decreases in post-surgery activity, interactive behaviours and changes in waking/sleeping patterns were observed. The most sensitive marker of psychological stress, cortisol, in comparison with basal values, showed a significant increase both during pre- and post-surgery confinement in the ICU cage. Prolactin values were characterised by a significant decrease early into the post-surgery period. The immune response was characterised by long-term neutrophilia and monocytosis, but by short-term lymphopaenia and eosinopaenia, limited to the early post-operative period. With regard to the acute phase response, both C-reactive protein and haptoglobin showed a long-term increase, post-surgery. Changes in behavioural, haematological and biochemical markers showed that perioperative stress represents a major challenge to dog welfare.

Long-lasting alteration in mesocorticolimbic structures after repeated social defeat stress in rats: time course of mu-opioid receptor mRNA and FosB/DeltaFosB immunoreactivity

Social defeat stress is a salient stressor that induces neuroadaptive changes in the mesocorticolimbic dopaminergic system. Substantial evidence indicates that mu-opioid receptors (MORs) modulate dopamine transmission in the ventral tegmental area (VTA). FosB/DeltaFosB protein accumulation in dopaminergic projections during repeated treatments is thought to be involved in long-term neuroplasticity. In this study we characterize the magnitude and time-course of MOR mRNA expression and FosB/DeltaFosB immunoreactivity in mesocorticolimbic regions following repeated social defeat stress. Effects of brief repeated social defeat stress or control handling procedures were studied in rats either 2 h after the last exposure, or 3, 7, 14, 21 and 28 days later. We found that MOR mRNA expression in the VTA doubled after the last stress compared with handling, and remained 30-70% higher until day 21. The number of FosB/DeltaFosB-labeled neurons in regions of the frontal cortex, nucleus accumbens (NAc) shell and core, and in the medial, central and basolateral amygdala increased significantly immediately after the last stress episode, and remained enhanced for 21 days. Another group of rats received bilateral intra-VTA infusion of the MOR agonist, DAMGO, 7 days after the last stress. Prior social defeat stress augmented DAMGO-induced Fos expression in the NAc shell, suggesting that Fos expression in this region might be the direct result of MOR activity in the VTA. Social defeat stress leads to an increased capacity for MOR activation in the VTA, which may be relevant to enduring FosB/DeltaFosB expression in mesocorticolimbic areas and to the behaviorally sensitized response to psychostimulant drugs.

Sex differences in the effects of two stress paradigms on dopaminergic neurotransmission

Sex differences in behavioral and neurobiological responses to stress are considered to modulate the prevalence of some psychiatric disorders, including major depression. In the present study, we compared dopaminergic neurotransmission and behavior in response to two different stress paradigms, the Forced Swim Test (FST) and the Chronic Mild Stress (CMS). Male and female rats were subjected to one session of swim stress for two consecutive days (FST) or to a variety of mild stressors alternating for six weeks (CMS). Subsequently, the tissue levels of dopamine (DA) and its metabolites (HVA and DOPAC) in the hippocampus, the hypothalamus, the prefrontal cortex and the striatum were measured using high-performance liquid chromatography (HPLC). The ratios HVA/DA and DOPAC/DA were also calculated as indices of the dopaminergic activity. Results from the FST determined that males exhibited lower immobility, higher climbing duration and increased dopaminergic activity in the prefrontal cortex and the hippocampus compared to females. CMS induced alterations in sucrose intake in both sexes, while it only decreased dopaminergic activity in the prefrontal cortex of females. These findings show that FST and CMS have different effects on the dopaminergic activity of discrete brain regions depending on the sex of the animal. These data support the growing evidence that females display a differential response and adaptation to stress than males.

5-HT2- and D1-mechanisms of the basolateral nucleus of the amygdala enhance conditioned fear and impair unconditioned fear

The inferior colliculus (IC) is involved in processing of auditory information, but also integrates acoustic information of aversive nature. In fact, chemical stimulation of the IC with semicarbazide (SMC) - an inhibitor of the GABA synthesizing enzyme glutamic acid decarboxylase - has been found to cause defensive behavior in an open-field test and functions as an unconditioned stimulus in the place conditioned aversion test (PCA). A question has arisen regarding whether the basolateral nucleus of the amygdala (BLA) is involved in the acquisition of the aversive information ascending from the IC and whether dopaminergic and serotoninergic mechanisms of the BLA regulate this process. Recent evidence has shown that inactivation of the BLA with muscimol inhibits the PCA and causes an increase in the aversiveness of the chemical stimulation of the IC. Based on this, we examined the effects of ketanserin and SCH-23390, antagonists of the 5HT(2) and D(1) receptors, respectively, on the conditioned and unconditioned fear elicited by IC stimulation with SMC. The results obtained confirm the crucial role of 5-HT(2)- and D(1)-mechanisms of the BLA on conditioned fear in that ketanserin and SCH-23390 injections into the BLA caused a reduction in the PCA. On the other hand, ketanserin and SCH-23390 injections into the BLA enhanced the aversiveness of the IC injections of SMC. These findings suggest that while 5-HT(2) and DA(1) mechanisms in the BLA appear to facilitate the conditioned fear they inhibit the unconditioned fear triggered by IC activation.

From Malthus to motive: how the HPA axis engineers the phenotype, yoking needs to wants

The hypothalamo-pituitary-adrenal (HPA) axis is the critical mediator of the vertebrate stress response system, responding to environmental stressors by maintaining internal homeostasis and coupling the needs of the body to the wants of the mind. The HPA axis has numerous complex drivers and highly flexible operating characterisitics. Major drivers include two circadian drivers, two extra-hypothalamic networks controlling top-down (psychogenic) and bottom-up (systemic) threats, and two intra-hypothalamic networks coordinating behavioral, autonomic, and neuroendocrine outflows. These various networks jointly and flexibly control HPA axis output of periodic (oscillatory) functions and a range of adventitious systemic or psychological threats, including predictable daily cycles of energy flow, actual metabolic deficits over many time scales, predicted metabolic deficits, and the state-dependent management of post-prandial responses to feeding. Evidence is provided that reparation of metabolic derangement by either food or glucocorticoids results in a metabolic signal that inhibits HPA activity. In short, the HPA axis is intimately involved in managing and remodeling peripheral energy fluxes, which appear to provide an unidentified metabolic inhibitory feedback signal to the HPA axis via glucocorticoids. In a complementary and perhaps a less appreciated role, adrenocortical hormones also act on brain to provide not only feedback, but feedforward control over the HPA axis itself and its various drivers, as well as coordinating behavioral and autonomic outflows, and mounting central incentive and memorial networks that are adaptive in both appetitive and aversive motivational modes. By centrally remodeling the phenotype, the HPA axis provides ballistic and predictive control over motor outflows relevant to the type of stressor. Evidence is examined concerning the global hypothesis that the HPA axis comprehensively induces integrative phenotypic plasticity, thus remodeling the body and its governor, the brain, to yoke the needs of the body to the wants of the mind. Adverse side effects of this yoking under conditions of glucocorticoid excess are discussed.

Changes in the biogenic amine content of the prefrontal cortex, amygdala, dorsal hippocampus, and nucleus accumbens of rats submitted to single and repeated sessions of the elevated plus-maze test

It has been demonstrated that exposure to a variety of stressful experiences enhances fearful reactions when behavior is tested in current animal models of anxiety. Until now, no study has examined the neurochemical changes during the test and retest sessions of rats submitted to the elevated plus maze (EPM). The present study uses a new approach (HPLC) by looking at the changes in dopamine and serotonin levels in the prefrontal cortex, amygdala, dorsal hippocampus, and nucleus accumbens in animals upon single or double exposure to the EPM (one-trial tolerance). The study involved two experiments: i) saline or midazolam (0.5 mg/kg) before the first trial, and ii) saline or midazolam before the second trial. For the biochemical analysis a control group injected with saline and not tested in the EPM was included. Stressful stimuli in the EPM were able to elicit one-trial tolerance to midazolam on re-exposure (61.01%). Significant decreases in serotonin contents occurred in the prefrontal cortex (38.74%), amygdala (78.96%), dorsal hippocampus (70.33%), and nucleus accumbens (73.58%) of the animals tested in the EPM (P < 0.05 in all cases in relation to controls not exposed to the EPM). A significant decrease in dopamine content was also observed in the amygdala (54.74%, P < 0.05). These changes were maintained across trials. There was no change in the turnover rates of these monoamines. We suggest that exposure to the EPM causes reduced monoaminergic neurotransmission activity in limbic structures, which appears to underlie the "one-trial tolerance" phenomenon.

Chronic mild stress (CMS) revisited: consistency and behavioural-neurobiological concordance in the effects of CMS

The chronic mild stress (CMS) model of depression has high validity but has in the past been criticized for being difficult to replicate. However, a large number of recent publications have confirmed that CMS causes behavioural changes in rodents that parallel symptoms of depression. This review summarizes studies from over sixty independent research groups that have reported decreases in reactivity to rewards, and a variety of other depression-like behaviours, in rats or mice, following exposure to CMS. Together, these changes are referred to as a 'depressive' behavioural profile. Almost every study that has examined the effects of chronic antidepressant treatment in these procedures has reported that antidepressants were effective in reversing or preventing these 'depressive' behavioural changes. (The single exception is a study in which the duration of treatment was too brief to constitute an adequate trial.) There are also a handful of reports of CMS causing significant effects in the opposite direction, termed here an 'anomalous' behavioural profile. There are six neurobiological parameters that have been studied in both 'anhedonic' and 'anomalous' animals: psychostimulant and place-conditioning effects of dopamine agonists; dopamine D2 receptor number and message; inhibition of dopamine turnover by quinpirole, and beta-adrenergic receptor binding. On all six measures, CMS caused opposite effects in animals displaying 'depressive' and 'anomalous' profiles. Thus, there is overwhelming evidence that under appropriate experimental conditions, CMS can cause antidepressant-reversible depressive-like effects in rodents; however, the 'anomalous' profile that is occasionally reported appears to be a genuine phenomenon, and these two sets of behavioural effects appear to be associated with opposite patterns of neurobiological changes.

Novel stressors affected catecholamine stores in socially isolated normotensive and spontaneously hypertensive rats

Catecholamines in some central (hypothalamus and hippocampus) and peripheral tissues (adrenal glands and heart auricles) of long-term socially isolated normotensive and spontaneously hypertensive rats exposed to novel immobilization stress were determined by a simultaneous single isotope radioenzymatic assay. Long-term isolation (21 days) produced depletion of hypothalamic norepinephrine (NE) stores and hippocampal dopamine (DA) stores in both normotensive and spontaneously hypertensive rats. Acute immobilization stress (2 h) significantly decreased NE and DA stores in hypothalamus and hippocampus of naive normotensive and spontaneously hypertensive rats controls. However, novel immobilization stress applied to normotensive rats previously subjected to long-term isolation produced no changes in catecholamine levels in hypothalamus, while resulting in somewhat higher depletion of NE stores in hypothalamus of spontaneously hypertensive rats treated in the same way. Novel immobilization stress decreased NE and DA stores in hippocampus of normotensive but was without effect on NE and DA stores of spontaneously hypertensive rats. Social isolation did not affect catecholamine stores in peripheral tissues but novel immobilization stress produced a significant decrease in catecholamine content. The results suggest that some central and peripherals tissues of spontaneously hypertensive rats and normotensive rats differ with regard to catecholamine content and that there are certain differences in their responsiveness to stress.

Increases in extracellular levels of 5-HT and dopamine in the basolateral, but not in the central, nucleus of amygdala induced by aversive stimulation of the inferior colliculus

Consistent evidence has shown that dopamine release in the prefrontal cortex is increased by electrical stimulation of the inferior colliculus (IC) as unconditioned stimulus. Recent reports have also demonstrated that inactivation of the basolateral nucleus of the amygdala (BLA) with muscimol enhances the behavioural consequences of the aversive stimulation of the IC and reduces the dopamine release in the prefrontal cortex. Moreover, neurotoxic lesions of the BLA enhance whereas those of the central nucleus of the amygdala (CeA) reduce the aversiveness of the electrical stimulation of the IC. Based on these findings the present study examined the effects of the electrical stimulation of the IC on the extracellular levels of serotonin and dopamine in the BLA and CeA. To this end, rats implanted with a stimulation electrode in the IC also bore a microdialysis probe in the BLA or CeA for determination of the release of dopamine and serotonin. IC electrical stimulation at the freezing and escape thresholds increased the levels of serotonin ( approximately 70%) and dopamine ( approximately 60%) in the BLA related to the basal values. Similarly, the metabolites DOPAC and 5-HIAA increased in a parallel fashion in BLA. No significant changes could be detected in these biogenic amines and metabolites in CeA following IC aversive stimulation. These findings point to a differential role of serotonergic and dopaminergic mechanisms of the BLA and CeA in the setting up of adaptive responses to fear states generated at the inferior colliculus level.

Dopaminergic mechanisms in the conditioned and unconditioned fear as assessed by the two-way avoidance and light switch-off tests

The involvement of dopaminergic mechanisms in fear and anxiety is still unclear. Behavioral studies aimed to disclose the involvement of dopamine in anxiety have reported anxiolytic-like, anxiogenic-like and lack of effects with the use of dopaminergic agonists and antagonists in animal models of anxiety. This work was an attempt to contribute to this field by providing evidence that these discrepancies may be due to the kind of aversive situation the animals experience in these models. The present study examined the effects of a dopaminergic agonist apomorphine, a dopaminergic D(1) antagonist SCH 23390 and a D(2) receptor antagonist sulpiride on the two-way avoidance response test (CAR) and on the switch-off responses to light (SOR). In both tests, learning was assessed by the performance of the animals across four blocks of 10 trials in which light was paired to footshocks (CAR) or only light was presented to the animals (SOR). The obtained data show that rats learn to make a shuttling response to avoid the shock in the CAR test and maintain a regular pace of switch-off responses in the SOR. While sulpiride and SCH 23390 administrations prevented learning of the avoidance responses, apomorphine injections produced a dose-dependent enhancement in the conditioned learning in the CAR test. The number of escape responses was unchanged by these drugs. In the light-induced switch-off test, apomorphine reduced the number of switch-off responses whereas sulpiride increased these responses. These findings suggest that the involvement of dopaminergic mechanisms in threatening situations depends on the nature of the aversive stimulus. Activation of D(1) and D(2) receptors seems to be implicated in the heightened aversiveness to conditioned stressful situations, as assessed by the CAR test. Thus, blockade of D(1) and D(2) receptors may be necessary for attenuating the aversiveness triggered by these conditioned fear stimuli. In contrast, mechanisms mediated by D(2) receptors seem to be involved in the setting up of adaptive responses to innate fear reactions. Therefore, the signal of the modulatory dopaminergic mechanisms on defensive behavior will depend on the type of emotional stimuli triggering the coping reaction.

Repeated swim stress leads to down-regulation of vesicular monoamine transporter 2 in rat brain nucleus accumbens and striatum

We assessed the impact of chronic swim stress in rats (daily for 3 weeks) on vesicular monoamine transporter 2 (VMAT2) in the nucleus accumbens and striatum. Exposure to repeated swim stress resulted in significant reduction in VMAT2 density in nucleus accumbens (20%, p<0.01) and striatum subregions (21-38%, p<0.001). The down-regulation of VMAT2 in this dopaminergic regions may serve as an adaptatory mechanism in the response to prolonged stress, and may be relevant to chronic stress-induced depression.

Aversive stimulation of the inferior colliculus changes dopamine and serotonin extracellular levels in the frontal cortex: modulation by the basolateral nucleus of amygdala

We have shown that stimulation of the neural substrates in the inferior colliculus (IC) causes a significant increase in the extracellular levels of dopamine (DA) in frontal cortex (FC). Also, it has been reported that the basolateral complex of the amygdala (BLA) serves as a filter for unconditioned and conditioned aversive information that ascend to higher structures from the brainstem. Linking these two kinds of information, this work examines whether inactivation of BLA interferes with the activation of cortical dopaminergic outputs produced by aversive stimulation of the IC. To this end, rats were implanted with an electrode in the IC for the determination of the threshold of escape responses. Each rat also bore a cannula implanted in the BLA for injections of lidocaine (10 microg/0.5 microL), muscimol (0.5 microg/0.5 microL), or its vehicle and a microdialysis probe in the FC for determination of the amount of DA and serotonin (5-HT). The data obtained show that IC electrical stimulation caused an increase in the DA release while it reduced the 5-HT release in the FC. BLA inactivation with both lidocaine or muscimol enhanced the aversiveness of the electrical stimulation of the IC and attenuated the increase in DA, while the reduction in 5-HT release in the FC remained unaffected. These findings suggest that ascending aversive information from IC on their way up to higher structures in the SNC courses with opposite modulation by DA/5-HT mechanisms in the FC. These processes are regulated by filters located in the BLA. It is proposed that the loss of these BLA regulatory mechanisms prevents the expression of these modulatory mechanisms in the FC that are adaptive responses in order to cope with unconditioned aversive stimulus triggered at the brainstem level.

A distinct neurochemical profile in WKY rats at baseline and in response to acute stress: implications for animal models of anxiety and depression

Wistar-Kyoto (WKY) rats exhibit hyperresponsive neuroendocrine and behavioral responses to stress that exceed normal controls and are especially prone to develop stress-induced depressive disorder. Pharmacological studies indicate altered serotonin (5-HT), norepinephrine (NE) and dopamine (DA) systems functioning in WKY rats, yet no attempt has been made to provide a comprehensive assessment of the neurochemical profile for WKY rats as compared to the outbred progenitor controls, Wistar rats. To this end, male, WKY and Wistar rats (N=6/group) were exposed to an acute forced-swim stress or were left untreated as controls. The prefrontal cortex (PFCtx), striatum, nucleus accumbens (NAS), and amygdala were assayed for levels of NE, DA and 5-HT, as well as major metabolites, by high-pressure liquid chromatography (HPLC) with electrochemical detection. In a separate experiment, designed to assess baseline and stress-induced neuroendocrine activation, male, Wistar and WKY rats (N=6/group) were exposed to an acute forced-swim stress of 15 min or were left untreated as controls. Animals were killed immediately after the test (T=0), 30 min after the test (T=30) or 60 min after the test (T=60), and control animals were killed immediately after weighing. After decapitation, trunk blood was collected and plasma was isolated by centrifugation and analyzed for corticosterone by immunoassay. The neurochemical results demonstrate distinct patterns of baseline and stress-induced monoamine turnover in WKY rats, including alterations to DA and 5-HT turnovers in prefrontal cortex and nucleus accumbens, two critical brain areas implicated in anxiety, depression and drug reward. The neuroendocrine results indicate that WKY rats exhibited a sustained corticosterone response to acute stress, as compared to Wistar controls. Overall, these data are predicted to be useful for understanding the anxiety- and depressive-like behavioral phenotype exhibited by these animals and for increased understanding of the role genetic background in altering neurochemical function.

Stimulus-specific plasticity of prefrontal cortex dopamine neurotransmission

Future planning and behavioral modification is thought to require experience-dependent plasticity in neuronal circuits involving the prefrontal cortex, nucleus accumbens and amygdala. Dopamine has been implicated in such plasticity; however, the nature of the adaptive response of dopamine systems to emotionally salient experiences is poorly understood. We determined whether the dopaminergic response to a given stimulus changes after the first exposure to that stimulus and whether this alteration is stimulus specific. Dopamine release was measured in the prefrontal cortex and the nucleus accumbens in response to two aversive but qualitatively distinct stimuli, physical restraint and electrical microstimulation of basolateral amygdala. In the prefrontal cortex, the first exposure to restraint or amygdala stimulation produced similar increases in dopamine release. The second exposure to restraint resulted in an attenuated response (- 36%) whereas the second exposure to amygdala stimulation produced a potentiated response (+ 110%). Cross-modal potentiation of response occurred with both stimuli. These adaptive changes were specific to the prefrontal cortex and were not observed in the nucleus accumbens. These findings demonstrate that prefrontal cortical dopamine output adapts after a single exposure to stimuli with emotional salience. The direction of this adaptation, however, is not uniform and depends on the nature of the stimulus.

Effect of stress on prefrontal cortex function

Stress is the major epigenetic factor that contributes to the etiology, pathophysiology, and treatment outcome of most psychiatric disorders. Understanding the mechanisms by which stress contributes to these processes can have important implications for improving therapeutic outcome. Considering that a dysfunctional prefrontal cortex has been implicated in many psychiatric disorders, such as schizophrenia and mood disorders, delineating mechanisms by which stress affects prefrontal cortex (PFC) function is critical to our understanding of the role of stress in influencing the disease process. This paper will review recent mechanistic information about the effects of stress on dopamine and glutamate neurotransmission in the PFC.

Corticotropin-releasing hormone receptor blockade fails to alter stress-evoked catecholamine release in prefrontal cortex of control or chronically stressed rats

Although it is well documented that stress can increase the activity of central dopamine and norepinephrine neurons, little is known about the role of other neurotransmitters in modulating this response. Previous studies have implicated corticotropin-releasing hormone in modulating stress-evoked changes in the activity of locus coeruleus neurons. The present study examines whether corticotropin-releasing hormone contributes to stress-evoked increases in extracellular norepinephrine and dopamine in rat medial prefrontal cortex, as monitored by in vivo microdialysis. As noted previously, 30 min of tail-shock increased extracellular levels of norepinephrine and dopamine in the medial prefrontal cortex of naïve rats, and this was enhanced in rats previously exposed to chronic cold ( approximately 5 degrees C for 2-3 weeks). Previous intraventricular administration of a corticotropin-releasing hormone antagonist (D-Phe-corticotropin-releasing hormone; 3 and 9 microg) did not alter the tail-shock evoked in increase in extracellular levels of norepinephrine and dopamine in either naïve or chronically cold-exposed rats. Intraventricular administration of 3 microg of D-Phe-corticotropin-releasing hormone attenuated the increase in extracellular norepinephrine induced by co-administration of 3 microg of corticotropin-releasing hormone, confirming the efficacy of this compound. Results of the present study suggest that endogenous corticotropin-releasing hormone does not play a role in modulating the release of norepinephrine and dopamine occurring in response to acute tail-shock or the expression of a potentiated response to tail-shock in rats exposed chronically to cold.

Effect of novel stressors on tyrosine hydroxylase gene expression in the adrenal medulla of repeatedly immobilized rats

The activity of the sympathetic-adrenomedullary system in rats submitted to novel stressors after prior repeated or chronic stress exposure is poorly understood. The purpose of the present work was to investigate changes in adrenomedullary (AM) tyrosine hydroxylase (TH) gene expression after a single or long-term repeated exposure of rats to immobilization stress (IMMO; 42 times), as well as in repeatedly immobilized rats (41 times) exposed once to various novel heterotypic stressors. Cold exposure for 5 h, administration of insulin (INS, 51U), or 2-deoxyglucose (2DG, 500 mg/kg) were used as novel stressors. A single exposure to cold, INS, or 2DG produced transient increases in TH mRNA levels in AM. Animals exposed to repeated homotypic IMMO stress showed permanently increased TH mRNA levels, TH activity, and protein levels; however, an exposure of such animals to heterotypic novel stressors did not induce any further changes. Thus the observed differences in TH mRNA levels in the AM of control rats and long-term repeatedly IMMO rats suggest that an adaptation to this stressor is displayed by a permanently increased TH gene expression, TH activity, and protein level. The exposure of repeatedly IMMO rats to a single episode of novel stressor does not induce exaggerated responses in TH gene expression, as some other stressors do. The mechanism of this finding could involve a central regulation and/or adrenomedullary signaling pathway(s), leading to additional modifications or accumulation of transcription factors. The precise mechanism(s) of this phenomenon remains to be elucidated.

Genetic susceptibility of mesocortical dopamine to stress determines liability to inhibition of mesoaccumbens dopamine and to behavioral 'despair' in a mouse model of depression

Clinical and preclinical research suggests a major role of mesocortical dopamine (DA) in psychopathology through regulation of subcortical, especially mesoaccumbens, DA functioning. In these experiments we demonstrate that the high vulnerability to stress-induced 'despair' and mesoaccumbens DA inhibition, exhibited by mice of the inbred strain C57BL/6 (C57) in a common animal model of depression, depends on their being highly susceptible to stress-induced mesocortical DA activation. Thus, C57 mice but not mice of the DBA/2 strain showed an extremely high level of immobility on their first experience with the forced swimming test (FST) as well as immediate and strong activation of mesocortical DA metabolism and inhibition of mesoaccumbens DA metabolism and release. In addition, the behavioral and the mesoaccumbens DA responses to FST in C57 mice were reduced and reversed, respectively, by bilateral mesocortical DA depletion. Finally, chronic treatment with the antidepressant clomipramine reduced immobility and eliminated both mesocortical DA activation and mesoaccumbens DA inhibition in response to FST. These results suggest that a genetically determined susceptibility to stress by the mesocortical DA system may favor the development of pathological behavioral responses through inhibition of subcortical DA transmission.

Cocaine sensitization in the mouse using a cumulative dosing regime

Many rodent models of cocaine sensitization use intermittent high doses of cocaine pretreatment followed by testing with a single moderate cocaine dose. The aim of the present study was to investigate the rate and extent of sensitization to the locomotor-stimulant effects of cocaine using multiple cocaine doses (5-40 mg/kg). Eight groups of male Swiss-Webster mice were pretreated with either single doses of cocaine (40 mg/kg) or saline in the home cage, or multiple doses in the test environment, for 4 days. On the fifth day they were tested for locomotor activity, following a single dose of saline and cumulative doses of cocaine (5-40 mg/kg at 10-minute intervals). All eight groups of mice developed context-dependent sensitization to the locomotor stimulant effects of cocaine. Subsequent testing, at 10-day intervals, revealed that sensitization was maximal after five test sessions of cumulative cocaine dosing, regardless of the pretreatment regime. The main determinant of the rate at which sensitization occurred was the frequency of cumulative cocaine dosing. However, both the potency and efficacy of cocaine were altered by different pretreatments associated with exposure to the locomotor activity chambers. This robust context-dependent sensitization was long lasting, and not abolished by a 5-day extinction procedure involving cumulative saline dosing in the locomotor activity chambers. In conclusion, cumulative dosing and its inherent handling, in combination with cocaine, induced marked sensitization not produced by cocaine alone.

Opposite imbalances between mesocortical and mesoaccumbens dopamine responses to stress by the same genotype depending on living conditions

The balance between mesocortical and mesoaccumbens dopamine (DA) response to stress may represent a major diathesis in psychopathology. These experiments evaluated the influence of variable living conditions on this phenotype and on behavioral coping. Mesocortical and mesoaccumbens DA responses to stress challenge (restraint) were analyzed in individually housed or food restricted mice of an inbred strain to control for genotype-dependent variability. Mice housed in groups with free access to food were used as controls. Little or no differences among the three conditions were found for basal mesoaccumbens and mesocortical DA and metabolite levels. Stress challenge promoted parallel activation of mesocortical and mesoaccumbens DA metabolism and release in group-housed mice. Individually housed mice showed enhanced mesocortical and reduced mesoaccumbens response to stress challenge. Instead, food restricted mice showed no response by the mesocortical DA system and enhanced mesoaccumbens DA response. Finally, the two differential housing conditions promoted opposite alterations of the behavioral profile exhibited by mice exposed to the forced swimming test. These results indicate opposite imbalances between mesocortical and mesoaccumbens DA responses to stress in intact, drug-naive animals, point to a strict relationship between these unbalanced responses and behavioral coping with aversive events and indicate that central and behavioral responses to stress are highly dependent on individual experiences.

Pharmacology and behavioral pharmacology of the mesocortical dopamine system

The prefrontal cortex (PFC) has long been known to be involved in the mediation of complex behavioral responses. Considerable research efforts are directed towards refining the knowledge about the function of this brain area and the role it plays in cognitive performance and behavioral output. In the first part, this review provides, from a pharmacological perspective, an overview of anatomical, electrophysiological and neurochemical aspects of the function of the PFC, with an emphasis on the mesocortical dopamine system. Anatomy of the mesocortical system, basic physiological and pharmacological properties of neurotransmission within the PFC, and interactions between dopamine and glutamate as well as other transmitters within the mesocorticolimbic circuit are included. The coverage of these data is largely restricted to what is relevant for the second part of the review which focuses on behavioral studies that have examined the role of the PFC in a variety of phenomena, behaviors and paradigms. These include reward and addiction, locomotor activity and sensitization, learning, cognition, and schizophrenia. Although the focus of this review is on the mesocortical dopamine system, given the intricate interactions of dopamine with other transmitter systems within the PFC and the importance of the PFC as a source of glutamate in subcortical areas, these aspects are also covered in some detail where appropriate. Naturally, a topic as complex as this cannot be covered comprehensively in its entirety. Therefore this review is largely limited to data derived from studies using rats, and it is also specifically restricted to data concerning the medial PFC (mPFC). Since in several fields of research the findings concerning the function or role of the mPFC are relatively inconsistent, the question is addressed whether these inconsistencies might, at least in part, be related to the anatomical and functional heterogeneity of this brain area.

Endogenous opiates: 1999

This paper is the twenty-second installment of the annual review of research concerning the opiate system. It summarizes papers published during 1999 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunologic responses.

Electrical stimulation of the midbrain tectum enhances dopamine release in the frontal cortex

One widely used animal model of anxiety is the electrical stimulation of a given structure supposed to be involved in the neural circuitry underlying emotional behavior. Indeed, electrical stimulation of midbrain structures with substrates for the processing of fear-like responses, such as the dorsal periaqueductal gray matter (DPAG) or the inferior colliculus (IC), produces behavioral, sensorial and autonomic responses very similar to the defense reactions observed in environmental threatening situations. It has also been proposed that the required level of integration of all these components of the defense reaction needs an integrative process situated at higher brain level, as the prefrontal cortex. As a matter of fact, substantial cortical inputs to the midbrain tectum have already been found. In view of this evidence, it seems important to know whether animals stimulated in the midbrain tectum would present neurochemical changes in the prefrontal cortex. To this end, we examined the temporal course of the effects of the electrical stimulation of the DPAG and IC on the dopamine (DA) release in the prefrontal cortex. Electrical stimulation of these structures was performed at the alertness (control) and escape thresholds. Electrical stimulation of the inferior colliculus at the escape threshold produced a long-lasting increase in the levels of corticofrontal dopamine in relation to these measurements in the control group. No significant changes in extracellular DA release in this cortical area could be observed following DPAG electrical stimulation. These findings bring evidence for the involvement of dopamine of the frontal cortex in the setting up of adaptive responses to stressful situations generated at the inferior colliculus level.

The adolescent brain and age-related behavioral manifestations

To successfully negotiate the developmental transition between youth and adulthood, adolescents must maneuver this often stressful period while acquiring skills necessary for independence. Certain behavioral features, including age-related increases in social behavior and risk-taking/novelty-seeking, are common among adolescents of diverse mammalian species and may aid in this process. Reduced positive incentive values from stimuli may lead adolescents to pursue new appetitive reinforcers through drug use and other risk-taking behaviors, with their relative insensitivity to drugs supporting comparatively greater per occasion use. Pubertal increases in gonadal hormones are a hallmark of adolescence, although there is little evidence for a simple association of these hormones with behavioral change during adolescence. Prominent developmental transformations are seen in prefrontal cortex and limbic brain regions of adolescents across a variety of species, alterations that include an apparent shift in the balance between mesocortical and mesolimbic dopamine systems. Developmental changes in these stressor-sensitive regions, which are critical for attributing incentive salience to drugs and other stimuli, likely contribute to the unique characteristics of adolescence.