Carotid sinus nerve section abolishes NMDA evoked respiratory effects in anaesthetised rats

Respiratory effects of NMDA injection into the right atrium were investigated in 11 urethane-chloralose anaesthetised and spontaneously breathing rats. The animals were initially vagotomised and six of them were subdued to the subsequent carotid sinus nerve section, and the other five were treated by NMDA antagonist. Bolus injection of NMDA (27 μmol/kg) induced the depression of ventilation in all rats, due to the decrease in tidal volume from a baseline of 2.98 ± 0.4 to 2.63 ± 0.3 ml (P < 0.01), and slowing down of the respiratory rate from a baseline of 56 ± 2.6 to 27 ± 2.0 breaths min−1 (P < 0.0001). Section of the carotid sinus nerves (CSNs) precluded the respiratory depression. Prolongation of the expiratory time was reduced by this neurotomy from 5.07 ± 2.6 to 1.04 ± 0.03 (P < 0.05). In five rats the blockade of NMDA receptors with the selective antagonist (AP-7) was likewise efficient in eliminating the post-NMDA respiratory response. NMDA increased mean arterial blood pressure and this rise occurred beyond the afferentation from the carotid bodies and the blockade of NMDA receptors. Results of this study indicate that inhibition of the respiratory drive evoked by NMDA administered via the peripheral circulation requires intact carotid bodies and activation of NMDA receptors.