Infusion of a monoamine oxidase inhibitor into the locus coeruleus can prevent stress-induced behavioral depression

Several stressors fail to reduce adult hippocampal neurogenesis

Neurogenesis in the dentate gyrus of the hippocampus of adult laboratory animals has been widely reported to be vulnerable to many psychological and physical stressors. However, we have found no effects of acute restraint stress, acute or subchronic tailshock stress, or acute, subchronic, or chronic resident-intruder stress on neural progenitor cell (NPC) proliferation, short or long term survival of newborn cells, or brain-derived neurotrophic factor (BDNF) mRNA expression in adult rats. In addition, we did not observe any effect of chronic resident-intruder stress on NPC proliferation in adolescent rats. A selectively bred stress-sensitive line was also found to exhibit no alterations in NPC proliferation following tailshock stress, although this line did exhibit a lower proliferation rate under baseline (unstressed) conditions when compared with non-selected rats. These results challenge the prevailing hypothesis that any stressor of sufficient intensity and duration has a marked negative impact upon the rate of hippocampal neurogenesis, and suggest that some yet unidentified factors related to stress and experimental conditions are crucial in the regulation of neurogenesis.

Catecholaminergic systems in stress: structural and molecular genetic approaches

Stressful stimuli evoke complex endocrine, autonomic, and behavioral responses that are extremely variable and specific depending on the type and nature of the stressors. We first provide a short overview of physiology, biochemistry, and molecular genetics of sympatho-adrenomedullary, sympatho-neural, and brain catecholaminergic systems. Important processes of catecholamine biosynthesis, storage, release, secretion, uptake, reuptake, degradation, and transporters in acutely or chronically stressed organisms are described. We emphasize the structural variability of catecholamine systems and the molecular genetics of enzymes involved in biosynthesis and degradation of catecholamines and transporters. Characterization of enzyme gene promoters, transcriptional and posttranscriptional mechanisms, transcription factors, gene expression and protein translation, as well as different phases of stress-activated transcription and quantitative determination of mRNA levels in stressed organisms are discussed. Data from catecholamine enzyme gene knockout mice are shown. Interaction of catecholaminergic systems with other neurotransmitter and hormonal systems are discussed. We describe the effects of homotypic and heterotypic stressors, adaptation and maladaptation of the organism, and the specificity of stressors (physical, emotional, metabolic, etc.) on activation of catecholaminergic systems at all levels from plasma catecholamines to gene expression of catecholamine enzymes. We also discuss cross-adaptation and the effect of novel heterotypic stressors on organisms adapted to long-term monotypic stressors. The extra-adrenal nonneuronal adrenergic system is described. Stress-related central neuronal regulatory circuits and central organization of responses to various stressors are presented with selected examples of regulatory molecular mechanisms. Data summarized here indicate that catecholaminergic systems are activated in different ways following exposure to distinct stressful stimuli.

Depression in an animal model: focus on the locus ceruleus

When rats are exposed to highly stressful events over which they have no control, they subsequently show many of the symptoms seen in depression in humans. In the attempt to discover neurochemical factors underlying depression, the neurochemical basis of stress-induced behavioural depression in rats has been studied extensively. Initial research (1968-1976) indicated that behavioural depression in this model was produced by alteration of noradrenaline (NA) concentrations in the brain. More recent research has indicated that the critical change may be a large depletion of NA in the locus ceruleus (LC). Behavioural depression may result when such NA depletion is sufficient to reduce NA release in the LC region, leading to a 'functional blockade' of inhibitory alpha 2-receptors in that brain region. Studies have now shown that behavioural depression after uncontrollable shock can be mimicked by pharmacological blockade of alpha 2-receptors in the LC region. Conversely, behavioural depression can be eliminated by either infusion of clonidine into the LC to replace at the alpha 2-receptors the NA depleted after uncontrollable shock, or infusion of pargyline into the LC to prevent the depletion of NA that otherwise follows uncontrollable shock. If alpha 2-receptors are functionally blocked in depression, then release of NA in regions innervated by the LC should be increased and stimulation of postsynaptic adrenoceptors outside the LC should be higher than normal. Thus, higher-than-normal stimulation of postsynaptic NA receptors should also produce behavioural depression; this has been demonstrated.

Behavioral control, the medial prefrontal cortex, and resilience

The degree of control that an organism has over a stressor potently modulates the impact of the stressor, with uncontrollable stressors producing a constellation of outcomes that do not occur if the stressor is behaviorally controllable. It has generally been assumed that this occurs because uncontrollability actively potentiates the effects of stressors. Here it will be suggested that in addition, or instead, the presence of control actively inhibits the impact of stressors. At least in part this occurs because (i) the presence of control is detected by regions of the ventral medial prefrontal cortex (mPFCv); and (ii) detection of control activates mPFCv output to stress-responsive brain stem and limbic structures that actively inhibit stress-induced activation of these structures, Furthermore, an initial experience with control over stress alters the mPFCv response to subsequent stressors so that mPFCv output is activated even if the subsequent stressor is uncontrollable, thereby making the organism resilient. The general implications of these results for understanding resilience in the face of adversity are discussed.

Depletion of brain norepinephrine does not reduce spontaneous ambulatory activity of rats in the home cage

6-Hydroxydopamine (6-OHDA) lesions of brain noradrenergic neurons and terminals were made in rats to assess the importance of forebrain norepinephrine (NE) for mediating circadian patterns of spontaneous ambulatory activity that rats show in the home cage. 6-OHDA was injected intracranially into the fibers of the ascending noradrenergic dorsal and ventral bundle pathways or infused into the lateral ventricle or both. Rats living in a 12/12 h light/dark cycle exhibit a marked increase in ambulatory activity during the dark period in comparison to the light period and a 'W-shaped' pattern of activity during the 12 h of the dark phase. Results showed that near-total depletion of brain NE did not impair the capacity to generate normal patterns of spontaneous ambulatory activity that occur in the home cage. In the animals that sustained the most complete NE lesions, the amounts of activity generated at times of peak activity were exaggerated in comparison to the control animals, which is consistent with the possibility that NE in the brain exerts a moderating influence on behavior.

Galanin: a significant role in depression?

This paper describes a hypothesis that attempts to account for how changes in noradrenergic systems in the brain can affect depression-related behaviors and symptoms. It is hypothesized that increased activity of the locus coeruleus (LC) neurons, the principal norepinephrine (NE)-containing cells in the brain, causes release of galanin (GAL) in the ventral tegmentum (VTA) from LC axon terminals in which GAL is colocalized with NE. It is proposed that GAL release in VTA inhibits the activity of dopaminergic cell bodies in this region whose axons project to forebrain, thereby resulting in two of the principal symptoms seen in depression, decreased motor activation and decreased appreciation of pleasurable stimuli (anhedonia). The genesis of this hypothesis, which derives from studies using an animal model of depression, is described as well as recent data consistent with the hypothesis. The formulation proposed suggests that GAL antagonists may be of therapeutic benefit in the treatment of depression.

Poststress glucose mitigates behavioral impairment in rats in the "learned helplessness" model of psychopathology

Three experiments examined the effects of poststress glucose treatment in the learned helplessness model of psychopathology in rats. In experiment 1, rats were given access to water or 40% aqueous glucose immediately following exposure to inescapable tailshocks or simple restraint in a 2 x 2 factorial design. Inescapably shocked rats failed to drink the glucose solution during the poststress interval and failed to show any improvement 24 hours after stress induction in shuttle-escape performance. Consequently, all rats received preexposure to a sweetened glucose cocktail in an attempt to increase poststress ingestion following inescapable shock treatment in experiment 2. Under these conditions, poststress intake of the glucose cocktail eliminated behavioral impairment in inescapably shocked rats relative to water-treated shocked rats and water- and glucose-treated restrained controls. Experiment 3 demonstrated that glucose prophylaxis occurs in the absence of sucrose when rats are preexposed to a 40% glucose solution prior to stress induction.

The protective effects of stress control may be mediated by increased brain levels of benzodiazepine receptor agonists

Control over stress protects against many of the deleterious effects of stress exposure, but the endogenous mediators responsible for these prophylactic effects have remained elusive. Using behavioral pharmacology, in vitro radioligand binding and neurochemical analyses, we demonstrate that exposure to escapable stress results in brain and behavior changes reminiscent of benzodiazepine administration. The stress control group shows significant protection against picrotoxinin-induced seizures, reductions in [35S]t-butylbicyclophosphorothionate (TBPS) binding and a 3-fold increase of benzodiazepine-like substances in brain in comparison to both yoked-inescapable shock and non-shock controls. These observations suggest that coping behavior leads to the release of endogenous benzodiazepine-like compounds in brain which protect the organism from stress pathology.

Depression and anxiety: role of the locus coeruleus and corticotropin-releasing factor

Based on studies of depression and anxiety using animal (rat) models, it is suggested that, contrary to a widely accepted theory, increased activity of locus coeruleus (LC) neurons does not appear to potentiate anxiety; instead, the influence of LC activity may be opposite to this. First, studies are described that indicate that behavioral changes resembling what is seen in human clinical depression occur in rats exposed to highly stressful conditions, and the research is then traced, which links this stress-induced depression to disturbance of normal noradrenergic regulation of LC activity. Second, the potential role of corticotrophin releasing factor (CRF) in stress-induced behavioral depression is explored. CRF infused into the LC did not produce behavioral depression in the swim test but did increase anxiety; by comparison, CRF infused into the parabrachial nucleus lateral to LC increased both depression and anxiety. Finally, to further explore the relationship between LC activity and anxiety, drugs were infused into LC region to attempt to specifically activate or depress firing of LC neurons. In contrast to expectations, infusion to decrease firing of LC cells increased anxious behavior, while infusion to increase firing decreased anxious behavior. Several other studies are discussed that point to a similar conclusion. It is suggested that, at least in rats, the capacity of stress-inducing or aversive stimuli to activate LC neurons does not potentiate anxiety under environmental conditions that elicit this response, but, rather, the increased activity of the LC/dorsal noradrenergic system under such conditions may exert a counterbalancing, antianxiety influence.

Multiple neurochemical and behavioral consequences of stressors: implications for depression

Animal models of clinical depression have frequently focused on the contribution of stressors to the induction of behavioral impairments and pharmacological intervention in the amelioration of these disturbances. Stressors provoke various behavioral disturbances and influence the activity of central neurotransmitters implicated in depression. It is our contention that those variables which favor the provocation of amine depletions or prevent the development of a neurochemical adaptation will increase vulnerability to behavioral disturbances. It is essential to consider, however, that marked interindividual and interstrain differences exist in the behavioral and neurochemical response to stressors, and in the effectiveness of antidepressant treatments.

An immunocytochemical study on the GABA-ergic and serotonin-ergic neurons in rat locus ceruleus with special reference to possible existence of the masked indoleamine cells

Alternate consecutive cryostat sections of the rat locus ceruleus (LC) were immunostained for either GABA or serotonin by the unlabelled peroxidase-antiperoxidase method. By identifying the same neurons in adjacent sections of this series, we confirmed that the GABA- and serotonin-ergic neurons are different cell groups. Serial sections of the LC were alternately stained either by the immunocytochemical method for serotonin or with cresyl violet for neuroanatomical observations, under normal and various experimental conditions using pharmacological manipulations. By using both a monoamine oxidase (MAO) inhibitor (pargyline) and a serotonin precursor, 5-hydroxytryptophan rather than L-tryptophan, with or without an inhibitor of serotonin synthesis, p-chlorophenylalanine, we have demonstrated that many LC neurons have the capacity to accumulate serotonin and not other indoleamines. It is suggested that the LC has numerous masked indoleamine cells that contain large amounts of MAO in the cytoplasm, so that under physiological conditions, the serotoninimmunoreactivity of these cells is masked.

The neuropathology of aminergic nuclei in Alzheimer's disease

Neuronal loss and the presence of neurofibrillary tangles (NFTs) within aminergic nuclei were examined in a series of patients with Alzheimer's disease (AD). Neuromelanin-containing neurons within the locus ceruleus and large nucleolus-containing neurons within the dorsal raphe nucleus and the central superior (raphe) nucleus were counted in 25 patients with AD and in 12 age-matched control subjects. Numbers of NFTs were quantified in the same regions. Counts were compared with clinical data, including psychiatric evaluations, available for 21 of the patients with AD. Within the locus ceruleus in the patients with AD, abnormalities were more severe at mid level than at caudal or rostral levels (p less than 0.01). Within the dorsal raphe nucleus, neuronal loss was most severe caudally (p less than 0.05). NFTs, but not neuronal loss, were demonstrated within the central superior nucleus. Neuronal and NFT counts did not correlate at individual levels; the relative severity of both pathological processes was consistent from level to level within nuclei but was less consistent between nuclei. Neuronal loss correlated inversely with age, particularly within the locus ceruleus. Duration of disease correlated inversely with counts of NFTs, particularly within the dorsal raphe nucleus, implying a correlation between NFT counts and rate of progression of disease as all but 3 patients had severe dementia. Significantly, patients with AD complicated by major depression had fewer neurons at the mid level of the locus ceruleus and at the rostral level of the central superior nucleus in comparison with nondepressed patients. There was a trend suggesting greater loss of neurons at all levels of the locus ceruleus and dorsal raphe nucleus in depressed individuals.

Neurochemical and electrophysiological events underlying stress-induced depression in an animal model

This paper has described an animal (rodent) model of depression in which depression-like characteristics are produced by exposure of animals to stressful events that they cannot control. This model, called "stress-induced depression," appears to be mediated by stress-induced changes in brain norepinephrine (NE), and evidence now indicates that a large depletion of NE in the Locus Coeruleus region of the brain stem (LC) is critical for producing the behavioral disturbance seen in this model. To explain the functional significance of NE depletion in the LC, it has been suggested that this change results in decreased stimulation, (i.e., functional blockade) of alpha-2 receptors that normally inhibit firing of LC neurons. Consequently, LC neurons should be disinhibited in stress-induced depression. The final part of this paper describes findings from electrophysiological measurement of LC activity. Prior to measuring changes occurring in stress-induced depression, studies examined the basic role played by alpha-2 receptors in LC activity. These receptors were found to regulate the responsivity of LC neurons to excitatory stimuli, so that pharmacological blockade of these receptors increased the firing of LC neurons to excitatory input. When LC activity was then measured in stressed animals showing behavioral depression, responsivity of LC neurons to excitatory input was elevated in comparison to animals not exposed to stress. Also, administration of an alpha-2 blocking drug could not increase responsivity of LC neurons in stressed animals, which further indicated that alpha-2 receptors in the LC region were functionally blocked in these animals. Finally, the amount that LC neurons increased their firing to an excitatory stimulus (i.e., an index of alpha-2 receptor blockade) was highly correlated with the amount of behavioral depression seen in an activity test conducted just prior to electrophysiological measurement. These electrophysiological findings indicate that LC neurons are disinhibited in stress-induced depression, and that this disinhibition is particularly characterized by increased responsivity of LC neurons to excitatory input, which is indicative of alpha-2 receptor blockade. These findings further support the view that the LC is involved in stress-induced depression, and are consistent with a proposed mechanism that attributes behavioral disturbance in the model to disinhibition of LC neurons arising from subnormal activation of inhibitory alpha-2 receptors.