Partial neuroprotection with low-dose infusion of the alpha2-adrenergic receptor agonist clonidine after severe hypoxia in preterm fetal sheep.

We have previously shown that brief alpha(2)-adrenergic receptor blockade increased neuronal injury after severe hypoxia in preterm fetal sheep. We now examine whether infusion of an alpha(2)-adrenergic receptor agonist, clonidine, is neuroprotective. Preterm fetal sheep (70% gestation) received either saline-vehicle or clonidine at either 10 microg/kg/h (low-dose) or 100 microg/kg/h (high-dose) from 15 min until 4 h after 25 min of umbilical cord occlusion. Both low- and high-dose clonidine infusions after sham-occlusion were associated with transient EEG suppression but no neuronal loss. Low-dose but not high-dose clonidine infusions after umbilical cord occlusion were associated with a significant overall increase in numbers of surviving neurons after three days' recovery. High-dose clonidine was associated with transient hyperglycemia and increased numbers of delayed electrographic seizures. These results provide further evidence that alpha(2)-adrenergic receptor activation shortly after perinatal hypoxia-ischemia can promote neural recovery, but highlight the complex dose-response of exogenous therapy.