Dynamic modulation of inspiratory drive currents by protein kinase A and protein phosphatases in functionally active motoneurons.

Plasticity underlying adaptive, long-term changes in breathing behavior is hypothesized to be attributable to the modulation of respiratory motoneurons by intracellular second-messenger cascades. In quiescent preparations, protein kinases, including cAMP-dependent protein kinase A (PKA), potentiate glutamatergic inputs. However, the dynamic role of protein kinases or phosphatases in functionally active and behaviorally relevant preparations largely remains to be established. Rhythmic inspiratory drive to motoneurons innervating inspiratory muscles is mediated by the release of glutamate acting predominantly on AMPA receptors. In rhythmically active brainstem slices from neonatal rats, we investigated whether synaptic AMPA receptor function could be modulated by changes in intracellular PKA activity, affecting inspiratory drive in hypoglossal (XII) motoneurons. Intracellular perfusion of the catalytic subunit of PKA potentiated endogenous synaptic and (exogenously applied) AMPA-induced currents in XII motoneurons. Conversely, when a peptide inhibitor of PKA was perfused intracellularly, inspiratory drive currents were depressed. Intracellular perfusion with microcystin, a potent phosphatase 1 and 2a inhibitor, increased both endogenous and exogenous AMPA receptor-mediated currents, further supporting a role of phosphorylation in modulating motoneuronal excitability affecting behaviorally relevant synaptic inputs. These findings suggest that PKA is constitutively active in XII motoneurons in vitro. Thus, endogenous synaptic AMPA currents in XII motoneurons are influenced by phosphorylation, specifically by PKA, and dephosphorylation. The role of this modulation may be to keep the activity of motoneurons within a dynamic range that aids in responding to different physiological challenges affecting breathing, such as exercise, hypoxia, and sleep.