Age-Dependent Influence of Hypoxia on Methionine-Enkephalin Concentration within Rabbit Brainstem Nuclei

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Abstract
We examined the effect of hypoxia (FIO2 = 0.10) on methionine-enkephalin concentrations in brainstem nuclei involved in the integration of cardiopulmonary control in 3− and 21-day-old rabbits. Rabbit pups were confined in environmental chambers for 6 h and exposed to one of four conditions. Control, 21% O2 for 6 h; intermittent hypoxia, 12 cycles of 20 min 21% O2 followed by 10 min of 10% O2; acute hypoxia, 4 h of 21% O2 followed by 2 h of 10% O2; recovery, 2 h of 10% O2 followed by 4 h of 21% O2. Methionine-enkephalin was measured by radioimmunoassay in the nucleus tractus solitarius, nucleus ambiguus, nucleus parabrachialismedialis, and nucleus reticulogigantocellularis. In 3-day-old rabbits, exposure to 10% O2 did not affect methionine-enkephalin concentrations in any brainstem nuclei studied. In contrast, 21-day-old pups demonstrated a decrease in methionine-enkephalin concentration in three of the four nuclei studied when exposed to intermittent hypoxia, as well as an apparent ability to recover from an acute hypoxic exposure (p < 0.05). These data support an age-, nucleus-, and stimulus-specific effect of hypoxia on methionine-enkephalin concentration within specific brainstem nuclei and suggest a possible mechanism for the newborn's increased cardiopulmonary instability under hypoxia.