Moxonidine inhibits excitatory inputs to airway vagal preganglionic neurons via activation of both α2-adrenoceptors and imidazoline I1 receptors.

As an imidazoline I1 receptor agonist with very weak binding affinity for α2-adrenoceptors, moxonidine is commonly used in the treatment of hypertension. Moxonidine also has been implicated to act centrally to reduce airway vagal outflow. However, it is unknown at which central sites moxonidine acts to affect airway vagal activity, and how moxonidine takes effect at synaptic and receptor levels. In this study, airway vagal preganglionic neurons (AVPNs) were retrogradely labeled in neonatal rats from the intrathoracic trachea; retrogradely labeled AVPNs in the external formation of the nucleus ambiguus (NA) were identified in rhythmically active medullary slices using whole-cell patch-clamp techniques; and the effects of moxonidine on the spontaneous excitatory postsynaptic currents (EPSCs) of AVPNs were observed at synaptic level. The results show that moxonidine (10 μmol·L-1) significantly inhibited the frequency of spontaneous EPSCs in both inspiratory-activated and inspiratory-inhibited AVPNs. This effect was partially blocked by SKF-86466 (10 μmol·L-1), a highly selective antagonist of α2-adrenoceptors, or AGN-192403, a selective antagonist of imidazoline I1 receptors, and was completely blocked by efaroxan (10 μmol·L-1), an antagonist of both α2-adrenoceptors and imidazoline I1 receptors. These results demonstrate that moxonidine inhibits the excitatory inputs to AVPNs via activation of both α2-adrenoceptors and imidazoline I1 receptors, and suggest that physiologically both of these two types of receptors are involved in the central regulation of airway vagal activity at preganglionic level. Moxonidine might be potentially useful in diseases with aberrant airway vagal activity such as asthma and chronic obstructive diseases.