Methods: 

We investigated the role of ionotropic glutamate receptors located within the caudal portions of the nucleus tractus solitarii (cNTS) and the caudal ventral respiratory group (cVRG) in the mediation of coughing evoked by citric acid inhalation in spontaneously breathing rabbits under pentobarbitone anaesthesia. Bilateral microinjections (30-50 nl) of 10 mM CNQX and 10 mM D-AP5 were performed to block non-NMDA and NMDA receptors, respectively. The effects of ionotropic glutamate receptor blockade within the cVRG on sneezing induced by mechanical stimulation of the nasal mucosa were also studied.

Results: 

CNQX microinjections into the cNTS abolished coughing and associated tachypneic responses, while D-AP5 microinjections reduced cough responses. CNQX microinjections into the cVRG always abolished spontaneous rhythmic abdominal activity as well as coughing and tachypneic responses; D-AP5 microinjections reduced spontaneous rhythmic abdominal activity and coughing. As to sneezing, CNQX microinjections into the cVRG suppressed the expiratory thrusts without affecting the inspiratory preparatory bursts, while D-AP5 microinjections only attenuated the expiratory thrusts.

Conclusion: 

This study shows that ionotropic glutamate receptors, and especially non-NMDA receptors, are involved in the mediation of coughing induced by citric acid inhalation. The cVRG appears to be involved in the generation of the whole cough motor pattern, but seems to represent merely an expiratory output system for sneezing.