NMDA receptor blockade unmasks novel gating and memory mechanisms in vagal control of respiratory rhythm

Respiratory activity of the neonatal dorsolateral pons in vitro

The lateral and medial parabrachial and the Kölliker-Fuse nuclei (NPB/KF) are well known respiratory modulating centers in adulthood, but their role in neonates is largely unknown. We examined the role of the NPB/KF using hemi-sectioned pons-brainstem-spinal cord preparations in neonatal rats. Electrical stimulation applied at various intensities and delays in relation to the onset of spontaneous inspiratory C4 bursts, evoked transient depression or termination of C4 activity. This depression/termination was greatly attenuated either after perfusion of the NMDA-receptor antagonists (MK-801 or APV) or after microinjecting MK-801 into NPB/KF. Furthermore systemic application of the GABA-A receptor antagonist bicuculline reduced NPB/KF evoked inhibition of the C4 burst. Finally, we identified inspiratory, tonic inspiratory, expiratory, and inspiratory-expiratory (I-E) neurons which was major in the recorded neurons in the NPB/KF using the whole-cell patch-clamp method. MK-801 significantly decreased the driving potential and burst duration of I-E neurons. We conclude that neonatal NPB/KF mediated inspiratory off-switch operates on similar synaptic mechanisms as an adult.

Postnatal emergence of synaptic plasticity associated with dynamic adaptation of the respiratory motor pattern

The shape of the three-phase respiratory motor pattern (inspiration, postinspiration, late expiration) is controlled by a central pattern generator (CPG) located in the ponto-medullary brainstem. Synaptic interactions between and within specific sub-compartments of the CPG are subject of intensive research. This review addresses the neural control of postinspiratory activity as the essential determinant of inspiratory/expiratory phase duration. The generation of the postinspiratory phase depends on synaptic interaction between neurones of the nucleus tractus solitarii (NTS), which relay afferent inputs from pulmonary stretch receptors, and the pontine Kölliker-Fuse nucleus (KF) as integral parts of the CPG. Both regions undergo significant changes during the first three postnatal weeks in rodents. Developmental changes in glutamatergic synaptic functions and its modulation by brain-derived neurotrophic factor may have implications in synaptic plasticity within the NTS/KF axis. We propose that dependent on these developmental changes, the CPG becomes permissive for short- and long-term plasticity associated with environmental, metabolic and behavioural adaptation of the breathing pattern.

Ventilation induced apnea and its effect on dorsal brainstem inspiratory neurones in the rat

The purpose of this study was to examine the effect of mechanical ventilation (MV) on inherent breathing and on dorsal brainstem nucleus tractus solitarius (NTS) respiratory cell function. In pentobarbitone-anaesthetised rats, application of MV at combined high frequencies and volumes (representing threshold levels) produced apnea. The apnea persisted as long as MV was maintained at or above the threshold frequency and volume. Following removal of MV, inherent breathing did not resume immediately, with the diaphragm exhibiting post-mechanical ventilation apnea. The fall in arterial P(CO2) (Pa(CO2)) levels evoked by MV-engendered hyperventilation was shown not to be the trigger for initiation of apnea. MV-induced apnea was immediately reversed by bilateral vagotomy. Further, MV-induced apnea could not be evoked in bilaterally vagotomized animals suggesting that vagal feedback is the critical pathway for its initiation. NTS inspiratory neurones were inhibited during both MV-induced apnea and post-mechanical ventilation apnea, implying the involvement of central neural mechanisms in mediating this effect.