Developmental modulation of mouse hypoglossal nerve inspiratory output in vitro by noradrenergic receptor agonists

The ontogeny of the noradrenergic receptor subtypes modulating hypoglossal (XII) nerve inspiratory output was characterized. Noradrenergic agents were locally applied over the XII nucleus of rhythmically active medullary slice preparations isolated from mice between zero and 13 days of age (P0–P13) and the effects on XII inspiratory burst amplitude quantified. The α₁ receptor agonist phenylephrine (PE, 0.1–10 μM) produced a dose-dependent, prazosin-sensitive (0.1–10 μM) increase in XII nerve inspiratory burst amplitude. The magnitude of this potentiation increased steadily from a maximum of 15±8% in P0 mice to 134±4% in P12–P13 mice. The β receptor agonist isoproterenol (0.01–1.0 mM) produced a prazosin-insensitive, propranolol-sensitive potentiation of XII nerve burst amplitude. The isoproterenol-mediated potentiation increased with development from 27±5% in P0–P1 slices, to 37±3% in P3 slices and 45±4% in P9–P10 slices. The α₂ receptor agonist clonidine (1 mM) reduced XII nerve inspiratory burst amplitude in P0–P3 slices by 29±5%, but had no effect on output from P12–P13 slices. An α₂ receptor-mediated inhibition of inspiratory activity in neonates (P0–P3) was further supported by a 19±3% reduction in XII nerve burst amplitude when norepinephrine (NE, 100 μM) was applied in the presence of prazosin (10 μM) and propranolol (100 μM). Results indicate that developmental increases in potentiating α₁ and, to a lesser extent, β receptor mechanisms combine with a developmentally decreasing inhibitory mechanism, most likely mediated by α₂ receptors, to determine the ontogenetic time course by which NE modulates XII MN inspiratory activity.