Alleviation of Ischemia-Induced Brain Edema by Activation of the Central Histaminergic System in Rats

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We have reported that facilitation of central histaminergic activity prevents the development of ischemia-induced brain injury. Since cerebral edema is a major cause of brain damage, we studied effects on brain edema of postischemic administration of l-histidine, a precursor of histamine, and thioperamide, a histamine H3–receptor antagonist, both of which enhance central histaminergic activity. Focal cerebral ischemia for 2 h was provoked by transient occlusion of the right middle cerebral artery in rats, and the water content and infarct size were determined 24 h after reperfusion. Changes in the extracellular concentration of histamine were examined in the striatum by a microdialysis procedure, and effects of these compounds were evaluated. Repeated administration of l-histidine (1000 mg/kg × 2, i.p.), immediately and 6 h after reperfusion, reduced the increase in the water contents in ischemic regions. Simultaneous administration of thioperamide (5 mg/kg, s.c.) with l-histidine (1000 mg/kg, i.p.) completely prevented edema formation and alleviated brain infarction, although a single dose of l-histidine, immediately after reperfusion, showed no benefits. The striatal histamine level was gradually increased after reperfusion as well as during ischemia. Simultaneous administration of thioperamide with l-histidine markedly increased the brain histamine concentration, and the value increased up to 230% of that in the saline group 5 – 6 h after reperfusion. l-histidine alone did not affect the increase in the histamine output after ischemia. These findings suggest that further activation of the central histaminergic system after initiation of cerebral ischemia prevents development of ischemia-induced brain edema.