Chronic orthostatic and antiorthostatic restraint induce neuroendocrine, immune and neurophysiologial disorders in rats

BION-M 1: First continuous blood pressure monitoring in mice during a 30-day spaceflight

Animals are an essential component of space exploration and have been used to demonstrate that weightlessness does not disrupt essential physiological functions. They can also contribute to space research as models of weightlessness-induced changes in humans. Animal research was an integral component of the 30-day automated Russian biosatellite Bion-M 1 space mission. The aim of the hemodynamic experiment was to estimate cardiovascular function in mice, a species roughly 3000 times smaller than humans, during prolonged spaceflight and post-flight recovery, particularly, to investigate if mice display signs of cardiovascular deconditioning. For the first time, heart rate (HR) and blood pressure (BP) were continuously monitored using implantable telemetry during spaceflight and recovery. Decreased HR and unchanged BP were observed during launch, whereas both HR and BP dropped dramatically during descent. During spaceflight, BP did not change from pre-flight values. However, HR increased, particularly during periods of activity. HR remained elevated after spaceflight and was accompanied by increased levels of exercise-induced tachycardia. Loss of three of the five mice during the flight as a result of the hardware malfunction (unrelated to the telemetry system) and thus the limited sample number constitute the major limitation of the study. For the first time BP and HR were continuously monitored in mice during the 30-day spaceflight and 7-days of post-flight recovery. Cardiovascular deconditioning in these tiny quadruped mammals was reminiscent of that in humans. Therefore, the loss of hydrostatic pressure in space, which is thought to be the initiating event for human cardiovascular adaptation in microgravity, might be of less importance than other physiological mechanisms. Further experiments with larger number of mice are needed to confirm these findings.

Terrestrial stress analogs for spaceflight associated immune system dysregulation

Recent data indicates that dysregulation of the immune system occurs and persists during spaceflight. Impairment of immunity, especially in conjunction with elevated radiation exposure and limited clinical care, may increase certain health risks during exploration-class deep space missions (i.e. to an asteroid or Mars). Research must thoroughly characterize immune dysregulation in astronauts to enable development of a monitoring strategy and validate any necessary countermeasures. Although the International Space Station affords an excellent platform for on-orbit research, access may be constrained by technical, logistical vehicle or funding limitations. Therefore, terrestrial spaceflight analogs will continue to serve as lower cost, easier access platforms to enable basic human physiology studies. Analog work can triage potential in-flight experiments and thus result in more focused on-orbit studies, enhancing overall research efficiency. Terrestrial space analogs generally replicate some of the physiological or psychological stress responses associated with spaceflight. These include the use of human test subjects in a laboratory setting (i.e. exercise, bed rest, confinement, circadian misalignment) and human remote deployment analogs (Antarctica winterover, undersea, etc.) that incorporate confinement, isolation, extreme environment, physiological mission stress and disrupted circadian rhythms. While bed rest has been used to examine the effects of physical deconditioning, radiation and microgravity may only be simulated in animal or microgravity cell culture (clinorotation) analogs. This article will characterize the array of terrestrial analogs for spaceflight immune dysregulation, the current evidence base for each, and interpret the analog catalog in the context of acute and chronic stress.

Contribution of social isolation, restraint, and hindlimb unloading to changes in hemodynamic parameters and motion activity in rats

The most accepted animal model for simulation of the physiological and morphological consequences of microgravity on the cardiovascular system is one of head-down hindlimb unloading. Experimental conditions surrounding this model include not only head-down tilting of rats, but also social and restraint stresses that have their own influences on cardiovascular system function. Here, we studied levels of spontaneous locomotor activity, blood pressure, and heart rate during 14 days under the following experimental conditions: cage control, social isolation in standard rat housing, social isolation in special cages for hindlimb unloading, horizontal attachment (restraint), and head-down hindlimb unloading. General activity and hemodynamic parameters were continuously monitored in conscious rats by telemetry. Heart rate and blood pressure were both evaluated during treadmill running to reveal cardiovascular deconditioning development as a result of unloading. The main findings of our work are that: social isolation and restraint induced persistent physical inactivity, while unloading in rats resulted in initial inactivity followed by normalization and increased locomotion after one week. Moreover, 14 days of hindlimb unloading showed significant elevation of blood pressure and slight elevation of heart rate. Hemodynamic changes in isolated and restrained rats largely reproduced the trends observed during unloading. Finally, we detected no augmentation of tachycardia during moderate exercise in rats after 14 days of unloading. Thus, we concluded that both social isolation and restraint, as an integral part of the model conditions, contribute essentially to cardiovascular reactions during head-down hindlimb unloading, compared to the little changes in the hydrostatic gradient.

Parvalbumin immunoreactivity and protein content alter in the hippocampus after adrenalectomy in seizure sensitive gerbils

OBJECTIVES

Neurons containing parvalbumin (PV), a calcium-binding protein, in the hippocampus, play an important role in hippocampal excitability in epilepsy. In this study, we examined temporal and spatial changes of PV immunoreactivity and protein content in the hippocampus after adrenalectomy (ADX) in seizure sensitive (SS) gerbils, which are hereditarily seizure-prone.

METHODS

PV distribution and change in SS gerbils after ADX were examined in the hippocampal CA1 region and in the dentate gyrus (DG) using immunohistochemistry and Western blot analysis.

RESULTS

PV immunoreactivity in sham-operated SS gerbils was detected in many CA1 pyramidal cells. Three hours after ADX, PV immunoreactivity significantly decreased in CA1 pyramidal cells and thereafter PV immunoreactivity began to increase by 4 days after ADX. Four days after ADX, PV immunoreactivity was significantly higher than that in the sham-operated SS gerbils. In the DG of sham-operated SS gerbils, PV immunoreactivity was mainly detected in polymorphic cells. Three hours after ADX, PV immunoreactivity in the DG significantly decreased in the polymorphic layer. Thereafter, PV-immunoreactive neurons decreased with time after ADX. Western blot analysis showed that change in PV protein content was similar to immunohistochemical data after ADX in SS gerbils.

CONCLUSION

Our results indicate that PV is changed in hippocampus after ADX and PV may be associated with the regulation of seizure activity.

Continuous i.c.v. infusion of brain-derived neurotrophic factor modifies hypothalamic-pituitary-adrenal axis activity, locomotor activity and body temperature rhythms in adult male rats

Brain-derived neurotrophic factor is a neurotrophin belonging to the nerve growth factor family, which is involved in the differentiation and survival of many types of neurons. It also participates in neuroprotection and neuronal plasticity in adult rats. Our previous studies showed that a single brain-derived neurotrophic factor injection modifies hypothalamic-pituitary-adrenal axis activity in adult male rats. To investigate the effect of chronic brain-derived neurotrophic factor administration on some physiological parameters, adult rats were implanted with osmotic micro-pumps to deliver brain-derived neurotrophic factor continuously for 14 days in the lateral ventricle (12 microg/day/rat). mRNA levels were evaluated by in situ hybridization analysis, peptide contents and plasma hormone concentrations by radioimmunoassay. Animals were also equipped with telemetric transmitters to study locomotor activity and temperature rhythms modifications, since hypothalamic-pituitary-adrenal axis is known to modulate these two parameters. Decreased body weight was used as a control of brain-derived neurotrophic factor access to hypothalamic areas as already documented. In the hypothalamus the continuous brain-derived neurotrophic factor treatment increases: (i) the mRNA steady state levels of corticotropin releasing hormone and arginin-vasopressin in the paraventricular nucleus, the supraoptic nucleus, and the suprachiasmatic nucleus; (ii) the surface of corticotropin releasing hormone and arginin-vasopressin mRNA signals in these nuclei as detected by in situ hybridization, and (iii) the corticotropin releasing hormone and arginin-vasopressin contents. The plasma concentrations of adrenocorticotropic hormone and corticosterone were decreased and increased, respectively. Finally, this treatment increased daily locomotor activity and temperature, and provoked some circadian perturbations. These results obtained after chronic brain-derived neurotrophic factor administration extend data on the brain-derived neurotrophic factor involvement in the hypothalamic-pituitary-adrenal axis regulation and illustrate its effects on the locomotor and temperature rhythms. They also allow demonstrating that the regulation of the hypothalamic-pituitary-adrenal axis by brain-derived neurotrophic factor differs according to the brain-derived neurotrophic factor administration mode, i.e. acute injection or chronic administration.

Inverse-Orthostasis May Induce Elevation of Blood Pressure due to Sympathetic Activation

Microgravity and simulated microgravity may cause cardiovascular deconditioning, but mechanisms of instantaneous responses to inverse-orthostasis are not studied. Hence, we investigated transient and steady state cardiovascular changes by combining the tilt technique with cardiovascular telemetry. Normotensive and NO-deprived hypertensive Wistar rats were used to analyze responses of mean arterial blood pressure, heart rate, contractility, spontaneous baroreflex sensitivity (sBRS), and autonomic balance. Inverse-orthostasis tests were carried out by 45 degrees head-down tilting (repeated 3 x 5 mins "R", or sustained for 120 mins "S"). In normotensive rats, horizontal control blood pressure was R111.3 +/- 1.7/S110.4 +/- 2.3 mm Hg and heart rate was R385.2 +/- 5.9/S371.1 +/- 6.1 BPM. Head-down tilt induced an increase in blood pressure by R5.9/S10.6 mm Hg, while heart rate, contractility, sBRS, and autonomic balance did not change. The hypertensive response was sustained, could be prevented by prazosin (10 mg/kgbw), and augmented by subanesthetic doses of chloralose (26 and 43 mg/kgbw). In NO-suppressed hypertension, control blood pressure and heart rate were R132.4 +/- 2.9/S130.0 +/- 4.1 mm Hg and R339.2 +/- 7.9/S307.2 +/- 23.6 BPM, respectively. Head-down tilt further increased blood pressure by R5.1/S10.5 mm Hg. These data demonstrate that conscious rats respond to inverse-orthostasis by sustained elevation of blood pressure independent of NO synthesis. This response is neither due to increased contractility and altered sBRS, nor due to non-specific stress, but probably due to sympathetic activation elicited by gravity-related reflexes, which increase peripheral resistance.

Selective Suppression of an Endothelin and Platelet-Derived Growth Factor Containing Vesicular System in Endothelium of Rat Saphenous Vein by Long-Term Orthostasis

Electron-dense vesicles were observed in rat vascular endothelium. The purpose of this study was to characterize their content(s), venous-arterial distribution and response to chronic orthostatic stress in extremity vessels. Saphenous and brachial vessels - saphenous vein (SV), saphenous artery (SA), brachial vein, brachial artery - were prepared for electron microscopy to quantitate the vesicle area within the endothelium following immunohistochemical and immunocytochemical identification. The effect of long-term orthostasis was assessed by exposure to head-up tilt for 2 weeks. The vesicular area in relation to the total cross-sectional area of the endothelial cells in the SV and SA of normal and confined control groups was 3.88 +/- 0.38 versus 0.89 +/- 0.06% (p < 0.05) and 4.92 +/- 0.25 versus 1.09 +/- 0.47% (p < 0.05), respectively. Head-up tilt suppressed the vesicle content of the SV to 2.26 +/- 0.39% (p < 0.05), but it remained low in the SA (1.29 +/- 0.45%), brachial vein (0.45 +/- 0.12%) and brachial artery (0.59 +/- 0.17%). Endothelin and platelet-derived growth factor, but not acidic phosphatase activity or lipid content, could be identified in the vesicles. Plasma endothelin levels were unchanged. We conclude that dense vesicles in the endothelium of extremity vessels are not cell degradation products. They may represent a vesicular secretory or storage system for endothelin and platelet-derived growth factor which participates in regional vascular adaptation to long-term orthostatic load.

Experimental Orthostasis Elicits Sustained Hypertension, Which Can Be Prevented by Sympathetic Blockade in the Rat

Incidence of orthostatic hypertension is estimated at 5% but is even more prevalent in borderline hypertension and autonomic neuropathies. The aim of this study was to develop a potential model to investigate orthostatic hypertension. We used normotensive and hypertensive Wistar rats to analyze responses and diurnal variations of arterial blood pressure, heart rate, temperature, and locomotor activity by telemetry. Orthostatic tests were carried out during 45 degrees head-up tilt (R, repeated 3 times for 5 minutes; or S, sustained for 120 minutes). Hypertension was induced by blockade of nitric oxide synthesis. In normotensives, horizontal control blood pressure was R115.4 +/- 1.4/S113.7 +/- 1.6 mm Hg and heart rate R386.4 +/- 7.0/S377.9 +/- 8.8 bpm. Head-up tilt increased blood pressure by R4.5/S8.4 mm Hg, including a 3.8 mm Hg hydrostatic component. The sustained hypertensive response was prevented by prazosin (10 mg/kgbw) and augmented by a subanesthetic dose of chloralose (26 mg/kgbw). In NO-deprived hypertension, horizontal control blood pressure and heart rate were R138.4 +/- 2.6/S140.3 +/- 2.7 mm Hg and R342.1 +/- 12.0/S346.0 +/- 8.3 bpm, respectively. Tilt increased blood pressure further by R4.2/S9.4 mm Hg. In both normo- and hypertensives, variables exhibited similar diurnal rhythms except for nighttime locomotor activity, reduced from 3.7 +/- 0.4 to 2.8 +/- 0.3 counts/s. These data demonstrate that conscious rats respond to sustained orthostasis with hypertension, probably as a result of increased sympathetic output. Decreasing stress using a subanesthetic dose of chloralose increased this response, reducing the inhibitory effect on hypertensive responses.

Gonadal, adrenal, and neuroactive steroids' role in ictal activity

Of the many people that have epilepsy, only about 70% achieve seizure control with traditional pharmacotherapies. Steroids have long been known to influence ictal activity and may have a therapeutic role. This review summarizes recent investigations that have enhanced knowledge of the effects and mechanisms of gonadal, adrenal, and neuroactive steroids on seizure processes. Progesterone, which varies across reproductive cycles, pregnancy, and as a function of aging, has been shown to have anti-seizure effects among women with epilepsy and in animal models of epilepsy. Further, data suggest that progesterone's anti-seizure effects may involve its metabolism to the neuroactive steroid, 5 alpha-pregnan-3 alpha-ol-20-one (3 alpha,5 alpha-THP), and its subsequent actions at GABA(A) receptors. Androgens also have anti-seizure effects. Androgens' anti-seizure effects may be mediated, in part, through actions of the testosterone metabolite, and neuroactive steroid, 5 alpha-androstane-3 alpha,17 alpha-diol (3 alpha-diol) at GABA(A) receptors. Stress can alter seizure susceptibility, suggesting a role of adrenal steroids on seizure processes. In animal models of epilepsy, acute or chronic stress can increase ictal activity. Notably, stress and seizures can alter levels of gonadal, adrenal, and neuroactive steroids, which may then influence subsequent seizure activity. Thus, this review summarizes recent progress in the role of gonadal, adrenal, and/or neuroactive steroids in seizure processes which suggest that greater understanding of these steroids' effects and mechanisms may ultimately lead to improved seizure control for people with epilepsy.

Effects of acute tilt from orthostatic to head-down antiorthostatic restraint and of sustained restraint on the intra-cerebroventricular pressure in rats

The tail-cast suspension rat model was developed to explore in ground laboratories the physiological effects of some of the stresses prevailing during space flight including and among them those of the headwards body fluid shifts. We recently showed in rats that an acute head-down tilt (45 degrees) from tail-cast orthostatic (OR) to antiorthostatic restraint (AOR) induced within 30 min and for 2 to 4 h an acute stress-like surge in plasma ACTH and corticosterone levels. Considering the proximity of the CRF producing neurons with the 3rd ventricle, we decided to explore the acute and longer-term effects of the OR/AOR tilt on the intra-cerebroventricular pressure (Picv) measured with an indwelling sensor-transmitter catheter stereotaxically implanted in the 3rd ventricle. At 1- or 10-min intervals the unit sent radiotelemetric signals for both Picv and motor activity (MA) to a receiver coupled with an automatic data analyser. The acute AOR-tilt induced within 10 min and for 60 min a 2.5-fold rise in Picv which receded to baseline between 60 and 90 min. During this time, the normally close correlation between Picv and MA was lost, as assessed by Spearman's rank coefficient. In a long-term experimental series we explored the evolution of both Picv and MA in individual rats subjected successively to a 7 day control phase (C). 7 days OR, and 3 days AOR. After the 1-h-long post-tilt rise of the Picv, the mean Picv levels measured for the next 3 days decreased significantly vs. both the preceding OR phase (-30%) and the initial C Phase (-40%). The circadian pattern of the diurnal Picv profile was impaired, as evidenced by a significant fall (i) in the night/day ratio (-25% vs. C). and (ii) even more in the spectral power of the circadian 1 c/24 h frequency (-85% vs. C). The simultaneously recorded MA fluctuations similarly displayed an altered diurnal pattern with a spectral power of the circadian frequency reduced to 7% of controls. However, contrary to the short-term experiment, in the long-term study the large alterations to both Picv and MA were strongly correlated, as during the control phase. The mechanisms involved in the swift post-tilt rise in the Picv together with an aroused corticotropic axis, and in the impact of sustained head-down restraint on CNS-controlled adaptive regulations including their circadian rhythms remain unknown.

Choroidal responses in microgravity. (SLS-1, SLS-2 and hindlimb-suspension experiments)

Fluid and electrolyte shifts occurring during human spaceflight have been reported and investigated at the level of blood, cardiovascular and renal responses. Very few data were available concerning the cerebral fluid and electrolyte adaptation to microgravity, even in animal models. It is the reason why we developed several studies focused on the effects of spaceflight (SLS-1 and SLS-2 programs, carried on NASA STS 40 and 56 missions, which were 9- and 14-day flights, respectively), on structural and functional features of choroid plexuses, organs which secrete 70-90% of cerebrospinal fluid (CSF) and which are involved in brain homeostasis. Rats flown aboard space shuttles were sacrificed either in space (SLS-2 experiment, on flight day 13) or 4-8 hours after landing (SLS-1 and SLS-2 experiments). Quantitative autoradiography performed by microdensitometry and image analysis, showed that lateral and third ventricle choroid plexuses from rats flown for SLS-1 experiment demonstrated an increased number (about x 2) of binding sites to natriuretic peptides (which are known to be involved in mechanisms regulating CSF production). Using electron microscopy and immunocytochemistry, we studied the cellular response of choroid plexuses, which produce cerebrospinal fluid (CSF) in brain lateral, third and fourth ventricles. We demonstrated that spaceflight (SLS-2 experiment, inflight samples) induces changes in the choroidal cell structure (apical microvilli, kinocilia organization, vesicle accumulation) and protein distribution or expression (carbonic anhydrase II, water channels,...). These observations suggested a loss of choroidal cell polarity and a decrease in CSF secretion. Hindlimb-suspended rats displayed similar choroidal changes. All together, these results support the hypothesis of a modified CSF production in rats during long-term (9, 13 or 14 days) adaptations to microgravity.