Delayed Increase in Neuronal Nitric Oxide Synthase Immunoreactivity in Thalamus and Other Brain Regions after Hypoxic–Ischemic Injury in Neonatal Rats

We examined the response of neuronal nitric oxide synthase (nNOS)-containing CNS neurons in rats exposed to a unilateral hypoxic–ischemic insult at 7 days of age. Animals were sacrificed at several time points after the injury, up to and including 7 days (Postnatal Day 14). Brain regions ipsilateral to the injury (including cerebral cortex, caudate-putamen, and thalamus) exhibited delayed, focal increases in nNOS immunoreactivity. The increase in nNOS immunoreactive fiber staining was prominent in areas adjacent to severe neuronal damage, especially in the cortex and the thalamus, regions that are also heavily and focally injured in term human neonates with hypoxic–ischemic encephalopathy. In cerebral cortex, these increases occurred despite modest declines in nNOS catalytic activity and protein levels. Proliferation of surviving nNOS immunoreactive fibers highlights regions of selective vulnerability to hypoxic–ischemic insult in the neonatal brain and may also contribute to plasticity of neuronal circuitry during recovery.