Relative magnitude of tonic and phasic synaptic excitation of medullary inspiratory neurons in dogs

The relative contribution of phasic and tonic excitatory synaptic drives to the augmenting discharge patterns of inspiratory (I) neurons within the ve ntral respiratory group (VRG) was studied in anesthetized, ventilated, para lyzed, and vagotomized dogs. Multibarrel micropipettes were used to record simultaneously single-unit neuronal activity and pressure micro-ejected ant agonists of GABAergic, glycinergic, N-methyl-D-aspartate (NMDA) and non-NMD A glutamatergic, and cholinergic receptors. The discharge patterns were qua ntified via cycle-trigger histograms. The findings suggest that two-thirds of the excitatory drive to caudal VRG I neurons is tonic and mediated by NM DA receptors and the other third is ramp-like phasic and mediated by non-NM DA receptors. Cholinergic receptors do not appear to be involved. The silen t expiratory phase is produced by phasic inhibition of the tonic activity, and approximate to 80% of this inhibition is mediated by gamma-aminobutyric acid receptors (GABA(A)) and approximate to 20% by glycine receptors. Phas ic I inhibition by the I decrementing neurons does not appear to contribute to the predominantly step-ramp patterns of these I neurons. However, this decrementing inhibition may be very prominent in controlling the rate of au gmentation in late-onset I neurons and those with ramp patterns lacking the step component.