Discharge Identity of Medullary Inspiratory Neurons is Altered during Repetitive Fictive Cough

Abstract

This study investigated the stability of the discharge identity of inspiratory decrementing (I-Dec) and augmenting (I-Aug) neurons in the caudal (cVRC) and rostral (rVRC) ventral respiratory column during repetitive fictive cough in the cat. Inspiratory neurons in the cVRC (n = 23) and rVRC (n = 17) were recorded with microelectrodes. Fictive cough was elicited by mechanical stimulation of the intrathoracic trachea. Approximately 43% (10 of 23) of I-Dec neurons shifted to an augmenting discharge pattern during the first cough cycle (C1). By the second cough cycle (C2), half of these returned to a decrementing pattern. Approximately 94% (16 of 17) of I-Aug neurons retained an augmenting pattern during C1 of a multi-cough response episode. Phrenic burst amplitude and inspiratory duration increased during C1, but decreased with each subsequent cough in a series of repetitive coughs. As a step in evaluating the model-driven hypothesis that VRC I-Dec neurons contribute to the augmentation of inspiratory drive during cough via inhibition of VRC tonic expiratory neurons that inhibit premotor inspiratory neurons, cross-correlation analysis was used to assess relationships of tonic expiratory cells with simultaneously recorded inspiratory neurons. Our results suggest that reconfiguration of inspiratory-related sub-networks of the respiratory pattern generator occurs on a cycle-by-cycle basis during repetitive coughing.

Keywords: breathing; cough; expiratory; inspiratory; medulla; respiratory pattern generator.

Figure 1

(A) Neuronal spike times (top) and firing rate histograms (firing rate vs. time; binwidth = 75.0 ms; bottom) of two VRC cells recorded during a triple-cough response episode; the discharge identities of these cells did not change during this cough episode. (B) Firing rate histograms (binwidth = 47.5 ms) of two I-Dec neurons illustrating their change to an augmenting discharge pattern during C1 of the cough response; cell I-Dec^a returned to a decrementing pattern by cycle C2, but cell I-Dec^b maintained an augmenting pattern throughout the cough episode before returning to a decrementing discharge during the recovery period. Gray vertical bars indicate inspiratory phases; the difference in shade delineates the first half of inspiration from the second. E, eupneic respiratory cycles; C, cough cycles; R, recovery cycles; *, amplitude of integrated phrenic (lumbar) nerve activity is ≥2 SD above the mean maximum amplitude during eupneic period.

Figure 2

Average log ratios of I-Dec neurons during (A) single-, (B) double-, and (C) triple-cough episodes. E represents the log ratio of the total spike counts in the five eupneic breaths immediately prior to cough onset; similarly, R depicts the log ratio of the counts in the five recovery breaths immediately following the cough episode. *, Significantly different from E; #, significantly different from R; 1, significantly different from C1. P-values are indicated by one (p < 0.02) or two (p < 0.001) symbols. Number of neurons (n) analyzed for each cough episode type is indicated on the plots. Some neurons were evaluated for more than one type of cough response. Error bars indicate the standard error of the mean.

Figure 3

Average log ratios of I-Aug neurons during (A) single-, (B) double-, and (C) triple-cough episodes. 1, Significantly different from C1 (p < 0.02). Number of neurons (n) analyzed for each cough episode type is indicated on the plots. Some neurons were evaluated for more than one type of cough response. Error bars indicate the standard error of the mean.

Figure 4

Average log ratios of I-Dec neurons in the cVRC and rVRC. The first (C1) and second (C2) cough cycles from all repetitive cough episodes were analyzed. *, Significantly different from E; 1, significantly different from C1. P-values are indicated by one (p < 0.02) or two (p < 0.001) symbols. Error bars indicate the standard error of the mean.

Figure 5

Average phrenic burst amplitude (in arbitrary units, au) during single-, double-, and triple-cough episodes (top, middle, and bottom panels, respectively). Values for a given cough cycle (e.g., C1) were not different among these three cough response types. *Significantly different from E; #, significantly different from R; 1, significantly different from C1. P-values are indicated by one (p < 0.02) or two (p < 0.001) symbols. Error bars indicate the standard error of the mean.

Figure 6

Average inspiratory duration (s) during single-, double-, and triple-cough episodes (top, middle, and bottom panels, respectively). Values for a given cough cycle (e.g., C1) were not different among these three cough response types. *, significantly different from E; #, significantly different from R; 1, significantly different from C1. P-values are indicated by one (p < 0.02) or two (p < 0.001) symbols. Error bars indicate the standard error of the mean.

Figure 7

Correlations among pairs of VRC I and E neurons responsive to cough-eliciting tracheal stimulation. (A) Firing rate histograms (binwidths = 375.0 ms) for 6 of 16 simultaneously recorded neuron spike trains during a double-cough response to tracheal stimulation. For each trace, the respiratory-modulated pattern, cell identification number, and maximum firing rate are shown. *, Amplitude of integrated phrenic (lumbar) nerve activity is ≥2 SD above the mean maximum amplitude during eupneic period. (B) Features in the CCHs are included in the correlation linkage map shown in (D). Bin width for all CCHs = 0.5 ms. The minimum and maximum bin values, normalized to spikes per second per trigger event, are shown for each correlogram. Feature description, detectability index (equal to the ratio of the maximum amplitude of feature departure from background activity divided by the SD of the correlogram noise), half-width, and numbers of trigger neuron and target neuron spikes for each CCH are as follows: 1: central peak, 15.0, 4.0 ms, 55,253, and 92,800; 2: offset trough, 4.1, 1.0 ms, 92,800, and 51,671; 3: offset trough, 5.0, 2.5 ms, 92,800, and 57,087; 4: offset peak, 4.3, 0.5 ms, 51,671, and 57,087. (C) STAs of contralateral phrenic nerve activity. Bin width for both STAs = 0.5 ms. a: Neuron 57, peak (lag to peak = 7.5 ms). b: Neuron 55, dip (lag to nadir = 6.0 ms). (D) Correlation linkage map summarizing features found in all CCHs calculated for the group of four neurons in (B). Each large “sphere” represents a neuron and contains that neuron’s ID code and direction of change in firing rate when tracheal stimulation elicited a cough; the cell’s respiratory-modulated firing pattern is indicated by the color of the sphere (see Key). White, black, and gray circles at the ends of the lines between spheres represent central or offset peaks or troughs (see Key). Circled numbers indicate corresponding CCHs shown in (B).