Neuromodulation: purinergic signaling in respiratory control.

The main functions of the respiratory neural network are to produce a coordinated, efficient, rhythmic motor behavior and maintain homeostatic control over blood oxygen and CO2/pH levels. Purinergic (ATP) signaling features prominently in these homeostatic reflexes. The signaling actions of ATP are produced through its binding to a diversity of ionotropic P2X and metabotropic P2Y receptors. However, its net effect on neuronal and network excitability is determined by the interaction between the three limbs of a complex system comprising the signaling actions of ATP at P2Rs, the distribution of multiple ectonucleotidases that differentially metabolize ATP into ADP, AMP, and adenosine (ADO), and the signaling actions of ATP metabolites, especially ADP at P2YRs and ADO at P1Rs. Understanding the significance of purinergic signaling is further complicated by the fact that neurons, glia, and the vasculature differentially express P2 and P1Rs, and that both neurons and glia release ATP. This article reviews at cellular, synaptic, and network levels, current understanding and emerging concepts about the diverse roles played by this three-part signaling system in: mediating the chemosensitivity of respiratory networks to hypoxia and CO2/pH; modulating the activity of rhythm generating networks and inspiratory motoneurons, and; controlling blood flow through the cerebral vasculature.