Tissue Oxygen is Reduced in White Matter of Spontaneously Hypertensive-Stroke Prone Rats: A Longitudinal Study with Electron Paramagnetic Resonance

Abstract

Small vessel disease is associated with white-matter (WM) magnetic resonance imaging (MRI) hyperintensities (WMHs) in patients with vascular cognitive impairment (VCI) and subsequent damage to the WM. Although WM is vulnerable to hypoxic-ischemic injury and O 2 is critical in brain physiology, tissue O 2 level in the WM has not been measured and explored in vivo. We hypothesized that spontaneously hypertensive stroke-prone rat (SHR/SP) fed a Japanese permissive diet (JPD) and subjected to unilateral carotid artery occlusion (UCAO), a model to study VCI, would lead to reduced tissue oxygen (pO 2 ) in the deep WM. We tested this hypothesis by monitoring WM tissue pO 2 using in vivo electron paramagnetic resonance (EPR) oximetry in SHR/SP rats over weeks before and after JPD/UCAO. The SHR/SP rats experienced an increase in WM pO 2 from 9 to 12 weeks with a maximal 32% increase at week 12, followed by a dramatic decrease in WM pO 2 to near hypoxic conditions during weeks 13 to 16 after JPD/UCAO. The decreased WM pO 2 was accompanied with WM damage and hemorrhages surrounding microvessels. Our findings suggest that changes in WM pO 2 may contribute to WM damage in SHR/SP rat model, and that EPR oximetry can monitor brain pO 2 in the WM of small animals. Keywords: brain oxygen; EPR oximetry; vascular cognitive impairment; white matter INTRODUCTION Brain cells poorly tolerate hypoxia, and even short periods of low oxygen (O 2 ) can initiate molecular pathways that are lethal to cells in both the gray and white matter (WM). Deep WM is a common site of hypoxic/ischemic injury in the elderly where it is associated with cognitive decline, gait disturbances, and focal ischemia leading to vascular cognitive impairment (VCI). Magnetic resonance imaging (MRI) reveals white-matter hyperintensities (WMHs) in the elderly located in the periventricular and subcortical regions, and considered to be due to strokes and secondary small vessel disease. An alternative mechanism is that the deep WM is vulnerable to hypoxia for a variety of reasons. Studies in humans suggest that there is impairment in the ability of the deep WM to respond to increased demand by raising cerebral blood flow or O 2 . 1 Although glutamate mediated excitotoxicity, inflammatory cytokines, and protease activation result in oligodendrocyte death in acute ischemic injury, the pathophysiology of WM injury in chronic vascular disease has not been fully explored. 2 Small vessel disease is associated with WMHs in patients with VCI; damage to small vessels causes an inflammatory response with disruption in the blood-brain barrier (BBB) associated with expression of matrix metalloproteinases (MMPs) and subsequent damage to the WM. Vascular dementia patients show expression of MMPs in regions of loss of myelin. [3][4][5]

Hypoxia-Induced Neuroinflammatory White-Matter Injury Reduced by Minocycline in SHR/SP

“…Electron paramagnetic resonance showed an increase in tissue oxygen levels, beginning at 9 weeks of age, reaching hyperoxic levels at 12 weeks before precipitously falling into the hypoxic range from 13 weeks to death at 16 weeks. 22 Thus, the direct measurements of oxygen in tissue corroborated the increase in HIF-1α in this study, which has been shown to also occur in acute stroke. 30 Prolyl hydroxylase has a complex pattern of expression.…”

“…21 Using an oxygensensitive crystal with electron paramagnetic resonance, we showed an increase in oxygen extraction in the WM at 12 weeks of life with a drastic decrease in oxygen from the thirteenth to the sixteenth week when the animals died. 22 The present report describes the molecular events leading from hypoxia to WM damage and death, which we hypothesize is a multistep process 1 proceeding from chronic hypoxia to MMP-9-mediated BBB disruption, resulting in myelin loss and behavioral changes. 23 In addition, we treated the SHR/SP/JPD/UCAO with the antiinflammatory agent, minocycline.…”

The neuropathology and cerebrovascular mechanisms of dementia

“…A previous study from our group has investigated the mechanisms of VCI in a hypertensive model of smallvessel disease (spontaneously hypertensive stroke-prone rats with carotid occlusion and a high-salt diet), using EPR (electron paramagnetic resonance) technology. 99 An EPR lithium crystal was implanted in vivo and used to monitor interstitial tissue O 2 levels in the spontaneously hypertensive stroke-prone rats model. Our results demonstrated a reduction in tissue oxygenation within the white matter, which was exacerbated by age and associated with neuroinflammation.…”

“…100 In summary, these findings suggest a strong mechanistic link between cerebrovascular disease caused by hypertension and cerebral hypoperfusion. 99 …”

Consensus statement for diagnosis of subcortical small vessel disease

“…Measurements of the level of oxygen in the white matter over an extended period of time have been performed using the minimally invasive technique of electron paramagnetic resonance. 125 The oxygen content in the white matter begins to gradually increase by the nineth week reaching a maximum at 12 weeks after birth. After unilateral carotid artery occlusion and abnormal diet, which are started at 12 weeks, the oxygen content in the white matter falls precipitously to hypoxic levels where it remains until the animal dies around week 16.…”