Received September 29, 2005
Revised November 7, 2005
Accepted after revision November 28, 2005

¹ University of Auckland

^* To whom correspondence should be addressed. E-mail: l.bennet{at}auckland.ac.nz.

	Abstract

Exposure to severe hypoxia commonly leads to delayed cerebral and peripheral hypoperfusion. There is evidence in the very immature brain that transient abnormal glutaminergic receptor activity can occur during this phase of recovery. We therefore examined the role of N-methyl-D-aspartate (NMDA) receptor activity in mediating secondary hypoperfusion in preterm fetal sheep at 70% of gestation. Fetuses received either sham-asphyxia or asphyxia and were studied for 12 h recovery. The specific, non-competitive NMDA receptor antagonist, dizocilpine maleate (2 mg kg-1 bolus plus 0.07 mg kg h-1 i.v.) or saline were infused from 15 min after asphyxia until 4 h. In the asphyxia-vehicle group abnormal epileptiform transients were observed during the first 4 h of reperfusion, the peak of which corresponded approximately with the nadir in peripheral and cerebral hypoperfusion. Dizocilpine significantly suppressed this activity (2.7±1.3 vs. 11.2±2.7 counts min-1 at peak frequency, P<0.05) and markedly delayed and attenuated the rise in vascular resistance in both peripheral and cerebral vascular beds observed after asphyxia, effectively preventing the initial deep period of hypoperfusion in carotid blood flow and femoral blood flow (P<0.01). However, while continued infusion did attenuate subsequent transient tachycardia, it did not prevent the development of a secondary phase of persistent but less profound hypoperfusion. In conclusion, the present studies suggest that in the immature brain the initial phase of delayed cerebral and peripheral hypoperfusion following exposure to severe hypoxia is mediated by NMDA receptor activity. The timing of this effect in the cerebral circulation corresponds closely with abnormal EEG activity, suggesting a pathological glutaminergic activation that we speculate is related to evolving brain injury.

Key Words: Hypoxia, NMDA (N-methyl-D-aspartate), Vascular blood flow