Comparison of Pyramidal and Magnocellular Neuroendocrine Cell Volume Responses to Osmotic Stress and Stroke - Like Stress

Abstract

Acute brain cell swelling (cytotoxic edema) can occur in the first minutes of stroke, presumably as a result of brain cells taking up water. In extreme hypo-osmotic situations such as excessive water-loading by patients, uptake by brain cells can expand the brain, causing seizures. But is ischemic brain cell swelling the same as hypo-osmotic swelling? Water can passively diffuse across the plasma membrane. However the presence of water channels termed aquaporins (AQP) facilitates passive water diffusion by 10-100 times. Unlike astrocytes, there is no evidence of water channels on neuronal plasma membrane. However, there is still much debate about which cells (neurons or astrocytes) swell during over-hydration or during stroke and if neurons and astrocytes can volume-regulate during osmotic stress. The purpose of this study was to examine and compare the volume responses of PyNs and magnocellular neuroendocrine cells (MNCs) to acute osmotic challenge and to OGD. We examined MNCs because they are intrinsically osmosensitive to small changes (2-3 mOsm) of plasma osmolality. We also examined if the same neurons behave similarly in brain slices or when dissociated and if they respond differently to acute osmotic stress and stroke-like stress. Our results indicate that during acute osmotic stress (±40 mOsm) half of dissociated PyNs and MNCs tended to show appropriate responses. MNCs in brain slices showed similar responses to when they were dissociated, while brain slice PyNs were less responsive than when dissociated. Exposure to OGD resulted in obvious differences between the two types of in vitro preparations. Dissociated PyNs and MNCs showed no consistency in their volume responses to 10 minutes of OGD. Dissociated neurons swelled, shrunk or were unchanged in about equal numbers. In contrast, brain slice PyNs underwent profound swelling whereas, brain slice MNCs showed minor volume decreases. We conclude that about half of our dissociated neurons were too variable and unpredictable in their osmotic volume responses to be useful for osmotic studies. They also were too resistant to stroke-like stress to be good models for ischemia. Brain slice neurons were similar in their osmotic responses to dissociated neurons but proved to have consistent and predictable responses to stroke-like stress.