The nitric oxide/cyclic guanosine monophosphate pathway modulates the inspiratory-related activity of hypoglossal motoneurons in the adult rat

Motoneurons integrate interneuronal activity into commands for skeletal muscle contraction and relaxation to perform motor actions. Hypoglossal motoneurons (HMNs) are involved in essential motor functions such as breathing, mastication, swallowing and phonation. We have investigated the role of the gaseous molecule nitric oxide (NO) in the regulation of the inspiratory-related activity of HMNs in order to further understand how neural activity is transformed into motor activity. In adult rats, we observed nitrergic fibers and bouton-like structures in close proximity to motoneurons, which normally lack the molecular machinery to synthesize NO. In addition, immunohistochemistry studies demonstrated that perfusion of animals with a NO donor resulted in an increase in the levels of cyclic guanosine monophosphate (cGMP) in motoneurons, which express the soluble guanylyl cyclase (sGC) in the hypoglossal nucleus. Modulators of the NO/cGMP pathway were micro-iontophoretically applied while performing single-unit extracellular recordings in the adult decerebrated rat. Application of a NO synthase inhibitor or a sGC inhibitor induced a statistically significant reduction in the inspiratory-related activity of HMNs. However, excitatory effects were observed by ejection of a NO donor or a cell-permeable analogue of cGMP. In slice preparations, application to the bath of a NO donor evoked membrane depolarization and a decrease in rheobase, which were prevented by co-addition to the bath of a sGC inhibitor. These effects were not prevented by reduction of the spontaneous synaptic activity. We conclude that NO from afferent fibers anterogradely modulates the inspiratory-related activity of HMNs by a cGMP-dependent mechanism in physiological conditions.