Synaptic mechanisms of inspiratory off-switching evoked by pontine pneumotaxic stimulation in cats

To elucidate synaptic mechanisms and the involvement of N-methyl-D-aspartate (NMDA) receptors in inspiratory off-switching (IOS) evoked by the stimulation of the nucleus parabrachialis medialis (NPBM), excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) were recorded from bulbar augmenting inspiratory (aug-I) and postinspiratory (PI) neurons in vagotomized cats. Stimulation of NPBM produced either transient inhibition or premature termination of inspiration (reversible or irreversible IOS), depending on the stimulus intensity. Each neuron displayed four-phasic postsynaptic responses during the reversible IOS, i.e. Phase 1 EPSPs, Phase 2 IPSPs, Phase 3 EPSPs and Phase 4 IPSPs in aug-I neurons, and Phase 1 plus 2 EPSPs, Phase 3 IPSPs and Phase 4 EPSPs in PI neurons. During the irreversible IOS, Phase 4 responses were replaced by sustained hyperpolarization in aug-I neurons and decrementing depolarization in PI neurons. Blockade of NMDA receptors by dizocilpine (0.3 mg kg(-1) i.v.) selectively increased Phase 4 potentials in both types of neurons and decreased the thresholds for evoking the irreversible IOS. The NPBM-induced responses had a pattern and time-course similar to those induced by vagal stimulation. The present results suggest that pneumotaxic and vagal inputs converge on the common IOS circuit, and the effectiveness of both inputs is modulated by NMDA receptors.