1. This study describes a novel vagal respiratory reflex in anaesthetized rabbits. In contrast to the well-known inspiratory (I) off-switching by vagal afferent excitation, this vagal reflex initiates and maintains the central I activity of phrenic nerve discharges in rabbits pre-treated with antagonists of N-methyl-D-aspartate-type excitatory amino acid receptors (NMDA-Rs).
2. Under NMDA-R blockade with either dizocilpine (0.025-0.3 mg kg(-1)), D-2-amino-5-phosphonopentanoic acid (AP5, 0.5-1 mg, 1 mg, I.C.V.) or ketamine (10 mg kg(-1)), vagal stimulation at low frequencies (5-40 Hz) during the I phase prevented or markedly delayed the spontaneous I termination. In contrast, stimulation of the same vagal afferent at the same intensity but at a higher frequency (100-160 Hz) during the I phase immediately terminated the I phase.
3. In non-vagotomized rabbits, maintaining the tidal volume at end-expiratory levels during the I phase prevented spontaneous I termination and maintained apneusis after NMDA-R blockade with dizocilpine.
4. Brief stimulation of vagal afferents at low frequency (5-40 Hz) during the expiratory (E) phase constantly initiated phrenic I discharge after NMDA-R block.
5. We conclude that low-frequency discharge of vagal pulmonary stretch receptor afferents, as when lung volume is near functional residual capacity: promotes central I activity under NMDA-R blockade.