Cerebral vasoconstriction during sustained ventricular tachycardia induces an ischemic stress response of brain tissue in rats

Effects of adrenaline and vasopressin on cerebral microcirculation at baseline and during global brain ischemia and reperfusion in rabbits

BACKGROUND

During cardiopulmonary resuscitation, the brain becomes ischemic. Adrenaline and vasopressin have been recommended for use during cardiopulmonary resuscitation. We aimed to investigate the direct effects of adrenaline and vasopressin on the cerebral microvasculature at baseline and during ischemia and reperfusion in rabbits.

METHODS

The closed cranial window method was used to visualize the cerebral microcirculation and changes in the pial arteriole diameter in rabbits. Adrenaline and vasopressin were administered topically on the brain tissue. First, the effects of adrenaline and vasopressin on pial arterioles were evaluated in 7 rabbits that were given 4 different concentrations of adrenaline, and another 7 rabbits that received 4 different concentrations of vasopressin. Second, the effects of adrenaline and vasopressin were determined during the global brain ischemia and reperfusion, which was induced by clamping the brachiocephalic, left common carotid, and left subclavian arteries for 15 min. An additional 21 rabbits were randomly assigned to receive artificial cerebrospinal fluid (aCSF) (n = 7), adrenaline 10^-5 mol/L (n = 7), or vasopressin 10^-7 mol/L (n = 7). Each drug was continuously infused from 5 min after the initiation of ischemia until 120 min after reperfusion. The pial arteriole diameters were recorded before and during ischemia, and after reperfusion.

RESULTS

At baseline, adrenaline and vasopressin did not affect the cerebral pial arterioles. During ischemia, vasopressin, but not aCSF and adrenaline constricted the pial vessels. Late in the reperfusion phase, pial diameter became reduced in the vasopressin and aCSF groups whereas pial diameter was higher in the animals treated with adrenaline.

CONCLUSIONS

Adrenaline and vasopressin did not affect pial arterioles at baseline. During reperfusion, adrenaline may counteract the cerebral vasoconstriction.

Young Sprague Dawley rats infected by Plasmodium berghei: A relevant experimental model to study cerebral malaria

Cerebral malaria (CM) is the most severe manifestation of human malaria yet is still poorly understood. Mouse models have been developed to address the subject. However, their relevance to mimic human pathogenesis is largely debated. Here we study an alternative cerebral malaria model with an experimental Plasmodium berghei Keyberg 173 (K173) infection in Sprague Dawley rats. As in Human, not all infected subjects showed cerebral malaria, with 45% of the rats exhibiting Experimental Cerebral Malaria (ECM) symptoms while the majority (55%) of the remaining rats developed severe anemia and hyperparasitemia (NoECM). These results allow, within the same population, a comparison of the noxious effects of the infection between ECM and severe malaria without ECM. Among the ECM rats, 77.8% died between day 5 and day 12 post-infection, while the remaining rats were spontaneously cured of neurological signs within 24-48 hours. The clinical ECM signs observed were paresis quickly evolving to limb paralysis, global paralysis associated with respiratory distress, and coma. The red blood cell (RBC) count remained normal but a drastic decrease of platelet count and an increase of white blood cell numbers were noted. ECM rats also showed a decrease of glucose and total CO2 levels and an increase of creatinine levels compared to control rats or rats with no ECM. Assessment of the blood-brain barrier revealed loss of integrity, and interestingly histopathological analysis highlighted cyto-adherence and sequestration of infected RBCs in brain vessels from ECM rats only. Overall, this ECM rat model showed numerous clinical and histopathological features similar to Human CM and appears to be a promising model to achieve further understanding the CM pathophysiology in Humans and to evaluate the activity of specific antimalarial drugs in avoiding/limiting cerebral damages from malaria.

Cerebral ischemia increases bone marrow CD4+CD25+FoxP3+ regulatory T cells in mice via signals from sympathetic nervous system

Recent evidence has shown that an increase in CD4(+)CD25(+)FoxP3(+) regulatory T (Treg) cells may contribute to stroke-induced immunosuppression. However, the molecular mechanisms that underlie this increase in Treg cells remain unclear. Here, we used a transient middle cerebral artery occlusion model in mice and specific pathway inhibitors to demonstrate that stroke activates the sympathetic nervous system, which was abolished by 6-OHDA. The consequent activation of β2-adrenergic receptor (AR) signaling increased prostaglandin E2 (PGE2) level in bone marrow. β2-AR antagonist prevented the upregulation of PGE2. PGE2, which acts on prostaglandin E receptor subtype 4 (EP4), upregulated the expression of receptor activator for NF-κB ligand (RANKL) in CD4(+) T cells and mediated the increase in Treg cells in bone marrow. Treatment of MCAO mice with RANKL antagonist OPG inhibited the increase in percent of bone marrow Treg cells. PGE2 also elevated the expression of indoleamine 2,3 dioxygenase in CD11C(+) dendritic cells and promoted the development of functional Treg cells. The effect was neutralized by treatment with indomethacin. Concurrently, stroke reduced production of stromal cell-derived factor-1 (SDF-1) via β3-AR signals in bone marrow but increased the expression of C-X-C chemokine receptor (CXCR) 4 in Treg and other bone marrow cells. Treatment of MCAO mice with β3-AR antagonist SR-59230A reduced the percent of Treg cells in peripheral blood after stroke. The disruption of the CXCR4-SDF-1 axis may facilitate mobilization of Treg cells and other CXCR4(+) cells into peripheral blood. This mechanism could account for the increase in Treg cells, hematopoietic stem cells, and progenitor cells in peripheral blood after stroke. We conclude that cerebral ischemia can increase bone marrow CD4(+)CD25(+)FoxP3(+) regulatory T cells via signals from the sympathetic nervous system.

d-ribose improves cardiac contractility and hemodynamics, and reduces expression of c-fos in the hippocampus during sustained slow ventricular tachycardia in rats

BACKGROUND

Moderate hypotension during hemodynamically stable ventricular tachycardia (VT), leads to cerebral ischemia. Supplementation of d-ribose has been shown to improve cardiac metabolism. We hypothesized that cerebral ischemia during slow VT may lead to the expression of immediate early genes related to neurodegeneration. This expression may be prevented by d-ribose substitution.

METHODS

Slow VT was induced over 20 min by external left ventricular pacing after infusion of physiologic saline or d-ribose (450 mg/kg) in 44 rats. Different coloured microspheres were used for tissue blood flow measurements. Histochemistry of c-fos in cerebral tissue sections was performed.

RESULTS

With the onset of VT, the mean arterial pressure (MAP) significantly dropped in both groups. However, the MAP in the d-ribose group was significantly higher (p<0.05) than in the control group (111+/-21 mm Hg vs. 80+/-40 mm Hg). The rate pressure product (RPP) during VT was significantly higher in the d-ribose group than in the control group (75,000 vs. 59,000, p<0.05). The occurrence of lethal VT was significantly higher in the control group and could be prevented by d-ribose. A stable activation of c-fos was observed in the control group. This ischemic stress response of the brain could not be seen after d-ribose infusion.

CONCLUSION

d-ribose improves hemodynamic parameters, cardiac contractility and prevents the activation of pro-apoptotic c-fos, demonstrating a neuroprotective effect of d-ribose during slow VT.

Sympathetic mechanisms in cerebral blood flow alterations induced by spinal cord stimulation

OBJECT

Cervical spinal cord stimulation (SCS) has been found to augment cerebral blood flow (CBF) in a number of animal models, although the mechanisms underlying the cerebrovascular effects of SCS are poorly described. In this study, the authors examined the role of sympathetic tone in CBF alterations induced by SCS in rats.

METHODS

Spinal cord stimulation was performed at three intervals while CBF was monitored with laser Doppler flowmetry (LDF). Either hexamethonium (5, 10, or 20 mg/kg), prazosin (0.25, 0.5, or 1 mg/kg), idazoxan (0.5, 1, or 2 mg/kg), propranolol (1, 2, or 4 mg/kg), or vehicle was administered intravenously before the second stimulation. Changes in LDF values due to SCS were recorded as the percentage of change from baseline values and were analyzed. In vehicle-treated animals, SCS increased LDF values by 60.5 +/- 1.8% over baseline, whereas both high-dose hexamethonium and prazosin completely abolished the SCS-induced increases in LDF values. On the other hand, LDF values increased by 50.9 +/- 4% and 61.4 +/- 4% after SCS in the presence of idazoxan or propranolol, respectively. Administration of sympathetic nervous system blockers resulted in a variable degree of systemic hypotension as well. Nevertheless, induced hypotension without sympathetic blockade had only a minimal effect on SCS-induced augmentation of LDF values (48 +/- 1.4% over baseline).

CONCLUSIONS

Sympathetic tone plays a major role in SCS-induced increases in CBF. This effect seems to be mediated primarily by alpha1-adrenergic receptors. Systemic hypotension alone cannot explain the effects of sympathetic blockade on the SCS response. Clinical use of SCS in the treatment of cerebral ischemia should take alpha1-adrenergic receptor sympathetic tone into account.

Development of a novel fluorescent microsphere technique to combine serial cerebral blood flow measurements with histology in the rat

The aim of the study was to evaluate the microsphere technique for the quantitative assessment of regional cerebral blood flow (rCBF) at different time points in the same animal. Yellow-green and red fluorescent microspheres with a diameter of 15 microm were injected into the rat at two different time points via a cannula inserted into the left ventricle of the heart. The reproducibility of the rCBF measurements in normocapnic conditions (n=7) and the responsiveness of the flow to hypercapnia induced by 7% CO(2) (n=7) was examined. The fluorescent spheres were counted on 100 microm vibratome sections of perfusion-fixed brains and rCBF was calculated. The median total CBF in normocapnic rats was 224 ml/min/100 g for the first microsphere injection and 216 ml/min/100 g for the second one. In the hypercapnic group CBF amounted to 400 ml/min/100 g and after 30 min of normocapnia decreased to 178 ml/min/100 g. No differences between the left and right hemisphere were found and there was no indication that the first injection might have influenced the second one. The described approach allows combining the assessment of rCBF at different time points in physiological or pathological conditions with histological evaluation of related morphological alterations in the same brain region of the same animal.

Case report: regional cerebral hypoperfusion induced by ventricular tachycardia - short-term hippocampal hypoperfusion and its potential relationship to selective neuronal damage

BACKGROUND

Focussing on regional cerebral hypoperfusion during hemodynamically stable, but borderline hypotensive, sustained ventricular tachycardia (VT) experimental studies show (1) a reduction of cerebral blood flow (CBF) during tachyarrhythmias in contrast to the concept of CBF autoregulation, (2) a mediation of hypoperfusion by neuronal and humoral mechanisms, and (3) an involvment of microcirculation due to an ischemic stress response of the cerebral tissue. The clinical relevance of these observations remains still unclear.

CASE REPORTS

Two patients with coronary artery disease, left ventricular dysfunction and sustained monomorphic VT underwent electrophysiological study. VT was induced and the tracer (99m)Tc-HMPAO was injected after 3 minutes of ongoing VT. Regional CBF during this life threatening arrhythmia was determined with brain SPECT. A scanning protocol was performed after termination of VT. The measurements were repeated at baseline during normofrequent sinus rhythm (SR) one week later. CBF during SR was significantly reduced in the temporal lobe in comparison to the conditions during stable VT, particularly in the left hippocampus.

CONCLUSION

The reduction of hippocampal CBF due to cerebrovascular vasoconstriction and neuronal reflex mechanism previously observed in experiments during stable, sustained VT can be confirmed in a clinical scenario by high resolution (99m)Tc-HMPAO brain SPECT. This supports the hypothesis that repetitive stable VT can play a role in the pathophysiology of cerebrovascular insufficiency. Further clinical studies are needed to analyze the impact of tachyarrhythmias on cognitive and mnemic function.