Nitric oxide synthase activation in nTS is essential to NMDA receptor dependent encoding of the cough reflex

Polymodal vagal afferent nerves terminating in the mucosa of the large conducting airways play essential roles in regulating cough. These cough receptors project bilaterally from the nodose ganglia, are activated by protons and by mechanical stimulation, and terminate centrally in the medial (SolM) solitary tract nuclei (nTS). Although many vagal afferent nerves utilize non-NMDA type glutamate receptors exclusively for transmission centrally, cough receptor signaling requires NMDA receptor activation. We hypothesized that nTS nitric oxide synthase (NOS) activation and a cGMP transduction cascade would act as downstream messenger systems during the encoding of cough. We also hypothesized that NOS expression may be a defining characteristic of cough receptor relay neurons. NADPH-diaphorase staining identified NOS-expressing neurons throughout the brain stem (especially in the trigeminal and cuneate nuclei and in the dorsal motor nuclei of the vagus nerves). NOS expressing nTS neurons were rare but found in SolM. Bilateral SolM microinjections of NOS inhibitors or the NMDA receptor blocker SDZ 220581 markedly reduced coughing evoked by tracheal citric acid challenges. But citric acid evoked coughing was neither attenuated by inhibiting soluble guanylate cyclase in SolM nor potentiated by inhibiting the cGMP-selective phosphodiesterase-5. No changes in basal respiratory patterns were observed with NOS inhibitor or nitric oxide (NO) donor microinjections, but NMDA microinjection into SolM induced respiratory responses including cough that were at least partially NOS dependent. We conclude that NO is an essential downstream regulator of NMDA receptor mediated encoding of cough in nTS but does not act via soluble guanylate cyclase or cGMP.<b>NEW & NOTEWORTHY</b> Nitric oxide (NO) acts downstream from NMDA receptor activation to encode the cough reflex but without involvement of soluble guanylate cyclase. We propose that the enhancement of glutamatergic signaling by NO, perhaps through nitrosylation mechanisms, is an essential component of signal transduction at the synapses between cough receptor afferent nerves and their solitary tract nuclei (nTS) relay neurons.