Central chemoreceptor modulation of breathing via multipath tuning in medullary ventrolateral respiratory column circuits

Abstract

Ventrolateral respiratory column (VRC) circuits that modulate breathing in response to changes in central chemoreceptor drive are incompletely understood. We employed multielectrode arrays and spike train correlation methods to test predictions of the hypothesis that pre-Bötzinger complex (pre-BötC) and retrotrapezoid nucleus/parafacial (RTN-pF) circuits cooperate in chemoreceptor-evoked tuning of ventral respiratory group (VRG) inspiratory neurons. Central chemoreceptors were selectively stimulated by injections of CO(2)-saturated saline into the vertebral artery in seven decerebrate, vagotomized, neuromuscularly blocked, and artificially ventilated cats. Among sampled neurons in the Bötzinger complex (BötC)-to-VRG region, 70% (161 of 231) had a significant change in firing rate after chemoreceptor stimulation, as did 70% (101 of 144) of the RTN-pF neurons. Other responsive neurons (24 BötC-VRG; 11 RTN-pF) had a change in the depth of respiratory modulation without a significant change in average firing rate. Seventy BötC-VRG chemoresponsive neurons triggered 189 offset-feature correlograms (96 peaks; 93 troughs) with at least one responsive BötC-VRG cell. Functional input from at least one RTN-pF cell could be inferred for 45 BötC-VRG neurons (19%). Eleven RTN-pF cells were correlated with more than one BötC-VRG target neuron, providing evidence for divergent connectivity. Thirty-seven RTN-pF neurons, 24 of which were chemoresponsive, were correlated with at least one chemoresponsive BötC-VRG neuron. Correlation linkage maps and spike-triggered averages of phrenic nerve signals suggest transmission of chemoreceptor drive via a multipath network architecture: RTN-pF modulation of pre-BötC-VRG rostral-to-caudal excitatory inspiratory neuron chains is tuned by feedforward and recurrent inhibition from other inspiratory neurons and from "tonic" expiratory neurons.

Peripheral chemoreceptors tune inspiratory drive via tonic expiratory neuron hubs in the medullary ventral respiratory column network

“…Short-timescale peak or trough features with half-widths ranging from 0.5 ms to 100 ms (consistent with synaptic mechanisms in the VRC; Ott et al 2012) were identified using cycle-shifted surrogates (Ott et al 2012) or with a Monte Carlo test using surrogate spike trains (Pauluis and Baker 2000) with gamma-distributed interspike intervals. The shape parameter of the gamma distribution was estimated from the data (Miura et al 2006).…”

“…Changes in firing rates of single neurons during effective stimuli were identified over the entire respiratory cycle as well as during just the I and E phases by a bootstrap-based statistical method with the false discovery rate controlled to be Ͻ5% (Benjamini and Hochberg 1995) and classified into one of several response categories: increase (1), decrease (2), biphasic [increase-decrease (12) or decrease-increase (21)], or no change (¡) (Nuding et al 2009b). Neurons were also assessed for significant changes in the depth of respiratory modulation [i.e., "rate ratio" (↕): the ratio of the maximum to the minimum mean firing rate; see Ott et al 2012]. The numbers used to calculate the rate ratio were obtained by dividing each respiratory cycle within a response evaluation period and its corresponding prestimulus control period into 20 slices and measuring the mean firing rate in a 7-slicewide window for each of the 20 possible starting locations of the window, yielding 20 numbers for each cycle.…”

“…Correlation linkage maps were generated to graphically represent sets of detected cross-correlogram features among simultaneously monitored neurons ). Significant features in spiketriggered averages of full-wave rectified phrenic signals were identified (method adapted from Poliakov and Schieber 1998) with a two-sided Wilcoxon signed-rank test with Bonferroni correction (P Ͻ 0.05) (for details, see Ott et al 2012).…”

“…In the course of these prior studies, we identified crosscorrelation feature sets indicative of distributed functional inhibitory connectivity of t-E neurons within the column (Ott et al 2012;Segers et al 2012) and noted that some pairs of t-E neurons exhibited short-timescale spike synchrony. One aim of the present work was to confirm and extend that result.…”

“…First, baroreceptor-evoked reductions in inspiratory drive are associated with increased firing rates in t-E neurons that have inhibitory functional connections with inspiratory premotor or motor neurons (Lindsey et al 1998). Second, central chemoreceptor-evoked enhancement of inspiratory drive occurs during suppressed inspiratory-phase activity in t-E neurons (Ott et al 2012). Third, such disinhibitory amplification of inspiratory drive also occurs during a bout of coughing ).…”