Magnesium sulphate induces preconditioning in preterm rodent models of cerebral hypoxia-ischemia

Background: Brain injury in preterm infants represents a substantial clinical problem associated with development of motor impairment, cognitive deficits and psychiatric problems. According to clinical studies, magnesium sulphate (MgSO4) given to women in preterm labor reduces the risk of cerebral palsy in the offspring but the mechanisms behind its neuroprotective effects are still unclear. Our aim was to explore whether MgSO4 induces tolerance (preconditioning) in the preterm rodent brain. For this purpose we established a model of perinatal hypoxia-ischemia (HI) in postnatal day 4 rats and also applied a recently developed postnatal day 5 mouse model of perinatal brain injury. Methods: Postnatal day 4 Wistar rats were exposed to unilateral carotid artery ligation followed by 60, 70 or 80 min of hypoxia (8% O-2). On postnatal day 11, brains were collected and macroscopically visible damage as well as white and grey matter injury was examined using immunohistochemical staining. Once the model had been established, a possible preconditioning protection induced by a bolus MgSO4 injection prior to 80 min HI was examined 7 days after the insult. Next, a MgSO4 bolus was injected in C57Bl6 mice on PND 4 followed by exposure to unilateral carotid artery ligation and hypoxia, (10% O-2) for 70 min on PND 5. Brains were collected 7 days after the insult and examined with immunohistochemistry for grey and white matter injury. Results: In rats, a 60 min period of hypoxia resulted in very few animals with brain injury and although 70 min of hypoxia resulted in a higher percentage of injured animals, the brains were marginally damaged. An 80 min exposure of hypoxia caused cortical tissue damage combined with hippocampal atrophy and neuronal loss in the C3 hippocampal layer. In the rat model, MgSO4 (1.1 mg/g administered i.p. 24 h prior to the induction of HI, resulting in a transient serum Mg2+ concentration elevation to 4.1 +/- 0.2 mmol/l at 3 h post i.p. injection) reduced brain injury by 74% in grey matter and 64% in white matter. In the mouse model, MgSO4 (0.92 mg/g) i.p. injection given 24 h prior to the HI insult resulted in a Mg2+ serum concentration increase reaching 2.7 +/- 0.3 mmol/l at 3 h post injection, which conferred a 40% reduction in grey matter injury. Conclusions: We have established a postnatal day 4 rat model of HI for the study of preterm brain injury. MgSO4 provides a marked preconditioning protection both in postnatal day 4 rats and in postnatal day 5 mice.