Differences in central noradrenergic and behavioural responses of Maudsley non-reactive and Maudsley reactive inbred rats on exposure to an aversive novel environment

Individual differences in inflammatory and oxidative mechanisms of stress-related mood disorders

Emotional stress leads to the development of peripheral disorders and is recognized as a modifiable risk factor for psychiatric disorders, particularly depression and anxiety. However, not all individuals develop the negative consequences of emotional stress due to different stress coping strategies and resilience to stressful stimuli. In this review, we discuss individual differences in coping styles and the potential mechanisms that contribute to individual vulnerability to stress, such as parameters of the immune system and oxidative state. Initial differences in inflammatory and oxidative processes determine resistance to stress and stress-related disorders via the alteration of neurotransmitter content in the brain and biological fluids. Differences in coping styles may serve as possible predictors of resistance to stress and stress-related disorders, even before stressful conditions. The investigation of natural variabilities in stress resilience may allow the development of new methods for preventive medicine and the personalized treatment of stress-related conditions.

Stress modulates cortical excitability via α-2 adrenergic and glucocorticoid receptors: As assessed by spreading depression

An increase in cortical excitability may be one of the factors mediating stress-induced vulnerability to neuropsychiatric disorders. Since stress increases extracellular glutamate and predisposes to migraine with aura attacks, we aimed to study the effect of stress on cortical spreading depression (CSD), the biological substrate of migraine aura and a measure of cortical excitability. CSD was induced by increasing concentrations of KCl applied over the dura with 5-minute intervals and recorded from parieto-occipital cortex to assess the CSD-induction threshold in acutely-stressed, chronically-stressed and naive mice. To study the mechanisms of acute stress-induced decrease in CSD threshold, we systemically administered clonidine, yohimbine, propranolol, CRH1 receptor antagonist NBI27914, mifepristone and spironolactone at doses shown to be effective on stress as well as a central noradrenergic neurotoxin (DSP-4) before acute stress. CSD threshold was significantly lowered by acute and chronic stress as well as central noradrenergic denervation. Clonidine and mifepristone further decreased the CSD threshold below the acute stress-induced levels, whereas yohimbine reversed the acute stress-induced decrease in CSD threshold compared to the saline-injected and stressed control groups. Propranolol, NBI27914 and spironolactone did not modify the effect of acute stress on CSD threshold. Stress increases cortical excitability as illustrated by a decrease in CSD-induction threshold. This action of acute stress is mediated by α2-adrenergic and glucocorticoid receptors. The decrease in CSD threshold may account for the stress-induced susceptibility to migraine. CSD may be used as a tool to study the link between stress-related disorders and cortical excitability.

Disruption of noradrenergic transmission and the behavioural response to a novel environment in NK1R-/- mice

The behaviour of neurokinin-1-receptor gene knockout (NK1R-/-) mice, which lack functional, substance P-preferring receptors, resembles that of NK1R+/+ mice treated with an antidepressant. Because all antidepressants increase central monoamine transmission, we have investigated whether noradrenergic transmission is increased in NK1R-/- mice and, if so, whether this could influence their behaviour. In anaesthetized subjects, the concentration of extracellular noradrenaline in NK1R-/- mice was two-fourfold greater than in NK1R+/+ mice. Systemic administration of the alpha2-adrenoceptor antagonist, 2-(2,3-dihydro-2-methoxy-1,4-benzodioxan-2-yl)-4,5-dihydro-1H-imidazoline (RX 821002), in anaesthetized or freely moving animals increased extracellular noradrenaline in NK1R+/+ mice only. This suggests that the function of alpha2a-autoreceptors, which modulate noradrenergic transmission, is impaired in NK1R-/- mice. Consistent with this, [35S]GTPgammaS binding to activated alpha2a-adrenoceptors was lower (-70%) in the locus coeruleus, but not the frontal cortex, of NK1R-/- mice compared with their NK1R+/+ counterparts. RX 821002-pretreatment, followed by retrodialysis of the noradrenaline reuptake inhibitor, desipramine, into the frontal cortex of anaesthetized mice increased extracellular noradrenaline to the same extent in the two genotypes. Western blots confirmed that there was no difference in the amount of noradrenaline transporter protein in NK1R-/- and NK1R+/+ mice. Finally, the effects of RX 821002 on certain behaviours in a light/dark exploration box were blunted in NK1R-/- mice, but there was no consistent effect on anxiety-like behaviour in the two genotypes. It is concluded that the greater basal efflux of noradrenaline in NK1R-/- mice is explained by increased transmitter release, coupled with desensitization of somatodendritic alpha2a-adrenoceptors. These changes could contribute to the difference in the behavioural phenotypes.

Attenuation of alpha2A-adrenergic receptor expression in neonatal rat brain by RNA interference or antisense oligonucleotide reduced anxiety in adulthood

Brain alpha2-adrenergic receptors (alpha2-ARs) have been implicated in the regulation of anxiety, which is associated with stress. Environmental treatments during neonatal development could modulate the level of brain alpha2-AR expression and alter anxiety in adults, suggesting possible involvement of these receptors in early-life programming of anxiety state. The present study was undertaken to determine whether the reduction of the expression of A subtype of these receptors most abundant in the neonatal brain affects anxiety-related behavior in adulthood. We attenuated the expression of alpha2A-ARs during neonatal life by two different sequence specific approaches, antisense technology and RNA interference. Treatment of rats with the antisense oligodeoxynucleotide or short interfering RNA (siRNA) against alpha2A-ARs on the days 2-4 of their life, produced a marked acute decrease in the levels of both alpha2A-AR mRNA and [3H]RX821002 binding sites in the brainstem into which drugs were injected. The decrease of alpha2A-AR expression in the neonatal brainstem influenced the development of this receptor system in the brain regions as evidenced by the increased number of [3H]RX821002 binding sites in the hypothalamus of adult animals with both neonatal alpha2A-AR knockdown treatments; also in the frontal cortex of antisense-treated, and in the hippocampus of siRNA-treated adult rats. These adult animals also demonstrated a decreased anxiety in the elevated plus-maze as evidenced by an increased number of the open arm entries, greater proportion of time spent in the open arms, and more than a two-fold increase in the number of exploratory head dips. The results provide the first evidence that the reduction in the brain expression of a gene encoding for alpha2A-AR during neonatal life led to the long-term neurochemical and behavioral alterations. The data suggests that alterations in the expression of the receptor-specific gene during critical periods of brain development may be involved in early-life programming of anxiety-related behavior.

Animal models of anxiety

Animal models for anxiety-related behavior are based on the assumption that anxiety in animals is comparable to anxiety in humans. Being anxious is an adaptive response to an unfamiliar environment, especially when confronted with danger or threat. However, pathological variants of anxiety can strongly impede the daily life of those affected. To unravel neurobiological mechanisms underlying normal anxiety as well as its pathologi- cal variations, animal models are indispensable tools. What are the characteristics of an ideal animal model? First, it should display reduced anxiety when treated with anxiolytics (predictive validity). Second, the behavioral response of an animal model to a threatening stimulus should be comparable to the response known for humans (face validity). And third, the mechanisms underlying anxiety as well as the psychological causes should be identical (construct validity). Meeting these three requirements is difficult for any animal model. Since both the physiological and the behavioral response to aversive (threatening) stimuli are similar in humans and animals, it can be assumed that animal models can serve at least two distinct purposes: as (1) behavioral tests to screen for potential anxiolytic and antidepressant effects of new drugs and (2) tools to investigate specific pathogenetic aspects of cardinal symptoms of anxiety disorders. The examples presented in this chapter have been selected to illustrate the potential as well as the caveats of current models and the emerging possibilities offered by gene technology. The main concepts in generating animal models for anxiety-that is, selective breeding of rat lines, experience-related models, genetically engineered mice, and phenotype-driven approaches-are concisely introduced and discussed. Independent of the animal model used, one major challenge remains, which is to reliably identify animal behavioral characteristics. Therefore, a description of behavioral expressions of anxiety in rodents as well as tests assays to measure anxiety-related behavior in these animals is also included in this chapter.

Modulation by sudden darkness of apomorphine-induced behavioral responses

Sudden darkness increases motor activity and decreases anxiety. In the present study, we focused on the role of dopaminergic mechanisms involved in the effects of sudden darkness. The influence of sudden darkness on the behavioral effects of low (0.05 and 0.1 mg/kg) and high (0.25, 0.45 and 0.6 mg/kg) doses of apomorphine (APO) was tested. We assayed the effects of low APO doses on yawning-penile erection syndrome (YES; 0.05 and 0.1 mg/kg) and on motor activity (0.05 mg/kg), and the effects of high APO doses on motor activity (0.25 mg/kg) and stereotyped behavior (0.45 mg/kg and 0.6 mg/kg). Spontaneous total and genital grooming of male and female rats were also recorded. Sudden darkness modified some spontaneous behaviors and also modulated several APO-induced behavioral effects. It increased spontaneous total grooming and genital grooming in male rats but had no effect on these parameters in female rats. These results show sexual dimorphism for total and genital grooming in both control and sudden darkness conditions. APO was able to induce YES in a dose-dependent manner. Sudden darkness decreased yawning elicited by both 0.05 and 0.1 mg/kg of the drug. No other parameter of YES was modified. In the open-field test, sudden darkness increased total locomotion and rearing and decreased immobility duration. APO at a dose of 0.05 mg/kg had the opposite effect on these parameters under light conditions; none of them were modified by sudden darkness. Animals treated with APO at 0.25 mg/kg, a dose that augmented total locomotion and rearing and diminished immobility duration, were clearly divided into two groups according to their responses, i.e., hypo- and hyper-responsive rats. Sudden darkness improved total locomotion and rearing, reduced immobility duration and total grooming in the hyporesponsive group, and showed no effects on the hyper-responsive group. Sudden darkness caused no modifications of stereotyped behavior. These results may be due to a sudden darkness-induced physiological release of dopamine that diminishes pre-synaptic responses to APO and increases low-intensity post-synaptic responses such as motor activity without modifying high-intensity post-synaptic responses such as stereotyped behavior.