Identification of deflation-sensitive inspiratory neurons in the dorsal respiratory group of the rat.

It has been well established that inspiratory neurons of the dorsal respiratory group (DRG) are classified into two types based on whether they receive inputs from slowly-adapting pulmonary stretch receptors (SARs) or not. Inspiratory neurons with SAR inputs are called Ibeta and the others are called Ialpha neurons. In this study, we identified a novel group of inspiratory neurons in Nembutal-anesthetized, paralyzed, and artificially-ventilated rats. (1) These DRG inspiratory neurons were activated characteristically by lung deflation. (2) They were orthodromically activated by electrical stimulation of the vagus nerve at a low intensity just above the threshold for afferents from SARs or rapidly-adapting pulmonary stretch receptors (RARs). (3) The orthodromic latencies (ranged from 1.9 to 2.5 ms) indicated that they receive direct inputs from low-threshold vagal afferents. (4) Unlike Ibeta neurons, they hardly responded to lung inflation and never exhibited tonic firing in response to maintained lung inflation. (5) The majority (92%) of them were antidromically activated by electrical stimulation of the cervical spinal cord. These deflation-sensitive inspiratory neurons clearly form a distinct group, and their firing pattern is consistent with the hypothesis that they receive inputs from RAR afferents as well as the central inspiratory drive. The results indicating that DRG inspiratory neurons are classified into at least three groups provide new insights into the organization and role of the DRG.