Mechanisms underlying the health benefits of intermittent hypoxia conditioning

Intermittent hypoxia (IH) is commonly associated with pathological conditions, particularly obstructive sleep apnoea. However, IH is also increasingly used to enhance health and performance and is emerging as a potent non-pharmacological intervention against numerous diseases. Whether IH is detrimental or beneficial for health is largely determined by the intensity, duration, number and frequency of the hypoxic exposures and by the specific responses they engender. Adaptive responses to hypoxia protect from future hypoxic or ischaemic insults, improve cellular resilience and functions, and boost mental and physical performance. The cellular and systemic mechanisms producing these benefits are highly complex, and the failure of different components can shift long-term adaptation to maladaptation and the development of pathologies. Rather than discussing in detail the well-characterized individual responses and adaptations to IH, we here aim to summarize and integrate hypoxia-activated mechanisms into a holistic picture of the body's adaptive responses to hypoxia and specifically IH, and demonstrate how these mechanisms might be mobilized for their health benefits while minimizing the risks of hypoxia exposure.

Intermittent Hypoxic Training as an Effective Tool for Increasing the Adaptive Potential, Endurance and Working Capacity of the Brain

This review is devoted to the phenomenon of intermittent hypoxic training and is aimed at drawing the attention of researchers to the necessity of studying the mechanisms mediating the positive, particularly neuroprotective, effects of hypoxic training at the molecular level. The review briefly describes the historical aspects of studying the beneficial effects of mild hypoxia, as well as the use of hypoxic training in medicine and sports. The physiological mechanisms of hypoxic adaptation, models of hypoxic training and their effectiveness are summarized, giving examples of their beneficial effects in various organs including the brain. The review emphasizes a high, far from being realized at present, potential of hypoxic training in preventive and clinical medicine especially in the area of neurodegeneration and age-related cognitive decline.

Comparative Analysis of Pathobiochemical Changes in Major Depression and Post-Traumatic Stress Disorder

Comparative analysis of available literature data on the pathogenetic neuroendocrine mechanisms of depression and post-traumatic stress disorder (PTSD) is provided in this review to identify their common features and differences. We discuss the multidirectional modifications of the activity of cortical and subcortical structures of the brain, levels of neurotransmitters and their receptors, and functions of the hypothalamic-pituitary-adrenocortical axis in depression and PTSD. The analysis shows that these disorders are examples of opposite failures in the system of adaptive stress response of the body to stressful psychotraumatic events. On this basis, it is concluded that the currently widespread use of similar approaches to treat these disorders is not justified, despite the significant similarity of their anxiety-depressive symptoms; development of differential therapeutic strategies is required.

Acclimatization to Middle Attitude Hypoxia Masks the Symptoms of Experimental Posttraumatic Stress Disorder, but Does Not Affect Its Pathogenetic Mechanisms

The effects of acclimatization to middle attitude hypoxia on the resistance to acute emotional stress were studied on the model of posttraumatic stress disorder in rats. Anxyolitic, but not anxiogenic effect was observed in acclimatized rats. However, acclimatized rats with posttraumatic stress disorder were characterized by hypofunction of the pituitary-adrenocortical axis, which is typical of this pathology, and reduction in corticosterone/dehydroepiandrosterone ratio. At the neuroendocrine level, up-regulation of glucocorticoid receptors and a decrease in the level of corticotropin-releasing hormone in the hippocampus were revealed. The observed modifications of regulatory mechanisms can underlie hypofunction of the pituitary-adrenocortical axis. It was concluded that acclimatization to middle attitude hypoxia masks behavioral symptoms of posttraumatic stress disorder, but does not alter its pathogenetic neuroendocrine mechanisms.

Pathogenetic Role of the Stress-induced Release of Glucocorticoid Hormones in the Development of Post-traumatic Stress Disorder: An Experimental Study

In the rat experimental model of posttraumatic stress disorder (PTSD), the level of blood corticosterone was at least eight-fold increased (an overrelease). The use of hypobaric hypoxic preconditioning or short-term inhibition of glucocorticoid synthesis by metyrapone injection prevented development of the experimental PTSD.

[Current Theory on the Cerebral Mechanisms of Hypoxic PRE- and Postconditioning]

An exposure of the organism to several episodes of mild hypoxia results in the development of brain hypoxic/ischemic tolerance, as well as cross-tolerance to the stressful factors of psychoemotional nature. Such kind of preconditioning by mild hypoxia functions as “alarm signalization” by I.P. Pavlov, preparing the organism and, in particularly, brain to the forthcoming harmful event. Dose-dependent action of hypoxia on the brain can be considered as one particular case of the general phenomenon termed hormesis, or neurohormesis. Endogenous defense processes launched by the hypoxic preconditioning and leading to the development of cerebral tolerance are associated with activation of intracellular signal cascades, transcriptional factors, regulatory proteins and expression of pro-adaptive genes and their products in the susceptible brain regions. Important mechanism of systemic adaptation induced by hypoxic preconditioning includes modifications of pituitary-adrenal axis aimed at enhancement of its adaptive resources. All these components are involved in the neuroprotective processes in three sequential phases - initiation, induction, and expression. Important role belongs also to epigenetic mechanisms controlling the activity of pro-adaptive genes. In contrast to the preconditioning, hypoxic postconditioning is comparatively novel phenomenon and therefore its mechanisms are less studied. The involvement of hypoxia-inducible factor HIF-1, and non-specific protective processes as up-regulation of anti-apoptotic factors and neurotrophines.

Effects of Hypobaric Hypoxia in Various Modes on Expression of Neurogenesis Marker NeuroD2 in the Dentate Gyrus of Rats Hippocampus

The expression of neurogenesis marker--NeuroD2 transcription factor--in the hippocampal dentate gyrus was studied in rats exposed to severe destructive hypoxia, a single or three episodes of moderate hypobaric hypoxia, preconditioned severe hypoxia, and severe hypoxia followed by 3 sessions of postconditioning by moderate hypobaric hypoxia. All the studied hypoxic exposure modes led to an increase of NeuroD2 level. Three-fold moderate hypoxia per se and in the preconditioning mode (followed by exposure to severe hypoxia) produced most pronounced up-regulatory effect on NeuroD2 expression. The results indicated that stimulation of neurogenesis processes seemed to be one of the aspects of the neuroprotective effect of three-fold preconditioning moderate hypoxia, but not of hypoxic postconditioning.

[A comparison of a neuroprotective effects of hypoxic postconditioning and cerebrolysin in the experimental model]

Hypoxic postconditioning using episodes of mild hypobaric hypoxia is a new neuroprotective technique. We compared the neuroprotective efficacy of hypoxic postconditioning and cerebrolysin in a model of posthypoxic pathology in rats. Animals that survived the severe hypoxia (180 Torr, 3 h) were exposed to hypoxic postconditioning or received cerebrolysin. Postconditioning prevented the injury and loss of hippocampal (fields CA1, CA4) and neocortical neurons whereas cerebrolysin was protective only for CA4 and the neocortex. Besides that, postconditioning, unlike cerebrolysin, led to the complete functional rehabilitation from the severe hypoxia by normalizing the level of anxiety and the pituitary-adrenal axis activity. The findings demonstrate that the elaborated postconditioning technique might provide useful tool for therapy of posthypoxic pathology and stroke.

[Involvement of transcriptional factor induced by hypoxia in the neuronal mechanisms of adaptation to psychoemotional and hypoxic stress]

Using quantitative immunohistochemistry, neuronal expression of alpha-subunit of the transcriptional factor HIF-1 in hippocampus and neocortex of rats in response to pathogenic psychoemotional (model of posttraumatic stress disorder, PTSD) and hypoxic (severe hypobaric hypoxia, 180 Torr, 3 h), as well as to neuroprotective exposures to hypoxic pre- and postconditioning has been studied. Elongated overexpression of HIF-1alpha in hippocampus and neocortex of rats in response to the psychoemotional stress in PTSD paradigm, but not hypoxic stress, has been observed. Hypoxic pre- and postconditioning with mild hypobaric hypoxia (360 Torr, 2 h, 3 trials spaced at 24 h), those induced adaptation to the psychoemotional stress, abolished the elongated HIF-1alpha overexpression. Hypoxic postconditioning which improved structure and functional rehabilitation following severe hypoxic stress up-regulated HIF-1alpha expression in the brain neurons of rats survived severe hypoxia. The findings indicate that transcription factor HIF-1 is particularly involved in the processes of adaptation/ maladaptation to the action of injurious stresses, but its role depends upon the nature of stressor.

[The effect of mild hypobaric hypoxia regime on expression of factor induced by hypoxia in the rat neocortex]

Using quantitative immunohistochemistry, modifications of HIF-1alpha expression in neocortex of rats exposed to various modes of mild hypobaric hypoxia (MHH) (1,3 and 6 episodes) differed in their neuroprotective efficacy have been studied. It has been shown that three-trial MHH being the most effective neuroprotective mode when used as a preconditioning produces most considerable changes in HIF-1 by substantial up-regulation of its regulatory alpha-subunit expression in the rat neocortex. Present findings support the hypothesis on important roles of HIF-1 in the mechanisms of brain hypoxic tolerance induced by the hypoxic preconditioning with three-trial MHH.

[Effects of several modes of hypobaric hypoxia on expression profiles of corticosteroid receptors in rat hippocampus]

Gluco- and mineralocorticoid receptors are believed to play important roles in mechanisms of the hypothalamic-pituitary-adrenal axis (HPA) regulation, neuronal death/survival, as well as learning and memory processes. Imbalanced levels of MR and GR result in impairment of HPA activity and can promote neuronal injury and loss following exposures to extreme factors. In the present study, using quantitative immunohistochemistry, the comparative analysis of the effects of hypobaric hypoxia in several modes on expression profiles of GR and MR in dorsal (CA1) and ventral (dentate gyrus) hippocampus was performed. According to the data obtained, severe injurious hypoxia induced prominent disturbances of GR and MR expression in the cells of CA1 and dentate gyrus that correlated to the remarkable neuronal injury/loss in CA1 and dysregulated HPA activity. Sets of three- or six-trial (but not one-trial) preconditioning using mild hypoxia prior to severe hypoxia prevented these abnormalities.

[Signal molecular and hormonal mechanisms of formation of the hypoxic preconditioning protective effects]

In the review, results of the long-standing authors'studies and literature data concerning one of the underresearched aspects of actual problem of induced brain tolerance to injurious factors - "preventive" signal function of the hypoxic preconditioning, as well as molecular and hormonal mechanisms underlying its protective effects are presented. Hypoxic preconditioning by using of mild hypobaric hypoxia in special mode mobilizes evolutionary acquired genome determined defense mechanisms of brain neurons and whole organism. This process involves an activation of multiple intracellular components, as well as hypothalamic-pituitary-adrenal axis. Cascade mechanisms of intracellular signaling including receptors, mitochondrial respiratory chain, key intracellular regulatory systems, early genes, superfamilies of the inducible and activation transcription factors are sequentially engaged in the processes of initiation, induction and expression of hypoxic tolerance. The determination of optimal modes of hypoxic preconditioning appears to be of significant importance to assure the effective activation of protective signal mechanisms.

[Hypoxic postconditioning corrects behavioral abnormalities in a model of post-traumatic stress disorder in rats]

Protective effects of the novel technique of hypoxic postconditioning with a hypobaric hypoxia paradigm were studied in "stress-restress" model ofposttraumatic stress disorder in rats. It was shown that repeated (3 times) exposure of rats that survived after severe traumatic stress to mild hypobaric hypoxia (postconditioning mode) efficiently abolished the development of stress-induced anxiety state. Postconditioning had a clear anxiolytic effect both when it was delivered after traumatic stress and after restress, but the intensity of this effect depended on the period ofpathogenesis of the posttraumatic stress disorder, when postconditioning was given. The results indicate that suggested postconditioning model with repetitive mild hypobaric hypoxia exerts potent anxiolytic and stress-protective action.

[Molecular-cellular and hormonal mechanisms of induced brain tolerance of extreme factors]

This review includes results of own studies and literature data on the topical problem of neurobiology and medicine: discovery of the mechanisms of increased brain resistance to extreme exposures. The emphasis is made on the molecular-cellular and hormonal mechanisms of hypoxic preconditioning-induced brain tolerance to injurious hypoxia, psychoemotional and traumatic stress. A role of basic hormonal and intracellular cascade pro-adaptive processes mediating the neuroprotective action of hypoxic preconditioning is reviewed. A dynamics of the mechanisms of development of induced susceptible brain areas (hippocampus, neocortex) tolerance which includes phases of induction, transformation and expression, is presented. New data on preconditioning-induced cross-tolerance providing increased brain resistance not only to hypoxia but also to other stresses are reported. For the first time neuroprotective effects of hypoxic postconditioning are described.

[A test for evaluation of pituitary-adrenal axis disregulation]

In rat models, a modification of the fast feedback paradigm for the pituitary adrenocortical system applied to detect posttraumatic stress disorder (PTSD) was developed. Both standard and modified methods were used. In contrast to the standard method (injection of exogenous cortisol), the new modification suggested measuring blood corticosterone, rather than adrenocorticotropic hormone, at the early stages of development of the stress reaction (3, 10, 30, and 60 min of the exposure to stress factors). With the suggested modification, the fast feedback phenomenon was studied in reliable rat models of PTSD (stress-restress) and depression (learned helplessness). Fast pathological inhibition of the pituitary adrenocortical system by the fast feedback mechanism was revealed only during the simulated PTSD but not in the depressive-like state, which supported the specificity and validity of the developed modification of the test. Thus, the proposed methodological modification is a valid tool for diagnostics of the PTSD-specific fast feedback inhibition of the pituitary adrenocortical system in the animal models of this psychopathology.

[The hypoxic preconditioning effect upon expression of the NGFI-A transcription factor in the rat brain following an unavoidable stess in the "learned helpnessness" model]

In this study, by an immunocytochemistry it is established that development of the depression after an inescapable stress in the learned helplessness model in rats is associated with stable induction of the transcription factor NGFI-A in the dorsal hippocampus (CA1 field) and in the paraventricular hypothalamic magnocellular nucleus (PVNm), as well as with rapid and transient stress-induced expression of NGFI-A in the dentate gyrus and, supported on high level till 5 days, in the neocortex. Hypoxic preconditioning using mild repetitive hypobaric hypoxia (360 Torr for 2 hrs each of 3 days) prevented development of the depressive state in rats, and considerably changed the dynamics of the NGFI-A immunoreactivity in the hippocampus: the stable increase of an expression in the dentate gyrus and only transitory and delayed (for 5 day) in the CA1 field was detected. In the neocortex (Layer II) the stress influence was levelled with preconditioning by preventing the prolongation of the first wave of expression NGFI-A untill 5 days, and in PVNm, on the contrary, was stimulated the second (delayed) wave of an expression of this transcription factor. The pattern of NGFI-A expression in the hippocampus, neocortex and hypothalamus of preconditioned rats revealed an obvious pathological response to aversive stress, which results in development of depressive frustration rather than the adaptation. Stress-induced modifications of early gene product NGFI-A expression in the brain caused by hypoxic preconditioning, possibly, play important role in tolerance to hard psychoemotional stresses and may be an important part of antidepressive mechanisms.

[Effect of mild hypobaric hypoxia in the preconditioning regime on expression of pCREB and NF-kappaB transcription factors in the rat hippocampus before and after severe hypoxia]

Preconditioning using threefold mild hypobaric hypoxia (HH) is known to increase the tolerance of vulnerable brain neurons to severe hypoxia and other damaging factors. In the present study, the changes of the expression of transcription factors NF-kappaB (nuclear factor kappa B) and CREB (cAMP response element binding protein) were studied in the hippocampus of rats preconditioned by mild hypoxia. Using immunocytochemical method, it was demonstrated that HH increased NF-kappaB and phosphorylated CREB (pCREB) immunoreactivity in CA1-CA4 fields of the hippocampus and gyrus dentatus. It also contributed to the up-regulation of the expression of these transcription factors in the hippocampus of preconditioned rats 3-24 h following severe HH. These findings suggest that NF-kappaB and CREB are involved in HH-activated mechanisms of brain tolerance development.

The possible use of hypoxic preconditioning for the prophylaxis of post-stress depressive episodes

The protective effects of hypoxic preconditioning on the development of depressive states in rat models were studied. Three episodes of intermittent preconditioning using hypobaric hypoxia (360 mmHg, 2 h) prevented the onset of depressive behavioral reactions, hyperfunction of the hypophyseal-adrenal system, and impairments in its suppression in the dexamethasone test in rats following unavoidable aversive stress in a model of endogenous depression. The anxiolytic and antidepressant actions of hypoxic preconditioning in experiments on rats were no less marked than those of the tetracyclic antidepressant ludiomil. The results obtained here provide evidence that preconditioning with intermittent hypobaric hypoxia increases resistance to psychoemotional stresses, has marked anxiolytic and antidepressant effects, and can be used for the prophylaxis of depressive episodes.

[Modifications of hypothalamic neurohormone expression in animal model of post-traumatic stress disorder]

Neuroendocrine mechanisms of development of anxiety-like state in rats in experimental model of human post-traumatic stress disorder (PTSD), referred to as stress-restress paradigm, have been studied. Immunocytochemistry has revealed significant increase of CRH expression in both parvo- and magnocellular subdivisions of paraventricular nuclei up to 10th post-stress day. Significant reduction of vasopressin expression in rats' hypothalamic paravntricular nuclei has been detected on the 1st post-stress day. Vasopressin immunoreactivity in hypothalamus of stressed animals was indistinguishable from that in control group on the 10th post-stress day only in parvocellular subdivision of the paraventricular nucleus, while it was increased in magnocellular subdivision of the nucleus in experimental group compared to controls. The results imply a plausible role of hypothalamic vasopressin along with CRH on the development of PTSD.

[The anxyolytic effect of mild hypobaric hypoxia in a model of post-traumatic stress disorder in rats]

The impact of mild hypobaric hypoxia on the development of anxiety-like state in rats in experimentally simulated human post-traumatic stress disorder was studied. Three-trial exposure to mild hypobaric hypoxia (360 mm Hg for 2 hours daily, for 3 days) in preconditioning or post-conditioning mode performed, respectively, before or after exposure to severe traumatic stress in the "stress-restress" model produced a significant anxiolytic effect on the rat open-field and plus-maze behavior. Anxiolytic effect of modem antidepressant Paxil (20 mg/kg daily, for 3 days) was weaker. This drug produced side-effects on particular behavioral characteristics in the open field. The conclusion was made on the efficacy of mild hypobaric hypoxia and the possibility of its implementation as a medication-free tool for prophylaxis and correction of post-traumatic stress disorder.

[The feasible applications of hypoxic preconditioning for prevention of post-stress depressive episodes]

The protective effect of hypoxic preconditioning on the development of depressive states in rat experimental models has been studied. Three-trial preconditioning by repetitive hypobaric hypoxia (360 mm Hg, 2h) prevented triggering of behavioral depression, pituitary-adrenal axis hyperactivity and impairment of negative feedback in dexamethasone test in rats following inescapable aversive stress in the model of endogenous depression. Anxyolytic and antidepressant-like effect of the hypoxic preconditioning was not less than that of tetracyclic antidepressant ludiomil. The findings reveal that the preconditioning by intermittent hypobaric hypoxia increases tolerance to psychoemotional stress, possesses potent anxiolytic and antidepressant-like effects and might be applied for prevention of depressive episodes.

[The extrahypothalamic level of the hypophyseal-adrenocortical system regulation appears to be involved in development of different types of depression-like state in rats]

Possible role of extrahypothalamic corticotrophin-releasing hormone (CRH) and vasopressin-producing centers in post-stress depression development were studied. We used genetically selected strains: KHA (Koltushi High Avoidance) and KLA (Koltushi Low Avoidance) rats developing different types of depression in the "learned helplessness" paradigm: the model analogues of endogenous (KHA strain) and exogenous (KLA strain) depression. Interstrain differences of control and stress-induced CRH- and vasopressin-expression in hippocampus and neocortex in the course of depression development in KHA and KLA rats were revealed using immuno-histochemical studies. It has been shown that a significant increase of CRH- and vasopressin-immune reactivity in hippocampus and neocortex of KHA rats occurred on the 10th post-stress day. We detected also decreased CRH- and vasopressin-expression in dorsal hippocampus, and increased CRH-immune reactivity in neocortex of KLA rats in the same post-stress period. These findings imply that extrahypothalamic CRH- and vasopressin-ergic systems appear to be involved in pathogenesis mechanisms of model analogues of endogenous and exogenous depression in different ways.

Expression of vasopressin in the hypothalamus of active and passive rats with poststress depression

No interstrain differences were revealed in vasopressin concentration in the hypothalamus of control and treated active and passive rats with poststress depression. Changes in vasopressin immunoreactivity corresponded to variations in corticotropin-releasing hormone concentration observed in this model of depression. These data suggest that vasopressin contributes to the development of this experimental psychopathology.

The effect of preconditioning on the Cu, Zn superoxide dismutase expression and enzyme activity in rat brain at the early period after severe hypobaric hypoxia

Severe hypoxia results in functional and structural injury of the brain. A preconditioning with repetitive episodes of mild hypoxia considerably ameliorates neuronal resistance to subsequent severe hypoxia. Activation of endogenous antioxidants including Cu, Zn-depending superoxide dismutase (Cu, Zn-SOD) (EC.1.15.1.1) is one of the main cell defense mechanisms against oxidative stress induced by hypoxia. Alterations of expression and enzyme activity of Cu, Zn-SOD 3 and 24h after severe hypobaric hypoxia in forebrain structures of preconditioned and non-preconditioned rats were investigated. We found that hypoxia without preconditioning suppressed the Cu, Zn-SOD enzyme activity at 3h time-point but preconditioning essentially modified the reaction to severe hypoxia by increasing the expression and activity of Cu, Zn-SOD during early stages of reoxygenation crucial for apoptosis initiation.

Expression of early gene proteins, structural changes in brain neurons in hypobaric hypoxia, and the correcting effects of preconditioning

The Nissl method and immunocytochemistry were used to study the effects of severe hypobaric hypoxia and its actions in combination with the preconditioning actions of moderate hypoxia on the expression of the early gene proteins c-Fos and NGFI-A as well as structural changes in hippocampal and neocortical neurons in the rat brain. Severe hypoxia was found to suppress c-Fos and NGFI-A synthesis (3-24 h after exposure) and to induce delayed (days 3-7) structural damage to neurons, of the "light" and predominantly the "dark" types, which appear to reflect the development of necrotic and apoptotic processes respectively. Preconditioning with the regime used here corrected these derangements, resulting in increases in the expression of early gene proteins and significant reductions in structural damage to neurons after severe hypoxia.

Preconditioning enhances the expression of mitochondrial antioxidant thioredoxin-2 in the forebrain of rats exposed to severe hypobaric hypoxia

The impact of severe hypoxia and preconditioning on the expression of the mitochondrial antioxidant thioredoxin-2 (Trx-2) in rat hippocampus (CA1, CA2, CA3 fields, and dentate gyrus) and neocortex was studied by immunocytochemistry. The preconditioning consisted of three trials of mild hypobaric hypoxia (360 Torr, 2 hr) spaced at 24 hr. The last trial was followed by severe hypobaric hypoxia (180 Torr, 3 hr) 24 hr later. Both in hippocampus and in neocortex, severe hypobaric hypoxia resulted in enhanced Trx-2 expression at 3 hr, followed by a slight decline in Trx-2 levels, which nevertheless remained increased at 24 hr elsewhere except for the CA1 region. The preconditioning considerably augmented severe hypoxia-induced Trx-2 immunoreactivity, affecting both the number of immunoreactive cells and the intensity of immunostaining. The findings suggest a role for Trx-2 in the formation of brain hypoxic/ischemic tolerance accomplished by the preconditioning.

The augmentation of brain thioredoxin-1 expression after severe hypobaric hypoxia by the preconditioning in rats

Induction of endogenous antioxidants is one of the key molecular mechanisms of cell resistance to hypoxia/ischemia. The effect of severe hypoxia on the expression of cytosolic antioxidant thioredoxin-1 (Trx) in hippocampus and neocortex was studied in preconditioned and non-preconditioned rats. The preconditioning consisted of three trials of mild hypobaric hypoxia (360 Torr, 2 h) spaced at 24 h. Twenty-four hours after the last trial rats were subjected to severe hypobaric hypoxia (180 Torr, 3 h). Trx expression was studied by immunocytochemistry. In hippocampus severe hypobaric hypoxia rapidly induced Trx expression, which remained elevated still at 24 h. In neocortex the enhanced expression appeared only at 24 h. The preconditioning significantly augmented severe hypoxia-induced Trx-immunoreactivity at 3 h but not at 24 h. These findings point out that Trx contributes to mechanisms of brain tolerance to hypobaric hypoxia, especially in early periods after the exposure.

[Dynamics of hypothalamic CRH immune reactivity in active and passive rats in the course of development of behavioural depression]

A possible relation between activity of the main CRH-producing centers of hypothalamus and depressive-like behavior of animals was studied. We used genetically selected strains--KHA (Koltushi High Avoidance) and KLA (Koltushi Low Avoidance) rats, demonstrating active and passive strategy of adaptive behavior in novelty situaltions, respectively. Rats were exposed to inescapable stress to develop a "learned helplessness". We observed considerable differences between two strains of animals in CRH-expression in parvo-, magno-cellular parts of the paraventricular nucleus and in the supraoptic nucleus in the course of behavioral depression development. Significant differences between control groups were seen only in paraventricular nucleus. On the 1st post-stress day in hypothalamus of KLA rats, we detected decreased CRH immune reactivity that remained unchanged up to the 10th day. In KHA rats, there were no notable changes of CRH expression in all studied nuclei. These findings, including previous results on different dynamics of behavioral changes and different hypothalamo-pituitary-adrenocortical system activity during development of depression in KLA and KHA rats, indicate that "learned helplessness" in these two groups of animals provides the model analogues of different types of depression. Besides, these findings indicate different implication of hypothalamus CRH-system in the behavioral depression development in rats with divergent strategy of adaptive behavior.

[Early genes expression, structural neuron changes in hypobaric hypoxia and correcting effect of preconditioning]

The effects of severe hypobaric hypoxia and preconditioned severe hypoxia on the expression of early genes products--proteins c-Fos and NGFI-A, and structural changes in rat hippocampal and neocortical neurons were studied using Nissl staining and immunocytochemistry. Severe hypoxia was found to induce a suppression of c-Fos and NGFI-A synthesis (at 3-24 h after exposure) and delayed (by day 3-7) destructive neuronal changes, which developed according to "light" and predominantly "dark" type, that obviously reflected necrotic or apoptotic processes, respectively. The preconditioning in the regime applied abolished these pathological changes, as expressed by stimulation of early gene products expression and marked reduction of neuronal damage following severe hypoxia.

[Corticotropin-releasing hormone in the regulation of adaptive behavior]

Recent findings on the role of corticotropin-releasing hormone (CRH) in the regulation of stress and its consequences are summarized and analyzed in the review. Being involved in stress-activating system this neurohormone is referred to as a neurochemical factor triggering and integrating both endocrine and behavioral functions. The CRH distribution in hypothalamus and extrahypothalamic brain regions relevant to its involvement in the controlling of endocrine processes and behavior is viewed in details. Distinct behavioral outcomes of stress and the contribution of amygdalar, hippocampal, and striatal CRH-structures, implicated in general organism response to external influences, are widely discussed. From this viewpoint the mechanisms involved in the development of post-stress psychopathology, as well as drug addiction and alcoholism are treated.

Localization of corticoliberin receptors in the rat brain

In situ hybridization was used to study the distribution of corticoliberin receptors of subtypes 1 and 2 (CL-R1 and CL-R2 respectively) in different structures of the rat brain. Levels of CL-R1 mRNA in the brain were significantly greater than levels of CL-R2 mRNA, and the most intense expression of the CL-R1 gene was seen in forebrain structures, especially various neocortical, archicortical, and paleocortical regions in the cerebellar cortex. In addition, significant levels of CL-R1 mRNA expression were noted in the red nucleus and the reticular nucleus of the tegmentum. Intense expression of CL-R2 mRNA was observed in structures of the olfactory system, corticomedial parts of the amygdala, fields CA1-CA4 of the hippocampus, the ventromedial hypothalamus, and several brain stem nuclei. Moderate levels of CL-R2 mRNA were seen in the dorsolateral neostriatum. These results provide evidence that corticoliberin receptors of both subtypes are widespread in the brain. The different patterns of expression of CL-R1 and CL-R2 in the brain probably provide the basis for the functional specificity of action of corticoliberin in brain structures.

Expression of early genes in the rat brain after administration of corticoliberin into the neostriatum

In situ hybridization using oligonucleotide probes was used to study the effects of intrastriatal microinjection of corticoliberin on the expression of the early genes c-fos, jun B, c-jun, and NGFIA in the rat brain. Administration of corticoliberin (0.25 microg) into the neostriatum induced the expression of mRNA encoded by the early genes c-fos, jun B, and NGFIA in both the neostriatum itself and in its efferent structures, particularly the nucleus accumbens and various parts of the cortex. Intrastriatal microinjection of corticoliberin had no effect on the expression of mRNA for the oncogene c-jun in the brain. These results suggest that neuronal activation in the neostriatum and its projection targets manifest as the expression of early genes is one of the mechanisms underlying the adaptive effects of corticoliberin in stress.

[Localization of corticoliberin receptors in the rat brain]

Using the in situ hybridization, a distribution of corticotropin-releasing hormone (CRH) receptors CRH-1 and CRH-2 in the rat brain has been studied. Brain levels of CRH-1 mRNA were higher than the those of CRH-2 mRNA. Most intensive CRH-1 gene expression was observed in the forebrain, including neocortex, archicortex, paleocortex and cerebellar cortex. In addition, significant CRH-1 mRNA expression was detected in the red nucleus, pontine nucleus, cochlear nucleus and reticular tegmental nucleus. CRH-2 mRNA was intensively expressed in the olfactory structures, corticomedial amygdala, CA1-CA4 of hippocampus, ventramedial hypothalamus and several modullar nuclei. Moderate CRH-2 mRNA level were seen in the dorsomedial neostriatum. The findings indicate that within the brain two types of CRH receptors are widely expressed. A distinct pattern of CRH-1 and CRH-2 expression in the brain appears to underline the functional specifics of the CRH actions in brain structures.

[Early gene expression in the rat brain following administration of corticoliberin in the neostriatum]

Using in situ hybridisation with oligonucleotide probes, an expression of immediate early genes c-fos, jun B, c-jun, and NGFIA in the rat brain was studied following intrastriatal microinjection of corticotropin-releasing hormone (CRH). The hormone induced expression of c-fos, jun B, and NGFIA mRNAs in the neostriatum as well as in its target brain areas, including nucleus accumbens and different cortical areas. The expression of c-jun mRNAs was unaffected. The findings indicate that neuronal activation of the neostriatum and its target brain areas provides one possible mechanism for mediating adaptive CRH actions in stress.

[Corticoliberinergic mechanisms of the neostriatum in the neuroendocrine stress regulation]

Neostriatum and its dopaminergic mechanisms were shown to be involved in mediation of some adaptive effects of the CRH (corticotropin-releasing hormone), both behavioural and endocrine. The data obtained suggest that neostriatum contributes to a coping response by selecting an appropriate behavioural strategy in stressful situation.

Involvement of the striatum in the central regulation of the hormonal functions of the gonads

Studies reported here show that intrastriatal administration of corticoliberin to rats decreases the blood testosterone level. However, in conditions of chemical deficiency of dopaminergic transmission in the dorsal striatum induced by injection of 6-hydroxydopamine, the effect of this neurohormone did not appear. It is concluded that extrahypothalamic corticoliberin is involved in regulating the hormonal reproductive system acting via dopaminergic mechanisms.

Dopaminergic mechanisms of neostriatum in the regulation of adaptive behavior by corticoliberin

A conditioned active avoidance response was developed in rats with high (KHA) and low (KLA) rates of learning and the effects of injection of corticoliberin into the dorsal striatum on orientational-investigative and avoidance behavior were studied in normal animals and after depletion of striatal dopamine by preliminary injection of 6-hydroxydopamine. These studies showed that corticoliberin, like 6-hydroxydopamine, produced similar trends in the animals' behavior. Their effects were mediated by opposite mechanisms in animals with initial active and passive learning strategies for adaptive behavior. The role of dopaminergic structures of the striatum in mediating the behavioral effects of corticoliberin is discussed.

[The participation of the striatum in the central regulation of gonadal hormonal function]

Intrastriatal corticotrophin-releasing hormone (CRH) was shown to induce a decrease in the plasma tectosterone concentration. Besides, the 6-OHDA pre-treatment completely prevented the suppression of plasma testosterone in response to intrastriatal CRH administration. The findings suggest that the striatum is involved in the extrahypothalamic regulation of the gonadal endocrine function.

[The effect of administering corticoliberin into the striatum on the open-field and shuttle-box behaviors of KHA and KLA strain rats]

The effects of corticotropin-releasing hormone (CRH) injected into the dorsal neostriatum on the open-field and shuttle-box behavior were studied in rats with high (Koltushi high avoidance, KHA) and low (Koltushi low avoidance, KLA) capability for avoidance learning. The effects of this hormone on the behavior of these rat strains were different. In KLA rats with passive strategy of behavior the CRH injection led to a rapid locomotor activation in the open field, while the rats with active behavioral strategy (KHA) reacted to the injection by a significant decrease in locomotion and change for the passive mode of behavior. The same CRH effects on locomotion were obtained in the shuttle-box experiments. Moreover, in the KLA rats the neurohormone injection resulted in an improvement of avoidance learning in contrast to the KHA rats, in which CRH substantially impaired avoidance learning. The obtained evidence is discussed in terms of the important role of striatal CRH in the choice of behavioral strategy in stress.

[Dopaminergic mechanisms of the neostriatum in the corticoliberin regulation of adaptive behavior]

Administration of corticotropin-releasing Hormone (CRH) to dorsal striatum in the course of active avoidance and open-field behaviour of genetically selected rats exerted different effects on adaptive behaviour of high-acquisition (KHA) and low-acquisition (KLA) rats. The findings suggest an important role of striatal dopamine in behavioural effect of the CRH.

Modification of the behavioral effects of corticoliberin by early post-natal administration of corticosteroid hormones

Experiments on rats in which hydrocortisone was given in the early postnatal period were used to study the effects of intrastriatal microinjection of corticoliberin on behavior in an open field test. Bilateral microinjection of corticoliberin into the neostriatum led to a sharp reduction in orientational-investigative activity. Rats given hydrocortisone in the first days of life had elevated movement activity, and the anxiogenic effect of corticoliberin was absent in these animals.